Low temperature magnetization and thermoremanence of FeCl2 center dot H2O

The latest member of the new series of transition metal dichloride monohydrates to exhibit novel magnetic behavior is FeCl2 . H2O. Reported here are field cooled and zero-field cooled magnetizations (M-FC and M-ZFC, respectively) for different fields as a function of temperature, and time-dependent thermoremanent magnetizations (TRM) for different temperatures and wait times. Near 20.4 K a peak appears in both M-FC and M-ZFC using H=106 G, as seen previously. In the smaller fields of 26.5 and 0.1 G, M-FC is more nearly constant for temperatures below the 20.4 K peak in M-ZFC which still appears. The irreversible magnetization, M-FC-M-ZFC, is more pronounced for lower measuring field. Unusual behavior is seen in the temperature and wait-time (t(w)) dependence of TRM(t). Data at 4.4 K (0.22T(c)) show no wait time, or aging, effects, using a cooling field of 9.95 G. At 15.1 K (0.74T(c)), for the same cooling field, differences occur among TRM(t) data for t(w)=10, 100, and 500 min. The data become essentially coincident, however, if the measuring time is scaled by the wait time. Although the general behavior is similar at 18.0 K (0.88T(c)), there is a noticeable shift of the t(w)=500 min data in this case. The results suggest that below the critical temperature the system must equilibrate among a huge number of metastable states. It takes an almost infinite time to probe all these states below 0.88T(c), which explains the t/t(w) scaling. At 0.22T(c), the system remains stuck in one specific state and the aging disappears. Two different time scales appear to control the relaxation. The observed properties are not altogether typical of spin glasses, and the system may better be viewed as a weakly and randomly coupled array of ferromagnetic chains, in which dynamic domains readily form and evolve, and in which the aging effects are associated with domain wall growth. (C) 1997 American Institute of Physics.