Theoretical study of rectifying properties in a terahertz regime with a zero-bias voltage for fully epitaxial Fe/ZnO/MgO/Fe magnetic tunnel junctions

Abstract

We conducted a theoretical study of high-frequency rectifying properties with a zero-bias voltage in epitaxial Fe(001)/ZnO(001)/MgO(001)/Fe(001) magnetic tunnel junctions (MTJs) in conjunction with the expectation of the element being applied as a square-low detector. By optimizing device parameters such as insulator thickness and the junction area, we obtained current responsivity of up to 0.12 A W−1 at 1 THz (with a cut-off frequency of 1.5 THz), which is comparable to the best results obtained in experiments for semiconductor-based diodes, performed under similar conditions. The results indicate that this MTJ system has great potential for use as a high-frequency rectifying element in a terahertz regime.

Estimation of Rectifying Performance for Terahertz Wave in Newly Designed Fe/ZnO/MgO/Fe Magnetic Tunnel Junction

“…Therefore, β0 does not necessarily represent the rectifying performance of the diode although it is in general referred to as the performance index. The obtained RDA (6.3 m 2 ) and β0 (0.20 A/W) were in good agreements with the theoretically estimated values in the identical MTJ (RDA = 8.9 m 2 and β0 = 0.23 A/W 19) ), indicating that the MTJ prepared in this study justifies the design concept, and also fabricated precisely in line with the concept for the THz rectification.…”

“…The MTJ film consisted of Au (5 nm) cap / Co (10 nm which is so called pseudo-spin valve structure. The tZnO and tMgO were determined, in accordance with our theoretical calculation 19) , so as to realize the maximum βeff in this MTJ system. The ZnO upper tunnel barrier was confirmed to have identical reflection high-energy electron diffraction patterns ( Fig.…”

“…The MTJ film consisted of Au (5 nm) cap / Co (10 nm) pinned layer / Fe (5 nm) top electrode / ZnO (0.6 nm) upper tunnel barrier / MgO (0.5 nm) lower tunnel barrier / Fe (30 nm) bottom electrode / MgO (5 nm) buffer layer on a MgO(001) substrate, which is so called pseudo-spin valve structure. The tZnO and tMgO were determined, in accordance with our theoretical calculation 19) , so as to realize the maximum βeff in this MTJ system. The ZnO upper tunnel barrier was confirmed to have identical reflection high-energy electron diffraction patterns (Fig.…”

“…Very recently, we found a strong diodic behavior in the current-voltage (I-V) characteristics in epitaxial MI(I)M device of Fe(001)/ZnO(001)/MgO(001)/Fe(001) magnetic tunnel junction (MTJs) at room temperature (RT) 16) , in which tunneling magnetoresistance (TMR) is caused by so called spin-polazied coherent tunneling 17,18) . We theoretically estimated the effective current responsivity (βeff) of the MTJ system, which correnponds to the conversion capability from a high-frequency to a DC current, in regards to the ZnO and MgO barrier thicknesses (tZnO and tMgO, respectively) as well as the junction area (A) 19) . The calculation predicted that βeff can reach up to 0.12 A/W at 1 THz with the conditions, tZnO ~ tMgO with low RDA (a few m 2 ) and small A (~ 0.01 m 2 ).…”

“…The obtained βeff is comparable to that of the best result obtained in semiconductor-based diodes in experiments for InP/(In,Ga)As system, performed under similar conditions (~ 0.2 A/W at 1 THz 20,21) ). However, most of the physical parameters of the tunnel barriers used in our previous calclation 19) , such as electron affinity and relative dielectric constant, were extracted from the experimetal I-V data of the MTJ having high RDA (an order of km 2 ) with large A (36 m 2 ) 16) . These values are actually three orders higher than desirable values for effective rectification of the THz wave.…”