[Paper Review] Impact of in-plane currents on magnetoresistance properties of an exchange-biased spin-valve with insulating antiferromagnetic layer

D. N. H. Nam, Nguyen Chi Thuan+5 more|ArXiv.org|Jan 9, 2008

Magnetic properties of thin films6 citations

TL;DR

This study investigates the impact of in-plane alternating currents on magnetoresistance in an exchange-biased spin-valve with an insulating Ni₀.₈₅Co₀.₁₅O antiferromagnetic layer. Despite the insulating antiferromagnet preventing spin-polarized current injection, Joule heating from in-plane currents suppresses pinned layer reversals and reduces magnetoresistance, while spin-transfer torques between ferromagnetic layers may still contribute at high currents.

ABSTRACT

The impact of in-plane alternating currents on the exchange bias, resistance, and magnetoresistance of a CoFe/NiCoO/CoFe/Cu/CoFe spin-valve is studied. With increasing current, the resistance is increased while the maximum magnetoresistance ratio decreases. Noticeably, the reversal of the pinned layer is systematically suppressed in both field sweeping directions. Since the NiCoO oxide is a good insulator, it is expected that the ac current flows only in the CoFe/Cu/CoFe top layers, thus ruling out any presence of spin-transfer torque acting on the spins in the antiferromagnetic layer. Instead, our measurements show clear evidences for the influence of Joule heating caused by the current. Moreover, results from temperature-dependent measurements very much resemble those of the current dependence, indicating that the effect of Joule heating plays a major role in the current-in-plane spin-valve configurations. The results also suggest that spin-transfer torques between ferromagnetic layers might still exist and compete with the exchange bias at sufficiently high currents.

Motivation & Objective

• To investigate the influence of in-plane alternating currents on exchange bias, resistance, and magnetoresistance in a spin-valve with an insulating antiferromagnetic layer.
• To determine whether spin-transfer torque effects can occur in current-in-plane (CIP) configurations despite the absence of direct current flow through the antiferromagnet.
• To distinguish between thermal (Joule heating) and non-thermal (spin-transfer torque) contributions to the observed magnetotransport changes.
• To compare current-induced effects with temperature-dependent behavior to isolate the role of heating.
• To assess the persistence of spin-transfer torques between ferromagnetic layers in CIP geometry under high current conditions.

Proposed method

• Fabricated a Co₈₅Fe₁₅/Ni₀.₈₅Co₀.₁₅O/Co₈₅Fe₁₅/Cu/Co₈₅Fe₁₅ spin-valve structure using magnetron sputtering with a base vacuum of 8×10⁻⁶ mbar.
• Used a high-resistance Ni₀.₈₅Co₀.₁₅O layer (estimated >100 MΩ) to prevent current flow into the antiferromagnetic layer, isolating Joule heating as the primary thermal mechanism.
• Performed current-in-plane (CIP) magnetotransport measurements with varying current amplitudes and durations to study resistance (R(H)) and magnetoresistance (MR(H)) behavior.
• Conducted temperature-dependent measurements from 300 K to 400 K to compare thermal effects with current-induced effects.
• Analyzed time-resolved resistance data under applied current and cooling cycles to verify thermal relaxation dynamics.
• Extracted exchange bias field (Hₑb) from R(H) curves and compared its dependence on current, temperature, and current duration.

Experimental results

Research questions

• RQ1 To what extent does Joule heating from in-plane currents affect the exchange bias and magnetoresistance in a spin-valve with an insulating antiferromagnetic layer?
• RQ2 Can spin-transfer torques between ferromagnetic layers occur in current-in-plane (CIP) configurations despite the absence of current flow through the antiferromagnet?
• RQ3 How do the current-induced changes in resistance and Hₑb compare quantitatively to purely thermal effects from temperature variation?
• RQ4 Why is the suppression of exchange bias under current greater than that expected from temperature alone, even when resistance values are matched?
• RQ5 What is the relative contribution of thermal effects versus spin-transfer torque to the suppression of pinned layer reversals?

Key findings

• Increasing in-plane current amplitude leads to a monotonic increase in resistance and a decrease in maximum magnetoresistance ratio.
• The reversal of the pinned layer is systematically suppressed with increasing current, even at low current levels where exchange bias is not yet significantly altered.
• Temperature-dependent measurements show qualitatively identical trends to current-dependent measurements, indicating that Joule heating is the dominant mechanism.
• At matched resistance values, the exchange bias field is more suppressed under current (Hₑb ≈ 110 Oe at 70 mA, 300 K) than under pure thermal effects (Hₑb ≈ 165 Oe at 0.05 mA, 320 K), indicating an additional non-thermal effect.
• The resistance values at 70 mA and 300 K are lower than at 0.05 mA and 340 K, suggesting that current has a stronger effect on reducing resistance than temperature alone, pointing to spin-transfer torque contributions in CIP geometry.