Modeling Magnetic Anisotropy in Single Chain Magnets
This thesis embodies work on modeling magnetic anisotropy in molecular magnets. Understanding the factors that govern the magnitude of anisotropy in these materials is crucial for tailoring molecules that can be used in magnetic devices. In chapter 1, we briefly discussed magnetism and magnetic materials, and in chapter 2, we have discussed the model Hamiltonian we have employed in our work. In chapter 3, we have studied the effect of on-site anisotropy on magnetic anisotropy, $D_M$. In principle, for strong anisotropy, we cannot define the molecular anisotropy parameters $D_M$ and $E_M$, which correspond to the axial and rhombic zero-field splitting parameters, respectively. However, the giant spin of the low-lying large $M_S$ states is very nearly an integer, and using this spin value; it is possible to construct an effective spin Hamiltonian and compute $D_M$ and $E_M$. We have also studied the effect of finite size, rotations of site anisotropies, and chain dimerization on the effective anisotropy of the spin chains. In chapter 4, we have studied the effect of exchange anisotropy as well as on-site anisotropy on molecular anisotropy, $D_M$. We find that the axial anisotropy parameter, $D_M$ is the sum of the individual contributions due to exchange and on-site anisotropies. We have also studied magnetic susceptibility, specific heat as a function of temperature, and magnetization as a function of the applied field. One of the possible applications of molecular magnets is in the area of the magnetocaloric effect. In chapter 5, we have studied two different spin models (i) spin chains with alternating ferro and antiferromagnetic interactions, and (ii) with an additional next nearest neighbor ferromagnetic interaction using both exact diagonalizations as well as the Monte Carlo technique. We have computed the magnetic Grüneisen parameter $(Γ_H )$, which is closely related to the magnetocaloric effect, for different parameters such as on-site anisotropy, exchange anisotropy as well as spin-dipolar interaction strength. We also show the dependence of $Γ_H$ on the dimensionality of the lattice for a fixed lattice constant. Modeling blocking temperature in molecular magnets has been a long-standing problem in the field of molecular magnetism. In chapter 6, we have studied the relaxation of magnetization on an assembly of 100,000 spin chains using an innovative rejection free kinetic Monte Carlo technique. We study this for different exchange anisotropy, on-site anisotropy, and strength of dipolar interactions. The magnetization relaxation times show non-Arrhenius behavior for weak on-site interactions. The energy barrier to magnetization relaxation increases with the increase in on-site anisotropy, strength of spin dipolar interactions, and exchange anisotropy. However, in all these cases, the barrier saturates at large on-site anisotropy. These studies throw light on why single ion rare-earth magnets have high blocking temperatures.