Magnetism And Large Magnetoresistance In Transition Metal Compounds

Owing to the unique d electrons, transition metal compounds exhibit richly colorful physical properties, such as high temperature superconductivity, large magnetoresistance, quantum mag-netism, and recently widely investigated topological protected surface state and Weyl semimetal etc In this dissertation, we investigated the magnetic properties and large magnetoresistance effect of several transition metal compounds. We synthesized three bulk diluted magnetic semiconductor(DMS) systems (La,Sr)(Cu, Mn)SO, (Ba, K)(Cd,Mn)2As2 and (Sr,K)(Zn,Mn)2As2,found gi-ant linear magneto-resistance (MR) induced by disorder in PtBi2, and observed pressure induced superconductivity and larger MR in NbTe4. The main original results are summarized as the fol-lowings.(1),we synthesized three bulk DMS systems (La,Sr)(Cu,Mn)SO,(Ba,K)(Cd,Mn)2As2 and(Sr,K)(Zn,Mn)2As2. As a wide band gap p-type oxide semiconductor, LaCuSO satisfies all the conditions forecasted theoretically to be a room temperature DMS. The Curie temperature (Tc) of La1-xSrxCu0.925Mn0.075SO (x = 0, 0.025, 0.05, 0.075 and 0.1) is around 200 K as x ? 0.05, which is among the highest TC records of known bulk DMS materials up to now. The system provides a rare example of oxide DMS system with p-type conduction, which is important for the formation of high temperature spintronic devices. K and Mn co-doped BaCd2 As2 system was also found to be a bulk diluted magnetic semiconductor. The Ba1-yKyCd2-xMnxAs2 system has a hexagonal CaAl2Si2-type structure with the Cd2As2 layer forming a honeycomb-like network. The Mn con-centration reaches up to x ?0.4. Magnetization measurements show that the samples undergo ferromagnetic transitions with Curie temperature up to 16 K. With low coercive field less than 10 Oe and large magnetoresistence of about -70%, the hexagonal structured Ba1-yKyCd2-xMnx As2 can be served as a promising candidate for spin manipulations. Moreover, We also synthesized(Sr,K)(Zn,Mn)2As2 bulk diluted magnetic semiconductor with decoupled charge and spin dop-ing. Similar with (Ba, K)(Cd, Mn)2As2, it owns a hexagonal CaAl2Si2-type structure with the(Zn,Mn)2As2 layer forming a honeycomb-like network. Magnetization measurements show that the sample undergoes a ferromagnetic transition with a Curie temperature of 12 K and magnetic moment reaches about 1.5?B/Mn under ?0H = 5 T and T = 2 K. Surprisingly, a colossal negative magnetoresistance, defined as [?(H) — ?(0)]/?(0), up to -38% under a low field of ?0H = 0.1 T and to -99.8% under ?0H=5 T,was observed at T= 2 K. The colossal magnetoresistance can be explained based on the Anderson localization theory.(2), We synthesized nonmagnetic PtBi2 single crystals and observed a giant linear magneto-resistance (MR) up to 684% under a magnetic field ?0H =15TatT = 2K. The linear MR decreases with increasing temperature, but it is still as large as 61% under ?0H of 15 T at room temperature.Such a giant linear MR is unlikely to be described by the quantum model as the quantum condition is not satisfied. Instead, we found that the slop of MR scales well with the Hall mobility, and it can be well explained by a classical disorder model.(3), Owing to the existence of charge density wave (CDW) and its competition with super-conductivity, transition-metal chalcogenides have long attracted much attention. Here we report pressured induced superconductivity in a large spin-orbit(SO) coupled Quasi-One-Dimensional(1D) transition-metal chalcogenide NbTe4. When we applied high pressure, the CDW transition temperature is strongly suppressed, and superconductivity emerges. We also find large magne-toresistance(MR) in NbTe4.Actually, only a few CDW materials show large positive MR. These foundings help us sight into Quasi-One-Dimensional (ID) transition-metal chalcogenides.At last, I also briefly present one other research during my Ph. D period: negative MR and non-trival ? Berry phase in Weyl semimetal NbAs. We report the electric transport properties of NbAs, which is a Weyl semimetal candidate proposed by recent theoretical calculations and confirmed by recent angle-resolved photoemission spectroscopy (ARPES) data. We detected the negative magneto-resistance generated by the chiral anomaly in NbAs. Clear Shubnikov de Haas(SdH) oscillations have been detected starting from very weak magnetic field. Analysis of the SdH peaks gives the Berry's phase accumulated along the cyclotron orbits close to ?, consistent with the existence of Weyl points. We provide transport evidences for the existence of Weyl Fermions in NbAs.