Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G53: Invited Session: Microscopic Understanding of Dynamics of Localized Spin Wave Modes |
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Sponsoring Units: GMAG DCMP Room: Grand Ballroom C3 |
Tuesday, March 3, 2015 11:15AM - 11:51AM |
G53.00001: Atomic spin chains as testing ground for quantum magnetism Invited Speaker: Sander Otte The field of quantum magnetism aims to capture the rich emergent physics that arises when multiple spins interact, in terms of elementary models such as the spin~$\frac{1}{2}$ Heisenberg chain. Experimental platforms to verify these models are rare and generally do not provide the possibility to detect spin correlations locally. In my lab we use low-temperature scanning tunneling microscopy to design and build artificial spin lattices with atomic precision. Inelastic electron tunneling spectroscopy enables us to identify the ground state and probe spin excitations as a function of system size, location inside the lattice and coupling parameter values. Two types of collective excitations that play a role in many dynamic magnetic processes are spin waves (magnons) and spinons. Our experiments enable us to study both types of excitations. First, we have been able to map the standing spin wave modes of a ferromagnetic bit of six atoms, and to determine their role in the collective reversal process of the bit (Spinelli {\it et al.}, Nature Materials 2014). More recently, we have crafted antiferromagnetic spin~$\frac{1}{2}$ XXZ chains, which allow us to observe spinon excitations, as well as the stepwise transition to a fully aligned phase beyond the critical magnetic field (Toskovic {\it et al.}, in preparation). These findings create a promising experimental environment for putting quantum magnetic models to the test. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:27PM |
G53.00002: Spatially-resolved measurement of spin transport across nanoscale interfaces Invited Speaker: Rohan Adur Spintronics uses spin for information processing and storage. Mechanisms for spin relaxation in bulk systems have been extensively studied. However, a clear understanding of few-spin systems remains challenging. We report spatially-resolved magnetic resonance studies of a ``spin nanowire'' formed by nitrogen vacancies in diamond. The result reveals that the lifetime of the spin ensemble is dominated by spin transport from the ensemble into the adjacent spin reservoir, which is in striking contrast to conventional spin-lattice relaxation measurements of isolated spin ensembles. Electron spin resonance spectroscopy corroborates spin transport in strong field gradients. These experiments, supported by microscopic Monte Carlo modelling, provide a unique insight into the intrinsic dynamics of pure spin currents needed for nanoscale devices that seek to control spins. In addition, we observe a dependence of the damping of a confined mode of precessing ferromagnetic magnetization on the size of the mode. The micron-scale mode is created within an extended, unpatterned YIG film by means of the intense local dipolar field of a micromagnetic tip. The damping of the confined mode scales like the surface-to-volume ratio of the mode, indicating an interfacial damping effect (similar to spin pumping) due to the transfer of angular momentum from the confined mode to the spin sink of ferromagnetic material in the surrounding film. Though unexpected for insulating systems, the measured intralayer spin-mixing conductance of $3\times 10^{19}$ m$^{-2}$ demonstrates efficient intralayer angular momentum transfer. [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 1:03PM |
G53.00003: Mode- and Size-Dependent Landau-Lifshitz Damping in Magnetic Nanostructures Invited Speaker: Thomas Silva At nanometer dimensions, magnetic excitation bands transform into discrete eigenmodes with nontrivial shape and size dependence. The eigenmode spectral peak positions are well-understood in terms of conventional micromagnetics [R. D. McMichael and M. D. Stiles, J. Appl. Phys. 97, 10J901 (2005)][H. T. Nembach, et al., Phys. Rev. B 83, 094427 (2011)]. However, the effect of finite size on the damping process is not yet settled. Conventional micromagnetics does not predict any effect, insofar as numerical formulations of magnetization dynamics are usually predicated on the assumption of local damping. However, nonlocal damping has been predicted for metals via nontrivial scattering between coherent excitations and uncorrelated spin-flip electron-hole pairs [Y. Tserkovnyak, et al., Phys. Rev. B 79, 094415 (2009)] [I. V. Baryakhtar and V. Baryakhtar, Ukr. Phys. Journ. 43, 1433 (1998)]. In particular, theory predicts a dependence of damping on the eigenmode curvature. We developed a novel Kerr microscope to measure ferromagnetic resonance in deep-sub-wavelength structures. We use heterodyne mixing for phase-sensitive detection of the magnetization dynamics with a signal-to-noise ratio proportional to the square-root of the scattered optical power. The heterodyne magneto-optic microwave microscope (H-MOMM) is optimized for cw measurements to extract the damping parameter. We measured damping in e-beam-patterned 10-nm-thick Permalloy nanomagnets ranging in size from 100 to 400 nm. We observe two eigenmodes; the end-mode, with an exponentially decaying amplitude for increasing distance from the ends along the applied field direction, and the center mode, with relatively uniform amplitude throughout much of the nanomagnet volume, though with two nodes near the ends. The center-mode damping increases with decreasing nanomagnet size, but the end-mode damping exhibits the opposite trend [H. T. Nembach, et al., Phys. Rev. Lett. 110, 117201 (2013)]. We quantitatively fit the data with the Barakhtar/Tserkovnyak theory, but obtain a much larger dependence on sample size than expected from microscopic considerations. Subsequent measurements by us of perpendicular standing spin waves in thick Permalloy films, as well as additional H-MOMM investigations of variable thickness Permalloy nanomagnets, strongly suggest that the observed non-local damping is enhanced with decreasing film thickness. Such thickness dependence is not theoretically predicted, and indicates that surface/interface scattering is important. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:39PM |
G53.00004: Full control of the spin-wave damping in a magnetic insulator using spin orbit torque Invited Speaker: Olivier Klein The spin-orbit interaction (SOI) has been an interesting and useful addition in the field of spintronics by opening it to non-metallic magnet. It capitalizes on adjoining a strong SOI normal metal next to a thin magnetic layer. The SOI converts a charge current, $J_c$, into a spin current, $J_s$, with an efficiency parametrized by $\Theta_{\mathrm{SH}}$, the spin Hall angle. An important benefit of the SOI is that $J_c$ and $J_s$ are linked through a cross-product, allowing a charge current flowing in-plane to produce a spin current flowing out-of-plane. Hence it enables the transfer of spin angular momentum to non-metallic materials and in particular to insulating oxides, which offer improved performance compared to their metallic counterparts. Among all oxides, Yttrium Iron Garnet (YIG) holds a special place for having the lowest known spin-wave (SW) damping factor. Until recently the transmission of spin current through the YIG|Pt interface has been subject to debate. While numerous experiments have reported that $J_s$ produced by the excitation of ferromagnetic resonance (FMR) in YIG can cross efficiently the YIG|Pt interface and be converted into $J_c$ in Pt through the inverse spin Hall effect (ISHE), most attempts to observe the reciprocal effect, where $J_s$ produced in Pt by the direct spin Hall effect (SHE) is transferred to YIG, resulting in damping compensation, have failed. This has been raising fundamental questions about the reciprocity of the spin transparency of the interface between a metal and a magnetic insulator. In this talk it will be demonstrated that the threshold current for damping compensation can be reached in a 5~$\mu$m diameter YIG(20nm)|Pt(7nm) disk. Reduction of both the thickness and lateral size of a YIG-structure were key to reach the microwave generation threshold current, $J_{c}^{*}$. The experimental evidence rests upon the measurement of the ferromagnetic resonance linewidth as a function of $I_{\mathrm{dc}}$ using a magnetic resonance force microscope (MRFM). It is shwon that the magnetic losses of spin-wave modes existing in the magnetic insulator can be reduced or enhanced by at least a factor of five depending on the polarity and intensity of the in-plane dc current, $I_{\mathrm{dc}}$. Complete compensation of the damping of the fundamental mode by spin-orbit torque is reached for a current density of $\sim 3 \cdot 10^{11}$A.m$^{-2}$, in agreement with theoretical predictions. At this critical threshold the MRFM detects a small change of static magnetization, a behavior consistent with the onset of an auto-oscillation regime. This result opens up a new area of research on the electronic control of the damping of YIG-nanostructures. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 2:15PM |
G53.00005: X-ray Imaging of Spin Wave Dynamics at the Nanoscale Invited Speaker: Hendrik Ohldag Imaging current induced magnetization dynamics has so far remained an elusive task, due to the lack of microscopy techniques with combined high spatial and temporal resolution, and elemental magnetic sensitivity. Using synchrotron radiation, we have been able to create the first x-ray images of spin waves generated in nano-contact spin torque oscillators realized on different ferromagnetic thin films, using the x-ray magnetic circular dichroism (XMCD) effect as a probe. In nanocontacts with perpendicular magnetic anisotropy, we directly observed the appearance of a highly localized mode beneath the nanocontact with large precession angle. In samples with an easy-plane magnetic anisotropy, we have been able to implement time-resolution in the measurements and record the first spin wave ``movie'' of the spin torque induced dynamics with 35 nm spatial resolution and 50 ps temporal resolution. We observe that the spin wave dynamics is excited on the side of the nanocontact where the internal field in the thin film is at a minimum. Around this field minimum, the spin wave extends with a radius of approximately 250 nm, demonstrating the localized nature of the mode. Finally, our measurements show that the spin wave has a node in amplitude, around which the magnetization points in two opposite out-of-plane directions.\\[4pt] In collaboration with Stefano Bonetti, Roopali Kukreja, Zhao Chen, Stanford University and SLAC National Accelerator Laboratory; Sergei Urazhdin, Emory University; Josef Frisch, SLAC National Accelerator Laboratory; Ferran Maci\`a, Dirk Backes, New York University; Anders Eklund, Gunnar Malm, The Royal Institute of Technology (KTH); Fred Mancoff, Everspin Technologies Inc.; Jordan Katine, HGST; Vasyl Tyberkevich, Andrei Slavin, Oakland University; Andrew Kent, New York University; and Joachim St\"ohr, Hermann Du\"urr, Hendrik Ohldag, SLAC National Accelerator Laboratory. [Preview Abstract] |
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