Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S2: Symposium on Exchange Bias |
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Sponsoring Units: GMAG Chair: Dan Dahlberg, University of Minnesota Room: Colorado Convention Center Four Seasons 4 |
Wednesday, March 7, 2007 2:30PM - 3:06PM |
S2.00001: Energy and Length Scales in the Exchange Bias Problem Invited Speaker: Exchange bias phenomenon in antiferromagnet/ferromagnet (AF/F) systems has been studied for over fifty years, however a quantitative theory of exchange bias is still lacking. Although main theoretical ideas necessary for a comprehensive exchange bias theory appear to be in place, reliable quantitative predictions of exchange anisotropy based on the properties of the ferromagnet and the antiferromagnet cannot be made at present. One reason for the difficulty of understanding of exchange bias is the wide range of qualitatively different magnetic behaviors exhibited by different exchange bias systems. In this talk I will argue that the wide range of magnetic behaviors exhibited by exchange bias systems results from the large number of energy and length scales in the exchange bias problem. Different hierarchies of energy and length scales can give rise to qualitatively different magnetic properties of exchange bias systems. Therefore, a classification scheme of exchange bias systems based on the hierarchy of the relevant energy and length scales would greatly facilitate the progress towards the comprehensive understanding of the exchange bias effect. In this talk I will discuss existing theoretical predictions for the magnitude and symmetry of exchange anisotropy in AF/F systems belonging to different energy and length scale hierarchy classes. These predictions will be compared to our experimental data on exchange anisotropy in Fe/MnF$_{2}$, Fe/FeF$_{2}$ and Co/CoO systems belonging to three different classes. I will also analyze exchange anisotropy data for other exchange bias systems reported in the exchange bias literature in the context of the energy and length scale hierarchy classification. The degree of success of the proposed classification scheme of exchange bias systems for analysis of the experimental data will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:42PM |
S2.00002: Using Exchange Bias to Control Magnetic Vortices Invited Speaker: Spintronics has spurred the interest in patterned magnetic nanostructures both for fundamental reasons and due to their applications. Moreover, exchange bias (i.e., the exchange coupling between ferromagnetic, FM, and antiferromagnetic, AFM, materials) constitutes an essential part of many spintronics devices (e.g., read heads or MRAM). However, exchange bias in nanostructures has not been extensively studied [1]. We have investgated the magnetic behavior of exchange coupled ferromagnetic (Permalloy) -- antiferromagnetic (IrMn) lithographed dots by magneto optic Kerr effect, magnetic force microscopy and micromagnetic simulations. We have recently demonstrated that vortex formation remains the reversal mode in these FM-AFM dots although the loops are shifted along the field axis [2]. In fact, the actual magnetization reversal mechanism (coherent rotation vs. vortex formation) is angle dependent [2] and can be controlled by varying the strength of the exchange bias or nucleation field [3]. Moreover, if the system is field cooled in an unsaturated state (i.e., using small fields) a new type of asymmetric hysteresis loop is found. This asymmetry is characterized by the appearance of curved, reversible, central sections in the hysteresis loops, with non-zero remanent magnetization [4]. The origin of the asymmetric loop shape is ascribed to the imprint [5] of displaced magnetic vortices in the AFM during the cooling process, which pin the vortex core away from the dot center. \\ {[}1] J. Nogu\'{e}s et al., Phys. Rep. \textbf{422, }65 (2005). \\ {[}2] J. Sort et al., Phys. Rev. Lett. \textbf{95}, 067201 (2005). \\ {[}3] J. Sort et al., Appl. Phys. Lett. \textbf{88}, 042502 (2006). \\ {[}4] J. Sort et al., Phys. Rev. Lett. \textbf{97, }067201 (2006). \\ {[}5] S. Br\"{u}ck et al. Adv. Mater. \textbf{17}, 2978 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 4:18PM |
S2.00003: Induced magnetic structure in exchange-coupled ferro-/antiferromagnet thin films Invited Speaker: The most prominent feature observed in exchange-coupled ferromagnetic/ antiferromagnetic (FM/AF) bilayers is the so-called exchange bias field ($H_{EB})$, i.e. the shift of the hysteresis loop along the magnetic field axis. However the exchange bias phenomenon can induce other interesting effects on the FM. In this talk we show two methods to establish a bi-domain state in the FM, due to the coexistence of domains with opposite sign of $H_{EB}$ [1-3]. Magneto-optical, polarized neutron and soft X-ray measurements show that this lateral structure becomes more complex for low magnetocrystalline anisotropy materials where a spin depth profile is created in the FM due to the exchange coupling with the AF [4-6]. The internal magnetic structure in the AF and its role on exchange bias has also been investigated using FM/AF/FM trilayers. These studies demonstrate that the bulk spin configuration in the AF plays a crucial role in the pinning of uncompensated spins at the interface thus determining the $H_{EB}$ . Supported by the US-DOE, European Marie-Curie-OIF and the Alfred P. Sloan Foundation. [1] O. Petracic et al. Appl. Phys. Lett. 87, 222509 (2005) [2] I. V. Roshchin et al. Europhys. Lett. 71, 297 (2005) [3] J. Olamit et al. Phys. Rev. B 72, 012408 (2005) [4] R. Morales et al. Appl. Phys. Lett. 89, 072504 (2006) [5] S. Roy et al. Phys. Rev. Lett. 95, 047201 (2005) [6] Z-P. Li et al. Phys. Rev. Lett. 96, 217205 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:54PM |
S2.00004: Understanding Thermal Activation Processes in Exchange Bias Systems Invited Speaker: The phenomenon of exchange bias has been of major scientific interest and technological importance since the 1980s following its discovery by Meiklejohn and Bean in 1956 [1,2]. Following initial seminal work by Fulcomer and Charap [3] it has recently become clear that a major contribution to the phenomena of exchange bias derives from the fact that the grains in the antiferromagnetic (AF) layer are capable of thermally activated reorientation due to the exchange field from the ferromagnetic (F) layer. In this work careful measurement protocols will be presented that enable the thermal activation process to be analysed in considerable detail. More recently Hoffman [4] has described a spin reorientation process that occurs after the AF layer is set which leads to a large shift in the forward going hysteresis loop on the first reversal of the F layer. This effect, coupled to the thermal activation process, gives rise to the phenomenon of training whereby the loop progressively shifts from its original set direction towards the origin. Lastly we have observed a spin freezing phenomena at the interface that can be induced by either temperature or applied field which results in a systematic variation of the exchange bias. We interpret this effect as being due to paramagnetic like spins at the interface whose ordering leads to a significant increase in the overall value of the exchange bias. Thus we show that exchange bias is a complex convolution of at least three distinct effects, all of which will be described in detail. This explains why single theories of how this effect arises have been so unsuccessful during the last 50 years. \newline \newline [1] Meiklejohn and Bean: Physical Review vol.102 p.1413 (1956) \newline [2] Nogues and Schuller: Journal of Magnetism and Magnetic Materials vol.192 p.203 (1999) \newline [3] Fulcomer and Charap: Journal of Applied Physics vol.43 p.4190 (1972) \newline [4] Hoffmann: Physical Review Letters vol.93 p.097203 (2004) [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:30PM |
S2.00005: Electric and Magnetic Field control of Exchange Bias Invited Speaker: Exchange bias (EB) and its accompanying training effect are fundamental interface phenomena in coupled magnetic thin films with significant impact in spintronic applications. Here we report on the electric field control of the EB in innovative antiferromagnetic (AF)/ferromagnetic (FM) heterostructures and the magnetic field control of the EB training effect in exchange coupled all FM bilayer systems. Electric control of the EB is realized in Cr$_{2}$O$_{3}$ (111)/(Co/Pt)$_{3}$ heterostructures by taking advantage of the magnetoelectric (ME) properties of the AF pinning layer [1]. An electric field induces excess magnetization in the ME Cr$_{2}$O$_{3}$ film. Exchange coupling between the induced magnetization and the CoPt thin film gives rise to electrically controlled perpendicular EB. Bias fields are measured by means of AGFM, SQUID-magnetometry and polar Kerr-rotation. Electrically controlled EB is proposed for novel spintronic applications such as pure voltage control of magnetic configurations in spin valve-type architectures. The latter provide an attractive alternative to current-induced switching of the magnetization [2]. In addition, training of the EB effect is studied in novel all FM heterostructures of exchange coupled soft and hard FM thin films [3]. FM bilayers show remarkable analogies to the conventional AF/FM EB systems. Not only do they exhibit a tunable EB effect, they also show a distinct training behavior upon cycling the soft layer through consecutive hysteresis loops. In contrast to conventional EB systems, all FM bilayers allow the observation of training induced changes in the bias-setting hard layer by means of simple magnetometry. Initialization of the EB is achieved at constant temperature exclusively by means of magnetic fields. Our experiments show unambiguously that EB training is driven by deviations from the equilibrium spin configuration of the pinning layer. The experimental data show excellent agreement with our theoretical predictions including the subtle dynamic enhancement of the EB training which evolves with increasing field sweep rates. \newline \newline [1] P. Borisov, A. Hochstrat, X. Chen, W. Kleemann, and Ch. Binek, Phys. Rev. Lett. 94, 117203 (2005). \newline [2] Ch. Binek, B.Doudin, J. Phys. Condens. Matter 17, L39 (2005). \newline [3] Ch. Binek, S. Polisetty, Xi He and A. Berger, Phys. Rev. Lett. 96, 067201 (2006). [Preview Abstract] |
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