Ex) Article Title, Author, Keywords
Ex) Article Title, Author, Keywords
New Phys.: Sae Mulli 2023; 73: 324-329
Published online April 30, 2023 https://doi.org/10.3938/NPSM.73.324
Copyright © New Physics: Sae Mulli.
Beopgil Cho, Jaemun Park, Keeseong Park*
Department of Physics and Chemistry, Daegu Gyeongbuk Institute of Science and Technology, Daegu 42988, Korea
Correspondence to:*E-mail: firstname.lastname@example.org
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Ni3Sn2 crystallizes into a hexagonal phase (HT-Ni3Sn2) at higher temperatures while it undergoes structural distortion and vacancy alignment to transform into an orthorhombic phase (LT-Ni3Sn2) at lower temperatures. LT-Ni3Sn2 has not been synthesized in a single-crystalline form, although spin heptamers of Ni atoms can constitute well-organized spin clusters and possibly influence various physical phenomena. This paper demonstrated a successful synthesis of both phases in single crystals using a flux method, and their electrical transport was measured. Both phases showed unusual metallic resistivity, which can be fitted via a power law. This power law behavior is frequently exhibited by compounds consisting of 3d transition metals because of their spin frustration. In magnetoresistance, LT-Ni3Sn2 showed an abrupt increase at a low temperature and weak magnetic field region, indicating an influence of the spin clusters or occurrence of a weak antilocalization due to a strong spin–orbit coupling.
Keywords: Single crystal, Ni3Sn2, Resistivity, Magnetoresistance
Topological materials have been researched extensively due to their exotic physical phenomena, and electrical transport is widely investigated for these materials for its use in observing the behavior of bands near the Fermi level[1, 2]. In particular, when combined with spin–orbit coupling, some spin states of topological materials could break the time reversal symmetry of the bands under the protection. Ferromagnetic Fe3Sn2 and antiferromagnetic Mn3Sn could be excellent examples because of their physical phenomena as magnetic topological materials, where Dirac and Weyl bands produce large anomalous Hall effects through interaction with their innate spin state[3, 4]. This is likely to be a case when a spin becomes unsettled by the nearest neighbors of the lattice. Gerard Toulouse introduced this concept of geometrical frustration, and topics on crystals consisting of triangles and hexagons of 3
Meanwhile, some crystals can possess topological properties from structural transformation. For instance, some IV–VI semiconducting crystals, such as SnTe, can transform from a cubic to a rhombohedral phase by decreasing the synthesis temperature. Though symmetry is reduced for such crystals, they are still able to generate topological semimetallic properties such as nodal lines or Weyl points. However, although such method of distortion could break symmetry or produce new topological properties that can lead potentially intriguing research fields, it is difficult to find proper samples to be investigated.
It has been reported that many T–Sn binary phases can reduce symmetry via synthesis conditions. For instance, cubic Cu3Sn undergoes transformation to an orthorhombic phase around 650 °C. Here, orthorhombic Cu3Sn could generate large magnetoresistance because of the broken symmetry. Likewise, Ni3Sn2 can have hexagonal (P63/mmc) and orthorhombic (Pnma) structures that are temperature dependent as shown in Table 1. The hexagonal structure contains random vacancies at two sites of the unit cell, which should require high-activation energy of migration. In contrast, when synthesizing at lower temperatures, an orthorhombic distortion is permitted by assigning one site as fully filled while leaving others vacant; they have been named HT-Ni3Sn2 and LT-Ni3Sn2.
HT-Ni3Sn2 grown via chemical vapor transport (CVT) is known to exhibit anisotropic and enhanced paramagnetic behavior and unusual temperature dependence of resistivity that are proportional to the power law. In fact, such dependence has frequently been observed in various materials consisting of 3
Furthermore, spin heptamers may occur when the vacant site is filled, where hexagonal isostructural Fe1.3Sb exhibits a topological Hall effect via scalar spin chirality derived from the Dzyaloshinskii–Moriya interaction of the spin heptamer. Similarly, stacked spin heptamers in LT-Ni3Sn2 function as spin clusters and exhibit intriguing physical properties, concerning geometrical frustration, when compared to HT-Ni3Sn2.
Here, we report that the synthesis of HT-Ni3Sn2 and LT-Ni3Sn2 using the flux method is possible, and we discuss some distinguishable properties observed in LT-Ni3Sn2 from the perspective of resistivity measurements.
All Ni3Sn2 single crystals in the study were synthesized using the flux method. We determined synthesis conditions using the Ni–Sn binary phase diagram. Ni, Sn, and Bi elements were mixed and placed in an alumina crucible. The silica ampoule containing the crucible was then sealed under partial Ar atmosphere. They were initially heated up to 1000 °C to ensure completely melting, HT-Ni3Sn2 single crystals were obtained at 800 °C, whereas LT-Ni3Sn2 single crystals were obtained at 450 °C via annealing for 7 days. Unlike the HT-Ni3Sn2 single crystals that were grown as long hexagonal rods, the as-grown LT-Ni3Sn2 single crystals resembled needles.
X-ray diffraction (Empyrean, Malvern Panalytical, United Kingdom) was conducted to identify the Miller indices of the single crystals, and energy-dispersive X-ray spectroscopy (S-4800, Hitachi, Japan) was used to determine its nominal composition. Electrical transport was measured using a home-built 14-T magnet (Oxford Instruments, United Kingdom). Magnetoresistance was defined via the formula MR = 100 ×
Figure 1(a) shows HT-Ni3Sn2 with one hexago-nal layer of a fully occupied Ni(I) site and another hexagonal layer of a half-filled Ni(II) and Sn sites. Meanwhile, lowering the synthesis temperature changes the unit cell orthorhombically by aligning the Ni(II) sites, as shown in Fig. 1(b). Here, the top-view schematics of Figs. 1(c) and 1(d) represent the emergence of spin clusters, as the ordered Ni(II) sites create a line of spin heptamers onto the
Figure 2 displays the X-ray diffraction patterns analyzed to determine the orientation of the planes of HT-Ni3Sn2 and LT-Ni3Sn2, which are shown in Figs. 1(e) and 1(f), respectively. Their sharp peaks guaranteed the crystal’s growth. Still, since HT-Ni3Sn2 and LT-Ni3Sn2 were almost indiscernible during the analysis, we intentionally synthesized the Ni3Sn2 crystals by pulling the ampoule at 450 °C without annealing. As they showed a mixed phase where the peaks were separated, our observation strongly suggests that the HT-Ni3Sn2 and LT-Ni3Sn2 phases, grown under distinct temperatures, had not indented each other. The energy-dispersive spectroscopy (EDS) result for each sample was nominally Ni2.98Sn2 and Ni3.24Sn2. This implies that Ni3Sn2 crystals could have nonstoichiometric Ni3+
Figure 3 depicts resistivity as a function of temperature. We found that the residual resistivity (
Fascinatingly, the order of the resistivity was similar in LT-Ni3Sn2, where
The resistivity of metals is usually described using Matthiessen’s rule,
We used the power law model to analyze HT-Ni3Sn2 over a temperature range between 50 and 300 K and found that
Notably, HT-Ni3Sn2 and LT-Ni3Sn2 also exhibited different behaviors in magnetoresistance, as depicted in Fig. 4. In Fig. 4(a), we found no clear dependence of magnetoresistance against the magnetic field for the HT-Ni3Sn2, in contrast to the reference. This could be interpreted that our flux-grown HT-Ni3Sn2 may contain fewer Ni atoms or it has highly anisotropic transport behavior.
In contrast, Fig. 4(b) shows the magnetoresistance of LT-Ni3Sn2, which is related to the increase in the magnetic field. It increased abruptly under 1 T with decreasing temperature and achieved 1.18% at 14 T and 5 K. Such a dip might imply fast short-range spin correlations via the occurrence of spin clusters in LT-Ni3Sn2. It could simply be a signal of the weak antilocalization derived from strong spin–orbit coupling, as demonstrated using several topological materials. This behavior should be investigated further to elucidate the issue.
We successfully synthesized both single crystals of HT-Ni3Sn2 and LT-Ni3Sn2 using the flux method. The observed X-ray diffraction peaks imply that the phases are not merged and that each crystal shows that, as with the EDS results, they are not contaminated by flux. We found that both crystals are metallic with similar resistivity orders of magnitude. However, our analysis using the power law suggests LT-Ni3Sn2 may have stronger short-range excitations than HT-Ni3Sn2. In magnetoresistance, while HT-Ni3Sn2 was almost independent against the applied magnetic field, LT-Ni3Sn2 showed a strong dip at lower temperatures and weaker magnetic fields. This phenomenon might have something to do with its hidden nature in innate spin clusters. We hence conclude that LT-Ni3Sn2 could possess unusual physical properties, and further investigation would be required to elucidate speculations.
This work was supported by the DGIST institution specific program (21-BRP-07) and the faculty startup funds from DGIST.