Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N42: Focus Session: Magnetic Nanoparticles, Nanostructures & Heterostructures VI |
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Sponsoring Units: DMP GMAG Chair: Ralph Skomski, University of Nebraska Room: LACC 150B |
Wednesday, March 23, 2005 8:00AM - 8:36AM |
N42.00001: Magnetic Properties of Chemically Synthesized FePt Nanoparticles Invited Speaker: J.W. Harrell Chemically synthesized FePt nanoparticles have attracted considerable attention in recent years because of their potential use in ultra-high density magnetic recording media. In the original procedure described by Sun et al., the as-synthesized nanoparticles have the fcc phase and must be thermally annealed to achieve the high-anisotropy L1$_{0}$ phase [1]. We have been addressing some of the materials problems associated with obtaining the L1$_{0}$ phase. These include lowering the ordering temperature, reducing sintering during annealing, orienting the easy axes, and understanding the size effect on chemical ordering. Additive Au and Ag significantly lower the ordering temperature, while additive Cr and Cu increase the ordering temperature; however, the onset of ordering is correlated with sintered grain growth. Sintering can be reduced by encapsulating the nanoparticles with a shell such as silicon oxide or copper. Easy-axis orientation has been achieved using L1$_{0}$ FePt nanoparticles that were directly synthesized using a high-temperature solvent [2]. The nanoparticles were dispersed in a PVC binder and oriented by drying the dispersion in a magnetic field. [1] S. Sun et al., Science 287, 1989 (2000). [2] S. Kang et al., Appl Phys. Lett. (in press). [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N42.00002: Magnetic Properties of Core-Shell FePt(CFx) Nanocluster Films Yingfan Xu, Minglang Yan, D.J. Sellmyer A core-shell FePt nanocluster system, in which the magnetic core is coated with a layer of a non-magnetic shell, is of great interest for study and tailoring magnetic properties such as magnetization, anisotropy and interparticle interactions. In this study core-shell FePt clusters with fluorocarbon (CF$_{x})$ shell are synthesized by a cluster-deposition system with gas-aggregation technique. Monodispersed core-shell structure FePt(CF$_{x}) \quad _{ }$clusters are produced with average diameter of 4 nm and with a uniform size distribution. High magnetic anisotropy L1$_{0}$ phase FePt(CF$_{x})$ cluster-assembled films were realized via post-deposition annealing. Crystal structure and nanostructure of the films were studied by XRD and TEM. Magnetic properties of the films were measured at temperatures between 10 K and 300 K. Results show that the FePt L1$_{0}$ ordering temperature is decreased by addition of CF$_{x}$.$_{ }$Interparticle interactions were studied by measuring the $\Delta $M curves. Thermal stability of the films was also studied by fitting the temperature dependence of coercivity with the Sharrock formula. Our results indicate that the magnetic properties of the core-shell FePt(CF$_{x})$ nanoclusters are tunable for various nanomagnetic applications. [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N42.00003: Magnetic and Transport Properties of Co Nanoparticles Fabricated With a Cluster Gun P. Liu, M.J. Bonder, Y. Huang, Y. Zhang, G.C. Hadjipanayis, D. Vlachos, S.R. Deshmukh Cluster guns have been found to be suitable for the fabrication of nanoparticles in a wide range of materials with the additional advantage of in-situ processing (annealing, surface passivation, etc.) of the nanoparticles inside the sputtering chamber [1, 2]. In this study, we look to optimize parameters for fabricating Co nanoparticles and embed them in a carbon matrix. Magnetic and transport properties are measured over a wide temperature range. The size and distribution of the nanoparticles can be controlled by varying the target-orifice distance, Argon pressure, sputtering time and Co magnetron power. At the lowest power used the Co nanoparticles are less than 5nm in size. At this size thermo-magnetic measurements indicate a blocking temperature of 115 K indicative of superparamagnetism in this sample. As the power is increased there is an increase of the blocking temperature commensurate with the increase in nanoparticle size as seen in bright field electron microscopy. The transport studies of these samples show a cross-over from metallic to semi-conducting behavior as the inter-particle spacing is varied. The origin of the cross-over is under investigation and the results will be reported. [1] Stoyanov S, et al., J. Appl. Phys., 93 (10): 7190 (2003). [2] Skumryev V, et al., Nature, 423 (6942): 850 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N42.00004: High coercivity in FePt nanoparticle assemblies V. Nandwana, K. E. Elkins, T.S. Vedantam, J.P. Liu Ultra-fine FePt nanoparticles have been synthesized via a novel chemical solution synthesis route. Without using a reducing agent, the stoichiometric FePt nanoparticles were produced by the decomposition of iron acetylacetonate and platinum acetylacetonate in octyl ether in the presence of oleic acid and oleyl amine. The particle size was found by transmission electronic microscopy observation to be around 2 nm. The particles were then deposited on a substrate to form thin-film-like assemblies and undertaken heat treatments. Upon annealing the as-synthesized nanoparticles were expected to transform from FCC structure to the high anisotropic FCT structure and therefore magnetic hardening was developed in the assemblies. Coercivity up to 2.7 T has been obtained in the samples with the Fe:Pt molar ratio of 1.2:1 after being annealed at 650\r{ }C for 1 hour in forming gas (Ar + 7{\%} H$_{2})$. The high coercivity indicates a highly completed phase transition from the FCC structure to the FCT structure. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N42.00005: Magnetism of Pd7Ni3 nano alloy particles Yung-Yuan Hsu, S. R. Sheen, M. K. Wu, K. W. Wang, T. P. Perng The magnetic properties of Pd$_{7}$Ni$_{3}$ alloy nano-particles, $\sim $10 nm diameter, prepared from chemical precipitation followed by reduction reaction are reported. Magnetic palladium alloys is interesting for the enhanced magnetic moments due to Ni doping into Pd, ordinarily paramagnetic, with ferromagnetic transition temperature T$_{C}$, for bulk, of about 327 K. While prepared in the nano size, depending on preparation procedures and dispersity, samples exhibit super-paramagnetism, spin-glass-like or even Curie-Weiss-like behaviors. However, enhancement of the magnetic moment remains with an enhancement magnitude of about 0.3 $\mu _{B}$ smaller and is preparation method dependent. The complicated magnetic behavior observed may suggest a surface spin effect. Possible core-shell magnetic structure for particles of such small diameter along with the chemically observed core-shell nonuniformity by EXAFS further complicated the observed magnetic behavior. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N42.00006: Magnetic Properties of CoFe2O4 Nanopillars Haimei Zheng, Rajesh Chopdekar, Yayoi Takamura, T. Zhao, Y. Suzuki, R. Ramesh, F. Zavaliche, L. Mohaddes-Ardahili, S. Shinde, S. Ogale, D. Schlom Ferrimagnetic CoFe$_{2}$O$_{4}$ spontaneously forms nanopillars embedded in a BaTiO$_{3}$ or BiFeO$_{3}$ matrix during thin film growth by pulsed laser deposition. Such thin film nanostructures show three dimensional heteroepitaxy. All the films have a large uniaxial magnetic anisotropy with an easy axis normal to the film plane. It is calculated that stress anisotropy is the main contribution to the anisotropy field. We studied the magnetic behavior of the CoFe$_{2}$O$_{4}$ nanopillars formed at different growth temperatures, with different film thickness and on various substrates. [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N42.00007: Magnetic Oxide nanoparticles D. Ruzmetov, Y. Seo, V. Chandrasekhar, L.J. Belenky, D.M. Kim, C.B. Eom, X. Ke, M.S. Rzchowski We have fabricated nanopillar arrays of epitaxial magnetic oxide thin films and heterostructures consisted of SrRuO$_{3}$, La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ and insulating barrier. The films were grown on TiO$_{2}$ surface terminated (001) SrTiO$_{3}$ substrates with atomic layer control by pulsed laser deposition with in situ high pressure RHEED, and were patterned into nanopillars using e-beam lithography and neutralized Ar ion milling with Ti and Au as milling mask materials. Scanning electron and atomic force microscopy measurements confirmed that we have produced well defined diameter 100 nm and 40 nm tall pillar arrays, which are, to our knowledge, the smallest pillars made from magnetic perovskite oxides. The LSMO pillars whose dimensions are smaller than the domain size ($\sim $150nm) and comparable to the exchange length ($\sim $50nm) are ferromagnetic at room temperature as shown by magnetic force microscopy. Using multilevel e-beam lithography we made single nano-ellipses from LSMO and SRO and wired them individually with Au leads. We performed electron transport measurements at 5K aiming to measure anisotropic magnetoresistance and coercive fields of single nano-ellipses ranging in size from 850 x 400 nm$^{2}$ to 400 x 150nm$^{2}$. Supported by NSF-ECS 0210449. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N42.00008: Volume Dependence of Thermoinduced Magnetization in Antiferromagnetic Nanoparticles Gregory Brown, Anderson Janotti, Markus Eisenbach, G. Malcolm Stocks Monte Carlo methods have been applied to classical Heisenberg models to study the thermoinduced magnetization in nanoparticles of antiferromagnetic materials. In the presence of uniaxial anisotropy, the average magnetization per spin of the individual particles is found to decrease as approximately the square-root of the particle volume, $M \sim V^{-1/2}$, which is significantly different from the $M \sim V^{-1}$ predicted by a recent theory [S. M{\o}rup and C. Frandsen, Phys.\ Rev.\ Lett.\ {\bf 92}, 217201 (2004)]. The exact value of the exponent depends on the strength of the uniaxial anisotropy, and approaches $-1/2$ from below as the strength of the anisotropy increases. In addition, the magnitude of the thermoinduced magnetization decreases as the strength of the anisotropy increases, and it vanishes in the infinite anisotropy, i.e. Ising, limit. This indicates that spin-waves are essential to thermoinduced magnetization. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N42.00009: Anomalous Temperature Dependence of Magnetic Moment in Monodisperse Antiferromagnetic Nanoparticles Dane Gillaspie, B. Gu, W. Wang, J. Shen 1 Condensed Matter Sciences Division, Oak Ridge National Laboratory*, TN 37831 2 Department of Physics and Astronomy, The University of Tennessee, TN 37996 3 Environmental Sciences Division, Oak Ridge National Laboratory*, TN 37831 Recent experiments [1] and theory [2] from AFM nanoparticles showed that they exhibit sizable net magnetization, which increases with increasing temperature. In order to further understand such peculiar temperature dependence, we have measured the magnetic properties of monodisperse hematite ($\alpha$-Fe2O3) nanoparticles, grown using a microemulsion precipitation technique, which minimizes the impact of the particle moment distribution on the measured properties of the samples. Our measured results indicate that the net magnetization of these nanoparticles, when small, indeed increases linearly with increasing temperature. This is in sharp contrast to the bulk-like behavior of $\alpha$-Fe2O3, which was observed in particles with size larger than 120 nm. [1] M. Seehra et al, Phys. Rev. B 61, 3513 (2000) [2] S. Mørup, C. Frandsen, Phys. Rev. Lett. 92, 217201 (2004) *Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725 [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N42.00010: Tailoring magnetic properties of Co-ferrite soft magnetic nanoparticles N. Poudyal, T.S. Vedantam, J.P. Liu Monodisperse Co-ferrite soft magnetic nanoparticles with particle size from 3 nm to 20 nm and different Co concentration have been synthesized by chemical solution methods. The composition was controlled by varying the mole ratios of the precursors in the solution. It has been found that magnetic properties of the nanoparticles can be tailored by changing the composition, particle size, as well as by subsequent heat treatments. Magnetization value of 223 emu/g was obtained after reduction. Particles with size less than 12 nm showed ferromagnetism-superparamagnetism transitions at temperatures between 10 K and 300K. The magnetic interaction of the nanoparticles was studied by zero-field-cooling and field-cooling experiments. The blocking temperature of the nanoparticles was found to increase with increasing particle size. The 4 nm and lesser sized particles showed exchange bias at 10 K. A coercivity value of 1.8 T was obtained at 10 K for the 20 nm particles. [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N42.00011: Magnetocaloric effect in ferrite nanoparticles D. Rebar, J. Gass, P. Poddar, H. Srikanth Miniaturization of the electronic devices for space, military and consumer applications requires cooling devices to be fabricated on a chip for power efficient, noise-free operations. Refrigeration based on the adiabatic-demagnetization has been used for several decades for cooling down to sub-kelvin temperatures. Superparamagnetic particles also hold tremendous potential towards this application. We have studied magnetocaloric effect (MCE) properties in chemically synthesized ferrite nanoparticles over a broad range in temperature and magnetic fields. Nanoparticles investigated include Fe$_{3}$O$_{4}$ (average size = 8 nm, synthesized using co-precipitation method), MnZnFe$_{2}$O$_{4}$ (average size = 15 nm, synthesized using reverse-micelle technique) and CoFe$_{2}$O$_{4}$ (average size 8 nm, synthesized using pyrolectic technique). The magnetic entropy change was calculated by applying Maxwell's relations to magnetization vs magnetic field curves at various temperatures. Our results indicate that the single-domain particles in their superparamagnetic state show a considerable entropy change near the blocking temperature. The influence of interactions on MCE effect will also be discussed. Work supported by NSF through Grant No. CTS-0408933 [Preview Abstract] |
Wednesday, March 23, 2005 10:36AM - 10:48AM |
N42.00012: Magnetic characterization of nano-sized iron oxide particles embedded in alginate hydrogel Ronald Tackett, Gavin Lawes, Elizabeth Buc, Upul Senaratne, Ratna Naik, Vaman Naik, Prem Vaishnava, Georgy Tsoi, Lowell Wenger We have examined the magnetic properties of a nanoparticle system (gamma-Fe2O3 alginate nanocomposite) prepared using eight iterations of a standard matrix-mediated precipitation reaction. Earlier measurements on the magnetic properties of lower generations of this system showed sensitivity to the number of iterations of the reaction[1]. We used DC magnetization measurements to determine the temperature and field dependence of the blocking temperature, saturation magnetization, and the coercive field. We find a low-field blocking temperature T$_B$=72K, higher than the lower generation samples. We probed the blocking dynamics of the system by measuring the temperature and frequency dependent complex AC susceptibility. These measurements yield a zero-field activation energy of approximately 2770K, which yields important information about the magnetic anisotropy in these systems. [1] R. Naik et al, J. Appl. Phys. (accepted for publication) [Preview Abstract] |
Wednesday, March 23, 2005 10:48AM - 11:00AM |
N42.00013: Reduction of Ordering Temperature in Substituted FePtM (M = Ni,Cu) Nanoparticles Formed by Chemical Synthesis Hongli Wang, Yunhe Huang, Yong Zhang, Karl Unruh, George Hadjipanayis, Dieter Weller, T. Simopoulos FePt and CoPt-type nanoparticles made by chemical synthesis have recently become promising candidates for ultra-high density magnetic recording media [1]. However, at high annealing temperatures, the particles sinter together and array formation is lost. Recent studies [2] have reported reduced transformation temperatures with the addition of Ag and Au in the FePt nanoparticles. In this study, we used M = Ni, Cu substitution to reduce the transformation temperature. The as-made FePtNi and FePtCu have the disordered fcc structure with zero coercivity at room temperature. After annealing at temperatures in the range of 300-600 $^{o}$C, the particles become fct with a coercivity of 4 kOe in FePtCu at 400 $^{o}$C and 6 kOe in FePtNi at 500 $^{o}$C. The high coercivity obtained at the lower annealing temperature suggests a lower transformation temperature in both alloys. Preliminary DSC studies showed a reduced transformation temperature in the Ni substituted samples. The structural transformations that occur after annealing and their effects on magnetic properties are currently being investigated by HREM and M\"{o}ssbausser spectroscopy. References [1] S. Sun, et al. Science, 287, 1989-1992 (2000) [2] S. Kang, et al., IEEE T MAGN, 39, 5, 2753 (2003) [Preview Abstract] |
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