Calorimetric and susceptibility investigations on the MnO⋅Al2O3⋅SiO2 spin-glass system (abstract)

Abstract

A systematic heat capacity and ac magnetic susceptibility study has been performed on a magnetic ternary glass system—MnO⋅Al2O3⋅SiO2—with Mn concentrations varying from 6 to 19 at. %. At temperatures above 70 K, the susceptibilities follow a Curie–Weiss behavior with negative paramagnetic Curie temperatures and Bohr magneton numbers corresponding nearly to the free ion values of Mn++. However, below 70 K, all glass samples show strong deviations from the Curie–Weiss behavior (a decrease in 1/χ) culminating in spin-glass susceptibility maxima at liquid-helium temperatures for Mn concentrations above 9 at. %. These results suggest a formation of antiferromagnetic clusters which grow in size with decreasing temperature and become ‘‘infinitely’’ large as the spin-glass freezing temperature is approached. Since the specific heats of glasses do not follow a simple temperature dependence in the temperature range of 1–60 K, calorimetric measurements on several nonmagnetic aluminosilicate glasses were performed in order to accurately determine the magnetic specific heat contribution Cm for the Mn aluminosilicate glasses. A broad maximum in Cm/T is observed in the vicinity of the spin-glass freezing temperature and nonnegligible Cm values are still present at temperatures greater than 50 K (>10Tf). Only a small fraction of the total magnetic entropy (∼7–8% of cR ln 6) is accounted for between 0 K and Tf, in agreement with the idea that spin-glass ordering in this system is dominated by short-range interactions.