GRAIN-SIZE DEPENDENCE OF COERCIVITY AND PERMEABILITY IN NANOCRYSTALLINE FERROMAGNETS

Amorphous ribbons of composition Pe74. 5-xCuxNb3Si13. 5B9 (x=0, l at%) have been annealed between about 500°C and 900°C. This produced a series of crystallized samples with grain sizes between about 10nm and 300nm and with coercivities Hcand initial permeabilities μivarying over several orders of magnitude. The best soft magnetic properties (Hc=0.01A/cm and μi=80xl03) were observed for the smallest grain sizes of about 10nm. With increasing grain size D coercivity steeply increases following a D6-power law (up to D=50nm). Hc then runs through a maximum of Hc=30A/cm and decreases again for grain sizes above 150nm according to the well-known l/D-law for poly-crystalline magnets. The initial permeability was found to vary in a similar manner, essentially being inversely proportional to coercivity. The variation of the soft magnetic properties with the average grain size is discussed and compared with the predictions of the random anisotropy model and other theories for the magnetization reversal. © 1990 IEEE