H. Yamamoto

Charles University in Prague, Praha, Hlavni mesto Praha, Czech Republic

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Publications (12)92.95 Total impact

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    ABSTRACT: We employ antiferromagnetic tunneling anisotropic magnetoresistance to study the behavior of antiferromagnetically ordered moments in IrMn exchange coupled to NiFe. Experiments performed by common laboratory tools for magnetization and electrical transport measurements allow us to directly link the broadening of the NiFe hysteresis loop and its shift (exchange bias) to the rotation and pinning of antiferromagnetic moments in IrMn. At higher temperatures, the broadened loops show zero shift, which correlates with the observation of fully rotating antiferromagnetic moments inside the IrMn film. The onset of exchange bias at lower temperatures is linked to a partial rotation between distinct metastable states and pinning of the IrMn antiferromagnetic moments in these states. The observation complements common pictures of exchange bias and reveals an electrically measurable memory effect in an antiferromagnet.
    Physical Review Letters 01/2012; 108(1):017201. · 7.73 Impact Factor
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    ABSTRACT: A spin valve is a microelectronic device in which high- and low-resistance states are realized by using both the charge and spin of carriers. Spin-valve structures used in modern hard-drive read heads and magnetic random access memoriescomprise two ferromagnetic electrodes whose relative magnetization orientations can be switched between parallel and antiparallel configurations, yielding the desired giant or tunnelling magnetoresistance effect. Here we demonstrate more than 100% spin-valve-like signal in a NiFe/IrMn/MgO/Pt stack with an antiferromagnet on one side and a non-magnetic metal on the other side of the tunnel barrier. Ferromagneticmoments in NiFe are reversed by external fields of approximately 50  mT or less, and the exchange-spring effect of NiFe on IrMn induces rotation of antiferromagnetic moments in IrMn, which is detected by the measured tunnelling anisotropic magnetoresistance. Our work demonstrates a spintronic element whose transport characteristics are governed by an antiferromagnet. It demonstrates that sensitivity to low magnetic fields can be combined with large, spin-orbit-coupling-induced magnetotransport anisotropy using a single magnetic electrode. The antiferromagnetic tunnelling anisotropic magnetoresistance provides a means to study magnetic characteristics of antiferromagnetic films by an electronic-transport measurement.
    Nature Material 03/2011; 10(5):347-51. · 35.75 Impact Factor
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    ABSTRACT: We investigated perpendicular magnetic tunnel junctions (p-MTJs) with a stepped structure for spin-transfer torque random access memory (SPRAM). In conventional p-MTJs, the retention time for storing “1” is shorter than that for storing “0”, because of the mangetostatic energy difference between the two states caused by dipole interaction. To counter this, one had to develop materials with low magnetization. Here we show, by employing a stepped structure, that the retention time can be made equivalent regardless of the stored information (“0” or “1”), without resorting to the employed materials. This is because this structure reduces the dipole interlayer coupling between two ferromagnetic layers as the diameter difference of the free and reference layers of MTJ increases.
    Digest of Technical Papers - Symposium on VLSI Technology 01/2011;
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    ABSTRACT: We have investigated the effect of stack structures on tunnel magnetoresistance (TMR) properties in perpendicular anisotropy MgO-based magnetic tunnel junctions (p-MTJs) with CoFe/Pd multilayer and CoFeB insertion. By adopting Ta and Ru cap-layers, the TMR ratios of 113 and 106% are obtained at annealing temperature (Ta) of 325 °C, respectively. Particularly, the Ru cap-layer is effective in realizing a TMR ratio of 100% at Ta = 350 °C. By replacing (Co25Fe75)80B20 with (Co25Fe75)85B15, the TMR ratio increased quickly at low Ta, reaching a maximum of 120% at Ta = 300 °C.
    Journal of Applied Physics 01/2011; 109. · 2.21 Impact Factor
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    ABSTRACT: Spin-valve is a microelectronic device in which high and low resistance states are realized by utilizing both charge and spin of carriers. Spin-valve structures used in modern hard drive read-heads and magnetic random access memories comprise two ferromagnetic (FM) electrodes whose relative magnetization orientations can be switched between parallel and antiparallel configurations, yielding the desired giant or tunneling magnetoresistance effect. In this paper we demonstrate >100$% spin-valve-like signal in a NiFe/IrMn/MgO/Pt stack with an antiferromagnet (AFM) on one side and a non-magnetic metal on the other side of the tunnel barrier. FM moments in NiFe are reversed by external fields <50mT and the exchange-spring effect of NiFe on IrMn induces rotation of AFM moments in IrMn which is detected by the measured tunneling anisotropic magnetoresistance (TAMR). Our work demonstrates a spintronic element whose transport characteristics are governed by an AFM. It demonstrates that sensitivity to low magnetic fields can be combined with large, spin-orbit coupling induced magneto-transport anisotropy using a single magnetic electrode. The AFM-TAMR provides means to study magnetic characteristics of AFM films by an electronic transport measurement. Comment: 8 pages, 4 figures
    11/2010;
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    ABSTRACT: Magnetic tunnel junctions (MTJs) with ferromagnetic electrodes possessing a perpendicular magnetic easy axis are of great interest as they have a potential for realizing next-generation high-density non-volatile memory and logic chips with high thermal stability and low critical current for current-induced magnetization switching. To attain perpendicular anisotropy, a number of material systems have been explored as electrodes, which include rare-earth/transition-metal alloys, L1(0)-ordered (Co, Fe)-Pt alloys and Co/(Pd, Pt) multilayers. However, none of them so far satisfy high thermal stability at reduced dimension, low-current current-induced magnetization switching and high tunnel magnetoresistance ratio all at the same time. Here, we use interfacial perpendicular anisotropy between the ferromagnetic electrodes and the tunnel barrier of the MTJ by employing the material combination of CoFeB-MgO, a system widely adopted to produce a giant tunnel magnetoresistance ratio in MTJs with in-plane anisotropy. This approach requires no material other than those used in conventional in-plane-anisotropy MTJs. The perpendicular MTJs consisting of Ta/CoFeB/MgO/CoFeB/Ta show a high tunnel magnetoresistance ratio, over 120%, high thermal stability at dimension as low as 40 nm diameter and a low switching current of 49 microA.
    Nature Material 09/2010; 9(9):721-4. · 35.75 Impact Factor
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    ABSTRACT: The authors fabricated double MgO barrier magnetic tunnel junctions (MTJs) with 3-nm-thick Co40Fe40B20 free layer. When annealed at 350 °C, tunnel magnetoresistance (TMR) ratio at room temperature was 130%, much lower than that (297%) of single MgO barrier MTJs processed and annealed under the same condition. The middle CoFeB free layer sandwiched between the two MgO barriers was found to be mostly amorphous. Replacement of the Co40Fe40B20 free layer by a highly oriented Co50Fe50 layer and a composite Co50Fe50/Co40Fe40B20 layer led to the enhanced TMR ratios up to 165% and 212% at annealing temperature of 350 °C, respectively.
    Applied Physics Letters 05/2010; 96(19):192507-192507-3. · 3.52 Impact Factor
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    ABSTRACT: A 32-Mb SPin-transfer torque RAM (SPRAM) chip was demonstrated with an access time of 32 ns and a cell write-time of 40 ns at a supply voltage of 1.8 V. The chip was fabricated with 150-nm CMOS and a 100 × 200-nm tunnel magneto-resistive (TMR) device element. A required thermal stability of 67 of the TMR device was estimated by taking into account the disturbances during read operations and data retention periods of 10 years for nonvolatile operation. The 32-Mb SPRAM chip features three circuit technologies suitable for a large-scale array: 1) a two-transistor, one-resistor (2T1R) type memory cell for achieving a sufficiently large write current despite the small cell size, 2) a compact read/write separated hierarchy bit/source-line structure with a localized bi-directional write driver for efficiently distributing write current, and 3) a '1'/'0' dual-array equalized reference scheme for stable read operation.
    IEEE Journal of Solid-State Circuits 05/2010; · 3.06 Impact Factor
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    ABSTRACT: A magnetic-tunnel-junction (MTJ) circuit model, which considers spin dynamics under finite temperature, electrical bias, a stochastic process, and spin-transfer torque, was developed. Switching behaviors simulated by this model were verified by experimental measurements. Moreover, a disturbance-free read scheme for Gbit-scale spin-transfer torque RAM (SPRAM) was also developed. The feasibility of this scheme was confirmed by circuit simulation using the model and on-chip measurement of switching probability.
    Electron Devices Meeting (IEDM), 2009 IEEE International; 01/2010
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    ABSTRACT: We studied the magnetic and magnetoresistance characteristics of pseudospin-valve magnetic tunnel junctions (MTJs) based on CoFe/Pd multilayer electrodes with perpendicular magnetic anisotropy and an MgO barrier. The MTJs at annealing temperature ( T <sub>a</sub>) of 473 K showed a tunnel-magnetoresistance (TMR) ratio of 1.5%. An fcc (111)-oriented texture of the bottom and top Co<sub>90</sub>Fe<sub>10</sub>/Pd multilayer electrodes, together with an imperfectly crystallized MgO, were revealed by cross-sectional TEM images. The TMR properties of perpendicular MTJs with a Co<sub>20</sub>Fe<sub>60</sub>B<sub>20</sub> or Co<sub>50</sub>Fe<sub>50</sub> layer inserted between the CoFe/Pd multilayer electrodes and the MgO barrier were also studied. The TMR ratio with Co<sub>20</sub>Fe<sub>60</sub>B<sub>20</sub> insertion was 1.7% at T <sub>a</sub>= 473 K and monotonically decreased at T <sub>a</sub> over 523 K. The TMR ratio with Co<sub>50</sub>Fe<sub>50</sub> insertion increased up to 3% at T <sub>a</sub>= 573 K and then decreased to 0.4% at T <sub>a</sub>= 598 K. The influence of the Pd layer on CoFeB was studied by using the simplified structures of Pd/CoFeB/MgO/CoFeB/Pd and Ta/CoFeB/MgO/CoFeB/Ta with inplane anisotropy. A former structure with Pd resulted in reduced TMR ratio which decreases with increasing T <sub>a</sub>, whereas MTJs with a Ta-based structure showed a monotonic increase of a TMR ratio. The low TMR ratio observed in Pd-containing structures appears to result from crystallization of CoFeB in an unfavorable crystal orientation.
    IEEE Transactions on Magnetics 11/2009; · 1.42 Impact Factor
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    ABSTRACT: The authors studied an effect of ferromagnetic (Co20Fe60B20 or Fe) layer insertion on tunnel magnetoresistance (TMR) properties of MgO-barrier magnetic tunnel junctions (MTJs) with CoFe/Pd multilayer electrodes. TMR ratio in MTJs with CoFeB/MgO/Fe stack reached 67% at an-nealing temperature (Ta) of 200 degree C and then decreased rapidly at Ta over 250 degree C. The degradation of the TMR ratio may be related to crystallization of CoFe(B) into fcc(111) or bcc(011) texture result-ing from diffusion of B into Pd layers. MTJs which were in-situ annealed at 350oC just after depo-siting bottom CoFe/Pd multilayer showed TMR ratio of 78% by post annealing at Ta =200 degree C. Comment: 12 pages, 4 figures
    Applied Physics Letters 10/2009; · 3.52 Impact Factor
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    ABSTRACT: A 32-Mb SPin-transfer torque RAM (SPRAM) chip was demonstrated with an access time of 32 ns and a cell write-time of 40 ns at a supply voltage of 1.8 V. The chip was fabricated with 150-nm CMOS and a 100 × 200 nm tunnel magnetoresistive device element. This chip features three circuit technologies suitable for a large-scale array: 1) a two-transistor, one-resistor (2T1R) type memory cell for achieving a sufficiently large writing current despite the small cell size, 2) a compact read/write separated hierarchy bit/source-line structure with a localized bi-directional write driver for efficiently distributing writing current, and 3) a ‘1’/‘0’ dual-array equalized reference cell for stable read operation.
    VLSI Circuits, 2009 Symposium on; 07/2009

Publication Stats

449 Citations
92.95 Total Impact Points

Institutions

  • 2012
    • Charles University in Prague
      • Matematicko-fyzikální fakulta
      Praha, Hlavni mesto Praha, Czech Republic
  • 2011
    • University of Cambridge
      Cambridge, England, United Kingdom
  • 2009–2010
    • Tohoku University
      • Institute for Materials Research
      Japan