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Publications (3)5.22 Total impact

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    ABSTRACT: In order to launch spin transfer torque-based magnetoresistive random access memory into mass production below the 40 nm technology node, high performance perpendicularly magnetized magnetic tunnel junctions (pMTJs) are crucial. One of the key issues for pMTJs is to ensure compatibility with conventional back-end-of-line (BEOL) heating process, where thermal tolerances over 350 C for standalone memory and 400 °C for embedded memory are typically required. In this work, we successfully demonstrated high thermal tolerance in CoFeB-based pMTJs with a synthetic antiferromagnet (SAF) pinned structure while keeping stray fields on the free layer low. The annealing temperature (Tanneal) dependence of spin torque switching (STS) and TMR characteristics revealed that the reduction of Ic0, the increase of Δ, and the increase of TMR ratio were achieved by elevating Tanneal from 250 °C up to 380 °C for 30 min. Thus, we attained high STS efficiency (Ic0/Δ)=0.88 (Ic0=69 μA, Δ=78), TMR ratio=120%, RA =12 Ωμm2 at Tanneal=380 °C for the pMTJs with dimensions of 44-46 nm in diameter. Moreover, by using specially tailored pMTJs with a SAF pinned structure, we achieved high thermal tolerance up to 400 °C for 30 min while keeping Ic0/Δ=1.12 (Ic0=56 μA, Δ=50), TMR ratio=150% , RA=13 Ωμm2.
    IEEE Transactions on Magnetics 01/2013; 49(7):4335-4338. · 1.42 Impact Factor
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    ABSTRACT: A novel nonvolatile memory utilizing spin torque transfer magnetization switching (STS), abbreviated spin-RAM hereafter, is presented for the first time. The spin-RAM is programmed by magnetization reversal through an interaction of a spin momentum-torque-transferred current and a magnetic moment of memory layers in magnetic tunnel junctions (MTJs), and therefore an external magnetic field is unnecessary as that for a conventional MRAM. This new programming mode has been accomplished owing to our tailored MTJ, which has an oval shape of 100 times 150 nm. The memory cell is based on a 1-transistor and a 1-MTJ (ITU) structure. The 4kbit spin-RAM was fabricated on a 4 level metal, 0.18 mum CMOS process. In this work, writing speed as high as 2 ns, and a write current as low as 200 muA were successfully demonstrated. It has been proved that spin-RAM possesses outstanding characteristics such as high speed, low power and high scalability for the next generation universal memory
    Electron Devices Meeting, 2005. IEDM Technical Digest. IEEE International; 01/2006
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    ABSTRACT: A calculation of the intrinsic spin transfer switching current Ic0 is realized in the magnetic tunnel junctions (MTJs). Ic0 is determined by the distribution of Ic in repeated measurements, assuming the linear decrease of the thermal stability by increasing the spin-polarized current. The spin transfer torque is found to be proportional to η(I)I with the applied current I and I-dependent spin polarization η(I). Even in case that I is limited less than Ic due to the tunnel barrier breakdown, Ic0 is calculated with an assisting external magnetic field and making better use of the magnetic phase diagram. In our MTJs, the average intrinsic switching current density was 6.4 MA/cm2.
    Applied Physics Letters 08/2005; 87(8):082502-082502-3. · 3.79 Impact Factor