Photon Factory Activity Report 2019 #37 (2020)

BL-7A / 2019G028
Relationship between degree of order and magnetic moments
in candidate of spin-gapless semiconductor CoFeCrAl studied by XMCDJun Okabayashi1 , Tomoki Tsuchiya2,3 , and Shigemi Mizukami3,2
1

Research Center for Spectrochemistry, The University of Tokyo, 113-0033 Tokyo, Japan
Center for Science and Innovation in Spintronics (Core Research Cluster), Organization
for Advanced Studies, Tohoku University, Sendai 980-8577, Japan
3
WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan2

Half-metals exhibit fully spin-polarized band structures and therefore are suitable for spintronic applications. The half-metallicity is seen in diﬀerent classesof materials, such as magnetic oxides, diluted magnetic semiconductors, and Heusler alloys. Cobaltbased Heusler alloys are very interesting due to their
theoretically predicted half-metallic electronic structures and experimentally observed high spin polarization with high Curie temperatures [1]. Spin-gaplesssemiconductors (SGS) are recognized as a candidateof ideal room-temperature ferromagnetic semiconductors. Bainsla et al. have investigated the growth andcharacterization of SGS in the quaternary Heusler alloy CoFeMnSi [2]. Other SGS candidates are also required. Here, we focus on the quaternary CoFeCrAl(CFCA) films.
Original and stoichiometric full-Heusler alloys have
A2 BC in their chemical formulas, where A and B aretransition-metal elements and C is a main group element and those alloys have a cubic L21 crystal structure [Fig. 1(a)]. A quaternary Heusler alloy witha diﬀerent structural symmetry is obtained when Ain A2 BC partially is replaced by another element À,i.e., AÀBC, as shown inFig. 1(b). The crystal structure is defined as a LiMgPdSn prototype or Y structure. Depending on the occupation of various latticesites, diﬀerent types of Y structure are possible for
CFCA. One is defined as AÀ consists of Fe, Co (BCis Cr, Al). The other is defined as AÀ consists ofCo, Cr (BC is Fe, Al) by swapping between AÀ andBC layers. The SGS is defined as a presence of apseudo energy band gap in the majority spin channel at the Fermi level. The density of states for halfmetals and SGS are given in Fig. 1(c) for comparison.
Due to their unique band structures, SGS have been
predicted to possess interesting properties, such as (i)spin-polarized current resulting from the electrons aswell as holes; (ii) high spin polarization; and (iii) tunable spin polarization by voltage varying the Fermi
level [3].
In this report, we focus on the structural and magnetic properties for CFCA films. We deduce theelement-specific magnetic moments of Fe and Co using
x-ray magnetic circular dichroism (XMCD) and dis-cuss the ordering types of CFCA by comparing withthe first-principles band structure calculations.
The samples of 30-nm-thick CFCA were grown on
MgO (001) substrates with Cr buﬀer. Sample surfaces
were capped by MgO layer. The MgO(001) substrate
was flushed thermally at 700◦ C in the UHV chamberprior to the deposition process. All of the layers weredeposited at room temperature. The Cr layer was insitu annealed at 700◦ C before the CFCA deposition.
One of the samples was in situ annealed at temperature of 700◦ C after the CFCA deposition [4].
XMCD was performed at BL-7A in the Photon
Factory (KEK). Photon helicity was fixed, and amagnetic-field switching between ±1 T was appliedalong the incident polarized soft x-ray. A totalelectron-yield mode was adopted. The measurements
were carried out in a grazing incidence setup to thesample surface normal in order to detect the in-plane
spin and orbital magnetic moments. All the measurements were performed at room temperature.

Fig. 1 — The schematic of the crystal structuresfor (a) the L21 structure for A2BC Heusler alloys
and (b) the Y structure for A ÀBC Heusler alloys.
(c) The schematic density of states near the Fermi
level for the half-metals and spin-gapless semiconductors [2].

Figure 2 shows the x-ray absorption spectra (XAS)
of Cr, Fe, and Co L2,3 edges, respectively, for thesample annealed at 700◦ C. XAS are normalized by
the photon fluxes. Clear metallic peaks are observed,
which confirms that there is no mixing of oxygen
atoms into CFCA. Shoulder structures appear in thehigher photon energy region of Co L3,2 XAS peaks,which correspond to the formation of Heusler alloys
due to the Co-Co bonding states within the molecular orbital calculations [5]. Finite XMCD signals at
the Cr L-edges are not detected. The spin and orbital magnetic moments are estimated by applying
the magneto-optical sum rules. The magnetic moment summing both spin and orbital components ofeach element is estimated to be 1.14 and 0.52 µB for
Fe and Co, respectively. Total magnetic moment is1.66 µB and 1.29 µB in the samples of 700◦ C annealing and no annealing, respectively, which is similar
to the value estimated from the magnetization measurements. The hysteresis loops taken at Fe L3 -edge
XMCD is also consistent with that by magnetization
measurements. We deduced the magnetic moments
depending on the annealing temperature. The results
clearly indicates the increase of magnetic momentswith annealing temperature, suggesting the improvement of the crystallinity by post annealing procedure.
In the case of highly ordered Y -type CFCA, the SGS
characteristics are anticipated in the first-principles
calculation. The highly ordered Y structure consistsof AÀ in Fe and Co, BC in Cr and Al. The Fe andCo atoms are coupled antiferromanetically. By introducing the disorder within the layer between Fe-Co,
the magnetic moment values remain unchanged. In
the case of Cr-Al disorder within the layer,the largest
moments are estimated in Cr sites and antiparallel
coupling between Fe and Co is kept. Furthermore, inthe case of including Fe-Cr swapped disorder betweenlayers, the calculation results in the parallel coupling
between Fe and Co and both positive and negativevalues of Cr magnetic moments which is canceled out
each other. Quite small magnetic moments are also
induced at the Al sites. We found that the disorderedstate can clearly explain the XMCD results and theSGS states are broken. Previous report in the calculation of CFCA also suggests the collapse of SGS byintroducing Coulomb interaction U within the schemeof GGA+U methods [6].
Considering these results, we discuss how the SGS
materials are designed. In order to achieve the SGS
possessing the advantages of ferromagnetic semiconductors, not only high ordering but also lattice strain
controlling which modulates the electron orbital hybridization is essential to obtain the ideal SGS states.
Further, antiparallel coupling between Fe and Co esti-

mated from the calculations might be originated fromthe exchange couping with antiferromagnetic Cr sites
in Cr-Al layer. These behaviors are analogous to theinterfacial exchange coupling between transition metals and antiferromagnetic or ferrimagnetic MnGa [7].
Further studies of ordered CFCA alloys are needed todevelop the SGS materials.
This work was partially supported by CREST (JST)(No. JPMJCR17J5).

Fig. 2 — The x-ray absorption spectra with different magnetic fields for the CFCA sample annealed at 700◦C measured at the Cr, Fe, and Co L3,2edges. The XMCD spectra for the corre-sponding
elements are also shown. Inset shows the XMCD
hysteresis curves taken at Fe L3-edge. Solid and dotlines show in-plane (easy) and out-of-plane (hard)
directions, respectively, to the film [4].

References[1] Y. Sakuraba and K. Takanashi, in Heusler Alloys,edited by C. Felser and A. Hirohata, Springer Seriesin Materials Science Vol. 222 (Springer, Berlin, 2015),
Chap. 16, pp. 389.
[2] L. Bainsla et al., Phys. Rev. B 96, 094404 (2017).
[3] L. Bainsla and K. G. Suresh, Appl. Phys. Rev. 03,031101 (2016).
[4] T. Tsuchiya et al., Phys. Rev. Mater. 3, 084403 (2019).
[5] I. Galanakis et al., Phys. Rev. B 66, 173329 (2002).
[6] G.Y. Gao et al., J. Alloy. Compounds 551, 539 (2013).
[7] J. Okabayashi et al., J. Mag. Mag. Mater. 460, 418(2018).

*jun@chem.s.u-tokyo.ac.jp