Structure and giant inverse magnetocaloric effect of epitaxial Ni-Co-Mn-Al films

Teichert, Niclas; Kucza, D.; Yıldırım, O.; Yüzüak, E.; Dinçer, I.; Behler, A.; Weise, Bruno; Helmich, Lars; Boehnke, Alexander; Климова, С. А.; Waske, Anja; Elerman, Y.; Hütten, Andreas

doi:10.1103/physrevb.91.184405

Cited by 37 publications

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References 43 publications

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“…Magnetic materials such as Fe 3 O4, CoFe 2 O 4 , BiFeO 3 , GaFeO 3 , Bi 3 Fe 5 O 12 , NiCoMnAl are being increasingly studied because of their specific properties, either their electronic properties [2], their magneto-electric properties [3], their magneto-optical [4] or their magneto caloric properties [5]. The study of these materials is often limited by the weakness of the magnetic interfacial effects that are of interest.…”

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confidence: 99%

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Polarized neutron channeling as a tool for the investigations of weakly magnetic thin films

et al. 2016

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We present and apply a new method to measure directly weak magnetization in thin films. The polarization of a neutron beam channeling through a thin film structure is measured after exiting the structure edge as a microbeam. We have applied the method to a tri-layer thin film structure acting as a planar waveguide for polarized neutrons. The middle guiding layer is a rare earth based ferrimagnetic material TbCo5 with a low magnetization of about 20 mT. We demonstrate that the channeling method is more sensitive than the specular neutron reflection method.PACS numbers: 3.75. Be, Magnetic nanomaterials are widely used in practical applications and in fundamental physics. Therefore the development of new methods providing enhanced sensitivity for the characterization of their magnetic properties is a challenge. One of the well-established methods for magnetic thin films and nanostructures characterization is the Polarised Neutron Reflectometry (PNR) technique [1]. Its strength is the unique ability to probe the magnitude and the direction of the magnetization vector as a function of the depth in magnetic heterostructures. For collinear spin configurations, the PNR method resolves the depth dependence of the scattering length densities (SLD) ρ(z) which depends on the neutron polarization: ρ ± (z) = ρ 0 (z) ± cB(z). Here ρ 0 (z) and B(z) are the depth profiles of the nuclear SLD and the magnetic induction. By measuring reflectivities of spin-up and spin-down neutrons and fitting them to theoretical values one can extract the SLD depth profile and the depth dependence of the induction in a film. The method is routinely used for the characterization of ferromagnetic systems with rather high magnetization (about 0.3 -2 T). Being a model dependent method it leads to the nonuniqueness of the resulting set of parameters describing the system. As a result, the accuracy of the fitted parameters depends both on the adequacy of a model and on the quality of the experimental data. For systems with weak magnetization (∼ 10 mT), PNR requires very long measurement times (∼ 10 2 hours) in order to achieve sufficient statistics. This is usually not compatible with standard neutron reflectometry experiments.Weak magnetizations, typically less than 100 mT, are characteristic of ferrimagnetic materials or magnetic oxides. Magnetic materials such as Fe 3 O4, CoFe 2 O 4 , BiFeO 3 , GaFeO 3 , Bi 3 Fe 5 O 12 , NiCoMnAl are being increasingly studied because of their specific properties, either their electronic properties [2], their magneto-electric properties [3], their magneto-optical [4] or their magneto caloric properties [5]. The study of these materials is often limited by the weakness of the magnetic interfacial effects that are of interest. While XMCD [6] is a very sensitive technique to probe surface magnetism, it is sometimes rather difficult to apply to complex heterostructures as used in spintronic devices. Also, the XRMS [7] technique cannot access thicker layer structure because the soft x-ray penetration depth is on the order...

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confidence: 99%

“…One may also reduce the background by using the beam-splitting effect in external fields for non-collinear arrangements. For example, in [5,6,30] beam-splitting was used to observe standing waves in off-specular region with higher ratio signal/background. In [24,25] a ratio signal/background≈10 for the microbeam outgoing from a planar waveguide.…”

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Polarized neutron channeling as a tool for the investigations of weakly magnetic thin films

et al. 2016

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“…For adaptive 14M martensite, which is found for example, in Ni–Mn–Ga and Ni–Co–Mn–Al, the martensitic nuclei are elongated and flat diamonds with habit planes inclined by few degrees from the [011] A direction and an aspect ratio of approximately 40:14:1 . The maximum size is determined by one tip of the diamond touching the substrate (see Figure ) because a larger nucleus requires a finite interface with the substrate, which is energetically vastly unfavorable.…”

Section: Stress Couplingmentioning

confidence: 99%

Coupling Phenomena in Magnetocaloric Materials

et al. 2018

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Strong coupling effects in magnetocaloric materials are the key factor to achieve a large magnetic entropy change. Combining insights from experiments and ab initio calculations, we review relevant coupling phenomena, including atomic coupling, stress coupling, and magnetostatic coupling. For the investigations on atomic coupling, we have used Heusler compounds as a flexible model system. Stress coupling occurs in first‐order magnetocaloric materials, which exhibit a structural transformation or volume change together with the magnetic transition. Magnetostatic coupling has been experimentally demonstrated in magnetocaloric particles and fragment ensembles. Based on the achieved insights, we have demonstrated that the materials properties can be tailored to achieve optimized magnetocaloric performance for cooling applications.

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“…Regions of austenite, which remain untransformed, are mostly filled by new martensitic nuclei. 5) Below the martensitic finish‐temperature ( T < M F ) there is a hierarchical twins‐within‐twins microstructure which is typical for Ni–Mn–X(‐Co) films . However, at the disturbed interfaces between differently oriented, macroscopic twinned regions, a certain amount of residual austenite (marked by a green ellipsoid) remains untransformed.…”

Section: Nucleation and Growthmentioning

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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Structure and giant inverse magnetocaloric effect of epitaxial Ni-Co-Mn-Al films

Cited by 37 publications

References 43 publications

Polarized neutron channeling as a tool for the investigations of weakly magnetic thin films

Polarized neutron channeling as a tool for the investigations of weakly magnetic thin films

Coupling Phenomena in Magnetocaloric Materials

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

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