Giant tunnel magnetoresistance up to 417% at room temperature using Fe/MgO/Fe magnetic tunnel junctions

Abstract

Magnetic tunnel junctions (MTJs) are a key element for various spintronic devices such as magnetic random-access memories including novel ones, i.e., brain-morphic devices and magnetic logic circuits. For future applications, room temperature (RT) TMR ratios of 1000% are targeted [1]. However, fabricating MTJs with a TMR ratio much larger than 200% is difficult because the spin-polarized tunneling current strongly depends on the ferromagnet (FM)/oxide interfaces quality. MTJs suffer from imperfect interfaces due to dislocations, formation of unfavorable oxide layers at the electrode/barrier interfaces or oxygen vacancies [2,3]. Especially, the fabrication of high-quality FM electrodes on the oxide barrier is difficult due to their different chemical properties which is challenging for fabrication of sharp interfaces. Such imperfections disturb highly spin polarized electron tunneling and lead to reduced TMR ratios experimentally. In fact, the TMR ratio of well-studied Fe/MgO/Fe merely exceed 200% at RT even though theoretical calculations predicted >1000% TMR ratios [2]. Here, we report giant enhancement in RT-TMR ratios of Fe/MgO/Fe by tuning the growth condition of each layer and optimization of barrier interfaces: maximum 417% at RT and 914% at 5 K in a single-crystal, spin-valve-type Fe/MgO/Fe MTJ [4].Exchange-spin-valve MTJ structures were fabricated in an ultrahigh vacuum magnetron sputtering system (base pressure: 4×10-7 Pa) with the structure: MgO(001) substrate//Cr (60)/Fe (30 or 50)/CoFe (0 or 2.24)/Mg (0.5)/wedge-shaped MgO (dMgO = 0.5 – 2.5)/Fe (5)/IrMn (10)/Ru (20) (number in brackets in nm). The MgO barrier was deposited using electron-beam evaporation method and a linear shutter for the wedge-shaped barrier. In-situ post-annealing was performed for each layer. After deposition, the wafers were ex-situ annealed in a 0.7 T magnetic field at 200°C and subsequently, the TMR ratio and resistance area product (RA) was measured using current in-plane tunneling (CIPT) method. The TMR ratio is defined as TMR = (RAP – RP)/RP × 100%, where RP (RAP) are the resistance for the parallel (antiparallel) state. Thereafter, the wafers were pattered into micrometer scale MTJs using photo- and EB-lithography. The transport properties were investigated using a standard dc 4-probe method.In Fig. 1 (a) and (b), RA and TMR ratio vs. dMgO for the MTJ with a 30 nm and 50 nm bottom-Fe is shown, respectively, for the patterned MTJs (dc 4-probe). For the MTJ with 30 nm bottom-Fe the unpatterned wafer (CIPT) results are also shown. As expected, a linear relationship between log(RA) and dMgO in a wide dMgO range was observed, suggesting good MgO barrier formation. For dMgO < 1 nm, the MgO barrier may be discontinuous. For dMgO > 1 nm, the TMR ratio increases with dMgO and it finally reaches >400%. The inset of Fig. 1 (b) shows the MR loop of the Fe (50 nm)/MgO/Fe MTJ with a maximum 417% TMR ratio which almost doubles previously reported values of Fe/MgO/Fe. Improved Fe/MgO interfaces and well-controlled (001)-orientation of our Fe/MgO/Fe resulted in this significant enhancement of the TMR ratio. In addition, it shows significant oscillatory behavior with dMgO. The oscillation period is 0.31 nm, which agrees with the previous Fe/MgO/Fe report [2]. The oscillation is clearly observed in both the patterned and unpatterned cases, which excludes possible errors of our measurements. The peak-to-valley difference of the oscillation reaches ~80%, which is ~7 times larger than the previous report [2]. The RA curves also possess an oscillatory component as seen in Fig.1 (a). Furthermore, the TMR ratio was increased to 497% at RT by using a Fe/CoFe/MgO/Fe structure (red square in Fig. 1 and MR loop in inset of Fig. 1(b)). This value is larger than other ones of exchange-spin-valve MTJs such as Co/MgO/Co (410%) [5] and Co2(FeMn)Si/MgO/Co2(FeMn)Si (429%) [6].In Fig. 2, the temperature dependence of the TMR ratio for an Fe (30 nm)/MgO/Fe MTJ is shown. The TMR ratio is 386% at RT and increases to 914% at 3 K. The respective MR loops are displayed in the inset of Fig. 2. The low temperature (LT) value is around three times larger as compared to other reported values of Fe/MgO/Fe MTJs (250-370%) [2,7]. Our Fe/MgO/Fe MTJ exhibits a giant TMR ratio close to 1,000% at LT, which is often referred as a theoretical value of Fe/MgO/Fe.The study demonstrated that there is more room for improving TMR ratios of Fe/MgO/Fe MTJs by the optimization of each layer and interface. Further improvement in TMR ratios using the technology in the present study can be expected as demonstrated by the CoFe insertion result.This work was partly supported by the ImPACT Program of the Council for Science, Technology and innovation (Cabinet Office, Government of Japan), JSPS KAKENHI Grant No. 16H06332, TIA collaborative research program “Kakehashi”, and JPNP16007, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). **