No abstract

No abstract

%'e report a correction of the Curie temperature, T" for our Mossbauer study of critical spin dynamics in Gd '6'Dy. The revised T, value, which. is 0.4 K lower than previously reported, leads to spin-correlation times that diverge with an exponent m=0. 49(5). Though based on a welldefined power law over reduced temperatures 10~t~10 ', this result remains anomalous: It agrees neither with the predictions for the Heisenberg model nor with those for the Ising model.In 1984 we published a Mossbauer study of critical slowing down in Gd ' 'Dy in which we asked the following Does Gd exhibit order-parameter-nonconserving spin dynamics such as the isotropic ferromagnets Fe, Ni, EuO, and EuS'? Because of its large localized magnetic moment, and the fact that it is an S-state ion, Gd should be a better Heisenberg system than either Fe or Ni, both of which are partly itinerant. On the other hand, since Gd is noncubic, with uniaxial spin alignment along the c axis below T" it is possible that it exhibits Ising critical behavior. As noted in our earlier paper' experimental values of static critical exponents in Gd do not provide a clear-cut distinction betweenIsing and Heisenberg behavior.To characterize the spin dynamics of Gd we converted measurements of the critical component of the Mossbauer linewidth to the wave-vector averaged spin autocorrelation time r, using the "motional narrowing" form to=v(z+2ds)),and where d is the lattice dimensionality and z, v, and ri are critical exponents defined in the usual manner.Measurements of r, versus T could not be fitted with a single power law, but yielded to=0.28 (2) and 0.21(3), depending on whether the reduced temperature was unrestricted or limited to t =(T/T, -1}~10 . These values of w, or corresponding values of z obtained via the scaling law of Eq. (3), were recognized as distinctly anomalous because they cannot be explained by either the d =3 iso-r, =D(T/T, -1) where the critical exponent w is given by the scaling law2 EI, =(hc/E")Chf r, =(8.01X 10' mm/s )~, , (1) where E& is the gamma-ray energy and Cht is the hyperfine coupling parameter derivable from Mossbauer linewidth theory. ' By recourse to the dynamic scaling form of the dynamic structure factor, S,(q,co}, we expressed v, in terms of the power law 0. 5--0.5-287.!5KTABLE I. Critical exponent predictions for d = 3 ferromagnets. Values of P, y, v, and g were taken from Ref. 3 and represent the most accurate predictions of renormalizationgroup theory. Values of a were derived via the scaling law a+2P+y=2. Valum of z are based on the predictions z= 2 {5g), z =2g/2, and z =2+a/v for the three columns left to right, as given in Ref. 4. Values of m were derived via the scaling law m =v(z+2dg). Spin Exponent conserved r 2 484(2) 1. 023(5) 0.3645(25) 1.386(4) 0.705(3) 0.033(4) -0.115(5) 1.984(2) 0.670(5) 0.3250{20) 1.2410(20) 0.6300(15) 0.031{4) + 0.109(5) 2.173(5) 0.718(95) Heisenberg model Spin nonconserved Ising model -05 I tao t(ns)FIG. 1. Typical PAC spectra belo~(top) and above (bottom) the Curie temperature. Below T, t...

Fe/Tb multilayers that display significant perpendicular magnetic anisotropy (PMA) up to an Fe layer thickness of 55 Å have been fabricated and examined for the first time (to the best knowledge of the authors). Two series of Fe/Tb multilayer samples with a wide range of Fe layer thickness ranging between 8 and 55 Å and with constant Tb layer thickness of 6.5 and 11.5 Å were fabricated by planar magnetron sputtering. The magnetic and structural properties of these samples were examined using x-ray diffraction, Mössbauer spectroscopy, and torque magnetometry. Mössbauer spectra show PMA of the Fe magnetic moments for samples that consist of Fe layers and mixed interfaces only. The formation of an unmixed Tb layer starting at about 12 Å Tb layer thickness leads to parallel alignment of the Fe magnetic moments. PMA in such multilayers is induced by a mixed interface and maintained by the exchange interaction between adjacent Fe layers. The interfaces that induce PMA in Fe/Tb multilayers are very close in composition to that of TbFe2 for which maximum PMA was found in thin amorphous Fe–Tb films.

Structural and magnetic ordering of the interfaces determines the physical properties of multilayered structures. Interfaces of Fe/Al multilayers were studied using Mössbauer effect spectroscopy. The samples were fabricated by dc planar magnetron sputtering at room temperature on polyester substrates. The observed spectra indicate that the interfaces have a common structural composition and for thin Fe layers, the whole Fe layer forms mixed phases at the interface. For larger Fe layer thickness, the interface is formed using an about 12-Å-thick Fe layer. Besides the hyperfine field component of bcc Fe, six different magnetic components were identified in all samples (with larger Fe layer thickness). The intensities of the components were determined from the area under the absorption peaks of the Mössbauer spectra of the corresponding phases. The average canting angle of the Fe magnetic moments, as obtained from the spectra, indicate parallel magnetic anisotropy for all phases.