Magnetic tri-axial orientation in (Y1−xErx)2Ba4Cu7O15−y superconductors

We report the tri-axial grain-orientation effects under a modulated rotation magnetic field for (Y_1?xEr_x)₂Ba₄Cu₇O_y [(Y, Er)247]. The magnetic easy axis at room temperature was drastically changed around x ～ 0.1; however, the Er-doping levels for the conversion of magnetic easy axes from the c-axis to the ab-direction and from the a- to b-axes were quite different. Tri-axial single-ion magnetic anisotropy of Er³⁺ was roughly 10 times greater than tri-axial magnetic anisotropy generated by both the superconducting CuO₂ plane and the blocking Cu–O chain layer. An appropriate choice of rare-earth (RE) ions in RE-based cuprate superconductors enables the reduction of the required magnetic field for the production of bulks and thick films based on the magnetic orientation technique.     

Width and temperature dependence of lithography-induced magnetic anisotropy in (Ga,Mn)As wires

In-plane magnetic anisotropy of 40-μm-long (Ga,Mn)As wires with different widths (0.4, 1.0, and 20 μm) has been investigated between 5 and 75 K by measuring anisotropic magneto-resistance (AMR). The wires show in-plane 〈1 0 0〉 cubic and [−1 1 0] uniaxial anisotropies, and an additional lithography-induced anisotropy along the wire direction in narrow wires with width of 0.4 and 1.0 μm. We derive the temperature dependence of the cubic, uniaxial, and lithography-induced anisotropy constants from the results of AMR, and find that a sizable anisotropy can be provided by lithographic means, which allows us to control and detect the magnetization reversal process by choosing the direction of the external magnetic fields.     

Magnetic phase transitions in the Nd(Mn0.9Me0.1)O3 (Me=Al, Fe, Cr, Zn) perovskites

It is shown that perovskite NdMnO₃ is a weak ferromagnet with an anomalous magnetization behavior due to Nd sublattice contribution. Ferromagnetic component drastically increases whereas T_N slightly decreases when a part of manganese ions is replaced with Cr, Al, Fe, Zn. It is suggested that the Mn³⁺–O–Mn³⁺ superexchange interaction changes a sign in the microdomains enriched with Me=Cr, Al, Fe, Zn ions due to removing static Jahn–Teller distortions. All these substituted perovskites show a sharp drop of the magnetization as temperature decreases. A large temperature hysteresis indicates first-order phase transition. Below this transition neodymium magnetic moments orient opposite to a moment of manganese magnetic sublattice. It is supposed that this phase transition results from a change of the ground state of Nd ions.     

Crystal growth and optical properties of Cr3+, Er3+, RE3+: Gd3Ga5O12 (RE=Tm,Ho, Eu) for mid-IR laser applications

In this paper we report on the optical properties of triply Cr³⁺, Er³⁺, and RE³⁺ (RE=Tm, Ho, Eu) doped Gd₃Ga₅O₁₂ crystals that were grown by the Czochralski method. Optical absorption, near-infrared (NIR), and mid-infrared (mid-IR) fluorescence spectra were characterized for the fabricated crystals and corresponding luminescence decay measurements under 654 nm excitation were also carried out. Based on the analysis of energy transfer process between Er and RE (RE=Tm, Ho, Eu) ions, the energy transfer efficiency (ETE) values were evaluated, correspondingly. From the spectral data of all the studied crystals, it is observed that the co-doped Cr³⁺ ion highly increases the absorption pump power and the three kinds of co-doped RE³⁺ ions depopulate the Er:⁴I_13/2 energy level effectively. The spectral analysis shows that titled rare earth doped crystals are promising materials for ～3.0 μm mid-IR laser applications and among them Cr,Er,Eu:GGG is relatively more suitable due to its excellent optical properties compared with others.     

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.     

Temperature dependence of the lattice parameters and magnetic properties of Er3Ir single crystals

Er₃Ir single crystals were grown by the Czochralski method from a levitated melt. The electrical resistivity thermal dependence exhibits ordering temperature of the erbium sublattice at 40 K and a spin reorientation process at 22 K. The DC and AC magnetic susceptibility show antiferromagnetic ordering in the form of an asymmetric peak. The magnetization in strong magnetic fields up to 140 kOe exhibits anisotropy. The lattice parameters’ thermal dependence of Er₃Ir and Er₃Ni show anisotropy and anomalous behaviour.     

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.     

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.     

Textured Co nanowire arrays with controlled magnetization direction

Textured HCP Co nanowires with preferentially oriented (1 0 0) along the growth direction were fabricated by electrodeposition at high potential. Further increase of deposition potential results in the formation of twist wires with circumferential strain. These conclusions are unambiguously reached by combining TEM, XRD rocking curves and pole figures. The magnetic properties are determined by the combination of magnetocrystalline, shape, and stress anisotropies and magnetostatic interaction. Consequently, the magnetic easy axis can be tuned with structure and temperature, thus paving the road for magnetic nanowire array use in applications where self-biasing of magnetization is necessary.     

Magnetic anisotropy,phase transitions,and domain structures in films with out-of-plane magnetization

A phenomenological theory of induced magnetic anisotropy in nanostructures is developed. It is shown that, in magnetic films with out-of-plane magnetization, a complex redistribution of the anisotropy energy-density within the layer volume causes nonlinear and nonmonotonic dependencies of the effective anisotropy on layer thickness. These distributions also account for the continuous character of the transition into the phase with perpendicular magnetization. In the thickness range above this transition, a phase with twisted magnetization favoring multidomain structures appears. Experimental data on Ni/Cu(0 0 1) layered systems are analyzed in terms of this theory.     

Luminescence properties of Eu3+ and Sm3+ coactivated Gd(III) tungstate phosphor for light-emitting diodes

Rare-earth ions coactivated red phosphors Gd_0.2RE_1.8(WO₄)₃ (RE=Eu³⁺ and Sm³⁺) were synthesized by conventional solid-state reaction using boric acid as a flux agent. The samples were characterized by X-ray diffractometer (XRD), energy-dispersive X-ray spectrometer (EDS) and luminescence spectrometer (LS). The results showed that the Eu–Sm system exhibits higher emission intensity than those of the Eu single-doped system and Sm separate-doped system under ultraviolet (UV) radiation. Samarium(III) ions are effective in broadening and strengthened absorptions around 400 nm. Furthermore, it exhibits enhanced luminescence emission. when the mole ratio of boric acid is about 0.16, the luminescence capability is optimum. Two strongest lines at ultraviolet (394 nm) and blue (465 nm) in excitation spectra of these phosphors match well with the output wavelengths of UV and blue GaN-based light-emitting diodes (LEDs) chips.     

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.     

Unusual magnetization anisotropy in amorphous Nd–Fe–Al ribbons

Nd₆₀Fe₃₀Al₁₀ ribbons has been prepared by chill-block melt-spinning with different wheel speeds from 5 to 30 m/s. Fully amorphous ribbons were obtained at wheel speeds of 25 and 30 m/s. These ribbons exhibited an unusually large anisotropy in magnetization. The effect of the magnetic anisotropy decreased with decreasing wheel speed, and nearly disappeared at the wheel speed of 5 m/s, at which the ribbon consisted of a mixture of a more stable Fe-rich amorphous phase and a crystalline Nd phase with a strong crystallographic texture.     

Transition of magnetic anisotropy in Ni/GaAs (0 0 1) observed by magnetization and ferromagnetic resonance

Polycrystalline thin Ni films deposited onto GaAs (0 0 1) show a transition of the magnetic anisotropy depending on its thickness. The anisotropy is perpendicular to the film plane for the thicknesses of the film ⩽12 nm. This becomes in-plane in the films having thicknesses ⩾15 nm. The films are deposited onto the n-type GaAs (0 0 1) substrate by the usual thermal evaporation method and also by the electron beam evaporation in ultra high vacuum onto a GaAs epilayer in the standard molecular beam epitaxy system. The magnetization and ferromagnetic resonance (FMR) are observed in the temperature range from 4.2 to 300 K. For the discussion of the microscopic origin of the anomalous properties in magnetization and FMR experiments, the experimental results are reviewed by introducing a uniaxial anisotropy, which is calculated from the easy-axis and hard-axis magnetization data. This calculated anisotropy is able to explain the temperature and angle dependency of the FMR spectra of the Ni films. Hence the magnetization and FMR spectra are in agreement with the type of the anisotropy and its temperature dependency. In addition to these, the temperature dependence of the in-plane magnetic anisotropy is able to explain the previously reported anomalous effect of reducing the squareness at low temperatures in Ni/GaAs.     

Nonlinear magnetoelastic effects in ultrathin epitaxial FCC Co(0 0 1) films:: an ab initio study

Those linear and nonlinear magnetoelastic coupling coefficients which determine the magnetostrictive stress and the strain-induced out-of-plane magnetic anisotropy in epitaxially grown FCC Co(0 0 1) films are calculated by the ab initio density functional electron theory. The nonlinear couplings have a strong effect on the change Δσ₁^m of the in-plane magnetostrictive stress resulting from a change of the magnetization direction from [0 1 0] to [1 0 0], but a negligibly small effect on the out-of-plane anisotropy e_MCA. The calculations confirm the experimental result that the measured out-of-plane anisotropy cannot be totally attributed to volume magnetoelastic effects. Estimates are given for the nonlinear magnetoelastic coupling coefficients m₁^γ,2 and m₂^γ,2.     

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.     

Anisotropic properties of rare-earth dibismites

We report measurements of magnetic, thermal and transport properties of single crystals of rare-earth dibismites RBi₂ (R=La–Nd, Sm), grown via self-flux method. All compounds are good metals, and those with magnetic ions order antiferromagnetically at low temperatures. Ce, Pr and Sm members of the series show single magnetic transition whereas NdBi₂ most likely exhibits two magnetic transitions. Significant magnetic anisotropy and a series of metamagnetic transitions in fields up to 55 kG are found in PrBi₂. Ordering temperatures range from 3 K to just above 16 K and they scale well with the de Gennes factor.     

LiMn2−xTixO4 spinel-type compounds (x ≤ 1): Structural,electrical and magnetic properties

LiMn_2−xTi_xO₄ compounds with 0 ≤ x ≤ 1 were prepared by solid state reaction and Pechini technique. Powder X-ray diffraction showed that all samples crystallize with the spinel crystal structure (S.G. Fd3¯m). The cubic unit-cell parameter increases with the Ti content. The influence of the Ti content and cationic distribution on the magnetic properties of the compounds was studied by measuring the temperature and magnetic field dependences of the magnetization: substitution by non-magnetic d⁰ Ti⁴⁺ ions appeared to weaken the magnetic interactions between the manganese ions. The electrical properties of LiMnTiO₄ were studied by AC impedance spectroscopy and DC polarisation measurements, which revealed the electronic character of the conduction process.     

Magnetic properties and Verwey transition of quasi-one-dimensional magnetite nanowire arrays assembled in alumina templates

Magnetite (Fe₃O₄) has been successfully assembled into anodic alumina templates by an electrochemical method followed by a heat-treating process. Here, we report on the magnetic properties of these so formed nanowires and the Verwey transition measured by vibrating sample magnetometer and SQUID. A Mössbauer spectrum was collected to verify the magnetic orientation of the wires, and a tilt of the moment of 45° with respect to the wire axis was found. These wires show perpendicular magnetic anisotropy mainly due to the average easy axis of the grains pointing along the wire axis. The temperature dependence of the coercity, remanence, and the magnetization undergo a major change at 50 K, induced by the Verwey transition, which occurs at a temperature much lower than for bulk materials (120 K). The behavior of the magnetization in the vicinity of 50 K as well as its field-dependent properties was interpreted using the magneto-electronic model.