Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session T33: Novel Magnetic Materials |
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Sponsoring Units: GMAG Chair: Yves Idzerda, Montana State University Room: E143 |
Wednesday, March 17, 2010 2:30PM - 2:42PM |
T33.00001: Strong crystal anisotropy of magneto-transport property in Fe$_{3}$Si epitaxial films S. Y. Huang, H. Y. Hung, P. Chang, W. C. Lin, S. F. Lee, M. Hong, J. Kwo We report transport study of the epitaxial Fe$_{3}$Si/GaAs heterostructures prepared by the molecular beam epitaxy growth. The temperature dependence of resistivity can be described as $T^{3}$ dependence in terms of Bloch-Gruneisen formula. Below 50 K, the $T^{3}$ term lent supports to the anomalous single-magnon scattering predicted for the half-metallic material. By changing the current direction from parallel to the magnetic hard axis to the magnetic easy axis, the anomalous zigzag magnetoresistance (MR) behavior can be altered to cos$^{2}$(\textit{$\theta $}$_{M})$ expected for the anisotropic MR behavior at high field $\sim $500 Oe, with a maximum and a minimum occurring at the magnetic field parallel and perpendicular to the current direction, respectively. However, at low field below 100 Oe, the MR showed an interesting step function-like dependence at a critical angle \textit{$\theta $}$_{critical, }$corresponding to the magnetic hard [110] axis. Our study showed unusually strong dependence of magneto-transport properties on crystal anisotropy in the plan of the epitaxial Fe$_{3}$Si/GaAs films. [Preview Abstract] |
Wednesday, March 17, 2010 2:42PM - 2:54PM |
T33.00002: Anomalous Hall Effect in Iron Silicide Thin Films Julie Karel, Hyeon-Jun Lee, Frances Hellman Iron silicide has attracted significant attention as a potential spin injector. Fe$_{3}$Si films can be grown epitaxially on various semiconductors, and recent results have demonstrated spin injection into Si with a non-local measurement. Theory has predicted Fe$_{3}$Si to be nearly half metallic, and the density of states can be tuned by small changes in the Fe concentration or the addition of small amounts of Mn. The metastable bcc composition range of Fe$_{x}$Si$_{1-x }$between 0.55$<$x$<$0.75 offers the potential to continuously tune the magnetic, structural and electronic properties, and we use thin film growth by electron beam co-evaporation of Fe and Si to probe these properties. We compare the magnetic, structural and electronic properties as a function of composition and growth conditions, with a focus on the observed Anomalous Hall Effect at both 2K and 300K. We discuss the origins of this effect and show evidence of Fe-spin polarized carrier exchange interaction in off-stoichiometry compositions. [Preview Abstract] |
Wednesday, March 17, 2010 2:54PM - 3:06PM |
T33.00003: Origin of Giant Saturation Magnetization in Fe$_{16}$N$_{2}$ thin film Jian-Ping Wang, Nian Ji, Xiaoqi Liu, Yunhao Xu, C. Sanchez-Hanke Can localized 3d electron exist in strong ferromagnetic metal because of some unusual correlation effect? This question is related to the controversy on whether $\alpha $''- Fe$_{16}$N$_{2}$ has giant saturation magnetization which has been debated for decades since its first observation$^{1,2}$. Here we report the synthesis of $\alpha $''- Fe$_{16}$N$_{2}$ thin films. The highest moment is obtained to be 3.0$\mu _{B}$/Fe. XMCD experiment is systematically performed on a series of iron nitrides samples. Among all the iron nitrides phases, it is found that there exist highly localized 3d electrons only in chemically disordered Fe$_{8}$N and ordered F$_{16}$N$_{2}$ phases$^{3}$. This discovery hints at the origin of the giant magnetic moment is correlated with the 3d electron localization in such system. First principle calculation (LDA+U) further verifies that the d electron localization is the key element to rationalize the high moment formation in iron nitrides system$^{4}$. We also provide a speculative outlook on the giant saturation magnetization formation based on ``cluster + atom'' concept. 1.Kim, T.K. and Takahashi. M, Appl. Phys. Lett., 20, 492 (1972) 2.Sugita, Y., et al., J. Appl. Phys. 76, 6637 (1994) 3.Liu, X. Q. et al., arxiv: 0909.4480 (2009) 4.Ji, N. et al., arxiv: 0909.4478 (2009) [Preview Abstract] |
Wednesday, March 17, 2010 3:06PM - 3:18PM |
T33.00004: Emergence of helimagnon bands in MnSi Marc Janoschek, Florian Bernlochner, Sarah Dunsiger, Christian Pfleiderer, Peter Boeni, Bertrand Roessli, Peter Link, Achim Rosch Recent theoretical studies predict that the broken inversion symmetry in the helical phase of MnSi will lead to a rich spectrum of helimagnons for wave vectors that are small compared to the helical propagation vector $\vec{k}$. Our extensive inelastic neutron scattering study in the helical phase shows the existence of broad dispersive excitations. Using a parameter free model we quantitatively establish that these excitations represent broad spin wave bands that are purely caused by the tiny magnetic propagation vector. The small magnetic Brillouin zone leads to multiple Umklapp interactions and thus many helimagnon modes. Our study provides a tractable show-case how collective spin excitations may be radically modified even in simple systems by seemingly harmless small magnetic propagation vectors. [Preview Abstract] |
Wednesday, March 17, 2010 3:18PM - 3:30PM |
T33.00005: Structural and Magnetic Study of Magnetic MnAl Thin Films by Reactive Ion Beam Deposition Wei Chen, J.R. Skuza, Jiwei Lu, Nam Dao, R.A. Lukaszew, Stuart Wolf The metastable magnetic $\tau $ phase of MnAl binary alloy with unique hard magnetic properties was discovered a long time ago followed by extensive investigation of its structure and magnetic properties but mainly on bulk samples instead of ultra thin films. Here we present a structural and magnetic study of $\tau $ phase MnAl thin films prepared by Reactive Biased Target Ion Beam Deposition. We found that the choice of substrate and the concomitant interfacial strain plays a very important role in determining the magnetic anisotropy of MnAl thin films. The effects of fabrication conditions and post-deposition annealing treatment on the film properties will be also discussed. [Preview Abstract] |
Wednesday, March 17, 2010 3:30PM - 3:42PM |
T33.00006: Variation of exchange interactions and magnetism with uniaxiality in MnAs thin films Magnus Wikberg, Ronny Knut, Sumanta Bhandary, Mikael Ottoson, Janusz Sadowski, Olof Karis, Biplab Sanyal, Peter Svedlindh The magnetic behavior of high quality MnAs thin films grown on GaAs(100) substrates by means of molecular beam epitaxy has been studied. The correlation between structural parameters such as film strain and orientation distribution of the MnAs hexagonal unit cell and the magnetic anisotropy and the ferromagnetic ordering temperature has been investigated. Uniaxialy oriented and relaxed films display a transition temperature (T$_{c}$) and anisotropy constants (K$_{1}$/K$_{2}$) close to bulk MnAs values while strained MnAs films show T$_{c}$) and K$_{1}$/K$_{2}$ values clearly deviating from the corresponding bulk values. The experimental findings from magnetic (SQUID magnetometry), structural (XRD) and element specific magnetic (X-ray magnetic circular dichroism) measurements have been complemented by theoretical calculations (full potential linearized muffin tin orbital method in the local spin density approximation) of the change of exchange and anisotropy energies with strain in the unit cell. [Preview Abstract] |
Wednesday, March 17, 2010 3:42PM - 3:54PM |
T33.00007: Magnetic Properties of Single Crystal Fe$_{1-x}$Ga$_{x}$ Thin Films Adam McClure, Hongyan Li, Paul Rugheimer, J.X. Cao, Ruqian Wu, Elke Arenholz, Yves Idzerda Single crystal thin films of the highly magnetostrictive Fe$_{1-x}$Ga$_{x}$ alloy have been prepared on GaAs(001), with a ZnSe buffer layer, and MgO(001) substrates by molecular beam epitaxy (MBE). The saturation magnetization of the samples, as determined by vibrating sample magnetometry (VSM) and Rutherford backscattering (RBS), shows a reduction as a function of Ga concentration which closely follows a simple dilution model up to a concentration of 25{\%} Ga where the magnetization falls more abruptly than simple dilution allows. X-ray magnetic circular dichroism (XMCD) performed at the Fe and Ga L$_{2,3}$-edges, along with \textit{ab-initio} density functional (GGA) calculations, ascribe this trend to a decrease in the elemental Fe moment and an induced moment in the gallium of 0.1 $\mu _{B}$ anti-aligned to the Fe, establishing Fe$_{1-x}$Ga$_{x}$ as a ferrimagnetic system. [Preview Abstract] |
Wednesday, March 17, 2010 3:54PM - 4:06PM |
T33.00008: Field-dependent Magnetic Anisotropy of Single Crystal Fe$_{1-x}$Ga$_{x}$ Films on ZnSe(100) Hongyan Li, Adam McClure, Ian Vrable, Galina Malovichko, Yves Idzerda Magnetoelastic alloys in the thin film form that are pinned to a substrate are of current interest as materials for controlled spin dynamic damping. When the single crystal magnetoelastic alloy material Fe$_{1-x}$Ga$_{x}$ (which has a large magnetostriction value in the bulk) is epitaxially deposited onto the non-magnetoelastic material ZnSe, a biaxial strain is generated at the interface because of lattice mismatch. Anisotropic mechanical strain relaxation will generate a uniaxial magnetic anisotropy in the thin film. The application of a magnetic field will modify the strain resulting in an additional field dependent uniaxial contribution. This has been demonstrated using multi-frequency, angle-dependent ferromagnetic resonance measurements on single crystal Fe$_{1-x}$Ga$_{x}$ thin films, ranging from 0{\%} to 60{\%} Ga concentration, deposited on ZnSe(001) surfaces that display a field independent cubic anisotropy while the uniaxial anisotropy is dependent on the applied field. [Preview Abstract] |
Wednesday, March 17, 2010 4:06PM - 4:18PM |
T33.00009: Thermodynamic measurements of iron-rhodium alloys David Cooke, Frances Hellman Iron-rhodium alloys undergo a unique metamagnetic AF$>$FM transition at just above room temperature, making it ideal for thermally-assisted storage applications. The details of this transition, however, are still much debated. Here we have measured the specific heat of equiatomic and slightly Fe-rich thin films grown on our unique microcalorimeters in order to shed light on this subject. Our data will be presented in the context of the thermal fluctuation model proposed by Gruner et al., \textit{Phys. Rev. B} \textbf{67}, 064415 (2003). In this model, the Fe-rich films where the transition is suppressed exhibit a Schottky-like two-state anomaly at a temperature far below the transition due to a competition between the magnetic and non-magnetic Rh states. Because the $J_{Fe-Rh}$ interaction responsible for this close competition in free energy cancels at the Rh atom in the equiatomic alloys, it does not exhibit this anomaly. Complementary work including magnetization data and magnetic x-ray microscopy imaging through the transition will be briefly discussed as well. [Preview Abstract] |
Wednesday, March 17, 2010 4:18PM - 4:30PM |
T33.00010: Magnetic Thermal Hysteresis in Dy nanolayers Ajani Ross, Ali Koymen, Fabio Sales, Artur Carrico, Ana Dantas Magnetic thermal hysteresis is observed when the temperature dependent magnetic properties of a material are reliant on the starting point of the measurement. Samples of pure Dysprosium (Dy) were grown on substrates of glass and sapphire. We observed magnetic thermal hysteresis in these thin film Dy samples at low values of constant external magnetic field strengths. The temperature is swept from 20K to 300K at constant field, then back (300K to 20K) under the same field. In these temperature sweeps differences in magnetic moment were observed near the low end of the temperature range. Experiments are being done, currently, to confirm the existence of alternate helicity (AH-state) and helical (H-state) states in Dy films, which are believed to be the cause of the observed thermal hysteresis according to theoretical calculations. In addition, the temperature dependence of entropy change for pure Dy films are also measured. [Preview Abstract] |
Wednesday, March 17, 2010 4:30PM - 4:42PM |
T33.00011: Metal-Insulator Transition in Ce$_{1-x}$Gd$_{x}$Fe$_{4}$P$_{12}$ (x $\sim $0.1) observed by ESR. M. Tovar, F.A. Garcia, P.G. Pagliuso, C. Rettori, P.A. Venegas, D.J. Garcia, P. Schlottmann, M.G. Passeggi, M.S. Torikachvili, S.B. Oseroff Measurements by Electron Spin Resonance (ESR) of Gd$^{3+}$ impurities doped into single crystals of CeFe$_{4}$P$_{12}$ show the evolution from fully resolved ESR spectra with Lorentzian (insulator) shape lines into a single broad ESR line with a Dysonian (metallic) shape as a function of temperature. The fine and hyperfine structure splitting of Gd$^{3+}$ is clearly observed at low temperature in the fully resolved ESR spectra. From the spectra we determined the crystal field and hyperfine structure parameters. With increasing T the ESR lines broaden and finally collapse into a single Dysonian line at about 180 K. As T increases further the linewidth becomes broader at a rate of $\sim $ 1.1 Oe/K. We discuss the origin of the ``Metal to Insulator'' transition observed. [Preview Abstract] |
Wednesday, March 17, 2010 4:42PM - 4:54PM |
T33.00012: The order parameter dependence of transition temperature in FeRhPd alloy films Hideo Sato, Jian Yu, Gary Mankey, Oleg Mryasov, Patrick LeClair FeRh alloys and FeRh-TM alloys have recently attracted great interest because well-ordered films exhibit a phase transition with antiferromagnetism observed at lower temperatures and ferromagnetism at higher temperatures.~ Here, the order parameter dependence of transition temperature in Fe$_{47}$Rh$_{47}$Pd$_{6}$ films is reported.~ FeRhPd/Co films were fabricated such that different order parameters were obtained. A higher transition temperature was observed for a film with lower order parameter in sharp contrast to prior experiments with FeRh that show that the transition temperature monotonically increasing with order parameter [1].~ The shift to lower transition temperature for the ordered film is accompanied by a sharpening of the phase transition.~ This is surprising, since disordered films are ferromagnetic throughout the temperature range of the measurements.~ These results, along with a possible explanation for the anomalous behavior including the effect of lattice constant on the transition temperature will be presented. Funded by the US DOE 1. Jiangwei Cao et al., J. Appl. Phys. 103, 07F501 (2008) [Preview Abstract] |
Wednesday, March 17, 2010 4:54PM - 5:06PM |
T33.00013: Electronic Investigation on Fe intercalated hard magnet Fe$_{0.25}$TaS$_{2}$ K.-T. Ko, J.-Y. Kim, J.-H. Park, Kyoo Kim, B.-I. Min, Sung Baek Kim, S-W. Cheong The electronic structure of Fe$_{0.25}$TaS$_{2}$ was investigated by using the x-ray absorption spectroscopy (XAS) at Fe $L_ {2,3}$- edge and density functional theory (DFT) calculation. The x-ray magnetic circular dichroism (XMCD) revealed that the intercalated Fe$^{2+}$ has extremely large orbital magnetic moment, consistently with a large magnetic coercive in this system. Additionally, the orbital states were determined from the polarization dependent XAS, in which the lowest orbital state is identified to have strong in-plane characters. The detailed XAS results were analyzed in terms of the theoretical cluster model calculations. We also found that the observed orbital occupation as well as the spin and orbital magnetic moments agree well with the predictions of the DFT calculations including on-site Coulomb interaction (U) and spin-orbit coupling (S.O.). Finally, we discuss the orbital anisotropy including local electronic structure and the giant magnetocrystalline anisotropy of Fe$_{0.25}$TaS$_{2}$. [Preview Abstract] |
Wednesday, March 17, 2010 5:06PM - 5:18PM |
T33.00014: Magnetic and Transport Studies in La$_x$Gd$_{1-x}$Rh$_3$B$_2$ and Ce$_x$Gd$_{1-x}$Rh$_3$B$_2$ Ulhas Vaidya, S.K. Dhar, A. Thamizhavel, S. Ramakrishnan, A.K. Grover The nonmagnetic rare earth ternary boride LaRh$_3$B$_2$, a hexagonal system of space group P6/mmm with CeCo$_3$B$_2$ type structure, is known to superconduct below T$_c$(0)$\sim$ 2.7 K. When doped with Gd the T$_c$(0) reduces and no superconductivity is observed above 1.2 K with more than 15\% (atomic) doping. The alloys, La$_{0.8}$Gd$_{0.2}$Rh$_3$B$_2$ and La$_{0.6}$Gd$_{0.4}$Rh$_3$B$_2$ show a jump at $\sim$ 70 K and rapid rise in magnetization below 10 K. However, no peaks are observed in at these temperatures in AC susceptibility which would have indicated magnetic ordering of the system. It is worth noting that GdRh$_3$B$_2$ orders ferromagnetically at $\sim $ 93 K. We shall present results of our investigations, above 2 K, in magnetic and transport properties of polycrystalline specimens La$_x$Gd$_{1-x}$Rh$_3$B$_2$ (0 $<$ x $ <$ 1). CeRh$_3$B$_2$ is a ferromagnet with T$_c$ $\sim$ 115 K. The results of Gd doping in this reveal the survival of antiferromagnetic coupling between Ce and Gd spins. [Preview Abstract] |
Wednesday, March 17, 2010 5:18PM - 5:30PM |
T33.00015: Frequency dependent susceptibility study for a ferromagnetic shape memory alloy Pratip Mukhopadhyay, B. Rajini Kanth, Sandeep Agarwal Ferromagnetic shape memory alloys (FSMA) display interesting interplay between thermal, structural and magnetic energies -- manifested by different transformations occurring in the system as a function of temperature. These changes are generally measured by XRD, resistivity, DSC and magnetization studies. However, these are basically static measurements, showing no frequency dependence, i.e. the time evaluation in the systems. However, the major structural change that occurs in these alloys is the martensitic transformation, occurring in only milli-second time scale. Thus frequency dependent a.c. susceptibility measurements would be a natural choice to look into the time domain picture of the processes occurring in such a system. We report here such a measurement first time on a CoNiAl based FSMA system that was characterized by XRD, SEM, resistivity and magnetization measurements. In the susceptibility measurements done under 100Oe, we varied the temperature from 400K to 80K and back and frequencies from 66 to 5000 Hz -- trying to match the slow dynamics in the system. The measurements indicated that there is a good amount of frequency dependence of the susceptibility, indicating that there are magnetic relaxations going on in the system in different time scales. We analyzed the results with Debye relaxation model. [Preview Abstract] |
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