Magnetic Properties And Magneto-Optical Kerr Activity Of La/Co Substituted Strontium Ferrites

The magneto-optical Kerr effect is important in magneto-optical recording technology. A vigorous of interest has been focused on this subject in the past couple of decades as it is used for optical readout of magnetically stored information in erasable video and audio disks. Other applications of the magneto-optical Kerr effect are microscopy for domain observation, study of the behaviour of nanodot chains, cocrcivity measurement and magneto-optics are widely used as a very sensitive and non destructive technique to investigate magnetism at the atomic scale, as its origin relies on the spin-orbit coupling. A strong magneto-optical Kerr activity and a large Kerr angle are required for the applications in high-density erasable magneto-optical memory devices and magneto-optics based sensors. The rare-earth elements in magneto-optical materials play an important role in the magneto-optical Kerr effect.M-type strontium hexaferrites are the most important ceramic permanent magnets and have attracted a great deal of interest. Their high magnetic coercive force, saturation magnetization and remanence enable them for different applications such as microwave devices, magnetic recording media and high frequency electromagnetic devices. A considerable amount of work has been done to improve the performance of strontium hexaferrites by various methods. The ion substitution technique is also used to improve the performance of the M-type hexaferrites. It is reported that by introducing rare-earth La3+ and transition Co2+ the performance of M-type strontium hexaferrites can be improved, but most of the works were focused on the magnetic properties, microwave absorption properties or thermoelectric power. However, no information is available on the magneto-optical Kerr effect studies of rare-carth La or transition Co substituted strontium hexaferrites in the literature.M-type strontium ferrites Sr1-xLaxFe12O19 and SrFe12-xCoxO19 were prepared by the ceramic process, where x varies from 0 to 0.20 with 0.05 increments. The process consists of seven main steps:weighing, milling, pre-sintering, milling, pressing, sintering and polishing. The X-ray diffractometer, vibrating sample magnetometer, scanning electron microscope and magneto-optical ellipsometer were used to characterize the samples. The influence of the amount of substituted La/Co on the structure, magnetic properties and magneto-optical Kerr effect were systematically studied. The results obtained as follows:1, La substituted strontium ferritcs Sr1-xLaxFe12O19(x=0,0.05,0.10,0.15,0.20)M-type hexagonal structure was already formed at 1000℃, but a-Fe2O3 phase and LaFeO3 phase were also found by XRD. The relative intensity of the diffraction peaks of M-phase became stronger with increasing sintering temperature, and pure phase was obtained as the sample was sintered at 1290℃. The lattice parameter a initially remains constant, up to a substitution of x=0.15, then decreases. The lattice parameter c decreases monotonically with increasing x.The suitable amount of La3+ substitution may remarkably influence the saturation magnetizationσs and coercivity Hc, both of them increase with increasing x. The coercivity of each sample is no more than 160 kA·m-1, which helps detecting the magneto-optical Kerr activity with the in-plane applied field used in this work.The magneto-optical activity of pure strontium ferrite is very poor. Rare earth La doping of M-type strontium ferrite is an effective route to induce a strong magneto-optical activity. The magneto-optical activity became stronger with increasing x, the magneto-optical activity of sample Sr0.80La0.20Fe12019 raises to almost 1% around 3 eV.2, Co substituted strontium ferrites SrFe12-xCoxO19 (x=0,0.05,0.10,0.15,0.20)All the samples sintered at 1290℃are single M-type hexagonal fcrrites and no other impurity was found in the final powders. The hexagonal lattice constant c decreases monotonically with the substituted amount x. The maximum variation is never over 0.3%. The lattice constant a initially shows a slight but smooth increase, then decreases at the largest amount of substituted Fe.The undoped sample exhibits a high coercivity. The cocrcivity Hc of Co-substituted samples rapidly decreases, and the coercivity remains stable around 56 kA·m-1 for x values between 0.05 and 0.20. The saturation magnetizationσs decreases at first, and then it increases slight gradually with the Co addition.The magneto-optical activity of SrFe11.8Co0.20O19 is strong compare to the poor magneto-optical activity of the pure ferrite. These results show that it is possible to induce a noticeable magneto-optical activity by substituting Fe ions by Co ions.