Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H14: Focus Session: Exchange Bias |
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Sponsoring Units: GMAG DMP Chair: Kai Liu, University of California, Davis Room: Colorado Convention Center Korbel 4D |
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H14.00001: Identification and separation of two distinct contributions to the training effect in polycrystalline exchange biased Co/FeMn bilayers M.K. Chan, J.S. Parker, P.A. Crowell, C. Leighton We show that polycrystalline Co/FeMn bilayers display two distinct forms of training and qualitatively explain their FeMn thickness ($t_{FeMn})$ dependence. The two types of training can be identified and separated via their distinctive field cycle and $t_{FeMn}$ dependences, and the degree of asymmetry between the ascending and descending branches of the hysteresis loops. Samples were prepared via UHV dc magnetron sputter deposition onto Si/SiO$_{2}$ substrates at room temperature. The Co thickness was 6 nm while $t_{FeMn}$ was varied between 0 and 20 nm. Upon field cooling, hysteresis loops display two distinct forms of training. The first is single cycle training accompanied by strong reversal asymmetry. The amount of training and degree of asymmetry are correlated and strongly dependent on $t_{FeMn}$. This effect is due to the biaxial anisotropy of the antiferromagnet$^{1}$. Subsequent loops are symmetric and exhibit multi-loop training that follows a $n^{-1/2}$ dependence, where $n$ is the loop number$^{2}$. This effect is attributed to thermally activated depinning of weakly coupled uncompensated interfacial antiferromagnet spins. 1. A. Hoffmann, Phys. Rev. Lett. \textbf{93} 97203, 2004. 2. D. Paccard, C. Schlenker, O. Massenet, R. Montmory, A. Yelon, Phys. Stat. Sol.16, 301, 1966. This work was supported by the NSF MRSEC program and NSF DMR 04-06029. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H14.00002: Evolution of exchange coupling in epitaxial Fe/SmCo during partial SmCo demagnetization* J. E. Davies, Kai Liu, J. S. Jiang, S. D. Bader, E. E. Fullerton In epitaxial Fe/SmCo, irreversible magnetization reversal occurs once the SmCo hard layer starts switching [1,2]. We investigated the evolution of the interlayer exchange coupling as the hard SmCo layer demagnetizes using the first and second order reversal curve methods (FORC and SORC, respectively). The FORC distribution [2,3] shows two distinct features during the hard layer reversal: a single positive peak at high fields and a negative/positive pair of features at low applied fields. The single peak corresponds to the reversal of the hard SmCo layer and yields a SmCo switching field distribution. The negative/positive pair is due to reversal of the soft Fe layer and its position traces the evolving interlayer exchange coupling strength. SORC measurements show that the Fe layer remains mostly reversible during SmCo demagnetization and allow direct determination of the exchange field. The measured exchange field evolution agrees with an analytical model [4]. [1].Fullerton, et al., PRB 58, 12193 (1998). [2].Davies, et al, APL 86, 262503 (2005). [3].Davies, et al, PRB 70, 224434 (2004); PRB 72, 134419 (2005). [4].Vlasko-Vlasov et al. PRL 86, 4386 (2001). * Supported by ACS-PRF, Alfred P. Sloan Foundation and DOE (BES-MS contract {\#}DE-AC02-06CH11357) [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H14.00003: Magnetic configuration in antiferromagnetically coupled [Co/Pd]$_{15}$/TbFeCo with out-of-plane anisotropy S.M. Watson, J.A. Borchers, T. Hauet, S. Mangin, E.E. Fullerton We have used Polarized Neutron Reflectometry (PNR) to investigate the magnetic properties of an exchange-coupled bilayer system with out-of-plane magnetization. These systems show potential for increasing storage densities in magnetic recording media. Magnetization measurements suggest the formation of an in-plane domain wall. The magnetic configuration inside such systems results from the competition between the magnetic field, short-range exchange coupling, and long-range dipolar interactions. This study involved [Co(0.5 nm)/Pd(X)]$_{15}$/TbFeCo(25 nm) (X=3.5, 5, 7 nm) structures. Both the [Co/Pd] and TbFeCo exhibit strong out-of-plane anisotropy and are exchange coupled antiferromagnetically due to the TbFeCo alloy concentration. The magnitude of exchange coupling between the Co layers may be modified by changing the Pd thickness. PNR measurements, which are sensitive to the in-plane component of the magnetization only, confirm the formation of an in-plane domain wall that varies with the exchange stiffness inside the Co/Pd and with the field. The extent of the in-plane domain wall decreases with increasing applied field for the films with Pd thicknesses of 7 and 5 nm whereas the behaviour of the thinnest film (Pd = 3.5nm) suggests the Co/Pd bilayers show no evidence of a domain wall. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H14.00004: Positive Exchange Bias in GdFe/NiCoO Thin Films Justin Olamit, Kai Liu Thin films of GdFe/NiCoO are one of the few systems that exhibit positive exchange bias [1-4]. In this study, we show that the positive bias in Gd$_{x}$Fe$_{1-x}$/NiCoO is sensitive to the GdFe composition and the field cooling sequence. In particular, the hysteresis loops are often bifurcated due to the existence of multiple phases: a low anisotropy phase with a single reversal in small fields and a higher anisotropy phase with a single or double loop, depending on the GdFe stoichiometry. In Fe-rich samples, increasing the cooling field causes the low anisotropy phase to shift from negative to positive bias and the double-loop high anisotropy phase to shift toward negative bias. In Gd-rich samples, the low anisotropy phase is always positively biased and the single-loop high anisotropy phase is always negatively biased for all cooling field strengths. These behaviors are a result of the parallel and antiparallel couplings between different magnetic phases of GdFe with the NiCoO layer. [1] J. Nogues\textit{, et al.}, Phys. Rev. Lett. \textbf{76}, 4624 (1996). [2] S. Mangin\textit{, et al.}, Phys. Rev. B \textbf{68}, 140404 (2003). [3] X. Ke\textit{, et al.}, Appl. Phys. Lett. \textbf{84}, 5458 (2004). [4] D. Z. Yang\textit{, et al.}, Phys. Rev. B \textbf{71}, 144417 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H14.00005: Exchange bias training effect in Co/CoO heterostructures with variation of the ferromagnetic film thickness Srinivas Polisetty, Tathagata Mukherjee, Sarbeswar Sahoo, Christian Binek The exchange bias (EB) training effect is studied in a Co/CoO heterostructure using low temperature longitudinal Kerr rotation. After field cooling the sample to below the N\'{e}el temperature of CoO the EB training effect manifests itself by a decrease of the EB field upon cycling the Co film through consecutive hysteresis loops. We explore the temperature dependence of the training effect and its dependence on the Co-thickness, t$_{Co}$. The latter is studied by locally probing the EB in a wedge Co/CoO system. The gradient of the Co film thickness allows to measure local t$_{Co}$-dependences in a range of $\Delta $t$_{Co}$ = 23.6nm varying over the substrate length of 8.5mm. The Co wedge is prepared by MBE taking advantage of the steep decrease of the Co flux when leaving the center of the Co beam. A wedge angle of $1.6\times 10^{-4}\;^{\circ}$is revealed by local small angle X-ray reflectivity. We compare the measured t$_{Co}$-dependence with our phenomenological theory predicting a t$_{Co}^{2}$-increase of the leading fitting parameter. This behavior is clearly distinguishable from the 1/t$_{Co}$ -decrease of the equilibrium EB field. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H14.00006: Tilted cores of magnetic vortices due to exchange bias K. Y. Guslienko, A. Hoffmann Recently, the influence of exchange bias on magnetic vortices has been investigated experimentally.\footnote{J. Sort {\em et al.}, Phys. Rev. Lett. \textbf{95}, 067201 (2005); Phys. Rev. Lett. \textbf{97}, 067201 (2006).} By generalizing the rigid vortex model\footnote{K. Y. Guslienko et al., Phys. Rev. B \textbf{65}, 024414 (2002).} we develop an analytic model of the magnetization reversal in an exchange-biased ferromagnetic dot. We account explicitly for a non-uniformity of the magnetization reversal mode along the direction perpendicular to the layers. This non-uniformity allows the vortex core position to vary throughout the thickness of the ferromagnetic layer. We show that the geometrical confinement in combination with the interface exchange field leads to new asymmetries of the hysteresis loops.\footnote{K. Y. Guslienko and A. Hoffmann, Phys. Rev. Lett. \textbf{97}, 107203 (2006)} Namely, the critical fields for vortex nucleation and annihilation respond differently to the interfacial exchange bias, resulting in an asymmetry of the irreversible parts of the hysteresis loops in addition to the overall shift due to the exchange bias. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H14.00007: Controlling the sign of the exchange bias in Fe$_{x}$Ni$_{1-x}$F$_2$/Co and Fe$_{x}$Zn$_{1-x}$F$_2$/Co bilayers Miyeon Cheon, Zhongyuan Liu, Hongtao Shi, David Lederman A correlation between the sign of the exchange bias and the sign of the uncompensated magnetization was observed in the Fe$_ {x}$Ni$_{1-x}$F$_2$/Co bilayer system. Due to this correlation and the fact that the uncompensated magnetization was reversed at high fields at low temperatures, the sign of the exchange bias was controlled by controlling the sign of the uncompensated magnetization in this system. The dilute antiferromagnet Fe$_{0.36}$Zn$_{0.64}$F$_2$/Co system, which was previously shown to also have a large uncompensated magnetization, also showed the same effect but at slightly higher temperatures. Using a micromagnetic simulation program (OOMMF) and comparing to the experimental data, the micromagnetic constants were obtained. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H14.00008: Dynamic enhancement of the exchange bias training effect Sarbeswar Sahoo, Andreas Berger, Srinivas Polisetty, Christian Binek Exchange bias in coupled magnetic films and its accompanying training effect are fundamental interface phenomena which impact spintronic applications. Training is referred to as a gradual change of the bias field, which evolves upon cycling the soft layer through consecutive hysteresis loops. We report on its dynamic enhancement in exchange coupled bilayers of soft and hard ferromagnetic materials. Dynamic effects are induced with increasing sweep rate of the applied magnetic field from quasi-static to the fully dynamic range. A dynamically generalized theory based on triggered and partially truncated relaxation is in excellent agreement with the data. Remarkable universality of our theoretical approach is evidenced when applying the approach to the dynamic training effect of a conventional exchange bias system involving an antiferromagnetic pinning layer. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H14.00009: Exchange bias inducing temperature Alexey Dobrynin, Ruslan Prozorov Characteristic temperatures governing behavior of ferromagnetic - antiferromangetic (F-AF) heterostructures are discussed. The inducing temperature, $T_{ind}$, at which the easy direction of magnetization is established, is in general case different from the maximum temperature at which exchange bias may exist $T_B$ (blocking temperature) and the N\'eel temperature of the antiferromagnet $T_N$. The case of $T_{ind} < T_N$ suggests presence of a frustrated interfacial AF spin structure in the system, otherwise $T_{ind} = T_N$. If $T_B = T_{ind} < T_N$, the interfacial F-AF interactions are stronger than that between the interfacial AF spins and the rest of the AF part, assuming rotation of those spins during the magnetization reversal. The exchange bias value in this case is determined by the latter AF exchange coupling. In the case of $T_B < T_{ind} < T_N$, the interfacial AF spins stay stable, and the exchange bias field is determined by the interfacial F-AF coupling. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H14.00010: FORC Study of Magnetization Reversal Asymmetry in Fe/FeF$_{2}$ Exchange Biased Thin Films Justin Olamit, Kai Liu, Zhi-Pan Li, Ivan K. Schuller Asymmetric magnetization reversal in exchange biased ferromagnet/antiferromagnet (FM/AF) thin films have attracted much interest. Recent work has shown that the asymmetry in Ni/FeF$_{2}$ films is due to local incomplete domain walls in the FM parallel to the interface [1, 2]. We have investigated reversal asymmetry in Fe/epitaxial-FeF$_{2}$ using a First Order Reversal Curve (FORC) technique [3]. The major hysteresis loop is asymmetrical. Along the decreasing-field sweep of the hysteresis loop, FORC measurements show that the nucleation of domain structures occurs gradually while the domain annihilations are abrupt. However, along the increasing-field reversal, the domain nucleations are abrupt and the annihilations occur gradually. Rotating the AF easy axis away from the applied field shows that the nucleation and annihilation field distributions also have different angular dependencies along the field sweeps. These different distributions lead to the asymmetry seen in the shape of the major loop. [1] Li, \textit{et al}., PRL \textbf{96}, 217205 (2006). [2] Morales, \textit{et al}., APL \textbf{89}, 072504 (2006). [3] Davies, \textit{et al}., PRB \textbf{70}, 224434 (2004)~; APL \textbf{86,} 262503 (2005); PRB \textbf{72}, 134419 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H14.00011: Magnetic coupling and training effects in Co/NiO/[Co/Pt] structures with orthogonal easy axes S. Adenwalla, A. Baruth In an attempt to broaden our understanding of the unexpected oscillatory coupling seen in perpendicularly magnetized [Co/Pt]/NiO[Co/Pt]$^{1}$ samples we have investigated a series of Co/NiO/[Co/Pt] samples in which the magnetization lies in-plane and perpendicular to plane for the Co and [Co/Pt] layers respectively. Although no preferred coupling is expected, we find a coupling that depends on a variety of parameters including strength of an in-plane magnetic field pulse (the ``set field''), the number of cycles (the training effect) and the NiO thickness. The strength of coupling, as measured by a shift in the Co in-plane hysteresis loop, is directly proportional to the in-plane set field, an effect of the nonzero remanence of the [Co/Pt] layer. On training, the coupling strength drops abruptly by a factor of nearly 3 on the first cycle and drops more slowly thereafter. We attribute this to the presence of domains in the [Co/Pt] layer. [1] A. Baruth et al. Phys. Rev. B \textbf{74}, 054419 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H14.00012: Domain overlap in antiferromagnetically coupled [Co/Pt]/NiO/[Co/Pt] multilayers A. Baruth, L. Yuan, J.D. Burton, K. Janicka, E.Y. Tsymbal, S.H. Liou, S. Adenwalla Antiferromagnetically coupled magnetic thin films with perpendicular anisotropy exhibit domain overlap regions originating from magnetostatic stray fields localized in the vicinity of the domain walls. Using high resolution magnetic force microscopy we investigate these overlap regions in [Co/Pt]/NiO/[Co/Pt] multilayers as a function of the antiferromagnetic interlayer exchange coupling between the two Co/Pt stacks. Our results agree both qualitatively and quantitatively with a simple model that looks at the external fields near the domain wall regions and their magnetostatic interaction. This model gives an overlap $\delta $=8M$_{top}$M$_{bottom}$t$^{2}$/J$_{IEC}$, where M$_{top}$ and M$_{bottom}$ are the respective saturation magnetizations, t is the thickness, and J$_{IEC}$ is the coupling strength. There is excellent quantitative agreement between the data and the model, which is substantiated by independent magnetization and thickness measurements which agree within 5{\%} to those obtained by the 2 parameter fit. Details can be found in [1]. This research was supported by NSF (grants Nos. MRSEC DMR-0213808 and DMR-0203359). [1] A. Baruth et al. Appl. Phys. Lett. \textbf{89}, 202505 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H14.00013: Probing the exchange-biased system of Co/ $\gamma $-Fe$_{50}$Mn$_{50}$ using a large scale micromagnetic model. Jyotirmoy Saha, Randall Victora We have built a micromagnetic model to study the exchange interaction between Co (a ferromagnet) and $\gamma $-Fe$_{50}$Mn$_{50}$ (an antiferromagnet) that has (111) texturing. The antiferromagnet thickness dependence of exchange bias and enhanced coercivity at two different temperatures were obtained and comparison to experiments showed reasonable agreement. Another aspect of this exchange interaction that we looked into was the grain size dependence of exchange bias. Here we obtain a linear relationship between exchange bias and inverse grain size for nearly thermally stable antiferromagnetic grains. This is the trend that is both predicted by theory and produced in experiments. The plot of exchange bias vs. in-plane field cooling angles exhibits a cosine like symmetry that is set by the thermal stability of the antiferromagnetic grains. Most of the values relating to material characteristics used in our model are taken from literature. One input parameter in our model that needs refinement either from experiments or from band structure calculations is the maximum anisotropy value of the antiferromagnet and its distribution amongst the antiferromagnetic grains. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H14.00014: Exchange bias measurement methodologies and the role of hysteresis loop asymmetry Ondrej Hovorka, Andreas Berger, Gary Friedman The phenomenon of exchange bias refers to the observation of a hysteresis loop field shift in ferromagnetic-antiferromagnetic (AFM) compound structures [1] and in all-ferromagnetic bilayer systems [2]. The exchange bias effect is typically quantified by determining the sum of the coercive fields from a hysteresis loop. Such a two-point (TP) measurement is, however, unambiguous only for time reversal symmetric hysteresis loops [3]. To account for the loop asymmetry, frequently observed in experiments, we recently proposed an alternative characterization scheme, called the center of mass method (CM) [3]. In the present study, we correlate the differences between TP and CM methods and the hysteresis loop asymmetry, using measurement data obtained from the all-ferromagnetic bilayer system, which are supported by model calculation results. We find the loop asymmetry to be a reliable indicator for the ambiguity of the conventional TP method. We will also discuss the applicability of the CM method to conventional AFM structures. [1] A. Berkowitz, K. Takano, J. Magn. Magn. Mater. 200, 552 (1999). [2] A. Berger et. al., Appl. Phys. Lett. 85, 1571 (2004). [3] O. Hovorka et. al., Appl. Phys. Lett. 89, 142513 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H14.00015: Ferrimagnetism in Gd-Ni bilayers Marta Marszalek, J. Jaworski, W.E. Evenson, A. Barth, F. Treubel, M. Albrecht, G. Schatz Physical properties of thin films play an increasingly important role in technical applications. With controlled growth and the production of layered systems, interesting and novel mechanical, optical, electrical and chemical characteristics can be obtained. Here we present studies of structural and magnetic propterties of Gd-Ni bilayers. Temperature-dependent SQUID magnetization measurements show antiferromagnetic coupling between Gd and Ni films, with compensation temperature determined for various bilayer structures. They were complemented by field-dependent magnetization measurements by SQUID and XMCD, revealing the typical switching behavior of an artificial ferrimagnet with two exchange-coupled layers. Structural investigations have been performed using STM, XRD and XRR to determine crystallinity and morphology of the system. [Preview Abstract] |
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