Dominant carrier of pseudo-gap antiferromagnet Cr3Al thin film
Introduction
Some alloys between transition metals and group 13 metals, for example, Nowotny chimney-ladder compounds (RuAl2, RuGa2, etc.) and Heusler alloys (Fe2VAl), exhibit semiconducting or semimetallic electrical properties, despite the fact that they consist entirely of metallic elements [[1], [2], [3], [4], [5]]. These systems have attracted much attention owing to their properties such as relatively high Seebeck coefficients and half-metallicity.
Antiferromagnetic Cr1-xAlx with approximately x = 0.25, is a pseudo-gap semimetal that differs from the typical systems mentioned above [[6], [7], [8], [9], [10], [11], [12], [13], [14]]. In the 1970s, it was known that Cr–Al alloys with approximately 25 at.%-Al exhibited semiconductor-like behaviors and had a band gap [6]. The pseudo-gap of Cr–Al has been reported to be 6–95 meV by resistivity measurements [6] and spectroscopic studies [11,12]. The characteristics of Cr3Al alloys are not only limited to pseudo-gap formation. It was also reported that, in the same composition range (x ≈ 0.25), Cr–Al alloys exhibited the highest Néel temperature and magnetic moments when doped with Al compared to those when other dopants, such as Fe, Mn, V, Si, and Ge, were used [[15], [16], [17]]. In addition, chemical ordering, called the X phase, has been suggested by a selected area electron diffraction of transmission electron microscopy [[18], [19], [20]]. The crystal structure is still not known for certain, but is mostly characterized as rhombohedrally-ordered BCC. The relationship between the pseudo-gap, magnetic, and chemical ordering has also been discussed. However, a definite interpretation has not yet been established.
Boekelheide et al. studied Cr3Al both theoretically and experimentally [[12], [13], [14]]. They compared two thin-film samples: one was deposited at a lower temperature (573 K) and exhibited semiconductor-like behavior, and the other one was deposited at a high temperature (873 K) and exhibited a positive temperature coefficient of resistivity (TCR). The latter had C11b ordering, which differed from the disordered BCC or X phase. Combined with the density-functional calculations, they concluded that antiferromagnetism was necessary for the pseudo-gap formation. They also reported that the X phase ordering was favorable for the pseudo-gap, unlike the C11b ordering with the same composition, which was rather unfavorable [13,14]. It was also indicated that the disordered Cr0.80Al0.20 could have a pseudo-gap according to the calculation [12]. Combined with the unambiguous crystal structure, it is still unclear whether the X phase ordering is a necessary condition for pseudo-gap formation. However, C11b ordering is a sufficient condition to suppress it. Thus, magnetic and chemical ordering should play essential roles in pseudo-gap formation in the Cr3Al system.
In this study, we examined the Hall and Seebeck effect measurements of Cr3Al thin films with and without C11b ordering. Both the Hall and Seebeck effects were used to determine the dominant carrier. It is beneficial to examine these two factors in systems with coexisting electrons and holes because these two coefficients are derived from different weights of mobility [[21], [22], [23]]. The purpose of this study is to determine the relationship between the dominant carrier change and the pseudo-gap formation due to the lack of C11b ordering. The previously reported calculations showed that the pseudo-gap formation of both disordered Cr0.80Al0.20 and X phase Cr3Al occurred as the hole pocket disappeared but the electron pocket survived [[12], [13], [14]]. This is in contrast to the case of pure Cr, where the dominant carrier is a hole [6,17,24]. This is supported by previous reports on the composition dependence of the Hall and Seebeck coefficients [6,7,10]. However, it has not been reported for comparison in samples of the same composition with different chemical ordering: the C11b-ordered and the non-ordered Cr3Al. In the first part of this report, the relationship between the existing phases and the pseudo-gap formation is investigated from the viewpoints of crystallographic evaluations, resistivity, and magnetism. In the following section, the dominant carriers are discussed for the two ordering states.
Section snippets
Experimental
The designed stacking structure of the samples was Cr3Al(50 nm)/MgO(001)-substrate. The composition ratio of the Cr3Al layers was controlled by the relative deposition rates of Cr and Al. In this study, no capping layer was used to acquire more accurate conductivity of the Cr–Al alloy film by eliminating the shunt to the capping layer in the conductivity measurements. The samples were deposited using molecular beam epitaxy (MBE). Cr and Al targets were heated by an electron beam and in the
Crystallographic evaluation
Fig. 2(a)–(c) show the RHEED patterns of the MgO substrate and Cr3Al layer. Because streaky patterns were observed, flat surfaces were obtained for the MgO substrate and Cr3Al layers deposited at 473 K and 873 K. The observed streaks for Cr3Al are equally separated, which suggests that the crystallographic orientation of the Cr3Al layer is aligned both parallel and perpendicular to the film plane, and the epitaxial growth of the Cr3Al layer on the MgO(001) substrate was confirmed.
Selected area
Summary
The dominant carriers of the Cr3Al thin films were evaluated based on the measurements of the Hall and Seebeck effects. The evaluations were conducted for semiconductor-like and nearly metallic Cr3Al films deposited at 473 K and 873 K, respectively. While the former film has the signature of the X phase formation according to the criteria adopted in previous reports, there is no direct evidence in the diffraction measurements. The latter film clearly exhibited C11b ordering.
The Hall coefficient
Credit author statement
Kentaro Toyoki: Methodology, Formal analysis, Investigation, Data Curation, Writing - Original Draft. Masayuki Hayashi: Formal analysis, Data Curation. Shunsuke Hamaguchi: Formal analysis, Data Curation. Noriaki Kishida: Resources. Yu Shiratsuchi: Conceptualization, Methodology, Writing - Review & Editing. Takafumi Ishibe: Formal analysis, Data Curation, Writing - Review & Editing. Yoshiaki Nakamura: Formal analysis, Data Curation, Writing - Review & Editing. Ryoichi Nakatani:
Declaration of competing interest
The authors declare no conflicts of interest associated with this manuscript.
Acknowledgements
This work was partly supported by JSPS KAKENHI (Grant No. 20K15109).
References (32)
- et al.
J. Phys. Chem. Solid.
(1971) - et al.
Solid Stat. Commun.
(1967) - et al.
Physica
(1971) - et al.
J. Magn. Magn Mater.
(1999) - et al.
IUCrJ
(2019) - et al.
Phys. Rev. B
(1998) - et al.
Phys. Rev. Lett.
(1997) - et al.
Phys. Rev. B
(2001) - et al.
Phys. Rev. B
(1998) - et al.
Phys. Rev. B
(2007)