Theoretical Study On Magnetic Properties Of Magnetic Nanowires And Nanorings

Since the size of magnetic nanomaterials is just at the same scale as some basic magnetic quantities,such as the coherence length of electrons,the size of single magnetic domains,and the length of exchange interaction,may exhibit some magnetic and thermodynamic properties different from bulk magnetic materials.Some magnetic nanomaterials,such as magnetic nanowires,magnetic nanotubes,and magnetic nanorings,usually have strong shape anisotropy due to their special spin structure,and has good application prospect in spintronic devices,high-density memory devices and high-frequency microwave magnetic devices.Therefore,the research on the magnetization behavior and dynamic behavior attracts much attention.In this paper,the magnetization and dynamic behavior of the core-shell magnetic nanowires,antiferromagnetic single-walled nanotubes and nanotube rings are studied by spin wave theory,double-time Green's function method and micromagnetic simulation method.The main contents of the paper are as follows:1.A cylindrical core/shell nano wire with antiferromagnetic core and ferromagnetic core described by quantum Heisenberg spin model are studied by using linear spin-wave theory and Green's function method.At zero temperature,it is found that the antiferromagnetic exchange interaction is the main cause of quantum fluctuation and the anisotropic exchange interaction has a frustrated effect on quantum fluctuation.The internal energy decreases as the exchange interaction and anisotropic exchange interaction increases,and the value of the internal energy of the system with antiferromagnetic core is always a little lower than the system with ferromagnetic core.At low temperatures,it is found that changing the magnetic structure of the system influences the quantum fluctuation very much but cannot affects the thermal fluctuation apparently.The internal energy and specific heat increases with the increase of temperature and the diminution of anisotropic exchange interaction.2.A cylindrical core-shell nanowire and a single walled nanotube were described by anisotropic XXZ Heisenberg model.The spontaneous magnetization,critical temperature and hysteresis behavior were calculated by using the double-time spin Green's function method.For the nanowire with antiferromagnetic interfacial exchange interaction,the quantum fluctuation of spins decreases with the increase of the exchange anisotropy and the quantum fluctuation of the core spin is more obvious than that of the shell,indicating the number of antiferromagnetic links plays more roles to the quantum fluctuation of the spins.When the temperature reaches the critical temperature,and as the anisotropy parameter increases from 0 to 1,the asymptotic value of the ratio of the magnetization of the shell to the core changes nonlinearly.Under the action of external magnetic field,when the interfacial exchange interaction is antiferromagnetic exchange interaction,the hysteresis loop of the system shows magnetization platform and multi-loop.When the interfacial exchange interaction is ferromagnetic,the system shows a simple loop.For the single-walled magnetic nanotubes.The sublattice magnetization and the critical temperature of the system were calculated by using the double-time spin Green's function method.At zero temperature,with the increase of the exchange interaction in the circumferential direction,a maximum value appears in the sublattice magnetization curves of antiferromagnetic and ferrimagnetic systems.As the diameter of the tube increases,the spin quantum fluctuations and thermal fluctuations are suppressed.3.A magnetic nanoring is described by the Heisenberg spin model with shape anisotropy,and the magnetization was calculated by using the spin Green's function method.It is found that the strong surface anisotropy can ensure the spins vortex state of the system.When there is no external DC magnetic field,with the increase of the radius,the transition temperature of the system decreases rapidly and then gradually tends to a finite value.Changing the direction of the current,the generated reverse toroidal magnetic field can switch the magnetic state of spins.The value of the current required for magnetization switching shows monotonic relationship to the radius of nanoring,surface anisotropy and the temperature which is far below the transition temperature.When the temperature is greater than the transition temperature,system shows the paramagnetic behavior.4.The dynamic susceptibility spectra of core/shell nanowires and torus nanoring are studied by means of micromagnetic simulation.For nanowires with core-shell structure,it is found that changing the interfacial exchange interaction has a great influence on the resonance frequency of the system with antiferromagnetic interfacial exchange interaction and little effect on the system with ferromagnetic interfacial exchange interaction.With the increase of external DC magnetic field,the resonance frequency and resonance line width of the system with ferromagnetic interface exchange gradually increase,but the resonance line width of the system with antiferromagnetic interfacial exchange interaction first becomes wider,then narrower,and then wider.With the increase of temperature,the resonance frequency of the system with ferromagnetic and antiferromagnetic interfacial exchange interaction decreases gradually,and the resonance line width become narrow.For the torus nanoring,the frequency response of the system with canted DC magnetic field was investigated.With the increase of DC magnetic field which is perpendicular to the plane of the ring,the resonant frequency decreases firstly and then increases monotonically.When the angle between the direction of the DC magnetic field and the center axis of the nanoring increases,the imaginary part of the magnetic susceptibility gradually shows a low frequency and three high frequency resonance peaks.In addition,it is found that changing the inner and outer radius of the ring only affects the high frequency resonance frequency of the ring with canted DC field.With the increase of temperature,under the vertical DC magnetic field,the resonance frequency increases slightly,and the resonance peak value gradually decreases;while under the DC magnetic field parallel to the ring plane,the four resonance peaks gradually merge into a single resonance peak because of thermal disturbance.