Magnetic susceptibility measurements of ultrathin films using the surface magneto-optic Kerr effect: Optimization of the signal-to-noise ratio

The magnetic susceptibility, chi = partial derivative M/partial derivative H, can be determined for ultrathin films using the surface magneto-optic Kerr effect and an ac technique (ac-MOKE) where the response to a small, modulated H field is measured. Optimization of the signal-to-noise ratio is imperative if the measurements are to be used to extract critical scaling exponents from the small tail of chi in the paramagnetic region. These optimal conditions are different than those appropriate for more conventional static H measurements (dc-MOKE) used to generate magnetic hysteresis loops, because the relative noise contributions from the laser stability, sample stability, and detector are changed. A simple method for determining optimum conditions is demonstrated, and the technique is used to show that a 1.8 ML Fe/W(110) film has the critical exponent gamma of the 2D Ising model. (C) 1997 American Institute of Physics. [S0034-6748(97)03111-0].