Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z29: Magnetostructural Effects and General Magnetism |
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Sponsoring Units: GMAG Chair: Sara Majetich, Carnegie Mellon University Room: 333 |
Friday, March 20, 2009 11:15AM - 11:27AM |
Z29.00001: Non-cubic, coherent nanoprecipitates observed by neutron diffuse scattering in highly magnetostrictive Galfenol (Fe$_{1-x}$Ga$_{x})$ alloys. Peter Gehring, Hu Cao, Chris DeVreugd, Jose Abelardo Rodriguez, Jie Fang Li, Dwight Viehland We report neutron diffuse scattering measurements on highly magnetostrictive Fe$_{1-x}$Ga$_{x}$ alloys with different thermal treatments. Diffuse scattering is seen for compositions 0.14$<$x$<$0.20 heat-treated to increase the magnetostriction that is otherwise absent. This diffuse scattering exhibits asymmetric peaks at the (100) and (300) reciprocal lattice positions that are consistent with the coexistence of short-range ordered, coherent nanoprecipitates embedded in a long-range ordered, body-centered cubic matrix. A large peak splitting is observed at (300) for x=0.19, which suggests that the nanoprecipitates are not cubic but have a lower symmetry and a large elastic strain. The strongest diffuse scattering occurs for x=0.19 where the maximum magnetostriction is found. This suggests a structural origin for the enhanced magnetostriction in these materials. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z29.00002: Magnetic and magnetoelastic properties of Ge/Co co-substituted cobalt ferrite Naresh Ranvah, Eugene Melikhov, John Snyder, David Jiles The highly magnetostrictive material cobalt ferrite and its derivatives based on substitution of cations have been shown to have extreme sensitivity of their magnetization to stress. In order to control their properties (e.g. magnetostriction, magnetic anisotropy, strain derivative, and hysteresis) substitution of specific cations are needed. We have shown Ge$^{+4}$/Co$^{+2}$ co-substituted cobalt ferrites (Co$_{1+x}$Ge$_{x}$Fe$_{2-2x}$O$_{4})_{ }$to have very interesting combinations of magneto-elastic properties. In the present study the variation of magnetic anisotropy with composition and temperature was investigated, for the new germanium/cobalt co-substituted cobalt ferrite Co$_{1+x}$Ge$_{x}$Fe$_{2-2x}$O$_{4}$.. Hysteresis loops were measured in the range T = 10 - 400 K, with \textit{$\mu $}$_{0}H_{max}$~=~0 - 5~T. The high field regions of these loops were then fitted to the Law of Approach to Saturation (LA) for cubic materials as given by $M=M_{s}$[1-(8/105)($K_{1}$/\textit{$\mu $}$_{0}M_{s}H)^{2}$], plus a linear forced magnetisation term. Values for first order cubic anisotropy constant $K_{1}$ were calculated and it was found that anisotropy increased as temperature decreases for all compositions. At most temperatures anisotropy decreased with increase in $x$. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z29.00003: Magnetostriction of Fe-based alloy Yanning Zhang, Juexian Cao, Ruqian Wu High magnetostrictive materials are important for sensor and actuator applications. Although the rare earth-3d metal compound, such like TbFe2, have large positive magnetostriction of $\sim $2600ppm, their practical application was hindered due to larger activation external magnetic field and their high material costs. Large-scale industrial applications require soft magnetic materials with high magnetostrictive materials with low material costs. Nanocrystalline Fe-based alloys without rareearths are very interesting candidates due to the fact doping some nonmagnetic element, such as Ga, Al, Zn and Be, strongly alters the anisotropy energy and enhances the magneticostiction, which indicated substituting Fe by nonmagnetic elements might be a new method to develop cheap and soft magnetic material with high magnetostriction. With First-principle calculations, we have investigated the Young's modulus, shear modulus, anisotropy energy and magnetostriction of Ga, Al, Zn, Ge and Be doped in bulk Fe. The magneto-crystalline anisotropy energy $E_{MCA}$ and the magnetostrictive coefficients ($\lambda _{001})$ strongly depend on the compositions and atomic arrangement. A rigid band picture is proposed to estimate the trend of magnetostrictive coefficients of Fe, Ga and Zn compounds. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z29.00004: The Magnetocaloric Effect in Single Crystal and Processed Polycrystalline MnP Ryan A. Booth, Sara A. Majetich Manganese Phosphide (MnP) is a promising magnetocaloric effect (MCE) material for use in room temperature-magnetic refrigeration because it exhibits a first-order ferromagnetic to paramagnetic phase transition at a Curie temperature of 290 K, possesses no measurable magnetic hysteresis, and has a saturation field of lower than 7.5 kOe along the c-axis. The magnetic entropy change ($\Delta$S) was measured in single crystals along the c-axis to be 2.2, 3.3, and 6.0 J / kg K in applied fields of 10 kOe, 20 kOe, and 50 kOe respectively. Cold-rolling followed by a short annealing of polycrystalline MnP was shown to create preferential crystallographic alignment that mimics the favorable properties of single crystals without the expenses associated with their growth. A comparison of the magnetocaloric effect between single crystals and rolled polycrystalline MnP is presented. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z29.00005: Magnetic anisotropy dependency on structural properties in strained MnAs thin films Magnus Wikberg, Mikael Ottoson, Janusz Sadowski, Ronny Knut, Olof Karis, Peter Svedlindh High quality thin films of MnAs (between 30 and 200 {\AA}) have been grown with molecular beam epitaxy (MBE) on GaAs(111)B and (001) substrates and under different growth conditions. The magnetic anisotropy of the MnAs layers has been investigated with SQUID magnetometry and magnetic force microscopy (MFM). A clear correlation between choice of substrate, growth temperature and film thickness can be seen in the magnetic anisotropy and T$_{c}$ measurements with a rapid transition towards bulk like anisotropy constants as the film thickness is increased. From X-ray diffraction, a relationship between T$_{c}$ and lattice strain has been established, where the film with increasing film thickness rapidly exhibits a transition from a highly strained to a fully relaxed film. A complex dependence on the structural transition from the ferromagnetic to the paramagnetic phase is also seen in X-ray magnetic circular dichroism (XMCD) measurements, where the orbital moment does not strictly follow the spin moment near the ferromagnetic-paramagnetic phase transition. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z29.00006: Coercivity of Melt-Spun Gd$_{100-x}$Fe$_{x}$ Paul Shand, Andrew Meyer, David Schmitter, Geoffrey Rojas, Jeffrey Shield, Jared Goertzen, Danial Haskel, Diandra Leslie-Pelecky We have measured the coercivity of melt-spun Gd$_{100-x}$Fe$_{x}$ (0$\le x\le $40) alloys over the temperature range 2 K$\le T\le $340 K. Previously performed structural measurements revealed that the system consists of crystalline hcp-Gd grains surrounded by a non-crystalline Gd or Gd-Fe phase composed of Gd$_{100-x'}$Fe$_{x'}$, where $x$'$>x$ is the iron concentration in the amorphous region. The two-phase structure is responsible for an unusual dependence of the coercivity on temperature in which non-zero coercivity is observed above the hcp-Gd $T_{c}$ with a peak near 320 K. The coercivity decreases as the hcp-Gd grains order, then increases with decreasing temperature. This behavior is explained by the presence of Fe-rich magnetically correlated regions. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z29.00007: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z29.00008: Epitaxial growth and spin dependent states of Co$_{x}$Mn$_{y}$Si$_{z}$ (111) thin films Liang He, Brian Collins, Frank Tsui, Yong Chu Epitaxial growth of Co$_{x}$Mn$_{y}$Si$_{z}$ on Ge (111) substrates has been studied using combinatorial MBE techniques, including that of the Heusler alloy Co$_{2}$MnSi. For Si concentration of 25 at. {\%}, in-situ RHEED and ex-situ X-ray diffraction experiments indicate that the epitaxial growth is coherent for atomic ratio Co:Mn between 1 and 9, while the film is microcrystalline for Co:Mn $<$ 1 and it is rough and of poor crystalline quality for Co:Mn $>$ 9. The crystalline quality is the highest around Co:Mn = 4, whereas it exhibits a plateau around a ratio of 2, i.e. the Heusler alloy, Co$_{2}$MnSi. Within the region of coherent growth, at coverages below 100{\AA}, the growth front is smooth 2D-like. As thickness increases, the surface morphology systematically changes from quasi-2D into 3D. The morphology transition also depends sensitively on composition, i.e. Co:Mn ratio, and temperature. Spin-dependent states as a function of composition at low coverages have been examined by tunneling spectroscopy using Al$_{2}$O$_{3}$ as the tunneling barrier and Fe and Nb as the detector layers. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z29.00009: Self-energy of half-metallic ferromagnet Mn$_5$Ge$_3$ calculated from infrared spectroscopy data S.V. Dordevic, N. Stojilovic, L.W. Kohlman, C. Petrovic We will report the results of our infrared and optical spectroscopy study of a half-metallic ferromagnet Mn$_5$Ge$_3$. This compound is currently being investigated as potential injector of spin polarized currents into germanium. Infrared measurements have been performed over a broad frequency (30 - 50000~cm$^{-1}$) and temperature (10 - 300 K) range. From the complex optical conductivity $\sigma(\omega)$ we extract the electron self-energy $\Sigma(\omega)$. The calculation of $\Sigma (\omega)$ is based on novel numerical technique which uses Levenberg--Marquardt algorithm for solution of systems of non- linear equations. Obtained self-energy provides a new insight into electron correlations in Mn$_5$Ge$_3$. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z29.00010: Electronic and Magnetic properties of Transition Metal Borides. Aditi Herwadkar, Yufeng Zhao Boron has three valance electron and a small covalent radius undergoing sp$_{2 }$hybridization in many boron clusters. It is also known to form large variety of crystal structures both planer and 3D caged molecules. Our current work is to study transition metal boride clusters. This would be useful towards designing metal boride nanoclusters with tunable optical, magnetic and electron transport properties. Different stoichiometries are studied. The metal to Boron ratio varies from 0.3 to 0.75. For each of these compositions the lowest energy geometries were determined by optimizing the bond length for several initial symmetric geometries. The lowest energy structure is then chosen as the equilibrium structure. Most strikingly, all these transition-metal boride clusters are cage-like although both boron and transition metal usually favor high coordination number. We also find that some of the clusters have an extremely high magnetic moment per unit mass. This renders metal boride nanoparticles as potentially promising light-weighted magnetic materials. All the calculations are done using the spin-polarized density functional theory method implemented in the Vienna ab initio simulation package. A plane wave basis set with (400 eV cutoff) was used in combination with an all electron like projector augmented wave potential and PBE exchange correlation functional with in the generalized gradient approximation. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z29.00011: Revisiting Classical Diamagnetism: A Surprise of Physics Narendra Kumar, Vijay Kumar KrishnaMurthy The Classic Bohr-van Leeuwen (BvL) theorem states that the orbital diamagnetism of a classical system of charged particles in thermal equilibrium is identically zero. This theorem is universally accepted and has entered textbooks. Physically, the theorem derives from the exact cancellation of the orbital diamagnetic moment associated with the completed cyclotron orbits of the charged particles by the paramagnetic moment subtended by the incomplete orbits skipping the boundary cuspidally in the opposite sense. In this work we have revisited the problem of this crucial but subtle role of the boundary by considering the case of a finite but unbounded system, namely that of a charged particle moving on a sphere in the presence of an externally applied magnetic field. The orbital moment calculated on the basis of the classical Langevin equation in the infinite time limit now indeed turns out to be non-zero, and has the diamagnetic sign. This violates the BvL theorem as stated in the literature. To the best of our knowledge, this is the first report of non-zero classical diamagnetism. It is explicitly owing to the above avoided cancellation. We also present possible experimental realization of the predicted classical diamagnetism. [Preview Abstract] |
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