Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B41: Novel Magnetic Structures and Excitations IFocus
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Sponsoring Units: GMAG DMP Chair: Adam Ahmed, Ohio State University Room: BCEC 209 |
Monday, March 4, 2019 11:15AM - 11:51AM |
B41.00001: Anatomy of Dzyaloshinskii–Moriya interaction at interfaces: from Fert-Levy type DMI at Co/Pt to Rashba effect induced DMI at Co/graphene Invited Speaker: Hongxin Yang The antisymmetric exchange interaction, Dzyaloshnskii-Moriya Interaction (DMI), has gained an intense research effort due to its crucial role in the creation, annihilation and the influence on the dynamics of magnetic Skyrmions and chiral domain wall. Here, we will present a general DMI calculation approach based on DFT total energy calculations and show the calculated DMI at Co/Pt interfaces and the comparison with experiments. We will address the main features and microscopic mechanism of typical Fert-Levy type DMI at Co/Pt interface from first-principles by clearing up several fundamental questions: how does the DMI extend away from the interface? Where is the corresponding electronic energy source located? [1] How to design enhanced DMI systems? [2,3] Moreover, we will go beyond Fert-Levy type DMI systems to avoid using heavy metals but with simple ferromagnet/graphene interfaces. [4] At the end, we will give a perspective on the topic with new systems which may also give large DMI but are different to FM/HM and FM/graphene. |
Monday, March 4, 2019 11:51AM - 12:03PM |
B41.00002: Few-layer dependence of Spin-Orbit torques in 1T’-MoTe2/ ferromagnet heterostructures. Gregory Stiehl, Ruofan Li, Vishakha Gupta, Ismail El Baggari, Shengwei Jiang, Hongchao Xie, Kin Fai Mak, Jie Shan, Lena F Kourkoutis, Robert Buhrman, Daniel Ralph Single-crystal materials with sufficiently low crystal symmetry can be used to generate novel forms of spin-orbit torques on adjacent ferromagnets. For example, we have previously shown that when WTe2 is used as a spin generation layer, an out-of-plane antidamping torque is generated in heterostructures of WTe2/Py that is consistent with the WTe2 crystal symmetries. Here, we present measurements of spin-orbit torques produced by another low-symmetry material, 1T’-MoTe2, which manifests different crystal symmetries as compared to WTe2. We perform systematic studies of the spin-orbit torques using spin-torque ferromagnetic resonance and second harmonic Hall techniques down to the monolayer limit. We report the presence of an out-of-plane antidamping torque in MoTe2/Py heterostructures when the MoTe2 is a monolayer or trilayer thick, but surprisingly find that the out-of-plane antidamping torque goes to zero in bilayer MoTe2. Finally, we compare these results to those found in the few-layer limit of WTe2. |
Monday, March 4, 2019 12:03PM - 12:15PM |
B41.00003: ABSTRACT WITHDRAWN
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Monday, March 4, 2019 12:15PM - 12:27PM |
B41.00004: Time-dependent ab initio insight to the ultrafast demagnetization mechanism Zhanghui Chen, Lin-Wang Wang Laser induced ultrafast demagnetization process has a great potential in future spintronic devices. Ever since its discovery, this subject has been studied intensively. However, the fundamental physics is still not well understood. There are several long-standing problems regarding the demagnetization mechanism. One major issue is the remarkable underestimation in ab initio simulation. At the same intensity of laser, the theoretical demagnetization rate can be ten times smaller than the experimental observation. Here, we present an ultrafast real-time time-dependent density functional theory method, together with atomic Landau-Lifshitz-Gilbert model, to investigate this problem. Our results show that one important reason for the underestimation is the missing of initial spin disorder, which can significantly enhance the demagnetization to the experimentally observed rate. This spin disorder connects the electronic structure theory with the phenomenological three-temperature model. We have also systematically studied the roles of various interactions which are heavily debated. In particular, we found that electron-electron interaction and spin-orbit interaction play extremely important roles, while electron-phonon and light-spin interactions are not essential. |
Monday, March 4, 2019 12:27PM - 12:39PM |
B41.00005: Photocontrol of magnetic structure in a metallic ferromagnet Atsushi Ono, Sumio Ishihara We study theoretically photoinduced phenomena in a metallic ferromagnet described by the double-exchange (DE) model, where conduction electrons mediate the ferromagnetic (FM) DE interaction between localized magnetic moments in equilibrium. We found that the DE interaction acts as the antiferromagnetic (AFM) interaction in highly photoexcited states, which is in sharp contrast to the conventional DE mechanism. Numerical computations of real-time dynamics described by the Schrödinger equation combined with the Landau-Lifshitz-Gilbert equation reveal that the FM metallic initial state is transformed into the AFM steady state through a transient state with emergent topological magnetic defects. We also calculated magnetic excitation spectra in the photoirradiated FM metallic state by using the Floquet Green function method and found that magnon is softened by photoirradiation, which indicates that the AFM instability is induced by the Stoner excitation in the Floquet state. |
Monday, March 4, 2019 12:39PM - 12:51PM |
B41.00006: All-Optical Deterministic Switching of Synthetic CoCrPt Ferrimagnets Bradlee K. Beauchamp, AVEEK DUTTA, Alexandra Boltasseva, Vladimir M Shalaev, Ernesto Marinero Ultrafast magnetic switching via femtosecond laser pulses is vital for increasing the speed of spintronic devices and has been demonstrated in various ferrimagnetic materials. However, this switching has not yet been demonstrated in synthetic ferrimagnetic hcp CoPtCr alloys. The synthetic ferrimagnetic trilayers consist of two CoCrPt layers antiferromagnetically coupled via a thin Ru spacer. Synthetic ferrimagnetic thin film trilayers have the potential to be switched in the sub-picosecond time regime. CoCrPt is a highly anisotropic magnetic material with low saturation magnetization, making it optimal for magnetic recording. Here we demonstrate all-optical helicity-dependent switching of CoCrPt/Ru/CoCrPt thin film trilayers using circularly polarized, 65 femtosecond, 800 nm KrF laser pulses. |
Monday, March 4, 2019 12:51PM - 1:03PM |
B41.00007: Optical Control of Magnetic Orientation in Epitaxial CoFe2O4 Films Yi-Chun Chen, Yi-De Liou, I-Tse Tsai, Jan-Chi Yang Complex oxides have caught significant attention for the development of the next-generation electronic devices due to their versatile functionalities and the tunability via the external stimuli, such as electrical and magnetic fields. To broaden the application of complex oxides, the new pathway to control the physical properties is on demand. Cobalt ferrite (CoFe2O4, CFO), a ferrimagnetic inverse spinel, which has advantages of high magnetization and coercivity for magnetic data storage, is investigated in this study. We introduce the high density laser to illuminate the epitaxial CFO film and control the magnetic orientation of the film at ambient temperature. Effects of various illumination conditions of CW and pulsed lasers are systematically investigated. Magnetic force microscopy (MFM) and X-ray magnetic circular dichroism coupled to photoemission electron microscopy (XMCD-PEEM) are used to investigate the magnetic orientation of the optical-modulated area. The possible mechanism for modulating magnetic structures is thus determined as the thermal effect combined with magnetostatic couplings. |
Monday, March 4, 2019 1:03PM - 1:15PM |
B41.00008: Ultrafast transfer of magnetization on magnetic sublattices in half-metallic Co2MnGe heusler alloys Phoebe Tengdin, Christian Gentry, Dmitriy Zusin, Adam Z Blonsky, Justin Shaw, Hans T. Nembach, Monika Arora, Thomas Silva, Yaroslav Kvashnin, Erna Delczeg, Olle Eriksson, Henry C Kapteyn, Margaret Mary Murnane Heusler alloys are exciting materials for future applications because they display a wide range of tunable electronic and magnetic interactions such as metallicity, ferromagnetism, superconductivity, and giant magneto-resistance. However, laser-induced spin dynamics in heuslers were expected to be slower than in conventional metallic ferromagnets, due to the presence of a blocked spin channel. Here we directly observe ultrafast spin transfer from one magnetic sublattice to another in half-metallic heusler alloy Co2MnGe, that occurs on the timescale of the femtosecond laser excitation. Ultrafast high harmonic pulses make it possible to simultaneously record the element-specific magnetic dynamics of Co and Mn as the material undergoes demagnetization. The magnetization of Co is transiently enhanced by 10% within 60 fs, while that of Mn rapidly quenches. By comparing our data to density functional theory, we show that optical excitation can directly transfer spin from one magnetic sub-lattice to another, due to spin-polarized optical excitation pathways. The observed transient enhancement of ferromagnetic ordering demonstrates fast manipulation of spin by light, thus providing a path towards spintronics logic devices that can operate on femtosecond or even attosecond timescales. |
Monday, March 4, 2019 1:15PM - 1:27PM |
B41.00009: Revisiting half-metallicity of Co-based full Heusler alloys from non-empirical DFT+U method Kenji Nawa, Yoshio Miura The half-metallic (HM) ferromagnet, which has 100% spin polarization at the Fermi level (EF), is great advantage to enhance efficiency of spin-dependent tunneling property. In particular, Co-based full Heusler alloys have been paid much attention because of the HM and high Curie temperature. The density functional theory (DFT) calculations are expected to play a key role for searching the HM materials and understanding an origin of HM. However, the DFT methods within the local density approximation often fail to predict the HM due to localized d electrons around EF. In this work, we revisit electronic structure of Co2MnSi by the DFT+U method, where +U parameters representing correlation effect are derived from linear response theory. We revealed important atomic orbital hybridizations, which mainly dominate energy gap at EF. Although the Co2MnSi is not HM, our energy diagram provides a new guideline to tune the HM gap. We extended our calculations to quaternary systems Co2(Y,Mn)Si, where Y is 3d transition metals. We found the systems with Y of Ti, V, Cr, and Fe show the HM when composition of Y is selected appropriately. |
Monday, March 4, 2019 1:27PM - 1:39PM |
B41.00010: Half-metallic surfaces in thin-film Ti2MnAl0.5Sn0.5 Pavel Lukashev, Sam Prophet, Rishabh Dalal, Parashu Kharel Materials exhibiting a high degree of spin polarization are in demand in spintronics. Room-temperature half-metals are considered ideal candidates, as they behave as an insulator for one spin channel and as a conductor for the other spin channel. In addition, for nano-size devices, one has to take into account possible modification of electronic structure in thin-film geometry, due to the potential presence of surface/interface states. Typically, these states have a detrimental impact on half-metallicity. Here, we employ density functional calculations to explore an inverse Heusler compound, Ti2MnAl0.5Sn0.5, which exhibits half-metallic electronic structure in bulk geometry. We show that in thin-film geometry, the type of termination surface has a decisive effect on half-metallicity of this material. In particular, we analyze six possible termination configurations, and show that for four of them, energy states emerge in the minority-spin band gap, significantly reducing the spin polarization. At the same time, two termination surfaces preserve half-metallic properties of this material. Thus, our results show that a judicious choice of the termination surface may be a crucial factor in nano-device applications, where highly spin-polarized current is needed. |
Monday, March 4, 2019 1:39PM - 1:51PM |
B41.00011: Why is the Curie temperature of Co2FeSi so high? Guanhua Qin, Wei Ren, David Singh We have investigated the Curie temperature of Co2FeSi and Co2TiSi by using first principle calculations. The full heusler alloy Co2FeSi has large magnetic moment per f.u. 6μB and very high Curie temperature ~1100K. And the value for Tc is 380K for half-metallic ferromagnetism Co2TiSi. We Compare these two systems using models and calculations of magnon dispersions. |
Monday, March 4, 2019 1:51PM - 2:03PM |
B41.00012: Electronic structure and magnetic properties in T2AlB2 (T = Fe, Mn, Cr, Co, and Ni) and their alloys Bruce Harmon, Matthew J. Kramer, Liqin Ke Fe2AlB2 is one of the promising candidates for magnetic refrigeration technology and has attracted great attention since the recent discovery of its substantial magnetocaloric effect around room temperature. The electronic structure and intrinsic magnetic properties of Fe2AlB2-related compounds and their alloys have been investigated using density functional theory. For Fe2AlB2, the crystallographic a axis is the easy axis, in agreement with experiment. The magnetic ground state of Mn2AlB2 is found to be ferromagnetic in the basal ab plane, but antiferromagnetic along the c axis. All 3d dopants considered decrease the magnetization and Curie temperature in Fe2AlB2. Electron doping with Co or Ni has a stronger effect on the decreasing of Curie temperature in Fe2AlB2 than hole doping with Mn or Cr. However, a larger amount of Mn doping in Fe2AlB2 promotes a ferromagnetic to antiferromagnetic transition. A very anisotropic magnetoelastic effect is found in Fe2AlB2: the magnetization has a much stronger dependence on the lattice parameter c than on a or b, which is explained by electronic-structure features near the Fermi level. (Ref: PRB 95, 104427) |
Monday, March 4, 2019 2:03PM - 2:15PM |
B41.00013: Magnetic and magnetocaloric properties of Co doped Ni-Mn-Sn Heusler alloys at high magnetic field and pressure Sudip Pandey, Yury Koshkid’ko, Igor Dubenko, Anil Aryal, Alexander B. Granovsky, Shane Stadler, NAUSHAD ALI The magnetic and magnetocaloric properties of Ni45Mn43CoSn11 have been investigated using direct method for magnetic field changes up to 14 T, heat capacity measurements, and magnetization with hydrostatic pressure applications. A large reversible magnetocaloric effect resulted in a ΔTad of about =−11 K and 5 K for a magnetic field change of 14 T in the vicinity of magnetostructural (TA~260 K) and magnetic (TC~320K) transition, respectively. The impact of thermomagnetic history on ΔTad at high magnetic fields has been revealed. Significant changes in the relaxation time of ΔTad depending on the type of phase transitions, magnetization, and demagnetization cycle are discussed. The density of states and Debye temperature have been estimated from heat capacity measurements. The mixed effects of pressure and magnetic field on the transition temperature is discussed. |
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