Abstract

The exchange coupling between ferromagnetic (FM)-antiferromagnetic (AF) interfaces is a key element of modern spintronic devices. We here introduce a new way of triggering exchange bias (EB) in swift heavy ion (SHI) irradiated FeCo-SiO2 films, which is a manifestation of spin-flipping at high irradiation fluence. The elongation of FeCo nanoparticles (NPs) in SiO2 matrix gives rise to perpendicular magnetic anisotropy at intermediate fluence. However, a clear shift in hysteresis loop is evident at the highest fluence. This reveals the existence of an AF exchange pinning domain in the NPs, which is identified not to be oxide shell from XANES analysis. Thermal spike calculations along with first-principles based simulations under the framework of density functional theory (DFT) demonstrate that spin flipping of 3d valence electrons is responsible for formation of these AF domains inside the FM NPs. EXAFS experiments at Fe and Co K-edges further unravel that spin-flipping in highest fluence irradiated film results in reduced bond lengths. The results highlight the possibility of miniaturization of magnetic storage devices by using irradiated NPs instead of conventionally used FM-AF multilayers.

Highlights

  • The ever-increasing demand for non-volatile memories and reading heads has led to the rapid development of anti-ferromagnetic (AF)-ferromagnetic FM) junctions, which are crucial elements of present day’s micro or nano electronics devices[1,2]

  • Despite exchange bias (EB) effect is observed in swift heavy ion (SHI) irradiated multilayers, for SHI irradiated NPs only magnetic anisotropy (MA) effects are reported but no specific EB effects are addressed till date

  • We can say that this enhanced MA along the SHI beam direction in 5e13 film is a direct consequence of NP elongation due to the shape anisotropy introduced in the system

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Summary

Experimental Section

Fe and Co foils, glued on a SiO2 target, were co-sputtered in a high vacuum sputtering chamber by 1.5 keV Ar fast atom beam (FAB) for depositing FeCo-SiO2 nanocomposite films on Si substrate. The substrate holder was rotated continuously for uniform deposition of the films. The relative area of the metal pieces w.r.t. the quartz plate exposed to the atom beam determines the amount of metal fraction (here 20%) in the film. Cross-sectional TEM (XTEM) samples were prepared following conventional procedure and images were recorded with FEI TITAN 80–300 microscope operating at accelerating voltage of 300 kV. Both in-plane and out-of-plane magnetic measurements were carried out in a Quantum Design MPMS SQUID magnetometer. X-ray absorption data at near and far edges (for both Fe and Co K-edges) were collected at XAFS beamline, Elettra, Italy

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