Current-induced manipulation of exchange bias in IrMn/NiFe bilayer structures

Abstract

The electrical control of antiferromagnetic moments is a key technological goal of antiferromagnet-based spintronics, which promises favourable device characteristics such as ultrafast operation and high-density integration as compared to conventional ferromagnet-based devices. To date, the manipulation of antiferromagnetic moments by electric current has been demonstrated in epitaxial antiferromagnets with broken inversion symmetry or antiferromagnets interfaced with a heavy metal, in which spin-orbit torque (SOT) drives the antiferromagnetic domain wall. Here, we report current-induced manipulation of the exchange bias in IrMn/NiFe bilayers without a heavy metal. We show that the direction of the exchange bias is gradually modulated up to ±22 degrees by an in-plane current, which is independent of the NiFe thickness. This suggests that spin currents arising in the IrMn layer exert SOTs on uncompensated antiferromagnetic moments at the interface which then rotate the antiferromagnetic moments. Furthermore, the memristive features are preserved in sub-micron devices, facilitating nanoscale multi-level antiferromagnetic spintronic devices.

Fig. 1. Current-induced manipulation of the exchange bias in an IrMn/NiFe structure.

a Left: Exchange-biased IrMn/NiFe structure, where the blue and red arrows represent the magnetization (m) of NiFe and IrMn, respectively, and B_EB is the exchange bias field. Middle: The Hall bar device used for electrical measurements. The Hall resistance (R_H) is measured after applying an in-plane current pulse J_P. Right: Schematics of the current-induced exchange bias switching, where the magnetization direction of the IrMn/NiFe bilayer (φ_m) is modulated by the spin current with spin polarization (σ). b Hysteresis loop of IrMn (5 nm)/NiFe (4 nm) structure measured with magnetic fields along the x-axis (solid blue) and y-axis (open red). c R_H versus azimuthal angle of a magnetic field (φ_B) of 100 mT. d The R_H versus J_P curves, where the arrows denote the sweeping direction of J_P. e, R_H as a function of a magnetic field along x-axis (B_x). R_H is initially set to ±0.14 by an J_P of $\mp$8.3 × 10¹¹ A/m², represented by the red and blue symbols. Open squares (lines) refer to increasing (decreasing) B_x. The inset illustrates the magnetization changes by B_x.

Fig. 2. Current-induced thermal effects in the IrMn (5 nm)/NiFe (4 nm) structure.

a Longitudinal resistance R_xx as a function of current density J_P ranging from 1.25 × 10¹¹ to 8.4 × 10¹¹ A/m². b R_xx measured as a function of temperature. c Estimated sample temperature T_sample as a function of current density J_P. The quadratic line represents a fitting curve of T = 300 + 98.6J_P². d Normalized magnetization loops at various temperatures between 100 and 385 K. Inset shows magnified magnetization loops measured at 360–385 K. e Exchange bias field B_EB versus temperature. f Planar Hall resistance R_H measured with external magnetic fields of 200 and 1 mT at 354 K.

Fig. 3. SOT-induced exchange bias switching for various IrMn/NiFe structures.

a, b Direction of the exchange bias (φ_EB) versus J_P curves for the NiFe (4 nm)/IrMn (5 nm) sample (a) and the IrMn (5 nm)/NiFe (4 nm)/Ta (1.5 nm) sample (b). The arrows denote the sweeping direction of J_P. The φ_EB values are extracted from the R_H values.

Fig. 4. Thickness dependence of SOT-induced exchange bias switching.

a–c. Hysteresis loop measured using a magnetic field along the x-axis, B_x (a), the φ_EB versus current density (J_P) curves, where the arrows denote the sweeping direction of J_P (b), Δφ_EB [ = φ_EB (−I) − φ_EB (+I)] and B_EB as a function of IrMn thickness t_IrMn (c) of the IrMn (t_IrMn)/NiFe (4 nm) samples with different t_IrMn’s ranging from 5 to 25 nm. d–f. Hysteresis loop measured using B_x (d), the φ_EB versus J_P curves, where the arrows denote the sweeping direction of J_P (e), the Δφ_EB and B_EB versus t_NiFe (f) of the IrMn (5 nm)/NiFe (t_NiFe) samples with different t_NiFe’s ranging from 3 to 20 nm.

Fig. 5. Memristive behavior of exchange bias switching.

a Minor φ_EB versus J_P curves for the IrMn (5 nm)/NiFe (4 nm)/Ta (1.5 nm) sample with a 4-μm-wide Hall bar. The arrows denote the sweeping direction of J_P, b Schematics of the measurement sequence; J_P,ini is the initializing current pulse of −7.4 × 10¹¹ A/m², and J_P(+max) is the positive maximum J_P. Minor loops are consecutively measured while sweeping J_P between −7.4 × 10¹¹ A/m² and J_P(+max), which increases from 5.2 × 10¹¹ to 7.4 × 10¹¹ A/m² mA. c Minor φ_EB versus J_P curves for the IrMn (5 nm)/NiFe (5 nm) sample with a 500-nm-wide Hall bar. The arrows denote the sweeping direction of J_P.