Structural Control, Microstructure And Magnetic Properties Of High Anisotropy Magnetic Recording Thin Films

Ferromagnetic nano-structural thin films(FePt based and SmCo based) have many advantages, such as, high magnetocrystalline anisotropy constant, small single domain size, thermal stability and excellent high temperature magnetic properties. For these reasons, they are the promising candidates for application in micro-magnetic devices, ultra-high density magnetic recording media, et al. However, due to its exceesively high magnetic anisotropy field, it is difficult to control thecoercivity and exchanged coupling effects among the magnetic grains. It is necessary to add some non-magnetic phase can isolate the magnetic grains. The direction of magnetic anisotropy and the easy axis(c axis) changing from out-ofplane to in-plane is also worth to investigating, which can be adjusted by annealing temperature. This part of work is rarely reported. Additionally, the current progress of synthesizing the thin films media with well magnetic anisotropy and low recording noise is rather complicated, since using the low cost buffer layer materials, which can enhance the magnetic properties, are appropriate for the industrial application. Moreover, the excessive grain growth after the post-annealing is also detrimental to minimize the storage unit size, which should utilize the quenching method to solve. At last, the composition adjustment is the key point to affect the magnetic propertiesof SmCo thin film because of the multi-phases for Sm-Co binary alloy. As the results, it needs to optimize the deposition pressure and annealing process.TiN material is widely used as buffer layer due to its large surface energy, good diffusion barrier and simple fabrication method. The preferred orientation and surface morphology of TiN buffer layer are investigated. Two of the different preferred orientations, the(111) and the(200), are found to be directly related to the variable substrate temperatures and negative bias voltages. As the temperature increases approaching 300℃, the grain sizes get larger, however if the temperature is increased beyond 400℃ and combined with high negative bias, the grain sizes become smaller and microstructure get smoother and denser. The effects of these two significant parameters on the crystal structure and the surface morphologies of these TiN layers reveal that the(111) orientation is preferred at higher substrate temperatures approaching 450 ℃ together with higher negative bias due to the higher surface energy, whereas the(200) orientation is preferred at lower negative bias voltages due to the higher strain energy at that condition.By introducing the TiN as the buffer layer for FePt thin film, it can be successfully obtain TiN/FePt films with controllable magnetic anisotropy followed by the different temperatures of annealing. The effects of TiN layer thickness on the microstructure and magnetic properties of FePt films are systematically investigated. It is found that TiN layer has significant effects on the magnetic anisotropy of the FePt films. The L10 phase transformation of the FePt films with TiN layer is more completely than that without a TiN layer. After annealing at 600 °C, TiN(111) buffer layer in thickness of 30 nm induces the out-of-plane anisotropy and reaches the highest out-of-plane coercivity(4.5 kOe). However, the coercivity is tended to decrease as the thickness of TiN(111) layer increasing to 50 nm. The anisotropy gradually switches from out-of-plane to in-plane when the annealing temperature is increased to 700 °C. The in-plane anisotropy of L10-FePt films may come from the relaxation of internal stress of the TiN layer after annealing at 700 °C, which is not able to provide sufficient in-plane tensile stress to induce the perpendicular growth of L10-FePt grains. Nevertheless, the in-plane coercivity enhances to 12 kOe when the thickness Ti N(111) intermediate layer is 30 nm.The effects of deposition temperature and the quenching rate on the magnetic properties and microstructure of the FePt/Ti N films have been investigated. By increasing the deposition temperature from room temperature to 450°C, both the L10-ordering parameter and the inplane anisotropy can be enhanced. The microstructure and phase structural results indicate that a high quenching rate is beneficial to refine the FePt grains despite the high deposition temperature. The highest in-plane coercivity(11.5kOe) is obtained by depositing at 300°C and quenching in air. However, when the film is deposited at 300°C followed by quenching in ice water, the average FePt grain size dramatically decreases to 22 nm and the in-plane coercivity still maintains at about 10.5 kOe.Moreover,(FePt)100-x(MgO)x granular thin films and [MgO/FePt]n multilayer are fabricated on single crystal MgO substrate at different deposition temperatures followed by a high-temperature annealing. The adding MgO is found to have the following effects on L10 phase transformation; exchange interaction between the FePt grains and the optimal parameters for the L10-FePt to gives the highest coercivity. Deposition temperature has also affect on the magnetic properties of FePt-MgO films. The mechanism for cercivity variations is attributed to the thermal expansion and competitively growth among FePt and MgO grains. The epitaxial growth of [MgO/FePt]n multilayer multilayer thin films is affected by the modulus. The random oriented FePt upper layer will break the epitaxial growth as the modulus increasing. Results also reveal that adding appropriate amount of MgO(55 vol.%) by finding a proper deposition temperature(200°C), well-defined granular L10-FePt films with fine grain, high coercivity and small domain size can be obtained.The Cr/SmCo/Cu/Cr thin films with high coercivity are prepared under different deposition pressures and annealing temperatures by using the magnetron sputtering. The asdeposited SmCo films show the amorphous structure while Sm2Co17 or SmCo5 hard magnetic phases appear upon annealing in vacuum furnace at temperature between 500 and 600 oC. It is also found that the SmCo phase transformation takes place from SmCo5 to Sm2Co17 with elevating deposition pressure from 0.2 to 0.6Pa. The post-annealing temperature has a great effect on the phase transition and the decomposition of the SmCo phase. It is found that the magnetic properties of the films including coercivity are dependent both on the deposition pressure and the annealing temperature. Thickness of Cu buffer layer also affects the crystallization of Sm Co phase and its magnetic properties. In-plane coercivity value is as high as 38 kOe when the SmCo film deposited at 0.2Pa pressue and annealed at 550 oC. However, the coercivity drops down to 15 kOe when the annealing temperature is 600 oC because of the decomposition of SmCo phase. The results suggest that single phase SmCo thin films with high coercivity and the composition control can be effectively achieved by simply adjusting the synthesis parameters in magnetron sputtering system.