Electric resistance and magnetoresistance of 30-nm-Thick La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films elastically compressed in the (110) plane

We have studied the structure, electric resistance, and magnetoresistance of 30-nm-thick (110)La_0.67Ca_0.33MnO₃ (LCMO) films grown by laser deposition on (001)-oriented LaAlO₃ substrates. The unit cell parameters a and b (along the [100] and [010]LCMO axes, respectively) of these manganite films are significantly (by ∼1.2%) increased as compared to the corresponding values in the pseudocubic unit cell of bulk stoichiometric LCMO crystals. At T < 150 K, the temperature dependence of the resistivity of LCMO films is well described by the relation ρ = ρ₁ + ρ₂ (H) T ^4.5. The value of ρ 2 decreases with increasing magnetic field and is close to the analogous coefficient for manganite films grown on substrates with small lattice misfit.     

Effect of low level substitution of Sr–Ba on transport and magnetic behaviour of La<Subscript>0·67</Subscript>Ca<Subscript>0·33</Subscript>MnO<Subscript>3</Subscript>

In this paper we report the investigation of transition metal oxide compound, La_0·67Ca_0·25Sr_0·04Ba_0·04MnO₃ (LCSBMO), along with La_0·67Ca_0·33MnO₃ (LCMO), synthesized by sol–gel route under identical conditions. The effect of simultaneous low level substitution of large size ions such as Sr²⁺ and Ba^2 + for Ca^2 + ions on the electronic transport and magnetic susceptibility properties are analysed and compared apart from microstructure and lattice parameters. The temperature dependent electrical transport of the polycrystalline pellets of LCSBMO and LCMO when obeying the well studied law, r = r₀ + r₂   T²\rho = \rho_{0} + \rho_{2} \;T^{2} for T < T _MI , is observed to differ by more than 50% from the values of ρ ₀ and ρ ₂, with the former compound showing enhanced electrical conductivity than the latter. Similarly in fitting the adiabatic small polaron model for resistivity data of both the samples for T > T _MI , the polaron activation energy is found to differ by about 11%. In addition, the temperature dependent a.c. magnetic susceptibility study of the compounds shows a shift of about 6% in the paramagnetic to ferromagnetic transition temperature (285 K for LCSBMO and 270 K for LCMO).     

Response of the electric resistance of 40-nm-thick La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films to an increase in the lattice mismatch between film and substrate

We have studied the structure and resistivity of 40-nm-thick La_0.67Ca_0.33MnO₃ (LCMO) films coherently grown on (001)-oriented La_0.29Sr_0.71Al_0.65Ta_0.35O₃ single crystal substrates bearing epitaxial interlayers of strontium titanate with a thickness of 7 or 70 nm. As the effective mismatch between the crystal lattice parameters of the film and substrate increases, the ρ value exhibits a sharp growth, while the maximum of the ρ(T) curve shifts by ∼40 K toward lower temperatures. At T < 150 K, the temperature dependence of the resistivity of the LCMO films obeys the relation ρ ∼ ρ₁ T ^4.5, where the coefficient ρ₁ decreases with increasing applied magnetic field strength and with decreasing lattice mismatch between the manganite film and the substrate.     

Impact of Tuning Molar Ratio in the Starting Materials: <Emphasis Type="Italic">Magneto</Emphasis>-Transport Features of Hybrid YSr<Subscript>2</Subscript>Ru<Subscript>0.9</Subscript>Cu<Subscript>2.1</Subscript>O<Subscript>7.9</Subscript>

The impact of slightly tuning molar ratio in the starting materials on the physical properties of 1212-type rutheno-cuprate, YSr₂Ru_0.9Cu_2.1O_7.9 (nominal) samples prepared under four synthesis approaches are reported. Interestingly, all samples clearly show the differences in the physical properties of the samples prepared under different synthetic protocols. However, neither XRD nor EDX reveal any notable differences in the crystal structure or sample composition. All the samples exhibit magneto-superconducting properties (H _ext=5 Oe) which are slightly varied with synthetic approaches. The high field (H _ext=10 kOe) temperature dependence of magnetization data shows a sharp ferromagnetic transition around 150 K and all the samples obey Curie–Weiss linear behavior above 180 K. The experimental effective paramagnetic moment for the various samples is in the range of 2.5 and 2.7μ _B/Ru which are in line with the literature report. The magnetization, M(H) isotherm curves measured at 5 K and −10 kOe≤H≤10 kOe conditions reveal weak ferromagnetic-like hysteresis loops for all samples with returning moment (M _r) and coercive field (H _c), whereas the high field M(H) loops indicate soft ferromagnetic behaviors with magnetic saturation. The saturation moment of the samples is slightly varied with the synthesis approaches. None of the samples showed bulk superconductivity (T_c^{R = 0})T_{\mathrm{c}}^{R = 0}) down to 2 K, while all samples show onset transitions (T_c^onsetT_{\mathrm{c}}^{\mathrm{onset}}) except the sample prepared by approach-3. The latter approach sample shows semiconducting behavior down to 2 K. The T_c^onsetT_{\mathrm{c}}^{\mathrm{onset}} noticed at 34 K, 12 K, and 6 K for the sample prepared by approach-1, 2, and 4, respectively. The nearly linear dependence suggests that hopping conduction is dominant in certain temperature range for all samples. The magneto-transport features of these samples exhibit maximum magnetoresistance (MR) at low temperatures. Remarkably, the sample prepared by approach-1 shows largest −MR about 77% at low temperature 2 K and H=90 kOe which stimulates for further investigations. Among the four synthesis approaches employed in the present study, we can probably suggest that the approach-1 (0.5Y₂O₃+0.5SrO₂+1.5SrCuO₂+0.9RuO₂+0.6CuO) is the preferable method to achieve the best sample (in terms of magneto-transport features).     

Ac and dc magnetotransport properties of the phase-separated La<Subscript>0.6</Subscript>Y<Subscript>0.1</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite

The physical properties of the La_0.6Y_0.1Ca_0.3MnO₃ compound have been investigated, focusing on the magnetoresistance phenomenon studied by both dc and ac electrical transport measurements. X-ray diffraction and scanning electron microscopy analysis of ceramic samples prepared by the sol–gel method revealed that specimens are single phase and have average grain size of ∼0.5 μm. Magnetization and 4-probe dc electrical resistivity ρ(T,H) experiments showed that a ferromagnetic transition at T _C ∼ 170 K is closely related to a metal-insulator (MI) transition occurring at essentially the same temperature T _MI . The magnetoresistance effect was found to be more pronounced at low applied fields (H ≤ 2.5 T) and temperatures close to the MI transition. The ac electrical transport was investigated by impedance spectroscopy Z(f,T,H) under applied magnetic field H up to 1 T. The Z(f,T,H) data exhibited two well-defined relaxation processes that exhibit different behaviors depending on the temperature and applied magnetic field. Pronounced effects were observed close to T _C and were associated with the coexistence of clusters with different electronic and magnetic properties. In addition, the appreciable decrease of the electrical permittivity ε′(T,H) is consistent with changes in the concentration of e _g mobile holes, a feature much more pronounced close to T _C .     

Thermodynamic properties of NaZr<Subscript>2</Subscript>(AsO<Subscript>4</Subscript>)<Subscript>3</Subscript>

The heat capacity C _p ⁰ of crystalline NaZr₂(AsO₄)₃ has been measured in the range 7–650 K using precision adiabatic calorimetry and differential scanning calorimetry. The experimental data have been used to calculate the standard thermodynamic functions of the arsenate: C _p ⁰, enthalpy H ⁰(T) − H ⁰(0), entropy S ⁰(T), and Gibbs function G ⁰(T) − H ⁰(0) from T → 0 to 650 K. The standard entropy of its formation from elements is Δ_f S ⁰(NaZr₂(AsO₄)₃, cr, 298.15 K) = −1087 ± 1 J/(mol K).     

Magnetoresistance of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films grown on substrates with orthorhombically distorted unit cells

We have studied the structure and magnetoresistance of 40-nm-thick epitaxial La_0.67Ca_0.33MnO₃ (LCMO) films grown by laser deposition on (001)-oriented NdGaO₃ (NGO) substrates. The manganite layers were oriented so that the b axis was perpendicular to the substrate plane and occurred under the action of inhomogeneous biaxial mechanical stresses. The negative magnetoresistance of the LCMO films in the vicinity of the ferromagnetic spin ordering was about 71% (μ₀ H = 1 T). The observed azimuthal anisotropy of the magnetotransport properties of 40-nm-thick LCMO/(001)NGO films can be explained within the framework of a model of anisotropic magnetoresistance taking into account the presence of the preferred orientation of the spontaneous magnetization.     

Magnetic cluster state in anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

It is established that the magnetic state of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite represents the spin-glass state of the cluster. The magnetic field at the beginning (H < 10 kOe) leads to the fragmentation of ferromagnetic clusters, then (H > 10 kOe) leads to the transition to a ferromagnetic state of an antiferromagnetic matrix and to increase in the degree of polarization of local spins of manganese. It is determined that the freezing temperature of magnetic moments varies as T _f = 65 − 6H ^0.21. The causes and mechanism of the magnetic phase separation are discussed.     

Pressure,temperature, and magnetic-field effects on the transport properties of Cd<Subscript>0.7</Subscript>Mn<Subscript>0.3</Subscript>GeAs<Subscript>2</Subscript>

The resistivity, Hall coefficient, and magnetoresistance ratio of the ferromagnetic semiconductor Cd_0.7Mn_0.3GeAs₂ have been measured as functions of pressure, temperature, and magnetic field. The ρ(p) and R _H(p) curves point to a structural insulator-metal phase transition under both increasing and decreasing pressure. The temperature dependences of ρ and R _H show anomalies, which are interpreted as a ferromagnetic-paramagnetic phase transition. Cd_0.7Mn_0.3GeAs₂ is shown for the first time to have a negative magnetoresistance under pressure.     

Revised heat capacity and thermodynamic functions of GdVO<Subscript>4</Subscript>

The heat capacity of GdVO₄ has been determined by adiabatic calorimetry in the range 5–345 K. The present experimental data and earlier results have been used to evaluate the thermodynamic functions of gadolinium orthovanadate (C _p ⁰(T), S ⁰(T), H ⁰(T) − H ⁰(0), and Φ⁰(T)) as functions of temperature (5–350 K). Its Gibbs energy of formation is determined to be Δ_f G ⁰(GdVO₄, 298.15 K) = −1684.5 ± 1.6 kJ/mol.     

Thermodynamic properties of Cu<Subscript>5</Subscript>SmSe<Subscript>4</Subscript>

The heat capacity of Cu₅SmSe₄ has been measured from 80 to 300 K. The results have been used to assess the main thermodynamic functions of Cu₅SmSe₄: entropy (S ⁰(T) − S ⁰(0)), enthalpy increment (H ⁰(T) − H ⁰(0)), and reduced Gibbs energy (Φ⁰(T)).     

Scaling of the Temperature Dependent Resistivity and Hall Effect in Ba(Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)As<Subscript>2</Subscript>

We show that the temperature-dependent resistivity ρ(T), Hall number n _H(T) and the cotangent of the Hall angle cot θ _H(T) of Ba(Fe_1−x Co _x )As₂ (x=0.0–0.2) can be scaled using a recently proposed model-independent scaling method (Luo et al. in Phys. Rev. B 77:014529, 2008). The zero field normal-state resistivity above T _c can be reproduced by the expresion r(T) = r₀ +cTexp(- \frac2\varDelta T )\rho(T) = \rho_{0} +cT\exp(- \frac{2\varDelta }{T} ) and scaled using the energy scale Δ, c and the residual resistivity ρ ₀ as scaling parameters. The scaling parameters have been calculated and the compositional variation of 2Δ and ρ ₀ has been determined. The 2Δ(x) dependence show almost linear decreasing in underdoped regime, minimum corresponding to the T _c maximum and increasing in overdoped regime. The latter is different from that reported for cuprates. The existence of a universal metallic ρ(T) curve which, however, is restricted for the underdoped compounds to temperatures above a structural and antiferromagnetic transition is interpreted as an indication of a single mechanism which dominates the scattering of the charge carriers in Ba(Fe_1−x Co _x )As₂ (x=0.0–0.2).     

Electrical Properties of Amorphous S<Subscript>0.60</Subscript>Ge<Subscript>0.40</Subscript>:H<Subscript>x</Subscript> Films

Amorphous Si_0.60Ge_0.40:H_x films containing 1.7, 3.9, 7.1, 12.1, and 17.3 at % H are prepared by magnetron sputtering at different hydrogen partial pressures, and their electrical conductivity is measured from 80 to 350 K. The conductivity of the films exhibits Arrhenius behavior in the range 250–350 K and satisfies the relation log(σT ^1/2) ∼ T^−1/4 between 80 and 250 K. The conductivity data are used to evaluate the electron localization radius, hop distance, and activation energy of hopping conduction at 80 K; the electron mobility at the Fermi level and in the conduction band; and the 300-K activation energy of conduction.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 787–791.Original Russian Text Copyright © 2005 by Najafov, Isakov.     

Controlled magnetoresistance in Y<Subscript>3/4</Subscript>Lu<Subscript>1/4</Subscript>Ba<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript>-CuO composites at 77 K

We have studied the low-temperature magnetoresistance of Y_3/4Lu_1/4Ba₂Cu₃O₇-CuO composites obtained by fast sintering technique and established a relation between the probing to critical current density ratio j/j _c and the shape of the magnetoresistance curve ρ(H). For j/j _c<1, the electric resistance arises at a threshold value of the magnetic field strength H _c. For j/j _c≥1, a linear variation of ρ(H) at 77 K in the range from 0 to 14 Oe can be provided by selecting the CuO content (in the 15–30 vol % interval) and the j value (in the 0.003–0.2 A/cm² range). In the latter case, the slope dρ/dH (i.e., the sensitivity of the electric resistivity with respect to the magnetic field) is 1–20 mΩ cm/Oe and the relative field-induced increase in the resistivity ρ₀=(ρ(H)−ρ(H=0))/ρ(H=0) amounts to 1320 and 685% at H=200 and 35 Oe, respectively. Composites possessing controlled magnetoresistance are promising materials for the active elements of magnetic field sensors capable of operating at a practically convenient liquid nitrogen temperature.     

Interplay of Superconductivity and Ferromagnetism in UGe<Subscript>2</Subscript>

The emergence of pressure induced superconductivity (SC) under the background of ferromagnetic state in 5f-electron based itinerant ferromagnetic superconductor UGe₂ is studied in the single band model by using a mean-field approximation. The solutions to the coupled equations of superconducting gap (Δ) and magnetization (m) are obtained using Green’s function technique and equation of motion method. It is shown that there generally exists a coexistent (Δ≠0, m≠0) solution to the coupled equations of the order parameters in the temperature range 0<T<min (T _C,T _FM), where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. The study of electronic specific heat (C/T), density of states, free energy, etc. are also presented. The specific heat capacity at low temperature shows linear temperature dependence as opposed to the activated behavior. Density of states increases as opposed to the case of a standard ferromagnetic metal. Free energy study reveals that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and, therefore, realized at low enough temperature. The agreement between theory and experimental results for UGe₂ is quite satisfactory.      

Ferromagnetism of Mn<Subscript>5</Subscript>Ge<Subscript>3</Subscript> Precipitates in Mn-implanted Self-organized Ge/Si Quantum Dots

One layer of self-assembled Ge quantum dots with Si barrier were grown on high resistivity (100) p-type Si substrates by rapid thermal chemical vapor deposition followed by Mn ion implantation and post-annealing. A presence of ferromagnetic structure was confirmed in the dilute magnetic quantum dots (DMQD). The DMQD through 10 min annealing was found to be homogeneous, and to exhibit p-type conductivity, insulating property, and ferromagnetic ordering with a Curie temperature, T _c=170 K. On the other hand, the DMQD through 30 and 60 min annealing was found to be semi-insulating and ferromagnetic ordering with a Curie temperature over 300 K. The XRD data show that there is a phase separation of Mn rich phases Mn₅Ge₃ from MnGe nanostructure. Therefore, it is likely that the ferromagnetic exchange coupling of sample with T _c=170 K is hole-mediated and the ferromagnetism in sample with T _c>300 K is due to Mn₅Ge₃ phase.     

Martensitic phase transformation in single crystal Co<Subscript>5</Subscript>Ni<Subscript>2</Subscript>Ga<Subscript>3</Subscript>

The magnetic, thermal, and transport properties of martensitic phase transformation in single crystal Co₅Ni₂Ga₃ have been investigated. The single crystal Co₅Ni₂Ga₃ shows martensitic transformation at 251 K on cooling and 254 K on warming. Large jumps in the temperature-dependent resistance curve, temperature-dependent magnetization curve, and temperature-dependent thermal conductivity curve are observed at martensitic transformation temperature (T _M). Negative magnetoresistance due to spin disorder scattering was observed in Co₅Ni₂Ga₃ single crystal at all temperature range. The temperature-dependent negative magnetoresistance shows a peak at T _M, which indicates that the spin disorder increases in the process of phase transition. Co₅Ni₂Ga₃ sample exhibits a temperature dependence of thermal conductivity κ(T) (dκ/dT > 0) due to electrons being above temperature 100 K.     

Magnetocaloric effect of Gd<Subscript>5</Subscript>Si<Subscript>2</Subscript>Ge<Subscript>2</Subscript> alloys in low magnetic field

The magnetocaloric effect of Gd₅Si₂Ge₂ alloys under heat treatment conditions are investigated in low magnetic fields. The magnetocaloric effect (MCE) is studied by measuring magnetic entropy change (ΔS _M) and adiabatic temperature change (ΔT _ad) in a magnetic field of 1·5 T using a vibrating sample magnetometer (VSM) and a home-made magnetocaloric effect measuring apparatus, respectively. The maximum ΔS _M of the alloys increases by 200% from 4·38 to 13·32 J kg⁻¹ K⁻¹, the maximum ·T _ad increases by 105% from 1·9 to 3·9 K when compared to the as-cast due to the homogeneous composition distribution and microstructure, while the magnetic ordering temperature is slightly reduced. These results indicate that the annealed Gd₅Si₂Ge₂ compounds are promising as high-performance magnetic refrigerants working room temperature in relatively low magnetic fields.     

Nano Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Induced Fluxoid Jumps and Low Field Enhanced Critical Current Density in MgB<Subscript>2</Subscript> Superconductor

Nano particle of Fe₃O₄ (nFe₃O₄) up to 6 at% were doped in the superconducting MgB₂ samples. Despite the strong ferromagnetic nature of Fe₃O₄, both the ac susceptibility and the resistivity measurements show that up to 4 at% of Fe₃O₄, T _c =38 K is not changed, whereas for 6% T _c decreases by 6 K. This indicates that a low concentration of Fe does not substitute either the Mg or B sites and probably occupies the intergrain spaces. For 0.5% doped Fe₃O₄, an increase in J _c with respect to the pure MgB₂ samples is observed in the lower field and temperature regions (H<2 T and 20 K) indicating an enhanced flux pinning and the magnetic activation, i.e., the interaction between the magnetic dipole of Fe ion and the vortices is weak in comparison to the effective pinning potential. Whereas, at H>2 T, J _c of the doped samples is always less than that of MgB₂, and the activation is dominant in comparison with the effective pinning potential provided by the doping. Flux jumps are observed in lower T and H regions for the samples doped up to 1% nFe₃O₄ only. Magnetization plots of higher Fe content samples exhibited clear paramagnetic background. Mossbauer measurements for the higher (4, 6 at%) nFe₃O₄ doped MgB₂ samples show that at RT, the hyperfine field for both samples is ∼100 kOe and ∼120 kOe at 90 K. This means that the nFe₃O₄ particles decompose and form possibly an intermetallic Fe-B phase in the matrix.     

Room-Temperature Photomagnetic Effect of Fe<Subscript>3</Subscript>Ga<Subscript>4</Subscript> Grown on GaAs Substrates

Ga–As–Fe composite films prepared by molecular beam epitaxy at 600°C on GaAs(100) substrates with the stacking sequence of [100-nm GaAs/50-nm Fe₃Ga_{2− x} As _x /100-nm GaAs] exhibit the distinct photo-enhanced magnetization at room temperature. Transmission electron microscopy reveals the formation of metamagnetic Fe₃Ga₄ grains on the sample surface. Illumination power dependence of the enhanced magnetization has been carefully compared with the antiferromagnetic-type magnetization–temperature (M–T) curve (Neel temperature of T _N = 340–390 K), from which we have discussed the existence of photon-mode photo-enhanced magnetization of some sort in addition with the enhancement due to the light-induced heating.