Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session R19: Magnetic Bulk MaterialsFocus
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Sponsoring Units: GMAG DMP Chair: James Lee, Lawrence Berkeley Laboratory Room: 318 |
Thursday, March 17, 2016 8:00AM - 8:12AM |
R19.00001: Field induced spin density and spiral phases in a layered antiferromagnet Matthew Stone, Mark Lumsden, Vasile Garlea, Beatrice Grenier, Eric Ressouche, Eric Samulon, Ian Fisher, Lisa Debeer-Schmitt, Alexander Hristov, Jorge Gavilano We will present neutron scattering measurements examining the low-field ordered magnetic phases of the $S=1$ dimerized antiferromagnet Ba$_3$Mn$_2$O$_8$. We have found that for magnetic both spin density wave order with incommensurate wave vectors and a higher field spiral phase with incommensurate wave vectors only along the $[hh0]$ direction. For both field induced ordered phases, the magnetic moments are lying in the plane perpendicular to the field direction. The nature of these two transitions is fundamentally different: the low-field transition is a second order transition to a spin-density wave ground state, while the one at higher field, toward the spiral phase, is of first order. More recent SANS measurements of the magnetic phases with H $||$ c will be presented if available at the time of the meeting. [Preview Abstract] |
Thursday, March 17, 2016 8:12AM - 8:24AM |
R19.00002: Incommensurate Spin Density Wave state in metamagnetic Fe$_3$Ga$_4$ Yan Wu, Huibo Cao, António dos Santos, Greg McCandless, Julia Chan, Amar Karki, Rongying Jin, John DiTusa Fe$_3$Ga$_4$ displays a rich competition between magnetic states without structural transitions: a ferromagnetic(FM) ground state transitions to an antiferromagnetic(AFM) intermediate state above 68 K followed by a reemergence of the FM state above room temprature(T). The reentrance of the FM state hints of a coupling of the magnetic degrees of freedom to other modes. To explore the nature of the magnetic states, we have performed extensive single crystal neutron diffraction measurements over a wide range of T and pressure. These measurements revealed two very different magnetic states with the low T FM state having magnetic moments along the c-axis while we discovered that the AFM state is in an incommensurate spin density wave(SDW) order with moments mostly along the a-axis. However, there is still considerable non-collinear and non-coplanar contributions along the b- and c-axial directions. This non-coplanar moment is likely to be the origin of the very large anomalous Hall effect(HE) including a substantial topological HE that we discovered in Fe$_3$Ga$_4$. Study of the effect of hydrostatic pressure indicates a reduction of the T$_c$ and a destabilization of the SDW phase. [Preview Abstract] |
Thursday, March 17, 2016 8:24AM - 8:36AM |
R19.00003: Increased operational temperature of Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$-based spintronic devices Michael Street, Will Echtenkamp, Takashi Komesu, Shi Cao, Jian Wang, Peter Dowben, Christian Binek Spintronic devices have been considered a promising path to revolutionizing the current data storage and memory technologies. This work is an effort to utilize voltage-controlled boundary magnetization of the magnetoelectric chromia (Cr$_{\mathrm{2}}$O$_{\mathrm{3}})$ to be implemented into a spintronic device. The electric switchable boundary magnetization of chromia can be used to voltage-control the magnetic states of an adjacent ferromagnetic layer. For this technique to be utilized in a spintronic device, the antiferromagnetic ordering temperature of chromia must be enhanced above the bulk value of T$_{\mathrm{N}} \quad =$ 307K. Previously, based on first principle calculations, boron doped chromia thin films were fabricated via pulsed laser deposition showing boundary magnetization at elevated temperatures. Measurements of the boundary magnetization were also corroborated by spin polarized inverse photoemission spectroscopy. Exchange bias of B-doped chromia was also investigated using magneto-optical Kerr effect, showing an increased blocking temperature from 307K. Further boundary magnetization measurements and spin polarized inverse photoemission measurements indicate the surface magnetization to an in-plane orientation from the standard perpendicular orientation. [Preview Abstract] |
Thursday, March 17, 2016 8:36AM - 8:48AM |
R19.00004: "Switching" of Magnetic Anisotropy in Magnets with Strong Spin-Orbit Coupling Hiroaki Ishizuka, Leon Balents Motivated by recent studies on heavy-element magnetic oxides, we theoretically study a spin model on a fcc lattice with bond-dependent anisotropic interactions. Strong spin-orbit coupling in heavy elements often gives rise to bond-dependent anisotropic interactions in magnetic compounds. Such anisotropic interactions are known to induce peculiar magnetic behavior such as quantum spin-liquid and order-by-disorder. In this study, we investigate magnetic anisotropy of a fcc lattice antiferromagnet with bond-dependent interactions. We show that, in this model, the magnetic anisotropy is induced by fluctuations in both high-temperature paramagnetic and low-temperature magnetically-ordered phases. Furthermore, they show strong temperature dependence and switching of the magnetic anisotropy as the temperature decreases; <111> direction is favored in high-temperature above magnetic transition, while <100> or <110> is favored in the ordered phase, depending on the parameter. This is in contrast to the magnetic anisotropy induced by crystal field, which is independent of temperature. Observation of this temperature dependent anisotropy may provide a way to experimentally determine the anisotropic interaction in heavy-element magnets. [Preview Abstract] |
Thursday, March 17, 2016 8:48AM - 9:00AM |
R19.00005: Lattice Dynamics and Magnetoelastic Coupling in a Frustrated Shape Memory Alloy Paul Stonaha, Mike Manley, Nick Bruno, Ibrahim Karaman, Raymundo Arroyave, Navdeep Singh, Eliot Specht, Doug Abernathy, Songxue Chi, Xiaoping Wang Magnetocaloric (MC) materials present an avenue for chemical-free, solid state refrigeration through cooling via adiabatic demagnetization. We have used inelastic neutron scattering to measure the lattice dynamics in the MC shape memory alloy Ni$_{\mathrm{45}}$Co$_{\mathrm{5}}$Mn$_{\mathrm{36.6}}$In$_{\mathrm{13.4}}$. In this presentation, we present the results of inelastic neutron scattering near the Curie temperature. We find that there is an unaccounted-for change in phonon entropy of 0.22 kB atom$^{\mathrm{-1}}$ (26 J kg$^{\mathrm{-1\thinspace }}$K$^{\mathrm{-1}})$, which we attribute to phonon-magnon coupling. We identify an anomalous softening of the TA[100] phonon mode. We also present the results of diffuse neutron and X-ray diffraction experiments and provide an interpretation on the implied local order. [Preview Abstract] |
Thursday, March 17, 2016 9:00AM - 9:12AM |
R19.00006: Magnetotransport anisotropy in microstructures of Yb$_2$Pt$_2$Pb Toni Helm, Philip J.W. Moll The Yb$^{3+}$ moments in Yb$_2$Pt$_2$Pb (YPP) form a strongly frustrated Shastry-Sutherland lattice (SSL) [1]. Below $2\,$K, a dimerized antiferromagnetic order consisting of two AF sublattices has been recently identified by neutron diffraction [2]. Unlike other quantum magnets, YPP is a highly conductive metal and the large Sommerfeld coefficient $\Gamma\sim 300\,$ mJ$/$molK$^{2}$ suggests hybridization of the Yb-4f states with the conduction band [3]. This opens the possibility to search for signatures of the metamagnetism associated with the plateaus at fractions of the saturation magnetization, a characteristic of SSL systems. To study the influence of YPP’s rich magnetic structure on the anisotropic charge transport, we fabricated micron-sized transport devices from single crystalline YPP by Focused Ion Beam etching. This technique enables thickness and length dependent magnetotransport measurements along the most relevant lattice directions. [1] M. S. Kim et al. PRB 77,144425 (2008) [2] W. Miiller et al. arXiv:1408.0209v1 (2014) [3] M. S. Kim, M. C. Aronson, PRL 110, 017201 (2013) [Preview Abstract] |
Thursday, March 17, 2016 9:12AM - 9:24AM |
R19.00007: Magnetic anisotropy of rare-earth magnets calculated by SIC and OEP Hisazumi Akai, Masako Ogura We have pointed out in our previous study that the chemical bonding between N and Sm plays an important role in the magnetic anisotropy change of Sm$_2$Fe$_{17}$ from in-plane to uniaxial ones caused by the introducing of N. This effect of N insertion was discussed in terms of change in the electronic structure calculated in the framework of LDA+SIC. The main issue here is whether the 4f states are dealt with properly in SIC. In the present study, we examine the applicability of SIC for the evaluation of the magnetic anisotropy of rare-earth (RE) magnets by comparing the results with various methods, in particular, the optimized effective potential (OEP) method. In this study, OEP is applied only on the RE sites. Admittedly, this is a drawback from the viewpoint of the consistent treatment of uncertainly inherent in the so-called KLI (Krieger-Li-Iafrate) constants. Putting this aside for the moment, we have calculated the electronic structure of RE magnets R$_2$Fe$_{17}$N$_x$ and RCo$_5$ (R=light RE), by OEP with exact-exchange (EXX) combined with Colle-Salvetti correlation. Our preliminary results have shown considerable differences between the SIC and OEP calculations. We will discuss the meaning of this discrepancy. [Preview Abstract] |
Thursday, March 17, 2016 9:24AM - 9:36AM |
R19.00008: Domain wall order and motion in Mn$_3$O$_4$ Alexander Thaler, Alexander Zakjevskii, Brian Nguyen, Yewon Gim, Adam Aczel, Lisa Debeer-Schmitt, S. Lance Cooper, Gregory MacDougall Mn$_3$O$_4$ is an orbitally ordered, magnetically frustrated spinel with strong spin-lattice coupling, which exhibits a series of low temperature magnetic and structural transitions. Transverse field $\mu$SR has shown that ordered and disordered volumes coexist within this material, while MFM measurements have further shown that the magnetic domain walls themselves order in specific crystallographic directions, with a typical length scale of 100's of nm. In order to directly study these phenomena, we have performed small angle neutron scattering (SANS) measurements at both zero and applied magnetic field. We will present the results of these measurements and discuss what they show as far as the formation of domains, as well as the motion of the domain walls. We will also discuss the effects of internal disorder on the behavior of the material. This work was sponsored by the National Science Foundation, under grant number DMR-1455264. [Preview Abstract] |
Thursday, March 17, 2016 9:36AM - 9:48AM |
R19.00009: Nanoscale Magnetic Structure of Non-Joulian Magnets Ravini Chandrasena, Weibing Yang, Andreas Scholl, Jan Minar, Padraic Shafer, Elke Arenholz, Hubert Ebert, Alexander Gray, Harsh Deep Chopra Strain dependence of magnetic anisotropy energy produces Joule magnetostriction that is a volume conserving process, whereas sensitivity of isotropic exchange energy to interatomic distance is the cause of volume magnetostriction. In a typical magnet, Joule magnetostriction dominates as the volume fraction occupied by regions of uniform spin alignment (domains) is 2-4 orders of magnitude higher than that which is occupied by regions with magnetoelastic gradients (domain walls). Recently, `giant' non-volume conserving or non-Joulian magnetostriction has been discovered in iron-gallium alloys. Here we show using high-resolution polarization-dependent photoelectron microscopy that non-Joulian magnetism arises from an unusual partition of the crystal into nm-scale lamellar domains and domain walls within highly periodic magnetic microcells. High-resolution x-ray circular dichroism measurements at the Fe and Ga L absorption edges further provide evidence of weak iron-induced magnetism on gallium atoms via negative exchange. The results are in excellent agreement with the state-of-the-art theoretical electronic-structure calculations. [Preview Abstract] |
Thursday, March 17, 2016 9:48AM - 10:00AM |
R19.00010: Control over magnetic properties in bulk hybrid materials Christian Urban, Adrian Quesada, Thomas Saerbeck, Miguel Angel de la Rubia, Miguel Angel Garcia, Jose Francisco Fernandez, Ivan K. Schuller We present control of coercivity and remanent magnetization of a bulk ferromagnetic material embedded in bulk vanadium sesquioxide (V2O3) by using a standard bulk synthesis procedure. The method generalizes the use of structural phase transitions of one material to control structural and magnetic properties of another. A structural phase transition (SPT) in the V2O3 host material causes magnetic properties of Ni to change as function of temperature. The remanent magnetization and the coercivity are reversibly controlled by the SPT without additional external magnetic fields. The reversible tuning shown here opens the pathway for controlling the properties of a vast variety of magnetic hybrid bulk systems. [Preview Abstract] |
Thursday, March 17, 2016 10:00AM - 10:12AM |
R19.00011: A Study of Phase Stability and Properties of TiO2 Polymorphs with Diffusion Monte Carlo Ye Luo, Anouar Benali, Luke Shulenburger, Jaron Krogel, Olle Heinonen, Paul Kent In the past decades, many studies have focused on the fundamental properties of TiO$_2$ due to its important role in effectively converting solar energy such as in photovoltaic batteries and photocatalic water splitting. TiO2 presents many stable and metastable phases of which, Rutile Anatase and Brookite are the most studied. Using density functional theory (DFT), the energy ordering of these phases depends strongly on the scheme describing the electronic correlation, for instance GGA+U and Hybrid functionals, often tied to an empirical parameter for reproducibility with no guarantee of predictability. We present the first analysis of the polymorphic energy ordering and properties of three naturally existing phases Rutile, Anatase and Brookite, by performing the highly accurate ab initio calculation with fixed node diffusion Monte Carlo (DMC) implemented in QMCPACK[1]. [1] QMCPACK, http://www.qmcpack.org [Preview Abstract] |
Thursday, March 17, 2016 10:12AM - 10:24AM |
R19.00012: On the Electronic and Magnetic Properties of the ionic superatomic solid Ni$_{9}$Te$_{6}$(PEt$_{3})_{8}$C$_{60}$ Vikas Chauhan, Sanjubala Sahoo, Shiv Khanna We have carried out first principles electronic structure studies to examine the atomic structure, stability, and electronic and magnetic properties of the recently synthesized Ni$_{9}$Te$_{6}$(PEt$_{3})_{8}$C$_{60}$ ionic material consisting of Ni$_{9}$Te$_{6}$(PEt$_{3})_{8\, }$superatoms and C$_{60}$. It is shown that the PEt$_{3}$ ligands result in an internal coulomb well that lifts the quantum states of the Ni$_{9}$Te$_{6}$ cluster lowering its ionization potential to 3.39 eV thus creating a superatomic alkali motif. The metallic core has a spin magnetic moment of 5.3 \textmu $_{b}$ in agreement with experiment. The clusters are marked by low magnetic anisotropy energy (MAE) of 2.72 meV and a larger intra-exchange coupling exceeding 0.2 eV indicating that the observed paramagnetic behavior around 10K is due to superparamagnetic relaxations. The magnetic motifs separated by C$_{60}$ experience a weak superexchange that stabilizes a ferromagnetic ground state as observed around 2K. The calculated MAE is sensitive to the charged state that could account for the observed change in magnetic transition temperature with size of the ligands or anion. [Preview Abstract] |
Thursday, March 17, 2016 10:24AM - 10:36AM |
R19.00013: Submicron sized R2Fe14B particles fabricated by mechanochemical process OZLEM KOYLU-ALKAN, Jose Manuel Barandiaran, Daniel Salazar, George C. Hadjipanayis In this work, we have synthesized submicron R$_{\mathrm{2}}$Fe$_{\mathrm{14}}$B particles by the mechanochemical process. Mechanical activation of oxides of rare earth, iron and boron was done by high energy ball milling in a CaO with a reduction agent (Ca). After a heat treatment at 900 \textdegree C the powder was washed with water and glycerol solution to remove the dispersant and other non-magnetic phases. Magnetic measurements showed that the as-synthesized unwashed powders had coercivity values of 10.3 kOe, 12.8 kOe, and 24.6 kOe for R$=$Nd, Pr, and Dy, respectively. During washing, H$_{\mathrm{2}}$ is released and absorbed by the 2:14:1 structure. After removing the H$_{\mathrm{2}}$, the submicron particles have coercivities of 3.3 kOe (Nd), 4.4 kOe (Pr) and 21.0 kOe (Dy) with average sizes 160 nm, 242 nm, and 107 nm, respectively. Fitting of high field M(H) measurements to the law of approach to saturation showed that the anisotropy constant of the Nd$_{\mathrm{2}}$Fe$_{\mathrm{14}}$B particles are 3.73x10$^{\mathrm{7\thinspace }}$erg/cm$^{\mathrm{3}}$ which is comparable to bulk. Work supported by DOE DE-FG02-04ERU4612 and Bizkaia Talent AYD-000-195. [Preview Abstract] |
Thursday, March 17, 2016 10:36AM - 10:48AM |
R19.00014: First-principles study of intrinsic magnetic properties of hexagonal and orthorhombic (Fe$_{1-x}$Co$_x$)$_2$P alloys Ivan Zhuravlev, V. P. Antropov, K. D. Belashchenko (Fe$_{1-x}$Co$_x$)$_2$P is a candidate rare-earth-free alloy for permanent-magnet applications, which is hexagonal ($h$) up to $x\approx0.12$ and orthorhombic ($o$) at larger $x$. The Curie temperature $T_C$, which is only 270 K in Fe$_2$P, raises sharply with $x$, peaking above 450 K in the $o$-phase [1]. The measurement [2] of magnetocrystalline anisotropy (MCA) in the $o$-phase is inconsistent with M\"ossbauer data suggesting a spin reorientation transition (SRT) at $x\approx0.3$ [1]. Here we report the results of $ab\ initio$ calculations of the magnetization, mean-field $T_C$, and MCA in $h$- and $o$-phases as a function of $x$, addressing the role of unequal site occupation, which is confirmed by total-energy calculations. The trends in the magnetization are reproduced, as well as MCA in the $h$-phase, and so is the SRT near $x\approx0.3$ (at odds with the results of Ref. 2). The trends in the mean-field $T_C$, obtained using the disordered-local-moment method, agree with experimental data. [1] R. Fruchart \emph{et al.}, J. Appl. Phys. 40, 1250 (1969). [2] T. Hokabe \emph{et al.}, J. Phys. Soc. Japan 36, 1704 (1974). [Preview Abstract] |
Thursday, March 17, 2016 10:48AM - 11:00AM |
R19.00015: Magnetic effects of H in Metals, the case of Iron Patricio Vargas, Andrea León, Juan Manuel FLorez A growing consensus on the possible role of hydrogen in future energy technology has incited worldwide efforts for the development of new hydrogen-storage materials and their application to rechargeable batteries and fuel cells. Meanwhile, research in the basic properties of metal-hydrogen systems has also been advanced. High-pressure experiments have unraveled new features of elemental hydrogen (phases of solid H2 and metallization of liquid H2 and superconductivity) as well as of many metal--hydrogen systems (superabundant vacancy formation, phase diagrams over wide p--x--T ranges) . In this work we address the magnetic changes induced by interstitial hydrogen in Fe. From the point of view of the Slater Pauli Curve, Fe alloys (Fe(1-x)Mx) show an increase of the magnetization (but always less than pure Fe) due substitutional non magnetic impurities like M $=$ V, Cr, Ti. For the magnetic impurity Cobalt, the Slater Pauli Curve reaches its maximum of about 2.5 Bohr magnetons per atom when x$=$0.4. For an interstitial impurity H, which adds one electron to the system, we observe an increasing of the magnetization too but less than the effect induced by the volume expansion. Therefore like the case of NiHx , one of the effects of interstitial hydrogen on a ferromagnetic material is to fill the minority spin states . [Preview Abstract] |
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