Spin valve devices are composed by two magnetic (FM) regions separated by a nonmagnetic spacer: one of the two FM films is magnetically "soft" (free layer), and the other behaves as a magnetic "hard" layer (pinned layer) thanks to the exchange coupling with an adjacent antiferromagnetic (AFM) layer. The orientation of the magnetization of the FM soft "free" layer may change with respect to that of the pinned layer that acts as a reference due to external applied fields, and the resistance of the whole system will vary due to its giant magnetoresistive (GMR) properties. In this contribution, we address the exchange bias (EB) phenomenon at the AFM/FM interface. We use Fe50Mn50 as AFM layer and Fe50Co50 as FM layer. The layered structure [Cu(t)/FeMn(t)/FeCo(tFM)/Cu(5nm)] was deposited at room temperature by dc-magnetron sputtering in Ar atmosphere on Si substrates, t changes from 10 to 30 nm and tFM is 3 nm. The growth conditions were chosen according both to what reported in literature [1] and to the results of a previous investigation performed on some preliminary samples. During the deposition a magnetic field is applied. We analyze the structural parameters that affect the interface exchange coupling, in particular, the thickness t of the Cu underlayer and of the AFM layer. As t changes, we measure field cooling magnetization loops at different temperatures and analyze the dependence of the exchange field (Hex) and of the coercive field (Hc) on temperature (T). For all the samples, Hex decreases with T and disappears at 300 K. More in detail, as t changes, both the value of Hex and its dependence on T are modified. We investigate also the role of the FeMn layer in the EB phenomenon. In particular, we find that a sample with t = 10 nm and tFM = 0 nm shows, at 10 K, Hex and Hc higher than those measured for t = 10 nm and tFM = 3 nm; for T > 75 K its behaviour becomes superparamagnetic-like. The data support the hypothesis that the FeMn layer consists of two different magnetic phases (one antiferromagnetic and one disordered ferro or ferri-magnetic). At low T, the exchange coupling between these two FeMn phases provides a contribution to the EB phenomenon. However, in the sample where also the FM layer is present, this contribution is reduced due to the competing action of the exchange anisotropy originating at the FM/AFM interface. [1] H. J. Kim, J. S. Bae, T. D. Lee and H. M. Lee, J. Magn. Magn. Mater. 241, 173 (2002)

Influence of the antiferromagnet magnetic structure on the exchange bias in the Fe50Mn50/Fe50Co50 system

TAMISARI, Melissa;SPIZZO, Federico;DEL BIANCO, Lucia;RONCONI, Franco
2011

Abstract

Spin valve devices are composed by two magnetic (FM) regions separated by a nonmagnetic spacer: one of the two FM films is magnetically "soft" (free layer), and the other behaves as a magnetic "hard" layer (pinned layer) thanks to the exchange coupling with an adjacent antiferromagnetic (AFM) layer. The orientation of the magnetization of the FM soft "free" layer may change with respect to that of the pinned layer that acts as a reference due to external applied fields, and the resistance of the whole system will vary due to its giant magnetoresistive (GMR) properties. In this contribution, we address the exchange bias (EB) phenomenon at the AFM/FM interface. We use Fe50Mn50 as AFM layer and Fe50Co50 as FM layer. The layered structure [Cu(t)/FeMn(t)/FeCo(tFM)/Cu(5nm)] was deposited at room temperature by dc-magnetron sputtering in Ar atmosphere on Si substrates, t changes from 10 to 30 nm and tFM is 3 nm. The growth conditions were chosen according both to what reported in literature [1] and to the results of a previous investigation performed on some preliminary samples. During the deposition a magnetic field is applied. We analyze the structural parameters that affect the interface exchange coupling, in particular, the thickness t of the Cu underlayer and of the AFM layer. As t changes, we measure field cooling magnetization loops at different temperatures and analyze the dependence of the exchange field (Hex) and of the coercive field (Hc) on temperature (T). For all the samples, Hex decreases with T and disappears at 300 K. More in detail, as t changes, both the value of Hex and its dependence on T are modified. We investigate also the role of the FeMn layer in the EB phenomenon. In particular, we find that a sample with t = 10 nm and tFM = 0 nm shows, at 10 K, Hex and Hc higher than those measured for t = 10 nm and tFM = 3 nm; for T > 75 K its behaviour becomes superparamagnetic-like. The data support the hypothesis that the FeMn layer consists of two different magnetic phases (one antiferromagnetic and one disordered ferro or ferri-magnetic). At low T, the exchange coupling between these two FeMn phases provides a contribution to the EB phenomenon. However, in the sample where also the FM layer is present, this contribution is reduced due to the competing action of the exchange anisotropy originating at the FM/AFM interface. [1] H. J. Kim, J. S. Bae, T. D. Lee and H. M. Lee, J. Magn. Magn. Mater. 241, 173 (2002)
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11392/1433111
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