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Low-temperature specific heat of and in magnetic fields to 7 T

Physical Review B - Condensed Matter and Materials Physics, ISSN: 1550-235X, Vol: 61, Issue: 1, Page: 538-548
2000
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The specific heats of samples of (Formula presented) and (Formula presented) on which magnetization and/or neutron-diffraction measurements had been made earlier, were measured for the temperature range (Formula presented) in magnetic fields to 7 T. (Formula presented) has a complex structure with 90 formula units in a bcc unit cell; 6 Cu are “lone spins,” 48 are in 8 (Formula presented) “ring clusters,” and 36 are in 2 (Formula presented) “sphere clusters.” The ring and sphere clusters have ferromagnetically ordered ground states with spins (Formula presented) and 9, respectively. Antiferromagnetic ordering of the ring clusters occurs with a Néel temperature (Formula presented) The specific heat of (Formula presented) shows a cooperative ordering anomaly associated with the antiferromagnetic ordering of the ring clusters. Schottky-like anomalies, having maxima at ∼5 and ∼0.7 K, are identified with the ordering of the sphere clusters and the lone spins, respectively. Only Schottky-like anomalies are observed for the specific heat of (Formula presented) It is suggested that the increase in the Cu oxidation state, due to the addition of 0.14 mol of O, increases Cu-O covalent bonding (spin compensation) and/or produces nonmagnetic (Formula presented) which in addition to the known increase in the Cu-O bond lengths, disrupts the superexchange paths that lead to the antiferromagnetic ordering of the ring clusters in (Formula presented) For (Formula presented) the magnetic entropy was 90% of that predicted for the ordering of the three Cu structures. On the other hand, the magnetic entropy for (Formula presented) was only 65% of that predicted, which suggests a relative large suppression of some magnetic entities due to the addition of 0.14 mol of O. Although (Formula presented) is an insulator, the specific heat has a T-proportional component that is magnetic field dependent and is presumably associated with the magnetic degrees of freedom. © 2000 The American Physical Society.

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