United States

Searching for the efficient manipulation of antiferromagnets for spintronic applications, we study the properties of the magnetic domain walls with due account of magnetoleastic effects. We show that magnetoelastic coupling induces anisotropy of the domain wall orientation which affects the shape and distribution of the domain walls in a sample. In case of non-1800 domains such anisotropy favours formation of the plane walls oriented along or perpendicular the hard magnetic axis. We study the pinning effects caused by magnetoelasticity and calculate the frequency of the localized oscillations and depinning field for an easy-plane antiferromagnet. We further consider dynamics of excitations of magnetoelastic domain wall and show that such a wall effectively scatters the long-wave magnons. The scattering and pinning effects are more pronounced in case of non-1800 domains, due to the strain-induced anisotropy inside domains. However, 1800 domain wall can also scatter the magnons due to the magnetoelastic strains inside the wall.To illustrate possible applications of our results we discuss the magnon propagation in multidomain hematite and NiO.

MMM 2020

Domain Walls in Antiferromagnets with Strong Magnetoelastic Coupling

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

Recently manipulation of antiferromagnetic (AFM) order has been gaining the attention of the spintronics community [1]. Magnetic switching has been reported in AFM thin films based on electronic transport methods, which provide only spatially averaged information on AFM states. Other techniques, such as x- ray magnetic linear dichroism (XMLD) microscopy, can provide local information on AFM domains. In this study, we report the direct observation of spin reorientation in response to current pulses in (0001)-oriented α-Fe2O3 films [2,3]. Our experiment combines the application of current pulses and spatially resolved photoemission electron microscopy (PEEM) with XMLD to detect AFM contrast. Pulses were applied in two different configurations A and B (Fig. 1a), which results in resistance changes (Fig. 1b), and XMLD images were taken before and after the application of a current pulse to reveal changes in the AFM domains (Fig. 1c). Our analysis shows that electrical pulses do not only reorient the Néel vector within the easy plane as previous work indicates [2,3]. Instead, we conclude that most of the change associated with the Néel vector is out of the film plane and different pulse directions can bring the AFM order in and out of the plane deterministically. This work is supported in part by the Air Force Office of Scientific Research under Grant FA9550- 19-1-0307.References: [1] V. Baltz et al., Rev. Mod. Phys. 90, 015005 (2018) [2] Y. Cheng et al., Phys. Rev. Lett. 124, 027202 (2020) [3] P. Zhang et al., Phys. Rev. Lett. 123, 247206 (2019)

X-ray Magnetic Linear Dichroism Studies of Electrical Switching of Antiferromagnetic Order in α-Fe2O3 Epitaxial Films

In this work, we study the canted antiferromagnet α-Fe2O3. At room temperature, the Dzyaloshinskii-Moriya interaction (DMI) induces canting of the two magnetic sublattices MA and MB giving rise to a weak ferromagnetic (FM) moment m. We employ spin Hall magnetoresistance (SMR) measurements to study electrical and magnetic field control of the Néel order in α-Fe2O3/Pt bilayers. The angle-dependent magnetoresistance (ADMR) shows the characteristic saturated negative SMR signal of AFMs (red curve in figure 1) agreeing with previous findings1–4. We note that the FM moment lifts the degeneracy of the sublattices and consequently the orientation of the Néel vector with respect to the applied magnetic field. The blue curve in figure 1 shows the remanent resistance as a function of the angle of the preceding saturating magnetic field. The data shows a simple scaling between the saturated ADMR signal and the remanent resistance. The lack of signatures of anisotropy in the remanent signal implies that the relaxation of the SMR signal is dominated by destressing effects favoring an equilibrium state with multiple domains distributing the orientation of the Néel vector along different directions. Applying the insights from investigating the remanence of α-Fe2O3 with an external magnetic field on our current-induced switching experiments, we find that the fundamental limit of the switching efficiency is given by the magnitude of the remanence Furthermore, we observe hysteresis in the SMR signal. The irreversibility upon switching the direction of the magnetic field in combination with investigating the transient effects of current-induced changes in the domain structure allows us to study the nature and formation of domain walls at low magnetic fields. Our work emphasizes the importance of the FM moment induced by DMI and the relaxation effects for the equilibrium domain structure in α-Fe2O3.References: 1. Lebrun, R. et al. Anisotropies and magnetic phase transitions in insulating antiferromagnets determined by a Spin-Hall magnetoresistance probe. Commun. Phys. 2, 50 (2019). 2. Cheng, Y., Yu, S., Zhu, M., Hwang, J. & Yang, F. Electrical Switching of Tristate Antiferromagnetic Néel Order in a-Fe2O3 Epitaxial Films. Phys. Rev. Lett. 124, 027202 (2020). 3. Fischer, J. et al. Large Spin Hall Magnetoresistance in Antiferromagnetic a-Fe2O3/Pt Heterostructures. Phys. Rev. Appl. 13, 014019 (2020). 4. Zhang, P., Finley, J., Safi, T. & Liu, L. Quantitative Study on Current-Induced Effect in an Antiferromagnet Insulator/Pt Bilayer Film. Phys. Rev. Lett. 123, 247206 (2019).

Importance of the Ferromagnetic Moment Induced by Dzyaloshinskii-Moriya Interaction in a Canted Antiferromagnet for Electrical and Magnetic Field Control of the Néel Order

Magnons and phonons are two elementary excitations associated with spin waves and lattice vibrations in solids. In the presence of magneto-elastic interactions, the coupling of the spin and lattice degrees of freedom leads to the formation of a new type of hybridized excitation, i.e., magnon-polarons. Manifested as anomalies in the spin Seebeck effect (SSE) in a ferrimagnet YIG, magnon-polarons have profound consequences in modifying the magnon and phonon dispersion relations and affecting spin transport properties in ferrimagnetic insulators. In antiferromagnetic insulators, the magnon spectrum is distinctly different due to the existence of two equivalent, antiferromagnetically coupled spin sub-lattices. Here we report an observation of antiferromagnetic magnon-polarons in Cr2O3, a prototypical uniaxial antiferromagnet, featured as anomalies in the SSE signals of Cr2O3/heavy metal heterostructures. Despite the relatively high magnon energy in the absence of an external magnetic field, near the spin-flop transition of ~ 6 T, the left-handed magnon spectrum shifts downward to intersect those of acoustic phonons (both longitudinal and transverse) to cause hybridization. We have measured the temperature dependence of the SSE magnon-polaron anomalies in Cr2O3 and revealed the strength of the magneto-elastic coupling. Our study indicates that the hybridized excitations can be employed to manipulate spin transport in antiferromagnetic systems. This work was supported as part of the SHINES, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under Award No. SC0012670.

Observation of magnon-polarons in a uniaxial antiferromagnetic insulator Cr2O3

The CoxFe3-xO4 (0.8≤x≤0) ferrites based magnetic nanoparticle with single domain nature have potential applications in the development ofthe high-density memory devices, magnetic fluid hyperthermia and biomedicine. The present work deals with the experimental investigation of oleic acid coated CoxFe3-xO4 (0.8≤x≤0) nano ferrite based ferrofluids synthesized by the wet chemical co-precipitation method and oleic acid used as surfactant to prevent the agglomeration of magnetic nanoparticles.Structural and magnetic properties of these ferrofluids have been investigated using different characterization techniques. The structural, cations distribution and Rietveld refinement was carried out by the XRD data and the result confirms that single cubic spinel phase belongs to Fd3m space group and the average crystallite size calculated from the XRD data varies from 11 to 14 nm.The Transmission Electron Microscopy (TEM) studies revealed that the particles are spherical in nature and mean particle size varies from 10 to 15 nm and all samples are polycrystalline in nature.The SEM result were also demonstrate the surface morphology of magnetic particle are spherical in shape nanoparticle with least agglomeration. FTIR spectroscopy showed the molecular dynamics and the formation of cubic spinel phase of frequency bands at 545 cm-1and 404 cm-1respectively.The UV-Visible measurement we have estimate the band gap of the materials lies between(2.2 to 2.8& 3.1 to 3.8 e V) of the all the sample form optical absorption spectra fitting by Tauc plot method. The room temperature Vibrating Sample Magnetometer(VSM) analysis revealed that the saturation magnetization and coercivity value varies from 26.18 to 61.78 emu/g and 31.45 to 128.36 Oe, respectively by the increasing the Co concentration. These magnetic properties can be enhanced by the distribution of cations on both A-site and B-site.References: 1.Clara Pereira et al., J. Phys. Chem. C 2016, 120, 2, 1328-1341. 2. R. Tackett et al., JMMM (2008), 320 2755– 2759.

Optimization of cobalt concentration & cation distribution for improved magnetic characteristics and stability of CoxFe3-xO4 mixed ferrite magnetic fluids.

An undulator is a device including two parallel arrays of magnets (separated by a gap) with a polarity in an alternating manner (↑↓↑↓↑↓…). The undulator is an essential part of a Free Electron Laser (FEL) upon which a large body of literature has expressed interest for the applications on various areas such as defence industry, material science and medicine. Also, sub-millimeter period undulators are another interesting approach. The current state of the art of X-FEL uses undulator period down to 3 cm which creates an output radiation wavelength down to 1.5 A using ~14GeV e- beam, ergo big and costly e-beam source is needed. By developing a sub-mm period undulator (i.e. a period of 300µm), it is possible to obtain the same output radiation wavelength with a much less e-beam energy of ~1.4 GeV which could be revolutionary since much smaller FELs could be constructed. Undulators are constructed using either electromagnets or permanent magnets. The latter is the choice due to the fact that no additional power supply is needed. On the other hand, there is a growing interest on L10 phase FePt thin films on the magnetic applications due to their high magnetic anisotropy. In this work, FePt thin films and micro magnets were synthesized by sputtering technique and their magnetic properties were extensively studied. FePt micromagnets could be utilized in next generation sub-mm period undulators. This work was supported by TUBITAK project: 118F319

Development of Sub-mm Period Undulator with L10 phase FePt Micromagnet Array

Magnetofluidic cooling (MFC, Figure 1a) is a passive heat transfer technique governed by the principle of thermomagnetic convection (TMC)1, 2. Under the effect of a thermal and magnetic field gradient, the magnetic fluid’s temperature-dependent response leads to TMC3. However, practical applications require specific device design and materials, mainly determined by the flow behavior of the magnetic fluid. However, the opaque nature of the magnetic fluid limits the experimental flow investigations. We addressed this challenge by a range of experimental setups to measure velocity profiles, vorticity, and temperature profiles. We developed a hybrid PIV method to measure velocity profiles by fabricating a bonding-free 3D PDMS device and analyzed by a MATLAB based image analysis code. The results of the PIV analysis are summarized in Figure 1. We utilized the experimental findings to develop a simulation model and studied the effect of magnetic and hydrodynamic properties of the fluid on the MFC process (Figure 2). Simulation-driven system design was performed4. A ferrofluid was developed and integrated with our MFC system, significantly enhancing device performance. We found that the MFC flow velocity, vorticity increases with increasing HL power, resulting in higher cooling at higher HL powers (Figure 2). Experiments and simulations demonstrated the role of magnetoconvection on device performance. The new device showed enhanced performance viz. cooling up to 200°C at a heat load of 10 W. Our superior device performance is attributed to increased fluid mixing and mass transfer during MFC. Acknowledgments This research is supported by grants from the National Research Foundation, Prime Minister’s Office, Singapore under its Campus of Research Excellence and Technological Enterprise (CREATE) programme.References: 1. V. B. Varma, Energy Efficient Green Magnetic Cooling, 2019. https://youtu.be/caInO_IxnHQ. CREATE Symp.- Climate Change, Singapore 2. R. V. Ramanujan, Pushing Frontiers, Issue 14, 32 (2018). 3. M. S. Pattanaik, V. B. Varma, S. K. Cheekati, G. Prasanna, N. M. Sudharsan and R. V. Ramanujan, Energy Conversion and Management, vol. 198, p. 111819 (2019). 4. V. B. Varma, M. S. Pattanaik, S. K. Cheekati and R. V. Ramanujan, Applied Energy, (2020- in Communication).

Study of Thermo-Magnetoconvection of a Magnetofluidic Cooling Device

This work aims at designing a closed-loop magnetic circuit to enhance the uniformity and intensity of the magnetic field based on an internal conductive square Magnetohydrodynamics (MHD) thruster. Figure 1(a) shows the diagram of the square MHD thruster consisting of a pair of NdFeB magnets and electrodes. Figure 1(b) shows the design of the permanent magnet set comprising two permanent magnet pieces and an iron shell. The magnetization of the magnet pair is along the +z axis, and the iron shell formed a closed-loop magnetic circuit which reduces the magnetic leakage and enhances the magnetic field at the central region i.e. z=0 mm. Theoretically, the field strength decreases when the distance from the magnet is increased. The accurate measured magnetic flux density along the y-axis on the central cross-section shown in figure 1(c) reveals that the maximum field strength increases from 0.116 T to 0.142 T after installing the 2-mm thick iron shell, which reduces the magnetic leakage and increases the uniformity of the field. We performed a systematic experiment to measure the field distribution under various width and thickness of iron casing in this work. It is noticed the thicker shell results in stronger magnetic strength inside the thruster, and the field strength becomes stronger as the width of the iron shell decreases. The thruster with an iron shell is fully submerged in the saline water and hanged on a sliding track with negligible friction to verify its performance, as shown in figure 1(d). The moving speed is increased from 0.0826m/s to 0.12 m/s when the iron shell is installed. In this paper, the effect of the material of the shell is also investigated to evaluate the optimal design to enhance the uniformity and strength of the magnetic field at the central region of square MHD thruster.

Enhancement for magnetic field strength of a magnetohydrodynamic thruster consisting of permanent magnets

Magnetic Josephson junctions, where a magnetic material is sandwiched by superconductors, show peculiar phenomena such as π junction and spin triplet due to the exchange energy of the magnetic material. Among the magnetic Josephson junctions, ferromagnetic materials have been commonly used, and the π state has been reported in ferromagnetic CuNi1). However, ferromagnet has stray field, which would affect to superconductivity. To avoid the effect of stray field, we focus on antiferromagnet Mn3GaN (MGN). MGN is a non-collinear antiferromagnetic material in which magnetic moments cancel each other out noncollinearly and has characteristic properties not found in collinear antiferromagnetic materials.2) By using MGN in the Josephson junction, not only suppress the deterioration of the superconducting characteristics, but also unconventional magnetic junction characteristics is expected. In this study, we fabricated NbN/MGN/NbN hetero-stacked structures and evaluated the crystal structure and junction characteristics. NbN and MGN layers were deposited on a MgO(001) substrate by reactive magnetron sputtering. Figure 1 shows the out-of-plane XRD pattern of NbN/MGN bilayers. Both NbN and MGN(002) and (004) peaks were observed, indicating that (001)-oriented NbN/MGN bilayers grow on the MgO(001) substrates. Square mesa junctions of NbN/MGN/NbN with in-plane size L = 20 - 100 μm were fabricated by a conventional photolithographic and liftoff process. Figure 2 shows the current-voltage properties of the junction with 2.5 nm MGN layer. Thanks to the suppression of deterioration of superconducting properties at the junction interface, IcRN is as high as 3.1 mV, which is on the same order as the theoretical value, indicating that the Josephson junctions using the non-collinear antiferromagnetic material MGN are feasible and good junction characteristics can be obtained. Our results show the possible further improvement of the magnetic junction characteristics by using non-collinear antiferromagnet and potential of non-collinear antiferromagnetic Josephson junctions.References: 1) T. Yamashita et al., Phys. Rev. Appl. 8, 054028 (2017) 2) T. Hajiri et al., Appl. Phys. Lett. 115, 052403 (2019)

Fabrication and characterization of NbN magnetic Josephson junctions using non-collinear antiferromagnet Mn3GaN

We present results of Monte Carlo study of non-equilibrium critical behavior in three-dimensional Heisenberg model in isotropic case and with anisotropy of easy axis type. We investigate relaxation Glauber-like dynamics of these models with evolution from different initial states. Usually, we can distinguish the high-temperature initial states created at temperatures T0 > Tc with initial magnetization m0 = 0 and low-temperature initial states with T0 < Tc and m0 ≠ 0. Initial states with m0 ≠ 0 give rise to a new time scale tm ~m0-k with the exponent k > 0. This time scale produces a pronounced effect on the temporal behavior of magnetization, autocorrelation function and response function [1-4]. Study of influence of initial states with 0 < m0 ≤ 1 on dynamical properties of model has been carried out. It is established realization of scaling relation for time dependence of magnetization. We calculated critical exponents for isotropic and anisotropic models with values β/zν= 0.2565(4) and z=2.035(4) for the anisotropic Heisenberg model which agree well with the exponents for the 3D Ising model. Aging has been revealed during study of two-time dependence of the autocorrelation function and dynamical susceptibility on different initial states which are characterized by the slowing down with growth of its age tw. We show that the slowing down of the correlation function during evolution from high-temperature initial state is considerably slower than in case of low-temperature initial state. Analysis of parametrical dependence of the dynamical susceptibility on the correlation function gave possibility to determine asymptotical values of the fluctuation-dissipation ratio (FDR) for isotropic X∞=0.383(6) and anisotropic X∞=0.392(7) Heisenberg models with evolution from high-temperature initial states. The obtained value of FDO for anisotropic model agrees very well with X∞=0.391(12) for the 3D Ising model calculated in [4] by the same method. This study is supported by RFBR (projects 18-42-550003, 20-32-70189), the Ministry of Education and Science of Russian Federation (state assignment 0741-2020-0002) and grant MD-2229.2020.2 of the President of the Russian Federation.References: 1. V.V. Prudnikov, P.V. Prudnikov and M.V. Mamonova, Phys. Usp., 60, 762 (2017). 2. V.V. Prudnikov, P.V. Prudnikov and E.A. Pospelov, J. Stat. Mech., 2016, 043303 (2016). 3. V.V. Prudnikov, P.V. Prudnikov, E.A. Pospelov and A N. Vakilov, Phys. Lett. A 379, 774 (2015). 4. P.V. Prudnikov, V.V. Prudnikov, E.A. Pospelov and P.N. Malyarenko, PTEP, 2015, 053A01 (2015).

Nonequilibrium Critical Behavior of the 3D Isotropic and Anisotropic Heisenberg Models

So far, finding Majorana fermion is one of the important research in condensed matter physics. In theory, zero energy Majorana modes resides on the edge or vortex core in topological superconductor, characterizing the topological phase of matter protected by particle hole symmetry. Zero bias conductance peak is one of the phenomena from Andreev reflection process in normal metal/topological interface. In this work, we use the non-equilibrium Green's function method to study the interface between the normal metal and two dimensional noncentrosymmetric superconductor which lack a center of inversion symmetry and exhibit spin orbital coupling. The tight binding hamiltonian[1] for noncentrosymmetric superconductor is used. Then, we simulate the selective equal spin Andreev reflection[2]. Equal spin Andreev reflection means if the electrons in normal metal has the same spin polarization with the electrons on the edge state of superconductor, they are reflected as holes with the same spin. Moreover, with opposite spin polarization between electrons in normal metal and electrons in the edge of superconductor, they are reflected as electrons with same spin. As the result, we can get the conductance as the function of the spin polarized angle of electron in normal metal to realize the properties of Majoranana edge state. In addition, the spin current can be injected to the noncentrosymmetric superconductor which allow both singlet and triplet cooper pairs, generating the new spintronic device.References: 1. M. Biderang, H.Yavari, Mohammad-Hossein Zare, PHYSICAL REVIEW B 98, 014524(2018) 2. PA. Lee, PHYSICAL REVIEW LETTERS 112, 037001(2014)

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Domain Walls in Antiferromagnets with Strong Magnetoelastic Coupling

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X-ray Magnetic Linear Dichroism Studies of Electrical Switching of Antiferromagnetic Order in α-Fe2O3 Epitaxial Films

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