Evolution of barrier-resistance noise in CoFeB/MgO/CoFeB tunnel junctions during annealing

Abstract

The low-frequency resistance noise in sputtered-deposited magnetic tunnel junctions with MgO barriers has been measured as a function of annealing time at different annealing temperatures. The noise has a 1 / f spectrum and it is quantified by a Hooge-like parameter ␣ given in units of m 2. Unannealed devices have the highest noise levels and their ␣ parameters exhibit a pronounced dependence on the voltage bias across the junction. A significant increase in tunneling magnetoresistance ͑TMR͒ is observed for short annealing times ͑on the order of minutes͒ at high temperatures and it is correlated with a large reduction in noise and in its bias dependence. The maximum TMR and minimum noise levels are reached at a later time that depends on temperature, being shorter at higher annealing temperatures. Devices annealed at 380 and at 430°C exhibit the same minimum noise levels, ␣ Ϸ 2 ϫ 10 −10 m 2. The origin of the resistance noise, its annealing time evolution, and its bias dependence are discussed and they are attributed to vacancy defects in the MgO barriers.

Reduced low frequency noise in electron beam evaporated MgO magnetic tunnel junctions

“…2 inset͒, although the magnetoresistance continues to increase after. This observation supports a recent report of Stearret et al, 18 who found a similar behavior of low frequency noise in MTJs with sputtered MgO tunnel barriers as a function of annealing time. All the annealing temperatures we used in this study are above 300°C.…”

“…Low frequency noise in MTJs in the parallel state is thought to be dominated by barrier noise. 18 Our results, therefore, indicate that the electron-beam evaporated MgO barrier is quieter than the rf sputtered one.…”

“…12,17 In conventional radio frequency ͑rf͒ sputtered MTJs with CoFeB electrodes and MgO tunnel barriers, ␣ was reported to decrease as a function of annealing time and reach a minimum value of 2 ϫ 10 −10 m 2 . 18 The lowest ␣ value ever observed in MTJs, between ͑1-5͒ ϫ 10 −11 m 2 , was achieved in fully epitaxial Fe͑100͒/MgO͑100͒/Fe͑100͒ junctions grown by molecular beam epitaxy ͑MBE͒. 19 Recently, some of us suggested electron-beam evaporation as an alternative to MBE or rf-sputtering for growing high quality MgO tunnel barriers.…”

1 / f noise in MgO double-barrier magnetic tunnel junctions

“…1 inset͒, with the AP state being much noisier than the P state. [15][16][17][19][20][21] Figure 2͑a͒ shows the T a dependence of ␣ in the P state ͑␣ P ͒ for representative DMTJs and SMTJs. The ␣ P value decreases with T a for both types, which reflects a gradual improvement of the interfaces between the ferromagnetic layers and the MgO tunnel barrier.…”

“…It has been previously reported that 1 / f noise dominates the low frequency response of MTJs. [15][16][17][18][19][20][21][22][23] The 1 / f noise can be characterized by a noise magnitude parameter ␣ = AfS V / V 2 , where A is the junction area, f is the frequency, S V is the noise power spectrum density, and V is the applied bias. 15 Recently, Guerrero et al 24 suggested that 1 / f noise can be greatly reduced in field sensors, when a large number of MTJs are connected either in series or in parallel.…”