Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A32: Magnetization and Spin Dynamics |
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Sponsoring Units: GMAG Chair: Ion Garate, University of Texas at Austin Room: 336 |
Monday, March 16, 2009 8:00AM - 8:12AM |
A32.00001: Nanomagnetic Spin Fluids Skomski Ralph, A. Enders, R. D. Kirby, D. J. Sellmyer The dynamics of conventional magnets is governed by the static micromagnetic response to an external magnetic field, with corrections due to thermal excitations. For example, permanent magnets undergo aging (magnetic viscosity), and magnetic recording media lose some of the stored bits due to thermal excitation. Essential deviations from this Arrhenius (or N\'{e} el- Brown) behavior occur on a length scale below about 2 nm. The relaxation no longer obeys Kramer's escape-rate theory and must be replaced by path-integral considerations with nontrivial activation-entropy contributions. This presentation investigates several theoretical and experimental aspects of unusual magnetization dynamics in small-scale wires, thin films and dots. The first explicit example is the formation of liquid-like droplets, observed in ultrathin films with perpendicular magnetic anisotropy and characterized by 180$^{\circ}$ domains of well-defined chirality. (The one-dimensional equivalent of this phenomenon is a hard-core gas with particle-number conservation.) The second example is of theoretical nature and links the phenomenon of slow magnetization dynamics to the concept of fractional kinetics. A general feature of the considered nanomagnets is their resemblance to fluids (liquids or gases), as opposed to the glassy dynamics of conventional magnets. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A32.00002: Interface Magnetization Switching and Demagnetization in \textit{Fe/Al} films and \textit{Fe/Pt} nanoparticles Wei Lai, Tetiana Nosach, Yu Gong, Yuhang Ren, Chaehyun Kim, Savas Delikanli, Hao Zeng We report on the reversal and demagnetization processes of the Fe interface layer magnetization in thin films and nanoparticles of \textit{Fe/Al} and \textit{Fe/Pt} by time-resolved magnetization-induced second-harmonic generation. The results are compared with those of the bulk magnetization as obtained from magneto-optic Kerr effect. We realize that switching and demagnetization characteristics are distinctly different between bulk and interface layers because of the interface-derived anisotropy and the dipole interactions. In particular, the surface and interface magnetism will dominate the behaviors of nanoscale structures. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A32.00003: Magnetization hysteresis studies in Sm$_{1-x}$Gd$_x$Al$_2$ alloys U.V. Vaidya, S. Venkatesh, V.C. Rakhecha, S. Ramakrishnan, A.K. Grover SmAl$_2$ (T$_c$ $\sim$ 125 K, $\mu_{sat}$= 0.23 $\mu_B$/f.u.) is known to exhibit magnetic compensation when doped with Gd ($<$ 3 at.\%). In such stoichiometries though the magnetization gets closer to zero, there exists a large spin polarization. This makes such materials attractive candidates for applications. We have performed detailed magnetization hysteresis and other studies in the series Sm$_{1-x}$Gd$_x$Al$_2$. In x=0.02 alloy, the loops are shifted (notion of exchange bias) along negative H-axis for temperatures just above T$_{comp}$ , and along positive H-axis for temperatures T $<$ T$_{comp}$. We argue that the change in the sign of exchange bias is due to the magnetic contribution of conduction electron polarization as well as that of local magnetic moments reversing the signs. At T$_{comp}$ the width of the hysteresis loop collapses. In the given series, one can set up the system in either spin-surplus or orbital-surplus state and control the exchange bias field. The compositions with 0.03 $\leq$ x $<$ 0.06 do not exhibit zero cross over of magnetization and remain spin surplus. Our various studies and analysis shall be presented. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A32.00004: New spin modes in itinerant ferromagnets John Feldmann, Kevin Bedell We are theoretically investigating new collective spin phenomena that could exist in itinerant ferromagnetic materials such as MnSi. We postulate a ground state that is in agreement with experimental findings for these materials and derive a set of new spin collective modes. These modes would affect properties of these materials such as specific heat and spin transport and would be experimentally observable. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A32.00005: Scaling collapse of the irreversible magnetization of ferromagnetic thin films R. Das, A.F. Hebard The irreversible magnetization, \textit{$\Delta $M}, defined as the difference of field-cooled magnetization $M_{FC}$ and zero-field-cooled magnetization $M_{ZFC}$, has been measured for a variety of ferromagnetic thin films as a function of magnetic field $H$ at different temperatures $T$. Isotherms of \textit{$\Delta $M} show maxima\textit{ $\Delta $M}$_{max}$ at characteristic temperature-dependent fields $H_{m}(T)$. At very low and high magnetic fields the values of $M_{FC}$ and $M_{ZFC}$ converge and \textit{$\Delta $M} is observed to approach zero in these limits. If \textit{$\Delta $M/$\Delta $M}$_{max}$ is plotted as a function of $H/H_{m}$ for a given ferromagnetic system, the graphs for different temperatures collapse onto the same curve. This scaling collapse is clearly seen for three different ferromagnetic thin-film systems: polycrystalline gadolinium, phase separated manganites, and single domain Ni nanomagnetic grains embedded in an insulating host. Similar scaling behavior has also been observed in spin-glass material [1]. These results represent a heretofore unrecognized scaling behavior that appears to apply to a broad range of ferromagnetic systems. [1] V. S. Zotev, G. G. Kenning, and R. Orbach, Phys. Rev. B \textbf{66}, 014412 (2006) [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A32.00006: Atomistic spin-dynamics simulations from first principles theory Olle Eriksson In this talk I will present recent developments in atomistic spin-dynamics simulations using first principles theory. Details of the implementation will be give and simulations of spin-glass materials (Cu-Mn) and diluted magnetic semiconductors (Mn doped GaAs) will be presented. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A32.00007: Dynamics of Einstein - de Haas Effect: Application to Magnetic Cantilever. Reem Jaafar, E.M. Chudnovsky, D.A. Garanin Local time dependent theory of Einstein - de Haas effect is developed. We show that internal elastic twists that accompany dynamics of spins enter equations of elasticity in the universal form that does not require precise knowledge of spin-lattice interactions. As long as the space-time dependence of the magnetization is known, local elastic deformations can be computed rigorously without any unknown parameters. The theory is applied to the description of the motion of a magnetic cantilever caused by the oscillation of the domain wall. Theoretical results are compared with a recent experiment on Einstein - de Haas effect in a microcantilever. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A32.00008: Colored thermal noise in spin valves Jiang Xiao, Gerrit Bauer, Sadamichi Maekawa, Arne Brataas We report a theoretical study of the thermal electrical noise in spin valves. There are two independent noise sources in spin valves: 1) thermal agitation of charge carriers causing Johnson-Nyquist noise, 2) thermal agitation of the magnetization that contributes to the electric noise by spin and charge pumping. The noise power spectrum from the latter consists of two absorption lines at zero frequency and at the ferromagnetic resonance on top of a white noise background. The relative intensities depend on the magnetization configuration. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A32.00009: Numerical evidence for unstable magnons at high fields in the square lattice Heisenberg antiferromagnet Olav F. Sylju{\aa}sen We have found numerical evidence for decaying magnons in the square lattice spin-1/2 Heisenberg antiferromagnet when it is exposed to a strong external magnetic field. The results are obtained using Quantum Monte Carlo simulations combined with a Bayesian inference technique to obtain dynamics, and are consistent with earlier predictions from spin wave theory. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A32.00010: Low-Frequency Magnetization Noise in Spin-Valve Structures Arif Ozbay, Aisha Gokce, Edmund Nowak, Thomas Flanagan, Ryan Stearrett, Cathy Nordman We report on 1/f resistance noise due to thermally driven fluctuations of the domain structure in GMR and MTJ sensors. Resistance noise from both the free layer (FL) and reference layer (RL) is evident. A near linear scaling of the normalized noise power with the sensor's sensitivity is observed. For a given sensitivity, the RL exhibits higher noise than the FL. This appears correlated to the larger imaginary (dissipative) component in the resistance susceptibility of the RL. In addition, we find that the imaginary component is larger for layers that exhibit pronounced magnetic hysteresis, suggestive of connection between the noise and hysteresis. A model based on equilibrium magnetization fluctuations is in good quantitative agreement with the measured noise power over most of the sensor's magnetoresistive response. A magnetic 1/f noise parameter is defined which can be used to compare magnetoresistive sensors having differing sizes, sensitivities, and under different biasing conditions. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A32.00011: Phase Diagram of Equilibrium Domain-Wall Solutions in Finite-Size ECC Media Sonali Mukherjee, Luc Berger Reversal in ECC media where hard and soft anisotropy magnetic material are exchange coupled has been studied because it has high thermal stability with low reversal field. Using Euler-Lagrange condition, we have studied the field evolution of domain-wall solutions in ECC for various anisotropy ratios of hard and soft phase and soft-phase length scales. We find that there exist 3 critical fields. At the field H1s, the domain-wall solution Es (surface domain-wall) and E1(soft-phase domain-wall) start existing. The nucleation field Hn,is the field where the energy of Es and the unreversed uniform solution E0 coincide. Above Hn, Es ceases to exist. The domain-wall propagation field Hdw is the field where the energy of soft domain wall E1 and hard domain wall E2 coincide. Above Hdw, E1 and E2 cease to exist. The reversal field is the field at which no domain-wall solutions exist anymore and is the maximum of Hn and Hdw fields. The field Hn is found to reduce with increasing soft-phase length ls, and Hdw is found to be independent of ls for ls greater than eh where eh is the domain- wall width of the hard phase. For hard/soft anisotropy ratio kh/ks less than 5, the nucleation field is always dominant. When kh/ks is greater than 5, there exits a soft-phase length lsc, at which the fields Hn and Hdw become equal. When ls is greater than lsc, Hdw dominates the reversal and, when ls is smaller than lsc, Hn is the reversal field. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A32.00012: Nature of magnetic ordering in Ni(OH)$_{2}$ nanoplates James Rall, Mohindar Seehra Nickel hydroxides are important for their potential applications in rechargeable batteries and as precursors for NiO and Ni catalysts. $\beta $--Ni(OH)$_{2}$ has the CdI$_{2}$ layered structure with Ni atoms forming a hexagonal unit cell. Here, we report on the magnetic ordering in 17 nm $\times $ 4 nm nanosheets of $\beta $--Ni(OH)$_{2}$ . Measurements of the magnetization M as a function of temperature (2K to 300K) and magnetic field H up to $\pm $65kOe are reported. M vs. T data in H =100 Oe for the ZFC case shows a peak in M at T$_{N}$ = 24 K characteristic of antiferromagnetic (AF) ordering; however for T $>$ T$_{N}$, the Curie-Weiss ($\chi $ = C/(T - $\theta ))$ fit yields $\theta $ = 26K characteristic of ferromagnetism. Following Takada (J. Phys. Soc. Jpn. 21, 2745, 1966), we measured M vs. H loops from T = 2K to 25K and observed a metamagnetic transition at H$_{c}$ = 56 kOe at 2K, with H$_{c}$ decreasing with increasing T. These results suggests strong ferromagnetic coupling among Ni within (001) sheets and a weaker antiferromagnetic coupling in the neighboring (001) sheets, and [001] as the easy axis. This model is used to determine the exchange constants consistent with the observed Curie-Weiss variation. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A32.00013: First-principles calculations of laser-induced spin manipulation in small magnetic clusters with CO Chun Li, Georgios Lefkidis, Wolfgang H{\"u}bner We present a fully {\it ab initio} controlled ultrafast magnetooptical switch and transfer mechanism in small magnetic clusters exploiting spin-orbit-coupling enabled $\Lambda$-processes [1-3]. Two-magnetic-center clusters with CO attached to one of the magnetic atoms are studied to achieve a mapping of the laser-induced spin manipulation to the IR spectrum of CO. The predicted spin-state-dependent CO frequencies can facilitate experimental monitoring of the processes. The lower electronic states of the clusters exhibit a very high degree of spin localization either at the Co or the Ni site. Spin flip on one magnetic atom and transfer from one magnetic center to the other are realized in structurally optimized magnetic clusters with fidelities that reach 99.8\%. \newline\newline[1] R. G\'{o}mez-Abal, O. Ney, K. Satitkovitchai and W. H\"{u}bner, Phys. Rev. Lett. 92, 227402 (2004)\newline[2] G. Lefkidis and W. H\"{u}bner, Phys. Rev. B 76, 014418 (2007)\newline[3] T. Hartenstein, C. Li, G. Lefkidis and W. H\"{u}bner, J. Phys. D: Appl. Phys. 41, 164006 (2008). [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A32.00014: Magnetic properties of a doped quasi-triangular lattice material, Cu$_{2(1-x)}$Zn$_{2x}$(OH)$_{3}$NO$_{3}$/(C$_{7}$H$_{15}$COO) Jian Wu, Anup K. Gangopadhyay, S.A. Solin Cu$_{2}$(OH)$_{3}$NO$_{3}$,is a geometrically frustrated layered compound in which spin S=1/2 Cu$^{2+}$ ions are arranged on a slightly distorted triangular lattice. The magnetic properties of the pure compound and of the compound intercalated with alkanecarboxylate have been extensively studied.[1] However, the effects of intralayer doping remain unexplored. The substitution of non-magnetic ions such as Zn$^{2+}$ for Cu$^{2+}$ will ultimately drive the ordering temperature toward zero [2] which may provide a candidate system possessing an exotic spin-liquid ground state. We have prepared powder samples of the Cu$_{2(1-x)}$Zn$_{2x}$(OH)$_{3}$NO$_{3}$ family and systematically investigated them by magnetic susceptibility measurements. The ordering temperature decreases from 11K to 5.6K while the C-W temperature increases from -5.1K to +2.8K as the Zn concentration increases from 0 to 65\%. To enhance the 2-dimensional characteristic and reduce the interlayer interaction, we introduce an alkanecarboxylate C$_{7}$H$_{15}$COO into the interlayer space. The experimental results we have obtained indicate that this new class of materials have much higher frustration levels $|\Theta_{cw}$ /T$_{c}|\sim$ 20 and order at a lower temperature than the doped parent compounds.\newline[1] M. A. Girtu et al, Phys Rev B 61,4117(2000).\newline[2] M. Mekata et al, J. Phys. Soc. Japan 56, 4544(1987). [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A32.00015: Coupling between optically-induced coherent spin and lattice dynamics in epitaxial Fe films Vladimir Stoica, Don Walko, Eric Landahl, Yuelin Li, Roy Clarke Spin dynamics excitation using femtosecond optical pulses in ferromagnetic thin films is a powerful technique to study spin dependent interactions in solids. One topic of interest is the temporal separation of relaxation processes related to fundamental interaction mechanisms, which include spin-orbit, spin-lattice and exchange coupling. Establishing experimentally the relaxation timescales for these couplings is an important step that assists the development of new spintronic applications. We employ time-resolved magneto-optical and X-ray diffraction probes to separate spin and lattice dynamics in epitaxial Fe samples grown by molecular beam epitaxy on Ge and MgO substrates. We study the connection between the magnetic and lattice relaxation transients excited by optical pulses. We find that coherent spin precession dynamics correlates well with thermo-elastic strain relaxation from picosecond to nanosecond time scales. [Preview Abstract] |
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