Enhancement of room-temperature magnetoresistance in La<Subscript>0.6</Subscript>Dy<Subscript>0.1</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>/Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The samples of La0.6Dy0.1Sr0.3MnO3/(Ag2O)x/2(x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10, 0.20, 0.25, and 0.30) were prepared by using the solid-state reaction method.Their magnetic property, transport behavior, transport mechanism and magnetoresistance effect were studied through the measurements of magnetization-temperature(M-T) curves, ρ-T curves and the fitting of ρ-T curves.The results indicated that Ag could take part in the reaction when the doping amount is small.However, when the doping amount is compar...     

Effect of yttrium doping on the resistivity and magnetoresistance of La<Subscript>0.80</Subscript>Sr<Subscript>0.20</Subscript>MnO<Subscript>3</Subscript>

The temperature and magnetic field dependence of resistivity of crystals of nominal compositions of La_0.75Y_0.05Sr_0.20MnO₃ grown by floating zone method at different growth rates are studied. The yttrium doping is found to result in the decrease of Curie temperature, suppression of the Pnma-R $ \bar 3 $ \bar 3 c structural transition, increase of resistivity and magnetoresistance. It is shown that the physical properties of the manganites studied are determined by not only the nominal composition but also the growth rate.     

<Superscript>63</Superscript>Cu NMR spectra,magnetic susceptibility,and transmission electron microscopy of the rapidly quenched alloy Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript>

Methods of transmission and scanning electron microscopy and nuclear magnetic resonance (NMR) at ⁶³Cu nuclei, as well as measurements of the static magnetic susceptibility χ(T) have been used to study a shape-memory alloy (SMA) Ti₅₀Ni₂₅Cu₂₅, which experiences a thermoelastic martensite transformation. The alloy was obtained from an amorphous ribbon in a bimodal nano- and submicrocrystalline state via a crystallization annealing for 1 h at 770 K with a subsequent quenching to room-temperature water. The resultant B2 austenite is characterized by a fine structure of the ⁶³Cu NMR spectra, which is connected with the different distribution of ⁶³Cu atoms on the second coordination shell. The evolution of the shape of the spectra with decreasing temperature reveals a structural transition B2 → B19. In addition, the ⁶³Cu NMR spectra, just as the transmission electron microscopy, indicate the presence of phase separation in the alloy, with the precipitation of a TiCu (B11) phase. The temperature dependence of the static magnetic susceptibility χ(T) also indicates the occurrence of a structural transition and has a hysteretic nature of “stepped” type. The discovered stepped nature of the χ(T) dependence is explained by the bimodal size distribution of grains of the B2 phase due to the size effect of the martensitic transformation.     

Fabrication and characterization of SmO_（0.7）F_（0.2）FeAs bulk with a transition temperature of 56.5 K

The superconductivity of iron-based superconductor SmO 0.7 F 0.2 FeAs was investigated. The SmO 0.7 F 0.2 FeAs sample was prepared by the two-step solid-state reaction method. The onset resistivity transition temperature is as high as 56.5 K. X-ray diffraction (XRD) results show that the lattice parameters a and c are 0.39261 and 0.84751 nm, respectively. Furthermore, the global J c was more than 2.3 × 10 5 A/cm 2 at T = 10 K and H = 9 T, which was calculated by the formula of J c = 20ΔM/[a(1-a/(3b))]. The upper critical fields, H c2 ≈ 256 T (T = 0 K), was determined according to the Werthamer-Helfand-Hohenberg formula, indicating that the SmO 0.7 F 0.2 FeAs was a superconductor with a very promising application.     

Effect of NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> doping on the structure,magnetic properties and electrical properties of La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>

The (La_0.7Ca_0.3MnO₃)_1x /(NiFe₂O₄) _x (x = 0 to 0.09) composites were prepared using a conventional solid state reaction method. The structural, magnetic properties, and electrical properties of LCMO/NFO composites were investigated using X-ray diffraction, scanning electron microscopy, field cooled DC magnetization, and magnetoresistance (MR) measurements. The resistivity measured as a function temperature demonstrates that the pure LCMO and x = 0.01 samples display metal to semiconductor transitions. However, the composites of x > 0.03 samples clearly present the electrical behavior as an insulator/semiconductor type behavior. It was observed that the resistivity of the samples increased systemically with an increase of the NFO content. From the MR measurements, it was found that the MR effect is enhanced for x = 0.01 with a NFO composition. In all, the spin-polarized tunneling and the spin-dependent scattering may be beneficial for an improved low-field magnetoresistance effect. These phenomena can be explained by the segregation of a new phase related to NFO at the grain boundaries or surfaces of the LCMO grains.     

Commensurate-incommensurate magnetic phase transition induced in TbNi<Subscript>5</Subscript> by an external magnetic field

Neutron-diffraction studies of a single crystal of TbNi₅ have been performed. Temperature and field dependences of the intensities of the Bragg reflection (001) and magnetic satellites (001)⁻ and (001)⁺ have been determined. In the temperature range of 7–10 K, sharp changes in the intensities of both the Bragg reflection and its satellites have been revealed. A conclusion is made that these changes are due to the existence of a magnetic transition from an incommensurate into a lock-in structure. It has been found that at 11 K an external magnetic field μ₀ H = 0.1 T oriented along the [100] axis induces the same magnetic state of the crystal as its cooling to 7 K in the absence of a field. In an external magnetic field μ₀ H = 0.4 T, there occurs a magnetic transition from the incommensurate phase with a wave vector k = 2π/c (0, 0, 0.18) into a commensurate ferromagnetic phase with k = 0. In the ferromagnetic phase, an increase in the field from 0.4 to 0.6 T is accompanied by an increase in the intensity of the ferromagnetic reflection (001) by a factor of 1.6. This indicates the existence of strong fluctuations of the magnetization at 0.4 T.     

Transport phenomena in the La<Subscript>0.72</Subscript>Ba<Subscript>0.28</Subscript>MnO<Subscript>3</Subscript> single crystal

Results of an experimental study of the temperature dependences of the magnetization M, electrical resistivity, magnetoresistance, thermo-and magnetothermo-emf, and Hall effect of the La_0.72Ba_0.28MnO₃ single crystal are presented. An analysis of the temperature dependences of kinetic properties shows that, at low temperatures, electrons are principal charge carriers in La_0.72Ba_0.28MnO₃ and the metallic conduction takes place. As the temperature increases to T ≈ 145 K, the sign of the ordinary Hall coefficient reverses; this indicates the change in the type of the majority charge carriers. Within a certain temperature range which lies substantially below the Curie temperature (T _C ), a metal-semiconductor transition occurs. Near the Curie temperature and within the paramagnetic range, the manganite under study is a semiconductor; the conduction is mainly effected by holes activated to the mobility edge. The critical behavior of the resistance and magnetoresistance is discussed.     

Kinetics of forward extraction of Ti（IV） from H_2SO_4 medium by P_（507） in kerosene using the single drop technique

The kinetics of forward extraction of Ti(IV) from H2SO4 medium by P507 in kerosene has been investigated using the single drop technique.In the low concentration region of Ti(IV),the rate of forward extraction at 298 K can be represented by F(kmol·m-2·s-1)=10-5.07 [TiO 2 + ][H+]-1 [NaHA 2 ](o)·Analysis of the rate expression reveals that the rate determining step is(TiO)(i)2+ +(HA 2)(i)-[TiO(HA2)](i)+.The values of Ea,H±,S±,and G±298 are calculated to be 22 kJ·mol-1,25 kJ·mol-1,-218 J·mol-1·K-1,and 25 kJ·mol-1,respectively.The experimental negative S± values indicate that the reaction step occurs via SN2 mechanism.     

Magnetic behavior of the nanophase of YbNi<Subscript>2</Subscript> alloys

Variations in magnetic properties of the heavy-fermion YbNi₂ alloy when milled in a high energy ball milling system have been investigated. The ferromagnetic transition (T _C = 10.4 K) in the initial sample almost vanishes after milling, which leads to the appearance of a magnetic transition at T* = 3.2 K in nanocrystallites. Before milling, processes of spin–lattice relaxation of the Orbach–Aminov type with the participation of the first excited Stark sublevel of the Yb³⁺ ion located at 75 K are dominating in the electron spin dynamics in the paramagnetic phase of the alloy. A comparative study of the temperature dependence of the magnetic properties and spectra of electron paramagnetic resonance in poly- and nanocrystalline samples indicates the existence of a magnetic inhomogeneity of the compound arising upon milling.     

Electronic structures of new tunnel barrier spinel MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript>: first-principles calculations

The electronic structures of spinel MgAl2O4 and MgO tunnel barrier materials were investigated using first-principles density functional theory calculations. Our results show that similar electronic structures are found for the MgAl2O4 and MgO tunneling barriers. The calculated direct energy gaps at the Γ-point are about 5. 10 eV for MgAl2O4 and 4. 81 eV for MgO, respectively. Because of the similar feature in band structures from Γ high-symmetry point to F point (△ band), the coherent tunneling effect might be expected to appear in MgAl2O4-based MTJs like in MgO-based MTJs. The small difference of the surface free energies of Fe (2. 9 J. m-2) and MgAl2O4 (2. 27 J·m-2) on the {100}orientation, and the smaller lattice mismatch between MgAl2O4 and ferromagnetic electrodes than that between MgO and ferromagnetic electrodes, the spinel MgAl2O4 can substitute MgO to fabricate the coherent tunneling and chemically stable magnetic tunnel junction structures, which will be applied in the next generation read heads or spintronic devices.     

Magnetoresistive properties of Ni-doped La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganites

La_0.7Sr_0.3Mn_1?x Ni _x O₃ (x = 0, 0.025, 0.050 and 0.075) ceramics were prepared by the conventional solid-state reaction method. The partial substitution of Mn by Ni²⁺ leads to a decrease in cell volume as well as a structural transition from the rhombohedral to the orthorhombic structure. Ni²⁺ doping increases the electrical resistivity, decreases the semiconductor–metal transition temperature (T _ms) and relatively enhances the room temperature magnetoresistance (MR), especially in x = 0.025 and around T _ms. With respect to conduction mechanism, the small polaron hopping (SPH) and the variable range hopping (VRH) models were used to examine conduction in the semiconducting region.     

Grain size dependent magnetization, electrical resistivity and magnetoresistance in mechanically milled La0.67Sr0.33MnO3

We have studied magnetization, ac susceptibility, resistivity and magnetoresistance in mechanically milled La_0.67Sr_0.33MnO₃. The material with grain size micron to nanometer scale has stabilized in rhombohedral crystal structure with space group R3C. We have found various grain size effects, e.g., decrease of ferromagnetic moment, increase of surface spin disorder, and appearance of insulator/semiconductor type resistivity. In addition to these conventional features, we have identified a magnetic anomaly at 45 K in bulk sample. Ferromagnetic to paramagnetic transition temperature (T_C) is above room temperature for all samples. The samples are typical soft ferromagnet that transformed from multi-domain state to single domain state in nanocrystalline samples. The remarkable observation is that low temperature freezing of ferromagnetic domains/clusters does not follow the conventional spin glass features. Experimental results clearly showed the enhancement of high field magnetoresistance in nanocrystalline samples below 200 K, whereas low field magnetoresistance gradually decreases above 200 K and almost absent at 300 K. We have discussed few more magnetic and electrical changes, highly relevant to the progress of nanomaterial research in ferromagnetic manganites.     

Determination of spin-orbit splitting Δ_0 of valence band at Γ for gallium phosphide nanoparticles using fluorescence and infrared spectroscopes

The value of spin-orbit splitting Δ 0 of gallium phosphide (GaP) nanoparticles was determined. The information concerning the spin-orbit splitting of the valence band at Γ was acquired using fluorescence and infrared spectroscopes. Detailed investigation on the fluorescence characteristics under ultraviolet photoexcitation reveals that two doublets of emission transitions are related to the spin-orbit splitting of the valence band. The origin of two broad violet emissions, 3.00 and 3.10 eV, can be attributed to the direct transitions near the Γ point of the Brillouin zone between the Γ 1 conduction band and Γ 15 valance band, that is, Γ 6c -Γ 8v and Γ 6c -Γ 7v , respectively. The origin of two blue emissions, 2.74 and 2.64 eV, can be attributed to the indirect transitions between the X 1 conduction band and Γ 15 valance band, that is, Δ 5c -Γ 8v and Δ 5c -Γ 7v , respectively. Based on these transitions, the spin-orbit splitting Δ 0 of the GaP nanoparticles is determined as 0.10 eV. The infrared spectrum of the GaP nanoparticles shows a band at 817 cm -1 which is assigned to the transition between the Γ 7v and Γ 8v valence band maxima. It follows therefore that the spin-orbit splitting Δ 0 is 0.10 eV.     

Temperature dependence of magnetic properties of a disordered Fe<Subscript>0.65</Subscript>Ni<Subscript>0.35</Subscript>

The method of calculation of average and local magnetic characteristics of transition metals at finite temperatures developed by the author and Grebennikov in previous works and based on the employment of a real band structure and spin fluctuations has been applied to a disordered invar alloy Fe_0.65Ni_0.35. The magnetization, Curie temperature, local magnetic moment, and uniform and local susceptibilities have been calculated for both ferromagnetic and paramagnetic states. The results are in agreement with experimental data in a wide temperature range. It has been shown that the effect of disorder in the filling of sites with Fe and Ni atoms on the magnetic properties of the Fe-Ni invar is insignificant.     

Microstructures of Ti<Subscript>50</Subscript>Al<Subscript>45</Subscript>Mo<Subscript>5</Subscript> alloy powders produced by plasma rotating electrode process

Microstructures of Ti₅₀Al₄₅Mo₅ (at.%) alloy powders produced by the plasma rotating electrode process (PREP) were investigated. The powders have inhomogeneous structures, which consist of dendrites and rounded grains. The dendrites, which show a “rosettelike” morphology, are formed on the powder surface and around the rounded grains. The rosettelike dendrites are of hexagonal α ₂ (D0₁₉) phase even though the dendrites have an equiaxial morphology, and a small amount of β ₂ (B2) phase is also contained inside. It is suggested that the solidification to α (hcp-A3) phase occurred by the peritectic reaction between the primary β (bcc-A2) dendrites and the liquid: L+β→L+β+α. The rounded grains, on the other hand, are of β ₂ phase in which acicular α or α ₂ laths are precipitated with the Burgers orientation relationship. Antiphase domain boundaries in the β ₂ matrix are intersected by α(α ₂) laths. It is interpreted that the α(α ₂) laths were formed by the solid-state transformations: β ₂→β ₂+α→β ₂+α ₂. The formation of the two different microstructures in the powder particles is rationalized in terms of the changes in local composition of the liquid phase during the rapid solidification process.     

Multiferroic BiFeO<Subscript>3</Subscript> thick film fabrication by aerosol deposition

BiFeO₃ (BFO) multiferroic materials in the crystalline phase require very delicate processing conditions. In order to fabricate a high quality BFO thick film, aerosol deposition (AD) was employed and the phase evolution and multiferroic properties of the film were investigated for different annealing temperatures. A BFO thick film annealed at 500 °C had a dielectric constant of 80 at 1 kHz and possessed ferroelectric characteristics. At an applied electric field of ∼900 kV/cm, the remaining polarization and coercive field (E_c) were approximately 7.5 μC/cm² and 370 kV/cm, respectively. In addition, the BFO thick film fabricated via AD and annealed at 500 °C showed weak ferromagnetic behavior between −1250 Oe and +1250 Oe and was saturated at the higher magnetic field strength, showing ferromagnetic behavior.     

VO<Subscript>2</Subscript> thin films: Defect mediation in room temperature ferromagnetic switching characteristics

Vanadium dioxide (VO₂) has tremendous potential in multifunctional device applications related to spintronics, switching, and magnetic recording. We have discovered that the room temperature ferromagnetism (RTFM) in undoped vanadium oxide epitaxial films can be switched on and off by altering the cooling ambient conditions which exhibit a sharp electrical transition at 341 K. By correlating the structural and ferromagnetic properties in VO₂, we envisage the potential for creation of novel multifunctional solid-state devices. High-quality epitaxial VO₂ thin films were grown on c-sapphire (0001) substrates, under different ambient conditions via the domain matching epitaxy paradigm. The observed RTFM has its origin in the valence charge defects with unpaired electrons in V⁺³ in VO₂ thin films, where the concentration of the defects could be varied with oxygen partial pressure. The VO₂ films-with a high ferro- to paramagnetic transition (Curie) temperature around 500 K estimated by fitting the magnetization data to the Bloch’s T^3/2 law, a saturated magnetization of 18 emu/cm³, and with a finite coercivity of 40 Oe at 300 K-can be useful for integrated smart sensors operable at room temperature.     

Investigation of the structure of compacts and sheets of an Al-Cu-Li alloy strengthened by Al<Subscript>2</Subscript>CuLi (T<Subscript>1</Subscript>) particles

X-ray diffraction and electron microscopy have been used to investigate structural states of the material of compacts and sheets of an aluminum-copper-lithium alloy with a lithium content corresponding to the field of equilibrium of a solid solution with the phase T ₁ (Al₂CuLi). In pressed strips, there arises a multicomponent texture (Bs “110”〈112〉, Cu “211”〈111〉, S “123”〈hkl〉) typical of pressed articles of aluminum-lithium alloys. Their microstructure was characterized by the presence of slip bands oriented in the rolling direction. The formation of lamellar precipitates of the T ₁ phase at slip-band boundaries in the process of deformation is supposedly stimulated by Shockley partial dislocations. In hot-rolled sheets, there arose an unusually intense nearly single-component texture of the Ex ₁ “011”〈111〉 type, whose appearance is mainly caused by the cross rolling of the sheets. The reduced strength characteristics of the sheets examined are connected with a sharply oriented character of the structure of both the matrix phase and the strengthening T ₁ phase. A quasicrystalline phase T ₂ present in the material and the precipitates of the δ′ phase exert no marked effect on the level of mechanical properties.     

Low-temperature properties of the orderable Cu<Subscript>3</Subscript>Pd alloy

The effect of atomic ordering on the transport properties of the Cu₃Pd alloy in the low-temperature range (T < 70K) in strong magnetic fields (H ≤ 150 kOe) has been studied. The temperature dependence of the resistivity during alloy disordering has been found to have a minimum at T ～ 10 K. The Hall effect has a negative sign. The magnetoresistance, on the contrary, is positive; the longitudinal effect equals ～50% of the transverse one.