Pseudo spin‐valve structures with NiO/Co as a hard magnetic layer

Abstract

PACS 75.47. De, 75.70.Cn A key problem in some applications of pseudo spin-valve structures (PSV) is the choice of appropriate materials for soft and hard ferromagnetic layers and their thickness to ensure a high difference in the switching fields. We show that this can be achieved using a NiO/Co structure as a hard magnetic layer. By varying the thickness of the constituent layers, good symmetrical switching characteristics have been shown in PSV structures with NiO/Co as a hard magnetic layer. The most important features of our PSV structures with suitable selected thickness of particular layers are: a relatively high value of GMR amplitude (up to 9% at RT) due to a possible electron specular reflection at the NiO/Co interfaces, a high coercivity (~500 Oe) of hard magnetic layer resulted from a specific interaction of a 15 nm thick antiferromagnetic NiO with a 2 nm thick Co layer, nearly symmetrical magnetoresistance transfer curves MR(H) due to negligible unidirectional anisotropy field of NiO/Co bilayer with NiO thickness <15 nm. 1 Introduction Pseudo spin-valves (PSV) comprising soft and hard ferromagnetic layers separated by a nonferromagnetic spacer are interesting due to their potential application in magnetoresistive sensors and magnetic random access memories [1]. The soft magnetic layer switches at a smaller external field than the hard layer, which gives rise to antiparallel alignment of magnetization correlated with a high resistance state [2]. For some applications a key problem in PSV is the choice of appropriate ferromagnetic materials for the hard and soft layers to ensure a high difference in the switching fields and large magnetoresistance (MR) ratios. There have been various attempts to optimize PSV by increasing the difference in coercivity [3], by changing the layer thickness [4], or by optimizing sputtering technology [5]. In the present study we show how to optimize the switching fields and MR ratio in NiO/Co/Cu/Co PSV by changing the thickness of constituent layers. A stress is put on optimizing the magnetic characteristics of the Co layer exchange coupled (pinned) to NiO so that the NiO/Co bilayer forms the hard magnetic layer. In particular, the influence of NiO is the most important since NiO is known to induce unidirectional anisotropy and to enhance the coercive field of the pinned Co layer [6] due to a specific ferromagnetic/antiferromagnetic coupling. Moreover, there is an increase of GMR effect due to specular electron reflection at NiO/Co interface [7].