Mössbauer study of a glassy ferromagnet: ${\mathrm{Fe}}_{40}$${\mathrm{Ni}}_{40}$${\mathrm{P}}_{14}$${\mathrm{B}}_6$

The glassy ferromagnet ${\mathrm{Fe}}_{40}$ ${\mathrm{Ni}}_{40}$ ${\mathrm{P}}_{14}$ ${\mathrm{B}}_6$ has been studied by Mössbauer spectroscopy from 4.2 to 1000 K. In the glassy state, the average hyperfine field $H_{\mathrm{eff}}\mathrm{}\left(T\right)\mathrm{}$ decreases with temperature more rapidly than that of crystalline ferromagnets due to a distribution of exchange interactions. At $T\lesssim \frac{1}{2}{T}_C$, $H_{\mathrm{eff}}\mathrm{}\left(T\right)\mathrm{}$ has a temperature dependence of $H_{\mathrm{eff}}\mathrm{}\left(0\right)\mathrm{}$ [$1-B_{\frac{3}{2}}{\left(\frac{T}{T_C}\right)}^{\frac{3}{2}}\cdots$], indicative of spin-wave excitations. The value of $B_{\frac{3}{2}}=0.47\mathrm{}\pm 0.07$ is four times larger than those of crystalline Fe and Ni. The Curie temperature $T_C=537\mathrm{}$ K is found to be well defined. At $T$ close of $T_C$, $H_{\mathrm{eff}}\mathrm{}\left(T\right)\mathrm{}$ behaves as a power law with a critical exponent of $\beta =0.32\mathrm{}\pm 0.05$. In the glassy state and at $T>\mathrm{}{T}_C$, the quadrupole spectra show asymmetrical doublets, suggesting that the electronic state of Fe may not be unique. A crystallization temperature ($T_{\mathrm{cr}}$) of 700 K has been determined under high heating rate. At much lower heating rate, crystallization occurs at 670 K. The crystallized sample of ${\mathrm{Fe}}_{40}$ ${\mathrm{Ni}}_{40}$ ${\mathrm{P}}_{14}$ ${\mathrm{B}}_6$ contains two ferromagnetic crystalline phases of ${\mathrm{Fe}}_{55}$ ${\mathrm{Ni}}_{45}$ ($T_C=750\mathrm{}$ K) and ${\mathrm{}(\mathrm{F}\mathrm{e}-\mathrm{N}\mathrm{i})\mathrm{}}_{30}$ ${\mathrm{P}}_7$