  A high-sensitive DC Magneto-Optical Kerr Effect (MOKE) apparatus is described
in this letter. Via detailed analysis on several dominating noise sources, we
have proposed solutions that significantly lower the MOKE noise, and a
sensitivity of $1.5\times10^{-7} \text{rad}/\sqrt{\text{Hz}}$ is achieved with
long-term stability. The sensitivity of the apparatus is tested by measuring a
wedge-shaped Ni thin film on $\text{SiO}_2$ with Ni thickness varying from 0 to
3 nm. A noise floor of $1.5\times10^{-8}$ rad is demonstrated. The possibility
of further improving sensitivity to $10^{-9}$ rad via applying ac modulation is
also discussed.

Instrumentation and Detectors;Optics