Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D29: Focus Session: Ferromagnetic Metals |
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Sponsoring Units: GMAG DMP FIAP Chair: Johan Akerman, University of Gothenburg Room: 206A |
Monday, March 2, 2015 2:30PM - 2:42PM |
D29.00001: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 2:42PM - 2:54PM |
D29.00002: High-frequency modes of a magnetic antivortex Martin Asmat-Uceda, Grant Riley, Arabinda Haldar, Kristen Buchanan Magnetic vortices have attracted considerable attention in recent years not only because of their interesting physical properties but also due to their potential for applications. The magnetic antivortex (AV), the topological counterpart of the magnetic vortex, possesses similarly rich dynamics and its spin configuration may prove advantageous for spin-wave-based devices, however, it has not been studied as intensely. Recent experiments show that AV's will form naturally at the intersections of patterned pound-key-like nanostructures that are magnetically soft. Here we present micromagnetic simulations of the dynamics of AV's in these structures. The simulations show that pound-key-like structures made of 30-nm thick Permalloy exhibit a complex dynamic profile that includes a number of discrete high-frequency modes (\textgreater 1 GHz). Spatial maps of the dynamic modes that were constructed using Fourier analysis of the simulation results show modes that are in similar in character to the radial and azimuthal modes observed for magnetic vortices but the spin dynamics also differ from those of a vortex due to the presence of the elongated nanowires in the pound-key-like structure. The frequencies of the observed modes tend to decrease with increasing sample size, however, the general features of the modes remains relatively unaffected by the structure size. The simulations will be compared to Brillouin Light Scattering (BLS) experimental results. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D29.00003: Control of magnetic damping and magnetic fluctuations by spin current Yong Pu, Chi Zhang, Sergei Manuilov, Ezekiel Johnston-Halperin, Fengyuan Yang, Chris Hammel We use spin hall effect in a non-magnetic film (NM) to generate spin current, which can excite magnetic precessions and manipulate magnetic properties of an adjacent ferromagnetic thin film (FM). We show that both magnetic damping and magnetic fluctuations can be sufficiently suppressed or enhanced by the spin current. We find that the magnetic damping is linear with spin current that is consistent with previous reports; on the other hand, the quasi-uniform precession and magnetic fluctuations show strongly nonlinear behaviors at driving current approaching critical value. The observations suggest that spin current interacts with all allowed spin-wave modes and induces strong nonlinear influence. Our results give an insight of the complex spin-torque driven dynamics in FM/NM systems and suggest a route to control the magnetic damping and magnetic fluctuations in nanodevices. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:42PM |
D29.00004: Co1.5Fe1.5Ge and Co2MnSi Half-Metal Magnetic behavior tested by spin-resolved photoemission and ferromagnetic resonance Invited Speaker: St\'ephane Andrieu In a magnetic spin-valve or tunnel junction, a crucial parameter to get both large magnetoresistance (MR) and a good Spin Transfer Torque (STT) efficiency is the spin-polarization of the magnetic electrodes. So-called ``Half-Metallic'' Magnetic (HMM) materials are of interest for such devices due to the existence of a spin-gap at the Fermi level for minority spins [1]. Recently, MR enhancements have been observed by different groups on Co2-xFe1$+$xGe [1] and Co2MnSi [2] Heusler materials, suggesting HMM behavior. A second consequence of that minority spin gap is that very low magnetic damping is expected. Combining both properties in a device is a challenge for decreasing the critical current necessary to switch the magnetization using STT. Up to now, many Heusler alloys are claimed to get this HMM property [3], but direct demonstration using spin-resolved photoemission is often missing. Here we focus on 2 systems, (i) Co1.5Fe1.5Ge for which a significant increase of the GMR was observed in spin valves [1], and (ii) Co2MnSi for which very large TMR values were observed in MgO-based MTJs [2]. The Co1.5Fe1.5Ge and Co2MnSi(001) films (noted CFG and CMS) were prepared by Molecular Beam Epitaxy coupled to the Spin-Resolved PhotoEmission (SR-PES) set-up on CASSIOPEE beamline at SOLEIL synchrotron. The L21 chemical ordering was confirmed in CFG films by using anomalous diffraction on SIXS beamline at SOLEIL. However, SR-PES experiments did not show any HMM behavior on our CFG films [4]. Similar PES experiments performed on CMS showed that the minority spin density of states (DOS) drops down to zero at -0.4eV below EF, leading to a 100{\%} spin polarization. However, we also observed an increase of the minority spin DOS at EF, not predicted by ab initio calculations on the bulk structure. The spin-gap is thus decreased due to the surface symmetry breaking. We will show however that this spin-gap can be enlarged when finishing the surface by 1 Mn atomic plane, or when covering with the MgO barrier. Extremely low damping (\textless 10-3) are observed, making CMS a very good candidate for spintronics devices.\\[4pt] [1] S. Maat et al, Appl. Phys. Lett. 93, 143505 (2008)\\[0pt] [2] Hong-xi Liu et al, App. Phys. Lett., 101, 132418 (2012)\\[0pt] [3] T. Graf, C. Felser, S. S. P. Parkin, Progress in Solid State Chemistry 39, 1 (2011)\\[0pt] [4] A. Neggache et al, Appl. Phys. Lett. 104, 252412 (2014) [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D29.00005: ABSTRACT MOVED TO Q31.00006 |
Monday, March 2, 2015 3:54PM - 4:06PM |
D29.00006: Temperature dependent ferromagnetic relaxation and gyromagnetic ratio in Ni80Fe20/Gd thin films Behrouz Khodadadi, Jamileh Mohammadi, Claudia Mewes, Tim Mewes, Tatiana Eggers, Casey Miller We report on the temperature dependence of the magnetization dynamics of NiFe thin films (5nm {\&} 10nm) capped with a 3nm Gd layer using broadband ferromagnetic resonance. We observe that the effective Gilbert damping parameter determined from the broadband measurements increases as the temperature approaches the Curie-temperature of the Gd layer. Part of the enhancement can be explained by an increase of the spin-pumping contribution to the relaxation [1,2] as the temperature approaches the Curie temperature of Gd. We also measure a strong increase of the gyromagnetic ratio with decreasing temperature which resembles the increase of the gyromagnetic ratio in rare earth containing transition metals near the compensation point [3,4]. This increase in the gyromagnetic ratio is expected to lead to an increased Gilbert type damping due to spin-orbit interaction [5,6], that likely also contributes to the increase in damping. References: 1. Y. Tserkovnyak et al. Phy. Rev. Lett., 88, 117601-1 (2002). 2. H. Lee et al. J.of. Phys. D, 41, 215001 (2008). 3. W. Ng et al. J. Appl. Phys., 53, 2359 (1982). 4. R.K. Wangsness, Am. J. Phys., 24, 60 (1956). 5. R.J. Elliott, Phys. Rev. 96, 266 (1954). 6. F. Schreiber et al. Solid State Comm. 93, 965 (1995). [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D29.00007: Kambersky Damping in L10 Magnetic Materials of Ordered and Disordered States with Substitutional Defects Tao Qu, Randall Victora L1$_{\mathrm{0}}$ phase alloys with high magnetic anisotropy play a key role in spintronic devices. The damping constant $\alpha $ represents the elimination of the magnetic energy and affects the efficiency of devices. However, the intrinsic Kambersky damping reported experimentally differs among investigators and the effect of defects on $\alpha $ is never investigated. Here, we apply Kambersky's torque correlation technique, within the tight-binding method, to L10 ordered and disordered alloys FePt, FePd,CoPt and CoPd. In the ordered phase, CoPt has the largest damping of 0.067 while FePd has the minimum value of 0.009 at room temperature. The calculated damping value of FePt and FePd agrees well with experiment. Artificially shifting Ef, as might be accomplished by doping with impurity atoms, shows that $\alpha $ follows the density of states (DOS) at Ef in these four L10 alloys. We introduce lattice defects through exchanging the positions of 3d and non-3d transition elements in 36 atom supercells. The damping increases with reduced degree of chemical order, owing to the enhanced spin-flip channel allowed by the broken symmetry. This prediction is confirmed by measurements in FePt. It is demonstrated that this corresponds to an enhanced DOS at the Fermi level, owing to the rounding of the DOS with loss of long-range order. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D29.00008: Nanoscale cluster dynamics in the martensitic phase of Ni-Mn-Sn shape-memory alloys Michael Hoch, Shaojie Yuan, Phillip Kuhns, Arneil Reyes, James Brooks, Daniel Phelan, Vijay Srivastava, Richard James, Chris Leighton The martensitic phases of Ni-Mn-Sn magnetic shape memory alloys exhibit interesting low temperature magnetic properties, including intrinsic superparamagnetism and exchange bias effects, which have previously been rationalized in terms of spin clusters. We show here that spin-echo NMR, involving $^{\mathrm{55}}$Mn hyperfine fields, permits ferromagnetic and antiferromagnetic nanoregions to be directly identified in these materials and yields estimates of their size distributions. Nuclear relaxation rate measurements, made as a function of temperature, provide information on both the dynamics and on the electronic structure of the nanoregions. The relaxation rates are analyzed using a combination of Redfield and Korringa mechanisms, the Korringa procedure providing information on the density of states at the Fermi level. Results will be presented for a number of these alloys. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D29.00009: Transport signature of spin gapless semiconducting properties in quaternary Heusler of CoFeCrAl Guizhou Xu, Xiaoming Zhang, Enke Liu, Wenhong Wang, Guangheng Wu Spin gapless semiconductors (SGS), since proposed by Wang in 2008, have attracted intensive attention due to its potential application in spintronics. In our previous works, we have predicted some quaternary Heusler alloys are promising to be candidates of SGS. In this presentation, we will report the transport signature of SGS properties for CoFeCrAl, one of SGS candidate. The results show that samples treated in different ways can present distinguished transport properties. On the one hand, the arc-melted bulk samples exhibit a negative temperature dependence of resistivity accompanying with a negative magnetoresistance from 5-300K, revealing a normal transport behavior signifying for a metallic magnetic system. On the other hand, for the melt-spun ribbon samples, a positive temperature dependence of the resistivity as well as positive sign of magnetoresistance were observed, which implies that a semiconducting-like transport mechanism dominate in this sample. Based on our first principles analysis, this difference can be attributed to the occurrence of anti-site occupation between Co/Cr atoms in the compound. Our findings raised the possibility to tune the properties of SGS through proper sample treatments due to its atomic-occupation sensitivity. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D29.00010: Thermophysical and thermomagnetic properties of Heusler compound Ni$_{45}$Co$_{5}$Mn$_{37}$In$_{13}$ Joseph P. Heremans, Sarah J. Watzman, Ajaya K. Nayak, Claudia Felser Ni$_{45}$Co$_{5}$Mn$_{37}$In$_{13}$ is a Heusler compound that develops a predominant ferromagnetic magnetic moment at temperatures in excess of 270 K, similar to Mn$_{2}$PtGa. Upon cooling Mn$_{2}$PtGa undergoes a paramagnetic-to-ferrimagnetic transition at 230 K, followed by a sharp drop in magnetization at 150 K due to a first-order ferrimagnet-to-antiferromagnetic transition. By analogy, the moment change observed in Ni$_{45}$Co$_{5}$Mn$_{37}$In$_{13}$ at 270 K is due to a first-order magneto-structural transition, where the sample undergoes a predominant ferromagnetic to an antiferromagnetic phase transition, accompanied by a structural phase transition. Experimental data will be given for the heat capacity and heat of magnetization of Ni$_{45}$Co$_{5}$Mn$_{37}$In$_{13}$, as a function of temperature and magnetic field. Thermomagnetic tensor elements will also be reported though the phase transition, i.e. thermal conductivity, thermopower, longitudinal and transverse magnetothermopower or Nernst effect, as a function of magnetic field and temperature. \\[4pt] [1] A. K. Nayak {\&} al., Phys. Rev. Lett. \textbf{110}, 127204 (2013) [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D29.00011: Symmetry-dependent electron-electron interaction in coherent tunnel junctions resolved by measurements of zero-bias anomaly Jian Wei, Liang Liu, Jiasen Niu, Li Xiang, D.-L. Li, J.-F. Feng, X.-F. Han, X.-G. Zhang, J.M.D. Coey We provide conclusive experimental evidence that zero-bias anomaly in the differential resistance of magnetic tunnel junctions (MTJs) is due to electron-electron interaction (EEI), clarifying a long standing issue. The magnon effect that caused confusion is now excluded by measuring at low temperatures down to 0.2 K and with reduced ac measurement voltages down to 0.06 mV. The normalized change of conductance is proportional to $\ln{(eV/k_{B}T)}$, consistent with the Altshuler-Aronov theory of tunneling that describes the reduction of density of states due to EEI, but inconsistent with magnetic impurity scattering. The slope of the $\ln{(eV/k_{B}T)}$ dependence is symmetry dependent: the slopes for parallel and antiparallel states are different for coherent tunnel junctions with symmetry filtering, while nearly the same for those without symmetry filtering (amorphous barriers). This observation may be helpful for verifying symmetry preserved filtering in search of new coherent tunneling junctions, and for probing and separating electron Bloch states of different symmetries in other correlated systems.\footnote{Liu et al., arXiv:1410.3636, accepted by Phys. Rev. B} [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D29.00012: Large resistivity modulation in mixed-phase metallic systems Yeonbae Lee, Zhiqi Liu, John Heron, James Clarkson, Jeongmin Hong, Changhyun Ko, Michael Biegalski, Ulrich Aschauer, Shang-Lin Hsu, Mark Nowakowski, Junqiao Wu, Hans Christen, Sayeef Salahuddin, Jeffrey Bokor, Nicola Spaldin, Darrell Schlom, Ramamoorthy Ramesh We have investigated the effect of an electric field to FeRh/PMN-PT heterostructures and report 8{\%} change in the electrical resistivity of FeRh films. Such a ``giant'' electroresistance (GER) response is striking in metallic systems, in which external electric fields are screened and thus only weakly influence the carrier concentrations and mobilities. We show that our FeRh films comprise coexisting ferromagnetic and antiferromagnetic phases with different resistivities, and the origin of the GER effect is the strain-mediated change in their relative proportions. The observed behavior is reminiscent of colossal magnetoresistance in perovskite manganites, and illustrates the role of mixed-phase coexistence in achieving large changes in physical properties with low-energy external perturbation. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D29.00013: Effect of annealing on the surface magnetic and magnetoimpedance properties of Co-based amorphous microwires V. Kalappattil, J. Devkota, E. Clements, S. Chandra, J.S. Liu, H.X. Shen, J.F. Sun, H. Srikanth, M.H. Phan Magnetic domains of negative magnetostrictive amorphous microwires (AWs) form a core-shell type structure with the core and shell domains preferred along axis and circumference, respectively. In this work, we have studied the effect of a combined current modulation annealing on the bulk and surface magnetic properties of melt-extracted Co$_{68.2}$Fe$_{4.3}$B$_{15}$Si$_{12.5}$ and Co$_{68.2}$Fe$_{4.3}$B$_{14}$Si$_{12.5}$Nb$_{1}$ AWs and their correlations with the magnetoimpedance (MI) effect. The AWs were annealed by a combination of ac (90 mA, 50 Hz) and dc (60, 63, and 65 mA) currents for 480 seconds each, and magnetic hysteresis loops were measured by VSM and a magneto-optic Kerr effect (MOKE) magnetometer. Compared to VSM loops, MOKE loops for the annealed AWs were observed to have higher coercive and anisotropy fields, indicating that the near-surface region was magnetically harder. The anisotropy fields of the AWs defined as the peaks of the MI spectra at 1 MHz and 500 MHz were found to have a correlation with those probed by VSM and MOKE, respectively. These findings are of practical importance in designing MI-based sensors. [Preview Abstract] |
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