Critical phenomena in amorphous ferromagnetic and spin-glass alloys

Four amorphous alloys of the type ${\left(T_{1-x}{T}_x^{\prime }\right)}_{75}{\mathrm{P}}_{16}{\mathrm{B}}_6{\mathrm{Al}}_3$ with transition metal $T=\mathrm{Fe}\mathrm{or}\mathrm{Co}$ and $T^{\prime }=\mathrm{Mn}\mathrm{or}\mathrm{Ni}$ 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 $T^{\prime }$ 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 $T_{\mathrm{FG}}\mathrm{}\left(x\right)\mathrm{}$ 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 ($\beta =0.4\mathrm{}$ and $\delta =5\mathrm{}$), 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.