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Tatsuya Yamamoto, Tomohiro Ichinose, [Jun Uzuhashi](https://orcid.org/0000-0003-2023-8158), Takayuki Nozaki, [Tadakatsu Ohkubo](https://orcid.org/0000-0003-3548-1951), Kay Yakushiji, Shingo Tamaru, Shinji Yuasa

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[Large Tunneling Magnetoresistance in Perpendicularly Magnetized Magnetic Tunnel Junctions Using Co75Mn25/Mo/Co20Fe60B20 Multilayers](https://mdr.nims.go.jp/datasets/48e7632e-a375-4784-a217-0cde45ea359a)

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APS/123-QEDLarge tunneling magnetoresistance in perpendicularly magnetizedmagnetic tunnel junctions using Co3Mn/Mo/CoFeB multilayerTatsuya Yamamoto,1, ∗ Tomohiro Ichinose,1 Jun Uzuhashi,2 Takayuki Nozaki,1Tadakatsu Ohkubo,2 Kay Yakushiji,1 Shingo Tamaru,1 and Shinji Yuasa11National Institute of Advanced Industrial Science and Technology (AIST),Research Center for Emerging Computing Technologies, Tsukuba, Ibaraki 305-8568, Japan2National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0047, Japan(Dated: January 12, 2023)AbstractWe study the magnetic and electrical transport properties of magnetic tunnel junctions (MTJs)consisting of a Co3Mn/Mo/CoFeB multilayer prepared using a mass-production-compatible mag-netron sputtering system. The Co3Mn/Mo/CoFeB multilayer sandwiched between two MgO layersexhibits remarkable perpendicular magnetic anisotropy, and a uniaxial magnetic anisotropy con-stant as large as 0.2 MJ/m3 is achieved by optimizing the Co3Mn layer thickness as well as theannealing temperature. The current-in-plane tunneling measurement reveals a large tunnelingmagnetoresistance of over 100% in perpendicularly magnetized MTJs. These experimental resultsindicate the applicability of Co3Mn alloy for magnetic random access memory devices.∗ yamamoto-t@aist.go.jp1I. INTRODUCTIONMagnetic tunnel junctions (MTJs) using perpendicularly magnetized ferromagnetic layershave been intensively studied to develop high-density and energy-efficient magnetic randomaccess memory (MRAM) devices [1–4]. In terms of MRAM application, a large tunnelingmagnetoresistance (TMR) is also required to improve the signal-to-noise ratio for a high-speed data readout. Among various MTJ materials, CoFe-alloys/MgO multilayers are themost promising material combination that enables a large TMR of over 100% [5–7] as well asa large perpendicular magnetic anisotropy (PMA) [8–11]. These remarkable TMR and PMAin CoFe-alloy/MgO MTJs originate from the coherent spin-polarized tunneling through the(001)-oriented MgO barrier [12, 13] and large interfacial PMA due to the hybridization ofFe(Co) 3d and O 2p orbitals [14, 15], respectively. The use of an amorphous CoFeB filmenables the fabrication of MTJs with a highly (001)-oriented MgO barrier on a polycrys-talline electrode using mass-production-compatible magnetron sputtering processes [9–11].The CoFeB-based perpendicularly magnetized MTJs (p-MTJs) are currently used for state-of-art MRAM memory cells. Continued development of MTJ materials remains necessaryfor next-generation MRAMs including those using spin-orbit torque (SOT) [16–18] and/orthe voltage-controlled magnetic anisotropy (VCMA) effect [19–21] to further improve theenergy efficiency.The use of bcc FeCoMn ternary alloys is a new approach for developing high-TMR MTJs[22, 23]. The bcc FeCoMn is a metastable phase that can be obtained from epitaxial growthon single crystalline substrates such as GaAs(001) [24] and MgO(001) [25–27]. An intriguingproperty of the bcc FeCoMn is the large magnetic moment that even exceeds the SlaterPauling limit depending on the alloy composition [27]. This unusual feature comes fromthe large magnetic moment of Mn atoms, which ferromagnetically aligns to that of Fe andCo atoms. First-principles calculation revealed that the band structure of bcc Co3Mn alloynear the Fermi level is similar to those of bcc Fe and Co, i.e., the ∆1 state only exists for themajority spin band at the Fermi level [23]. Kunimatsu et al. experimentally demonstrateda large TMR ratio of over 200% in an epitaxial Co3Mn/MgO/Co3Mn MTJ prepared on asingle crystalline MgO(001) substrate [23]. Therefore, the next step toward practical use is toachieve large PMA in sputter-deposited polycrystalline films. In this article, we evaluate themagnetic and electrical properties of MTJs consisting of an ultrathin (< 1 nm) Co3Mn film2FIG. 1. Schematic illustration of MTJ and representative magnetization curve obtained fromMTJ consisting of a Co3Mn (0.8 nm)/Mo (0.3 nm)/CoFeB (0.6 nm) multilayer annealed at 350◦C.Magnetization curve was measured under application of out-of-plane magnetic field.deposited using a mass-production-compatible magnetron sputtering system. The Co3Mnultrathin film deposited on an MgO barrier layer exhibits PMA, and p-MTJ films consistingof a Co3Mn/Mo/CoFeB multilayer is deposited on 300-mm Si wafers with a polycrystallineelectrode. TMR ratios of over 100% are also demonstrated by optimizing the Co3Mn layerthickness and post annealing temperature.II. EXPERIMENTThe MTJ films were prepared on 300-mm Si wafers with a 100-nm-thick thermally ox-idized Si layer using an ultrahigh-vacuum magnetron sputtering system manufactured byTokyo Electron Ltd (TEL-EXIM). After the deposition, the wafers were cut into 20 × 20mm2 square chips then annealed at Ta = 200− 380◦C for 1 h in a vacuum. Figure 1 showsa schematic of the MTJ stack along with a representative out-of-plane magnetization curvemeasured using a vibrating sample magnetometer (VSM). A strong antiferromagnetic cou-pling in the Co/Pt-based synthetic antiferromagnetic reference layer was obtained using anIr spacer layer [28]. The Ta/Cu bottom electrode was used to evaluate the TMR as well asthe resistance-area product of MTJ films by using the current-in-plane tunneling magnetore-sistance (CIPT) technique [29]. The free layer consists of an MgO (2 nm)/Co3Mn (t nm)/Mo3FIG. 2. Out-of-plane minor magnetization curves representing magnetization process of free layer:(a) Co3Mn (0.8 nm)/Mo (0.3 nm)/CoFeB (0.6 nm) and Co3Mn (0.8 nm)/Mo (0.3 nm)/Co3Mn(0.6 nm) free layers annealed at Ta = 350◦C, and (b) Co3Mn (0.8 nm)/Mo (0.3 nm)/CoFeB (0.6nm) free layer annealed at various Ta.(0.3 nm)/CoFeB (0.6 nm)/MgO (0.8 nm)/capping layer structure if not otherwise specified.The Mo spacer layer was chosen because Mo exhibits small intermixing with CoFe, and largePMA of over 0.3 MJ/m3 has been reported for MgO/CoFeB/Mo/CoFeB/MgO multilayers[30].A Co3Mn alloy target was used for depositing the Co3Mn thin films, and the sput-tering rate of Co3Mn, Mo and CoFeB were calibrated using X-ray fluorescence (XRF).The compositional ratio in the deposited Co3Mn films was also determined by XRFto be Co 75.5 at.% versus Mn 24.5 at.%. To quantitatively evaluate the PMA in theMgO/Co3Mn/Mo/CoFeB/MgO multilayer, we carried out vector network analyzer ferro-magnetic resonance (VNA-FMR) measurement [31]. Scanning transmission electron mi-croscopy (STEM), nano-beam electron diffraction (NBD), and energy dispersive X-rayspectrometry (EDS) were also carried out using FEI Titan G2 80-200 to investigate thenanostructural properties and atomic distribution of the MTJ. Thin foil specimens for thenanostructural analyses were prepared using a focused ion beam with a scanning electronmicroscopy system (FEI Helios G4UX) through a standard lift-out method.4III. RESULTS AND DISCUSSIONFigure 2(a) displays a minor magnetization curves obtained from the MTJ films con-sisting of Co3Mn (0.8 nm)/Mo (0.3 nm)/CoFeB (0.6 nm) and Co3Mn (0.8 nm)/Mo (0.3nm)/Co3Mn (0.6 nm) free layers. The MTJ films were annealed at Ta = 350◦C. The satu-ration magnetization of the Co3Mn/Mo/Co3Mn free layer is considerably smaller than thatof Co3Mn/Mo/CoFeB free layer even if the difference in the saturation magnetization be-tween bcc CoFe (∼ 2.2 T [32]) and bcc Co3Mn (∼ 2.0 T [22, 26]) is taken into account.Therefore, it is expected that the fcc Co3Mn phase with a much smaller magnetic momentis formed in the Co3Mn/Mo/Co3Mn free layer. It should be noted that the saturation fieldof the Co3Mn/Mo/Co3Mn free layer (∼ 0.42 T) is smaller than the demagnetizing field(∼ 0.68 T), which reveals finite positive PMA at the MgO/Co3Mn interface. As shown inFig. 2(a), the use of CoFeB instead of the upper Co3Mn substantially enhances PMA andsaturation magnetization, enabling a perpendicularly magnetized Co3Mn layer. Therefore,it is considered that the formation of the fcc Co3Mn phase is effectively prevented in theCo3Mn/Mo/CoFeB multilayer.Figure 2(b) displays minor magnetization curves obtained from the Co3Mn (0.8 nm)/Mo(0.3 nm)/CoFeB (0.6 nm) free layer annealed at different Ta. Note that all these MTJfilms including the as-prepared one underwent a thermal treatment at 80◦C for about 3 minduring the wafer-cut process. The as-prepared Co3Mn/Mo/CoFeB free layer has an in-planeeasy axis and exhibits a small saturation magnetization, indicating that the Co3Mn layer isdominated by the paramagnetic phase(s). Interestingly, annealing at Ta ≥ 200◦C leads toa remarkable enhancement in the saturation magnetization. This suggests that the Co3Mnlayer crystalizes into the ferromagnetic bcc phase upon annealing. The Co3Mn/Mo/CoFeBfree layer also exhibits perpendicular magnetization for 250◦C ≤ Ta ≤ 350◦C. Annealing ata higher Ta degrades both the PMA and saturation magnetization.Figure 3(a) displays a bright field (BF-)STEM image of the MTJ films consisting of theCo3Mn (0.8 nm)/Mo (0.3 nm)/CoFeB (0.6 nm) multilayer after annealing at Ta = 350◦C.The BF-STEM image as well as the NBD patterns [Fig. 3(b)] reveal the formation of (001)-oriented MgO layer and the partial crystallization of the Co3Mn/Mo/CoFeB multilayer afterannealing at Ta = 350◦C. Although the crystalline structure of the Co3Mn/Mo/CoFeB mul-tilayer cannot be distinguished from the BF-STEM image and the NBD patterns shown in5FIG. 3. Nanostructural analysis results on Co3Mn (0.8 nm)/Mo (0.3 nm)/CoFeB (0.6 nm) freelayer annealed at Ta = 350◦C: (a) BF-STEM image (b) NBD patterns, and (c) EDS elemental lineprofile.FIG. 4. Post annealing effect on Co3Mn/Mo/CoFeB free layer: (a) Co3Mn thickness dependenceof µ0Mst, and (b) Ta dependence of µ0Ms and td. ttotal = t+ 0.6.Fig. 3, the large saturation magnetization revealed from the VSM measurements suggeststhe bcc crystallization of Co3Mn. Figure 3(c) displays the EDS elemental line profile ob-tained from the rectangle region drawn in Fig. 3(a) along the direction of the arrow. Dueto the considerable film roughness, the atomic ratio at each position may contain a certainamount of uncertainty. However, it is noteworthy that we achieved perpendicular magneti-zation using a Co-rich ferromagnet, which is beneficial for improving the VCMA efficiency[33–35]. Note also that there is no significant change in the line profile of Mn atoms associ-ated with the annealing, i.e., most Mn atoms remain on top of the MgO barrier layer. Theinterfacial Mn atoms can exhibit a large PMA as well as VCMA. [36].6FIG. 5. VNA-FMR measurement results: (a) FMR frequency as function of applied out-of-planefield. (b) Ku values estimated from Hk and Ms values.To estimate the saturation magnetization of Co3Mn in our MTJ films, we evaluated theµ0Msttotal values by varying the Co3Mn thickness t in the MTJs, and the results are shownin Fig. 4(a). The total free layer thickness, ttotal, is defined as the sum of the Co3Mnand CoFeB thicknesses, i.e., ttotal = t + 0.6 nm. The slope and ttotal-intercept of linearfits to the experimental data correspond to the saturation magnetization µ0Ms and deadlayer thickness in the Co3Mn/Mo/CoFeB stack td, respectively. Figure 4(b) summarizes theestimated values of µ0Ms and td as a function of Ta. Annealing at Ta ≥ 200◦C remarkablyenhanced the saturation magnetization, and µ0Ms values as large as 2.0 T were obtainedfor 250◦C ≤ Ta ≤ 350◦C. For this Ta range, the td value was constant around 0.6 nm.These results suggest that the ferromagnetic (bcc) phase is stabilized in the Co3Mn layerfor 250◦C ≤ Ta ≤ 350◦C. Annealing at Ta = 380◦C led to a slight increase in td, which canbe explained by the formation of paramagnetic fcc phase [23] and interlayer atomic diffusionin the Co3Mn/Mo/CoFeB stack. These magnetic phase transition in Co3Mn may be usedto further improve the efficiencies of VCMA- and SOT-driven magnetization switching byreplicating the phase transition using the Joule heating and/or an electric field.Figure 5(a) displays the FMR frequency f0 determined from the VNA-FMR measure-ments as a function of the applied out-of-plane field µ0H. For all MTJ films, the f0 linearlyincreases with increasing µ0H; thus, one can estimate from the linear dependence the sat-uration field Hk as well as PMA energy density Ku (= MsHk/2). Figure 5(b) summarizesthe Ta dependence of Ku for the MTJ films with different t. Regardless of t, Ku max-imizes at around Ta = 250 − 300◦C, and the largest Ku of 0.2 MJ/m3 is achieved for7FIG. 6. CIPT measurement results: (a) TMR as function of applied out-of-plane field. (b) TMRas function of Ta.Ta = 300◦C with t = 0.7 nm. Although this Ku value is smaller than that reported forMgO/CoFeB/Mo/CoFeB/MgO [30, 37] and Mo buffer/CoFeB/MgO multilayers [38–40], itis comparable to the standard Ta buffer/CoFeB/MgO junctions [10, 35].Finally, we discuss the TMR characteristics of the Co3Mn/Mo/CoFeB MTJ films deter-mined from CIPT measurements under an out-of-plane magnetic field. Figure 6(a) displaysthe TMR curves obtained from the Co3Mn (0.8 nm)/Mo (0.3 nm)/CoFeB (0.6 nm) MTJfilms annealed at various Ta. In accordance with the VSM and FMR measurement results,square-shaped TMR curves, which represent the switching of the perpendicularly magnetizedCo3Mn/Mo/CoFeB free layer are obtained for Ta = 200 − 350◦C. The Co3Mn/Mo/CoFeBMTJ film exhibits a large TMR ratio of over 100% after annealing at Ta = 350◦C. Figure6(b) displays the Ta dependence of the TMR ratio determined from the CIPT measurement.The largest TMR of 105% is obtained for the Co3Mn (0.8 nm)/Mo (0.3 nm)/CoFeB (0.6 nm)MTJ film annealed at Ta = 350◦C. Although this TMR ratio is smaller than that reportedfor an in-plane-magnetized MTJ using epitaxially grown bcc-Co3Mn electrodes [22, 23], itshould be stressed that we obtained a large TMR ratio of over 100% in perpendicularly-magnetized MTJ films using polycrystalline Co3Mn electrodes. Note that the smaller TMRfor the MTJ films consisting of 9- and 10-Å-thick Co3Mn films is due to the unsaturatedmagnetic moment in the Co3Mn/Mo/CoFeB layer with insufficient PMA. Further enhance-ments in the TMR as well as PMA will be readily achieved by improving the flatness of thebottom electrode.8IV. SUMMARYTo summarize, we studied the magnetic and electrical transport properties of MTJ filmsconsisting of Co3Mn/Mo/CoFeB multilayers prepared on a polycrystalline electrode. TheCoMn3/Mo/CoFeB multilayers exhibited remarkable PMA and a fairly large Ku of 0.2MJ/m3 was achieved after annealing at Ta = 300◦C. A TMR ratio of 105% was also ob-tained for perpendicularly magnetized Co3Mn/Mo/CoFeB MTJ films. These experimentalresults will contribute to the development of advanced MRAMs including voltage-controlledMRAM and SOT-MRAM.ACKNOWLEDGMENTSThe authors thank T. Nozaki, M. Konoto, A. Sugihara, S. Tsunegi, Y. Hibino, M. Endo,H. Ohmori, Y. Higo, Y. Kageyama and M. Hosomi for their fruitful discussions, and E.Usuda, M. Toyoda and K. Suzuki for assisting with the experiments. This work is basedon results obtained from a project, JPNP20017, commissioned by the New Energy andIndustrial Technology Development Organization (NEDO), Japan.[1] H. Yoda, T. Kishi, T. Nagase, M. Yoshikawa, K. 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