Monte Carlo Simulations Of Magnetic Properties Of Spin Frustrated Systems

Competing or frustration interaction is a common and important feature of condensed matter systems. The frustrated systems may be applied in many hot fields due to their novel and complex phenomena. Nowadays room-temperature magnetic refrigeration and ultrahigh-density magnetic recording are two hot subjects in the magnetism field, and have been extensively studied. In recent years, theoretical and experimental investigations predict that frustrated systems would become new good candidates for magnetic refrigeration and magnetic recording. In this paper, the magnetic properties of two models with geometrical frustration and nearest neighbor exchange interaction are calculated by using Monte Carlo simulation, based on Ising and classical Heisenberg models, respectively. The main jobs and conclusion are as follows.1) A modified Monte Carlo Metropolis algorithm is performed to simulate the hysteresis loops of three-dimensional classical Heisenberg models. In order to obtain the minimum energy of spins, we completely consider the thermal fluctuation of rotated spins at low temperature. Therefore, in this method the flipping probability depends not only on the energies of initial and final states but also on the saddle-point energy between two states. If the saddle-point energy is higher than the energy of initial and final states, the flipping probability depends on the energy difference between the initial state and saddle point. The judgment problems of energy barrier are resolved by using the Stoner-Wohlforth model.2) The magnetization behavior and the magnetic entropy change of frustrated Ising antiferromagnets with spin-1/2 on two-dimensional triangular and three-dimensional hexagonal closed-packed lattices by using standard Monte Carlo simulation. The results indicate that the normalized magnetization as a function of external field shows a 1/3 plateau on the two-dimensional system, while it shows 0 and 1/2 plateaus on the three-dimensional system at low temperature. Moreover, we study the change of magnetization behavior and the spin configuration. The reason of differences between two-dimensional and three-dimensional magnetization plateaus is that the interlayer exchange interaction is introduced on the three-dimensional lattice. It is observed that the values of magnetic entropy change may be positive at low temperature, namely, an endothermic phenomenon appears in an external field. However, as the temperature is elevated to an extent, the positive magnetic entropy change no longer appears. It indicates that frustration plays a crucial role. We also find a mapping of the magnetization plateaus to the magnetic entropy changes at low temperature. It is found that the value of external field at which the magnetization plateau begins is just corresponding to the place where the magnetic entropy change achieves a negative maximum, and at which the magnetization plateau ends is just corresponding to the place where the positive magnetic entropy change begins. It indicates that the field-induced metamagnetic transition occurs in the systems in an external field at low temperature. The study on systems with frustrated antiferromagnetic phases may open an important field in searching new materials for room-temperature magnetic refrigeration.3) The effect of field-cooling strength and interfacial coupling on exchange bias and coercivity of two ferromagnetic/antiferromagnetic systems with cores embedded in a matrix is simulated by using a modified Monte Carlo Metropolis algorithm, based on three-dimensional classical Heisenberg model. The results show that exchange bias changes from negative value to positive value with increasing cooling field as the interfacial coupling is antiferromagnetic. However, for the case of ferromagnetic interfacial coupling, the change of exchange bias as a function of cooling field is not evident, and the value of exchange bias is constantly negative. The pinning effect of the net magnetization of antiferromagnetic surface on ferromagnetic spins is the main reason. After cooling in a weak field, the exchange bias may be also negative due to forming the negative net magnetization in the antiferromagnetic surface as the interfacial coupling is large and antiferromagnetic, while the cooling field is strong, the exchange bias is positive and increasing with increasing values of interfacial coupling. However, for the case of ferromagnetic interfacial coupling, the difference of trends of exchange bias between weak and strong cooling fields is obscure, that is, the values of exchange bias are below zero and increasing for larger interfacial coupling. The coercivity in our model is almost unchanged with the cooling field strength and interfacial coupling. The effective field is produced by the action of the antiferromagnet on the ferromagnet through the interfacial coupling and influences the reversal of the ferromagnetic spins, so the coercivity as a function of interfacial coupling is nearly symmetric about the axis of zero interfacial coupling.4) Compare with two systems with different magnetic structures, it is found that the trends of exchange bias and coercivity as functions of cooling field and interfacial coupling in two systems are almost identical. The phenomenon of exchange bias is evident in the ferromagnetic cores-antiferromagnetic matrix, but the value of coercivity is small. The inverse results are obtained in the antiferromagnetic cores-ferromagnetic matrix, namely, the value of coercivity is big and the phenomenon of exchange bias is relatively obscure. In the former, the ferromagnetic cores are nearly in a single-domain state due to their small sizes, and the antiferromagnetic matrix is big enough to pin the ferromagnetic spins. On the contrary, in the latter the ferromagnetic matrix is in a multi-domain state due to its big size, and the small antiferromagnetic cores couldn't pin the ferromagnetic spins completely. Based on the mentioned above, we can fabricate the models according to the practical need in the magnetic recording. However, when the interfacial coupling is too large, an abnormality of exchange bias and coercivity may occur because the antiferromagnetic intrinsic configuration has been changed after cooling.