Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T16: Focus Session: Magnetic Nanostructures, Materials & Effects |
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Sponsoring Units: DMP GMAG Chair: Jiwei Lu, University of Virginia Room: D173 |
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T16.00001: Magnetocaloric effect and refrigerant capacity in Eu$_{8}$Ga$_{16}$Ge$_{30}$ clathrates and Eu$_{8}$Ga$_{16}$Ge$_{30}$-EuO composites H. Srikanth, A. Chaturvedi, M.H. Phan, S. Stefanoski, G.S. Nolas, V. Franco Eu$_{8}$Ga$_{16}$Ge$_{30}$ clathrates are widely known for their excellent thermoelectric properties. Recently, we have discovered the giant magnetocaloric effect (MCE) in Eu$_{8}$Ga$_{16}$Ge$_{30}$ type-VIII clathrates. The tunable MCE and refrigerant capacity (RC) have also been achieved in Eu$_{8}$Ga$_{16}$Ge$_{30 }$type-I clathrates by partial substitution of Eu with non-magnetic Sr. As an interesting host matrix the type-I clathrates are combined with EuO at different portions (80{\%}/20{\%}, 70{\%}/30{\%}, 65{\%}/35{\%}, 60{\%}/40{\%}, 40{\%}/60{\%}) for making novel composites with enhanced RC over a tunable temperature range (10-100K). We have achieved a very large RC of 794 J/kg at 5T over a 70K in the clathrate type I -- EuO (40{\%}/60{\%}) composite, which is the largest value ever achieved among the existing materials for magnetic refrigeration around 70K. This composite is very attractive for magnetic refrigeration for nitrogen liquefaction. A new potential of using the type VIII clathrate -- EuO composite (50{\%}-50{\%}) to produce refrigeration in two different temperature ranges has been proposed. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T16.00002: Exploration of thermal conductivity, Seebeck coefficient, and Lorenz number deviations in Ni-Fe alloy films B.L. Zink, A.D. Avery, R. Sultan, D. Bassett, G. Cotteril As electronic and spintronic systems continue to shrink, exploration of the fundamental physics affecting thermal transport in prospective materials becomes increasingly essential. For example, the potential use of spin-torque driven domain wall motion in ferromagnetic nanowires as a memory element requires application of large current densities to these tiny structures. The resulting heating could have both helpful and harmful effects, and is in general not yet well-understood. This is partly due to a gap in the fundamental knowledge of thermal properties of nanoscale systems that is due to the challenging nature of the necessary measurements. We have recently developed a micromachined thermal isolation platform that allows measurement of thermal conductivity, electrical conductivity, and thermopower (or Seebeck effect) in thin film systems. In this talk we present our recent data on thermal conductivity, resistivity, and Seebeck coefficient, for Ni-Fe alloy films with thicknesses varying from 25-100 nm. We compare our results to the predictions of the Wiedemann-Franz law and discuss variations represented by deviations from the Sommerfeld value of the Lorenz number, and conclude with our plans to extend the technique to yet smaller structures. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T16.00003: Thermodynamics and Magnetocaloric properties of Fe/Cr Superlattices T. Mukherjee, S. Michalski, R. Skomski, D.J. Sellmyer, Ch. Binek We explore MC properties of tailored Fe/Cr superlattices involving simple 3d metals. Our multilayers are fabricated by pulsed laser deposition with emphasis on maximizing magnetic entropy changes near room temperature. We use nanostructuring\footnote{Phys. Rev. B~\textbf{79}, 144406 (2009).} to tailor magnetic interaction and exploit geometrical confinement in order to fit the FM to paramagnetic transition temperature of the FM constituent films. In concert this leads to an optimized global metamagnetic transition maximizing the isothermal entropy change. Thermodynamic and MC properties of such Fe/Cr superlattices are studied with the help of SQUID magnetometry. Entropy changes are deduced via the Maxwell relation in single phase regions and via the Clausis-Clapeyron relations at first order metamagnetic transitions, X-ray diffraction and X-ray reflectivity are used to correlate structural data with the magnetic properties. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T16.00004: Magnetocaloric effect in heterostructures of Ni$_{x}$Cu$_{(1-x)}$ alloys C.A. Bauer, P.B. Jayathilaka, R.V. Rupani, D.D. Belyea, Casey W. Miller We used 99.9{\%} compound targets to co-sputter Ni$_{x}$Cu$_{(1-x)}$ thin film multilayers composed of sub-layers with differing alloying compositions on silicon and oxidized silicon substrates. Each system had a Ta underlayer and capping layer. XRD was used to determine structural properties, showing a (111) preferred orientation for all Ni$_{x}$Cu$_{(1-x)}$ layers. In-Plane XRD was used to check polycrystallinity. Energy-dispersive X-ray scattering is used to determine the relative concentrations of Nickel and Copper, with XRD data corroborating the EDX results from Vegard's law. The magnetic properties of the systems are characterized and the magnetic entropies are calculated near the relevant critical temperature. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T16.00005: Thermal conductivity behaviour of magnetic fluids R.A. Medina-Esquivel, J. Mendez-Gamboa, J. Tapia, J.J. Alvarado-Gil We study the thermal conductivity of five kinds of Magnetic Fluids (MFs) by varying the magnetic material volume fraction and the direction and intensity of an homogeneous magnetic field: The studied MFs are: Magnetorheological fluids (MRF), carbon coated and uncoated Fe nanofluids (CcFeNF, FeNF), ferrofluis (FF); and two kind of composed fluids; ferrofluids loaded with carbon nanotubes (FFCNTs) and ferrofluids loaded with carbon nanofibers (FFCNFs). MRF and FFCNFs fluids increase its thermal transport along the field direction; the thermal enhancement in MRF was dramatically overtaken by the FFCNFs, but in contrast; the rest of the fluids did not present thermal conductivity enhancement under the field. Theoretical models show that thermal resistance at the nanoscale level presents a very important role in the thermal transport among linked particles, this is the reason why FFCNTs, FF, FeNF, and CcFeNF did not present an increase in its thermal conductivity under the action of the magnetic field, although its chain-like structuring. We believe that these experimental finding may have significant application in the area of thermally tailored materials. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T16.00006: Large Seebeck coefficient in frustrated doped Mott insulators Louis-Fran\c{c}ois Arsenault, B. Sriram Shastry, Patrick S\'{e}mon, Andr\'{e}-Marie Tremblay Since calculations based on the standard Kubo formula have proven extremely difficult for electric and thermal transport, Shastry and co-workers [1] suggested two novel approximate ways to obtain the thermopower (S) in interacting systems. One method is based on the high-frequency limit. The other, based on ideas of Kelvin, is purely thermodynamical. With these we study the Hubbard model on a 3d FCC lattice, a frustrated lattice. The high dimensionality of the problem justifies the use of dynamical mean field theory (DMFT). CTQMC in the hybridization expansion and the fast IPT are the impurity solver. The Seebeck coefficient is obtained as a function of doping and temperature for different U. Within DMFT, vertex corrections vanish for transports coefficients, hence the bubble suffices. This enables us to further assess how both approximate methods compare with each other and with the DC Kubo approach. At low T, results can be interpreted in terms of effective Fermi temperatures and carrier number.\\[4pt] [1] B.S. Shastry, Rep. Prog. Phys. 72, 016501 (2009) [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T16.00007: Spin dynamics simulations for a nanoscale Heisenberg antiferromagnetic film Zhuofei Hou, David Landau, G. Malcolm Stocks Thermoinduced magnetization(TiM) is a novel response predicted to occur in nanoscale antiferromagnetic (AF) materials. Extensive Monte Carlo simulations\footnote{G. Brown, A. Janotti, M. Eisenbach, and G. M. Stocks, Phys.Rev.B {\bf 72}, 140405(2005)} have shown that TiM is an intrinsic property of the AF classical Heisenberg model. To obtain a fundamental understanding of TiM, spin dynamics (SD) simulations are performed to study the spin wave behavior, which seems to be the cause of TiM. A classical Heisenberg model with an AF nearest-neighbor exchange interaction and uniaxial single-site anisotropy is studied. Simple-cubic lattices with two free-surfaces and periodic boundaries parallel to the surfaces are used. We applied fast SD algorithms with 4th-order Suzuki-Trotter decompositions of the exponential operator. Discrete spin wave modes due to spin wave confinement\footnote{{\em Spin Wave Confinement}, edited by S. O Demokritov (Pan Stanford Publishing, Singapore, 2008)} are found in transverse S(q,\,$\omega$) in the perpendicular direction to free surfaces. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T16.00008: Thermomagnonic spin transfer in textured magnets Alexey A. Kovalev, Yaroslav Tserkovnyak We study interplay between the spin-energy transport and magnetization dynamics in ferromagnetic insulators with magnetic textures. With the help of the Onsager reciprocity principle we construct a phenomenological theory capable of describing various thermomagnonic effects. Motion of domain walls by thermal gradients and generation of heat flows by magnetization dynamics are suggested. By estimating the kinetic coefficients (such as $\beta$ like viscous coupling) for realistic materials (e.g. Yttrium iron garnet), we analyze the feasibility of mentioned effects for energy related applications. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T16.00009: Rotating magnon wavepackets in ferromagnets and thermal Hall effect Ryo Matsumoto, Shuichi Murakami We theoretically construct the semiclassical equation of motion of the magnon wavepacket in an insulating ferromagnets, in analogy with the electron systems. We find that the magnon wave packet has nonzero angular momentum, which consists of two parts: the self-rotational motion and the revolving motion (edge current). We show that these are expressed in terms of the Berry curvature in \textbf{k}-space, i.e., these arise from the magnon band structure. Furthermore, we find that the thermal Hall effect of the magnon is totally due to the magnon edge current, and present an intuitive picture of the thermal Hall effect. We also construct the linear response theory for the thermal Hall effect, and compare the results with the previous works with an example of Lu$_2$V$_2$O$_7$. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T16.00010: Magnetocaloric Properties of Thin Film Heterostructures H. Kirby, C. Bauer, B.J. Kirby, J. Lau, C.W. Miller In an effort to understand the impact of nanostructuring on the magnetocaloric (MC) effect, we have studied gadolinium in MgO/W(50 {\AA})/[Gd(400 {\AA})/W(50 {\AA})]$_{8}$ heterostructures [Miller et al., J. Appl. Phys. 107, 09A903 (2010)]. The entropy change peaks at a temperature of 284 K with a value of 3.4 J/kg K for a 0--30 kOe field change. Polarized neutron reflectometry was used to determine the depth profile of the magnetic moment per Gd atom, m$_{Gd}$ in a Gd/W multilayer. Our results suggest that creating materials with Gd-ferromagnet interfaces may increase the m$_{Gd}$, leading to enhanced MC properties. Therefore SiOx/Fe(50 {\AA})/Gd(300 {\AA})/Fe(50 {\AA}) heterostructures have been investigated. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T16.00011: Antiferromagnetism in BaF$_{2}$/Fe$_{x}$Ni$_{1-x}$F$_{2}$ bilayers Felio Perez, Trent Johnson, David Lederman A series of crystalline BaF$_{2}$/epitaxial (110) Fe$_{x}$Ni$_{1-x}$F$_{2}$ samples were deposited on MgF$_{2}$ (110) via molecular beam epitaxy. The Fe concentration x was determined from x-ray diffraction measurements of the [110] lattice parameter. The actual thickness of each layer and the roughness of each interface were determined by fitting x-ray reflectivity data. The antiferromagnetic ordering of samples with x = 0.04, 0.27, 0.40, 0.46, 0.50, and 0.85 were studied and compared with BaF$_{2}$/NiF$_{2}$ (x=0) and BaF$_{2}$/FeF$_{2}$ (x=1) bilayers, via standard magnetometry measurements. A significant enhancement of the N\'{e}el temperature in alloys and evidence of spontaneous magnetization along c-axis were found. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T16.00012: MnCu$_{4}$In: a new high temperature ferromagnet Alessia Provino, Durga Paudyal, Sudesh K. Dhar, Maria Luisa Fornasini, Pietro Manfrinetti, Vitalij K. Pecharsky, Karl A. Gschneidner Jr. The new intermetallic compound MnCu$_{4}$In has been synthesized and investigated. The crystal structure, studied by single crystal and powder X-ray diffractions, shows that the compound crystallizes into its own hexagonal prototype (\textit{hP}12-$P$6$_{3}$\textit{mc}) derived from the MgZn$_{2}$-type. The measured magnetic and physical properties indicate that, in contrast to the antiferromagnetic MnCu$_{4}$Sn (MgCu$_{4}$Sn-type), MnCu$_{4}$In is a high temperature ferromagnet with $T_{C}$ = 540$^{\circ}$C. In order to understand the physics involved, the first principles calculations have been performed and compared with the MnCu$_{2}$Al-type MnCu$_{2}$In(Sn) phases and the rare earth representatives GdCu$_{4}$In and GdCu$_{2}$In. Work partially supported by the US DOE, Division of Materials Science and Engineering (Office of Basic Energy Sciences). [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T16.00013: The layer-by-layer growth of ferromagnetic $\tau $ phase MnAl thin films by Bias Target Ion Beam Yishen Cui, Wenjing Yin, Jiwei Lu, Stuart Wolf It is well known that the metastable $\tau $ phase of MnAl has a L10 structure (chemical ordering along [001] directions) and is the only ferromagnetic phase of this binary intermetallic. In our study, alternating Al/Mn quasi-monolayer deposition was developed using a novel Bias Target Ion Beam deposition technique, that enabled precise control of the microstructural growth. We have obtained epitaxial $\tau $ phase MnAl thin films ($\sim $10 nm thick) on single crystal MgO substrates with improved saturation magnetization and anisotropy in comparison with co-sputtered ultra thin films. We will discuss the microstructure and magnetic behaviors of MnAl films in detail. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T16.00014: Field induced first order phase transition in the antiferromagnet Yb$_{3}$Pt$_{4}$ L.S. Wu, Y. Janssen, M.S. Kim, C. Marques, K.S. Park, M. Bennett, M.C. Aronson, S.X. Chi, J.W. Lynn Yb$_{3}$Pt$_{4}$ is an antiferromagnet that orders at T$_{N}$=2.4K. Magnetic fields B suppress T$_{N}$, and the B-T phase line T$_{N}$(B) terminates almost vertically at T=0, B$_{C}$=2.0 T. Specific heat measurements find a mean-field transition at T$_{N}$(B), and the magnetocaloric effect shows that the antiferromagnetic transition is continuous at all fields, with no associated latent heat. However, neutron diffraction measurements performed for B$\sim $B$_{C}$ find that a distinct step in the magnetization $\Delta $M occurs near the transition, with a magnitude that increases for T$<$1 K. The field dependent magnetization M(B) similarly has a metamagnetic-like step at T$_{N}$(B) below 1 K, accompanied by a sharp peak in the susceptibility whose magnitude increases but does not diverge as T$\to $0. We argue that a nonzero magnetization step $\Delta $M is required to give $\Delta $S=0 for T=0, since the vertical phase line at T=0 implies dT$_{N}$/dB=-$\Delta $M/$\Delta $S$\to -\infty $. We argue that T$_{N}$ (B) terminates at B$_{C}$ in a T=0 first order transition. [Preview Abstract] |
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