Critical phenomena in amorphous ferromagnetic and spin-glass alloys

Abstract

Four amorphous alloys of the type ${({T}_{1\ensuremath{-}x}{T}_{x}^{\ensuremath{'}})}_{75}{\mathrm{P}}_{16}{\mathrm{B}}_{6}{\mathrm{Al}}_{3}$ with transition metal $T=\mathrm{Fe}\mathrm{or}\mathrm{Co}$ and ${T}^{\ensuremath{'}}=\mathrm{Mn}\mathrm{or}\mathrm{Ni}$ are studied by low-field ac susceptibility bridge and vibrating sample magnetometer techniques. All systems under study have a ferromagnetic (FM) behavior for low concentrations $x$ of ${T}^{\ensuremath{'}}$ and a spin-glass (SG) behavior for high $x$. In the intermediate concentration regime alloys are characterized by a sharp drop in the ac susceptibility below the Curie temperature, at a temperature ${T}_{d}$, and by a vanishing of the magnetization at a lower temperature ${T}_{\mathrm{FG}}(x)$ which we identify as a transition temperature. Critical exponents have been extracted for the paramagnetic (PM)-FM and for the FM-SG transitions by using a scaling procedure. The PM-FM critical exponents have non-Heisenberg values ($\ensuremath{\beta}=0.4$ and $\ensuremath{\delta}=5$), while the FM-SG critical exponents seem to depend on $x$. The PM-SG line can also be viewed as a critical line as the peaks in the susceptibility obey scaling laws. Thus, we can identify for each system three lines of transitions intersecting at a multicritical point (MCP). The phase diagram and the nature of the MCP are similar to the predictions of the Sherrington-Kirkpatrick model.