Nonlinear magneto-optical probing of magnetic interfaces

Abstract

Magnetization induced effects in the nonlinear optical (second harmonic generation) response appear to be a very sensitive probe of magnetic interface properties. This leads to attractive applications for the study of magnetic multilayer structures and also leads to new modes of nonlinear magneto-optical imaging. PACS: 42.65.-k; 78.20.Ls; 75.70.Cn Some of the most exciting recent discoveries in magnetism, such as the giant magneto-resistance (GMR) and the oscillatory exchange coupling, are related to the properties of multilayers of alternating ferromagnetic and paramagnetic layers. For magnetic recording on the other hand, antiferromagnetic biasing of ferromagnetic films is used (spin-valve structures). The interfaces between these layers appear to play an essential role for these phenomena and consequently for the device properties based on them [1]. The well known magneto-optical Kerr effect (MOKE) is based on the changes in the linear susceptibility as a function of the magnetization. As this results in a (small) rotation of the polarization of light traveling through a magnetic material, it yields a probe for the bulk magnetization. Though very sensitive and even applicable to monolayers, MOKE is not interface specific. Magnetization induced second harmonic generation (MSHG) is a new nonlinear magneto-optical technique that combines interface sensitivity with huge magneto-optical effects [2] (up to three orders of magnitude times their linear equivalent). These effects are due to the simultaneous breaking of inversion symmetry (at interfaces) and time-reversal symmetry (by the magnetization), which lead to the appearance of even and respectively, odd tensor components in the magnetization that are of comparable magnitude. Because of the latter, the relative phase between these contributions plays an important role [3]. The higher-order nonlinear optical tensor is also responsible for the appearance of essentially new magneto-optical effects that have no equivalent in the linear case [4]. Since the initial theoretical predictions of Ru-Pin Pan et al. [5] and Hubner et al. [6], there have been a large number of experimental studies that have demonstrated the interface sensitivity as well as new and large magneto-optical effects (for example, Kerr rotations close to 90◦). An overview of these results can be found in a number of review papers [7– 9]. In this paper I will concentrate on some of the more recent developments and future challenges of this new technique. Because most magnetically ordered materials are centrosymmetric in their bulk form, MSHG is a particularly useful probe to study the magnetic properties of the interfaces in magnetic multilayer systems. A great challenge for this nonlinear magneto-optical technique is to correlate structural and electronic interface properties with their magnetic properties. Using MSHG, we have found that the spin orientation at the interface of CoNi/Pt multilayers can be different from the bulk due to specific preparation conditions. By a careful analysis of the experimental data, a correlation between increasing interface roughness and a canting spin orientation at the interface could be observed. MSHG has also been shown to be extremely sensitive to the appearance of quantum well states in ultra thin overlayer or spacer films [10]. We have studied several combinations of noble metal overlayers (Au, Ag, Cu) on a variety of magnetic materials (Co, Fe). Spectroscopic MSHG studies of these quantum well states show strong peculiarities of the oscillatory MSHG response. We have also observed QWS within thin Fe films, though here the effects on the MSHG response appear to be much weaker. These combined results reveal the complexity and the role of the interplay between electronic bandstructure, dipole matrix elements and the symmetry of the wavefunction of the quantum well states [11]. Nonlinear magneto-optics also appears to give promising new possibilities for nonlinear optical imaging [12]. We have used this to study the switching behaviour in thin CoNi films which may be used for magnetic recording. 1 Magnetization induced second harmonic generation The optical second harmonic polarization P (2ω) of a magnetic medium is generally described by a third rank polar tensor χcr ijk for the crystallographic contribution and a fourth