| Because of the potential applications such as ultrafast optomagnetic excitation, spin switching and multiferroics, Rare-earth orthoferrites R=rare earth) have attracted much attentions. It is well known that there are two magnetic subsystems of R3+and Fe3+in RFeO3system. Plenty of interesting phenomena are due to the competition of Fe-Fe, R-Fe, and R-R interactions in these materials. The system has gained considerable interest as a result of their superfast domain wall motion velocity, superior magneto-optical property, and ultrafast optomagnetic response.Many difficulties are encountered in growing RFcO3single crystal because of their melting characteristics and high melting point (over1600℃). To get high quality RFeO3single crystal, many previous attempts were conducted by several conventional methods such as flux method, hydrothermal method and IPE method. It is a pity that the obtained RFeO3crystals are still suffered from many kinds of problems. In this work, the influence of the growth parameters such as crystal size (diameter), growth rate, shape of solid-liquid interface, rotating rate and atmosphere environment were systematically investigated. Some problems such as growth striation, cracks and composition segregation are also studied.Optical floating-zone technique was successfully used to prepare high quality NdFeO3single crystal. Experiment with different growth rates show that higher growth rate is in favour of the NdFeO3crystal growth. With reducing the growth speed, stabilization of molten zone for NdFeO3crystal is very hard to maintain and thus evident cracks and defects resulted, in accordance with the metallurgical microscope and SEM images, which were obtained from the cross-sections cut at the same distance as the beginning of growth for the crystal samples at different growth rates. EDX analysis indicates that the cracks and defects were a result of deficiency in Fe element. A lower zoning speed results in more duration of evaporation of the Fe element. The best crystalline quality was observed in NdFeO3crystal grown at the rate of9mm/h, which was further confirmed by FWHM of the high-resolution X-ray rocking curve. Although there are many factors such as atmosphere, thermal stress and so on, growth rate plays an key role in single crystal growth for the rare-earth orthoferrites system.We designed a twice melting method to improve the crystalline quality of RFeO3system. By comparing the high-resolution X-ray rocking curves of the crystal via traditional optical floating-zone technique to that of the crystal via the twice melting method, it is found that the quality of the single crystal was improved by the obviously enhanced characteristic peak and the decreasing of FWHM.Powder X-ray diffraction (XRD) and Laue analysis show that the crystal prepared by Optical floating-zone technique is single phase and high quality. There is an obvious spin reorientation transition with the gradual transition of the Fe3+magnetic moment ordering from (Gz,Mx)-type ordering at low temperatures to (Gx,Mz)-type ordering at high temperatures between100K and170K. The hysteresis loops are very narrow with much small coercivity in the spin reorientation region. At low temperatures, the decrease of magnetization may attribute to the appearance of magnetic compensation. A peak induced by the antiferromagnetic ordering of Nd3+sublattice at2.4K in the specific heat curve, and the kink-like behavior of specific heat over temperature versus temperature shows the thermal variation in the spin reorientation region. |
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