Observation of an unusual magnetic anomaly in the superconducting mixed state of heavy-fermion compound UBe13 by precise dc magnetization measurements.

We have performed precise dc magnetization measurements for a single crystal of UBe(13) down to 0.14 K, up to 80 kOe. We observed a magnetic anomaly in the superconducting (SC) mixed state at a field, named H(Mag)(*) (~ 26 kOe, at 0.14 K), implying that UBe(13) has a magnetically unusual SC state. We studied the magnetization curves of UBe(13), assuming that the H(Mag)(*) anomaly originates from (1) and unusual SC diamagnetic response, or (2) a peculiarity of the normal-state magnetization due to vortices in the SC mixed state. The origin of the H(Mag)(*) anomaly is discussed.