W. J. M. de Jonge

A novel, all-optical method to excite and detect spin waves in magnetic materials is presented. By exploiting the temperature dependence of the magnetic anisotropy, an ultrashort laser pulse is efficiently converted in a picosecond "anisotropy field" pulse that triggers a coherent precession of the magnetization. Recording the temporal evolution of the precessing spins by a time-delayed probe-pulse provides a quantitative method to study locally the magnetic anisotropy, as well as switching and damping phenomena in micromagnetic structures. Applications to nickel and permalloy ( Ni80Fe20) films are discussed, particularly showing the possibility to explore standing spin waves in thin films.

We present a microscopic model that successfully explains the ultrafast equilibration of magnetic order in ferromagnetic metals at a time scale ${\ensuremath{\tau}}_{M}$ of only a few hundred femtoseconds after pulsed laser excitation. It is found that ${\ensuremath{\tau}}_{M}$ can be directly related to the so-called Gilbert damping factor $\ensuremath{\alpha}$ that describes damping of GHz precessional motion of the magnetization vector. Independent of the spin-scattering mechanism, an appealingly simple equation relating the two key parameters via the Curie temperature ${T}_{C}$ is derived, ${\ensuremath{\tau}}_{M}\ensuremath{\approx}{c}_{0}\ensuremath{\hbar}/{k}_{B}{T}_{C}\ensuremath{\alpha}$, with $\ensuremath{\hbar}$ and ${k}_{B}$ the Planck and Boltzmann constants, respectively, and the prefactor ${c}_{0}\ensuremath{\sim}\frac{1}{4}$. We argue that phonon-mediated spin-flip scattering may contribute significantly to the sub-ps response.

In this paper we reinterpret the magnetic-susceptibility data and present and discuss specificheat data on MEM-${(\mathrm{TCNQ})}_{2}$ in terms of a spin-Peierls transition theory. We find that the data can be described reasonably well by a mean-field spin-Peierls transition theory which suggests that at low temperatures the TCNQ chain should be tetramerized. The magnetic susceptibility above the transition temperature is shown to behave like a one-dimensional Heisenberg antiferromagnet. The consequences of this behavior on the relative magnitude of the on-site Coulomb interaction are discussed.

Pd/Co and Pd/Fe multilayer films containing ultrathin Co and Fe layers were prepared by vapor deposition on substrates at room temperature. Their modulated structure, even for films containing 2-Å-thin Co and Fe layers, was proved by x-ray diffraction and transmission electron microscopy. Below a Co layer thickness of about 8 Å, the Pd/Co multilayers acquire an easy magnetic axis perpendicular to the film, which is mainly caused by magnetic interface anisotropy. This leads for multilayers containing Co monolayers to almost rectangular hysteresis loops, by which these films may be very suitable as a perpendicular magnetic recording medium. Pd/Fe multilayers also have a perpendicular interface anisotropy, but the shape anisotropy dominates. Per unit Co volume the Pd/Co multilayers have a higher saturation magnetization than pure Co, which is attributed to an induced ferromagnetism on Pd interfacial atoms.