Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P32: Focus Session: Magnetic Media and Hard Magnetic Materials |
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Sponsoring Units: GMAG FIAP Chair: Ping Liu, University of Texas at Arlington Room: Morial Convention Center 225 |
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P32.00001: Magnetism of FePt nanoparticles and nanodot arrays. Invited Speaker: L1$_{0}$ structured FePt materials show great potential for magnetic data storage media applications.$^{1}$ The first part of this talk concerns the magnetism in chemically synthesized FePt nanoparticles. Discrete FePt nanoparticles with L1$_{0}$ structure have recently been realized by salt annealing, making it possible to study their size dependent magnetic properties.$^{2}$ We have discovered a strong reduction of magnetization with decreasing FePt particle size and an unusual temperature dependent magnetization that deviates from the Bloch's T$^{3/2}$ law at low temperatures. A model based on competing exchange interactions is proposed to explain the unusual behavior, considering explicitly the nanoparticle shape. FePt system has complicated exchange interactions, with interaction in the (100) plane being strongly ferromagnetic and inter-plane much weaker. The ferromagnetic and antiferromagnetic exchange interactions contribute differently at the nanoparticle surface and interior, leading to reduced ferromagnetic order at the surface terminated by certain facets. The model correctly explains the magnetization reduction with decreasing particle size, a surface paramagnetic phase as evidenced by Mossbauer spectroscopy and the unusual temperature dependent magnetization behaviors. The second part of this talk will report our recent efforts in developing ordered FePt nanodot arrays using self-assembled porous templates as evaporation masks. The arrays possess perpendicular anisotropy, large coercivity and extremely high density, all of which are desirable features for future data storage media. \newline $^{1}$S. Sun \textit{et al}., Science, 287, 1989 (2000). \newline $^{2}$C. Rong, \textit{et al.}, Adv. Mater. 18, 2984 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P32.00002: Permanent-Magnetic Fe-Pt Nanosparticles Ralph Skomski, D.J. Sellmyer Recently, it has become possible to produce magnetically stable small-scale single-phase [1] and hard-soft [2] Fe-Pt nanoparticles with potential applications in permanent magnetism. The coercivity is largely determined by the degree of ${L}$1$_{0}$ ordering and the presence of the soft phase, respectively, and affected by surface anisotropy. We model the coercivity of the particles as a function of composition, structure, and particle diameter. The smallest particles reverse coherently, with renormalized anisotropy constants, but with increasing size, micromagnetic corrections become important. The reversal modes in the two-phase particles are reminiscent of ${p} $-state wave functions in atomic physics [2] and well described by second-order perturbation theory. We also discuss extensions involving semihard phases, which may be created by substitutions or coatings using heavy transition metals, such as Pt and W. One example is hexagonal Co$_{1-x}$Pt$_{x}$, which exhibits a huge anisotropy per Pt atom and a substantial net anisotropy. - [1] C. B. Rong ${et al.}$, Adv. Mater. 18, 2984 (2006); R. Skomski ${et al.}$, JAP 103, in press (2008). - [2] J. E. Shield ${et al.}$, JAP 99, 08B508 (2006). - [3] R. Skomski, ${Simple Models of Magnetism}$, University Press, Oxford 2008. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P32.00003: FePt Nano-particles and Nano -wires Levent Colak, George Hadjipanayis In this work, we have studied the microstructure and magnetic properties of FePt nano-particles, nano-rods, and nano-wires synthesized by a modified chemical synthesis route described elsewhere$^{[1],[2]}$ The effect of synthesis parameters on the particle shape has been investigated for nanoparticles with sizes of 5-7 nm, for nano-rods and nano-wires with a diameter of 2-3 nm and a length of 20 and 100 nm, respectively. Low injection temperature for the iron precursor and usage of surfactants as reaction solvents resulted in 7 nm nano-cubes whereas a high heating rate to refluxing temperature and high injection temperature resulted in spherical shapes with 5 nm diameter. Nano-rods and nano-wires are formed by simply adjusting the relative ratios of surfactants to reaction solvents (oleic acid, oleyl amine and octadecene/benzylether) and the refluxing time. Transmission electron microscope (TEM) studies show that usage of high concentrations of oleyl amine and longer refluxing times induce nano-wire formation. HRTEM and magnetometry studies are currently in progress to investigate the development of particle morphology and microstructure during the synthesis and determine their influence on the magnetic properties. 1. C.Wang et. al. Angew. Chem. Int. Ed. 2007, 46,1-4. 2. M. Chen et. al. J. Am. Chem. Soc. 2007, 129, 6348-6349. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P32.00004: Nanostructure and Magnetic Properties $L$1$_{0}$ FePt Films with Additions of Au and Cu T. George, M. Yan, Y. Xu, R. Skomski, R. Kirby, D. J. Sellmyer Non-epitaxially grown $L$1$_{0}$ FePt:Au and FePt:Cu films have been fabricated and investigated.~ All films are initially deposited with the structure [FePt/X]$_{n}$ and have individual layer thicknesses from about 0.1 nm to 1 nm. The $L$1$_{0}$ phase is achieved by post-deposition annealing at temperatures from 500 to 600 \r{ }C for varying times.~ XRD and TEM show that Cu enters the $L$1$_{0}^{ }$lattice whereas Au segregates at the grain boundaries. Both types of films exhibit a decrease in $M_{s}$, due to magnetic dilution. The coercivity ($H_{c})$ increases and decreases with the addition of Au and Cu, respectively. These changes are due to reduced anisotropy (Cu) and to reduced inter-granular exchange coupling (Au). In the FePt:Au films, MFM shows a decrease in magnetic coherence length (L$_{M})$ from 90 to 74 nm and the $M-H$ slope $\alpha $ = (d$M$/d$H)_{Hc}$ decreases from 5.7 to 0.9 for Au contents from zero to 32 vol{\%}. A simple interaction model quantifies these trends by considering that interparticle exchange cooperatively enhances both $\alpha $ and L$_{M}$. In the FePt:Cu films, the addition of Cu yields a decrease in Curie temperature (574 K at 20 vol{\%}). Mean-field calculations qualitatively reproduce this decrease in $T_{c}$ but indicate deviations from random solid-solution behavior. - This research is supported by INSIC, NSF-MRSEC and NCMN. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P32.00005: Electric field assisted magnetization reversal in FePt films. Pavel Lukashev, Kirill Belashchenko, Renat Sabirianov We propose to use strain assisted reduction in anisotropy of FePt in order to make magnetization reversal easier in the writing of the magnetic storage devices. We performed first-principles calculations of the magnetocrystalline anisotropy of FePt under bi-axial stress using full-potential LAPW implemented in FLEUR code. Magnetocrystalline anisotropy decreases by 25{\%} with application of 1.5{\%} tensile biaxial strain. This is partially due to the reduction of the c/a ratio by about 1.5{\%} (calculated Poisson ratio is 0.33) in the tetragonal cell and partially due to the increase in volume by about 1.5{\%}. Biaxial strain can be obtained by placing piezoelectric film under FePt layer, and by applying electric field on the system. Modern ferroelectric systems can provide stress up to 2{\%}. Besides, we propose using thin ferroelectric films with asymmetric interfaces, which provides a simple way to generate bias field in the polarization reversal and related properties. The existence of the polar interfaces results in a different average polarization in the film upon reversal. As a result, the strain in the film depends on the direction of polarization. This asymmetric strain can be used do modulate magnetic properties. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P32.00006: Influence of interfacial non-magnetic materials on soft-hard bilayer interaction A. Zambano, H. Oguchi, I. Takeuchi, S. Lofland, J. Liu, D. Josell, L. Bendersky, Y. Liu, Z. Wang Among the factors that affect the hard magnet-soft magnet interaction, interfacial nonmagnetic impurities could play a significant role. Resembling 1-dimensional models, magnetic multilayer systems are simple tools to probe it. We have used the high-throughput approach [1] to study thickness gradient effects of a Cu nonmagnetic impurity layer on the interaction between a hard magnetic CoPt layer and a soft magnetic Fe layer. On single chips, multiple samples were grown by e-beam evaporation varying the impurity layer thickness (t$_{Cu})$. Magnetic hysteresis loops were taken by the magneto-optical Kerr effect, and the layer interaction was characterized by the nucleation field (H$_{N})$. H$_{N}$ vs. t$_{Cu}$ curves indicate that the hard-soft phase interaction is described by a RKKY oscillatory exchange coupling contribution plus a dipolar exponential one. We will discuss how the hard layer crystalline characteristics affect this behavior and how the interface nonmagnetic material can significantly alter the nature of the interaction. Such behavior can have a pronounced effect on hard-soft bulk nanocomposite magnets. ONR MURI N00014-05-1-0497. \newline [1] Zambano \textit{et al}., Phys. Rev. B \textbf{75}, 144429 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P32.00007: Depth dependence of anisotropy in graded Co/Pd multilayers S.M. Watson, J.E. Davies, K. Liu, G.T. Zimanyi , B.J. Kirby, J.A. Borchers As the magnetic recording industry looks beyond perpendicular recording [1] multilayered media such as exchange coupled composite [2] and graded media [3] have the potential for increasing storage density by combining low and high anisotropy materials. The soft layer reduces the required write field while the hard layer helps to maintain the thermal stability. Recent work has shown further enhancements when the anisotropy is gradually increased up to the hard layer anisotropy [3]. Grading the media in this manner is difficult to do experimentally. Equally difficult is accurately measuring the properties that make these materials unique, namely the depth dependence of the anisotropy. In this study we used polarized neutron reflectometry to measure the in-plane magnetization depth profile of graded Co/Pd multilayers with perpendicular-to-plane easy axis as a function of in-plane applied field. This technique allowed us to observe the depth-dependent response of the spins as they were pulled away from their easy axis, thus allowing us to determine the depth dependence of the anisotropy field. [1] M. Mallary, \textit{et al.} IEEE Trans. Magn. \textbf{38}, 1719 (2002). [2] R. Victora, \textit{et al.} IEEE Trans. Magn. \textbf{41}, 2828 (2005). [3] D. Suess, Appl. Phys. Lett. \textbf{89}, 189901 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:24AM |
P32.00008: Fast Reversal in Multilayer Exchange Spring Media Invited Speaker: Hard disk media that support ultra high densities require small grains in order to obtain high signal to noise ratios. The use of high coercive materials such as alloys in the L$_{1}$0 phase allow for thermally stable grains at grain diameters in the order of 4nm . However state of the art write heads produces too small fields to reverse these extremely hard magnetic grains. Recently composite media and exchange spring were proposed in order to decrease the write field requirements [1,2]. In exchange spring media an ultra hard magnetic storage layer is strongly exchange coupled to a softer magnetic nucleation host layer. The nucleation host decreases the switching field of the storage layer up to a factor of five without lowering the thermal stability of the entire structure. If the nucleation host is composed of multiple magnetic layers where the anisotropy increases from layer to layer it was shown that the resulting structure has a high thermal stability whereas at the same time the coercive field decreases with one over the total layer thickness [3]. Besides the previous results which were obtained in the quasi static limit, where the external field was applied slowly (several nanosecond) further surprising effects occur if the field rise time is in the order of several hundred picoseconds. These fast field rise times together with small damping constants in the media allow for precessional switching in composite media. It was demonstrated that precessional switching significantly lowers the coercive field [4] and also leads to ultra fast reversal modes [5]. We will present results on the reversal time of magnetic bilayers and magnetic trilayers in the precessional switching regime. Micromagnetic simulations show that a magnetic bilayer with a total thickness of 25 nm (hard layer anisotropy is $K_{1}$ = 1 MJ/m$^{3}$ ) can be reversed with a field pulse of 20 ps. Interestingly the reversal time increases to 0.5 ns as the field rise time is decreased from 0.1 ns to 0.01 ns. \newline \newline [1] R. H. Victora et al., IEEE Trans. Magn. 41, 2828 (2005). \newline [2] D. Suess et al. J. Magn. Magn. Mater, 290-291, 551 (2005). \newline [3] D. Suess, Appl. Phys. Lett. 89, 113105 (2006). \newline [4] B. Livshitz, et al Appl. Phys. Lett. 91, 182502 (2007) \newline [5] D. Suess, J. Magn. Magn. Mater., 41, 183 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P32.00009: Role of dipolar interactions in the determination of intrinsic switching field distributions in perpendicular recording media Yang Liu, Karin Dahmen, Andreas Berger The $\Delta H(M,\Delta M)$ method and its ability to determine the intrinsic switching field distributions of perpendicular recording media are numerically studied with the coupled hysteron model. It is found that the presence of dipolar interactions with strength of practical recording media enhances the reliability of the $\Delta H(M,\Delta M)$ method. The correlation between the fit quality measure and the deviation from redundancy measure indicates that the latter, which can be determined from experimental data alone, is a good predictor of the reliability. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P32.00010: Chiral electrical DW injection and single shot detection for ultra-high density data storage L. O'Brien, D.E. Read, D. Petit, A.-V. Jausovec, E.R. Lewis, H.T. Zeng, R.P. Cowburn Ultra high density data storage devices based on magnetic domain walls (DWs) propagating through Permalloy (Py) nanowires have recently been proposed [Cowburn et al., Science 2005]. Controlling the chirality (defined as the sense of rotation of the magnetic moments within the DW) of the DW is of vital importance for proper functioning of these devices. Chiral DW injection can be achieved using global magnetic fields; however, technological applications require this to be carried out using independent, localised fields in multiple wires simultaneously. Using the Oersted field from pulsed electrical currents passing through gold wires ($\sim $4$\mu $m wide, $\sim $200nm thick) fabricated at an angle over Py nanowires (100nm wide, 10nm thick) we inject transverse DWs. The chirality of the DWs is probed using spatially resolved MOKE measurements of their chirality dependent interaction with a cross-shaped trap. The results are consistent with chirally controlled DW injection. In addition, we are able to individually address four parallel wires and detect DW propagation using single shot MOKE measurements. Electrical readout was separately demonstrated by detecting the presence of single DWs at the end of a wire using Anisotropic Magnetoresistance (AMR) measurements. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P32.00011: Effect of Grain size on the Giant Intrinsic Coercivity of High-Energy Milled Sm(Co,Cu,Fe)5 Alloys Dilara Sultana, Alexandar Gabay, George Hadjipanayis The giant intrinsic magnetic hardness of Sm(Co,Cu)5 alloys have been known for a long time [1]. Previous studies suggested that this behavior is due to the crystal site disorder [2]. Our previous work has explained that the room-temperature intrinsic coercivity of 37 kOe after low-temperature aging is rather due to the intrinsic change in the Co atomic site occupation [3]. In this study, we investigated the effect of grain refinement through the high energy milling on the intrinsic coercivity of the Sm(Co,Cu,Fe)5 alloys. We have found that grain refinement does not affect the high coercivity of homogenized alloys, but strongly influences the onset of the giant coercivity during low-temperature aging. The microstructures of the samples are examined with TEM. [1] E.A. Nesbitt, R.H. Willens, R.C. Sherwood, E. Buehler, J.H. Wernick 1968 Appl.. Phys. Lett. 12, 361. [2] H. Oesterrier , F.T. Parker, M. Misroach 1979 J. Appl. Phys. 50, 4273. [3] A.M. Gabay, P. Larson, I.I. Manzin, G.C. Hadjipanayis 2005, J. Phys. D: Appl. Phys. 38, 1. [Preview Abstract] |
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