Giant perpendicular exchange bias with antiferromagnetic MnN

Abstract

We investigated an out-of-plane exchange bias system that is based on the antiferromagnet MnN. Polycrystalline, highly textured film stacks of Ta / MnN / CoFeB / MgO / Ta were grown on SiO x by (reactive) magnetron sputtering and studied by x-ray diffraction and Kerr magnetometry. Nontrivial modifications of the exchange bias and the perpendicular magnetic anisotropy were observed both as functions of film thicknesses as well as field cooling temperatures. In optimized film stacks, a giant perpendicular exchange bias of 3600 Oe and a coercive field of 350 Oe were observed at room temperature. The effective interfacial exchange energy is estimated to be J eff = 0.24 mJ/m 2 and the effective uniaxial anisotropy constant of the antiferromagnet is K eff = 24 kJ/m 3 . The maximum effective perpendicular anisotropy field of the CoFeB layer is H ani = 3400 Oe. These values are larger than any previously reported values. These results possibly open a route to magnetically stable, exchange biased perpendicularly magnetized spin valves.Spin electronics allows to realize nonvolatile fast lowpower computer memory and is well established in hard disk drive read heads and magnetic sensors.1,2 The key component in spin electronic devices, a magnetoresistive element using either giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR), is composed of two magnetic films: a free sense layer and a fixed reference layer. The magnetization of the ferromagnetic free layer follows external magnetic fields or can be switched by a current via the spin transfer torque. The reference layer has to be stable against external fields to allow for different magnetic alignments of the two layers, which give rise to the magnetoresistance. The reference layer is typically created by pinning a thin ferromagnetic (FM) film to an antiferromagnetic (AFM) film via the exchange bias (EB) effect.3-9 In a typical device, the magnetic hysteresis loop of the reference layer is shifted by the exchange bias to fields that are not encountered during normal device operation.Thin films with perpendicular magnetic anisotropy (PMA) are of great interest for spintronic devices. The tunable anisotropy energy allows to enhance the thermal stability of the magnetization and lower critical current densities for the spin-transfer torque switching are achievable as compared to in-plane magnetized systems.10-12 Thus, interest in systems showing perpendicular EB (PEB) increased as well. There are several studies about (Co/Pt) n and (Co/Pd) n multilayer systems coupled with an AFM such as IrMn or FeMn. 13-17However, the reported perpendicular exchange bias field values H eb are similar to the coercive field H c , making these systems not attractive for practical applications that require H eb H c . Chen et al. In the present article, we report on an exchange bias system that is based on antiferromagnetic MnN. It crystallizes in the θ-phase of the Mn-N phase diagram, 20 which crystallizes in the tetragonal variant of the NaCl structure with a = 4.256Å and c = 4.189Å ...

Spin-orbit torque induced electrical switching of antiferromagnetic MnN

“…However, the spin orientation is controversial and might depend critically on the lattice constants. In previous studies, some of the authors of this article have shown its utility for exchange bias applications with large exchange bias fields at room temperature [22][23][24][25][26][27]. However, the critical thickness for the onset of exchange bias was observed to be around 10 nm at room temperature, leading to the conclusion that MnN has a small magnetocrystalline anisotropy energy density [22].…”

Recent Developments on MnN for Spintronic Applications

“…Hence, the possibility of achieving a large out-of-plane loop shift using MnN is of great interest. Zilske et al [23] reported a giant perpendicular exchange bias of 3.6 kOe at room temperature in MnN/CoFeB bilayers. The samples had a similar composition and identical deposition conditions to some of the in-plane samples described earlier.…”

Study the exchange bias field in ferromagnetic film on antiferromagnetic substrate

“…Для ультратонкого тонкого слоя CoFeB толщиной 0.65 нм в системе MnN / CoFeB / MgO / TA может быть достигнуто значение h EB = 3600 Э [24].…”