Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y4: New insight into Exchange Bias from Advanced Scattering Techniques |
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Sponsoring Units: GMAG Chair: Julie Borchers, National Institute of Standards and Technology Room: 306/307 |
Friday, March 20, 2009 8:00AM - 8:36AM |
Y4.00001: Neutron and X-Ray Scattering Studies of the Exchange Bias Problem Invited Speaker: Exchange Bias, i.e. the shift of the hysteresis loop of a ferromagnet in juxtaposition to an antiferromagnet, is a phenomenon that has been known for fifty years, and has already been put to wide commercial use in devices such as magnetic read-heads and other devices. Nevertheless a detailed understanding of the effect has proved to be fairly controversial, notwithstanding much research on this problem over the years, and the development of several alternative theoretical models. This is partly due to the necessity of understanding the details of the interactions and the magnetic structure across and in the vicinity of the interface between the ferromagnet and the antiferromagnet. The details of how interface roughness and other defects affect exchange bias and the details of how magnetic domains are established on both sides of the interface are still not well understood. Non-destructive probing of such buried interfaces is conveniently accomplished with neutron scattering or synchrotron X-ray techniques such as X-Ray Magnetic Circular Dichroism, Photoemission imaging or Resonant Magnetic X-ray Scattering, and these types of experiments have been increasingly employed over the last decade. We shall attempt to discuss what has been learned from such experiments and what crucial issues remain unresolved, with particular emphasis on recent studies of the Co/FeF$_2$ and Permalloy/CoO exchange bias bilayer systems. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 9:12AM |
Y4.00002: Measuring interfacial magnetic configurations with Polarized Neutron Reflectometry Invited Speaker: Polarized neutron reflectivity (PNR) is ideally suited for imaging both vertical structural and magnetic variations in the complex magnetic multilayers [1]. During the talk will be described particularly how this technique allows obtaining the magnetic depth-profile of exchange-coupled bilayer. For instance, Gd40Fe60/ Tb12Fe88 is a model system to study exchange-bias phenomena origin in anti-ferromagnetically coupled AF/FM system, like FeF2/Fe. In these systems, unusual properties are observed such as a transition from positive to negative exchange bias field HE as the cooling field Hcf is swept from small to large positive value [2]. Combining complementary techniques that are macroscopic magnetization measurements and PNR, we have demonstrated that the above properties, e.g. the cooling field dependence of HE, come from an interfacial domain wall (iDW) frozen in the TbFe as the sample is cooled down under a field [3, 4]. Moreover, PNR measurements have recently revealed that the frozen iDW is metastable and that the exchange bias training effect in TbFe/GdFe results from the thermally assisted relaxation of the iDW, with field cycling [4, 5]. Overall, PNR studies concerning the TbFe/GdFe have brought strong insights into the exchange bias mechanisms in exchange coupled hard/soft systems with in-plane anisotropy. However we have demonstrated as well that this powerful technique can be applied to systems with perpendicular magnetic anisotropy (PMA). Although, in that case, the perpendicular moments are parallel to the scattering vector and do not give rise to scattering via the neutron selection rules, we have used a unconventional geometry to obtain a depth-dependent magnetic profile of a PMA exchange-coupled system. Specifically, we have characterized antiferromagnetically-coupled, TbFeCo/[Co/Pd] multilayers [6]. \\[4pt] [1] K.V. O'Donovan et al., Phys. Rev. Lett. 88, 067201 (2002). \\[0pt] [2] J. Nogues and al. Phys. Rev. Lett. 76, 4624 (1996) \\[0pt] [3] Y. Henry et al., Phys. Rev. B 73, 134420 (2006) \\[0pt] [4] T. Hauet et al., Phys. Rev. Lett. 96, 067207 (2006) \\[0pt] [5] T. Hauet et al., Appl. Phys. Lett. 91, 022505 (2007) \\[0pt] [6] S. Watson et al., Appl. Phys. Lett. 92, 202507 (2008) [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:48AM |
Y4.00003: Inelastic neutron scattering studies of exchange biased core-shell nanoparticles Invited Speaker: Inelastic neutron scattering (INS) measurements of nanoparticle systems are very few, and we report here the first investigation of exchange biased core-shell nanoparticles. We present a study of spin dynamics in core-shell Co/CoO nanoparticles, which display an exchange bias field of 0.6T. We have used INS measurements to determine how the magnetic dynamics are affected, both by the onset of antiferromagnetic (AF) order at 250K and the subsequent onset of the exchange bias effect at 200K. At the highest temperatures, the scattering consists of two quasielastic peaks. The narrower peak is ascribed to superparamagnetic reorientations of the Co core. The broader peak originates with moments in the CoO shell. Surprisingly, their dynamics speed up with decreased temperature, suggesting that the CoO shell absorbs some of the magnetic energy of the core as exchange blocking is approached. Below 200K, the scattering is dominated by an inelastic peak at $\sim $3meV. The integrated spin wave intensity grows when the temperature is reduced below 200K, reaches a maximum near 150K, and nearly vanishes at the low temperatures. We attribute this peak to AF spin waves in the CoO shell, and their lack of dispersion and overall energy scale are consistent with predictions for low energy spin waves in bulk CoO [3,4]. It is remarkable that bulk-like spin wave behavior is observed in the CoO shell, which is only 4 nm thick. \\[3pt] References \\[0pt] [1] M. Feygenson et al., in preparation \\[0pt] [2] Inderhees et al., Phys. Rev. Lett., 101, 117202 (2008) \\[0pt] [3] Daniel et al., Phys. Rev. 177, 932 (1969) \\[0pt] [4] Sakurai et al., Phys. Rev. 167, 510 (1968) [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:24AM |
Y4.00004: Exchange bias in core/shell magnetic nanoparticles: experimental results and numerical simulations Invited Speaker: In this talk, we will review some of the main experimental observations related to the occurrence of exchange bias in magnetic systems, focusing the attention on the phenomenology appearing in nanoparticles with core/shell structure as compared to thin film bilayers [1]. The main open questions posed by the experimental observations will be discussed and contrasted to existing theories and models for exchange bias [1]. We will also present some recent numerical simulations [2-4] based on a simple model of a core/shell nanoparticle, showing evidence that the magnetic order of interfacial spins accounts for most of the experimental observations. Finally, we will discuss the occurrence of exchange bias on laser-ablated granular thin films composed of Co nanoparticles embedded in amorphous zirconia matrix [5]. The deposition method allows controlling the degree of oxidation of the Co particles by tuning the oxygen pressure at the vacuum chamber. The nature of the nanoparticles embedded in the nonmagnetic matrix may be monitored from metallic, ferromagnetic (FM) Co to antiferromagnetic (AFM) CoO$_{x}$, with a FM/AFM intermediate regime for which the percentage of the AFM phase can be increased at the expense of the FM phase, leading to the occurrence of exchange bias in particles of about 2 nm in size. This is a model system to study some of the features of exchange bias in nanoparticles, such as particle size dependence, induced exchange anisotropy on the FM leading to high irreversible hysteresis loops, and blocking of the AFM clusters due to proximity to the FM phase. \\[4pt] [1] For a recent review see, for example, ``Exchange bias phenomenology and models of core/shell nanoparticles''; Iglesias, O.; Labarta, A.; and Batlle, X. Journal of Nanoscience and Nanotechnology \textbf{8}, 2761 (2008). \\[0pt] [2] ``Microscopic origin of exchange bias in core/shell nanoparticles''; Iglesias, O.; Batlle, X.; Labarta, A.; Physical Review B\textbf{ 72}, 212401 (2005). \\[0pt] [3] ``Modelling exchange bias in core/shell nanoparticles''; Iglesias, O.; Batlle, X.; Labarta, A., Journal of Physics-Condensed Matter \textbf{19}, 406232 (2007). \\[0pt] [4] ``Particle size and cooling field dependence of exchange bias in core/shell magnetic nanoparticles''; Iglesias, O.; Batlle, X.; Labarta, A.; Journal of Physics D: Applied Physics \textbf{41}, 134010 (2008). \\[0pt] [5] ``Exchange coupling in Co-CoO$_{x}$ nanoparticles in zirconia matrix''; M. Kovylina, M. Garcia del Muro, Z. Konstantinovic, O. Iglesias, M. Varela, A. Labarta and X. Batlle (submitted). [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 11:00AM |
Y4.00005: Small-Angle Neutron Scattering Studies of Magnetic Correlation Lengths in Nanoparticle Assemblies Invited Speaker: Small-angle neutron scattering (SANS) measurements of ordered arrays of surfactant-coated magnetic nanoparticle reveal characteristic length scales associated with interparticle and intraparticle magnetic ordering. The high degree of uniformity in the monodisperse nanoparticle size and spacing leads to a pronounced diffraction peak and allows for a straightforward determination of these length scales [1]. There are notable differences in these length scales depending on the particle moment, which depends on the material (Fe, Co, Fe3O4) and diameter, and also on whether the metal particle core is surrounded by an oxide shell. For 8.5 nm particles containing an Fe core and thick Fe3O4 shell, evidence of a spin flop phase is seen in the magnetite shell when a field is applied , but not when the shell thickness is $\sim $0.5 nm [2]. 8.0 nm particles with an e-Co core and 0.75 nm CoO shell show no exchange bias effects while similar particles with a 2 nm thick shell so significant training effects below 90 K. Polarized SANS studied of 7 nm Fe3O4 nanoparticle assemblies show the ability to resolve the magnetization components in 3D. \\[4pt] [1] M. Sachan, C. Bonnoit, S. A. Majetich, Y. Ijiri, P. O. Mensah-Bonsu, J. A. Borchers, and J. J. Rhyne, \textit{Appl. Phys. Lett. }\textbf{92}, 152503 (2008). \\[0pt] [2] Yumi Ijiri, Christopher V. Kelly, Julie A. Borchers, James J. Rhyne, Dorothy F. Farrell, Sara A. Majetich, \textit{Appl. Phys. Lett.} \textbf{86}, 243102-243104 (2005). \\[0pt] [3] K. L. Krycka, R. Booth, J. A. Borchers, W. C. Chen, C. Conlon, T. Gentile, C. Hogg, Y. Ijiri, M. Laver, B. B. Maranville, S. A. Majetich, J. Rhyne, and S. M. Watson, Physica B (submitted). [Preview Abstract] |
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