This study reports on the modulation frequency dependence of the photothermally modulated magnetic resonance signal of a set of magnetic samples in the form of foils, layers, and thin films: $50\mu \mathrm{m}$ Fe and Ni foils; $5\mu \mathrm{m}$ magnetic layer of $\gamma \text{-}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$ in a cassette tape; and $150\mathrm{nm}$ Co and Permalloy films deposited on glass and Si (111) substrates, besides the naked Si substrate. It is shown, both by analytical calculation and by measurements, that the skin depth of the microwaves deeply influences the signal behavior by selecting the portion of the sample that is probed. Clear differences in the frequency dependence are observed between the metallic Ni and Fe foils and the dielectric $\gamma \text{-}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$ cassette tape. Furthermore, the thermal mismatch between the magnetic films (Co and Permalloy) and substrates (glass and Si) also plays a crucial role, once the modulation of the temperature is strongly dependent on the substrate thermal parameters at low modulation frequencies. The non-resonant signal from the diamagnetic Si is also analyzed. It is produced by the absorption of microwaves by the photo-injected free carriers and presents characteristic behavior in the investigated frequency range.

This report describes the photoacoustic and electron paramagnetic resonance investigations of Brazilian and Cuban zeolites. Photoacoustic optical absorption measurements indicate the presence of iron (Fe3+) ions with their respective transition bands for both zeolites. Two species of manganese (Mn2+ and Mn3+) were identified in the Cuban sample and the electronic transitions assigned. Iron and manganese ions were confirmed through nonradiative relaxation (τ) and characteristic diffusion (τβ) times evaluation, whose values were found to be τBRA = 5.40 ms, τCUB = 4.60 ms, τβBRA = 387 μs and τβCUB = 305 μs. Crystal field (Dq-BRA/Dq-CUB = 1048 cm-1/945 cm-1) plus Racah (B-BRA/B-CUB = 457 cm-1/813 cm-1 and C-BRA/C-CUB = 3655 cm-1/2496 cm-1) parameters were assessed as well. Paramagnetic resonance corroborated Fe3+ ions present in the Brazilian zeolite occupying sites showing axial and/or rhombic symmetry distortions. For the Cuban sample, results reveal the characteristic hyperfine sextet lines of Mn2+ overlapping the Fe3+ line. Values of Landé factor and isotropic hyperfine splitting constant were found to be 2.0 and 9.7 mT, respectively. This tells us that the Mn2+ lies in octahedral symmetry probably replacing calcium ions and point towards an ionic bonding character of the Mn2+ and its surroundings.

Marble waste is produced on a large scale in many countries, resulting in serious pollution problems. This investigation aimed to study the valorization potential of marble waste from the ornamental rock industry used in the synthesis of a novel calcium niobate–magnesium niobate composite powder prepared by a solid-state reaction between 1000 °C and 1200 °C. The chemical and mineralogical characteristics of the marble waste were determined. Structural and morphological characterizations of the synthesized calcium niobate–magnesium niobate composite powders were conducted by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The thermophysical properties were measured using open photoacoustic cell and photothermal techniques. Structurally, at all synthesis temperatures, the calcium niobate–magnesium niobate powders were found to be composed of a complex mixture of CaNb₂O₆/Ca₂Nb₂O₇/MgNb₂O₆/CaMg_0.33Nb_0.67O₃. In addition, the calcium niobate–magnesium niobate composite powders exhibited low values of thermal diffusivity (1.88–2.15 × 10⁻⁷ m²/s) and thermal conductivity (0.12–0.16 W/mK). The findings of this investigation highlight the potential of marble waste as a promising sustainable source of carbonate for obtaining calcium niobate–magnesium niobate composite powder, which has thermophysical properties that should be explored in low-thermal-conductivity applications.