Critical phenomena in amorphous ferromagnetic and spin-glass alloys

Four amorphous alloys of the type (T_1-xTₓ^')₇₅P₁₆B₆Al₃ with transition metal T=FeorCo and T^'=MnorNi 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^' 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 Td, and by a vanishing of the magnetization at a lower temperature TFG(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 (β=0.4 and δ=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.