C. E. Zaspel

Gyrotropic mode frequency of vortex-state permalloy disks

Submicron permalloy magnetic dots have a vortex ground state because of competition between the exchange and magnetostatic interactions, and the application of an in-plane magnetic pulse will result in precession of the vortex about the dot axis at a frequency in the sub-GHz range. The precession frequency of this mode is calculated using a perturbation technique based on vortex–magnon scattering including the magnetostatic interaction. These calculations show that the frequencies vary between 0.2 and 0.8 GHz for 60 nm disks or radii between 250 nm and 1000 nm, which agrees with recent experimental data.

ELECTRON PARAMAGNETIC RESONANCE DETECTION OF SOLITONS IN TWO-DIMENSIONAL MAGNETS

It has previously been assumed that spin waves were the dominant excitations in lower-dimensional magnets. Recently, however, it has been shown that nonlinear excitations or solitons rather than spin waves influence the dynamic thermal quantities such as the spin correlation function which can be investigated experimentally through the electron paramagnetic resonance linewidth. In this review the influence of both spin waves and solitons on the temperature-dependent linewidth in the fluctuation region immediately above the ordering temperature is discussed. It is seen that both excitations result in a theoretical Arrhenius temperature-dependence, (∆H~exp(E/T) where E=6πJs2 for spin waves and E=4πJs2 for solitons, J is the nearest neighbor exchange constant, and s is the value of the spin. In experiments, quantum (s=1/2) layered copper compounds exhibit the temperature dependence expected from spin waves even though nonlinear excitations have been shown to exist in these systems. On the other hand nearly classical (s=5/2) manganese compounds have the temperature dependence expected from solitons. The calculation of the linewidth from both spin waves and solitons is reviewed and compared with experimental data to show that solitons dominate the dynamics of the layered, nearly classical magnet.

Magnetic properties of the copper trimer chains 4‐picolinium2 Cu3Cl8 and 4‐picolinium2 Cu3Br8

(4‐pic)2Cu3Cl8 and (4‐pic)2Cu3Br8 contain planar, bibridged copper halide trimers. The trimers are then stacked with longer semicoordinate Cu–halide–Cu bonds between adjacent trimers. The magnetic susceptibilities show that trimers are in a spin‐1/2 ground state at low temperatures with coupling constants J/k=30 and −100 K, respectively, determined by fitting the data to a new high‐temperature expansion with only exchange constants and external field as expansion parameters. We show that even though the spin‐1/2 trimers are antiferromagnetically exchange coupled along the chain, they order ferromagnetically at low temperature providing examples of pseudo‐one‐dimensional, S=1/2, Heisenberg ferromagnets. Electron paramagnetic resonance measurements at the X band were also performed and confirm the susceptibility measurements.

Small impurity-pinned solitons in layered antiferromagnets

Layered manganese-halide compounds exhibit quasi-two-dimensional magnetic behavior in the temperature region immediately above the ordering temperature where soliton excitation can be experimentally detected as an Arrhenius, exp(E/T), temperature-dependent electron paramagnetic resonance linewidth, where E is the soliton energy. When a nonmagnetic impurity such as magnesium is introduced into the Mn lattice, experimental linewidth data indicate that the excitation energy is dramatically reduced and the temperature range over which Arrhenius behavior is observed is widened to include higher temperatures. These effects occur for very small (less than 1%) impurity concentrations.

Modeling of the power-dependent velocity of microwave magnetic envelope solitons in thin films

Recently obtained single soliton solutions to the nonlinear Schrodinger equation with additional higher order nonlinear and dispersion terms have been used to model the properties of microwave magnetic envelope solitons in thin films and, in particular, the recently observed power-dependent soliton velocity in these films. The modeling is based on an empirical scaling between the dimensionless amplitude parameters in the higher order nonlinear Schrodinger (HONLS) equation and the pulse power levels in the experiment. Based on this scaling, the amplitude-dependent soliton velocity from the HONLS equation solution is shown to match the velocity versus power response from the experiment.

Exchange constants in the copper trimer system 3CuCl2·2dx☆

Abstract A high temperature series expansion for the susceptibility of an interacting trimer chain is obtained to third order J ′/ T where J ′ is the intertrimer exchange constant. This is compared with experimental data obtained by Willetts group to estimate the intratrimer exchange, J / k B = 85 K and the intertrimer exchange, J ′/ k B = −14 K.

Structure and magnetic studies of bis‐piperidinium tetrachlorocuprate

X‐ray, magnetic susceptibility, and epr investigations of bis‐piperidinium tetrachlorocuprate (C5H10NH2)2CuCl4 indicate that it is a complicated structure of CuCl4= chains and ribbons separated by piperidinium groups and that it exhibits weak antiferromagnetism with J/k approximately −0.8 °K. The space group is Pc and the unit cell data are a=8.218 A, b=16.924 A, c=12.186 A, and β=100.5°. The possibilities of magnetic dimensionality transitions and disordering as a function of temperature are discussed.

A new model for dark solitons in magnetic films

In magnetic thin films, the nonlinear Schroedinger (NLS) equation predicts the formation of a pair of dark solitons with equal and opposite phase shifts from an input pulse without phase modulation. To consider the effects of higher input power, a modified NLS equation is derived including higher order dispersive and nonlinear terms. An analytic solution to the modified equation is obtained which has the form of a single dark soliton in agreement with recent experiments using higher input power.

Vortex energies in the two-dimensional easy-plane ferromagnet

Abstract In the core region of a planar vortex in the two-dimensional Heisenberg model, the continuum approximation breaks down and a cut-off radius must be introduced as the lower limit of the energy integral. By expressing the total vortex energy as the sum of discrete core and continuum contributions it is possible to determine this cut-off radius as well as an accurate estimate of the vortex energy.

Temperature dependence of the electron‐spin‐resonance linewidth of copper (II) trimers

The temperature dependence of the electron‐paramagnetic‐resonance linewidth is calculated for a one‐dimensional system of copper (II) trimers. The second moment of the resonance line is calculated from the following effective spin Hamiltonian H=Hs +Ha +Hh +H’s , where Hs is the intratrimer isotropic exchange interaction, Ha is the intratrimer anisotropic exchange interaction, Hd is the dipole–dipole interaction, and H’s is the intertrimer anisotropic exchange interaction. With the assumption that Hs is the dominant term in the Hamiltonian, the second moment has an approximate (T)1/2 temperature dependence. This is compared with experimental data for the copper trimer chain (4‐picolinium) 2Cu3 Cl8.