Investigation Of Ferromagnetic Coupling Mechanism In Mn Doped SnO₂

Diluted magnetic semiconductors (DMSs) have become a focus of considerable interests in recent years due to their potential application in the new field of research and application known as spintronics. However, the origin of the ferromagnetism challenges the researcher's understanding of magnetic order in solids. In this thesis, we have investigated the influence of charge carrier on the structural, magnetic and electrical properties of Sb and Cu doped Sn_1-xMn_xO₂. The main results are generalized as follows.Sn_1-xMn_xO₂ (x = 0.01, 0.03, 0.05), Sn_1-x-yMn_xSb_yO₂ (x = 0.01, 0.03; y = 0, 0.005, 0.02, 0.04), Sn_1-x-zMn_xCu_zO₂ (x = 0.01, 0.03, 0.05; z = 0.005, 0.02) polycrystalline samples were prepared by a solid-state reaction method. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and physical property measurement system (PPMS) were used to measure the crystal structure, the chemical binding state, the electrical properties and the magnetic properties, respectively.It is found that all Sn_1-x-yMn_xSb_yO₂ samples are crystallized in single phase with a rutile structure which is the characteristic of SnO₂. The lattice expansion and reducing ferromagnetism are observed with increasing Sb concentration. In particular, at a fixed Mn composition of x = 0.03, it is found that the ferromagnetic behaviors observed in undoped Sn0.97Mn0.03O₂ are suppressed due to the antimony doping; and the resistivity decreased drastically with Sb concentration up to 0.04 in Sn0.97-yMn0.03SbyO₂. Combined with all the results, we conclude that the ferromagnetic interaction between Mn³⁺ ions is destroyed by the mobile charge electrons which are introduced by Sb⁵⁺ substitution for Sn⁴⁺.To investigate the role of hole in ferromagnetism, Cu is co-doped into the Sn_1-xMn_xO₂. It is found that the doping of copper, which acts as Cu²⁺ in SnO₂, brings significant enhancement of the ferromagnetism in Sn_1-xMn_xO₂, while the samples remain electrical insulating. In particular, at a fixed Mn composition of x = 0.03, Curie temperature (TC) increases up to 40.0 K at Cu concentration z = 0.02 from TC = 27.0 K at undoped Sn0.97Mn0.03O₂.The observed ferromagnetism in Mn-doped SnO₂ is discussed on the basis of the formation of bound magnetic polarons (BMP): a BMP consists of a localized hole and many Mn³⁺ ions around the localized center, the overlapping and interaction of neighboring BMPs produce long-range ferromagnetic coupling. In Sn_1-x-yMn_xSb_yO₂, the localized holes will be compensated by the electrons introduced by the Sb doping, which will induce a decrease of the concentration of BMPs and suppression of the ferromagnetism. For the Cu doped Sn0.97Mn0.03O₂, the concentration of localized hole will be increased by substituting Cu²⁺ for Sn⁴⁺, which would introduce more BMPs with the proximate Mn, hence the ferromagnetism are enhanced.