Improved exchange bias and blocking temperature of PtCr/PtMn bilayer antiferromagnets

Abstract

Herein, we investigated exchange coupling in PtCr/CoFe, PtCr/PtMn/CoFe, and PtMn/CoFe film structures, demonstrating that Pt51Cr49(30 nm)/Co90Fe10(10 nm), Pt51Cr49(27.2 nm)/Pt50Mn50/(2.8 nm)/Co90Fe10(10 nm), and Pt50Mn50(30 nm)/Co90Fe10(10 nm) structures annealed at 350 °C for 20 h in a 10-kOe field featured unidirectional anisotropy constants (Jk) of 0.09, 0.56, and 0.32 erg/cm2, respectively. In the case of the Ni50Fe12Cr38(4 nm)/Pt51Cr49(30 − X nm)/Pt50Mn50(X nm)/Co90Fe10(10 nm)/Ta(10 nm) [X = 0–30 nm] film system, Jk linearly and steeply increased with increasing X to reach a maximum of 0.56 erg/cm2 at X = 2.8 nm and then decreased in a complicated nonlinear fashion as X further increased to 30 nm. On the other hand, the blocking temperature of Pt51Cr49(28 nm)/Pt50Mn50/(2 nm)/Co90Fe10(10 nm) was determined as 500 °C and nearly equaled that of Pt51Cr49(30 nm)/Co90Fe10(10 nm), significantly exceeding the value of 400 °C determined for Pt50Mn50(30 nm)/Co90Fe10(10 nm). These results imply that the exchange bias field of the Ni50Fe12Cr38(4 nm)/Pt51Cr49(30 − X nm)/Pt50Mn50/(X nm)/Co90Fe10(10 nm)/Ta(10 nm) system is ultimately determined by the chemical composition and the modification of the antiferromagnetic spin structure at the ferromagnet–antiferromagnet interface, while the blocking temperature is almost entirely determined by the volume fraction of the PtCr component.