Reversible processes and magnetic viscosity of nanocrystalline permanent magnets

The accurate determination of the magnetic viscosity parameter Sv requires the calculation of a new parameter eta=(partial derivative M-rev/partial de rivative M-irr)(Hi) which describes the dependence of reversible magnetizat ion on irreversible magnetization. Measurements of the magnetic viscosity w ere performed at 300 K on the major demagnetization curves of two different nanocrystalline permanent magnetic materials: (i) melt-spun and flash-anne aled Pr4Fe78B18 and (ii) mechanically alloyed and heat-treated Sm18Fe33Co49 . We show that eta is proportional to the intrinsic reversible susceptibili ty chi(rev)(i), in good agreement with predictions of the moving Preisach m odel. Measurements of the reversible susceptibility were made in order to d etermine eta. Experimental values of eta are positive on the major demagnet ization curve in both materials, resulting in substantial corrections to S- v near H-c. (C) 1998 American Institute of Physics.