Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D7: Focus Session: Magnetic Anisotropy |
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Sponsoring Units: GMAG DMP Chair: Weigang Wang, University of Arizona Room: 106 |
Monday, March 3, 2014 2:30PM - 2:42PM |
D7.00001: Magnetic anisotropy properties of CoFeB/W bilayers and W/CoFeB/MgO trilayers Yongxi Ou, Yun Li, L.H. ViLeLa Leao, Chi-Feng Pai, D.C. Ralph, R.A. Buhrman The development of highly scaled spin torque MRAM cells requires that the ferromagnetic free layer in a magnetic tunnel junction, generally a CoFeB layer in combination with an MgO tunnel barrier, have strong perpendicular magnetic anisotropy (PMA) in as thick a free layer as possible. Currently this PMA is believed to arise from interfacial anisotropy energy at the CoFe/MgO interface due to Fe-O bonds. We have found that strong PMA can also be achieved in W/CoFeB/MgO trilayers under certain growth and annealing conditions. Control experiments with amorphous-substrate/CoFeB/W bilayer structures indicate that PMA can be induced by the interface between CoFeB and W and that the effective interfacial anisotropy energy density is quite large in comparison to that found with the CoFeB/MgO PMA system. We have used spin torque ferromagnetic resonance to study the angle dependent anisotropy in these structures and find that there is a quite strong second order component competing with the first order term. We have used second-harmonic anomalous Hall voltage measurements to determine the strength of the spin-orbit torques in these W based systems. We will report on these measurements as well as on the spin-Hall-effect induced switching in our samples. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D7.00002: Magneto-Ionic Control of Interfacial Magnetic Anisotorpy Uwe Bauer, Satoru Emori, Geoffrey Beach Voltage control of magnetism could bring about revolutionary new spintronic memory and logic devices. Here, we examine domain wall (DW) dynamics in ultrathin Co films and nanowires under the influence of a voltage applied across a gadolinium oxide gate dielectric that simultaneously acts as an oxygen ion conductor. We investigate two electrode configurations, one with a continuous gate dielectric and the other with a patterned gate dielectric which exhibits an open oxide edge right underneath the electrode perimeter. We demonstrate that the open oxide edge acts as a fast diffusion path for oxygen ions and allows voltage-induced switching of magnetic anisotropy at the nanoscale by modulating interfacial chemistry rather than charge density. At room temperature this effect is limited to the vicinity of the open oxide edge, but at a temperature of 100$^{\circ}$C it allows complete control over magnetic anisotropy across the whole electrode area, due to higher oxygen ion mobility at elevated temperature. We then harness this novel ``magneto-ionic'' effect to create unprecedentedly strong voltage-induced anisotropy modifications of 3000 fJ/Vm and create electrically programmable DW traps with pinning strengths of 650~Oe, enough to bring to a standstill DWs travelling at speeds of at least 20~m/s. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D7.00003: Origins of voltage effect on interfacial magnetic anisotropy at nanoscale: ab-initio simulations Roman Chepulskyy, Dmytro Apalkov We estimate the effect of electric field on Fe|MgO interfacial perpendicular magnetic anisotropy (PMA) from first principles with and without oxygen at interface. Segregation profile of oxygen is constructed. The possible origins of effect are analyzed by comparison of simulations with published experimental data. Previously it was often assumed that voltage controlled anisotropy (VCA) primarily originates from the modifications of electron density of states. We conclude that such mechanism as well as lattice distortions and undiffusive oxygen atoms at interface cannot explain the experimentally observed effects. The oxygen ion electromigration is suggested as a primary possible mechanism of large PMA change in electric field leading to asymmetrical and time dependent effect. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D7.00004: Perpendicular Magnetic Anisotropy Driven by Antiferromagnetic Layers Invited Speaker: Minn-Tsong Lin We show a novel effect of an antiferromagnet (AFM) with the interfacial unpinned moments, which can form an intrinsic perpendicular anisotropy and switch the magnetization of adjacent ferromagnetic layer from in-plane into perpendicular orientation, providing a new feature of AFM. AFM has been known that its interfacial moments can lead to crucial effects of induced coercivity enhancement and exchange bias field on adjacent ferromagnet (FM), both of which are important for the design of state-of-the-art magnetic logic devices. In the study on the system of Fe/Mn bilayers, the unpinned moments of the Mn can form an intrinsic perpendicular anisotropy that drives the magnetization of an adjacent Fe layer from the in-plane into out-of-plane direction [1]. In the systematic measurements with variations of temperature and FM and AFM thickness, a phenomenological analysis shows that the perpendicular anisotropy is correlated to AFM/FM exchange coupling, and can be modulated according to the finite size effect of AFM ordering [2]. Our x-ray magnetic circular dichroism (XMCD) experiment [1] indicates further that the magnitude of perpendicular anisotropy of the system is enhanced proportionally to the out-of-plane oriented orbital moment of the unpinned Mn layer, rather than that from the Fe layer, providing evidence for the unpinned Mn moments as the origin of the established perpendicular magnetization. The result presented here shows functional characteristics other than the well-investigated phenomena of coercivity enhancement and exchange bias, and renews our knowledge on the role of the AFM layer [1-3], providing a new angle for the design of future perpendicular spintronic nanodevices. \\[4pt] [1] B. Y. Wang, J. Y. Hong, K. H. Ou Yang, Y. L. Chan, D. H. Wei, H. J. Lin, and Minn-Tsong Lin, Phys. Rev. Lett. 110, 117203 (2013).\\[0pt] [2] B. Y. Wang, N. Y. Jih, W. C. Lin, C. H. Chuang, P. J. Hsu, C. W. Peng, Y. C. Yeh, Y. L. Chan, D. H. Wei, W. C. Chiang, and Minn-Tsong Lin, Phys. Rev. B 83, 104417 (2011). \\[0pt] [3] B. Y. Wang, C. C. Chiu, W. C. Lin, and Minn-Tsong Lin, Appl. Phys. Lett. 103, 042407 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D7.00005: FMR Linewidth divergence in V$_{2}$O$_{3}$/Ni bilayers Jose de la Venta, Juan Gabriel Ramirez, Thomas Saerbeck, Siming Wang, Ivan K. Schuller The effects of stress on the magnetic properties of ferromagnetic thin films are dramatic when the ferromagnets are in proximity with materials undergoing structural phase transitions (SPT) [1]. Here we report on Ferromagnetic Resonance (FMR) measurements on V$_{2}$O$_{3}$/Ni bilayers across the SPT of V$_{2}$O$_{3}$. The SPT occurs on V$_{2}$O$_{3}$ at 160 K from a metallic/rhombohedral to an insulating/monoclinic phase. Our results reveal a rotation of the anisotropy axis in Nickel films when cooled below the SPT of V$_{2}$O$_{3}$. The obtained anisotropy axis will be compared to the underlying structural morphology obtained from x-ray diffraction. More interestingly, the FMR linewidth as a function of the temperature shows a divergence across the SPT. This suggests a breakdown of the uniform precession of the Ni magnetization caused by the induced strain across the SPT. Discussion among linewidth-broadening mechanisms will be addressed. \\[4pt] [1] J. de la Venta, S. Wang, J. G. Ramirez, and I. K. Schuller, Appl. Phys. Lett. 102, 122404 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D7.00006: Ferroelectric Influence of Magnetic Anisotropy in Organic Ferroelectric/Co Heterostructures Keith Foreman, Celeste Labedz, Valeria Lauter, Artur Glavic, Haile Ambaye, Stephen Ducharme, Shireen Adenwalla Magnetoelectric coupling between ferroelectric (FE) and ferromagnetic (FM) thin films can influence the magnetic anisotropy of the FM film. This coupling is difficult to measure, requiring careful selection of the FE material. The organic FE poly(vinylidene fluoride-tri fluoroethylene), P(VDF-TrFE), provides high electric fields, compensating for the short penetration depth in the metallic FM layer, and is soft enough to minimize strain coupling. Here, we report on recent Polarized Neutron Reflectometry (PNR) experiments on a P(VDF-TrFE)/Co heterostructure indicating subtle changes in the magnetic anisotropy of the Co as a function of the FE polarization. To improve the magnetoelectric coupling in FE/FM heterostructures, further refinement of the layer interface is required. Typical deposition methods of P(VDF-TrFE) thin films expose the sample to atmosphere resulting in an ill-defined interface between FE/FM layers. Therefore, we have designed and constructed a thermal evaporation system capable of depositing thin films of the ferroelectric oligomer vinylidene difluoride (VDF), allowing us to create entire FE/FM heterostructures \textit{in situ}. We also report on recent magnetic measurements on these clean interface heterostructures. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D7.00007: Magnetocrystalline anisotropy of L10 FePt nanoparticles Alamgir Kabir, Jun Hu, Volodymyr Turkowski, Ruqian Wu, Talat S. Rahman We perform theoretical investigation of Magneto Crystalline Anisotropy (MCA) of L10 FePt nanoparticles. Structural relaxation and magnetic moment of the clusters are evaluated using spin polarized \textit{ab initio} density functional theory, and the MAE is calculated by using two approaches: (i) self-consistent inclusion of spin-orbit coupling and (ii) the torque method.[1] The clusters studied have 3(4) planes of Fe and 2(3) plane of Pt atoms and vice versa. We find an enhancement of MCA for the FePt clusters as compared to that of pure Fe nanoparticles and of bulk L10 FePt. We trace this enhancement to the increased spin and orbital moment of Pt atoms which raises the spin-orbit coupling. We also find that nanoparticles with Pt atoms in the central layer have larger MCA than the corresponding ones whose central layer is Fe. This is due to the fact that when Pt atom is the central layer it has more Fe atoms around so it more strongly hybridized resulting in higher orbital moments then Pt atoms on other layers. Detailed investigation of electronic structure of atoms on the clusters is also performed. Our finding can give useful insight to experimentalist for their studies of high density magnetic recording media. 1. X. D. Wang et al. Phys. Rev. B 54, 61(1996) [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D7.00008: Interatomic exchange in Mn-based alloys Priyanka Manchanda, Ralph Skomski, Arti Kashyap, David Sellmyer The ongoing quest for new rare-earth free permanent magnets includes the search for new magnetic phases with high magnetization and magnetic anisotropy. Manganese alloys could be used because Mn$^{2+}$ ion has a moment of 5 $\mu_{\mathrm{B}}$ per atom. However, manganese is in the middle of the 3$d$ transition-metal series, and it is well-known and easily explained in terms of general electronic structure trends that such elements prefer to form antiferromagnetic (AFM) rather than ferromagnetic (FM) spin structures. Most of the Mn compounds are antiferromagnetic and the few existing ferromagnetic compounds, such as MnAl and MnBi, have low magnetization, of the order of 1 $\mu_{\mathrm{B}}$ per atom. In this presentation, we used first-principle calculations to study interatomic exchange in Mn based alloys. As a model system, we use $L$1$_{\mathrm{0}}$-ordered MnAl. Interestingly, we find a strong ferromagnetic interatomic exchange in the Mn planes of the alloy, in spite of the short Mn-Mn interatomic distances. Furthermore, we study modifications of the $L$1$_{\mathrm{0}}$ structure, such as the effect of Fe substitution on the exchange interactions in MnAl derivatives. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D7.00009: Constituents of magnetic anisotropy and a screening of spin-orbit coupling Liqin Ke, Aleksander Wysocki, Mark van Schilfgaarde, Vladimir Antropov Using perturbation theory (PT) we analyze how the different orders of perturbation affect the energy in solids. We test the validity of PT analysis by considering spin-orbit coupling (SOC) as a perturbation. We show how the atomic SOC is screened in different magnets and how it affects the magnetic anisotropy. The dependence of magnetic anisotropy on the ratio between the strengths of SOC and crystal field is studied using an impurity model. We carried out density functional calculations for FePt, CoPt, FePd, MnAl, MnGa, FeNi, and tetragonally strained FeCo. The relativistic energy and magnetic anisotropy in those compounds from the perturbation approach and self-consistent relativistic calculations had been compared. In addition using decomposition of anisotropy into contributions from individual sites and different spin components we explain the microscopic origin of high anisotropy in most popular magnets. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D7.00010: Electronic structure and magnetic anisotropy of Sm$_2$Fe$_{17}$N$_x$ Hisazumi Akai, Masako Ogura Electronic structure and magnetic properties of Sm$_2$Fe$_{17}$N$_x$ are studies on the basis of the first-principles electronic structure calculation in the framework of the density functional theory within the local density and coherent potential approximations. The magnetic anisotropy of the system as a function of nitrogen concentration $x$ is discussed by taking account not only of the crystal field effects but also of the effects of the f-electron transfer from Sm to the neighboring sites. Also discussed is the magnetic transition temperature that is estimated by mapping the system into a Heisenberg model. The results show the crystalline magnetic anisotropy changes its direction from in-plane to uniaxial ones as x increases. It takes the maximum value near $x\sim 2.8$ and then decreases slightly towards $x=3$. The mechanism for these behaviors is discussed in the light of the results of detailed calculations on the bonding properties between Sm and its neighboring N. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D7.00011: Monte Carlo Simulation of GdFeCo Amorphous Films with Perpendicular Magnetic Anisotropy Xiaopu Li, Chung T. Ma, S. Joseph Poon, Nattawut Anuniwat, Jiwei Lu Amorphous ferrimagnetic Gd$_{\mathrm{x}}$Fe$_{\mathrm{93-x}}$Co$_{\mathrm{7}}$ alloy films have been reported with a tunable perpendicular magnetic anisotropy in both the low-Gd region (20 \textless x \textless 34) and the high-Gd region (52 \textless x \textless 59) [1]. The compositional and temperature dependence of their saturation magnetization is attributed to the competition between antiferromagnetic coupling of rare-earth (RE) with transition-metal (TM) ions and ferromagnetic interaction between the TM ions. Here, we present a computational model of the RE-TM amorphous structure using the Monte Carlo simulation method. The classical atomistic spin Hamiltonian has been used considering a model of random crystalline alloy. To obtain a consistent magnetization with the experimental data, we find it necessary to assume Gd spins having a non-collinear sperimagnetic structure, which origins from the anisotropy term. The calculated saturation magnetizations exhibit compensation phenomena for the low-Gd region and ferromagnetic transition behavior for the high-Gd region, in agreement with experiment. The results are analyzed in light of the sublattice magnetizations. [1] Manli Ding, S. Joseph Poon, J. Magn. Magn. Mater. 339, 51-55 (2013). [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D7.00012: First principles investigation of magnetocrystalline anisotropy at Full Heusler / MgO interfaces Rajasekarakumar Vadapoo, Ali Hallal, Mairbek Chshiev Magnetic tunnel junctions with perpendicular magnetic anisotropy (PMA) have the potential for realizing next generation high density nonvolatile memories and logic devices [1]. The origin of high PMA in these interfaces has been explained by orbital hybridizations at interface along with spin-orbit interactions [2]. Here we present a systematic study of PMA in Heusler alloy [X$_{2}$YZ]/ MgO interfaces using first principle methods with X$=$Co, YZ$=$FeAl, MnGe and MnSi. Among the interfaces studied, we found that Co terminated interface of Co$_{2}$FeAl/MgO gives rise to PMA value of 1.2erg/cm$^{2}$ in agreement with recent experimental observations [3]. On the contrary, FeAl terminated interfaces of the same structure shows in-plane magnetic anisotropy (IMA). We also found that the most of PMA contribution originates from d$_{\mathrm{yz}}$ and d$_{\mathrm{z}}^{2}$ orbitals of Co atoms at the interface. Finally, Co$_{2}$MnGe and Co$_{2}$MnSi structures tend to favor IMA for any termination. \\[4pt] [1] S. Iked et al., Nature materials 9, 721 (2010)\\[0pt] [2] H. X. Yang et al., Physical Review B 84, 054401 (2011).\\[0pt] [3] M. Belmeguenai et al., Cond-mat.mtrl-sci, may 2013, arXiv:1305.0714. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D7.00013: Ground States in Thin Ferromagnetic Nanorings with Four-Fold In-Plane Anisotropy Gabriel Chaves-O'Flynn, Cyrill Muratov We present results of micromagnetic simulations based on the optimal grid algorithm for the study of metastable states in thin nano-rings with four-fold anisotropy. Previous work have demonstrated a rich energy landscape for these structures resulting from competition between shape and crystalline anisotropies [1]. We present a quasi-1D framework for the analysis of nanorings. First, we calculate the energy of domain walls of different windings for magnetic strips oriented at different angles with respect to the easy anisotropy axes. We consider the dependence of wall energy on material parameters. With these numbers we build a reduced-model for the micromagnetic energy on the rings which allows to treat the micromagnetic energy minimization as a combinatorial problem: the walls in the ring are treated as separate entities each with an intrinsic energy calculated from the strip case and interacting with each other via dipole-dipole interactions. A comparison with the phase diagram for ground states provides information on the limits of validity of this simplified model. [1] G.D. Chaves-O'Flynn, C. Muratov. IEEE Trans. Mag. 49, p. 3125 (2013) [2] C. Muratov and V. Osipov. IEEE Trans. Mag, 45, p.3207 (2008) [Preview Abstract] |
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