Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B9: Magentic Recording/Exchange Bias/Magnetic Properties |
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Sponsoring Units: GMAG Chair: William Egelhoff, NIST Room: LACC 153A |
Monday, March 21, 2005 11:15AM - 11:27AM |
B9.00001: New Perpendicular Media by Engineering Thermal Stability and Writing Capability Separately Jian-Ping Wang, W.K. Shen, J.M. Bai, N.M. Khan We experimentally demonstrated two novel approaches to tune the thermal stability ($K_{u}$V/k$_{B}$T) and coercivity of perpendicular media separately to alleviate writing field limitation of perpendicular magnetic recording time by combining a nano-granular FeSiO soft layer and a [Co/PdSi]$_{n}$ hard layer. 0-4 nm PdSi spacing layer was used to tune the exchanged-coupling strength. The first approach, namely composite media, is to control the exchange coupling strength to a proper value thus the soft region of a grain will switch first with an external field and apply a torque to help the switch of the hard region [1,2]. The coercivity drop at this zone is mainly due to the dynamic tilted switching mechanism. The maximum benefit obtainable is half of the coercivity value of the perpendicular media. The second approach, which can be named as exchange-spring media, is to use Zeeman energy contribution from soft region of a grain to help the switching of hard region of grain. The maximum benefit depends on the moment ratio of soft and hard regions. The thermal stability factor was found the same for all the cases within measurement tolerance. [1] R. Victora and X. Shen, TMRC 2004 [2] J. P. Wang, et al, Appl. Phys. Lett. 2004, submitted {\&} MMM 2004, HB-04, Jacksonville [Preview Abstract] |
Monday, March 21, 2005 11:27AM - 11:39AM |
B9.00002: Improved Patterned Media for Probe-Based HAMR Emmanuelle Algre, Gilles Gaudin, Ahmad Bsiesy, Jean-Pierre Nozieres The superparamagnetic limit prevents the increase of areal density of informations in HDD storage systems. One way to overcome this limitation is to use very high coercive materials with heat assisted recording (HAMR). The major challenge lies in the spatial localization of heat, which needs to be of the order of the bit cell size for appropriate SNR. In the light of a thermal modeling, we present a new media well suited for probe- based HAMR. It combines simultaneous use of low thermal conductivity nanoporous silicon and patterned media, the latter allowing both ultra-high storage densities and a quasi perfect lateral (e.g. inter bits) thermal insulation. We show promising experimental results on the media developpment, as well as the first writing tests.. [Preview Abstract] |
Monday, March 21, 2005 11:39AM - 11:51AM |
B9.00003: Domains and Magnetization Rotation in Exchange Biased Ni/FeF$_2$ Justin Olamit, Kai Liu, Elke Arenholz, Zhi-Pan Li, Oleg Petracic, Igor Roshchin, Ivan Schuller Exchange biased Ni/epitaxial-FeF$_{2}$ films have been investigated using vector coil vibrating sample magnetometry as a function of the cooling field strength $H_{FC}$, applied along the FeF$_{2}$ easy axis. At low $H_{FC}$ a single longitudinal hysteresis loop is observed, negatively biased with a large exchange field. With increasing $H_{FC}$, the loop divides into two sub-loops shifted oppositely from zero field by the same amount. The positively biased sub-loop grows in size with $H_{FC}$ until only a single positively shifted loop is found. Throughout this process, the negative/positive (sub)loop shift has maintained the same \textit{discrete} value. This is in sharp contrast to films with twinned FeF$_{2}$ where the exchange field \textit{gradually} changes from negative to positive values with increasing $H_{FC}$.$^{1}$ The transverse magnetization shows clear correlations with the longitudinal sub-loops. Interestingly, over 90{\%} of the Ni reverses its magnetization by rotation, either in one step or through two successive rotations. These results are due to the single crystal nature of the antiferromagnetic FeF$_{2}$, which breaks down into two opposite regions of large domains. $^{1}$ J. Nogu\'{e}s, D. Lederman, T. J. Moran, and I. K. Schuller, Phys. Rev. Lett., \textbf{76}, 4624 (1996). Work supported by NSF, DOE, Cal-IT$^{2}$ and NEAT IGERT. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:03PM |
B9.00004: Asymmetric Reversal in Exchange Bias: experiment and micromagnetic simulations Zhi-Pan Li, Oleg Petracic, Ivan K. Schuller Asymmetric reversal process in several exchange biased systems is a yet unexplained problem. Recent experimental results on epitaxial FeF$_{2}$/Ni bilayers reveal strongly asymmetrical hysteresis loops. A systematic micromagnetic study based on the assumption of pinned AF moments at the ferromagnetic interface is performed. The simulation result suggests a winding and unwinding of \textit{local} incomplete domain walls in the ferromagnet parallel to the ferro-/antiferromagnet interface at only one side of the saturation is responsible for the asymmetry observed. The simulations are in very good agreement with the experiment results. [Preview Abstract] |
Monday, March 21, 2005 12:03PM - 12:15PM |
B9.00005: Magnetization depth dependence and reversal processes in exchange coupled FeF$_{2}$/F (F=Fe,Ni,Py) bilayers R. Morales, Zhi-Pan Li, O. Petracic, X. Batlle, Ivan K. Schuller Antiferromagnet-ferromagnet (AF-F) bilayers exhibit exchange bias effect, i.e. a shift of the F hysteresis loop after a cooling down below the Ne\'{e}l temperature of the AF. We use SQUID magnetometry and magneto-optical Kerr effect (MOKE) to study the magnetization depth dependence and reversal processes in the F layer of FeF$_{2}$(70nm)/F(70nm) (F=Fe, Ni, Py) samples. Since the MOKE penetration depth is about 35nm for F layer, it is possible to achieve depth dependent information on the F probing both sides of the layer. Analyzing the SQUID response arising from the whole sample and MOKE hystesis loops from both AF-F and air interfaces we show that a magnetic structure perpendicular to the interface is created in the F layer near the AF-F interface. The compression of this magnetic structure with the external field is a reversible process. Work supported by US-DOE. [Preview Abstract] |
Monday, March 21, 2005 12:15PM - 12:27PM |
B9.00006: Intrinsic exchange bias effect in a charge-ordered manganite Dario Niebieskikwiat, Myron Salamon Pr$_{1/3}$Ca$_{2/3}$MnO$_{3}$ is a charge-ordered (CO) and antiferromagnetic (AFM) manganite, with a N\'{e}el temperature $T_{N}\sim 160$K. However, for temperatures below $T_{N}$ the magnetization vs. field ($M$-$H$) loops exhibit hysteresis and a shift towards a negative field ($-H_{E}$) when the sample is cooled in a positive field. Both the exchange bias field ($H_{E} $) and the width of the hysteresis loop present a strong dependence on the value of the cooling field ($H_{cool}$). The observed dependence is successfully described in terms of nanometer-sized ferromagnetic (FM) inclusions, interacting via exchange coupling with a disordered shell at the interface with the CO/AFM matrix. The existence of a disordered spin layer around the FM bubbles is also consistent with the observed training effect of the exchange bias. The calculated size of the FM domains, $D\sim 1.9$nm, is similar to that found by neutron scattering in other electron doped manganites. We acknowledge useful discussions with Chris Leighton. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B9.00007: Influence of the antiphase boundaries density on the anomalous magnetic behaviour of epitaxial Fe$_3$O$_4$ (111) thin films Alexandre M. Bataille, Jean-Baptiste Moussy, Susana Gota, Marie-Jo Guittet, Martine Gautier-Soyer We report on the link between the structural and magnetic properties of Fe$_3$O$_4$ (111) thin films systematically studied as a function of the film thickness (ranging from 8 to 50 nm). The films are epitaxially grown onto $\alpha$-AlO3 (0001) single crystals by atomic-oxygen-assisted molecular beam epitaxy. Despite the high structural order, sharp interfaces and low roughness of the films, the magnetic properties deviate from bulk: the magnetization is reduced and the approach to saturation is very slow. Structural analysis, performed using high resolution transmission electron microscopy, reveals the presence of antiphase boundaries (APBs), the density of which decreases when the thickness increases. Using a model of ferromagnetic domains separated by antiferromagnetically sharp interfaces, we show that the anomalous magnetic properties of the films are driven by the APBs density. [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B9.00008: Improvements in magnetomechanical properties of highly magnetostrictive ferrites due to magnetic annealing Chester C.H. Lo, Andy P. Ring, John E. Snyder, David C. Jiles Magnetostrictive composites based on cobalt ferrite and newer compounds in which 3d elements such as Mn and Cr are substituted for some of the iron, hold promise for use in stress sensors due to their large magnetostriction and high sensitivity of magnetization to stress which depends on the piezo-magnetic coefficient d = d$\lambda $/dH. We report substantial increases of both magnetostriction and d$\lambda $/dH of cobalt ferrite by magnetic annealing under a field of 0.4 T at a temperature of 300 C for 36 hours. The annealed sample showed a uniaxial anisotropy, with the easy axis being along the annealing field direction. The maximum magnetostriction increased from -200~$\times $~10$^{-6}$ for the as-fabricated sample to -252~$\times $~10$^{-6}$ (field and strain along the hard axis) after magnetic anneal, whereas the maximum d$\lambda $/dH increased from 1.3~$\times $~10$^{-9}$~A$^{-1}$m to 3.9~$\times $~10$^{-9}$~A$^{-1}$m. This is attributed to the induced anisotropy, which resulted in increased rotation and non-180\r{ } domain wall processes as domain magnetizations re-oriented from the induced easy direction towards the applied field along the hard axis. This research was supported by the National Aeronautical and Space Administration (NASA) under award No NAG-1-02098. [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B9.00009: Magnetization and Phase Transitions in Single-Crystal Tb$_5$(Si$_{2.2}$Ge$_{1.8}$) Andrew Ring, Hattie Ziegler, T. Lograsso, D. Schlagel, John Snyder, David Jiles The Tb$_{5}$(Si$_{x}$Ge$_{4-x})$ alloy system is similar to the Gd$_{5}$(Si$_{x}$Ge$_{4-x})$ system although it has a more complex magnetic and structural phase diagram. We report on the magnetic anisotropy and magnetic phase transitions in single crystal Tb$_{5}$(Si$_{2.2}$Ge$_{1.8})$ by measurements of M-H and M-T along the a, b, and c axes. The variation of 1/$\chi $ vs T indicates a transition from paramagnetic to ferromagnetic at T$_{c}$ = 110 K. Below this transition temperature, M-H curves show strong anisotropy. It is believed this is due to the complex spin configuration. M-H measurements at T = 110 K show that the \textbf{\textit{a }}axis is the easy axis, and that the saturation magnetization is 200 emu/g. The $b$ axis is the hard axis, which needs an external magnetic field higher than 2 T to saturate the magnetization in that direction, indicating a high magnetocrystalline anisotropy. The c axis is of intermediate hardness. This research was supported by the U.S. Department of Energy under contract number W-7405-ENG-82. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B9.00010: Magnetization-induced second-harmonic generation of La0.7Sr0.3MnO3 thin films grown by RF magnetron sputtering method Y.-M. Chang, Y. J. Hsu, T. M. Liu, S. L. Cheng, J. G. Lin, C. H. Chen Magnetization-induced second-harmonic generation (MSHG) scanning microscopy and spectroscopy are used to explore the nonlinear magneto-optical properties of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ thin films at room temperature. The thin film is deposited on single crystalline LaAlO$_{3}$ (100) substrate by a RF magnetron sputtering system at room temperature and then annealed at 900\r{ }C in flowing O$_{2}$ for one hour. The temperature dependent magnetization measurement indicates its ferromagnetic transition temperature T$_{c}$ around 360 K. The X-ray diffraction shows that its crystal structure is orthorhombic and its c-axis is along the surface normal. However, our MSHG measurements reveal the existence of individual LSMO micrograins, which have their c-axis orientation different from the surface normal. These particular LSMO micrograins can proceed a large nonlinear Kerr rotation with a low external magnetic field. We attribute this phenomenon to the probable anisotropic spin-flipping process inside the single micrograin. [Preview Abstract] |
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