We report on the new polycrystalline exchange bias system MnN/CoFe, which shows exchange bias of up to 1800 Oe at room temperature with a coercive field around 600 Oe. The room temperature values of the interfacial exchange energy and the effective uniaxial anisotropy are estimated to be J eff = 0.41 mJ/m 2 and K eff = 37 kJ / m 3 . The thermal stability was found to be tunable by controlling the nitrogen content of the MnN. The maximum blocking temperature exceeds 325• C, however the median blocking temperature in the limit of thick MnN is 160• C. Good oxidation stability through self-passivation was observed, enabling the use of MnN in lithographically defined microstructures. As a proof-of-principle we demonstrate a simple GMR stack exchange biased with MnN, which shows clear separation between parallel and antiparallel magnetic states. These properties come along with a surprisingly simple manufacturing process for the MnN films.

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 ...

Electrical switching and readout of antiferromagnets allows to exploit the unique properties of antiferromagnetic materials in nanoscopic electronic devices. Here we report experiments on the spin-orbit torque induced electrical switching of a polycrystalline, metallic antiferromagnet with low anisotropy and high Néel temperature. We demonstrate the switching in a Ta / MnN / Pt trilayer system, deposited by (reactive) magnetron sputtering. The dependence of switching amplitude, efficiency, and relaxation are studied with respect to the MnN film thickness, sample temperature, and current density. Our findings are consistent with a thermal activation model and resemble to a large extent previous measurements on CuMnAs and Mn2Au, which exhibit similar switching characteristics due to an intrinsic spin-orbit torque.

We report an exchange bias of more than 2700 Oe at room temperature in MnN/CoFe bilayers after hightemperature annealing. We studied the dependence of exchange bias on the annealing temperature for different MnN thicknesses in detail and found that samples with t MnN > 32 nm show an increase of exchange bias for annealing temperatures higher than T A = 400 • C. Maximum exchange bias values exceeding 2000 Oe with reasonably small coercive fields around 600 Oe are achieved for t MnN = 42, 48 nm. The median blocking temperature of those systems is determined to be 180 • C after initial annealing at T A = 525 • C. X-ray diffraction measurements and Auger depth profiling show that the large increase of exchange bias after hightemperature annealing is accompanied by strong nitrogen diffusion into the Ta buffer layer of the stacks.

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We investigated the influence of doping antiferromagnetic MnN in polycrystalline MnN/CoFe exchange bias systems, showing high exchange bias of up to 1800 Oe at room temperature. The thermal stability of those systems is limited by nitrogen diffusion that occurs during annealing processes. In order to improve the thermal stability, defect energies of elements throughout the periodic table substituting Mn were calculated via density functional theory. Elements calculated to have negative defect energies bind nitrogen stronger to the lattice and could be able to prevent diffusion. We prepared exchange bias stacks with doping concentrations of a few percent by (reactive) co-sputtering, testing doping elements with defect energies ranging from highly negative to slightly positive. We show that doping with elements calculated to have negative defect energies indeed improves the thermal stability. Y doped MnN layers with doping concentrations below 2% result in systems that show exchange bias fields higher than 1000 Oe for annealing temperatures up to 485 • C.In spinelectronics, the exchange bias effect 1-5 is used to pin a ferromagnetic electrode to an antiferromagnetic layer. This is crucial in GMR or TMR stacks to allow for distinct stable resistance states 6 . For several years, the search for new antiferromagnetic materials for exchange bias has been going on in order to find rare-earth free alternatives for commonly used MnIr 7 or MnPt 8,9 . For integration into spinelectronic devices, the antiferromagnet should be easy to prepare, generate exchange bias fields that are clearly higher than corresponding coercive fields and be thermally stable at typical device operation temperatures. As we recently reported 10-12 , antiferromagnetic MnN is a very promising candidate. MnN crystallizes in the Θ−phase of the Mn-N phase diagram 13 , a tetragonal variant of the NaCl structure with a = b = 4.256Å and c = 4.189Å at room temperature 14 . The exact lattice constants depend on the nitrogen content in the lattice. With increasing nitrogen content, increasing lattice constants are observed 14,15 . Optimized polycrystalline MnN/CoFe bilayer systems show exchange bias of up to 1800 Oe at room temperature with an effective interfacial exchange energy of J eff = 0.41 mJ/m 2 and an effective uniaxial anisotropy constant of K eff = 37 kJ/m 3 10 . They yield ratios of H eb /H c significantly larger than one and are easy to prepare with sputter deposition at room temperature, satisfying earlier mentioned requirements for integration into spintronic devices. The Néel temperature of MnN is around 660 K 16 and MnN/CoFe systems show a median blocking temperature of 160 • C 10 . However, nitrogen diffusion at high temperatures, respectively long annealing times, limits the thermal stability of the system. In the course of our previous investigations 10 we already found that preparing MnN with a higher nitrogen concentration can slightly increase the thermal stability but at the same time lowers the exchange bias. In the present article, w...

Ta/MnN/CoFeB systems show high exchange bias of about 1800 Oe at room temperature; however, their thermal stability is limited by nitrogen diffusion that occurs during annealing processes [Quarterman et al., Phys. Rev. Mater. 3, 064413 (2019) and Dunz et al., AIP Adv. 8, 056304 (2018)]. In this study, we investigate the consequences of nitrogen diffusion in Ta/MnN/CoFeB exchange bias stacks in dependence on the Ta buffer layer thickness. Furthermore, we test the effects of introducing a TaNx layer between MnN and Ta as a diffusion barrier. Our findings show that the Ta buffer layer plays a decisive role in determining the exchange bias in the Ta/MnN/CoFeB system. It acts as a crystallographic seed layer for better growth of MnN and as a nitrogen sink during the annealing process. We show that both of these functions are crucial for the outcome of high exchange bias. Additionally, our results reveal that the measures decreasing nitrogen diffusion, even though being beneficial in terms of thermal stability, mostly lead to decreased crystallinity and thus weaker exchange bias.

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