Anomalous magnetic viscosity in the bulk-amorphous ferromagnet Nd60Fe20Co10Al10

Full text: Magnetic viscosity is the time dependence of the magnetisation, M(t), in a constant magnetic field and is the result of thermal activation of irreversible magnetisation processes. On the major hysteresis loop, and over short times, M(t) can be described by M(t) = A + S ln(t +t0), where A and t0 are fitting parameters and S is the magnetic viscosity parameter. In the second, or de-magnetisation, quadrant of the hysteresis loop S is negative and reaches its largest value near the intrinsic coercivity. On the lower branch of a recoil curve, S can sometimes be initially positive before becoming negative after a sufficiently long waiting time; this is called anomalous magnetic viscosity. Recently, it has been reported that bulk-amorphous hard magnets can be obtained in the multi-component alloy systems Nd-Fe-Al and Nd-Fe-Co-Al. Wang et al. have observed anomalous magnetic viscosity on the lower branch of minor hysteresis loops in Nd60Fe30Al10. We have studied magnetisation and de-magnetisation processes in a suction-cast rod of bulk amorphous Nd60Fe20Co10Al10. Remanence curves were measured starting from three different states: ac de-magnetised, thermally de-magnetised and fully saturated. Magnetic viscosity was measured on the major loop and on both branches of minor loops for times extending to 30 hours. A rich variety of magnetic behaviour was observed, including anomalous viscosity on the minor loops, and our results will be presented and discussed in terms of activation energy distributions