Structural and magnetic properties of Ni₇₈Fe₂₂ thin films sandwiched between low-softening-point glasses and application in spin devices

Graphical abstract: This paper presents the first demonstration of the formation of Ni₇₈Fe₂₂ thin films sandwiched between low-softening-point (LSP) glasses used in spin quantum cross (SQC) devices and the theoretical prediction of spin filter effect in Ni₇₈Fe₂₂-based SQC devices. The fomation of the LSP-glass/Ni₇₈Fe₂₂/LSP-glass structures was successfully demonstrated using a newly proposed thermal pressing technique. Interestingly, this technique gives rise to both a highly-oriented crystal growth in Ni₇₈Fe₂₂ thin films and a 100-fold enhancement in coercivity, in contrast to those of as-deposited Ni₇₈Fe₂₂ thin films. This remarkable increase in coercivity can be explained by the calculation based on two-dimensional random anisotropy model. These excellent features on structural and magnetic properties allowed us to achieve that the stray magnetic field was uniformly generated from the Ni₇₈Fe₂₂ thin-film edge in the direction perpendicular to the cross section of the LSP-glass/Ni₇₈Fe₂₂/LSP-glass structures. As we calculated the stray magnetic field generated between the two edges of Ni₇₈Fe₂₂ thin-film electrodes in SQC devices, a high stray field of approximately 5 kOe was generated when the gap distance between two edges of the Ni₇₈Fe₂₂ thin-film electrodes was less than 5 nm and the thickness of Ni₇₈Fe₂₂ was greater than 20 nm. These experimental and calculated results suggest that Ni₇₈Fe₂₂ thin films sandwiched between LSP glasses can be used as electrodes in SQC devices, providing a spin-filter effect, and also our proposed techniques utilizing magnetic thin-film edges will open up new opportunities for the creation of high performance spin devices, such as large magnetoresistance devices and nanoscale spin injectors. Our paper is of strong interest to the broad audience of Applied Surface Science, as it demonstrates that the proposed and established technique, especially, the thermal pressing technique, is a significant advance in the surface and interface engineering and spin-device application. - Highlights: • Ni₇₈Fe₂₂ films between low-softening-point glasses were formed by thermal pressing. • The thermal pressing technique gives rise to a 100-fold enhancement in coercivity. • The stray magnetic field is uniformly generated from the Ni₇₈Fe₂₂ thin-film edge. • The calculation revealed the generation of a high stray field of 5 kOe in SQC device. - Abstract: We investigate the structural and magnetic properties of Ni₇₈Fe₂₂ thin films sandwiched between low-softening-point (LSP) glasses, which can be used in spin quantum cross (SQC) devices utilizing stray magnetic fields generated from magnetic thin-film edges. We also calculate the stray magnetic field generated between the two edges of Ni₇₈Fe₂₂ thin-film electrodes in SQC devices and discuss the applicability to spin-filter devices. Using the established fabrication technique, we successfully demonstrate the formation of LSP-glass/Ni₇₈Fe₂₂/LSP-glass structures with smooth and clear interfaces. The coercivity of the Ni₇₈Fe₂₂ thin films is enhanced from 0.9 to 103 Oe by increasing the applied pressure from 0 to 1.0 MPa in the thermal pressing process. According to the random anisotropy model, the enhancement of the coercivity is attributed to the increase in the crystal grain size. The stray magnetic field is also uniformly generated from the Ni₇₈Fe₂₂ thin-film edge in the direction perpendicular to the cross section of the LSP-glass/Ni₇₈Fe₂₂/LSP-glass structures. Theoretical calculation reveals that a high stray field of approximately 5 kOe is generated when the distance between two edges of the Ni₇₈Fe₂₂ thin-film electrodes is less than 5 nm and the thickness of Ni₇₈Fe₂₂ is greater than 20 nm. These experimental and calculation results indicate that Ni₇₈Fe₂₂ thin films sandwiched between LSP glasses are useful as electrodes for SQC devices, serving as spin-filter devices.