Random-field-crossover scaling in Mn0.35Zn0.65F2.

Abstract

The weakly anisotropic and highly diluted antiferromagnet ${\mathrm{Mn}}_{0.35}$${\mathrm{Zn}}_{0.65}$${\mathrm{F}}_{2}$ has been studied by dc magnetization measurements under a uniform magnetic field and by ac susceptibility at zero field. Earlier experiments in ${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{F}}_{2}$ for xg0.4 have shown that under an external field applied parallel to the easy direction [001] the critical phase boundary ${\mathit{T}}_{\mathit{c}}$(H) is governed by a random-exchange Ising model (REIM) to random-field Ising model (RFIM) crossover scaling. This crossover scaling stipulates that ${\mathit{T}}_{\mathit{N}}$-${\mathit{T}}_{\mathit{c}}$(H)\ensuremath{\sim}${\mathit{H}}^{2/\mathrm{\ensuremath{\varphi}}}$, where \ensuremath{\varphi}\ensuremath{\simeq}1.4 is the universal REIM-RFIM crossover exponent. We present magnetization results which show that for x=0.35, ${\mathit{T}}_{\mathit{c}}$(H) has a different curvature following the above scaling law but with \ensuremath{\varphi}\ensuremath{\simeq}3.4 (similar to the de Almeida--Thouless line in spin glasses). Zero-field ac susceptibility (${\mathrm{\ensuremath{\chi}}}_{\mathrm{ac}}$) measurements performed on this sample reveal a freezing of spatial fluctuations at low temperatures. The magnetic features of ${\mathrm{Mn}}_{0.35}$${\mathrm{Zn}}_{0.65}$${\mathrm{F}}_{2}$ are distinct in some aspects from the spin-glass behavior found in the isostructural but more anisotropic compound ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{F}}_{2}$ close to the percolation threshold.