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    Magnetic phase diagram of antiferroquadrupole ordering in HoB2C2

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    The magnetic phase diagram for antiferro-quadrupole (AFQ) ordering in tetragonal HoB2_2C2_2 has been investigated by measurements of elastic constants C11C_{11}, C44C_{44} and C66C_{66} in fields along the basal xx-yy plane as well as the principal [001]-axis. The hybrid magnet (GAMA) in Tsukuba Magnetic Laboratory was employed for high field measurements up to 30 T. The AFQ phase is no longer observed above 26.3 T along the principal [001] axis in contrast to the relatively small critical field of 3.9 T in fields applied along the basal [110] axis. The quadrupolar intersite interaction of OxyO_{xy} and/or O22O_2^2 is consistent with the anisotropy in the magnetic phase diagram of the AFQ phase in HoB2_2C2_2.Comment: Phys. Rev. B. (2005) in press. approx 8 pages, 10 figure
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