Improved thermal stability in doped MnN/CoFe exchange bias systems

Abstract

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...

Improving thermal stability of MnN/CoFeB exchange bias systems by optimizing the Ta buffer layer

“…Possibly, those samples exhibit a maximum of exchange bias at temperatures T A > 550 • C, followed by a collapse, that might also be shifted to higher temperatures. The minimum of exchange bias marking the transition from temperature range I to II has also been observed in all our previous studies on MnN exchange bias systems [10,12,19]. At this point we are not sure what causes this behavior, but it is likely that a magnetic ordering transition is happening at annealing temperatures around T A = 250 • C. The fact that all samples show similar values of exchange bias in the first regime could hint at the formation of a spin glass structure in the MnN [21,22].…”

“…In the course of our previous investigations [10] we already found that preparing MnN with a higher nitrogen concentration can * mdunz@physik.uni-bielefeld.de slightly increase the thermal stability but at the same time lowers the exchange bias. Similar results are obtained when doping MnN with elements that have stronger bonds with nitrogen [19].…”

“…1a) displays the dependence of exchange bias on the annealing temperature for samples with Ta thicknesses varying between 1 nm and 15 nm. As we chose CoFeB as a ferromagnetic layer instead of CoFe, the total exchange bias values are somewhat smaller and the optimized sample with the commonly used 10 nm Ta slightly deviates from the results observed in our previous studies [10,12,19]. Notably, independent of the Ta thickness, all samples follow a similar behavior at low annealing temperatures.…”

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

“…Doping has been shown to enhance the thermal stability of MnN/CoFe exchangebias systems [19]. Undoped samples with structure Ta(10 nm)/MnN(30 nm)/Co 70 Fe 30 / (1.6 nm)/Ta(0.5 nm)/Ta 2 O 5 (5 nm) were deposited on thermally oxidized Si wafers.…”