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An alternate method of measuring anisotropy fields in thin film ferromagnets is demonstrated. The method relies on the magnetic susceptibility in a small a.c. magnetic field, measured \txtit{in situ} using the magneto-optic Kerr effect (MOKE), and will be useful in situations where more specialized apparatus are not available, or constraints discourage the use of a large, static magnetic field. The method is demonstrated for Co/W(110) films, where it yields anisotropy fields in agreement with previous studies using more conventional torque magnetometry. The sensitivity of the method is demonstrated using CoO/Co/W(110) bilayer films, where the anisotropy due to interfacial exchange coupling is detected and used to find the N\'{e}el temperature of the thin CoO layer., Comment: 8 pages, 5 figures

The influence of long-range dipole interactions on two dimensional magnetism has been studied extensively in the spin-reorientation transition of ferromagnetic ultrathin films. Although there is a great deal of experimental information on the perpendicular domain phase that is stabilized by dipole interactions, the transitions to or from the domain phase are subtle and difficult to characterize experimentally. Magnetic susceptibility measurements show no divergence in the vicinity of the spin-reorientation transition as a function of thickness -- a null result that is difficult to interpret with confidence. This article reports separate dynamical and equilibrium versions of the reorientation transition in Fe/2ML Ni/W(110) films, using measurements of the magnetic susceptibility as the films are grown. The dynamical version occurs when increasing the film thickness causes the domain walls to depin, and the system moves from a configuration that minimizes local energetics to one that minimizes global energetics. The dynamical transition \textit{is} marked by a divergent magnetic susceptibility measured with a field applied along the in-plane W[001] direction. A comparative study of the two versions of the same spin-reorientation transition aids in the experimental characterization of the effects of dipole interactions on the phase transitions. This comparison confirms the original null result found in magnetic susceptibility measurements of the equilibrium transition; despite its name, the spin-reorientation transition in ferromagnetic ultrathin films has no critical phase transition in either the magnetization or its orientation., Comment: 11 pages, 4 figures

A two dimensional (2D) percolation transition in Fe/W(110) ultrathin magnetic films occurs when islands in the second atomic layer percolate and resolve a frustrated magnetic state to produce long-range in-plane ferromagnetic order. Novel measurements of the magnetic susceptibility $\chi(\theta)$ as the films are deposited at a constant temperature, allow the long-range percolation transition to be observed as a sharp peak consistent with a critical phase transition. The measurements are used to trace the paramagnetic-to-ferromagnetic phase boundary between the $T=0$ percolation magnetic transition and the thermal Curie magnetic transition of the undiluted film. A quantitative comparison to critical scaling theory is made by fitting the functional form of the phase boundary. The fitted parameters are then used in theoretical expressions for $\chi(T)$ in the critical region of the paramagnetic state to provide an excellent, independent representation of the experimental measurements., Comment: 9 pages, 5 figures

The spin reorientation transition of an ultrathin film from perpendicular to in-plane magnetization is driven by a competition between dipole and anisotropy energies. \textit{In situ} measurements of the magnetic susceptibility of Fe/2 ML Ni/W(110) films as a function of Fe coverage, made as the films are deposited at constant temperature, show two clear peaks; one at the long-range and one at the local realization of the transition. In the long-range realization, the susceptibility probes the striped domain pattern that is formed in response to the balance of energetics on a mesoscopic scale. Here the reorientation transition occurs at a non-integer layer thickness. In the local realization, the susceptibility probes the response of small islands with in-plane anisotropy in the 3rd atomic Fe layer that are grown on the 2nd atomic Fe layer, which has perpendicular anisotropy. It is a response to the local finite-size, metastable energetics due to discrete steps in thickness. An excellent quantitative description of the susceptibility data is obtained when both local and long-range aspects of the spin reorientation transition are included., Comment: 9 pages, 5 figures