Tunnel Magnetoresistance above 170% and Resistance–Area Product of 1  (m) 2 Attained by In situ Annealing of Ultra-Thin MgO Tunnel Barrier Hiroki Maehara 1;2;3 , Kazumasa Nishimura 1;3 , Yoshinori Nagamine 1;3 , Koji Tsunekawa 1;3 , Takayuki Seki 2;4 , Hitoshi Kubota 2;3 , Akio Fukushima 2;3 , Kay Yakushiji 2;3 , Koji Ando 2 , and Shinji Yuasa 2;3 1 Process Development Center, Canon ANELVA Corporation, Kawasaki 215-8550, Japan 2 Spintronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan 3 CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan 4 Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan Received January 27, 2011; accepted February 23, 2011; published online March 10, 2011 CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) prepared by sputtering deposition and in situ annealing exhibited a high magnetoresistance (MR) ratio (above 170%) and an ultra-low resistance–area (RA) product [about 1.0  (m) 2 ]. The MgO barrier, which was about 1 nm thick, was initially amorphous. In situ annealing of the barrier at 300  C promoted crystallization of the MgO with (001) orientation, which resulted in the high MR ratio at the ultra-low RA product. The present achievements will enable the development of highly sensitive tunnel magnetoresistive (TMR) read heads for hard disk drives with a recording density of about 1 Tbit/in. 2 . # 2011 The Japan Society of Applied Physics A highly sensitive magnetic sensor based on current- perpendicular-to-plane (CPP) geometry is indispen- sable for the read out of ultrahigh-density hard disk drives (HDDs). Tunnel magnetoresistance (TMR) read heads with magnetic tunnel junctions (MTJs) have been widely used since 2004. 1,2) CPP-type read heads for high-density HDDs must meet three requirements: (i) high magneto- resistance (MR) ratio to compensate for the reduction in signal-to-noise ratio due to the shrinking size of recording bits; (ii) low resistance–area (RA) product to provide the impedance matching indispensable for high-speed read out; and (iii) a stacking structure (including antiferromagnetic layer) with a total thickness of less than the bit length so that it can be fabricated between the two magnetic shield layers. CPP giant magnetoresistive (GMR) devices cannot satisfy requirement (iii) so far because the nonmagnetic spacer layer is too thick (although they might satisfy the other two requirements if half-metallic Heusler alloys are used). 3,4) The stacking structure of MTJs, on the other hand, can basically satisfy requirement (iii) because the nonmagnetic spacer layer (i.e., a tunnel barrier) is only about 1 nm thick. MgO-based MTJs with a crystalline MgO(001) tunnel barrier theoretically exhibit MR ratios well above 1000% at room temperature (RT). 5,6) Experimentally, giant MR ratios of up to several hundred percent at RT have been attained — first in epitaxial and textured MgO-based MTJs 7–9) and later in textured CoFeB/MgO/CoFeB MTJs 10,11) that are suitable for device application. To attain a giant MR ratio in the CoFeB/MgO/CoFeB MTJs, a highly textured MgO(001) layer should be grown on an amorphous CoFeB bottom electrode layer. The CoFeB layers should then be crystal- lized as a bcc(001) textured structure by post annealing. 2,12) For read head application, CoFeB/MgO/CoFeB MTJs were previously fabricated by depositing an ultrathin magnesium layer below the MgO 13) or by using the tantalum getter process. 14) These techniques promoted the (001)-oriented crystallization of an MgO barrier grown on the amorphous CoFeB and resulted in MR ratios above 100% at RT for low RA products [below a few  (m) 2 ]. For example, the authors previously demonstrated an MR ratio of about 100% and an RA product of 1.0  (m) 2 . 14) On the basis of these technologies, MgO-based TMR read heads have been developed that are currently used in HDDs with recording densities of 300 to 500 Gbit/in. 2 . 1,2) For a recording density of 1 Tbit/in. 2 , however, MR ratio higher than 70% at RA product of about 0.6  (m) 2 is required. 15) It should be noted here that in the current TMR read heads, a shunt resistance is usually connected to the MTJ in parallel. Shunt resistance can be used to reduce the resistance of the read head (i.e., the total resistance of the parallel circuit) at the cost of a reduction in MR signal intensity. 15) Shunt resistance is particularly effective for improving the reliability (i.e., break-down voltage) of the TMR read head because a thicker tunnel barrier can be used. To get a sufficient reliability for read head application, the MgO tunnel barrier should be thicker than about 1 nm, which corresponds to the RA product higher than about 1  (m) 2 . From the viewpoint of read head application, therefore, it is very important to enhance the MR ratio for an RA range of about 1 to 2  (m) 2 in order to compensate the reduction in the net MR ratio due to the shunt resistance. To achieve this goal, it is necessary to fabricate a highly textured MgO(001) barrier with a thickness of about 1 nm. A technical difficulty, however, in regard to the fabrication is that a 1-nm-thick MgO layer grown on an amorphous CoFeB tends to become amorphous in the as-grown state. 2,12) As a result, the (001) orientation of MgO deteriorates when it is crystallized by post annealing. To improve the (001) orientation of the ultra-thin MgO barrier, Isogami et al. performed in situ annealing just after depositing MgO on CoFeB and achieved an MR ratio of 206% for RA product of 2.1  (m) 2 . 16) However, they did not demonstrate an RA product below 2  (m) 2 , which is necessary for attaining a recording density of 1 Tbit/in. 2 . Besides, the detailed mechanism of the improvement of (001) orientation has not been clarified yet. In this study, we performed in situ annealing of the MgO barrier layer by using a sputtering machine for mass-manufacturing and attained MR ratios above 170% for RA  1 (m) 2 . We also investigated the origin of the improvement by using in situ reflective high energy electron diffraction (RHEED) observation. The thin films for the MTJs were deposited on a thermally oxidized Si(001) wafer with a diameter of 200 mm using a magnetron-sputtering system (Canon ANELVA C-7100)  E-mail address: yuasa-s@aist.go.jp Applied Physics Express 4 (2011) 033002 033002-1 # 2011 The Japan Society of Applied Physics DOI: 10.1143/APEX.4.033002