Percolation effects in the conductivity and magnetoresistance of compacted chromium dioxide powder

The resistive and magnetoresistive properties of a submicron (120 nm) compacted CrO₂ powder with a Curie temperature of ∼385 K were investigated in the temperature range of 5–430 K for magnetic fields of up to 1.6 T. The specimen revealed a nonmetallic temperature dependence of resistance and high negative magnetoresistance (MR) (20%) in a low-temperature range. The MR magnitude rapidly decreased with an increase in temperature and was less than 0.3% for T > 200 K. Such MR behavior is shown to be characteristic for granular systems with spin-dependent intergranular tunneling. Some features of MR behavior in a low-temperature range (lower than 40 K) can be attributed to the percolation nature of the tunnel conductivity of such a granular system in the presence of a limited number of dominant conducting current paths.     

New opportunities of variation in the particle sizes and properties of chromium dioxide

It is experimentally found that clusters containing 24 W or Mo atoms act as initial nuclei in CrO₂ crystallization. Joint application of W or Mo compounds with Sb(III) compounds makes it possible to change the size of CrO₂ particles in a very wide range. Thin (10–13 nm) CrO₂ particles have a coercivity of about 450 Oe and are not superparamagnetic.     

Thermal stability of modified chromium dioxide nanopowders with various magnetic properties obtained by hydrothermal route

Melt modification is a method commonly used when enhancing mechanical and microstructural properties of various alloys. The addition of heterogeneous nucleation sites generally refines the given microstructure and enhances the toughness of the material. Ferroalloys based on high-melting elements require a longer incubation time in the melt which makes them liable candidates for modification agents. A research on the effect of melt modification was conducted on an indefinite chill cast iron alloy commonly used for working layers in rolls. The goal of the research was to determine the effect of ferrovanadium and ferrotungsten melt treatment with the use of thermal analysis along with thermodynamic modelling using the CALPHAD approach. The results show a decrease in the measured liquidus temperature of the modified alloy along with a possible second eutectic reaction hidden inside the main graphite/cementite eutectic reaction. Scanning electron microscopy revealed small and evenly distributed MC type carbides rich in V with W and other carbide-forming elements. The results presented in this work enable a better understanding of the impact melt modification has on the development of microstructure and subsequently the improvement in mechanical properties of the alloy.     

Low-temperature spin relaxation in compacted powders of CrO<Subscript>2</Subscript> solid solutions with iron impurities

The effect iron impurities have on the low-temperature behavior of the magnetoresistance of compacted powders of CrO₂ ferromagnetic half-metal with anisotropic particle shape is investigated. The strong dependence of magnetoresistance on the spin relaxation rate during specimen magnetization reversal is revealed. It is shown that the addition of Fe impurities increases the powders’ coercive force and reduces tunnel magnetoresistance. A resonance mechanism of carrier tunneling in powders of CrO₂ solid solutions with iron impurities is proposed.     

Influence of iron ions on morphology and magnetic and electrical properties of vanadium dioxide

Russian Journal of General Chemistry -     

Magnetic behavior of doped VO2 nanoparticles

Nanoparticles of vanadium dioxide doped with chromium and iron are obtained using the hydrothermal technique. Their morphology is studied by means of TEM and XRD. The temperature dependence of their magnetic characteristics is investigated using the magnetic susceptibility method, and the parameters of exchange interaction between magnetic atoms are estimated. It is shown that the introduction of a doped element changes the mechanism of nanoparticle growth and lowers the temperature of the semiconductormetal transition to 150–200 K.     

The Effect of Biocompatible Coating Layers on Magnetic Properties of Superparamagnetic Iron Oxide Nanoparticles

The g-factor of the exteremely proton-rich nucleus ²³Al(T _{1 / 2} = 0.47 s) has been measured for the first time, applying β-NMR technique on this nucleus implanted in Si. The obtained ∣g∣ = (1.58 ± 0.2) suggests that the spin of the ground state of ²³Al is 5 / 2. The magnetic moment is determined as ∣μ∣ = (3.95 ± 0.55) μ _N .     

Synthesis and Optical Properties of Boehmite Nanoparticles Doped with 3d-Metal Ions

Russian Journal of General Chemistry - Nanorods and nanoplates of boehmite doped with copper, cobalt, and chromium ions were obtained by hydrothermal method. The resulting samples were...     

Magnetic anisotropy and anisotropic tunneling magnetoresistance of compacted half-metal CrO<Subscript>2</Subscript> nanopowders

Low-temperature magnetic and magnetoresistive properties of compacted ferromagnetic halfmetal CrO₂ powders with nanoparticle shape anisotropy are studied. Magnetic anisotropy induced by the formation of magnetic texture during compaction is revealed. It is shown that tunneling magnetoresistance anisotropy is associated with the difference between a sample’s rates of magnetization in the longitudinal and transverse fields.