Development of a confocal spin-noise spectroscopy and magneto-optical Kerr effect imaging platform for probing magnetic fluctuations in two-dimensional magnetic heterostructures.

We present a confocal optical platform integrating three complementary modules, spin-noise spectroscopy (SNS), conventional magneto-optical Kerr effect (MOKE) magnetometry, and wide-field MOKE imaging, sharing a common focal plane. The system employs a wavelength-tunable (700-1000 nm), ultra-narrow-linewidth Ti:sapphire laser and achieves sub-micron spatial resolution. The wavelength-dependent MOKE measurements of Co20Fe60B20 (∼1 nm thick) heterostructures identify the optimal probe wavelength for SNS detection. At room temperature, spatially resolved spin-noise spectra reveal that magnetic noise arises primarily from domain-wall fluctuations, while noise inside domains is strongly suppressed. Temperature-dependent zero-field measurements across the spin reorientation transition reveal a suppression of low-frequency spin noise, reflecting the evolution of magnetic fluctuations as the magnetization rotates from out-of-plane to in-plane. This three-module SNS-MOKE platform enables magnetic-structure-selective studies of spin fluctuations in complex magnetic textures and provides a versatile framework for investigating fluctuation dynamics in two-dimensional magnetic materials, spin glasses, and artificial spin ice.