United States

Exotic massless fermions have been predicted in a family of transition metal silicides and germanides, and experiments have provided evidence for these chiral fermions and their associated long Fermi-arc surface states.1,2 Additionally, a recent report has predicted that the topological properties in the electronic structure may be preserved even in the presence of complete structural disorder (e.g. amorphous materials).3 In this talk, we will discuss studies of the magnetism and transport signatures in a series of amorphous transition metal silicide and germanide thin films, MxZ1-x (M=Fe, Co; Z=Si, Ge; x=0.45-0.71). In the amorphous CoxZ1-x thin films, we will show a positive magnetoresistance at low temperature which crosses from a quadratic dependence at low field to a non-saturating linear dependence at high field (up to 14T). We will suggest these results may be an indication of Abrikosov’s quantum magnetoresistance. We will also show that not only do the amorphous thin films of FexZ1-x exhibit Stoner model-type itinerant ferromagnetism over the whole range of x studied, the equiatomic films have a larger moment per Fe than their crystalline counterparts.4,5 In contrast, the CoxZ1-x amorphous thin films only become weakly magnetic at x=0.6 and the moment per Co atom in these films scales non-linearly with Co concentration, unlike in the case of Fe. The origin of this difference between M=Fe, Co as well as the differences between Z=Si,Ge on the ferromagnetic properties will be discussed. Ultimately we show that these amorphous MxZ1-x alloys are highly tunable magnetic materials which have potential for the development of novel magnetic memory technologies. This work was supported primarily by the Department of Energy, Materials Sciences and Engineering Division Contract DE-AC02-05-CH11231 within the LBNL magnetism (NEMM) program, and in part through the Australian Research Council Centre of Excellence in Future Low Energy Electronics Technologies CE170100039 and the Australian Research Council Discovery Project DP200102477, using the Melbourne Centre for Nanofabrication of the Australian National Fabrication Facility.References: 1P. Tang et al., Phys. Rev. Lett. 119, 206402 (2017) 2D. Takane et al., Phys. Rev. Lett. 122, 076402 (2019) 3Y.B. Yang et al., Phys Rev Lett 123 076401 (2019) 4D.S. Bouma et al., Phys Rev B 101 014402 (2020) 5J. Karel et al., in preparation

MMM 2020

Magnetism and Transport Signatures in Amorphous Transition Metal Silicide and Transitional Metal Germanide Thin Films

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

The increasing demand of energy is requiring the continuous diversification of the production sources and the consideration of alternative potential resources such as the waste heat from industrial processes or residential heating systems. New devices and materials must be designed to recover efficiently the waste heat and among them, the thermomagnetic materials are an interesting alternative. The ideal thermomagnetic materials for energy harvesting devices should have a sharp ferro-paramagnetic phase transition, with minimal hysteresis and minimal latent heat and be accurately tuned to the available waste-heat temperature. The main objective of this work is to thoroughly investigate the thermomagnetic characteristics and their connection with the microstructural features for Fe79.7-xETMxNb0.3B20 (ETM = Cr, Mn; x = 11.5÷20 at.%) rapidly quenched alloys, depending on the ETM content. The possibility to tailor TC around room temperature and their minimal hysteresis recommends these glassy alloys for energy conversion applications. UHR-TEM studies indicate glassy structures with clusters of 2-3 nm embedded within an amorphous matrix. Mn additions double the saturation magnetization (80 emu/g) compared with Cr compositions (40 emu/g). The larger the ETM content (up to 20 at.%), the more reduced the Curie temperature is (from +450C down to -1000C). However, the Curie temperature is strongly dependent on the addition element. Mn is antiferromagnetic and behaves in a similar way as Cr does. Thus, the larger the Mn content the more numerous the antiferromagnetic interactions are, suggesting a transition temperature evolving more towards a Néel temperature for Mn contents over 16 at.%. Our aim is to study the complex interrelation among structure, mechanical, magnetic and thermal properties and, subsequently, to design new energy conversion devices with enhanced efficiency. 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.

Fe-ETM-Nb-B (ETM = Cr, Mn) glassy alloys for energy recovery

Magnetostriction plays a key role in magnetization dynamics inside soft magnetic materials, which are widely used in electronic devices for inductor cores. In general, the performance of the soft magnetic device is governed by magnetization dynamics such as domain-wall motions. Thus, it is indispensable to understand how magnetostriction affects the magnetization dynamics. We performed micromagnetic simulations in order to clarify the effects of the magnetostriction. To simulate the magnetostriction, we have to solve the elastic equations of the crystal structure.[1,2] The magnetization generates strains of the crystal structure, and deformations of the crystal are affected by the distant magnetization. We solved equations of motions for the strains, and we obtained the magnetostrictive field. We parallelized simulations using Message Passing Interface.[3] Figure 1 shows a simulation model containing two grains. Inside the grains, we consider strip-shape magnetic domains. The crystal axes are rotated -20 and 20 degrees from the x-direction in the grain A and B, respectively. We applied AC magnetic field along the x-direction. The frequency of the AC magnetic field is 100MHz. We also applied static external stress parallel to the crystal axis of the grain A. The magnetic material is Fe. Figure 2 shows energy losses as a function of the external stress using the simulation results. When the tensile strain is 20 MPa, the energy loss is 1.8 times larger than that without stress. The results of this work show that the energy loss strongly depends on the stress and the microstructure of the soft magnetic materials.References: [1] J.X. Zhang, and L.Q. Chen, Acta Materialia 53, 2845 (2005). [2] S. Bhattacharyya, T.W. Heo, K. Chang, and L.-Q. Chen, Modelling Simul. Mater. Sci. Eng. 19 035002 (2011). [3] H. Tsukahara, K. Iwano, C. Mitsumata, T. Ishikawa, K. Ono, Comput. Phys. Commun, 207, 217 (2016).

Effects of magnetostriction on domain wall dynamics inside multi-grains soft magnet revealed by micromagnetic simulations

The modification in magnetic films of the local magnetic properties is a fruitful strategy to achieve or optimize application required functionalities as it was shown on the development of spin waves phase shifters [1] and that of magnetic field sensors [2]. This work focuses on the use of synchrotron nanobeams to explore the possibility of modifying, at length scale of the beam size, the magnetic anisotropy of materials suitable for implementing spin waves guides. Our samples were amorphous Fe80B20 stripes (15 nm thick, 700 µm long and 15 µm wide) that were prepared using UV lithography from films grown by using pulsed laser ablation deposition. Irradiation, at the ID16B-ESRF, was performed by using a 56 nm x 56 nm spot size, an energy maximum at E = 17.5 keV and a flux of 2.21 x 1011 photons/s. Our characterization of the irradiation outcome, Figure 1, revealed that the local coercive force, measured by means of a Kerr effect device at points in between different irradiated regions (IR), in the same stripe and separated by 100 µm, increased with the IR width (varied by scanning in different areas the nanobeam). That increase can be understood by recalling that the reversal mechanisms of our stripes corresponds to the wall propagation/pinning type [3]. The wall involved in the reversal gets pinned when it finds a region at which it lowers its energy (as, for instance, a reduced anisotropy region). The depinning field (that is, the local coercivity) is related both to the local anisotropy decrease and to the domain wall width that is, approximately, 350 nm [3]. When the width of the IR is lower than that of the domain wall the local coercivity can be approximated to the local anisotropy reduction induced by the irradiation (local amorphous relaxation) multiplied by the ratio of the IR width to the wall one. Differently, when the wall is narrower than the IR the coercivity gets almost constant and measures, to a first approach, the difference in anisotropy energy between the irradiated and non-irradiated zones. We conclude about the need of implementing IR widths larger than ca. 500 nm in order to adequately stabilize a propagating wall.References: [1] R. Hertel, W. Wulfhekel, and J. Kirschner, "Domain-wall induced phase shifts in spin waves", Physical Review Letters, vol. 93, no. 25, p. 257202 (2004) [2] Isaac Koh and Lee Josephson, “Magnetic Nanoparticle Sensors”, Sensors, vol. 9, p. 8130-8145 (2009) [3] U. Urdiroz et al., "Antiphase resonance at x-ray irradiated microregions in amorphous Fe80B20 stripes," Journal of Magnetism and Magnetic Materials, https://doi.org/10.1016/j.jmmm.2020.167017

Local Coercivity at X-rays Nanobeam Irradiated Regions in Amorphous Fe80B20 stripes

The interesting magnetic behavior of chemically resistant and biologically inert glass-coated amorphous magnetic microwires has been of enduring interest since the mid-1980’s. However, their response under different mechanical loading conditions, of great interest for numerous sensor applications, has not been thoroughly explored. Here we present data connecting the magnetoelastic response of glass-coated amorphous ferromagnetic microwires with mechanical loading, revealing an exceptional magnetomechanical response. A selection of amorphous glass-coated Fe73Si11B13Nb3 microwire samples (100-110 μm outer diameter, 4 cm in length) were evenly covered with a thin layer of electrically conducting silver paint (Coating 1), electrically insulating nitrocellulose (Coating 2), or were left uncoated. The magnetomechanical response (resonant frequency and amplitude) of the wires was probed in the frequency range 30-100 kHz using a wireless pickup coil set-up of fixed 1.5 V voltage and a modulated current. It was found that the microwire resonant frequency systematically decreased on the order of ~3 kHz/mg, Fig. 1(a), independent of coating type, Fig. 1(b). These data confirm a shift in the magnetoelastic resonant frequency response of microwires that is approximately 100 times greater than that reported for Metglass amorphous magnetic ribbons (ref = Grimes et al.). These findings quantify the lower detection limits of mass loading in glass-coated amorphous magnetic microwires, where microgram differences in mass can be detected wirelessly and with no applied bias field. This exceptional responsiveness to mass loading, along with their small diameter, highlights the promise of magnetic microwires for a variety of sensor applications, including as biosensors, for civil infrastructure monitoring, and for anti-counterfeit measures. Acknowledgements – Fulbright España, I-Link A20074 (CSIC), Spanish Ministry of Science and Innovation RTI2018-095856-B-C21 and Comunidad de Madrid NANOMAGCOST S2018/NMT-4321References: [1] C.A. Grimes, S.C. Roy, S. Rani, Q. Cai, Theory, Instrumentation and Applications of Magnetoelastic Resonance Sensors: A Review, 18 (2011) 2809–2844. doi:10.3390/s110302809.

Exceptional Magnetomechanical Response of Glass-coated Amorphous Microwires

Magnetic Tunnel Junction (MTJ) consisting of two ferromagnetic electrodes separated by thin oxide layer are used as magnetic field sensors in read-write heads as well as in spintronic logic devices. High tunnel magnetoresistance, of the order of 1000 percent is a prerequisite for such applications. Till date highest tunnel magnetoresistance is observed in MTJs consisting of CoFeB as magnetic electrode with thin MgO tunnel barrier. Post-deposition thermal annealing is an important step in which amorphous CoFeB is partially crystallized to form bcc-CoFe phase, resulting in enhancement of TMR by several orders of magnitude. In recent years buffer layers of refractory metals (W, Mo, Hf) have been used to induce perpendicular magnetic anisotropy in CoFeB films [1]. Since in a MTJ multilayer, we would like to obtain depth-resolved information about structural changes occurring in CoFeB layer with temperature. X-ray diffraction understanding wave conditions were used for this purpose. X-ray standing waves are generated using total external reflection (TER) of X-rays by the W, Mo & Hf buffer layer. We used an x-ray waveguide structure: substrate/W(30nm)/CoFeB(25nm)/ W(2nm). Various waveguide modes (TE0, TE1 etc.) can be excited in the multilayer by varying the angle of incidence of x-rays. TE0 and TE1 provide information about the center of sample and both the interfaces, respectively. In this view, GIXRD measurements were performed at PF, KEK, JAPAN, at various grazing angles to see the transformation of amorphous matrix into nano-crystalline phase. Crystallization starts from the center of film. Also, there is one additional peaks present near CoFe(110) peak at the interface. In addition, one can see that the samples have uniaxial magnetic anisotropy at its as-deposited state and show a large increase in the coercivity. Also, 250°C annealed film shows the different behavior at hard axis(figure 1).References: [1] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H. D. Gan, M. Endo, S. Kanai, J. Hayakawa, F. Matsukura, and H. Ohno, Nature Mater. 9, 721 (2010).

Depth profiling study of magnetic CoFeB thin film

Magnetization reversal in 1D structures attracts increasing interest due to its connection with the development of novel spintronics, logic devices, actuators and manipulators for nano-sized objects. Amorphous micro-scale objects (stripes, films, rods) are attractive as materials with an extremely high magnetic softness, but difficult to be studied because of amorphous state and macro scale. Magnetic domain structure and magnetization reversal mechanism in amorphous fibers are defined by magnetoelastic energy, which leads to (i) very sensitive tunability of magnetic anisotropy and (ii) unusually fast domain wall propagation when reversing the magnetization. The role of relaxation processes under current annealing and conventional annealing in initially stressed amorphous materials is considered as a part of the complex process involving both magnetostriction constant change and stress components redistribution [1, 2]: a sign of the magnetostriction can be changed through annealing, which leads to the change of a type of magnetic domain structure. Moreover, the relationships between anisotropy constant and characteristics of domain wall dynamics are established: anisotropy value at the fiber axis enters the expressions for domain wall width, length, slope, velocity and mobility, whereas rate of anisotropy radial variation influences only on the length of domain wall [3]. Configuration of the head-to-head domain wall is visualized as a particular solution of Landau-Lifshitz-Gilbert equation. 1D amorphous fibers are considered as prospective magnetic materials suitable for multiferroic applications especially when they are incorporated into piezo-polymer matrices [4]. Due to the sensitivity of soft magnetic materials to low magnetic field, this approach can be used for the design of new self-biased magnetoelectric composites that provide large ME coupling under an external AC magnetic field in the absence of a DC magnetic field.References: 1. K. Chichay, V. Rodionova et al., Journal of Alloys and Compounds 835 (2020) 154843; 2. G. Nematov, I. Baraban et al., Journal of Alloys and Compounds 837 (2020) 155584; 3. M. Vereshchagin, I. Baraban et al., Journal of Magnetism and Magnetic Materials 504 (2020) 166646; 4. A. Amirov, V. Rodionova et al., Materials, 13, 916 (2020).

Tunable Magnetic Anisotropy In Amorphous-Fiber-Based Structures For Multiferroic Applications INVITED

Two-dimensional MoSe is a multipurpose material that can be made into valleytronics and spintronic devices, quantum dots led, hydrogen evolution reaction catalyst, and ion-batteries. All the above functions are sensitive in any structural variation. The Cr-doped effect turns the MoSe2 from diamagnetism to paramagnetism. Room temperature Raman spectra show different behaviors between pure (x=0) and half Cr-doped (x=0.5) samples. The out-of-phase A1g vibration mode along c-axis of the crystal, shows an enhancement in the Crdoped MoSe2 but weaken in pure one. Both of two samples will return to their original status as magnetic field removed. In contrast, the applying external electric field broadening the FWHM of A1g mode in both samples and will not narrow down again as electric field removed. Furthermore, the Cr-doped MoSe2 also shows less influenced by the electric field. Varied-temperature x-ray diffraction (xrd) and Raman spectroscopy were studied. The temperature profile of xrd {002} peak position is associated with that's of the FWHM of A1g mode of Raman spectra. This observation shows how the MoSe2 interlayer's spacing influent the phonon softening behavior. The varied-temperature Raman spectra of MoSe2 sample display two peak broadening maxima of the A mode at Ta =150 K and Tb =220 K. Similar phenomena also appeared in Cr-doped one, but the Ta' and Tb' shifted to the higher temperature of 180 K and 260 K, respectively. This observation refers to both phonon softening temperatures are influent by Crdoped effect. As the 200 mT magnetic field applied, all of Ta , Tb , Ta' , and Tb' disappeared in both samples. The magnetic field reduces the phonon softening no matter Cr-doped or not. This study can be a reference for how the lattice and energy gap be affected under electric and magnetic filed as these materials are made as and electronic devices.References: 1. John R. Schaibley, Hongyi Yu, Genevieve Clark, Pasqual Rivera, Jason S. Ross, Kyle L. Seyler, Wang Yao & Xiaodong Xu, Nature Reviews Materials 1, 16055 (2016). 2. Wenwu Guo, Quyet Van Le, Amirhossein Hasani, Tae Hyung Lee, Ho Won Jang, Zhengtang Luo, and Soo Young Kim, Polymers 10, 1309 (2018). 3. Philipp Tonndorf, Robert Schmidt, Philipp Boettger, Rudolf Bratschitsch, Optics Express 21, 4908-4916 (2013). 4. Dahyun Nam, Jae-Ung Lee, and Hyeonsik Cheong, Scientific Reports 5, 17113 (2015).



Effect of magnetic and electric fields work on Cr-doped Mo1-xCrxSe2 (x=0, 0.5) nanosheet

Since the discovery of graphene, researches on two-dimensional (2D) materials have attracted much attention because of the good controllability of material properties. In 2018, 2D ferromagnetic metal Fe3GeTe2 devices have been developed [1], where the Curie temperature TC is originally 220 K and enhanced above room temperature by gating. In the next year, a cleavable ferromagnetic metal Fe5GeTe2 with TC = 310 K has been synthesized [2]. Although this material could be useful for future van der Waals (vdW) spintronic devices, magnetic properties of the thin Fe5GeTe2 films are not well established yet. In this work, we have fabricated thin film devices of both quenched (Q) and non-quenched (NQ) Fe5GeTe2 with various thicknesses and measured the Hall resistivities in a wide temperature range [3]. Figure 1 shows typical anomalous Hall effect (AHE) curves measured with both Q- and NQ- Fe5GeTe2 thin film (less than 10 atomic layer (L)) devices at T = 50 K. While any significant differences between Q and NQ devices have not been observed for thicker film (more than 10 L) devices (not shown here), the coercive field Hc of 5L Q-Fe5GeTe2 is about two times larger than that of 6L NQ-Fe5GeTe2. To investigate more details, we have plotted Hc as a function of temperature with both Q- and NQ-Fe5GeTe2 in Fig. 2. For comparison, Hc of thicker Q- and NQ-Fe5GeTe2 are also plotted. For all the devices, Hc starts to increase below 200 K, but the enhancement of Hc for thinner Q-Fe5GeTe2 can be clearly seen below 120 K, where one of the Fe atoms would be ferrimagnetically ordered. This work has demonstrated that the coercive field can be controlled by the film thickness and the synthesis method, and such a controllability of the perpendicular magnetic anisotropy should be useful for future vdW spintronic devices.References: [1] Y. Deng et al., Nature 563, 94 (2018). [2] A. F. May et al., ACS Nano 13, 4436 (2019). [3] T. Ohta et al., Appl. Phys. Express 13, 043005 (2020).

Enhancement of coercive field in van der Waals ferromagnet Fe5GeTe2

The recent discoveries of intrinsic ferromagnetism (FM) in two-dimensional (2D) dilute magnetic semiconductors with high Curie temperature (> 300 K) capable of spintronic applications are among the most impressive tour de force achieved by theoretical and experimental physicists in the field [1,2]. However, the origin of FM in these systems has remained an open issue. The power function proposed by Bloch, M(T) = M(0)[1 - (T/TC)α] with α = 3/2, has successfully described the temperature dependence of saturation magnetization MS(T) for bulk ferromagnetic materials [3]. The extended Bloch’s law for ferromagnetic nanoparticles (α-Fe) embedded in a non-magnetic matrix (SiO2) has yielded higher values of α associated with the reduced dimensionality effect [4]. In this study, we demonstrate that V doping into WS2 monolayer induces ferromagnetic couplings between the vanadium and its tungsten neighbors, forming nanosized ferromagnetic clusters that behave like ferromagnetic nanoparticles. The MS(T) dependence of the nanoclusters in V-WS2 monolayer samples with different V concentrations (0, 0.4, 2, and 8 at.%) is well described by the modified Bloch’s law at temperatures higher than the blocking temperature. Relative to the Bloch coefficient (α = 3/2), the higher values of α (up to 5.96) and its dopant concentration dependence reveal the dimensional effect and the magnetic interaction between the nanoclusters. This method has been successfully applied to describe the M(T) dependence of other 2D magnetic systems such as VSe2 monolayers that exhibit similar magnetic behavior. Our findings also offer a new way to predict the Curie temperatures of low-dimensional magnetic materials that may not be measured experimentally.References: 1. S. J. Yun, D. L. Duong, D. M. Ha, K. Singh, T. L. Phan, W. Choi, Y. M. Kim, Y. H. Lee, Adv. Sci. 7, 1903076 (2020). 2. F. Zhang, B. Y. Zheng, A. Sebastian, H. Olson, M. Z. Liu, K. Fujisawa, Y. T. H. Pham, V. J. Ortiz, V. Kalappattil, T. Zhang, R. Pendurthi, Y. Lei, A. Laura Elias, Y. Wang, P. E. Hopkins, S. Das, V. H Crespi, M. H. Phan, M. Terrones, Adv. Sci. 2020 (https://arxiv.org/abs/2005.01965) 3. F. Bloch, Z. Phys. 61, 206 (1930). 4. G. Xiao, C.L. Chien, J. Appl. Phys, 61, 8 (1987).

Extended Bloch’s law description of two-dimensional dilute magnetic semiconductors

Magneto-optical (MO) thin films can be used for applications in various ways such as MO microscopy, optical isolaters, and information storage devices. Although it is widely investigated, growth of high Faraday rotation magnetic thin films on glass substrate is still considered as a primary issue in MO thin films. Here, we have investigated enhancement of MO properties in praseodymium-substituted bismuth yttrium iron garnet (PrxBi1Y2-xFe5O12) thin film on a glass substrate by using a metallo-organic decomposition (MOD) method. To improve the MO performance of the as-prepared PrxBi1Y2-xFe5O12 thin film, different concentrations of Pr-substituted bismuth yttrium iron garnet were prepared. It shows that PrxBi1Y2-xFe5O12 thin film with an optimal concentration of x=1 exhibited high crystallinity, high saturation magnetization and the highest Faraday rotation (FR) angle of – 6.50/μm at a wavelength of 510 nm as shown in figure below. The Pr1Bi1Y1Fe5O12 thin film fabricated by the modified MOD method displayed excellent MO performance and is a potential candidate for application in optical devices.

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