Magnetic Behavior of Iron-Oxoclusters Prepared in an Organosilica Sol–Gel Matrix

The crystal structure and magnetization of nanoscale enTMOS–Fe2O3 sol–gel composites with weight iron concentration x, varying from 0.003 to 0.065, have been studied by the transmission electron microscopy technique and a superconducting quantum interference device magnetometer. The clusters are crystallized in a hexagonal crystal structure. All the samples demonstrate a superparamagnetic behavior with antiferomagnetic cluster–cluster coupling at low temperature. The effective paramagnetic moment, μeff, has been found to vary in the range from 5.9 (S=5/2) to 2.5 μB per iron ion. The concentration dependence of the μeff shows a minimum for x∼0.01. At a low iron concentration x\u3c0.01, μeff is practically independent of x and equals about 6 μB per Fe ion. The concentration interval 0.01\u3cx\u3c0.07 is characterized by a monotonical increase of μeff from 2.5 to about 3 μB per Fe ion. Thus, an abrupt variation of μeff (about two times) is observed at x≈0.01. It has been shown that such behavior can be caused by competition between the uncoupled “surface” and antiferromagnetically coupled “bulk” Fe magnetic moments