Analysis of low temperature specific heat in Nd0.5Sr0.5MnO3 and R0.5Ca0.5MnO3 (R=Nd,Sm, Dy and Ho) compounds

We report on the specific heat C(T) of doped manganites Nd_0.5Sr_0.5MnO₃, Nd_0.5Ca_0.5MnO₃, Sm_0.5Ca_0.5MnO₃, Dy_0.5Ca_0.5MnO₃ and Ho_0.5Ca_0.5MnO₃ in the temperature range 2?T?300 K using modified rigid ion model (MRIM). The present specific heat results are in general satisfactory agreement with experimental data except at very low temperatures (i.e. T?12 K). Also a sharp peak observed in the experimental results for these compounds around 5 K could not be revealed by our computed results as they arise due to Schottky-like anomaly. Besides, we have reported the cohesive and the thermal properties of these compounds. The results obtained by us are discussed in detail.     

Anomalous thermoelectric power behaviour in PrSn3 and NdSn3

Results of the thermoelectric power (TEP) measurements done on monocrystalline samples of RESn₃ compounds (RE=La, Pr, Nd, and Gd) are presented for the temperature range of 5.5-300 K. It was found that the TEP is positive and weakly temperature dependent at temperatures T>100 K. For T<100 K pronounced anomalies have been observed for the PrSn₃ and the NdSn₃ compounds in the vicinity of 10 K.We argue that the Kondo and crystal field effects cause these anomalies. A shape of the TEP anomaly found for PrSn₃ resembles very much that observed in the electrical resistivity.     

The effect of pressure on the spin density wave transition in the layered cobaltites [Ca2CoO3]0.62[CoO2] and [Ca2Co4/3Cu2/3O4]0.62[CoO2]

In order to investigate the pressure effect on the magnetism in the layered cobaltites, positive muon spin rotation and relaxation μ⁺SR experiments have been carried out up to 1.3 GPa using c-aligned polycrystalline samples of [Ca₂CoO₃]_0.62[CoO₂] and [Ca₂Co_4/3Cu_2/3O₄]_0.62[CoO₂]. A transverse field μ⁺SR experiment indicates that the transition temperature to an incommensurate spin density wave IC-SDW state is independent of hydrostatic pressure up to 1.3 GPa for the both compounds. Furthermore, there are no changes in the spontanious muon precession frequency in zero field at 5 K even under 1.3 GPa. These results strongly suggest that the IC-SDW exists not in the rocksalt-type block ([Ca₂CoO₃] and/or [Ca₂Co_4/3Cu_2/3O₄]) but in the CoO₂ plane.     

Study of the magnetostrictive strains in Pr6Fe11Ga3 alloy

Magnetoelastic properties of the Pr₆Fe₁₁Ga₃ alloy are studied by magnetostriction and thermal expansion measurements. The effects of short- and long-range magnetic ordering processes about Curie temperature clearly appear in the temperature dependence of the spontaneous magnetostriction as two increasing steps with decreasing temperatures. Thermal variations of the total magnetocrystalline anisotropy introduce pronounce changes in the isofield curves of the forced magnetostriction as a negative minimum below 200 K, a compensation phenomena about 250 K, and a positive maximum between 250 K and T_c=320 K. The observed behavior of magnetostriction is discussed in terms of the competitive anisotropies of Pr and Fe sublattices and coupling magnetostrictive constants.     

AC conduction mechanisms of Gd1/3Sr2/3FeO3 ceramic

In this study, the AC conductivity of insulating Gd_1/3Sr_2/3FeO₃ was analyzed within the framework of the quantum-mechanical tunneling mechanism (QMT) and the hopping of barrier mechanism (HOB). Experimental data were taken from 20 Hz to 1 MHz and from 80 to 300 K. Observation revealed that the small polaron QMT model is the more suitable mechanism for modeling the AC conductivity of Gd1/3Sr2/3FeO3 at low temperatures.     

Influence of microstructures on exchange bias behaviors of La0.7Sr0.3MnO3/La0.33Ca0.67MnO3 bilayers

Ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO) and antiferromagnetic La_0.33Ca_0.67MnO₃ (LCMO) layers were grown on SrTiO₃ (STO) substrates by the pulsed laser deposition technique. LSMO films had rougher surfaces and larger grain sizes than LCMO films. Fully strained bilayers, in which each layer was as thin as 10 nm, were prepared by changing their stacking sequences, i.e. LSMO/LCMO/STO and LCMO/LSMO/STO. The former had higher T_C (350 K) than the latter (300 K), and exchange bias effects were only observed in the former bilayers. This revealed that microstructures could play an important role in the transport and magnetic properties of manganese oxide thin films.     

Valence fluctuation in Ce2Co3Ge5 and crystal field effect in Pr2Co3Ge5

Polycrystalline samples of ternary rare-earth germanides R₂Co₃Ge₅ (R=La, Ce and Pr) have been prepared and investigated by means of magnetic susceptibility, isothermal magnetization, electrical resistivity and specific heat measurements. All these compounds crystallize in orthorhombic U₂Co₃Si₅ structure (space group Ibam). No evidence of magnetic or superconducting transition is observed in any of these compounds down to 2 K. The unit cell volume of Ce₂Co₃Ge₅ deviates from the expected lanthanide contraction, indicating non trivalent state of Ce ions in this compound. The reduced value of effective moment (μ_eff≈0.95 μ_B) compared to that expected for trivalent Ce ions further supports valence-fluctuating nature of Ce in Ce₂Co₃Ge₅. The observed temperature dependence of magnetic susceptibility is consistent with the ionic interconfiguration fluctuation (ICF) model. Although no sharp anomaly due to a phase transition is seen, a broad Schottky-type anomaly is observed in the magnetic part of specific heat of Pr₂Co₃Ge₅. An analysis of C_mag data suggests a singlet ground state in Pr₂Co₃Ge₅ separated from the singlet first excited state by 22 K and a doublet second excited state at 73 K.     

Superparamagnetic behavior of La0.67Sr0.33MnO3 nanoparticles prepared via sol-gel method

Magnetic nanoparticles of La_0.67Sr_0.33MnO₃ (LSMO) manganite were prepared by sol-gel method. Phase formation and crystal structure of the synthesized powder were examined by the X-ray diffraction (XRD) using the Rietveld analysis. The mean particle size was determined by the transmission electron microscopy (TEM). Infrared transmission spectroscopy revealed that stretching and bending modes are influenced by calcinations temperature. The temperature dependence of the ac magnetic susceptibility was measured at different frequencies and ac magnetic fields in the selected ranges of 40-1000 Hz and 80-800 A/m, respectively. The temperature dependence of ac susceptibility shows a characteristic maxima corresponding to the blocking temperature near room temperature. The frequency dependence of the blocking temperature is well described by the Vogel-Fulcher law. By fitting the experimental data with this law, the relaxation time τ₀=1.7×10⁻¹² s, characteristic temperature T₀=262±3 K, anisotropy energy E_a/k=684±15 K and effective magnetic anisotropy constant k_eff=2.25×10⁴ erg/cm³ have been obtained. dc Magnetization measurement versus magnetic field shows that some of LSMO nanoparticles are blocked at 293 K. The role of magnetic interparticle interactions on the magnetic behavior is also investigated.     

Detailed infrared spectroscopic study of thermal transitions in new hybrid [(CH3)2CHNH3]4Cd3Cl10 crystal

Phase transitions of tetra(isopropylammonium)decachlorotricadmate(II) [(CH₃)₂CHNH₃]₄Cd₃Cl₁₀ crystal have been studied by infrared, far infrared and Raman measurements in wide temperature range, between 11 K and 388 K. The temperature changes of wavenumber, center of gravity, width and intensity of the bands were analyzed to clarify cationic and anionic contributions to the phase transitions mechanism. The results of investigation showed earlier by differential scanning calorimetry (DSC), thermal expansion and dielectric measurements clearly confirmed the sequence of phase transitions at T₁=353 K, T₂=294 K and T₃=260 K. The current results derived from DSC and infrared measurements revealed additional phase transition at T₄=120 K.     

μSR investigation of the magnetic ordering in EuAs3

We have investigated the magnetic ordering and the incommensurate-commensurate phase transition in EuAs₃ by zero-field (ZF) and longitudinal-field μSR. In the commensurate phase, stable at temperatures below T_L=10.3 K, the ZF muon signal exhibits oscillations corresponding to four muon precession frequencies the lowest of which behaves anomalously. The muon signal shows no oscillation but exponential decay in the incommensurate phase stable in temperature range from T_L≈10.3 K up to T_N≈11 K. The temperature dependence of the fitted relaxation rate shows divergence-like behaviour at the ordering temperature T_N≈11 K and also at the lock-in transition T_L≈10.3 K. The results are in qualitative agreement with those previously obtained by neutron and X-ray magnetic scattering investigations except for the anomalous temperature dependence of the lowest frequency in the commensurate phase. We propose a model for this anomalous behaviour.     

Electrical behavior of BaZr0.1Ti0.9O3 and BaZr0.2Ti0.8O3 thin films

BaZr_0.1Ti_0.9O₃ and BaZr_0.2Ti_0.8O₃ (BZT) thin films were deposited on Pt/Ti/LaAlO₃ (1 0 0) substrates by radio-frequency magnetron sputtering, respectively. The films were further annealed at 800 °C for 30 min in oxygen. X-ray diffraction θ-2θ and Φ-scans showed that BaZr_0.1Ti_0.9O₃ films displayed a highly (h 0 0) preferred orientation and a good cube-on-cube epitaxial growth on the LaAlO₃ (1 0 0) substrate, while there are no obvious preferential orientation in BaZr_0.2Ti_0.8O₃ thin films. The BaZr_0.1Ti_0.9O₃ films possess larger grain size, higher dielectric constant, larger tunability, larger remanent polarization and coercive electric field than that of BaZr_0.2Ti_0.8O₃ films. Whereas, BaZr_0.1Ti_0.9O₃ films have larger dielectric losses and leakage current density. The results suggest that Zr⁴⁺ ion can decrease dielectric constant and restrain non-linearity. Moreover, the enhancement in dielectric properties of BaZr_0.1Ti_0.9O₃ films may be attributed to (1 0 0) preferred orientation.     

Effect of Y-doping on the magnetic and charge orderings in Nd2/3Ca1/3MnO3

The shifts of the magnetic and charge ordering transition temperatures caused by Nd substitution for Y in Nd_2/3Ca_1/3MnO₃ CMR narrow-band perovskite manganite have been studied. At low temperatures, three different long-range magnetic orderings consistent with a phase separation scenario have been observed in the doped compound (Nd_0.9Y_0.1)_2/3Ca_1/3MnO₃ by neutron-diffraction study: the antiferromagnetic orderings of PCE and DE types existing below ∼110 and ∼60 K, respectively, and the ferromagnetic one of B type existing below ∼42. Magnetic phase transformations temperatures as well as those of charge ordering have been found to be structural-dependent. Y-doping leads to the decrease of the anisotropy of the orthorhombic Pnma crystal lattice b/√2c, which causes a decrease of the indirect exchange parameters in the system and thus a decrease in the magnetic transformation temperatures for 20-30 K in the doped compound. Doping leads as well to the higher level of the coherent Jahn-Teller distortions of the MnO₆ octahedra in the 200-300 K temperature region, which results in the increase of the charge ordering temperature for ∼80 K.     

Magnetic phase coexistence in Tb- and Sr-doped La0.7Ca0.3MnO3 manganite:A temperature-dependent neutron diffraction study

We report the results of the temperature-dependent neutron diffraction measurements on the nearly half-doped (La_0.325Tb_0.125)(Ca_0.3Sr_0.25)MnO₃ manganite sample. The simultaneous doping of magnetic Tb³⁺ and divalent Sr²⁺ in the La_0.7Ca_0.3MnO₃ system results into a large A-site size disorder. Rietveld refinement of neutron diffraction data reveal that the single phase sample crystallizes in a distorted orthorhombic structure. Increased 〈r_A〉 value affects the transport behavior that results into an insulating-like behavior of the sample. Under application of 1 T field sample exhibit insulating-like behavior while insulator-metal transition (T_IM) is exhibited under 5 and 8 T fields. Variable range hoping (VRH) mechanism of charge carriers is exhibited in the insulating region. Field cooled and zero field cooled magnetization measurement shows the Curie temperature (T_C)~47 K. The refinement of the ND data collected at various temperatures below 300 K shows that there is no structural phase transition in the compound. Around 100 K, a magnetic peak appears at lower angle that can be ascribed to the presence of the A-type antiferromagnetic (AFM) phase. Two more peaks are observed around 50 K at lower angles that can be fitted in CE-type antiferromagnetic phase. Splitting of the peaks at lower temperatures is the signature of orbital ordering in the presently studied nearly half-doped manganite system. Results of the detailed structural analysis of the temperature-dependent ND measurements on (LaTb)_0.45(CaSr)_0.55MnO₃ sample has been discussed in the light of coexisting A-type and CE-type antiferromagnetic phases present in the sample at low temperature.     

Magnetic resonances spectroscopy of nanosize particles La0.7Sr0.3MnO3

Using a co-precipitation method, perovskite-type manganese oxide La_0.7Sr_0.3MnO₃ nanoparticles (NPs) with particle size 12 nm were prepared. Detailed studies of both ⁵⁵Mn nuclear magnetic resonance and superparamagnetic resonance spectrum, completed by magnetic measurements, have been performed to obtain microscopic information on the local magnetic structure of the NP. Our results on nuclear dynamics provide direct evidence of formation of a magnetically dead layer, of the thickness ≈2 nm, at the particle surface. Temperature dependences of the magnetic resonance spectra have been measured to obtain information about complex magnetic properties of La_0.7Sr_0.3MnO₃ fine-particle ensembles. In particular, electron paramagnetic resonance spectrum at 300 K shows a relatively narrow sharp line, but as the temperature decreases to 5 K, the apparent resonance field decreases and the line width considerably increases. The low-temperature blocking of the NPs magnetic moments has been clearly observed in the electron paramagnetic resonances. The blocking temperature depends on the measuring frequency and for the ensemble of 12 nm NPs at 9.244 GHz has been evaluated as 110 K.     

Neutron diffraction studies of Tb2Rh3Si5 compound

In this work neutron diffraction studies of Tb₂Rh₃Si₅ compound are reported. The compound crystallizes in the monoclinic crystal structure of Lu₂Co₃Si₅-type. At 1.5 K an antiferromagnetic ordering with a propagation vector k=(1/2;1/2;1/2) was observed. The Tb magnetic moments of 9.8(2) μ_B form a non-collinear magnetic structure. In the vicinity of Néel temperature of 8 K a change of the magnetic ordering is evidenced. The change seems to be connected with phase transition from commensurate to incommensurate sine-wave modulation of the Tb magnetic moments.     

Metastable structure and properties of (La0.73Bi0.27)0.67Ca0.33MnO3

The electrical transport and magnetic properties of high Bi doped (La_0.73Bi_0.27)_0.67Ca_0.33MnO₃ are studied at the temperature and magnetic field ranges from 10 to 300 K and 0 to 3 T. Significant temperature and magnetic field hystereses are observed in both resistivity and magnetization measurements. Meanwhile, an enhanced magnetoresistance effect, within a wide temperature window, is obtained in the (La_0.73Bi_0.27)_0.67Ca_0.33MnO₃. The hysteresis and enhanced magnetoresistance are discussed based on an inhomogeneous metastable structure related to the Bi dopant.     

PLD of Fe3O4 thin films: Influence of background gas on surface morphology and magnetic properties

Ablation of Fe₃O₄ targets has been performed using a pulsed UV laser (KrF, λ = 248 nm, 30 ns pulse duration) onto Si(100) substrates, in reactive atmospheres of O₂ and/or Ar, with different oxygen partial pressures. The as-deposited films were characterised by atomic force microscopy (AFM), X-ray diffraction (XRD), conversion electron Mössbauer spectroscopy (CEMS) and extraction magnetometry, in order to optimise the deposition conditions in the low temperature range. The results show that a background mixture of oxygen and argon improves the Fe:O ratio in the films as long as the oxygen partial pressure is maintained in the 10⁻² Pa range. Thin films of almost stoichiometric single phase polycrystalline magnetite, Fe_2.99O₄, have been obtained at 483 K and working pressure of 7.8 × 10⁻² Pa, with a high-field magnetization of ∼490 emu/cm³ and Verwey transition temperature of 112 K, close to the values reported in the literature for bulk magnetite.     

Static susceptibility and heat capacity studies on V3O7·H2O7 nanobelts

V₃O₇·H₂O nanobelts were prepared by a hydrothermal method at 190 °C using V₂O₅·nH₂O gel and H₂C₂O₄·2H₂O as starting agents. The obtained nanobelts have diameters ranging from 40 to 70 nm with lengths up to several micrometers. Measurements of the static magnetic susceptibility and the specific heat show a discontinuous phase transition at around T=145 K, which separates two regions of paramagnetic behavior.     

Raman study of BiFeO3 with different excitation wavelengths

Raman spectra of bismuth ferrite (BiFeO₃) over the frequency range of 100-1500 cm⁻¹ have been systematically investigated with different excitation wavelengths. The intensities of the two-phonon modes are enhanced obviously under the excitation of 532 nm wavelength. This is attributed to the resonant behavior when incident laser energy closes to the intrinsic bandgap of BiFeO₃. The Raman spectra of BiFeO₃ excited at 532 nm were measured over the temperature range from 77 to 678 K. Besides the abnormal changes of the peak position and the linewidth of the A₁ mode at 139 cm⁻¹, the prominent frequency shift, the line broadening and the decrease of the intensity for the two-phonon mode at 1250 cm⁻¹ were observed as the temperature increased to Néel temperature (T_N). All these results indicate the existence of strong spin-phonon coupling in BiFeO₃.     

Singularities of magnetic and elastic characteristics of La2/3Ba1/3MnO3: Analysis of martensitic kinetics

A coordinated temperature behavior of magnetic susceptibility and internal friction has been observed in the La_2/3Ba_1/3MnO₃ manganite in the temperature region of the crystal phase separation 5–340 K. Stepwise temperature behavior of the susceptibility of the single crystal sample and corresponding singular behavior of the internal friction in the polycrystalline manganite have been found. These small-scale features of the temperature dependences of the susceptibility and the internal friction are considered to be a reflection of martensitic kinetics of the structural phase transformation R3¯c↔Imma in the 200 K temperature region.