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Energy barrier analyses on highly oriented Nd-Fe-B thin films with and without Nd overlayer

59th Annual Magnetism and Magnetic Materials (MMM) Conference

R.Goto、S.Okamoto、N.Kikuchi、O.Kitakami ( IMRAM、Tohoku Univ. )

概要/Abstract

It has long been discussed that Nd-Fe-B permanent magnets exhibit very small coercivity compared its anisotropy field, that is, so-called coercivity problem1. Study on magnetization reversal process of Nd-Fe-B magnets is essentially important to solve the coercivity problem and also to improve the permanent magnet performance.
It is well known that magnetization reversal at a finite temperature is the thermal activation process with an energy barrier EB which varies with the magnetization reversal mechanism. In this study, we analyze the thermal activation process of Nd-Fe-B thin films and discuss the reversal process.
To simplify the analyses, highly (001) oriented thin Nd-Fe-B films were used.
Samples were prepared by co-sputtering technique with Fe, Nd, and B targets.
Film composition was tuned to be Nd12.5Fe78.4B9.1 (at%).
Sample structure was MgO(001) sub./Mo(30 nm)/Nd(0.25 nm)/Nd-Fe-B(50 nm)/Nd(tNd)/Mo(10 nm), where tNd is the thickness of Nd overlayer.
By adding Nd overlayer, magnetic properties of films can be significantly modified. Fig. 1 shows magnetization curves for tNd = 0 and 1 nm. Both magnetization curves exhibit very high squarenes, but their coercivity are quite different.
Fluctuation field Hf is a good measure of EB and is accurately evaluated from the slope of ln (S/t) against the applied field H2, as shown in Fig. 2, where S is the viscosity coefficient.
Thus determined Hf are about 7 mT for both films. Moreover, time-dependent Hc analyses also give the information on EB.
Elaborately combining Hf and time-dependent Hc, the power index of EB function against H is determined. As a result, the power indexes of EB function for both films are 1.2 insensitive to tNd.
This fact indicates that the magnetization reversal processes of the Nd-Fe-B films with and without Nd overlayer would be both assigned to be domain wall pinning type , whereas their coercivity are quite different.

References

1. H. Kronmüller et al., J. Magn. Magn. Mater., 74 (1988) 291.
2.M. El-Hilo et al., J. Magn. Magn. Mater., 248 (2002) 360.



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