Magnetic properties and microstructure of LaCo_(5)-based powders produced by mechanical milling and subsequent annealing

摘要

The magnetic and structural properties of LaCo_(5)-based La_(x)Co_(100-x) (x=17-22) alloys have been studied in powders synthesized by mechanical milling and subsequent annealing. A record high coercivity (over 1.7 T) with a M_(r)/M_(s) ratio about 0.7 (measured at room temperature) has been obtained in La_(22)Co_(78) powders. Their microstructure consists of major LaCo_(5) phase grains with the grain size of about 150-350 nm, coexisting with LaCo_(13) phase particles (30-70 nm) and La/La oxide grains (100-200 nm). The observed magnetic properties originate from the high anisotropy field and uniform grain size and shape of the LaCo_(5) phase. The demagnetization curve, however, often exhibits a shoulder, which was correlated with the appearance of the magnetically soft 1:13 phase and with the inhomogeneity in the microstructure. All the analyses are consistent with a nucleation-type LaCo_(5) magnet, where the coercivity is attributed to the difficulty in the formation of reversed magnetic domains. The observed phases and microstructure are briefly discussed based on a possible nonequilibrium phase transition resulting in chemical inhomogeneity and oxidation.