Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The choice of proper materials for magnetic tunnel junctions (MTJs) for storage and communication applications (like MRAM or spin-transfer-torque nano-oscillators (STNOs)) is always an issue. On the one hand, the magnetic layers should exhibit as little stray field as possible and be mostly insensitive to the external magnetic field. On another hand, in order to maximize the tunneling magnetoresistance (TMR) ratio, these materials should provide high spin polarization, or even ideally possess half-metallic properties. An option which satisfies both criteria are the compensated half metallic ferrimagnets (CHFMs) — a class of materials predicted in 1995 by van Leuken and de Groot. CHFMs are materials which behave as antiferromagnets (AFMs) with respect to external magnetic fields, since the magnetic moments of the two sublattices compensate, while simultaneously exhibiting half-metal behavior from the point of view of magnetotransport. Experimentally, the first identified zero-moment half-metal was Mn₂RuₓGa (MRG) in 2014. It was already known that Mn-based Heusler compounds possess huge uniaxial anisotropy fields (exceeding tens of teslas); this, together with their vanishing magnetization, lead to resonance frequencies of several hundred GHz in such materials, making them very attractive candidates for STNOs in the sub-THz range. Such devices, due to the much higher bandwidth accessible, are expected to open the way for remote hospitals, 3-D remote meetings and much more.
Earlier MRG studies have already shown that these materials exhibit tunable magnetic properties. Indeed, the compensation temperature varies between 2 and 450K, depending on the Ru concentration. They also yield giant spontaneous Hall angle (7.7%). MRG has also been successfully integrated into perpendicular MRG/MgO/CoFeB MTJs, with low-bias TMR reaching up to 40 % at 10 K and 7 % at 300 K.
As the low value of TMR was attributed to diffusion of Mn atoms inside the MgO barrier, here, we investigate the effect of different insertion layers introduced between MRG and MgO on the magnetic properties and transport of MTJs. Mn₂Ruₓ Ga (23)/insertion layer(t)/MgO(1.7)/CoFeB(1)/Ta(0.3)/CoFeB(0.9)MgO(0.7)/Ta(3)/Ru(4) multilayers were deposited using a “Shamrock” fully automated sputter deposition tool (thickness given in nm). Mn₂RuₓGa was grown by co-sputtering from a Mn₂Ga and a Ru target. Different MRG compositions (Mn₂Ru1.1Ga, Mn₂Ru0.9Ga, Mn₂Ru0.75Ga, and Mn₂Ru0.65Ga) have been obtained by varying the sputtering power of Mn₂Ga while keeping the sputtering power of Ru constant. Changing Ru concentration in MRG allows adjusting the compensation temperature Tcomp from 2 to 450 K.
We fabricated MTJs without insertion layers, as well as stacks with Ta (0.3 nm, 0.6 nm, 0.9 nm) and Al (0.3 nm, 0.6 nm, 0.9 nm) insertion layers. The switching properties of MTJs were analyzed through magnetotransport measurements as a function of applied bias voltage at room temperature. Al 0.6 nm acts as the best diffusion barrier. Magnetic properties of the multi-layers were characterized by the quantum design superconducting quantum interference device (SQUID) with a maximal applied field of 7n T at the range of temperatures from 60 K to 300 T.
The magnetometry data was extracted from the typical out-of-plane hysteresis loop of the investigated MTJs (Fig.1). As the magnetic field is swept from +7 T to – 7 T, the magnetic moment of CoFeB starts to rotate first and switches close to 0 T. The sharp jump observed at -0.4 T is attributed to the reversal of MRG magnetization. With conducting the same measurements at different temperatures, it is possible to detect the compensation temperature of MRG, which will lead to a decrease of its magnetic moment and a divergence of the coercive field. The temperature with zero magnetic moment and extremely high Hc corresponds to the compensation point of MRG. In Fig.2 the temperature dependence of magnetic properties of MTJs with the same MRG composition, but different diffusion barriers, is presented. For different insertion layers, Tcomp can shift over a large range, showing that the choice of insertion layer can have a dramatic effect on the properties of MRG. For instance, in MTJs with no insertion layer 100 K < Tcomp < 160 K; the shift to the higher temperatures is observed for Ta 0.3 nm insertion (140 K < Tcomp < 200 K), and to the lower temperatures with Al 0.6 nm insertion (Tcomp < 120 K). Moreover, we demonstrate that Tcomp can also be altered by post-annealing, as a 20 K shift is observed after annealing at 325°C for 1 hour.
Mn₂RuₓGa integrated into MTJs demonstrates a low magnetic moment, high coercivity, and thereby high immunity to the applied magnetic field over a broad temperature range (60 K – 300 K). At the same time, these MTJs show TMR even at the compensation temperature, highlighting a fundamental difference between an AFM and a CHFM. All these make MRG extremely attractive for spintronics applications, and for the excitation of magnetic resonances in STNOs.