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Atomically thin, magnetic materials have gained a lot of attention since 2017, when the first 2D ferromagnet was reported [1]. This breakthrough has triggered research on 2D magnetic materials [2]. They are not only important from a fundamental point of view -to understand the theory of magnetism in reduced dimensions-, but also for technological applications. However, probing the magnetic order of the 2D systems by conventional magnetic experimental setups is very challenging. On the other hand, it is well known, that even in the single layer limit, semiconducting two-dimensional materials strongly absorb light. Therefore, optical spectroscopy is a good method for their characterization. In order to shed light on the intriguing phenomena of 2D magnetism, we present comprehensive theoretical investigations of the optical properties of the van der Waals layered material MnPS3, which is one important example from the large family of transition metal phosphorus trisulfide (MPS3) [3]. Our study reveals, that the interband absorption spectrum, which is proportional to the imaginary part of the dielectric function, is very similar for different possible magnetic states of MnPS3. On the other hand, the calculated effective masses of electrons and holes exhibit an anisotropic behaviour and they strongly depend on the magnetic order. Aforementioned properties are reflected in the binding energy of excitons in the studied systems. The impact of the magnetic order on electronic band gap is also revealed, and indirect character of electronic band gap is confirmed as the groundstate of bulk system. Our results demonstrate that the magnetic order of 2D materials can be indirectly inferred via optical measurements. The study was accomplished thanks to the funds allotted by the National Science Centre, Poland within the framework of the research project ‘SONATA12’ no. UMO-2016/23/D/ST3/03446. Access to computing facilities of TU Dresden ZIH for the project ”TransPheMat”, PL-Grid Polish Infrastructure for Supporting Computational Science in the European Research Space, and of the Interdisciplinary Center of Modeling (ICM), University of Warsaw are gratefully acknowledgedReferences: [1] B. Huan et al., Nature 546, p. 270-273 (2017) [2] M. Gibertini et al., Nat. Nanot. vol. 14, p. 408 (2019). [3] R. Brec, Solid State Ionics vol. 22, p. 3-30 (1986).

MMM 2020

Controlling magnetic properties of the van der Waals crystal MnPS3 via the absorption spectrum.

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The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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The miniaturization and integration is important market requirements for emerging electronic devices. Magnetic passive components are one of the most bulky components on the electronic boards. Therefore, a significant challenge for magnetic devices is miniaturization and integration towards system in package (SiP) and system on chip (SoC) [1]. Increasing the switching frequency in the applications is a way for miniaturization of magnetic components. On-chip inductors and transformers with magnetic materials fabricated on silicon are potential candidates for high switching frequency applications [2-4]. Developments of thin-film magnetic components in the last years show significant improvement in integration technology. In many works, newely developed microinductors and microtransformers were implemented in different electrical applications [5-10]. This paper will show the improved microtransformer on silicon with a bar magnetic core for data transmission applications [11]. The microtransformer can be used for higher frequencies data transmission application up to 25 MHz. The size of microtransformer device is 1.6 mm x 0.8 mm. The microtransformer consists of a bar magnetic core and interleaved primary and secondary windings Both windings feature 12 turns each. One turn has a length of 640 µm, width of 40 µm and thickness of 1 µm. A copper was applied for the windings. For magnetic core, a Co60Fe20B20 alloy is used. The magnetic core consists of two magnetic layers with dimensions 1400 µm x 500 µm x 0.4 µm. As insulating material SiO2 and Si3N4.are implemented. The inductance of the device is about 40 nH. The breakdown voltage is in the range of 1.5 kV. Using sputtering deposition a copper layer with only 1 µm is deposited, therefore the microtransformer device shows high electrical resistance of about 7 Ω. The coupling factor of the transformer has a value of over 0.6. For this reason, the microtransformer is suitable for data transmission applications. Fig. 1 shows a completed device.References: [1] C. O’Mathuna et al., IEEE Trans. Power Electron., 27(11), 4799–4816 (2012). [2] D. S. Gardner et al., IEEE Trans. on Magnetics, vol. 45, no. 10, pp. 4760-4766, 2009. [3] D. S. Gardner et al., IEEE Trans. on Magnetics, vol. 43, no. 6, pp. 2615-2617, 2007. [4] P. R. Morrow et al., IEEE Trans. on Magnetics, vol. 47, no. 6, pp. 1678-1686, 2011. [5] H. Wu et al., IEEE Trans. on Magnetics, vol. 52, no. 7, pp. 1-4, June 2016, Art. No. 8401204. [6] D. Dinulovic et al., IEEE Trans. on Magnetics, vol. 53, no. 11, pp. 1-7, Nov. 2017, Art no. 4700107. [7] H. T. Le et al., IEEE Trans. on Power Electronics, vol. 34, no. 1, pp. 74-85, 2019. [8] D. V. Harburg et al., IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 6, no. 3, pp. 1280-1294, 2018. [9] J. Michel et al., IEEE Trans. on Magnetics, vol. 55, no. 7, pp. 1-7, 2019, Art no. 8401207. [10] Z. Yue et al., 2019 IEEE Int. Solid- State Circuits Conference - (ISSCC), San Francisco, CA, USA, 2019, pp. 244-246. [11] D. Dinulovic et al., AIP Advance, vol. 10, no. 1, 2020, p. 015206.

On-Chip Thin-Film Microtransformer with Improved Coupling Factor for High Frequency Data Transmission

The performance of protection current transformers (CTs), which are one of the key components of the measurement chain for any protection device, have a significant impact on the performance of power system protection. The demagnetization of protection CTs is vital to ensuring the proper performance of power system protection [1]. However, the most difficulty of demagnetization is how to detect the remanent flux. This paper proposes a fluxgate theory based remanent flux detection method for protection CTs. The proposed circuit for remanent flux detection is shown in Fig. 1. The high-frequency square-wave voltage source, composed by a DC voltage source (VDC) and a full-bridge DC/AC converter, is connected to a resistor (R1) at the secondary side of the CT, and causes the core of the CT saturated slightly in the positive or negative direction. The polarity of the remanent flux is determined by the sign of integration of the voltage across R1. The level of the remanent flux is calculated with the linear relationship between the remanent flux and the second harmonic of the high-frequency component of the voltage across R1, as seen in Fig. 2. The linear relationship can be obtained in advance considering the fluxgate theory. The proposed method is tested for different hysteresis characteristics, different levels of remanent flux, with and without primary current, and errors in the assumed element parameters using EMTP-RV simulations. The results indicate that the proposed method can quickly and effectively detect the remanent flux of a protection CT in situ. The CT is not required to be disconnected from the substation, which means that the remanent flux can be detected in situ. The maximum detection time in all cases is smaller than 20ms. In addition, it is also confirmed that the slight distortion of the secondary current can be clearly filtered by a low pass filter, which consequently insure any protection device against maloperation.References: [1] T. Zheng, E. Hu, H. Yang, R. Zhao, Y. C. Kang, P. Wall and V. Terzija, “Fast, in Situ Demagnetization Method for Protection Current Transformers,” IEEE Trans. on Magnetics, Vol. 52, No. 7, July 2016, pp. 1-4.

Fast and in Situ Remanent Flux Detection Method for a Protection Current Transformer based on the Fluxgate Theory

Parallel control of milli-scale robots (milli-robots) is important for biomedical applications because it can improve fluidic delivery systems, which interface with cells and tissues. Systems based on diamagnetic levitation are advantageous because of frictionless transport around the workspace. The most common technique for contactless actuation in these systems is achieved through current traces in printed circuit boards or electromagnets to drive a milli-scale permanent magnet array levitated on graphite [1, 2]. Such systems have segmented work surfaces, limited motion in close proximity, and require extensive changes to expand the workspace. Automated optical actuation of diamagnetic pyrolytic graphite (PyG) levitating over permanent magnet arrays has been previously demonstrated using single laser sources for individual control [3, 4] and commercial projectors for parallel control [5]. Optical actuation allows accurate control of multiple PyG milli-robots that are able to move within close proximity to each other throughout an unsegmented work surface. Parallel control is demonstrated using three 17mmΦ levitating PyG milli-robots, which were simultaneously controlled using spotlights moving along pre-programmed paths forming the letters ‘U’, ‘S’, and ‘N’ as shown in Fig. 1. In this work, we demonstrate the potential use of multiple diamagnetically levitated milli-robots for delivering biological samples. As a proof of concept, we demonstrate loading, transporting, and releasing water into agar gel with microscale precision. End effectors, in the form of micropipettes, are placed on top of the PyG milli-robots. The fluid sample loads into the micropipette, which behaves as a horizontal capillary, through the aid of capillary action. The dimesions of the micropipettes and the properties of the fluid use dictate the time to fully load the capillary. A micropipette resting on top of a PyG milli-robot is shown in Fig. 2.References: [1] E. E. Hunter, et al., IEEE Robotics and Automation Letters 4.2 (2019): 997-1004. [2] A. Hsu, et al., IEEE Robotics and Automation Letters 5.2 (2020): 2913-2920. [3] M. Ewall-Wice, et al., IEEE Transactions on Magnetics 55.7 (2019): 1-6. [4] J. Young, et al. AIP Advances 9.12 (2019): 125038. [5] J. Young, et al. MOEMS and Miniaturized Systems XIX, 2020.

Optically Powered Diamagnetically Levitated Robots for Biomedical Applications

Permanent magnet dipole can generate magnetic field higher than the Br of the material with structures such as the Halbach array [1] and Stelter array [2]. These arrays usually generate magnetic field inside a cavity. And the magnetic field is perpendicular to the axis of the cavity. For applications such as Faraday Effect and erasing hard disks perpendicular media, a strong in-plane magnetic field is required. In the case of erasing hard disks, the disk needs to rotate in the air gap with magnetic field perpendicular to the surface of the disks. This requires that the dipole must open along the field direction. And it is often difficult to generate strong magnetic field when magnets can only be placed around the path of the magnetic flux. This paper presents a permanent magnet dipole design for strong in-plane magnetic field. The design starts with a Halbach structure that generates strong magnetic field in the cavity. To create access to the cavity from the in-plane field direction, a pair of magnets with orientation opposite to the magnetic field of the Halbach dipole are substituted along the field direction. The magnetic field from the substitute magnets is analyzed using current sheet equivalent method. This field is then subtracted from the Halbach magnet to generate the magnetic field from the new structure. We then used Bound Analysis software to simulate the structure. For the prototype, we choose an 8-segment Halbach magnet as the basic structure and used triangular magnets so that the magnets fit inside a rectangular steel housing. The magnetic material is NdFeB 48 MGOe with Br of 13600 G. Fig 1 shows the prototype dipole and the magnetic field in the air gap. In conclusion, this paper proposes a permanent magnet dipole with strong in-plane magnetic field. The prototype dipole generates strong in-plane magnetic field that is in line with the predictions from the analytical equation as well as our BEA analysis. This strong dipole can be used in many applications such as Faraday Effect and hard disk erase. The design is granted with US patent 10529362 [3]References: [1] K.Halbach, “Design of permanent multipole magnets with oriented rare earth cobalt materials”, Nucl. Instru. and Methods, Vol.169, pp 1-10 (1980) [2] Richard E. Stelter, US patent 5635889, 1997 [3] Chun Li, US patent 10,529,362, 2020

Permanent Magnet Dipole Design with Strong In-plane Magnetic Field

I Introduction Accurate modeling of the hysteresis characteristics of magnetic materials is of great significance to analysis the electromagnetic performance of electrical equipment[1]. The Preisach model is widely used in the simulation of the magnetic characteristic of transformers through the FEM calculation, while its parameter identification problem is still a complicated task[2]. This paper proposed a new parameter identification method based on an intelligent optimization algorithm with the explicit-form Everett function, which can speed the identification process. II Method and Discussion Firstly, the hysteresis loops of the B30P105 steel under quasi-static conditions are measured according to the IEC60404-2; Secondly, the Preisach model is built through the explicit-form Everett function. The explicit-form Everett function transformed the integral on the Preisach plane into the sum of a series of exponential functions which gives an explicit expression of the Everett function. To simulate the hysteresis loops of the material by the proposed method, the parameters identification problems of the explicit function need to be fully investigated. The optimization process of calculating the Everett function is shown in Figure 1 (where a, b, c, γ, Hs, Ms are the parameters of the proposed model); Finally, a Velocity-Controlled Particle Swarm (VCPSO) algorithm, which only needs the limiting hysteresis loop of the experimental data is developed and implemented to find the optimal parameters of the model. The simulation results are in good agreement with the experimental data as shown in Figure 2. III Conclusion A new parameter identification method of the Preisach model with explicit Everett function is proposed. By using the VCPSO algorithm, a fast parameter identification process, which only needs the limiting loop of the experimental data is developed. The proposed method are proved to be very efficient and accurate to construct the Preisach model, which has great significance on fast simulating magnetic performance of the transformer when considering hysteresis properties.References: [1] X. Zhao, X. Liu, Z. Zhao, X. Zou, Y. Xiao, and G. Li, "Measurement and modeling of hysteresis characteristics in ferromagnetic materials under DC magnetizations," Aip Advances, vol. 9, no. 2, Feb 2019, Art no. 025111, doi: 10.1063/1.5054948. [2] S. Hussain and D. A. Lowther, "An Efficient Implementation of the Classical Preisach Model," IEEE Trans. Magn., vol. 54, no. 3, Mar 2018, Art no. 7300204, doi: 10.1109/tmag.2017.2748100.

Parameter Identification of Preisach Model Based on Velocity-Controlled Particle Swarm Optimization Method

Millimeter-wave electronics are crucial for next-generation (5G) electronic communication systems. However, the development of desirable millimeter (mm) wave absorbers remains an ongoing challenge due to the difficulties of fabricating suitable magnetic absorbers in mm regimes. For ideal mm wave magnetic absorbers, materials should have 1) large anisotropy field (HA) and 2) saturation magnetization (Ms). The resonance frequency is proportional to HA and thus materials must have a large HA are essential for their resonance frequency to be located in mm wave range (20-100 GHz)1. However, the permeability decreases with increasing resonance frequency according to Snoek’s limit2. Thus, materials with larger Ms, are also required to mitigate this challenge. Combing both properties will enable us to achieve enhanced absorption intensity and broaden the bandwidth at mm waves. Rather than focusing on optimization of a single material, here we employ exchange coupled core/shell strategies to generate composite structures with both large Ms and HA. Here, we have fabricated novel magnetic core/shell nanostructures with the goal to realize a highly efficient mm wave absorber. Specifically, we have successfully synthesized ε-Fe2O3 core particles and ε-Fe2O3/Co core/shell nanoparticles using wet-chemistry methods specifically via sol-gel and thermal decomposition of metal precursors. The structural analysis of core and core/shell structures was performed by TEM, HRTEM and EDX, revealing well-defined interfaces and good crystal structure. Furthermore, the coercivity of resulting ε-Fe2O3 core particles is in the range of 2 – 18 kOe, which is significantly affected by the synthetic conditions. Details of the magnetic and the frequency response from the mm wave range of ε-Fe2O3 core and ε-Fe2O3/Co core/shell nanostructures performed by DC-magnetometry, X-ray magnetic circular dichroism and custom built vector network analyzer system will be discussed in the presentation. Prepared by LLNL under Contract DE-AC52-07NA27344.References: 1. S. Ohkoshi, S. Kuroki, S. Sakurai, K. Masumoto, K. Sato, S. Sasaki, Angew. Chem. 119, 8544-8547 (2007). 2. J. V. I. Timonen, R. H. A. Ras, O. Ikkala, M. Oksanen, E. Seppälä, K. Chalapat, J. Li, G.S. Poraoanu, Magnetic Nanocomposites at Microwave Frequencies. In Trends in Nanophysics: Theory, Experiment and Technology; Bârsan, V., Aldea, A., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, pp 257-285 (2010).

Development of magnetic core/shell nanostructures for 5G mm wave absorbers.

We have investigated the control of effective magnetic anisotropy in Permalloy (Ni80Fe20) linear chain arrays by tuning the symmetry arrangements of the ellipsoidal nanomagnets and the film thickness. The coupled ellipsoidal nanomagnets are arranged in two distinct configurations (Fig. 1 (a) and (b)), namely: type-I, where elements are arranged along the major axis of the ellipsoid, and type-II, in which the neighboring elements are coupled along the minor axis of the ellipsoid [1, 2]. Hence, the shape anisotropy of each nanomagnet and the configurational anisotropy due to the lattice arrangements favor the same direction of magnetization for type-I arrays while competition exists between the shape and configurational anisotropy for type-II arrays. The hysteresis loops depicted in Fig. 2 (a) and (c) clearly show the variation of the effective anisotropy with thickness which is also supported by the ferromagnetic resonance (FMR) spectra, shown in Fig. 2 (b) and (d) for type-I and type-II arrays respectively. A clear transition from a single domain states to a combination of complex flux closure states such as vortex, double vortices are observed at different applied field angles (φ) when the film thickness is varied in the range from 20 nm to 100 nm. Tunable microwave absorption spectra established the complex interplay between the shape anisotropy and magnetostatic interactions at different film thicknesses and applied field angles. Micromagnetic simulations are in good agreement with the experimental results. Our results demonstrate the possible ways of manipulating the effective magnetic anisotropy in the arrays of nanomagnets for magnonics and microwave applications.References: [1] S. Jain, A. O. Adeyeye, N. Singh, Nanotechnology 2010, 21 (28), 285702. [2] A. Talapatra, A. O. Adeyeye, Linear Chains of Nanomagnets: Engineering the Effective Magnetic Anisotropy (submitted).

Linear Chains of Nanomagnets: Engineering the Effective Magnetic Anisotropy

For next generation wireless systems requirements for multi-gigabit speed communication, researchers are focusing on millimeter wave frequency band up to 100 GHz for potential solution. Meanwhile hexagonal barium and strontium ferrite material are attracting increasing attention because its Ferromagnetic Resonance (FMR) frequency is at the vicinity of aforementioned frequency band of interest. Other features such as large saturation magnetization (4πMs), high anisotropy field (HA), low dielectric loss and most importantly, its potential of achieving self-bias, paves the way for fabrication of integrated, planar ferrite devices that can play a crucial role in next generation wireless system. BaFe12O19 (BaM) and SrFe12O19 (SrM) are M-type (magnetoplumbite structure) hexaferrites, anisotropic hard magnetic oxides which have great technological importance as low-cost permanent magnets and maintain great scientific interest due to potential use in magnetic recording and millimeter wave (MMW) filters and absorbers. Many electromagnetic properties of materials change as particle sizes are reduced to the nanoscale. Due to high magneto crystalline anisotropy, both BaM and SrM hexaferrites are expected to have narrow (20 Oe) spin resonances in millimeter-wave frequency range, while YIG and nickel ferrites have rather wide (300 Oe and wider) ferromagnetic resonance characteristics. Measurements conducted confirm the idea of using BaM and SrM ferrite materials as sharp, narrow ferromagnetic resonance-based devices, operating in millimeter waves. Our recent study reveal that the grain size of commercially available nano ferrite powders must be greater than single domain dimension (about 80-100 nanometers in diameter) to have any sharp ferromagnetic resonance to appear in millimeter waves. These broadband MMW transmittance measurements were performed using a quasi-optical MMW spectrometer and a series of backwards wave oscillators encompassing the 30–120 GHz range. BaM and SrM powders can be further investigated by DC magnetization, electron microscopy, x-ray diffraction, and light scattering to assess magnetic behavior, size, size distribution, and structure.References: Millimeter wave hexagonal nano-ferrite circulator on silicon CMOS substrate Liu Chao; Hassan Oukacha; Enjin Fu; Valencia Joyner Koomson; Mohammed N. Afsar 2014 IEEE MTT-S International Microwave Symposium (IMS2014) Year: 2014 Pages: 1 - 4, DOI: 10.1109/MWSYM.2014.6848467 Effects of particle size on magnetic and dielectric properties of ferrites at microwave and millimeterwave frequencies Anjali Sharma and Mohammed N. Afsar 2013 European Microwave Conference Year: 2013 Pages: 786 - 789

Self-Biased Hexagonal Ferrite Thin Film on GaN Substrate for Millimeter-Wave Devices

Thermomagnetic materials are a new type of magnetic energy materials, which enable the conversion of low temperature waste heat to electricity by three routes: Thermomagnetic motors, oscillators, and generators. Taking our recent work on thermomagnetic generators with different magnetic field topologies (TMG, Fig. 1 and 2) [1] as a starting point, in this talk we also analyse the material requirements for a more energy and economic efficient conversion. We present experiments of our TMG using different thermomagnetic materials (La-Fe-Si-Co with different transition temperatures and Gd). We compare the suitability of these materials and from this derive a generalized approach to evaluate the suitability of different thermomagnetic materials, which is different compared to magnetocaloric materials. We show that the thermodynamic efficiency of the best thermomagnetic materials outperforms thermoelectric materials when harvesting low temperature waste heat. Furthermore, the low raw materials cost results in a price per watt more than one order of magnitude lower than today’s power plants and thermoelectrics, which makes them also economically competitive. As summary, we present a materials library, which allows selecting the best available thermomagnetic materials in two Ashby plots as figures of merit and gives guidelines for future development of this material class [2]. At the end of this talk, approaches towards micro systems for harvesting low temperature waste heat are described.References: [1] A. Waske et al. Nature Energy 4, 68 (2019) [2] D. Dzekan et al, arXiv: 2001.03375 (2020)

Thermomagnetic harvesting of low temperature waste heat INVITED

Ten years ago, it was shown that ultrafast laser-induced heating of a thin film of amorphous Gd(FeCo)3 can magnetically switch the irradiated area [1-2]. A train of light pulses toggles the magnetization between two equivalent directions perpendicular to the film. Since then an intense research effort, both experimental and theoretical, has sought to unravel the underlying mechanisms of this single-pulse, all-optical switching (SP-AOS). Empirically it was established that the material needs to be ferrimagnetic with two magnetic lattices that demagnetize at different rates. As the initial, purely thermal demagnetisation rate is proportional to the local atomic moments on the sublattices, efforts were therefore focused on the gadolinium/transition metal alloys where the moments differs by a factor of almost four. It is therefore surprising that a second material, where we recently discovered SP-AOS [3], was the XA-ordered ferrimagnetic Heusler alloy Mn2RuxGa (MRG) where both magnetic sublattices are composed of manganese, with similar atomic moments. In this talk, we discuss how ~200 femtosecond light pulses toggle the magnetization of MRG.[3]. MRG is a half-metallic ferrimagnet with two antiparallel sublattices made up of manganese occupying inequivalent 4a and 4c positions in the F-43m space group[4]. On account of its unusual electronic structure, 4c electrons at the Fermi level dominate both magneto-optic and magneto-transport properties [5-6]. The sublattice moments compensate at a temperature Tcomp that depends on x and the thin film preparation conditions. Usually, at low temperatures the moment of the 4c sublattice is bigger than that of the 4a sublattice, but as the temperature rises, it falls off faster than 4a, and the two cross at Tcomp where they are equal but opposite. Unlike Gd(FeCo)3, which exhibits toggle switching for a range of initial temperatures ranging from 100 K below to 100 K above its compensation temperature, because the difference in atomic moments ensures the TM sublattice is demagnetised before the RE one, MRG will only switch when the initial temperature lies below Tcomp. This suggests that exchange relaxation plays a much more important role in the switching of MRG. MRG is a versatile material magnetic material. Its compensation temperature can easily be tuned and it is impervious to high magnetic fields in the vicinity of Tcomp, where the coercive field rises to 10 T. Furthermore, it possesses a large anomalous Hall effect and a very high intrinsic spin-orbit torque that may be useful for spintronic applications [7-8]. The observation of SP-AOS in a material with these properties opens both new technological prospects, and new challenges for understanding the fundamental non-equilibrium electronic processes involved.References: 1. Radu et al., Nature, Vol. 472, p. 205 (2011) 2. Ostler at al., Nature Communications, Vol. 3, p. 666 (2020) 3. C. Banerjee et al., arXiv, Vol. 1909, p. 09085 (2020) 4. H. Kurt et al., Physical Review Letters, Vol. 112, p. 027201 (2014) 5. K. Fleisher et al., Physical Review B, Vol. 98, p. 134445 (2018) 6. K. E. Siewierska et al., IEEE Transactions on Magnetics, Vol. 55, p. 2 (2019) 7. N. Thiyagarajah et al., Applied Physics Letters, Vol. 106, p. 122402 (2015) 8. S. Lenne et al., arXiv, Vol 1903, p. 04432 (2019)

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