Critical magnetic properties of disordered polycrystallineCr75Fe25andCr70Fe30alloys

Abstract

Critical magnetic properties of disordered polycrystalline ${\mathrm{Cr}}_{75}{\mathrm{Fe}}_{25}$ and ${\mathrm{Cr}}_{70}{\mathrm{Fe}}_{30}$ alloys are investigated employing bulk magnetization and, for the first time to our knowledge, low-field ac susceptibility measurements. Contrary to the earlier claims, an elaborate analysis of high-precision magnetization and ac susceptibility data reveals that long-range ferromagnetic order exists, and that a true ferromagnetic to paramagnetic phase transition takes place in the chemically homogenized alloys. Chemical clustering leads to superparamagnetic behavior above and inhomogenous magnetization below the Curie temperature, complicating the experimental determination of the asymptotic critical exponents, particularly when high-field extrapolation methods are used. The effective critical exponents possess the values ${\ensuremath{\beta}}_{\mathrm{eff}}=0.30\ifmmode\pm\else\textpm\fi{}0.01,$ ${\ensuremath{\gamma}}_{\mathrm{eff}}=1.39\ifmmode\pm\else\textpm\fi{}0.01,$ and ${\ensuremath{\delta}}_{\mathrm{eff}}=5.5\ifmmode\pm\else\textpm\fi{}0.1$ for chemically homogenized samples. These values deviate substantially from the estimates based on three- or two-dimensional isotropic ``short-range'' models. The presently determined values for the critical exponents instead indicate that the critical behavior of ${\mathrm{Cr}}_{75}{\mathrm{Fe}}_{25}$ and ${\mathrm{Cr}}_{70}{\mathrm{Fe}}_{30}$ is akin to that of a $d=2,n=1$ ferromagnet in which the attractive long-range interactions between spins decay with distance (r) as $J(r)\ensuremath{\sim}{r}^{\ensuremath{-}(d+\ensuremath{\sigma})}$ with \ensuremath{\sigma}=1.4.