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Thorough characterization of magnetic nanoparticles (MNP) is a demanding task. Still, due to growing usability and demand, there is a constant pressure to develop better means of MNP characterization, especially in a solution, which is a typical environment for majority of MNP applications. Techniques well developed so far are well suited to probe nanopowders or thin films, but are scarce in case of magnetic nanoparticles embedded in a matrix, due to short probing distance, weak selectivity, or incompatibility of method’s characteristic time and MNP’ relaxation time.&lt;br/&gt;It has been shown that RIXS-MCD (Magnetic Circular Dichroism in Resonant Inelastic X-ray Scattering) is a sensitive, element selective probe of structure, environment and magnetic moment of transition metals in solids, nanomaterials and frozen solutions of MNP [1-3]. 1s2p RIXS-MCD is a high-resolution second-order optical transition spectroscopy (K absorption followed by Kα relaxation) resulting in dichroism effect comparable to L-edge XMCD. Any material embedded in a hard X-ray transparent matrix can be studied volumetrically with weak self-absorption. By the use of bend crystal analyzers Ka emission can be recorded free of background, omitting constituents not containing studied element, what results in a high sensitivity often required for nanoparticles in solutions. Well-resolved spectral features in the K pre-edge energy range holds information on ion coordination and MCD signal in that region can be used to quantitatively determine magnetic moment projection onto the beam/magnetic field direction. MNP solution magnetization can be measured free of diamagnetic or paramagnetic contributions from carrier or chemical additives in the solution, what is especially important for the evaluation of the smallest nanoparticles that are very hard to magnetize and easy to disregard as background (fig. 1) [4].&lt;br/&gt;We demonstrate applicability of RIXS-MCD to study structural and magnetic state of ferrites nanoparticles being synthesized via decomposition reaction starting from pure precursors. Growth of MNP by means of thermal decomposition of iron/cobalt acetylacetonates syntheses were studied on-the-fly both on iron and cobalt edges using a special cell designed for &lt;i&gt;in-situ&lt;/i&gt; synchrotron experiments on solutions (fig.2) [5]. Recorded spectra are used to discern main components taking part in the MNP growth process and to follow evolution of local atomic coordination and valence of transition metal ions. Development of magnetization in iron and cobalt ferrites MNP are linked to changes in atomic structure and chemical state. RIXS-MCD supported by ex-situ characterization (TEM, SAXS, Mössbauer spectroscopy) provides a comprehensive characterization of magnetic nanoparticles in solution. It opens a new pathway for controlled synthesis of MNP with defined magnetic response.

**References:**

1. A. Juhin, A. Lopez-Ortega, M. Sikora, et al., Nanoscale, Vol. 6, 11911 (2014)&lt;br/&gt;2. N. Daffé, M. Sikora, M. Rovezzi, et al., Advanced Materials Interfaces, Vol. 4, 1700599 (2017)&lt;br/&gt;3. N. Daffé, J. Zečević, A. Juhin, et al., Nanoscale, Vol. 12, 11222 (2020)&lt;br/&gt;4. J. Kuciakowski, M. Sikora, D. Koziej, et al., Nanoscale, Vol. 12, 12640 (2020)&lt;br/&gt;5. M. Staniuk, O. Hirsch, N. Kränzlin, et al., Chemistry of Materials, Vol. 26, p. 2086-2094 (2014)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/9eebf768e8201652248821acd3ad6a19.jpg)
Fig. 1. Magnetization profiles of Zn&lt;sub&gt;x&lt;/sub&gt;Fe&lt;sub&gt;3-x&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; MNP in solution measured using VSM (black squares) and RIXS-MCD (orange circles). Lines correspond to VSM signal corrected for diamagnetic background using different approaches [4].

![](https://assets.underline.io/uploads/markdown_image/1/image/4c8a57800b76886a7ecb28cd4308d35c.jpg)
Fig. 2. Sketch of RIXS-MCD experimental setup with reaction cell for in-situ experiments on solutions [5].

INTERMAG 2021

RIXS-MCD as selective in-situ probe of structure and magnetization of nanoparticles throughout synthesis

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Thorough characterization of magnetic nanoparticles (MNP) is a demanding task. Still, due to growing usability and demand, there is a constant pressure to develop better means of MNP characterization, especially in a solution, which is a typical environment for majority of MNP applications. Techniques well developed so far are well suited to probe nanopowders or thin films, but are scarce in case of magnetic nanoparticles embedded in a matrix, due to short probing distance, weak selectivity, or incompatibility of method’s characteristic time and MNP’ relaxation time. It has been shown that RIXS-MCD (Magnetic Circular Dichroism in Resonant Inelastic X-ray Scattering) is a sensitive, element selective probe of structure, environment and magnetic moment of transition metals in solids, nanomaterials and frozen solutions of MNP [1-3]. 1s2p RIXS-MCD is a high-resolution second-order optical transition spectroscopy (K absorption followed by Kα relaxation) resulting in dichroism effect comparable to L-edge XMCD. Any material embedded in a hard X-ray transparent matrix can be studied volumetrically with weak self-absorption. By the use of bend crystal analyzers Ka emission can be recorded free of background, omitting constituents not containing studied element, what results in a high sensitivity often required for nanoparticles in solutions. Well-resolved spectral features in the K pre-edge energy range holds information on ion coordination and MCD signal in that region can be used to quantitatively determine magnetic moment projection onto the beam/magnetic field direction. MNP solution magnetization can be measured free of diamagnetic or paramagnetic contributions from carrier or chemical additives in the solution, what is especially important for the evaluation of the smallest nanoparticles that are very hard to magnetize and easy to disregard as background (fig. 1) [4]. We demonstrate applicability of RIXS-MCD to study structural and magnetic state of ferrites nanoparticles being synthesized via decomposition reaction starting from pure precursors. Growth of MNP by means of thermal decomposition of iron/cobalt acetylacetonates syntheses were studied on-the-fly both on iron and cobalt edges using a special cell designed for in-situ synchrotron experiments on solutions (fig.2) [5]. Recorded spectra are used to discern main components taking part in the MNP growth process and to follow evolution of local atomic coordination and valence of transition metal ions. Development of magnetization in iron and cobalt ferrites MNP are linked to changes in atomic structure and chemical state. RIXS-MCD supported by ex-situ characterization (TEM, SAXS, Mössbauer spectroscopy) provides a comprehensive characterization of magnetic nanoparticles in solution. It opens a new pathway for controlled synthesis of MNP with defined magnetic response.

**References:**

1. A. Juhin, A. Lopez-Ortega, M. Sikora, et al., Nanoscale, Vol. 6, 11911 (2014) 2. N. Daffé, M. Sikora, M. Rovezzi, et al., Advanced Materials Interfaces, Vol. 4, 1700599 (2017) 3. N. Daffé, J. Zečević, A. Juhin, et al., Nanoscale, Vol. 12, 11222 (2020) 4. J. Kuciakowski, M. Sikora, D. Koziej, et al., Nanoscale, Vol. 12, 12640 (2020) 5. M. Staniuk, O. Hirsch, N. Kränzlin, et al., Chemistry of Materials, Vol. 26, p. 2086-2094 (2014)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/9eebf768e8201652248821acd3ad6a19.jpg)
Fig. 1. Magnetization profiles of ZnxFe3-xO4 MNP in solution measured using VSM (black squares) and RIXS-MCD (orange circles). Lines correspond to VSM signal corrected for diamagnetic background using different approaches [4].

![](https://assets.underline.io/uploads/markdown_image/1/image/4c8a57800b76886a7ecb28cd4308d35c.jpg)
Fig. 2. Sketch of RIXS-MCD experimental setup with reaction cell for in-situ experiments on solutions [5].

technical paper

Investigations the current-driven domain wall (DW) motion in cylindrical nanowires (CNWs) are more valuable if performed at zero applied external field for their use in prospective electronic devices. Here, we present an all-magnetoelectrical method to pin DWs in multi-segmented CNWs, maintain them pinned at zero field and de-pin them using current pulses. We test this in compositionally modulated nickel/cobalt (~ [10/14] μm) and nickel/cobalt/nickel (~ [3/7/3] μm) CNWs of c.a. 150 - 190 nm diameter where DWs can be pinned in a predictable and reproducible way and their soft/hard magnetic texture has been fairly studied [1]–[3]. Furthermore, we perform micromagnetic simulations to interpret and study the behavior of the magnetoresistance as a function of segment length. The method is based on the anisotropic magneto-resistance (AMR) measurement of a CNW with a pinned DW [4] for which we show the relevant range of one branch in Figure 1 (solid line). This branch starts from a saturated magnetization state in the “right” direction (3000 Oe, not shown) and moves to a saturated state in the “left” direction (-3000 Oe, not shown), as indicated by the arrow at the bottom. The CNW’s long axis is parallel to the applied field. Simplifying, AMR gives an overall measure of the portion of the CNW’s magnetization that is not along the direction of the applied electrical current: at ±3000 Oe, most of the CNW’s magnetization is in the same direction as the electrical current, i.e., along the CNW’s long axis, resulting in a saturated high resistance state. As the field decreases, magnetization vortexes appear at the ends of the CNW, driving magnetization away from the axial direction and resulting in a low resistance state. When the field is ramped in the opposite direction, a DW propagates from one of the ends and is pinned at some point within the nickel segment [1], increasing the axial component of the magnetization as indicated by the rapid increase in resistance in Figure 1 (“Domain wall pinning”). Finally, when the field is further increased, the DW de-pins and propagates to the opposite end from where it was initially nucleated leaving most of the magnetization in the axial direction, as indicated by the second resistance change in Figure 1 (“Domain wall de-pinning”). Note that pinned DWs and vortexes at the ends of the CNW lower the resistance as they are mainly composed of magnetization textures out of axis. To test current-driven DW motion at zero field, a DW has to be pinned first by saturating the magnetization to +3000 Oe, decreasing the field, inverting it and increasing it only until the first resistance jump is reached (red, open squares in Figure 1). After this, the field is reduced to 0 (blue, open triangles in Figure 1), at which point current pulses are applied. To verify whether the DW is still pinned after reducing the field to zero and/or that a current pulse has de-pinned it, the field is ramped up again in the negative direction (towards -3000 Oe). If the DW had de-pinned as a consequence of reducing the field, the CNW’s magnetization would return to a “right” state and the domain wall pinning resistance jump would be registered again, which we never observed. If the DW remained pinned, even after the application of a pulse, only the DW de-pinning resistance jump was observed, as seen in Figure 2 (solid red squares), where a current pulse of 9.21x1011 A/m2 and 10 ns width was applied. On the other hand, if a current pulse de-pinned the DW, neither of the resistance jumps would be observed, as seen in Figure 2 (solid blue triangles), where a pulse of 1.38x1012 A/m2 and 20 ns width was used. This method resembles recent ones in the sense that the state of the CNW is known after the current pulse is sent, for example using MFM [5] or TEM [6]. As the DW is de-pinned at high enough current densities, the width of the pulse can be used to estimate the DW speed by successively reducing the pulse width until no de-pining event is found. With this, the velocity is estimated as the length of the cobalt segment over the current pulse width. We find that the de-pinning event is dependent on the direction of the applied current and that the magnetoresistance value for pinned domain walls increases as the length of the segments increases, which is consistent with our simplified view of AMR. This suggests that, for long enough CNWs, the DW width remains constant as the segment length increases, which we also confirmed with micromagnetic simulations. We believe this method can be used to study current-induced DW de-pinning at zero applied field in soft/hard magnetic CNWs with known magnetic configurations.

**References:**

[1] Y. P. Ivanov, et al, ACS Nano, vol. 10, no. 5, pp. 5326–5332 (2016) [2] S. Lopatin, Y. P. Ivanov, J. Kosel, and A. Chuvilin, Ultramicroscopy, vol. 162, pp. 74–81 (2016) [3] C. Bran et al., ACS Nano, vol. 14, no. 10, pp. 12819–12827, (2020) [4] H. Mohammed, E. V. Vidal, Y. P. Ivanov, and J. Kosel, IEEE Trans. Magn., vol. 52, no. 7 (2016) [5] M. Schöbitz et al., Phys. Rev. Lett., vol. 123, no. 21, pp. 1–5 (2019) [6] Y. P. Ivanov, et al, ACS Appl. Mater. Interfaces, vol. 9, no. 20, pp. 16741–16744 (2017)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/58702b89496d700156b28fb30ad38e88.jpg)
Figure 1. Anisotropic magneto-resistance measurement of a cylindrical nanowire with a pinned domain wall rescaled to the domain wall pinning and de-pinning field range and the subsequent reduction of external field to zero where nanosecond square current pulses are applied.

![](https://assets.underline.io/uploads/markdown_image/1/image/fbc20f594c582e2ebe95d27127378af1.jpg)
Figure 2. Magnetoresistance profile after two different current pulses are applied at zero field: 9.21x1011 A/m2, 10 ns width (red squares), the de-pinning resistance jump indicates the domain wall was not de-pinned; 1.38x1012 A/m2, 20 ns (blue triangles) results in the de-pinning of the DW as the depinning resistance jump is not observed.

Anisotropic Magnetoresistance Zero-Field Domain Wall Depinning in Cylindrical Nanowires

I Introduction The iron core residual flux density is one of the two reasons that cause the no-load closing of the large power transformer to generate magnetizing inrush current[1]. In order to effectively weaken the residual flux density to reduce the occurrence probability of magnetizing inrush current, it is necessary to measure the magnitude and direction of the residual flux density. In the existing research, the empirical estimation method[2] and the voltage integration method[3] are mainly used to estimate the residual flux density in the core, but these methods have some limitations in the residual flux measurement of large power transformer. Therefore, the accurate measurement of the residual flux density in the core is of great theoretical significance for the efficient operation of the whole power grid. The main purpose of this paper is to study the most effective method to measure the residual flux density of transformer. II Theoretical analysis of measurement methods When the transformer core structure is fixed, the excitation inductance is mainly affected by differential permeability. Residual flux will lead to the change of core differential permeability. By using the method of field-circuit coupling, the magnetic field problem with closed magnetic circuit structure is transformed into the circuit problem with equivalent inductance. Based on the electromagnetic transient characteristics of iron core materials, this paper proposes a residual flux density measurement method of externally applied voltage. The residual flux results in different transient current waveforms under positive and reverse voltage applied in the windings. The relation expression between the extreme value of transient current difference and residual flux is derived theoretically. In order to improve the accuracy of residual flux density measurement, modeling and analysis were carried out on the measured square core, and a set of appropriate test circuit parameters were optimized based on the differential permeability characteristics of the core material. It can be seen from Fig.1 that the transient current differential extremum parameter ts, Δ Imax change with residual flux changes obviously, but the parameter ts is more sensitive to the change of residual flux in the core. Therefore, ts is selected as the basis for quantitative detection of residual flux. III Conclusion By using the finite element method, the effects of different circuit parameters on the transient process can be compared, and some valuable conclusions are obtained, which will provide a theoretical reference for the subsequent application of the measurement method. Finally, experimental verification was carried out on the measured core, and the estimated residual flux in the core was compared with the actual value. As shown in Figure 2, the maximum measurement error did not exceed 5.4%. Therefore, it was considered that the measurement method proposed in this paper was reliable. This method can be used to test the residual flux of large power transformer without any requirement for transformer, which laid a foundation for the quantitative detection of residual flux in transformer core.

**References:**

[1] Y. Wang, Z. Liu and H. Chen, "Research on Residual Flux Prediction of the Transformer", IEEE Transactions on Magnetics, Vol. 53, no. 6, pp. 1-4, June 2017. [2] Colombo E, Santagostino G. "Results of the enquiries on actual network conditions when switching magnetizing and small inductive currents and on transformer and shunt reactor saturation characteristics", Electra, 1984, 94: 35—53. [3] W. Wei, Y.Liu, S.W.Mei and et al. "Study on Residual Flux Evaluation Method Based on Variable-Regional Integral during the Voltage Attenuation Process," 2019 IEEE Innovative Smart Grid Technologies - Asia (ISGT Asia), Chengdu, China, 2019, pp. 954-959.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ffc6d5c2761c3b560b36db929ce09c40.jpg)
Fig.1 The curves of transient current difference extremum parameters versus Br

![](https://assets.underline.io/uploads/markdown_image/1/image/8fcc1ffc5fecf23644df6c2889571d89.jpg)
Fig.2 The comparison of experimental residual flux density and estimated residual flux density

Residual Flux Measurement of Power Transformer Based on Transient Current Difference

Spin transfer torque magnetic random-access memory (STT-MRAM) is one of the promising non-volatile memory (NVM) technology resolving scalability issue, current consumption, and current leakages of traditional transistor-based memories. Due to the intrinsic STT-switching stochasticity and thermal fluctuation dependance of MRAM technology, potential transitions and retention faults may happen during chip operating life [1]. Consequently, each bit writing and reading should be tested many times (&gt; 106 iterations) to quantify the fault event occurrence, and validate that each fault is well detected and addressed by the error correction code (ECC) algorithms. For large memory arrays (&gt;MB), non-persistent faults with ppm occurrences are difficult to test. Extrapolating the transition faults of large array in 10 years operation within few microseconds is challenging and does require the use of faults acceleration factors to limit the testing time [2], as well as the use of on-chip BIST (Built In-Self-Test) circuitry with algorithms optimized for testing MRAM arrays at higher speed than with external electrical tester. The MRAM switching behavior dependance with temperature is usually used to compress the retention time through application of weak but long disturb current [1,4], and application of temperature while testing [3]. Although these approaches are helping in reducing the retention testing time for each bit cell of the array, it is still very problematic for addressing full testing of large arrays. As example, the testing of each bit using different proposed algorithms leads to test time of 0.5 to 3 sec per bit cell, which is way too long for large array testing [1-4]. To further improve testing time, it is interesting to apply a magnetic field as an additional acceleration factor, as applying an offset field in opposite direction to the anisotropic field of free layer will help in further compressing the retention [2]. Importantly, testing MRAM arrays under magnetic field also allows to evaluate the immunity of the bit cells to parasitic external field [5]. The difficulty for testing MRAM arrays at wafer level for such purpose is the need of using an external magnetic field, perpendicular to the wafer and covering the entire array surface area in the range of 1 to 10mm2 while testing electrically at high temperature. It requires both a good field strength and uniformity on the covered area. In this presentation, we propose a new magnetic head designed for the testing of large arrays, aiming in both optimizing the identification of non-persistent faults related to STT-switching stochasticity and reducing retention testing time. The system can also be used for evaluating bit cells persistence to magnetic interference through magnetic disturbance test. The proposed magnetic test head system is integrating a new 5-poles design from Hprobe proprietary 3D magnetic generator. In test operation, the magnetic head is docked on the head plate of 300mm wafer probing station (figure 1) with the probe card placed underneath. The system is driven by external software interface or operating with triggered signals enabling field application synchronized with electrical measurements. The test head integrates two robotic systems enabling three modes of operations. (i) Field calibration mode to generate and optimize the field patterns to be used in testing; (ii) testing mode for electrical probing and under static or variable magnetic field; and (iii) field monitoring mode to verify that the field stimulus is under the specification limits required by application. A hexapod robot is handling the electro-magnet with six degrees of freedom (X, Y, Z, roll, pitch and yaw). Other two axis system is used for controlling the position of the 3D hall sensor used to sense the magnetic field, scanning the entire area of the array in 3D with step resolution of less than 10µm. The system enables to work with standard probe cards having hundreds of needles and is designed to do the field calibration at the exact DUT position above the chuck. STT-MRAM anisotropy is oriented perpendicular to the wafer, and compression scheme for non-persistent fault testing requires magnetic field strength at the upper range of the free layer switching field (100-400mT). The proposed system can generate a perpendicular field up to 700mT (Fig.1) and is sufficient for time acceleration testing of bit cell switching and retention, as well as for field immunity testing. The last would be beneficial to be made also by applying disturbance field in the planar direction to the wafer, the system can generate 3D fields and a maximum planar field amplitude of 100mT is obtained. The achieved non-uniformity is less than 5% over an area above 5mm2 (Fig.2), which is enough to cover arrays of &gt;64Mb considering an average density of &lt; 0.08 mm2/Mb [6].

**References:**

[1] H. Naeimi et al., “STTRAM scaling and retention failure,”Intel Technology Journal, vol. 17, 2013, https://www.intel.com/ content/dam/www/public/us/en/documents/research/2013-vol17-iss-1n-intel-technology-journal.pdf#page=54. [2] Wu, Lizhou & Taouil, M. & Rao, Siddharth & Marinissen, Erik & Hamdioui, Said. (2020). Survey on STT-MRAM Testing: Failure Mechanisms, Fault Models, and Tests. (https://arxiv.org/abs/2001.05463) [3] A. Iyengar, S. Ghosh, and S. Srinivasan, “Retention testing methodology for STTRAM,” IEEE Design & Test, vol. 33, pp. 7–15, 2016, doi:10.1109/MDAT.2016.2591554. [4] I. Yoon and A. Raychowdhury, “Test challenges in embedded STTMRAM arrays,” in Int. Symp. Quality Electronic Design, Mar. 2017, doi:10.1109/ISQED.2017.7918289. [5] Y. Chih et al., "13.3 A 22nm 32Mb Embedded STT-MRAM with 10ns Read Speed, 1M Cycle Write Endurance, 10 Years Retention at 150°C and High Immunity to Magnetic Field Interference," 2020 IEEE International Solid- State Circuits Conference - (ISSCC), San Francisco, CA, USA, 2020, pp. 222-224, doi: 10.1109/ISSCC19947.2020.906295 [6] S.H. Han et al, “28-nm 0.08 mm2/Mb Embedded MRAM for Frame Buffer Memory”, in International Electron Device Meeting 2020

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/67ac20f6011a1a46800ed12d99483a54.jpg)
Fig. 1 – Perpendicular field versus the gap between 3D generator and DUT and an illustration of the test head mounted on wafer probing station

![](https://assets.underline.io/uploads/markdown_image/1/image/fde7b8a2d8ec18f3f26560cab9038494.jpg)
Fig. 2 – Field area uniformity (at 4mm gap) of perpendicular field

System for testing non-persistent switching and retention faults of STT-MRAM arrays

Magnetic susceptibility methods particularly, AC susceptibility measurement have shown high potential to provide a fast characterization and standardization technique of magnetic nanoparticles (MNPs) [1]–[4]. Depending on the applications, AC magnetometers have been developed to measure magnetic responses from MNPs where Neel and Brown relaxation parameters are mostly characterized and utilized in these applications. From the acquired Brownian and Neel relaxation, physical and magnetic properties of the MNPs can be estimated such as the hydrodynamic size and magnetic anisotropy energy ratio. However, the characterization frequency range of the Neel and Brownian relaxations plays an important role where a wider frequency range may reveal further information on the MNP properties. In this study, to realize a simple, wideband, and sensitive AC magnetometer for bio-sensing applications and characterization of magnetic nanoparticles (MNPs), we report the improvements in the detection unit of our previously developed induction-based AC magnetometer [5] for achieving a larger usable range of the detection frequency. In order to obtain a linear sensitivity response, the induction coil is typically operated below its self-resonance frequency where its inductive property becomes dominant and not affected by the parasitic capacitance. This self-resonance frequency is governed by the coil’s inductance and parasitic impedance, and increasing the self-resonance frequency by reducing the coil’s inductance will extend the usable frequency range of the coil. However, it should be noted that there is a trade-off between the coil’s usable frequency range and sensitivity where an increase of the self-resonance frequency may reduce the coil’s sensitivity at the low-frequency region. In the attempt to achieve a wide frequency range yet sufficiently sensitive pickup coil, we compared the sensitivity and usable frequency range of 6 different designs of pickup coils by investigating the effect of inductance and parasitic capacitance. The number of turns and coil section separation were varied from 400 to 200 turns, and 1 to 4 sections, respectively. We found that the usable frequency range was greatly affected by the pickup coil’s inductance due to the self-resonance phenomena compared to parasitic capacitance. A low noise amplification circuit was designed based on two AD8429 instrument amplifiers and fabricated on a 2-layer printed circuit board as shown in Fig. 1 (a). For the field excitation unit, the excitation coil was designed to produce a field uniformity of more than 90% and constructed using a Litz wire with 60 strands of 0.1-mm Cu wire. The acquisition of the magnetization signal was realized by implementing a generalized Goertzel algorithm to achieve fast signal and phase extractions [6] in comparison to the conventional use of Fast Fourier Transform and lock-in amplifier techniques. As a result, the developed magnetometer showed a sensitivity of 10-8 Am2/sqrt(Hz) at 6 Hz and a frequency range of up to 158 kHz. We then demonstrated the developed AC magnetometer by evaluating the effect of viscosity on the frequency response of thermally blocked, single-core nanoparticles (SHP30, Ocean Nanotech, USA). The viscosity of the carrier liquid was varied by different weights of glycerol mixed in a constant volume of purified water and the Fe-concentration was fixed at 0.5 mg/mL. The excitation frequency was swept from 5 Hz to 158 kHz at a maximum field of 0.55 mTpp to obtain discrete 51 measurement points within the acquisition time of 5 minutes. As a result, the peak for the imaginary part was observed to be shifted towards lower frequency values when the wt/V% of the glycerol solution was increased. From the real and imaginary magnetization curves, the hydrodynamic size and the average anisotropy energy ratio s were estimated to be 60.6 nm and 25, respectively. It can be expected that the developed AC magnetometer can be a valuable tool in providing a simple, fast, and reliable assessment of MNPs for bio-sensing applications. Acknowledgments This work was supported by the Ministry of Higher Education of Malaysia under the grant number of FRGS/1/2019/TK04/UMP/02/4 (RDU1901154), the Research Management Center of Universiti Malaysia Pahang under the grant number of RDU1903100, and the “Strategic Promotion of Innovative R&D” of the Japan Science and Technology Agency (JST).

**References:**

[1] J. Wells et al., “Standardisation of magnetic nanoparticles in liquid suspension,” J. Phys. D. Appl. Phys., vol. 50, no. 38, p. 383003, Sep. 2017, doi: 10.1088/1361-6463/aa7fa5. [2] P. Schier et al., “European Research on Magnetic Nanoparticles for Biomedical Applications: Standardisation Aspects,” Curr. Trends Biomed. Eng. Bioimages Anal. PCBEE 2019. Adv. Intell. Syst. Comput., vol. 1033, no. May, pp. 316–326, 2020, doi: 10.1007/978-3-030-29885-2_29. [3] M. M. Saari et al., “Influence of Viscosity on Dynamic Magnetization of Thermally Blocked Iron Oxide Nanoparticles Characterized by a Sensitive AC Magnetometer,” J. Supercond. Nov. Magn., vol. 32, no. 9, pp. 2765–2772, Sep. 2019, doi: 10.1007/s10948-019-5031-6. [4] S. Ota et al., “Effects of size and anisotropy of magnetic nanoparticles associated with dynamics of easy axis for magnetic particle imaging,” J. Magn. Magn. Mater., vol. 474, no. September 2018, pp. 311–318, 2019, doi: 10.1016/j.jmmm.2018.11.043. [5] M. M. Saari, N. S. Suhaimi, N. A. C. Lah, K. Sakai, T. Kiwa, and K. Tsukada, “A Sensitive AC Magnetometer using A Resonant Excitation Coil for Magnetic Fluid Characterization in Nonlinear Magnetization Region,” in 2018 IEEE International Magnetics Conference (INTERMAG), Apr. 2018, pp. 1–4, doi: 10.1109/INTMAG.2018.8508784. [6] P. Sysel and P. Rajmic, “Goertzel algorithm generalized to non-integer multiples of fundamental frequency,” EURASIP J. Adv. Signal Process., vol. 2012, no. 1, p. 56, Dec. 2012, doi: 10.1186/1687-6180-2012-56.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/31298569e8861862e56ebd7954d7d29b.jpg)
Fig. 1. a) Diagram of the developed AC magnetometer. b) The arrangement and configuration of the excitation coil with respect to the sample and pickup coil.

![](https://assets.underline.io/uploads/markdown_image/1/image/250b9fca4d1de27aa2318159dbd55abe.jpg)
Fig. 2. (a) The normalized real and imaginary susceptibilities of the SHP30 particles at different glycerol wt/V % in the solutions with respect to the frequency. (b) The ratio between the imaginary and real susceptibilities of the SHP30 solutions with respect to the frequency normalized by each Brownian frequency fB.

Towards A Wideband Induction-Based AC Magnetometer for A Fast Characterization of Magnetic Nanoparticles.

Magnetic nanoparticles are powerful tools in biomedical applications, where they are employed in both diagnosis and therapy. A prerequisite for the efficiency of these applications are precisely characterized particles. To this end, several magnetic measurement techniques are established. They all have in common that they measure the magnetic response of the particles exposed to an externally applied magnetic field, which may change the aggregation state of the particles [1]. To overcome this limitation, the method of Thermal Noise Magnetometry (TNM) has been developed to characterize magnetic nanoparticle ensembles without the use of an external magnetic excitation [2]. Amplitude variations in the net magnetic moment of the sample are measured, which are caused by thermal fluctuations in the system. Rather than measuring the dissipation or impedance in the system as a result of the application of the external field, the thermal fluctuations are studied in the absence of the external excitation. The dissipation-fluctuation theorem describes the accordance between the conventional dissipation characterization techniques such as Magnetorelaxometry (MRX) or AC-Susceptibility (ACS) measurements [4, 5], and the fluctuation-based TNM technique. The total switching rate of the magnetic moments depends on volume, anisotropy, aggregation of the particles and the viscosity and temperature of the suspension. TNM measurements have been proven to be feasible, and complementary to other characterization techniques due to its diminutive impact on the sample [2, 5]. In this contribution we present a numerical framework which we use to mirror our TNM measurements, in order to further establish this characterization method. First, we numerically investigate and explain the dependence of the stochastic TNM signal on basic parameters such as the number of particles and their volume. The linear dependence on the number of particles is experimentally verified. Next, we designed a sample holder geometry by solving an optimization problem, maximizing the noise power in our setup (see Fig. 1). Its performance is compared to that of a cylindrical sample holder, typically used for MRX measurements. This was done by measuring the noise power as a function of the sample distance from the detector (i.e. the depth profile of the experimental setup), both computationally and experimentally (Fig. 2). We obtained an increase in noise signal by a factor of 3.5 for the optimized sample holder, while the volume of the sample decreased by more than half. The superior signal amplitude of our optimized sample holder and the match between computed and measured depth profile demonstrate the accuracy and importance of our TNM simulations. The theoretical framework allows us to study aspects of TNM which are hard to accomplish experimentally, making it possible to visualize them and gain insight in the experiment. Our results therefore contribute to the further establishment of TNM as a magnetic nanoparticle characterization method.

**References:**

[1] Myrovali, E. et al., Sci. reports, Vol. 6, p. 37934 (2016) [2] Leliaert, J. et al., Appl. Phys. Lett., Vol. 107, p. 222401 (2015) [3] Wiekhorst, F., Steinhoff, U., Eberbeck, D. & Trahms, L., Pharm. Res., Vol. 29, p. 1189 (2012) [4] Ludwig, F., Guillaume, A., Schilling, M., Frickel, N. & Schmidt, A. M., J. Appl. Phys., Vol. 108, p. 33918 (2010) [5] Leliaert, J. et al., J. Phys. D: Appl. Phys., Vol. 50, p. 085004 (2017)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/7ea209e345c17a0e6c8b74d2e36a0635.jpg)
The geometry for the regular cylindrical and the optimized 200 µl sample in the experimental setup

![](https://assets.underline.io/uploads/markdown_image/1/image/a05936f4f0f2c5859de5571080a6414a.jpg)
The measured and simulated noise power as a function of the depth in the warm bore of both sample geometries

Thermal noise power properties of magnetic nanoparticles: theory and experiment

Modern MRAM technology relies on the magnetic tunnel junction (MTJ) consisting of the state-of-the-art CoFeB/MgO heterostructure. Although current studies support the importance of this heterostructure to the technology, the concern of MgO dielectric breakdown limits MRAM lifetime. We found that inserting an ultra-thin Mg (0.4nm) layer between CoFeB and MgO could promote lifetime and change polarity dependence on applied voltage (Fig.1). However, it traded off the perpendicular magnetic anisotropy and tunneling magneto-resistance. In this work we visualized the modified CoFeB/MgO with Mg insertion against dielectric breakdown by using an in-situ x-ray absorption spectroscopy technique. We find this technique very helpful to understand the origin of interface reconstruction in association with the changes of energy barrier-height and electronic occupation, which are invisible but essential to the MTJ’s performance. This is due to the uniqueness of the element-/orbital specificity and the real-time design of the setup. We established a bridge correlating the heterostructure’ microscopic and macroscopic properties, by cross-utilizing the x-ray technique with spin transport measurements and atomistic magnetic simulation (Vampire). This thus offers us comprehensive insights which may improve contact engineering of the MTJ technology.

**References:**

[1] Van Beek, Simon, et al. "Impact of processing and stack optimization on the reliability of perpendicular STT-MRAM." 2017 IEEE International Reliability Physics Symposium (IRPS). IEEE (2017). [2] Van Beek, Simon, et al. "Understanding and empirical fitting the breakdown of MgO in end-of-line annealed MTJs." 2020 IEEE International Reliability Physics Symposium (IRPS). IEEE (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/706c071b51b55198e8156662bc6a6fd7.jpg)
Fig. 1: Welbull distribution versus breakdown voltage for the pMTJ stack consisting of (a) SAF/CoFeB(8Å)/MgO(10Å)/CoFeB(11.5Å)/Ta(3Å) (referred to w/o Mg sample), (b) SAF/CoFeB(8Å)/MgO(10Å)/Mg(4Å)/CoFeB(11.5Å)/Ta(3Å) (referred to with Mg sample). Blue and red data correspond to positive and negative voltages applied to the MTJ, respectively. SAF refers to synthetic antiferromagnetic layer which consists of [Co(4Å)/Pt(3Å)]7/Co(6Å)/Ru(8Å)/[Co(4Å)/Pt(3Å)]3/Co(4Å). β and η refer to slope parameter and extrapolated breakdown voltage, respectively.

Virtualizing CoFeB/MgO Reconstruction Effects on the STT-pMTJ's Performance

We present a simple device that measures the optical transmission of a suspension of magneto-tactic bacteria under application of a magnetic field. The system consists of a measurement head with a RGB LED, a photodetector and three sets of orthogonal coils that generate a field up to 3.5 mT. In addition we realized a module based on an ESP32 microprocessor that sets the coil currents, performs data collection, and features a touchscreen and Wifi access. Source files such as source code for the software as well as links to the designs for hardware (3D prints, circuit boards) are available online at https://github.com/LeonAbelmann/MagOD, so that anyone with average electronic skills (for instance EE students) can construct such a magnetic Optical Density meter (MagOD) for under 1000 Eu. We demonstrate to application of the MagOD system on two types of magneto-tactic bacteria. Magnetospirillum bacteria (MSR-1) are elongated and have very different scattering properties depending on the direction of the incident light. Figure 1 shows that there is a clear change in transmitted light when the magnetic field is rotated from parallel to perpendicular to the light path. The signal difference (red lines) is a measure for the fraction of magnetic bacteria in the culture. Since the MagOD meter measures at data acquisition rates up to 800 Hz, we can also time-resolve the rotation of the bacteria (blue lines). The time constant for rotation is a measure for the ratio between bacteria length and their magnetic moment. Unlike MSR-1, which swim along the magnetic field lines but reverse frequently in direction, Magnetococcus bacteria (MC-1) swim in one direction only. This allows us to perform marathon experiments, as shown in figure 2. First, the bacteria are collected at the bottom of the container by a vertical magnetic field. After reversal of the field, the entire groups swim up and passes the photodetector after some time. From the reduction in transmission, we can obtain the distribution in swimming velocity. These are just two examples of the wealth of information one can obtain with such an inexpensive system. Not all possibilities have been explored yet. Next to magneto-tactic bacteria, also suspensions of magnetic nanoparticles can be analyzed with respect to sedimentation and clustering. Since the LED used for the light beam has three different wavelengths, which can be individually tuned in intensity, simple spectral analysis methods can be applied. Moreover next to transmitted light also the scattered light can be detected. Since the housing is 3D printed, other geometries can be easily realized. By making the source code available, we hope that researchers in other fields will make use of and expand on our efforts.

**References:**



**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/34803d5b19fbdfb0bfe99155bb581fe1.jpg)
Fig 1. The scattering properties of magnetospirillum bacteria is dependent on their orientation with respect to the incident light beam. By rotating the magnetic field, we can obtain information on the amount of magnetic bacteria in the suspension from the amplitude of the signal (Cmag) and information on the magnetic moment from the time response (gamma).

![](https://assets.underline.io/uploads/markdown_image/1/image/7ceb7ab967a7f5ec83236c2905f337fb.jpg)
Fig. 2. Magnetococcus bacteria can be sent to the bottom of the container by a vertical magnetic field. When the field is reversed, the entire group swims up (like in a marathon). From the resulting dip in transmission, we can determine the velocity distribution of the bacteria.

Analysis of magneto-tactic bacteria using an open-source optical density meter

Electronic structure and magnetism of complex materials have been a field for decades extensively investigated in materials science. Although the scientific community has in hands outstanding tools to probe such properties in a microscopic level, resonant x-ray techniques are still a unique instrument for probing electronic and magnetic properties with chemical and site selectivity. In particular, x-ray techniques are ideal for providing additional information for AC/DC susceptibility, electrical resistivity, neutron scattering, etc. In order to grant the community with a powerful instrument for investigating 4d/5d transition metals, lanthanide and actinide complex magnetic materials, beamline P09 at Petra III/DESY has employed diverse x-ray scattering, diffraction and absorption techniques. [1] Particular attention has been drawn to x-ray methods that employ circular radiation and able to probe interfacial magnetism. A few recent examples include measurements carried out by means of x-ray magnetic circular dichroism (XMCD)[2-6] and x-ray resonant magnetic reflectivity (XRMR)[6], in which a wide variety of samples, such as powder, single crystals, thin films can be investigated. Using an in vacuum double phase retarder system, P09 is able to provide full polarization control (e.g. linear or circular polarization) between 2.7 and 15 keV [7]. Dichroic magnetic x-ray data combined with magneto-optical sum rules [8] are able to individually provide spin and angular momentum of the probed ions. Hereby I will present a few experimental set-ups currently available at P09 along with some of novel scientific results obtained mainly by means of XMCD and XRMR methods.

**References:**

[1] J. Strempfer, S. Francoual, D. Reuther, J. Synchrotron Rad. 20, 541 (2013) [2] J. Mardegan, S. Francoual, G. Fabbris, Phys. Rev. B 93, 024421 (2016) [3] A. Khanas, S. Zarubin, A. Dmitriyeva, Adv. Mater. Interfaces, 2000411, 1 (2020) [4] Y. Liu, A. Luchin, S. M. Sanchez, ACS Appl. Mater. Interfaces 12, 8780 (2020) [5] Z. Liu, X. Wang, X. Ye, accepted Phys. Rev. B (2021) [6] D. Graulich, J. Krieft, A. Moskaltsova, Appl. Phys. Lett. 118, 012407 (2021) [7] S. Francoual, J. Strempfer, D. Reuther J. Phys.: Conf. Ser. 425 132010 (2013) [8] B. T. Thole, P. Carra, F. Sette, Phys. Rev. Lett. 68, 1943 (1992)

**Images:**

Exploring magnetic materials by means of XMCD and XRMR methods at beamline P09 at Petra III/DESY

Magneto-rheological materials are composites consisting of hard or soft magnetic particles embedded in a cross-linked elastomer with various filling ratios. They belong to the class of smart materials whose stiffness and damping can be controlled by an external stimuli as magnetic field. Due to their great flexibility and potential strain amplitude under a various magnetic field, they can find an interest in the field of mechatronic systems such as, soft-robotic or medical technologies. Magneto-mechanical coupling of flexible magnet needs to be clearly understood for development of applications in which they can be submitted to magnetic and mechanical loads. Some researchers have already worked on a full magneto-mechanical experimental characterization for soft magneto-rheological elastomer in case of large deformation [1-3]. However, due to high deformation range, the distances between magnetic particles increase. Thus, the magneto-rheological effect, which is defined by an increase of the stiffness under various magnetic field, is lowered. In the first part of this work we present the recent development of a magneto-mechanical bench for magneto-rheological elastomer filled with hard or soft ferromagnetic particles (figure 1). Concerning the mechanical part, the setup is composed by a system which consist of a displacement loading with a stress controller. By monitoring the displacement, we are able to work up to 1% of its deformation. This deformation mode allows to avoid non-linearities in case of large deformation range. The magnetic loading consists of a reluctant magnetic circuit which is able to produce homogeneous and large magnetic field (up to 40kA/m) in a variable air gap. Mechanical stress and magnetic loading are oriented toward the same direction. A 3D simulation analysis has been performed with a finite element simulation software (Altair Flux ©) for sizing and optimization of the magnetic loading such as intensity and homogeneity. The goal of this study is to expand as large as possible the magnetic field range to which our material would be submitted. For the numerical study, we have chosen a permanent magnet made of NdFeB (Br = 1.3T) and the magnetic flux is driven through a pure iron core. Various magnetic strength field can be reached by adjusting the air-gap thickness of the device. In case of hard magneto-rheological elastomer, composed by silicon-based elastomer filled with 36% vol of NdFeB particles, we are able to explore 10% of the demagnetization curve. The second part of this work focuses on the experimental characterization of magneto-mechanical coupling based on the setup developed. Thanks to this characterization tool, we are able to perform mechanical loading and unloading cycles with and without magnetic field. We observe, without any magnetic field applied, a hysteretic behavior and a remanent deformation specific to elastomeric material (figure 2) [4]. From this unloading stress state, a magnetic field strength of 40kA/m is applied. As it can be seen (figure 2), a tensile stress around 6.5kPa is induced due to magnetic particles interaction in response of the magnetic field applied. These first results are very promising and confirm the ability of the setup to characterize the magneto-mechanical coupling of hard flexible magnet. Further tests will be performed on composites with different elastomeric matrices with various Young’s moduli, different filling ratios and polarization orientation.

**References:**

[1] C.bellan, & Bossis, Georges. (2012). Field dependence of viscoelastic properties of MR Elastomers. Int J Mod Phys B;16(17n18):2447-2453 [2] Schubert, Gerlind & Harrison, Philip. (2015). Large-strain behaviour of Magneto-Rheological Elastomers tested under uniaxial compression and tension, and pure shear deformations. Polym Test; 42:122-134 [3] Pössinger, Tobias. (2015). Experimental Characterization, Modeling and Simulation of Magneto-Rheological Elastomers. PhD THESIS. Ecole Polytechnique, France. [4] Chagnon, Gregory. (2003). Modélisation de l'effet Mullins dans les élastomères. PhD THESIS. Ecole Centrale de Nantes, France.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/d30aa3cd104288673cbffe08ee05f8b5.jpg)
Figure 1: Illustration of the magneto-mechanical bench

![](https://assets.underline.io/uploads/markdown_image/1/image/254d7b2a27c44c2635c7592b269ecf07.jpg)
Figure 2: Experimental characterization of the mechanical behavior of a hard magneto-rheological elastomer

Development of a magneto-mechanical bench and experimental characterization for magneto-rheological elastomers

The prospect of thermal imaging and control in magnetic particle imaging (MPI) is an important advancement for non-invasive medical diagnostics and therapeutics. Current MPI systems focus on imaging only, from measurement of magnetic nanoparticle concentrations, while leaving out the important in vivo diagnostic of temperature. Here, we report on our progress toward accurate thermal imaging with high resolution MPI and discuss our achievement of 410 mK total temperature uncertainty at 100 ms integration time near room temperature using a high-precision magnetic field-stabilized magnetic particle spectrometer. The magnetic field is stabilized to ~ 14 nTRMS uncertainty, which allows for precise and accurate determination of magnetic nanoparticle temperature by measurement of magnetization under AC driving fields. Figure 1 shows the obtained temperature Allan deviation plot extracted from the measurement of magnetization in 10 nm ferrite magnetic nanoparticles driven by a 2.5 kHz magnetic field [1]. It has been reported that the imaging resolution of MPI is critically impacted by nanoparticle relaxation dynamics [2]; specifically, spin relaxation under AC drive fields causes a lagged response that results in image blurring. The dynamic response of magnetic nanoparticles in magnetic fields is intricate, depending strongly on the particles’ inherent magnetic and structural properties, inter-particle interactions, as well as the local environment in which they are suspended. Thus, comprehensive understanding of these effects requires high sensitivity over a broad parameter space. We have recently developed an arbitrary-wave magnetic particle spectrometer for characterizing relaxation dynamics for particles ranging in sizes from 10 – 70 nm. The combined advantage of arbitrary drive waveforms (sine, pulse, chirp, composite, etc.) and high drive amplitudes in our system allows for facile characterization over a large parameter space, using magnetic field frequencies from DC to 50 MHz at amplitudes 0-10 mTRMS. This capability to drive and observe magnetization dynamics occurring on the order 20 ns is required for isolating independent Brownian and Néel relaxation processes, as well as their dependence on particle environment and magnetic field amplitude. We have characterized the dynamic AC susceptibility of magnetic nanoparticles ranging in diameter from 10 nm to 70 nm and in composition (ferrite, cobalt-doped ferrite, zinc-doped ferrite, core-shell, etc.), and we have observed peaks in the imaginary part of the AC susceptibility (χ’’) at frequencies from as low as 50 Hz to &gt; 50 MHz, depending on particle size and composition. As an example, Figure 2 show the χ’’ peak frequency for 15 nm (blue arrow) and 20 nm (green arrow) Fe3O4 nanoparticles suspended in water. This peak is an indication of the effective relaxation timescale (τ ~ 16 ms for 20 nm, and τ ~ 160 ns for 15 nm), which will impact the spatial resolution of MPI, depending on the excitation frequency used. This knowledge will eventually inform strategies for design and synthesis of magnetic nanoparticles with properties targeted for accurate and sensitive thermal imaging using MPI.

**References:**

[1] Thinh Q. Bui, Weston L. Tew, Solomon I. Woods. AC magnetometry with active stabilization and harmonic suppression for magnetic nanoparticle spectroscopy and thermometry. J. Appl. Phys. 128, 224901 (2020). [2] Zhi Wei Tay, Daniel W Hensley, Erika C Vreeland, Bo Zheng, Steven M Conolly. The Relaxation Wall: Experimental Limits to Improving MPI Spatial Resolution by Increasing Nanoparticle Core size. Biomed Phys. Eng. Express, 3 (3), 035003 (2017)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/99ecda540b1e19510881aae0f2006451.jpg)
Figure 1: Temperature Allan deviation for 10 nm ferrite nanoparticles at 2.5 kHz and 2.5 mTRMS

![](https://assets.underline.io/uploads/markdown_image/1/image/778f9916b80bb72d4d72ad20e1dbea97.jpg)
Figure 2: AC susceptibility for 15 and 20 nm ferrite nanoparticles. χ’ and χ” are the real and imaginary components, respectively.

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