The size effect on transport properties of colossal magnetoresistance materials La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

High quality La_0.67Ca_0.33MnO₃ (LCMO) film was deposited via a novel pulsed electron deposition technique on SrTiO₃(100) single crystal substrate. The micro-bridge with different widths was fabricated by using electron beam lithography (EBL) technique and their transport properties were studied. For the micro-bridges with width of 2 and 1.5 μm, the insulator-to-metal transition temperature (T _P) keeps unchanged compared with the film. For the micro-bridges with width of 1 μm, the T _P shifts towards the lower temperature by 50 K. When the width decreases down to 500 nm, the insulator-to-metal transition disappears. The magnetoresistance behavior of these micro-bridges was studied, and the results show that the low field magnetoresistance (LFMR) decreases and the high field magnetoresistances (HFMR) keep almost unchanged as the width of micro-bridge is reduced. Supported by the Key Project of the Natural Science Foundation of Zhejiang Province of China (Grant No. Z605131), the National Natural Science Foundation of China (Grant No. 60571029), and W. H. Tang was supported by the Creative Research Group of National Natural Science Foundation of China (Grant No. 60321001)     

The giant volume magnetostriction and colossal magnetoresistance in Eu<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganite

A doped manganite with the composition Eu_0.55Sr_0.45MnO₃ exhibits giant negative magnetostriction and colossal negative magnetoresistance at temperatures in the vicinity of the magnetic phase transformation (T～41 K). In the temperature interval 4.2 K≤T ≤40 K, the isotherms of magnetization, volume magnetostriction, and resistivity exhibit jumps at the critical field strength H_c1, which decreases with increasing temperature. At 70 K ≤T ≤120 K, the jumps on the isotherms are retained, but the shapes of these curves change and the H_c1 value increases with the temperature. At H<H_c1, the magnetoresistance is positive and exhibits a maximum at 41 K; at H>H_c1, the magnetoresistance becomes negative, passes through a minimum near 41 K and then reaches a colossal value. The observed behavior is explained by the existence of three phases in Eu_0.55Sr_0.45MnO₃, including a ferromagnetic (in which the charge carriers concentrate due to a gain in the s-d exchange energy) and two antiferromagnetic phases (of the A and CE types). The volumes of these phases at low temperatures are evaluated. It is shown that the colossal magnetoresistance and the giant volume magnetostriction are related to the ferromagnetic phase formed as a result of the magnetic-field-induced transition of the CE-type antiferromagnetic phase to the ferromagnetic state.     

Magnetic properties of a quasi-one-dimensional NaFeGe<Subscript>2</Subscript>O<Subscript>6</Subscript> polycrystal

The magnetic properties of a synthesized dielectric NaFeGe₂O₆. polycrystal have been studied. The antiferromagnetic ordering of this compound below 15 K has been established. The Mössbauer spectrum at 300 K is a quadrupole doublet; it is characterized by an isomeric shift typical of the high-spin Fe³⁺ ion in the octahedral coordination and quadrupole splitting, which indicates distortion of the oxygen octahedron around the iron cation. Quasi-one-dimensionality of the sample magnetic structure is proved.     

Li<Superscript>+</Superscript> ion conductivity and transport properties of LiYP<Subscript>2</Subscript>O<Subscript>7</Subscript> compound

A lithium yttrium diphosphate LiYP₂O₇ was prepared by a solid-state reaction method. Rietveld refinement of the X-ray diffraction pattern suggests the formation of the single phase desired compound with monoclinic structure at room temperature. The infrared and Raman spectrum of this compound was interpreted on the basis of P₂O₇ ^4? vibrations. The AC conductivity was measured in the frequency range from 100 to 10⁶ Hz and temperatures between 473 and 673 K using impedance spectroscopy technique. The obtained results were analyzed by fitting the experimental data to the equivalent circuit model. The Cole–Cole diagram determined complex impedance for different temperatures. The angular frequency dependence of the AC conductivity is found to obey Jonscher’s relation. The temperature dependence of σ _AC could be described in terms of Arrhenius relation with two activation energies, 0.87 eV in region I and 1.36 eV in region II. The study of temperature variation of the exponent(s) reveals two conduction models: the AC conduction dependence upon temperature is governed by the correlated barrier hopping (CBH) model in region I (T < 540 K) and non-overlapping small polaron tunneling (NSPT) model in region II (T > 540 K). The near value of activation energies obtained from the equivalent circuit and DC conductivity confirms that the transport is through ion hopping mechanism dominated by the motion of the Li⁺ ion in the structure of the investigated material.     

Giant magneto-optical response of ferromagnetic EuB<Subscript><Emphasis Type="Bold">6</Emphasis></Subscript>

Intercalation of polyatomic molecules into a superconductor can drastically affect the properties of the compound. A mechanism responsible for a large increase in T _c for such systems is proposed. It explains the recent remarkable observation of high T _c superconductivity in the hole-doped C₆₀/CHX₃ (X≡Cl, Br) compounds and the large shift in their T _c upon Cl↦Br substitution. The increase in T _c is due to contribution to the pairing arising from the interaction of electrons with the vibrational manifold of the molecule. The proposed mechanism opens up the possibility to observe a site-selective isotope effect. We also suggest that intercalating CHI₃ would further increase the critical temperature to T _c≃ 140 K. Received 23 January 2002     

Magnetic properties of DyFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The magnetic properties of an easy-axis trigonal DyFe₃(BO₃)₄ antiferromagnetic crystal have been theoretically studied. On this basis, recent experimental data [1] on the field and temperature dependences of magnetization and the temperature dependence of the initial magnetic susceptibility for three crystallographic directions in this antiferromagnet have been interpreted. The characteristics of the trigonal crystal field for the rare earth ion and the parameters of the Fe-Fe and Fe-Dy exchange interactions are determined. Limitations imposed by features of the magnetic characteristics (anisotropic magnetization in the three crystallographic directions, Schottky-type anomalies in the magnetic susceptibility, etc.) on the possible splitting of the ground-state multiplet in the crystal field and the splitting of the lowest doublet due to the f-d interaction for Dy³⁺ ions are established.     

Magnetic structure and magnetic excitations in the two-dimensional spin system of the Cu<Subscript>3</Subscript>B<Subscript>2</Subscript>O<Subscript>6</Subscript> compound

This paper reports on the results of measurements of the magnetic susceptibility, heat capacity, neutron scattering, muon spin relaxation, and electron paramagnetic resonance in Cu₃B₂O₆ for the study of the ground state of the spin system of this compound. The results obtained suggest that, at a temperature of 10 K, the spin subsystem of the crystal, which consists of single spins and clusters of pairs and fours of spins interacting with one another, undergoes a transition to a state representing a superposition of the singlet (for clusters) and magnetically ordered (for single spins) states.     

Iron-induced magnetic,transport and magnetoresistance behavior in Nd<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films

The effects of Fe doping on Mn site in the colossal magnetoresistive film, Nd_0.67Sr_0.33MnO₃ have been studied by preparing the series Nd_0.67Sr_0.33Mn_1-xFe_xO₃ (x=0,0.05 and 0.1). Upon doping, no structural changes have been found. However, the Curie temperature, the associated metal-to-insulator transition temperature and the magnetization decrease drastically with Fe doping. The resistivity in the paramagnetic regime for all the samples follows Emin–Holsteins theory of small polaron. The polaron activation energy, W_p and resistivity coefficient, A increase with Fe doping. This effect may be ascribed to the fact that upon Fe doping, the long-range ferromagnetic order is destroyed and, therefore, W_p is enhanced in the system. As compared to the La-based system, Fe doping has a stronger tendency to destabilize the long-range ferromagnetic order in the Nd-based system. Large MR (as high as 90%) observed in the epitaxial NSMFO film may be attributed to the good lattice-matching between the grown film and substrate. PACS 75.47.Gk; 75.47.Lx; 75.70.-i     

Magnetic properties of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> surface

The magnetic properties of the magnetite Fe₃O₄(110) surface have been studied by spin resolved Auger electron spectroscopy (SRAES). Experimental spin resolved Auger spectra are presented. The results of calculation of Auger lines polarization carried out on the basis of electronic state density are presented. Problems related to magnetic moments of bivalent (Fe²⁺) and trivalent (Fe³⁺) ions on the Fe₃O₄(110) surface are discussed. It is established that the deposition of a thin bismuth film on the surface results in significant growth of polarization of iron Auger peaks, which is due to additional spin-orbit scattering of electrons by bismuth atoms.     

Investigation of ion transport in Li<Subscript>8</Subscript>ZrO<Subscript>6</Subscript> and Li<Subscript>6</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> solid electrolytes

Lithium ionic conductivity and spin-lattice relaxation rates were measured in Li₈ZrO₆ and Li₆Zr₂O₇ solid electrolytes. It was found that the Li₈ZrO₆ solid electrolyte undergoes a transition to the superionic state in the temperature range 673–703 K. It was shown that Li⁺ ions are mobile in particular lattice positions of the Li₆Zr₂O₇ phase, and that ionic conductivity is monotonic at an activation energy of 79.4 kJ/mol.     

Magnetic and thermal properties of single-crystal NdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The neodymium ferroborate NdFe₃(BO₃)₄ undergoes an antiferromagnetic transition at T _N = 30 K, which manifests itself as a λ-type anomaly in the temperature dependence of the specific heat C and as inflection points in the temperature dependences of the magnetic susceptibility χ measured at various directions of an applied magnetic field with respect to the crystallographic axes of the sample. Magnetic ordering occurs only in the subsystem of Fe³⁺ ions, whereas the subsystem of Nd³⁺ ions remains polarized by the magnetic field of the iron subsystem. A change in the population of the levels of the ground Kramers doublet of neodymium ions manifests itself as Schottky-type anomalies in the C(T) and χ(T) dependences at low temperatures. At low temperatures, the magnetic properties of single-crystal NdFe₃(BO₃)₄ are substantially anisotropic, which is determined by the anisotropic contribution of the rare-earth subsystem to the magnetization. The experimental data obtained are used to propose a model for the magnetic structure of NdFe₃(BO₃)₄.     

Calculation of the electronic structure,lattice dynamics,and optical and magnetic properties of europium tetraborate EuB<Subscript>4</Subscript>O<Subscript>7</Subscript>

The electronic band structure, lattice vibration frequencies, and optical and magnetic properties of EuB₄O₇ crystal with α-SrB₄O₇-type structure are calculated within the density functional method. It is found that this compound is a dielectric with a band gap of the order of 4 eV. It is found that the ground state of this crystal is the state with ferromagnetic ordering of Eu²⁺ spins. Exchange interaction constants are calculated; the ferromagnetic ordering temperature is estimated within the molecular field approximation (T_c ≈ 1 K). The EuB₄O₇ compound is a magnetic pyroelectric and, hence, can exhibit magnetoelectric properties. The calculated polarization change during ferromagnetic ordering of this crystal is 3973 μC/m².     

Magnetic properties of aerugite Co<Subscript>10</Subscript>Ge<Subscript>3</Subscript>O<Subscript>16</Subscript>

This paper reports on the first study of the temperature and field dependences of the magnetization, heat capacity, and electrical properties of synthesized polycrystalline samples of aerugite Co₁₀Ge₃O₁₆, as well as on x-ray diffraction analysis of this compound. It is shown that the cobalt ions in this compound occupy three nonequivalent positions. The results of the experimental and theoretical studies suggest that aerugite is a ferrimagnet with two uncompensated magnetic moments of the cobalt atom per formula unit.     

Magnetic properties and the magnetoimpedance effect of nanostructured Fe<Subscript>73.5</Subscript>Si<Subscript>16.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> ribbons with induced magnetic anisotropy

The structure and magnetic properties and the magnetoimpedance effect of nanocrystalline Fe_73.5Si_16.5B₆Nb₃Cu₁ alloy ribbons, obtained from the amorphous state by annealing under different conditions, were comparatively analyzed. Despite the similarity of the samples’ structural states and the processes of their quasi-static magnetization reversal, the features of the magnetoimpedance effect are indicative of significant differences in the processes of their dynamic magnetization.     

Magnetic anisotropy and anisotropic tunneling magnetoresistance of compacted half-metal CrO<Subscript>2</Subscript> nanopowders

Low-temperature magnetic and magnetoresistive properties of compacted ferromagnetic halfmetal CrO₂ powders with nanoparticle shape anisotropy are studied. Magnetic anisotropy induced by the formation of magnetic texture during compaction is revealed. It is shown that tunneling magnetoresistance anisotropy is associated with the difference between a sample’s rates of magnetization in the longitudinal and transverse fields.     

Magnetic and electrical properties of Fe<Subscript>1.91</Subscript>V<Subscript>0.09</Subscript>BO<Subscript>4</Subscript> warwickite

We have performed a complex investigation of the structure and the magnetic and electrical properties of a warwickite single crystal with the composition Fe_1.91V_0.09BO₄. The results of Mössbauer measurements at T=300 K indicate that there exist “localized” (Fe²⁺, Fe³⁺) and “delocalized” (Fe_2.5+) states distributed over two crystallographically nonequivalent positions. The results of magnetic measurements show that warwickite is a P-type ferrimagnet below T=130 K. The material exhibits hopping conductivity involving strongly interacting electrons. The experimental data are analyzed in comparison to the properties of the initial (unsubstituted) Fe₂BO₄ warwickite. The entire body of data on the electric conductivity and magnetization are interpreted on a qualitative basis.     

Effect of B-site dopants on magnetic and transport properties of LaSrCoRuO<Subscript>6</Subscript>

Effect of Co, Ru and Cu substitution at B and B’ sites on the magnetic and transport properties of LaSrCoRuO₆ have been investigated. All the doped compositions crystallize in the monoclinic structure in the space group P2₁/n indicating a double perovskite structure. While the magnetization and conductivity increase in Co and Ru doped compounds, antiferromagnetism is seen to strengthen in the Cu doped samples. These results are explained on the basis of a competition between linear Co-O-Ru-O-Co and perpendicular Co-O-O-Co antiferromagnetic interactions and due to formation of Ru-O-Ru ferromagnetic networks.     

Superconductivity in Ti-doped iron-arsenide compound Sr<Subscript>4</Subscript>Cr<Subscript>0.8</Subscript>Ti<Subscript>1.2</Subscript>O<Subscript>6</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>

Superconductivity was achieved in Ti-doped iron-arsenide compound Sr₄Cr_0.8Ti_1.2O₆Fe₂As₂ (abbreviated as Cr-FeAs-42622). The X-ray diffraction measurement shows that this material has a layered structure with the space group of P4/nmm, and with the lattice constants a = b = 3.9003 Å and c = 15.8376 Å. Clear diamagnetic signals in ac susceptibility data and zero-resistance in resistivity data were detected at about 6 K, confirming the occurrence of bulk superconductivity. Meanwhile we observed a superconducting transition in the resistive data with the onset transition temperature at 29.2 K, which may be induced by the nonuniform distribution of the Cr/Ti content in the FeAs-42622 phase.     

Magnetic properties of Pb<Subscript>2</Subscript>Fe<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals

Single crystals of Pb₂Fe₂Ge₂O₉ have been grown. They were subjected to X-ray diffraction, magnetic, neutron diffraction, Mössbauer and spin resonance studies. It has been established that Pb₂Fe₂Ge₂O₉ is a weak ferromagnet with a Néel temperature T _N = 46 K, and the exchange and spin-flop transition fields have been estimated. It has been demonstrated that the weak ferromagnetic moment is actually the result of the single-ion anisotropy axes for the magnetic moments of different magnetic sublattices being not collinear.     

Synthesis and properties of NaFeGe<Subscript>2</Subscript>O<Subscript>6</Subscript> polycrystals

NaFeGe₂O₆ polycrystals were synthesized and their x-ray diffraction, magnetic, electrical, and Mössbauer characteristics were measured. It is established that this monoclinic compound is a dielectric with a temperature of antiferromagnetic ordering of 15 K. The Mössbauer spectrum at 300 K is a quadrupole doublet. The isomer shift is 0.40 mm/s, which is characteristic of the high-spin Fe³⁺ ion in the octahedral coordination. The quadrupole splitting is 0.34 mm/s, which indicates that the oxygen octahedron around the iron cation is distorted. The exchange interactions are estimated, and the crystal magnetic structure is discussed.