Magnetism and Superconductivity in Er_<1-x>Tb_xNi_2B_2C (x=0.2 and 0.3)(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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A stable and microscopic coexistence state between weak ferromagnetism (WFM) and super-conductivity (SC) is realized in the type II superconductor ErNi_2B_2C (T_c=10.5K) below its WFM transition temperature, T_<WFM>〜2.3K. An internal magnetic field (H_<int>) mediated by WFM moments seems to be slightly higher than its lower critical magnetic field (H_<c1>), and thus a spontaneous vortex phase is expected to be realized. However, since T_c≥T_<WFM>, a pinning effect makes it difficult to verify the spontaneous vortex phase experimentally. To find a system with T_c≤T_<WFM>, we investigated magnetism and superconductivity in Er_<1-x>Tb_xNi_2B_2C by magnetization, resistivity and neutron diffraction measurements. In contrast to our expectations, the results show that T_c drops steeply even in a small Tb doping region, where T_<WFM> also decreases due to a magnetic competition in anisotropy, and the relationship T_c>T_<WFM> is always reserved. In the system with x=0.2, an upper critical field (H_<c2>) shows hysteresis behavior below 〜1K, which was attributed to the WFM order below it.
社団法人日本物理学会の論文
2008-10-15