Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T19: Focus Session: Spin Transport & Magnetization Dynamics in Metals VII |
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Sponsoring Units: GMAG DMP Chair: Frances Hellman, University of California, Berkeley Room: D170 |
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T19.00001: Direct observation of strain-induced magnetic domain evolution in Heusler shape memory compounds C.A. Jenkins, A. Scholl, A. Doran, T. Omori The magnetic domain structure in single crystals of a novel Heusler magnetic shape memory (MSM) compound Fe$_2$MnGa was observed to undergo strain-induced evolution by synchrotron-based photoelectron emission microscopy (PEEM) at Beamline 11.0.1 of the Advanced Light Source. PEEM can produce high resolution, surface-sensitive images reflecting the spatial distribution of magnetism, and a custom flexure rig has been designed for in situ actuation and simultaneous observation of MSM. System energy of an MSM compound is lowered when the volume fraction of the favorably oriented martensitic variant is increased: intermartensitic twin boundary motion is energetically easier than magnetic rotation so magnetic domains evolve not by domain wall motion but by propagation of planar structural defects called twin boundaries. In this ternary MSM intermetallic the direction of tetragonal distortion, which is coupled to the magnetic easy axis, will lie favorably along the axis of compression as defined by the action of the custom rig. The disappearance of unfavorably oriented orthogonal magnetic domains with strain is then observed as expected. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T19.00002: Thin film growth and characterization of full Heusler alloys Rh$_{2-x}$Co$_{x}$FeSn and RhCoMnSn Li Gao, Mingyang Li, Mahesh G. Samant, Brian P. Hughes, Kevin P. Roche, Claudia Felser, Stuart S.P. Parkin Heusler alloys can be designed and prepared with high spin polarization, high Curie temperature, very low magnetization damping, as well as, tunable magnetic magnetization and anisotropy. Therefore, this family of compounds has great potential for applications such as spin-transfer-torque magnetic random access memory. The growth and characterization of epitaxial thin films of the Rh-based full Heusler compounds, Rh$_{2-x}$Co$_{x}$FeSn and RhCoMnSn, are presented. The magnetization, Curie temperature and crystal structure of these compounds have been investigated and are compared with bulk materials. An important known property of many Heusler alloys is low magnetization damping. Ferromagnetic resonance (FMR) studies using a strip line transmission technique reveals Gilbert damping values of $\sim $0.015 at room temperature in films of RhCoMnSn. These films have Curie temperatures well above room temperature whereas the Curie temperature of Rh$_{2}$FeSn is $\sim $350 K. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T19.00003: Electronic structure modifications in Cu doped Ni$_{2}$MnGa Sujoy Roy, R. Qiao, P.-A. Glans, A. Pathak, I.S. Dubenko, N. Ali, E. Blackburn, W. Yang Ni$_{2}$MnGa Heusler alloy is a multifunctional ferromagnetic alloy that exhibits various interesting properties. The compounds typically exhibit a high temperature magnetic and a low temperature martensitic transition. Stoichiometry changes or elemental substitution makes it possible to merge the two transitions to a unique magnetostructural one. We have used resonant inelastic x-ray scattering to study the effects of $d-d$ interactions and charge transfer effects in 25{\%} Cu doped Ni$_{2}$MnGa. We find distinct charge transfer effects in the Ni absorption and inelastic x-ray spectrum that are significantly modified by Cu doping. Mn on the other hand shows $d-d$ interaction effects but no charge transfer. Multiplet calculations have been performed and will be compared to the experimental data. These results provide an insight into the origin of multifunctional properties of Ni based Heusler alloys. Work at LBNL is supported by U.S DOE. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T19.00004: Approach towards full Heusler alloy based CPP-GMR: from Ag and non-magnetic Heusler to binary intermetallic spacers Oleg Mryasov, Sergey Faleev, S.V. Karthik Recently, it has been demonstrated that GMR response can be significantly enhanced by incorporating high spin polarization ferromagnetic (FM) full Heulser alloy into spin valve nano-structures. Experimental results for two types of non-magnetic spacers (i) elemental metal [1] and (ii) non-magnetic Heusler alloy spacers [2] deserve careful comparison. More practical (110) textured combination of Co$_{2}$MnGe (CMG) and non-magnetic Heusler alloy Rh$_{2}$CuSn (RCS) [2] have been used to build test hard disk drive [3]. In this work, we investigate the mechanism of spin dependent interface scattering for (001) CMG/Ag/CMG (Case1) and (110) CMG/RCS/CMG (Case2) models on the basis of ab-initio electronic structure calculations. We find that in both cases GMR has significant contribution from the spin dependent interface scattering. We propose new binary intermetallic spacer materials Al$_{2}$Au and Cu$_{3}$Sn as an alternative to Ag and RCS spacers. \\[4pt] [1] T.Iwase et.al. Appl.Phys.Express, \textbf{2}, 063003 (2009).\\[0pt] [2] K. Nikolaev et.al. App.Phys. Lett., \textbf{94}, 222501 (2009) [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T19.00005: Spin fluctuations and doping trends in the itinerant magnet Pd$_{2}$TiIn$_{x}$ Jessica Missaghian, Andrew LaForge, David Matthews, Garrett Rogren, Zack Schlesinger, Arthur Ramirez The intermetallic compound Pd$_{2}$TiIn is one of several Heusler-type materials which are remarkable for possessing a significant magnetic moment despite being composed of elements which have no spontaneous local moment. We investigate the nature of the magnetic order and the role of spin fluctuations by studying the magnetic, transport and heat capacity properties of two series of polycrystalline samples derived from Pd$_{2}$TiIn$_{x}$: one in which the indium content is varied from $x$ = 0.87 to 1.22, and another in which 3d and 4d metals are substituted in small quantities for Pd and Ti. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T19.00006: Thermal effects on magnetic resonance in single crystal $Co_{1-x}Fe_xS_2$ B. Kaster, M. Pechan, M. Manno, A. Baruth, C. Leighton Many spintronic applications require spin injection from spin polarized ferromagnetic materials. A promising model system for fundamental studies of such processes is $CoS_2$, which has -56\% spin polarization at the Fermi level and is tunable with Fe doping in $Co_{1-x}Fe_xS_2$ to over 85\%. Single crystals of $Co_{1-x}Fe_xS_2$ have been successfully prepared with close to ideal sulfur stoichiometry using chemical vapor transport methods. We have employed variable temperature ferromagnetic resonance (FMR) at 9.2 GHz to investigate the magnetodynamic properties of this system for x=0, 0.05, 0.12 and 0.17. Resonance signals are observed upon cooling to 160 K, well above the Curie temperature, suggesting short-range order enhancements to the susceptibility lead to observable resonances. All concentrations exhibit increasing resonance position, and decreasing damping, with increasing temperature. Both the resonance position and the damping decrease with increasing Fe concentration - the former revealing Fe concentration effects on the moment and anisotropy, while the latter reflects the spin polarization influence on the damping. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T19.00007: Spin-dependent intergranular hopping transport in very thin highly spin-polarized CoS$_{2}$ thin films M. Manno, A. Gunawan, A. Baruth, R. Frakie, A. Mkhoyan, C. Leighton The Co$_{1-x}$Fe$_{x}$S$_{2}$ alloy system has been shown to exhibit high, composition tunable, spin polarization (-56 {\%} $< \quad P <$ +85 {\%}) in bulk, demonstrating great promise for fundamental studies in spintronics. Incorporation in heterostructures requires reliable thin film deposition routes, which have recently been developed. We present here a detailed study of the thickness ($t)$ dependence of the structural, magnetic, and electronic properties of polycrystalline CoS$_{2}$ thin films (70 -- 1600 {\AA}). As $t$ is decreased, we observe a suppression in magnetic properties accompanied by a metal-insulator transition. A distinct 3D to 2D crossover is evident in the conductance-voltage curves and intergranular tunneling magnetoresistance. At $t$ of order 70 {\AA} we observe granular metal conduction, in the presence of a Coulomb charging penalty. We demonstrate quantitative agreement between experiment and proposed models. The very thin film data are understood in terms of enhanced grain boundary resistance, due to S accumulation, which is evidenced via several modes of structural characterization. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T19.00008: Synthesis and basic characterization of the itinerant ferromagnet Cr$_{11}$Ge$_{19}$ Nirmal Ghimire, Michael McGuire, David Mandrus Cr$_{11}$Ge$_{19}$ is a member of the tetragonal, but structurally complex family of materials known as Nowotny chimney-ladder phases.~These materials have composition T$_{n}$X$_{m}$~where 2$>m$/$n>$1.25 (T=transition~element, X=Si, Ge, Sn or Ga). Although Cr$_{11}$Ge$_{19}$~was reported to be an itinerant ferromagnet, its basic properties have not been well characterized. Here we present resistivity, magnetization, and heat capacity results on polycrystalline Cr$_{11}$Ge$_{19}$ . [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T19.00009: Unusual magnetism of Gd$_{5}$Ge$_{4}$ with non magnetic rare earths substitutions Durga Paudyal, Y. Mudryk, V.K. Pecharsky, K.A. Gschneidner, Jr. We present first principles and experimental studies on the small substitutions of Gd atoms by Lu, La, Y, and Sc atoms in Gd$_{5}$Ge$_{4}$. While replacing the Gd atoms located inside the slabs with the Lu or Y atoms leads to a substantial loss of ferromagnetism, the identical substitutions of other Gd locations preferred by La atoms have essentially no effect on the magnetostructural transitions. On the other hand, the Sc atoms prefer the same locations as the Lu and Y atoms. This substitution, however, changes the crystal structure of Gd$_{5}$Ge$_{4}$ from Sm$_{5}$Ge$_{4}$ type to Pu$_{5}$Rh$_{4 }$type at 25{\%} of Sc which was not observed with the former substitutions. The Pu$_{5}$Rh$_{4}$-type has structural parameters that are intermediate between the Gd$_{5}$Si$_{4}$ and Sm$_{5}$Ge$_{4}$ types. The exchange interactions in this substitution are positive which indicates that the Sc substituted Gd$_{5}$Ge$_{4}$ has the ferromagnetic ground state. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T19.00010: Examining the AF$>$FM transition in Fe-Rh thin films through specific heat, photoemission, and Mossbauer spectrometry measurements David Cooke, Catherine Bordel, Frances Hellman Iron-rhodium alloys near equiatomic composition undergo a metamagnetic antiferromagnetic-to-ferromagnetic (AF$>$FM) transition at just above room temperature. This material has been proposed as an exchange layer in thermally-assisted magnetic recording, using the ferromagnetic phase to reduce the switching field of a high-anisotropy storage layer, so clearly being able to control this transition is crucial to implementation. However, theoretically there is still much debate as to the precise mechanism of the AF$>$FM transition, primarily centered on the contributions of electronic and magnetic entropy differences in the two phases. Through thermodynamic measurements on epitaxially- grown ferromagnetic and antiferromagnetic Fe-Rh alloy films, we test two different thermal fluctuation models of the transition. We also discuss complementary photoemission and Mossbauer spectrometry data above and below the transition to examine the magnetic behavior and electronic densities of state in the two phases and compare these to theoretical calculations. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T19.00011: Transport Spin Polarization of High-Curie Temperature MnBi Films Pushkal Thapa, Parashu Kharel, Pavel Lukashev, Renat Sabirianov, Evgeny Tsymbal, David Sellmyer, Boris Nadgorny We report on the study of the structural, magnetic and transport properties of highly textured MnBi films with the Curie temperature of 628 K. In addition to detailed measurements of resistivity and magnetization, we measure transport spin polarization of MnBi by Andreev reflection spectroscopy and perform first-principles electronic structure calculations. A transport spin polarization of up to 63{\%} is observed, consistent with the calculations and with a recent observation of a large magnetoresistance in MnBi contacts. The band structure calculations indicate that, in spite of almost identical densities of states at the Fermi energy, the large disparity in the Fermi velocities leads to high transport spin polarization of MnBi. The correlation between the values of magnetization and spin polarization observed in MnBi will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T19.00012: Anomalous electron transport in ferromagnetic MnBi films Parashu Kharel, D.J. Sellmyer Materials having high spin polarization, large perpendicular magnetic anisotropy and high Curie temperature hold great potential for a range of spintronic applications.\footnote{S. Mangin, D. Ravelosona, J. A. Katine, M. J. Carey, B. D. Terris and Eric E. Fullerton, Nature Mater. \textbf{5}, 210 (2006).} MnBi has the hexagonal NiAs structure and possesses strong permanent magnet and magneto-optical properties. Our recent research shows that MnBi exhibits a high transport spin polarization of 63{\%}, so it is useful to investigate the electron transport properties of this material. We have found that MnBi is a metallic conductor but the resistivity shows an anomalous temperature dependence at low temperature. Analysis of the Hall data for various samples shows that the extraordinary Hall effect is the dominant part in the transverse Hall effect and a Hall angle of 2.8{\%} has been measured. An experimental investigation on the origin of the observed large extraordinary Hall effect in MnBi thin films will be discussed. This research is supported by NSF-MRSEC Grant DMR-0820521, the DOE Grant DE-FG02-04ER46152 and NCMN. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T19.00013: SrMnBi$_2$, a new transition metal compound with metallic spacer layer Jiakui J. Wang, Liang L. Zhao, Q. Yin, G. Kotliar, Emilia Morosan To explore the correlation between superconductivity and crystal structure in transition metal-pnictides systems, we investigate the band structure and physical properties of SrMnBi$_2$ single crystals. This compound is isostructural with the superconducting Fe-pnictides. In this talk, magnetization, resistivity and specific heat data will be compared with band structure calculations. Both the experimental results and the density functional theory (DFT) calculation are consistent with this material being a bad metal with large residual resistivity, similar to the well-studied Fe-pnictides. The key difference is that the Sr-Bi blocking layer in SrMnBi$_2$ is metallic, which may be more favorable to the occurrence of superconductivity upon doping, likely with a higher transition temperature, commensurate with the high T$_N$ $\approx$ 280 K. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T19.00014: Doping - induced Quantum Phase Transition in Sc$_{3.1}$In Eteri Svanidze, Emilia Morosan Sc$_3$In is a known itinerant ferromagnet with a reported T$_{\rm{c}}~\approx~$6 K. In this talk we will show that Lu doping induces a quantum phase transition in this compound. Temperature and field - dependent magnetization measurements on (Sc$_{1-\rm{x}}$Lu$_{\rm{x}}$)$_{3.1}$In polycrystalline samples were performed, where 0 $\leq$ x $\leq$ 0.08. The 3.1 : 1 stoichiometry was chosen because it showed the highest T$_{\rm{c}}~\approx$ 10 K for x = 0 when Arrott plots were employed to determine the Curie temperature. In this study we use modified Arrott plots M$^2$ vs. (H/M)$^{1/\alpha}$. For $\alpha = 1.5$, the corresponding isotherms were linear over larger field ranges, and, for the critical composition x$_c~\approx$ 0.02, the isotherm was linear down to (M,H) = (0,0). The Curie temperature determined using this method was close to 6 K. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T19.00015: On the origin of the magnetic susceptibility anomaly in nearly ferromagnetic alloys Romeo de Coss, Aar\'on Aguayo, Filiberto Ortiz-Chi The magnetic susceptibility of the Ni-Rh and Ni-Cu alloys shows an anomaly near the transition from ferromagnetism to paramagnetism. In order to contribute to understand this phenomenon, we have studied the electronic and magnetic properties of the Ni$_{1-x}$Cu$_{x}$ alloy by means of first principles calculations. The ground state properties were obtained using the Full-Potential Linear Augmented Plane Waves method. The alloying was modeled using the self-consistent virtual crystal approximation. The spin magnetic susceptibility is calculated from the total energy as a function of the spin moment, obtained using the Fixed Spin Moment methodology. We found that the calculations predict correctly the reduction of the magnetic moment with the Cu concentration and that the critical concentration where the magnetic moment goes to zero is x$_{c}$ = 0.5, in excellent agreement with the experimental data. The calculated magnetic susceptibility is in good agreement with the experimental data in the whole range of concentrations for the Ni$_{1-x}$Cu$_{x}$ alloy, in particular the anomaly present at x $\approx $ 0.4 is reproduced by the calculations. This research was supported by Conacyt-M\'{e}xico under Grant No. 83604. [Preview Abstract] |
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