Low-Dimensional Magnetic Characteristics Of Several Non-Ferromagnetic Materials

The discovery of room temperature ferromagnetism in low-dimensional non-ferromagnetic materials not only overturns the traditional impression that ferromagnetism is originates from the exchange interaction between the magnetic ions, but also provides opportunities for searching novel functional materials in magnetism field. Focusing on the low-dimensional magnetic characteristics of non-ferromagnetic materials, studies of ferromagnetism and magnetoresistance are carried out on several zero- and two-dimensional non-ferromagnetic systems. The main innovative conclusions are as follows:1. Anion defects introduce room temperature ferromagnetism into oxide and sulfide nanoparticles. NiO, ZnO, sphalerite CdS, and wurtzite CdS nanoparticles have been fabricated by thermal decomposition, hydrolysis-deposition, and hydrothermal method, respectively. All types of nanoparticles behave distinct ferromagnetism at room temperature. The saturation magnetization increases with the decreasing particle size. There are anion defects present in particle surface through X-ray photoelectron spectroscopy, Raman spectrum, and photoluminescence characterizations. The defect concentrations increases with the decreasing particle size, which agrees with the magnetization variation. First principle calculation and post-annealing results further confirm that room temperature ferromagnetism is induced by anion defects.2. Defects as well as boundary effects are the origins of high Curie temperature ferromagnetism in transition metal disulfides nanosheets. Atomically thin MoS₂ and WS₂ nanosheets have been synthesized by a novel chemical vapor deposition method. Intrinsic ferromagnetism is present in both samples, and the saturation magnetization reaches 0.8 emu/g and 0.2 emu/g, respectively. The measurement and fitting of magnetization at different temperatures reveal that the Curie temperatures are 865 K for MoS2 and 820 K for WS2. High resolution transmission electron microscopy images confirm that defects and dislocations present inside of both nanosheets. Combining with the fitting critical exponent, the observed two-dimensional ferromagnetism is attributed to the defects and boundaries of the nanosheets.3. Weak localization and Lorentz force lead to magnetoresistance behavior in defect-rich graphene. Large area graphene has been grown by chemical vapor deposition method. Raman characterization reveals that atomically sharp defects exist in monolayer graphene. The relationship between resistivity and temperature behaves as a semiconductor, and the majority carrier is hole. The defect-rich graphene shows obvious magnetoresistance effect. Deducing from the temperature and anisotropic characteristics of magnetoresistance curves, positive magnetoresistance originates from Lorentz force, and negative magnetoresistance originates from weak localization. Moreover, the weak localization magnetoresistance can be tuned by bias voltage, which is attributed to the dephasing effect of carriers in strong drift field.