Mn2Au: body centered tetragonal bimetallic antiferromagnet grown by
molecular beam epitaxy
Han-Chun Wu*, Zhi-Min Liao, R.G.S. Sofin, Gen Feng, Xiu-Mei Ma, Alexander B. Shick,
Oleg N. Mryasov, and Igor V. Shvets
CRANN, School of Physics, Trinity College Dublin, Dublin 2, Ireland
Abstract: Mn2Au, a layered bimetal, has been successfully grown using MBE technique in AFM/FM
bilayer setting. Our experiments and first principle calculations suggest that Mn2Au film is an
antiferromagnet with a much lower critical temperature compared with the theoretical prediction for
bulk Mn2Au. First principle calculations indicate that the reduced critical temperature is likely due to
the interface between Mn2Au and Fe which was affected by local increase of d-states concentration.
Our experimental and theoretical studies establish a primary basis for further research of this new
antiferromagnet with bct structure in the context of different spin-electronic device applications such
as novel spin valves with PMA free layer and antiferromagnetic spin-electronic devices.
Antiferromagnetic (AFM) materials are broadly used in magneto-electronic devices such as
spin-valves (SVs). Despite their technological and research importance, AFM materials are much less
investigated than their ferromagnetic (FM) counterparts and are much more difficult to identify due to
zero net magnetization. Recently, significant effort has been devoted to harness the potential of these
materials in so-called antiferromagnetic spintronics where the ferromagnetic electrodes are replaced
by antiferromagets. Hals et al. theoretically studied current-induced dynamics in an antiferromagnet.
Park et al. experimentally demonstrated a more than 100% spin-valve-like signal in an IrMn-based
tunnel junction by measuring the tunnelling anisotropic magnetoresistance. Shvets et al. discussed
spin-dependent tunneling via an antiferromagnetic dielectric layer. Due to large spin-orbit coupling
on the 5d shell of its Au atoms, Mn2Au, a layered bimetallic material, has been proposed as an
interesting candidate for this emerging antiferromagnetic spintronic devices. In addition, Mn2Au
unlike other well established AFMs has a body centered tetragonal (bct) structure which may benefit
the thin film growth. The tetragonal structure of this material may be suitable for development of
novel spin valves incorporating, for example, a low symmetry tetragonal free layer with perpendicular
magnetic anisotropy (PMA) such as Mn(3-x)Ga. In this case the exchange bias effect in FM/AFM
bilayers due to lower symmetry (bct) antiferromagnet is also of significant interest both from applied
and fundamental research points of view.
Mn2Au has been recently discussed and determined, on the basis of first-principles calculations
of exchange coupling constants, to be a robust antiferromagnet with a very large Néel temperature in
excess of 1500 K. However, there are no experimental studies so far confirming AFM nature of
magnetic ordering in this material. It is interesting to note that in early reports the Mn2Au alloy was
experimentally identified to be a nonmagnetic material based on
197
Au Mössbauer spectra and
magnetization measurements. In order to utilize Mn2Au as an antiferromagnet for the purposes
mentioned above and other applications, it is important to prepare Mn2Au thin film samples of high
quality, to clarify its magnetic nature.
Figure 1 Temperature dependent M(H) loops for Mn2Au (10 nm)/Fe bilayers with (a) 3 nm, (b) 5 nm, (c) 8 nm,
and (d) 10 nm Fe on MgO substrate. Temperature dependent M (H) loops (e) for 10 nm thick Fe on MgO
substrate and (f) M (H) loop of a Fe (10 nm) /MgO (3 nm)/Fe (10 nm)/Mn2Au (10 nm) spin valve structure
measured at 5 K. Inset, M (H) loop for a Mn2Au (10 nm)/Fe bilayer with 10 nm thick Fe at 5 K.
The Mn2Au/Fe bilayers were grown on MgO (001) single crystal substrates using a MBE
system (DCA MBE M600, Finland) with a base pressure of 5×10-10 Torr. The structural properties of
the Mn2Au were further characterized by XRD and XPS. Figures 2(a)-(d) show the temperature
dependent M (H) loops for Mn2Au (10 nm)/Fe bilayers with 3 nm, 5 nm, 8 nm, and 10 nm thick Fe
layers, respectively. The magnetic field during M (H) measurements was applied in the plane of films
along the field cooling direction (<001> of MgO). One can clearly see from Figures 2(a)-(d) that all
M (H) loops shift to the negative field side and the coercivity depends on temperature and also on Fe
thickness. It is clear that the coercivity enhancement is due to the exchange interaction between
Mn2Au and Fe. Thus, the Mn2Au thin film is an antiferromagnet with a negative exchange bias. The
blocking temperature estimated from the exchange bias effect is around 350 K.
This work is published in: Han-Chun Wu, Zhi-Min Liao, R.G.S. Sofin, Gen Feng, Xiu-Mei Ma,
Alexander B. Shick, Oleg N. Mryasov, and Igor V. Shvets, Mn2Au: body centered tetragonal
bimetallic antiferromagnet grown by molecular beam epitaxy, Advanced Materials 24, 6374 (2012)