Role of disorder when upscaling magnetocaloric Ni-Co-Mn-Al Heusler alloys from thin films to ribbons

Research in functional magnetic materials often employs thin films as model systems for finding new chemical compositions with promising properties. However, the scale-up of thin films towards bulk-like structures is challenging, since the material synthesis conditions are entirely different for thin films and e.g. rapid quenching methods. As one of the consequences, the type and degree of order in thin films and melt-spun ribbons are usually different, leading to different magnetic properties. In this work, using the example of magnetocaloric Ni-Co-Mn-Al melt-spun ribbons and thin films, we show that the excellent functional properties of the films can be reproduced also in ribbons, if an appropriate heat treatment is applied, that installs the right degree of order in the ribbons. We show that some chemical disorder is needed to get a pronounced and sharp martensitic transition. Increasing the order with annealing improves the magnetic properties only up to a point where selected types of disorder survive, which in turn compromise the magnetic properties. These findings allow us to understand the impact of the type and degree of disorder on the functional properties, paving the way for a faster transfer of combinatorial thin film research towards bulk-like materials for magnetic Heusler alloys

Shape memory Heusler alloys for thin film applications

Teichert N. Shape memory Heusler alloys for thin film applications. Bielefeld: Universität Bielefeld; 2016.This thesis addresses the examination of shape memory Heusler alloy thin films for applications in spintronics and magnetocalorics. In the first experimental chapter, we investigate the potential of Ni-Mn-Sn films as pinning layers in magnetic tunnel junctions and the second chapter we elucidate the potential of Ni-Co-Mn-Al films for magnetic refrigeration. The underlying physical phenomenon for the first project is an intrinsic exchange bias effect (EB) caused by a cluster spin-glass state at low temperature, whereas for the second project the giant inverse magnetocaloric effect corresponding to the magnetostructural martensitic phase transformation is decisive. We integrated a Ni52Mn34Sn14 Heusler compound film on into an MgO (substrate)/Ni-Mn-Sn/CoFeB/MgO/CoFeB magnetic tunnel junction and have shown that the intrinsic exchange bias causes a shift on the switching field of the magnetic electrode. For the study of magnetocaloric Ni-Co-Mn-Al films we fabricated a series of films with different composition in order to obtain a set of different transformation temperatures. With this we compared the structural, magnetic and magnetocaloric properties of substrate constrained and freestanding films. The structural examination reveals an adaptive 14M martensite which coarsens into mesoscopic variants of tetragonal ‘NM’ martensite for substrate constrained films. In contrast to that, freestanding films exhibit only NM martensite with the peculiarity that the c-axis is exclusively in-plane oriented. Therefore, the martensite is not self-accommodating and a large misfit between the austenite and martensite film area is present. To compensate for this the film bulges out and as a consequence the martensitic film shows high waviness. Magnetocaloric measurements were conducted on one substrate constrained film (Ni40Co9.3Mn32.9Al17.8) and one freestanding film (Ni39.4Co9.2Mn32.3Al19.1) where different compositions were chosen because the martensitic transformation shifts to higher temperatures in freestanding films. Giant inverse magnetocaloric effects were found with up to DeltaS_maxwell=7.3 J/(kg K) for both substrate constrained and freestanding films. The entropy difference between austenite and martensite DeltaS_cc was found to decrease with increasing magnetic field which leads to saturation of the field induced entropy change and increasing field dependence of the transformation temperatures (dT_M/A/ dH) at high field. DeltaS_cc decreases with decreasing temperature which causes kinetic arrest and suppression of the martensitic transformation in films with low expected transformation temperature. The most striking limitations that should be addressed in future work are the temperature dependence of the intrinsic EB in Ni-Mn-Sn and the structural hysteresis in Ni-Co-Mn-Al

Synthesis of NiCoMnX (X = In, Al) Heusler-type Magnetic Shape Memory Alloy Thin Films

Magnetic shape memory alloys are a class of shape memory alloys, and therefore exhibit a thermoelastic martensite phase transformation between symmetric and asymmetric crystalline states induced by appropriate temperature and/or stress changes. Shape memory alloys are able to recover strain when stress is applied, which can generate higher actuation forces and displacements compared to piezoelectrics and magnetostrictive materials when the material is constrained. While shape memory alloys have found applications in biomedical and aerospace industries, actuator applications are limited to relatively low frequencies compared to piezoelectric materials. The slow response of shape memory alloys is associated with heating or cooling the material from an external source. Compared to traditional shape memory alloys, the coupling of structural and magnetic ordering result in magnetic and structural transformations that increase the functional properties in magnetic shape memory alloys, such as magnetic field-induced rapid martensite transformation (forward and reversed), giant magnetoresistance, and the magnetocaloric effect. While bulk MSMAs can be used for structural components, in many cases MSMA thin films are preferred for device applications, such as miniaturized actuators, small scale propulsion devices, and micro-electro-mechanical systems (MEMS). This thesis focuses on the synthesis of NiCoMnX (X=In, Al) Heusler-type magnetic shape memory alloy thin films via physical vapor deposition, and details the challenges associated with controlling film composition, precipitation, microstructure, residual stress, and mechanical properties. As-deposited films were found to contain a mixture of amorphous and nanocrystalline microstructure, and thus, did not exhibit a martensitic transformation. Appropriate post-deposition heat treatments were required to crystallize the films, tailor the grain size, and reduce the formation of precipitates. Crystallized films exhibited martensitic transformations that showed a grain size dependence. An analytical model that uses a thermodynamic framework was developed to explain the suppression of the martensitic transformations for films with submicron-sized grains. Hence, in addition to chemical composition, sub-micron grain size can be used to tailor the martensitic transformation temperature of NiCoMnX (X=In, Al) thin films for device applications. Additionally, the analytical model may reduce the uncertainty associated with a direct scale-up of thin film compositions used for combinatorial investigations of magnetic shape memory alloys

Entropy Change during Martensitic Transformation in Ni50−xCoxMn50−yAly Metamagnetic Shape Memory Alloys

Specific heat was systematically measured by the heat flow method in Ni50-xCoxMn50-yAly metamagnetic shape memory alloys near the martensitic transformation temperatures. Martensitic transformation and ferromagnetic–paramagnetic transition for the parent phase were directly observed via the specific heat measurements. On the basis of the experimental results, the entropy change was estimated and it was found to show an abrupt decrease below the Curie temperature. The results were found to be consistent with those of earlier studies on Ni-Co-Mn-Al alloys