S. V. Naumov

Temperature transition from 3D to quasi-1D antiferromagnetism in CuO single crystals

Abstract The susceptibility of CuO 1-δ and Cu 0.99 Li 0.01 O single and polycrystals has been investigated in the temperature range 56–600 K in magnetic fields of up to 13 kOe. It is shown that defects produce a strong effect on the magnitude and behaviour of X ( T ). Anisotropy of susceptibility in CuO single crystals both below and above T N has been revealed. The analysis of susceptibility has shown that in the T S= 1 2 directed along the b axis. A small X minimum was observed for the direction H ⊥ b in the vicinity of T =212 K. The anomalous behaviour of susceptibility above T N is explained by the quasi-1D magnetic structure comprising Cu-O-Cu antiferromagnetic chains oriented along [10 1 ]. The dependence X ( T ) for a polycrystal is well described by the Heisenberg antiferromagnetic chain model with S= 1 2 , I k =430 K and g =1.97. It has been found not possible to describe the dependences X ∥ ( T ) and X ⊥ ( T ) in single crystals by means of Heisenberg and Ising models: this may be due to both g -factor and I k anisotropies.

Effect of doping on the magnetic properties of the low-dimensional antiferromagnet CuO

The effect of doping with Li+, Zn2+, Ni2+, and Ga3+ ions on the magnetic susceptibility of the low-dimensional antiferromagnet CuO (TN=230 K) has been studied within a broad temperature range of 77–600 K. The solubility of impurity ions in the CuO lattice is low, ⩽3%. Impurity ions, similar to intrinsic defects, distort antiferromagnetic coupling and can shift the long-and short-range magnetic-order regions toward lower T.

Giant electric conductivity at the CuO-Cu interface: HTSC-like temperature variations

The temperature dependence of electric conductivity and current-voltage characteristics were studied in CuO single crystals with Cu films deposited onto natural faces by thermal evaporation in vacuum or by electrolysis. After electric (resistive) or thermal annealing of the samples, the conductivity of Cu films in this system significantly increases (by a factor of up to 1.5×105 and above) as compared to that of the control Cu films on a glass-ceramic substrate. The effect is attributed to an interfacial layer formed between CuO and Cu, the high conductivity mechanism in which is unclear. It is suggested that the giant electric conductivity and its HTSC-like temperature dependence, as well as nonlinear current–voltage characteristics of the samples can be due to the formation of superconducting regions with the critical temperatures significantly higher than 400 K.

Size effect of the thermal expansion of nanostructural copper oxide

The temperature dependence of the thermal expansion ΔL/L of samples of high-density nanostructural CuO ceramics with a crystallite size of 20, 70, and 90 nm was measured. The nanoceramics were obtained from coarse-grained CuO powders under converging spherical shock waves. It is found that, at temperatures T > 50 K, the thermal expansion coefficient α(T) of the nanoceramic samples increases with decreasing crystallite sizes and exceeds the value of α(T) of the CuO single crystal by a factor of 3.5 to 4.5. At temperatures T < 50 K, regions with zero and negative values of α(T) were revealed. The possible reasons for the increase in the thermal expansion coefficient of nanoceramics based on 3d-metal oxides are discussed.

A Magnetic resonance study of La1−xSrxMnO3 manganites

Single crystals of La0.9Sr0.1MnO3 and La0.8Sr0.2MnO3 manganites are examined using magnetic resonance in the temperature range 80–370 K. It is found that magnetostatic oscillations arise near the Curie temperature. The possible reasons for the appearance of additional lines in the ferromagnetic resonance (FMR) spectrum are considered, and the anisotropy field and the type of crystalline magnetic anisotropy in the La0.8Sr0.2MnO3 compound are determined. It is shown that the crystalline magnetic anisotropy in the La0.9Sr0.1MnO3 compound exhibits specific features associated with its type of crystal structure.

Charge segregation and a nonuniform magnetic state in donor-and acceptor-doped LaMnO3

A coordinated study of the magnetic, electrical, optical, and EPR properties of LaMnO3 single crystals doped by donors (7 at. % Ce) and acceptors (7 at. % Sr) revealed that in all cases, except undoped LaMnO3, charge segregation associated with large-scale crystal-field fluctuations occurs and the magnetic properties originate from the existence of ferromagnetic phase inclusions and localized ferrons in the matrix with a canted magnetic structure.

Magnetic properties of electron-irradiated quasi-layered manganites La2−2xSr1+2xMn2O7 (x=0.3, 0.35, 0.4)

The magnetic properties of La2−2xSr1+2xMn2O7 polycrystals (x=0.3–0.4) are studied over a broad temperature range 80–600 K. Quasi-two-dimensional manganites have a complex magnetic structure that undergoes several transitions from one type of magnetic ordering to another. A specific feature of these manganites is a hyperbolic dependence of inverse susceptibility in the transition region from the magnetically ordered to paramagnetic state for T>360 K. This suggests the onset of ferrimagnetism. Electron irradiation to a fluence Φ=1×1018 electrons/cm2 is shown to have no effect on the long-range magnetic order while favoring the formation of paramagnetic polarons and of an inhomogeneous paramagnetic state.

The evolution of the optical spectra of LaMnO3 under light n-and p-type doping and the separation of phases

The optical absorption spectra of LaMnO3 single crystals, pure and slightly doped with Ce and Sr (7%), were studied in the middle IR region. Ellipsometry measurements were performed in the visible region to determine real and imaginary permittivity parts. The energies of the onset of indirect transitions in LaMnO3 at 293 and 80 K were found to be 0.30 and 0.40 eV, respectively. Impurity absorption bands corresponding to transition in hole and electron clusters were observed. The anomalous temperature dependence of IR absorption was explained by the existence of conducting droplets in the insulated doped crystal matrices at temperatures below the temperature of the appearance of a ferromagnetic contribution. The existence of such a contribution was proved by studying the equatorial Kerr effect. Because of the separation of phases, manganites slightly doped by a nonisovalent admixture can be treated as optically nonuniform media whose properties are similar to those of a composite system with metallic inclusions.

Magnetorefractive effect in La{sub 0.7}Ca{sub 0.3}MnO{sub 3} in the infrared spectral range

The reflection and magnetic reflection spectra, magnetic resistance, electrical properties, and equatorial Kerr effect in La0.7Ca0.3MnO3 crystals have been complexly investigated. The measurements have been performed in wide temperature and spectral ranges in magnetic fields up to 3.5 kOe. It has been found that magnetic reflection is a high-frequency response in the infrared spectral range to the colossal magnetore-sistance near the Curie temperature. Correlation between the field and temperature dependences of the magnetic reflection and colossal magnetoresistance has been revealed. The previously developed theory of the magnetorefractive effect for metallic systems makes it possible to explain the experimental data at the qualitative level. Both demerits of the theory of the magnetorefractive effect in application to the magnets and possible additional mechanisms responsible for the magnetic reflection are discussed.

Intercluster conduction in lightly doped La1 − xCaxMnO3 manganites in the paramagnetic temperature range

The magnetotransport and magnetic properties of La 1 − xCaxMnO3 polycrystalline samples (x = 0–0.3) annealed under vacuum and in the oxygen environment are investigated in the temperature range from 77 to 400 K. The magnetic studies of lightly doped manganites reveal persistence of short-range magnetic order up to a temperature T* ≈ 300 K, which is about 2–3 times higher than their Curie temperature TC. The temperature dependence of the electrical resistivity measured from T* down to nearly T ≈ TC is fitted by the relation logρ ∼ T−1/2, which is characteristic of granular metals with electrons tunneling among nanoclusters of magnetic metals embedded in a dielectric host. The magnetoresistance of polycrystalline samples annealed in the oxygen environment has been observed to increase. The electrical, magnetic, and magnetotransport properties of the manganites can be accounted for by the formation of magnetic nanoclusters below T*, tunneling (or hopping) of carriers among the nanoclusters, variation in the magnetic cluster size, and tunneling barrier thickness with variations in temperature and magnetic field strength, as well as by the effect of annealing in different media on the cluster properties.