70 research outputs found
Charge order and tilt modulation in multiferroic (KxM(II)xM(III)1-xF3)(0.4 < x < 0.6) transition metal fluorides with tetragonal tungsten bronze structure
No full textTransition metal fluorides with tetragonal tungsten bronze (TTB) structure, having general formula (KxMxM1-xF3)-M-II-F-III with 0.4 < x < 0.6 and M belonging to transition metals, have been studied over the past decades because of the frustrated magnetic ordering arising at low temperature. We investigated the structural properties of TTB fluorides by transmission electron microscopy (TEM). The results, coupled with an accurate structure analysis performed by single-crystal XRD on K0.53FeF3, revealed ferroelectricity and a unified structural description of these materials, where charge order and tilt modulations coexist, each one with its own periodicity differently commensurate to the conventional TTB cell. Whereas the charge order is peculiar of the mixed valence fluorides,the nature of the tilt modulation is clearly associable to the structural features of TTB niobates, suggesting also for the fluoride bronzes the existence of a generalized ferroelectric-ferroelastic behavior at room temperature
Influence of the transition width on the magnetocaloric effect across the magnetostructural transition of Heusler alloys
No full textWe report a complete structural and magneto-thermodynamic characterization of four samples of the Heusler alloy Ni-Co-Mn-Ga-In, characterized by similar compositions, critical temperatures and high inverse magnetocaloric effect across their metamagnetic transformation, but different transition widths. The object of this study is precisely the sharpness of the martensitic transformation, which plays a key role in the effective use of materials and which has its origin in both intrinsic and extrinsic effects. The influence of the transition width on the magnetocaloric properties has been evaluated by exploiting a phenomenological model of the transformation built through geometrical considerations on the entropy vs. temperature curves. A clear result is that a large temperature span of the transformation is unfavourable to the magnetocaloric performance of a material, reducing both isothermal entropy change and adiabatic temperature change obtainable in a given magnetic field and increasing the value of the maximum field needed to fully induce the transformation. The model, which is based on standard magnetometric and conventional calorimetric measurements, turns out to be a convenient tool for the determination of the optimum values of transformation temperature span in a trade-off between sheer performance and amplitude of the operating range of a material
Tuning the magnetic and magnetocaloric properties of austenitic Ni-Mn-(In,Sn) Heuslers
No full textIn this work, we highlight the occurrence of different physical mechanisms that independently control the saturation magnetization and the ferro-paramagnetic transition temperature of Ni-Mn-based Heusler compounds, opening new possibilities in mastering the functional properties of this wide class of magnetic materials. We present the magnetic, structural and magnetocaloric features of a complete Ni48Mn36In16-xSnx (x= 0-16) series. The observed different trends of the critical temperature and of the saturation magnetization on varying the Sn to In ratio are discussed with the help of first-principles calculations of the electronic structure and magnetic interactions of the compound
In-operando test of tunable Heusler alloys for thermomagnetic harvesting of low-grade waste heat
No full textThermomagnetic generation stands out as a promising technology for harvesting and converting low-grade waste heat below 100°C. Despite the exponential growth in research on thermomagnetic materials and prototypes over the last decade, there remains, to unlock the full potential of this technology, a critical gap between fundamental research on materials and the design of advanced devices.
In this study, we present the in-operando assessment of thermomagnetic performance of three representative Ni,Mn-based Heusler alloys optimized for harvesting low-grade waste heat below 373 K. These materials were tested under operational conditions using a specially designed laboratory-scale prototype of a thermomagnetic motor. The mechanical power output of the motor, operated with NiMnIn, NiMnSn and NiMnCuGa alloys, was correlated with the magnetic properties of the materials, highlighting the critical role of the magnetic transition temperature and saturation magnetization in determining the efficiency of thermomagnetic energy conversion. Austenitic Heusler alloys were confirmed to be promising thermomagnetic materials due to their highly tunable Curie temperature and significant magnetization changes in the 300–360 K temperature range. Among the tested materials, the Ni48Mn36In16 alloy demonstrated the highest thermomagnetic performance, surpassing the benchmark material Gd in the 320–340 K range. From an experimental perspective, the developed prototype of thermomagnetic motor serves as a flexible test-bench for evaluating and comparing the thermomagnetic performance of small amounts (less than 0.3 g) of new materials under variable conditions. Additionally, its modular design facilitates testing and optimization of its various components, thereby contributing to the advancement of thermomagnetic motor technology
Long-range antiferromagnetic interactions in Ni-Co-Mn-Ga metamagnetic Heusler alloys: A two-step ordering studied by neutron diffraction
No full textWe report on the experimental observation of a long-range antiferromagnetic structure in the metamagnetic Ni-Co-Mn-Ga Heusler alloys. The accurate magnetic symmetry analysis based on experimental neutron diffraction data, exploiting the Shubnikov theory, allows the determination of the correct magnetic space group of the system. A two-step process, featuring the ordering of the Ni and Mn sublattices at different temperatures, leads to the antiferromagnetic structure in martensite. A perfect, constrained by the symmetry, antiferromagnetic ordering of the Ni sublattice in the "paramagnetic gap" is observed, followed by the ordering of the Mn sublattice at lower temperatures. The observation of such antiferromagnetic structure clarifies the current debate on the presence of antiferromagnetic interactions in the (Ni,Co)-Mn-X (X=Ga, Sn, Sb, and In) ferromagnetic shape memory alloys and yields new insights in understanding the magnetostructural properties of this relevant class of materials
Strong magneto-volume effects and hysteresis reduction in the In-doped (NiCo) 2 MnGa Heusler alloys
No full textWe report the effect of high hydrostatic pressure (up to 1.1 GPa) on magnetization and critical temperatures of the magnetic and magneto-structural transitions in the off-stoichiometric In-doped (Ni,Co)2Mn(Ga,In) alloys. The magnetization MM(5 K) of their martensitic phase significantly decreases under pressure and the value of dlnMM(5 K)/dP = −55 × 10−3 GPa−1 was observed in the Ni41Co9Mn33Ga14In3 alloy. The temperature of structural transition of the alloys from martensite to austenite, TM-A, shows a strong dependence on both pressure (dTM-A/dP = 60 K/GPa) and magnetic field. The complex mutual dependence of TM-A to both pressure and magnetic field produces a remarkable reduction of the transformation hysteresis
Convergence of direct and indirect methods in the magnetocaloric study of first order transformations: the case of Ni-Co-Mn-Ga Heusler alloys
No full textThe study of two aspects of the magnetocaloric effect (MCE), namely, the matching between isothermal
entropy change and direct adiabatic temperature change, is not straightforward since huge differences between
these two quantities have often been reported. Here we put in relation the direct and indirect measurements on the
first order magnetostructural martensitic transformation occurring in Ni-Co-Mn-Ga alloys. In order to complete
the characterization of the MCE and to find an explanation of these mismatches, differential scanning calorimeter
measurements have been performed at different applied magnetic fields
Curie temperature effect on the inverse MCE in Ni2MnIn based Heusler alloys near room temperature
No full textWe investigate the magneto caloric properties in Ni2MnIn based Heusler alloys having the martensitic to austenitic transition temperatures close to the Curie temperature of the austenitic phase. We tuned the structural transition by composition in order to encounter three possible cases as a function of the increasing temperature: I) the martensite to austenite transition completes before the drop of magnetization associated to the Curie temperature of the austenite, ii) the martensite to austenite transition evolves across the Curie temperature of the austenite and iii) the martensite to austenite transition lies almost completely above the Curie transition of the austenite. When the martensitic transition temperature overlaps with the Curie temperature of the austenite we observe, at constant temperature, the coexistence of direct and inverse magneto caloric effects. In order to understand this behaviour we design a layered structure composed by a 1st order inverse MC material and a 2nd order direct MC material. In this structure, part of the heat exchanged by the direct MCE cooperates with the magnetic field to drive the structural transition further
Grain size reduction and magnetic properties improvement by in-situ annealing of FePt epitaxial thin films
No full textFePt thin films were epitaxially grown by RF sputtering on MgO (1 0 0) substrates at 550 1C, followed by in situ annealing at the sametemperature. The in situ annealing was found to improve the magnetic characteristics: i.e., the increase in magnetic squareness up to 0.9and increase in the ratio between anisotropy and coercivity (e 1⁄4 anisotropy field/coercive field 1⁄4 35.9 for 115 min annealing).Remarkably, a decrease in grain size was also found to occur by increasing the annealing time.r 2007 Elsevier B.V. All rights reserved
- …
