Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session A15: Focus Session: Exchange Bias and Magnetic Interfaces |
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Sponsoring Units: GMAG DMP Chair: Igor Roshchin, Texas A&M University Room: 317 |
Monday, March 18, 2013 8:00AM - 8:12AM |
A15.00001: Influence of magnetic annealing and interdiffusion on the exchange bias of CoFe/IrMn Waldemar Macedo, Luis Fernandez-Outon, Mario Araujo Filho, Raphael Araujo, Jose Ardisson Magnetic annealing is broadly used to set exchange bias (EB). The EB field depends on the magnetic field and the temperature at which the F/AF exchange interaction is set. Atomic interdiffusion is also expected to have strong influence on EB. For systems containing IrMn, different results have been reported regarding the effect of setting EB between 200 and 400 $^{\circ}$C. We study the effect of atomic interdiffusion on the exchange bias of polycrystalline IrMn/($^{57}$Fe$+$CoFe) multilayers due to the magnetic annealing between 225 and 500 $^{\circ}$C. The samples have been prepared by magnetron sputtering, and $^{57}$Fe probe layers (10 {\AA} thick) were grown at the F/AF interface, and 1 nm and 2 nm above it, inside the CoFe layer. Depth-resolved $^{57}$Fe conversion electron M\"{o}ssbauer spectroscopy (CEMS) was used to quantify atomic interdiffusion, and vibrating sample magnetometry was used to monitor the variation of exchange bias and magnetisation. We found that interface sharpness is only affected above $\sim$350 $^{\circ}$C. Three different stages for the setting of exchange bias can be inferred from our results. At temperatures \textless\ 350 $^{\circ}$C, no interdiffusion is observed and the F/AF exchange coupling establishes partial spin alignment of interfacial and bulk AF spins. At intermediate setting temperatures (350-450 $^{\circ}$C) interfacial spin order is dominant over chemical intermixing effects, and both exchange field and coercivity increase up to 450 $^{\circ}$C. Above 450 $^{\circ}$C, severe chemical intermixing reduces significantly ($\sim$50{\%}) the F/AF coupling. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A15.00002: Controlling Exchange Bias in FeMn with Cu Igor V. Roshchin, Dogan Kaya, Pavel N. Lapa, Priyanga Jayathilaka, Hillary Kirby, Casey W. Miller One of the puzzles of exchange bias (EB) that remains unsolved is the origin and role of uncompensated magnetization (UM) in the antiferromagnet (AF). We offer a way of controlling the \textit{intrinsic} EB in FeMn by growing it in contact with Cu. The multilayers of Ta(5~nm)/[Cu(5~nm)/FeMn(\textbf{\textit{t}})]$_{\mathrm{10}}$/Ta(5~nm) with 5~nm \textless \textbf{\textit{t}} \textless 15~nm are deposited by RF and DC magnetron sputtering on top of Si/SiO$_{\mathrm{2}}$. The hysteresis loops at 10 K for field-cooled Cu/FeMn multilayers are EB-shifted, while the samples without Cu exhibit no EB. Unlike the ``classical'' EB observed at the interface of AF-ferromagnet (FM) bilayer systems, this EB is ``intrinsic'' to this system with no separate FM layer. The exchange bias field, $H_{\mathrm{E}}$ scales with the inverse thickness of FeMn. This fits Malozemoff's model,\footnote{ A. P. Malozemoff, Phys. Rev. B \textbf{35}, 3679 (1987), \textbf{37}, 7673 (1988).} where the thickness of the FM is replaced with the thickness of FeMn, which supports that the role of the FM is played by the UM which scales with the thickness of the FeMn film. Coercivity ($H_{\mathrm{C}})$ and $H_{\mathrm{E}}$ dependences on the FeMn thickness and temperature are similar to those for Cu/FeMn/Co samples.$^{\mathrm{\thinspace }}$\footnote{ B. T. Bolon, \textit{et al.}, J. Magn. Magn. Mat. \textbf{309}, 54 (2007).} This suggests that the \textit{intrinsic} EB in Cu/FeMn may be determining the EB in AF-FM samples. The role of Cu in the intrinsic EB in FeMn will be discussed. Work is supported by TAMU-CONACyT Collaborative Research Program, and by NSF (at USF). [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A15.00003: Exchange Bias in Ferromagnetic/Antiferromagnetic/Ferromagnetic Co/FeF$_{2}$/Co Trilayers Trent Johnson, David Lederman We have measured the magnetic properties of Co(20 nm)/FeF$_{2}$(2 nm)/Co(5 nm) trilayers grown on Al$_{\mathrm{2}}$O$_{3}$ substrates via e-beam evaporation. The layers were polycrystalline and the samples were capped with 5 nm of Pd to avoid oxidation. The sample surface was very smooth, as indicated by AFM images where the underlying substrate's atomic terraces were visible, while the interface roughness parameters were on the order of 1 nm determined from x-ray reflectivity. After field-cooling to below the N\'{e}el temperature of FeF$_{2}$ in either 1 kOe and 5 kOe, magnetic hysteresis loops were measured as a function of temperature. We found that both layers have a negative exchange bias, with the exchange bias of the thinner layer larger than that of the thicker layer. In addition, the coercivity below the blocking temperature T$_{\mathrm{B}}$ of the thinner layer was significantly larger than that of the thick layer, even though the coercivity of the two layers is the same for T\textgreater\ T$_{\mathrm{B}}$. The drastic difference in coercivities for T\textless\ T$_{\mathrm{B}}$ illustrates the importance of the interface magnetic order on the reversal mechanism of the ferromagnet. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A15.00004: Probing boundary magnetization through exchange bias in heterostructures with competing anisotropy Yi Wang, Christian Binek Cr$_{2}$O$_{3}$ (chromia) is a magnetoelectric antiferromagnet with a bulk T$_{\mathrm{N}}$ of 307 K. It has been utilized for electrically controlled exchange bias (EB) by taking advantage of voltage-controllable boundary magnetization (BM) occurring as a generic property in magnetoelectric single domain antiferromagnets.\footnote{Xi He, et al., Nature Mater.\textbf{9}, 579-585 (2010)} In the perpendicular Cr$_{2}$O$_{3}$(0001)/CoPd EB system the EB-field shows an order parameter type T-dependence close to T$_{\mathrm{N}}$ reflecting the T-dependence of the BM. At about 150 K a decrease of the EB-field sets in with decreasing temperature suggesting canting of the BM. To evidence this mechanism we use EB as a probe. Specifically, we investigate EB in Permalloy(5nm)/Cr$_{2}$O$_{3}$ (0001)(100nm) with Permalloy and chromia having competing anisotropies. We measure easy axis magnetic hysteresis loops via longitudinal magneto-optical Kerr effect for various temperatures after perpendicular and in-plane magnetic field-cooling. The T-dependence of the EB field supports the canting mechanism. In addition to the all thin film EB system, we explore a Permalloy(10nm)/Cr$_{2}$O$_{3}$(0001 single crystal) heterostructure where magnetoelectric annealing allows selecting Cr$_{2}$O$_{3}$ single domain states. Here the effect of T-dependent canting of the BM is compared with findings in the complementary perpendicular EB system. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A15.00005: Positive exchange bias in thin film multilayers produced with nano-oxide layer Byong Sun Chun, Mohamed Abid, Han-Chun Wu, In Chang Chu, Chanyong Hwang We report a positive exchange bias in thin film multilayers produced with nano-oxide layer.The positive exchange bias, obtained for our system results from an antiferromagnetic coupling between the ferromagnetic CoFe and the antiferromagnetic CoO layers, which spontaneously from on top of the nano-oxide layer. The shift in the hysteresis loop along the direction of the cooling field and the change in the sign of exchange bias are evidence of antiferromagnetic interfacial exchange coupling between the CoO and CoFe layers. Our calculation indicates that uncompensated oxygen moments in the nano-oxide layer results in antiferromagnetic interfacial exchange coupling between the CoO and CoFe layers. One of the interesting features observed with our system in that it displays the positive exchange bias even above the bulk Neel temperature of CoO [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A15.00006: Isothermal electric field-tuning of Exchange bias training in Cr$_{2}$O$_{3}$/PdCo Will Echtenkamp, Christian Binek Voltage-controlled exchange bias (EB) is investigated in a Cr$_{2}$O$_{3}$/PdCo EB heterosystem where a ferromagnetic and perpendicular anisotropic Pd/Co multilayer has been deposited on a (0001) Cr$_{2}$O$_{3}$ (chromia) single crystal. The EB of the system arises from chromia's electrically controllable boundary magnetization (BM) which is switched isothermally and at room temperature by magnetoelectric means [1]. The BM couples to the bulk AF order parameter and follows the latter during switching. In the work reported here, we electrically and isothermally tune chromia into distinct AF multi-domain states. As a result, exchange bias training, which originates from triggered rearrangements of the AF domain state of the pinning system during consecutively cycled hysteresis loops, can be tuned in a controlled manner between zero and sizable effects. We quantify the training effect through best fits of our Landau-Khalatnikov analytic expression [2] to the EB vs loop number. The electric field dependence of the fitting parameters is interpreted in terms of the hysteretic E-field dependence of the AF order parameter.\\[4pt] [1] Xi He, et al., Nature Mater.9, 579--585 (2010).\\[0pt] [2] Ch. Binek, Phys. Rev. B. 70, 014421 (2004). [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A15.00007: Depth profiling of interfacial spin complexities in magnetic heterostructures Invited Speaker: Sujoy Roy Attentively restrained interfaces or superlattices between two materials can lead to emergent functionalities not shown by either constituents in their bulk form. Direct quantitative investigation of spatio-temporal correlations of magnetic and electronic properties of such interfaces is crucial in controlling and tailoring the close proximity of competing energy landscape that naturally exist in these systems. Due to the smallness and buried nature of the magnetization, characterizing these materials at the appropriate length scale is of considerable challenge. In this talk I will give examples from a variety of systems where the unique sensitivity of soft x-ray beams in reflection geometry have been exploited to obtain a quantitative description of the complex magneto-chemical depth profile across the interface between ferromagnetic (F) and antiferromagnetic (AF) thin films. In exchanges bias Co/FeF$_{\mathrm{2}}$ heterostructures we have found antiferromagnetic coupling across the interface with the net magnetization having a twisted ``fan-like'' structure near the F/AF interface. For Py/CoO we observed a redox reaction driven novel interfacial layer that has magnetic properties very different from bulk. We found that 10{\%} of the net spins in this layer get pinned antiparallel to the cooling magnetic field at low temperatures. In complex oxide BiFeO$_{\mathrm{3}}$-La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ we have obtained direct experimental evidence of transitory layers, ionic rearrangements and depleted magnetism at the BiFeO$_{\mathrm{3}}$-La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ interface. Our examples show that interface-selective probing of magnetism in thin film heterostructures can provide vital understanding needed for rational design of future nanoelectronic devices. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A15.00008: Temperature Dependence of Current Induced Magnetic Domain Wall Motion in a Multilayered Co/Ni Nanowire with MgO Cap Kohei Ueda, Ryo Hiramatsu, KabJin Kim, Daichi Chiba, Takahiro Moriyama, Hironobu Tanigawa, Eiji Kariyada, Tetsuhiro Suzuki, Yoshinobu Nakatani, Teruo Ono Current-induced magnetic domain wall motion (CIDWM) has been investigated not only for the fundamental physics but also for its potential application for nonvolatile magnetic random access memory. Our group reported that adiabatic spin transfer torque (STT) dominates the DW motion in nanowires made of a perpendicularly magnetized Co/Ni multilayer with symmetrical top and bottom non-magnetic layers (Ta/Pt and Pt/Ta). Recently, new aspect of the DW motion was reported that DW moves against electron flow direction in asymmetric AlO/Co/Pt system, which is in contrast to STT theory that predicts the DW motion along electron flow direction. We found, in a nanowire made of an Co/Ni multilayer with asymmetric top (MgO) and bottom (Pt/Ta) layers, that the DW moves against electron flow direction as reported in AlO/Co/Pt system. We also investigated the temperature dependences of the threshold current density for DW displacement (Jth). It was found that Jth increases with decreasing device temperature whereas it is almost independent of temperature in a symmetric Co/Ni system, suggesting that the observed DW motion was not simply dominated by the adiabatic STT brought by the electron flow in the Co/Ni multilayer. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A15.00009: Electrical Probing of Magnetic Phase Transition and Domain Wall Motion in Single-Crystalline Mn$_5$Ge$_3$ Nanowire Jianshi Tang, Chiu-Yen Wang, Kang L. Wang, Lih-Juann Chen We studied the magnetic phase transition and domain wall motion in single-crystalline Mn$_{5}$Ge$_{3}$ nanowires fabricated by thermally germaniding Ge nanowires with Mn contacts. The R-T curve showed a clear slope change near 300 K accompanied by a magnetic phase transition from ferro- to para-magnetism. Near this phase transition, the critical behavior was characterized by a power-law relation with a critical exponent of about 0.07. Besides, a cusp revealed in the dR/dT curve at about 67 K was attributed to a possible magnetic transition between non-collinear and collinear ferromagnetic states. Furthermore, temperature-dependent magneto-transport measurements demonstrated a hysteretic, symmetric and stepwise axial magnetoresistance. The interesting features of abrupt jumps indicated the presence of multiple domain walls in the Mn$_{5}$Ge$_{3}$ nanowire and the annihilation of domain walls driven by the magnetic field. The fitting on the temperature-dependent depinning fields yielded an energy barrier of 0.166 eV based on the Kurkijarvi model describing the domain wall depinning as thermally assisted escape from a single energy barrier. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A15.00010: Topological classification of domain walls in a cylindrical nanowire Se Kwon Kim, Christopher Mogni, Oleg Tchernyshyov We classify possible configurations of domain walls in a cylindrical nanowire [1-3] using topology. Dipolar interactions induce effective shape anisotropy so that magnetization tends to be tangential to the surface locally and is parallel to the axis of the wire in the ground states. Topological defects in the bulk are Bloch points with integer skyrmion numbers [second homotopy group $\pi_2(S^2)$]. The surface anisotropy gives rise to surface defects (boojums) with integer vorticity [first homotopy group $\pi_1(S^1)$] and half-integer skyrmion number [relative second homotopy group $\pi_2(S^2,S^1)$]. These defects are weakly bound by the easy-axis anisotropy into composite domain walls. Thus transformations and mergers of domain walls are constrained by the topological conservation laws. Long-lived textures left behind after annihilation of domain walls are classified by the third homotopy group $\pi_3(S^2)$.\\[4pt] [1] R. Hertel, Physica B \textbf{343}, 206 (2004).\\[0pt] [2] R. Wieser, U. Nowak, and K. Usadel, Phys. Rev. B \textbf{69}, 1 (2004).\\[0pt] [3] N. Cooper, Phys. Rev. Lett. \textbf{82}, 1554 (1999). [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A15.00011: Domain wall remote pinning in magnetic nano wires Dan Read, Jorge Miguel, Francesco Maccherozzi, Stuart Cavill, Sarnjeet Dhesi In the current race for information storage media with ever increasing density the position of magnetic domain walls, the region in a magnetic system where the local magnetization continually rotates its direction between adjacent magnetic domains, is one of the most promising routes for future storage media devices. Information storage requires ultrafast read-out and writing operations, but domain walls need to be pinned so that the information is safely stored in the long term. Here we investigate the use of remote magnetostatic charges to trap domain walls. By using X-ray photoelectron emission microscopy we have followed the position of domain walls of opposite charge being pinned or repelled by pinning potentials of increasing strength. Micromagnetic simulations show an excellent agreement with the experimental results. We demonstrate the attractive or repulsive character of the interaction between domain wall and trap depending upon the sign of their magnetic charges. These quasi-static experiments are the antecedent to ultrafast time-resolved XMCD-PEEM experiments where the spin-transfer torque effect will be studied dynamically by applying picosecond-long current pulses across the magnetic nanowire. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A15.00012: ABSTRACT WITHDRAWN |
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