On the pressure and temperature dependence of ordering temperature and magnetization of Y1-xThxCo4B compounds

International audienceWe present here a detailed investigation of the pressure dependence of the structural and magnetic properties in a wide temperature range from 5 to 340 K for the Y1-xThxCo4B series. The unit cell lattice compressibility has been determined from powder neutron diffraction technique in the 0 to 0.5 GPa pressure range. Substitution of Th for Y is found to reduce significantly the compressibility. The Curie temperature decreases dramatically upon the Th for Y substitution. Studies were performed on polycrystalline samples under hydrostatic pressure up to 1.1 GPa and in magnetic field up to 5 T. The evolution of the saturation magnetization under pressure is presented as well as the pressure dependence of the Curie temperature. Remarkable pressure effects on the Curie temperature and on the critical field at which the first order magnetization process occurs for Y0.8Th0.2Co4B are analyzed and discussed. The pressure derivatives values of both the Curie temperature and the magnetization are negative for all the studied compounds. This is an opposite tendency than that expected from chemical pressure effects. This supports the dominant role of valence electrons in the magnetic properties of the substituted Y1-xThxCo4B compound in the case of the chemical pressure effect. The magnetization and the Curie temperature exhibit different pressure dependence along the Y1-xThxCo4B series of compounds

Pressure dependence of magnetism and martensitic properties in Co-doped NiMnGa alloys

NiMnGa alloys display several "giant" effects due to the interplay between magnetic and structural degrees of freedom. A large magnetization difference, between martensitic phases and austenitic phases iso f great importance to obtain higher performances due to the improbe possibilit? of driving structural trasformations by magnetic fields. A deltaM enhancement was recently achieved by adding Co to Mn-rich NiMnGa alloy

From direct to inverse Giant Magnetocaloric Effect in Co-doped NiMnGa multifunctional alloys

We report the magnetic and magnetocaloric properties in Co-doped Ni–Mn–Ga Heusler alloys around the Mn-rich composition Ni50Mn30Ga20. The presence of Co affects profoundly the critical temperatures and alters the exchange interactions of martensite and austenite to different extents; by varying the composition it is possible to tune the critical temperatures and to induce a paramagnetic gap between the magnetically ordered martensite and magnetic austenite, thus giving rise to a reverse magnetostructural transformation. Contrary to the Co-free alloys, the saturation magnetization moment of austenite is strongly enhanced by Co with respect to the martensitic one: thus the magnetocaloric effect turns from direct into inverse. Remarkable values of the magnetic properties related to the magnetocaloric effect, e.g. the saturation magnetization jump at the transformation (DM) and the field dependence of the transformation temperature (dT/dH) are reported, together with high positive values of the isothermal magnetic entropy change (DS). The conditions for enhancing the magnetocaloric properties and triggering the sign reversal of the magnetocaloric effect are discussed

Hydrostatic pressure control of the magnetostructural phase transition in Gd5Si2Ge2 single crystals

Magnetic and structural properties of single crystalline Gd5Si2Ge2 under hydrostatic pressure have been characterized by using magnetization, linear thermal expansion, and compressibility measurements. A strong dependence of Curie temperature on pressure, dTC∕dP=+4.8 K∕kbar, is observed in contrast with the smaller values of about 3 K∕kbar found in polycrystalline specimens. This difference reflects the role the microstructure may play in pressure-induced magnetic-crystallographic phase changes, likely related to stress relaxation at the grain boundaries, domain pinning and/or nucleation of defects. The pressure dependence of the critical magnetic field, d(dHC∕dT)∕dP, drops at the rate −0.122(5)kOe∕K kbar, which points to an enhancement of the magnetoelastic coupling with pressure. The latter affects the magnetocaloric behavior of the material at the rate d(ΔSM)∕dP≅1.8 J∕kg K kbar. The linear thermal expansion confirms the strongly anisotropic change of the lattice parameters through the orthorhombic to monoclinic crystallographic transformation with Δa∕a=+0.94%, Δb∕b=−0.13%, and Δc∕c=−0.22%. The structural transition temperature varies with pressure synchronously with the Curie temperature, and the size and shape of the strain anomalies remain nearly unaffected by the hydrostatic pressure, indicating, respectively, that the structural and magnetic transformations remain coupled, and the anisotropic behavior of the lattice is preserved as pressure increases. The room temperature linear compressibility data show that the magnetostructural transformation can be triggered isothermally at ∼6 kbar and that the compressibility is anisotropic

Pressure dependence of the magneto-transport properties in Fe/MgO granular systems

The effect of hydrostatic pressure at room temperature on the resistance and magnetoresistance (MR) of a discontinuous metal-insulator multilayer (DMIM) of nominal composition [Fe(t Fe = 0.7 nm)/MgO(t MgO = 3 nm)]15 has been studied. The resistivity of the DMIM, ρ, decreases linearly with pressure indicating an increase in conduction via tunneling effect. The value of coefficient (1/ρ 0)dρ/dP = -3.9 × 10-2 kbar-1 is higher than reported values in other granular films implying that the electronic state of the DMIM is close to the iron percolation threshold. At the maximum applied magnetic field (3 kOe) the MR ratio increases from 0.6% at ambient pressure to 1.1% at 7 kbar. This result can be explained by a reduction of the tunnel barrier width induced by the hydrostatic pressure. © 2013 The Korean Physical Society.Financial support by the Spanish Ministry of Economia y Competitividad (through project MAT-2011-27553-C02, including FEDER funding) and by Aragon Regional Government (through project E26) is acknowledged. Work of A. G.-G. was supported by Portuguese FCT (through post doctoral grant SFRH/BPD/81710/2011).Peer Reviewe

Transport properties of Lu₂Fe₁₇ single crystals under extreme conditions

Presented are the measurement results of Lu₂Fe₁₇ single crystals electrical resistivity under hydrostatic pressure up to 1.1 GPa and magnetoresistance in magnetic fields up to 12 T in temperature range 5-300 K. A large increase of resistivity has been observed due to transition from ferromagnetic phase to pressure-induced helical antiferromagnetic one. The simultaneously measured resistivities along the a and c crystallographic directions confirm the strong anisotropic behavior of Lu₂Fe₁₇ resistivity, a remarkably lower resistivity being observed along the a axis. The magnetoresistance values measured along c and a axes under identical conditions also are different. The magnetoresistance ratio for the a direction was found to be smaller than for the c one.Представлены результаты измерений удельного электрического сопротивления монокристаллов Lu₂Fe₁₇ под гидростатическим давлением до 1,1 ГПа и измерений магнитного сопротивления в магнитных полях до 12 Т в температурном диапазоне 5300 К. Наблюдалось значительное повышение удельного сопротивления вследствие перехода из ферромагнитной фазы в возникающую под давлением спиральную антиферромагнитную фазу. Одновременное измерение удельного сопротивления вдоль кристаллографических направлений c и a свидетельствует о сильной анизотропии электрического сопротивления Lu₂Fe₁₇, причем вдоль оси а наблюдается значительно меньшее сопротивление. Значения магнитного сопротивления, измеренные вдоль осей а и с в одинаковых условиях, также различны. Обнаружено, что отношение магнитного сопротивления для направления а меньше, чем для направления с.Представлено результати вимiрювань питомого електричного опору монокристалiв Lu₂Fe₁₇ під гідростатичним тиском до 1,1 ГПа та вимiрювань магнітного опору в магнітних полях до 12 Т в температурному діапазоні 5-300 К. Спостерігалося значне збільшення питомого опору внаслідок переходу з феромагнітної фази у спіральну анти-феромагнітну фазу, що утворюється під тиском. Одночасне вимірювання питомого опору вздовж кристалографічних напрямів а та с посвідчує сильну анізотропію електричного опору Lu₂Fe₁₇, причому вздовж осі а спостерігається значно менший опір. Значення магнітного опору, виміряні вздовж вісей а та с в однакових умовах, також різні. Виявлено, що відношення магнітного опору для напряму а менше, ніж для напряму с

Adipocyte size in morbidly obese women and its relation to type 2 diabetes

Aim of the study: The aim was to perform a morphometric analysis of subcutaneous and visceral adipose tissue of morbidly obese women and to determine the relationship between adipocyte size and the development of type 2 diabetes (T2D)