IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014 4401004 Transport Properties in Sputtered CoFeB/MgAl 2 O 4 /CoFeB Magnetic Tunnel Junctions Bingshan Tao, Dalai Li, Houfang Liu, Hongxiang Wei, Jia-Feng Feng, Shouguo Wang, and Xiufeng Han Beijing National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China CoFeB/MgAl 2 O 4 /CoFeB magnetic tunnel junctions (MTJs) with the barrier sputtered from sintered MgAl 2 O 4 target have been successfully fabricated. Dependence of tunneling magnetoresistance (TMR) ratio on both MgAl 2 O 4 deposition pressure and postannealing temperature has been studied. The TMR ratio of more than 50% at room temperature was obtained with an annealing temperature of 325 °C and MgAl 2 O 4 deposition pressure of 1.3 Pa. Temperature dependence of resistance in both parallel and antiparallel configurations can be well fitted by the model based on direct elastic tunneling and magnon-assisted inelastic tunneling. Inelastic electron tunneling spectroscopy (IETS) at low temperature, exhibiting three peaks originating from zero-bias anomaly, interface magnons, and barrier phonons, were measured and compared with the results of AlO x and MgO-based MTJs. The IETS for all three types of MTJs shows quite similar peak positions for all kinds of elementary excitations except barrier phonons. Index Terms—Direct elastic tunneling, inelastic electron tunneling spectroscopy, magnon-assisted inelastic tunneling, tunneling magnetoresistance (TMR). I. I NTRODUCTION M AGNETIC tunnel junctions (MTJs), consisting of two ferromagnetic layers separated by a thin tunnel barrier, have been extensively studied due to the giant tunneling magnetoresistance (TMR) effect at room temperature (RT) [1]–[6] and their important applications in spintronic devices [7], [8]. The TMR effect at RT was first discovered in amor- phous AlO x barrier-based MTJs [1], [2] and then improved tremendously in crystallized MgO barrier-based MTJs [3], [4] due to the spin filtering effect [5], [6]. However, the lattice mismatch between MgO and typical bcc ferromagnetic metals introduces defects at the interfaces, resulting in a rapid TMR decrease with bias voltage and low breakdown voltage. Thus, further improvement in performance of MTJs and new barrier exploration is urgent for industrial applications. Recently, spinel oxide MgAl 2 O 4 has attracted great interests due to its small lattice mismatch with typical ferromagnetic metals and the same spin-filter effect as MgO [9], [10]. Epitaxial MgAl 2 O 4 -based MTJs, which were grown on single crystalline MgO substrate with the barrier formed by plasma oxidation of Mg/Al bilayer or Mg–Al alloy, exhibit weak bias voltage dependence and high TMR ratio [11]–[14]. Above results show that MgAl 2 O 4 is a promising candidate for tunnel barrier in MTJs. However, the single-crystal substrate is not beneficial for commercial applications. In our previous work, CoFeB/MgAlO x /CoFeB MTJs were fabricated on thermally oxidized Si wafer with MgAlO x barrier formed by plasma oxidation of Mg/Al bilayer [15]. However, under or over oxidation can be easily induced by plasma oxidation of metallic layer, which have been demonstrated in AlO x -based MTJs [16], [17]. A possible solution to the problem is to use Manuscript received March 6, 2014; revised April 25, 2014; accepted April 29, 2014. Date of current version November 18, 2014. Corresponding author: X. Han (e-mail: xfhan@aphy.iphy.ac.cn). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2014.2321494 sintered MgAl 2 O 4 target to achieve stoichiometric MgAl 2 O 4 . Therefore, it is worthy to fabricate CoFeB/MgAl 2 O 4 /CoFeB MTJs on thermally oxidized Si substrate with the barrier sputtered from sintered MgAl 2 O 4 target, considering that extremely high TMR ratio has been achieved in sputtered CoFeB/MgO/CoFeB MTJs [18]. In this paper, MgAl 2 O 4 sintered target was utilized to form stoichiometric MgAl 2 O 4 tunnel barrier by RF sputtering and the magneto-transport properties of MgAl 2 O 4 -based MTJs were investigated and compared with results of AlO x and MgO-based MTJs. TMR of 53% at RT have been obtained by optimizing the growth conditions of MgAl 2 O 4 barrier. Temperature dependence of resistance in the parallel (P) and antiparallel (AP) states and inelastic electron tunneling spectroscopy (IETS) at low temperature have been studied. II. EXPERIMENTAL METHOD The MTJ stack, with the structure of Ta(5)/Ru(30)/Ta(5)/CoFeB(5)/MgAl 2 O 4 (2)/CoFeB(3)/Ru(0.9)/ CoFe(2.5)IrMn(12)/Ta(5)/Ru(5) (units in nm), was deposited on thermally oxidized Si wafer using an ULVAC magnetron sputtering system at a base pressure of 1.0 × 10 -6 Pa. The MgAl 2 O 4 tunnel barrier was formed by RF sputtering from sintered MgAl 2 O 4 target with deposition pressure ranging from 1.0 to 1.8 Pa. For comparison, AlO x -based MTJs with the stack of Ta(5)/Ru(30)/Ta(5)/ CoFeB(4)/AlO x (2)/CoFeB(4)/IrMn(12)/Ta(5)/Ru(5) (units in nm) were deposited in the same sputtering system with AlO x barrier formed by plasma oxidation of an Al layer in mixture atmosphere Ar + O 2 with pressure 1.0 Pa, and MgO-based MTJs with the stack of Ta(5)/ Ru(30)/Ta(5)/NiFe(5)/IrMn(10)/CoFe(2.5)/Ru(0.9)/CoFeB(3)/ MgO(2.5)/CoFeB(3)/Ta(5)/Ru(5) (units in nm) were also fabricated using the high vacuum Shamrock cluster deposition tool with a base pressure of 1 × 10 -7 torr, where MgO barrier is formed by RF sputtering from MgO target. All the MTJs were patterned into junctions with size of 10 × 20 μm 2 using 0018-9464 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.