Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H6: Perpendicular Anisotropy Multi-layers and Hard MagnetsFocus
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Sponsoring Units: GMAG DMP Chair: Mazin Almaqablah, Western Digital Corporation Room: 302 |
Tuesday, March 15, 2016 2:30PM - 2:42PM |
H6.00001: Electric-field-induced modification in Dzyaloshinskii-Moriya interaction of Co monolayer on Pt(111) Kohji Nakamura, Toru Akiyama, Tomonori Ito, Teruo Ono, Michael Weinert Magnetism induced by an external electric field ($E$-field) has received much attention as a potential approach for controlling magnetism at the nano-scale with the promise of ultra-low energy power consumption. Here, the $E$-field-induced modification of the Dzyaloshinskii-Moriya interaction (DMI) for a prototypical transition-metal thin layer of a Co monolayer on Pt(111) is investigated by first-principles calculations by using the full-potential linearized augmented plane wave method that treats spin-spiral structures in an $E$-field. With inclusion of the spin-orbit coupling (SOC) by the second variational method for commensurate spin-spiral structures, the DMI constants were estimated from an asymmetric contribution in the total energy with respect to the spin-spiral wavevector. The results predicted that the DMI is modified by the $E$-field, but the change is found to be small compared to that in the exchange interaction (a symmetric contribution in the total energy) by a factor of ten. [Preview Abstract] |
(Author Not Attending)
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H6.00002: Random Field effects in perpendicular-anisotropy multilayer films Jian Xu, Daniel Silevitch, Thomas Rosenbaum With the application of a magnetic field transverse to the magnetic easy axis, randomly-distributed 3D collections of dipole-coupled Ising spins form a realization of the Random-Field Ising Model. Tuning the strength of the site-specific random field, and hence the disorder, via the applied transverse field regulates the domain reversal energetics and hence the macroscopic hysteresis loop. We extend this approach to two dimensions, using sputtered Perpendicular Magnetic Anisotropy (PMA) Co/Pt multilayer thin films. We characterize the coercive fields and hysteresis loops at a series of temperatures and transverse fields. [Preview Abstract] |
Tuesday, March 15, 2016 2:54PM - 3:06PM |
H6.00003: Polarization dependent soft x-ray spectro-microscopy of local spin structures MacCallum Robertson, Christopher Agostino, Mi-Young Im, Sergio Montoya, Eric Fullerton, Peter Fischer Quantitative information about element-specific contributions to local magnetic spin and orbital moments is readily available by XMCD spectroscopy and images of magnetic domain patterns with a few tens of nanometer spatial resolution. We show that the x-ray spectroscopic analysis of x-ray microscopy images provides quantitative information about local spin structures. We have investigated two prototypical multilayered PMA film systems prepared by sputtering, specifically (Co 0.3 nm/Pt 0.5 nm)x30 and (Fe 0.7nm/Gd 0.4nm)x100 systems. A spectroscopic sequence of full-field magnetic transmission soft x-ray microscopy (MTXM) images covering about 8mm field-of-views with a spatial resolution of about 20nm were recorded across the Co and Fe L edges, resp. To modulate the magnetic contrast, two sets of images were obtained with left and right circular polarization. Standard XMCD spectroscopy analysis procedures were applied to retrieve the local spectroscopic behavior. We observe a decrease of the L3/L2 ratio when approaching the domain walls, indicating a non-uniform spin configuration along the vertical profile of a domain, which we will discuss in view of both systems' magnetic anisotropies. [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:18PM |
H6.00004: \textbf{Nanoparticulate CoPt Thin Films} Yasaman Barekatain, George Hadjipanayis Equiatomic FePt and CoPt alloys are very attractive for application in high density recording media because of the high magnetocrystalline anisotropy K of their \textbf{fct }(L1$_{\mathrm{0\thinspace }})$ structure with values exceeding 2MJ/m$^{\mathrm{3}}$.The aim of this study is to fabricate a nanoparticulate CoPt film consisting of CoPt nanoparticles embedded in a matrix. To obtain this we have used co-sputtering of CoPt with different materials M$=$ BN,C, Cu and SiO$_{\mathrm{2}}$. Our first experiments were done on CoPt films with thickness of 200 nm. The as-sputtered films had the \textbf{fcc} structure and a coercivity of 150 Oe. Annealing at 700 $^{\mathrm{o}}$C for 30 min led to an increase in coercivity to 4 kOe. Optimization studies are under way to find the optimum sputtering conditions to obtain a fully ordered tetragonal structure with the highest value of coercivity which can then be used in the nanoparticulate composites. Work supported by DOE BES- FG02-04ERU4612 [Preview Abstract] |
Tuesday, March 15, 2016 3:18PM - 3:30PM |
H6.00005: Microscopic evidence of strain-mediated magnetoelectric coupling in Co/Pt multilayers/PMN-PT(011) heterostructures Ying Sun, Wenbo Wang, Weida Wu, Xiaoli Zheng, Jianwang Cai, Yonggang Zhao, Ming Liu A promising way to control magnetization(M) via an electric field(E-field) is using magnetoelectric(ME) effect in FM/FE heterostructures. We use magnetic(electric) force microscopy(M(e)FM) to study the strain-mediated E-field modulation of M in (Co/Pt)$_{n}$ with perpendicular magnetic anisotropy(PMA) or in-plane anisotropy on PMN-PT(011) substrates. MFM were performed on (Co/Pt)$_{n}$ with an DC E-field applied to PMN-PT. In MeFM, we superimpose an AC modulation on a DC one and utilize lock-in technique to detect weak ME effect. For (Co/Pt)$_{n}$ with PMA, MFM images show stripe domains with no obvious changes at varied DC E-fields. However, MeFM shows interesting structures and the image contrast reverses sign at opposite strain slopes of the PMN-PT substrate. For sample with in-plane anisotropy, both MFM and MeFM images show dipole-like domains. Interestingly, the MeFM image contrast reverses sign at opposite strain slopes of the substrate. The sign reversal of MeFM contrast indicates that features revealed by MeFM are intrinsic local ME effect. Our MeFM data are consistent with the ferromagnetic resonance results showing that strain-induced anisotropy change will cause part of M switching to the in-plane direction. Possible scenarios will be discussed. [Preview Abstract] |
Tuesday, March 15, 2016 3:30PM - 3:42PM |
H6.00006: Effect of perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction on the enhancement of domain wall creep velocity in Pt/Co thin films by piezoelectric strain Philippa M. Shepley, Gavin Burnell, Thomas A. Moore We investigate piezoelectric strain control of domain wall creep motion in perpendicularly magnetized Pt/Co thin films. Domain wall (DW) motion has potential applications in data storage and spintronics, where the use of voltages rather than magnetic fields to control magnetization reversal could reduce power consumption. Materials with perpendicular magnetic anisotropy (PMA) are of particular interest due to their narrow domain walls and potential for efficient current-induced DW motion. Sputtered Ta/Pt/Co(t)/X films (t=0.78-1.0nm, X= Pt, Ir/Pt or Ir) on thin glass substrates were bonded to biaxial piezoelectric transducers, to which 150V was applied to produce a tensile out-of-plane strain of 9x10$^{-4}$. This reduced the PMA by 10kJ/m$^{3}$ and increased the DW creep velocity by up to 90\%. DW energy can be calculated from the PMA and the Dzyaloshinskii-Moriya interaction (DMI) field. DW creep measurements of DMI field found no change with strain. The change in DW velocity with strain is linear with the change in DW energy for Pt/Co DWs with a mixed Bloch-N\’{e}el structure. Pt/Co/Pt films with higher DW velocity changes were found to have purely Bloch DWs. We conclude that the velocity of Bloch DWs is more sensitive to strain-induced changes than that of Bloch-N\’{e}el DWs. [Preview Abstract] |
Tuesday, March 15, 2016 3:42PM - 4:18PM |
H6.00007: Thermal Stability of Magnetic States in Circular Thin-Film Nanomagnets with Large Perpendicular Magnetic Anisotropy Invited Speaker: Gabriel Chaves-O'Flynn The scaling of the energy barrier to magnetization reversal in thin-film nanomagnets with perpendicular magnetization as a function of their lateral size is of great interest and importance for high-density magnetic random access memory devices. Experimental studies of such elements show either a quadratic or linear dependence of the energy barrier \footnote{ J. Z. Sun \textit{et al.}, Phys. Rev. B \textbf{8}, 104426 (2013). } \footnote{ H. Sato \textit {et al.}, Appl. Phys. Lett. {\textbf 105}, 062403 (2014) } on element diameter. I will discuss a theoretical model we developed to determine the micromagnetic configurations that set the energy barrier for thermally activated reversal of a thin disk with perpendicular magnetic anisotropy as a function of disk diameter \footnote{ G. D. Chaves-O'Flynn, G. Wolf, J. Z. Sun and A. D. Kent, Phys. Rev. Applied {\textbf 4}, 024010 (2015). }. We find a critical length in the problem that is set by the exchange and effective perpendicular magnetic anisotropy energies, with the latter including the size dependence of the demagnetization energy. For diameters smaller than this critical length, the reversal occurs by nearly coherent magnetization rotation and the energy barrier scales with the square of the diameter normalized to the critical length (for fixed film thickness), while for larger diameters, the transition state has a domain wall, and the energy barrier depends linearly on the normalized diameter. Simple analytic expressions are derived for these two limiting cases and verified using full micromagnetic simulations with the string method. Further, the effect of an applied field is considered and shown to lead to a plateau in the energy barrier versus diameter dependence at large diameters. Based on these finding I discuss the prospects and material challenges in the scaling of magnetic memory devices based on thin films with strong perpendicular magnetic anisotropy. [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:30PM |
H6.00008: Large electric-field control of perpendicular magnetic anisotropy in strained [Co/Ni] / PZT heterostructures Daniel Gopman, Cindi Dennis, P. J. Chen, Yury Iunin, Robert Shull We present a piezoelectric/ferromagnetic heterostructure with PMA - a Co/Ni multilayer sputtered directly onto a Pb(Zr,Ti)O$_{\mathrm{3}}$ (PZT) substrate. Chemical-mechanical polishing was used to reduce the roughness of PZT plates to below 2 nm \textit{rms}, enabling optimal magnetoelectric coupling via the direct interface between PZT and sputtered Co/Ni films with large PMA ($K_{\mathrm{eff}}$~$=$ (95 \textpm 9 kJ/m$^{\mathrm{3}}))$. We grew the following layer stack: Ta(3)/Pt(2)/[Co(0.15)/Ni(0.6)]$_{\mathrm{x4}}$/Co(0.15)/Pt(2)/Ta(3); numbers in parentheses indicate thicknesses in nm. Applied electric fields up to $+$/- 2 MV/m to the PZT generated 0.05{\%} in-plane compression in the Co/Ni multilayer, enabling a large electric-field reduction of the PMA ($\Delta K_{\mathrm{eff}} \quad \ge $10$^{\mathrm{3}}$ J/m$^{\mathrm{3}})$ and of the coercive field (35{\%}). Our results demonstrate that: (i) heterostructures combining PZT and [Co/Ni] exhibit larger PMA ($K_{\mathrm{eff}}$ \textasciitilde 10$^{\mathrm{5}}$ J/m$^{\mathrm{3}})$ than previous magnetoelectric heterostructures based on Co/Pt and CoFeB, enabling thermally stable hybrid magnetoelectric/spintronic devices only tens of nm in diameter and (ii) electric-field control of the PMA is promising for more energy efficient switching of spintronic devices. [Preview Abstract] |
Tuesday, March 15, 2016 4:30PM - 4:42PM |
H6.00009: Properties of easy-plane/ perpendicular magnetic anisotropy bilayers with varied interlayer exchange coupling Lorenzo Fallarino, Volker Sluka, Bartek Kardasz, Mustafa Pinarbasi, Andrew D. Kent We explore the possibility of an easy-cone ground state in coupled easy plane/easy axis magnetic bilayers. The samples consist of a Co/Ni multilayer with perpendicular magnetic anisotropy and a CoFe layer with easy-plane anisotropy separated by a variable thickness Ru layer. Using ferromagnetic resonance spectroscopy, we characterize the magnetic behavior of the coupled thin films for different Ru thicknesses by determining the resonance fields for both the acoustic and optical FMR modes. In particular, we observe a gap in the resonance field opening up between the two modes in angular-dependent FMR, which is direct evidence for the presence of interlayer coupling. Quantitative comparisons with a theoretical model indicate that by varying the Ru thickness the coupling strength can be tuned continuously from ferromagnetic to the anti-ferromagnetic. These results are consistent with a canted magnetic ground state in zero field, a state of interest for applications in spin-torque devices, such as current tunable spin-torque oscillators. [Preview Abstract] |
Tuesday, March 15, 2016 4:42PM - 4:54PM |
H6.00010: Giant magnetic anisotropy of Co, Ru, and Os adatoms on MgO (001) surface. Hongbo Wang, Xuedong Ou, Fengren Fan, Zhengwei Li, Hua Wu Large magnetic anisotropy energy (MAE) is desirable and critical for nanoscale magnetic devices. Here, using ligand-field level diagrams and density functional calculations, we well explain the very recent discovery [I. G. Rau et al., Science 344, 988 (2014)] that individual Co adatom on MgO (001) surface has a large MAE of more than 60 meV. More importantly, we predict that a giant MAE up to 110 meV could be realized for Ru adatoms on MgO (001), and even more for the Os adatoms (208 meV). This is a joint effect of the special ligand field, orbital multiplet, and significant spin-orbit interaction, in the intermediate-spin state of the Ru or Os adatoms on top of the surface oxygens. The giant MAE could provide a route to atomic scale memory. [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:06PM |
H6.00011: ABSTRACT WITHDRAWN |
Tuesday, March 15, 2016 5:06PM - 5:18PM |
H6.00012: Perpendicular Magnetic Anisotropy of Tb/Fe and Gd/Fe Multilayers Studied with Torque Magnetometer Ataur Chowdhury Perpendicular magnetic anisotropy (PMA) of multilayers critically depend on the magnetic and structural ordering of the interface. To study the effect of interface on PMA, Tb/Fe and Gd/Fe multilayers with varying Fe (0.8-9.0 nm) and Gd (0.5-2.8 nm) or Tb (0.3-6.3 nm) layer thicknesses were fabricated by planar magnetron sputtering. The magnetometer results of spin orientation clearly reveals that samples with Gd or Tb layer thickness of more than 1.2 nm display no PMA, regardless of the Fe layer thickness. Tb/Fe and Gd/Fe multilayers with thin (\textless 1.2 nm) Tb or Gd layers display large PMA, but no PMA is observed when the Fe layer thickness is increased to 4.0 nm and higher. The bulk magnetization and anisotropy energy constant of the samples are found to increase with increasing Fe layer thickness. Torque measurement also reveals that there are two distinctly different axes of spin alignment at different energy. Tb/Fe and Gd/Fe multilayers with similar composition reveal similar magnetic and structural characteristics, and it may imply that single-ion-anisotropy of rare-earth element, which is quite large for Tb ions and very small for Gd ions, may not be the dominating cause of PMA in Td/Fe and Gd/Fe multilayers. A detailed explanation of the results will be provided based on exchange interaction at the interface. [Preview Abstract] |
Tuesday, March 15, 2016 5:18PM - 5:30PM |
H6.00013: Stripe glasses in ferromagnetic thin films Alessandro Principi, Mikhail Katsnelson Domain walls in magnetic multilayered systems can exhibit a very complex and fascinating behavior. The magnetization of thin films of hard magnetic materials is in general perpendicular to the thin-film plane, but its direction changes periodically, forming an alternating spin-up and spin-down stripe pattern. The latter is stabilized by the competition between the ferromagnetic coupling and dipole-dipole interactions, and disappears when a moderate in-plane magnetic field is applied. It has been suggested that such a behavior may be understood in terms of a self-induced stripe glassiness. In this paper we show that such a scenario is compatible with the experimental findings. The strong out-of-plane magnetic anisotropy of the film is found to be beneficial for the formation of both the stripe-ordered and glassy phases. At zero magnetic field the system can form a glass only in a narrow interval of fairly large temperatures. An in-plane magnetic field, however, shifts the glass transition towards lower temperatures, therefore enabling it at or below room temperature. In good qualitative agreement with the experimental findings, we show that a moderate in-plane magnetic field of the order of $30~{\rm mT}$ can lead to the formation of defects in the stripe pattern. [Preview Abstract] |
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