Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V16: Focus Session: Magnetic Nanostructures, Exchange Coupled System |
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Sponsoring Units: DMP GMAG Chair: Rafael Morales, UPV/EHU, Spain Room: D173 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V16.00001: Mu metal exchage bias Priyanga Jayathilaka, Scott Campbell, Casey Miller The exchange bias of the soft ferromagnet mu-metal, Ni77Fe14Cu5Mo4, with the metallic antiferromagnet Fe50Mn50 has been studied. Two series of multilayer heterostructures were grown with (111) texture induced by different buffer layer materials: Cu(300 A)/Ni77Fe14Cu5Mo4(200 or 400 A)/Fe50Mn50 (100 A)/Cu(300 A) and Ta(50 A)/Ni77Fe14Cu5Mo4(60--400 A)/Fe50Mn50(150 A); control samples were grown without Fe50Mn50. The samples have a clear unidirectional anisotropy induced by depositing in a magnetic field, the exchange bias magnitude is inversely proportional to the mu-metal thickness, and the interfacial coupling energy of 0.045 erg/cm2 agrees with previous results for FeMn antiferromagnets. While the Cu-buffered samples reveal a significant increase in coercivity and saturation field when exchange biased, the Ta-buffered samples retain the soft magnetic properties of the mu-metal simultaneously with the exchange bias. The ability to preserve soft ferromagnetic behavior in an exchange biased heterostructure may be useful for device and sensing applications. [Preview Abstract] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V16.00002: Intrinsic Exchange Bias and Origin of Uncompensated Magnetization in FeF$_{2}$ Igor V. Roshchin, Karie E. Badgley, K.D. Belashchenko, M. Zhernenkov, M.R. Fitzsimmons, Ivan K. Schuller After more than 50 years since the discovery of Exchange Bias, its microscopic mechanism remains unknown. Several experimental findings demonstrate and many models agree that uncompensated magnetization (UM) in the antiferromagnet (AF) plays an important role in exchange bias. However, the origin of the UM is unknown. Magnetometry and polarized neutron reflectometry (PNR) measurements indicate that the UM is present even in the AF-only, (110)-FeF$_{2}$ grown on MgF$_{2}$, samples, and the PNR reveals the spatial distribution of the UM. Exchange bias in the AF-only sample is reported. Coupling of the UM to the bulk antiferromagnetic order parameter is supported by several experimental results, including high value of exchange bias field, its temperature dependence and the absence of training effect. We will discuss the origin of the UM based on general symmetry arguments. [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V16.00003: Tuning the Magnetic Properties in Exchange Coupled FeO/Fe$_{3}$O$_{4 }$Core-Shell Nanoparticles Natalie Frey Huls, Xiaolian Sun, Shouheng Sun Chemically synthesized FeO with a native oxide shell has recently received attention due to the large exchange bias effects observed in this system. The magnetic properties reported thus far have been highly dependent upon the aging affects of the system given the vulnerability of FeO to further oxidation resulting in degradation of the exchange bias effects. We report on the magnetic properties of FeO nanoparticles chemically synthesized to form several base diameters (10 nm, 20 nm, and 30 nm) which have each been annealed at various temperatures to obtain a variety of core/shell FeO/Fe$_{3}$O$_{4}$ size ratios. This controlled oxidation method has also given excellent chemical stability to the particles. XRD analysis confirms the existence of polycrystalline phases of FeO and Fe$_{3}$O$_{4}$, and magnetometry experiments reveal the existence of large exchange bias (up to 230 mT) as well as coercivity enhancements (up of 250 mT) which persist up the N\'{e}el temperature (which scales with core size). Other exchange coupling effects such as a large vertical shift in the field cooled hysteresis loops and asymmetric magnetization reversal are observed. Our results further advance the understanding of this exchange coupled system and imply that the properties can be chosen utilizing as-synthesized particle size and annealing temperatures. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V16.00004: Tuning exchange bias in Ni/FeF$_{2}$ heterostructures using antidot arrays Xavier Batlle, M. Kovylina, A. Labarta, R. Morales, J.E. Villegas, M. Erekhinsky, Ivan K. Schuller The transition from positive to negative exchange bias can be systematically tuned with antidot arrays artificially introduced into Ni/FeF$_{2}$ ferromagnetic (FM)/antiferromagnetic (AF) heterostructures. This is a consequence of the energy balance between the Zeeman coupling of the AF spins to the cooling field, and the AF exchange coupling at the FM/AF interface. The nanostructure plays a key role in the formation of pinned uncompensated spins in the AF: the antidot carving produces regions of locally pinned uncompensated spins throughout the antidot faces of the FeF$_{2}$ and these \textit{non} interfacial magnetic moments favor the onset of positive exchange bias at lower cooling fields, by increasing the Zeeman energy of AF domains and favoring the alignment with the latter. Those \textit{non} interfacial AF spins, and the pinned uncompensated interfacial AF spins responsible for the exchange bias (loop shift), align simultaneously with the cooling field since they belong to the same AF domain and become pinned below the N\'{e}el temperature. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V16.00005: Depth Profiles of Exchange Stiffness and Anisotropy in a Spring Magnet with Intermixed Interfaces Yaohua Liu, S.G.E. te Velthuis, J.S. Jiang, Y. Choi, S.D. Bader, A.A. Parizzi, H. Ambaye, V. Lauter With complementary studies of Polarized Neutron Reflectometry (PNR) and micromagnetic simulations, we determined the depth profiles of the intrinsic magnetic properties in an Fe/Sm-Co spring magnet with intermixed interfaces, including saturated magnetiztion, exchange Stiffness and magnetic anisotropy. We found that intermixed region at the Fe/Sm-Co interface is about 8 nm wide, where the magnetic properties change gradually. We compared the results to a model based on a simple mixture of the Fe phase and the Sm-Co phase, as determined from the chemical depth profile using x-ray and neutron reflectivities. In the intermixed region, the saturation magnetization is slightly lower than the value estimated from the model but the exchange stiffness is higher. The magnetic anisotropy is also lower than the expected value from the model. Therefore the intermixed interface yields superior exchange coupling between the Fe and the Sm-Co layers but at the cost of total magnetization. [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V16.00006: Geometric Structure of Magnetic Domains in CoPd/IrMn Multilayer Films Run Su, Sujoy Roy, Keoki Seu, Daniel Parks, Jimmy Kan, Eric Fullerton, Stephen Kevan Using coherent x-ray resonant scattering in a transmission geometry, we collected magnetic scattering signal from CoPd/IrMn exchange biased multilayer films. The incident photon energy was tuned to the Co L3 edge allowing the magnetic domain configuration in Q space to be probed. Rotational autocorrelation functions of the resulting speckle diffraction patterns manifest the local geometric character of domain structure in Q-dependent fashion. These results are compared to microscopic magnetic domain memory probed by cross correlating different patterns. The memory under two different cooling conditions, with saturating field or with zero field was investigated, as well as the dependence of memory on external parameters, such as applied magnetic field and temperature. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V16.00007: Correlation between bias fields and magnetoresistance in CoPt biased NiFe/Ta/NiFe heterosystems Yi Wang, Xi He, Tathagata Mukherjee, Sarbeswar Sahoo, Michael Fitzsimmons, Christian Binek Exchange coupled magnetic hard layer/soft layer (SL) thin films show SL biasing in close analogy to exchange bias systems with antiferromagnetic pinning. Here we study CoPt(35nm)/NiFe(450nm)/Ta(d)/NiFe(450nm) heterostructures with 0.7 $<$d$<$5nm. We use alternating gradient force magnetometry to measure the overall magnetization reversal and minor loop behavior. Magnetoresistance (MR) is measured by four-point methodology and modeled using magnetization data thus confirming the assumptions of uniform rotation of the top layer and exchange spring behavior of the pinned NiFe layer. In addition, Polarized Neutron Reflectometry (PNR) provides an independent data set for magnetization depth profiles. We compare and contrast results from our magnetometry and MR technique with PNR results. The objective of this comparison is to show that single-component magnetometry in concert with MR and modeling reveals the full vector and depth profile information of the distinct magnetization reversal mechanisms. Financial support by NSF through Career, MRSEC, DOE-OBES [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V16.00008: Growth and magnetism of highly (001)-oriented [Fe/Pt]$_{n}$/Pt films T.A. George, X.Z. Li, L. Yue, David J. Sellmyer Highly (001) textured non-epitaxial $L$1$_{0}$ FePt films have been fabricated on SiO$_{2}$ substrates by post-deposition annealing 11 nm magnetron sputtered multilayers of Fe and Pt with an additional overlayer of 1 nm Pt. An identical series of films was made without the thick Pt terminating layer for comparison. All films were post-deposition annealed at 600 \r{ }C for 300 s in a rapid thermal processor and show a high degree of chemical order. The ordered films without a Pt overlayer include a mixture of (001) and randomly oriented grains. In the samples with a Pt overlayer only the (00$l)$ peaks are visible, demonstrating an enormous enhancement in the degree of (001) texture. Structural analysis reveals a decrease in surface roughness from over 2 nm to less than 1 nm, elimination of voided regions, and an increase in average grain size from 50 to 150 nm with the inclusion of a Pt overlayer. Magnetic hysteresis loops show a high squareness ratio for Pt-overlayer samples with coercivities much smaller than their no-overlayer counterparts. The effects of Fe:Pt stoichiometry and bilayer thickness are investigated along with the involved grain-growth process. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V16.00009: Exchange bias and magnetic anisotropy in ultrathin iron films grown on (001) GaAs Kritsanu Tivakornsasithorn, Xinyu Liu, Malgorzata Dobrowolska, Jacek Furdyna Ultrathin iron films grown by MBE on GaAs substrates were studied by SQUID and by ferromagnetic resonance (FMR). Exchange bias (EB) was observed in this system at temperatures below 20 K, but disappeared at higher temperatures. The angular dependence of asymmetric hysteresis loops of the sample were understood as resulting from the coexistence of the cubic and uniaxial magnetic anisotropy fields of the Fe film and the EB field arising from a yet unidentified inter-layer between Fe and GaAs (possibly Fe2As). Magnetic anisotropy of the Fe films was investigated by FMR in a manner similar to that described by Aktas et al. [1]. By fitting the angular dependence of the FMR field we have obtained magnetic parameters of the sample, which are similar to those reported in Ref. [1]. However, the g- factor obtained from the fitting shows an unexpected anomalous increase in the low temperature range. Since this behavior occurs exactly in the range where EB appears, it is tempting to speculate that these two effects are causally related.\\[4pt] [1] B. Aktas et al., J. Appl. Phys. 102, 013912 (2007). [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V16.00010: Measuring Exchange Bias in Patterned Films using Ferromagnetic Resonance Rohan Adur, Inhee Lee, Yuri Obukhov, Christine Hamann, Jeffrey McCord, Denis V. Pelekhov, P. Chris Hammel Exchange bias exploits the exchange interaction at the interface between a~ferromagnet and an adjacent antiferromagnet to create a preferred orientation for the ferromagnet. He-ion bombardment has been used to create stripe-patterned films displaying anti-parallel exchange bias in adjacent stripes. As the width of these stripes approaches micron-size, magnetization reversal within individual stripes can be hindered by dipolar fields from magnetic charges at boundaries, making magnetometry measurements difficult~to interpret. Here we report Ferromagnetic Resonance measurements of the magnitudes of the two opposing exchange bias fields perpendicular to the stripe axis, the dipolar fields experienced by neighboring stripes, and we quantify the effect of ion irradiation on the saturation magnetization of the Ni$_{81}$Fe$_{19}$ films. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V16.00011: Scanning Probe Ferromagnetic Resonance Imaging of Stripe Patterned Exchange Bias IrMn-NiFe Film Using Nanoscale Confined Modes Inhee Lee, Rohan Adur, Christine Hamann, Yuri Obukhov, Jeffrey McCord, Denis Pelekhov, Bernd Buchner, Chris Hammel We report scanned probe Ferromagnetic Resonance (FMR) imaging of the spatially modulated internal exchange-bias field in the exchange coupled ferromagnet (FM)/antiferromagnet (AF) Ni$_{81}$Fe$_{19}$ /Ir$_{23}$Mn$_{77}$ bilayer material using Magnetic Resonance Force Microscopy (MRFM). The exchange bias is spatially modulated by ion beam irradiation into a periodic stripe pattern having 2 or 20 micron periods. Adjacent stripes have oppositely aligned exchange bias fields. Our new method of FMR imaging employs the locally confined FMR modes created by a strong, non-uniform probe tip field on the out-of-plane saturated Ni$_{81}$Fe$_{19}$ film. We image the spatial variation of the inhomogeneous internal field with spectroscopic precision clearly resolving two exchange bias regions. Analysis of the local magnetic properties and their transition at the boundary of two exchange bias regions will be presented. This work was supported by the U.S. Department of Energy through Grant No. DE-FG02-03ER46054. [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V16.00012: Large exchange bias after zero-field cooling from an unmagnetized state Lan Wang, Baomin Wang, Yong Liu, Peng Ren, Bin Xia, Kaibin Ruan, Jiabao Yi, Jun Ding, Xiaoguang Li Exchange bias (EB) is usually observed in systems with interface between different magnetic phases after \textit{field cooling}. Here, we report an unexpected finding that a \textit{large} EB can be realized in Ni-Mn-In bulk alloys after \textit{zero-field cooling from an unmagnetized state}. We propose that the size of superparamagnetic domains in the alloys can grow up under external magnetic fields, which induces a transition from a superspin glass to a superferromagnetic (SFM) state. The SFM unidirectional anisotropy, which is the origin of EB effect, can be created at the \textit{newly} formed SFM-antiferromgantic interface during the initial magnetizing process. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V16.00013: High energy product of Sm$_{2}$Co$_{7}$/FeCo nanocomposites prepared by severe plastic deformation Narayan Poudyal, Chuanbing Rong, J. Ping Liu, Ying Zhang, M.J. Kramer Nanocomposite magnets, consisting of exchange coupled hard magnetic and soft magnetic phases exhibit enhanced remanent magnetization and therefore high energy product \textit{(BH)}$_{ max}$. In addition to the high performance, nanocomposites magnets are of commercial interest because the alloys require less expensive rare-earth elements. Here, we report Sm$_{2}$Co$_{7 }$+ x wt {\%} FeCo (x = 0 to 50) nanocomposites prepared by high energy ball-milling and subsequent heat treatments. The evolution of structure and magnetic properties with soft phase fraction was systematically studied. Effect of the soft phase composition Fe$_{100-X}$ Co$_{X}$ ( x = 20, 35 and 50) was also investigated. Microstructural studies by energy filter transmission electron microscopy revealed a homogeneous distribution of -Fe phase in the matrix of hard magnetic Sm-Co phase with grain size less than 20 nm after severe plastic deformation. Enhanced remanence and \textit{(BH)} $_{max}$ (up to 17 MGOe) in the nanocomposites with 40 {\%} of the soft phase are obtained. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V16.00014: Microstructure refinement in Nd$_{2}$Fe$_{14}$B/(Fe,Co) nanocomposite ribbons produced by melt-spinning in a magnetic field Vuong Van Nguyen, Chuanbing Rong, J. Ping Liu Nd$_{2}$Fe$_{14}$B/(Fe,Co) ribbons were prepared by melt-spinning in a magnetic field perpendicular or parallel to the wheel surface. The starting alloy Nd$_{15}$Fe$_{77}$B$_{8}$ was mixed with soft magnetic Fe, or Co, or Fe$_{65}$Co$_{35}$. The amount of the soft phases was varied from10 to 40 wt.{\%} of the hard phase. The wheel was reconstructed to provide the surface magnetic fields in the range of 1 to 4 KG perpendicular or parallel to the wheel surface. The obtained results show that grain size in the ribbons was significantly reduced while the texture was enhanced. The mechanism remains to be fully understood, though it may be related to a change of the C-shaped diagram. The observed results suggest that a magnetic field can be used to control and optimize microstructures of nanocomposite ribbons. The effect of field strength and configuration is also discussed in details. [Preview Abstract] |
Thursday, March 24, 2011 10:48AM - 11:00AM |
V16.00015: Structure and magnetism of epitaxial NiMn single layers and Co/NiMn bilayers on Cu$_{3}$Au(100) Waldemar Macedo, Wolfgang Kuch, Pedro Gastelois, Jorge Miguel, Maximiliano Martins, Yaqoob Khan The structure of single-crystalline NixMn100-x (NiMn) ultrathin films on Cu3Au(100) and also the magnetic properties of Co films on the NiMn/Cu3Au(100) films have been investigated by multiple techniques. For 10 $\le $ x $\le $ 77, our results revealed good epitaxial, layer-by-layer growth at a substrate temperature of 300 K for all NiMn films with near equiatomic composition. The results indicate a face-centered tetragonal (fct) structure for NiMn, as expected for the L1o phase, and with the c-axis along the film normal. For the Co/NiMn bilayers, MOKE hysteresis loops show a thickness independent coercivity, suggesting no magnetic coupling at the Co/NiMn interface. Although the structural results indicate the formation of ftc NiMn in the equiatomic concentration range, we have no indication of antiferromagnetism for NiMn on Cu3Au(100) at room temperature. This is contrary to the observations for Co/NiMn on Cu(100). [Preview Abstract] |
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