Tunneling Magnetoresistance with Zero-moment Half-metallic electrodes

In recent years, great attention has been paid to the research of intermetallic Heusler compounds. These materials have widely tuneable properties. They display high spin polarisation , low magnetic moment , low Gilbert damping α and high effective magnetic anisotropy field. All of the above-mentioned characteristics play key role at the choice of materials for integration in spin-transfer-torque oscillators. Here we have successfully integrated a compensated half-metallic ferrimagnet as a fixed layer in magnetic tunnel junctions (MTJ). Theoretically this class of materials was predicted in 1995 by van Leuken and de Groot, but experimentally the zero-moment half-metal was realised only in 2014 for a near-cubic Heusler alloy of Mn, Ru and Ga (MRG). Here, MTJs with different insertion layers between MRG and the tunnel barrier were studied. Sufficient tunnelling magnetoresistance (TMR) ratios were demonstrated for Mn2RuGa / Al 0.6 nm / MgO / CoFeB MTJs. We measured the switching properties of MTJ as a function of applied bias voltage and ex-situ annealing temperature. At low bias (U ≈ 10mV), the as-grown sample shows TMR ratio ≈ 1.6% at room-temperature, annealing at 375°C leads to increasing of TMR to ≈ 7.5%. At higher negative bias (U ≈ - 0.5 V), the TMR varies from -2.9 to -6.3%, for the as-grown sample and the sample annealed at 375°C, respectively. Low temperature measurements on the same device show in excess of 40% TMR close to zero bias. Moreover, we demonstrate non-zero TMR while cooling through the compensation temperature (when the magnetic moment is zero). Finally, by changing the electrode composition from Mn2Ru0.65 Ga through to Mn2Ru1.1Ga we also demonstrate finite TMR at ambient temperature with an electrode designed to be compensated at room temperature.