Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B42: Focus Session: Magnetic Nanoparticles, Nanostructures & Heterostructures I |
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Sponsoring Units: DMP GMAG Chair: Axel Hoffmann, Argonne National Laboratory Room: LACC 150B |
Monday, March 21, 2005 11:15AM - 11:51AM |
B42.00001: On the Life and Death of High Energy Surface Magnons Invited Speaker: Surface magnons may be excited by inelastic scattering of low energy electrons. They are distinguished from surface phonons of similar energy by using spin-polarized primary electrons. We studied high-energy magnons with large wavevector, for which no other analysis technique exists. For fcc Co films on Cu(100) and hcp Co films on W(110) we found the dispersion to be determined by the surface Brillouin zone. By Fourier analysis of the energy- and momentum-distribution of experimental spectra we found the magnons to be short lived (of order of a few 10 fs). During their lifetime they move in real space by about 1 nm. Thus we probe elementary magnetic excitations on a linear space-time scale of 10 yocto-metersec. This puts the applicability of the conventional picture of spinwaves into question. * This work was done in collaboration with R. Vollmer, M. Etzkorn, A. Kumar, and H. Ibach. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:03PM |
B42.00002: Magnetic Anisotropy and Quantized Spin Waves in Hematite Nanoparticles S.N. Klausen, K. Lefmann, P.-A. Lindg{\aa}rd, L. Theil Kuhn, C. Frandsen, S. M{\O}rup Nanoparticles of the canted antiferromagnet hematite ($\alpha $-Fe$_{2}$O$_{3})$ are particularly interesting due to a complicated magnetic structure, which gives rise to rich dynamics. Further, the spin-flop Morin transition observed in the bulk is suppressed in nanoparticles. We present inelastic neutron scattering studies that exemplify aspects of magnetic dynamics of hematite nanoparticles with an average size of 11 nm. Both superparamagnetic relaxation and collective magnetic excitations with a resonance frequency of $\sim $0.26 meV have been studied. Thus detailed information on the relevant anisotropy, relaxation times and life times as well as variations with particle size have been achieved. Further, we report on a previously unobserved magnetic excitation mode with q=0 and a resonance energy of $\sim $1.1 meV, i.e. a second collective magnetic excitation. The lack of dispersion in the mode is a clear evidence of spin wave quantization in the nanoparticles. The related anisotropy remains negative with decreasing temperature, in contrast to the change of sign at the Morin transition in the bulk. This explains the suppression of the Morin transition in hematite nanoparticles. [Preview Abstract] |
Monday, March 21, 2005 12:03PM - 12:15PM |
B42.00003: Magnetic Switching In Permalloy Ellipses By Field Pulse Hyuk-Jae Jang, Pete Eames, E. Dan Dahlberg, Doug Stone The switching behavior of patterned Ni$_{81}$Fe$_{19 } $(permalloy) particles induced by a magnetic field pulse has been studied using magnetic force microscopy (MFM). Arrays of permalloy elliptical elements, 340 nm$\times $130 nm with a thickness of 40 nm, were patterned directly onto a copper microstrip line. The magnetization of the ellipses was first saturated in one direction and then a pulsed magnetic field was applied along their hard axis with a constant bias field (348 Oe), which was lower than their switching field $H_{s}$ ($\sim $ 0.85$H_{s})$, along their easy axis. The switching probabilities $P(t)$ of the elements were measured repeating the process 100 times at room temperature with various durations and amplitudes of the field pulse. The ellipses showed quite different behaviors in $P(t)$ even though their static switching fields had a similar angular dependence. The data will be discussed in terms of a model for the thermally activated switching and the Stoner-Wohlfarth model. [Preview Abstract] |
Monday, March 21, 2005 12:15PM - 12:27PM |
B42.00004: Current induced Spin Torque in a nanomagnet Xavier Waintal, Olivier Parcollet In a nanomagnet (whose total spin $S_0 \leq 1000$), very small polarized currents can lead to magnetic reversal. Treating on the same footing the transport and magnetic properties of a nanomagnet connected to magnetic leads via tunneling barriers, we derive a closed equation for the time evolution of the magnetization. The interplay between Coulomb blockade phenomena and magnetism gives some additional structure to the current induced spin torque. In addition to the possibility of stabilizing uniform spin waves, we find that the system is highly hysteretic: up to three different magnetic states can be simultaneously stable in one region of the parameter space (magnetic field and bias voltage). [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B42.00005: Multiple Reversal Paths in Magnetization Switching for Models of Iron Nanopillars S.H. Thompson, G. Brown, P.A. Rikvold Stochastic micromagnetic simulations are employed to study switching in three-dimensional magnetic nanopillars exposed to highly misaligned fields. The switching proceeds through two different reversal modes, with very different average lifetimes and average values of the transverse magnetization components. We present projective-dynamics and phase-plot analyses that clearly demonstrate the existence of these two paths, but which do not expose the underlying mechanism that determines the reversal path. Information provided by quenching or annealing the system to $T=0$ while in the metastable state is also presented. Upon decreasing the temperature in this regime, the system magnetization settles into a local energy minimum which one expects to be characteristic of the particular reversal mode. [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B42.00006: Switching field of ultra-thin films dependence on edge roughness and external magnetic field orientation Adebanjo Oriade, Siu-Tat Chui There is a demand in the physics and the technological application of micromagnetics to thin films nanostructures to achieve a high degree of control over the switching process of its magnetization. To attain high bit selectivity, that is a robust control of millions of magnetoresistive random access memory (MRAM) devices, it is important to understand how edge roughness and the orientation of these ultra-thin films affect the switching process. In MRAM elements, for example, the hard and easy axis switching fields can be effected by word and digit lines. We compute the switching field $H_c$ in permalloy films, dimensions $0.2\mu m \times 1\mu m \times 50 \AA$, considering different edge roughness for different orientation $\psi$ of the sample in external field $H_{ext,\psi}$. We investigate edge roughness parameters and their effect on the switching process making comparison with a ``perfect'' sample. Edge roughness parameters like amplitude - depth of the roughness (maximum of 100\AA), and frequency - number of defects in the edge, are considered. We present plots of hard axis switching field $H_{cy}$ against easy axis switching field $H_{cx}$ for different roughness parameters, to show how edge roughness affects the switching process. External magnetic fields $H_{ext,\psi}$ that switch a ``perfect'' sample but not samples with a defined maximum roughness fall in a region of our plot, prescribing control of switching. [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B42.00007: Effect of surface structure on the dynamic magnetic response in Ni-Zn ferrite nanoparticles P. Poddar, H. Srikanth, R. Swaminathan, M. E. McHenry Surface magnetic spin structure plays a dominant role in determining the effective anisotropy in magnetic nanostructures. To probe the anisotropy in nanoparticle systems, we have developed an RF transverse susceptibility technique based on a resonant tunnel-diode oscillator (TDO). Transverse susceptibility measurements were performed on NiZn ferrite nanoparticles (synthesized using a RF induction plasma torch) over a wide temperature range (10K to 300K) and magnetic fields (-10kOe to 10kOe). As-synthesized polydisperse nanoparticles showed broad peaks at the characteristic anisotropy fields that are attributed to the presence of both the (100) and (111) surfaces and their respective surface anisotropy contributions. The peak positions and heights were found to be sensitive to the particle size dispersion and surface magnetic spin structure. The polydisperse particles were coated with oleic acid and size-selected using ultra-centrifugation. The anisotropy peaks are conspicuously absent in the size-selected smaller nanoparticles. This is understood within the framework of a surface structure model based on isotropic canted triangular spin structures on the dominant (111) surfaces in the smallest nanoparticles. Work at USF supported by NSF through Grant No. CTS-0408933. RS and MEM thank the Institute of Complex Engineering Systems (ICES), CMU for support. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B42.00008: Ferromagnetic resonance studies of Co and Pt/Cu/Co/Cu/Pt layered ultrathin films J.-M. L. Beaujour, A. D. Kent Ferromagnetic resonance studies of polycrystalline Co thin films and Pt/ Cu/Co/Cu/Pt layered structures have been carried out with Co layer thickness varying from 1 to 70 nm. The resonance field and linewidth were studied as a function of the ferromagnetic layer thickness at room temperature and as a function of applied field. The films were grown by e-beam and thermal evaporation in UHV using an in-situ wedge growth mechanism. The absorption line was measured using a strip-coil by sweeping the in-plane field at fixed frequency ranging from 5 GHz to 10 GHz. It was found that the width of the absorption line, which has a Lorentzian shape, has a linear dependence on frequency. The intrinsic Gilbert damping constant $\alpha$ was extracted from the slope of the line. For both single films and layered structures, $\alpha$ decreases with decreasing Co thickness. In addition, the resonance field is larger than expected based on the Kittel formula. We are presently performing FMR studies of films where the thickness of the Co layer is well below 10 nm. A broadband coplanar transmission line has been designed to work in the frequency range of 5GHz to 20GHz to measure the complex susceptibility of ultrathin magnetic films. In addition, the resonance frequency and the Gilbert damping constant will be studied as a function of the Cu layer thickness and interface nature (Co/Cu, Co/Pt, Co/Cu/Pt). [Preview Abstract] |
Monday, March 21, 2005 1:15PM - 1:27PM |
B42.00009: Spin Dynamics in Single Domain Structures J.P. Park, R.L. Compton, P.A. Crowell Spin waves at nonzero wave vector can exist in nonellipsoidal single domain particles due to the dynamic demagnetization field. In order to investigate spin dynamics in a single domain state, we have fabricated 20 nm thick Permalloy ellipses with a minor axis \textit{b} ranging from 50 nm to 500 nm and the ratio \textit{a}/\textit{b} of 2, 3, 4, and 5, where \textit{a} is the major axis. We used ion-milling to transfer 30 nm thick Ti patterns made by e-beam lithography onto a Permalloy film grown on a 150 $\mu$m glass substrate, which allows a 430 nm probe beam to pass through for a time-resolved Kerr microscopy measurement. The ellipses relax from saturation into either a single domain state or a vortex state, depending on the magnitude of \textit{b} and the ratio \textit{a}/\textit{b}. The spin dynamics in isolated Permalloy ellipses in a single domain state have been investigated using micromagnetic simulations with a 150 ps wide in-plane magnetic field pulse along \textit{b}. We have identified two distinct modes in the single domain state corresponding to the backward volume magnetostatic spin-wave modes (BWVMS) with different wave vectors, where the dynamic demagnetization field plays a major role. As the ratio \textit{a}/\textit{b} for a fixed \textit{b} increases, the frequency separation between these two BWVMS modes remains the same, while an overall blue shift occurs due to the increase in shape anisotropy and the decrease in wave vector. This work was supported by the University of Minnesota MRSEC (NSF DMR-02-12032) and a Doctoral Dissertation Fellowship. [Preview Abstract] |
Monday, March 21, 2005 1:27PM - 1:39PM |
B42.00010: Magnetization reversal in arrays of nanodots Randy Dumas, Kai Liu, Igor Roshchin, Chang-Peng Li, Ivan K. Schuller Macroscopic arrays of Fe nanodots have been fabricated by using a nanoporous alumina shadow mask technique.$^{1}$ Such nanomagnets have potential applications as high density patterned magnetic recording media. We have investigated the magnetization reversal in Fe nanodots with diameters of 67, 60, and 53nm, using a first order reversal curve (FORC) method.$^{2}$ With increasing size the nanodots undergo a single domain to vortex state transition. Striking differences in the FORC diagrams have been observed. The 53nm nanodots exhibit single domain behavior and the FORCs uniformly fill the interior of the hysteresis loop. The resultant FORC distribution is a narrow ridge along the local coercivity axis with zero bias. The 60 and 67nm nanodots exhibit vortex states. Their magnetization reversal, from nucleation to annihilation of the vortex, displays clear stages of reversible and irreversible behavior as manifested in the FORC distribution. Furthermore the FORC method gives a quantitative measure of the switching and annihilation field distributions. -- Work supported by NSF and AFOSR. $^{1}$ Liu et al. APL \textbf{81,} 4434 (2002). $^{2}$ Katzgraber et al. PRL \textbf{89}, 257202 (2002); Davies et al. PRB \textbf{70}, (22), Dec. 1$^{st}$ (2004). [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 1:51PM |
B42.00011: Paramagnetic iron spin dynamics simulations Xiuping Tao, D. P. Landau, T. C. Schulthess, G. M. Stocks The contradiction\footnotemark[2]\footnotetext[2]{J. W. Lynn, Phys. Rev. B {\bf 11}, 2624 (1975).}\footnotemark[3]\footnotetext[3]{G. Shirane, O. Steinsvoll, Y. J. Uemura and J. Wicksted, J. Appl. Phys. {\bf 55}, 1887 (1984).}\footnotemark[4]\footnotetext[4]{H. A. Mook and J. W. Lynn, J. Appl. Phys. {\bf 57}, 3006 (1985).} about paramagnetic BCC iron spin dynamics is investigated using a classical Heisenberg model with four shells of interacting neighbors and exchange parameters derived from electronic structure calculations. For $T\geq T_c$ (up to at least $1.2T_c$), the spin dynamics simulated dynamic structure factor $S(\vec{q},E)$\footnotemark[5]\footnotetext[5]{Shan-Ho Tsai, Alex Bunker and D. P. Landau, Phys. Rev. B {\bf 61}, 333 (2000).}, with $\vec{q}$ fixed, has two symmetric peaks and a third peak at zero energy. Fitting results show that the symmetric peaks are due to spin-waves. The dispersion curves soften in energy with increasing temperatures and generally lie lower than those of the experiments\footnotemark[2], but both sets have the same qualitative features and suggest that spin wave excitations persist above $T_c$. [Preview Abstract] |
Monday, March 21, 2005 1:51PM - 2:03PM |
B42.00012: Thermally Randomized Magnetization Dynamics Ralph Skomski, Jian Zhou, David Sellmyer The effect of thermodynamic fluctuations on magnetization processes in ferromagnets is investigated. In addition to Neel-Brown contributions, which assume local equilibrium [1], thermal excitations amount to local magnetic fields that disproportionally facilitate the nucleation of reverse domains. Explicit solutions are obtained for transition-metal rich rare- earth intermetallics, where the leading contribution to the temperature dependence reflects 4f intramultiplet excitations. The single-ion character of the 4f anisotropy leads to relatively transparent anisotropy distribution functions. A static random-field approximation is then used to analyze the temperature dependence of the coercivity. The modes affect the magnetization reversal of nanostructures including high-density recording media, where they affect the thermal stability of the stored information. We present quasi-static simulations describing this effect for hard-soft nanoparticles and derive an approximate analytical solution for the time dependence of the effect. In the static approximation, thermal fluctuations are modeled as snapshots of time-dependent random magnetic fields. Physically, the thermal excitations switch the magnetization of the soft phase, which then exerts a destabilizing bias field on the phase with the higher anisotropy. - This research is supported by USDOE, NSF-MRSEC, ARO, the W.M. Keck Foundation, INSIC, and CMRA. [1] R. Skomski, J. Phys. Condens. Matter \textbf{15}, R841 (2003). [Preview Abstract] |
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