Spin glass and ferrimagnetism in YFe_xAl_12-x(4≤x≤6)

Magnetization and ⁵⁷Fe Mössbauer studies of YFe_xAl_12-x, x=4.0, 4.5, 5.0, 5.5 and 6.0 of the tetragonal ThMn₁₂ structure, at temperatures 4.1 to 350 K have been performed. While YFe₄Al₈ is an antiferromagnet (T_N = 170 K) and YFe₆Al₆ 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 YFe₄Al₈ iron occupies mainly the (f) site and thus all Fe ions are equivalent and the crystal orders antiferromagnetically. In YFe₆Al₆ the Fe occupy mainly the (j) and (i) sites and the high concentration of iron in the sites leads to ferrimagnetism. In YFe_xAl_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.