The Electrical Transport Behavior And Electronic Structure Of CrO₂under High Pressure

Electrons, which bound on the solid atoms, load charge and spin at the same time,and accompanied by the orbital degrees of freedom. Therefore the electronic transportand magnetic properties of the solid are closely related. Ferromagnetic half-metaloxides, perovskite manganite oxides such spintronic materials are prototypes ofstrongly-correlated electron systems. They have the properties of spin polarization,spin-dependent scattering and spin-dependent tunneling effect etc, which are a seriesof quantum interference effects, accordingly becoming the frontier and hot point ofcondensed matter physics. New functional materials for spintronics as two differentbehaviors （metal and non-metallic） in half-metallic ferromagnetic oxides CrO₂, thefurther study and breakthroughs will provide the reference value for the research onspintronics.Volume decreased, distance shortened between the atoms or molecules, theoverlap between the electron wave function enhancement for substances underpressure, which are the driving force for the occurrence of structural phase transitionand the new band structure. Consequently, the study for electrical transition behaviorof material internal carrier behavior on high-pressure effect, will provide the mostdirect and clear physical picture for the new discovery of structural phase transition.Due to the sophisticated and limitation of high pressure experimental technique, mostresearches on the material of half-metallic ferromagnetism CrO₂were carried out withtheoretical calculations or under low pressure.In this paper, both in-situ electrical transport properties measurement andCASTEP were employed to investigate the CrO₂variation about structural phasetransition under high pressure. High pressure in-situ resistivity measurement, in-situHall effect measurement and first-principles calculation were employed to conduct ananalysis and research synthetically on resistivity, magnetoresistance, Hall coefficient,carrier concentration, Hall mobility and electronic structure of CrO₂along withpressure.High pressure in-situ resistivity measurement, in-situ Hall effect measurementand magnetoresistance measurement indicate: the carrier behavior of CrO₂have discontinuous changes, which is consistent with the pressure for structural phasetransition from rutile-type to CaCl₂-type in many articles of theoretical studies. And itindicates that the phase transition is accompanied by mutations of the carrier behavior.These mutations were associated with the d-electron band splitting of Cr4+ion andCrO₂double exchange interaction.Meanwhile, the result also shows the negative magnetoresistance phenomenon,and magnetoresistance of CrO₂presents linearly decreases as the magnetic fieldincreases. Magnetoresistance and field present the characteristic of thequasi-symmetry. At low pressure, the variation of magnetoresistance with field showsa good linear relationship. In low magnetic field, spin tunneling effect in grainboundary is the leading causes for magnetoresistance. At high pressure and magneticfield, magnetoresistance is affected by two effects, including the interactions in grainboundary under high pressure and spin-dependent scattering aroused by straymagnetic, while the latter plays a main role. As a result, the magnetic moments ofgrain tend to ordered arrangement with magnetic field increasing, and resistancereduces.The first principle theory was used to calculate the material properties of CrO₂under high pressure. The reasons for the half-metallic behavior of CrO₂sample werefurther expatiated: spin-up electrons across the Fermi level, so that CrO₂samplesshowed metallic, while spin-down electrons formed an energy gap of about1.2eV atthe Fermi level, causing the semiconductor or insulator property. As the pressureincreasing, the energy gap of CrO₂spin-down minority carrier was narrowed whichled to the weaken of CrO₂semiconductor property, however majority carrier stillmaintained metallicity, so in macroscopic view the resistivity decreased with thepressure increasing, which further explained the reasons for the decrease of theresistivity with increasing pressure in theory.