Spin glass and ferrimagnetism in YFexAl12- x(4⩽ x⩽6)

Abstract

Magnetization and 57Fe Mössbauer studies of YFe x Al 12- x , x=4.0, 4.5, 5.0, 5.5 and 6.0 of the tetragonal ThMn 12 structure, at temperatures 4.1 to 350 K have been performed. While YFe 4Al 8 is an antiferromagnet ( T N = 170 K) and YFe 6Al 6 is a ferrimagnet ( T c = 340 K), the systems with x = 4.5, 5.0 and 5.5 exhibit below T G ( T G is 84, 82 and 128 K, respectively) all the phenomena associated with spin glass behaviour, yet they order ferrimagnetically at a higher temperature ( T c = 105, 165 and 225 K) as evidenced by the Mössbauer spectra. In the spin glass region large field cooled and zero field cooled remanence are observed. The remanent magnetization is time dependent. The magnetization curves strongly depend on cooling rates and the hysteresis loops have unusual shapes. The spin glass transition temperature decreases as the external magnetic field increases, following very accurately the 2/3 power law. The observations can be understood in terms of the relative occupation of the three inequivalent crystallographic sites ((f), (j) and (i)) available to the Fe ions. Mössbauer spectroscopy of 57Fe at 4.1 K yields detailed information on the iron population and the hyperfine field distribution in these three crystallographic sites. In YFe 4Al 8 iron occupies mainly the (f) site and thus all Fe ions are equivalent and the crystal orders antiferromagnetically. In YFe 6Al 6 the Fe occupy mainly the (j) and (i) sites and the high concentration of iron in the sites leads to ferrimagnetism. In YFe x Al 12- x , x = 4.5, 5.0, 5.5, the iron ion are distributed among the three crystallographic sites and their concentration in at least one site is low enough to allow the formation of the spin glass state. In each site there is a dominant hyperfine field (110, 144 and 172 kOe for the (f), (j) and (i) sites, respectively) with a tail toward a peak at zero field.