Giant magnetoresistance in the Ge-rich magnetocaloric compound,Gd5(Si0.1Ge0.9)4

We have measured the zero-field electrical resistivity in the temperature range 5–295 K and magnetoresistance in magnetic fields of up to 12 T of Gd₅(Si_0.1Ge_0.9)₄. The resistivity changes drastically at the magnetostructural first-order transition (T_C≅80 K on heating). This transition can be induced reversibly by the application of an external magnetic field above T_C, producing a concomitant giant magnetoresistance (GMR) effect, Δρ/ρ≅−50%. This study demonstrates that (in addition to giant magnetocaloric and magnetoelastic effects) GMR can be tuned between ∼20 and ∼290 K in Gd₅(Si_xGe_1−x)₄ with x⩽0.5 by simply adjusting the Si : Ge ratio.     

Low-temperature magnetic ordering in the perovskites Pr1−xAxCoO3 (A=Ca,Sr)

The magnetic and electrical properties of polycrystalline Pr_1?xA_xCoO₃ cobaltites with A=Ca, Sr and 0≤x≤0.5 were studied in the temperature range 4 K≤T≤1000 K and field up to 7 T. The X-ray analyses show the presence of only one phase having monoclinic or orthorhombic symmetry. The magnetic measurements indicate that the Ca-doped samples have at low temperatures, similar properties to the frustrated magnetic materials. PrCoO₃ is a paramagnetic insulator in the range from 4 to 1000 K. The Sr-doped cobaltites exhibit two phase transitions: a paramagnetic–ferromagnetic (or magnetic phase separated state) phase transition at about 240 K and a second one at about 100 K. The magnetic measurements suggest the presence of magnetic clusters and a change in the nature of magnetic coupling between Co ions at low temperatures. A semiconducting type behavior and high negative magnetoresistance was found for the Ca-doped samples, while the Sr-doped ones were metallic and with negligible magnetoresistance. The results are analyzed in the frame of a phase separation scenario in the presence of the spin-state transitions of Co ions.     

Effect of field-induced magnetic ordering on the electrical resistance of the frustrated magnet Au80Co20

Au₈₀Co₂₀ platelets were produced by solid-state quenching followed by furnace annealing and characterized by X-ray diffraction. Magnetization and magnetoresistance (MR) were measured from 2 to 265 K. Minima of the zero-field electrical resistance R₀(T) were observed at low temperatures (T_m≈20 K) in both as-quenched and annealed samples. These minima disappear upon applying a magnetic field. At low T, a negative MR is associated to strong frustration and a very short magnetic coherence length; at higher temperatures, a positive MR is also observed.     

Magnetotransport properties of ferromagnetic LaMnO3+δ nano-sized crystals

Transport and magnetic properties of LaMnO_3+δ nanoparticles with average size of 18 nm have been investigated. The ensemble of nanoparticles exhibits a paramagnetic to ferromagnetic (FM) transition at T_C～246 K, while the spontaneous magnetization disappears at T≈270 K. It was found that the blocking temperature lies slightly below T_C. The temperature dependence of the resistivity shows a metal–insulator transition at T≈192 K and low-temperature upturn at T<50 K. The transport at low temperatures is controlled by the charging energy and spin-dependent tunnelling through grain boundaries. The low temperature I–V characteristics are well described by indirect tunnelling model while at higher temperatures both direct and resonant tunnelling dominates.     

Giant magnetic refrigerant capacity in Ho3Al2 compound

Magnetic properties and magnetocaloric effects (MCEs) of the intermetallic Ho₃Al₂ compound are investigated by magnetization and heat capacity measurements. Two successive magnetic transitions, a spin-reorientation (SR) transition at T_SR=31 K followed by a ferromagnetic (FM) to paramagnetic (PM) transition at T_C=40 K, are observed. Both magnetic transitions contribute to the MCE and result in a large magnetic entropy change (ΔS_M) in a wide temperature range. The maximum values of ?ΔS_M and adiabatic temperature change (ΔT_ad) reach 18.7 J/kg K and 4.8 K for the field changes of 0–5 T, respectively. In particular, a giant value of refrigerant capacity (RC) is estimated to be 704 J/kg for a field change of 5 T, which is much higher than those of many potential refrigerant materials with similar transition temperatures.     

The effect of chemical pressure in rutheno-cuprates

We have studied the effect of negative chemical pressure in the RuGd_1.5(Ce_0.5?xPr_x)Sr₂Cu₂O_10?δ with Pr content of 0.0 ? x ? 0.2. This is also investigated using the bond length results obtained from the Rietveld refinement analysis. The c parameter and cell volume increase with x for 0.0 ? x ? 0.15. The width of the resistivity transition also increases with Pr concentration, indicating higher inhomogeneity and oxygen deficiency. The difference in the ionic valences of Pr^3+,4+ and Ce⁴⁺ causing different hole doping, the difference in the ionic radii, and oxygen stoichiometry affect the superconducting transition. The magnetoresistance shows a cusp around 135 K which lies between the antiferromagnetic and ferromagnetic transition temperatures, which is probably due to the presence of a spin glass region. There exist two magnetic transition temperatures for 0.0 ? x ? 0.2 which respectively change from T_M = 155 K to 144 K and from T_irr = 115 K to 70 K. The magnetization versus applied magnetic field isotherms at 77 K and 300 K show that the remanent magnetization and coercivity are lower for samples with higher Pr content.     

Giant magneto-caloric effect around room temperature at moderate low field variation in La0.7(Ca1−xSrx)0.3MnO3 perovskites

Among the perovskite manganites, a series of La_1?xCa_xMnO₃ has the largest magneto-caloric effect (MCE) (|ΔS_m|_max=3.2–6.7 J/kg K at ΔH=13.5 kOe), but the Curie temperatures, T_C, are quite low (165–270 K). The system of LaSrMnO₃ has quite high T_C but its MCE is not so large. The manganites La_0.7(Ca_1?xSr_x)_0.3MnO₃ (x=0, 0.05, 0.10, 0.15, 0.20, 0.25) have been prepared by solid state reaction technique with an expectation of large MCE at room temperature region. The samples are of single phase with orthorhombic structure. The lattice parameters as well as the volume of unit cell are continuously increased with the increase of x due to large Sr²⁺ ions substituted for smaller Ca²⁺ ions. The field-cooled (FC) and zero-field-cooled (ZFC) thermomagnetic measurements at low field and low temperatures indicate that there is a spin-glass like (or cluster glass) state occurred. The Curie temperature T_C increases continuously from 258 K (for x=0) to 293 K (for x=0.25). A large MCE of 5 J/kg K has been observed around 293 K at the magnetic field change ΔH=13.5 kOe for the sample x=0.25. The studied samples can be considered as giant magneto-caloric materials, which is an excellent candidate for magnetic refrigeration at room temperature region.     

Magnetic properties of GdCo12B6 compound under high pressures

The effects of hydrostatic pressure up to 10 kbar on Curie temperature T_C, compensation temperature T_COMP and spontaneous magnetization M_S of ferrimagnetic GdCo₁₂B₆ compound have been studied. Two antiferromagnetically coupled sublattices that are carrying magnetization of typically 0.42 μ_B/Co atom and 7 μ_B/Gd cancel out at compensation temperature at about 50 K and magnetic ordering temperature T_C=163±2 K. The volume dependence of intrinsic magnetic properties of the GdCo₁₂B₆ compound has been determined by studying it under hydrostatic pressure. The observed increase of M_S with pressure (dM_S/dp=+0.005 μ_B kbar^?1 at 5 K) is attributed predominantly to the pressure induced decrease of Co magnetic moments. The crucial role of Co in this behavior is confirmed by the change of sign of the pressure slope at temperatures above T_COMP and by the fact that the estimated decrease of m_Co is also quite comparable with pressure induced decrease of M_S in YCo₁₂B₆ (dM_S/dp=?0.007 μ_B kbar^?1). The decrease of m_Co is also responsible for the increase of T_COMP with pressure (dT_COMP/dp=+0.06 K kbar^?1). The decrease of T_C with pressure (dT_C/dp=?0.55 K kbar^?1) is comparable to the decrease observed on RCo₁₂B₆ compounds with non-magnetic R and can be attributed to the volume dependence of Co–Co exchange interactions. The remarkable role of the hybridization as a consequence of small distances between Co and B atoms could be a background of this rather unexpected volume stability of magnetic properties.     

Magnetic frustration in the antiferromagnetic Kondo lattice YbPtAl: Anomalous magnetic behavior at high pressures

We have investigated the magnetic and electronic properties of the antiferromagnetic Kondo lattice YbPtAl using the ¹⁷⁰Yb Mössbauer effect at ambient pressure (1.8<T<10 K), electrical resistance (1.8<T<300 K) and X-ray diffraction (T=300 K) techniques at high pressures up to 26 GPa. We find a complex magnetic state in YbPtAl at ambient pressure and an unusual volume-induced change of T_N. It is suggested, that the anomalous volume dependence of T_N is due to the interplay between frustrated anisotropic exchange interactions and magnetocrystalline anisotropy. The magnetic frustration originates from the topology of the crystal lattice.     

Defect-induced magnetic fluctuations in YbB12

We present the results of nuclear spin–lattice relaxation rate (1/T₁) measurements in a typical Kondo insulator YbB₁₂ for ^10,11B and ¹⁷¹Yb nuclei. Above 20 K, 1/T₁ at the B sites shows thermally activated temperature dependence with a gap of about 100 K. However, it shows anomalous enhancement below 15 K, which is partially suppressed by magnetic field up to 16 T. No such anomaly was observed at the Yb sites. The ratio of 1/T₁ for ¹¹B and ¹⁰B nuclei indicates that the anomaly below 15 K is caused by dilute magnetic moments assisted by nuclear spin diffusion. The origin and the nature of the low temperature magnetic fluctuations are discussed.     

Pressure effect on magnetic properties of RCo12B6 (R=Y,Ce) compounds

The effect of pressure on magnetic properties of YCo₁₂B₆ and CeCo₁₂B₆ was studied in temperature range 5–300 K at pressures up to 9 kbar. The Curie temperature T_C and spontaneous magnetization M_S decrease with pressure for both compounds. The decrease can be attributed mostly to the volume dependence of both, the Co magnetic moment and the exchange interactions. The hybridization of the p–d states as a consequence of small distances between the Co and B atoms can be one reason of the relatively low pressure effects (ΔT_C/Δp=?0.39±0.02 K/kbar, d ln M_S/dp=?0.0013±0.0002 kbar^?1) in YCo₁₂B₆. Higher volume sensitivity of magnetic properties of CeCo₁₂B₆ in comparison with YCo₁₂B₆ can be attributed to the pressure induced changes of the Ce f- and Co d-states.     

Pressure-induced magnetic transition in MnRhAs

The magnetic properties of a Fe₂P-type intermetallic compound MnRhAs have been investigated under high pressure up to 8.0 GPa by AC susceptibility measurement. Initially, both the antiferromagnetic (AF(I)) to the canted state magnetic transition temperature T_t and the canted state to another antiferromagnetic one (AF(II)) transition temperature T_C increase with compression. At 4.0 GPa, however, T_t decreases abruptly, while the increasing rate of T_C becomes larger above this pressure. A pressure-induced magnetic phase transition was seen at around this pressure when T_t and T_C are plotted in the pressure–temperature phase diagram. The transition from the antiferromagnetic to the ferromagnetic state observed below 160 K with increasing pressure is not frequently observed.     

Magnetic properties of iron nanoparticles in a polymer film

We systematically synthesized self-aggregated iron nanoparticles in the perfluorinated sulfo-cation membrane (MF-4SK) by ion-exchange method. Our experimental results show that iron nanoparticles in MF-4SK exhibit superparamagnetic properties above the blocking temperature. Field-cooled and zero-field-cooled magnetization data show the blocking temperature, T_B≅120 K for the iron concentration of 5×10¹⁹ atoms per 1 g of polymer film at 500 Oe applied field. This result is well matched with the calculation based on the temperature dependence of the coercivity, which shows T_B≅110 K, with the zero temperature coercivity (H_C0) ≅ 420 Oe. The radius of the typical iron particle is determined to be ∼2 nm from transmission electron microscopy (TEM), showing good agreement with the value acquired by Langevin function fit. These experimental evidences suggest that iron nanoparticles in the polymer film obey a single-domain theory.     

Superconductivity and thermal properties of sulphur doped FeTe with effect of oxygen post annealing

Here, we report the synthesis and characterization of sulphur-substituted iron telluride i.e. FeTe_1?xS_x; (x = 0–30 %) system and study the impact of low temperature oxygen (O₂) annealing as well. Rietveld analysis of room temperature X-ray diffraction (XRD) patterns shows that all the compounds are crystallized in a tetragonal structure (space group P4/nmm) and no secondary phases are observed. Lattice constants are decreased with increasing S concentration. The parent compound of the system i.e. FeTe does not exhibit superconductivity but shows an anomaly in the resistivity measurement at around 78 K, which corresponds to a structural phase transition. Heat capacity C_p(T) measurement also confirms the structural phase transition of FeTe compound. Superconductivity appears by S substitution; the onset of superconducting transition temperature is about 8 K for FeTe_0.75S_0.25 sample. Thermoelectric power measurements S(T) also shows the superconducting transition at around 7 K for FeTe_0.75S_0.25 sample. The upper critical fields H_c2(10%), H_c2(50%) and H_c2(90%) are estimated to be 400, 650 and 900 kOe respectively at 0 K by applying Ginzburg Landau (GL) equation. Interestingly, superconducting volume fraction is increased with low temperature (200 °C) O₂ annealing at normal pressure. Detailed investigations related to structural (XRD), transport [S(T), R(T)H], magnetization (AC and DC susceptibility) and thermal [C_p(T)] measurements for FeTe_1?xS:O₂ system are presented and discussed.     

Surface and interface effects on structural transformation of vapor-deposited ethylbenzene films

We have investigated how the structures of vapor-deposited glassy films change with increasing temperature by using time-of-flight secondary ion mass spectrometry and ion scattering spectroscopy. It is found that intermixing of the topmost layer of an ethylbenzene film occur at temperature (~ 80 K) considerably lower than the glass transition temperature (T_g = 118 K) when the film is deposited at 20 K. This phenomenon can be interpreted as the occurrence of a two-dimensional liquid that diffuses into pores of the film, which is evidenced from comparison with surface diffusivity measurements using a porous silicon layer. For nonporous films deposited at higher temperatures, the molecules intermix gradually prior to the abrupt film morphology change at T_g. This phenomenon can be interpreted as decoupling between translational diffusivity and viscosity in the bulk. The film thickness has no significant effects on the evolution of supercooled liquid at T_g except for the monolayer film, whereas crystallization is quenched for the films thinner than 8 monolayers. The roles of the 2D liquid on the surface and an immobilized layer formed at the interface are discussed in finite-size effects on the glass-liquid transition and crystallization.     

Surface electromigration of In on vicinal Si(0 0 1)

Surface mass transport of In film on vicinal Si(0 0 1) has been systematically investigated by a scanning Auger electron microscopy (SAM), low energy electron diffraction (LEED) and atomic force microscopy (AFM). It was observed that the temperature dependence of the mass transport shows the critical phenomenon. Above a critical temperature T_c, surface electromigration of the In film toward the cathode side dominated the surface mass transport on the vicinal Si(0 0 1) surface. The LEED and AFM observations revealed that the In film surface on the vicinal Si(0 0 1) consists of 3×4 terraces and (3 1 0) facets. The area ratio of the facet to the terrace exhibited abrupt an increase at T_c. It is believed that the change of the mass transport is related to the abrupt change of the area ratio of the facet to the terrace. Both the critical temperature T_c and the spread due to the surface electromigration of the In film depended on the configuration of the DC current direction and the step edge.     

Synthesis and characterization of CaLaFe11O19

A new ferrimagnetic compound with the chemical formula CaLaFe₁₁O₁₉ has been synthesized by solid state reaction between the respective oxides and their structural, electrical and magnetic properties have been studied. One magnetic Fe³⁺ ion in CaLaFe₁₂O₁₉ is replaced by La³⁺ ion. The crystallographic results show the compound is hexagonal magnetoplumbite. The electrical conductivity has been measured from 300 to 800 K. The activation energy changes at Curie temperature (653 K). The compound is ferrimagnetic from 300 to 653 K and above T_c it acts as a paramagnetic. Variation of inverse molar susceptibility has been measured at various temperature in paramagnetic region and Curie molar constant (C_m) is calculated. AC susceptibility measurements are made at room temperature. The Seebeck coefficient (S) measurements show that the compound is n-type.     

Structural and magnetic properties of thin epitaxial Fe films on (1 1 0) GaAs prepared by metalorganic chemical vapor deposition

Iron films have been grown on (1 1 0) GaAs substrates by atmospheric pressure metalorganic chemical vapor deposition at substrate temperatures (T_s) between 135°C and 400°C. X-ray diffraction (XRD) analysis showed that the Fe films grown at T_s between 200°C and 330°C were single crystals. Amorphous films were observed at T_s below 200°C and it was not possible to deposit films at T_s above 330°C. The full-width at half-maximum of the rocking curves showed that crystalline qualities were improved at T_s above 270°C. Single crystalline Fe films grown at different substrate temperature showed different structural behaviors in XRD measurements. Iron films grown at T_s between 200°C and 300°C showed bulk α-Fe like behavior regardless of film thickness (100–6400 Å). Meanwhile, Fe films grown at 330°C (144 and 300 Å) showed a biaxially compressed strain between substrate and epilayer, resulting in an expanded inter-planar spacing along the growth direction. Magnetization measurements showed that Fe films (>200 Å) grown at 280°C and 330°C were ferromagnetic with the in-plane easy axis along the [1 1 0] direction. For the thinner Fe films (⩽200 Å) regardless of growth temperature, square loops along the [1 0 0] easy axis were very weak and broad.     

Magnetocaloric heat-pump cycles based on the AF–F transition in Fe–Rh alloys

The proposal involves a heat-pumping scheme based upon the first-order antiferromagnetism–ferromagnetism transition in FeRh alloy. Using the model S–T diagram for this alloy, the heat-pump cycles, are drawn up based on the transition latent heat absorption and emission when the transition is induced by applying magnetic field. The calculated values of heat coefficient ϕ for the cycles are ∼39 at ΔT=5 K and ∼30 at ΔT=10 K, where ΔT is the difference between the temperature surrounding and that of the heat receiver. These values are achieved using the comparatively low magnetic fields of ∼2×10⁶ A m⁻¹. The high values of ϕ, together with high value of cooling capacity, make it possible to consider Fe–Rh alloys as an effective magnetic heat-pump working body near the room temperature.     

Intrinsic pinning properties of FeSe0.5Te0.5

The intrinsic pinning properties of FeSe_0.5Te_0.5, which is a superconductor with a critical temperature T_c of approximately 14 K, were studied through the analysis of magnetization curves obtained using an extended critical state model. For the magnetization measurements carried out with a superconducting quantum interference device (SQUID), external magnetic fields were applied parallel and perpendicular to the c-axis of the sample. The critical current density J_c under the perpendicular magnetic field of 1 T was estimated using the Kimishima model to be equal to approximately 1.6 × 10⁴, 8.8 × 10³, 4.1 × 10³, and 1.5 × 10³ A/cm² at 5, 7, 9, and 11 K, respectively. Furthermore, the temperature dependence of J_c was fitted to the exponential law of J_c(0) × exp(?αT/T_c) up to 9 K and the power law of J_c(0) × (1 ? T/T_c)ⁿ near T_c.