Fe-Cr multilayers with very thin Cr layers are unusual because the moments of the Fe layers are antialigned in zero magnetic field.[1,2] The suppression of the antiferromagnetic alignment by an applied field results in a large negative magnetoresistance.[3-5] In simple sandwich structures with only two Fe layers, both the magnetization and the magnetoresistance show well-defined steps in their field dependence.[4,5] The large size and sharpness of the magnetoresistive steps make the Fe-Cr system a promising one for applications. A recent theoretical study calculates the magnetoresistance as a function of the electron mean-free path and of the number of Fe repeats.[6] An alternative approach is to study the transport properties as a function of the Cr layer thickness since the Cr thickness should strongly influence the magnetic coupling between the Fe layers. An understanding of the relationship between the interlayer coupling and the magnetoresistance will be a necessary element in any fundamental theory.
In order to study the Cr-thickness dependence of the magnetic and transport properties, a series of single-crystal bcc Fe-Cr-Fe sandwiches was prepared using molecular beam epitaxy. Starting with a well-polished GaAs(001) substrate, a ZnSe(001) epilayer about 2000 Å thick was grown to achieve a flat surface. Subsequent single crystal Fe, Cr, and Fe films were grown using a substrate temperature near 20 °C and growth conditions similar to those of Baibich et al.[3] Cr thicknesses ranged from 7 to 85 Å. Finally, a polycrystalline ZnSe layer about 1000 Å thick was deposited on the stack for symmetry and for protection against the atmosphere. Details of the sample characterization are reported elsewhere.[7] Gold contacts sputtered onto the ZnSe surface show good linearity. The room-temperature resistivity of the sandwiches is typically 50 µOhm-cm, similar in magnitude to previously reported values.[3] All measurements reported here were made at room temperature with an excitation current I of 20 µamp applied at 219 Hz along an in- plane [110] direction.