Magnetoresistance with half-metallic Heusler electrodes
Big amount of data creates new social demand - to increase the speeds of the transferring information. For this reason, technology operating in the terahertz (THz) gap is required. Nowadays THz-transmitter/receiver solutions are either based on superconductors (requiring cryogenic temperatures) or have a tabletop-sized devices (lasers), and low-cost, chip scale alternatives are needed. The main goal of TRANSPIRE project to integrate a new class of magnetic materials – zero-moment half-metals – into broadband, tunable spin-torque nano-oscillator devices in order to breach the terahertz gap. 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. Mn2RuXGa (MRG) is close to zero net moment state within a broad compensation region. Additionally, it possesses high Fermi-level spin polarization, which enables them to be driven at resonance by spin-polarized currents. According to the Kittel formula, the antiferromagnetic resonance frequency is dependent on the sub-lattice anisotropy (HKsl) and the inter-lattice exchange field (Hex), f=γ√(HKsl*Hex). As the MnGa alloys are very anisotropic  the resonant frequencies in these materials can be pushed up to 1Thz without requiring an external magnetic field.
Before fabricating the sub-µm devices for detection of spin-transfer induced dynamics, its magnetotransport properties have to be understood. We demonstrated that MRG can be implemented in perpendicular anisotropy magnetic tunnel junctions (MTJ) stacks which exhibit TMR of about ten percents. The magnetic properties of the MTJs were analyzed by magnetotransport measurements as a function of applied bias voltage at room temperature in Mn2RuXGa / MgO / CoFeB MTJs (Fig. 2). Low-temperature measurements on the same device show TMR in excess of 40% close to zero bias . 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 (Fig.1).
The same hypothesis is further supported by the fact that samples with Tcomp above room temperature exhibit inverted TMR as compared to samples that compensated below. (Fig.3) 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 a partial annihilation of Mn anti-sites, and Tcomp may pass from above room temperature to below, giving rise to an inverted TMR response.
Based on the Hall bar structure, current in plane (CIP) spin valves devices with MRG electrodes are also investigated. This includes the study of giant magnetoresistance (GMR) in MRG / Cu / NiFe multilayer structures versus the spacer layer Cu thickness, which enhances the understanding of the magnetotransport properties in the high spin-polarization MRG material. Various techniques like Electrically Detected Ferromagnetic Resonance (ED-FMR) will be conducted to study the spectral properties of spin waves in the ferromagnetic layer in the spin valves. These results will then be used to optimize the materials and geometries for THz spin transfer oscillators.
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Fig. 1. Crystal structure of C1b half-Heusler Mn2RuXGa
Fig. 2. TMR (voltage) scans at 300K and 10K for a sample annealed at 350⁰C (Tcomp ≈ 400K).
Fig. 3. TMR measurements on a sample with Tcomp ≈ 200K.