Interfacial magnetization in exchange-coupled Fe/Cr/Fe structures investigated by second harmonic generation

We have studied the magnetic field dependences of magnetic optical second harmonic generation (SHG) in MBE-grown Fe/Cr/Fe/Ag/GaAs(100) heterostructures displaying both bilinear and biquadratic interlayer exchange coupling. The magnetic field H was applied in the (100) surface plane along both easy ([001]) and hard ([110]) axes of the in-plane fourfold magnetic anisotropy. The SHG has been measured in reflection at near normal incidence for different polarization combinations (pp,ps,ss,sp) of the fundamental and second harmonic light in longitudinal and transversal geometries. The magnetic field variation of the SHG signal clearly reflects the field-induced transformations of the magnetic state at the interfaces in the trilayer. It strongly depends on the configuration of light polarization, experimental geometry (longitudinal or transversal), and orientation of the magnetic field H relative to the crystal axes. In contrast to linear magneto-optical Kerr effect, which is odd in magnetic field, magnetic SHG is either even in H or does not display a definite parity at all, depending on the polarization configuration. We interpret the data based on a model accounting for nonmagnetic and magnetic contributions to SHG from the surface and interfaces described by C-4v point symmetry. Taking into account the changes of the mutual orientation of interfacial magnetizations allows us to describe the general features of the measured field dependences of SHG.