Temperature Coefficient of Coercivity for M-Type Sr-Ferrite Substituted by Lanthanum and Cobalt

The relationship between the temperature coefficient of coercivity {α(HcJ)} and the ratio Sv to HcJ was investigated for M-type Sr-ferrite substituted by La and Co (Sr1-xLaxFe12-xCoxO19) in the temperature range from 173to 423 K, where Sv and HcJ were the magnetic viscosity constant, which essentially determined the thermal stability, and the coercivity, respectively. The value of x was varied between 0, 0.3, and 0.6. The value of α(HcJ) for x = 0 with Sv/HcJ = 0.038 was 0.35%/K, whereas that for x = 0.3 with Sv/HcJ = 0.034 was 0.1%/K at 298 K. Furthermore, α(HcJ) for x = 0.6 with Sv/HcJ = 0.026 was almost zero at 298 K. The following relation of α(HcJ) to Sv/HcJ was also found to hold in these M-type Sr-ferrite magnets substituted by La and Co, as in the case of Nd-Dy-Fe-Al-B-Sn sintered magnets. α(HcJ) was expressed as {28.5k − (δE/δT)H}/kT ·(Sv/HcJ), where, k, E, T, and H were the Boltzmann constant, the activation energy, the absolute temperature, and the magnetic field, respectively. The equation shows that smallerSv/HcJ values are necessary for ensuring small α(HcJ) values, which are very important for permanent magnets.