Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q30: Focus Session: Magnetic Films and Magnetic Anisotropy |
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Sponsoring Units: GMAG DMP Chair: David Sellmyer, University of Nebraska Room: 206B |
Wednesday, March 4, 2015 2:30PM - 2:42PM |
Q30.00001: Magnetic properties and structures of fibrous R$_{11}$Ni$_{4}$In$_{9}$ intermetallics (R $=$ heavy rare earths) Alessia Provino, Clemens Ritter, Karl A. Gschneidner, Pietro Manfrinetti, Sudesh K. Dhar, Vitalij K. Pecharsky The existence and the unusual self-assembled nano/microfibrous morphology of the R$_{11}$T$_{4}$In$_{9}$ (R $=$ rare earth, T $=$ Ni, Pd, Pt) phases has been recently studied [1,2,3]. All the rare earths (but Sc, Eu, Yb) form this ternary compound (orthorhombic Nd$_{11}$Pd$_{4}$In$_{9}$-type, \textit{oC}48, \textit{Cmmm}). The bundles of fibers grow parallel to the temperature gradient and along the short $c$-axis. In this presentation we describe the results of a detailed investigation of the physical properties (electrical resistivity, heat capacity, magnetization measurements) of Tb$_{11}$Ni$_{4}$In$_{9}$, Dy$_{11}$Ni$_{4}$In$_{9}$ and Ho$_{11}$Ni$_{4}$In$_{9}$ by orienting the fibers parallel and orthogonal, respectively, to the electric current and magnetic field. The unusual fibrous microstructure of these compounds leads to a strong anisotropy in their physical properties, with the $c$-axis of the orthorhombic cell being the easy magnetization and high electrical-conductivity direction. The magnetic structures of Tb$_{11}$Ni$_{4}$In$_{9}$ and Ho$_{11}$Ni$_{4}$In$_{9}$, which have multiple magnetic orderings, have been investigated by neutron diffraction. The complex magnetic behavior found in these phases is a result of the competing ferrimagnetic (along the $c$-axis) and antiferromagnetic (on the $a-b$ plane) orderings of the five R sublattices. [Preview Abstract] |
Wednesday, March 4, 2015 2:42PM - 2:54PM |
Q30.00002: Structural, Magnetic and Electron Transport Properties of Rapidly Quenched CoFeCrAl Nanostructures P. Kharel, R. Fuglsby, S. Gilbert, Y. Huh, W. Zhang, S. Valloppilly, R. Skomski, D.J. Sellmyer Materials with moderate magnetization, high spin polarization at the Fermi level and high Curie temperature well above room temperature have huge potential for spin-based electronic devices. Several Heusler compounds including a quaternary compound CoFeCrAl are predicted to have these interesting materials properties. We have used a rapid quenching technique to prepare single-phase CoFeCrAl nanostructured ribbons in a cubic L21 crystal structure and have investigated the magnetic and electrical properties. As-quenched ribbons are ferrimagnetic at room temperature with a Curie temperature of about 500 K. The saturation magnetization is 1.9 $\mu_{\mathrm{B}}$/f.u, which is very close to the value predicted by the Slater-Pauling Rule. The ribbons are conducting with a room temperature resistivity of about 80 m$\Omega $cm, but the resistivity is almost independent of temperature. The thermal coefficient of resistivity is very small and it is negative. These ribbons show a small positive magnetoresistance (1.5{\%} at 5 K) between 5 K and 300 K. We will also discuss the effect of vacuum annealing on the structural and magnetic properties of this material. [Preview Abstract] |
Wednesday, March 4, 2015 2:54PM - 3:06PM |
Q30.00003: Control of anisotropy and magnetism of MnBi nanomaterials Wenyong Zhang, David Sellmyer High-anisotropy MnBi nanostructures have been fabricated by in-situ annealing of Bi/Mn/Bi multilayers and magnetic-field annealing of melt-spun Mn$_{\mathrm{x}}$Bi$_{\mathrm{100-x}}$ ribbons. The ratio of Mn to Bi affects the concentration of NiAs-type MnBi, the degree of c-axis orientation, and phase distribution. For x $=$ 50, the Mn$_{\mathrm{x}}$Bi$_{\mathrm{100-x}}$ film exhibits the optimum nanostructure in which MnBi grains are uniformly separated by a thin layer of Bi. This has produced a record value of \textit{(BH)}$_{max} = $ 16.3 MGOe for this compound. A good c-axis texture has been developed for Mn$_{50}$Bi$_{50}$ ribbons with a remanence ratio of 0.94 after magnetic-field annealing and this result subsequently leads to \textit{(BH)}$_{max} = $ 9.2 MGOe, the highest value for bulk MnBi materials. The reason for the much higher energy product for the Mn$_{50}$Bi$_{50}$ film compared to the Mn$_{50}$Bi$_{50}$ ribbon is that the ribbon has a comparatively lower coercivity induced by inhomogeneous distribution of intergranular Bi. The effect of element doping, optimization of preparation parameters, and temperature dependence of properties also will be discussed. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q30.00004: Well-isolated FePt grains with high coercivity on TiN underlayers for heat-assisted magnetic recording media Tiffany Santos, Shikha Jain, Akemi Hirotsune, Olav Hellwig MgO is the underlayer material of choice for granular FePt thin film media for heat assisted magnetic recording, because MgO (001) seeds L1$_{\mathrm{0}}$-ordered FePt with c-axis perpendicular to the film plane and high perpendicular magnetic anisotropy. MgO is also an effective diffusion barrier between the FePt grains and the metallic underlayers beneath the MgO. However, there are possible concerns associated with using MgO in the media structure. MgO is highly sensitive to moisture, and hydration of MgO could potentially degrade film properties. In addition, many particulates are incorporated into the film during the RF-sputter process, which can be sources of delamination, pinholes and damage to the low-flying recording heads. TiN is an attractive alternative to MgO because it is chemically and mechanically robust, and TiN can be DC-sputtered, which produces fewer particles and has a faster deposition rate. Even though TiN has the same rocksalt crystal structure and lattice constant as MgO, the higher surface energy of TiN causes more wetting of the FePt grains on the TiN surface. As a result, deposition of granular FePt on TiN most often produces inter-connected, worm-like grains with low coercivity. We will show that by optimizing the deposition of FePt and segregant material on the TiN underlayer, we are able to fabricate FePt media with well-isolated grains and high coercivity reaching nearly 4 Tesla. In addition, the FePt has excellent structural properties with a high degree of L1$_{\mathrm{0}}$ atomic ordering and minimal c-axis in-plane oriented grains. [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q30.00005: Thickness Dependent Magnetoelastic Effects and Perpendicular Magnetic Anisotropy in the Ta/CoFeB/MgO system Gregory Stiehl, Praveen Gowtham, Daniel Ralph, Robert Buhrman We report the observation of strong thickness-dependent in-plane magnetoelastic coupling in Ta/CoFeB(x$=$0.7-2 nm)/MgO multilayers. Measurements are made using a four-point bend test strain fixture, revealing the emergence of large effective surface and volume magnetoelastic couplings after post-deposition annealing. When such surface and volume magnetoelastic interactions are included in the standard Neel model of surface anisotropy, they provide a natural explanation for the nonmonotonic K$_{\mathrm{eff}}$t$_{\mathrm{eff}}$ vs t$_{\mathrm{eff}}$ curves measured for CoFeB films in the thickness range that yields perpendicular magnetic anisotropy (PMA). The large magnitude of the magnetoelastic coupling terms suggest that enhanced control of thin film strains could be used to beneficially manipulate the PMA in CoFeB/MgO magnetic tunnel junctions and other thin film multilayer nanostructures. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q30.00006: Tunable magnetic anisotropy in perpendicular exchange-coupled CoFeB/(Co/Pt) films Long You, OukJae Lee, Terrell Glenn, Haron Abdel-Raziq, Sayeef Salahuddin Spintronic materials with strong perpendicular magnetic anisotropy (PMA), such as Co/Pd, Co/Pt and Co/Ni multilayers, have been introduced to improve the functional performance of STT devices (e.g. enhanced thermal stability, scalability and switching speeds of spin memory/logic). Furthermore, by coupling magnetic layers with PMA and longitudinal magnetic anisotropy (LMA), added benefits such as a variable magnetization tilt angle and tunable damping have been shown. In our study, we discuss how to precise control the anisotropy tilt angle by coupling the PMA hard layer (Co/Pt) with an in-plane soft layer (IMA, CoFeB). Due to the competition between the PMA and IMA, the tilted angle can be tuned by varying thickness of IMA. The stack of Pt(5nm)/Co (1nm)/CoFeB(Xnm)/MgO (2nm) (x varied from 0 to 1nm) was deposited by magneto-sputtering system. The magnetic properties were investigated by vibrating sample magnetometer and anomalous Hall effect . The electric transport of microscale devices comprised of that stack were also studied by our probe station with electromagnet. The experiments show the magnetic anisotropy can be tuned well by changing thickness of in plane layer and open a promising new avenue to next generation spintronics devices. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q30.00007: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q30.00008: Isothermal tuning of magnetic coercivity in NiFe/NiO/[Co/Pt] heterostructures with orthogonal easy axes Andrew Baruth Heterostructures of NiFe/NiO/[Co/Pt] with mutually orthogonal easy axes allow for isothermal tuning of the magnetic coercivity at room temperature with no associated shift in the hysteresis loop along the applied field axis. This is in contrast to what is typically seen in exchange biased heterostructures. The application of moderate dc magnetic fields of \textless 3 kOe enhances the NiFe coercivity from 14.5 to 105 Oe. The application of a similarly sized dc magnetic field perpendicular to the film completely resets this enhancement back to 14.5 Oe. We propose that the in-plane magnetization of both the NiFe and [Co/Pt] adjacent layers greatly influences the pinning of the antiferromagnetic NiO interlayer (with a blocking temperature expected to be well below 50 K at this thickness). In addition, these heterostructures show unique high and low-field training effects due to alignment of [Co/Pt] stripe domains during field cycling. The dynamic, yet predictable, behavior of isothermally tuning the magnetic coercivity without any permanent structural/chemical modifications has potential uses in advanced magnetic logic/storage, as well as tuning the interfacial coupling in spintronic applications. [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q30.00009: Controllable magnetic phase front in a vertically graded Ni$_{x}$Cu$_{1-x}$ alloy film Brian Kirby, H.F. Belliveau, D.D. Belyea, T. Eggers, P.A. Kienzle, A.J. Grutter, P. Riego, A. Berger, C.W. Miller We have used polarized neutron reflectometry to study the temperature and magnetic field dependent magnetization depth profile of a ferromagnetic 100 nm Ni$_{x}$Cu$_{1-x}$ alloy film with $x$ that varies linearly from 0.61 - 0.70 along the growth axis. Modeling the data in terms of a mean-field exchange strength gradient theory, we find that with increasing temperature, the magnetized thickness of the film continuously decreases, indicating a continuous vertical distribution of effective ferromagnetic transition temperatures. For temperatures corresponding to a partially magnetized film, increasing the applied field from 5 mT to 500 mT is observed to significantly alter the shape of the profile, consistent with magnetization of an effectively paramagnetic region. Thus, we demonstrate that this system exhibits a vertical magnetic phase boundary that can be moved continuously along the growth axis with temperature. Such temperature and field control of the magnetized phase boundary could have important implications for our understanding of metamagnetic transitions, as well as for magnetocaloric and thermomagnetic device applications. [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q30.00010: Strain effect on magnetocrystalline anisotropy of 5$d$ TM/Fe and TM/Fe/MgO(001) (TM$=$Ta, Ir, Pt, Au): A first principles study Purev Taivansaikhan, Dorj Odkhuu, Sung-Hyon Rhim, Soon Cheol Hong Strain effect on magnetization and magnetocrystalline anisotropy (MCA) of TM/Fe(001) [TM$=$Ta, Ir, Pt and Au] with and without MgO(001) substrate has been investigated using first-principles calculations. It is found that perpendicular MCA of Pt/Fe(001) changes to in-plane MCA in the presence of MgO substrate, where lattice is extended by 3.8{\%} with respect to that without MgO. For Ta/Fe(001) and Au/Fe(001), PMCA is significantly enhanced by the MgO substrate, whereas MCA of Ir/Fe(001) remains in-plane. Furthermore, thickness dependence of MCA on both the TM and Fe layers will be also discussed. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q30.00011: Multilayer FeRh/MgO: controllable magnetocrystalline anisotropy for an antiferromagnetic system Guohui Zheng, Odkhuu Dorj, Sanhuang Ke, Rammoorthy Ramesh, Maosheng Miao, Nickolas Kioussis Controlling the magnetocrystalline anisotropy (MCA) of ferromagnetic (FM) thin films by tunable strain and electric field has been pursued as an effective method of achieving low-power and highly scalable memory. Comparing with FM materials, AFM are much less sensitive to external magnetic field, a substantial advantage for memory devices. Inspired by recent work on AFM memory resistors based on FeRh, we carried out a systematic first principles study of the MCA of multi-layer FeRh, either stand alone, or combined with MgO layers. FeRh is a unique material that undergoes a transition from AFM (type-II) to FM at elevated temperature of 370 K. Our calculations for thin films of FeRh from 5-15 atomic layers reveal that AFM is always the most stable configuration for Fe terminated films; while for Rh terminated films, there is a transition from FM to a configuration featured AFM at the center layers and FM at the surface layers (reconstructed). While applying the spin-orbit interactions (SOI) for the valence electrons, we found Fe-terminated films exhibit a relatively small MCA that varies and may change sign with film thickness, substrate and strain, providing a possibility of spin reorientation via the control of strain and electric field. The k-resolved MCA values reveals that the region around Gamma point adds the major contribution to the MCA. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q30.00012: Thickness and surface-termination effects on magnetocrystalline anisotropy of FeM (M$=$Rh, Cr, and Pt) (001) thin films Soyoung Jekal, Sung-Hyon Rhim, Soon-Cheol Hong FeM (M$=$Rh, Cr, and Pt) alloys show diverse interesting physical properties,\footnote{X. Mart et al., Nature Mater. \textbf{13}, 367 (2014)}$^,$\footnote{S. L. Qiu, P. M. Marcus, and V. L. Moruzzi, J. Appl. Phys. \textbf{85}, 4839 (1999).}$^,$\footnote{G. Brown, B. Kraczek, A. Janotti, T. C. Schulthess, G. M. Stocks, and D. D Johnson, Phys. Rev. B \textbf{68}, 052405 (2003).} which attracted these alloys as a promising candidate for spintronics application.\footnote{X. Mart, et al.} In this study, we investigate effects of thickness and surface-termination on magnetism and magnetocrystalline anisotropies (MCAs) on the CsCl-structured FeM(001) thin films, using Vienna Ab initio Simulation Package. Two surface terminations (Fe-terminated and M-terminated) and thicknesses from 3 to 15 monolayers are taken into account. We find that magnetism and MCAs are sensitive to the surface termination and the thickness, whose relevance to electronic structures will be discussed. [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q30.00013: ROTMOKE study of step-induced magnetic anisotropy in vicinal Cu/Py/Ni/Cu(001) J.X. Deng, Song Ma, A. Tan, J. Li, Z.D. Zhang, C. Hwang, Z.Q. Qiu Py/Ni films were epitaxially grown on a 6$_{\mathrm{o}}$ vicinal Cu(001) substrate with steps parallel to the [110] axis. The addition of Py film increases the Ni film spin reorientation transition (SRT) thickness to permit a study of the step-induced in-plane magnetic anisotropy in a wider Ni thickness range. Rotation MOKE (ROTMOKE) was applied to determine the step-induced magnetic anisotropy in the vicinal Cu/Py/Ni/Cu(001) as a function of both the Py and Ni film thicknesses. We found that the atomic steps from the vicinal Cu(001) induce an in-plane uniaxial magnetic anisotropy that favors both Py and Ni magnetizations perpendicular to the steps. In addition, thickness-dependent ROTMOKE measurement allows a separation of the Py and Ni volume-type step-induced magnetic anisotropies. We show that Ni films exhibit different step-induced magnetic anisotropies below and above $\sim$ 5-6ML Ni thickness. [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q30.00014: Atomistic modeling of L1$_0$ FePt: path to HAMR 5Tb/in2 Tianran Chen, Mourad Benakli, Chris Rea Heat assisted magnetic recording (HAMR) is a promising approach for increasing the storage density of hard disk drives. To increase data density, information must be written in small grains, which requires materials with high anisotropy energy such as L1$_0$ FePt. On the other hand, high anisotropy implies high coercivity, making it difficult to write the data with existing recording heads. This issue can be overcome by the technique of HAMR, where a laser is used to heat the recording medium to reduce its coercivity while retaining good thermal stability at room temperature due to the large anisotropy energy. One of the keys to the success of HAMR is the precise control of writing process. In this talk, I will propose a Monte Carlo simulation, based on an atomistic model, that would allow us to study the magnetic properties of L1$_0$ FePt and dynamics of spin reversal for the writing process in HAMR. [Preview Abstract] |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q30.00015: Ferromagnetic thickness dependence of exchange bias: breaking of the inverse proportionality law Rafael Morales, Ali C. Basaran, J.E. Villegas, D. Navas, N. Soriano, B. Mora, C. Redondo, X. Batlle, Ivan K. Schuller The exchange coupling between antiferromagnetic/ferromagnetic (AF/FM) materials shifts the hysteresis loop along the field axis by an amount known as exchange bias field. It is believed that the ferromagnetic thickness dependence of the exchange bias field follows an inverse proportionality law. This has experimentally and theoretically been confirmed for FM thicknesses below the FM domain wall width. In this work we demonstrate that this exchange bias dependence is broken for certain FM spin structures, even though in FM layers thinner than the FM domain wall width. We present experimental data of FeF$_{2}$/FeNi bilayers that deviate from the inverse proportionality law, as well as a theoretical calculation that accounts for the results. [Preview Abstract] |
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