Permalloy Dots Research Articles

The Arrhenius law prefactor in permalloy mesoscale systems

The Arrhenius equation was used to describe the dynamics of two-state switching in mesoscale, ferromagnetic particles. Using square permalloy dots as an idealized two-state switching system, measurements of the prefactor of the Arrhenius law changed by 26 decades over barrier heights from 30 to 700 meV. Measurements of the prefactor ratios for a two well system revealed significant deviations from the common interpretation of the Arrhenius law. The anomalous Arrhenius prefactors and the prefactor ratios can be fitted to a modified model that includes entropic contributions to two-state transitions. Similar considerations are likely for the application of the Arrhenius law to other mesoscale systems.

Vortex Annihilation Dependence of Chirality in Asymmetric Permalloy Dots

Geometric asymmetry effect on vortex annihilation was investigated in series of submicro-scaled permalloy dots. By introducing the one-side-flat asymmetric shape into circular magnetic disks, the original degenerate signals of vortex annihilation for different chirality are separated into two different trajectories, where the asymmetry level is quantified by an excised angle. The separation of annihilation field for clockwise and counterclockwise vortex shows as a function of the excised angle. The asymmetry influence on the separation of annihilation field is discussed from the view of disk aspect ratio and flat boundary effect. Comparison of experimental results with numerical simulations is also presented.

Effects of nanodots shape and lattice constants on the spin wave dynamics of patterned permalloy dots

Micromagnetic simulations studies on Permalloy (Ni80Fe20) nanodots with different shapes and edge-to-edge separations (s) down to 25nm arranged in square lattice are reported. We observe the significant variation of spin-wave (SW) dynamics of nanodots of different shapes (triangular, diamond and hexagon) and of fixed dot diameter 100nm with varying s. Modes for single dot are transformed in an array into multiple quantized, edge and centre modes for different shapes and edge-to-edge separations, with different spin wave frequencies and peak intensities. Specifically, in the triangular dot sample, a broad range of mode frequencies is observed with highest SW frequency 14.7 GHz. For separation less than 100nm, the SW frequencies undergoes significant modification due to the varying nature of the magnetostatic and dipolar interaction in the array while for separations above 100nm, the SW frequency mostly remain constant. The power profiles confirm the nature of the observed modes. The spatial profiles of magnetostatic field are determined by a combination of internal magnetic-field profiles within the nanodots and the magnetostatic fields within the lattice. The inter-dots interaction of magnetostatic field shows dipolar and quadrupole contributions for all the shapes. Interestingly, vortex states with shifted core and polarity are observed in the array for all the shapes at Hbias = 0. Our results provide important understanding about the tunability of SW spectra in the array of triangular, diamond and hexagon shaped nanoelements.

Magnonic band gap and mode hybridization in continuous permalloy films induced by vertical dynamic coupling with an array of permalloy ellipses

We investigate magnonic band structure in thin homogeneous permalloy film decorated with periodic array of elliptically shaped permalloy dots and separated by non-magnetic Pt spacer. We demonstrated experimentally formation of the magnonic band structure for Damon-Eshbach wave propagating in permalloy film with the band gap opened at the Brillouin zone border and band splitting at smaller wavenumbers, due to the Bragg interference and interaction of propagating wave of the continuous film with a standing resonant mode of the nano-ellipses, respectively. The shape anisotropy of the permalloy nanodots allows to control the spin wave dynamics through the switch between two states of the magnetization with respect to the underneath film magnetization, thus enabling magnonic band structure reprogrammability. With numerical analysis we show, that predominant role in formation of the magnonic band structure is played by a vertical dynamic coupling between propagating wave in the film and magnetization oscillations in the nanodots.

Spin-wave dynamics in permalloy/cobalt magnonic crystals in the presence of a nonmagnetic spacer

In this paper, we theoretically study the influence of a non-magnetic spacer between ferromagnetic dots and ferromagnetic matrix on the frequency dispersion of the spin wave excitations in two-dimensional bi-component magnonic crystals. By means of the dynamical matrix method we investigate structures inhomogeneous across the thickness represented by square arrays of Cobalt or Permalloy dots in a Permalloy matrix. We show that the introduction of a non-magnetic spacer significantly modifies the total internal magnetic field especially at the edges of the grooves and dots. This permits the manipulation of the magnonic band structure of spin waves localized either at the edges of the dots or in matrix material at the edges of grooves. According to the micromagnetic simulations two types of end modes were found. The corresponding frequencies are significantly influenced by the end modes localization region. We also show that, with the use of a single ferromagnetic material, it is possible to design a magnonic crystal preserving properties of bi-component magnonic crystals and magnonic antidot lattices. Finally, the influence of the non-magnetic spacers on the technologically relevant parameters like group velocity and magnonic band width are discussed.

Fine structure of excitation spectra of magnetic vortex dot clusters

Low-frequency gyrotropic dynamics (100 MHz range) in clusters of interacting magnetic vortex state dots are considered. The clusters consist of 3 or 4 laterally placed cylindrical ferromagnetic dots on a dielectric substrate in the form of equilateral triangles or squares. The interdot dynamical magnetostatic interactions are considered in the form of explicit multipole decompositions on the inverse dot center-to-center distance. The eigenfrequencies of collective magnetic vortex oscillations are calculated analytically and compared with the results of recent experiments conducted on clusters of permalloy dots.

Magnetic normal modes of bicomponent permalloy/cobalt structures in the parallel and antiparallel ground state

Two-dimensional arrays of bicomponent structures made of cobalt and permalloy elliptical dots with thickness of 25 nm, length 1 \ensuremath{\mu}m, and width of 225 nm, have been prepared by a self-aligned shadow deposition technique. Brillouin light scattering has been exploited to study the frequency dependence of thermally excited magnetic eigenmodes on the intensity of the external magnetic field, applied along the easy axis of the elements. Several modes have been observed while sweeping the field along the major and minor hysteresis loops, encompassing both the parallel and the antiparallel alignment of the magnetization in adjacent permalloy and cobalt dots. Micromagnetic simulations based on the dynamic matrix method enabled us to successfully reproduce the measured evolution of the frequencies with the field, as well as to identify the spatial profiles of the modes. A marked difference in the field dependence of the frequency of some modes has been observed for parallel and antiparallel magnetization configurations, suggesting the possibility of tuning the dynamic response in a reprogrammable way. The role of both static and dynamic magnetic coupling in determining the mode frequency is discussed in detail by studying the frequency evolution as a function of the gap size between the dots.

From micro- to nanomagnetic dots: evolution of the eigenmode spectrum on reducing the lateral size

Brillouin light scattering experiments and micromagnetic simulations have been exploited to investigate the spectrum of thermally excited magnetic eigenmodes in 10 nm-thick elliptical Permalloy dots, when the longer axis D is scaled down from about 1000 to 100 nm. It is shown that for D larger than about 200 nm the characteristics of the spin-wave eigenmodes are dominated by dipolar energy, while for D in the range of about 100 to 200 nm exchange energy effects cause qualitative and quantitative differences in the spin-wave spectrum. In this ‘mesoscopic’ regime, the usual classification scheme, involving one fundamental mode with large average magnetization and many other modes collected in families with specific symmetries, no longer holds. Rather, one finds the simultaneous presence of two modes with ‘fundamental’ character, i.e. with a significant and comparable value of the average dynamical magnetization: the former is at larger frequency and has its maximum amplitude at the dot's centre, while the latter occurs at lower frequency and is localized at the dot's edges. Interestingly, the maximum intensity swaps from the higher frequency mode to the lower frequency one, just when the dot size is reduced from about 200 to 100 nm. This is relevant in view of the exploitation of nanodots for the design of nanomagnetic devices with lateral dimensions in the above interval, such as memory cells, logic gates, reading heads and spin-torque oscillators.

Microwave absorption properties of permalloy nanodots in the vortex and quasi-uniform magnetization states

When the in-plane bias magnetic field acting on a flat circular magnetic dot is smaller than the saturation field, there are two stable competing magnetization configurations of the dot: the vortex and the quasi-uniform (C-state). We measured microwave absorption properties in an array of non-interacting permalloy dots in the frequency range 1–8 GHz when the in-plane bias magnetic field was varied in the region of the dot magnetization state bi-stability. We found that the microwave absorption properties in the vortex and quasi-uniform stable states are substantially different, so that switching between these states in a fixed bias field can be used for the development of reconfigurable microwave magnetic materials.

Geometrical optimization of a local ballistic magnetic sensor

We have developed a highly sensitive local magnetic sensor by using a ballistic transport property in a two-dimensional conductor. A semiclassical simulation reveals that the sensitivity increases when the geometry of the sensor and the spatial distribution of the local field are optimized. We have also experimentally demonstrated a clear observation of a magnetization process in a permalloy dot whose size is much smaller than the size of an optimized ballistic magnetic sensor fabricated from a GaAs/AlGaAs two-dimensional electron gas.

SPIN WAVES ALONG THE EDGE STATES

Spin waves (SWs) have been studied experimentally and by simulations in 1000 nm side equilateral triangular Permalloy dots in the Buckle state (B, with in-plane field along the triangle base) and the Y state (Y, with in-plane field perpendicular to the base). The excess of exchange energy at the triangles edges creates channels that allow effective spin wave propagation along the edges in the B state. These quasi one-dimensional SWs emitted by the vertex magnetic charges gradually transform from propagating to standing due to interference and (as pointed out by simulations) are weakly affected by small variations of the aspect ratio (from equilateral to isosceles dots) or by interdot dipolar interaction present in our dot arrays. SWs excited in the Y state have mainly a two-dimensional character. Propagation of the SWs along the edge states in triangular dots opens possibilities for creation of new and versatile spintronic devices.

Observation of propagating edge spin waves modes

Broadband magnetization response of equilateral triangular 1000 nm Permalloy dots has been studied under an in-plane magnetic field, applied parallel (buckle state), and perpendicular (Y state) to the triangles base. Micromagnetic simulations identify edge spin waves (E-SWs) in the buckle state as SWs propagating along the two adjacent edges. These quasi one-dimensional spin waves emitted by the vertex magnetic charges gradually transform from propagating to standing due to interference and are weakly affected by dipolar interdot interaction and variation of the aspect ratio. Spin waves in the Y state have a two dimensional character. These findings open perspectives for implementation of the E-SWs in magnonic crystals and thin films.

Wide range tuning of resonant frequency for a vortex core in a regular triangle magnet

A magnetic vortex structure stabilized in a micron or nano-sized ferromagnetic disk has a strong potential as a unit cell for spin-based nano-electronic devices because of negligible magnetostatic interaction and superior thermal stability. Moreover, various intriguing fundamental physics such as bloch point reversal and symmetry breaking can be induced in the dynamical behaviors in the magnetic vortex. The static and dynamic properties of the magnetic vortex can be tuned by the disk dimension and/or the separation distance between the disks. However, to realize these modifications, the preparations of other devices with different sample geometries are required. Here, we experimentally demonstrate that, in a regular-triangle Permalloy dot, the dynamic properties of a magnetic vortex are greatly modified by the application of the in-plane magnetic field. The obtained wide range tunability based on the asymmetric position dependence of the core potential provides attractive performances in the microwave spintronic devices.

25aPS-18 2端子を狭窄したホールクロスによるパーマロイドットの磁化測定(領域3ポスターセッション(スピントロニクス・表面・界面磁性・マルチフェロイクス・遍歴磁性・磁性一般),領域3(磁性,磁気共鳴))

25aPS-18 2端子を狭窄したホールクロスによるパーマロイドットの磁化測定(領域3ポスターセッション(スピントロニクス・表面・界面磁性・マルチフェロイクス・遍歴磁性・磁性一般),領域3(磁性,磁気共鳴))

Erratum: Brillouin light scattering investigation of dynamic spin modes confined in cylindrical Permalloy dots [Phys. Rev. B68, 184409 (2003)

with q2 n = q2 nx + q2 ny + q2 nz and n = 1,2, . . . . After Eq. (4) the symbols qx , qy , and qz should be replaced by qnx , qny , and qnz, respectively. The changes do not affect the numerical calculations presented in Sec. V performed at qnx = 0 for the generic nth backward modes that have a qny component much larger than qnx . The article J. Jorzick, S. O. Demokritov, C. Mathieu, B. Hillebrands, B. Bartenlian, C. Chappert, F. Rousseaux, and A. N. Slavin, Phys. Rev. 60, 15194 (1999) is cited twice, both as Ref. 12 and as Ref. 15.

Measurements of configurational anisotropy in isolated sub-micron square permalloy dots

We measure a shape dependent anisotropy energy in isolated square permalloy dots with characteristic dimensions as small as 200 nm. The magnetization behavior is characterized using the anisotropic magnetoresistance to determine the magnetic state of the sample. The behavior is consistent with the sample adopting a quasi-single-domain magnetization with a direction determined by an anisotropy intrinsic to the square geometry. We quantify the magnitude of this anisotropy through two independent measurements and confirm the observed magnetic behavior with micromagnetic simulations.

Magnetization evolution in a Permalloy dot driven by spin-polarized currents in multi-nanocontact geometry

Magnetization evolution in a Permalloy dot with radius R=200nm and thickness L=10nm driven by spin polarized currents was studied by micromagnetic simulations with three point contacts on a diameter. We obtained rich phase diagrams through changing the directions, magnitudes and the inter-distances of the spin-polarized currents. The current must exceed a minimum value before changes in the magnetic state occurred. As the current density was increased, the ground state changed from a single vortex state to complex magnetic states via nucleation and annihilation of vortex–antivortex pairs. However, these rich magnetization phases finally recovered into two different stable vortex states after the current was powered off.

Standing spin waves in perpendicularly magnetized circular dots at millimeter waves

Spin wave spectra of 40-nm thick perpendicularly magnetized circular Permalloy dots of 250 nm radius were measured using ferromagnetic resonance technique in 70-80 GHz range at 4.2 K and in 10 GHz at room temperature. The five sharp resonance peaks were observed for both frequency ranges. The resonance fields can be well described by a magneto-exchange dispersion relation, implying that the observed resonances correspond to circular “drumhead” modes with Bessel-function profiles. The relative distances between neighbor peaks for different frequency ranges were almost the same, while the absolute interpeak distances in millimeter range were ∼30% bigger than at 10 GHz, as predicted by the theory.

Spin Wave Excitations and Winter’s Magnons in Vertically Coupled Vortex State Permalloy Dots

Circular soft magnetic dots are the main elements of many proposed novel spintronics devices, capable of fascinating spin-based electronics applications, from extremely sensitive magnetic field sensors, to current-tunable microwave vortex oscillators. Here, we investigate static and broadband dynamic magnetization responses of vertically coupled Permalloy (Py) magnetic dots in the vortex state in layered nanopillars (experiment and simulations), which were explored as a function of in-plane magnetic field and interlayer separation. Under reduction of magnetic field from saturation for the field range just above vortex-vortex ground state. We observe a metastable double vortex state for each of the dots. In this state, novel kinds of spin waves (Winter’s magnons along domain walls between vortex cores and half-edge antivortex) are excited. For dipolarly coupled circular Py(25 nm)/Cu(20 nm)/Py(25 nm) trilayer nanopilars of diameter 600 nm, a small in-plane field splits the eigenfrequencies of azimuthal spin wave modes inducing an abrupt transition between acoustic (in-phase) and optic (out-of-phase) kinds of the low-lying coupled spin wave modes. Qualitatively similar changes (although more gradual and at higher values of in-plane fields) occur in the exchange coupled Py(25 nm)/Cu(1 nm)/Py(25 nm) trilayer nanopillars. These findings are in qualitative agreement with micromagnetic dynamic simulations.

Phononic dispersion of a two-dimensional chessboard-patterned bicomponent array on a substrate

Brillouin measurements of the dispersion relations of surface acoustic- and optical-like waves along Γ-M and Γ-X symmetry directions in a two-dimensional bicomponent nanostructured crystal are reported. The sample, in the form of a periodic chessboard array of alternating Permalloy and cobalt square dots on a SiO2/Si substrate, was fabricated using high-resolution electron-beam lithographic, sputtering, etching, and lift-off techniques. The measured phononic band structures exhibit diverse features, such as a partial hybridization bandgap and unusual surface optical-like phonon branches, where there are out-of-phase vibrational characteristics between nearest-neighbor dots. Numerical simulations, based on the finite element analysis, reproduced the experimental dispersion relations.