Microstructure, Magnetic And Electronic Transport Properties Of Mn_x(M=Ti, Cr, Gd) Films

Due to the potential application in spintronics devices,3d transition-metal nitridesand rare-earth nitrides have been research hotspots in the field of material science. Sincetheir cations have partially filled3d and4f shells, these elements have a net moment.Among3d transition-metal nitrides, TiN is superconducting, CrN is antiferromagnetic,and the Ti1xCrxN solutions are ferromagnetic. Furthermore, the physical mechanism onthe metal-insulator transition of CrN near Néel temperature is still unclear. Amongrare-earth nitrides, GdN has a large moment of7μB/Gd3+and has potential applicationsin spin-filtering since it is theoretically half-metallic. However, the half-metallicity hasnot been experimentally confirmed.Epitaxial and polycrystalline Ti1xCrxN films, CrNxand GdNxfilms are fabricatedby facing-target reactive sputtering. Their phase composition, microstructure, valencestate, magnetic and electronic transport properties are investigated systematically.Ti1xCrxN films are metallic. At0.17≤x≤0.51, the films are ferromagnetic, whichoriginates from Cr2+N3Cr3+double-exchange interaction, rather than thecarrier-mediated mechanism. Ti0.53Cr0.47N film has the highest Curie temperature TCof~120K. The films with0.35≤x≤0.47show a metal-insulator transition near TCand thelow-temperature resistivity upturn in the films with0.10≤x≤0.47can be attributed to theeffects of weak localization and electron-electron interaction. At0.10≤x≤0.51, themechanism of negative magnetoresistance (MR) effect is similar to that of colossalmagnetoresistance (CMR) effect in the manganites, which is from the suppression ofmagnetic filed on the metal-insulator transition. MR in polycrystalline Ti0.22Cr0.78N filmis related to the localized magnetic moment scattering at grain boundaries. Theanomalous Hall effect (AHE) of epitaxial films is from the side-jump mechanism, whileAHE of polycrystalline films is scattering-independent.With the increase of nitrogen gas flow rates (fN2), CrNxfilms show a phase changefrom Cr2N+CrN mixture to single CrN. Meanwhile, there is a conductancetransformation from metallic to semiconducting. At fN2=100sccm, low-temperatureconductance mechanism can be described by Mott and ES variable-range hopping.Polycrystalline CrNxfilms fabricated at fN2=30and50sccm exhibit a metal-insulator transition at260280K, suggesting that the transition is affected by N-vacancy densityas well as substrate strain and grain boundaries.GdNxfilms are ferromagnetic with TCof3545K. The films are semiconductingand show a metal-insulator transition near40K. MR is dominated by the effect ofmagnetic field on metal-insulator transition that is also similar with CMR. Interestingly,the film fabricated at fN2=20sccm shows low-field magnetoresistance, which isattributed to the spin-polarized intergrain tunneling. With the increase of temperature,the ordinary Hall coefficient transforms from positive to negative, while the anomalousHall resistivity changes from negative to positive. The carrier densities are in the rangeof1.0×10202.5×1021cm3.