C. Larica

Magnetic properties of NiMn-based Heusler alloys influenced by Fe atoms replacing Mn

The influence of the Mn substitution by Fe atoms on the magnetocaloric and magnetic properties of the martensitic Ni50Mn36Sn14 Heusler-type compound has been investigated using magnetization measurements. The insertion of Fe atoms reduces the Mn-Mn AF interactions resulting in (i) a systematic decrease in the martensitic transition temperature, down to its disappearance at 15 at. % of Fe, (ii) an enhancement of the saturation magnetization, and (iii) a monotonic increase in the L21-type phase Curie temperature. The Fe substitution also induces metamagnetic transition from an incipient AF to a noncollinear spin configuration for applied magnetic fields higher than 3 T in the case of 3 and 7 at. % Fe substitutional. The exchange-bias effect is only found in compounds with a well-defined martensitic phase transition (Fe content lower than 10 at. %). The maximum of the inverse magnetic entropy change, for a field variation of 5 T, is about +12 J kg−1 K−1 and it is nearly constant for Fe content up to 7 at. %....

Magnetic studies of Fe-doped martensitic Ni2Mn1.44Sn0.56-type Heusler alloy

Localization and magnetism of Fe replacing either Mn or Ni in the Ni2Mn1.44Sn0.56-type Heusler alloy have been systematically investigated using magnetization, scanning electron microscopy and Mossbauer spectroscopy. It has been shown that the addition of Fe either in Mn or in Ni sites reduces the fraction of the Mn-rich NiMnSn-type Heusler alloys that has short-range antiferromagnetic interactions; consequently it reduces the martensitic–austenitic transition temperature and increases the thermal hysteresis width due to an increase in atomic disorder caused by Fe replacements. The Fe atoms in Mn sites have two magnetic configurations with magnetic moments of 0.8μB/Fe and 1.4μB/Fe in the martensitic orthorhombic structure, while Fe in Ni sites have magnetic moments smaller than 0.1μB/Fe. These results indicate that the Fe atoms are distinctly substituting either Mn or Ni and the decrease in the martensitic phase transition temperature for increasing Fe content can be mainly attributed to the Fe atoms in the Mn sites in both cases.

Magnetocaloric properties of (La, RE)Fe11.4Si1.6 compounds (RE=Y,Gd)

The mechanosynthesis process has been applied in the LaFe11.4Si1.6 compound to reduce the undesirable segregated rich-Fe phases that impair its application as a solid magnetic refrigerant. The influence of La substitution (5 at. %) by Y or Gd atoms on the magnetic and magnetocaloric properties has been also studied. Y- and Gd-substituted compounds have a magnetic ordering temperature higher than the pure La compound. While the Y-substituted compound keeps a first-order-like magnetic transition feature, the Gd-substituted one seems to suppress it. The maximum value of the magnetic entropy change of the Y compound is roughly the same as the La compound (−18 J∕kg K) but with a magnetic entropy change peak significantly broader. For the Gd-compound case a drastic reduction of the magnetic entropy change (−7 J∕kg K) is found.

Influence of grain refinement and induced crystal defects on the magnetic properties of Ni50Mn36Sn14 Heusler alloy

The effect of high energy ball milling on the structural, magnetic and magnetocaloric properties of Ni50Mn36Sn14 Heusler-type alloy has been studied. X-ray diffraction results have revealed a reduction in the crystalline grain size concomitantly with defect inclusions in the crystalline lattice, favouring a chemical disorder effect that transforms the L21–B2-type disordered structure to a simple cubic B2-type structure for increasing milling time. From magnetometry and Mossbauer spectroscopy results, a decrease in the ferromagnetic exchange interaction contribution, an enhancement of the effective exchange bias field and a significant reduction in the magnetic entropy change of the milled alloy are observed.

La(Fe1−xCox)11.44Al1.56: A composite system for Ericsson-cycle-based magnetic refrigerators

The La(Fe1−xCox)11.44Al1.56 system, with x values of 0, 0.04, 0.08, and 0.12, was investigated for its magnetocaloric potential. For selected values of Co doping, it was possible to cover a wide magnetic ordering temperature range, from 200 up to 370K, and reduce substantially the metamagnetic critical fields. At applied magnetic fields up to 5T, the maximum magnetic entropy change approaches a nearly constant value of about 5J∕kgK for Co-doped compounds, with x varying from 0.04 to 0.12. The magnetic entropy change of a prototype composite was calculated in order to obtain a constant value in a wide temperature span. The results indicate that this material can be a good candidate for magnetic refrigeration using the Ericsson cycle.

Thermal studies and magnetic properties of mechanical alloyed Fe2B

In the present paper, a high energy milling process has been used to alloy Fe and B in the proportion 2:1. X-ray diffraction and Mossbauer spectroscopy were used to follow the solid state reaction among the alloy components and phase formation during thermal treatments of the final milled alloy, at distinct temperatures and environments. X-ray, DSC and Mossbauer data of the 310 h milled alloy show the presence of three phases: small fractions of α-Fe (n-Fe) and Fe2B (n-Fe2B) nanocrystalline phases and a large amount of amorphous Fe2B (a-Fe2B) phase (73%). Heat treatment of the milled alloy was done in a high vacuum furnace, followed by in situ Mossbauer measurements, or by annealing the sample sealed in a quartz tube. It is shown that segregation and crystallization effects thermally induced in the samples have been enhanced by the presence of oxygen in the residual atmosphere, being less effective in the case where the sample was continuously pumped during the annealing. These effects may have occurred due to boron oxidation. Also, the presence of a tet-Fe3B phase in the annealed samples has been observed. The high temperature Mossbauer spectra for the sample annealed at 823 K indicate that the n-Fe2B, n-Fe3B and n-Fe phases exhibit superparamagnetic behaviour, with estimated blocking temperatures of 723 and 823 K, respectively. The amorphous a-Fe2B phase has a TC value estimated at about 823 K that fits into the system reported for amorphous Fe/B ribbons.

Ferromagnetic properties of fcc Gd thin films

Magnetic properties of sputtered Gd thin films grown on Si (100) substrates kept at two different temperatures were investigated using X-ray diffraction, ac magnetic susceptibility, and dc magnetization measurements. The obtained Gd thin films have a mixture of hcp and fcc structures, but with their fractions depending on the substrate temperature TS and film thickness x. Gd fcc samples were obtained when TS = 763 K and x = 10 nm, while the hcp structure was stabilized for lower TS (300 K) and thicker film (20 nm). The fcc structure is formed on the Ta buffer layer, while the hcp phase grows on the fcc Gd layer as a consequence of the lattice relaxation process. Spin reorientation phenomenon, commonly found in bulk Gd species, was also observed in the hcp Gd thin film. This phenomenon is assumed to cause the magnetization anomalous increase observed below 50 K in stressed Gd films. Magnetic properties of fcc Gd thin films are: Curie temperature above 300 K, saturation magnetization value of about 175 emu/cm3, and coercive field of about 100 Oe at 300 K; features that allow us to classify Gd thin films, with fcc structure, as a soft ferromagnetic material.

Enhancement of magnetocaloric properties near room temperature in Ga-doped Ni50Mn34.5In15.5 Heusler-type alloy

A martensitic Ni50Mn34.5In15.5 Heusler-type alloy doped with Ga was studied by x-ray diffractometry and magnetization measurements. Ga-doping does not affect the austenitic phase transition but shifts the martensitic phase transformation towards room temperature, producing an enhancement of the magnetic entropy change (ΔSM) in that temperature region. Large ΔSM-values in the Ga-doped samples are attained for an applied field of 30 kOe as opposed to the field of 50 kOe commonly found for the un-doped cases. These effects (enhancement of ΔSM-values, shift to temperatures close to 300 K, and large ΔSM-values at lower applied fields) make the Ga-doped Ni50Mn34.5In15.5 Heusler-type alloys good candidates for technological applications as a solid refrigerant.

Single magnetic domain precipitates of Fe/Co and Fe and Co in Cu matrix produced from (Fe-Co)/Cu metastable alloys

Structural and magnetic properties of nanocrystalline Fe2Co and (Fe2Co)0.30 Cu0.70 alloys prepared by high energy ball milling have been studied basically by x-ray, Mossbauer spectroscopy and magnetization measurements. For the Fe2Co alloy case, the Mossbauer measurements indicate that the sample with 160 hours of milling has two magnetic components with the same average hyperfine parameters: one magnetic crystalline component associated with the bcc Fe2Co phase and another component attributed to the small particles of the same bcc Fe2Co phase (SP-Fe2Co). (Fe2Co)0.30Cu0.70 alloys have been prepared by milling in two different ways: (1) starting from the mixture of Fe2Co milled alloy and pure Cu powders (sample I) and (2) milling of the elemental powder mixture of Fe, Co and Cu (sample II). The x-ray diffraction and bulk magnetization results of samples I and II indicate the formation of a (Fe2Co)0.30Cu0.70 supersaturated solid solution, with features of a ferromagnetic material and Tc at about (420±1) K. High temperature magnetization measurements of the (Fe/Co)Cu milled materials show particle precipitation effects. Heat treatment at 675 and 875 K of the final milled materials leads to different results: in the sample I case to the precipitation of single magnetic Fe/Co particles into the Cu matrix, and in the case of sample II the precipitation of single magnetic particles of Fe and of Co into the Cu matrix.

Mechanical alloying of Fe and Cr: Structural and magnetic studies

Mechanical milling of a mixture of pure iron and chromium powders with a nominal composition of 28 at. % of Fe and 72 at. % of Cr was performed using two kinds of tools. The alloying process was followed by X ray diffraction and57Fe Mössbauer spectroscopy. The room temperature Mössbauer spectra revealed the formation of a broad single line at the expense of the area of α-Fe sextet. At 4.2 °K most of the singlet still remains unsplit. This behavior may be due to some metastable structure induced by the preparation method.