Terahertz-frequency AC signal source based on antiferromagnetic tunnel junction

Abstract

Spintronic nano-oscillators [1], [2]are now considered as promising nanoscale AC signal sources for the future energy-efficient electronics. However, nowadays such sources mainly utilize ferromagnetic materials (FM), and, therefore, their operation frequencies are limited to the interval of 1–50 GHz [1], [2], which is not sufficiently high for many practical applications. To substantially increase the frequency of the spintronic AC signal sources one could use antiferromagnets (AFMs), where characteristic operation frequencies, typically, lie in the terahertz range (0.1–10 THz) [3]–[5]. The theoretically proposed AFM-based nano-oscillators can operate in the absence of a bias dc magnetic field, and can be driven by a dc electric current [6]–[8]. However, the problem of a practical development of AFM-based AC signal generators, and, in particular, the problem of a power extraction from such AFM generators, has not been solved yet. In [7]an AFM-based spin Hall oscillator (SHO), where the power of the generated AC signal is extracted through the inverse spin-Hall effect (ISHE), has been proposed. The other power extraction mechanism, based on the reception of the magneto-dipolar radiation from a current-driven canted AFM attached to the high-Q resonator has been analyzed in [8]. Both these methods of the THz-frequency signal extraction have some disadvantages: for an SHO utilizing ISHE the AFM material must be bi-anisotopic with a weak perpendicular anisotropy [7], while in the case of a canted AFM embedded in a resonator, the device has a large size $( \sim 10 \mu \mathrm {m})$defined by the wavelength of the generated signal [8]. In this paper we consider an alternative mechanism of the THz-frequency signal extraction from an AFM-based SHO, where the signal power is collected through the AC variation of the tunneling anisotropic magnetoresistance (TAMR) in an AFM tunnel junction (ATJ), the electrical switching of which has been experimentally observed in the recent works [9], [10]. We consider an AFM SHO based on an IrMn/Pt bilayer structure, where the driving dc current $I_{drive}$flowing in the Pt layer forces a flow of a spin current $I_{SH}$into the AFM layer. This spin current excites the rotation of magnetizations of the IrMn AFM sublattices through the spin-Hall effect (SHE) [5], which gives rise to the AC variations of the TAMR [10]. Thus the junction resistance changes in time as $R(t) \quad = R_{0} + \Delta {R} \sin(\omega{t})$(Fig. 1). At the same time, the simultaneously supplied bias dc current $I_{dc}$, traversing the junction cross-section, results in the generation of the AC voltage of the magnitude $U_{ac} \quad = I_{dc} \Delta {R}$across the whole structure. Thus, when such a dc biased ATJ is connected through a bias tee to a load, having the resistance of $R_{L} \quad = 50 \Omega $, the AC voltage generated in the ATJ excites an AC current in the load, allows one to extract an AC power $P_{L}$of the excited signal from the load. Our theoretical model of the proposed AC source based on an ATJ is very simple. We consider an ATJ as a circuit consisting of an AC voltage source (generating AC voltage of the magnitude $U_{ac})$with the internal resistance $R_{0}$shunted by a capacitor of the capacitance $C$. This equivalent circuit is connected through an ideal bias tee to a resistive load of the resistance $R_{L}$. Using Kirchhoff's laws, we found the following expressions for the powers: $P_{dc} \quad = \quad I_{dc}^{2}R_{0}$is the dc power injected in the ATJ, $P_{L} \quad = ( R_{L} U_{ac}^{2})/ [ 2 ( R_{L} \quad + R_{0})^{2} \quad + \quad R_{L}^{2}\beta ^{2})]$is the power emitted in the load, where $\beta \quad = \omega R_{0}C$. Then, the efficiency of the AC power extraction can be estimated as $\zeta \quad = \quad P_{L}/ P_{dc}$. The frequency dependences of the $P_{L}$and $\zeta $, calculated for a junction having experimental parameters (resistance-area product, tunnel magneto-resistance ratio, etc.) taken from [10], are shown in Fig. 2. They demonstrate that both these characteristics are decreasing with the increase of the generated frequency. The calculations also demonstrated, that the output power $P_{L}$, injected into the load in the frequency range 0.1 – 1 THz, is comparable to, or may even exceed, the power extracted via ISHE and magneto-dipole emission mechanism, while the efficiency of the power extraction mechanism via AC TAMR variations is about 1%, which may be sufficient for some practical applications. Also, it should be noted, that a substantial advantage of the proposed AFM-based AC signal source with signal extraction through TAMR lies in the simplicity and reliability of its experimental realization at micro- and nano-scale. In conclusion, we proposed a method of the AC signal extraction from an ATJ-based SHO based on TAMR, which can be easily experimentally realized at micro- and nano-scale, and can provide an output AC power of the order of $P_{L} \sim 10 \mu \mathrm {W}$and an efficiency of about 1%.