A. Neudert

Dynamic anisotropy in amorphous CoZrTa films

The high-frequency response of amorphous CoZrTa thin films was measured by using a pulsed inductive microwave magnetometer. The anisotropy of the magnetic films was varied by magnetic field annealing. Static anisotropy field values ranging from Hk=100 to 1920 A/m were obtained. The dynamically determined anisotropy field is shifted to higher values compared to the static anisotropy by an additional isotropic internal field Hadd. This internal field is independent of the strength of the static anisotropy field. We determined a value of about Hadd=510 A/m.

Time-resolved investigation of magnetization dynamics of arrays of nonellipsoidal nanomagnets with nonuniform ground states

We have performed time-resolved scanning Kerr microscopy (TRSKM) measurements upon arrays of square ferromagnetic nano-elements of different size and for a range of bias fields. The experimental results were compared to micromagnetic simulations of model arrays in order to understand the non-uniform precessional dynamics within the elements. In the experimental spectra two branches of excited modes were observed to co-exist above a particular bias field. Below the so-called crossover field, the higher frequency branch was observed to vanish. Micromagnetic simulations and Fourier imaging revealed that modes from the higher frequency branch had large amplitude at the center of the element where the effective field was parallel to the bias field and the static magnetization. Modes from the lower frequency branch had large amplitude near the edges of the element perpendicular to the bias field. The simulations revealed significant canting of the static magnetization and the effective field away from the direction of the bias field in the edge regions. For the smallest element sizes and/or at low bias field values the effective field was found to become anti-parallel to the static magnetization. The simulations revealed that the majority of the modes were de-localized with finite amplitude throughout the element, while the spatial character of a mode was found to be correlated with the spatial variation of the total effective field and the static magnetization state. The simulations also revealed that the frequencies of the edge modes are strongly affected by the spatial distribution of the static magnetization state both within an element and within its nearest neighbors.

Domain wall induced modes of high-frequency response in ferromagnetic elements

The influence of domain wall density on the magnetization dynamics of amorphous CoZrTa thin-film elements was investigated by a combination of microwave magnetometry and quasistatic plus time-resolved wide-field Kerr microscopy. In addition to domain wall motion, permeability rolloff at low frequencies occurs due to rotational processes. The dominating ferromagnetic resonance modes depend on the domain wall density due to the formation of a zone of magnetization curling at the domain walls, which results from a phase lag of domain and domain wall response. Both the amount of permeability reduction and the increase in precessional frequency, can be varied with magnetic history. All effects are avoided by lamination of the ferromagnetic films. The results demonstrate the importance of detailed domain control for the optimization of patterned films for high-frequency applications, beyond the elementary adjustment of material’s high-frequency properties.

Subnanosecond vortex transformation in ferromagnetic film elements observed by stroboscopic wide-field Kerr microscopy

The magnetization dynamics of a Ni81Fe19 disk of 16μm diameter and 50nm thickness was measured by time-resolved stroboscopic wide-field Kerr microscopy with a temporal resolution of less than 25ps. Time resolution is obtained by illumination with a frequency-doubled high-power Nd:YVO4 solid-state laser. The ground state of the magnetization is excited with a 700‐ps-wide pulse of about 1.2kA∕m amplitude. After a fast rotation of magnetization and vortex displacement due to the pulsed magnetic field the magnetization slowly relaxes back to the ground state. During relaxation of the vortex we observe in the center of the element a stretched vortex structure that could be a 180° wall or a superposition of two vortices with switched core magnetization.

Phase-resolved x-ray ferromagnetic resonance measurements in fluorescence yield

Phase-resolved x-ray ferromagnetic resonance (XFMR) has been measured in fluorescence yield, extending the application of XFMR to opaque samples on opaque substrates. Magnetization dynamics were excited in a Co50Fe50(0.7)/Ni90Fe10(5) bilayer by means of a continuous wave microwave excitation, while x-ray magnetic circular dichroism (XMCD) spectra were measured stroboscopically at different points in the precession cycle. By tuning the x-ray energy to the L3 edges of Ni and Fe, the dependence of the real and imaginary components of the element specific magnetic susceptibility on the strength of an externally applied static bias field was determined. First results from measurements on a Co50Fe50(0.7)/Ni90Fe10(5)/Dy(1) sample confirm that enhanced damping results from the addition of the Dy cap.

Time- and vector-resolved magneto-optical Kerr effect measurements of large angle precessional reorientation in a 2×2μm2 ferromagnet

The precessional dynamics of a 2×2μm2 CoFe∕NiFe (4.6nm) element stimulated by an in-plane pulsed magnetic field have been investigated using time- and vector-resolved Kerr microscopy measurements and micromagnetic simulations. The time-resolved signals were normalized to in-plane hysteresis loops obtained from the patterned material, and suggest that the magnetization reorients through an angle of 100°±10°. The simulations reveal that only the magnetization of the center region undergoes large angle reorientation, while the canted magnetization at the edges of the element remains pinned. An enhanced Gilbert damping parameter of 0.1 was required to reproduce the experimentally observed Kerr signals.

Influence of a Dy overlayer on the precessional dynamics of a ferromagnetic thin film

Precessional dynamics of a Co50Fe50(0.7)/Ni90Fe10(5)/Dy(1)/Ru(3) (thicknesses in nm) thin film have been explored by low temperature time-resolved magneto-optical Kerr effect and phase-resolved x-ray ferromagnetic resonance measurements. As the temperature was decreased from 300 to 140 K, the magnetic damping was found to increase rapidly while the resonance field was strongly reduced. Static x-ray magnetic circular dichroism measurements revealed increasing ferromagnetic order of the Dy moment antiparallel to that of Co50Fe50/Ni90Fe10. Increased coupling of the Dy orbital moment to the precessing spin magnetization leads to significantly increased damping and gyromagnetic ratio of the film while leaving its magnetic anisotropy effectively unchanged.

Excitation and Imaging of Precessional Modes in Soft-Magnetic Squares

We investigated the high-frequency modes of 40 mum wide, 160 nm thick Fe 70Co8B12Si10 squares using time-resolved scanning Kerr microscopy. Two modes were identified using pulsed field excitation while the spatial character of the out-of-plane and in-plane magnetization component was investigated using harmonic field excitation. The field dependence of the two modes has been fitted using the Damon-Eshbach model.

Bloch-line generation in cross-tie walls by fast magnetic-field pulses

The magnetization dynamics of a constrained cross-tie wall in a 50-nm-thick Ni81Fe19 element is investigated using static and time-resolved wide-field Kerr microscopy. By applying a series of short magnetic-field pulses to the sample the creation of additional vortex antivortex pairs within the cross-tie wall is observed. The cross-tie spacing decreases by a factor of 2–4 relative to the initial equilibrium state. By using sinusoidal and pulsed magnetic-field excitations, a discrimination between the influence of the repetition rate and the rise time of the applied fields is possible. We found a logarithmic increase of the Bloch-line number with decreasing rise time. A resonant excitation with increased repetition rate decreases the cross-tie spacing even further. From direct stroboscopic observation of the precessional remagnetization process, a mechanism for vortex multiplication is proposed.

Effect of wheel speed and boron content on microstructure and crystallographic texture of boron substituted Sm-Co melt spun ribbons

The Sm(Co0.71Fe0.1Cu0.12Zr0.04B0.03)7.5 (2:17 type magnet) melt spun ribbons have been produced from bulk as cast samples at low (5 m/sec) to medium (40 m/sec) wheel speed by the melt spinning technique. The crystallographic texture on wheel side, the microstructural characteristics and magnetic properties have been investigated. The soft magnetic fcc-Co forms a very high degree of texture especially at low velocities but for the first time a degree of texturing has been remarked on fcc-Co grains. Diffraction patterns have been traced by x-ray scattering using Cu-Kα radiation on the wheel and free side of the ribbons. In the pattern of ribbons which have been produced at 5 m/sec the (002) plane of fcc-Co is almost the dominant peak while at 40 m/sec this peak diminishes in parallel to the appearance of the structure type TbCu7 and (111) plane of fcc-Co structure. Scanning electron microscopy on the wheel side of the ribbons has been used to observe microstructural characteristics and showed that the formation of texture is attributed to the appearance of dendrites, with their long axis parallel to the longitudinal direction of the ribbons. Dendrites’ density depends on the wheel speed of the roller and boron content. It decreases as the velocity increases while for constant velocity of about 40 m/sec, higher boron content stabilizes higher degree of texturing. Magnetic properties are also examined from low to medium wheel speed by also using the magnetooptical Kerr microscopy. Therefore coercive field as high as 3.4 kOe and reduced remanence (mr) of ~0.76 has been detected from the hysteresis curve for as spun Sm Co0.71Fe0.1Cu0.12Zr0.04B0.03)7.5 ribbons at 5 m/sec.