Modification of Magnetic Anisotropy at the Interface between Antiferromagnet and Ferromagnet

Abstract

In this thesis, electrical modulation of magnetic anisotropy has been investigated by using various methods including applying a mechanical strain, vibration and spin current. These external stimuli were mainly applied on exchange coupled antiferromagnet (AFM)/ferromagnet (FM) bilayers. In our present work, it was demonstrated that the magnitude of anisotropy as well as the direction of easy axis is determined by the applied DC piezoelectric strain. Furthermore, mechanical vibration, which is generated when AC electric field is applied on a piezoelectric substrate, can repeatedly realign or readjust the AFM spins along a desired direction in the single specimen. Finally, the AFM spins, which are distributed randomly in as-grown state, can be aligned in one direction by injecting spin current into AFM layer without an external magnetic field. This thesis is organized in three parts. In the first section, coupling of magnetoelastic and piezoelectric effects is achieved with the FM Co60Fe20B20/AFM Co0.7Ni0.3O bilayers deposited on the surface of piezoelectric single crystalline substrate. Piezoelectric strain of the substrate is successfully transferred to the magnetic layers, and controllable variation of magnetic anisotropy in response to the elastic deformation is demonstrated in the M-H loops measured at various azimuthal angles with respect to the direction of AFM spin alignment. Magnetic behaviors of the exchange coupled bilayers can be understood by considering the competition of many magnetic anisotropy energies within the magnetic film based on the modified Meiklejohn and Bean (MB) model. From the comparison of experimentally measured results with the modified MB model, it is found that the strain in the film from the substrate effectively introduces additional uniaxial anisotropy in the FM layer with its anisotropy direction determined by the direction of the applied strain. In the second section, alignment of the AFM spin structure in a uniaxial direction usually required a field annealing procedure, that heats the AFM material to a temperature above Néel temperature followed by cooling inside the magnetic field. Ferromagnet (FM) coupled to the uniaxially aligned AFM spins exhibits unidirectional anisotropy conventionally measured by exchange bias effect along the cooling field direction. In the present study, for the first time, we report that an alternating mechanical vibration in kHz frequency range can replace the heating procedure. AFM/FM bilayers were deposited on the surface of piezoelectric single crystalline substrate and mechanical vibration was applied via the AC electric field across the substrate at room temperature. It was found that the exchange bias effect of the bilayers can be set in any desired direction along the externally applied magnetic field with the help of vibration. The simple technique presented in the present study exclude the heating procedure, hence enables the local control of exchange bias effect in the micro-structured devices. Furthermore, repeated post-resetting of exchange bias direction as well as its magnitude can also be achieved. In the third section, trilayers structure consisting of FM alloy CoFeB/AFM IrMn/heavy metal Pt layers was prepared and the manipulation of its exchange bias effect was achieved without applying an external magnetic field. Spin polarized current, generated by the spin Hall effect of the electrical current through Pt layer, was shown to be able to control the spin alignment state in the neighboring antiferromagnetic material as well as the corresponding exchange bias effect in the ferromagnet coupled to the antiferromagnet. The results demonstrated stable and reliable switching of spin structure in antiferromagnets providing a useful route to all-electrical manipulation of antiferromagnetic states without a need for bulky global application of external magnetic field. Therefore, with the results presented in the present work, precise control of magnetism including both ferromagnet and antiferromagnet in submicron to nanometer length scales can be expected.