Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A22: Spin Polarization and Spin Order in Heterostructures and OscillatorsFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Silvia Tacchi, Istituto Officina dei Materiali del CNR (CNR-IOM) Room: LACC 402A |
Monday, March 5, 2018 8:00AM - 8:12AM |
A22.00001: Easy-Plane Spin Hall Nano-Oscillator Chris Safranski, Eric Montoya, Andrew Smith, Jen-Ru Chen, Ilya Krivorotov In spin Hall nano-oscillators (SHNO) based on ferromagnet (FM) and heavy non-magnetic metal (NM) bilayers, a dc current in NM induces a spin current from the spin Hall effect (SHE) that drives auto-oscillations of FM. Large power, low phase noise SHNOs require large precession cone angle. FMs with easy-plane anisotropy are predicted to have cone angle near 90o when spin current is injected with polarization perpendicular to the easy-plane. Although thin FMs exhibit natural easy-plane anisotropy, the spin Hall current polarization lies in this easy-plane. Here we present a SHNO with FM easy-plane normal to the sample plane, allowing injection of spin Hall current with desired polarization. This SHNO device is a 50 nm wide Pt|Co|Ni|Ta nanowire with two electric leads separated by a 150 nm gap. Interfacial perpendicular magnetic anisotropy is used to tune out-of-plane anisotropy while the nanowire width is used to tune in-plane anisotropy. Careful control of anisotropies allows us to manufacture an easy-plane for the Co|Ni magnetization that is defined by the wire axis and the sample normal. We have found that such easy-plane Pt|Co|Ni|Ta nanowire SHNOs support large-amplitude auto-oscillations in low external field. |
Monday, March 5, 2018 8:12AM - 8:24AM |
A22.00002: Spin-polarized ballistic conduction through correlated Au-NiMnSb-Au heterostructures Christian Morari, Wilhelm Appelt, Andreas Östlin, Andreas Prinz-Zwick, Ulrich Eckern, Udo Schwingenschlogl, Liviu Chioncel We examine the ballistic conduction through Au-NiMnSb-Au heterostructures consisting of up to four units of the half-metallic NiMnSb in the scattering region, using density functional theory (DFT) in combination with dynamical mean-field theory (DMFT). For a single NiMnSb unit the transmission function displays a spin polarization of around 50% in a window of 1 eV centered around the Fermi level. By increasing the number of layers, an almost complete spin polarization of the transmission is obtained in this energy range. Supplementing the DFT calculations with local electronic interactions, of Hubbard-type on the Mn sites, leads to a hybridization between the interface and many-body states. The significant reduction of the spin polarization seen in the density of states is not apparent in the spin-polarization of the conduction electron transmission, which suggests that the hybridized interface and many-body induced states are localized.1 |
Monday, March 5, 2018 8:24AM - 8:36AM |
A22.00003: Junction size dependence of the spin-waves modes in perpendicular magnetic tunnel junction nanopillars Jinting Hang, Christian Hahn, Sebastien Petit-Watelot, Stephane Mangin, Andrew Kent Perpendicularly magnetized tunnel junctions (pMTJs) nanopillars with CoFeB free and reference layers are being intensely explored for applications as spin-torque switched magnetic memories. They also provide interesting structures to study spin-waves in confined geometries. Here we study spin waves modes as a function of nanopillar diameter (50 to 100nm) and applied in-plane field by measuring thermal ferromagnetic resonance (FMR) spectra at room temperature. For 50 and 60 nm diameters only one mode is observed. While for larger diameters, 2 modes with a spacing of about 2GHz is observed. At field large enough to saturate free layer, the 2 modes have almost the same variation in frequencies with applied field, which suggested they are both associated with the free layer. Micromagnetic simulation show that for small devices only the fundamental spin-wave mode is accessible in our experiments, while for larger devices higher order modes with radial nodes can be excited, but with a very small amplitude. |
Monday, March 5, 2018 8:36AM - 9:12AM |
A22.00004: Beller Lectureship: Ultrahigh anisotropy Heusler alloys for THz spin-torque oscillators Invited Speaker: Alina DEAC We investigate the potential of the recently discovered class of almost compensated ferrimagnetic Mn-Ga pseudo-Heusler alloys1 for high frequency applications. To estimate the frequency ranges where such devices could operate, we first conducted high-field magnetotransport measurements2 on selected films with different composition and, therefore, different compensation temperatures (Tc) and anisotropies. In manganese ruthenium gallium (MRG), for instance, both the transverse Hall resistivity and longitudinal resistivity were recorded in magnetic fields up to 58T, at variable temperature. MRG exhibits a large spontaneous Hall angle of ~2%, coercivity exceeding 1T at room temperature (RT) and very low net magnetisation of 25kA/m. Despite having no net moment at Tc, the magnitude of the Hall signal does not become zero, indicating both a half-metallic nature of the material and that the magnetotransport is dominated by one sublattice only. By comparison to analytical and mean-field calculations of the sublattice magnetisation directions, we can estimate both the sublattice anisotropy and interlayer exchange coupling. From these values, the out-of-phase and in-phase resonance modes are expected to lie around 0.3THz and 2THz, respectively. The out-of-phase resonance mode was directly measured for ferrimagnetic Mn3-xGa thin films as function of anisotropy and applied magnetic fields up to 10T. At low applied fields, we find that the resonance frequency ranges between 200 and 350GHz for films with different compositions/anisotropy3. Furthermore, MRG-based magnetic tunnel junctions demonstrate magnetoresistance ratios as high as 40% at 4.2K and 12% at RT4, providing proof of concept for efficient on-chip emitters of coherent, narrow-band light pulses in the THz gap. |
Monday, March 5, 2018 9:12AM - 9:24AM |
A22.00005: QSGW Calculations on Electronic Structures of Co- and Mn-based Heusler Alloys Okumura Haruki, Kazunori Sato, Tomoyuki Kakeshita, Takao Kotani Heusler alloys show various functionalities depending on the chemical composition. Particularly, half-metallicity or spin gapless electronic structure is considered to be important for spintronics applications. So far, first-principles methods based on the density functional theory (DFT) are conveniently used for designing spintronics materials. The DFT accurately predicts ground state properties, but its reliability for the calculation of excited states is rather limited. In this study, we systematically calculate the electronic structures of Co- and Mn-based Heusler alloys by using the Quasi-particle self-consistent GW (QSGW) method, the local density approximation (LDA), and the Hybrid method (HSE06). We found that the QSGW and the LDA predict very similar electronic structure for Co2MnSi except for a slight difference in the minority spin gap 0.77 eV (QSGW) and 0.59 eV (LDA). In addition, the QSGW predicts the Fermi level (EF) at 0.20 eV from the VBM, but in case of the LDA the EF is at 0.32 eV from the VBM. Such a small difference may affect magnetic excitation in this material. In contrast, the HSE06 doesn't predict half-metallic electronic structure. For typical Co- and Mn-based Heusler alloys, we discuss the difference of the calculated electronic structures around EF. |
Monday, March 5, 2018 9:24AM - 9:36AM |
A22.00006: Ultra-broadband spin wave propagation in Co2(Mn0.6Fe0.4)Si thin films Jianyu Zhang, Tobias Stückler, Chuanpu Liu, Haiming Yu, Florian Heimbach, Jilei Chen, Junfeng Hu, Sa Tu, Youguang Zhang, Simon Granville, Mingzhong Wu, Weisheng Zhao, Dapeng Yu, Zhi-Min Liao, Tao Liu Ferromagnetic Heusler alloys with low magnetic damping are highly promising materials for magnonic devices, which rely on excitation and detection of spin waves. Using all-electrical spin wave spectroscopy we report spin wave propagation in sputtered Co2(Mn0.6Fe0.4)Si Heusler alloy thin films with thickness of 50 nm. We integrated a nanostructured microwave antenna to locally excite and detect propagating spin waves in a Damon-Eshbach configuration. We estimate the group velocity to be up to 12.0 km/s and we observe spin wave propagation with a frequency band as broad as 15 GHz. From the experimental frequency dependence of group velocity we calculate the spin wave dispersion.Our results show that all-electrical measurements are a powerful method for determining the fundamental spin wave characteristics of Heusler alloys, over a broad and tunable range of frequencies, and with group velocities an order of magnitude higher than in conventional materials. |
Monday, March 5, 2018 9:36AM - 9:48AM |
A22.00007: Magnetotransport properties in the noncentrosymmetric itinerant ferromagnet Cr11Ge19 Nan Jiang, Yoichi Nii, Rieko Ishii, zenji hiroi, Yoshinori Onose We have investigated the anomalous Hall effect and magnetoresistance of the noncentrosymmetric itinerant ferromagnet Cr11Ge19. We have found a characteristic temperature (∼30 K) below the ferromagnetic transition temperature TC = 85 K. While a conventional anomalous Hall conductivity that is proportional to the magnetization smoothly grows below TC, the anomalous Hall conductivity shows an additional enhancement below the characteristic temperature. Concomitantly, the AMR emerges and the magnitude of negative magnetoresistance begins to increase toward low temperature. Because there is no anomaly in the temperature dependence of magnetization around the characteristic temperature, the origin of these observations in transport properties is ascribed to some electronic structure with the energy scale of 30 K. We speculate this is caused by the spin splitting due to breaking of spatial inversion symmetry. |
Monday, March 5, 2018 9:48AM - 10:24AM |
A22.00008: Tailoring metallic ferromagnet heterostructures for non-collinear spin states and skyrmions Invited Speaker: Kirsten Von Bergmann
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Monday, March 5, 2018 10:24AM - 10:36AM |
A22.00009: Magnetism and Magneto-Electric Transport in Amorphous Iron-Germanium Thin Films Dinah Simone Bouma, Frank Bruni, Frances Hellman In amorphous iron-germanium films (a-FexGe1-x), local spin-orbit coupling and broken inversion symmetry yield a local Dzyaloshinskii-Moriya (DM) interaction, which manifests itself in distinctly non-square hysteresis loops. We present magnetization, Hall effect, and electrical resistivity measurements of thin films (50-80nm in thickness) of a-FexGe1-x with 0.45 < x < 0.65. The films are magnetic over the entire composition range investigated, and exhibit a significant anomalous Hall effect. As with films of amorphous iron-silicon, we ascribe these findings to the local DM interaction, and to the increased importance of the orbital magnetism in amorphous materials, which is equivalent to a localized Berry phase curvature. |
Monday, March 5, 2018 10:36AM - 10:48AM |
A22.00010: Magneto-thermal imaging of exchange bias in Pt/FeRh bilayers Isaiah Gray, Gregory Stiehl, John Heron, Daniel Ralph, Gregory Fuchs We use a recently-developed technique, magneto-thermal microscopy, to image exchange bias at the interface between coexisting antiferromagnetic (AF) and residual ferromagnetic (FM) order below the Neél temperature in 10 nm Pt/20 nm FeRh /MgO(001) bilayers. The 1st-order AF-FM phase transition in FeRh near 100 °C may provide a simple way to deterministically switch the AF order by field-cooling from the FM phase. However, weak residual FM order below TN complicates the process. We image this FM order with the anomalous Nernst effect, using 3 ps laser pulses focused to 700 nm diameter to generate local thermal gradients. Imaging as a function of field, we observe spatially-dependent magnetization-like (vertical) shifts in hysteresis loops at 25 °C, indicating exchange bias at a disordered AF/ FM interface. The exchange bias and the coercivity decrease at higher temperature until we observe an abrupt increase in signal at 100 °C from exchange-biased emergent FM order in the transition region. By indirectly imaging the AF order through exchange bias, our results provide new insight into the spatial structure of AF order in FeRh. |
Monday, March 5, 2018 10:48AM - 11:00AM |
A22.00011: Berry phase contribution to the spin-orbit torque in an antiferromagnet with weak ferromagnetic order. Suik Cheon, Hyun-Woo Lee The previous theoretical [1] and experimental [2] antiferromagnet-generated spin-orbit torque studies are yet limited to ideal antiferromagnets where magnetization is strictly collinear so that anti-unitary symmetries, such as the combinations between time-reversal and spatial inversion or time-reversal and lattice translation, are preserved. Even in antiferromagnets with collinear spin configurations in equilibrium, however, spin configurations become noncollinear during their magnetization dynamics and the anti-unitary symmetries are broken [3]. In this talk, I show theoretically that the noncollinearity, which breaks anti-unitary symmetry, of antiferromagnet due to magnetization dynamics, can significantly modify properties of Berry phase contribution to the spin-orbit torque in clean antiferromagnets. |
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