Optical ferromagnetic resonance studies of thin film magnetic structures

Abstract

We have used a magneto-optical pump-probe technique to stimulate and characterize small amplitude precession of the magnetization in a variety of thin film magnetic structures. The sample is stimulated by an optically triggered magnetic field pulse and its response probed by means of a stroboscopic measurement of the magneto-optical Kerr effect. We demonstrate that the experimental technique is sensitive to small amplitude deflections of the magnetization and present formulae that describe the frequency of precession in continuous films containing either one or two magnetic layers. We show how the dependence of the precession frequency upon the orientation and magnitude of the static field may be used to characterize a 100 Å Co thin film, a Ni81Fe19(50 Å)/Cu(20 Å)/Co(50 Å)/IrMn(100 Å) spin-valve structure, and a square Ni81Fe19 element of 10 µm side and 150 nm thickness. Information may be obtained about the g factor, demagnetizing field, exchange bias effect and magnetic anisotropy within an individual ferromagnetic layer, and about the coupling between layers. We deduce the presence of a four-fold anisotropy in the square element that is associated with non-uniformity of the magnetization. Time resolved images show that the dynamic magnetization is generally non-uniform. A stripe pattern is observed when the static field is significantly larger than the anisotropy field. Measurements at low field show that the dynamic magnetization initially has a regular pattern associated with the underlying domain structure, but then evolves into a highly disordered state. Finally we discuss the outlook for further development and application of the experimental technique.