Session X15: Focus Session: Spin and Dynamics in Metal, Resonance Phenomena II: FMR in Magnetic Nanostructures

Two dimensional spectroscopic imaging of individual ferromagnetic nanostripes

We report on high resolution imaging of the center and edge modes of individual Permalloy nanostripes using ferromagnetic resonance force microscopy (FMRFM). Fabrication of future spintronic devices requires an understanding of how edge damage affects a device's magnetic properties, and the highly localized edge modes of a nanostripe provide a direct method to investigate edge properties. While previous studies have measured spectra of the edge modes in large arrays of stripes, in this work we use FMRFM to image and probe the center and edge modes in individual NiFe stripes, 400 and 700 nm in width and 20 nm in thickness. Our spectroscopic measurements reveal different resonance fields for opposite edges of a stripe, which also exhibit different structural profiles. 2-D spatial imaging of the bulk and edge mode resonances demonstrates the inhomogeneity of the edge modes along their lengths with a resolution of 300 nm.   

Size dependent nonlinear effects of magnetization dynamics in Permalloy disks

We use ferromagnetic resonance force microscopy (FMRFM) to probe nonlinear magnetization dynamics in individual Ni$_{80}$Fe$_{20}$ disks with diameters ranging from 100 nm to 500 nm. The nonlinear behavior of magnetic nanostructures has important implications for rapid switching of memory devices and for frequency stability of spin torque oscillators. In the studied size range, micromagnetic modeling predicts a transition from complex power dependent behavior in the larger disks to simple behavior in the smallest disks where only a few modes are observed. At low power levels in the measurements, precession produces a force-detected signal that is linear in power and that displays a Lorentzian line shape. At higher powers, we observe nonlinear effects, including asymmetric line shapes and foldover where the resonance shifts with power. In addition, complex, jagged line shapes appear at high powers. We report that the onset power level of the nonlinear regime was found to be size dependent. In smaller disks, higher microwave power is required to drive nonlinear precession, and we see fewer complex peaks. Furthermore, the direction of foldover also depends upon the disk size. We have modeled our results using micromagnetic simulations and they display good correspondence with our experimental data.   

Electrically Detected Broadband Ferromagnetic Resonance in Individually Defined Permalloy Nanowires

We report measurements of electrically detected broadband ferromagnetic resonance (FMR) in lithographically defined Permalloy nanowires. For these measurements, the Permalloy nanowire is placed in close proximity to the short of a gold coplanar strip waveguide. The microwave power applied to the waveguide drives the magnetization precession in the wire and the four-point resistance of the wire is measured as a function of DC external magnetic field. The time-averaged resistance of the wire depends on the amplitude of the magnetization precession via anisotropic magnetoresistance (AMR), and thus peaks and dips with resistance dependence on the bias magnetic field arise from resonant excitation of spin wave modes in the wire. Using this electrically detected FMR technique, we measure the frequency and linewidth of the quasi-uniform mode of magnetization precession as well as bulk modes and edge modes that exist for the magnetic field applied in the plane of the sample perpendicular to the wire. We will present measurements of the resonance frequency and linewidth of various modes for several values of the wire width and compare our results to theoretical predictions of the field dependence of the mode frequency.   

Spin torque phenomena originating from the spin Hall effect: resonance, magnetic switching, and magnetic dynamics

The spin Hall effect (SHE) generates a transverse spin current with the passage of a current through a non-ferromagnetic metal (NM) film. Quite different results have been obtained for the magnitude of this effect. Here I will discuss a new technique where, by applying an RF current to a NM/FM thin film bilayer, the spin current onto the FM layer can induce a spin torque ferromagnetic resonance (ST-FMR). This enables the determination of the SHE strength with precision and without the need to assume the values of unmeasured parameters. The large magnitude of the SHE that we have established in several types of NM is sufficient to reversibly switch the magnetic orientation of a FM layer and I will discuss two different implementations of this. In the first the FM layer has a perpendicular-to-plane magnetic moment at equilibrium and the SHE injected spins are orthogonal to the moment. The spin torque can overcome the anisotropy (coercive) field restoring torque, with the polarity of the current-induced switching being determined by the sign of a small external field applied along the current direction. In the second approach, the FM moment lies in plane and the spins injected by the SHE exert a negative damping just as in conventional ST switching of a spin valve or magnetic tunnel junction (MTJ). We have fabricated three-terminal devices that incorporate a MTJ and a SHE layer to induce in-plane reversal switching. The simple architecture of this three terminal device and the high efficiency of the SHE induced switching made it a promising technique for future memory and non-volatile logic applications. We have also used this three terminal device to demonstrate DC induced dynamics in the magnetic layer due to the SHE.   

Magnetostatics and magnetodynamics in single crystal Ni on MgO

We present an investigation of the magnetic properties of 120 nm thick (100) and (110) oriented single crystal Ni films grown on MgO via evaporative deposition. Xray diffraction analysis was used to confirm the single crystal nature and crystallographic orientations of the films. Magnetization measurements reveal anisotropy and magnetic moment consistent with bulk Ni values. Ferromagnetic resonance measurements have been made as a function of in-plane angle and temperature at 36 GHz. Resonance field maps confirm the anisotropy expected for high quality single crystal films. Both the anisotropy and damping are presented at temperatures ranging from 50 K to room temperature. Additional FMR results at 10 GHz are also presented from 4 K to room temperature. These results are discussed in the context of the temperature-dependent magnetocrystalline anisotropy of Ni and magnetoelastic anisotropy resulting from thermal mismatch between the Ni and MgO.   

Localized Edge Modes in Permalloy Square Antidot Arrays

We have carried comprehensive experiments on and simulations of the ferromagnetic resonance spectrum of various thin films of permalloy patterned with a periodic array of holes on a square lattice. These so-called antidot lattices show a rich multi-line spectrum covering a wide range of frequencies and magnetic fields; they also exhibit striking angular dependences. One mode that is predicted in our simulations is highly localized at the edges of the holes. Our simulations further show that for a fixed field or excitation frequency the angular dependence of this mode strongly depends on the shape of the holes in the antidot array. It has been suggested\footnote{S. Neusser, B. Botters and D. Grundler, Phys. Rev. B, 78, 054406 (2008).} that, in comparison to other more extended modes, this localized mode would be difficult to find experimentally due to: i) the effects of varying shape and roughness present in an actual array, and ii) the fact that the mode is concentrated within a small fraction of the unit cell of the sample. Our experiments, performed with a broadband meanderline-based ferromagnetic resonance (FMR) spectrometer,\footnote{C. C. Tsai, J. Choi, S. Cho, B. K. Sarma, C. Thompson, O. Chernyashevskyy, I. Nevirkovets, and J. B. Ketterson, Rev. of Sci. Instr. \textbf{80}, 023904 (2009).} show a very weak resonance (requiring extensive signal averaging) that is in rough agreement with the simulations, which we propose as a candidate for this elusive edge mode.   

Topological Localization of FMR Modes by Antidot Arrays Patterned into Permalloy Thin Films

We have observed novel broad-band FMR spectra for 25-nm-thick Permalloy films patterned with square arrays of diamond antidots with axes D$_{x }$= 1430 nm and D$_{y}$ = 860 nm. The y-axis lattice spacing was held constant at d$_{y}$ = 2000 nm, and variable x-axis lattice spacings d$_{x }$= 1730, 2000, 2267 and 2730 nm. The applied DC magnetic field H (in-plane at angle $\theta $ with respect to x-axis) spanned the hysteretic regime $\vert $H$\vert \quad \le $ 150 Oe, to $\vert $H$\vert $ = 3 kOe in the saturated regime, corresponding to mode frequencies f $\approx $ 250 MHz to 14 GHz. In spite of hysteretic evolution of domain walls in the low-field regime, highly reproducible absorption peaks appear at f $<$ 3 GHz. Static and dynamic micromagnetic simulations agree with DC magnetization and FMR dispersion curves, and show domain pinning by the antidot edges is responsible for the reproducible spectra in the hysteretic regime. For H = 1 kOe along the x-axis, we observe two localized modes: one (f = 9 GHz) in a narrow gap between the accute vertices, and another (f = 10.25 GHz) between the oblique vertices, of adjacent diamonds. For $\theta $ = 45$^{\circ}$, one mode (f = 8.7 GHz) extends along the (-1,1) direction with strong angular variation of f, and a standing mode (f = 9.87 GHz) is localized between nearly parallel edges of adjacent antidots.   

Ferromagnetic Resonance Studies of Individual Ferromagnetic Nanowires

We investigate magnetization dynamics in ferromagnetic nanowires using two techniques: Ferromagnetic Resonance Force Microscopy (FMRFM) and Vector Network Analyzer Ferromagnetic Resonance (VNA-FMR). Using FMRFM we report our attempts at scanned probe FMR imaging of individual ferromagnetic nanowires. By placing ferromagnetic nanowires on permalloy we can use the capability of localized mode imaging (I. Lee et. al, Nature 2010) to measure local fields both in and around the ferromagnetic nanowire. In addition, we use VNA-FMR to study dipolar interactions in densely packed nanowire arrays. The dipolar fields between neighboring nanowires introduce an anisotropy field that can dominate over the shape anisotropy expected for isolated nanowires, and this effect is observed in the angular dependence of VNA-FMR spectra.   

Damping Dependence in Microwave Assisted Magnetization Reversal

Microwave assisted magnetization reversal (MAMR) is one possible technique to mitigate the writability problem in ultrahigh density magnetic recording. In the presence of microwaves, the magnetization reversal of a magnetic recording material could be triggered at significantly reduced switching field. The damping constant is one of the critical parameters in magnetization switching for the magnetization precession spiraling down to the direction along the external field. We demonstrate microwave assisted magnetization reversal (MAMR) in a CoFeB film and the damping dependence in MAMR through the measurement of ferromagnetic resonance (FMR). Spin-pumping in non-ferromagnetic/ferromagnetic films provides a large range variation of Gilbert damping constants in magnetic samples when changing the thickness of non-ferromagnetic layers without changing the ferromagnetic film. An evident dependence of switching fields on the damping constant is observed in the presence of microwaves. The trend of the experiment data is well reproduced by a numerical simulation based on the Landau-Lifshitz-Gilbert equation. The result indicates that the large damping decreases the efficiency of microwaves in reducing the magnetization switching field.   

Theory of the ac spin valve effect: a new method to measure spin relaxation time

Parallel (P) and antiparallel (AP) configurations of FNF junctions have, in a dc regime, different resistivities ($R_{AP}>R_{P}$), giving rise to the giant magnetoresistance (GMR) effect, which can be explained within the spin injection drift-diffusion model. We extend the model to include ac phenomena and predict new spin dynamical phenomenon; the resonant amplification and depletion of spin accumulation in the P and AP configurations, respectively. As the major new effect, the spin valve magnetoimpedance of the FNF junction oscillates with the driving ac frequency, which leads to negative GMR effect ($|Z_{AP}|<|Z_{P}|$). We show that from the spin-valve oscillation periods, measured all electrically in the GHz regime, the spin relaxation times could be extracted without any magnetic field and sample size changes (contrary to other techniques). For thin tunnel junctions the ac signal becomes pure Lorentzian, also enabling one to obtain the spin relaxation time of the N region from the signal width. This work, was published in Physical Review Letters,10, 176604 (2011).   

Vortex Resonance in Coupled Ferromagnetic Disks

Advances in nanolithography and thin film growth techniques offer the unique opportunity to prepare a variety of laterally confined nanostructured magnets. Of particular interest are lithographically patterned micron and submicron disk-shaped particle arrays. The magnetic ground state in confined geometries consists of a curling spin configuration, known as a vortex state. Studies of vortex dynamics have mainly focused on circular or elliptical dots at remanence. In this work, we investigate the dynamic response of vortex gyration in interacting systems where two circular dots are statically exchange coupled. The induced coupling due to the interacting area forces the disks to have antiparallel chirality of magnetization. Apart from different vortex polarity combinations, various frequency modes were observed as a function of external magnetic field and contact length. Moreover, due to the induced configurational anisotropy in the system, vortex resonance in the two disks was found to be strongly dependent on the orientation of the static magnetic field.   

Study of the coupling of vortices in magnetic nanodisks

There is a marked interest nowadays in the dynamic behavior of magnetic nanodisks that present magnetic vortices [1]. The tailoring of vortex features, including their gyrotropic frequency, critical vortex core switching velocity and strength of the coupling between nanodisks, is very desirable for future applications, such as vortex magnetic memories (VRAMs) and spin transfer nanooscillators (STNOs) [2]. An original way to tune some of the static vortex properties (specially the vortex core diameter) is simply to introduce a uniaxial perpendicular magnetic anisotropy, as has been recently shown [3]. Here we have studied the coupling between vortices as a function of the magnetic properties of the disks and their separation, using micromagnetic simulations. We analyzed the motion of a vortex core caused by the motion of the second one, excited by static or rotating magnetic fields. A splitting of the gyrotropic frequency is also observed. Exciting one of the disks, it is possible to switch the core polarity of the other. These results open new possibilities for applications of magnetic vortices.\\[4pt] [1] Ruotolo et al. Nat Nano, 4(8):528-532, (2009);\\[0pt] [2] Jung et al. Sci. Rep. 1 ,59;DOI:10.1038/ srep00059 (2011);\\[0pt] [3] Garcia et al. APL. 97, 022501 (2010).