Experimental Study on the Mdssbauer Effect of 57Fe in Several Iron Compounds

The MOssbauer absorption spectra of some iron compounds have been observed using the 14.4 keV gamma-ray photons of "Fe. The quadrupole effect and isomer shift of FeSO4 •7H2O have been measured.The internal magnetic field in Fe2B has also been estimated. The effect of spin flopping at the Morin temperature has been confirmed for normal a-Fe203 (>1000A). The absorption spectra of ultra-fine a-Fe2O3 particles (about 50A and 150A) have been observed at 120°K and 300°K, and no Morin transition has been found above at least 120°K with such ultra-fine particles. The spectrum of the particles of 50A taken at 120°K showed six-line splitting, while at 300°K showed only two lines. This phenomenon may be explained by a motional narrowing due to the rapid relaxation of antiferromagnetic spins compared with the nuclear Larmor precession (---10-e sec). The critical volume of particles and critical temperature for this transition have been found to be between 50A and 150A at 300°K and between 120°K and 300°K for particles of 50A, respectively. It has also been found that for ultra-fine particles the recoilless fraction would decrease from that obtained with larger particles. Three iron oxyhydrate isomers, a-, 3- and 7-FeOOH, were also used as absorbers and the following facts have been found: the Neel temperature for a-FeOOH may be close to 400°K and that for 0-FeOOH may lie between 110°K and 300°K, while 7-FeOOH is paramagnetic above at least 110°K. I