Thermal activation effects on the exchange bias in ferromagnetic-antiferromagnetic nanostructures

V. Baltz, J. Sort, B. Rodmacq, B. Dieny, and S. Landis

Phys. Rev. B 72, 104419 – Published 9 September 2005

Abstract

The hysteresis loop shift $H_{E}$ of sub- $100 − nm$ ferromagnetic- (FM-) antiferromagnetic (AFM) nanostructures is found to be strongly influenced by thermal activation effects. These effects, which tend to reduce $H_{E}$ , are more pronounced in the nanostructures than in continuous films with the same composition, particularly for thin AFM layers. In addition, the reduced dimensions of the nanostructures also impose spatial constraints to the AFM domain size, particularly for thick AFM layers. This favors an enhancement of $H_{E}$ . Due to the interplay between these two competing effects, the loop shift in the dots can be either larger or smaller than in the continuous films with the same composition, depending on both the AFM thickness and temperature. A temperature-AFM thickness phase diagram, separating the conditions resulting in larger or smaller $H_{E}$ in the nanostructures with respect to continuous film is derived.

Received 20 April 2005

DOI:https://doi.org/10.1103/PhysRevB.72.104419

Authors & Affiliations

V. Baltz^*, J. Sort, B. Rodmacq, and B. Dieny

SPINTEC (URA 2512), CEA/CNRS, 17 Avenue des Martyrs, 38054 Grenoble Cedex 9, France

S. Landis

LETI/D2NT, CEA, 17 Avenue des Martyrs, 38054 Grenoble Cedex 9, France

^*Electronic mail: baltz@drfmc.ceng.cea.fr

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Vol. 72, Iss. 10 — 1 September 2005

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Images

Figure 1
Scanning electron microscopy (SEM) image of the array of $90 \times 90 {nm}^{2}$ square dots fabricated by first patterning a $Si ∕ Si O_{2}$ wafer and subsequently depositing the FM-AFM bilayer structure.Reuse & Permissions
Figure 2
Hysteresis loops of (a) the continuous films and (b) the nanostructures, with compositions $Ta (5 nm) ∕ Py (12 nm) ∕ Ir Mn (5 nm) ∕ Pt (2 nm)$ , for different temperatures [ $T_{heat 2} = 298 K$ (-엯-), $328 K$ (-x-), and $343 K$ (-▴-)] from which the continuous film and the nanostructures, previously field cooled in $H_{FC} = 2.4 kOe$ from above $T_{B}$ , have been field cooled again in $H_{FC} = - 2.4 kOe$ . All measurements have been performed at room temperature by longitudinal Kerr effect along the field cooling direction.Reuse & Permissions
Figure 3
(a) Dependence of the exchange bias field $H_{E}$ on the temperature $T_{heat 2}$ from which the continuous film (-엯-) and the nanostructures (-∎-), with compositions $Ta (5 nm) ∕ Py (12 nm) ∕ Ir Mn (t_{Ir Mn}) ∕ Pt (2 nm)$ , with $t_{Ir Mn} = 5$ , 9, and $16 nm$ , initially field cooled in $H_{FC} = 2.4 kOe$ from above $T_{B}$ , have been field cooled in $H_{FC} = - 2.4 kOe$ . (b) Derivative $d H_{E} ∕ d T_{heat 2}$ , of the curves plotted in (a), which are typically representative of the blocking temperature distributions. The lines are guides to the eye.Reuse & Permissions
Figure 4
Dependence of the exchange bias field $H_{E}$ on the IrMn thickness $t_{Ir Mn}$ for both continuous films (-엯-) and nanostructures (-∎-) with compositions $Ta (5 nm) ∕ Py (12 nm) ∕ Ir Mn (t_{Ir Mn}) ∕ Pt (2 nm)$ , (a) after a first field cooling $(H_{FC} = 2.4 kOe)$ from $550 K$ and (b)–(d) after a second field cooling $(H_{FC} = - 2.4 kOe)$ from different heating temperatures ( $T_{heat 2} = 313$ , 333, and $353 K$ ). The lines are guides to the eye. The arrows show the intersection of the evolutions in the nanostructures and in the continuous film. Note that Fig. 4a has been taken from Fig. 2 of Ref. 23.Reuse & Permissions
Figure 5
Dependence of the exchange bias field $H_{E}$ on the temperature $T$ for both the continuous films (-엯-) and the nanostructures (-∎-), with compositions $Ta (5 nm) ∕ Py (12 nm) ∕ Ir Mn (9 nm) ∕ Pt (2 nm)$ . The lines are guides to the eye. The inset shows the corresponding hysteresis loops measured at $10 K$ by SQUID along the field cooling direction, after cooling from $T = 550 K$ in the presence of a $2.4 kOe$ field.Reuse & Permissions
Figure 6
Phase diagram showing the temperature $T$ and IrMn thickness $t_{Ir Mn}$ conditions for which the exchange bias field of sub- $100 nm$ nanostructures $(H_{E, dots})$ is either larger or smaller than that of continuous films $(H_{E, cont})$ with the same composition, at the same temperature. Note that the (-x-) data is deduced from room temperature Kerr effect measurements after the cooling in negative field from $T_{heat 2}$ , whereas the (-▴-) data is deduced from low-temperature SQUID measurements. The line is a guide to the eye.Reuse & Permissions

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