Study Of The Physical Properties Of Low-Dimensional Magnetic Nanostructures By Monte Carlo Simulation

Low-dimensional magnetic nanomaterials have attracted the attention of scientists because of their excellent physical properties and wide application prospects.Among these materials,magnetic core-shell nanomaterials,such as magnetic core-shell nanowires,nanotubes and nanoislands as well as magnetic nanofilms,have been widely used in the fields of environmental catalysis,photodetectors,solar cells and magnetic storage devices because of their special structural characteristics,which make them exhibit rich magnetic properties and diversified surface properties.Compared with bulk materials,magnetic nanofilms can show significant size effect,interface effect and quantum effect.The compensation behavior and the law of phase transition have important application value in the magnetic recording.At present,a large number of low-dimensional magnetic nanomaterials have been prepared by scientists,which provide important basis for the theoretical study of the physical properties of low-dimensional magnetic nanomaterials.In this paper,based on the Monte Carlo method,four kinds of low-dimensional magnetic structures have been selected,such as,magnetic core-shell nanowire,magnetic core-shell nanotube,magnetic core-shell nanoisland and three-layer magnetic nanofilms with a graphene structure.Four mixed-spin Ising models are established.From the microscopic perspective of quantum mechanics,the magnetic and thermodynamic properties of four low-dimensional magnetic nanostructures are studied.The effects of physical parameters such as single-ion anisotropy,exchange coupling,longitudinal magnetic field and temperature on the magnetization,susceptibility,hysteresis loop,coercivity,remanence,internal energy and specific heat are studied.The phase diagrams including phase transition temperatures,compensation temperatures and blocking temperatures under the influence of different physical parameters are obtained.When applied in the temperature field and the longitudinal field,the mechanisms of magnetic properties of low-dimensional magnetic nanostructures are explored,the effects of various physical parameters are revealed,and the origin of the magnetic step effect is illustrated.The results show that the magnetic core-shell nanowire and nanotube systems can exhibit the rich phase diagrams including the first-order and second-order phase transition temperatures,the compensation point and the tricritical point for certain physical parameters.The existence of the compensation point depends on certain critical values of the anisotropies and exchange couplings.The compensation behavior of the system originates from different temperature dependences of subalttice magnetizations.Two compensation points phenomenon is also found in the magnetic core-shell nanotube.The nanowire system can display the types of magnetization curves predicted by the Neel theory,such as N,Q,P and L types,which come from the competition among the anisotropy,exchange coupling and temperature.Both the susceptibility and the specific heat curves can exhibit the singularity phenomena at the phase transition temperature.In addition,the double-peak phenomena also exist in the susceptibility and the specific heat curves.For certain parameter range,the system exhibits the multiple hysteresis loops such double loops,triple loops and quintuple loops besides single loop.It is worth mentioning that multiple hysteresis loops behaviors play a potential role in multiple magnetic recording.Decreasing the absolute values of anisotropy and temperature or increasing the exchange coupling will increase the coercivity of the system.For the magnetic core-shell nanoisland,the influence of the anisotropy on the magnetization,susceptibility and internal energy is greater than that of the exchange coupling.The system also shows the double-peak susceptibility phenomenon as well as the double and triple hysteresis loops behaviors.The increase of temperature can cause the hysteresis behavior of the system disappear and eventually turns to a super-paramagnetic state.For the triple-layer magnetic nanofilm with a graphene structure,the saturation values of magnetization are more influenced by the anisotropy.The double-peak phenomenon is also found in the susceptibility curves,one peak in high temperature corresponds to the blocking temperature TB,while the other in low temperature reflects the non monotonic change of the internal energy.The TB increases with the increase of the exchange couplings and longitudinal magnetic field,but increases with the absolute values of the anisotropies increasing,and the TB is almost unchanged when the external magnetic field is large.Under the competition of the longitudinal magnetic field and the other physical parameters,the system shows an interesting step effect about magnetization,reflecting the change of the spin configuration at low temperature and whether to satisfy the 2S+1 rule.The nanofilm system can also exhibit the single loop and triple loops hysteresis behaviors.Some results are compared with other theoretical and related experimental results,and similarities and differences are obtained,at meanwhile,some reasonable explanations are provided.This dissertation will enrich people’s understanding of the law of phase transition and compensation behavior for the ferrimagnetic systems in order to further clarify the mechanism of magnetic origin of low-dimensional magnetic nanostructures.It will improve the theoretical system of low-dimensional magnetic nanostructures,and provide scientific theoretical guidance for the preparation and application of low-dimensional magnetic nanostructures in the future experiment.