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.
