Magnetic properties of amorphous ${\mathrm{Fe}}_x{\mathrm{B}}_{100-x}(72\mathrm{}<~x<~86)$ and crystalline ${\mathrm{Fe}}_3$B

Amorphous samples of ${\mathrm{Fe}}_x{\mathrm{B}}_{100-x}(72\mathrm{}<~x<~86)$ have been studied by ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer spectroscopy and in some cases by magnetization measurements. The magnetic ordering temperature ($T_C$) decreases sharply with increasing Fe concentration; from 760 K (${\mathrm{Fe}}_{72}$ ${\mathrm{B}}_{28}$) to 552 K (${\mathrm{Fe}}_{86}$ ${\mathrm{B}}_{14}$). The value of $T_C$ of amorphous pure Fe has been extrapolated to about 220 K. Well-defined hyperfine field distributions $\mathrm{}\left\{P\right(H\left)\right\}$ have been found. The mean hyperfine field is found to be proportional to the average Fe moment with a ratio of about 130 kOe/${\mathrm{\mu }}_{\mathrm{B}}$. The shape of $P\left(H\right)\mathrm{}$ for each alloy is practically independent of temperature. At low temperatures, the effective hyperfine field ($H_{\mathrm{eff}}$) shows a temperature dependence of $H_{\mathrm{eff}}\mathrm{}\left(T\right)=H_{\mathrm{eff}}\mathrm{}\left(0\right)(1-B{T}^{\frac{3}{2}}\cdots )$ due to spin-wave excitations. The value of $B_{\frac{3}{2}}=B{\left(T_C\right)}^{\frac{3}{2}}$ increases with Fe concentration. As the Fe concentration is increased in these alloys, the reduced hyperfine field decreases faster with reduced temperature due to a systematic change in the distribution of exchange interactions. A correlation of $H_{\mathrm{eff}}\mathrm{}\left(0\right)\mathrm{}$ and the isomer shift exists for crystalline and amorphous Fe-B systems. Crystalline ${\mathrm{Fe}}_3$B($T_C\sim 800$ K) has been found after crystallizing amorphous samples with $x>\mathrm{}75\mathrm{}$ under high heating rates. At $T<\mathrm{}{T}_C$, ${\mathrm{Fe}}_3$B shows at least three magnetically inequivalent sites, whereas at $T>\mathrm{}{T}_C$, electric quadrupole interactions are observed. The crystal structure of ${\mathrm{Fe}}_3$B is likely to be tetragonal rather than orthorhombic.