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

Abstract

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.

Magnetism and exchange-bias effect at the MnN/Fe interface

“…Interestingly, a sizable Dzyaloshinskii-Moriya interaction appears that gives rise to large EB. Studying the thickness dependence of the EB effect, we find that a rather large AFM thickness is needed to stabilize the EB, in agreement with recent experiments [17].…”

“…FeMn and NiMn are known alternatives that also produce an EB effect, but the ratios of the exchange bias to the coercive fields are less ideal for technological applications [14]. Because of this technological relevance, the quest for new antiferromagnetic materials avoiding scarce elements is an important topic of modern material research [16], and, in a recent work of Meinert et al [17], it was shown that in the case of polycrystalline MnN/CoFe interfaces a strong exchange-bias effect exists. Although this material combination displays a nonmonotonic dependence of the EB field on both the AF and FM thicknesses, this work indicates the possibility to use MnN as antiferromagnetic material in future magnetic devices.…”

“…Motivated by the experimental study in Ref. [17], we investigate the magnetic properties of the θ -phase of bulk MnN and the MnN/Fe interface at zero and finite temperatures using a multiscale model: we first calculate spin-model parameters ab initio and subsequently perform atomistic spindynamic simulations. We find that the magnetic ground state of MnN is AF-I type, in agreement with previous theoretical investigations [18,21], and the calculated Néel temperature agrees well with the experimental value.…”

Exchange interactions and Curie temperatures of the tetrametal nitrides Cr4N, Mn4N, Fe4N, Co4N, and Ni4N

“…Exchange bias stacks with γ -Fe 4 N and rocksalt CoN as the antiferromagnet show negative exchange bias. Very large exchange bias at room temperature was observed in stacks with body-centered tetragonal MnN as the antiferromagnet [5]. Ferrimagnetic Mn 4 N has a low magnetic moment and a high Curie temperature [6] and exhibits perpendicular magnetization on some substrates [7], making it an ideal candidate for an electrode material in STT switching devices.…”

Giant perpendicular exchange bias with antiferromagnetic MnN

“…27 Large exchange bias up to 1800 Oe at room temperature was observed with an effective interfacial exchange energy of J eff = t CoFe M CoFe µ 0 H eb = 0.41 mJ/m 2 , an effective uniaxial anisotropy constant of K eff = J eff /t crit = 37 kJ/m 3 , and a median blocking temperature of 160…”