Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Intermetallic Heusler compounds can possess high spin polarization, low magnetic moment, low Gilbert damping constant α, and huge uniaxial anisotropy fields, of the order of tens of tesla. Such a wide range of properties, most of them tunable, make these materials very attractive for spin-transfer-torque oscillators in the (sub-) THz range. A particularly suitable candidate is the near-cubic Heusler alloy of Mn, Ru, and Ga (MRG) [1]. Here, we show that tunneling magnetoresistance (TMR) of about ten percent can be achieved in MRG-based magnetic tunnel junctions (MTJs), and that the TMR can be improved by integrating different insertion layers acting as diffusion barriers between the half-metallic electrode and the tunnel barrier.

MRG-based stacks were deposited using a “Shamrock” fully automated sputter deposition tool by co-sputtering from a Mn2Ga and a Ru target. Changing the Ru concentration allows tuning the compensation temperature Tcomp between 2 and 450 K. The thin-film stacks were subsequently patterned into 20 × 20 μm2 junctions using standard UV lithography, prior to annealing in temperatures ranging from 250 °C to 350 °C. Selected samples were investigated by transmission electron microscopy (TEM).

The magnetic properties of the MTJs were analyzed by magnetotransport measurements as a function of applied bias voltage at room temperature. We found that 0.6 nm of Al acts as good diffusion barrier in Mn2RuXGa / MgO / CoFeB MTJs. Low-temperature measurements on the same device show TMR in excess of 40% close to zero bias [2]. In addition, we demonstrate non-zero TMR while cooling through the compensation temperature (where the magnetic moment is zero), indicating that magnetotransport in MRG is governed by one Mn sublattice only. This hypothesis is further supported by the fact that samples with Tcomp above room temperature exhibit inverted TMR as compared to samples that compensated below. The precise value of Tcomp is the result of a delicate balance between the moments carried by Mn ions on the 4c and 4a sites. Upon thermal annealing, this balance is slightly shifted due to partial annihilation of Mn anti-sites, and Tcomp may pass from above room temperature to below, giving rise to an inverted TMR response. The next step is to fabricate sub-µm devices based on MRG for detecting spin-transfer induced dynamics, which should occur at frequencies of several hundred GHz, given the ultra-high anisotropy of these.

This work is supported by the Helmholtz Young Investigator Initiative Grant No. VH-N6-1048.