Theoretical Study On The Remanence Enhancement And Reversal Mechanism In Nanomagnets

The performance of a permanent magnet is largely dependent on the maximum magnetic energy product (B·H) max, the remanence Mr and the coercivity Hc. In the past 20 years, many magnetic thin films of the rare-earth transition metal have been investigated intensively. In particular, remanence enhancement discovered in the rare-earth nanomagnets has attracted much attention. Remanence enhancement in composite permanent nanomagnets has also been demonstrated, due to the exchange-coupling interaction between magnetically soft and hard phases. In this thesis, remanence enhancement and reversal mechanism in nanomagnets have been studied based on a hybrid model proposed by G. P. Zhao and Brown's standard micromagnetic model.1. Remanence enhancement in single-phased permanent nanomagnets has been investigated using Nd2Fe14B as an example. A universal law for remanence has been derived within the analytical hybrid model proposed by G. P. Zhao. The formula for remanence is Mr = Ms / 2 + cWMs / L, where Mr, Ms, W, L are the remanence, saturation magnetization, domain wall width and average grain size respectively, while c is a dimensionless constant. In addition, Mr, Ms, W, L and constant c can be obtained by fitting the experimental data.2. The influence of surface anisotropy on remanence has been studied within a micromagnetic method, assuming that the surface anisotropy is normal to the film surface and the volume anisotropy is parallel to surface. The surface anisotropy doesn't change the magnetization angular distribution and the remanence in the film when it is smaller than a critical value. When the surface anisotropy is comparable to the volume anisotropy, however, it changes the orientation of the magnetization near the surface and the remanence significantly.3. The magnetization reversal process of perpendicularly orientated Nd2Fe14B /α-Fe trilayers has been studied with the demagnetization energy considered. The nucleation field of the perpendicularly orientated case is lower than that of the parallelly orientated case and the nucleation appears before the reversal of the applied field. As the thickness of the soft phase increases, the theoretical coercivity changes from the nucleation field to the pinning field. Finally, the effect of surface anisotropy on the nucleation field has also been discussed.