STUDIES ON TEMPERATURE COMPENSATED RARE-EARTH COBALT MAGNETIC MATERIALS

During the past decade rare-earth cobalt alloys of the composition RCo(,5) (R = rare-earth) have become important permanent magnet materials owing to their high Curie temperatures and large magnetic anisotropy, resulting in high coercive forces and energy products, (BH)(,max). Among RCo(,5) alloys, SmCo(,5) has attractive properties. However, these magnets have a temperature coefficient of magnetization which is not useful in some specialized applications.; The heavy rare-earth RCo(,5) alloys have positive slopes and SmCo(,5)(' )hasa negative slope for M vs. T curves. The systems of the form Sm(,1-x)R(,x)Co(,5)(R = Gd, Tb, Dy, Ho and Tm) were studied to obtain temperature; compensated materials, i.e., an alloy with zero slope for M vs. T curve. Efforts to prepare ternaries with Yb were unsuccessful. The materialswere characterized by x-ray diffraction and thermomagnetic analysis.The magnetization measurements were performed on powders from -195to 250(DEGREES)C in a field of 4 k0e. The hysteresis loops of aligned powders set in wax are traced using a vibrating sample magnetometer, in fields up to 20 k0e. Among the ternaries studied, Sm(,0.6)Gd(,0.4)Co(,5) and Sm(,0.6)Dy(,0.4)Co(,5.04) showed promising properties. The values of the magnetic properties for; Sm(,0.6)Gd(,0.4)Co(,5) are: 4(pi)M(,s) = 7.37 kG, B(,r) = 7.27 kG, (BH)(,max) = 12 MGOe, H(,c) = 6 k0e and (alpha) = -0.034 (-195 to -56(DEGREES)C), -0.007 (-56 to 250(DEGREES)C) and for Sm(,0.6)Dy(,0.4)Co(,5.04) are: 4(pi)M(,s) = 8.13 kG, B(,r) = 7.94 kG, (BH)(,max) = 9 MGOe, H(,c) = 4.k0e and (alpha) = 0.0157 (-106 to 104(DEGREES)C), -0.0104 (104 to 250(DEGREES)C). The temperature coefficient of magnetization, (alpha) is defined as rate of per cent change of magnetization with respect to temperature.(,); The substitution of Co by Fe in Sm(,0.57)Gd(,0.43)Co(,5) was studied. Thisdid not improve the properties significantly except for increase in the saturation magnetization. Sm(,0.6)Gd(,0.4-x)Dy(,x)Co(,5) and Sm(,0.6)Gd(,0.4-x)Ho(,x)Co(,5); were also studied. Among these, Sm(,0.6)Gd(,0.2)Dy(,0.2)Co(,5) has the lowest value for (alpha) and is equal to 0.002 from -71 to 60(DEGREES)C. The other properties are: 4(pi)M(,s) = 8.16 kG, B(,r) = 8.05 kG, (BH)(,max) = 13 MGOe and H(,c) = 5.9; k0e. Sm(,0.6)(Gd,Dy)(,0.4-x)(Pr,Nd)(,x)Co(,5) were also prepared to study the effect of substitution of Gd and Dy by Pr and Nd. The energy products for Sm(,0.6)Gd(,0.4-x)Pr(,x)Co(,5) (x = 0.1, 0.2 and 0.3) exceeded 12 MGOe and for Sm(,0.6)Dy(,0.4-x)Pr(,x)Co(,5) (x = 0.1, 0.2 and 0.3) exceeded 10 MGOe, and the value of (alpha) for these is rather high for specialized applications.