Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A24: Spintronic Materials |
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Sponsoring Units: GMAG DMP Chair: George Kioseoglou, University of Crete Room: D133-D134 |
Monday, March 15, 2010 8:00AM - 8:12AM |
A24.00001: Magnetocrystalline anisotropy in single crystal Gd$_{5}$Si$_{2.7}$Ge$_{1.3}$ David Jiles, Ravi Hadimani, Yevgen Melikhov, John Snyder We have determined the magnetocrystalline anisotropy, easy and hard axes and the anisotropic constants for single crystal Gd$_{5}$Si$_{2.7}$Ge$_{1.3}$ using magnetization vs. angle of rotation and magnetization vs. magnetic field measurements carried out on a vibrating sample magnetometer (VSM). Magnetization was measured as a function of angle with an applied field of 0.12~MA/m (1500~Oe) in the `ab', `bc' and `ca' planes with angles measured from the `a', `c' and `c' axes respectively. It was determined from the measurements that the `b' axis has the highest susceptibility and hence is the easy axis, while the `a' and `c' have lower and approximately equal susceptibilities and hence are the hard axes. The magnetic anisotropy energy E$_{an}$ was calculated using E = $\mu _{0}$HM. The first order uniaxial anisotropic constant K$_{1}$ was determined by fitting the E$_{an}$ vs. $\theta $ curve to uniaxial anisotropy energy equation E$_{an}$ = K$_{1}$sin$^{2}\theta _{ }$and was determined to be 1.451$\times $10$^{4}$~J/m$^{3}$. This research is supported by the Royal Society under a Wolfson Research Merit Award. [Preview Abstract] |
Monday, March 15, 2010 8:12AM - 8:24AM |
A24.00002: Optimization of Giant Magnetoimpedance in amorphous (Co$_{1-x}$Fe$_{x})_{89}$Zr$_{7}$B$_{4}$ ribbons A. Chaturvedi, M.H. Phan, H. Srikanth, A. Leary, M.E. McHenry The giant magnetoimpedance (GMI) effect is observed upon the application of a static magnetic field on a soft ferromagnetic material carrying an ac current. GMI is a tool that can be used in a number of applications and especially important for magnetic sensors. There is currently a need for optimizing the effect in new materials and fine tune the material parameters for enhanced GMI and sensitivity. We report systematic studies of the GMI effect and its field sensitivity in (Co$_{1-x}$Fe$_{x})_{89}$Zr$_{7}$B$_{4}$ amorphous ribbons with x=0.00, 0.025, 0.05, 0.075 and 0.10. Magnetoimpedance measurements were carried out along the ribbon axis in applied dc magnetic fields up to 120 Oe over a frequency range of 0.1 $\sim $ 13 MHz. We find that while the highest value of GMI is achieved for the x = 0 sample, the highest field sensitivity of GMI is achieved for the x=0.025 sample. The Fe doping causes a significant modification of a transverse domain structure thus reducing the GMI effect. We have also studied systematically the influence of Fe doping on field hysteresis and symmetry in GMI profiles, as well as relative contributions of resistance and reactance to magnetoimpedance for all the samples. [Preview Abstract] |
Monday, March 15, 2010 8:24AM - 8:36AM |
A24.00003: Ferromagnetism and large magnetocaloric effect in Eu$_{8}$Ga$_{16}$Ge$_{30 }$ clathrates M.H. Phan, A. Chaturvedi, S. Stefanoski, H. Kirby, G.S. Nolas, H. Srikanth, V. Franco Semiconductors with the clathrate hydrate crystal structure are widely known for their excellent thermoelectric properties. The presence of Eu with large magnetic moment (7.94 $\mu _{B})$ also makes them very interesting for magnetic and magnetocaloric studies. We report large magnetocaloric effect (MCE) in Eu$_{8}$Ga$_{16}$Ge$_{30}$ type-VIII clathrates and on the influence of Sr doping on the magnetic properties and MCE in Eu$_{8}$Sr$_{8-x}$Ga$_{16}$Ge$_{30}$ (x = 0, 4) type-I clathrates. Experimental results reveal a correlation between the long-range ferromagnetism and giant MCE in Eu$_{8}$Ga$_{16}$Ge$_{30}$ type-VIII clathrates. The substitution of Sr for Eu increases the Eu-Eu distance which consequently decreases the Curie temperature (T$_{C})$, saturation magnetization, and MCE in Eu$_{8}$Sr$_{8-x}$Ga$_{16}$Ge$_{30}$ (x = 0, 4) type-I clathrates. In addition to the paramagnetic-ferromagnetic transition at T$_{C}$, a new low temperature magnetic transition is observed in MCE experiments. This transition is likely associated with the ordering of the magnetic moments of Eu. The excellent magnetocaloric properties of the clathrate materials make them very interesting for cryogenic magnetic refrigeration applications. [Preview Abstract] |
Monday, March 15, 2010 8:36AM - 8:48AM |
A24.00004: Intrinsic-Vacancy Mediated, Room-Temperature Ferromagnetism in a Silicon-Compatible Dilute Magnetic Semiconductor: Cr-doped Ga$_2$Se$_3$ E.N. Yitamben, T.C. Lovejoy, A.B. Pakhomov, S.M. Heald, F.S. Ohuchi, M.A. Olmstead Room temperature ferromagnetism (RTFM) and silicon compatibility are key criteria for useful spintronic materials. We present RTFM in a new class of dilute magnetic semiconducor (DMS) involving intrinsic vacancies: epitaxial Cr-doped Ga$_2$Se$_3$ on Si(001). With Ga$_2$Se$_3$'s ordered defects responsible for band-edge states, Cr:Ga$_2$Se$_3$ lies at the intersection between traditional DMS materials, where FM is mediated by dopant-induced carriers, and ones in which defects play a predominate role. Semiconducting Cr:Ga$_2$Se$_3$ films have a RT magnetic moment of 4 $\mu_B$/Cr and Cr-induced states overlapping the vacancy-derived states. Cr substitution for Ga, combined with a rotation of bonds around a single Se, can explain the observed octahedral local environment without long-range lattice disruption and lead to FM mediated by states associated with intrinsic vacancies. Cr incorporates into laminar Ga$_2$Se$_3$ films up to about 8 \% Cr, disaggregating into Cr-rich islands at higher concentration; islanded films remain FM, but with altered magnetic moment per Cr. [Preview Abstract] |
Monday, March 15, 2010 8:48AM - 9:00AM |
A24.00005: Ferromagnetic order at 298 K in Cr$_{0.005}$Sn$_{0.995}$Te Jason C. Lashley, James L. Smith, James E. Gubernatis, Roland K. Schulze, Bogdan Mihaila, Robert D. Field, Cyril P. Opeil, Peter B. Littlewood, Eli Rotenberg, Aaron Bostwick We observe ferromagnetic ordering at 298 K by magnetic susceptibility measurements. In addition we observe a structural transition at 98 K that has been attributed to ferroelectricity. Transmission electron microscopy at 300 K shows a modulated cubic structure thereby challenging the heretofore accepted B1 rocksalt structure. Ramifications of these results plus Fermi surface measurements made by angle resolved photoemission using synchrotron radiation will be given. [Preview Abstract] |
Monday, March 15, 2010 9:00AM - 9:12AM |
A24.00006: Absence of localized-spin magnetic scattering in the narrow-gap semiconductor FeSb$_{2}$ Andrei Savici, Igor Zaliznyak, Ovidiu Garlea, Rongwei Hu, Cedomir Petrovic We report inelastic neutron scattering measurements aimed at investigating the origin of the temperature-induced paramagnetism in the narrow-gap semiconductor FeSb$_2$. We find that inelastic response for energies up to 60 meV and at temperatures $\approx 4.2$ K, $\approx 300$ K and $\approx 550$ K is consistent with scattering by the lattice phonon excitations. Hence, we observe no evidence for a well-defined magnetic excitation corresponding to transitions between the non-magnetic ground state and states of magnetic multiplet in the localized spin picture. However, a broad magnetic scattering continuum in the 15 meV to 35 meV energy range is not ruled out by our data. Our findings make description in terms of the localized Fe magnetic states unlikely and suggest that paramagnetic susceptibility of itinerant electrons is at the origin of the temperature-induced magnetism in FeSb$_2$. [Preview Abstract] |
Monday, March 15, 2010 9:12AM - 9:24AM |
A24.00007: M\"{o}ssbauer Spectrometry study of Fe$_{x}$Si$_{1-x}$ thin films Catherine Bordel, Julie Karel, Jean Juraszek, Frances Hellman Fe$_{3}$Si is ferromagnetic and potentially half-metallic and therefore of high interest for spintronics applications. Below 75 at.{\%} Fe, a homogeneous metastable structure can be grown and leads to a tunable set of magnetic and electrical properties. We are studying Fe-Si thin films with an Fe concentration ranging from 55 to 80 at.{\%} to determine the different structural phases that can form, their role on the Fe magnetic moment, and combined influence on the electronic band structure of the alloy. This work presents the $^{57}$Fe Conversion Electron M\"{o}ssbauer Spectrometry measurements obtained on 3 different samples: two epitaxial samples (Fe$_{78}$Fe$_{22}$ and Fe$_{65}$Si$_{35})$ grown on (100) MgO substrates, and an amorphous Fe$_{65}$Si$_{35}$ sample grown on amorphous SiN$_{x}$/Si substrates. Different Fe sites and Fe magnetic moments are observed. These studies enable differentiation between the role of composition and that of the long-range structural order. [Preview Abstract] |
Monday, March 15, 2010 9:24AM - 9:36AM |
A24.00008: Positive and Negative magnetoresistance of semiconduting Fe$_{x}$-C$_{1-x}$/Si thin film Xiaozhong Zhang, Lihua Wu, Caihua Wan, Xili Gao We synthesized amorphous C film with thickness of 60-80 nm on n-type Si (100) substrate by pulsed laser ablation. When magnetic field is applied perpendicular to the film plane, the C/Si sample shows a positive MR of 40{\%} at room temperature and 5T. This MR is anisotropic and is mainly attributed to the Si substrate. Addition Fe into carbon film enhances the MR and distribution of Fe in carbon film affects the MR sensitivity, i.e. when we distributed Fe into some special positions, the MR of Fe-C/Si sample can reach 30{\%} at H=300 Oe. At bias of 0.4V Fe-C/Si sample shows a linear MR-T relationship at 40-160K and H$>$1T. At negative bias the MR at low temperature can be positive or negative depending on the direction of magnetic field applied. In addition, the Fe-C/Si films also show some other novel physical properties, such as giant electroresistance, photoconductivity, and gas pressure sensitivity. Study of Fe-C/Si nanostructures, which have characters of multifunctional and simple structure, may shed a light on discovery of new spintronic material. [Preview Abstract] |
Monday, March 15, 2010 9:36AM - 9:48AM |
A24.00009: Strain and magnetism in epitaxial MnSi thin films T.L. Monchesky, E.A. Karhu, M.D. Robertson Epitaxial MnSi grown on Si(111) provides new opportunities to explore spin-dependent transport in helical magnets. We investigated the influence of strain on the Curie temperature, T$_{C}$, in MnSi thin films grown by MBE and solid phase epitaxy. The substrate introduced a positive volume strain in the MnSi(111) film, as determined by x-ray diffraction and TEM. The strain can be controlled by a combination of growth temperature and film thickness and leads to an enhanced Curie temperature that is 50{\%} larger than bulk. This volume strain is correlated with $T_{C}$ measurements and is found to deviate from that expected for bulk MnSi. The magnetic properties measured by SQUID magnetometry also show departures from bulk behavior. The helical magnetic order was expected to create a remanent magnetization, $M_{r}$, which oscillates as a function of the thickness with a wavelength equal to that of the helix. Instead a single peak in $M_{r}$ was observed at a thickness of 7.5nm. Departures from Curie Weiss behavior are also observed above $T_{C}$. [Preview Abstract] |
Monday, March 15, 2010 9:48AM - 10:00AM |
A24.00010: Electronic structure and magnetism in EuN Ben Ruck, Joe Trodahl, Jan Richter, Andrew Preston, Franck Natali, Natali Plank, Claire Meyer, Julio Cezar, Laurent Ranno The rare-earth nitrides exhibit unusually strong coupling between their spin and charge degrees of freedom. Some rare-earth nitrides, such as GdN, have been shown to be ferromagnetic semiconductors, in agreement with band structure calculations. However, similar calculations are far less clear regarding the electronic structure of EuN, mainly due to the propensity of Eu to exist in either 2+ or 3+ charge states. Here we present the first evidence that EuN is ferromagnetic with a Curie temperature of about 30 K, and it is metallic, likely half-metallic. X-ray magnetic circular dichroism experiments indicate the Eu ions are predominantly in the 3+ charge state, but a small fraction are in the 2+ state. The nature of the exchange interaction remains unclear. [Preview Abstract] |
Monday, March 15, 2010 10:00AM - 10:12AM |
A24.00011: Spin-dependent processes in silicon-rich silicon-nitride thin film solar cells Sang-Yun Lee, Seoyoung Paik, Dane McCamey, Christoph Boehme, Jian Hu, Feng Zhu, Arun Madan Silicon-rich silicon-nitride (SiN$_{x}$:H) is attracting attention due to its relevance for light emitting diodes, electrolytic devices and solar cells. Charge transport and recombination in this material are key factors for its optoelectronic properties. Since SiN$_{x}$:H is highly disordered and exhibits weak spin-orbit coupling similar to amorphous silicon (a-Si:H), many of the electronic transitions in this material take place through localized electronic states and are governed by spin-selection rules. Thus, it is possible to study these processes using electrically detected magnetic resonance spectroscopy (EDMR). Using coherent, pulsed EDMR, we observed a variety of qualitatively different spin-dependent processes. We present a mapping of the properties of these processes regarding coupling and defect type and show that the nature of many of these processes is similar to previously studied mechanisms in a-Si:H as they involve a variety of different states (dangling bonds, tails states), with various spin coupling modes (exchange, dipolar coupling). However, in contrast to a-Si:H, many of the transitions between strongly spin-coupled pairs influence the conductivity and hence they do not represent geminate recombination processes. [Preview Abstract] |
Monday, March 15, 2010 10:12AM - 10:24AM |
A24.00012: GW study of the local field effect in half metallic CrAs Liam Damewood, Ching Fong We determined the semiconducting gap of zinc blende (ZB) CrAs within the GW approximation (GWA). This is the first GW calculation of a half-metal. Previous calculations using density functional theory within the generalized gradient approximation (GGA) determined a gap of 1.8 eV, but the GGA is known to give too small of a value for this quantity in semiconductors. Additionally, we studied the role of the local field effect in CrAs. Due to the simultaneous metallic and insulating properties of half metals, the screening in CrAs is weaker compared to insulating screening. Preliminary results suggest that half metallic screening increases the semiconducting gap as much as 0.54 eV from the fully insulating value of 1.92 eV. [Preview Abstract] |
Monday, March 15, 2010 10:24AM - 10:36AM |
A24.00013: Measurement of the Transport Spin polarization of Ru Doped CrO2 Using Point-contact Andreev reflection Michael Osofsky, Kevin West, Stuart Wolf, Jiwei Lu Point contact Andreev reflection (PCAR) has proven to be a versatile tool for evaluating the transport spin polarization of ferromagnetic materials for spintronics applications. I will review the principles behind PCAR and results from a variety of materials that verify its usefulness. It now well known that CrO2 is 100{\%} spin polarized but very difficult to grow in thin film form. We recently demonstrated that high quality films of Ru doped CrO2 can be grown using reactive bias ion beam deposition. Some of these films have Curie temperatures above room temperature and exhibit unusual magnetic behavior including an intrinsic tunable exchange bias. PCAR results on these films will be presented that show that these films are also highly spin polarized compared to conventional metals like Fe. [Preview Abstract] |
Monday, March 15, 2010 10:36AM - 10:48AM |
A24.00014: Magnetoelastic coupling through the antiferromagnetic - to - ferromagnetic transition in quasi-two-dimensional [Cu(pyz)$_2$HF$_2$]BF$_4$ using infrared spectroscopy J.L. Musfeldt, L.I. Vergara, T.V. Brinzari, C. Lee, L.C. Tung, J. Kang, Y.J. Wang, J.A. Schlueter, J.L. Manson, M.-H. Whangbo We investigated magneto-elastic coupling through the field-driven transition to the fully polarized magnetic state in quasi-two-dimensional [Cu(HF$_2$)(pyz)$_2$]BF$_4$ by magneto-infrared spectroscopy. This transition modifies out-of-plane ring distortion and bending vibrational modes of the pyrazine ligand. The extent of these distortions increases with field, systematically tracking the low temperature magnetization. These distortions weaken the antiferromagnetic spin exchange, a finding that provides important insight into magnetic transitions in other copper halides. [Preview Abstract] |
Monday, March 15, 2010 10:48AM - 11:00AM |
A24.00015: Investigations of interactions between low-dimensional electron systems and magnetic surface species by antilocalization measurements Yao Zhang, R.L. Kallaher, J.J. Heremans, J. Gilpin Weak antilocalization (AL) is a sensitive probe of quantum states in two-dimensional electron systems (2DESs). In order to experimentally investigate interactions between conduction electrons and magnetic species, we compare measurements of AL in 2DESs where magnetic species have been deposited on the surface with those performed on samples where no magnetic species are present, and give two examples. AL yields a characteristic positive magnetoresistance in weak applied magnetic fields. We find that a square lattice of lithographically prepared 40 nm diameter, 25 nm thick CoFe discs on the surface of an InGaAs 2DES reduces the AL signal as compared to a InGaAs 2DES with bare surface. The reduced magnitude is a consequence of increased magnetic scattering from the CoFe discs. In contrast, randomly distributed Dy3+ ions on the surface of InAs result in a more pronounced AL signal in the InAs surface electron accumulation layer. Dy3+ ions are expected to increase both the spin-orbit scattering rate and the magnetic scattering rate. The increase in the AL signal is here attributed to a decrease in the spin coherence time. [Preview Abstract] |
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