Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T17: Magnetic Alloys and Multilayers |
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Sponsoring Units: GMAG Chair: Masatoshi Onoue, Northwestern University Room: 319 |
Thursday, March 21, 2013 8:00AM - 8:12AM |
T17.00001: Phase stability, ordering, and magnetism of single-phase fcc Fe-Au alloys Joonhee M. An, Sergey V. Barabash, Kirill D. Belashchenko Motivated by experimental evidence of L1$_0$ ordering in single-phase fcc Fe-Au nanoparticles, we study the structural thermodynamics of Fe-Au alloys. First, separate cluster expansions for fcc and bcc lattices are constructed for fully optimized ferromagnetic structures using density functional theory calculations. The optimized structures were assigned to fcc or bcc lattice by a structural filter. Although the lowest formation enthalpy at 50\% Au is reached in the bcc lattice, the fcc lattice is preferred for the random alloy. Dynamical stability of specific orderings strongly depends on the magnetic configuration. To analyze the ordering tendencies of the fcc alloy, we restrict uniform lattice relaxations and separate the contributions of chemical interaction and local relaxations. By using the effective tetrahedron model (Ruban \emph{et al.}, Phys. Rev. B 67, 214302 (2003)) and explicit calculations for ordered and special quasi-random structures, we find that the local relaxation energies depend weakly on the magnetization. Although the L1$_0$ ordering is the ground state at 50\% Au on the ideal lattice, local relaxations make it unfavorable compared to the random alloy. Moderate compression due to the size effect tends to slightly stabilize the L1$_0$ ordering. [Preview Abstract] |
Thursday, March 21, 2013 8:12AM - 8:24AM |
T17.00002: First-principles study of magnetic properties of Fe-Ni based alloys M. Onoue, G. Trimarchi, A.J. Freeman Investigations of the magnetic properties of Fe-Ni based alloys are important from the fundamental as well as technological points of view. Furthermore, the magnetization at saturation and Curie temperature ($T_{\rm C}$) of FeNi can be tuned for specific applications by alloying with other metallic species. We have performed electronic structure calculations on Fe-Ni-$M$ alloys, where $M$ are 3d transition metals, to determine how the magnetization depends on the species $M$ and alloy composition. Electronic band structure and total energies are calculated by the Korringa-Kohn-Rostoker method within the coherent-potential-approximation (KKR-CPA). For the KKR-CPA calculations, we use the generalized gradient approximation of the exchange and correlation functional. In the case of Fe$_{0.50}$Ni$_{0.45}M_{0.05}$ ($M$=Sc, Ti, V, Cr, Mn, and Co), the early 3$d$ atoms have antiparallel magnetic moments to the Fe or Ni, whereas the late ones, Mn and Co, have a parallel magnetic moment. [Preview Abstract] |
Thursday, March 21, 2013 8:24AM - 8:36AM |
T17.00003: FeCo-based permanent magnet materials search by genetic algorithm Cai-Zhuang Wang, Manh Cuong Nguyen, Xin Zhao, Kai-Ming Ho FeCo alloy is well-known soft magnetic material with high magnetic moment, 2.5 $\mu_B$/atom at $\sim$ 30 wt. \% Co. However, doping FeCo alloys with heavy 5d transition metal and mix FeCo phase with nomagnetic structure of AlNi (e.g., Alnico) would increase the coercivity of the alloys. In order to gain more insight into the enhancement of the magnetic anisotropy in FeCo by doping or mixing, we have investigated the stable and metastable crystal structures of Fe-Co-W and Fe-Co-Al-Ni systematically over a wider range of composition by adaptive genetic algorithm method. Our search results show that the Fe-rich FeCoW alloys are all in bcc structures with W prefer substituting Fe sites. The Fe-Co-Al-Ni structures are also found to be in bcc lattice with broad chemical variation across the FeCo and AlNi interface. The magnetic properties in these stable and metastable structures are also calculated and the microscopic mechanism for the enhancement of magnetic anisotropy is discussed. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 8:48AM |
T17.00004: Unusually sharp paramagnetic phase transition in thin film Fe$_{3}$Pt invar Jasper Drisko, John Cumings Invar alloys, typically 3d transition metal rich systems, are most commonly known for their extremely low coefficients of thermal expansion (CTE) over a wide range of temperatures close to room temperature. This anomalous behavior in the CTE lends Invar to a variety of important applications in precision mechanical devices, scientific instruments, and sensors, among others. Many theoretical models of Invar have been proposed over the years, the most promising of which is a system described by two coexisting phases, one high-spin high-volume and the other low-spin low-volume, that compete to stabilize the volume of the material as the temperature is changed. However, no theory has yet been able to explain all experimental observations across the range of Invar alloys, especially at finite temperature [1]. We have fabricated thin films of a Fe$_{3}$Pt Invar alloy and investigate them using Lorentz Transmission Electron Microscopy (TEM). 23nm films are deposited onto SiN membrane substrates via radio-frequency magnetron sputtering from a pure Fe target decorated with Pt pieces. We observe novel magnetic domain structures and an unusually sharp phase transition between ferromagnetic (FM) and paramagnetic (PM) regions of the film under a temperature gradient. This sharp transition suggests that the FM-to-PM transition may be first order, perhaps containing a structural-elastic component to the order parameter. However, electron diffraction reveals that both the FM and PM regions have the same FCC crystal structure. \\[4pt] [1] Kakehashi, Y., \textit{Phys. Rev. B.} \textbf{38}, 474 (1988). [Preview Abstract] |
Thursday, March 21, 2013 8:48AM - 9:00AM |
T17.00005: Predicting magnetostructural trends in equiatomic FeRh-based ternary systems Radhika Barua, Felix Jimenez-Villacorta, Laura H. Lewis A phenomenological model is proposed to predict the influence of elemental substitution on the magnetostructural transition temperatures and Curie temperatures of nominally-equiatomic FeRh-based compounds with the B2 (CsCl)-type crystal structure. Clear trends in the characteristic magnetic transition temperatures, as reported in the literature, are found as a function of the averaged weighted valence band electrons (($s+d)$ electrons/atom) in compounds of composition Fe(Rh$_{\mathrm{1-x}}$M$_{\mathrm{x}})$ or (Fe$_{\mathrm{1-x}}$M$_{\mathrm{x}})$Rh (M $=$ 3$d$, 4$d$ or 5$d$ transition metals). Substitution of 3$d$ or 4$d$ elements ($\le $ 6.5 atomic {\%}) into B2-type FeRh causes the magnetostructural transition temperature $T_{t}$ to increase to a maximum around a critical valence band electron concentration of 8.5 electrons/atom and then decrease. Substitution of 5$d$ transition metal atoms echoes this trend but shifts it to higher transition temperatures. These data and associated trends allow deductions that the stability of the ground state antiferromagnetic phase of the FeRh-based system depends both on the size of the constituent atoms as well as the character of the valence electrons. [Preview Abstract] |
Thursday, March 21, 2013 9:00AM - 9:12AM |
T17.00006: Development of Magnetic Materials Based on the Ordered Fe$_{50}$Ni$_{50}$ Phase: Methodologies and Results Eric Poirier, Misle M. Tessema, Martin S. Meyer, Frederick E. Pinkerton The L1$_{0}$ FeNi structure known as tetrataenite, usually found in meteorites, is reported to possess significant magnetocrystalline anisotropy suitable for hard magnetic properties. As part of the ongoing Advanced Research Project Agency-Energy project on FeNi-based magnets, melt-spinning was used to synthesize new FeNi precursors. The melt-spinning conditions were established in terms of wheel speed, ejection pressure, and atmosphere composition and pressure. The as-spun ribbons have a cubic crystal structure with a$=$3.584 $\pm$ 0.002 {\AA}, and (100) preferred grain orientation perpendicular to the ribbon. They also behave like soft magnetic materials, with coercitivities below 0.3 kOe. DSC response curves were essentially featureless, except for a thermal signature at about 515 $^{\circ}$C associated with the Curie temperature. In contrast, melt-spun FeNi ribbons that were subsequently ball-milled and annealed exhibited a more complex thermal behavior compared to the as-spun ribbons with a weak endotherm in the 300-350 $^{\circ}$C range followed by an exotherm at higher temperatures. These results are discussed in the context of a search for an order-disorder phase transition associated with the L1$_{0}$ phase, and preferred properties for permanent magnet applications. Although L1$_{0}$ phase formation was not observed at this point, the techniques established for processing FeNi will be further studied on modified FeNi alloys as a promising route to obtain the L1$_{0}$ phase. [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:24AM |
T17.00007: Combinatorial Approach for High-efficiency Magnetization Measurements of Co-Fe-Ni Alloys with a Scanning Hall Probe Microscope Girfan Shamsutdinov, Debabrata Mishra, Boris Nadgorny, Peng Zhao, Ji-Cheng Zhao, Sreenivas Bhattiprolu A Scanning Hall Probe Microscope with a submicron scale Hall probe (HP) was used for high efficiency measurements of magnetic properties of Co-Fe diffusion couples. Co-Fe couples were made by placing Co and Fe blocks in an intimate contact and annealing at high temperature to allow thermal interdiffusion to create solid-solution with a composition varying gradually from pure Fe to pure Co. The magnetic field in the vicinity of these variable composition Fe-Co alloys, with the width of approximately 400 microns, was measured continuously as the HP was scanned across the interdiffusion region. Using a simple model, we determined the composition dependent saturation magnetizations of Co-Fe alloys. The values of the saturation magnetization were in good agreement with the known values for pure Fe and Co. The composition variation and the crystal structure along the scan line were measured independently using Energy Dispersive X-ray Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). Similar measurements were performed for the Fe-Ni and Co-Ni interfaces. This study demonstrates that Scanning Hall microscopy can be used for high efficiency and high accuracy measurements of saturation magnetization in variable composition alloys. [Preview Abstract] |
Thursday, March 21, 2013 9:24AM - 9:36AM |
T17.00008: Tuning magnetic anisotropy in Fe/Pt multilayers on Pt(001) by surface charging Pedro Ruiz-Diaz, Valeri Stepanyuk Magnetic anisotropy of nanoscale systems has recently received considerable attention from both experimentally and theoretically perspectives. Diverse ways of manipulating the anisotropy have been sought and found. Those include alloying, external electric field exposure and electrolyte charging. However, the hunt for a system that would exhibit a large anisotropy and be easy to manipulate at the same time is still on. By using density functional theory tools, we study the magnetic anisotropy of Fe/Pt multilayers on Pt(001). Our fully relativistic {\it ab initio} calculations demostrate that the value of magnetic anisotropy energy (MAE) strongly depends on the composition of Fe/Pt multilayers, achieving remarkable large values for systems featuring Fe layers capped with Pt. For instance, positive charging of a Fe slab capped with Pt enhances significantly the MAE. More intriguing is the behavior of Fe bilayers, for which surface charging does not only change the value of the anisotropy but can also lead in the switching of the easy axis. To understand the physics underlying this behavior of MAE, we analyze the electronic structure of the system by means of the second-order perturbation theory linking MAE to the local density of electronic states near the Fermi level. [Preview Abstract] |
Thursday, March 21, 2013 9:36AM - 9:48AM |
T17.00009: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T17.00010: Investigation of the atomic structure of Zr$_{2}$Co$_{11}$ Xin Zhao, Manh Cuong Nguyen, Liqin Ke, Vladimir Antropov, Cai-Zhuang Wang, Kai-Ming Ho The compound known as Zr$_{2}$Co$_{11}$ is a ferromagnet with high uniaxial anisotropy. Although a lot of experimental work has been done on this compound, its crystal structure is still unsolved. We performed adaptive Genetic Algorithm (GA) search on its atomic structure, in order to have a better understanding of this compound. The validity of our method was verified by locating all the stable phases in Zr-Co alloy system. The search for Zr$_{2}$Co$_{11}$ was performed with up to 117 atoms per unit cell and a narrow composition window near 15.38{\%} Zirconium was explored. We found that Zr$_{2}$Co$_{11}$ compound has a structure derived from CaCu$_{5}$ prototype and complex mixed phases can be formed. Simulated XRD and TEM patterns of our models are in agreement with the experimental results. Calculated magnetic properties provide explanations of the high uniaxial anisotropy in this system. [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T17.00011: Atomic structure and magnetic properties of HfCo$_7$ alloy Manh Cuong Nguyen, Xin Zhao, Liqin Ke, Vladimir Antropov, Cai-Zhuang Wang, Kai-Ming Ho Low energy atomic structures of HfCo$_7$ alloys were searched by adaptive generic algorithm with unit cell up to 48 atoms. We found some different motifs existing in other magnetic systems in low energy structures for unit cell with 16 and 32 atoms. When the unit cell size is bigger than 40 atoms, we observed structures with phase separation into pure hcp Co and Hf$_2$Co$_7$ in agreement with phase diagram. Magnetic properties calculations were performed to investigate the relationship between the structure motifs and magnetic properties. The magnetization and Curie temperature of low energy structures are close to those of hcp Co and for some structures, a magnetic anisotropy larger than that of hcp Co were found. We will discuss more on how calculated intrinsic magnetic properties can explain the observed permanent magnet properties and how to improve the magnetic properties of HfCo$_7$ alloy. [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T17.00012: Magnetic properties of doped Ce$_2$Co$_{17}$ alloys Liqin Ke, Denis Kukusta, Vladimir Antropov Substitutional alloys Ce$_2$Co$_{17-x}$T$_{x}$, where T is $d-$atom or Al, Si, Ga, have been analyzed using electronic structure calculations with a focus on the influence of doping on such properties as magnetization, magnetic anisotropy and Curie temperature. A complication arises because we need to improve all three of these key properties of magnets. We found that a system with small levels of doping has a strong site preference effect. This effect, when combined with site decomposition of magnetic anisotropy and Curie temperature leads to the specific scenario of producing desirable new magnetic materials with better properties as permanent magnets. We show that in order to obtain a better set of these three key magnetic properties, one has to consider dopings by two elements, with one element responsible for changes to magnetic anisotropy and another for improving the magnetization and Curie temperature. Obtained theoretical results have been compared favorably with a large amount of available experimental data for certain systems. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T17.00013: Ferromagnetism in Single Crystal MoS$_2$ Sima Saeidi Varnoosfaderani, Sefaattin Tongay, Bill Appleton, Junqiao Wu, Arthur Hebard We report on the magnetic properties of MoS$_2$ flakes measured from room temperature down to 10 K and magnetic fields up to 5 Tesla. Molybdenum disulfide (MoS$_2$) is one of the most stable layered transition metal dichalcogenides, which has a finite band gap and is regarded as a complementary (quasi-) 2D material to graphene. We find that single crystals of MoS$_2$ display ferromagnetism superimposed onto a large temperature-dependent diamagnetism and observe that ferromagnetism persists from 10 K up to room temperature. We attribute the existence of ferromagnetism partly to the presence of zigzag edges in the magnetic ground state at the grain boundaries. Since the magnetic measurements are relatively insensitive to the interlayer coupling, these results are expected to be also valid in the single layer limit. [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T17.00014: Structural and electronic properties of the half-Heusler phases PtFeBi, PtMnBi, PdFeBi and PdMnBi Xiaofang Wang, Xiaoshuang Chen, ChingYao Fong, Wenchao Huang, Wei Lu First-principles calculations based on density functional theory have been performed to study the structural and electronic properties of the PtFeBi, PtMnBi, PdFeBi and PdMnBi half-Heusler alloys. The results reveal that all the alloys show metallic properties at the ground state configuration. We further investigated the dependence of electronic band structures by applying hydrostatic pressure. It is found that the PtMnBi and PdMnBi are half-metallic with the same magnetic moment of 4.0 $\mu$B per formula-unit when their lattice constants are reduced (from -3.0{\%} to -11.2{\%} and -6.1{\%} to -7.9{\%}, respectively). For PtMnBi, its band gap of the minority spin channel increases with compression due to the noticeable strong p-d hybridization, which is the reason for the formation of bonding and antibonding states. It is obvious that the high spin polarization of PtMnBi is over a large range of its lattice constant and with a wide band gaps in the PtMnBi. However, the PdFeBi and PtFeBi are quasi-half-metallic with magnetic moment to be 3.0 $\mu$B at -6.9{\%} and -8.3{\%} uniform strain, respectively. They are sensitive to the changes of lattice constants. [Preview Abstract] |
Thursday, March 21, 2013 10:48AM - 11:00AM |
T17.00015: Complex magnetic ordering and spin glass behavior as a driving mechanism of multifunctional properties of Heusler alloys from first principles Anna Gruenebohm, Peter Entel, Heike C. Herper, Markus E. Gruner, Alfred Hucht, Denis Comtesse, Raymundo Arroyave First-principles calculations are used to study the structural, electronic and magnetic properties of (Pd, Pt)-Mn-Ni-(Ga, In, Sn, Sb) alloys which display multifunctional properties like the magnetic shape-memory, magnetocaloric and exchange bias effect. The ab initio calculations give a basic understanding of the underlying physics which is associated with the complex magnetic behavior (also spin glass) arising from competing ferro- and antiferromagnetic interactions with increasing number of Mn excess atoms in the unit cell. This information allows to optimize, for example, the magnetocaloric effect by using the strong influence of compositional changes on the magnetic interactions. Thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. We present guidelines of how to improve the functional properties. For Pt-Ni-Mn-Ga alloys, a shape memory effect with 14\% strain can be achieved in an external magnetic field. [Preview Abstract] |
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