Temperature-dependent magnetoresistance of magnetic tunnel junctions with ultraviolet light-assisted oxidized barriers

Abstract

Co(10 nm)/AlO x ( nominally 2 nm)/Co(20 nm) tunnel junctions have been prepared under ultrahigh vacuum conditions applying a shadow mask technique. An ultraviolet light-assisted oxidation process of the AlOx barrier has been optimized by in situ x-ray photoelectron spectroscopy, in conjunction with temperature-dependent tunneling magnetoresistance measurements. Optimum-oxidized tunnel junctions show a magnetoresistance of 20% at 285 K, and up to 38% at 100 K. For under-oxidized samples, with a remaining Al layer between the Co bottom electrode and the AlOx barrier, the tunneling magnetoresistance decreases more rapidly with increasing temperature than observed for the over-oxidized samples. The resistance × area product of optimum-oxidized tunneling junctions exhibits a minimum, and increases for under- and over-oxidized samples.

Temperature dependent resistance of magnetic tunnel junctions as a quality proof of the barrier

“…Also modestly overoxidized samples ͑t ox ϭ85 min͒ can show a TMR of approximately 10%. 22 The observed strong decrease of the TMR of underoxidized samples as function of temperature can have different origins. For an interpretation one has to distinguish between the temperature dependence of the spin-dependent and spinindependent contribution to the resistivity.…”

“…XPS Co core level ͑2 p 1/2 and 2p 3/2 ͒ spectra also support that 60 min oxidation time is the optimum oxidation time achieving a fully oxidized Al layer without oxidizing the underlying Co electrode. 22 For junctions oxidized during t ox ϭ60 min the TMR at Tϭ100 K exhibits a sharp maximum of 36%. Slightly underand overoxidized samples ͑45 and 75 min oxidation time͒ reveal a reduction of the TMR to approximately 20% symmetric about t ox ϭ60 min.…”

“…In the presence of a nonuniformly oxidized barrier ͑Al/AlO x double layer͒ or nonoxidized Al inclusions within the oxide barrier the temperature dependence of a spin-independent tunneling contribution can be larger than in the case of an overoxidized sample with an additional CoO x layer separating the Co bottom electrode and the AlO x barrier. 22 Additionally, a remaining Al layer on top of the Co bottom electrode can significantly reduce the surface spin polarization of the Co interface layer close to the Fermi energy E F . Spin-polarized band structure calculations have shown that an Al termination of the Co bottom electrode strongly reduces the spin polarization of Co at the interface.…”

Recent developments in magnetic tunnel junctions

“…The energy profile of the barrier is particularly important in determining the bias dependence in MTJs. The barrier structures have been intensively investigated by many analytical methods, such as the optical polarization modulation technique [6], Fourier transform infrared spectroscopy [7], Auger spectroscopy [8], and X-ray photoelectron spectroscopy [9]. Recently, we used electron holography (EH) to investigate the energy profile of the barrier in MTJs.…”

Growth and morphology of the epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) Trilayers

“…MTJs consisting of amorphous aluminium oxide (AlO x ) tunnel barrier and ferromagnetic electrodes with high spin polarization [4][5][6][7] show relatively low tunneling magnetoresistance (TMR) ratio, up to about 70% at room temperature (RT), which seriously limits the implementation of such structures in spintronic devices. On the other hand, first-principle theories predicted an extremely high MR ratio (over 1000%) for epitaxial Fe(1 0 0)/MgO(1 0 0)/Fe(1 0 0) MTJs with abrupt interfaces between MgO and Fe [8,9].…”