Banca de QUALIFICAÇÃO: LAURA JINETH ZIPA ROMERO

Uma banca de QUALIFICAÇÃO de DOUTORADO foi cadastrada pelo programa.
DISCENTE: LAURA JINETH ZIPA ROMERO
DATA : 02/10/2026
LOCAL: Programa de Pós-Graduação em Ciência de Materiais
TÍTULO:

Electron paramagnetic resonance as a universal probe of local distortions, spin dynamics, and external stimuli in functional oxides: A comparative study of 𝑍𝑛𝑂: 𝐶𝑜, 𝑃𝑑 and 𝐵𝑖25𝐹𝑒𝑂40

 

PALAVRAS-CHAVES:

Zinc oxide, Cobalt oxide, Palladium oxide, Diluted magnetic semiconductors, electron paramagnetic resonance, Iron silenite, chemical pressure, spin-lattice relaxation, crystal field distortions, EasySpin simulations.

PÁGINAS: 141
RESUMO:

 

 

Diluted magnetic semiconductors (DMS) and complex functional oxides have attracted significant attention due to their potential applications in spintronics, magneto-optics, and photocatalysis. In this work, we present a comprehensive and integrated study based on three complementary investigations that employ X-band electron paramagnetic resonance (EPR) as the primary experimental tool to probe local magnetic environments, spin dynamics, and crystal field distortions in two distinct material systems: ZnO doped with Co2+ and Pd2+, and Bi25FeO40 (iron sillenite). In the first study, samples with nominal compositions Zn1−𝑥−𝑦Co𝑥Pd𝑦O (𝑥 = 0.03,
𝑦 = 0.00 and 0.03–0.05) were synthesized via the solid-state reaction method at 950 °C. X-ray diffraction confirmed the hexagonal wurtzite structure, and no secondary phases were detected. The EPR spectra revealed two characteristic resonances arising from Co2+ ions (3d7, 𝑆 = 3/2) substituting Zn2+ at tetrahedral sites: a low-field resonance at 𝑔 ≈ 4.42 (perpendicular component, 𝑔⊥) and a high-field resonance at 𝑔 ≈ 2.25 (parallel
component, 𝑔‖). These resonances originate from the lowest-lying Kramers doublet |𝑆,±1/2⟩, split from the |𝑆,±3/2⟩ state by a large positive axial zero-field splitting parameter 𝐷 ≈ 100 GHz. Upon Pd co-doping, a selective suppression of the resonance intensity was observed: the 𝑔 ≈ 4.42 signal remained essentially unchanged, whereas the 𝑔 ≈ 2.25 resonance exhibited a significant linewidth broadening of approximately 30% (from 541 Oe to 653 Oe at 100 K). Temperature-dependent EPR measurements from 100 K to 200 K showed that the linewidth follows a combination of direct (Δ𝐻 ∝ 𝑇) and Orbach relaxation mechanisms, described by Δ𝐻 = 𝐴 + 𝐵 · exp(−𝐶/𝑇 ) + 𝐸 · 𝑇. For the 𝑔 ≈ 2.25 resonance, Pd doping increased the constant term 𝐴 from 326 Oe to 410 Oe, the Orbach pre-factor 𝐵 from 2350 Oe to 9667 Oe, and the activation energy parameter 𝐶 from 631 K to 837 K, while the linear coefficient 𝐸 increased from 2.12 Oe/K to 2.31 Oe/K. These results indicate that Pd acts as a source of chemical pressure, selectively altering spin-lattice relaxation and local anisotropy along the c-axis. In the second study (Bi25FeO40), the material was synthesized by solid-state reaction at 750 °C, and Rietveld refinement confirmed a cubic sillenite structure (space group 𝐼23) with lattice parameter 𝑎 = 10.1863(2) Å. EPR spectra at room temperature exhibited a main asymmetric resonance centered around 330 mT (𝑔 ≈ 2.15), with a linewidth of approximately 58 mT. Simulations using EasySpin showed that the left portion of the resonance could be well reproduced by Fe3+ (𝑆 = 5/2) with axial zero-field splitting 𝐷 = 0.020 cm−1 and rhombic distortion 𝐸 = 0.0065 cm−1, while the right portion (𝑔 ≈ 2.00) was attributed to defect-related paramagnetic centers, likely oxygen vacancies. Upon continuous illumination with 365 nm UV light (3.40 eV, above the optical band gap of 2.80 eV), a selective and progressive suppression of the 𝑔 ≈ 2.00 component was observed, without significantly affecting the Fe3+-related 𝑔 ≈ 2.15 signal. This photoinduced
effect was most pronounced at an intermediate microwave power of 10 mW and persisted down to 100 K. The suppression is interpreted as a result of UV-induced delocalization of electrons trapped at vacancy sites, which shortens the transverse relaxation time 𝑇2 and broadens the resonance beyond detection, effectively reducing the EPR intensity. No new paramagnetic species were generated, indicating a reversible redistribution of spin populations. In the third study (ZnO:Co,Pd under hydraulic pressure), the same ZnO:Co and ZnO:Co,Pd samples were subjected to uniaxial ex situ mechanical pressure of 686 ± 20 MPa using a hydraulic press for 24 hours. XRD analysis revealed a reduction in lattice parameters and a marked increase in microstrain: from 0.58×10−3 to 2.22×10−3 for the Co-doped sample, and from 0.47 × 10−3 to 1.18 × 10−3 for the Co-Pd codoped sample. Raman spectroscopy showed that the 𝐸2(high) and 𝐸2(low) modes, associated with inplane lattice vibrations, increased in intensity under pressure, indicating the development of rhombic distortions within the basal plane. EPR measurements at 104 K demonstrated that, contrary to the effect of chemical pressure from Pd, uniaxial hydraulic pressure primarily broadened the perpendicular resonance (𝑔⊥ ≈ 4.42), while leaving the parallel resonance (𝑔‖ ≈ 2.25) almost unaffected. The linewidth of the 𝑔⊥ resonance increased from 119 Oe to 207 Oe in ZnO:Co, and from 100 Oe to 172 Oe in ZnO:Co,Pd. Simulations of the spin Hamiltonian with 𝑆 = 3/2, an isotropic 𝑔 = 2.24, and a fixed axial parameter 𝐷 = 24000 MHz showed that introducing a rhombic term 𝐸 = 690 MHz accurately reproduced the pressure-induced broadening and line shape changes. The ratio 𝐸/𝐷 ≈ 3 × 10−2 correlates well with the microstrain values (≈ 10−3), indicating that EPR is approximately an order of magnitude more sensitive to local distortions than XRD. Taken together, these three studies demonstrate that Co2+ and Fe3+ ions act as versatile local probes capable of distinguishing between different types of external perturbations. In ZnO:Co, chemical pressure from Pd selectively affects the parallel resonance (𝑔‖), while hydraulic pressure selectively broadens the perpendicular resonance (𝑔⊥). This orthogonal sensitivity allows Co2+ to function as a dual sensor for distinguishing chemical and mechanical stress. In Bi25FeO40, UV light selectively modulates defect-related resonances without affecting the Fe3+ signal, opening possibilities for optically controlled magnetic centers. The combination of experimental EPR, structural characterization (XRD, SEM, Raman), and numerical simulations (EasySpin) provides a consistent microscopic picture of how axial and rhombic crystal field distortions, spin-lattice relaxation mechanisms, and photoinduced charge redistribution govern the magnetic response of these functional materials.

MEMBROS DA BANCA:
Externa à Instituição - LETICIE MENDONÇA FERREIRA - UFABC
Presidente - 2457389 - EDUARDO PADRON HERNANDEZ
Externa ao Programa - 3310101 - LIDIA CARVALHO GOMES - UFPE