T. I. Arbuzova

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.

Magnetic susceptibility of nanostructural manganite LaMnO3 + δ produced by mechanochemistry method

The mechanochemical method is shown to be a relatively simple method for producing nanostructural manganites LaMnO3 + δ with crystallite size D ≥ 10 nm. An increase in the treatment duration in a planetary mill from 1 to 13 h decreases the size D and increases microstrains. The Curie temperature of the nanostructural manganites decreases insignificantly and the phase transition is smeared as D decreases. A decrease in the unit-cell volume and the temperature dependences of the inverse magnetic susceptibility 1/χ(T) indicate an increase in the Mn4+ ion concentration with the milling duration. The variation of the magnetic properties of LaMnO3 + δ nanostructural powders is explained by the competition of a number factors, such as variations of the composition, the cation-sublattice defect structure, the size effect, and the microstrain level.

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.

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.

Magnetic susceptibility anisotropy and low-dimensional antiferromagnetism of CuO

The anisotropy of the magnetic susceptibility χ and the influence of oxygen vacancies in CuO single crystals on it are investigated. The temperature dependences of χ(T) along the a, b, and c axes in the range 60<T<600 K and the behavior of the field dependence of the magnetization σ(H) above and below the Néel temperature TN are plotted for a crystal before and after heat treatment. The χ(T) curves have the form characteristic of low-dimensional systems, which become three-dimensional when the temperature is lowered. The character of the χ(T) curves remains unchanged after annealing. Oxygen vacancies have practically no influence on the a-axis magnetic susceptibility, but they alter the absolute values of the b-and c-axis susceptibilities. The significant effects of reducing the oxygen concentration include a decrease in the magnitude of the low-temperature anomaly (increase) in χ and an increase in the minimum value of χ. The results of the calculations of the exchange parameter I/k and the g factor are discussed in terms of the Heisenberg and Ising models for a one-dimensional system.

Effect of electron irradiation on the paramagnetic state of La0.67Ca0.33MnO3

The effect of point defects on the magnetic properties of La0.67Ca0.33MnO3 polycrystals and single crystals has been studied. The magnetic susceptibility χdc of the initial samples and samples irradiated by electrons to the maximum dose F = 9 × 1018 cm−2 has been measured in the temperature region 80 K < T < 650 K. Local variations of Mn-O-Mn bond angles and lengths result in a nonmonotonic dose dependence of the Curie temperature TC. At high doses of electron irradiation, F ≥ 5 × 1018 cm−2, the temperature of the transition from the ferromagnetic to polaron state in a single crystal is found to increase. In the paramagnetic region close to TC, ferromagnetically ordered polarons are observed to exist, while at T > 1.2TC, localization of eg electrons initiates formation of paramagnetic polarons with a higher magnetic moment. Electron irradiation stimulates persistence of magnetic polarons up to higher temperatures T > 2TC.

Multiphase magnetic state of Ca1 − x La x MnO3 − δ single crystals (x = 0.03, 0.05, 0.07) containing oxygen vacancies

Magnetic properties (magnetization, dynamic and static susceptibility) and transport properties (resistance and magnetoresistance) have been studied in a temperature range of 2–600 K in magnetic fields to 90 kOe for single crystals of Ca1 − xLaxMnO3 − δ with a weak electron doping (x ≤ 0.07) grown in argon and oxygen atmospheres. The magnetic state of Ca1 − xLaxMnO3 − δ single crystals is multiphase. Below T = TN(G) ∼ 110 K, in all the crystals there coexists an AFM G phase with an FM contribution and an AFM C phase. In crystals with x = 0.07, a transition from the paramagnetic phase into the AFM C phase occurs in part of their volume below T ∼ 130–150 K. In crystals with x = 0.05 annealed in oxygen, an anomaly of paramagnetic susceptibility is observed near T* ∼ 270 K, which is related to the formation of FM clusters near defects. At x = 0.05 and 0.07, AFM correlations are retained in the paramagnetic state (to 600 K). The differences in the magnetic and transport properties of single crystals grown in argon and oxygen are explained by the various content of oxygen vacancies and by their possible ordering.

Surface magnetism of nanocrystalline copper monoxide

The effect of the surface on the magnetic susceptibility of nanopowders of the CuO semiconducting antiferromagnet was studied. Single-phase nanopowders with nanoparticles 15, 45, and 60 nm in size were prepared through copper vapor condensation in an argon environment, with subsequent oxidation of the copper. The temperature dependences of the magnetic susceptibility of the nanopowders differ qualitatively from the χ (T) relations for bulk samples. In the region 80≤T≤600 K, the magnetic susceptibility of nanopowders is inversely proportional to temperature and is described by the sum of contributions due to the bulk part of CuO and to the Cu2+ paramagnetic ions localized in surface layers. The paramagnetic contribution to the total susceptibility is shown to increase with decreasing particle size and sample density. A comparison of the χ (T) relations is made for nanopowders and for a dense CuO nanoceramic with grain size 5≤d≤100 nm prepared using the shock wave technique.

An inhomogeneous paramagnetic state of the LaMnO3 + δ nanoceramics prepared by shock-wave loading

The structural (at T = 300 K) and magnetic properties of LaMnO3 + δ nanoceramic materials prepared by shock-wave loading have been investigated in the paramagnetic region. The samples contain a mixture of the orthorhombic and rhombohedral phases in different ratios. The Curie-Weiss law is satisfied in the temperature range T > 440 K > 2TC, and magnetic polarons are generated in the vicinity of defects at temperatures in the range 300 K < T < 440 K. An increase in the concentration of Mn4+ ions leads to a decrease in the Curie temperature TC due to the decrease in the total number of Mn ions, the size effects of small particles, and the long-range elastic stresses.