Determination of the tricritical point,H-Tphase diagram and exchange interactions in the antiferromagnet MnTa2O6.

Using the analysis of the temperature and magnetic field dependence of the magnetization (M) measured in the temperature range of 1.5 K to 400 K in magnetic fields up to 250 kOe, the magnetic field-temperature (H-T) phase diagram, tricritical point and exchange constants of the antiferromagnetic MnTa2O6are determined in this work. X-ray diffraction/Rietveld refinement and x-ray photoelectron spectroscopy of the polycrystalline MnTa2O6sample verified its phase purity. Temperature dependence of the magnetic susceptibilityχ(=M/H) yields the Néel temperatureTN= 5.97 K determined from the peak in the computed ∂(χT)/∂TvsTplot, in agreement with theTN= 6.00 K determined from the peak in theCPvsTdata. The experimental data ofCPvsTnearTNis fitted toCP=A|T-TN|-αyielding the critical exponentα= 0.10(0.13) forT>TN(T 25 K fits well with the modified Curie-Weiss law:χ=χ0+C/(T-θ) withχ0= -2.12 × 10-4emu mol-1 Oe-1yieldingθ= -24 K, andC= 4.44 emu K mol-1 Oe-1, the later giving magnetic momentµ= 5.96 µBper Mn2+ion. This yields the effective spinS= 5/2 andg= 2.015 for Mn2+, in agreement withg= 2.0155 measured using electron spin resonance spectroscopy. Using the magnitudes ofθandTNand molecular field theory, the antiferromagnetic exchange constantsJ0/kB= -1.5 ± 0.2 K andJ⊥/kB= -0.85 ± 0.05 K for Mn2+ions along the chainc-axis and perpendicular to thec-axis respectively are determined. TheχvsTdata when compared to the prediction of a Heisenberg linear chain model provides semiquantitative agreement with the observed variation. TheH-Tphase diagram is mapped using theM-Hisotherms andM-Tdata at differentHyielding the tricritical pointTTP(H,T) = (17.0 kOe, 5.69 K) separating the paramagnetic, antiferromagnetic, and spin-flop phases. At 1.5 K, the experimental magnitudes of the exchange fieldHE= 206.4 kOe and spin-flop fieldHSF= 23.5 kOe yield the anisotropy fieldHA= 1.34 kOe. These results for MnTa2O6are compared with those reported recently in the isostructural MnNb2O6.

Magnetic field-temperature phase diagram, exchange constants and specific heat exponents of the antiferromagnet MnNb2O6.

This work presents the magnetic field-temperature (H-T) phase diagram, exchange constants, specific heat (CP) exponents and magnetic ground state of the antiferromagnetic MnNb2O6polycrystals. Temperature dependence of the magnetic susceptibilityχ(=M/H) yields the Néel temperatureTN= 4.33 K determined from the peak in the computed ∂(χT)/∂TvsTplot in agreement with the transition in theCPvsTdata atTN= 4.36 K. The experimental data ofCPvsTnearTNis fitted toCP=A|T-TN|-αyielding the critical exponentα= 0.12 (0.15) forT>TN(T 50 K toχ=χ0+C/(T-θ) withχ0= -1.85 × 10-4emu mol-1Oe-1yieldsθ= -17 K, andC= 4.385 emu K mol-1Oe-1, the latter giving magnetic momentµ= 5.920µBper Mn2+ion. This confirms the effective spinS= 5/2 andg= 2.001 for Mn2+and the dominant exchange interaction being antiferromagnetic in nature. Using the magnitudes ofθandTNand molecular field theory (MFT), the exchange constantsJ0/kB= -1.08 K for Mn2+ions along the chainc-axis andJ⊥/kB= -0.61 K as the interchain coupling perpendicular toc-axis are determined. These exchange constants are consistent with the expectedχvsTvariation for the Heisenberg linear chain. TheH-Tphase diagram, mapped using theM-Hisotherms andM-Tdata at differentHcombined with the reported data of Nielsenet al, yields a triple-pointTTP(H,T) = (18 kOe, 4.06 K). The spin-flopped state aboveTTPand the forced ferromagnetism forH> 192 kOe are used to estimate the anisotropy energyHA≈ 0.8 kOe.

Generalized Brewster Angle Effect in Thin-Film Optical Absorbers and Its Application for Graphene Hydrogen Sensing.

The generalized Brewster angle (GBA) is the incidence angle at polarization by reflection for p- or s-polarized light. Realizing an s-polarization Brewster effect requires a material with magnetic response, which is challenging at optical frequencies since the magnetic response of materials at these frequencies is extremely weak. Here, we experimentally realize the GBA effect in the visible using a thin-film absorber system consisting of a dielectric film on an absorbing substrate. Polarization by reflection is realized for both p- and s-polarized light at different angles of incidence and multiple wavelengths. We provide a theoretical framework for the generalized Brewster effect in thin-film light absorbers. We demonstrate hydrogen gas sensing using a single-layer graphene film transferred on a thin-film absorber at the GBA with ∼1 fg/mm2 aerial mass sensitivity. The ultrahigh sensitivity stems from the strong phase sensitivity near the point of darkness, particularly at the GBA, and the strong light-matter interaction in planar nanocavities. These findings depart from the traditional domain of thin films as mere interference optical coatings and highlight its many potential applications including gas sensing and biosensing.

Catalyst-Free Plasma Enhanced Growth of Graphene from Sustainable Sources.

Details of a fast and sustainable bottom-up process to grow large area high quality graphene films without the aid of any catalyst are reported in this paper. We used Melaleuca alternifolia, a volatile natural extract from tea tree plant as the precursor. The as-fabricated graphene films yielded a stable contact angle of 135°, indicating their potential application in very high hydrophobic coatings. The electronic devices formed by sandwiching pentacene between graphene and aluminum films demonstrated memristive behavior, and hence, these graphene films could find use in nonvolatile memory devices also.