Development of point-contact spectrometer for spin polarization measurements

We construct a "needle-anvil" type point-contact spectrometer for spin polarization measurements, which is installed in a ⁴He-cryostat. Two types of piezo devices are used to control the contact size precisely between a sample ferromagnet and a superconductor. An attocube piezo-based positioner is mounted for coarse movement of the tip, while a stacked-type piezo device is used for fine control of the contact size. This enables to change the contact size between the tip and sample from sub micro-meters to atomic-size contacts continuously. By suppressing thermal flow into the sample space and mechanical vibration, we can keep the contact over hours, enabling the precision measurements. To examine the performance of the spectrometer, we study the spin polarization of polycrystalline SrRuO₃ with the point-contact Andreev reflection measurements. The polarization is estimated to be ∼0.59 at the clean limit of the interface, which is consistent with previous study.