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Magnetic anisotropies of MBE-grown fcc Co(110)-films on Cu(110) single crystal substrates have been determined by using Brillouin light scattering(BLS) and have been correlated with the structural properties determined by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Three regimes of film growth and associated anisotropy behavior are identified: coherent growth in the Co film thickness regime of up to 13 Å, in-plane anisotropic strain relaxation between 13 Å and about 50 Å and inplane isotropic strain relaxation above 50 Å. The structural origin of the transition between anisotropic and isotropic strain relaxation was studied using STM. In the regime of anisotropic strain relaxation long Co stripes with a preferential [ 110 ]-orientation are observed, which in the isotropic strain relaxation regime are interrupted in the perpendicular in-plane direction to form isotropic islands. In the Co film thickness regime below 50 Å an unexpected suppression of the magnetocrystalline anisotropy contribution is observed. A model calculation based on a crystal field formalism and discussed within the context of band theory, which explicitly takes tetragonal misfit strains into account, reproduces the experimentally observed anomalies despite the fact that the thick Co films are quite rough.
Absract: We report on measurements of the two-dimensional intensity distribtion of linear and non-linear spin wave excitations in a LuBiFeO film. The spin wave intensity was detected with a high-resolution Brillouinlight scatteringspectroscopy setup. The observed snake-like structure of the spin wave intensity distribution is understood as a mode beating between modes with different lateral spin wave intensity distributions. The theoretical treatment of the linear regime is performed analytically, whereas the propagation of non-linear spin waves is simulated by a numerical solution of a non-linear Schrödinger equation with suitable boundary conditions.
We report results of the switching properties of Stoner-like magnetic particles subject to short magnetic field pulses, obtained by numerical investigations. We discuss the switching properties as a function of the external field pulse strength and direction, the pulse length and the pulse shape. For field pulses long compared to the ferromagnetic resonance precession time the switching behavior is governed by the magnetic damping term, whereas in the limit of short field pulses the switching properties are dominated by the details of the precession of the magnetic moment. In the latter case, by choosing the right field pulse parameters, the magnetic damping term is of minor importance and ultrafast switching can be achieved. Switching can be obtained in an enlarged angular range of the direction of the applied field compared to the case of long pulses.
The static and spin wave properties of regular square lattices of magnetic dots of 0.5-2 microm dot diameter and 1-4 microm periodicity patterned in permalloy films have been investigated by Brillouin light scattering. The samples have been structured using x-ray lithography and ion beam etching. The Brillouin light scattering spectra reveal both surface and bulk spin wave modes. The spin wave frequencies can be well described taking into account the demagnetization factor of each single dot. For the samples with smallest dot separation of 0.1 microm a fourfold in-plane magnetic anisotropy with the easy axis directed along the pattern diagonal is observed, indicating anisotropic coupling between the dots.
A computer control for a Sandercock-type multipath tandem Fabry-Perot interferometer is described, which offers many advantages over conventionally used analog control: The range of stability is increased due to active control of the laser light intensity and the mirror dither amplitude. The alignment is fully automated enabling start of a measurement within a minute after start of align, including optionally finding the optimum focus on the sample. The software control enables a programmable series of measurements with control of, e.g., the position and rotation of the sample, the angle of light incidence, the sample temperature, or the strength and direction of an applied magnetic field. Built-in fitting routines allow for a precise determination of frequency positions of excitation peaks combined with increased frequency accuracy due to a correction of a residual nonlinearity of the mirror stage drive.
An experimental study of spin wave quantization in arrays of micron size magnetic Ni80Fe20 islands (dots and wires) by means of Brillouin light scattering spectroscopy is reported. Dipolar-dominated spin wave modes laterally quantized in a single island with quantized wavevector values determined by the size of the island are studied. In the case of wires the frequencies of the modes and the transferred wavevector interval, where each mode is observed, are calculated. The results of the calculations are in a good agreement with the experimental data. In the case of circular dots the frequencies of the lowest observed modes decrease with increasing distance between the dots, thus indicating an essential dynamic magnetic dipole interaction between the dots with small interdot distances.
Hexagonal BN films have been deposited by rf-magnetron sputtering with simultaneous ion plating. The elastic properties of the films grown on silicon substrates under identical coating conditions have been de-termined by Brillouin light scattering from thermally excited surface phonons. Four of the five independent elastic constants of the deposited material are found to be c11 = 65 GPa, c13 = 7 GPa, c33 = 92 GPa and c44 = 53 GPa exhibiting an elastic anisotropy c11/c33 of 0.7. The Young's modulus determined with load indenta-tion is distinctly larger than the corresponding value taken from Brillouin light scattering. This discrepancy is attributed to the specific morphology of the material with nanocrystallites embedded in an amorphous matrix.
We report on the observation of spin wave quantization in square arrays of micron size circular magnetic Ni80Fe20 dots by means of Brillouin light scattering spectroscopy. For a large wavevector interval several discrete, dispersionless modes with a frequency splitting of up to 2.5 GHz were observed. The modes are identified as magnetostatic surface spin waves laterally quantized due to in- plane confinement in each single dot. The frequencies of the lowest observed modes decrease with increasing distance between the dots, thus indicating an essential dynamic magnetic dipole interaction between the dots with small interdot distances.
Epitaxial growth of metastable Pd(001) at high deposition temperatures up to a critical thickness of 6 monolayers on bcc-Fe(001) is reported, the critical thickness being depending dramatically on the deposition temperature. For larger thicknesses the Pd film undergoes a roughening transition with strain relaxation by forming a top polycrystalline layer. These results allow to make a correlation between previ-ously reported unusual magnetic properties of Fe/Pd double layers and the crystallographic structure of the Pd overlayer.
An overview of the current status of the study of spin wave excitations in arrays of magnetic dots and wires is given. We describe both the status of theory and recent inelastic light scattering experiments addressing the three most important issues: the modification of magnetic properties by patterning due to shape aniso-tropies, anisotropic coupling between magnetic islands, and the quantization of spin waves due to the in-plane confinement of spin waves in islands.
We investigate the temperature dependence of the magnetization reversal process and of spinwaves in epi-taxially grown (001)-oriented [Fem/Aun]30 multilayers (m = 1, 2; n = 1- 6). Both polar magneto-optic Kerrr effect and Brillouin light scattering measurements reveal that all investigated multilayers, apart from the [Fe2/Au1]30-sample, are magnetized perpendicular to the film plane. The out-of-plane anisotropy constants are obtained. At high temperature, the magnetization curves are well described by an alternating stripe domain structure with free mobile domain walls, and at low temperature by a thermal activation model for the domain wall motion.
An experimental study of spin wave quantization in arrays of micron size magnetic Ni80Fe20 wires by means of Brillouin light scattering spectroscopy is reported. Dipolar-dominated Damon-Eshbach spin wave modes laterally quantized in a single wire with quantized wavevector values determined by the width of the wire are studied. The frequency splitting between quantized modes, which decreases with increasing mode number, depends on the wire sizes and is up to 1.5 GHz. The transferred wavevector interval, where each mode is observed, is calculated using a light scattering theory for confined geometries. The frequen-cies of the modes are calculated, taking into account finite size effects. The results of the calculations are in a good agreement with the experimental data.
Collisions of Spin Wave Envelope Solitons and Self-Focused Spin Wave Packets in Magnetic Films
(1999)
Head-on collisions between two-dimensional self-focused spin wave packets and between quasi-one-dimensional spin wave envelope solitons have been directly observed for the first time in yttrium-iron garnet (YIG) films by means of a space- and time-resolved Brillouin light scattering technique. We show that quasi-one-dimensional envelope solitons formed in narrow film strips ("waveguides") retain their shapes after collision, while the two-dimensional self-focused spin wave packets formed in wide YIG films are destroyed in collision.
High frequency switching of single domain, uniaxial magnetic particles is discussed in terms of transition rates controlled by a small transverse bias field. It is shown that fast switching times can be achieved using bias fields an order of magnitude smaller than the effective anisotropy field. Analytical expressions for the switching time are derived in special cases and general configurations of practical interest are examined using numerical simulations.
We present detailed studies of the enhanced coercivity of exchange-bias bilayer Fe/MnPd, both experimentally and theoretically. We have demonstrated that the existence of large higher-order anisotropies due to exchange coupling between different Fe and MnPd layers can account for the large increase of coercivity in Fe/MnPd system. The linear dependence of coercivity on inverse Fe thickness are well explained by a phenomenological model by introducing higher-order anisotropy terms into the total free energy of the system.
Static and dynamic properties of patterned magnetic permalloy films are investigated. In square lattices of circular shaped permalloy dots an anisotropic coupling mechanism has been found, which is identified as being due to intrinsically unsaturated parts of the dots caused by spatial variations of demagnetizing field. In arrays of magnetic wires a quantization of the surface spin wave mode in several dispersionless modes is observed and quantitatively described. For large wavevectors the frequency separation between the modes becomes smaller and the frequencies converge to the dispersion of the dipole-exchange surface mode of a continuous film.
Mn-Si-C alloy films are prepared by e-beam coevaporation onto a Si substrate held at 600 °C. Ferromagnetism is observed below T = (360 +/- 5) K with SQUID magnetometry and magneto-optical Kerr effect. This is the highest Curie temperature T yet observed for a Mn-based alloy. Although the composition determined by Auger depth profiling varies appreciably for different films, their T is the same indicating that ferromagnetism is caused by an alloy of well-defined composition independent of precipitations.
Annual Report 1998
(1999)
A new advanced space- and time-resolved Brillouin light scattering technique is used to study diffraction of two-dimensional beams and pulses of dipolar spin waves excited by strip-line antennas in tangentially magnetized garnet films. The technique is an effective tool for investigations of two-dimensional spin wave propagation with high spatial and temporal resolution. Nonlinear effects such as stationary and nonstationary self-focusing are investigated in detail. It is shown, that nonlinear diffraction of a stationary backward volume magnetostatic wave (BVMSW) beam, having a finite transverse aperture, leads to selffocusing of the beam at one spatial point. Diffraction of a finite-duration (non-stationary) BVMSW pulse leads to space-time self-focusing and formation of a strongly localized two-dimensional wave packet (spin wave bullet).
We report on the observation of spin wave quantization in tangentially magnetized Ni80Fe20 discs by means of Brillouin light scattering spectroscopy. For a large wave vector interval several discrete, dispersionless modes with a frequency splitting up to 2.5 GHz were observed. The modes are identified as being magne-tostatic surface spin wave modes quantized by their lateral confinement in the disc. For the lowest modes dynamic magnetic dipolar coupling between the discs is found for a disc spacing of 0.1microm.
We report on investigations of the crystallographic structure and the magnetic anisotropies of epitaxial iron films deposited onto periodically stepped Ag(001) surfaces using low energy electron diffraction, x-ray diffraction, second harmonic generation (SHG), as well as the Brillouin light scattering (BLS) technique. The focus of the present study lies on the interrelation between the surface morphology of the buffer layers and the magnetic properties of the Fe films, epitaxially grown onto them. Especially the symmetry breaking at the atomic steps is found to create an uniaxial magnetic anisotropy measured by BLS and a very strong anisotropic signal in magnetic SHG.