Magnetic Properties of Alnico Alloy Phases and Temperature Instability of Permanent Magnets
Authors: Sergeyev V. , Bulygina T. 
Published: February 2004

Analysis of the structural constituents of Alnico alloys with high coercive force showed that the saturation magnetization of the α′ phase is quite high, and for Alnico VIII and Alnico VS55 is close to the saturation of binary Fe–Co (Is∼1800 emu) At the same time, the saturation magnetization of the α phase is only several scores of emu. The relative volume of α′-phase decreases from Alnico V to Alnico VIII and VS55. This result is in qualitative agreement with the corresponding decrease of Is and increase in Hc for these alloys. Comparison of the Ku(T) and Hc(T) experimental curves for these alloys reveals their correspondence: a maximum in Ku and Hc in the temperature interval of +20°-–+100°C and a relatively small decrease of Ku and Hc over a wide temperature range. Calculation of the Ku(T) dependence from the equation Ku = ½(Is1 − Is2)2N·ν1·ν2, taking into account the temperature dependence of Is1 and Is2, shows good agreement between the experimental and calculated curves. These results prove that it is the shape anisotropy of the strongly magnetic phase particles that explains the high coercive force of the Alnico alloys. Anisotropy fields HA=(2Ku/Is) were calculated using the Ku and Is values of the alloys measured at room temperature. Comparison of the HA values with the coercive force shows that the anisotropy field is three-times higher than the coercive force values. Causes of the variation in anisotropy field in various alloys may be explained by analyzing the data. The increase in HA in Alnico VIII in comparison with Alnico V is explained by changing alloy-phase characteristics: an increase in ΔIs causes a 35% increase in HA, and an increase in ΔN causes a 17% increase; an increase in ν1+ν2 results in a 9% increase of HA, and decreased saturation Is causes a 9% increase in HA. All this results in a value of HA for Alinco VIII 70% higher than for Alnico V. When comparing corresponding characteristics of Alnico VIII and VS55, one can see that the ΔIs value is very small, and the increase in ΔN is negligible. The increase in HA is mainly due to a decrease in Is in Alnico VS55 in comparison with Alinco VIII, which increases HA by 30%. The decrease in Is is mainly due to a smaller ν1 (relative volume of the α′-phase). Thus, the Ku and HA values achieved in single-crystal alloys Alnico VIII and VS55 are nearly as large as possible because of the following: (a) further increase in Is (at a given composition) and increase in ΔN are practically impossible; (b) Is2 values are low enough so that further decrease has only a small effect. Part II. Special features of temperature changes in the induction of permanent magnets are qualitatively explained on the basis of analysis of results of investigation of the temperature dependence of the main alloy magnetic properties: saturation magnetization, coercive force and uniaxial anisotropy constant. Irreversible induction losses during the first temperature cycles may be related to the process of irreversible rotation in some precipitates of strongly magnetic phase under the influence of the internal field because of a decrease of anisotropy constant. Reversible magnet induction changes may be accounted for by the following: (a) Reversible temperature dependence of alloy saturation; (b) Temperature dependence of reversible rotation of magnetization in single-domain particles in the internal demagnetizing field. Induction changes when cooling magnets in the interval of +100° to −180°C are probably caused by two opposite effects, namely: (a) an increase in the saturation of the alloy results in an increase in induction, and (b) increasing reversible rotation of magnetization of strongly magnetic phase particles decreases the induction.1 Thus, a maximum is observed on the curve of reversible induction vs temperature. Reversible induction changes with temperatures above +100° may be accounted for by the influence of the following factors: (a) the decrease in alloy saturation results in a decrease of induction; (b) diminishing reversible rotation of particle magnetization causes some increase in induction. Because of the second mechanism, reversible temperature changes in magnet induction are always less than relative temperature changes in alloy saturation. It can be shown that the higher the internal demagnetizing field, the greater is this effect. This explains, first, the decrease in ΔB/B rev and temperature coefficient of induction with lowering the magnet working point, and second, the relatively high stability (i.e., small ΔB/B rev and temperature coefflcient values) of Alnico VIII and VS55 magnets. The (BH)max point for these high coercive-force alloys corresponds to high values of the internal demagnetizing field. This causes a significant reduction in the decrease of induction with temperature. Both parts of this abstract will be published in the I.E.E.E. Transactions on Magnetics as individual papers.