A. Szewczyk

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...

Crystal structure and magnetic properties of potassium erbium double tungstate KEr(WO4)2

Results of structural, magnetic and specific heat investigations of the potassium erbium double tungstate, KEr(WO 4 ) 2 , are presented. Potassium erbium double-tungstate KEr(WO 4 ) 2 single crystals have been grown by the top-seeded solution growth method (TSSG) and modified Czochralski techniques. It crystallizes in the monoclinic crystal structure (C2/c space group). The unit cell contains four formula units and is described by parameters a = 10.615(2) A, b = 10.316(2) A, c = 7.534(2) A, β = 130.73(3)°. From the x-ray diffraction measurements the fractional atomic coordinates, displacement parameters and interatomic distances have been determined. The specific heat C(T) of the KEr(WO 4 ) 2 crystal has been measured over a temperature range of 0.6-300 K. The susceptibility has been studied at T = 0.25-4.0 K. The magnetic phase transition was observed at a temperature of 0.48 K. The magnetization has been measured in the temperature region from 4.2 to 60 K and in magnetic field up to 1.6 T. A strong anisotropy of magnetic properties was found. The temperature and field dependences of susceptibility and magnetization data were used for both elucidation of character of the magnetic ordering and calculation of the exchange and dipole-dipole interaction energies as well as for determination of the possible magnetic structure of KEr(WO 4 ) 2 .

Specific heat in CeNi4Cu and YbNi4Cu

The temperature dependence of specific heat for the compounds CeNi4Cu and YbNi4Cu is analysed. These studies are supported by magnetic susceptibility and x-ray photoemission spectroscopy measurements. The scheme of the energy levels created by the splitting due to the crystal electric field is determined from the Schottky contribution to the specific heat. Anomalies observed at low temperatures are discussed in the framework of heavy-fermion/Kondo physics. It is found that an external magnetic field has a strong influence on the low temperature part of the specific heat.

Direct and specific heat study of magnetocaloric effect in La0.845Sr0.155MnO3

Magnetocaloric effect and specific heat of a La0.845Sr0.155MnO3 sample were measured over a temperature range of 4.2–300 K, to verify whether large entropy changes induced by magnetic field make possible the use of manganites for magnetic refrigeration and whether a magnetostrictive contribution to magnetocaloric effect is significant. Isothermal entropy change and adiabatic temperature change induced by ΔB=7 T were determined as functions of temperature. Near to the Curie temperature, TC≈234 K, they reach, respectively, ∼−6.6 J/(kg K) and ∼3.4 K, i.e., the values ∼2 and ∼4 times smaller than for Gd. Despite the presence of large magnetostriction, its contribution to magnetocaloric effect was found negligible.

Temperature dependence of the domain wall energy in SmNi5 crystals

Abstract The domain structure in SmNi 5 plates was studied in the range of 5–25 K, using the oxygen cryocondensation method. The temperature dependence of the domain wall energy was determined. This was found to agree qualitatively with the theoretical dependence calculated using the Ising domain wall model.

Ferromagnetic resonance in (La0.7Ca0.3)1−xMn1+xO3 films

Measurements of ferromagnetic resonance (FMR) in both in-plane (H‖) and out-of-plane (H⊥) magnetic field configurations have been performed on a series of epitaxial (La0.7Ca0.3)1−xMn1+xO3 films. The FMR linewidths have a local maximum near the Curie point and increase continuously at low temperatures. The effective anisotropy of the films has the easy-plane character. The angular dependences of Hres were found to be in reasonable agreement with the theoretical ones, taking into account the second- and fourth-order terms of the magnetic anisotropy energy. A reasonable agreement between ΔHres(α) and Bloch type dissipation was obtained only for deviations of the external field from the normal to the film plane up to α=20°. From the temperature and angular dependences of the FMR spectra, the main magnetic parameters such as the effective magnetization, effective field Heff, anisotropy field HA2, and FMR linewidth were estimated.

Magnetic phase transition in MnCr2-2xIn2xS4crystals

The influence of In for Cr substitution on the para- to ferrimagnetic phase transition has been investigated for MnCr2-2xIn2xS4 spinel crystals by means of DC magnetization measurements. The critical exponents and calculated from the high field extrapolation of the modified Arrott plots were found to be unaltered with substitution and close to 3D Heisenberg ones. A considerable expansion of the temperature range of the upward curvature of the inverse initial susceptibility of the samples with disorder increase has been found. Non-monotonic temperature dependences of Kouvel-Fisher effective susceptibility exponent * for disordered samples and the increase of the * value with the In concentration growth were also observed.

Low-temperature anomalies in resistance and magnetoresistance of amorphous FeCrB ribbons. Coexistence of ferromagnetism and local superconductivity?

Investigation of the conductivity mechanisms in ferromagnetic Fe(67)Cr(18)B(15) metallic glasses with clusterized structure reveals anomalies in the behaviour of resistance and magnetoresistance (MR) in a narrow temperature interval, T = 3.6-3.1 K. The anomalies are seen as a sharp decrease of the sample resistivity in this range, with a rate equal to 3.6% K(-1), i.e. 200-500 times more than the rate 0.008-0.021% K(-1) in the range of 300-4 K. MR in the same range increases with a rate 1000 times larger (4% K(-1) at T ∼ 3.1-3.6 K) than in the 300-4 K range (<0.0015% K(-1)). We explain this result by the appearance of local superconductivity in the large-scale layered clusters of metallic Fe-Cr phase, 150-200 Å in size, with ferromagnetic Fe(2)Cr core and nonmagnetic FeCr(2) superconducting shell. The superconducting phase, which occupies 0.4-0.5% of the sample volume, provides a resistance jump Δρ/ρ≈1.5% that corresponds to calculation. The superconducting state of the clusters collapses if the magnetic field exceeds 20 kOe.

Multiple magnetic phase transitions in Tb3Cu4Si4

A polycrystalline sample of Tb(3)Cu(4)Si(4) was investigated by means of magnetometric, electrical resistivity, thermopower and heat capacity measurements. This paper also includes reanalyses of former neutron diffraction data. The compound crystallizes in a Gd(3)Cu(4)Ge(4)-type orthorhombic structure (Immm space group), in which the Tb ions occupy two inequivalent sites (2d and 4e). The bulk results indicate that these two magnetic sublattices order antiferromagnetically at different temperatures, in agreement with the neutron diffraction data.

Heat capacity of GdNi5

Abstract The heat capacity of GdNi 5 was measured from 1.3 to 48 K in a fixed magnetic field (up to 5 T). The electronic, phonon, and magnetic contributions were separated. Temperature dependences of magnetic specific heat and entropy were calculated in the molecular field approximation. They are in good agreement with the experimental results.