  Spin fluctuation and transition have always been one of central topics of
magnetism and condensed matter science. Experimentally, the spin fluctuation is
found transcribed onto scattering intensity in the neutron scattering process,
which is represented by dynamical magnetic susceptibility and maximized at
phase transitions. Importantly, a neutron carries spin without electric charge,
and it can bring spin into a sample without being disturbed by electric energy,
although large facilities such as a nuclear reactor is necessary. Here we show
that spin pumping, frequently used in nanoscale spintronic devices, provides a
desktop micro probe for spin transition; spin current is a flux of spin without
an electric charge and its transport reflects spin excitation. We demonstrate
detection of antiferromagnetic transition in ultra-thin CoO films via frequency
dependent spin-current transmission measurements, which provides a versatile
probe for phase transition in an electric manner in minute devices.
