Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session X27: Focus Session: Exchange Bias and Magnetic Interactions |
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Sponsoring Units: GMAG Chair: Christian Binek, University of Nebraska-Lincoln Room: Morial Convention Center 219 |
Friday, March 14, 2008 8:00AM - 8:12AM |
X27.00001: Uncompensated moments in antiferromagnets: Origin and role in exchange bias. Igor V. Roshchin, Zhi-Pan Li, Casey W. Miller, Maria Varela, Stephen J. Pennycook, Ivan K. Schuller Exchange bias (EB) is a ferromagnet (FM) -- antiferromagnet (AF) proximity effect. The depth profile of the magnetization across the interface between a FM (Co) and an AF (FeF$_{2})$ in an EB system has been measured. [1] It was found that both uncompensated and compensated magnetic moments are present in FeF$_{2}$. The origin of these moments remains an open question. Our high-resolution STEM study confirms that FeF2 grows epitaxially on MgF$_{2}$, and its structural quality is very high. It also reveals that the substrate surface imperfections do not necessarily affect the quality of the FeF$_{2}$ layer. In magnetization measurements of just a thin film of FeF$_{2}$, without a FM, we find an uncompensated magnetization. This magnetization demonstrates temperature dependence and horizontal hysteresis loop shift, typical for EB. Dependence of this magnetization on the substrate, film parameters and cooling conditions will be discussed. Work supported by US DOE. \\ $[1]$ M. R. Fitzsimmons \textit{et al}., Phys. Rev. B \textbf{75}, 214412 (2007). [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X27.00002: Strong anisotropy suppression at the CoO Ne\'{e}l temperature in perpendicularly exchange-biased CoO/CoPt multilayers Erik Shipton, Keith Chan, Olav Hellwig, Eric E. Fullerton We have performed high-field torque magnetometry measurements on perpendicularly exchange-biased CoO/CoPt multilayers. From magnetometry measurements we observe perpendicular exchange bias that goes to zero at a blocking temperature of 200 K. Torque measurements were performed from 2 to 400 K in fields up to 9 T. There is significant hysteresis even at 9 T fields suggesting instabilities in the AF layer. This hysteresis persists up to the blocking temperature. Surprisingly, there is a large reduction in the uniaxial anisotropy with increasing temperature with a minimum at 250 K, the estimated Ne\'{e}l temperature of the CoO layers, followed by an increase in the anisotropy with increasing temperature. The anisotropy of Co/Pt multillayers without the CoO layers monotonically decreases with increasing temperature. This suggests that there is an additional planar anisotropy arising from the CoO that counters the anisotropy of the Co/Pt layers, and that the contribution from the CoO is maximized at the Ne\'{e}l temperature as observed by Grimsditch et al. [1]. These results show that the addition of antiferromagnetic layers may be used to tune the temperature dependence anisotropy response of magnetic systems. [1] M. Grimsditch et al., Phys. Rev. Lett. \textbf{90}, 257201 (2003). [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X27.00003: Pinned magnetization in the exchange bias system Permalloy/CoO Sujoy Roy, Elizabeth Blackburn, Cecilia Sanchez-Hanke, Sunil Sinha, Ami Berkowitz Interfacial effects are understood to be crucial in the development of exchange bias. In particular, the role of uncompensated spins is important, although because the interface is buried these uncompensated spins can be difficult to measure. Penetrating radiation such as neutrons or x-rays are one of the few tools available to do this. The problem of exchange bias is further complicated by the myriad differences observed from system to system, indicating that the local environment of the magnetic ions has a strong effect on the type of coupling that dominates across the interface. In turn, understanding this coupling is vital in understanding the microscopic origin of exchange bias in a given system. In this paper, we present soft x-ray reflectivity data that show that in the exchange biased state (i.e. below the Neel temperature 289 K for CoO) there is an interfacial layer between the Py and CoO that possesses a net magnetization at room temperature. In the exchange biased state, this contains the uncompensated spins from the CoO layer, and a significant fraction of the spins in this layer are pinned. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X27.00004: Ultrafast frustration of the magnetization in exchange biased Ni/FeF2 Amit Porat, Ivan K. Schuller, Shimshon Bar-Ad We used the magneto optical Kerr effect to study fast optically-induced magnetization dynamics in a Ni/FeF$_{2}$ exchange bias bilayer. We find that sub-picosecond laser pulses trigger an unexpected out of plane precession of the Ni magnetization, surprisingly in external magnetic fields that overcome the exchange bias (unlike previously reported precessions in bilayers). Even more surprisingly the precession persists for excitation intensities that completely decouple the Ni from the FeF$_{2}$. The experimental results suggest that the FeF$_{2}$ layer at the thermally-excited interface is frustrated by the opposing anisotropy fields created by the external field, the Ni layer, and the underlying thermally unexcited FeF$_{2}$ layer. The frustrated FeF$_{2}$ layer reorients, which in turn triggers the precession of the Ni. This implies that the decoupling at high excitation intensities does not only involve the Ni, but also the interfacial FeF$_{2}$ layer, which decouples from the cold underlying bulk FeF$_{2}$. The decoupling thus leads to a reversal of the exchange bias, as we found experimentally. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X27.00005: Antiferromagnetic Domain Size Measurement in Fe$_{0.70}$Zn$_{0.30}$F$_2$/Co Bilayers David Lederman, Hongtao Shi, Michael Fitzsimmons The size of the antiferromagnetic domains of an epitaxial (110) Fe$_{0.70}$Zn$_{0.30}$F$_2$ dilute Ising antiferromagnetic layer 68 nm thick with a polycrystalline Co overlayer 27 nm thick was studied via neutron diffraction. The sample's exchange bias changed sign from negative to positive as the temperature was increased, with the switching temperature, at which the exchange bias was zero, occurring at $T=20$ K. The width of the (100) antiferromagnetic peak of the Fe$_{0.70}$Zn$_{0.30}$F$_2$ layer was significantly narrower at the switching temperature than at either $T=5.5$ K or $T=30$ K. This result is consistent with models that predict an inverse relationship between the antiferromagnetic domain size and exchange bias. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X27.00006: Scaling behavior of the exchange-bias training effect Srinivas Polisetty, Sarbeswar Sahoo, Christian Binek The dependence of the exchange-bias training effect on temperature and ferromagnetic film thickness is studied in detail and scaling behavior of the data is presented.$^{\dag }$ Thickness-dependent exchange bias and its training are measured using the magneto-optical Kerr effect. A focused laser beam is scanned across a Co wedge probing local hysteresis loops of the Co film which is pinned by an antiferromagnetic CoO layer of uniform thickness. A phenomenological theory is best fitted to the exchange-bias training data resembling the evolution of the exchange-bias field on subsequently cycled hysteresis loops. Best fits are done for various temperatures and Co thicknesses. Data collapse on respective master curves is achieved for the thickness and temperature-dependent fitting parameters as well as the exchange bias and coercive fields of the initial hysteresis loops. The scaling behavior is strong evidence for the validity and the universality of the underlying theoretical approach based on triggered relaxation of the pinning layer towards quasi-equilibrium. $^{\dag }$Srinivas Polisetty, Sarbeswar Sahoo, Christian Binek, Phys. Rev. B \textbf{76}, 184423 (2007). [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X27.00007: Interlayer coupling in Co/NiO/Fe trilayers studied by element-specific XMCD and XMLD effects J. Wu, J. Choi, C. Won, A. Scholl, A. Doran, E. Arenholz, Y.Z. Wu, X.F. Jin, Z.Q. Qiu Co/NiO/Fe trilayers are grown on Ag(001) substrate using Molecular Beam Epitaxy (MBE). Element-specific magnetic domain images on both ferromagnetic (FM) Co and Fe layers and antiferromagnetic NiO layer are obtained using X-ray Magnetic Circular Dichrism (XMCD) and X-ray Linear Dichrism (XMLD), respectively. By comparing these magnetic domain images, we find that the Co-Fe interlayer coupling across the NiO spacer layer exhibits a transition from a 90$^{o}$-coupling to a collinear coupling as the NiO film thickness increases. This observation is shown to be directly related to the NiO magnetic structure and its interaction at the Co/NiO and NiO/Fe interfaces. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X27.00008: Thermodynamics of Co/Cr superlattices T. Mukherjee, S. Sahoo, R. Skomski, D.J. Sellmyer, Ch. Binek Progress in ultra thin film growth has resulted in many novel surface and interface induced properties of artificial heterostuctures. Here, we study magnetic superlattices of ultrathin Co and Cr films grown by Molecular Beam Epitaxy methodology at a base pressure below 1$\times $10$^{-10}$ mbar. Our approach is based on controlling two distinct magnetic degrees of freedom. First, the critical temperature, T$_{c}$, of individual Co films is tailored via geometrical confinement of the correlation length perpendicular to the film. Various thickness dependent values, T$_{c}$(d), between zero and the bulk Curie temperature of 1388 K are realized. Second, the T$_{c}$-tailored Co films are antiferromagnetically coupled through Cr interlayer films. The oscillating coupling strength is tailored via the Cr interlayer thickness. The resulting thermodynamic properties of such Co/Cr superlattices are studied with the help of SQUID magnetometry. Particular emphasis is laid on tailoring magnetic entropy changes in the vicinity of room temperature. X-ray diffraction and X-ray reflectivity are used to correlate structural data with the magnetic properties. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X27.00009: Cooling Field Dependence of Magnetization Depth Profiles in Exchange-coupled Superlattices Michael Fitzsimmons, Karine Dumesnil, Catherine Dufour In DyFe$_{2}$/YFe$_{2}$ superlattices, competition between ferromagnetic exchange coupling of adjacent Fe spins and antiferromagnetic coupling of Fe spins with rare earth spins leads to an antiparallel arrangement (confirmed with XMCD and neutron scattering) of magnetization across the DyFe$_{2}$/YFe$_{2}$ interfaces in low fields at 300 K. After cooling this simple structure to 12 K, the DyFe$_{2}$ magnetization becomes pinned and the sample exhibits very large exchange bias ($\sim $2 T) and a large (35{\%}) negative shift of the sample magnetization along the magnetization axis. However, when a large magnetic field is applied at room temperature, the magnetization adopts a spin-flop configuration across the DyFe$_{2}$/YFe$_{2}$ interfaces (confirmed with XMCD and neutron scattering). When cooled in a large field, the sample yields neither exchange bias nor a shift of the sample magnetization along the magnetization axis. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X27.00010: In-plane anisotropy of NiCo multilayers P. Panyajirawut, M.S. Rzchowski We have grown NiCo magnetic multilayers on Si substrates by sputter deposition. The Ni$_{0.6}$-Co$_{0.4}$ bilayer that repeats to form the multilayer ranges from 2~nm to 12~nm in thickness. The total multilayer thickness ranges from 50~nm to 75~nm. Room temperature vibrating sample magnetometer (VSM) measurements show that the multilayers have in-plane uniaxial magnetic anisotropy with K$_{u}\sim $1.2x10$^{5}$ erg/cc. This is apparently induced during growth by the sputtering geometry, as we also see uniaxial in-plane anisotropy in individual 30~nm thick Ni (K$_{u}\sim $0.3x10$^{5}$ erg/cc) and Co (K$_{u}\sim $2.9x10$^{5}$~erg/cc) sputtered films. However the multilayer anisotropy is more complex as it arises from an interaction between the Ni and Co layers, with the Ni and Co layer magnetizations to first approximation rotating together. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X27.00011: Determination of inhomogeneous magnetic profiles in an asymmetric Fe/Gd multilayer Evgeny Kravtsov, Daniel Haskel , Suzanne G.E. te Velthuis , Yongseong Choi, J. Samuel Jiang We have studied the dependence of the detailed magnetization depth profile in a [Fe(35 A)/Gd(50 A)]$_{5}$ multilayer on the applied magnetic field and temperature. Utilizing the complementarity of x-ray resonant magnetic reflectivity (element-specificity and high spatial resolution) and polarized neutron reflectivity (large magnetic scattering cross sections), we applied a unified approach by simultaneous refinement and resolved the complex magnetization profiles. It was found that the small number of periods together with the asymmetric termination (Fe-top, Gd-bottom) lead to unique inhomogeneous magnetic phases, which are characterized by significant twisting of Fe and Gd magnetic moments and non-uniform distribution of magnetization density within the Gd layers. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X27.00012: Magnetic Compensation in Ferrimagnetic Bimetallic Oxalates Peter Reis, Randy Fishman, Fernando Reboredo, Juana Moreno Bimetallic oxalates are layered organic magnets with the chemical formula A[M(II)M'(III)(ox)$_{3}$], where M(II) and M'(III) are transition metal ions, A is an organic cation, and ox = C$_{2}$O$_{4}$ is the oxalate molecule. For some ferrimagnetic bimetallic oxalates, the magnetization changes sign at a compensation temperature below the ferrimagnetic transition temperature. We have initiated a systematic study of these compounds by examining the possibility of magnetic compensation for any possible combination of transition metal ions. Our model includes spin-orbit coupling for both M(II) and M(III) ions, the antiferromagnetic exchange between neighboring metal ions mediated by the oxalate bridges, and the effects of the ligand fields. Using mean-field theory, we predict candidates that may exhibit magnetic compensation for certain choices of the intercalated cation A. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X27.00013: Giant Negative Magnetization in a Class of Layered Molecular-Based Magnets Randy Fishman, Fernando Reboredo Bimetallic oxalates are a class of layered molecular-based magnets with transition metals M(II) and M'(III) coupled by oxalate molecules in an open honeycomb structure. Energy, structure, and symmetry considerations are used to construct a reduced Hamiltonian, including exchange and spin-orbit interactions, that explains the magnetic compensation and giant negative magnetization in some of the ferrimagnetic Fe(II)Fe(III) compounds. By shifting the Fe(II) ions with respect to the oxalate molecules, the organic cation between the magnetic layers alters the C$_{3}$-symmetric crystal field and the orbital angular momentum of the ground-state doublet at the Fe(II) sites. We provide new predictions for the spin-wave gap, the effects of uni-axial strain, and the optical flipping of the negative magnetization in Fe(II)Fe(III) bimetallic oxalates [1]. [1] R.S. Fishman and F.A. Reboredo, \textit{Physical Review Letters }\textbf{99}, 217203 (2007). [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X27.00014: Effect of thermal fluctuation on the recoil loops of exchange-coupled nanocomposite magnets Chuanbing Rong, Yuzi Liu, J. Ping Liu Exchange-coupled hard/soft nanocomposite magnets have attracted great attention due to the very high potential energy product. One of the most effective ways to characterize the exchange-coupling strength in the nanocrystalline magnets is the measurement of recoil loops. It is often noted that the recoil curves are widely open for the hard/soft nanocomposite magnets but are narrow and even close for the single-phase magnets. In this work, we studied recoil loops of the FePt/Fe$_{3}$Pt nanocomposite magnets. It was interesting to find that the parameter that describes the openness of the loops, $\Delta m_{rc}^m$, is significantly dependent on the sweep rate of applied field, especially for the nanocomposite magnets with high soft-phase content, where $\Delta m_{rc}^m$ is maximum difference between upper and lower magnetization curves of the recoil loops. The quantitative analysis shows a reciprocally linear relation between $\Delta m_{rc}^m$ and the activation volume, which means that the recoil loops are intimately related to the thermal fluctuation. The large open area of the nanocomposite magnets compared to that of single phase magnets is attributed to the more unstable magnetization process in the exchange-hardened soft phase. [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X27.00015: Effects of additives on hydrogen absorption and desorption characteristics of Nd(Fe,Mo)12 alloys Jingzhi Han, Changsheng Wang, Honglin Du, Haiying Chen, Yingchang Yang Effects of additives such as Co, Zr, Nb and Ga on hydrogen absorption and desorption characteristics of Nd(Fe,Mo)$_{12}$ alloys are investigated. The results show that Zr or Nb addition increases the temperature of the disproportionation, and Co or Ga addition decreases the temperature of the recombination. This indicates that Zr or Nb addition makes the disproportionation sluggish while Co or Ga addition is effective for enhancing the recombination, which is similar to the effects of the above additives on the hydrogen absorption and desorption characteristics of Nd$_{2}$Fe$_{14}$B alloys. However, according to XRD measurement of the magnetic oriented samples, Anisotropic HDDR NdFe$_{10.5-X}$M$_{X}$Mo$_{1.5}$ (M=Co,Zr,Nb or Ga) powders is not found. In order to analyze the reason leading to isotropic Nd(Fe,Mo)$_{12}$ and anisotropic Nd$_{2}$Fe$_{14}$B powders, the similarities and differences of the Nd(Fe,Mo)$_{12 }$and Nd$_{2}$Fe$_{14}$B systems and their corresponding HDDR phenomena are discussed, and whether leading to anisotropic powders or not is suggested to be related to crystal growth direction of the Nd-Fe-B and Nd(Fe,Mo)$_{12}$ system. At the same time, it is also found that the Nb addition is helpful for improving the magnetic properties of the HDDR Nd(Fe,Mo)$_{12}$N$_{X}$ magnetic materials, and the HDDR NdFe$_{10.3}$Nb$_{0.2}$Mo$_{1.5}$N$_{X}$ powders have a Br of 54 emu/g, a iHc of 3900 Oe. Moreover, the Mr/Ms ratio of the above powders reaches 0.60. [Preview Abstract] |
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