Epitaxial Ni-Mn-Ga-Co thin films on PMN-PT substrates for multicaloric applications

Schleicher, Benjamin; Niemann, Robert; Diestel, Anett; Hühne, Ruben; Schultz, L.; Fähler, S.

doi:10.1063/1.4927850

Cited by 25 publications

(23 citation statements)

References 46 publications

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“…59 There, we expect the mechanical stress imposed by the substrate onto the film to have a similar effect as a magnetic field in the presence of magnetic anisotropy would have-both lead to a variant selection during the transformation. The analogy of stress and magnetic field regarding variant selection was proven in recent work on Ni-Mn-Ga. 45 …”

Section: B Microstructural Origin Of Hysteresis Asymmetry and Field mentioning

confidence: 99%

Field-temperature phase diagrams of freestanding and substrate-constrained epitaxial Ni-Mn-Ga-Co films for magnetocaloric applications

Diestel

Niemann

Schleicher

et al. 2015

Journal of Applied Physics

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Ferroic cooling processes that rely on field-induced first-order transformations of solid materials are a promising step towards a more energy-efficient refrigeration technology. In particular, thin films are discussed for their fast heat transfer and possible applications in microsystems. Substrate-constrained films are not useful since their substrates act as a heat sink. In this article, we examine a substrate-constrained and a freestanding epitaxial film of magnetocaloric Ni-Mn-Ga-Co. We compare phase diagrams and entropy changes obtained by magnetic field and temperature scans, which differ. We observe an asymmetry of the hysteresis between heating and cooling branch, which vanishes at high magnetic fields. These effects are discussed with respect to the vector character of a magnetic field, which acts differently on the nucleation and growth processes compared to the scalar character of the temperature. V C 2015 AIP Publishing LLC.

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Section: B Microstructural Origin Of Hysteresis Asymmetry and Field mentioning

confidence: 99%

Field-temperature phase diagrams of freestanding and substrate-constrained epitaxial Ni-Mn-Ga-Co films for magnetocaloric applications

Diestel

Niemann

Schleicher

et al. 2015

Journal of Applied Physics

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“…We use the inflection point method and not the tangent method because in a minor loop only a limited temperature range is available. The summarized temperatures T trans relate to austenitic transformation temperatures T A (blue circles) from cooling cycles and to martensitic transformation temperatures T M (red squares) from heating cycles.…”

Section: Resultsmentioning

confidence: 99%

“… and further information about structure of Ni–Mn–Ga–Co films are specified in Ref. . Before performing the present experiments, this film had experienced more than 20 major loops in order to avoid the training effects reported previously …”

Section: Methodsmentioning

confidence: 99%

Reducing Hysteresis Losses by Heating Minor Loops in Magnetocaloric Ni–Mn–Ga–Co Films

et al. 2018

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Magnetocaloric materials relying on first‐order phase transitions give the highest entropy changes compared to second‐order transitions. However, they have the drawback of a transformation hysteresis, which reduces the efficiency of a cooling cycle. Here an approach to reduce the thermal hysteresis losses by performing incomplete transformation cycles, so called minor loops, is presented. A freestanding, epitaxially grown Ni–Mn–Ga–Co film is used as a model system to analyze the direct martensitic and reverse transformation paths in detail. The additional residual phase fraction only facilitates the forward martensitic transformation, but not the reverse transformation. This pronounced asymmetry between cooling and heating demonstrates that the forward transformation to martensite is controlled by nucleation and the associated excess energy contributes to hysteresis losses. For the reverse transformation, however, nuclei of austenite are always present and accordingly this transformation is controlled by growth of the austenitic phase. The experiments reveal that only the use of heating minor loops is useful to improve the efficiency of magnetocaloric epitaxial films and by this, the ratio of usable magnetization difference divided by hysteresis loss can be increased by 64 %.

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“…The Ni−Mn−Ga−Co films are around 350 nm thick. A more detailed description of the sample preparation is provided elsewhere . The composition of the sample was determined by EDX measurements using a JEOL JSM‐6510 with an XFlash detector with an accuracy of about 1–2 at %.…”

Section: Methodsmentioning

confidence: 99%

“…Thin films are of particular interest for ferroic cooling technology as they allow for a fast heat transfer, which increases the possible cycling frequencies and thus raises the specific cooling power of the material. Finally, the possibility to choose different substrates allows us to grow Ni−Mn−Ga−Co films on ferroelectric Pb(Mg 1/3 Nb 2/3 ) 0.72 Ti 0.28 O 3 (PMN‐PT), which can also be utilized to investigate the response to biaxial stress. In a recent article on this particular system, a perceptible change in magnetization under stress is obtained only in the austenitic phase .…”

Section: Introductionmentioning

confidence: 99%

Probing the Martensitic Microstructure of Magnetocaloric Heusler Films by Synchrotron Diffraction

et al. 2018

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First‐order, magnetocaloric materials are promising for energy efficient solid‐state cooling because of their high entropy changes, but they have the drawback of a hysteresis due to the nucleation and growth of the involved phases. Here, synchrotron based X‐ray diffraction is used to investigate epitaxial Ni−Mn−Ga−Co thin films grown on Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (PMN‐PT). This integral method is used to test several hypotheses regarding structure and orientation of the martensite, whose nucleation was proposed to be the decisive contribution to hysteresis in Heusler materials. The measurements are compared to a recently proposed model of the martensitic nuclei, combining the phenomenological martensite theory with the adaptive concept. This investigation demonstrates that the diffraction patterns are consistent with a martensitic nucleus built from 8 variants forming a‐b laminates. Additionally, it is shown that in accordance with the surface morphology of these films only a subset of nuclei occur, namely those needed to form type X martensite.

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Epitaxial Ni-Mn-Ga-Co thin films on PMN-PT substrates for multicaloric applications

Abstract: Articles you may be interested inField-temperature phase diagrams of freestanding and substrate-constrained epitaxial Ni-Mn-Ga-Co films for magnetocaloric applications

Cited by 25 publications

References 46 publications

Field-temperature phase diagrams of freestanding and substrate-constrained epitaxial Ni-Mn-Ga-Co films for magnetocaloric applications

Field-temperature phase diagrams of freestanding and substrate-constrained epitaxial Ni-Mn-Ga-Co films for magnetocaloric applications

Reducing Hysteresis Losses by Heating Minor Loops in Magnetocaloric Ni–Mn–Ga–Co Films

Probing the Martensitic Microstructure of Magnetocaloric Heusler Films by Synchrotron Diffraction

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