Persistent interlayer coupling by an antiferromagnetic spacer above its Neel temperature (a Monte Carlo study)

It has been demonstrated by neutron diffraction experiments\footnote{J.A. Borchers et al., Phys. Rev. Lett. {\bf 70}, 1878 (1993).} that if a thin film of antiferromagnetic (AFM) material of bulk Neel temperature $T_N$ is placed between two AFM layers or between two ferromagnetic (FM) layers with much higher transition temperatures, then a short-range AFM ordering in the ``sandwiched'' layer may persist well above $T_N$, and it may maintain magnetic coupling between the two adjacent layers which are still in their ordered phase. We report MC simulations of exchange-coupled FM/AFM/FM trilayers with an even number (4, 6, or 8) of AFM monolayers. In these systems the magnetization vectors of the FM blocks are antiparallel, but an external magnetic field $B$ tends to incline them toward its direction. By varying $B$, we investigated the strength of the interlayer coupling between the FM films. In a system with a spacer consisting of 4 AFM layers the FM blocks remain coupled even at temperatures 50\% higher than $T_N$. We believe that such trilayers may be used for making new types of TMR sensors with ``temperature-tunable'' sensitivity to the magnetic field.