MAGNETIC BLOCKING IN VERY DILUTE (EUXSR1-X) S - EXPERIMENT VERSUS THEORY

Both ac and dc magnetic susceptibilities have been measured for Eu concentrations x=0.025,A 0.05,A andA 0.10 over temperatures in the range from 0.007 K to 4 K. While the dc susceptibility is found to exhibit no peak down to 7 mK, the ac susceptibility shows two distinct maxima around 15 mK and 200 mK. The frequency dependence of the two-peak positions Ta(ν) and Tb(ν) can be adequately described by an Arrhenius law in the range of our measurements from 10-106 Hz. The results for the peak at the higher-temperature Tb are explained quantitatively for x=0.025 and x=0.05, without adjustable parameters, by a model of small random Eu clusters. They consist of Eu atoms coupled by strong exchange forces between nearest-and next-nearest neighbors, and experience energy barriers mainly due to the intracluster dipolar energy. One particular configuration, two nearest-neighbor Eu atoms bonded ferromagnetically, produces the anomaly near Tb. Thus Tb(ν) arises from ordinary superparamagnetic behavior. In spite of the very low temperatures investigated, quantum-mechanical cluster reorientation is shown to be negligible due to the high-spin quantum number of Eu, even for the smallest clusters. The newly found maximum at the lower-temperature Ta is interpreted as a result of intercluster dipolar energy, giving good agreement for the order of magnitude of Ta and its concentration dependence. Its dependence on frequency remains to be explained. © 1979 The American Physical Society.