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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.

Magnetic properties of KGd(WO4)2, KYb(WO4)2 and KTm(WO4)2 single crystals have been studied. Temperature and field dependences of the magnetization along the main magnetic axes and angular dependence of the magnetization in the planes settled on the magnetic axes were recorded in the temperature range from 2 up to 100 K and in magnetic field up to 9 T. Paramagnetic Curie temperatures, g-factors and exchange interaction parameters were determined. Different theoretical models were used to describe magnetic properties of KGd(WO4)2 and KYb(WO4)2 crystals.

Magnetic and electron paramagnetic resonance (EPR) properties of EuFe3(BO3)4 single crystals have been studied over the temperature range of 300–4.2 K and in a magnetic field up to 5 T. The temperature, field and orientation dependences of susceptibility, magnetization and EPR spectra are presented. An antiferromagnetic ordering of the Fe subsystem occurs at about 37 K. The easy direction of magnetization perpendicular to the c axis is determined by magnetic measurements. Below 10 K, we observe an increase of susceptibility connected with the polarization of the Eu sublattice by an effective exchange field of the ordered Fe magnetic subsystem. In a magnetic field perpendicular to the c axis, we have observed an increase of magnetization at T < 10 K in the applied magnetic field, which can be attributed to the appearance of the magnetic moment induced by the magnetic field applied in the basal plane. According to EPR measurements, the distance between the maximum and minimum of derivative of absorption line of the Lorentz type is equal to 319 Gs. The anisotropy of g-factor and linewidth is due to the influence of crystalline field of trigonal symmetry. The peculiarities of temperature dependence of both intensity and linewidth are caused by the influence of excited states of europium ion (Eu3+). It is supposed that the difference between the g-factors from EPR and the magnetic measurements is caused by exchange interaction between rare earth and Fe subsystems via anomalous Zeeman effect.

The specific heat of ARE(WO4)2, where A=K, Rb and RE=Y, Nd, Gd, Yb, and Lu single crystals, has been studied. The samples were grown using the Top Seeded Solution Growth – Slow Cooling (TSSG-SC) method, and the resulting crystal structure data was tabulated. The specific heat measurements as a function of temperature in the range from 0.35 to 385 K are presented. Above 2 K, the specific heat behavior can be described by the phonon contribution while below 2 K, an anomalous increase in specific heat, attributed to the proximity of magnetic phase transitions, was observed.

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...

The magnetocaloric effect in La0.6Ca0.4MnO3 manganite has been investigated. The isothermal magnetization versus applied magnetic field at various temperatures in the vicinity of Curie temperature was measured, and the temperature dependence of magnetic entropy change was determined using Maxwell’s relation. This value is comparable to that in Gd. Nevertheless, the relative cooling power of La0.6Ca0.4MnO3 was shown to be considerably lower than that of Gd. The experimental results have been analyzed in frames of a phenomenological statistical model. This model considers explicitly Jahn-Teller interactions and allows prediction of the field dependences of the magnetic entropy change and the relative cooling power.

The influence of the kinematic interaction and the indirect exchange of electrons on the magnetic structure of a narrow-gap Hubbard magnet is investigated using the diagram technique of the perturbation theory. The summation of an infinite series of like diagrams makes it possible to separate the effective self-consistent filed. In contrast to the previous studies, the proposed theory contains only two self-consistent parameters, i.e., the chemical potential and the average spin. The obtained system of equations for these parameters admits the analytical solution at zero temperature. The competition of the simplest kinematic contribution and the indirect antiferromagnetic exchange is clearly demonstrated using the concentration dependences of the Neel temperature as an example. The boundaries of the existence of the homogeneous ferromagnetic, antiferromagnetic, and paramagnetic phases as a function of the electron concentration and the indirect exchange parameter are determined by numerically analyzing the internal energy.

The influence of the kinematic interaction and indirect exchange of d electrons on the magnetic structure of a narrow-band Hubbard magnet is investigated using diagrammatic perturbation theory. All contributions to the combined population and the spin Green’s function are examined to the first order of perturbation theory. A self-consistent equation is found for the order parameter of the phases. All contributions to the mass operator are determined with effective “dressing” of the interaction lines taken into account. The effect of indirect anti-ferromagnetic exchange on the spin wave spectrum and the damping of elementary excitations are investigated for ferromagnetic and paramagnetic phases. The correction to the frequency of spin-wave excitations at zero temperature and for hole concentrations c⪡1, proportional to c4∕3, is calculated for a ferromagnetic structure. As indirect exchange increases in magnitude, the spin-wave excitation frequencies decrease linearly, in agreement with the results obtained...

The pressure, magnetic-field, and excess manganese effects on transport and magnetoresistance effect (MRE) are studied in both epitaxial films and bulk ceramics of the manganites (La0.7Ca0.3)1−xMn1+xO3−y (x=0–0.2). A comparison of the electrical behavior in both kinds of samples of similar composition at hydrostatic pressures of up to 1.8GPa and in magnetic fields of up to 8kOe is performed. The pressure and magnetic-field effects are shown to increase with increasing manganese content. Experimental data show that the effects of pressure and magnetic field on the temperatures of both the metal-insulator transition (TMD) and the MRE peak (TMR) are considerably stronger in the films than in the ceramics. The hydrostatic pressure increases TMD and TMR. It was shown that for both types of samples the magnetoresistance effect is affected in opposite ways by pressure and magnetic field. A direct correlation is established between TMD and conductivity bandwidth as well as between MRE and concentration of charge ...

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.

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

The effect of particle size on the magnetic properties of RMnO3 (R = Pr, Nd, and Tb) has been investigated by magnetometric and X-ray and neutron diffraction methods. The samples were obtained by a sol–gel method. The investigated compounds with a grain size < 100 nm crystallize in the orthorhombic crystal structure described by the Pnma space group. The crystal parameters for the nanoparticle compounds are slightly smaller than those for bulk materials Low temperature magnetic and neutron diffraction data indicate the difference in their magnetic properties. For the nanoparticle PrMnO3 (annealed at 900 °C) and NdMnO3 samples, the magnetic ordering of CxFy-type exists in Mn sublattice while in PrMnO3 annealed at 800 °C it is missing. The Nd moments order below T ≈ 13 K according to a ferromagnetic arrangement of Fy-type.

Magnetic phase transition in the CsDyW2O8 magnet has been studied by means of low temperature specific heat C(T) measurements. The magnetic ordering temperature of the Dy3+ sublattice was established to be 1.34 K. The experimental results indicate on the antiferromagnetic character of interactions between Dy3+ ions. The behavior of the C(T) dependencies above and below TN is discussed in frames of different theoretical models. The measurements data on temperature and field dependencies of magnetization are used to calculate the exchange and dipole-dipole interactions energy and to determine the possible magnetic structure of the ground state.

M.T. Borowieca,∗, V.P. Dyakonov, K. Woźniak, Ł. Dobrzycki, A. Majchrowski, E. Michalski, E.E. Zubov, E.N. Khatsko, T. Zayarnyuk, A. Szewczyk, M.U. Gutowska, A.I. Rykova, M. Baranski, V. Domukhovski, V. Shtyrkhunova, P. Iwanowski, J. Żmija and H. Szymczak Institute of Physics, Polish Academy of Sciences, al. Lotnikow 32/46, 02-668 Warsaw, Poland A.A. Galkin Donetsk Institute for Physics and Engineering, National Academy of Sciences of Ukraine

The results of measuring the temperature and field dependences of the magnetization of a RbDy(WO4)2 single crystal in the temperature range from 4.2 to 50 K and in magnetic fields up to 1.6 T are presented. The energies of the exchange and dipole-dipole interactions are estimated. The magnetic structure of the ground state is determined.

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 well as the magnetic moment values for the nanosize samples was found to be smaller than those for polycrystalline sample.

The paper presents the results of complex investigations of the potassium-erbium double tungstate. The following properties have been measured: specific heat, specific heat in magnetic field, thermal expansion, thermal expansion in magnetic field, magnetization and X-ray structural data, reflectance and optical absorption spectra. The experiments were performed in the broad temperature range from 0.4 K to 300 K, spectral range from 4000 cm-1 to 40000 cm-1 and magnetic field up to 10T. The magnetic phase transition was found at temperature T=0.84K. In thermal expansion experiment the strong anomaly was found at T=7.6 K in magnetic field 10T. The KEr(WO4)2 belongs to the optical class of biaxial and pleochroic (trichroic) crystals. Using spectroscopy methods, the orientation of optical axes and refractive indices have been found. The main optical absorption spectra have been measured. The temperature dependence of optical absorption was determined.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Magnetic ordering of in low-dimensional magnet CsDy(W04)2, (CsDyW) has been first studied by means ofmeasurements ofthe low temperature specific heat over a temperature range ofO.5-23 K. The ordering temperature (IN) of the Dy sublattice was established to be 1.34 K. The experimental data indicate on the antiferromagnetic (AFM) characterofDy ions interactions. The magnetic behavior above and below TN is discussed in the framework ofdifferent theoreticalmodels.Keywords: rare-earth tungstate; specific heat; antiferromagnetic ordering; low-dimensional magnet. 1. INfRODUCTION Investigations of the alkaline (M) - rare-earth (Re) double tungstates MRe(W04)2 are of a special interest because of apossible realization ofboth magnetic and structural phase transitions in them. The existence ofrare-earth ions with closelyspaced energy levels in these low-symmetry compounds results in the occurrence of structural phase transitions connectedwith the manifestation of the cooperative Jahn-Teller effect. Indeed, such structural phase transitions (SPT) have beenrecorded in a monoclinic KDy(W04)2 (T= 6.38 K) and RbDy(W04)2 (T 4.9 and 9.0 K) single crystals. The doubletungstates are also considered to be a model system to check the conceptions of the low-dimensional magnetism. Thesestudies are-also strongly stimulated by possible applications ofthese materials as active component in solid state lasers. Itis shown [1] that there is possibility of application of rubidium-dysprosium double tungstate as a cooling agent in anadiabatic demagnetization method for obtaining very low temperatures.

We report on the pressure, field and temperature dependencies of magnetization and resistance in manganites of (La1-xCax)1-yMn1+y O3 type. The M(T) dependencies is established to exhibit the following peculiarities: the first is connected with the paramagnet (PM)-ferromagnet (FM) transition at 267 K; the peak of M(T) dependence eat 263K is presumable due to an existence of AFM clusters; an anomaly in magnetization is connected with the FM-canted phase transition at 42K and the large irreversibility in the field-cooled and zero field- cooled magnetization. As field is increased above 200 Oe, the sign of the M(T) anomaly changes, namely, the M(T) magnetization increases with increasing field at 42K. The character of M(T) dependencies does not change under pressure. However, both the Curie temperature, Tc, and the resistance peak corresponding to metal-insulator transition shift in the direction of high temperatures with increasing pressure. A pressure stabilizes the ferromagnetic metallic phase. The temperature of the FM-canting phase transition does not change under pressure. The magneto resistance effect increase by 15 percent under pressure of 12.6 kbar.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report on the temperature and pressure dependences of the magnetization in the KDy(W04)2 single crystal. The directions ofmagnetic axes in the ciystal under study were established. It has been shown that at the point ofthe structural phase transition theCurie-Weiss temperature changes considerably. Under pressure ofabout 0.8 GPa the temperature ofthe structural phase transitionincreases nearly by a factor oftwo. As a result ofthe pressure effect the low temperature phase area is expanded. Keywords: KDy(W04)2, double tungstates, phase transition, cooperative Jahn-TeIler effect, magnetization, magnetizationunder pressure, constants ofexchange interactions, Curie-Weiss temperature 1. INTRODUCTION Investigations ofmagnetic resonance and thermal properties ofthe KDy(W04)2 double tungstate ' have shown that this compound is ofinterest because of structural and magnetic phase transitions observed in this low-dimensional magnetic material. The Jahn-Teller type structural phase transition (SPT) at Tp1- 6.38 K and the antilerromagnetic (AFM) phase transition at

Thermodynamics of a weakly doped Hubbard magnet with infinite repulsion has been investigated. By means of perturbation theory the stability of ferromagnetic state at temperature T=0 is proved.

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.