The dependence of electronic conductivity on the electron spin state was observed in nanogranular systems where magnetic (M) nanoparticles are dispersed into a non-magnetic (NM) matrix. The nanogranular systems display a spin-dependent electronic resistivity that remarkably decreases if an external magnetic field (H) is applied, i.e. they show the so called giant magnetoresistance (GMR) [1]. GMR is ascribable to the magnetic ordering effect induced by H, so the higher the degree of disorder at zero field the larger the GMR effect. When the magnetoresistive properties of an ensemble of independent nanogranular particles [2] are considered, GMR is proportional to (m/msat)^2, where m is the sample magnetization. The proportionality constant γ can be interpreted as the GMR efficiency, i.e. as the overall GMR variation in correspondence with a unit change of reduced square magnetization, so it can be seen as an indication of how effective is the granular structure in producing GMR. We investigate how γ and the GMR intensity change with the Fe concentration. We focused on FexAg1-x nanogranular films, where x is the Fe atomic relative concentration and ranges from 0 up to 0.50, as measured by Rutherford Backscattering Spectrometry. The samples were deposited on Si substrates using dc-magnetron sputtering in cosputtering configuration and Ar atmosphere. The Fe-Ag phase diagram indicates that the two elements are not mutually soluble for any relative concentration but thanks to that out-of-equilibrium deposition technique it is possible to produce a system that at room temperature behaves like a magnetic nanogranular one [3]. X-ray diffraction measurements have been performed to investigate the structural properties of the samples. The granular films exhibit three different kind of structures: for x < 0.20, there is a Fe-Ag not-saturated solid solution; for 0.20 < x < 0.32 there are very small Fe clusters and Fe-Ag not-saturated solid solution; finally, for x > 0.32 there are bcc Fe cluster and Fe-Ag saturated solid solution. On the other hand, for all the concentrations, magnetization data show the presence of Fe precipitates whose size increases with x and the Mössbauer investigation confirms this picture. The GMR intensity is maximum for x = 0.32, while the maximum of γ is observed for x = 0.26. The maximum GMR effect is the best arrangement between a structure that displays the γ maximum, a not-saturated solid solution with very small Fe clusters and a structure with a high concentration of magnetic material. In particular, when the maximum GMR effect is observed, the distance between the magnetic clusters is of the order of the electron spin diffusion length. [1] A. E. Berkowitz et al., Phys. Rev. Lett. 68 (1992) 3745. [2] S. Zhang and P. M. Levy, J. Appl. Phys. 73 (1993) 5315 [3] J.Q. Wang, G. Xiao, Phys. Rev. B 49 (1994) 3982.

Interplay between GMR intensity and efficiency in the FeAg nanogranular system

TAMISARI, Melissa;SPIZZO, Federico;SACERDOTI, Michele;RONCONI, Franco
2009

Abstract

The dependence of electronic conductivity on the electron spin state was observed in nanogranular systems where magnetic (M) nanoparticles are dispersed into a non-magnetic (NM) matrix. The nanogranular systems display a spin-dependent electronic resistivity that remarkably decreases if an external magnetic field (H) is applied, i.e. they show the so called giant magnetoresistance (GMR) [1]. GMR is ascribable to the magnetic ordering effect induced by H, so the higher the degree of disorder at zero field the larger the GMR effect. When the magnetoresistive properties of an ensemble of independent nanogranular particles [2] are considered, GMR is proportional to (m/msat)^2, where m is the sample magnetization. The proportionality constant γ can be interpreted as the GMR efficiency, i.e. as the overall GMR variation in correspondence with a unit change of reduced square magnetization, so it can be seen as an indication of how effective is the granular structure in producing GMR. We investigate how γ and the GMR intensity change with the Fe concentration. We focused on FexAg1-x nanogranular films, where x is the Fe atomic relative concentration and ranges from 0 up to 0.50, as measured by Rutherford Backscattering Spectrometry. The samples were deposited on Si substrates using dc-magnetron sputtering in cosputtering configuration and Ar atmosphere. The Fe-Ag phase diagram indicates that the two elements are not mutually soluble for any relative concentration but thanks to that out-of-equilibrium deposition technique it is possible to produce a system that at room temperature behaves like a magnetic nanogranular one [3]. X-ray diffraction measurements have been performed to investigate the structural properties of the samples. The granular films exhibit three different kind of structures: for x < 0.20, there is a Fe-Ag not-saturated solid solution; for 0.20 < x < 0.32 there are very small Fe clusters and Fe-Ag not-saturated solid solution; finally, for x > 0.32 there are bcc Fe cluster and Fe-Ag saturated solid solution. On the other hand, for all the concentrations, magnetization data show the presence of Fe precipitates whose size increases with x and the Mössbauer investigation confirms this picture. The GMR intensity is maximum for x = 0.32, while the maximum of γ is observed for x = 0.26. The maximum GMR effect is the best arrangement between a structure that displays the γ maximum, a not-saturated solid solution with very small Fe clusters and a structure with a high concentration of magnetic material. In particular, when the maximum GMR effect is observed, the distance between the magnetic clusters is of the order of the electron spin diffusion length. [1] A. E. Berkowitz et al., Phys. Rev. Lett. 68 (1992) 3745. [2] S. Zhang and P. M. Levy, J. Appl. Phys. 73 (1993) 5315 [3] J.Q. Wang, G. Xiao, Phys. Rev. B 49 (1994) 3982.
2009
giant magnetoresistance; nanogranular magnetic materials; SQUID magnetometry
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/1390401
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