Enhanced exchange bias in MnN/CoFe bilayers after high-temperature annealing

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

Improved thermal stability in doped MnN/CoFe exchange bias systems

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{\deg} C

Spin-orbit torque induced electrical switching of antiferromagnetic MnN

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 N\'eel 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 Mn$_2$Au, which exhibit similar switching characteristics due to an intrinsic spin-orbit torque.Comment: 7 pages, 5 figure

Role of the Ta buffer layer in Ta/MnN/CoFeB stacks for maximizing exchange bias

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

Enhanced exchange bias in MnN/CoFe bilayers after high-temperature annealing

Dunz M, Schmalhorst J-M, Meinert M. Enhanced exchange bias in MnN/CoFe bilayers after high-temperature annealing. AIP ADVANCES. 2018;8(5): 5.We report an exchange bias of more than 2700 Oe at room temperature in MnN/CoFe bilayers after high-temperature 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) > 32nm show an increase of exchange bias for annealing temperatures higher than T-A = 400 degrees 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 degrees C after initial annealing at T-A = 525 degrees C. X-ray diffraction measurements and Auger depth profiling show that the large increase of exchange bias after high-temperature annealing is accompanied by strong nitrogen diffusion into the Ta buffer layer of the stacks. (c) 2017 Author(s)

Giant perpendicular exchange bias with antiferromagnetic MnN

Zilske P, Graulich D, Dunz M, Meinert M. Giant perpendicular exchange bias with antiferromagnetic MnN. Applied Physics Letters. 2017;110(19): 192402.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 SiOx 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 as functions of both film thicknesses and field cooling temperatures. In optimized film stacks, a giant perpendicular exchange bias of 3600Oe 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. Published by AIP Publishing

Large exchange bias in polycrystalline MnN/CoFe bilayers at room temperature

Meinert M, Bueker B, Graulich D, Dunz M. Large exchange bias in polycrystalline MnN/CoFe bilayers at room temperature. Physical Review B. 2015;92(14): 144408.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 MnN. The maximum blocking temperature exceeds 325 degrees C, however the median blocking temperature in the limit of thick MnN is 160 degrees 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 giant magnetoresistance 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

Effects of field annealing on MnN/CoFeB exchange bias systems

Quarterman P, Hallsteinsen I, Dunz M, et al. Effects of field annealing on MnN/CoFeB exchange bias systems. PHYSICAL REVIEW MATERIALS. 2019;3(6): 064413.We report the effects of nitrogen diffusion on exchange bias in MnN/CoFeB heterostructures as a function of MnN thickness and field-annealing temperature. We find that competing effects occur in which high-temperature annealing enhances exchange bias in heterostructures with thick MnN through improved crystallinity, but in thinner samples this annealing ultimately eliminates the exchange bias due to nitrogen deficiency. Using polarized neutron reflectometry and magnetic x-ray spectroscopy, we directly observe increasing amounts of nitrogen migration from MnN into the underlying Ta seed layer with increased annealing temperature. In heterostructures with thin MnN layers, the resulting nitrogen deficiency becomes significant enough to alter the antiferromagnetic state before the Ta seed layer is nitrogen saturated. Furthermore, we observe intermixing at the MnN/CoFeB interface which is attributed to the nitrogen deficiency creating vacancies in the MnN layer after annealing in a field. This intermixing of Mn with Co and Fe is not believed to be the cause for loss of exchange bias when the MnN layer is too thin but is instead a secondary effect due to increased vacancies after nitrogen migration