Magnetic tunnel junctions for spintronic memories and beyond

In this paper, recent developments in magnetic tunnel junctions (MTJs) are reported with their potential impacts on integrated circuits. MTJs consist of two metal ferromagnets separated by a thin insulator and exhibit two resistances, low (R_P) or high (R_AP), depending on the relative direction of ferromagnet magnetizations, parallel (P) or antiparallel (AP), respectively. Tunnel magnetoresistance (TMR) ratios, defined as (R_AP - R_P)/R_P as high as 361%, have been obtained in MTJs with Co₄₀Fe₄₀B₂₀ fixed and free layers made by sputtering with an industry-standard exchange-bias structure and postdeposition annealing at T_a = 400 °C. The corresponding output voltage swing ΔV is over 500 mV, which is five times greater than that of the conventional amorphous Al-O-barrier MTJs. The highest TMR ratio obtained so far is 500% in a pseudospin-valve MTJ annealed at T_a = 475 °C, showing a high potential of the current material system. In addition to this high-output voltage swing, current-induced magnetization switching (CIMS) takes place at the critical current densities ( J_co) on the order of 10⁶ A/cm² in these MgO-barrier MTJs. Furthermore, high antiferromagnetic coupling between the two CoFeB layers in a synthetic ferrimagnetic free layer has been shown to result in a high thermal-stability factor with a reduced J_co compared to single free-layer MTJs. The high TMR ratio enabled by the MgO-barrier MTJs, together with the demonstration of CIMS at a low J_co, allows development of not only scalable magnetoresistive random-access memory with feature sizes below 90 nm but also new memory-in-logic CMOS circuits that can overcome a number of bottlenecks in the current integrated-circuit architecture.