Voltage-controlled magnetic tunnel junctions with synthetic ferromagnet free layer sandwiched by asymmetric double MgO barriers

Abstract

Memory and computing applications utilizing voltage-controlled magnetic random-access memory (MRAM) require perpendicular magnetic tunnel junctions (pMTJs) capable of high thermal stability and large write efficiency at advanced technology nodes. We first discuss the scaling requirements for voltage-controlled MRAM to replace various existing memory applications at an advanced CMOS technology node, including cell size, thermal stability, interfacial perpendicular magnetic anisotropy (PMA), and voltage-controlled magnetic anisotropy (VCMA). For replacing volatile memories including SRAM, eDRAM, and DRAM, we employ the retention relaxation technique along with a DRAM-style refresh scheme in the analysis. To enhance both PMA and VCMA at scaled nodes, we explore pMTJs with asymmetric double MgO barriers sandwiching a synthetic ferromagnet (SyF) free layer. In a thin MgO/SyF free layer/thick MgO/fixed layer MTJ stack, the SyF free layer is realized in various ways: single layer (X = Ta, W, Mo, Ir) and bilayers (X = Ta/W, Mo/Ir) of heavy metals insertions in CoFeB/X/CoFeB and Co2FeAl/X/Co2FeAl structures, and multiple insertions in [CoFeB/X]n/CoFeB structures for n = 1–4. A VCMA coefficient of 36 fJ (V m)−1 is achieved in a MgO/CoFeB/Ta/CoFeB/MgO-based MTJ which is comparable to that of the single MgO barrier MTJ. We find PMA enhancement, although with VCMA reduction for an increasing number n of [CoFeB/X]n repetitions. In addition, we demonstrate large TMR of 78%, large interfacial PMA of 1.45 mJ m−2, and VCMA of 65 fJ (V m)−1 in a MgO/Co2FeAl/W/Co2FeAl/MgO-based MTJ annealed above 400 °C.

Correlation of interfacial perpendicular magnetic anisotropy and interlayer exchange coupling in CoFe/W/CoFe structures

“…Noting that the MAE of MgO/CoFe/W/CoFe/MgO structure originates from the CoFe/W/CoFe structure and the CoFe/MgO interface, we add up the MAEs of these two parts to get the MAE of the MgO/CoFe/W/CoFe/MgO structure [10,11]. In figure 2 W structures agree well with experimental results [31,32]. Moreover, for the structure with W thickness of 7 MLs, a strong FMC as well as a strong PMA can be obtained simultaneously, which is promising for high-density STT-MRAM applications with strong thermal stability.…”

A 3 pJ/bit free space optical interlink platform for self-powered tetherless sensing and opto-spintronic RF-to-optical transduction

“…The self-powered nature of the optical tag makes it ideal for implantable devices, or for working with other sensors that can also be operated without significant external power. An example is the spin-torque microwave detector (STMD) based on magnetic tunnel junctions 17 , 18 , 20 , 30 , 34 – 36 , which can operate as an RF detector without the need for external power or external magnetic fields 18 , 19 . Hence, as a proof of concept demonstration of the potential of self-powered sensor tags, we combined the opto-tag with an MTJ-based STMD to produce an RF-to-optical transducer.…”

“…2 a. The bottom CoFeB layer was the free layer, featuring an interfacial perpendicular magnetic anisotropy (PMA) originating from the 3d-2p hybridization of Fe and O orbitals at the CoFeB/MgO interface 34 , 36 . The top CoFeB layer was the in-plane reference layer of the MTJ.…”

Perpendicular magnetic anisotropy and its voltage control in MgO/CoFeB/Mo/CoFeB/MgO junctions

“…Very recently, K u t as high as 0.4 mJ m −2 has been reported for a MgO/CF-B/Mo/CFB/MgO structure [22]. Cheng et al [22] have also demonstrated that the MgO/CFB/Mo/CFB/MgO structure sustains a large PMA even after the annealing at 500 • C. However, there are only a few studies on the VCMA effect in these MgO/CFB/X/CFB/MgO structures [23,24] because it is difficult to reduce the total thickness of CFB/X/CFB for quantitatively evaluating ξ. Therefore, in this study, we investigate in detail the PMA in the MgO/CFB/Mo/CFB/MgO structure as a function of CFB and Mo thicknesses in order to achieve a large PMA while reducing the CFB/X/CFB total thickness.…”