Low-temperature magnetic softening in sputtered FePtAg granular multilayers

Marco Coisson 3Franca Albertini 2Paolo Allia 1Francesca Casoli 2Federica Celegato 1Simone Fabbrici 2Paola Tiberto 3Franco Vinai 3

1. INFM and Dipartimento di Fisica, Politecnico di Torino, Corso Duca degli Abruzzi, 29, Torino 10129, Italy
2. IMEM- CNR (IMEM), Parma, Italy
3. Istituto Elettrotecnico Nazionale "Galileo Ferraris" (IEN), Strada delle Cacce, 91, Torino 10135, Italy

Abstract

Multilayered granular FePtAg films have been prepared by rf sputtering on a MgO (100) substrate covered by a 20 nm Ag undelayer. The basic unit of each multilayer is a FePt block (nominal thickness: 3.55 nm, atomic composition Fe53Pt47), topped with a Ag layer (nominal thickness: 5 nm). The basic unit is repeated 5 times. The substrate temperature was kept at 550 C during the whole deposition process. The as-deposited multilayers were annealed in situ at 550 C for 85 minutes in order to develop a tetragonal FePt phase with perpendicular anisotropy.

Magnetisation loop measurements perpendicular and parallel to the film plane have been performed in the 5-300 K temperature range by alternating gradient magnetometry under a field up to 18 kOe. Magnetoresistance (MR) measurements have been performed in the 2-300 K temperature interval by a standard four-contact method under a field up to 70 kOe. The magnetic field and the electrical current were applied parallel to each other and to the film plane. Magnetic measurements show that a substantial fraction of the magnetisation is spontaneously directed out of plane at all temperatures. At room temperature, saturation of MR is not achieved up to 70 kOe, while below 50 K a clear saturating trend is observed around 50 kOe. Above 10 K, the MR displays the usual negative behaviour with increasing field, and can be interpreted in terms of field-induced alignment of the magnetic regions governing spin-dependent electron scattering. A sharp downward jump of the zero-field electrical resistance has been observed on reducing T below 10 K. Such a jump is associated with the onset of a pronounced dip which opens at low fields in the R(H) curves, and is ascribed to anisotropic magnetoresistance. These results are explained in terms of partial magnetic softening of the alloy, related to competition between perpendicular and shape anisotropy, resulting in a local magnetisation which can be easily oriented by the applied field.

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Presentation: oral at E-MRS Fall Meeting 2005, Symposium D, by Marco Coisson
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-05-20 10:12
Revised:   2009-06-07 00:44
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