Multiple magnetic phase transitions, electrical and optical properties of FeTe₂ single crystals

Abstract

Single-crystalline FeTe 2 in marcasite phase with orthorhombic structure was prepared via chemical vapor transport. Cooling FeTe 2 single crystals from room temperature down to 2 K, multiple magnetic phase transitions were observed. Paramagnetic (PM) to antiferromagnetic (AFM) and then to ferromagnetic (FM) occurred at 79 K and 35 K for in-plane, 73 K and 29 K for out-of-plane, respectively. A strong uniaxial magnetic anisotropy was found due to FeTe 6 octahedron distortion and structural modulation in FM region. The novel negative volume expansion (NVE) initiated in the vicinity of AFM to FM transition. An abrupt frequency shift of the most intense mode at 118 cm −1 and evolution of the Te-Te stretching mode near 40 K, corresponding to the phase transition from AFM to FM were observed. The temperature-dependent resistance revealed an anomaly (semiconductor to metallic transition) around AFM-FM transition, which can easily be suppressed and move to high temperature by the applied magnetic field. The results from XRD, Raman and resistivity indicated that the structural parameters, vibration frequency and transport are sensitive to the phase transition from AFM to FM. The nature of direct band gap with 0.67 eV was identified through UV-Vis-NIR spectrum of FeTe 2 single crystals at room temperature.

Synthesis and Characterization of Ultrathin FeTe2 Nanocrystals

“… 30 The first two peaks are attributed to the B 1g mode associated with antiphase Te–Te vibration, whereas the 154 cm –1 mode is associated with A 1g in-phase mixed Te–Te stretching related to mutual Fe–Te motion within FeTe 6 octahedron and is sensitive to changes in bond lengths. 7 , 30 Raman spectra in this work ( Figures 7 and 8 ) support the X-ray diffraction and energy-dispersive X-ray conclusion that FeTe 2 phase crystals were formed in this experiment.…”

“… 5 Early studies show that FeTe 2 crystals have an anisotropic antiferromagnetic ground state below about 80 K and a possible ferromagnetic (FM) state below approximately 30 K. 6 − 8 The electronic structure features a reported band gap of 0.35–0.67 eV, as deduced from electrical transport measurements. 7 , 9 FeTe 2 thin films have been found to feature a low thermal conductivity value of 1.20 W m –1 K –1 . 10 Electron-doped marcasite FeTe 2 crystals are a good candidate for thermoelectric applications due to the sharp increase in density of states around the band edges, which favors a large increase in the thermoelectric power factor S 2 σ, where S is the thermopower and σ is the electrical conductivity.…”

Synthesis, Transfer, and Properties of Layered FeTe2 Nanocrystals

“…27,30−32 Marcasite FeTe 2 (m-FeTe 2 ) is considered as antiferromagnetic, 33,34 and antiferromagnetic to ferromagnetic transition accompanied by semiconductormetallic transition was observed recently. 35 Different from bulk m-FeTe 2 , monolayer FeTe 2 is calculated to be ferromagnetic. 36−38 Hence, synthesis layered FeTe 2 , particularly using a simple method, for example, chemical vapor deposition (CVD), is highly desired, and their properties and applications call for studies.…”

“…Layered iron chalcogenides such as selenide (FeSe) − and tellurium-doped FeSe (FeTe x Se 1–x ) , are well-known superconductors, although the parent compound FeTe is nonsuperconducting and has an antiferromagnetic ground state. ,, A minor change in their structure or element composition will strongly affect their electronic and magnetic properties, which are closely correlated. − The study of low-dimensional materials with simple crystal structure and high anisotropy is helpful for understanding the superconductivity and magnetism of these compounds and the interplay between them. Iron dichalcogenides FeX 2 (X = S, Se, Te) are typically crystallized in either nonlayered pyrite or nonlayered marcasite phase structures. − Pyrite phase FeS 2 , marcasite phase FeSe 2 and FeTe 2 are the most stable structures at ambient condition, and all are semiconductors with the band gap decreasing as the chalcogen atom number increases. − Those materials would be potential candidates for solar cells, optoelectronic devices, photocatalysis, and thermoelectronics due to their narrow band gap, large optical absorption coefficient, and large thermoelectric power. ,− Marcasite FeTe 2 ( m -FeTe 2 ) is considered as antiferromagnetic, , and antiferromagnetic to ferromagnetic transition accompanied by semiconductor-metallic transition was observed recently . Different from bulk m -FeTe 2 , monolayer FeTe 2 is calculated to be ferromagnetic. − Hence, synthesis layered FeTe 2 , particularly using a simple method, for example, chemical vapor deposition (CVD), is highly desired, and their properties and applications call for studies.…”

Disorder-induced time effect in the antiferromagnetic domain state of Fe1+Te

“…We assign this ferromagnetic signal to minor ferromagnetic FeTe2 nanoplatelets known to form during sample growth. No anomalies appear in our specific heat measurements at reported phase transition temperatures of large-scale single crystalline FeTe2 [29,30]. Disorder generally introduces nucleation centers for domain walls, allowing them to restore the multi-domain state on faster time scales, fast enough to be detectable via synchrotron-based XAS experiments under UHV conditions.…”