Spin-torque diode radio-frequency detector with voltage tuned resonance

Abstract

We report on a voltage tunable radio-frequency (RF) detector based on a magnetic tunnel junction (MTJ). The spin-torque diode effect is used to excite and/or detect RF oscillations in the magnetic free layer of the MTJ. In order to reduce the overall in-plane magnetic anisotropy of the free layer, we take advantage of the perpendicular magnetic anisotropy at the interface between ferromagnetic and insulating layers. The applied bias voltage is shown to have a significant influence on the magnetic anisotropy, and thus on the resonance frequency of the device. This influence also depends on the voltage polarity. The obtained results are accounted for in terms of the interplay of spin-transfertorque and voltage-controlled magnetic anisotropy effects.Due to spin wave excitations, magnetic thin films can emit and/or absorb electromagnetic signals in a microwave frequency range determined by ferromagnetic resonance (FMR). In turn, radio-frequency (RF) signals can be produced in magnetic tunnel junctions (MTJs) via the spin-transfer-torque (STT) effect [1,2], where a spin polarized current excites magnetization precession with a frequency that depends, among others, on the magnetic anisotropy. This precession gives rise to an electric signal that can be used in various devices, like spin wave generators [3] or spin-torque oscillators [4,5]. The inverse effect, i.e., absorption of RF spin currents by a MTJ produces a detectable DC signal [6]. This phenomenon, called the spin-torque diode effect, can be used as an RF-detector of very high sensitivity [7]. Recently, it has been shown that similar behavior can be achieved with alternatingvoltage-controlled magnetic anisotropy (VCMA) [8,9]. In this paper we propose a voltage tunable spin-torque diode, that is capable of sensing RF signals of different frequencies. We achieved such a functionality by using a combination of the STT and VCMA effects. The former effect produces the DC voltage in response to AC current, while the latter one changes the resonance detection regime. In addition, using a specially designed MTJ, we are able to measure STT components in a volt- * Electronic address: skowron@agh.edu.pl age range of ±1 V, which to our knowledge is beyond the STT measurements published to date (up to ±0.5 V in Ref. [10]) and reveals a non-linear behavior of the in-plane STT component vs. bias voltage [11,12]. To achieve the above objective, we have investigated MTJs with the following multilayer structure: SiO 2 (substrate) / 5 Ta / 30 CuN / 3 Ta / 30 CuN / 3 Ta / 16 Pt 38 Mn 62 / 2.1 Co 70 Fe 30 / 0.9 Ru / 2.3 Co 40 Fe 40 B 20 / 1.6 MgO / 1-2 Co 40 Fe 40 B 20 / 10 Ta / 7 Ru (thickness in nm). By varying thickness of the CoFeB free layer (FL), we observed a transition from in-plane to perpendicular anisotropy at a critical thickness of t c ≈ 1.35 nm [13]. After deposition, the MTJs were annealed at 250 • C in an in-plane magnetic field of 0.4 T in order to set the exchange bias direction and to improve the crystallization of the ferromagnetic electrodes. For the transpor...

Underlayer material influence on electric-field controlled perpendicular magnetic anisotropy in CoFeB/MgO magnetic tunnel junctions

“…This behavior is attributed to the nonlinear effects induced by the shift in the precession center from the equilibrium point due to the dc voltage-induced torque under the FMR excitation [30,31]. We note that in the case of MTJs with thinner MgO tunnel barriers, the spin-transfer torque effect or dc voltage-dependent reflection coefficient can explain the change in the FMR signal symmetry [32,33]. Moreover, bottom layer FMR frequency depends on the bias voltage.…”

Microwave detection based on magnetoresistance effect in spintronic devices

“…Depending on the relative orientation between magnetization vectors of the top and bottom layer, spin polarized carriers exert torque on the top ferromagnet, which leads to the resistance change. Because the TMR effect measured in the fabricated device reached 100%, a higher sensitivity is also expected [10]. In case of TMR device, the maximum voltage applied to the tunneling barrier cannot exceed 1.5 V, because of a possibility of the electrical breakdown [11].…”

Magnetic field, current and voltage-induced ferromagnetic resonance in magnetic tunnel junctions: Micromagnetic spatial analysis

“…In order to perform an advanced analysis of complex magnetization dynamics induced by phenomena such as Spin-Transfer-Torque (STT) [1,2] or voltage-controlled anisotropy [3,4] changes, a spatial distribution of power spectral density is often needed [5] . We use a specially designed open-source tool for creating spectral density maps directly from a set of subsequent magnetization files [6], an output typically available in micromagnetic simulations [7] .…”