Thermal-magnetic noise measurement of spin-torque effects on ferromagnetic resonance in MgO-based magnetic tunnel junctions

TL;DR

This study uses thermal-magnetic noise measurements at ferromagnetic resonance to determine the magnetic anisotropy of MgO-based magnetic tunnel junctions, revealing a reduced effective demagnetization field in nanoscale free layers.

Contribution

It demonstrates that perpendicular T-FMR provides a reliable method for quantitative magnetic characterization of nanoscale magnetic tunnel junctions, especially for measuring effective demagnetization fields.

Findings

Perpendicular T-FMR yields consistent effective demagnetization field values.

In-plane T-FMR shows less consistent results due to complex mode excitations.

Effective demagnetization field for 20 A CoFeB free layer is about 6.1 kOe.

Abstract

Thermal-magnetic noise at ferromagnetic resonance (T-FMR) can be used to measure magnetic perpendicular anisotropy of nanoscale magnetic tunnel junctions (MTJs). For this purpose, T-FMR measurements were conducted with an external magnetic field up to 14 kOe applied perpendicular to the film surface of MgO-based MTJs under a dc bias. The observed frequency-field relationship suggests that a 20 A CoFeB free layer has an effective demagnetization field much smaller than the intrinsic bulk value of CoFeB, with 4PiMeff = (6.1 +/- 0.3) kOe. This value is consistent with the saturation field obtained from magnetometry measurements on extended films of the same CoFeB thickness. In-plane T-FMR on the other hand shows less consistent results for the effective demagnetization field, presumably due to excitations of more complex modes. These experiments suggest that the perpendicular T-FMR is preferred for quantitative magnetic characterization of nanoscale MTJs.