Crossover of tip-sample magnetic couplings in an in-field spin-polarized scanning tunneling microscopy

We report a crossover between long- and short-range magnetic couplings in an atomic scale magnetic tunnel junction (MTJ), measured from a tip-sample distance d_TS-resolved spin-polarized scanning tunneling microscopy of a Co nanoisland with Fe tips. We show that superparamagnetic response of tip magnetization [1] quantitatively measures the sample-induced field at the tip position μ₀H_S(d_TS), which results in non-zero field offset H_off in the field-dependent differential conductance hysteresis. The H_off(d_TS) with a varying d_TS quantitatively characterizes a dipole filed of ~60 mT. On the other hand, a short range coupling, which is characterized as an antiferromagnetic tip-sample coupling, is observed at a small d_TS. This strongly increases for a decreasing d_TS and competes with the dipole field, resultantly, leading to a crossover of the magnetic couplings from parallel to antiparallel. Our works demonstrate a novel method for quantitative measurement of magnetic couplings in an atomic scale MTJ under a simple and precise control of its tunnel barrier thickness.

References:
[1] S. Phark et al., Appl. Phys. Lett. 103, 032407 (2013).