MgO barrier-perpendicular magnetic tunnel junctions with CoFe/Pd multilayers and ferromagnetic insertion layers

Applied Physics Letters (Impact Factor: 3.52). 10/2009; DOI: 10.1063/1.3265740
Source: arXiv

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

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    ABSTRACT: We investigated magnetic anisotropy and tunnel magnetoresistance (TMR) properties in MgO/(Co0.25Fe0.75)100-xBx stack structures with x=0, 15, 20, and 25 (in at.%). After annealing at 350°C, the easy axis of magnetization switches from in-plane to perpendicular direction in 1.5-nm-thick CoFeB with the B composition near x=15 . The effective magnetic anisotropy energy density (Keff) shows a maximum of 1.9×105 J/m3 in the 1.5 nm-thick CoFeB film with x=20 annealed at 350 °C. Keff is determined by the competition between contributions of interface anisotropy energy per effective CoFeB thickness (Ki/t*, where t* is the effective CoFeB layer thickness) and demagnetization energy (-MS2/2μ0) . Bulk magnetic anisotropy energy (Kb) is negligibly small with comparison to those two terms. To obtain MgO/ferromagnetic stack structure with a high Keff, materials and structures that reduce demagnetization energy while maintaining a high Ki and a thin t* have to be explored. In MTJs with the higher B compositions, high TMR ratio is obtained at higher annealing temperature. High TMR ratio of 136% is observed in a MTJ with x=25 annealed at 350 °C.
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    ABSTRACT: A series of MgO perpendicular magnetic tunneling junction (p-MTJ) films with soft/hard composite electrodes is prepared by RF-sputtering, where amorphous CoFeB is chosen as soft layer. The modulation of MTJs film magnetic properties on CoFeB thickness is investigated. The critical thickness for composite free layer transform from rigid magnet (RM) to exchange spring system (ES) is indentified. Besides, an unexpected in-plane exchange bias is observed which is attributed to the formation of closure domains. The evolution of domain structures on CoFeB is examined by magnetic force microscope. Coexistence of two distinguished magnetic domains of different sizes is observed. It is found that the evolution of domain morphology as varying CoFeB thickness is due to the modulation of effective anisotropy.
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    ABSTRACT: Junction size (D) dependence of thermal stability (Δ) factor and intrinsic critical current (IC0) were investigated for MgO/CoFeB/Ta/CoFeB/MgO recording structure in magnetic tunnel junctions (MTJs) having a CoFeB reference layer and a synthetic ferrimagnetic (SyF) reference layer. Δ of the recording structure shows almost constant value down to 40 nm, whereas IC0 shows a linear dependence on the recording layer area, as similarly observed in recording structure with single-interface. Average absolute intrinsic critical current density is 3.5 MA/cm2, which is comparable to previously reported value for recording structure with single-interface. A MgO/CoFeB(1.4)/Ta(0.4)/CoFeB(1.0)/MgO double-interface recording structure in MTJ with SyF reference layer shows Δ of 59 at D = 29 nm.
    IEEE Transactions on Magnetics 07/2013; 49(7):4437-4440. · 1.42 Impact Factor

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