P. A. Algarabel

GIANT MAGNETORESISTANCE NEAR THE MAGNETOSTRUCTURAL TRANSITION IN GD5(SI1.8GE2.2)

Zero-field electrical resistivity over the temperature range of 4–300 K and magnetoresistance in magnetic fields of up to 12 T have been measured in Gd5(Si1.8Ge2.2). This system undergoes a first-order magnetostructural transition at TC≅240 K, from a high-temperature paramagnetic to a low-temperature ferromagnetic phase, accompanied by a large drop in the resistivity. The application of an external magnetic field above TC can induce this transition, and a giant negative magnetoresistance effect (Δρ/ρ≅−20%) is observed associated with this first-order field-induced transition.

Observation of the spin Seebeck effect in epitaxial Fe3O4 thin films

We report the experimental observation of the spin Seebeck effect in magnetite thin films. The signal observed at temperatures above the Verwey transition is a contribution from both the anomalous Nernst (ANE) and spin Seebeck (SSE) effects. The contribution from the ANE of the Fe3O4 layer to the SSE is found to be negligible due to the resistivity difference between Fe3O4 and Pt layers. Below the Verwey transition, the SSE is free from the ANE of the ferromagnetic layer and it is also found to dominate over the ANE due to magnetic proximity effect on the Pt layer.

A systematic study of structural, magnetic and electrical properties of perovskites

A thorough study of the series has been carried out. All of the samples have the same orthorhombic crystallographic structure. However, the magnetic, magnetoelastic and electrical properties show drastic changes with the Tb content. The decrease in the intensity of the ferromagnetic double-exchange interaction has been correlated with the increase in the Tb content and the decrease in the Mn - O - Mn bond angle, which leads to a decrease in the electron transfer between Mn 3d and O 2p orbitals. Two magnetic phases have been detected at low temperatures: an insulator spin-glass phase in samples with high Tb contents and a metallic ferromagnetic phase in samples with low Tb contents.

Composition and temperature dependence of the magnetocrystalline anisotropy in Ni2+xMn1+yGa1+z (x+y+z=0) Heusler alloys

The composition dependence of magnetic and structural transformations, as well as the temperature and composition dependence of the magnetocrystalline anisotropy, were studied in the martensitic Ni2+xMn1+yGa1+z (x+y+z=0) Heusler alloys. Moving away from the stoichiometric composition, a slight variation, a large reduction, and a marked enhancement, were, respectively, observed for the Curie temperature, the magnetic anisotropy, and the structural (martensitic–austenitic) transformation temperatures.

Magnetoelastic effects and magnetic anisotropy in Ni2MnGa polycrystals

Linear thermal expansion and magnetostriction measurements under a magnetic field up to 20 kOe, applied parallel and perpendicular to the measuring direction and in the temperature range of 10–300 K, have been performed on a Ni2MnGa polycrystal. The effects of zero-field cooling and field cooling on both the self-strain (H=0) and the magnetic-field-induced strain (MFIS) have been studied. We have found that the MFIS strongly depends on whether a magnetic field was applied during the cooling process. The applied magnetic field facilitates the growth of specific orientation variants along the field direction as the sample is cooled down through martensitic transformation. However, the application of a field in the martensitic phase induces a negligible motion of twin boundaries. On this basis, the singular point detection technique performed on polycrystalline specimens results in being the easiest and most direct way by which to determine the anisotropy field in this class of materials. The temperature beh...

Giant room‐temperature magnetoresistance in the FeRh alloy

The magnetoresistance of the FeRh alloy has been measured in the temperature range 200–300 K using high pulsed magnetic fields up to 36 T. This compound shows a first‐order magnetic transition from the antiferromagnetic (AF) low‐temperature state to the ferromagnetic (F) high‐temperature state at TAF–F=405 K. The F state is very close in energy to the AF state and can be reached by applying a magnetic field. The measurements made within the AF state show that the AF–F transition is accompanied by a giant magnetoresistance effect (ΔR/R≊50%).

Magnetocaloric effect in Tb5(SixGe1−x)4

The magnetocaloric effect has been investigated in a series of the R5(SixGe1−x)4 pseudobinary alloys with R=Tb and x=0.0, 0.5, and 1.0. In Tb5Si4 and Tb5Ge4, a maximum magnetic entropy change of 9.8 and 3.7 J/kg K, respectively, has been found at their ordering temperatures for a magnetic field change of 50 kOe. In the case of Tb5Ge4, we have observed the existence of a previously unreported magnetic transition below the Neel temperature. Tb5(Si0.5Ge0.5)4 has been successfully synthesized, and a large entropy change of 21.8 J/kg K (ΔH=50 kOe) is achieved at a first-order para- to ferromagnetic transition, TC≅105 K. This study demonstrates that Tb5(SixGe1−x)4 alloys are good candidates for magnetic refrigeration and that very exciting phenomenology in these systems awaits further investigation.

Structural, magnetic and transport properties of Sr2Fe1−xCrxMoO6−y

Abstract The series of Sr 2 Fe 1− x Cr x MoO 6− y has been synthesized and studied by means of a wide set of experimental techniques. These samples belong to the family of A 2 BB′O 6 double perovskites with a ferrimagnetic ground state. The replacement of Fe by Cr leads to samples with a high oxygen deficiency and to increasing cation disorder between B and B′ sites. The magnetic moments of these samples decrease as the iron content does. Electrical properties change as well strongly along the series: while Sr 2 FeMoO 6 is metallic, the rest of the samples show a semiconducting behavior with an activation energy that increases as the content of Cr does. Only Fe-rich samples exhibit large negative magnetoresistance with the low-field response characteristic of a half-metallic system. The Cr-rich samples display instead a small linear negative magnetoresistance.

Magnetoelastic behaviour of Gd5Ge4

A complete investigation of the complex magnetic behaviour of Gd5Ge4 by means of linear thermal expansion and magnetostriction measurements (5–300 K, 0–120 kOe) has been carried out. Our results support the suggested existence in this system of a coupled crystallographic–magnetic transition from a Gd5Ge4-type Pnma (antiferromagnetic) to a Gd5Si4-type Pnma (ferromagnetic) state. Strong magnetoelastic effects are observed at the field-induced first-order magnetic–martensitic transformation. A revised magnetic and crystallographic H– T phase diagram is proposed.

Grain-boundary magnetoresistance up to 42 T in cold-pressed Fe3O4 nanopowders

The magnetoresistance (MR) in cold-pressed magnetite nanopowders has been studied using pulsed magnetic field up to 42 T and steady field up to 12 T. Ball milling in air produces pure and stoichiometric Fe3O4 grains of nanometric size coated by a thin layer of Fe2O3, which electrically isolates the magnetite and acts as a tunnel barrier. Therefore, the intergrain magnetoresistance of magnetite grain boundaries can be analyzed regardless of the bulk transport properties. At high fields and high temperature, the MR depends linearly on the field, whereas at lower fields a direct tunneling contribution governed by the surface magnetization appears. Below the Verwey transition (T<120K) the linear high-field MR disappears. We interpret these results in terms of the grain-boundary properties.