We have performed precise dc magnetization measurements for a single crystal of UBe13 down to 0.14K, up to 80kOe. We observed a magnetic anomaly in the superconducting (SC) mixed state at a field, named HMag* (∼26kOe, at 0.14K), implying that UBe13 has a magnetically unusual SC state. We studied the magnetization curves of UBe13, assuming that the HMag* 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 HMag* anomaly is discussed.
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