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Magnetic and magnetoelastic properties of NiTi shape memory alloy and NiTi/Ni bilayer

Abstract : The initial goal of the thesis is to investigate the manipulation of magnetic anisotropy of a ferromagnetic thin layer through strain by means of shape memory effect in NiTi/ferromagnetic bilayer. First, the structural features and paramagnetic nature of NiTi are broadly studied. Then, magnetoelastic coupling in NiTi/Ni bilayer is investigated. In the first part, paramagnetic features of a freestanding 20µm thick NiTi polycrystalline samples grown by magnetron sputtering are studied with commercial magnetometers, as a function of a magnetic field, temperature, crystal structure, and mechanical strain. NiTi structure is characterized by X-ray diffraction, particularly in relation with its strain state. The samples are deformed in tension in the broad range of deformation magnitudes, the strain distribution is characterized by digital image correlation technique. Magnetometry is demonstrated to be an efficient technique for probing of shape memory alloys transition and strain state. Variations of NiTi magnetic susceptibility are explained based on considerations that NiTi is Pauli paramagnetic material by comparing experimental data with ab-initio calculations. It is found that magnetic susceptibility depends on particular trends in NiTi lattice parameters induced by tensile strain. In the second part, we prove that shape memory effect in freestanding NiTi is viable to induce and reversibly switch by 90° a uniaxial magnetic anisotropy of the Ni layer deposited on top of it. Switching is accomplished by thermal cycling between 300K and 400K. The magnitude of magnetic anisotropy change during a thermal cycle is shown to be dependent on the pre-strain value, that is tunable. The interface quality of the bilayer is investigated with transmission electron microscopy (TEM) technique. Consistent strain transmission rate to Ni from NiTi is independently estimated from strain distribution in NiTi, from Ni magnetic anisotropy magnitude and verified by TEM.
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Submitted on : Thursday, April 30, 2020 - 3:10:23 PM
Last modification on : Sunday, June 26, 2022 - 2:49:28 AM


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Anton Kyianytsia. Magnetic and magnetoelastic properties of NiTi shape memory alloy and NiTi/Ni bilayer. Physics [physics]. Université de Lorraine, 2019. English. ⟨NNT : 2019LORR0299⟩. ⟨tel-02559546⟩



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