The structural, magnetic, and magnetocaloric properties of epitaxial Ni-Co-Mn-Al thin films with different compositions have been studied. The films were deposited on MgO(001) substrates by co-sputtering on heated substrates. All films show a martensitic transformation, where the transformation temperatures are strongly dependent on the composition. The structure of the martensite phase is shown to be 14M. The metamagnetic martensitic transformation occurs from strongly ferromagnetic austenite to weakly magnetic martensite. The structural properties of the films were investigated by atomic force microscopy and temperature dependent x-ray diffraction. Magnetic and magnetocaloric properties were analyzed using temperature dependent and isothermal magnetization measurements. We find that Ni 41 Co 10.4 Mn 34.8 Al 13.8 films show giant inverse magnetocaloric effects with magnetic entropy change of 17.5 J kg −1 K −1 for μ 0 H = 5 T.
A thermal gradient as the driving force for spin currents plays a key role in spin caloritronics. In this field the spin Seebeck effect (SSE) is of major interest and was investigated in terms of in-plane thermal gradients inducing perpendicular spin currents (transverse SSE) and out-of-plane thermal gradients generating parallel spin currents (longitudinal SSE). Up to now all spincaloric experiments employ a spatially fixed thermal gradient. Thus, anisotropic measurements with respect to well defined crystallographic directions were not possible. Here we introduce a new experiment that allows not only the in-plane rotation of the external magnetic field, but also the rotation of an in-plane thermal gradient controlled by optical temperature detection. As a consequence, the anisotropic magnetothermopower and the planar Nernst effect in a permalloy thin film can be measured simultaneously. Thus, the angular dependence of the magnetothermopower with respect to the magnetization direction reveals a phase shift, that allows the quantitative separation of the thermopower, the anisotropic magnetothermopower and the planar Nernst effect.Adding the spin degree of freedom to conventional charge-based electronics opens the field of spintronics 1,2 with promising advantages such as decreased electric power consumption and increased integration densities. While spinelectronics use only voltages as driving force for currents, thermal gradients and the interaction between spins and heat currents have already been shown to provide new effects. Spin caloritronics investigate these interactions and promotes the search for applications such as heat sensors or waste heat recyclers 3,4 , that can improve thermoelectric devices.One of the most important and well established phenomena in spin caloritronics is the longitudinal spin Seebeck effect (LSSE) [5][6][7][8][9][10][11] , which uses typically out-of-plane thermal gradients in magnetic thin films for the generation of a spin current parallel to the thermal gradient. This pure spin current is then injected into an adjacent non-magnetic conductor with high spin-orbit coupling, e.g. Pt, which transforms the spin current into an electric voltage via the inverse spin Hall effect (ISHE). In very recent investigations, the LSSE was even detected without any Pt and ISHE by the use of the anomalous Hall effect in Au 12 and by time-resolved magnetooptic Kerr effect in Au and Cu 13 . Besides the application of an out-of-plane thermal gradient, effects driven by in-plane thermal gradients were also investigated. The transverse spin Seebeck effect (TSSE), the spin current generation perpendicular to an in-plane thermal gradient, was reported for metals 14 , semiconductors 15 and insulators 16 . However, it has been noted that TSSE experiments in metals and semiconductors can be influenced by parasitic effects like the planar
Lab-on-a-chip immuno assays utilizing superparamagnetic beads as labels suffer from the fact that the majority of beads pass the sensing area without contacting the sensor surface. Different solutions, employing magnetic forces, ultrasonic standing waves, or hydrodynamic effects have been found over the past decades. The first category uses magnetic forces, created by on-chip conducting lines to attract beads towards the sensor surface. Modifications of the magnetic landscape allow for additional transport and separation of different bead species. The hydrodynamic approach uses changes in the channel geometry to enhance the capture volume. In acoustofluidics, ultrasonic standing waves force µm-sized particles onto a surface through radiation forces. As these approaches have their disadvantages, a new sensor concept that circumvents these problems is suggested. This concept is based on the granular giant magnetoresistance (GMR) effect that can be found in gels containing magnetic nanoparticles. The proposed design could be realized in the shape of paper-based test strips printed with gel-based GMR sensors.
The present study is dedicated to the influence of different substrate and buffer layer materials on the martensitic transformation in sputter deposited epitaxial shape memory Heusler alloys. For this, the magnetocaloric Heusler alloy Ni-Co-Mn-Al 1 is grown on MgO(001), MgAl 2 O 4 (001), and MgO(001)/V substrates, which exhibit a lattice misfit to the Ni-Co-Mn-Al between −1.2% and 3.6%. By temperature dependent X-ray diffraction measurements it is shown that the optimum buffer layer for shape memory Heusler films is not one with minimum lattice misfit, but one with minimum Young's modulus and moderate misfit because an elastic buffer layer can deform during the martensitic transformation of the Heusler layer. Furthermore, epitaxial strain caused by a moderate lattice misfit does not significantly change the martensitic transformation temperatures.
We report on the preparation of Gadolinium thin films by means of sputter deposition on Silicon Oxide wafers. A series of samples with different buffer layers and various substrate temperatures has been produced. The film on an amorphous Tantalum buffer deposited at 773 K shows the highest increase of magnetization during the phase transition at the Curie temperature. Further detailed analysis of the magnetic properties has been conducted by VSM.
Magnetocaloric composites made from La−Fe−Co−Si particles and an epoxy binder matrix exhibit mechanical stability and good magnetocaloric properties, but also a large characteristic time for thermal transport. Here, the origin of this large time constant is examined by comparing two measurement techniques, direct and contactless, to finite‐element simulations based on a tomographic dataset of the sample. The combination of the low thermal conductivity of the epoxy matrix and a thermal resistance at the interface between epoxy and La−Fe−Co−Si is shown to be in good agreement in simulations and experiments. The findings help to disentangle the role of the thermal conductivity and the interfacial thermal resistance for the heat flow in magnetocaloric composites. It is shown that the low thermal conductivity of the epoxy alone cannot explain the large time constant and possibilities for using the interfacial thermal resistance to tailor anisotropic thermal conductivity for directional heat transfer in magnetocaloric composites are presented.
M(T) curves of Ni-Co-Mn-Al measured at 1 T (solid lines) and 20 mT (dashed lines) for FC (black) and FH (red). The blue squares denote T M , defined by the inflection point of the FC curve.Ni-Co-Mn-Al shows a strong metamagnetic martensitic transition near room temperature. Magnetic entropy change of Ni-Co-Mn-Al, calculated using numerical evaluation of the integratedMaxwell equation. The inset shows the change of the RCP with external field.
In this study we report a method for the preparation of freestanding magnetocaloric thin films. Nonstoichiometric Heusler alloys Ni-Mn-Sn, Ni-Co-Mn-Sn and Ni-Co-Mn-Al are prepared via sputter deposition. A sacrifial vanadium layer is added between the substrate and the Heusler film. By means of selective wetchemicals etching the vanadium layer can be removed. Conditions for the crystallization of Vanadium layers and epitaxial growth of the Heusler films are indicated. Magnetic and structural properties of freestanding and as-prepared films are compared in detail. The main focus of this study is on the influence of substrate constraints on the Martensitic transistion.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.