Training effect in specular spin valves

Abstract

Specular spin valves show an enhanced giant magnetoresistive (GMR) ratio due to specular reflection in nano-oxide layers (NOLs) formed by the partial oxidation of CoFe pinned and free layers. The oxides that form the (pinned layer) NOL were recently shown to antiferromagnetically order at $T\ensuremath{\sim}175\text{ }\text{K}$. Here, we study the training effect (TE) in MnIr/CoFe/NOL/CoFe/Cu/CoFe/NOL/Ta specular spin valves in the 300--15 K temperature range. The exchange bias direction between the MnIr and CoFe layers impressed during annealing is taken as the positive direction. The training effect is observed in antiferromagnetic (AFM)/ferromagnetic (FM) exchange systems and related to the rearrangement of interfacial AFM spin structure with the number of hysteretic cycles performed $(n)$, resulting in the decrease of the exchange field $({H}_{\text{exch}})$. Here, in the studied specular spin valve, TE was only observed for $T<175\text{ }\text{K}$ and is thus related to the pinned layer NOL-AFM ordering and to the evolution of the corresponding spin structure with $n$. We show that FM spins that are strongly coupled to AFM domains do not align with the applied positive magnetic field $(H)$, giving rise to a residual MR at $H\ensuremath{\gg}0$. Such nonsaturating MR will be related with a spin-glass-like behavior of the interfacial magnetism induced by the nano-oxide layer. The observed dependence of the training effect on the field cooling procedure is also likely associated with the existence of different spin configurations available in the magnetically disordered oxide. Furthermore, anomalous magnetoresistance cycles measured after cooling runs under $\ensuremath{-}500\text{ }\text{Oe}$ are here related to induced NOL exchange bias/applied magnetic field misalignment. The temperature dependence of the training effect was obtained and fitted by using a recent theoretical model.