Stability diagram of perpendicular magnetic tunnel junction with a composite free layer

• Main Authors: Skowronski, W., Frankowski, M., Zietek, S., Checinski, J., Rzeszut, P., Wrona, J., Czapkiewicz, M.
• Format: Conference Proceeding
• Language: English
• Subjects: Magnetic field measurement; Magnetic tunneling; Perpendicular magnetic anisotropy; Stability analysis; Temperature measurement; Thermal stability; Tunneling magnetoresistance
• ISSN: 2150-4601
• DOI: 10.1109/INTMAG.2017.8007568

Magnetic tunnel junctions (MTJs) have become a basic building block for various types of spintronics devices, such as magnetic random access memory (MRAM) cells, magnetic field sensors and microwave generators or detectors. There are a few ways to realize perpendicularly magnetized MTJ, which are characterized by greater thermal stability than their in-plane magnetized counterparts. Taking advantage of the interface anisotropy yields the best results, especially in terms of high tunnel magnetoresistance (TMR) measured typically in MTJs with CoFeB/MgO/CoFeB trilayer. Recent studies on perpendicular MTJ resulted in the TMR ratio exceeding 200 % [1] thanks to careful optimization of both the free layer (FL) and reference layer (RL) structure. We report on the perpendicular MTJ with a composite CoFeB/W/CoFeB FL [2,3] which is characterized by high perpendicular magnetic anisotropy and spin polarization resulting in up to 180 % TMR measured at room temperature and above 280 % TMR at low temperature. The multilayers with the following structure were deposited: buffer / SyF / spacer / CoFeB(1) / MgO(1.2) / CoFeB(t FL ) / W(0.3) / CoFeB(0.5) / MgO(1) / capping (thicknesses in nm), with t FL ranging from 1 up to 1.6 nm and RA product equal to 10 Ωμm 2 . For the 1.5 nm thick CoFeB sublayer, the TMR of 180 % at room temperature and 280 % at 20 K was measured. The stability diagram was modeled based on Ref. [4]. Additionally, it was assumed that in-plane all (out-of-plane al) torque component scales linearly (quadratically) with the applied current [5]. To account for additional physical effects that contribute to the stability diagram, namely VCMA and temperature effect, the coercive field HC is scaled by the factor: HC = HK(1-V K V-(T/ kT)1/2) where V is the applied voltage, HK is the anisotropy field, V K is the VCMA coefficient and Ta is the ambient temperature. kT represents the dependence of the temperature on the coercive field. The damping factor was measured independently by the broadband FMR technique. An increase of the TMR from 140% for t FL = 1.1 nm up to TMR = 180% for t FL = 1.5 nm is explained by an increase of the spin polarization for thicker ferromagnetic layer. A rapid reduction of the TMR for t FL = 1.6 nm is caused by the transition of the FL magnetization vector to the sample plane. Electronic transport properties were measured in perpendicular magnetic field (see Fig. 1). It has been already established that apart from the conventional STT effe