Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W14: Focus Session: Nanomagnetic Oxides |
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Sponsoring Units: GMAG DMP Chair: Christian Binek, University of Nebraska Room: Colorado Convention Center Korbel 4D |
Thursday, March 8, 2007 2:30PM - 2:42PM |
W14.00001: Magnetic Properties of Epitaxial Cr/Cr$_{2}$O$_{3}$/ Cr Multilayers Tathagata Mukherjee, Sarbeswar Sahoo, Christian Binek We study Cr/Cr$_{2}$O$_{3}$/Cr trilayer structures grown by Molecular Beam Epitaxy on (111) oriented Al$_{2}$O$_{3 }$substrates. X-ray diffraction reveals perfect single crystalline (110) Cr and stoichiometric single crystalline Cr$_{2}$O$_{3}$ (111) films. Both, Cr and Cr$_{2}$O$_{3 }$order antiferromagnetically with bulk N\'{e}el temperatures of 311 and 307K, respectively. Cr is an itinerant antiferromagnet where the antiferromagnetic (AF) order establishes as an incommensurate spin density wave. Cr$_{2}$O$_{3 }$in contrast is an AF insulator with localized magnetic moments where magnetoelectric and piezomagnetic effects are both symmetry allowed. Its insulating, magnetoelectric and piezoelectric properties make Cr$_{2}$O$_{3}$ an interesting material for extrinsically controlled tunnel barriers in TMR type structures. The lattice mismatch of $\sim $1.2{\%} at the Cr -- Cr$_{2}$O$_{3}$ interface creates a strong stress induced piezomagnetic moment revealed by SQUID measurements. The interaction between the piezomoment and the spin distribution at the Cr- interface gives rise to a rich scenario of magnetic proximity effects which we study by SQUID magnetometry, magneto-optical Kerr effect and electrical transport measurements. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W14.00002: Piezomagnetism in Epitaxial Cr$_{2}$O$_{3}$ Thin Films Yi Wang, Sarbeswar Sahoo, Christian Binek Recently, the magnetoelectric material Cr$_{2}$O$_{3}$ attracted renewed interest due to its potential for future spintronics applications which can be realized by novel magnetic thin film heterostructures [1]. Here we study thin films of Cr$_{2}$O$_{3}$ (111) on c-Al$_{2}$O$_{3}$ (111) substrate which are grown by thermal evaporation of Cr metal in an O$_{2}$ atmosphere. X-ray diffraction data reveal stoichiometric epitaxially grown Cr$_{2}$O$_{3}$ (111) films. Owing to a lattice mismatch of $\sim $4{\%} at the interface between the Al$_{2}$O$_{3 }$substrate and the film we observe a strong stress induced piezomagnetic moment in the Cr$_{2}$O$_{3}$ film. We measure the temperature dependence of this piezomoment by Superconducting Quantum Interference Device (SQUID) magnetometry and Kerr rotation. The presence of high inherent stress, a significant piezomagnetic moment and the possibility to realize high electric fields makes our Cr$_{2}$O$_{3}$ thin films ideal candidates for the challenging quest of the symmetry allowed but hitherto undiscovered piezomagnetoelectric effect. [1] Ch. Binek, B. Doudin, J. Phys. Condens. Matter\textbf{ 17}, L39 (2005). \newline [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W14.00003: Magnetoelectric thin films for electrically controlled exchange bias in spintronic applications Xi He, Sarbeswar Sahoo, Srinivas Polisetty, Yi Wang, Tathagata Mukherjee, Christian Binek Epitaxial (111) oriented thin films of magnetoelectric (ME) Cr2O3 are grown by MBE. These films are the key component of Cr2O3(111)/(Co/Pt)3 heterostructures allowing for electrically controlled exchange bias (EB) and novel spintronic applications [1]. Pure voltage control of magnetic configurations in TMR-type devices is proposed as an alternative to current-induced switching. Basic effects of electrically controlled EB and its ME switching are studied by magnetometry and polar Kerr rotation. Exchange coupling between the ME antiferromagnet Cr2O3 and the ferromagnetic CoPt multilayer gives rise to perpendicular EB. The latter is controlled by axial electric fields inducing excess magnetization at the interface which controlls the EB field. Recently, the sign of the EB field has been tuned via a field cooling procedure where a Cr2O3(111) bulk/(Co/Pt)3 system is exposed to either parallel or antiparallel axial magnetic and electric fields [2]. Here we study this ME switching effect in an all thin film heterostructure. [1] Ch.Binek, B.Doudin, J. Phys. Condens. Matter 17, L39 (2005). [2] P.Borisov et al., Phys. Rev. Lett. 94, 117203 (2005). [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W14.00004: Coupling Magnetism to Electricity in Multiferroic Heterostructures Invited Speaker: Complex perovskite oxides exhibit a rich spectrum of functional responses, including magnetism, ferroelectricity, highly correlated electron behavior, superconductivity, etc. The basic materials physics of such materials provide the ideal playground for interdisciplinary scientific exploration. Over the past decade we have been exploring the science of such materials (for example, colossal magnetoresistance, ferroelectricity, etc.) in thin film form by creating epitaxial heterostructures and nanostructures. Among the large number of materials systems, there exists a small set of materials which exhibit multiple order parameters; these are known as multiferroics. Using our work in the field of ferroelectric (FE) and ferromagnetic oxides as the background, we are now exploring such materials, as epitaxial thin films as well as nanostructures. Specifically, we are studying the role of thin film growth, heteroepitaxy and processing on the basic properties as well as magnitude of the coupling between the order parameters. In our work we are exploring the switchability of the antiferromagnetic order using this coupling. What is the importance of this work? Antiferromagnets (AFM) are pervasive in the recording industry. They are used as exchange biasing layers in MTJ's etc. However, to date there has been no antiferomagnet that is electrically tunable. We believe that the multiferroic BiFeO3 is one compound where this can be observed. The next step is to explore the coupling of a ferromagnet to this antiferromagnet through the exchange biasing concept. Ultimately, this will give us the opportunity to switch the magnetic state in a ferromagnet (and therefore the spin polarization direction) by simply applying an electric field to the underlying antiferromagnetic ferroelectric. In this talk, I will describe our progress to date on this exciting possibility. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W14.00005: Coercivity and Nano-structure in Magnetic Spinel Mg(Mn,Fe)$_{2}$O$_{4}$ Chenglin Zhang, S. Yeo, S.-W. Cheong We discovered that the micro-to-nano-structure of Mg(Mn,Fe)$_{2}$O$_{4}$ drastically changes with different thermal treatment. This extraordinary structural evolution is associated with spinodal chemical decomposition associate with the Jahn-Teller structural distortions around Mn ions. The magnetic properties of the polycrystalline Mg(Mn,Fe)$_{2}$O$_{4}$ vary with the structural progress. Particularly, the Curie temperature and magnetic coervcivity considerably change with the structural evolution. The significantly-enhanced coercivity in the system with elongated nanostuructre stems from the large shape anisotropy of the nanostructure. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W14.00006: Magnetic Properties of Cobalt-Ferrite Nanoparticles Prepared by a Sol-Gel Synthesis Technique Thomas Ekiert, Karl Unruh, E. Carpenter, K. Pettigrew, J. Long, D. Rolison Cobalt-ferrite nanoparticles have been prepared as highly porous aerogels using a sol-gel technique and characterized by XRD, TEM, and nitrogen-sorption porosimetry measurements. The XRD patterns for calcined Co-ferrite aerogels corresponded to a cubic structure with a lattice parameter near that of bulk Co-ferrite and a particle size of about 6 nm. TEM images indicated a similar particle size and a morphology similar to that of silica aerogels. The magnetic properties of these materials have been studied from 5 K to 340 K. Hysteresis loop measurements indicated that the coercivity and saturation magnetization of these materials evolves from nearly 19 kOe and 56 emu/g at 5 K to less than 10 Oe and 40 emu/g at 340 K. ZFC magnetization curves displayed a broad maximum that smoothly varied between about 300 K in an applied field of 100 Oe to about 180 K in a 10 kOe field. These measurements have been interpreted in terms of a distribution of effective particle sizes arising from a distribution in interparticle interactions. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W14.00007: Investigations of a New Diluted Magnetic Oxide with Room Temperature Ferromagnetism in Co-doped HfO$_{2}$ Y.H. Chang, W.C. Lee, M.L. Huang, S.F. Lee, Y.L. Soo, M. Hong, J. Kwo The structural, chemical, and magnetic properties of HfO$_{2}$ epitaxial films $\sim $100nm thick grown on YSZ at varying growth temperature have been systematically investigated. Nearly cobalt cluster-free films with RT ferromagnetic behaviors can be obtained via low T growth. \textit{In-situ} XPS analysis during growth indicated the formation of metallic cobalt at the initial growth stage under a low O$_{2}$ partial pressure $\sim $10$^{-10 }$Torr, and that the metallic cobalt can be mostly eliminated by raising the pressure to 10$^{-7}$ Torr. In conjunction with EXAFS local structural analysis and post annealing experiments, we infer that cobalt ions in low T grown films are located at interstitial site and appeared to be stable after being annealed in O$_{2}$ at 350$^{\circ}$C. Further Hall measurements are now underway to measure the carrier concentration, and to elucidate their role to the apparent ferromagnetism. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W14.00008: Magnetocaloric effect (MCE) in Nickel Ferrite nanoparticles J. Gass, M.B. Morales, N.A. Frey, M.J. Miner, S. Srinath, H. Srikanth We report on the magneto caloric effect (MCE) in a Nickel ferrite (NiFe2O4) nanoparticle system. The nanoparticles were synthesized using chemical co-precipitation. Extensive characterization of structural and magnetic properties was done using XRD, TEM, DC and AC magnetization, and transverse susceptibility. The change in entropy was calculated using the thermodynamic Maxwell relation from the family of M-H curves taken at different temperatures. Maximum entropy change in nanoparticle systems is influenced by particle size, anisotropy, and collective dipolar behavior. While the MCE is not as large as that reported in bulk systems, there are advantages as ferrite nanoparticles are easily produced and the operational temperature is tuned by the average particle size. In our studies, we observed a sharp peak in M-T curves at around 60K in addition to the blocking transition which occurs at 120 K. This results in a larger entropy change in comparison with the MCE results on other reported ferrite nanoparticles. The origin of this anomalous MCE is analyzed in the context of surface anisotropy and other possible contributions in the NiFe2O4 system. Work supported by NSF through grant CTS-0408933. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W14.00009: Verwey transition and magnetic irreversibility in nano-crystalline magnetite produced by magnetotactic bacteria T. Prozorov, R. Prozorov, T.J. Williams, D.A. Bazylinski, S.K. Mallapragada, B. Narasimhan Magnetic properties of 50 nm magnetite nanocrystals from different strains of magnetotactic bacteria are compared to high quality single crystal. It is found that the Verwey transition depends mostly on the particle shape. It is sharpest and occurs at a temperature approaching bulk values in elongated nanoparticles from MV-1 bacteria. This result contradicts previous reports of the Verwey temperature reduction in nanoparticles. Magnetic irreversibility below the Verwey transition shows thermal-history dependence and, in nanoparticles, is strongly influenced by the interparticle interactions. Collected data are analyzed in terms of the interplay between crystalline and shape anisotropies as well as collective behavior of the nanoparticles. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W14.00010: Magnetic properties of Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$ ferrofluids M.B. Morales, J. Gass, S.L. Morrow, H. Srikanth We study in detail the magnetic properties of ferrofluids consisting of $\sim $0.02 g/cm$^{3}$ volume concentration of surfacted Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$ nanoparticles suspended in two solvents, hexane and dodecane, with different room temperature viscosities. DC and AC magnetization measurements were done using a Physical Properties Measurement System (PPMS). Hysteresis loops of Fe$_{3}$O$_{4}$ in both liquids at different temperatures show that the particles are superparamagnetic having low coercivities even at low temperature. CoFe$_{2}$O$_{4}$-based ferrofluids, on the other hand, have a wide range of grain sizes and show high coercivity of 10 kOe at low temperature. From the ZFC-FC curves, the blocking temperature was determined to be 76 K and 223 K for Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$ in dodecane, respectively. To probe the dynamic relaxation effects, temperature-dependent complex AC susceptibility of all the ferrofluids were measured at frequencies of 100 Hz and 10 kHz. From these data sets, the relaxation contributions due to Neel and Brownian mechanisms were identified. We will also report on systematic magnetic measurements and analysis of ferrofluids with different nanoparticle concentrations. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W14.00011: Quantum Monte-Carlo Study of Mn and Mn-oxide clusters. Hiori Kino, Lucas K. Wagner, Lubos Mitas Many molecules and clusters of Mn and Mn-oxide have not only interesting physical properties but also can be found in enzymes as important components in biochemical reactions. The electronic structure calculations of these systems are difficult and, for example, choice of exchange-correlation functionals in Density Functional Theory can significantly influence both ground state geometries and spin-state predictions. Therefore, highly accurate calculation is very desirable for these systems. Experimentally, it is established that the Mn dimer is a van der Waals system with weak binding, however, the spin multiplicity has not been settled unambiguously with possibilities covering a range from singlet, triplet, etc, up to 2S+1=11. On the other hand, Mn$_n$O$_n$ molecules are quite well understood as being a high-spin system, but their geometries depend on the exchange-correlation functionals. We will present our recent results from the fixed-node quantum Monte Carlo calculations of these systems. We will also report on recent progress in modeling the [4Mn-4O-Ca] cluster structural prototypes for the oxygen evolving center in green plants Photosystem II. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W14.00012: Magnetic Isomerization of Chromium Clusters Wei Jiang, Forrest Payne, Louis Bloomfield We have used the Stern-Gerlach deflection technique to study magnetic properties of chromium clusters of different sizes (N=20-133) at different temperatures (T=60K-120K) and magnetic fields. Though chromium bulk is antiferromagnetic, we observed that nearly all these clusters are magnetic. And the deflection profiles of them suggest that two or more magnetic isomers exist in the beam, which have significantly different magnetic moments. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W14.00013: Simulation of interacting nanoparticles with random anisotropy axes Julio F. Fernandez, Juan J. Alonso We report Monte Carlo simulation results for the collective behavior of single-domain nanoparticles with randomly oriented easy magnetization axes. Such randomness may follow from a random orientation of the crystalline axes within each nanoparticle. Dipole-dipole interactions, as well as nearest neighbor exchange interactions of various strengths are taken into account. We report on the effect random anisotropy has on long range order as well as on magnetic relaxation at low temperature. [Preview Abstract] |
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