Bias field mediated vertical hysteresis loop shift in Ni80Fe20/SrRuO3 heterostructure for novel memory switching

Abstract

Exchange Bias (EB), based on the concept of hysteresis (MH) loop shift, investigated almost 70 years back, is one of the most studied phenomenon. The shift in the MH loop has garnered a good amount of interest due to the huge range of applications in spintronic devices. Vertical magnetization shift (VMS) of the MH loops has gained attention recently in exchange bias (EB) regime. But no concrete results have been found yet. We present a novel counter-intuitive approach to realizing the vertical MH loop shift in soft/hard heterostructure. A thin film of 13 unit cells (t= 5nm) of SrRuO3 (SRO) is grown epitaxially by pulsed laser deposition on SrTiO3 (STO) (100) substrate. On top of that, a polycrystalline Ni80Fe20 (Py) thin film of t=2nm is grown by DC/RF sputtering. We observe a clear and significant vertical MH loop shift at 2K after cooling from room temperature (300K) under a bias field (i.e. field cooled-FC) (Figure. a). There is a positive (negative) shift of the MH loop on applying a positive (negative) bias field of around 1T. Essentially, there is no vertical shift for zero field cooling (ZFC). This was only observed when the MH loop is measured with low field range (±500mT). For higher field range (e.g. ±7T) MH measurement, no loop shift is found for both ZFC and FC measurements. There is a threshold to field tracing range (< SRO’s saturation field) below which we can observe a vertical shift phenomenon only. The vertical shifts are observed only below 100K as well which is nearby to the Curie temperature of SRO (TC~160K) below which SRO becomes ferromagnetic. The reason of the shift is also ascribed to the anisotropy play of the two layers. Surprisingly, the coupling between Py and SRO here is exchange spring (ES) type rather than EB type, but simultaneously exhibiting strong EB like shift but in vertical direction. Moreover, the micromagnetic OOMMF simulations are performed, which are in broad agreement with the experimental results (Figure. b). We simulate for the same ratio of thicknesses of the two layers to manifest a generalized ground. The computational results with modelling serve as a verification of the experimental findings qualitatively and quantitatively. We make use of ES tenets to effectuate the novel phenomena of vertical shift. The vertical shift due to the ES interactions between the soft and the hard layer expands the territory of ES applications.