Tunnel magnetoresistance using full-Heusler alloys

We grew Co₂(Cr_1-xFe_x)Al Heusler alloy films at room temperature (RT) without any buffer layers as well as Co ₂MnZ (Z=Si and Ge), which were fabricated using a Cr buffer at RT and elevated temperatures, on thermally oxidized Si substrates using a magnetron sputtering system. The X-ray diffraction pattern of the Co₂MnZ (Z=Si and Ge) films exhibited L2₁ structure when deposited at an elevated temperature, while it showed amorphous or nanocrystalline nature for the films deposited at RT. Co₂(Cr_1-xFe_x)Al films deposited at RT, on the other hand, revealed the B2 and A2 structures, depending on the Fe concentration x, in which the structure tends to become the A2 with increasing x, which is different from the L2₁ structure as expected in the bulk. The magnetic moment and the Curie temperature monotonically increased with increasing x. Spin-valve-type tunneling junctions consisting of Co₂Cr_1-xFe_xAl (100nm)/AlO_x (1.4nm)/CoFe (3nm)/NiFe (5nm)/IrMn (15nm)/Ta (10nm) were fabricated at RT on thermally oxidized Si substrates without any buffer layers using metal masks. The maximum tunneling magnetoresistance obtained is 19% for x=0.4 at room temperature.