The Coherent Rotation Model In Composite Nanoferromagnets And Composite Nanoferroelectrics

The reversal process and coercivity mechanism in nanomagnets has always been an hot topic in magnetization. In most calculations of hysteresis loops, higher order terms of the magnetocrystalline anistropy are ignored. In this thesis, the effect of the higher order term of the magnetocrystalline anistropy on the reversal process has been investigated, and the analytical formula for the coercivity has been obtained. The composite nanomagnets have high coercivity and high remanence, provided by hard and soft phases respectively. In this thesis, the reversal process in composite nanomagnets has been investigated also. The reversal process and coercivity mechanism in ferroelectric material have also attracted much attention always. In this thesis, the reversal process in ferromagnet and ferroelectric have been investigated within a coherent rotation model. The main studies are listed as following:1. The hysteresis loops and coercivity mechanism in single phased permanent nanomagnets have been investigated. The effect of the higher order term of the magnetocrystalline anistropy on the demagnetization process has been analysed. The shape of hysteresis loop changes withλ1 andλ, whereλ1 is the ratio of the sixth order term of the magnetocrystalline aniaotropy coefficient and the second order term whileλis the ratio of the fourth order term and the second order term. The effect of the higher order term of the magnetocrystalline anistropy on the coercivity has been investigated, with the analytical formula for the coercivity as a function of λderived.2. The hysteresis loops and coercivity mechanism in single phased nano ferroelectrics has been investigated. When the easy axis parallel to the applied field, we study the ferroelectrics with the first order phase transition and the second order phase transition respectively. The effect of the higher order term of the energy on the depolarization process has been investigated. The shape of hysteresis loop changes with temperature.3. The reversal process in composite nanomagnet (Nd₂Fe₁₄B/α-Fe) and single phased many-layer nanomagnet (Nd₂Fe₁₄B) have been investigated. The coercivity and hysteresis loop of composite nanomagnet change with layer thickness. The coercivity hc of single phased nanomagnets increases with n (n is number of layer) and approaches to a constant hc=0.32. The effect of the layer thickness on the hysteresis loops and coercivity in composite nanomagnets is very large.4. The reversal process in single phased double-layer nanoferroelectric films has been investigated. The coercivity and hysteresis loop of single phased double-layer nanoferroelectric films change with the layer thickness and temperature. The effect of the layer thickness and temperature on the hysteresis loops and coercivity in double-layer nanoferroelectric films is important. The relation about theoretical model of ferromagnet and ferroelectric has also been investigated.