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Magnetic skyrmions are topological spin textures that can be stabilized by the Dzyaloshinskii-Moriya interaction [1,2]. There are particularly suitable for next generation spintronics devices, like the skyrmion-based racetrack memory [3]. Recent studies confirmed their current-driven skyrmion dynamics in ultrathin ferromagnets via spin-orbit torques [4], where the chirality and spin texture of the skyrmions are key. However, the topological Magnus effect leads to a transverse motion of ferromagnetic skyrmions due to their non-zero topological charge [5], which is disadvantageous for devices. Antiferromagnetically exchange-coupled skyrmions or compensated ferrimagnets could suppress this effect owing to an overall zero topological charge [6]. Especially at the angular momentum compensation temperature skyrmion propagation is predicted to be collinear with the current [7]. Here we explore a GdFeCo-based ferrimagnet system with perpendicular magnetic anisotropy. We demonstrate that in this system chiral worm domains as well as magnetic skyrmions can be observed using scanning electron microscopy with polarization analysis (SEMPA) (Figure 1). The high spatial resolution magnetic imaging technique reveals the domain wall spin structure as a function of temperature, with the observed structures found to be pure Néel-type spin textures, which is promising for devices. From the images we extract the domain wall width, which allows us to calculate the exchange stiffness for the system across the full temperature range.References: [1] I. Dzyaloshinsky, J. Phys. Chem. Solids 4, 241 (1958) [2] T. Moriya, Phys. Rev. 120, 91 (1960) [3] R. Tomasello et al., Sci. Rep. 4, 6784 (2015) [4] S. Woo et al., Nat. Mater. 15, 501 (2016) [5] K. Litzius et al., Nat. Phys. 13, 170 (2017) [6] Y. Hirata et al., Nat. Nanotechnol. 14, 232 (2019) [7] J. Barker et al., Phys. Rev. Lett. 116, 147203 (2016)

MMM 2020

Direct imaging of chiral domain walls due to the Dzyaloshinskii-Moriya interaction in ferrimagnetic alloys at different temperatures

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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technical paper

Magnetic domain walls (DWs) in ferromagnetic nanowires are candidates for both Boolean [1–3] and non-Boolean computing architectures such as neuromorphic/stochastic computing [4]. In this context, interconnected nanorings are particularly interesting as they show emergent behaviour which make them interesting for reservoir computing [5-6]. Since these approaches use magnetic DW motion for logic operations, it is crucial to understand the dynamics of DWs in these structures for device realisation. We used micromagnetic simulations (using MuMax3 [7]) to simulate DWs passing through junctions of 2 μm diameter, 5 nm-thick Permalloy bi-ring structures with different ring widths and degrees of ring overlap. An example structure is shown in the inset of Fig. 1. The system was initialized with DWs in the upper and lower halves of the left and right ring. A rotating field was then applied to move the DWs towards the junction following previous experimental approaches [8]. The field amplitude was varied to record the depinning field HDP when the DWs separated from the junction. The variation of HDP for different overlap and wire widths is shown in Fig. 1. A minimum in HDP is observed for most widths indicating that competing mechanisms are involved in the depinning mechanism. In order to better understand these competing mechanisms, we look at the magnetic energy density [7] E=-½M.Beff (1) for a wire width of 100 nm with 12 (Fig. 2a) and 100 nm (Fig. 2b) overlap . In Fig. 2 (a), the DWs form a ‘diamond’ state which forms an energy minima requiring a high HDP to depin the DW. For the largest overlap in Fig. 2 (b), the DWs get ‘pinned’ before and after the junction and a high HDP is required to overcome this pinned energy threshold. Thus the intermediate overlap states (of 25 & 50 nm) require lower HDP due to competition between these states. We expect that the understanding of these dynamics is crucial for the development of ring based computing applications.References: [1] S. S. P. Parkin, M. Hayashi, L. Thomas, Science 320, 190 (2008). [2] D. A. Allwood et. al., Science 309, 1688 (2005). [3] K. A. Omari et. al., Adv. Func. Mater. 29, 1807282 (2019). [4] J. Grollier, et. al., Nat. Electron. 1-11 (2020). [5] R. Dawidek et al., MMM conference (2017). [6] R. Dawidek et al., submitted [7] A. Vansteenkiste et. al., AIP Adv. 4 (10), 107133 (2014). [8] M. Negoita et. al., J. Appl. Phys. 114.1, 013904 (2013).

Domain Wall Dynamics in Junctions of Bi-Ring Structures

The focus of lab-on-a-chip technology centers on small, low power devices that can be integrated on a microscopic scale to perform laboratory functions within one unit or “chip”. In order to achieve this, one has to understand how to transfer fluid at this scale. Due to the low Reynolds number limit, such devices need to break time-reversible symmetry to produce a net flow. There has been a degree of interest and research into using magnetic materials to achieve this. In this work, we propose a new model employing ferromagnetic elements for construction of a practical microfluidic pump. The design is based on a repeated chain of magnetic discs connected together via links with hinges at the centers of the discs, which allow the discs to rotate (Fig 1a). While the model is based on that of ferromagnetic swimmers[1]. The device is surrounded by fluid and is activated by an external AC magnetic field. The magnetic and elastic forces in this device can work together against the fluidic forces to create a time-irreversible movement, which results in a net flow. While magnetisation of all the discs are oriented in the same direction, the relative rotational stiffness of the discs (or hinge stiffness) is varied to produce a lag in the response to the magnetic torque. This collective effect is similar to that of a metachronal wave, which is ultimately responsible for the flow. We found that if both discs had the same rotational stiffness, then no flow was produced. Depending on the chosen frequency, there is an optimal rotation stiffness of the two discs where a maximum flow rate is achieved. We observed as the frequency increases so does the flow, until the frequency is too large for the discs to turn to their maximum angle before the direction of the magnetic field reverses. To conclude, we demonstrated that an elasto-magnetic device can produce a net flow at low Reynolds number. The flow direction can be controlled by altering the frequency of the driving magnetic field or by using different rotational stiffness. These results could lead to new types of ferromagnetic pumping devices.References: [1] A. D. Gilbert, et al., Q. J. Mech. Appl. Math., 64, 239 (2011).

Investigation of a Coupled Elasto-Magnetic Discs for Low Reynolds Number Pumps

Introduction: Transcranial magnetic stimulation (TMS) is an emerging method for depression and neuropathic pain therapy with a painless and noninvasive neuromodulation function [1]. The winding path of the TMS coil can be realized in several ways [2]. For instance, the H1 coil is designed for deep brain stimulation [3], the “hat-like” coil is designed to suppress temperature rise during stimulation [4], and the fdTMS coil comprehensively considers energy, depth, and focality [5]. However, a practical TMS coil satisfying both high energy efficiency and robustness against individual variabilities in head shape has not yet been realized. Purpose: To optimize an energy-efficient TMS coil that is robust against individual differences in head shape. Methods: The optimization scheme is shown in Fig. 1. Multiple MRI anatomical images are used to optimize the shape to facilitate fitting of the scalp at the target stimulation position. Subsequently, the winding path of this shape is optimized through the following steps. First, the forward model Ε=AΨ for the scalar stream function Ψ and target-induced electric field Ε is established. Second, the Tikhonov regularization is used to solve the stream function Ψ numerically, given Ε. Finally, the coil winding path is extracted from the contour lines of Ψ [4]. This coil was compared with two commercial TMS coils by simulating the eddy current distribution in the digital brain model. Result: The results are shown in Fig. 2. Under the same energy consumption condition, compared with planar fig-8 and butterfly coils, this “head-fit” coil has stimulus area increases of 87% and 67%, respectively, with the corresponding efficiency drops of only 2% and 3%. The expansion of the stimulation area indicates that stimulation to the target area can be ensured even with a slight movement of the head during ~30min treatment. Furthermore, considering the shape of the scalp of different subjects facilitates close fitting of the head. Conclusion: Our method allows the design of TMS coils suitable for practical TMS therapy.References: [1] J. P. Lefaucheur et al., “Evidence-based guidelines on the therapeutic use of repetitive transcranial magnetic stimulation (rTMS),” Clin. Neurophysiol., vol. 125, no. 11. Elsevier Ireland Ltd, pp. 2150–2206, 2014. [2] Z. De Deng, S. H. Lisanby, and A. V. Peterchev, “Electric field depth-focality tradeoff in transcranial magnetic stimulation: Simulation comparison of 50 coil designs,” Brain Stimul., vol. 6, no. 1, pp. 1–13, Jan. 2013. [3] H. M. Gellersen and K. K. Kedzior, “Antidepressant outcomes of high-frequency repetitive transcranial magnetic stimulation (rTMS) with F8-coil and deep transcranial magnetic stimulation (DTMS) with H1-coil in major depression: a systematic review and meta-analysis,” BMC Psychiatry, vol. 19, no. 1, pp. 139, 2019. [4] L. M. Koponen, J. O. Nieminen, and R. J. Ilmoniemi, “Coil optimisation for transcranial magnetic stimulation in realistic head geometry,” Brain Stimul., vol. 10, no. 4, pp. 795–805, 2017. [5] L. J. Gomez, S. M. Goetz, and A. V. Peterchev, “Design of transcranial magnetic stimulation coils with optimal trade-off between depth, focality, and energy,” J. Neural Eng., vol. 15, no. 4, pp. 1–31, 2018. [6] G. N. Peeren, “Stream function approach for determining optimal surface currents,” J. Comput. Phys., vol. 191, no. 1, pp. 305–321, 2003.

Optimization of transcranial magnetic stimulator coil using stream function for reducing the influence of individual variabilities of head geometry

Recently, we have introduced a new type of magnetic particles (MPs), prepared by wet milling of superferromagnetic Fe-Cr-Nb-B precursor glassy ribbons, for cancer treatment by magneto-mechanical actuation in low magnetic fields (1÷20 Oe) [1]. In this work we studied the possibility of using adipose derived stem cells (ADSCs) for the transport of Fe-Cr-Nb-B MPs to areas with MG-63 human osteosarcoma (HOS), and the effect of MPs incorporated into ADSCs on the viability of HOS following the magneto-mechanical actuation of MPs with dimensions between 5 and 100 nm. The viability tests performed 24 hours after the magneto-mechanical actuation by MTT assay indicated values of 24÷25% for HOS and up to 10% for ADSCs. We also investigated the capacity of ADSCs to incorporate Fe-Cr-Nb-B MPs, and we found that large amounts of MPs were ingested as confirmed by a colorimetric ferrozine-based assay that quantitatively evaluates the Fe content in cells. We used a time-lapse cell imaging method to record the cellular migration of ADSCs loaded with MPs towards tumor cells in vitro. The recorded films demonstrated that MPs-loaded ADSCs were able to target osteosarcoma cells. We performed in vitro tests, where ADSCs were incubated with Fe-Cr-Nb-B MPs, then transferred onto an osteosarcoma cell culture and exposed to the rotating magnetic field. Magneto-mechanical actuation led to the destruction of ADSCs and to the release of MPs on HOS culture, which incorporated the released MPs. By magneto-mechanic actuation, the HOS viability dropped down to 21÷22%. The irregular shape of the MPs combined with their rotational motion in the rotating magnetic field produce irreversible damages on the cells membranes and organelles, and, subsequently, the cells death. Work supported by the Nucleu Programme (Project PN 19 28 01 01) and Contract No. 11PFE/16.10.2018 financed by the Romanian Ministry of Education and Research.)References: [1] H. Chiriac, E. Radu, M. Tibu, et al., Sci. Rep. 8 (2018) 11538.

Fe-Cr-Nb-B magnetic particles and STEM cells, triggers for cancer cells apoptosis by magneto-mechanical actuation

Magnetically actuated micro- or milli-scale gripper robots have been widely studied as a promising means to manipulate a variety of small objects, such as biological cells, medical drugs, and other functional materials, in an untethered and uncontaminated manner [1-3]. In this research, we propose a magnetic gripper robot (MGR) composed of a simple flexible body with three permanent magnets inserted into different parts of the body (Fig. 1). A cube magnet is fixed into the base of the flexible body, whereas two freely rotatable cylindrical magnets are inserted into each gripper arm of the body. Without an external magnetic field, the two cylindrical magnets in the gripper arm tend to align in the same lateral direction, which can make the MGR form a closed posture due to the attractive magnetic force. Under a specific magnetic field, on the other hand, the two magnets may align into a vertical direction parallel to the fixed magnet, in which the MGR can make an open posture due to the repulsive magnetic force. In this research, we derived an equation that can calculate the equilibrium postures of the MGR's magnets under an external magnetic field and the corresponding structural deformation of the flexible body by using the MGR’s magnetic and elastic potential energy (Fig. 1). From this equation, we could establish a method to generate the MGR’s gripper motions and its two-dimensional (2D) translational motion at the same time by using an efficient coil system comprised of only a triad of electromagnetic coils (TEC) shown in Fig. 2a [4]. We then fabricated a prototype MGR and a TEC to demonstrate the proposed MGR’s gripper motions such as the pick-and-place motions shown in Fig. 2. The results showed that the proposed MGR could play a role as an effective untethered gripper which can be utilized for various biomedical purposes such as cell manipulation, drug delivery, biopsy, etc.References: [1] T. R. Ger, H. T. Huang, W. Y. Chen, and M. F. Lai, Lab on a Chip, Vol. 13, p. 2364-2369 (2013) [2] E. Diller and M. Sitti, Advanced Functional Materials, Vol. 24, p. 4397-4404 (2014) [3] T. Xu, J. Zhang, M. Salehizadeh, O. Onaizah, E. Diller, Science Robotics, Vol. 4 (2019) [4] H. J. Lee and S. M. Jeon, AIP Advances, Vol. 10 (2020)

A Two-Dimensional Magnetic Gripper Robot Driven by a Triad of Electromagnetic Coils

The blood's acid-base balance is precisely controlled because even a minor deviation from the normal range can severely affect many organs. Due to Warburg effect, in tumor cells, an imbalance between oxygen supply and consumption, and accumulation of lactate cause increasing the concentration of H+. As a consequence, intracellular pH decreases, resulting in acidity. Since the pulsed magnetic field (PMF) influences human physiology, many studies have been reported its effect on anti-nociception as well as improvement of blood circulation[1-2]. Our study has tried to investigate the influence of PMF on blood's acid-base balance in microvascular channels. In addition, the applicability of PMF to H+ pump inhibitor in tumor cells was evaluated using negatively- charged magnetic beads(MB). Tert-butyl hydroperoxide (tBHP) was used to oxidizing agents, in order to make acidic environment similar to tumor cells. All experiments were done with red blood cells(RBCs) and plasma suspension with hematocrit (Ht) of 45%. Our PMF stimulator has the maximum intensity of 0.27T at a transition time of 102 μs with pulse intervals of 1Hz. The changes in pH of blood and the number of RBCs with MB attached were examined before and after PMF stimulus. The blood treated with tBHP on RBCs is more acidic than that of normal blood, but it was observed that the pH is restored after PMF stimulus. Also it was confirmed that MB adheres more to the RBCs after PMF stimulus, which could be inferred that PMF stimulus not only increases the zeta potential, but also reduces the H+ in the blood by combining MB with plasma cations. Further study needs to optimize PMF stimulation conditions such as pulse shape, duration, or repetition rate for fast homeostasis recovery in acid-base unbalanced blood.References: [1] N. M. Shupak, F. S. Prato & A. W. Thomas, Neurosci. Lett. Vol. 363, p.157–162(2004). [2] J. Mok, S. Han, & H. Lee, AIP Advances, 10(1), 015005 (2020).

Study on the Acid-Base Balance in Blood under Pulsed Magnetic Field using Magnetic Beads

Cell Sorting has become an indispensable technique for a variety of biological and biomedical applications. Moreover, with the development of fields such as personalized medicine and new breakthroughs in cancer treatments, cell sorting devices that have high throughput and single-cell manipulation capabilities are needed now more than ever1. Magnetically activated cell sorting (MACS) has quickly become one of the most widely used techniques due to its extremely high performance in throughput and ease of use, unfortunately these systems lack the much needed single-cell resolution that modern biomedical problems require. To address this, there has been much research and development aimed at creating MACS devices in the microfluidic regime where microscale fabrication can allow for single-cell sorting mechanisms to be implemented. In addition to increased resolution, these microfluidic MACS devices have other benefits that include low cost, the ability to run fast low volume experiments, and the ability to capture and release cells while simultaneously culturing them within a device. In this paper we present a novel method of capturing and releasing magnetically activated cells using Spin Orbit Torque (SOT) to change the magnetization of micromagnet structures patterned within a microfluidic channel. SOT has been shown to be a reliable and fast method of manipulating the magnetization of magnetic memory devices2 and for this reason it was chosen as the method of operation. This device is a significant improvement on current cell sorting technologies because it has both single cell manipulation capabilities and high throughput whereas most current cell sorting technologies suffer a tradeoff between these two properties. This paper includes both modeling and experimental data to verify the theoretical mechanism proposed.References: 1X. Wang, S. Chen, D. Sun. Lab on a Chip., Vol. 11, p. 3656 (2011) 2W. Kwak, J. Kwon, B. K. Cho. Scientific Reports.,Vol 8 (2018)

High Throughput Magnetic Single-Cell Sorting Using Spin Orbit Torque

Complex oxide perovskites are attractive for spintronics applications due to the ease of integration into functional devices through epitaxial, isostructural growth. However, this epitaxial integration may cause unexpected changes and interfacial phenomena. Here, we report on modifications to the magnetic properties of ferromagnetic (La2/3Sr1/3)MnO3 (LSMO) films on LSAT substrates by an epitaxial layer of CaRuO3 (CRO). Using ferromagnetic resonance (FMR) as a sensitive magnetic probe, we find that the presence of CRO can modify the magnetic anisotropy of the LSMO. By increasing CRO thickness, we find a reduction of the out-of-plane anisotropy and simultaneous rotation of the easy axis within the plane, from the <110> to <100> axes (see Fig. 1a). The anisotropy strength determined by FMR disagrees with bulk SQUID magnetometry. This is consistent with a depth-dependent structural change, such as oxygen octahedral rotations imposed on rhombohedral LSMO [1] by orthorhombic CRO. FMR is also used to study spin pumping across this LSMO/CRO interface. Spin pumping manifests as an increase in Gilbert damping with CRO thickness. The frequency dependence of the FMR linewidth with an in-plane field indicates two-magnon scattering is significant. By varying the field direction in-plane, we find the in-plane anisotropy of the FMR linewidth – and therefore strength of two-magnon scattering – increases as a function of increasing CRO thickness (see Fig. 1b). These results suggest that while LSMO and CRO remain promising candidates for efficient pure spin current generation and detection, respectively, the epitaxial integration of perovskites for spintronics applications will cause additional changes which must be accounted for.References: [1] Z. Liao, M. Huijben, Z. Zhong et al., Nature Materials, Vol. 15, p.425 (2016) [2] H. K. Lee, I. Barsukov, A. G. Swartz et al., AIP Advances, Vol. 6, p.055212 (2016)

Magnetic Anisotropy and Spin Scattering in (La2/3Sr1/3)MnO3/CaRuO3 Bilayers

The La-based double layered (DL) managnites La2-2xSr2+2xMn2O7 have gained importance due to colossal magnetoresistance (CMR) [1] analogous to 3D perovskite manganite. However, In incongruity with the three-dimensional (3D) half doped perovskite manganites (La0.5Sr0.5MnO3), which shows charge exchange (CE) spin/charge ordering (CO) [2], the A-type of antiferromagnetic (AFM) state is dominant in Ruddlesden –Propper half doped bilayered manganites (La1Sr2Mn207) [3]. The MR observed in the layered system is much higher than observed in 3D perovskites manganites because of the high spin polarisation of conduction electron. This abstract reports on the structural and magneto-transport properties of La2-2xSr2+2xMn2O7 (0.42≤x≤0.52) synthesized by solid state reaction method. The Rietveld refined X-ray diffraction pattern suggests that the samples crystallizes into tetragonal structure (I4/mmm) without any trace of impurity phase. The temperature dependent resistivity plot of the samples in the magnetic field 0-5T is shown in Fig 1. At 0T, the samples 0.42≤x≤0.48 show insulator to metal (TMI) transition and later a resistivity mimima (Tmin) with the lowering of temperature, whereas x=0.50 and 0.52 shows CO with trace of metal-insulator transition. The application of magnetic field reduces the resistivity to a large order and TMI shifts towards the higher temperature side suppressing the resistivity minima. This can be understood due to reduction of the spin fluctuation by the field [4]. The resistivity of the samples increases with Sr subsitution upto x=0.48, and later decreases for higher Sr substitution, which may be due to phase transformation from canted AFM to the CO insulator [5]. The conduction mechanism at high temperature follows hopping polaron model. At 5K and 8T, 0.46≤x≤0.50, shows nearly similar MR (58%) and decreases for (x=0.42, 0.52). MR is dominated by the spin-polarised tunnelling across the interface or grain separated by the insulating barrier. We intend to study the magnetic behaviour of all the samples. This work is supported by UGC-DAE-CSR, Indore (grant No. CSR-IC/CRS-89), Ames Lab and MRF(MITM)References: 1. Moritomo Y., Asamitsu A., Tokura Y., Nature, 380 , 141.(1996) 2. C. N. R. Rao and B. Raveau, Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, edited by (World Scientific, Singapore, (1998). 3. M. Kubota, H. Yoshizawa, Y. Moritomo, Y. Endoh, J. Phys. Soc. Jpn. 68, 2202 (1999). 4. C. Zener, Phys. Rev. 82 403 (1951) 5. Mitchell J F, Ling C D, Millburn Medarde M J. Appl. Phys. 89, 6618 (2001)

Structural and Magneto Transport Properties of Ruddlesden –Propper La2-2xSr2+2xMn2O7 (0.42≤x≤0.52) Layered Manganites

The considerable spin diffusion length and high electron mobility of graphene at room temperature makes it a suitable candidate to improve the spin transport properties of magnetic semiconductors when incorporated in a suitable stoichiometry. Here, we report observation of room temperature magnetoresistance properties of reduced graphene oxide (rGO) / cobalt ferrite (CFO) nanocomposite, which were synthesized by chemical methods followed by mixing in different ratios. The structural properties and morphology of rGO-CFO mixed states have been investigated from the x-ray diffraction and TEM imaging. The Raman spectroscopy shows the presence of various vibrational bands of CFO in addition to the D and G bands at ~ 1354 and 1591 cm-1 respectively, which indicates the formation of rGO. The magnetic field dependent electrical resistivity shows a decrease in resistivity with the increase in magnetic field from 0 to 0.5 T, representing a negative magnetoresistance in our samples. The highest MR of around – 1.4 % was observed in the 1:1 ratio CFO and rGO nanocomposite, whereas the samples with 2:1 and 3:1 ratio show the MR of maximum value about 1%. In the rGO-CFO nanocomposite, the charge transport takes place mainly through the rGO flakes due to the insulating nature of the CFO nanoparticles. However, the ferromagnetic moments of the CFO nanoparticles induce electron-spin scattering during the charge transport. The reduction in electron-spin scattering due to the parallel alignment of the magnetic moments results in negative magnetoresistance in our sample. The alignment of ferromagnetic moments under the external magnetic field has been further confirmed from the observation of the magnetocapacitance effect of CFO nanoparticles. Our results show the possibility to utilize the wide band gap magnetic oxides for room temperature magnetotransport applications when incorporated with the 2D materials like graphene.

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Direct imaging of chiral domain walls due to the Dzyaloshinskii-Moriya interaction in ferrimagnetic alloys at different temperatures

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