Magnetic Properties of Perpendicular Magnetic Tunnel Junctions

Perpendicular magnetic tunnel junctions (p-MTJs) represent a class of spintronic devices in which the easy axis of magnetisation is oriented out of the film plane, conferring superior thermal stability and scalability compared with in-plane architectures. At their core is a thin insulating barrier—most often MgO—sandwiched between two ferromagnetic electrodes, typically CoFeB alloys, whose interfacial electronic hybridisation gives rise to strong perpendicular magnetic anisotropy (PMA). This PMA underpins large tunnelling magnetoresistance (TMR) ratios by favouring coherent tunnelling through symmetry-filtered Δ1 electronic states, while also enabling efficient spin-transfer torque (STT) switching at reduced current densities. Critical parameters include the interfacial anisotropy energy density, damping constant, resistance-area product and thermal robustness under annealing up to CMOS back-end-of-line temperatures. Materials innovations—such as the introduction of heavy-metal spacers or capping layers (W, Ta, Nb, Hf)—and precise heat treatments have been pivotal in tuning anisotropy, magnetisation dynamics and magnetoresistance for applications ranging from non-volatile memory to emerging logic and neuromorphic circuits.

Research from Nature Portfolio

Recent studies have demonstrated that incorporating Nb as a heavy-metal underlayer in CoFeB/MgO stacks boosts interfacial PMA energy densities to nearly 1.9 mJ m⁻², yielding room-temperature TMR ratios of around 120 % and a low damping parameter of 0.011. Spin-transfer torque switching in these Nb-based p-MTJs can be achieved at current densities below 7.5 × 10⁵ A cm⁻². Complementary work on atom-thick W layers has shown that double MgO/CoFeB interfaces flanking a single monolayer of tungsten produce TMR ratios up to 249 % and resistance-area products as low as 7 Ω·µm², while retaining sub-3 MA cm⁻² switching currents in nanopillar devices. Advances in ultrathin dual-MgO junctions have further revealed that the insertion of a few monolayers of Mg within CoFeB enables functional perpendicular anisotropy in films as thin as five monolayers; these devices switch in under 1 ns at voltages below 500 mV, combining high thermal stability with high-speed, energy-efficient operation.

Magnetic Properties of Perpendicular Magnetic Tunnel Junctions publication trend

The graph below shows the total number of articles in magnetic properties of perpendicular magnetic tunnel junctions across all publications each year (not limited to Nature Index journals).

Technical terms

Perpendicular magnetic anisotropy (PMA): A magnetic property whereby the energy minimum for magnetisation lies perpendicular to the film plane, often arising from interface-induced orbital hybridisation.

Magnetic tunnel junction (MTJ): A trilayer device consisting of two ferromagnetic electrodes separated by an insulating barrier, whose electrical resistance depends on the relative orientation of electrode magnetisations.

Tunnelling magnetoresistance (TMR): The change in resistance between parallel and antiparallel magnetisation states of an MTJ, quantified as (R_AP − R_P)/R_P.

Spin-transfer torque (STT): A phenomenon in which a spin-polarised current exerts a torque on a magnetic layer, enabling reversal of its magnetisation without external fields.

Resistance-area product (RA): The product of an MTJ’s electrical resistance in its equilibrium state and its junction area, used to assess impedance matching and power consumption.

Damping parameter (α): A dimensionless factor describing the rate at which a disturbed magnetisation returns to equilibrium, affecting switching speed and energy dissipation.

References

1. Perpendicular magnetic anisotropy, tunneling magnetoresistance and spin-transfer torque effect in magnetic tunnel junctions with Nb layers. Scientific Reports (2023).
2. Current-induced magnetization switching in atom-thick tungsten engineered perpendicular magnetic tunnel junctions with large tunnel magnetoresistance. Nature Communications (2018).
3. Tunnel Junction with Perpendicular Magnetic Anisotropy: Status and Challenges. Micromachines (2015).
4. Effect of annealing conditions on the perpendicular magnetic anisotropy of Ta/CoFeB/MgO multilayers. AIP Advances (2016).
5. Ultrathin perpendicular magnetic anisotropy CoFeB free layers for highly efficient, high speed writing in spin-transfer-torque magnetic random access memory. Scientific Reports (2019).