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.     

Large magnetoresistance in Ni1–xVxS

Large magnetoresistance (MR) was observed in Ni_1−x V_xS(x=0, 0.02, 0.04, 0.06 and 0.08), MR=1530% at 268 K for x=0, MR=1180% at 255 K for x=0.02, MR=980% at 248 K for x=0.04, MR=810% at 224 K for x=0.06 and MR=490% at 198 K for x=0.08 in magnetic field 4 T. The large MR is due to magnetic field-induced magnetic and electrical transition from antiferromagnetic (AFM) nonmetal phase to paramagnetic (PM) metal phase.     

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 structural disorder on magnetic frustrations in the Fe80Nb6B14 amorphous alloy

The paper presents measurements of magnetic frustrations for the Fe₈₀Nb₆B₁₄ amorphous alloy preliminary annealed for 1 h at temperatures ranging from 300 to 770 K in comparison with other parameters characterizing structural relaxation. It was shown that annealing out of free volume and internal stresses cause a reduction in number of frustrated states observed in the range 400>T>50 K. In the range 2<T<50 K additional frustrations due to iron clusters or nanograins were detected.     

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.     

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.     

Electron channel with high carrier mobility at the interface of type-II broken-gapp-GaInAsSb/p-InAs single heterojunctions

We have studied experimentally the magneto-transport properties of type-II broken-gap Ga_1 − xIn_xAsSb/p-InAs heterostructures with various doping levels of the quaternary layer by Te or Zn. A strong electron channel with high electron mobility was observed at the interface of the heterostructures. Interface roughness scattering was found to dominate the electron mobility atT = 4.2–47 K in samples with an undoped or a slightly doped quaternary layer. A drastic mobility drop with increasing Zn doping level was observed. Shubnikov–de Haas oscillations at low temperatures (1.5–20 K) were studied and a weak anisotropy of magnetoresistance was found. Some important parameters of the heterostructures under study were determined.     

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.     

Exchange bias effects in ferromagnetic wires

We have investigated the exchange bias effect in micron-sized ferromagnetic wires made from Co and Ni₈₀Fe₂₀ films. The wires were fabricated using optical lithography, metallization by sputtering and lift-off technique. Magnetotransport measurements were performed at temperatures ranging from 3 to 300 K. We observed marked changes in the magnetoresistance (MR) properties as the temperature is varied. At 300 K, the field at which the sharp peak occurs corresponding to the magnetization reversal of the Co wires is 167 Oe and is symmetrical about the origin. As the temperature was decreased to 3 K, we observed a shift in the peak positions of the MR characteristics for both the forward and reverse field sweeps corresponding to a loop shift of 582 Oe in the field axis. The asymmetric shift in the MR loops at low temperatures clearly indicates the exchange bias between ferromagnetic (Co) and antiferromagnetic parts (Co-oxide at the surfaces) from natural oxidation. Ni₈₀Fe₂₀ wires of the same geometry showed similar effect with a low exchange bias field. The onset of exchange biasing effect is found to be 70 and 15 K for the Co and Ni₈₀Fe₂₀ wires, respectively. A striking effect is the existence of exchange biasing effect from the sidewalls of the wires even when the wires were capped with Au film.     

AC-susceptibility of Ni/W(1 1 0) ultrathin magnetic films: determination of the Curie temperature and critical behavior

The temperature dependence of the magnetic susceptibility of 6–8 ML Ni/W(1 1 0) is measured in situ in UHV by means of an AC-susceptibility mutual inductance bridge. At sufficient small driving magnetic fields ⩽11 A/m a susceptibility maximum and an interval of constant susceptibility on the high temperature side of the peak are observed. The Curie temperature is identified as the low temperature limit of this region of constant susceptibility. The appearance of the maximum at lower temperatures is interpreted as a ferro-magnetic response at T<T_C. The critical exponent γ is extracted from a power law fit at T>T_C. For the smallest field of 3 A/m, the determined γ of 1.26 (7) is consistent with γ of the three-dimensional Ising model. For larger fields, the exponent depends on the field and presents an effective value.     

Effect of Cr substitution on the superconducting and magnetic properties of RuSr2Eu1.5Ce0.5Cu2O10−δ

In this paper we investigate the properties of polycrystalline series of Ru_1?xCr_xSr₂Eu_1.5Ce_0.5Cu₂O_10?δ (0.0 ? x ? 0.40) by resistivity, XRD and dc magnetization measurements. EuRu-1222 is a reported magneto superconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity occurs in Cu–O₂ planes below T_c ? 40 K. The exact nature of Ru spins magnetic ordering is still being debated and no conclusion has been reached yet. In this work, we found the superconducting transition temperature T_c = 20 K from resistivity and dc magnetization measurements for pristine sample. DC magnetization measurements exhibited ferromagnetic like transition for all samples.     

Magnetic properties of RPdIn (R=Gd–Er) compounds

AC susceptibility and DC magnetization measurements were performed for the RPdIn (R=Gd–Er) compounds both in the paramagnetic and in the ordered state. In opposite to GdPdIn, which is a ferromagnet (T_c=102 K), the other samples show a complex ferrimagnetic behavior with the additional transition at T_t<T_c. In the high-temperature phase (for T_t<T<T_c), a ferromagnetic interaction dominates, while in the low-temperature phase (for T⩽T_t) antiferromagnetic interactions with the magnetocrystalline anisotropy, especially strong for TbPdIn, come into play. The ordering temperatures are T_c=70, 34, 25 and 12.3 K for Tb-, Dy-, Ho- and ErPdIn respectively, while transition temperatures are T_t=6, 14 and 6 K for Tb-, Dy- and HoPdIn respectively. TbPdIn reveals an additional transition at 27 K connected with the intermediate ferrimagnetic phase. The critical fields for the magnetization process of the low-temperature phase are high (52 and 150 kOe for TbPdIn and 32 kOe for DyPdIn at T=4.2 K) yet these values decrease remarkably with increasing temperature. Results of the study are compared with magnetic and neutron diffraction data hitherto available. We state that irreversibility of the zero-field cooled–field cooled magnetization is not connected with the spin-glass phase claimed elsewhere.     

Specific heat of TbMn2(H,D)2

Specific heat (SH) measurements on TbMn₂(H,D)₂ powders have been performed in the temperature range from 2 to 350 K, in zero magnetic field and in 9 T. Due to the low heat conductivity of the samples, the measurements were carried out on a mixed Cu- and sample-powder pellet. For TbMn₂, the anti-ferromagnetic phase transition was manifest by a single SH peak at T_N=47 K, whereas a double SH peak at 281 and 288 K and an upturn below 5 K were observed for the hydride sample. Upon applying the magnetic field of 9 T, the SH upturn was suppressed, whereas no visible influence was found on the specific heat in the whole temperature range above 10 K as well as on the double peak.     

Anisotropic vortex pinning in the β-pyrochlore oxide superconductor KOs2O6

Vortex pinning in the β-pyrochlore oxide superconductor KOs₂O₆ with T_c = 9.6 K is investigated by measuring magnetic torque. A large anisotropy of magnetic torque is observed in the superconducting state below T_p = 7.6 K, where a first-order structural transition takes place, in spite of the inherent isotropic nature of the structural and electronic properties. Magnetic torque is enhanced at external magnetic fields parallel to the [1 1 1] and [0 0 1] directions. Moreover, a pronounced peak effect is also observed in the magnetic field dependence of the torque in these two directions. We consider that the observed anisotropy is related to a microstructure associated with the structural transition.     

Thermoelectric power of CuCrxVySe4 p-type spinel semiconductors

Structural, electrical and magnetic measurements of polycrystalline CuCr_xV_ySe₄ spinels with x=1.79, 1.64 and 1.49 and y=0.08, 0.22 and 0.45, respectively, are presented. The compounds under study crystallize in regular system of a normal spinel type MgAl₂O₄ structure with the space group symmetry Fd3m. The chromium spins are coupled ferromagnetically and show both strong long- and short-range magnetic interactions evidenced by the large values of the Curie (T_C) and Curie–Weiss (θ_CW) temperatures, decreasing from T_C=407 K and θ_CW=415 K for y=0.08, via T_C=349 K and θ_CW=367 K for y=0.22 to T_C=283 K and θ_CW=293 K for y=0.45, respectively. In all the studied spinels a change of the electrical conductivity character from the semiconductive into the metallic one above 230 K was observed. A detailed thermoelectric power analysis showed a domination of diffusion thermopower component, maximum of phonon drag component at 230 K, a decrease of impurity component with increasing V content, as well as the weak magnon excitations at 40 K.     

La0.7Sr0.3MnO3 film prepared by dc sputtering on silicon substrate: Effect of working pressure

La_0.7Sr_0.3MnO₃ films were prepared by dc sputtering on Si (100) substrate at different working pressure. The possibility of controlling the magnetic and transport properties of colossal magnetoresistance film is investigated, which has attracted great research interest for practical application. The as-grown film shows different magnetic, transport and magnetoresistance change at different working pressure at room temperature, which is quite attractive from technological point of view. Maximum magnetoresistance (MR) of ?5.56%, Curie temperature (T_c) of 325 K and metal insulator transition temperature (T_MI) of 278 K was achieved at room temperature.     

Neutron studies of the iron-based family of high TC magnetic superconductors

We review neutron scattering investigations of the crystal structures, magnetic structures, and spin dynamics of the iron-based RFe(As, P)(O, F) (R = La, Ce, Pr, Nd), (Ba,Sr,Ca)Fe₂As₂, and Fe_1+x(Te–Se) systems. On cooling from room temperature all the undoped materials exhibit universal behavior, where a tetragonal-to-orthorhombic/monoclinic structural transition occurs, below which the systems become antiferromagnets. For the first two classes of materials the magnetic structure within the a–b plane consists of chains of parallel Fe spins that are coupled antiferromagnetically in the orthogonal direction, with an ordered moment typically less than one Bohr magneton. Hence these are itinerant electron magnets, with a spin structure that is consistent with Fermi-surface nesting and a very energetic spin wave bandwidth ～0.2 eV. With doping, the structural and magnetic transitions are suppressed in favor of superconductivity, with superconducting transition temperatures up to ≈55 K. Magnetic correlations are observed in the superconducting regime, with a magnetic resonance that follows the superconducting order parameter just like the cuprates. The rare earth moments order antiferromagnetically at low T like ‘conventional’ magnetic superconductors, while the Ce crystal field linewidths are affected when superconductivity sets in. The application of pressure in CaFe₂As₂ transforms the system from a magnetically ordered orthorhombic material to a ‘collapsed’ non-magnetic tetragonal system. Tetragonal Fe_1+xTe transforms to a low T monoclinic structure at small x that changes to orthorhombic at larger x, which is accompanied by a crossover from commensurate to incommensurate magnetic order. Se doping suppresses the magnetic order, while incommensurate magnetic correlations are observed in the superconducting regime.     

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.     

Coexistence of magnetism and superconductivity in R1.4Ce0.6RuSr2Cu2O10 − δ(R = Eu,Sm and Gd)

The superconducting R_1.4Ce_0.6RuSr₂Cu₂O_10 − δ(R = Sm, Eu and Gd) withT_c ∼ 28, 32 and 42 K are also magnetically ordered atT_N ∼ 220, 122 and 180 K, respectively, thus,T_N ⪢ T_c. This is in contrast to intermetallic magnetic superconductors (such as RNi₂B₂C) in whichT_c ⪢  T_N. Magnetic susceptibility and Mossbauer spectroscopy show that superconductivity is confined to the CuO₂planes, whereas magnetism is due to the Ru sublattice. Irreversibility phenomena and magnetic anomalies, observed at low magnetic fields originate from antisymmetric exchange coupling of the Dzyaloshinsky–Moria type, and from spin reorientation of the Ru moments. The shielding fraction is about 100%, supporting the conclusion that the materials consist of a single phase, manifesting both magnetism and superconductivity at once.     

Stable barium compounds in YBa2Cu3Oy superconductors

The stability of various amounts of Ba₃Cu₃In₄O₁₂ (334) or BaTbO₃ (BTO) in a sintered YBa₂Cu₃O_y (YBCO) matrix was examined. Samples with added 334 or BTO exhibited critical temperatures (T_c) above 90 K for up to 20 vol.% addition and improved critical current densities (J_c) under a magnetic field. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis indicated that 334 and BTO did not react with the YBCO matrix under the sintering conditions used. The normalized J_c under a magnetic field of 1 T reached a maximum at 14 vol.% of 334 addition and 20 vol.% BTO addition. YBCO thin films with added BTO showed a gradual decrease in the T_c with increasing BTO content. YBCO films with added 334 showed a constant T_c of 87 K up to a 334 content of 4 vol.%.