While the family of layered pnictides ABX2 (A—rare or alkaline earth metals, B—transition metals, and X—Sb/Bi) can host Dirac dispersions based on Sb/Bi square nets, nearly half of them have not yet been synthesized for possible combinations of the A and B cations. Here, we report the fabrication of EuCdSb2 with the largest B-site ionic radius, which is stabilized for the first time in thin film form by molecular beam deposition. EuCdSb2 crystallizes in an orthorhombic Pnma structure and exhibits antiferromagnetic ordering of the Eu magnetic moments at TN = 15 K. Our successful growth will be an important step for further exploring novel Dirac materials using film techniques.

Dirac materials with linear energy dispersions have attracted growing attention in light of exploring new compounds and elucidating their magnetotransport.1 In particular, Dirac dispersions interacting with magnetism have significant potential for producing rich quantum transport.2–4 Recently, ternary layered pnictides ABX2 (A—rare or alkaline earths, B—transition metals, and X—Sb/Bi) have been reported to host highly anisotropic Dirac or Weyl dispersions,5–22 and unprecedented quantum transport has been observed originating from the interplay between the Dirac dispersion and magnetic ordering, as exemplified by the bulk half-integer quantum Hall effect in EuMnBi2,5,6 BaMnSb2,7,8 and SrMnSb2.9 Therefore, exploring new ABX2 compounds, especially with magnetic A and/or B ions, is important for systematically investigating their unique magnetotransport.

As summarized in Fig. 1(a), there are four space groups (I4/mmm, Imm2, P4/nmm, and Pnma) for ABSb2. ABSb2 have two types of Sb atoms in the unit cell: one with −1 valence (defined as Sb1) consisting of the Sb net, which hosts Dirac or Weyl dispersions, and the other with −3 valence (defined as Sb2) forming Sb tetrahedra around the B-site cations, which are separated by A layers. The A-site cations are coincidently stacked across the Sb1 net in I4/mmm and Imm2, while they are staggeredly stacked in P4/nmm and Pnma. The Sb1 square nets are slightly distorted in Pnma and Imm2, resulting in a zig-zag chainlike structure along the b-axis.

FIG. 1.

(a) Crystal structures of the three space groups in ABSb2 systems (I4/mmm, Imm2, P4/nmm, and Pnma). They have two types of Sb atoms (valence states of −1 for Sb1 and −3 for Sb2), which are separated by A layers. One unit cell of each structure is represented by black lines. (b) Mapping of previously synthesized ABSb2 bulks (open squares)9–14,23–37 and the newly stabilized EuCdSb2 film in this study (filled square) as a function of A (= Ca, Yb, Eu, Sr, and Ba) and B (= Mn, Zn, and Cd)-site ionic radii. Depending on their space groups, the squares are indicated in blue (I4/mmm), black (Imm2), green (P4/nmm), or red (Pnma). This map clearly shows that the P4/nmm or Pnma structure becomes stable for smaller A-site ionic radius, while I4/mmm and Imm2 become stable for larger A-site ionic radius. Regarding BaMnSb2, its detailed structure has been recently found, Imm2,7,8 while it was originally characterized as I4/mmm.30–32 

FIG. 1.

(a) Crystal structures of the three space groups in ABSb2 systems (I4/mmm, Imm2, P4/nmm, and Pnma). They have two types of Sb atoms (valence states of −1 for Sb1 and −3 for Sb2), which are separated by A layers. One unit cell of each structure is represented by black lines. (b) Mapping of previously synthesized ABSb2 bulks (open squares)9–14,23–37 and the newly stabilized EuCdSb2 film in this study (filled square) as a function of A (= Ca, Yb, Eu, Sr, and Ba) and B (= Mn, Zn, and Cd)-site ionic radii. Depending on their space groups, the squares are indicated in blue (I4/mmm), black (Imm2), green (P4/nmm), or red (Pnma). This map clearly shows that the P4/nmm or Pnma structure becomes stable for smaller A-site ionic radius, while I4/mmm and Imm2 become stable for larger A-site ionic radius. Regarding BaMnSb2, its detailed structure has been recently found, Imm2,7,8 while it was originally characterized as I4/mmm.30–32 

Close modal

As shown in the map in Fig. 1(b), it becomes difficult to stabilize ABSb2 with smaller A-site and larger B-site ionic radii, and nearly half of ABSb2 has not been synthesized yet for all the possible combinations of the A and B cations.9–14,23–37 In this lower right region of the map in Fig. 1(b), phase separation may occur, for example, from EuCdSb2 to EuSb2 with −1 valence38,39 and EuCd2Sb2 with −3 valence,40 because ABSb2 has different −1 and −3 valence states in one structure. Especially in the case of B = Cd, its high vapor pressure also makes the fabrication difficult.

In this context, developing techniques for stabilizing new ABSb2 compounds is strongly called for advancing this research field. Here, we report the fabrication of its new member EuCdSb2, which is stabilized for the first time in thin film form by molecular beam deposition. EuCdSb2 crystallizes in the orthorhombic Pnma structure and exhibits unique nonmetallic transport.

Single-crystalline (0001) Al2O3 substrates were annealed at 850 °C in a base pressure about 3 × 10−7 Pa and then EuCdSb2 films were grown on it in an Epiquest RC1100 chamber.41 The molecular beams were simultaneously provided from conventional Knudsen cells containing 3N Eu, 6N Cd, and 6N Sb. The growth temperature was set at 390 °C, and the beam equivalent pressures, measured by an ionization gauge, were set to 1.2 × 10−5 Pa for Eu, 5.0 × 10−4 Pa for Cd, and 8.5 × 10−6 Pa for Sb (for details, see the supplementary material). To avoid Cd deficiency, the Cd flux was set 40 times higher than the Eu flux. The film thickness was typically set at 25 nm for structural characterization and magnetotransport measurements and 50 nm for magnetization measurements. The growth rate was about 0.07 Å/s. Magnetotransport was measured by using a standard four-probe method for 200 μm-width multi-terminal Hall bars. Longitudinal and Hall resistivities were measured up to 22.4 T using a non-destructive pulsed magnet42 at the International MegaGauss Science Laboratory in the Institute for Solid State Physics at the University of Tokyo. Temperature dependence of the resistivity and magnetization was measured using a Quantum Design Physical Property Measurement System and a Magnetic Property Measurement System, respectively.

Figure 2 shows structural characterization of an obtained EuCdSb2 film. As shown in the x-ray diffraction (XRD) θ–2θ scan in Fig. 2(a), the reflections from the (200) EuCdSb2 lattice planes are observed without any impurity phases. Its out-of-plane lattice constant along the a-axis is calculated to be 22.53 Å. As confirmed in Fig. 2(b), clear Laue fringes indicate highly coherent lattice ordering along the out-of-plane direction. A rocking curve taken for the (800) film peak in Fig. 2(c) is very sharp with a full width at half maximum of 0.11°, ensuring high crystallinity of the film. On the other hand, the in-plane XRD reciprocal space map shown in Fig. 2(d) suggests that the b- and c-axes are rather randomly oriented forming domain structures. In-plane (020) and (002) EuCdSb2 peaks are also indiscernible, indicating that the in-plane lattice constants of the b- and c-axes are almost the same. The atomic force microscopy image in Fig. 2(e) reveals a flat surface with a root mean square (rms) roughness of 0.27 nm.

FIG. 2.

(a) XRD θ–2θ scan of a EuCdSb2 film grown on a (0001) Al2O3 substrate. Al2O3 substrate peaks are marked with an asterisk. (b) Enlarged view of the θ–2θ scan and (c) rocking curve around the (800) EuCdSb2 film peak. (d) In-plane reciprocal space mapping measured by grazing incidence. (e) Surface morphology of the film taken by atomic force microscopy.

FIG. 2.

(a) XRD θ–2θ scan of a EuCdSb2 film grown on a (0001) Al2O3 substrate. Al2O3 substrate peaks are marked with an asterisk. (b) Enlarged view of the θ–2θ scan and (c) rocking curve around the (800) EuCdSb2 film peak. (d) In-plane reciprocal space mapping measured by grazing incidence. (e) Surface morphology of the film taken by atomic force microscopy.

Close modal

The crystal structure of this new compound is closely examined by taking high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. A cross-sectional image of the EuCdSb2 film in Fig. 3(a) shows clear stacking of the Sb square nets and other layers. Its in-plane lattice constant along the c-axis is calculated to be 4.47 Å by comparing with the a-axis length determined by the XRD θ–2θ scan. The higher resolution HAADF-STEM image and the corresponding elemental maps in Figs. 3(b)3(e) reveal additional details of the atomic arrangement. Staggered stacking of Eu atoms across the Sb1 layers rules out the I4/mmm and Imm2 structures among the four possible space groups, as shown in Fig. 1(a). As confirmed in the intensity profiles in Fig. 3(f), moreover, alternate left and right shifts of the Sb atoms are clearly resolved in the Sb1 layers, in contrast to the equally spaced Cd atoms. These Sb shifts correspond to the distorted Sb square nets, as shown in Fig. 3(g), and thus, we conclude that the EuCdSb2 crystal structure is Pnma. The Pnma structure is a nonsymmorphic structure, which protects band crossings of Dirac dispersions.21 

FIG. 3.

(a) Cross-sectional high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) image of the EuCdSb2 film. (b) Higher resolution HAADF-STEM image for the boxed region in (a) and energy dispersive x-ray spectrometry map for (c) Eu L, (d) Cd L, and (e) Sb L edges. The Eu, Cd, and Sb atoms are aligned as expected for the ABSb2 compounds shown in Fig. 1(a). (f) Intensity line profiles of Cd and Sb1 layers extracted from dotted horizontal lines in (b). In contrast to the equally spaced Cd atoms, alternate left and right shifts are clearly resolved in the Sb1 layers, as marked with arrows. (g) Cross-sectional Pnma structure viewed along the b-axis. Arrows represent the atomic shift or distorted square nets in the Sb1 layers.

FIG. 3.

(a) Cross-sectional high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) image of the EuCdSb2 film. (b) Higher resolution HAADF-STEM image for the boxed region in (a) and energy dispersive x-ray spectrometry map for (c) Eu L, (d) Cd L, and (e) Sb L edges. The Eu, Cd, and Sb atoms are aligned as expected for the ABSb2 compounds shown in Fig. 1(a). (f) Intensity line profiles of Cd and Sb1 layers extracted from dotted horizontal lines in (b). In contrast to the equally spaced Cd atoms, alternate left and right shifts are clearly resolved in the Sb1 layers, as marked with arrows. (g) Cross-sectional Pnma structure viewed along the b-axis. Arrows represent the atomic shift or distorted square nets in the Sb1 layers.

Close modal

Figure 4 summarizes fundamental transport and magnetic properties of the EuCdSb2 films. Temperature dependence of the magnetization in Fig. 4(a) shows a kink at TN = 15 K, which is ascribed to antiferromagnetic (AFM) ordering of the Eu magnetic moments, as detailed in the following. This kink is observed to be much sharper for the in-plane field, while almost constant behavior below TN is found for the out-of-plane field. This indicates that the Eu magnetic moments are antiferromagnetically ordered in the in-plane. Similar to other EuBX2 compounds,5,6,14,25,26,36 the Eu magnetic moments of EuCdSb2 probably form the A-type AFM structure where ferromagnetic (FM) in-plane layers are antiferromagnetically stacked along the out-of-plane direction. Figure 4(b) shows the nonmetallic temperature dependence of the longitudinal resistivity. A drastic upturn at 15 K coincides with the AFM ordering, as also confirmed by its shift upon increasing the magnetic field. This nonmetallic temperature dependence is a common feature only with EuMnSb2,14,25,26 and the further upturn below TN appears only in EuCdSb2, while other ABSb2 compounds shown in Fig. 1(b) exhibit metallic behavior.9,10,27–37

FIG. 4.

(a) Temperature dependence of magnetization M taken by applying the out-of-plane or in-plane magnetic field of 0.1 T. Both curves exhibit a kink at TN = 15 K. (b) Temperature dependence of longitudinal resistivity ρxx. The inset shows an enlarged view at low temperatures around TN. A drastic upturn observed below 15 K shifts to lower temperatures for 9 T. (c) Magnetoresistance ratios [MRR = (Rxx(B) − Rxx(0 T))/Rxx(0 T)] taken by sweeping the out-of-plane field at various temperatures. The saturation field is estimated to be Bs = 12.7 T at 1.4 K. The inset compares low-field MRR taken for the out-of-plane (solid line) and in-plane (dashed line, IB) fields at 1.4 K. For the in-plane field, transition from positive to negative magnetoresistance is found at 1.7 T. (d) Hall resistivity ρyx taken at various temperatures.

FIG. 4.

(a) Temperature dependence of magnetization M taken by applying the out-of-plane or in-plane magnetic field of 0.1 T. Both curves exhibit a kink at TN = 15 K. (b) Temperature dependence of longitudinal resistivity ρxx. The inset shows an enlarged view at low temperatures around TN. A drastic upturn observed below 15 K shifts to lower temperatures for 9 T. (c) Magnetoresistance ratios [MRR = (Rxx(B) − Rxx(0 T))/Rxx(0 T)] taken by sweeping the out-of-plane field at various temperatures. The saturation field is estimated to be Bs = 12.7 T at 1.4 K. The inset compares low-field MRR taken for the out-of-plane (solid line) and in-plane (dashed line, IB) fields at 1.4 K. For the in-plane field, transition from positive to negative magnetoresistance is found at 1.7 T. (d) Hall resistivity ρyx taken at various temperatures.

Close modal

Figure 4(c) presents magnetoresistance (MR) taken by out-of-plane magnetic field sweeps at various temperatures. Negative MR at the base temperature of 1.4 K saturates at Bs = 12.7 T, which shifts to lower fields with the increase in temperature and then disappears above TN. Therefore, it can be understood that this saturation corresponds to the phase transition from an AFM to a forced ferromagnetic (FM) phase and electron scattering by the Eu magnetic moments is suppressed through this canting process. For the in-plane field as shown in the inset, on the other hand, a transition from positive to negative MR is observed. This is ascribed to a spin–flop transition, consistent with the in-plane AFM ordering in the ground state (see the supplementary material). After the spin–flop transition, a similar negative MR is observed also for the in-plane field.

Finally, Fig. 4(d) shows Hall resistivity measured at various temperatures. While more than one type of carriers contribute to the conduction in most of the ABX2, an almost linear Hall resistivity is observed for EuCdSb2 in the entire temperature range. The carrier density and mobility are estimated at 1.9 × 1019 cm−3 and 86 cm2/Vs by single-carrier fitting at 1.4 K. Despite the large Eu magnetic moments, there are no clear indications of the anomalous Hall effect for EuCdSb2, similar to other magnetic ABSb2.7–10,25,29–36

In summary, we have demonstrated the fabrication of a new ABSb2 compound using molecular beam deposition. EuCdSb2 crystallizes in the orthorhombic Pnma structure and exhibits unique nonmetallic transport properties in addition to in-plane AFM ordering. Our successful growth of EuCdSb2 by the film-technique paves the way for further exploring ABX2 systems and designing their heterostructures.

See the supplementary material for detailed characterization in the phase diagram for optimizing growth condition, wide-range TEM image, and magnetization curves confirming the spin flop transition.

The authors thank H. Sakai for helpful discussions. This work was supported by the JST PRESTO (Grant No. JPMJPR18L2); the JST CREST (Grant No. JPMJCR16F1), Japan; and the Grant-in-Aid for Scientific Research (B) from the MEXT, Japan (Grant No. JP18H01866).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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