Magnetic static and scaling properties of the weak random-axis magnet (DyxY1-x)Al2.

Abstract

The effects of a random component of the magnetocrystalline anisotropy on the magnetic properties and critical behavior of polycrystalline ${\mathrm{DyAl}}_{2}$ have been investigated using dc magnetic measurements. Random magnetic anisotropy (RMA) is produced by site-diluting ferromagnetic ${\mathrm{DyAl}}_{2}$ with the nonmagnetic, isomorphic intermetallic ${\mathrm{YAl}}_{2}$. Dilution distorts the cubic Laves-phase unit cell because of a slight lattice mismatch thereby lowering the local crystal symmetry in a random fashion. Additional contributions to the RMA come from spin-orbit scattering by the conduction electrons. Hysteresis loops display little remanence and very small coercive fields, suggesting a weak RMA. This is consistent with estimates of the RMA strength D obtained using an approach of Chudnovsky et al. The magnetization at high temperatures (T>4${\mathit{T}}_{\mathit{c}}$) is well described by a Curie-Weiss law. The paramagnetic Curie temperatures are positive, implying an average ferromagnetic exchange coupling between Dy ions, and increase with x. The paramagnetic moment shows no evidence of quenching across the series, thus confirming the well-localized nature of the 4f electronic orbitals. Low-field thermal scans of the bulk dc magnetization show no sign of a spontaneous moment for Dy concentrations 0.10\ensuremath{\le}x\ensuremath{\le}0.90, yet a sharp increase in the magnetization occurs at a temperature ${\mathit{T}}_{\mathit{c}}$ that increases with x. A ferromagnetic scaling analysis applied to the line of transitions at ${\mathit{T}}_{\mathit{c}}$ results in a surprisingly good collapse of the magnetization data. By extension of prior theoretical work of Aharony and Pytte, a direct connection can be made between pure and RMA exponents, which gives remarkable agreement with the experimental values.