Unusual Meissner state of the superconducting ferromagnet UCoGe revealed by differential susceptibility measurements with a polycrystalline sample

UCoGe is a ferromagnet with a Curie temperature TC∼3K at ambient pressure and superconductivity appears within the ferromagnetic phase with a superconducting transition temperature TSC∼0.8K. We have investigated magnetic properties of the ferromagnetic superconductor UCoGe in superconducting states by the differential susceptibility measurements with a polycrystalline sample. We observed unusual diamagnetic signals for magnetic field dependence of the differential susceptibility in superconducting state. We have revealed the sharp peak for the butterfly-shaped hysteresis loop of ferromagnetism observed even in Meissner state and point out the possible coexistence of ferromagnetism and superconductivity inhomogeneously in real space.