Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V9: Magnetic Anisotropy |
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Sponsoring Units: GMAG Chair: Kai Liu, University of California-Davis Room: LACC 153A |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V9.00001: Novel Contributions to Magnetic Anisotropy of Inhomogeneous Materials: Exchange Mediated Magnetic Anisotropy Markus Eisenbach, G. Malcolm Stocks, Balazs Ujfalussy, Balazs Gyorffy Calculating and understanding the magnetic behavior of non-homogeneous systems requires careful consideration of of the relevant magnetic states. This is especially true in the case of systems that include induced magnetic moments. To study this we consider monatomic magnetic impurities embedded in non-magnetic metals. By performing fully relativistic self consistent calculations for these systems we can investigate the spin orbit coupling effects leading to anisotropies. In contrast to what would be accessible by force theorem calculations, which are useful for systems with robust magnetic moments, this approach allows us to take into account the possibilities of qualitative change in the induced moments. We find that in the case of a monatomic iron impurity in fcc Cu the induced magnetic moments on neighbouring Cu sites align parallel with the Fe moment, whereas for the hard directions some of the induced moments are oriented anti-parallel. This effect, which arises from slight differences in the energies associated with different $m$ quantum numbers, leads to a significant contribution to the magnetic anisotropy due to the resulting exchange energy that can not be captured by a force theorem approach. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V9.00002: Magnetic anisotropy of isolated Cobalt nanoplatelets Tor Olof Strandberg, Carlo M. Canali, Allan H. MacDonald Motivated by experiments performed by M.H. Pan et al. [1], we have undertaken a theoretical study of the the magnetic properties of two-monolayer-thick Co nanoplatelets with an equilateral triangular shape. We are using a microscopic Slater-Koster tight-binding model with atomic exchange and spin-orbit interactions, that has been designed to realistically capture the salient magnetic features of large magnetic nanoclusters [2]. Two different truncations of the fcc lattice have been studied, in which the nanoplatelet surface is aligned parallel to the [111] and [001] planes respectively. We find that the higher coordination number in the [111] truncation is more likely to reproduce the perpendicular easy direction found in experiment. Qualitatively, the most important model parameter governing the anisotropy is found to be the intra-atomic exchange integral J. If we set the value of J so as to reproduce the experimentally observed magnitude of the magnetic moments, we find both quasi-easy-planes and perpendicular easy directions. Increasing J, we find that, in agreement with experiment, the easy-axis of magnetization is predominantly perpendicular to the surface, and the magnetic anisotropy energy is anomalously large. The possible role of hybridization with substrate surface states in the experimental systems will be discussed. [1] M.H. Pan et al, Nanoletters V5, no 1, 87-90 (2005) \newline [2] A. Cehovin et al, Phys. Rev. B, 66, 094430 (2002) [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V9.00003: Formation of a Magnetically Hard FePt Alloy by Diffusional Mixing of Fe and Pt Nanoparticles* Brian Patterson, Thomas Ekiert, Karl Unruh The formation of a bulk FePt alloy by diffusional mixing of nanometer-sized particles of elemental Fe and Pt has been studied by differential scanning calorimetry, x-ray diffraction, and vibrating sample/DC extraction magnetometry. A strong exothermic signal and a rapid increase in the magnetization at a temperature of about 475 \r{ }C (both at a heating rate of 10 \r{ }C/min) indicate the formation of a new magnetic phase from the as-prepared dispersion. The coercivity of the resulting alloy was found to be in excess of 10 kOe at room temperature and about 12.5 kOe at 5 K. X-ray diffraction measurements confirmed the presence of a face centered tetragonal (FCT) phase with an $a$ lattice parameter similar to, and a $c$ lattice parameter somewhat smaller than, the corresponding values in bulk FePt. * This work has been supported by AFRL DARPA METAMATERIALS contract no. F33615-01-2-2166. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V9.00004: Preparation of monodisperse FCT FePt nanoparticles K.E. Elkins, Daren Li, J.P. Liu Chemically synthesized FePt nanoparticles have drawn great attention for their potential applications in magnetic recording, nanocomposite magnetic materials and biomedical technology. However, the as-synthesized particles have FCC structure and therefore have low magnetic anisotropy. To obtain FCT structured FePt phase with high magnetocrystalline anisotropy, heat treatment is necessary for the FCC-FCT phase transition. The heat treatments, however, usually cause particle agglomeration and sintering. We use a controlled heat treatment with stable media in which the particles are isolated during the heat treatments. With this technique, complete FCC-FCT transition was realized and no considerable particle growth and agglomeration were observed. Magnetic hardening is obtained in the FCT structured nanoparticles. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V9.00005: Magnetic-field-induced Crystallographic Texture Enhancement in Cold-deformed FePt/Fe3Pt Nanocomposites B. Z. Cui, D. S. Li, K. Han, H. J. Schneider-Muntau , J.P. Liu, H. Garmestani, N.M. Dempsey We introduce a unique approach to obtaining anisotropic FePt/Fe3Pt nanocomposites with crystallographic texture of the hard phase: roll-bonding and magnetic annealing. Magnetic-field- induced crystallographic texture and magnetic property enhancement in cold-deformed Fe62Pt38 nanostructured magnets have been studied. Compared with the samples annealed without a magnetic field, the annealing in the presence of both out-of- plane and in-plane 19 T fields enhances texture of the hard FePt phase by about 50 %. Consequently, annealing samples in a 19 T field improves iHc, Br, and (BH)max. Especially, (BH)max was increased by 21 - 25 %. Additionally, magnetically anisotropic behavior and better squareness of the demagnetization curves were observed in the magnetically annealed samples. It is suggested that cold-rolling-induced textures in both Fe and Pt phases and magnetic-field-assisted phase transformation should be responsible for the notice improvement of the crystallographic texture and magnetic properties in the magnetically annealed samples. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V9.00006: Crystallinity Dependence of Irreversible Switching in Fe/SmCo and FeNi/FePt Spring Magnets J.E. Davies, G.T. Zimanyi, Kai Liu, Olav Hellwig, E.E. Fullerton, J.S. Jiang, S.D. Bader The effect of hard layer crystallinity on irreversible switching in Fe/SmCo and FeNi/FePt spring magnets was investigated by a First Order Reversal Curve (FORC) technique and vector magnetometry. In Fe/epitaxial-SmCo samples, the switching fields of the individual layers can be conventionally determined from steps in the major loop. The FORC diagram shows a distinct onset of irreversible switching at the hard layer switching field. Vector magnetometry reveals that most of the soft layer switches by rotation before reversal of the hard layer. In FeNi/polycrystalline-FePt samples, the switching fields are masked by the smooth and step-free major loop. The FORC diagram displays a single onset of irreversible switching, suggesting that the soft (FeNi) and hard (FePt) layers switch together. The co-rotation is confirmed by vector magnetometry. Despite the random anisotropy of the hard layer, an appreciable transverse moment bigger than just the soft layer contribution is observed. The successive vs. simultaneous rotation of the soft/hard layer is mainly due to different hard layer crystallinity.-- Work is supported by NSF and DOE (BES-MS contract {\#}W-31-109-ENG-38). [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V9.00007: Bulk magnetic hardening in Cu-added (SmCo5)1-x(Sm2Co17)x cast alloys George Hadjipanayis, Alexander Gabay, Yong Zhang, Melania Marinescu, Kyriakos Christodoulides In attempt to establish a possible link between the magnetic hardening in Sm(Co,Cu)$_{5}$ magnets and the commercial "2:17" magnets, we have investigated a series of homogenized ternary Sm-Co-Cu alloys with composition between Sm(Co,Cu)$_{5}$ and Sm$_{2}$Co$_{17}$. The coercivity of the alloys with x $<$ 0.75 can be increased by isothermal aging without separation into 1:5 and 2:17 phases. The increase in coercivity is accompanied by an increase in the Curie temperature. A similar effect of low-temperature aging has also been observed in Sm-Co-Ni, Sm-Co-Fe-Cu and Pr-Co-Fe-Cu alloys. A different pattern of magnetic hardening can be observed in the alloys with $x \quad >$ 0 which were slowly cooled from 800 to 350 $^{o}$C. In this case, the initial hexagonal structure transforms into the cellular structure of the 1:5 and 2:17 (rhomb.) phases. In the alloy with x = 0.25, the two types of magnetic hardening may lead to the same room temperature coercivity despite the very different microstructures. The correlation between the observed microstructures and coercivity will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V9.00008: XAS and XMCD study of $3d$-$4d$ hybridization in Ni/Pd films J.-S. Lee, B. H. Seung, B.-G. Park, J.-H. Park, J.-Y. Kim, K.-B. Lee X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD) measurements at the Ni $L_{\rm 3;2}$- and Pd $M_{\rm 3;2}$-edge were carried out to reveal changes in intrinsic electronic and magnetic structures of Ni-Pd films prepared in various configurations, such as Ni, Ni/Pd, Ni/Pd- multilayer, and NiPd-alloy. All films were grown by $in~situ$ e- beam evaporation system in the measurement chamber. The satellite structures (6 eV and 4 eV) of bulk Ni, which are present in XAS and XMCD spectra, disappear as portion of Ni/Pd interface increases such as multilayer, while XMCD signal of Pd became enhanced. Effects of the increased portion of Ni/Pd interface were also revealed in NiPd alloy film. The findings strongly imply that increased intermixing at the interface gives rise to the $d$-electron charge transfer at the interface and results in the modified orbital moments, which is a direct evidence for a close relation between the modified magnetic property and the interfacial $3d$-$4d$ hybridization in Ni-Pd films. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V9.00009: Layered Magnetic Thin Films for High Susceptibility William Egelhoff, Mark Stiles, Frank Johnson, Alex Shapiro, P.J. Chen, Ced Powell, Robert McMichael Thin-film magnetic sensors require sense layers with high magnetic susceptibility in order to achieve high sensitivity to low magnetic fields. We have carried out an extensive investigation into a variety of soft magnetic materials including varieties of Permalloy and mu-metal, depositing them by both magnetron and ion-beam sputter deposition. We have found that two of the most important characteristics for achieving high susceptibility are the use of metallic seed layers and layering the magnetic films with non-magnetic films. Some of the best results come from using two layers of the mu-metal alloy known as Co-netic (each film several tens of nm thick) with a 10 nm Cu film as a seed layer and spacer layer. In such samples, we have been able to achieve susceptibilities of $\sim $100,000 and hard-axis coercivities of 0.12 A/m $\pm $ 0.12 A/m (0.0015 Oe $\pm $ 0.0015 Oe). Such films should provide excellent sensitivity to very small magnetic fields when used as the sense layer in magnetic tunnel junctions. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V9.00010: Anhysteretic magnetization of thin Ni-Fe films under stress and plastic deformation Peter Finkel, Edward Garrity, Samuel Lofland The magnetostriction contribution to anhysteretic magnetization under elastic stress and the effect of the plastic strain on the hysteresis loops in thin film Ni-Fe are discussed. Also, a role of the plastic deformation interrelated with the elastic stress in the magnetization process is established. Anhysteretic permeability was extracted from the anhysteretic B-H curves constructed by ac demagnetization of a sample at given longitudinal dc fields. Observations indicate that the large positive magnetostriction constant of Fe-Ni samples leads to higher susceptibility and lower coercivity with increased tensile stress, while the large volume magnetostriction results in reduced saturation magnetization. Above a critical stress, however, while stress remains far below the macroscopic elastic limit, this trend reverses direction. An irreversible change in magnetic properties is observed when the stress is removed, and is exacerbated by subsequent applications of increasing stress. A mechanism for this higher stress effect is hypothesized to be related to increased dislocation density and domain wall pinning due to locallized plastic strain. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V9.00011: Anisotropy of Manganese-Substituted Cobalt Ferrite Yevgen Melikhov, Chester Lo, John Snyder, David Jiles, Jason Paulsen, Andy Ring Study of the anisotropy of magnetostrictive cobalt ferrite composites and its temperature dependence helps better understand the magnetomechanical properties, which will allow tuning the properties of the materials for magnetoelastic stress and torque sensors applications. We have investigated the temperature dependence of anisotropy of Mn-substituted cobalt ferrite CoMn$_{x}$Fe$_{2-x}$O$_{4}$ (0~$\le $~x~$\le $~0.8). Magnetization measurements of the hysteresis loops were made at temperatures over the range 200-300 K using a SQUID magnetometer. To determine the anisotropy constant $K_{1}$, the high field regime of the loops was fitted using the law of approach approximation $M(T)$~=~$M_{s}\cdot $(1-\textit{$\alpha $K}$_{1}^{2}$/$H^{2})$, where $\alpha $~=~8/105~\textit{$\mu $}$_{0}^{-2}M_{s}^{-2}$ for the samples which have spinel structure and cubic anisotropy. It was found that anisotropy decreases with increasing Mn content and with increasing temperature for all compositions. However, at higher temperatures the anisotropy of the Mn-substituted specimens converges to approximately the same value $K_{1}$=200 kJ/m$^{3}$. The pure cobalt ferrite sample appears to have considerably higher anisotropy $K_{1}$ than the Mn-substituted samples over the whole temperature range. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V9.00012: Magnetic Anisotropy Induced by Deposition Angle Michael Umlor, Timothy Carrier A series of 30 {\AA} cobalt thin films were prepared in an ultra-high vacuum chamber using a high energy electron bombardment evaporation source. The deposition angle between the incoming adatom flux relative to the substrate normal was varied from 0$^{\circ}$ to 60$^{\circ}$. The substrates used were Si (111), either hydrogen terminated or native oxide. The reflection high energy electron diffraction (RHEED) pattern was observed during the 25$^{\circ}$ deposition on hydrogen terminated Si (111) and found to decay steadily through sample growth, showing no signs of oscillations. No RHEED patterns were observable for any samples following the deposition indicating no long-range crystalline order. An ex-situ magneto-optic Kerr effect hysteresis loop tracer was used to observe the in-plane magnetic anisotropy of each sample. All samples with a deposition angle of 15$^{\circ}$ and more resulted in uni-axial anisotropy with the hard-axis found to be parallel to the deposition direction and the easy-axis perpendicular to this direction. Also, the coercivity field of the easy-axis was found to increase sharply with deposition angle above 30$^{\circ}$ from a value of 15 Gauss at normal incidence to 100 Gauss for 60$^{\circ}$ sample deposition. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V9.00013: Hall Effect in LaCrSb$_3$ Tuan Hoang, Myron Salamon, Damon Jackson Measurements of the Hall effect in LaCrSb$_3$ are reported. LaCrSb$_3$ is a quasi-two-dimensional system, where a-axis is highly resistive while b and c-axes are conducting [1]. Measurements were carried out with the current along the b, and c-axis in the temperature range $4$-$300$K. The anomalous Hall effect dominates at low fields. After the magnetization saturated, we observed a T-dependent deviation from the expected behavior of the ordinary Hall coefficient. The existence of two types of carriers has been suggested by Granado {\it et al.} [2] to explain this deviation. Our data for the current along c suggested that the first type of carriers, which arises from the isotropic Sb plane, is electron-like. The data for the current along b implied that the b-axis conductivity is dominated by hole-like carriers. These hole-like carriers originate from the DE interaction within the Cr chains, which line up along the b-axis, and contribute to the b-axis conductivity mainly at temperatures lower than $T_C$.\\[4pt] [1] D. D. Jackson {\it et al.}, Phys. Rev. B {\bf 65}, 014421 (2001).\\[0pt] [2] E. Granado {\it et al.}, Phys. Rev. Lett. {\bf 89}, 107204 (2002). [Preview Abstract] |
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V9.00014: Magnetic Anisotropy in Epitaxial SrRuO$_3$ and Ba$_{0.5}$Sr$_{0.5}$RuO$_3$ on (001) SrTiO$_3$ substrates Xianglin Ke, Kyoung-jin Choi, Chang-Beom Eom, Mark Rzchowski Epitaxial thin films exhibit quite different properties from the corresponding bulk materials due to strain induced by the substrate. For instance, a bulk SrRuO$_{3}$ (SRO) single crystal, has a Curie temperature near 160K with two magnetic easy axis. However, there shows unianxial magnetic anisotropy in single domain epitaxial SRO grown on SrTiO$_{3}$ (STO) substate[1]. In this study, we have grown epitaxial SRO and Ba$_{0.5}$Sr$_{0.5}$RuO$_{3}$ (BSRO) on 4$^{\circ}$ miscut (001) STO substrates using pulsed laser deposition (PLD) with in-situ high pressure reflection high energy electron-diffraction (RHEED). Through detailed X-ray diffraction, magnetization and angle-dependent magneto-transport measurements, we explore the magnetic properties of SRO and BSRO. However, in contrast to non-magnetic properties of bulk BSRO and uniaxial anisotropy in SRO film, BSRO film shows ferromagnetism with Tc about 140K and non-uniaxial properties. We discuss the mechanism in terms of lattice distortion due to strain effect and spin-orbit coupling. 1] Q. Gan, R. A. Rao, C. B. Eom, L. Wu, and F. Tsui, J. Appl. Phys. \textbf{85}, 5297 (1999). [Preview Abstract] |
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