In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

https://www.andrew.cmu.edu/user/dl0p/laughlin/pdf/PMS.pdf

Amorphous and nanocrystalline materials for applications as soft magnets

Amorphous metallic alloys

REVIEW ARTICLE: Glassy metals

Water purification using magnetic assistance: a review.

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.

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

Amorphous alloys with nominal composition of Ni 40 Fe 40 P 14 B 6 are shown to respond to annealing in a magnetic field. Coercive forces are reduced by a factor of 10 to 50 during annealing of straight ribbons to values of 0.003 Oe, as low as ever reported for potentially useful materials. Concurrently the ratio of the magnetization in 1 Oe applied field, to saturation, increases from about 0.5 to 0.95. These changes during annealing correlate with measured stress relief changes. It thus appears that most of the strain-magnetostriction contribution to the anisotropy is removed during annealing. Magnetic annealing at temperatures as low as 100°C results in noticeable changes in properties. From measurements transverse to the magneticaliy induced anisotropy axis, the induced anisotropy is calculated to be about 800 ergs/cm3, considerably smaller than obtained in crystalline Ni 50 Fe 50 . This field-induced anisotropy is reversible in direction and magnitude by reheating the sample to its Curie temperature and then cooling in a field. Annealing of 1.5 cm diameter toroids, made from 50 μm thick tapes, increases the initial permeability by more than a factor of 10 and decreases losses by more than a factor of 10. Losses and permeabilities after heat treatment compare favorably to the Permalloys with similar saturation magnetizations.

Magnetic annealing of amorphous alloys

The uniaxial magnetic anisotropy K u induced by a magnetic anneal has been determined, after stress relief, for a series of alloys given by (Fe y Ni 1-y ) 80 B 20 . For all of these alloys K u depends on anneal temperature as predicted by directional order theory. The concentration dependence of K u in these alloys is also consistent with directional order theory. The maximum K u , corrected for its temperature dependence, occurs at the composition y = 0.5 . However, K u does not fall to zero at y = 1 as predicted if directional order is assumed to arise only from Fe-Ni pair ordering. These results are interpreted as suggesting a role of the glass former, boron, in the directional ordering, perhaps as an "interstitial." The interaction energy derived from the results is negative as expected for interactions leading to precipitation. Its large value of \approx -7.5 \times 10^{-14} ergs corresponds to a critical temperature for precipitation of 3000 K.

Magnetic anneal anisotropy in amorphous alloys

Crystallization of some FeNi metallic glasses

The magnetic properties and crystallization temperatures of alloys in the ternary Fe-B-Si system are reported. The Curie temperature increases slightly on replacement of boron by silicon. This results in a sharp ridge of relatively constant room-temperature saturation magnetization extending from Fe 80 B 20 to Fe 82 B 12 Si 6 . The coercivity exhibits a broad minimum, both before and after stress relief annealing, in the region around Fe 81 B 15 Si 4 and extending at least to Fe 77 B 13 Si 10 . The crystallization temperature increases with increasing silicon and with decreasing iron and boron. The alloys with silicon are generally easier to prepare in the amorphous state than the binary Fe-B alloys. Thus for the highest saturation magnetization alloy combined with ease of preparation, stability, and lowest losses, the alloys between Fe 81 B 17 Si 2 and Fe 82 B 12 Si 6 are preferred.

Fred E. Luborsky

Papers

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

Magnetic annealing of amorphous alloys

Magnetic anneal anisotropy in amorphous alloys

Crystallization of some FeNi metallic glasses

Formation and magnetic properties of Fe-B-Si amorphous alloys