Magnetic semiconductors are being extensively investigated because of their promise to provide new functionalities and device concepts for spintronic applications. Nevertheless, it remains a challe...

Magnetic Properties of Semiconducting Spinel CdCr 2 S 4 Nanostructured Films Grown by Low-Pressure Metal–OrganicChemical Vapor Deposition

We report here a study on the evolution of structural and electronic properties of the lacunar spinel compounds GaV4S8 with charge doping. In this ferromagnetic (FM) Mott insulator, the heterovalent substitutions of Ga3+ by Zn2+ or Ge4+ allow induction of charge doping either by holes or by electrons. We show that electron-doped GaV4S8 displays a bulk, negative, and colossal magnetoresistance (CMR) with a relative drop of resistivity reaching −80% at 7 T in the vicinity of the Curie temperature. Conversely, hole-doped GaV4S8 does not display any negative CMR but a classical positive magnetoresistance. This asymmetric electron–hole doping effect challenges the common view stating that CMR effects in doped FM Mott insulators depends only on the density of carrier and not on their electron/hole nature. We show that a simple model based on multiorbital effects and Hund’s rule is able to capture the presence (absence) of negative CMR in electron- (hole-) doped GaV4S8.

Negative Colossal Magnetoresistance Driven by Carrier Type in the Ferromagnetic Mott Insulator GaV4S8

The origin of colossal magnetoresistance and colossal magnetocapacitance in a CdCr2S4 system was investigated. Thermoelectric-power and electronic spin resonance spectra reveal that the magnetic polaron is responsible for the colossal magnetoresistance in the n-type sample. The existence of magnetic polarons in the paramagnetic insulting matrix forms an intrinsic Maxwell-Wagner system, leading to the appearance of colossal magnetocapacitance. Being consistent with the evolution of magnetic polarons upon cooling, the Maxwell-Wagner system is valid around insulator-metal transition, where the resistance derived from impedance spectroscopy matches perfectly with DC resistance.

http://iopscience.iop.org/article/10.1209/0295-5075/104/17005/pdf

Magnetic-polaron–induced colossal magnetocapacitance in CdCr2S4

The p-type CdCr2Se4 exhibits semiconducting conductivity while n-type CdCr2Se4 displays a semiconductor–metal transition and a concomitant colossal magnetoresistance near the Curie temperature. Here, we investigate the pressure effect on the conductivity for both types of compounds. We show that the resistance of p-type sample decreases continuously upon compression, while the semiconducting behavior dominates up to 27.9 GPa. For the n-type sample, the semiconductor-metal transition is suppressed gradually with the increase in pressure; meanwhile, the magnetoresistance becomes less and less pronounced and is negligible at 9.2 GPa. Combined with in situ high-pressure magnetization, x-ray diffraction, and Raman spectra investigations, a ferromagnetic ground state is deduced in the pressurized n-type CdCr2Se4 before the structural transition, which is in stark contrast to the pressure effect on n-type HgCr2Se4.

Pressure-tuned colossal magnetoresistance effect in n-type CdCr2Se4

CdCr2S4 single crystal was reported by Hemberger et al. to be multiferroic with the evidences of relaxor ferroelectricity and colossal magnetocapacitance (CMC), but whether these effects are intrinsic is under debate. Recently, we reported a one-to-one correlation between CMC and colossal magnetoresistance (CMR) in CdCr2S4 polycrystalline samples, and argued that CMC could be explained by the superposition of the CMR and the Maxwell–Wagner effects. In this paper, we further examined the magnetic, dielectric, and electric transport properties of CdCr2S4 and CdCr1.8In0.2S4 single crystals before and after annealing in cadmium vapor. The CdCr2S4 single crystal sample has no relaxor ferroelectricity and CMC, and in contrast to the CdCr2S4 single crystal reported by Hemberger et al. , only the annealed CdCr1.8In0.2S4 displays CMC, but still does not exhibit the relaxor behavior. At the same time, it also shows CMR. All these results are in accordance with the results of our polycrystalline samples, and further confirm the resistive origin of the CMC in the CdCr2S4 system.

Magnetocapacitance in CdCr 1.8 In 0.2 S 4 Single Crystal Annealed in Cadmium Vapor

The correlation between colossal magnetocapacitance (CMC) and colossal magnetoresistance (CMR) in CdCr2S4 system has been revealed. The CMC is induced in polycrystalline Cd0.97In0.03Cr2S4 by annealing in cadmium vapor. At the same time, an insulator-metal transition and a concomitant CMR are observed near the Curie temperature. In contrast, after the same annealing treatment, CdCr2S4 displays a typical semiconductor behavior and does not show magnetic field dependent dielectric and electric transport properties. The simultaneous occurrence or absence of CMC and CMR effects implies that the CMC in the annealed Cd0.97In0.03Cr2S4 could be explained qualitatively by a combination of CMR and Maxwell-Wagner effect.

Y. L. Huang

Papers

Annealing induced colossal magnetocapacitance and colossal magnetoresistance in In-doped CdCr2S4

Annealing induced colossal magnetocapacitance and colossal magnetoresistance in In-doped CdCr2S4