3.3 Magnetic Properties of High-Coercivity Sm(Co, Cu, Fe, M)₇.₄ Permanent Magnetic Alloy from 1.5K to 523K
High coercivity \(Sm(Co, Cu, Fe, Zr)_{7.4}\) permanent magnetic alloy is widely used because of its high Curie temperature (\(T_c = 810°C\)) and high saturation magnetization intensity (Pan, Ma, Li, 1993; Pan, Ma, Ping, et al, 1991; Pan, Jin, 1990; Pan, Zhao, 1988). With the special requirement in the areas of aviation and navigation material properties must be measured at high temperature and low temperature. High properties material, especially rare earth permanent magnetic alloys with high coercivity, used in power device, magnetic force damper, magnetic force driver, etc. operating in media of liquid hydrogen and liquid nitrogen of spaceflight and operating at low temperature being specified by user. The designer must give the magnetism and the magnetism under repeated variation through high temperature to low temperature in order to effectively and reasonably selecting magnet. The permanent magnetic alloy used for aerospace permanent magnetic motor must be made with good stability in temperature and time at 523K. Therefore measuring and studying the magnetism of material at 1.5 - 523K are necessary.
Specimen Preparation and Magnetic Measurement Techniques for Sm(Co, Cu, Fe, M)₇.₄
Composition range of alloy: 23.5%-27.5%(wt.) Sm, 13.5%-25.5%Fe (mass fraction), 4.0%-8.2%Cu (mass fraction), 1.0%-4.1%Zr (mass fraction), the surplus was cobalt. For example, Sm(Co, Cu, Fe, M)7.4 specimen composition is 26.01% Sm, 51.38% Co, 4.36% Cu, 15.19% Fe, 2.99% Zr. The purity of raw metal materials was 99.5%-99.8%. Put above material in vacuum induction furnace and melt in argon atmosphere, cast into water cooling copper crucible, the melted alloy ingot was crushed and pulverized grossly, intermediately and finely to powder of 3 - 5μm; the powder was formed in magnetic field greater than 1.5T and the forming pressure was 2.5 - 3T/cm2 with a direction being perpendicular to magnetic field; the above pressed billet was sintered for 1h at 1150 - 1210°C, sintered billet was through solid solution treatment at 1130 - 1200°C for 1 - 2h, then through isothermal aging (720 - 900°C) or step aging. The billet was cut as per requirements, then was magnetized in a pulse magnetic field greater than 5T and measured magnetism with CL - 6 direct current parameter measurement instrument.
Measuring sequence is as follows: 25°C → 100°C → 150°C → 200°C → 250°C → 25°C → 60°C → -196°C → 25°C. The specimen was measured by vibrating specimen magnetometer in condition of 1.5K and 40K (Zhang, Cheng, 1982; Pan, Ping, Liu, Ma, 2003; Ma, et al, 1999).
Measurement Results and Discussion on the Temperature-Dependent Magnetism of Sm(Co, Cu, Fe, M)₇.₄
Specimens with different composition and heat treatment system were measured. The curve of one composition of specimens measured at -196°C to 200°C is shown in Fig. 3.31, the demagnetization curve under 1.5K is described in Fig. 3.32 and magnetic properties under 1.5K are given in Table 3.4 and Table 3.5.
Magnetic properties at 25°C are as follows:
Residual magnetic induction intensity
\(_{m}H_c=1,592\text{kA}/\text{m}(20\text{kOe})\)
\(_{b}H_c = 716\text{kA}/\text{m}(9\text{kOe})\)
\((BH)_{max}=206.96\text{kJ}/\text{m}^3(26\text{MGs}\cdot\text{Oe})\)
Magnetic properties at -196°C:
\(_{b}H_c = 765\text{kA}/\text{m}(9600\text{Oe})\)
\((BH)_{max}=214(27\text{MGs}\cdot\text{Oe})\)
Magnetic properties at 200°C:
\(_{m}H_c = 119.4\text{kA}/\text{m}(15\text{kOe})\)
\(_{b}H_c = 655.0\text{kA}/\text{m}(8200\text{Oe})\)
\((BH)_{max}=195.0(24.5\text{MGs}\cdot\text{Oe})\)
Conclusions: Key Findings on the Magnetic Behavior of Sm(Co, Cu, Fe, M)₇.₄ Across Different Temperatures
Conclusions are drawn based on the above researches as follows:
- Magnetic properties of \(Sm(Co, Cu, Fe, M)_{7.4}\) varied along with change of heat treatment system.
- The \(Sm(Co, Cu, Fe, M)_{7.4}\) permanent magnetic alloy with different ingredient has different magnetic properties. Zirconium content has large influence on coercivity, iron content impacts on \(B_r\) and \((BH)_{max}\), copper can promote precipitation of the 1:5 phase and improve coercivity.
- Properties of permanent magnetic alloy at low temperature are better than that at room temperature.
- Properties of \(Sm(Co, Cu, Fe, M)_{7.4}\) permanent magnetic alloy can be reversed to original value at room temperature through reversing low temperature to room temperature.
- It is testified from demagnetization curve that spinning and then orientation appeared in measurement of \(Sm(Co,Cu,Fe,M)_{7.4}\) permanent magnetic alloy under 1.5K.
