Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P42: Focus Session: Magnetic Nanoparticles, Nanostructures & Heterostructures VII |
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Sponsoring Units: DMP GMAG Chair: Xiaofeng Jin, Fudan University, China Room: LACC 150B |
Wednesday, March 23, 2005 11:15AM - 11:27AM |
P42.00001: The spatial profile of unpinned and pinned uncompensated spins in the antiferromagnetic layer of an exchange bias heterostructure determined with polarized neutron reflectometry M.R. Fitzsimmons, S. Park, A. Misra, X. Zhang, S. Roy, M. Dorn, O. Petracic, I.V. Roshchin, Z.P. Li, X. Batlle, R. Morales, S.K. Sinha, Ivan K. Schuller The spin structure of an antiferromagnet plays a critically important role as a means to establish a reference state in magnetic devices; however, the spin structure at the surface and interior of an antiferromagnetic thin film remains unknown. We have used the unique spatial sensitivity of polarized neutron reflectometry to measure the depth dependence of magnetization across the interface between a ferromagnet and an antiferromagnet. The net uncompensated magnetization near the interface responds to applied field, while uncompensated spins in the antiferromagnet bulk are pinned. A new model is proposed for exchange bias. This work was supported in part by the Office of Basic Energy Science, U.S. Department of Energy, BES-DMS, the University of California Campus Laboratory Collaborative program, and Laboratory Directed Research and Development program funds. [Preview Abstract] |
Wednesday, March 23, 2005 11:27AM - 11:39AM |
P42.00002: Resonant X-ray Scattering Studies of Positively Exchange Biased Co/FeF2 S. Roy, M. Dorn, S.K. Sinha, O. Petracic, Z.P Li, I.V. Roshchin, R. Moralles, X. Batlle, I.K. Schuller, K. Chesnel, J.B. Kortright, S. Park, M.R. Fitzsimmons We here report on our element sensitive resonant soft x-ray scattering studies on a positively exchange biased MgF$_{2}$/FeF$_{2}$/Co/Al sample at T = 20 K performed using circularly polarized incident X-ray beam. The data is analyzed in the Distorted Wave Born Approximation and the depth dependent magnetic density profiles of pinned and unpinned moments in both the ferromagnet and the antiferromagnet is quantitatively determined. Co and FeF$_{2}$ were found to be antiferromagnetically coupled and the net magnetization in the ferromagnet has a structure near the Co/FeF$_{2}$ interface. Diffuse scattering measurements indicated stripe-like domain structure with oppositely directed moments in Co and Fe at the interface that are correlated to interface roughness. Work of I.K.S is supported by DOE. [Preview Abstract] |
Wednesday, March 23, 2005 11:39AM - 11:51AM |
P42.00003: Tailoring the spin direction of antiferromagnetic NiO thin films grown on vicinal Ag(001) B. Sinkovic, Y.Z. Wu, Y. Zhao, Z.Q. Qiu NiO is one of the most intensively studied antiferromagnetic materials to study the exchange bias effect. It was shown recently that the spin direction of a NiO thin film can be manipulated between out-of-plane and in-plane directions by growing NiO on MgO(001) and Ag(001). Here we report that we realized the manipulation of NiO spin direction\textit{ in the plane} of the film by growing NiO on a vicinal Ag(001) surface. X-ray Magnetic Linear Dichroism (XMLD) was used to measure the NiO spin directions. We found that the NiO film grown on vicinal Ag(001) surface has an in-plane uniaxial magnetic anisotropy which favors the NiO spin parallel to the steps for $<$100$>$ vicinal surface or perpendicular to the steps for $<$110$>$ vicinal surface. Moreover, we observed the XMLD effect from the NiO L$_{3}$ edge, which further support the magnetic origin of the XMLD effect in our sample. [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:03PM |
P42.00004: Influences of lateral domains and interfacial domain wall formation on exchange bias phenomena in TbFe/GdFe bilayers Stephane Mangin, Thomas Hauet, Francois Montaigne, Yves Henry, Eric Fullerton We have studied the exchange bias (EB) in a ferrimagnetic TbFe/GdFe bilayer with antiferromagnetic interfacial coupling as a function of magnitude and angle of the cooling field. The magnetic configuration inside both layers has been followed by ultra sensitive magnetoresistance measurements. For increasing cooling field strength or angle from the GdFe easy axis we observe a continuous transition from negative to positive exchange bias. These effects are explained by the quenching of the magnetic configuration inside the random-anisotropy TbFe layer which leads to the presence of a frozen partial interface domain wall. The transition from positive to negative EB results from the continuous rotation of the direction of the interfacial pinning acting on the GdFe layer. Effects of the chirality of this domain wall on the EB have also been indentify. Finally the effect of lateral magnetic domains on EB have been investigated. The occurrence of multi-domain state is found to give rise to exotic and tunable hysteresis loops at low temperature that depend strongly on whether the domains are present in the TbFe or GdFe layers during cooling. The results are fully understood by taking into account both the presence of lateral domains and an interfacial domain wall. [Preview Abstract] |
Wednesday, March 23, 2005 12:03PM - 12:15PM |
P42.00005: Exchange Bias of Single-Crystalline Fe$_x$Zn$_{1-x}$F$_2$/Co Bilayers David Lederman, Hongtao Shi The exchange bias of polycrystalline Co films grown on epitaxial, 67 nm thick single-crystalline films of (110) Fe$_x$Zn$_{1-x}$F$_2$ was measured as a function of Fe concentration. A set of samples was grown with a pure, 1.0~nm thick FeF$_2$ layer at the interface, and another set was grown without the interface layer. Unlike previous measurements of \textit{twinned} Fe$_x$Zn$_{1-x}$F$_2$ films, the exchange bias of samples with the pure interface layer remains constant as the Fe concentration is decreased from $x=1.0$ to $x=0.35$. A decrease in samples without the pure interface layer was also observed, which can be explained by a weakening of the Co/Fe$_x$Zn$_{1-x}$F$_2$ exchange interaction as the Fe concentration is decreased. These results imply that, at least in highly anisotropic systems like FeF$_2$, the proposed domain state model does not explain the experimental data.\\ \\ Work supported by NSF grant 0400578. [Preview Abstract] |
Wednesday, March 23, 2005 12:15PM - 12:27PM |
P42.00006: Parallel Versus Antiparallel Interfacial Coupling In Exchange-biased Co/FeF$_2$ Hendrik Ohldag, Joachim Stohr, Hongtao Shi, David Lederman The nature of exchange bias in FeF$_2$ (110) remains elusive due to it's nominally compensated surface. Other interesting phenomena include positive exchange bias and an enhancement of the the coercivity near $T_N$. In order to address these issues, soft x-ray dichroism absorption spectroscopy was used to investigate the direction of interfacial exchange coupling in a antiferromagnetic/ferromagnetic exchange-coupled Co (2.5 nm) /FeF$_2$ (68 nm) bilayer. The FeF$_2$ was epitaxially grown on MgF$_2$ (110) and the Co layer was polycrystalline. The sample was capped with a 2.0 nm layer of Pd to protect it from oxidation. For comparison, a nominally identical sample without Co was also grown. A small portion of interfacial Fe spins couples antiparallel to the ferromagnet, causing the positive exchange bias for cooling fields. A larger potion of interfacial spins, coupled more strongly and parallel to the ferromagnet, increases the degree of antiferromagnetic order and plays an important role in the observed coercivity increase at high temperatures.\\ \\ Work supported by DOE at SSRL and by NSF grant DMR-0400578 at WVU. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 12:39PM |
P42.00007: Micromagnetic simulation to observe the reversal mechanism in exchanged biased system of NiFe/NiMn Jyotirmoy Saha, Randall Victora Understanding the mechanism of exchange interaction between a ferromagnet (FM) and an antiferromagnet (FM) has been both a scientific and technological endeavor in recent years. However, the theoretical and computational efforts so far have rarely predicted important parameters such as exchange bias (H$_{E})$ and enhanced coercivity (H$_{c})$ for any particular system. In our attempt to explain the same, we have simulated the behavior of a FM (NiFe) in the presence of a polycrystalline AF (NiMn) through a moment-moment interaction. To incorporate the surface roughness of the AF grains, the surface spins were selected using a random number generator. This assigned a net moment to each AF grain at the interfacial surface. Our design incorporates about a quarter million cubes which has been the key factor to our understanding of the magnetization reversal mechanism. The time evolution of the FM moments is governed by the solution to the Landau Lifshitz Gilbert equation. The hysteretic behavior of the AF grain includes the effect of thermal excitation. Our results indicate that the reversal mechanism can be either domain nucleation or uniform rotation depending on field direction and training. The hysteresis loop yields an exchange bias of 120 Oe and enhanced coercivity of 100 Oe. [Preview Abstract] |
Wednesday, March 23, 2005 12:39PM - 12:51PM |
P42.00008: Lateral length scales and local character of exchange bias. Ivan K. Schuller, Igor V. Roshchin, O. Petracic, R. Morales, Zhi-Pan Li, Xavier Batlle Exchange bias (EB) is a ferromagnet (F) -- antiferromagnet (AF) proximity effect. EB manifests itself as a horizontal shift of a single hysteresis loop. In our studies, an untwinned 38--100 nm-thick layer of (110) FeF$_{2}$ is epitaxially grown on (110) MgF$_{2}$, followed by a 4--70 nm-thick layer of Co, Ni or Fe. Easy axis magnetization curves (SQUID and spatially resolved MOKE) for different cooling fields and remanent magnetizations for zero-field cooled samples exhibit negatively or positively shifted single or tunable double hysteresis loops (DHL). In the untwinned epitaxial FeF$_{2}$, the AF domains can be much larger than the grains, and, hence, as large as the F domains. When each F domain is in contact with only one AF domain, it does not average the direction and the magnitude of EB. In this regime, inhomogeneity of an AF-F sample, either structural or magnetic, can result in two subsystems formed upon cooling through the AF transition temperature. Each subsystem exhibits EB of the same magnitude but of the opposite sign, which gives rise to DHL. We conclude that when the domain size in the AF is larger than or comparable to that in the F, the local, non-averaging character of EB can be observed. Work supported by DOE and AFOSR. [Preview Abstract] |
Wednesday, March 23, 2005 12:51PM - 1:03PM |
P42.00009: Bi-domain state in the exchange bias system FeF2/Ni Oleg Petracic, Zhi-Pan Li, Igor V. Roshchin, M. Viret, R. Morales, X. Batlle, Ivan K. Schuller Independently exchange biased subsystems are observed in FeF$_{2} $/Ni bilayers after various field cooling protocols. For intermediate cooling fields a double hysteresis loop is found, while negatively or positively shifted single loops for small or large cooling fields respectively are encountered. Both the subloops and the single loops have the same absolute value of the exchange bias field. This suggests that the antiferromagnet breaks into two subsystems in such a way that the ferromagnet does not experience an average bias but is exchange biased in opposite directions with equal magnitude (`bi-domain'). This idea is confirmed by micromagnetic studies including the effect of the antiferromagnet. We also present experiments where thermally activated motion of these antiferromagnetic `domain' boundaries can be achieved. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:15PM |
P42.00010: Angular Dependence of Exchange Anisotropy on Cooling Field in Exchange Biased Films Kai Liu, Justin Olamit, Zhi-Pan Li, Ivan Schuller Exchange anisotropy in ferromagnet/antiferromagnet (FM/AF) films is usually introduced along the cooling field or FM magnetization direction. Here we investigate the angular dependence of the exchange anisotropy on the cooling field$^{1}$ with vector magnetometry. Three types of (FM=Fe,Ni /AF = FeF$_{2}$, MnF$_{2})$ samples have been studied where the AF layer is polycrystalline, epitaxial (110) and twinned (110). With a polycrystalline AF, the exchange field direction is always the same as the cooling field. With an epitaxial AF which has one spin axis, the exchange field direction is selected by the cooling field to be along the spin axis. With a twinned AF where there are two orthogonal spin axes, the exchange field direction is always along the bisector of the spin axes that encompass the cooling field. Transverse loops show that when the exchange field has a component perpendicular to the applied field, the magnetization reversals occur by coherent rotations in the direction of the perpendicular component. Our results demonstrate systematically the dependence of the exchange field direction on the cooling field direction. $^{1}$H. Shi and D. Lederman, Phys. Rev. B \textbf{66}, 094426 (2002). Work supported by NSF, DOE, Cal-IT$^{2}$ and NEAT IGERT. [Preview Abstract] |
Wednesday, March 23, 2005 1:15PM - 1:27PM |
P42.00011: Interplay between Exchange Bias and Uniaxial Anisotropy S.-H. Chung, A. Hoffmann, M. Grimsditch The effect of the relative orientation and magnitudes of the exchange bias and the uniaxial shape anisotropy has been systematically investigated in nanometer sized strips of NiFe/FeMn bilayers. The magnetic behavior of these patterned lines was characterized using magneto-optic Kerr effect for different orientations of the applied magnetic field. The samples exhibit peculiar magnetic behavior when the exchange bias, the uniaxial anisotropy, and the applied magnetic field are not collinear. In the case when the exchange bias and the uniaxial anisotropy are parallel, the shift of the hysteresis loop changes non-monotonically with the orientation of the applied magnetic field and exhibits a maximum loop shift that exceeds the value that would be expected from the interface coupling alone. Furthermore, when the applied magnetic field is perpendicular to the exchange bias, the magnitude and the orientation of the uniaxial anisotropy determines the magnitude and the sign of the loop shift. A simple modified coherent rotation model provides a quantitative prediction of the hysteretic behavior in these patterned exchange bias systems. These results show clearly that, in order to understand interfacial coupling, in addition to the loop shift, it is also necessary to characterize all other magnetic anisotropies. [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P42.00012: Temperature dependence of the training effect in exchange bias heterostructures Christian Binek, Xi He, Srinivas Polisetty Recently, the training of the exchange bias (EB) effect in antiferromagnetic (AF)/ferromagnetic (FM) heterostructures has been considered in the framework of activated spin configurational relaxation [1]. The EB field, $\mu _{0}$H$_{e}$, is determined from hysteresis loops of the magnetization which are measured by SQUID-magnetometry after field-cooling the sample below the N\'{e}el temperature of the pinning layer. The evolution of $\mu _{0}$H$_{e}$ in terms of the number of consecutively cycled loops is derived from a discretized Landau-Khalatnikov (LK) equation. Here the time parameter is replaced by the loop index n. Mapping the LK equation onto an implicit sequence allows to describe the training effect, $\mu _{0}$H$_{e}$ vs. n for all n $\ge $1, of various EB heterostructures. In the limit n$>$1, our sequence approaches the empirical $\mu _{0}$H$_{e}$(n)$\propto $1/$\surd $n behavior. The best fit of the sequence to a data set $\mu _{0}$H$_{e}$ vs. n provides the essential fitting parameter $\gamma $ which combines properties of the free energy and the damping with the exchange coupling at the AF/FM interface. We study $\gamma $ vs. T by analyzing the T-dependence of the training effect in a CoO/Co bilayer. Various data sets of $\mu _{0}$H$_{e}$ vs. n are determined from hysteresis loops after in-plane field-cooling at $\mu _{0}$H=0.3T from T=320K to temperatures 5K$<$T$<$T$_{B}\approx $150K, respectively. $\gamma $ vs. T increases with increasing temperature which provides insight into the T-dependence of the free energy. [1] Ch. Binek, Phys. Rev. B \textbf{70}, 014421 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 1:51PM |
P42.00013: Exchange biasing of (Ga,Cr)N thin films using a MnO layer Hongxue Liu, Stephen Wu, Rakesh Singh, Nathan Newman Recently, significant advances have been made in synthesizing various dilute magnetic semiconductors. To develop practical applications, and in particular MRAM type devices, it is important to formulate methods to manipulate the magnetic properties. In this study we report the observation of exchange biaing of ferromagnetic (FM) Cr-doped GaN films by an antiferromagnetic (AFM) MnO overlayer. The magnetic hysteresis loop shows a clear shift to negative magnetic field by $\sim $ 70 Oe when measured after field cooling, which is absent in single Cr GaN layers. Enhancement of the coercive field of the exchange biased Cr-dopedÊGaN, as compared to films without a MnO overlayer, is alsoÊfound. The observation of the exchange bias in this system is attributed to the exchange coupling at the FM Cr-GaN/AFM MnO interface. [Preview Abstract] |
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