Thin Fe50Co50 (FeCo) layers have recently attracted great attention due to their high saturation magnetization, spin polarization factor [1] and possible application in magnetic devices, showing perpendicular magnetization, as well. Thanks to the high FeCo magnetostrictive coefficient, growth induced stress (s) may represent an additional parameter useful to tailor the thin films magnetic properties, in particular the magnetization reversal process. Indeed, a compressive or tensile stress, namely a negative or positive s, may induce an additional in-plane or out-of-plane magnetic anisotropy, respectively, whose strength is proportional to s [2]. We present FeCo layers grown by dc-magnetron sputtering in Ar atmosphere on Si substrates and having a thickness, t, ranging from 5 nm up to 100 nm. The layers were covered with a 5 nm Cr layer to protect them from oxidation. Room temperature magnetization (M) measurements were performed using a SQUID magnetometer with a magnetic field (H) applied in the plane of the film. The dependence of the three reduced M, m=M/Msat, components (in plane and parallel to H; in plane and perpendicular to H (mt); out of plane) as a function of H was accessed at room temperature, for different in-plane directions, d ̂, of the applied field, using a MOKE apparatus for vector magnetometry [3]. Samples resistance was measured with the four probes method at room temperature whilst s was measured using an optical profilometer with 1µm lateral and 1 nm vertical resolution. For t ≤ 20 nm, the shape of the in-plane M loops is squared and the coercivity increases with t for t up to 15 nm. This increase is possibly ascribable to the change in grain size; however, the coercivity values are larger than those expected for the bcc FeCo alloy. For t = 5 nm mt is about 0.1, and it decreases with increasing t, indicating that in-plane anisotropy changes slightly with d ̂. For t > 20 nm, coercivity smoothly decreases and the shape of the loops changes, as the approach to saturation is slower and the shape of the whole loop gets less and less squared. The large coercivity values suggest the presence of an in-plane tensile stress, and profilometry measurements support this hypothesis. s is found to be always positive, and its value monotonically decreases with t. This result supports the fact that for large t values coercivity decreases. The s decrease for large t values, besides being due to thickness increase, could be also due to the partial development of a compressive stress due to structural defects [4]. Indeed, resistivity measurements show that samples resistivity increases with t (the resistivity of the sample with t = 50 nm is nearly four times larger than that of the sample with t = 5 nm), thus supporting the conclusion that the presence of structural defects may increases with t. The partial development of a compressive stress could induce an out-of-plane magnetic anisotropy, thus explaining the slower approach to saturation observed for t > 20 nm. These results will be discussed and compared to MOKE vector magnetometry data. [1] L. Platt et al, J. Appl. Phys. 88 (2000) 2058. [2] P. Zou et al., J. Appl. Phys. 91 (2002) 7830. [3] P. Vavassori, App. Phys. Lett. 77 (2000) 1605. [4] T. Pienkos et al., Microel. Eng. 70 (2003) 442.

Magnetic effects of growth induced stress in FeCo thin films

SPIZZO, Federico;BONFIGLIOLI, Edgar;GUIDI, Vincenzo;NERI, Ilaria;TAMISARI, Melissa;VAVASSORI, Paolo
2013

Abstract

Thin Fe50Co50 (FeCo) layers have recently attracted great attention due to their high saturation magnetization, spin polarization factor [1] and possible application in magnetic devices, showing perpendicular magnetization, as well. Thanks to the high FeCo magnetostrictive coefficient, growth induced stress (s) may represent an additional parameter useful to tailor the thin films magnetic properties, in particular the magnetization reversal process. Indeed, a compressive or tensile stress, namely a negative or positive s, may induce an additional in-plane or out-of-plane magnetic anisotropy, respectively, whose strength is proportional to s [2]. We present FeCo layers grown by dc-magnetron sputtering in Ar atmosphere on Si substrates and having a thickness, t, ranging from 5 nm up to 100 nm. The layers were covered with a 5 nm Cr layer to protect them from oxidation. Room temperature magnetization (M) measurements were performed using a SQUID magnetometer with a magnetic field (H) applied in the plane of the film. The dependence of the three reduced M, m=M/Msat, components (in plane and parallel to H; in plane and perpendicular to H (mt); out of plane) as a function of H was accessed at room temperature, for different in-plane directions, d ̂, of the applied field, using a MOKE apparatus for vector magnetometry [3]. Samples resistance was measured with the four probes method at room temperature whilst s was measured using an optical profilometer with 1µm lateral and 1 nm vertical resolution. For t ≤ 20 nm, the shape of the in-plane M loops is squared and the coercivity increases with t for t up to 15 nm. This increase is possibly ascribable to the change in grain size; however, the coercivity values are larger than those expected for the bcc FeCo alloy. For t = 5 nm mt is about 0.1, and it decreases with increasing t, indicating that in-plane anisotropy changes slightly with d ̂. For t > 20 nm, coercivity smoothly decreases and the shape of the loops changes, as the approach to saturation is slower and the shape of the whole loop gets less and less squared. The large coercivity values suggest the presence of an in-plane tensile stress, and profilometry measurements support this hypothesis. s is found to be always positive, and its value monotonically decreases with t. This result supports the fact that for large t values coercivity decreases. The s decrease for large t values, besides being due to thickness increase, could be also due to the partial development of a compressive stress due to structural defects [4]. Indeed, resistivity measurements show that samples resistivity increases with t (the resistivity of the sample with t = 50 nm is nearly four times larger than that of the sample with t = 5 nm), thus supporting the conclusion that the presence of structural defects may increases with t. The partial development of a compressive stress could induce an out-of-plane magnetic anisotropy, thus explaining the slower approach to saturation observed for t > 20 nm. These results will be discussed and compared to MOKE vector magnetometry data. [1] L. Platt et al, J. Appl. Phys. 88 (2000) 2058. [2] P. Zou et al., J. Appl. Phys. 91 (2002) 7830. [3] P. Vavassori, App. Phys. Lett. 77 (2000) 1605. [4] T. Pienkos et al., Microel. Eng. 70 (2003) 442.
2013
magnetostriction; Magnetic properties; Magneto optic Kerr effect; Optical profilometry
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/1893401
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