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Rich variation of magnetic texture attracts attention in connection with topology of electronic structures. The temperature range slightly below Curie temperature (Tc) is interesting, because the contributing micromagnetic energies (demagtization, exchange, Dzyaloshinsky-moriya and anisotropy) change rapidly with temperature. Recently we reported bubble domain formation immediately below Tc within a kagome ferromagnet Co3Sn2S2 (Tc=176 K) in which anomalous high-temperature, low-field magnetic phase occurred [1]. Self-reorganization of domain walls (DWs) occurred even at zero field, and the domain evolution was dependent not only on temperature and field but also on temperature variation rate [2]. To examine such domain evolution associated with slow spin dynamics, we developed a method to record Lorentz images continuously at a fixed time interval when a specimen is heated/cooled at a constant rate, not like an usual manner to wait until thermal drift settles down. We applied it to examine domain evolution within another uniaxial kagome magnet, Fe3Sn2 (Tc= 657 K), in the temperature range at 303-673 K under no external field. A psuedo-three-dimensional (x-y-time (temperature)) image was constructed from a series of drift-corrected Lorentz images. Fig. 1 shows a cross-section image including time-axis, exhibiting time(temperature) evolution of the magnetic domains. The stripe domain stricture was very stable between room temperature (RT) and 620 K at which DW contrast disappeared due to reduced magnetization both on heating and cooling across Tc, although interesting temperature-dependent texture such as skyrmionic bubble was observed for the alloy below RT. It was also found that the domain structure was nearly independent of cooling/heating rate. The differece in domain evolution between Fe3Sn2 and Co3Sn2S2 is explained in terms of DW creep associated with thermal depinning in Co3Sn2S2.References: [1] M. A. Kassem et al., Phys. Rev. B 96, 014429 (2017). [2] Akira Sugawara et al., Phys. Rev. Mater. 3, 104421 (2019)

MMM 2020

Magnetic texture evolution within uniaxial kagome ferromagnets at constant heating/cooling rates

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

We present successful fabrication and demonstration of magnetic vortex core tip (VC) for magnetic force microscopy (MFM). The design of VC probe is based on permalloy disk with diameter of 330nm and thickness of 55nm fabricated on intentionaly blunted tip (see the Fig.1). The ground state of this permalloy disk is the magnetic vortex state with out-of-plane magnetic moment determined by well spatially localized vortex core. The novelty of this design is creation of spatially well localized magnetic out-of-plane field in mechanically blunt tip. This design offers two advantages: First, due to large area of the disk, VC probe has higher wear resistance and longer lifetime than sharp commercial MFM probes. Second, it has very good stability of the out-of-plane magnetic moment, because vortex core is almost independent on the edge imperfections. However, VC probe has also some limitations: the sample should be flat and it should not produce quasi-uniform field in large volume nearby the tip, because the polarization affects vortex core position. We analyze the permalloy tip polarization effect by micromagnetic calculations of MFM images of periodic magnetic strips. By this approach we identify the limits on samples for this new kind of MFM probe.References: J. Soltys, J. Feilhauer, I. Vetrova, J. Tobik, K. Bublikov, T. Scepka, J. Fedor, J. Derer, and V. Cambel, Appl. Phys. Lett. 116, 242406 (2020).

New Vortex Core Probe for Magnetic Force Microscopy

Since it was first demonstrated in 1987, magnetic force microscopy (MFM) has become a truly widespread and commonly used characterization technique that has been applied to a variety of research and industrial applications. Some of the main advantages of the method includes its high spatial resolution (typically ∼50 nm), ability to work in variable temperature and applied magnetic fields, versatility, and simplicity in operation, all without almost any need for sample preparation. However, the technique has historically provided only qualitative information, and the number of available modes was typically limited, thus not reflecting the experimental demands. We present the recent progress and development of MFM as well as a summary of the current state-of-the-art techniques and objects for study. Aspects including quantitative MFM, the accurate interpretation of the MFM images, new instrumentation, probe-engineering alternatives, and applications of MFM to new (often interdisciplinary) areas of the materials science, physics, and biology are discussed. We outline the importance of the technique in emerging fields including skyrmions, 2D-materials, and topological insulators.References: O. Kazakova, et al., ‘Frontiers of Magnetic Force Microscopy’ J Appl. Phys. 125, 060901 (2019) H. Corte-León, et al., ‘Magnetic imaging using geometrically constrained nano-domain walls’ Nanoscale 11, 4478 (2019) V. Panchal, et al., ‘Calibration of multilayered magnetic force microscopy probes’ Sci Reports 7, 7224 (2017) F. Moro, et al., ‘Room temperature uniaxial magnetic anisotropy induced by Fe-islands in the InSe semiconductor van der Waals crystal’, Advanced Science, 5, 1800257 (2018) H. Corte-León, et al., ‘Comparison and validation of different magnetic force microscopy calibration schemes’ Small 20, 1906144 (2020) X. Hu, et al., ‘Round Robin Comparison on Quantitative Nanometer Scale Magnetic Field Measurements by Magnetic Force Microscopy’ J. Magn. Magn Mater. 511, 166947 (2020)

Frontiers of magnetic force microscopy INVITED

Topological insulator BiSb is a promising candidate for spin-orbit-torque (SOT) magnetoresistive random-access memory (MRAM), due to its giant spin Hall effect (spin Hall angle θSH ∼ 52 for BiSb(012) surface) [1] and high electrical conductivity (σ ~ 2.5 × 105 Ω-1m-1) [2] at room temperature. Recently, we demonstrated that high-quality BiSb thin films with quasi single crystal structure could be grown by sputtering deposition, underlying its potential for mass production of SOT-MRAM [3]. In this work, we show that in all-sputtered BiSb-(Co/Pt)n multilayers structure, the BiSb layer with (110) surface has large θSH = 12.3 and high σ ~ 1.5 × 105 Ω-1m-1, which lead to ultrahigh efficient SOT magnetization switching of (Co/Pt)n multilayers. We deposited (0.4 nm Co/0.4 nm Pt)2 - 10 nm BiSb multilayers on c-plane sapphire (0001) substrate at room temperature (Fig. 1a). The (Co/Pt)2 multilayers are perpendicularly magnetized with a large anisotropy field, as shown in Fig. 1b. We employed the second harmonic Hall measurement to evaluate θSH in 50 μm × 25 μm Hall bars. A representative 2nd harmonic Hall resistance (R2ω) is plotted as a function of in-plane magnetic field (H) in Fig. 1c, and the antidamping-like field HAD is evaluated by fitting to R2ω at high field. Fig. 1d shows the HAD at various current density JBiSb, from which we obtained θSH = 12.3. Thanks to the large θSH, the magnetization of (Co/Pt)2 can be efficiently switched by a small DC threshold current density JthBiSb = 1.5 × 106 Acm-2 at H = ±2.75 kOe (Fig. 2a and 2b). Fig. 2c shows robust switching by 75 consecutive pulses at ±1.83 kOe (JBiSb = 4.4 × 106 Acm-2, tpulse = 100 μs). Our results demonstrate the feasibility of BiSb for ultralow power SOT-MRAM and other SOT-based spintronics devices.References: [1] N. H. D. Khang, Y. Ueda and P. N. Hai, Nat. Mater. 17, 808-813 (2018). [2] Y. Ueda et al., Appl. Phys. Lett. 110, 062401 (2017). [3] T. Fan et al., Jpn. J. Appl. Phys. 59, 063001 (2020).

Ultrahigh Efficient Spin-Orbit-Torque Magnetization Switching in Sputtered Topological Insulator BiSb and (Co/Pt)2 Multilayers

Spintronics has opened a new paradigm through the use of the spin variable as the vector of information and has been largely applied from hard drives read-heads to the STT-MRAMs. While very recent, the introduction of 2D materials in Magnetic Tunnel Junctions (MTJs) has already shown some promising properties[1]. Graphene and the 2D insulator h-BN have been the first 2D materials to show strong impact on spin transport in MTJs. The recent advent of the wide TMDC family of 2D semiconductors opened new opportunities for further tailoring of spintronics properties. We will present results on the scarcely studied WS2. We will detail a protocol using laser lithography technology to fabricate spin valves based on CVD grown WS2, with step by step characterizations in support (Raman spectroscopy, photoluminescence, and AFM measurements). We will finally show our first spin transport measurements obtained in a CVD WS2 based MTJ. Our measured MR signals, above state of the art for 2D semiconductor based MTJs, validates our integration approach. We observe that the spin signal extracted from a ferromagnetic electrode can be tuned by placing atomically thin WS2 on top of it. Furthermore the thickness of WS2 significantly affects the extracted spin polarization of the 2D/FM interface. We discuss that trend using the peculiar band structure of WS2, supported by DFT calculations, leading to thickness dependent spin filtering. Our work opens the way to the integration of different members of the very large TMDCs family, in order to reveal their spin transport properties in MTJs[2].References: [1] M. Piquemal-Banci, R. Galceran, M-B. Martin et al. J. Phys. D: Appl. Phys., 50 , 203002 (2017). [2] V. Zatko, M. Galbiati, S-M-M. Dubois et al. ACS Nano., 13 , 14468 (2019).

Band-Structure Spin-Filtering in Vertical Spin Valves Based on CVD Grown WS2

Recently, possible applications for thermoelectrics and spin caloritronics utilizing topological materials have been studied. Thermoelectric (TE) conversion that can convert heat into electricity has been attracting attention as a clean power generation technology without greenhouse gases in energy harvesting. The conversion efficiency is evaluated by the dimensionless figure of merit ZT, which is needed to be larger than 1 (energy conversion efficiency is 10%) in practice. Recently, a bismuth chalcogenide which shows the best TE conversion on the nanoscale has been also known as a three-dimensional topological insulator (3D TI). TI has a linear electronic dispersion on the surface. In the presence of a surface gap induced by magnetic dopants, the surface electronic transport strongly depends on the chemical potential, promising very large TE conversion efficiency [1]. Here, we propose a TE energy conversion device based on the surface of 3D TIs that is magnetically gap-opened and ionically disordered [2]. This device can achieve the thermocouple p-n junction by a single TI film, in which a pair of top and bottom surfaces of a 3D TI takes a role of the vertical p-n junction, which can be considered a thermocouple consisting of two dissimilar TE materials (see Fig. 1). Based on the Boltzmann theory at finite temperatures, we compute TE transport coefficients taking into account electron scattering by screened disorder potentials on the surface. By tuning the surface carrier screening by means of electric gating, we find that ZT of the present device achieves more than 1 in the wide temperature-regime from 100 K to 300 K.References: [1] T. Chiba and S. Takahashi, J. Appl. Phys. 126, 245704 (2019). [2] T. Chiba, S. Takahashi, and T. Komine, Appl. Phys. Lett. 115, 083107 (2019).

Spin caloritronic device based on asymmetric surfaces of magnetic topological insulators

Efficient and versatile spin-to-charge current conversion is crucial for the development of spintronic applications, which strongly rely on the ability to electrically generate and detect spin currents. In this context, the spin Hall effect has been widely studied in heavy metals with strong spin-orbit coupling. While the high crystal symmetry in these materials limits the conversion to the orthogonal configuration [1], unusual configurations are expected in low symmetry transition metal dichalcogenide semimetals [2], which could add flexibility to the electrical injection and detection of pure spin currents. Here, we report the observation of spin-to-charge conversion in MoTe2 flakes, which are stacked in graphene lateral spin valves [3]. We detect two distinct contributions arising from the conversion of two different spin orientations. In addition to the conventional conversion where the spin polarization is orthogonal to the charge current, we also detect a conversion where the spin polarization and the charge current are parallel. Both contributions, which could arise either from bulk spin Hall effect or surface Edelstein effect, show large efficiencies comparable to the best spin Hall metals and topological insulators. Our finding enables the simultaneous conversion of spin currents with any in-plane spin polarization in one single experimental configuration [3].References: [1] Sinova, J.; Valenzuela, S. O.; Wunderlich, et al., Rev. Mod. Phys., Vol. 87, p.1213-1260 (2015) [2] MacNeill, D.; Stiehl, G. M.; Guimaraes, et al., Nat. Phys. Vol. 13, p.300-305 (2017) [3] Safeer, C. K.; Ontoso, N.; Ingla-Aynés, et al., Nano Lett., Vol.19, p8758-8766 (2019)

Large multi-directional spin-to-charge conversion in low symmetry semimetal MoTe2 at room temperature

Metallic-based nanoparticles have been investigated extensively for their robustness, effectiveness and the potential functionalization of their surface. Especially, magnetic nanoparticles with their distinctive characteristics are widely used in bioassays and magnetic drug delivery systems in biomedical applications as an anti-cancer agent. In this study, Fe/Fe3O4 core-shell nanoparticles (13-20nm) (Figure 1), coated with PEG, have been successfully synthesized by Y-junction flow tube technique. The room temperature coercivity was found to be 350 Oe. A549 non-small cell lung cancer and NIH3T3 mouse embryonic fibroblast cell lines were used to monitor cellular toxicity (IC50) and proliferation effects of PEG-ylated Fe/Fe3O4 nanoparticles. The inhibitory potential on the cell motility of A549 cancer cells were higher than NIH3T3 cells. The results showed that the toxic effect of particles was higher on cancer cells. Upon examination, IC50 values were found to be 27.5 μg/ml and 1 μg/ml for NIH3T3 cell and A549 cell, respectively. (Figure 1 a and b) The nanoparticles were found to be 100 times more toxic on cells when compared to the previous studies. According to the results, PEG-ylated Fe/Fe3O4 nanoparticles not only have significantly lower IC50 values compared to similar research but also they could be used in diagnosis, magnetic hyperthermia and various drug systems as a treatment method against cancer. This work was supported by BAP.2018-2.05References: Domac, B. H., AlKhatib, S., Zirhli, O., Akdogan, N. G., Öcal Dirican, S. C., Bulut, G., & Akdogan, O., Effects of PEGylated Fe–Fe3O4 core-shell nanoparticles on NIH3T3 and A549 cell lines. Heliyon, 6(1), (2020).

Effects of PEGylated Fe/Fe3O4 Core Shell Nanoparticles on Cancer Cells

Combining two materials having different functional properties has become a current research area for biomedical applications. The progress of nanoplatforms brings a new non-invasive imaging and therapeutic tools for cancer. Here, biocompatible and multifunctional magnetic Fe3O4-ZnO core–shell nanostructure has been developed by using a soft chemical approach. Fe3O4-ZnO core–shell is well characterized by various sophisticated characterization techniques such as X-ray diffraction (XRD), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-visible absorption, photo-luminescent spectroscopy and physical properties measurement system (PPMS). X-ray diffraction (XRD) confirms the presence of both Fe3O4 and ZnO phases. XPS analysis confirms the magnetite phase of Fe3O4 nanodots in the core-shell structures. TEM micrograph clearly reveals that 5 nm Fe3O4 nanodots are well decorated with ZnO nanoparticles shell. This multi-functional approach will facilitate magnetic targeting and generation of reactive species under an external AC magnetic field, providing therapy a smart therapeutic agent for cancer. Fe3O4-ZnO core–shell conserves the intrinsic superparamagnetic behaviour of its constituent Fe3O4 with a magnetization value of ~ 36.5 emu/g. The aqueous colloidal stability, optimal magnetization, and good specific absorption rate (under external AC magnetic field) of Fe3O4-ZnO core–shell particles perform as an effective heating source for thermal therapy. Cytotoxicity studies exhibit the reduction of cellular viability for human cervical cancer cells (HeLa). Further, A combined effect of thermo-therapy and generation of reactive oxygen species under AC magnetic field results in a synergistic effect on the cancer cell death. Therefore, these results indicate that core–shell is sufficient materials for magnetic hyperthermia and bio-imaging.References: 1. J. Wan, H. Li, and K. Chen, Materials Chemistry and Physics, 114, 2009, 30-32. 2. H. Zhang, L. Guo, S. Ding, J. Xiong, and B. Chen, Oncotarget. 7, 2016, 36602–36613. 3. K. Patel, B. S. Raj, Y. Chen, and X.Lou,Colloids and Surfaces B: Biointerfaces, 150, 2017, 317–325.

Core-Shell ZnO@Fe3O4 Nanostructure for Magnetic Hyperthermia and Bio-Imaging Applications

Transcranial Magnetic Stimulation (TMS) is a non-invasive technique of modulating the neurons of the brain that functions on the principle of electromagnetic induction to treat disorders.1 TMS, which has been employed in research and clinically, has proven to be a useful tool for the treatment for certain neurological and psychiatric disorders.2 The modulation of the neurons of the brain is dependent on the magnitude and direction of the electric field (E-Field) induced from the time-varying magnetic field generated from TMS coils. The coil geometry and orientation are known to define the E-Field magnitude and direction and to achieve high effectiveness, and these parameters are required to be optimized.3 In recent years, researchers have proposed several coil designs for TMS with the aim of achieving focality and deep penetration.4 Amongst this design is the commercially available Figure-of-Eight Coil and the novel Quadruple Butterfly Coil (QBC).5 The QBC consists of two sets of coils that are inclined at an angle of 450 to the vertical axis.6 In this research, the QBC orientation in steps of 150 within a range of ± 600 over the vertex location was modeled using a finite element tool, Sim4life, the maximum electric field intensity (E-Max) and stimulated volume of the brain (V-Half) were computed and compared to determine the optimal coil orientation. Fig. 1 shows the different QBC orientation and the E-Max distribution in the grey matter of the brain.References: 1Zhong, X., Rastogi, P., Wang, Y., Lee, E. G., & Jiles, D. C. (2019). IEEE Transactions on Magnetics, 55(7), 1–5. https://doi.org/10.1109/TMAG.2018.2890069 2Chail, A., Saini, R. K., Bhat, P. S., Srivastava, K., & Chauhan, V. (2018). Industrial Psychiatry Journal, 27(2), 172–180. https://doi.org/10.4103/ipj.ipj_88_18 3Reijonen, J., Säisänen, L., Könönen, M., Mohammadi, A., & Julkunen, P. (2020). Journal of Neuroscience Methods, 331, 108521. https://doi.org/10.1016/j.jneumeth.2019.108521 4Deng, Z.-D., Lisanby, S. H., & Peterchev, A. V. (2013). Brain Stimulation, 6(1), 1–13. https://doi.org/10.1016/j.brs.2012.02.005 5Rastogi, P., Lee, E. G., Hadimani, R. L., & Jiles, D. C. (2017). AIP Advances, 7(5), 056705. https://doi.org/10.1063/1.4973604 6Rastogi, Priyam (2019), Iowa State University, Ames, IA, USA, https://lib.dr.iastate.edu/etd/17547

Effect of Coil Positioning and Orientation of the Quadruple Butterfly Coil during Transcranial Magnetic Stimulation

Co1-xMgxFe2O4（x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0）nanoparticles surrounded by amorphous SiO2 with particle sizes around 5 nm were prepared by an original wet chemical method. Furthermore, in order to develop the particles for conjugation with proteins, thiol group was modified. These particles were further confirmed as an agent for hyperthermia treatment. The heat dissipation characteristics of the particles were investigated by measuring AC magnetic susceptibility and the relationship between the imaginary part of AC magnetic susceptibility χ” and the increase in temperature in the AC field was estimated. We have carried out spin echo MRI measurements for Co1-xMgxFe2O4 nanoparticles using a 0.3-T MRI system. The particles showed drastic T2 shortening effect comparing to agarose as background depending on the composition. All the particles showed effective relaxivity, R2 values and significant contrast was observed in the phantom image. These particles are expected for future agents for theranostics.References: [1] T. Kondo, K. Mori, M. Hachisu, T. Yamazaki, D. Okamoto, M. Watanabe, K. Gonda, H. Tada, Y. Hamada, M. Takano, N. Ohuchi, and Y. Ichiyanagi, J. Appl. Phys. 11717D157 (2015). [2] D. Shigeoka, T. Yamazaki, T. Ishikawa, K. Miike, K. Fujiwara, T. Ide, A. Usui, Y. Hosokai, H. Saito and Y. Ichiyanagi, IEEE Trans. Magn., 54 6100707 (2018).


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Magnetic texture evolution within uniaxial kagome ferromagnets at constant heating/cooling rates

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