Anomalous magnetic behavior in the critical region of an amorphous magnetic alloy containing chromium

Abstract

A close scrutiny of the Mossbauer data previously taken on amorphous Fe₃₂ Ni₃₆Cr₁₄P₁₂B₆ (Metglas 2826A) alloy and on the other Fe-Ni-P-B alloys containing similar transition-metal concentrations by different workers reveals that Fe atoms in Metglas 2826A have two types of environment; one in which Fe atoms have primarily Fe, Ni, and a very small amount of Cr as their neighbors and the other in which Fe atoms are surrounded by Cr atoms. In the present investigation, Curie temperatures for these so-called "FeNi" and "FeCr" magnetic phases are found to be 249 and 320 ± 10 K, respectively. The magnetic study performed on Metglas 2826A in the temperature range 220 to 300 K in fields up to 10 kOe gives the Curie temperature T_C and critical exponents β , γ , and δ as 250 ± 1 K, 0.43 ± 0.02, 1.33 ± 0.05, and 4.43 ± 0.17, respectively, for the "FeNi" phase. The significantly large error limits in the determination of these exponent values result from the presence of the "FeCr" phase. The above-mentioned values for the critical exponents are seen to follow very closely the predictions of a three-dimensional Heisenberg model and thereby suggest dominance of the short-range forces in the critical region (long-range forces, e.g., dipolar forces are shown to have negligible influence on the critical fluctuations of magnetization). The data satisfy the magnetic equation of state characteristic of a second-order phase transition over the entire temperature range. Furthermore, the observation that the magnetic order for the "FeCr" phase persists well above the Curie temperature of the "FeNi" phase is shown to provide a straightforward explanation for the anomalies observed in various properties of Metglas 2826A and permits us to conclude that it is not necessary to invoke superparamagnetism in this system.