Photon Factory Activity Report 2018 #36 (2019)

BL-7A/2017G060

Relationship between degree of order and magnetic momentsin spin-gapless semiconductor CoFeMnSi Heusler alloy studied by XMCDJun Okabayashi1, Lakhan Bainsla2, Kazuya Suzuki2, 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, Japan1

Half-metals exhibit fully spin-polarized band
structures and therefore are suitable for spintronic
applications. The half-metallicity is seen in different
classes of materials, such as magnetic oxides, diluted
magnetic semiconductors, and Heusler alloys.
Cobalt-based Heusler alloys are very interesting due
to their theoretically predicted half-metallic electronicstructures and experimentally observed high spinpolarization with high Curie temperatures. Recently,
the quaternary Heusler alloys also have attracted
much attention because of their half-metallic and/or
spin gapless band structures [1]. Original andstoichiometric full-Heusler alloys have A2BC in theirchemical formulas, where A and B aretransition-metal elements and C is a main groupelement and those alloys have a cubic L21 crystalstructure. A quaternary Heusler alloy with a different
structural symmetry is obtained when A in A2BC
partially is replaced by another element A’, i.e.,
AA’BC. They also are named equiatomic quaternary
Heusler alloys. We focus on the structural andmagnetic properties for CoFeMnSi (CFMS) films,
which is a candidate of spin-gapless semiconductors.
We deduce the element-specific magnetic momentsof Mn, Fe, and Co using x-ray magnetic circulardichroism (XMCD) and discuss the ordering types ofCFMS.
The sample of 30-nm-thick CFMS were grownon MgO (001) substrates with Cr buffer. Sample
surfaces were capped by MgO layer. The MgO(001)substrate was ushed thermally at 700℃. All of thelayers were deposited at room temperature. The Cr
layer was in situ annealed at 700℃ for 1 h before theCFMS deposition. Samples with CFMS in situ
annealing at temperatures (Ta) of 300, 400, 500, and600℃ were prepared [2].
XMCD was performed at BL-7A in the Photon
Factory (KEK). Photon helicity was fixed, and amagnetic-field switching between ±10 kOe wasperformed along the incident polarized soft x-ray. A
total-electron-yield mode was adopted. The
measurements were carried out in a grazing incidence
setup to the sample surface normal in order to detect
the in-plane spin and orbital magnetic moments. All
the measurements were performed at room
temperature.
Figure 1 shows the x-ray absorption spectra(XAS) of Mn, Fe, and Co L2,3 edges, respectively, forTa =600℃. XAS are normalized by the photon fluxes.
Clear metallic peaks are observed, which confirmsthat there is no mixing of oxygen atoms in the thin
CFMS layer. Shoulder structures appear in the higher

photon energy region of Co L3 XAS peaks. Thesestructures correspond to the Heusler alloys due to theCo-Co bonding states within the molecular orbital
calculations. Element-specific hysteresis curves taken
at L3-edge photon energies are also plotted, whichindicates the in-plane easy axis in 30-nm-thick CFMS
film. The spin and orbital magnetic moments areestimated by applying the magneto-optical sum rules.
The magnetic moment summing both spin and orbital
components of each element is estimated to be 0.94,0.90, and 2.22 µB for Co, Fe, and Mn, respectively.
Total magnetic moment is 4.05 µB in this sample,
which is similar to the value for type-I ordered case in
the calculation [3]. We deduced the magnetic
moments depending on the annealing temperature.
The results clearly indicate the increase of magnetic
moments with annealing temperature.

Fig. 1, The x-ray absorption spectra with different
magnetic fields for the sample annealed at Ta =600℃ measured at the Mn, Fe, and Co L3,2 edges.
The XMCD spectra for the corresponding elements
are also shown. Insets show the element-specifichysteresis curves taken at each L3-edge photon
energy.
References[1] L. Bainsla and K. G. Suresh, Appl. Phys. Rev. 03,031101 (2016).
[2] L. Bainsla, J. Okabayashi et al., Phys. Rev. B 96,094404 (2017).
[3] X. Dai et al., J. Appl. Phys. 105, 07E901 (2009).
*jun@chem.s.u-tokyo.ac.jp