Structural and magnetic properties of BCC Fe/Co (0 0 1) superlattices

In thin layered Fe/Co (0 0 1), grown on MgO (0 0 1), both Fe and Co crystallize in the body-centered cubic (BCC) structure, as seen in a series of superlattices where the layer thickness of the components is varied from two to twelve atomic monolayers. These superlattices have novel magnetic properties as observed by magnetization and polarized neutron reflectivity measurements. There is a significant enhancement of the magnetic moments of both Fe and Co at the interfaces. Furthermore, the easy axis of the system changes from [1 0 0] for films of low cobalt content to [1 1 0] for a Co content exceeding 33%. No indication of a uniaxial anisotropy component is found in any of the samples. The first anisotropy constant (K₁) of BCC Co is found to be negative with an estimated magnitude of 110 kJ/m³ at 10 K. In all cases, the magnetic moments of Fe and Co have parallel alignment.     

Uniaxial pressure effect on magnetic susceptibility of perovskite manganite La0.66Ba0.34MnO3

Magnetization of La_0.66Ba_0.34MnO₃ and its temperature behavior under a uniaxial pressure of 0.1 kbar are measured between 5 and 270 K in magnetic fields 0<H<120 Oe. The magnetization represents nearly linear dependence on an external magnetic field. Temperature dependence of the magnetic susceptibility found represents a plateau, that is considered as an evidence of the formation of a long period magnetic structure (probably a sort of helix) below the Curie point. Pressure derivative of magnetization displays a sharp minimum at 200 K, pointing to an instability of electronic structure of the compound near this temperature.     

Ultrasound assisted synthesis of TiO2–WO3 heterostructures for the catalytic degradation of Tergitol (NP-9) in water

TiO₂–WO₃ heterostructures were synthesized at room temperature, ambient pressure, and short reaction time via a sonochemical approach. TEM and EDX images show that the prepared TiO₂–WO₃ heterostructures consist of globular agglomerates (∼250 nm in diameter) composed of very small (<5 nm) dense particles (WO₃) dispersed inside the globules. The observed less intense monoclinic WO₃ diffraction peak (around 2θ = 22° belonging to (0 0 1) plane) and the high intense hexagonal WO₃ diffraction peak (around 2θ = 28° belonging to (2 0 0) plane) in XRD indicate that there may be phase transition occurring due to the formation of intimate bond between TiO₂ and WO₃. In addition, the formation of such new phase was also observed from Raman spectra with a new peak at 955 cm⁻¹, which is due to the symmetric stretching of W = O terminal. The catalytic activity of TiO₂–WO₃ heterostructures was tested for the degradation of wastewater pollutant containing Tergitol (NP-9) by a process combined with ozonation and it showed two-fold degradation rate compared with ozone process alone.     

Superparamagnetic properties of C60Co3 complexes

Preparation of fullerites containing cobalt and analyses of reactions based on semiempirical quantum calculations are described. The magnetic properties of thermally treated C₆₀Co₃ samples: Curie constant (C≈3500 emu K/mol Oe) temperature and field dependencies of magnetization and nonequilibrium effects of magnetization are interpreted in terms of superparamagnetic blocking model of the compound.     

Magnetization reorientation in Ni (1 1 1) ultrathin films studied by low-temperature hysteresis measurements

Surface magneto-optical Kerr effect (SMOKE) magnetometry in the temperature range 10–300 K was exploited to investigate the magnetic properties of high-quality Cu/Ni/Cu/Si(1 1 1) epitaxial heterostructures with thickness of the Ni layer, d_Ni, between 10 and 60 Å. For a fixed temperature, the equilibrium direction of the magnetization is parallel or perpendicular to the film surface, depending on the Ni thickness, because of the competition among shape anisotropy, magnetoelastic anisotropy and interface anisotropy. No reorientation of the magnetization could be observed as a function of temperature, for any of the specimens analyzed, while a large variation of the loop squareness and coercivity was found. This last variation has been qualitatively explained using a theoretical model based on a Green's function technique, valid for a monodomain film with a coherent rotation of the magnetization.     

The magnetization processes,spin reorientation transitions and magnetic domain structure in DyFe10CoTi single crystal

The tetragonal ThMn₁₂-type, single crystalline DyFe₁₀CoTi sample has been investigated by torque and magnetization measurements and observation of domain structure at various temperatures between 10–300 K and in magnetic field from B=0 to 0.15 T. These examinations showed that the magnetic structure of DyFe₁₀CoTi changes from “easy axis” (c-axis) type to conical at 225 K and to “easy plane” (ab plane) type at 100 K.     

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.     

Growth of Sr2CrReO6 epitaxial thin films by pulsed laser deposition

We report the growth, structural, magnetic, and electrical transport properties of epitaxial Sr₂CrReO₆ thin films. We have succeeded in depositing films with a high crystallinity and a relatively large cationic order in a narrow window of growth parameters. The epitaxy relationship is Sr₂CrReO₆ (SCRO) (0 0 1) [1 0 0]∥SrTiO₃ (STO) (0 0 1) [1 1 0] as determined by high-resolution X-ray diffraction and scanning transmission electron microscopy (STEM). Typical values of saturation magnetization of M_S (300 K)=1 μ_B/f.u. and ρ (300 K)=2.8 mΩ cm have been obtained in good agreement with previous published results in sputtered epitaxial thin films. We estimate that the antisite defects concentration in our thin films is of the order of 14%, and the measured Curie temperature is T_C=481(2) K. We believe these materials be of interest as electrodes in spintronic devices.     

Magnetic anisotropy of epitaxial Fe layers grown on Si(0 0 1)

The magnetic properties of epitaxial iron films up to 80 monolayers (ML) thickness grown on Si(0 0 1) by using a template technique were investigated by means of superconducting quantum interference device and magneto-optic Kerr effect techniques. The thinnest films investigated (∼3 ML) exhibit a composition close to Fe₃Si with a Curie temperature below room temperature (RT) and strong out-of-plane remanent magnetization that reflects the presence of a dominant second order surface anisotropy term. Thicker films (⩾4 ML) are ferromagnetic at RT with remanent magnetization in film-plane and a composition closer to pure Fe with typically 8–10% silicon content. When deposited at normal incidence such films show simple in-plane fourfold anisotropy without uniaxial contribution. The relevant fourth-order effective anisotropy constant K₄^eff was measured versus film thickness and found to change its sign near 18 ML. The origin of this remarkable behavior is investigated by means of a Néel model and mainly traced back to fourth-order surface anisotropy and magneto-elastic effects related to the large biaxial in-plane compressive strain up to 3.5% in the thinnest (⩽25 ML) films.     

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.     

Perpendicular magnetization in Fe/Ni bilayers on GaAs(0 0 1)

Following the concept of spin-injection into a semiconductor-based device, a ferromagnetic element (like a GMR multilayer structure) can be used as a spin filter. A high spin-polarization of the electrons can be realized by the preparation of a monocrystalline multilayer structure consisting of ultrathin films of a high magnetic polarization. In the case of ultrathin films, the manipulation of the easy-axis of magnetization is possible, by changing the anisotropy terms contributing to the effective anisotropy of the structure. We report on the structural and magnetic properties of Ni/Fe and Fe/Ni bilayers epitaxially grown on GaAs(0 0 1). By a proper choice of Fe and Ni sequences (Fe/Ni/GaAs) and their thickness (up to 3 ML of Fe on the top of Ni), the rotation of magnetization from the in-plane to the out-of-plane direction was achieved.     

Investigation of magnetic properties of interacting Fe2O3 nanoparticles

The magnetic properties of Fe₂O₃ nanoparticles (average diameter ∅≅3 nm) in alumina (68% Fe₂O₃ in weight) have been investigated by magnetization measurements. The results indicate a superparamagnetic behavior of interacting particles, which block with decreasing temperature (the zero-field-cooled susceptibility shows a maximum at T≅145 K) with a distribution of relaxation times. A change of magnetic regime is observed below ∼60 K, due to the increasing interparticle interactions and local surface anisotropy.     

Biquadratic exchange coupling in an unequal Fe/Cr/Fe(1 0 0) trilayer

We have investigated the magnetic properties of a (1 0 0)-oriented unequal trilayer, Fe(45 Å)/Cr(30 Å)/Fe(15 Å), by means of Brillouin light scattering and magnetization measurements. The experimental results show that this sample highlights the effect of biquadratic coupling which aligns the magnetization of the Fe layers at 90° to each other. We extracted the bilinear and biquadratic coupling strengths by fitting the experimental results with a theory that treats the static and dynamic responses on an equal footing. Our results confirm that the model describes both the static and dynamic properties even when the magnetization of the layers is aligned at 90°. The coupling strengths, and their temperature dependence, are discussed and compared with other results reported in the literature.     

Magnetostriction and magnetization measurements on a DyNi2B2C single crystal in a magnetic field

In order to investigate the interactions between lattice properties, magnetic ordering and superconductivity of DyNi₂B₂C, thermal expansion, magnetostriction and magnetization measurements were performed for T=2–15 K and for μ₀H=0–3 T on a single crystal in the crystallographic [1 1 0] direction. A magnetic phase diagram is derived that shows two phases (AF1 and AF2) in the narrow region between the zero-field antiferromagnetic AF and the induced ferromagnetic state FM. Moreover, it is characterized by a large-field hysteresis. This behaviour can be described by a two domain magnetic state. The metamagnetic structure AF1 with about a quarter of the saturated magnetization is responsible for suppressing the superconductivity in DyNi₂B₂C because of its ferromagnetic component.     

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.     

Magnetic properties of rare-earth transition metal aluminides R6T4Al43 with Ho6Mo4Al43-type structure

The magnetic properties of 53 aluminium-rich intermetallic compounds R₆T₄Al₄₃ with R=rare-earth elements and T=Ti, V, Nb, Ta, Cr, Mo, W were investigated using polycrystalline samples and a SQUID magnetometer in the temperature range from 2 to 300 K with magnetic flux densities up to 5.5 T. The yttrium and lutetium compounds are Pauli paramagnetic, indicating that the transition metal atoms do not carry magnetic moments. The samarium compounds show van Vleck behavior and antiferromagnetic order with Néel temperatures of less than 12 K. Of these Sm₆Ti₄Al₄₃ becomes metamagnetic. The ytterbium compounds show a mixed or intermediate valent behavior and no magnetic order down to 2 K. All other compounds obey the Curie–Weiss law above 30 K. Their effective magnetic moments correspond to the theoretical moments of the rare-earth ions. They show ferromagnetic or metamagnetic behavior with ordering temperatures all below 20 K. The magnetization curves of most compounds (recorded up to 5.5 T) reach about 50% of the theoretical magnetization already at 0.5 T. The gadolinium compounds are exceptional in that they reach at 0.5 T only about 10% of their theoretical magnetization. The crystal structures of the isotypic compounds Yb₆V₄Al₄₃ and Yb₆Ta₄Al₄₃ were refined from single-crystal X-ray data.     

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.     

Irradiation induced modifications in morphology and magnetic property of Mn/Si structure

Mn films of ∼50 nm has been deposited by electron beam evaporation technique on cleaned and etched Si [(1 0 0), 8–10 Ω cm] substrates to realize a Mn/Si interfacial structures. The structures have been irradiated from energetic (∼100 MeV) ion beam from Mn side. The irradiated and unirradiated structures have been characterized from atomic force microscopy, X-ray diffractometry, magnetic force microscopy, and vibrating sample magnetometer facilities. It has been found that surface/interfacial granular silicide phases (of Mn_xSi_y) are formed before and after the irradiation with a irradiation induced modifications of surface morphology and magnetic property. The surface/interface roughness has been found to increase on the irradiation from the atomic force microscopy data. The magnetic property on the irradiation shows an interesting and significant feature of an increased coercivity and a ferromagnetic like behavior in the Mn–Si structure. The observed increased coercivity has been related to the increased roughness on the irradiation. The ferromagnetism after the irradiation is a curious phenomenon which seems due to the formation of Mn–C–Si compound from the carbon dissolved in silicon.     

Effect of polarization roughness scattering (PRS) on two-dimensional electron transport of MgZnO/ZnO heterostructures

Quantum transport properties of two-dimensional electron gas (2DEG) in undoped MgZnO/ZnO heterostructures with polarization charge effect have been investigated theoretically. Polarization roughness scattering (PRS) combining polarization charge and interface roughness scattering was proposed as a new scattering mechanism. It was found that the carriers confined in the heterostructures (HSs) would be scattered from polarization charges when they were moving along the in-plane and PRS played a very important role for the low-temperature electron mobility when the electron density N_s exceeded 1.0e11 cm⁻², especially in a higher electron density region. With PRS, the experimental data on the density dependence of 2DEG mobility in the MgZnO/ZnO HSs under study can be well reproduced. The study indicates that the improved processing techniques providing a smooth interface and a good separation between the 2DEG electrons and the polarization charges should be significant for the quantum device’s performance.     

Large magnetocaloric effect in HoFeO3 single crystal

Magnetocaloric properties of HoFeO₃ single crystal are investigated along the direction [100]. Magnetic field dependent magnetization isotherms at different temperatures undergo a metamagnetic transition, entropy change as large as 19.2 J/kg K and 15.8 J/kg K are obtained at 7 T in the vicinity of antiferromagnetic ordering temperature of Ho³⁺ and the metamagnetic transition, respectively. The coupling of Ho and Fe spins generates the compensation behavior at 6.5 K, separating the two large magnetic entropy change. Its refrigeration capacity (RC) value, as high as 220 J/kg, is appreciable and can be considered as a promising magnetic refrigerant. New evidence for spin reorientation of Fe³⁺ in HoFeO₃ is also provided by the change of magnetic entropy.