Spin-Torque Diode Measurements of MgO-Based Magnetic Tunnel Junctions with Asymmetric Electrodes

Abstract

We present a detailed study of the spin-torque diode effect in CoFeB/MgO/CoFe/NiFe magnetic tunnel junctions. From the evolution of the resonance frequency with magnetic field at different angles, we clearly identify the free-layer mode and find an excellent agreement with simulations by taking into account several terms for magnetic anisotropy. Moreover, we demonstrate the large contribution of the out-of-plane torque in our junctions with asymmetric electrodes compared to the in-plane torque. Consequently, we provide a way to enhance the sensitivity of these devices for the detection of microwave frequency.Spin-transfer torque (STT) in MgO-based magnetic tunnel junctions (MTJs) [1,2] is under development for device applications such as STT random access memories (STT-RAM) [3,4], domain-wall-motion MRAM, [5] racetrack memory [6] and spintronic memristors. [7,8] Recently, spin-torque diodes [9] have attracted much attention because their sensitivity for the detection of microwave frequency may exceed that of semiconductor diodes. [10,11] In the spin diode effect, an applied rf current to the MTJ exerts an oscillating spin torque on the magnetization of the free layer, leading to excitation of the ferromagnetic resonance (FMR) mode. The dynamics of the free layer cause oscillations of the tunnel magnetoresistance (TMR). As a result, the oscillating resistance partially rectifies the rf current and dc voltage is obtained (V diode ). The spin diode effect depends on the relative amplitudes (a J and b J ) of the classical inplane torque (T IP ) and the out-of-plane field-like torque (T OOP ). [12,13] Here, the torques are expressed as T IP = -γ(a J /M s )m×(m×M ref ) and T OOP = -γb J m×M ref with m (M s ) being the magnetization vector (the saturation magnetization) of the free layer, M ref the magnetization vector of the reference layer, and γ the gyromagnetic ratio. In spin diode spectra (V diode as a function of frequency (f )), the contribution of T IP (resp. T OOP ) results in a peak with a Lorentzian component (resp. an anti-Lorentzian component);where A and B are the amplitudes of the anti-Lorentzian component and Lorentzian component, f 1 is the resonance frequency, ∆ is the peak linewidth, H d is the outof-plane demagnetization field, and θ is the relative angle between the free layer and the reference layer. Experimentally evaluated T OOP was reported to reach over 25% of T IP in conventional CoFeB/MgO/CoFeB MTJs with symmetric electrodes. [12,13] However, in these MTJs, the dc bias dependence of T OOP is quadratic and symmetric with respect to the polarity of bias, leading to A = 0 at zero dc bias voltage. For MTJs with asymmetric electrodes, on the other hand, the bias dependence of T OOP is expected to be asymmetric and linear at low bias, [15,16] leading to larger A at zero dc bias voltage. In this study, we perform spin diode measurements of MgO-based MTJs with asymmetric electrodes. We also measure its dependence on magnitude and angle of the in-plane external magnetic field (H ext ) to identify the...

Spin-torque building blocks

“…The current dependence of T OOP is generally more complex than T IP . While T OOP is practically zero in metallic spin-valves, it can reach 40 % of T IP in magnetic tunnel junctions [14].…”

Influence of MgO tunnel barrier thickness on spin-transfer ferromagnetic resonance and torque in magnetic tunnel junctions

“…Especially, it has been shown that the shape of τ ⊥ (V ) curves can change from quadratic to linear. 25,26 However, such effects were only measured in asymmetric MTJs with different FL and RL electrodes. In our junctions, the composition and thickness of the CoFeB electrodes are the same.…”

Tuning the frequencies of the normal modes of a nanopillar oscillator through the magnetostatic interaction

“…(1) is the normalised spin transfer torque term. We neglected the out plane spin transfer torque due to the non-magnetic spacer is a metal 30 .…”