Low-temperature magnetization and thermoremanence of CoCl2 center dot H2O

Reported here are field-cooled (FC) and zero-field-cooled (ZFC) magnetizati ons (M-FC and M-ZFC, respectively) in smaller and larger fields than used i n previous work, and thermoremanent magnetization data (TRM) for various te mperatures and wait times. For fields of 12.8 and 0.1 G a separation betwee n M-FC and M-ZFC develops near 9 K, well below the 16.1 K magnetization max imum or the similar to 14 K antiferromagnetic transition; thus, reentrant b ehavior occurs as for the higher fields used previously. Maxima in M-ZFC(T) are observed at 7.2 and 8.0 K for 12.8 and 0.1 G, respectively. Similar me asurements in fields of from 3.0 to 15.5 kG show an evolution in the form o f M-FC(T) and M-ZFC(T); the separation between these develops at somewhat h igher temperatures than 9 K and no maximum appears in M-ZFC. The field depe ndence of (M-IRR/H)(MAX), where M-IRR = M-FC-M-ZFC, is determined and shows a change in regime between 2 and 3 kG. The TRM as a function of time is me asured for temperatures of 1.6, 4.3, and 5.8 K, with wait times ranging fro m 10 to 500 min. The relaxation rate is quite small, consistent with glassy dynamics with a wide distribution of relaxation times, and increases with increasing temperature. Remarkably, however, aging effects are absent even at 5.8 K, although this is 69% of the splitting temperature between M-FC an d M-ZFC. An explanation may reside in randomized antiferromagnetic coupling s between chemical chains which are individually ordered ferromagnetically, and in the formation of a structure of almost independent clusters. (C) 19 99 American Institute of Physics. [S0021-8979(99)74608-6].