Amorphous Yttrium Research Articles

Determination of the sign distribution of electric field gradient in amorphous yttrium iron garnet

An interpretation of the line-shape asymmetry for the $^{57}\mathrm{Fe}$ M\"ossbauer Zeeman spectrum in speromagnetically ordered amorphous yttrium iron garnet ($a$-YIG) at 4.2 K provides the first reported measurement of the distribution of electric-field-gradient (EFG) sign in an amorphous material. The number of iron sites with positive EFG (${n}_{+}$) is found to exceed those with negative EFG (${n}_{\ensuremath{-}}$) in $a$-YIG in the ratio $\frac{{n}_{+}}{{n}_{\ensuremath{-}}}\ensuremath{\approx}1.16\ifmmode\pm\else\textpm\fi{}0.04$. This finding is in quantitative agreement with the prediction of a computer model of randomly packed spherical ions. Since $\frac{{n}_{+}}{{n}_{\ensuremath{-}}}=0$ for crystalline YIG, the possibility that $a$-YIG retains any vestige of its crystalline local environment in distorted form can therefore be completely excluded.

Abstract: Quadrupole energy shifts of low temperature iron Mössbauer lines in amorphous magnets

The 57Fe Mössbauer effect has been used to study the hyperfine Zeeman pattern below the spin ordering or spin freezing temperature in amorphous magnets. At very low temperatures, for which the electronic magnet moments approach their saturation values, it is a common dictum of the amorphous Mössbauer literature that the effects of quadrupole energy are now contained only in the linewidths and line shapes of the six Mössbauer lines and not in their positions.1,2 A previously unsuspected quadrupole shift of the nuclear Zeeman lines in amorphous magnets has been observed by 57Fe Mössbauer spectroscopy in speromagnetic amorphous yttrium iron garnet of 4.2 K. The distinctive shift pattern, with the symmetry −3, +1, −1, +1, −1, +3, is shown to arise theoretically as a second-order perturbation of the Zeeman levels by the distribution of electric field gradients in the amorphous state. It is observed to have a magnitude which conforms quantitatively with the theoretical expectation. 1T. E. Sharon and C. C. Tsuei, Phys. Rev. B 5, 1047, (1972). 2C. L. Chien and R. Hasegawa, Phys. Rev. B 16, 3024 (1977).

Hard-sphere random-packing model for an ionic glass: Yttrium iron garnet

A computer model for amorphous yttrium iron garnet has been constructed using a modification of a method pioneered by Bennett involving the sequential addition of hard spheres. The method is embellished to allow for the presence of local electrostatic forces which are assumed to keep neighboring ions of like charge as far apart as the basic building algorithm will allow. This distance (between ion centers), for amorphous YIG, is $d=2.4$ \AA{}. The electric-field-gradient distribution at the iron sites is calculated in a point-charge approximation and provides an excellent interpretation of the observed M\"ossbauer quadrupole line shape in the paramagnetic phase. Ion-pair correlation densities out to a range of 10 \AA{} have been computed for all combinations of the constituent species. Finally the distribution of iron-oxygen-iron bond angles is computed and from this, via superexchange theory, a qualitative knowledge of the exchange distribution is obtained. This suggests that the magnetic properties of vitreous YIG may resemble those of a dilute antiferromagnet near its critical concentration.