The optimization of the fabrication techniques for obtaining multi-layered magnetic systems modulated at the nanoscale is nowadays a great challenge due to the large number of technological applications (e.g. Bit Patterned Magneto-recording media, MRAMs, sensors…) that exploit arrays of exchange coupled dots [1, 2]. Focused ion beam (FIB) is a powerful technique for the fabrication of prototypical devices with high resolution [2]. Low ion dose FIB processing was exploited to induced controlled modification of material surfaces, such as in the case of CVD diamond surface which become conductive in a thickness of the order of few to ~ 20nm [3]. However, in the FIB fabrication on very thin films, i.e. ultra thin magnetic layers, as effect of tails in the ion beam distribution a low (but non negligible) ion dose may be received by the areas surrounding the pattern which may undesirably result damaged. This effects may be noticeable in particular when the fabricated structures have a lateral size in the sub-micron range. In order to assess the feasibility of FIB fabrication on stacked thin magnetic films and study the effects of a non intentional ion dose received by the sample, we investigated the morphological, magnetic, and chemical properties of FIB processed areas, by means of atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), and magneto-optic Kerr effect (MOKE) magnetometry. The investigated sample was a Si/Ta/Pt/[Pt/Co]4/Pt/FeCo/Pt stack showing out-of-plane magnetization; the sample was deposited on Si substrates by sputtering technique at an Argon pressure of 3.5 μbar [4]. Ga+ based dual-beam FIB was used at 30kV acceleration voltage and 95pA current to expose the surface in the 4÷350 pC/μm2 ion doses the range. As effect to exposure to ions the surface experience both roughening and material removal, which is more apparent for high doses. The EDX analysis shows that the intensity from the elements of the stack is reduced as the ion dose is increased while the signal from Ga increases due to inclusion of atoms from the ion beam. The intensity of the Si peak also increases as effect of thinning of the magnetic film. The surface morphology of the sample evolve from a flat behavior with few point defects to a rougher one at increasing ion dose. The normalized out-of plane MOKE magnetization loops recorded on the continuous film after FIB processing at 4 pC/μm2 and 40 pC/μm2 indicate that even the smallest dose produces a strong reduction of the magnetic signal, that is mirrored by the large increase of the signal to noise ratio. With increasing FIB dose, due to the combination of roughness increase and film thickness removal, the magnetic signal gradually decreases and finally disappears for a 40 pC/μm2 dose. References [1] Spizzo et al. Phys. Rev. B, 91 (2015) 064410 [2] Laureti et al. Phys. Rev. Lett. 108 (2012) 077205 [3] A. A. Tseng, Small. 1(10), (2005) 924-39.

Effects of focused ion beam treatments on stacked magnetic films

SPIZZO, Federico;DEL BIANCO, Lucia;
2016

Abstract

The optimization of the fabrication techniques for obtaining multi-layered magnetic systems modulated at the nanoscale is nowadays a great challenge due to the large number of technological applications (e.g. Bit Patterned Magneto-recording media, MRAMs, sensors…) that exploit arrays of exchange coupled dots [1, 2]. Focused ion beam (FIB) is a powerful technique for the fabrication of prototypical devices with high resolution [2]. Low ion dose FIB processing was exploited to induced controlled modification of material surfaces, such as in the case of CVD diamond surface which become conductive in a thickness of the order of few to ~ 20nm [3]. However, in the FIB fabrication on very thin films, i.e. ultra thin magnetic layers, as effect of tails in the ion beam distribution a low (but non negligible) ion dose may be received by the areas surrounding the pattern which may undesirably result damaged. This effects may be noticeable in particular when the fabricated structures have a lateral size in the sub-micron range. In order to assess the feasibility of FIB fabrication on stacked thin magnetic films and study the effects of a non intentional ion dose received by the sample, we investigated the morphological, magnetic, and chemical properties of FIB processed areas, by means of atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), and magneto-optic Kerr effect (MOKE) magnetometry. The investigated sample was a Si/Ta/Pt/[Pt/Co]4/Pt/FeCo/Pt stack showing out-of-plane magnetization; the sample was deposited on Si substrates by sputtering technique at an Argon pressure of 3.5 μbar [4]. Ga+ based dual-beam FIB was used at 30kV acceleration voltage and 95pA current to expose the surface in the 4÷350 pC/μm2 ion doses the range. As effect to exposure to ions the surface experience both roughening and material removal, which is more apparent for high doses. The EDX analysis shows that the intensity from the elements of the stack is reduced as the ion dose is increased while the signal from Ga increases due to inclusion of atoms from the ion beam. The intensity of the Si peak also increases as effect of thinning of the magnetic film. The surface morphology of the sample evolve from a flat behavior with few point defects to a rougher one at increasing ion dose. The normalized out-of plane MOKE magnetization loops recorded on the continuous film after FIB processing at 4 pC/μm2 and 40 pC/μm2 indicate that even the smallest dose produces a strong reduction of the magnetic signal, that is mirrored by the large increase of the signal to noise ratio. With increasing FIB dose, due to the combination of roughness increase and film thickness removal, the magnetic signal gradually decreases and finally disappears for a 40 pC/μm2 dose. References [1] Spizzo et al. Phys. Rev. B, 91 (2015) 064410 [2] Laureti et al. Phys. Rev. Lett. 108 (2012) 077205 [3] A. A. Tseng, Small. 1(10), (2005) 924-39.
focused ion beam (FIB), magnetic nanostructures, exchange coupling, perpendicular magnetic anisotropy, AFM, EDX, MOKE
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/2368960
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