Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V14: Focus Session: High Anisotropy Magnetic Nanoparticles and Composite Materials |
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Sponsoring Units: GMAG FIAP Chair: Andreas Berger, Hitachi GST Room: Colorado Convention Center Korbel 4D |
Thursday, March 8, 2007 11:15AM - 11:51AM |
V14.00001: Magnetic and chemical ordering properties of FePt nanoparticles Invited Speaker: The presence of anisotropic L1$_0$ type chemical order in self- assembled, monodispersed FePt nanoparticles produces an extremely high magnetic anisotropy thus making these particles potentially useful for information storage. However, it has been difficult in experiments to achieve a high degree of L1$_0$ order in 3.5-nm-diam nanoparticles. To explore the possible reasons for this observed low degree of order, we studied theoretically the equilibrium chemical ordering of L1$_0$ type for spherical FePt nanoparticles of different sizes (2.5 to 6-nm- diam) in a wide temperature range. In our study we used first- principles calculations together with the cluster expansion technique and Monte Carlo simulation. Our results indicate that the theoretical equilibrium chemical order is higher than that observed experimentally in 3.5-nm-diam nanoparticles annealed at 600 C or below. Using first-principles calculations we considered one of the possible reasons for this differentce - the surface segregation in FePt and Fe-Pt-X (X=Ag, Au, Cr, and Cu) nanoparticles. It was established that surface segregation has only a small effect on ordering. We conclude that the experimental absence of (relatively) high L1$_0$ order is primarily a problem of kinetics rather than equilibrium. The recent experimental data supporting such a conclusion are discussed. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V14.00002: Array Formation and Size Effects in Chemically Synthesized FePt Nanoparticles Levent Colak, George Hadjipanayis FePt nanoparticles with controlled size have been synthesized following a route given by Shukla et. al.[1] The effect of particle size on the magnetic properties has been investigated for nanoparticles with sizes of 3.0 and 6.0 nm. With the addition of the surfactants at a later stage of preparation, a long range self-assembled array of particles was obtained as evidenced by transmission electron microscope (TEM). By comparing bright field images of the samples with projected potential image simulations$^{ }$[2], the packing structures and stacking sequences of the arrays were identified. Well-aligned mono and multi layered hcp to bcc nanostructures are formed from 5 nm FePt nanoparticles. Subjecting the NP's to thermal processing at 800 $^{o}$C results in a transformation of the nanoparticles from the disordered fcc phase to the ordered L1$_{0}$ phase. HRTEM studies have been carried out to investigate the development of particle morphology and microstructure during the synthesis and subsequent annealing of nanoparticles. 1. N. Shukla, C. Liu, A. G. Roy, Matt. Lett. 60, 2006, 995-998. 2. S. Yamamuro, D. F. Farrell and S. A. Majetich, Phys. Rev. B 65, 224431 (2002). [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V14.00003: Role of disorder and structural inhomogeneity on the magnetic structure of FePt nanoparticles Paul Kent, Don Nicholson, Markus Eisenbach, Thomas Schulthess Recent experiments (e.g. [1]) have demonstrated a substantial increase in Fe magnetic moment on annealing of FePt nanoparticles, underscoring the likely importance of local composition variations at the nanoscale. To test these ideas, we have calculated the magnetic structure of FePt nanoparticles up to 3nm in size using density functional theory. We investigate the role of disorder and the influence of explicitly constructed Fe and Pt rich regions, both at the center and at the surface of the particles. The size and disorder dependent spin-moment distributions are related back to changes in the electronic structure of the materials relative to the bulk. Even for 3nm nanoparticles, the magnetic structure deviates significantly from the bulk due to the large fraction of near-surface atoms. Structural relaxation is shown to significantly influence the magnetic structure, particularly reducing the magnetic moments of surface atoms. This work used resources of the National Center for Computational Sciences at Oak Ridge National Laboratory (ORNL) and is supported in part by the Division of Scientific User Facilities, U. S. Department of Energy as well as the Laboratory Directed Research and Development program at Oak Ridge National Laboratory. [1] C. Antoniak \emph{et al.} Phys. Rev. Lett {\bf 97} 117201 (2006). [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V14.00004: Magnetic Properties and Chemical Synthesis of FePt and CoPt Nanoclusters Yao Zhao, Yucheng Sui, David Sellmyer Much interest has been paid to self-assembled L1$_{0}$ FePt and CoPt nanoclusters for basic science and potential applications in ultra-high density magnetic recording media. In this study, thermal decompostion of a Fe and Pt acetylacetonates mixture in trioctylamine was employed to produce FePt nanoclusters with average size about 4 nm and a narrow size distribution. The atomic ratio of Fe and Pt for the as-synthesized FePt clusters is 50:50. Using the same procedure, CoPt nanoclusters were obtained with the atomic ratio of 40:60. With the addition of oleic acid into the reaction solution the atomic ratios were adjusted to 50:50. As-synthesized FePt and CoPt nanoclusters have fcc structures, but after rapid thermal annealing in forming gas the nanoclusters transform into hard magnetic L1$_{0}$ ordered structures. The nanostructures of the clusters were characterized by TEM and XRD. The magnetic properties were measured by a SQUID magnetometer. The coercivity of FePt and CoPt clusters reached the maximum value of 23 kOe and 17kOe respectively after annealing at 750 $^{o}$C for 5mins. Beyond 750 $^{o}$C, the coercivity decreased slightly because of the sintering effect and a different magnetic reversal mechanism. This work is supported by NSF-MRSEC, DOE, INSIC, NRI and NCMN. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V14.00005: Synthesis and Magnetic Properties of L1$_{0}$FePt / Silica Core Shell Nanoparticles Andrew Heitsch, Doh Lee, Brian Korgel FePt nanocrystals were coated with silica (SiO$_{2})$ shells in an inverse micelle microemulsion by tetraethyl orthosilicate (TEOS) hydrolysis and condensation. The shell thickness can be varied (from 6 to 25 nm), along with the FePt loading (per silica shell) to a limited extent, by changing the FePt:TEOS ratio. The silica-coated FePt nanocrystals can be heated up to $\sim $850$^{o}$C without shell layer decomposition or FePt sintering. Annealing under forming gas (7{\%}H2/ 93{\%}N2) at 700\r{ }C for 2 hours transforms the as-synthesized fcc FePt nanocrystals to the L1$_{0}$ phase with at least 90{\%} conversion. Magnetic measurements of annealed FePt nanocrystals confirm their phase transformation, with blocking temperatures exceeding room temperature. However, the hysteresis loops exhibit a constriction at low fields and zero field cooled (ZFC) magnetization scans show an intermediate plateau at temperatures between 50K$\sim $200K. Temperature and time-dependent remanance relaxation measurements reveal a short and fast, $\sim $10$^{4}$ and 10$^{5}$ seconds, relaxation of the remanance, which might be due to the presence of an additional ``soft''magnetic phase in the sample. The possible origins of the soft magnetic component will be presented and discussed. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V14.00006: Surface anisotropy enhancement of Co nanoparticles by capping with Ag. Juan Bartolome, Fernando Luis, Luis Miguel Garc\'ia Vinuesa, Fernando Bartolome, Frederic Petroff, C. Deranlot, Anatoly Rogalev, P. Bencok, F. Wilhelm, Nick Brookes The present work deals with the observed enhancement of the magnetic anisotropy of Co nanoparticles by capping with Ag, and its origin. The studied samples are granular multilayers prepared by sequential sputtering deposition of Co on amorphous Al$_{2}$O$_{3}$. For low Co deposition rates, fcc Co nanoparticles with average diameter 0.7$<<$ 5 nm are formed, and self-organized in a quasi-regular spatial order. We report on the enhancement of anisotropy induced by capping with Ag (4d$^{10})$, which represents an interesting case due to its intermediate spin orbit coupling and Co-interface metal binding energy, lying between those of Cu and Au [1]. We present magnetic measurements and X-ray Magnetic Circular Dichroism (XMCD) data on the L$_{2,3 }$edges of Co, showing an increase in the mean orbital moments of Co, and a polarization of the 3d bands of Ag observed ayt the L$_{2,3}$ edges of Ag. From the XMCD K-edge spectra of Co we verify that the induced magnetic moments of the 3d and 4p bands of Co have opposite sign. We prove therefore that surface anisotropy enhancement is caused by the increase of surface Co orbital moments due to hybridization of the 3d Co and 4d Ag bands. [1] F. Luis et al., Europhys Lett. 76, 142 (2006). [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V14.00007: Synthesis and characterization of hard/soft bimagnetic nanoparticles Vikas Nandwana, Girija Shankar Chaubey, Kazuaki Yano, J. Ping Liu Bimagnetic nanoparticles are synthesized from high-temperature solution method by growing soft magnetic phases on a hard magnetic phase. The hard phase is chosen as the FePt phase and the soft phases include Fe$_{3}$O$_{4, }$CoFe$_{2}$O$_{4}$ and FeCo. The soft phases can be coated or attached to the hard phase by changing reaction conditions. When the soft phases are coated on the hard phase, core/shell structured bimagnetic nanoparticles are formed; when the soft phases are attached to the hard phase, brick-like bimagnetic nanoparticles are formed. Magnetic properties of these nanoparticles are affected by dimensions of the soft and hard components due to the exchange coupling between them. Upon reductive annealing, an assembly of the bimagnetic nanoparticles is transformed into a hard magnetic nanocomposite with enhanced energy product which is 35{\%} higher than single FePt phase. With proper choice of materials and dimension of both phases, these bimangetic nanoparticles may be used as building blocks for novel functional nanomaterials for various applications. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V14.00008: Element- and depth resolved magnetic study of Sm-Co/Fe exchange spring magnet films with enhanced effective coupling Y. Choi, J. S. Jiang, J. E. Pearson, S. D. Bader, Y. Ding, Z. L. Wang, A. Zambano, M. Murakami, I. Takeuchi, J. P. Liu Sm-Co/Fe exchange-spring bilayers with intermixed interfaces exhibit enhanced exchange coupling effectiveness [1]. We have examined the element- and depth-resolved magnetization reversal process using x-ray resonant magnetic scattering (XRMS) magnetometry. The XRMS measurements indicate significant Co diffusion into the Fe layer, and electron microscopy observations give consistent results in the composition profiles. Using model concentration profiles in combination with micromagnetic codes, we simulated demagnetization curves that are in good agreement with the XRMS results. The results reveal that the enhanced exchange coupling effectiveness is due to the intermixing in the interfacial region and that the diffused Co behaves similarly to the surrounding Fe. [1] J. S. Jiang et al., Appl. Phys. Lett. 85, 5293 (2004). [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V14.00009: Bulk nanocomposite magnets with enhanced exchange coupling Chuan-bing Rong, Vikas Nandwana, J. Ping Liu Bulk FePt/Fe$_{3}$Pt nanocomposite magnets have been prepared by compaction of chemically synthesized nanoparticles. Advanced compaction techniques, warm compaction, shock-wave compaction, spark plasma sintering and microwave sintering, have been adopted in our research. Density of the bulk samples up to 95{\%} theoretical value has been obtained while the nanostructured morphology is retained. It is found that the density increases with the compaction pressure and temperature and is also affected by the phase transition of the FePt compound from the face-centered cubic structure to the face-centered tetragonal structure. Pressure is more effective in reaching high densities. Comparison of different compaction techniques has been made. It is also found that compaction of the particles and post-annealing lead to enhanced intergrain exchange coupling. Energy products up to 16.3 MGOe of the isotropic bulk nanocomposite magnets have been achieved, which is significantly higher than the theoretical limit for fully dense single-phase FePt magnets. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V14.00010: High-throughput Investigation of Exchange Coupling Interaction in Soft/Hard Magnetic Bilayer Systems A. Zambano, H. Oguchi, I. Takeuchi, Y. Choi, J. S. Jiang, J. P. Liu, S. Lofland, D. Josell, L. Bendersky To enhance the maximum energy product of exchanged coupled hard/soft phase nanocomposite, we need to establish the parameters that govern the exchange coupling interaction (ECI). To this end, on single chips, up to a hundred thin film bilayer samples were grown by e-beam evaporation varying composition and soft layer thicknesses (t$_{s})$. Magnetic hysteresis loops were taken by magneto-optical Kerr effect measurements. The simultaneous analysis of the samples allows us to delineate subtle variations of the exchange coupling behavior. We will show examples of studies of CoPt/(Fe, Co or Ni) and SmCo/(Fe, Co or Ni) libraries. We characterized ECI by measuring the variations of the coupling length ($\lambda )$ and the nucleation field (H$_{N})$ on various magnetic parameters, t$_{s}$, and interface conditions. The trend indicates that the dominant factors determining $\lambda $ and H$_{N }$are the hard layer magnetic constants and the saturation magnetization of the soft layer. We will also discuss the role played by other magnetic parameters. The results allow us to predict the behavior of coupled hard/soft magnetic layers in general. Supported by ONR MURI N00014-05-1-0497. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V14.00011: First-principles study of interface exchange coupling in SmCo$_{5}$/Co nanocomposite materials Dangxin Wu, Qiming Zhang, J.P. Liu, Renat F. Sabirianov Understanding the interface exchange coupling is very important in designing and enhancing the performance of exchange-coupled hard/soft phase nanocomposite magnets, which were proposed to increase the maximum energy product by combining the large anisotropy of hard phase materials and the high saturation magnetization of soft phase materials. In this talk, we will present our results of our recent first-principles calculations in investigate the interface exchange coupling between hard phase SmCo$_{5}$ and soft phase Co using superlattice model. The calculations were based on Density Functional Theory. The atomic structures were optimized and the electronic ground state was obtained. Then the noncollinear magnetic order calculations were performed to study the exchange interactions. We found that the change of the total energy is a quadratic function of angle $\theta $ between the directions of magnetic moments of hard phase and middle layer of soft phase. We also performed calculations on a SmCo$_{5}$/CoFe system in which the soft phase was doped with Fe. Comparison and discussions will be made between these two systems. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V14.00012: Simulation of magnetic coupling in die-upset composite magnets George Hadjipanayis, Alexander Gabay Die-upset composite magnets fabricated from blends of melt-spun Nd-Fe-B ribbons and coarse (micron-size) soft magnetic powders (Fe, Fe-Co) exhibit uniform magnetization behavior despite the fact that the soft magnetic inclusions are too large to be magnetically coupled through inter-phase exchange interactions. In this study, we present the results of numerical simulation showing that in the case of a layered microstructure (typical of the die-upset composites) the long-range magnetostatic interactions assure smooth demagnetization curves. Still, at least partial exchange coupling is required to have an increased remanence. It is argued that the effect of magnetostatic coupling in composite magnets with a layered morphology considerably relaxes the strict requirements for the size of the soft inclusions and it may facilitate the future development of high-performance composite anisotropic permanent magnets. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V14.00013: High performance isotropic R-(Co,Fe)-C magnets by melt spinning (R= Sm, Pr) Dilara Sultana, Alexander Gabay, George Hadjipanayis In our search of high performance isotropic magnets for high temperature applications a large variety of quasiternary R$_{x}$(CoFe)$_{100}$-x-y$_{ }$C$_{y}$ alloys with x=10.5-15,y = 0-4.5 were produced by melt-spinning. The alloys were characterized by X-ray diffraction and Scanning and transmission electron microscopy. The Fe substitution for Co and Pr substitution for Sm increases magnetization, while the C addition increases the coercivity. Remanence values of more than 100 emu/g and coercivity of 13 kOe were obtained. The hard magnetic properties are associated with the disordered hexagonal 2:17 compound. The microstucture properties are studied to optimize the properties for high temperature applications. The effects of the alloying elements (Pr,Fe,C) , quenching rate and heat treatments are analyzed and coercivity mechanism are discussed. [Preview Abstract] |
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