E. Zubov

Phase transitions in TbMnO3 manganites

X-ray diffraction and magnetic measurements of polycrystalline and nanosize TbMnO3 manganites were performed. All the compounds studied crystallize in the orthorhombic crystal structure (space group Pnma) at room temperature. Nanosize manganites were synthesized via the sol-gel method at different (800 and 900 °C) temperatures. The average size of the synthesized nanoparticles (from 45 to 70 nm) was estimated by x-ray diffraction and low-temperature adsorption of argon. Information about the evolution of properties of TbMnO3 with changing grain size, temperature, and magnetic field was obtained. Crystal structure parameters of nano-samples change slightly with changing nanoparticle size. Peculiarities of magnetic ordering in polycrystalline and nanosize TbMnO3 were compared. Magnetization and the Neel temperature corresponding to antiferromagnetic ordering of the Tb3+ sublattice decrease as the particle size is reduced. Inverse magnetic susceptibility of the nanoparticle samples deviates from the Curie–We...

Specific features of magnetic and transport properties of La1−xMn1+xO3 manganites

The magnetic and transport properties of La1−xMn1+xO3 manganites with excess manganese are studied. It is shown that magnetic and charge ordering heavily depends on the superstoichiometric manganese content, magnetic field, and pressure. The magnetoresistive effect (MRE) is enhanced as the manganese concentration increases. In addition to the paramagnet-ferromagnet transition, the temperature dependences of the magnetization exhibit anomalies at low temperatures in samples with x=0.1–0.4. The magnetization decreases at T<45 K in fields H<0.2 kOe and increases as H changes from 0.2 to 10 kOe. An analysis shows that the features observed at low temperatures are most probably related to the transition from the ferromagnetic state to the canted spin structure in clusters of mixed-valence manganese ions. The temperature dependences of the magnetization and resistivity remain unchanged as the pressure increases. It is demonstrated that the Curie and metal-dielectric transition temperatures shift to higher values as the manganese concentration increases under pressure. The temperature of the MRE peak increases under pressure, while the MRE decreases.

Noncollinear cluster ferromagnetism in lanthanum manganite perovskites with an excess of manganese

A theoretical substantiation is provided for realization of the spin structure in a weakly doped subsystem based on LaMnO3, which is observed in compounds with an excess of manganese, (La1−yCay)1− xMn1+xO3, where y=0 and 0.3, x=0–0.4. It is proved experimentally that samples with x>0.1 exhibit an anomalous behavior of magnetization at T<45 K. The magnetization decreases in fields H<100 Oe and increases for H>200 Oe. It is assumed that this is associated with the emergence of a canted phase in clusters of manganese ions with difference valences. A theoretical analysis indicates that a competition between double and superexchange interaction is in principle possible in such clusters, since the charge carrier concentration in them is considerably lower than in the host matrix. It is shown that the inclusion of quantum properties of spin leads to a considerable modification of the thermodynamic behavior of a magnet with collectivized electrons. The results of analysis are compared with analogous results in the de Gennes classical theory. The possibility of formation of a state with a canted magnetic sublattice in the weakly doped subsystem in the low-temperature region is substantiated on the basis of calculations. An analysis of the thermodynamic behavior of the weakly doped subsystem based of LaMnO3 taking into account quantum-mechanical properties of spin shows that relaxation phenomena determined, to a considerable extent, by the relation of parameters of intra-and interplanar indirect exchange, as well as the electron transport energy, can take place in the region of phase transition to the canted state. The microscopic parameters of interactions are estimated quantitatively. The results of calculations are in qualitative agreement with experimental data.

Magnetic Properties of the Nanocrystalline DyMnO_{3} Compound

V. Dyakonov, A. Szytuła, S. Baran, Z. Kravchenko, E. Zubov, O. Iessenchuk, W. Bazela, M. Dul, A. Zarzycki and H. Szymczak Institute of Physics, PAS, al. Lotników 32/46, 02-668 Warsaw, Poland A.A. Galkin Donetsk Physico-Technical Institute, NANU, R. Luxembourg St. 72, 83114 Donetsk, Ukraine M. Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059, Kraków, Poland Institute of Physics, Technical University, Podchorążych 1, 30-084 Kraków, Poland

Nanoparticle size effect on the magnetic and transport properties of (La0.7Sr0.3)0.9Mn1.1O3 manganites

Magnetic and transport thermal measurements of nanosize (La0.7Sr0.3)0.9Mn1.1O3 manganite are reported. The nanoparticles are synthesized with use of the co-precipitation method at different (800, 900, and 950°C) temperatures. Their crystal structure is determined to be perovskite-like with a rhombohedral distortion (the space group R3¯c). The phase composition and specific surface nanopowders are determined. The average size of synthesized nanoparticles (from 40to100nm) is estimated by both the method of low-temperature adsorption of argon and x-ray diffraction measurements. All the nanosize samples show ferromagnetic-like ordering with close phase transition temperatures. Their magnetization decreases with decreasing particle size. Comparison of experimental and calculated temperature dependences of the spontaneous magnetic moment shows that the spontaneous magnetization both in magnetic field and without field is well described in the framework of the double exchange model. The decrease of the magnetiza...

Magnetocaloric effect in (La0.6Ca0.4)0.9Mn1.1O3

The results of investigations of the magnetization, susceptibility, and magnetic-field-induced changes in the entropy of polycrystalline manganite (La0.6Ca0.4)0.9Mn1.1O3 near the magnetic phase transition have been presented. Magnetic measurements have been carried out at temperatures in the range from 210 to 310 K in magnetic fields of up to 9 T. The magnetocaloric effect has been revealed by measuring the magnetic-field dependences of magnetization. The magnitude of the magnetocaloric effect is compared with similar results obtained for other manganites.

EPR of Dy3+ ions in YAl3(BO3)4 and EuAl3(BO3)4 aluminoborates

The EPR spectra of impurity Dy3+ ions in YAl3(BO3)4 and EuAl3(BO3)4 aluminoborates were investigated. The signals from the ground and excited doublets were observed and the g-factors and the hyperfine interaction constants were determined. From the temperature dependence of the signal intensity of the excited doublet, the energy distances between the ground and excited doublets were obtained as (3.26 ± 0.13) cm–1 and (2.54 ± 0.16) cm–1 for YAl3(BO3)4 and EuAl3(BO3)4, respectively. A broadening of the absorption line with increasing temperature is related with the strong spin-phonon interaction and is described by the Orbach–Aminov process via the excited doublet. It was found that the decrease in g-factor is caused by the spin-phonon interaction.

Grain size effect on magnetic properties of REMnO3 (RE = Pr, Nd)

X-ray diffraction and magnetic using dc and ac methods measurements of the polycrystalline and nanosize REMnO3 (RE = Pr, Nd) powdered samples have been performed. The nanosize manganites were synthesized with a sol-gel method at different (800, 850 and 900 °C) temperatures. The average size of synthesized nanoparticles (from 56 to 89 nm) and polycrystalline powders (above 200 nm) was estimated using the x-ray diffraction data. All the compounds studied crystallize in the orthorhombic crystal structure (space group Pnma) at room temperature with smaller values of the lattice parameters in the nanosamples. The temperature-dependent ac magnetic susceptibilities show a sharp high-temperature peak connected with Mn magnetic moments ordering. The low-temperature maximum of magnetic susceptibility is proposed to be due to the polarization of the rare-earth sublattice by an effective exchange field of the Mn ordered sublattice. The antiferromagnetic ordering of Mn sublattice and paramagnetic Curie temperatures as...

The magnetic properties of potassium holmium double tungstate

The magnetic investigations of potassium holmium double tungstate KHo(WO4)2 have been performed. The results of measurements of magnetic susceptibility and magnetization as a function of temperature (T from 0.3 K up to 100 K) and magnetic field (up to 1.5 T) are presented. A strong anisotropy of magnetic properties was found. The magnetic measurements data were used to calculate the interaction energy. It was shown that the interactions between nearest neighbors Ho3+ ions have antiferromagnetic character.

Magnetic phase transition in KGd(WO4)2 double tungstate

Investigations of the specific heat of the potassium gadolinium double tungstate KGd(WO4)2 have been performed over the temperature range from 0.05 K up to 4 K in zero magnetic field. The λ -type anomaly observed at T = 0.42 K was interpreted as an indication of a second order magnetic phase transition within the Gadolinium sublattice. The critical exponent α was determined from experimental data.