Magneto‐Optical Characterization of Magnetic Thin Films, Surfaces, and Interfaces at Small Length and Short Timescales

Abstract

AbstractThe understanding of magnetism at its fundamental length and timescales related to the exchange interaction, that is, at nanometer (nm) length and femtosecond (fs) timescales, is becoming essential as future magnetic recording aims at Tbit densities switched at THz rates, which is exactly this regime. Magnetization‐sensitive second harmonic generation (MSHG) is a nonlinear optical technique that, due to the dipole selection rules, is specifically sensitive to surfaces and interfaces of centrosymmetric media. This surface/interface sensitivity of MSHG in combination with very large magneto‐optical effects has lead to a fast development of this technique over the past decade, demonstrating atomic scale resolution in the direction perpendicular to the interfaces. To increase the lateral resolution of magneto‐optics beyond the diffraction limit, scanning near‐field optical microscopy techniques are more appropriate. The development of novel probes that preserve the polarization has led to the possibility of magneto‐optical characterization of magnetic nanostructures with spatial resolution beyond 100 nm. The combination of these and other magneto‐optical techniques with pump–probe approaches finally allows one to include time resolution down to the femtosecond range in the magneto‐optical characterization of magnetic structures. The main attention of the present chapter is on the application of the MSHG, SNOM, and pump–probe techniques to magnetic thin films and surfaces, with a focus on the achieved progress in understanding of magnetic phenomena at the smallest length and shortest timescales. On the one hand, an extreme sensitivity of MSHG to the electronic and magnetic structure of clean surfaces has been successfully demonstrated. On the other hand, the penetration depth of light has allowed to use this sensitivity to study buried interfaces in multilayer systems. Various phenomena, such as surface states on magnetic metals, enhanced magnetic moments of low‐coordinated atoms, and quantum well states, have been studied. Complimentary to these developments, we show how SNOM gives in‐plane information about submicron magnetic domain structures. Finally, we demonstrate how magneto‐optical pump–probe techniques can be used to study the dynamics of magnetic phenomena with a time resolution down to femtoseconds.