Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session B14: Focus Session: Magnetic Nanowires, Nanodots, Multilayers |
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Sponsoring Units: GMAG DMP Chair: Justin Shaw, National Institute of Standards and Technology, Boulder Room: Colorado Convention Center Korbel 4D |
Monday, March 5, 2007 11:15AM - 11:51AM |
B14.00001: Enhancement of AMR in Permalloy Point Contacts due to Quantum Interference. Invited Speaker: We measure the low-temperature resistance of mechanically-stable permalloy break junctions as a function of contact size and the magnetic field angle in applied fields large enough to saturate the magnetization. We show that the size of the anisotropic magnetoresistance (AMR) signal at low temperature can increase dramatically as the contact cross section is narrowed to the nanometer-scale regime. For metallic devices with $R$ larger than $\sim 1$\ k$\Omega$ we observe AMR effects larger than in bulk devices, with an angular variation that can deviate from the sinusoidal bulk dependence, and which are associated with fluctuations in $dV/dI$ of similar magnitude as a function of $V$. Even more strikingly, we find that point contacts which are completely broken, so as to enter the tunneling regime, also exhibit a tunneling anisotropic magnetoresistance effect (TAMR) as large as 25\% when the magnetic-moment directions in the two contacts are rotated together while remaining parallel. We propose that these large AMR and TAMR effects are the result of mesoscopic quantum interference which depends on the orientation of the magnetization, leading to fluctuations of conductance and the spin-dependent local density of states. These fluctuations should affect a broad variety of nanoscale devices that contain magnetic components, producing strong perturbations in measurements of low-temperature spin-dependent transport. This work was done in collaboration with F. Kuemmeth and D. C. Ralph. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B14.00002: Temperature dependence of anisotropic magnetoresistance fluctuations and observations of atomic motion in ferromagnetic metal break junctions Sufei Shi, Kirill Bolotin, Ferdinand Kuemmeth, D.C. Ralph Recent studies of the resistance of nanometer-scale magnetic junctions as a function of the angle of an applied magnetic field have found very large anisotropic magnetoresistance (AMR) signals, compared to bulk samples. One proposed mechanism is that coupling between the ferromagnetic moment and electron orbits may lead to conductance fluctuations due to quantum interference when the moment is rotated. Here we report that the large AMR signals are suppressed with increasing temperature ($T)$, consistent with a quantum-interference mechanism. We also note that as $T$ is increased to 32 K, most of our samples exhibit an increasing amount of time-dependent two-level resistance noise due to atomic rearrangements. We conclude that higher-$T$ measurements of magnetic nanocontacts generally involve averages over many atomic configurations. In some samples, the atomic switching rates depend strongly on the magnetic field angle, so that even at low $T$ the resistance can sometimes switch abruptly between repeatable values as the field angle is rotated. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B14.00003: Magnetic Stripe Domains in Thermally Evaporated Ni Strips Soo Hyung Lee, Frank Zhu, Chia-Ling Chien, Nina Markovic We have studied thermally evaporated thin Ni strips with varying widths and geometry. Magnetic force microscope images showed the presence of magnetic stripe domains. Wide Ni strips in their as-prepared-state exhibited stripe domains oriented perpendicular to the edge of the samples. In contrast, narrow Ni strips showed stripe domains that were parallel to the edge. Changes to the geometry of the strips caused competition of different stripe orientations. After we demagnetized the strips with an in-plane field, stripe domains followed the field's direction, which was at an arbitrary angle to the edge of the sample. We will discuss these results in terms of existing theoretical models. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B14.00004: Oscillatory Curie Temperature of Fe/ Cu-wedge/ Fe/ Cu(111) nanodots. Noppi Widjaja, W.C. Lin, K. Fuchigami, M.A. Torija, M.-T. Lin, E.W. Plummer, J. Shen The interactions between two layers of Fe nanodots were studied as a function of the thickness of a Cu spacer layer. The bottom Fe-dot layer was grown on a Cu(111) substrate cleaned \textit{in situ} by cycles of Ne-ion sputtering and annealing, employing a inert gas buffer layer assisted growth (BLAG) technique. Subsequently, a wedge-shaped Cu spacer layer was evaporated to cap the Fe-dots, followed by the growth of the top Fe-dot layer using the same BLAG method. Wedge-shaped samples are crucial for this study because the effects we are looking for are often subtle and would have been obscured by fluctuations in preparation conditions if we had to prepare a separate sample for each thickness. In-situ magneto-optical Kerr effect (MOKE) measurements were utilized to determine the local Curie temperature (T$_{C})$ at various positions on the wedged samples, and regular oscillations in T$_{C }$ as a function of the spacer layer thickness were observed. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B14.00005: Temperature dependence of the uncompensated magnetization in Fe$_{x}$Ni$_{1-x}$F$_2$/Co bilayers David Lederman, Miyeon Cheon, Zhongyuan Liu A giant uncompensated magnetization in Fe$_{x}$Ni$_{1-x}$F$_2
$/Co was observed in the hysteresis loops at low temperatures
($T |
Monday, March 5, 2007 12:39PM - 12:51PM |
B14.00006: Magnetic properties of one dimensional Ni/ Cu (Al) /Ni nanowires: Role of non-magnetic spacer Partha Pratim Pal, Ranjit Pati One dimensional (1-D) magnetic multilayered nanowires with alternating ferromagnetic and non-magnetic structures arranged in sequence have been the subject of intense research in recent years for their potential applications in magneto-electronics or spintronics. We have used first-principles periodic density functional theory to study the stability, electronic, and magnetic properties of Ni/Cu/Ni and Ni/Al/Ni nanowires. The thickness of the non-magnetic spacer layer is systematically changed to explore the role of non-magnetic spacer in controlling the interlayer magnetic coupling and hence the magnetic properties of these 1-D nanowires. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B14.00007: Dipole-Exchange modes in ferromagnetic nano-wires of arbitrary cross sections Rodrigo Arias, Douglas Mills We present a method that allows to calculate the eigen- frequencies of dipole-exchange modes in ferromagnetic nano- wires of arbitrary cross sections. The method is setup for calculating modes of long wavelength along the main direction of the nano-wire, but it could be extended to short wavelengths. The basis of the theoretical approach is the extinction theorem, under a form appropriate for the inclusion of the exchange interaction. Appropriate integral equations in the form of contour integrals around the periphery of the wire are obtained for the magnetostatic potentials and magnetization of the modes. We perform a numerical analysis of the eigen- frequencies of geometries of interest, like rectangular and elliptical cross sections, recovering the appropriate limiting values of the magnetostatic or exchange dominated regimes [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B14.00008: High Magnetization FeCo/Pd multilayers Michael Walock, Frank Klose, Mairbek Chshiev, Gary Mankey, William Butler A high saturation magnetization is advantageous in magnetic recording. ~Currently, the peak of the Slater-Pauling curve is the BCC FeCo system with a saturation magnetization of 2.45 T.~ Recently, a magnetization of 2.57 T in the FeCo layers of a [40 nm Fe$_{30}$Co$_{70}$ /1.7 nm Pd]x25 superlattice has been reported [1, 2]. ~This behavior may be attributed to an enhanced Fe moment in the expanded FCC matrix, and an accompanying induced moment in the Pd. Our theoretical calculations show an atomic moment enhancement, but this is not great enough to overcome the overall magnetization density reduction caused by the incorporation of Pd in the matrix. The overall effect is a reduced magnetization. Through variation of the FeCo composition and Pd layer thickness, and the combinatorial methods of structural and magnetic characterization, we will gain insight into the magnetic structure of this tertiary thin film system. [1] K. Noma, M. Matsuoka, H. Kanai, Y. Uehara, K. Nomura, and N. Awaji. IEEE Trans. Magn. \textbf{42}, 140 (2006). [2] \textit{ibid}. \textbf{41}, 2920 (2005). [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B14.00009: Modulating magnetic properties of the one dimensional (1-D) Fe/Pt /Fe multilayered Nanowires: A first principles study Puspamitra Panigrahi, Ranjit Pati Using first-principles density functional theory within the Local Spin Density Approximation (LSDA), we have predicted the stability, electronic and magnetic properties of 1-D ferromagnetic Fe/Pt/Fe multilayered nanowires. The thicknesses of the magnetic and non-magnetic spacer layers are systematically varied to study the evolution of magnetic properties with the spacer size. The stability of the nanowire is found to increase with the increase of the thickness of the Pt spacer. Furthermore, by increasing the thickness of the Pt layer in the nanowire, we found that the average magnetic moment per Fe atom in the ferromagnetic configuration can be enhanced significantly. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B14.00010: Imprinting magnetic vortices into heterostructure multilayer rings Volker Rose, Vitali Metlushko, Bojan Ilic, John W. Freeland While thin (5 nm) microscale single-layer ferromagnetic rings usually only exhibit a one-step switching between opposite bidomain states, also referred to as onion states, we show that in a trilayer (NiFe/Cu/Co) ring structure the interlayer dipolar interactions can lead to the stabilization of flux-closure vortex states. Using X-ray resonant magnetic scattering we have studied magnetic interactions in a series of single- (5 nm NiFe and Co) and multilayer (NiFe (5 nm)/Cu (3 nm)/Co (5 nm)) continuous films and patterned ring arrays. Each ring has a width of 0.75 $\mu $m and an outer diameter of 2 $\mu $m. In the NiFe and Co single-layer rings the spin switching occurs from an initial onion state to a final reverse onion state. By contrast, in a NiFe/Cu/Co multilayer ring the magnetostatic coupling strongly affects the reversal and gives rise to the nucleation of a well-defined vortex state in the NiFe layer. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B14.00011: Size dependence and metastability of the vortex state in magnetic nanodots Igor V. Roshchin, Chang-Peng Li, X. Batlle, J. Mejia-Lopez, D. Altbir, A. H. Romero, Ivan K. Schuller Magnetization reversal in structures with sizes comparable to or smaller than ferromagnetic domains has received much attention recently. We report on the magnetization switching as a function of size in sub-100 nm magnetic nanodots fabricated using anodized nanoporous alumina masks.[1] The hysteresis loops for the samples where a vortex is observed exhibit a non-zero remanence in accordance with Monte Carlo simulations.[2] Even if the vortex state is the ground state the dot may get stuck in a metastable single domain state. The size range for this metastability is determined from the energy diagrams for various vortex and non-vortex states obtained from micromagnetic simulation. Effects of commensurability and other parameters affecting stability of the vortex state will be discussed. Work is supported by AFOSR, US DOE, Spanish MECD, Catalan DURSI, FONDECYT, and CONACYT. \\ $[1]$ C.-P. Li et al., J. Appl. Phys. \textbf{100}, 074318 (2006). \\ $[2]$ J. Mejia-Lopez, J. Appl. Phys. \textbf{100}, in press (2006). [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B14.00012: Optical and magneto-optical properties of composite noble-metal-ferromagnetic thin films Michelle Sestak, Jonathan Skuza, R. Alejandra Lukaszew We will report on the optical and magneto-optical properties of thin films composed of a magneto-optically active part and a noble metal that acts as a plasmonic counterpart: magnetoplasmonic materials. We will show that such combination leads to structures with novel properties that can be used to achieve more efficient and sensitive nanophotonic devices based on surface plasmon resonance. We will compare a material consisting of a noble/ferromagnetic/noble metal (ie. Au/Co/Au) trilayer, in which the optical and magneto-optical response can be tailored by varying the thickness of the different layers, with a nanocomposite material made with magnetic nanoclusters embedded on a noble metal matrix. The clusters are ion-implanted onto the noble metal matrix and their size and penetration depth can be tailored by choosing the ion-implantation conditions. We will show how sub-nanometer modifications can enhance the magneto-optical response of the system. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B14.00013: The importance of the dipolar interaction strength in magnetization hysteresis curves of two-dimensional nanomagnet arrays Richard Klemm, Marisol Alcantara Ortigoza, Talat Rahman Recently, Takagaki and Ploog [Phys. Rev. B {\bf 71}, 184439 (2005)] used a fourth-order Runge-Kutta technique to integrate the Landau-Lifschitz-Gilbert equations for square lattices of $N\times N$ magnetic nanodots with dipolar interdot interactions. Some of their results appeared to differ qualitatively from the second-order Runge-Kutta results obtained for the same systems by Kayali and Saslow [Phys. Rev. B {\bf 70}, 174404 (2004)], both in the hysteresis area $A_N$ and in the number of steps of the magnetization hysteresis loops. We [Phys. Rev. B {\bf 74} (22), xxxxxx (2006), in press] show that these differences are not due to inaccuracies in either calculation or to the potentially different magnetic induction sweep rates used, but can be attributed entirely to different choices of the dipolar interaction strength $h_{\rm dip}\propto a^{-3}$, where $a$ is the two-dimensional lattice constant. [Preview Abstract] |
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