Shape memory Heusler alloys for thin film applications

Teichert N (2016)
Bielefeld: Universität Bielefeld.

Download
OA
Bielefeld Dissertation | English
Author
Abstract
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.
Year
PUB-ID

Cite this

Teichert N. Shape memory Heusler alloys for thin film applications. Bielefeld: Universität Bielefeld; 2016.
Teichert, N. (2016). Shape memory Heusler alloys for thin film applications. Bielefeld: Universität Bielefeld.
Teichert, N. (2016). Shape memory Heusler alloys for thin film applications. Bielefeld: Universität Bielefeld.
Teichert, N., 2016. Shape memory Heusler alloys for thin film applications, Bielefeld: Universität Bielefeld.
N. Teichert, Shape memory Heusler alloys for thin film applications, Bielefeld: Universität Bielefeld, 2016.
Teichert, N.: Shape memory Heusler alloys for thin film applications. Universität Bielefeld, Bielefeld (2016).
Teichert, Niclas. Shape memory Heusler alloys for thin film applications. Bielefeld: Universität Bielefeld, 2016.
Main File(s)
Access Level
OA Open Access
Last Uploaded
2016-04-26T11:46:18Z
MD5 Checksum
d11662db92c4546d178ecea2a5c27379

This data publication is cited in the following publications:
This publication cites the following data publications:

Export

0 Marked Publications

Open Data PUB

Search this title in

Google Scholar