Priyanga Jayathilaka

Enhanced giant magnetoimpedance effect and field sensitivity in Co-coated soft ferromagnetic amorphous ribbons

A 50 nm-thick Co film has been grown either on the free surface (surface roughness, ∼6 nm) or on the wheel-side surface (surface roughness, ∼147 nm) of Co84.55Fe4.45Zr7B4 amorphous ribbons. A comparative study of the giant magnetoimpedance (GMI) effect and its field sensitivity (η) in the uncoated and Co-coated ribbons is presented. We show that the presence of the Co coating layer enhances both the GMI ratio and η in the Co-coated ribbons. Larger values for GMI ratio and η are achieved in the sample with Co coated on the free ribbon surface. The enhancement of the GMI effect in the Co-coated ribbons originates mainly from the reduction in stray fields due to surface irregularities and the enhanced magnetic flux paths closure. These findings provide good guidance for tailoring GMI in surface-modified soft ferromagnetic ribbons for use in highly sensitive magnetic sensors.

Impact of ultrathin transition metal buffer layers on Fe3O4 thin films

3000 A Fe3O4 (magnetite) thin films were simultaneously grown on (001) MgO single crystal substrates with and without 30 A buffer layers of Fe, Cr, Mo, and Nb. For all samples, the Verwey transition temperature (TV) occurs between 119 and 125 K, indicating good oxygen stoichiometry. We observe highly oriented (001) Fe3O4 with Mo and no buffer layer, reduced (001) texture with Nb and Fe, and polycrystalline growth with Cr. Mo, Cr, and unbuffered magnetite show typical magnetic behavior, whereas Nb and Fe buffers lead to anomalous magnetic properties that may be due to interfacial reactivity.

Magnetic force microscopy of epitaxial magnetite films through the Verwey transition

Magnetic force microscopy was performed on 300 nm thick magnetite films grown epitaxially on MgO (001) at temperatures ranging from well below to well above the Verwey transition temperature, TV. Frequency shift images were acquired at different locations on the sample as temperature was increased through the Verwey transition. The magnetic domain features are persistent at all temperatures, which indicates that the domains are pinned across the phase transition, probably due to antiphase boundaries. An enhancement of magnetic contrast below TV indicates the moments tilt out of the plane below TV, which is corroborated by superconducting quantum interference device magnetometry.

Influence of Growth Field on NiFe, Fe

Thin films of Ni80Fe20, Fe3O4, as well as Ni80Fe20/Cr/Fe3O4 spin valves, have been grown with and without magnetic fields applied during the deposition, and their magnetotransport properties have been studied at room temperature. The applied field induces an anisotropy in both single layer films, which causes notable differences in their anisotropic magnetoresistance. In the spin valve system, the applied field enables the parallel and antiparallel states to be more well-defined, which reveals a possible giant magnetoresistance in the system. The origin of this signal is likely the interaction of electrons that have been polarized by spin-dependent reflection from the Cr/Fe3O4 interface with the Ni80Fe20 interface.

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To study the effect of non-magnetic layer (Cu) on magnetic properties of antiferromagnetic FeMn, multilayers of Ta(5 nm)/[FeMn(t)/Cu(5 nm)]10/Ta(5 nm), where t is varied in the range of 5–15 nm, are fabricated by a combination of RF and DC magnetron sputter deposition. Magnetization curves for these samples exhibit magnetic hysteresis, and when the samples are cooled in an applied magnetic field, the hysteresis loops are shifted. This shift is attributed to an “intrinsic” exchange bias effect (i.e., it is observed without a separate ferromagnetic layer). Presented temperature and thickness dependences of the coercive field, magnetic moment, and exchange bias field provide insights into the origin and mechanism of the observed intrinsic exchange bias.

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Abstract The exchange bias of the soft ferromagnet mu-metal, Ni 77 Fe 14 Cu 5 Mo 4 , with the metallic antiferromagnet Fe 50 Mn 50 has been studied as a function of ferromagnet thickness and buffer layer material. Mu-metal exhibits classic exchange bias behavior: the exchange bias ( H EB ) and coercive fields scale inversely with the ferromagnets thickness, with H EB varying as the cosine of the in-plane applied field angle. Ta buffers, rather than Cu, allow the mu-metal to retain more of its soft magnetic character while exhibiting exchange bias. The ability to preserve soft ferromagnetic behavior in an exchange biased heterostructure may be useful for low field sensing and other device applications.

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This work reports efforts fabricating heterostructures of different materials relevant for the realization of magnon-induced spin transfer torques. We find the growth of high-quality magnetite on MgO substrates to be straightforward, while using transition metal buffer layers of Fe, Cr, Mo, and Nb can alter the structural and magnetic properties of the magnetite. Additionally, we successfully fabricated and characterized Py/Cr/Fe3O4 and Fe3O4/Cr/Fe3O4 spin valve structures. For both, we observe a relatively small giant magnetoresistance and confirm an inverse dependence on spacer layer thickness. Thus, we have shown certain materials combinations that may form the heterostructures that are the building blocks necessary to achieve magnon-induced spin transfer torque devices.