Altering the atomic composition ratio of magnetic tunnel junction interfaces

A magnetic tunnel junction (MTJ) consists of an ultrathin insulator separated by two ferromagnetic (FM) metal electrodes. Depending on the tunneling magnetoresistance (TMR) of the MTJ — the relative alignment of magnetizations in the electrodes — the ability of electrons to tunnel through the structure changes. In this way, the strength of an applied magnetic field affects the TMR of an MTJ, enabling the development of advanced sensors.

The fabrication of the tunnel material/FM metal electrode interface must fulfill delicate conditions, as contaminations and surface oxidations disturb spin polarizations and result in inaccurate TMR signals. There are fabrication options available that avoid this issue, but they produce significantly different MTJ interfaces.

Matsumoto et al. demonstrated the importance of the fabrication process on MTJ properties through density functional theory (DFT) calculations.

“There are two fabrication processes of the graphene/ferromagnetic alloy interface: one is the transfer of graphene, and the other is the evaporation of alloys onto graphene,” said author Naohiro Matsumoto. “Our systematic DFT calculations derived universal findings in the graphene/NiFe alloy interface that the surface compositions, which dominate the magnetic moments, are controlled by these fabrication processes.”

Results showed that for the fabrication of graphene/NiFe alloy interfaces, the graphene transfer process forms Ni-rich interfaces, while the alloy-evaporation process favors Fe-rich interfaces, indicating atomic composition ratio change at the interface depending on the fabrication method.

The authors hope their findings will inspire novel interface structures based on two-dimensional materials as well as the development of new MTJ devices with higher magnetic ordering.

“The developments of new magnetoresistive random-access memory devices, for example, are expected to drive innovations in low-power, high-density memory devices in the information society,” said Matsumoto.

Source: “Theoretical investigation of interface atomic structure of graphene on NiFe alloy substrate,” by Naohiro Matsumoto, Ryusuke Endo, Mitsuharu Uemoto, and Tomoya Ono, Journal of Applied Physics (2025). The article can be accessed at https://doi.org/10.1063/5.0283881 .