Magnetic properties of amorphous Fe x B 1 0 0 − x ( 72 x 8 6 ) and crystalline Fe 3 B

Amorphous samples of ${\mathrm{Fe}}_{x}{\mathrm{B}}_{100\ensuremath{-}x}(72l~xl~86)$ have been studied by $^{57}\mathrm{Fe}$ M\"ossbauer 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 ${P(H)}$ have been found. The mean hyperfine field is found to be proportional to the average Fe moment with a ratio of about 130 kOe/${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$. The shape of $P(H)$ for each alloy is practically independent of temperature. At low temperatures, the effective hyperfine field (${H}_{\mathrm{eff}}$) shows a temperature dependence of ${H}_{\mathrm{eff}}(T)={H}_{\mathrm{eff}}(0)(1\ensuremath{-}B{T}^{\frac{3}{2}}\ensuremath{\cdots})$ due to spin-wave excitations. The value of ${B}_{\frac{3}{2}}=B{({T}_{C})}^{\frac{3}{2}}$ 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 ${H}_{\mathrm{eff}}(0)$ and the isomer shift exists for crystalline and amorphous Fe-B systems. Crystalline ${\mathrm{Fe}}_{3}$B(${T}_{C}\ensuremath{\sim}800$ K) has been found after crystallizing amorphous samples with $xg75$ under high heating rates. At $Tl{T}_{C}$, ${\mathrm{Fe}}_{3}$B shows at least three magnetically inequivalent sites, whereas at $Tg{T}_{C}$, electric quadrupole interactions are observed. The crystal structure of ${\mathrm{Fe}}_{3}$B is likely to be tetragonal rather than orthorhombic.