Magnetic and magnetotransport properties of layered TaCoTe2 single crystals

Ming Mei(梅明), Zheng Chen(陳正), Yong Nie(聶勇), Yuanyuan Wang(王園園),Xiangde Zhu(朱相德), Wei Ning(寧偉), and Mingliang Tian(田明亮)

1Anhui Key Laboratory of Condensed Matter Physics at Extreme Conditions,High Magnetic Field Laboratory,HFIPS,Anhui,Chinese Academy of Sciences,Hefei 230031,China

2Department of Physics,University of Science and Technology of China,Hefei 230031,China

3School of Physics,Anhui University,Hefei 230601,China

Keywords: two-dimensional materials,magnetism,electronic transport,nanoflakes

1.Introduction

Since the discovery of graphene, two-dimensional (2D)materials have proved to be a fertile ground to explore novel electrical,[1-4]optical,[5-7]mechanical,[8,9]and magnetic properties.[10-13]In 2D materials, it has been confirmed that the decrease in the crystal dimension makes it sensitive to carrier concentration, magnetic field, pressure and electric field, and novel phenomena that are different from the bulk crystal can be observed, such as the observation of the quantum Hall effect in graphene,[14]the strongly thickness-dependent metallic to semiconductor,charge density wave, or superconducting phases in transition metal dichalcogenides,[15,16]the layer-dependent ferromagnetism in 2D magnetic materials,[17,18]the observation of the quantum anomalous Hall effect in few-layer MnBi2Te4,[19]and the enhancement of a weak-antilocalization signature in Nb3SiTe6.[20]During the past decade, a number of important potential applications have been proven in 2D materials.[15,21-28]Many new 2D materials have been predicted that are worth further experimental studies.

Theoretic investigation has predicted that the Dirac points can be stabilized in 2D materials.However,it remains a challenge to identify concrete 2D materials that host such magnetic Dirac points.Recently, the layered ternary telluride TaCoTe2was predicted to be a new topological semimetal.[29]It is suggested that the monolayer TaCoTe2is stable with an antiferromagnetic (AFM) ground state.It hosts a pair of 2D AFM Dirac points on the Fermi level in the absence of spin-orbit coupling (SOC) and emerged another pair of magnetic Dirac points below the Fermi level when SOC is included.An angleresolved photoelectron spectroscopy study reported the experimental signature of topological Dirac antiferromagnetism in this material.[30]However, the physics properties of this material have not been well studied.In this work, we have synthesized high-quality single crystal TaCoTe2and performed a detailed investigation into both the bulk crystals through magnetization and magnetotransport measurements.We also obtained nanoflakes by the Scotch tape-based micromechanical exfoliation method and fabricated nanodevices based on these nanoflakes.We found that when the thickness is reduced to 18 nm,the crystal shows the same properties as the bulk crystal.

2.Experiment

2.1.Crystal growth and nanodevice fabrication

TaCoTe2single crystals were grown by the chemical vapor transport(CVT)method using TeCl4as a transport agent.Stoichiometric elemental agents Ta (powder), Co (powder),and Te (chunk) were mixed and sealed in a vacuumed silica tube.The tube was 20 cm in length and placed in a twozone furnace.The transport was carried out in the temperature gradient of 930?C to 850?C for 7 days.Then the furnace was cooled naturally to room temperature after the reaction.Shiny black plate-like TaCoTe2single crystals were collected at the cold end.TaCoTe2thin flakes with different thicknesses were obtained by mechanically exfoliating the bulk crystal,followed by directly transferring them onto Si/SiO2substrate.Hall-bar devices were fabricated by standard electron-beam lithography followed by an Au (80 nm)/Ti (10 nm) evaporation and a liftoff process.

2.2.Material characterization

X-ray diffraction(XRD)was used to determine the structure of the as-grown single crystals.The XRD data were obtained at room temperature on a Rigaku-TTR3(CuKαradiation,λ=1.5418 ?A)operated at 20 kV voltage and 4 mA current with a flat single-crystal specimen mounted on the sample holder in a reflection mode (step size=0.02?).The composition analysis of single crystals was done by an energy dispersive spectroscopy(EDS)upon a dual beam system(Helios nanolab600i),operated at 20 kV voltage and 0.17 nA current.The resistivity measurement was measured by the four-probe method and was carried out by a physical properties measurement system (PPMS-14T, QD Inc.).Magnetization measurement was performed upon a magnetic properties measurement system (MPMS3-7T, QD Inc.).Electron beam lithography was used to prepare devices and was also performed upon a dual beam system(Helios nanolab600i).The measurement of the sample thickness was performed using an atomic force microscope(NX10,PARK Inc.).

3.Results and discussion

In previous work,[31]polycrystalline XRD has determined the structure of the TaCoTe2.TaCoTe2has crystallized in space groupP21/c(No.14) witha= 8.1524 ?A,b=6.2649(4) ?A,c=7.7945 ?A,andα=116.789?.The crystal structure of TaCoTe2is shown in Figs.1(a) and 1(b).As illustrated,the crystal has a layered structure aligned along thea-axis.Figure 1(c)depicts the room temperature XRD pattern of our single crystal.All the peaks can be indexed as (L00)reflections.The inset of Fig.1(c) show an optical image of an as-grown TaCoTe2single crystal.This indicates that the largest naturally grown surface of the crystals belongs to thebcplane.As shown in Fig.1(d), the obtained chemical composition from the EDS of three elements Ta, Co, and Te is 1:1:2.

Figure 2(a) depicts the temperature-dependent magnetization curves (M-T) of single crystal TaCoTe2under fieldcooled (FC) and the zero-field-cooled (ZFC) modes, with magnetic fieldH=1 T applied along thea-axis (H‖a-axis)and parallel to thebcplane(H‖bcplane),respectively.It can be found that theM-Tcurves of both the FC and ZFC modes almost overlap at low temperature with the magnetic field applied parallel to thebcplane, while the two curves are not coincident at a high temperature range.Figure 2(b)shows the magnetic-field dependent magnetization at different temperatures at theB‖bcplane.When the field is applied along thea-axis, it exhibits similar magnetic behavior at low temperature range.While at a high temperature range,a rapid increase in magnetization can be observed with temperature decreasing and a broad peak appears at about 250 K.DFT calculations suggest this compound should be an AFM material; thus, the peak at 250 K might be a signature magnetic ordering transition related to the predicted AFM order.[30]However, the magnetic moment is small and local lattice defects can also induce the presence of magnetization, especially at low temperature range.Further measurements are required to study the magnetic properties of TaCoTe2.

Fig.1.(a)Schematic of a unit cell of TaCoTe2 (inside solid line).(b)A layered structure of TaCoTe2 that is aligned parallel to the a-axis(space group P21/c).(c) Single-crystal XRD results along the (L00) plane for TaCoTe2.The inset shows an optical image of a typical as-grown TaCoTe2 single crystal.(d) Typical EDS data collected on a flat clean surface of crystal showing elemental composition.

Fig.2.(a)Temperature-dependent magnetization as function of temperature at H=1 T with H applied along the a-axis(solid lines)and parallel to the bc plane(dotted lines).The inset shows the enlargement area above 150 K.(b)and(c)The magnetic-field dependent magnetization at different temperatures for the H‖bc plane and a-axis,respectively.

Figure 3(a) shows the temperature-dependent resistivityρof bulk crystal measured with the current applied parallel to thebcplane.The resistivityρexhibits a semiconducting behavior at high temperature range.As the temperature decreases,it shows a semiconducting to metallic transition with a broad peak around 38 K.When the magnetic field is applied,the curves show an upturn at low temperature.Figure 3(b)showsR-Tcurves for a nanodevice with thicknessd=40 nm.It exhibits behavior similar to that of the bulk crystal, except the temperature of the peak is a little lower than that of the bulk crystal,i.e.,at 32 K.In two samples,the peak moves slightly to lower temperatures under magnetic fields.The upturn behavior is more obvious in the nanodevice.It can be expected that the resistivity may turn fully to semiconductor behavior when the magnetic field is high enough.Similar nonmetallic transport has been observed in the work,[30]which showed that the TaCoTe2crystal shows an even broader around 50 K.

Fig.3.(a)Temperature-dependent resistivity ρ measured with the current applied parallel to the bc plane for bulk crystal.The inset shows a ρ-T curve for the whole temperature range from 2 K-300 K.(b)Temperaturedependent resistivity ρ of a nanodevice with thickness d=40 nm under different magnetic fields.The inset shows an image of a nanodevice obtained with AFM.

Figure 4(a)shows temperature-dependent MR,defined as MR(%)=[R(B)-R(0)]/R(0)×100%, of bulk crystal with magnetic field applied along thea-axis.The value positive MR is 0.625%at 2 K at 9 T.With an increase in temperature,it reduces to about 0.05%at 40 K,9 T.As displayed in Fig.4(b),the MR behavior with theH‖bcplane at a different temperature is almost the same as theH‖a-axis, suggesting a weak anisotropy between the two directions.Figures 4(c) and 4(d)show MR behavior of the nanodevice atd=40 nm,measured at different temperatures with theH‖bcplane andH‖a-axis,respectively.The value of MR is 0.61%at 2 K and 0.045%at 40 K at 9 T.Obviously, the magnetotransport property is the same as bulk crystal when the thickness is lowered down to 40 nm.

In order to further investigate the physical properties,we fabricated a nanodevice to even lower thickness.Figure 5(a)shows theρ-Tcurves of the bulk crystal and three nanodevices with different thicknesses, i.e.,d=40 nm, 25 nm, and 18 nm for samples S1, S2, and S3, respectively.Theρ-Tcurves are shifted for clarify.Similar temperature dependence resistivity of all four samples can be observed.The temperature of the transition from semiconductor to metal moved to a lower temperature gradually,as indicated by the arrow.Figure 5(b)presents the MR behavior att=2 K for the three samples.All the samples exhibit similar unsaturated linear MR behavior, which indicates that the physical properties remain nearly the same even when the thickness is lowered to 18 nm.To further explore the predicted AFM behavior in monolayer TaCoTe2, further work on nanoflakes with lower thickness is required.

Fig.4.(a)The MR as a function of magnetic field measured at different temperatures with H applied along the a-axis for bulk crystal.(b)Single crystal:the MR as a function of magnetic field measured at different temperatures with the H‖bc plane for bulk crystal.(c)The MR as function of magnetic field measured at different temperatures with H‖a-axis for sample S1(d=40 nm).(d)The MR as function of magnetic field measured at different temperatures with the H‖bc plane for sample S1(d=40 nm).

Fig.5.(a)Temperature-dependent resistivity ρ of different samples measured with the current applied parallel to the bc plane.The curves of different samples are shifted for clarify.(b)The MR of different samples as a function of magnetic field measured at 2 K with the magnetic field applied along the a-axis.

We note that upturn behavior in resistivity below 10 K has been observed in other materials.A possible reason is the Kondo effect.[32]However, the applied magnetic field could suppress the upturn.This is not the case in our work and thus this cause could be ruled out.Another reason is the existence of a metallic surface state in the crystals.The broad hump of the temperature-dependent resistance was then considered to be the competition of the conductivity between the surface state and the bulk.[33]When the temperature decreases, the contribution of the surface conductivity dominates the transport and leads to the downturn in theR-Tcurves.However,in this case, the upturn behavior in thinner samples at low temperature should not appear due to the enhanced surface conductivity.This is not consistent with our observation and thus the existence of a metallic surface state is also not the origin of the upturn at the low temperature region.We will aim to get more information on these observations in our future work.

4.Conclusion

In summary, we have successfully synthesized layered TaCoTe2crystals using the CVT method.We performed a detailed study of magnetic and magnetotransport properties on both bulk crystal and nanosheets with thickness down to 18 nm.The bulk TaCoTe2shows magnetic ordering even at room temperature.The temperature-dependent resistivity of TaCoTe2shows a transition from semiconductor to metal near 38 K.Comparing the magnetotransport behavior of samples with different thicknesses, we found that the magnetotransport properties of bulk crystal and the nanoflakes even down to 18 nm are the same.To further explore the predicted 2D Dirac AFM behavior,nanoflakes with even lower thicknesses are needed.

Acknowledgments

Project supported by the National Key Research and Development Program of China(Grant No.2021YFA1600201),the National Natural Science Foundation of China (Grant Nos.U19A2093, U2032214, and U2032163), Collaborative Innovation Program of Hefei Science Center,CAS(Grant No.2019HSC-CIP 001), Youth Innovation Promotion Association of CAS (Grant No.2021117), the HFIPS Director’s Fund (Grant No.YZJJQY202304), and the CASHIPS Director’s Fund (Grant No.E26MMG71131).A portion of this work was supported by the High Magnetic Field Laboratory of Anhui Province.