Influence Of Off-stoichiometry Research Articles

Effect of off-stoichiometry and Ta doping on Fe-rich (Mn,Fe)2(P,Si) based giant magnetocaloric materials

The influence of off-stoichiometry and of doping with the 5d transition metal Ta has been studied in the quaternary (Mn,Fe)2(P,Si)-based compound, which is one of the most promising materials systems for magnetic refrigeration. It is found that Ta substitution can decrease the transition temperature Ttr, while the thermal hysteresis ∆Thys remains about constant. A low Ta doping enhances the magnetocaloric effect (MCE). For Mn0.6Fe1.27-yTayP0.64Si0.36 with y = 0.01 the magnetic entropy change ∆Sm shows and enhancement of 30.7% compared to the undoped material for a low magnetic field change of 1 T. The occupancy of substitutional Ta atoms is determined by XRD and DFT calculations. The Ttr shift and enhanced MCE upon Ta doping are ascribed to the competition between a weakening of the magnetic exchange interactions and a strengthening of the hybridization. Our studies provide a good strategy to further optimize the MCE of this material family.

Dependence of martensite transformation temperature on magnetic field in Ni2MnGa and Ni2MnGa0.95In0.05 single crystals

The shift of martensite transformation temperatures with magnetic field is an essential parameter for magnetic shape memory effect as well as for magnetocalorics. We compare the dependence of martensite transformation temperature on magnetic field in Ni2MnGa and Ni2MnGa0.95In0.05 single crystals. Choosing the stoichiometric and isoelectronic compound enables to observe the effect of In alloying free from the influence of off-stoichiometry. The martensite start temperature, Ms, decreases several kelvins in low magnetic fields (< 1 T). In high field (> 2 T), Ms increases with the magnetic field and the Ms(H) dependence is about linear with the slope of 0.26 K/T (Ni2MnGa) and 0.36 K/T (Ni2MnGa0.95In0.05). Modest 1.25 at% indium alloying decreases not only transformation temperature Ms by about 100 K but also considerably decreases the latent heat of martensite transformation from 1.99 kJ/kg (Ni2MnGa) to 0.29 kJ/kg (Ni2MnGa0.95In0.05) determined from Clausius-Clapeyron equation. The latent heat of premartensitic transformation also decreases, approximately five times.

Understanding complex multiple sublattice magnetism in double double perovskites

Understanding magnetism in multiple magnetic sublattice system, driven by the interplay of varied nature of magnetic exchanges, is on one hand challenging and on other hand intriguing. Motivated by the recent synthesis of AA^{prime }BB^{prime }O_6 double double perovskites with multiple magnetic ions both at A- and B-sites, we investigate the mechanism of magnetic behavior in these interesting class of compounds. We find that the magnetism in such multiple sublattice compounds is governed by the interplay and delicate balance between two distinct mechanisms, (a) kinetic energy-driven multiple sublattice double exchange mechanism and (b) the conventional super-exchange mechanism. The derived spin Hamiltonian based on first-principles calculations is solved by classical Monte Carlo technique which reproduces the observed magnetic properties. Finally, the influence of off-stoichiometry, as in experimental samples, is discussed. Some of these double double perovskite compounds are found to possess large total magnetic moment and also are found to be half-metallic with reasonably high transition temperature, which raises the hope of future applications of these large magnetic moment half-metallic oxides in spintronics and memory devices.

Robust perpendicular magnetic anisotropy of Co3Sn2S2 phase in sulfur deficient sputtered thin films

Perpendicular magnetic anisotropy (PMA) in magnetic thin films is a fundamental key feature in the design of spintronic devices. As one of magnetic Weyl semimetals, ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ has been studied for its large anomalous Hall effect (AHE), uniaxial crystalline magnetic anisotropy, and half metallicity. In this study, we investigated the effect of off-stoichiometric composition on the PMA and AHE of ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{x}$ thin films fabricated by the sputtering technique. The prepared thin films have off-stoichiometric S compositions $x$ of 1.54 (S poor) and 3.27 (S rich) as well as the nearly stoichiometric one of 2.02. In addition to the ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ phase, the segregated Co metal is found to contribute to the measured magnetization in the S-poor and S-rich films. The coercive field of perpendicular magnetization in all the films is much larger than that in the ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ bulk crystals despite the fact that effective perpendicular magnetic anisotropy constants (${K}_{\mathrm{u}}^{\mathrm{eff}}$) between the prepared films are significantly different. In addition, the ${K}_{\mathrm{u}}^{\mathrm{eff}}$ values of two samples with $x=2.02$ and 2.22 are comparable to those of the bulk crystals. In contrast to the isotropic magnetization behavior in the S-rich film, the S-poor film holds the PMA feature. This result means that the PMA is more robust in the S-poor film than in the S-rich film. For the electrical transport properties, a large tangent of Hall angle of about 0.2 is observed for both the nearly stoichiometric and the S-poor films. This large tangent of Hall angle demonstrates that the Weyl feature of ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ phase is well maintained even in the S-poor thin films as well as the nearly stoichiometric films although the amount of Co segregation in both S-poor and S-rich films is similar. Our findings on the influence of off-stoichiometry on the PMA and AHE are beneficial to design magnetic devices incorporated with the Weyl features of ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$.

Studying the effect of the controlled off-stoichiometry on the properties of Zn2SnO4 nanoparticles for DSSC applications

In this paper, the influence of off-stoichiometry and annealing temperature on the structural, electrical and optical properties of the zinc stannate nanoparticles prepared by conventional solid-state reaction method were studied. The structural studies revealed that the phase purity of the synthesized nanoparticles increases with annealing temperature. Furthermore, the reduction of the molar ratio of ZnO to SnO2 from 2 to 1.8 increased the electrical conductivity of the nanoparticles. The electrical conductivity improvement was ascribed to the excess Sn atoms in the nanoparticles which can fill the place of zinc atoms (Zn2+) or occupy interstitial sites in the zinc stannate lattice and lead to creation of more free electrons. Subsequently, the resultant nanoparticles were used as photoanode material. The fabricated dye-sensitized solar cells with 1.8 molar ratio of ZnO to SnO2 displayed better photovoltaic performance. Dye loading measurement on the Zn2SnO4-based photoelectrode revealed that the excess Sn in this structure not only increases the electrical conductivity of the Zn2SnO4 nanoparticles but also enhances their dye loading.

Influence of film composition in quaternary Heusler alloy Co2(Mn,Fe)Si thin films on tunnelling magnetoresistance of Co2(Mn,Fe)Si/MgO-based magnetic tunnel junctions

The influence of off-stoichiometry on the half-metallic character of quaternary Heusler alloy thin films of Co2(Mn,Fe)Si (CMFS) was investigated by studying the composition dependence of the tunnelling magnetoresistance (TMR) ratio of fully epitaxial CMFS/MgO/CMFS magnetic tunnel junctions (CMFS MTJs) having Co2(Mnα′Feβ′)Si0.84 electrodes with various Mn and Fe compositions. It was found that with (Mn + Fe)-rich electrodes had higher TMR ratios than ones with (Mn + Fe)-deficient electrodes at 4.2 and 290 K. These results indicate that the suppression of Co antisites at nominal Mn/Fe sites is critical to obtaining half-metallic quaternary Co2(Mn,Fe)Si in a similar way as in ternary alloy Co2MnSi. CMFS MTJs with Mn-rich and lightly Fe-doped CMFS electrodes showed giant TMR ratios of 2610% at 4.2 K and 429% at 290 K. These results suggest that Co-based Heusler alloy thin films would be highly applicable to spintronic devices because of their half-metallicity and material diversity arising from not only ternary alloy but also quaternary alloy systems.

Co2+xFe1−xSi/MgO(001) Heusler alloys: Influence of off-stoichiometry and lattice distortion on the magnetic properties in bulk and on MgO(001)

We investigate the influence of lattice distortion and off-stoichiometry on the electronic and magnetic properties of Co2+xFe1−xSi Heusler alloys in their L21 bulk phase and on MgO(001) using the density functional theory. Our investigations show that Co excess does not significantly change the magnetic properties, whereas an increase of the Fe content can reduce the spin polarization. In addition, the influence of off-stoichiometry on x-ray absorption spectra is studied. For comparison, x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements have been carried out for Co2FeSi/MgO(001) at the Co and Fe L2,3 edges.

Lattice parameters and orthorhombic distortion of CaMnO3

CaMnO3is a parent compound for numerous multicomponent manganese perovskite oxides. Its crystallographic data are of primary importance in the science and technology of functional CaMnO3-based materials. In the present study, data were collected for a CaMnO3sample at 302 K. The crystal structure refinement yields accurate absolute values of lattice parameters,a=5.281 59(4) Å,b=7.457 30(4) Å, andc=5.267 48(4) Å, leading to orthorhombic distortion of (c/a, √2c/b)=(0.997 33,0.998 95). The orthorhombic distortion of the CaMnO3structure is discussed on the basis of comparison of our unit-cell size with data already published. At a graphical representation of the distortion, it is observed that there is a considerable scatter of the distortion values among the literature data but, interestingly, a considerable fraction of experimental results (including the present one) for stoichiometric samples are grouped around the distortion (c/a, √2c/b)=(0.9973,0.9990), which lies close to a maximum in the extent of orthorhombicity. The influence of off-stoichiometry on the orthorhombic distortion is discussed on the basis of available experimental data. Simulations, employing a mean-field approach for low temperatures, predict an increase in cell volume and structural distortions with the concentration of oxygen vacancies when the additional electrons are localized on the manganese. A simple model of delocalization produced the opposite effect, which is expected to combine with lattice vibrations to recover the cubic phase at high temperatures.

Influence of off-stoichiometry on the properties of the heavy fermion superconductors UNi 2Al 3 and UPd 2Al 3

We have investigated the influence of deviations from stoichiometry on the metallurgical, superconducting and magnetic properties of UNi 2Al 3 and UPd 2Al 3. Deviations from stoichiometry in UNi 2Al 3 generally increase the width of the superconducting transition and can lead to a complete suppression of the anomalies related to the antiferromagnetic (AF) ordering. By contrast, in UPd 2Al 3 we have observed a reduction in the superconducting transition temperature T c, whereas the AF transition is not affected. We discuss the implication of these results on the metallurgical phase diagram and on the nature of both the superconducting and the AF state of these compounds.