Permalloy Platelets Research Articles

Magnetization dynamics in polycrystalline Permalloy and epitaxial Co platelets observed by time‐resolved photoemission electron microscopy

AbstractWe studied the dynamic magnetization response in rectangular polycrystalline Permalloy and also epitaxial Co structures (lateral sizes comprised tens of microns at a thickness of tens of nanometers) during the action of a magnetic field pulse, using time‐resolved X‐ray photoemission electron microscopy with a time resolution of 10 ps. In the case of Permalloy platelets the restoring torque that is necessary for the stroboscopic image acquisition is provided by the Landau flux closure structure representing a minimum of the free energy. We investigated the dynamic response of 90° Néel domain walls. The main results are: the maximum velocity of the domain wall is 1.5 × 104 m/s, the intrinsic frequency of the magnetization change in these structures is estimated to be several Gigahertz. For the case of epitaxial Co platelets grown on Mo(110) the magnetic uniaxial anisotropy with an easy axis along Mo[1$ \bar 1 $0] restores the homogeneous magnetization structure after each field pulse. We observed a rotation of the mean magnetization direction within the first 100 ps of the field pulse. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Transient spatio-temporal domain patterns in permalloy microstructures induced by fast magnetic field pulses

The response of multidomain flux-closure structures (Landau states) in micrometer-scale magnetic thin-film elements upon fast magnetic field pulses leads to the excitation of magnetic eigenmodes and to short-lived domain patterns that do not occur in quasi-static remagnetisation. Such transient spatio-temporal patterns and particular detail features are discussed. Examples are presented for permalloy platelets of various shapes and sizes. Dynamic series of domain patterns with variable delay between field pulse and photon pulse (synchrotron radiation) have been taken using stroboscopic XMCD-PEEM. Precessional remagnetisation starts at the domain boundaries. The damped precessional motion propagates into the domains magnetised perpendicular to the field pulse. Further, an apparent dynamical broadening of domain boundaries, the formation of transient vortices and domain walls as well as the fast formation of blocking patterns are discussed. An important feature is the action of the demagnetising field that develops via the damped precessional motion of the magnetisation on the nanosecond time scale. The effective field is constituted by the sum of the instantaneous external pulse field and the delayed response of the demagnetising field.

Self-Trapping of Magnetic Oscillation Modes in Landau Flux-Closure Structures

We investigated the magnetodynamics in rectangular Permalloy platelets by means of time-resolved x-ray photoemission microscopy. 10 nm thick platelets of size 16 x 32 microm were excited by an oscillatory field along the short side of the sample with a fundamental frequency of 500 MHz and considerable contributions of higher harmonics. Under the influence of the oscillatory field, the Néel wall in the initial classical Landau pattern shifts away from the center, corresponding to an induced magnetic moment perpendicular to the exciting field. This phenomenon is explained by a self-trapping effect of the dominating spin-wave mode when the system is excited just below the resonance frequency. The basic driving mechanism is the maximization of entropy.

Micromagnetic Dissipation, Dispersion, and Mode Conversion in Thin Permalloy Platelets

Micron-sized ferromagnetic Permalloy disks exhibiting an in-plane ferromagnetic vortex structure are excited by a fast rise time perpendicular magnetic field pulse and their modal structure is analyzed. We find azimuthal and axial modes. By a Fourier filtering technique we can separate and analyze the time dependence of individual modes. Analysis of the experimental data demonstrates that the azimuthal modes damp more quickly than the axial modes. We interpret these results as mode conversion from low-frequency azimuthal modes to the fundamental mode which is higher in frequency, i.e., mode-mode coupling in a system with a single Landau-Lifshitz-Gilbert phenomenological damping constant alpha.

Investigation of magnetization behavior in nanoelements using the finite element method

A model of thin film permalloy using an efficient finite element variational approach to the magnetostatic field calculation is described. The material is discretized into a nanoelement structure at the micron level which enables us to investigate material properties due to patterning. Predicted domain structures agree well with experimental data. Interactions between elements are significant and it will be shown that the domain structure in the central element differs from that of its neighbors. As expected, the addition of pointed ends stabilizes the single-domain state. Elements with two pointed ends exhibit pseudo-single-domain behavior. We have studied the single-domain/pseudo-single-domain transition for permalloy platelets, results for noninteracting platelets are given as a function of the elongation. Interactions are shown to increase or decrease the critical size depending on the geometry.

Equilibrium analysis of magnetization reversal in cobalt and permalloy platelets (abstract)

The local equations of stable equilibrium for the magnetization distribution in a ferromagnetic system in the micromagnetic approximation are derived from the Gibbs free energy, by application of the principle of minimum total potential. The resulting boundary value problem consists of a system of second order integrodifferential equations and boundary conditions for the magnetization distribution. These are solved iteratively using a finite-difference method and applied to the study of thin regularly shaped cobalt particles of sub and supra micron sizes with various aspect ratios and exchange coupling strengths. Permalloy platelets are also studied for size and aspect ratio dependence. Magnetization curves and distributions are calculated and the effect of aspect ratio and exchange coupling on the distributions are deduced. Comparison with previous calculations using finite element techniques1,2 are made. Variational methods using Hamilton’s principle suggest that a dynamic extension can be developed3,4 to model the transient processes associated with the irreversible phases of the magnetization reversal. Such a model based on equations of motion in the Hamiltonian form is currently being developed.

Observations of magnetostatic modes in single crystal permalloy platelets

Magnetostatic modes have been observed for parallel and perpendicular (⊥) magnetic resonance in single crystal Permalloy platelets. The ⊥ resonant field positions are approximately fitted to Akhiezer's magnetostatic theory. The intensity of the modes decrease as 1/ n, where n is the mode number. The line width is 80 oe for n = 1 and increases with n for n > 4 to a value of 100 oe for n = 13.

Helical and Other Stripe Structures in Single-Crystal Permalloy Platelets

Four primary stripe patterns are observed in Permalloy single-crystal platelets with {100} surfaces in different regions of thickness and composition. All four structures involve magnetic moment rotations between easy 〈111〉 axes parallel to {110} planes. The first to appear with increasing thickness (0.3<D<1.0 μm) has 〈100〉 stripes and 〈100〉 net magnetization. The second and third coexist from 0.8–1.4 μm, have fine 〈110〉 and broad 〈100〉 stripes, respectively, and have different magnitudes of net magnetization. Both involve 71° rotations parallel to vertical {110} planes. The fourth (D>0.9 μm) achieves appreciable internal flux closure by forming a helical structure through rotations toward alternate 〈111〉 directions in passing from top to bottom surfaces at any stripe, and through having the planar surface component in alternate stripes perpendicular to the stripes. Models for 180° and 90° walls readily follow from the above scheme in conjunction with applied-field observations.

Ferromagnetic Domains in Single-Crystal Permalloy Platelets

Thin single-crystal Permalloy platelets with up to 30% iron have been grown nonepitaxially by the reduction of mixed Ni-Fe halides. Those with weak negative magnetocrystalline anisotropy, 〈100〉 edges, and (100) surfaces display a “tulip” closure-domain pattern that reveals the mode of formation of the more complicated echelon and chevron patterns observed in pure nickel platelets. The tulip pattern may appear as a modified form of the Néel-strip closure structure that Bates and co-workers have studied extensively to obtain quantitative information concerning low-field domain processes. The platelets are thus useful for extending this work to smaller, thinner, and more nearly ideal specimens. Platelets in a region near 5000 Å thick and 5% iron have 〈100〉 domains and stripes even though K1<0.