Photon Factory Activity Report 2018 #36 (2019)

BL-7A/2017G060

Interfacial Exchange Coupling between Co and MnGa
Studied by X-ray Magnetic Circular Dichroism
Jun Okabayashi,1* Kazuya Suzuki,2 and Shigemi Mizukami2
Research Center for Spectrochemistry, The University of Tokyo, 113-0033 Tokyo, Japan
2
WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
1

Magnetic ordered alloys have attracted significant
attention for use as spintronics materials because they
are highly likely to exhibit perpendicular magneticanisotropy (PMA). As promising spintronics materials,tetragonal L10-type and D022-type Mn3Ga compounds
have been extensively investigated originating from
their
ferrimagnetic
properties
that
cause
anti-ferromagnetic coupling at the different Mn sites;
thus, they have high potentials for use in magnetic
tunnel junctions integrated in tunnel magnetoresistance(TMR) devices [1].
Using the advantage that Mn3-δGa is a hardmagnetic film, the deposition of other ferromagnetic
materials on Mn3-δGa layers can be used to induce
perpendicular exchange coupling through exchangeinteractions without the use of heavy metal elements.
Ultra-thin Fe1-xCox layers deposited on Mn3-δGa couple
ferromagnetically or anti-ferromagnetically, dependingon their compositions [2]. Antiferromagnetic couplinghas been demonstrated in high Co concentration
regions, while low Co concentration regions havebeen shown to exhibit ferromagnetic coupling. However,
the abruptness and element-specific magnetic properties
at the interfaces between Mn3-δGa and Fe1-xCox layers
must be clarified explicitly. Here, X-ray magnetic
circular dichroism (XMCD) is employed to investigate
the element-specific magnetic properties at a Co/
Mn1.5Ga interface. In particular, we discuss theinterfacial coupling, which may be ferromagnetic orantiferromagnetic depending on the annealing of thesamples.
The samples were prepared by magnetron
sputtering on MgO substrates. On the 30-nm-thick
MnGa, 1-nm Co were deposited at room temperatureand capped by 2-nm MgO. We prepared two samples ofas-grown and annealed at 350 C after the growth. The
X-ray absorption spectroscopy (XAS) and XMCD wereperformed at BL-7A in the Photon Factory (KEK). The
total-electron-yield mode was adopted, and allmeasurements were performed at room temperature andthe geometries were set to normal incidence
configuration.
Figure 1 shows the polarization dependences of theXAS and XMCD results for the Mn and Co L2,3 edges
after the annealing. By comparing these spectral line
shapes with those in the as-grown case, it is evident that
the XAS intensity ratios between Mn and Co aremodulated, which suggests that the Co atoms diffuse
chemically into the MnGa layer within the probing
depth of approximately 5 nm. Opposite XMCD signs
are observed in Co as shown in Fig. 1 because of theanti-parallel coupling between Co and MnGa layers.

We can discuss the magnetic properties at theinterfaces before and after the annealing in terms of
three aspects: the exchange coupling at the interfaces,
the PMA at the Co/MgO interface, and the chemical
reactions at the interfaces. When parallel coupling isdominant, the strong perpendicular magnetization in theCo layer is induced by exchange coupling with Mn3-δGaaccompanied by the large coersive fields of the Co
hysteresis curves. By annealing, anti-parallel exchange
coupling becomes dominant through the interfacial layer,that separates the two magnetic layers. The annealing
causes interfacial secondary phase formation within-plane anisotropy, which is not observable in atransmission electron microscopy image but clearly
detected by the XMCD line shapes and element-specifichysteresis curves. Heusler-type alloys such as Mn2CoGa
might be stabilized. Although the annealing process at
350℃ is necessary to obtain a high quality Co/MgO
interface, it inevitably produces an interfacial layer at
the Mn1.5Ga /Co boundary, which contributes slightly tothe TMR properties [3].

Fig. 1. (a) XAS spectra of Mn and Co L-edges in
Co/Mn1.5Ga annealed sample. (b) XMCDs of Mnand Co L-edges. Insets show the hysteresis curves
taken at L3-edge energies.
References[1] S. Mizukami et al., Phys. Rev. B 84, 014416(2012).
[2] Q. L. Ma et al., Phys. Rev. Lett. 112, 157202(2014)[3] J. Okabayashi et al., J. Mag. Mag. Mater. 460,418 (2018).
*jun@chem.s.u-tokyo.ac.jp