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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - PiezoSpin (Antiferromagnetic straintronics: towards an non-volatile all-voltage controlled memory device)

Teaser

Summary

Original outline of the project: The holy grail of magnetic storage research is the discovery of novel all-voltage controlled magnetic metamaterials that enable to develop a universal memory device that simultaneously meet high-power-efficiency and ultra-high storage capacity. Antiferromagnetic (AFM) materials could represent the future of spintronic applications because of the numerous interesting features they combine, e.g. they are robust against perturbation due to magnetic fields, produce no stray fields, display ultrafast dynamics and are capable of generating large magneto-transport effects. The PiezoSpin project sought to demonstrate a novel proof-of-concept for an innovative universal ultra-high power-efficient AFM-based spintronics memory device, which has the potential for transforming the ferromagnetic-dominated magnetic data storage technology. We envisaged to produce hybrid FeRh-based alloy/ferroelectric (FE) heterostructures, where the FeRh-based alloy overlayer would act as a resistive-switch driven by the FE underlayer acting as a voltage-controlled actuator. We aimed to investigate the strain-dependent electrical resistivity of the FeRh-based alloy on the applied voltage and to optimize such piezo-magnetoresistivity effect. This novel hybrid device concept would exploit the strain-dependent giant magnetoresistance that appears in spin-orbit coupled (anisotropic) AFMs. Such piezo-magnetoresistivity effect is likely shared by numerous AFMs, opening up a new research field, i.e. AFM-based strain-electronics, where the material combinations and functionalities to explore are immense.

Work performed

Unforeseen issue found: The work here presented shows that sputter-grown FeRh thin films deposited directly onto FE substrates like PMN-PT present a deficient crystallinity and a degraded first-order magnetic phase transition (FOMPT), which makes these FeRh/FE hybrid heterostructures unsuitable for investigating the foreseen piezo-magnetoresistivity effect. In order to promote the onset of the B2 ordered crystallographic phase in FeRh alloys, which is the only one that undergoes the FOMPT, these films have to be annealed at a temperature of around 700C or above. At high temperatures, the Ti-O bonds in the PMN-PT stressor weaken and hence the oxygen diffusion coefficient massively increases, which ultimately causes the chemical degradation of the metallic overlayer. Attempts to grow a buffer layer in between the PMN-PT substrate and the FeRh alloy film, aiming to block the oxygen diffusion from the stressor into the metallic layer, did not produce the expected results.
Overall project aim, contingency measures implemented to deliver objectives and work undertaken: An alternative manner to investigate the piezo-magnetoresistivity consists of growing FeRh alloys films with different thicknesses onto (001)-MgO and (0001)-Al2O3 substrates. These substrates induce dissimilar strained states in the deposited FeRh layers, so that the former produces a tetragonal distortion of the cubic FeRh lattice and introduces an in-plane compressive stress, whilst the later imposes an orthorhombic distortion to the FeRh overlayer and gives rise to an in-plane tensile stress. The strain introduced into the FeRh film can be finely tuned by varying its thickness.
Firstly, the Fellow produced a series of sputter-grown FeRh films onto MgO in order to optimize the sputter gas pressure. A large series of FeRh films deposited onto (001)-oriented MgO and (0001)-oriented single crystals substrates were sputter-grown, whose thickness ranged from a 3.5 nm up to 170 nm. Another set of FeRh films deposited onto MgO substrates with a constant thickness, ie approx. 42 nm, and variable thickness (from 20 nm up to 80 nm) were produced as a function of the deposition rate. To determine film thickness, lattice parameters, strain, microstrain, grain size, mosaicity, chemical order parameter and overall crystallinity, all films were characterized using x-ray reflectometry and diffraction. The FOMPT was characterized using a resistivity technique in all films. Atomic and magnetic force microscopy at room temperature was used on a set of ultrathin films to characterize surface morphology and magnetic domains.

Final results

1.â€” The existence of the piezo-magnetoresistance effect in strained FeRh thin films, ie the strain-dependent resistivity change associated to the onset of AF order, has been demonstrated and quantified.
The Fellow has demonstrated that the resistivityâ€™s change across the FOMPT is thickness dependent, increasing by more than a 70% as the tetragonal distortion increases in FeRh/MgO, and decreasing by more than a 20% as the rhombohedral distortion increases in FeRh/Al2O3.
This research output paves the way for, after some cultivation of the piezo-magnetoresistivity effect that emerge in AFMs, its implementation in tailored energy-efficient AFM-based memory devices.
2.â€”The origin of the giant magnetoresistance change across the first-order AF-FM phase transition in FeRh alloys has been demonstrated that is the arising of superzone band-gaps, resulting from the onset of the AF phase.
The Fellow conceived experiments that demonstrate the linear relationship between the change in resistivity and the temperature at which the FOMPT occurs, the anisotropy of the resistivity change across the FOMPT and performed the modelling of the resistivity in FeRh using the Elliott-Wedgwood model, which are all robust evidence of the superzone band-gaps origin of the magnetoresistance in FeRh alloys.
This discovery is instrumental to optimize and engineer the piezo-magnetoresistivity effect in AFM materials.
A poster reporting on these results was presented at the IoP Magnetism Conference 2019 held in Leeds, UK.
3.â€”Atom-peening engineering preserves the functionality of the first-order AF-FM phase transition in ultrathin sub-15-nm-thick FeRh alloy films.
The functionality of the first-order transition in these ultrathin films is preserved even for films as thin as 3.7 nm. The magneto-electric and structural characterization reveals that the magnetic properties of ultrathin FeRh alloys are dominated by the strain relaxation process resulting from the competition between surface and epitaxial volume-alike strains in the system, which is dislocation-free.
This research output is central for the implementation of FeRh/FePt heterostructures in heat-assisted-magnetic-recoding technology.
A paper reporting on the sputter-engineering of the FOMPT in ultrathin FeRh alloy films has been submitted for publication to Physical Review Materials and is currently under review. A paper reporting on the surface-induced-disorder origin of the surface FM layer at the FeRh/vacuum interface and magnetic domains in ultrathin FeRh films is currently been drafted. A poster reporting on these results was presented at the JEMSâ€™2019 Conference held in Uppsala, Sweden.

Figure 1 caption: Atomic force micrographs (AFM) taken on ultrathin FeRh alloy films with FeRh thickness 14.1 nm, (a) and (d), 9.2 nm, (b) and (e), and 3.7 nm, (c) and (f). AFM scan size is 50 um (top row) and 0.5um (bottom row); micrographs edges are aligned along the [110] MgO [100] FeRh and [-110] MgO [010] FeRh directions.

Figure 2 caption: Magnetisation as a function of temperature for FeRh alloy films with thickness 9.2, 8.1, 5.2 and 3.7 nm for an applied magnetic field, u0Happ= 0.1 T, so that H || [110].