We report dielectric spectroscopy of single-crystalline NdMgAl11O19, a magnetoplumbite hexaaluminate in which localized Nd^3+ moments coexist with a polarizable AlO5 bipyramidal network. The real part of the permittivity, ε'_c(T), measured along the crystallographic c axis, increases as the temperature is lowered from 275 K to 30 K and is frequency-independent between 4 Hz and 50 kHz. At lower temperatures, a frequency-dependent decrease in permittivity is observed, followed by a further upturn below 2 K. The high-frequency ε'_c(T) is described by a Barrett formula supplemented by an effective two-level contribution, yielding a robust gap of Δ= 25.85 ± 0.32 K consistent with the lowest Nd^3+ crystal-electric-field (CEF) splitting. Below ∼ 30 K, the dielectric response becomes strongly frequency and magnetic-field dependent. Isothermal ε_c'(H) measurements reveal a reproducible low-field crossover near μ_0H_c ≃ 0.85 T, which we attribute to the competition between antiferromagnetic correlations and Zeeman splitting of the ground-state Kramers doublet. NdMgAl11O19 thus provides a Kramers reference system in which dielectric signatures of the excited-state CEF manifold can be distinguished from those of the field-tuned, correlation-dominated ground-state doublet sector in a centrosymmetric frustrated magnetoplumbite host

Voltage-tunable capacitors (varactors) are key to microwave circuits. Tunable dielectric varactors outperform competing technologies in almost every relevant metric but usually suffer from high dielectric loss. In contrast, Ruddlesden-Popper (RPs) dielectric thin films have remarkably low microwave loss. Unfortunately, their crystallographic symmetry has until recently dictated an in-plane device structure, precluding the favorable out-of-plane parallel-plate varactor design for minimized size and maximized electric field in the tunable dielectric. Guided by theory, we report RPs akin to the widely studied tunable microwave dielectric BaxSr1-xTiO3. Assembling these same atoms into the first RP phase with broken out-of-plane symmetry, we achieve a low-loss, out-of-plane tunable dielectric thin film. The highest performing film, (ATiO3)nAO film with A = Ba0.45Sr0.55 and n = 8, unlocks a tenfold improvement in the figure of merit for out-of-plane tunable dielectrics at 10 GHz, paving the way for a new generation of tunable monolithic microwave integrated circuits.

Voltage-tunable capacitors (varactors) are key components in microwave circuits. Tunable dielectric varactors can outperform competing technologies but typically suffer from high dielectric loss. Ruddlesden-Popper dielectric thin films can, by contrast, offer low microwave loss. Unfortunately, their crystallographic symmetry is usually not compatible with an out-of-plane parallel-plate varactor design, which can minimize size and maximize the electric field in the tunable dielectric compared with an in-plane device design. Here we show that a low-loss and tunable Ruddlesden-Popper dielectric thin film that is compatible with the parallel-plate varactor design can be created by breaking this crystallographic symmetry. We study films that are similar to the widely studied tunable microwave dielectric Ba0.45Sr0.55TiO3 but have a Ba0.45Sr0.55O rock-salt layer for every n-perovskite unit cells. The film with n = 8 exhibit an optimum combination of tunability and loss, with a material quality factor of around 200 and a relative tunability of 51% at an applied electric field of 250 kV cm-1, which results in a dielectric tuning figure of merit of 100 at 10 GHz.

We report broadband dielectric spectra of the non-Kramers hexaaluminate PrMgAl11O19, revealing a pronounced interplay between permittivity and magnetization at cryogenic temperatures. The zero-field dielectric response follows a Barrett-type quantum-paraelectric form, while a broad dielectric anomaly near 5 K shows a complex field dependence that mirrors the multi-hump behavior of the magnetic specific heat, evidencing robust magnetoelectric coupling. The inverse permittivity epsilon('-1) (T, H) scales linearly with M-2, consistent with a biquadratic (PM2)-M-2 term in a Landau framework. Fits yield a temperature-dependent coupling constant lambda(T) that decreases with heating from (1.07 +/- 0.01) x 10(-4) mu B-2 (at 5 K) to (4.77 +/- 0.02) x 10(-5) mu B-2 (at 10 K), reflecting the thermal population of low-lying energy levels of Pr3 +. Consistently, the uniaxial thermal expansion develops an additional low-temperature hump below similar to 30 K that is progressively suppressed by magnetic field, recovering an approximately saturated response by 9 T. These results identify PrMgAl11O19 as a paradigmatic non-Kramers hexaaluminate where quantum paraelectricity and magnetoelectric interactions are intrinsically entangled, establishing hexaaluminates as a tunable platform for magnetoelectric physics in frustrated quantum materials.

Raman scattering measurements confirmed the theoretical prediction that the structural phase transition from the achiral tetragonal to the chiral tetragonal phase, which occurs near 400 K, is induced by a doubly degenerate soft phonon at the Z point of the Brillouin zone. In the low-temperature chiral phase, the soft mode activates in Raman spectra, splits into two components with A1 and B1 symmetries and harden with cooling according to Cochran law. Circularly polarized Raman scattering did not reveal the angular momentum of these singly degenerate phonons at the Gamma point, which is consistent with theory. We also calculated the phonon branches in the whole Brillouin zone for both crystalline phases and compared the results with the phonons observable in the Raman spectra. The calculations revealed that some phonons with nonzero k have angular momentum in the chiral phase. A pronounced circular motion of atoms can be observed, for example, in a Dirac-type topological phonon at the M-point of the Brillouin zone with a frequency of 168 cm-1.

Magnetic control of correlated spin systems is central to the development of next-generation spin-based technologies. Rare-earth orthoferrites provide an interesting platform in which exchange coupling between rare-earth 4f and transition-metal 3d moments generates competing magnetic interactions and multiple metastable states. Here, we show that the orientation of the applied magnetic field drives different magnetic phase transition sequences in NdFeO3 across a broad temperature range. Using Raman and polarized terahertz spectroscopies, supported by magnetization and specific-heat measurements, we track the temperature- and field-dependent evolution of the different magnetic phases and the successive spin rearrangements, driven by 4f - 3d magnetic anisotropic interactions. For fields applied along the crystallographic c-axis, a spin-reorientation transition is followed by spin-flop and spin-flip processes, producing an unexpectedly complex magnetic phase sequence at low temperatures. Below 8 K, precursor effects associated with ordering of the Nd-sublattice strongly modify the transition pathway. Our results demonstrate how anisotropic 4f-3d coupling enables magnetic-field control of coupled spin excitations and provide a route to accessing novel spin configurations in rare-earth orthoferrites.

We report dielectric spectroscopy of single-crystalline NdMgAl 11 O 19 , a magnetoplumbite hexaaluminate in which localized Nd 3 + moments coexist with a polarizable AlO 5 bipyramidal network. The real part of the permittivity ɛ c ′ ( T ) , measured along the crystallographic c axis, increases as the temperature is lowered from 275 K to 30 K and is frequency-independent between 4 Hz and 50 kHz. At lower temperatures, a frequency-dependent decrease in permittivity is observed, followed by a further upturn below 2 K. The high-frequency ɛ c ′ ( T ) is described by a Barrett formula supplemented by an effective two-level contribution, yielding a robust gap of Δ = 25.9 ( 3 ) K consistent with the lowest Nd 3 + crystal-electric-field (CEF) splitting. Below ∼ 30 K , the dielectric response becomes strongly frequency and magnetic-field dependent. Isothermal ɛ c ′ ( H ) measurements reveal a reproducible low-field crossover near μ 0 H c ≃ 0.85 T , which we attribute to the competition between antiferromagnetic correlations and Zeeman splitting of the ground-state Kramers doublet. NdMgAl 11 O 19 thus provides a Kramers reference system in which dielectric signatures of the excited-state CEF manifold can be distinguished from those of the field-tuned, correlation-dominated ground-state doublet in the centrosymmetric frustrated magnetoplumbite host.

Recent dielectric and magnetic studies of (Ca-0.Mn-5(1).(5))MnWO6 ceramics [A.A. Belik, Chem. Mater. 36, 7604 (2024)] have classified this material as a rare hybrid multiferroic, with both antiferromagnetic and (anti)ferroelectric ordering occurring at the same temperature of 22 K. The pronounced dielectric anomaly observed at this temperature indicated that the structural change is primarily induced by a phonon soft mode and not by a spin arrangement, as is usually the case in type II multiferroics. However, our comprehensive investigation involving new ceramic samples as well as the sample from the above-mentioned reference does not support this conclusion. Low-temperature polarization measurements revealed no evidence of either ferroelectric or antiferroelectric order in both sample series. The dielectric permittivity exhibits only a slight change at the antiferromagnetic transition, and phonon modes observed in IR and Raman spectra show no indication of a symmetry change at low temperatures. In the new samples the N & eacute;el temperature is shifted to T-N = 18 K. XRD, SEM, EDS and WDS analyses confirmed the composition (Ca-0.Mn-5(1).(5))MnWO6 of both ceramics, but also indicated a small amount (percentage points) of MnO and CaO impurities in the sample from the previous publication and Mn3O4, CaWO4 secondary phases (<4%) in the new ceramics. The differences in dielectric and magnetic properties of the two samples can therefore be explained by their different chemical purity. The small dielectric anomaly of the new sample at the antiferromagnetic transition temperature is explained by a spin-phonon coupling. We conclude that (Ca-0.Mn-5(1).(5))MnWO6 is not a multiferroic, but a paraelectric antiferromagnet.

Bi2O2Se belongs to a group of quasi-2D semiconductors that can replace silicon in future high-speed/lowpower electronics. However, the correlation between crystal/band structure and other physical properties still eludes understanding: carrier mobility increases non-intuitively with carrier concentration; the observed T2 temperature dependence of resistivity lacks explanation. Moreover, a very high relative out-of-plane permittivity of about 150 has been reported in the literature. A proper explanation for such a high permittivity is still lacking. We have performed infrared (IR) reflectivity and Raman scattering experiments on a large perfect single crystal with defined mosaicity, carrier concentration, and mobility. Five of the eight phonons allowed by factor group theory have been observed and their symmetries determined. The IR spectra show that the permittivity measured in the tetragonal plane is as high as epsilon r approximate to 500, and this high value is due to a strong polar phonon with a low frequency of '34 cm-1 ('1 THz). Such an unusually high permittivity allows the screening of charge defects, leading to the observation of high electron mobility at low temperatures. It also allows effective modulation doping providing a platform for high-performance 2D electronics. DFT calculations suggest the existence of a very low-frequency acoustic phonon '14 cm-1 ('0.4 THz). Both the low-frequency phonons cause anomalous phonon DOS, which is reflected in the unconventional temperature dependence of the heat capacity, Cp approximate to T3.5. The temperature-dependent, two-component group velocity is proposed to explain the unusual temperature dependence of the thermal conductivity, kappa approximate to T1.5.

The role of ambient gas in the epitaxial growth of manganese oxides on MgO by pulsed laser deposition was investigated by performing complex morphological and structural characterization of thin films while simultaneously monitoring the growth process with electrical probes. The growth in N2 atmospheres leads to the formation of highly oriented MnO coatings that present compressive in-line stress and contain highly oriented defects, which are induced during growth by the high kinetic energy of the plasma. The presence of O2 during unlocks the growth of highly crystalline Mn3O4 rotated by 45 deg to MgO, with X-ray photoelectron spectroscopy (XPS) measurements showing the contribution of the MnO bonding states on the surface on Mn3O4 due to ambient contamination of the sample. The ability of each atmosphere to promote the growth of two selected oxide phases was shown by both ellipsometry and infrared spectroscopy measurements. The in situ monitoring of growth highlights the ion acceleration caused by the use of N2 compared with O2 and the formation of a plasma environment that is optimal for the formation of molecular species as well as a strong oxide contribution to growth.

Nanostructured black metal (BM) layers represent a valuable material for many technological applications. Their unique properties resulting from their structure have led to their use as electromagnetic absorbers, gas sensing devices, and electronics. Although black gold films have been extensively studied, there is still a gap in the use of evaporated BM films for sensor applications. This paper compares the material and optical properties of reflective and black gold films, both of which were prepared by applying the thermal evaporation technique in a residual argon atmosphere. The impact of the nanostructured surface is discussed from the perspectives of morphology, chemical composition, emissivity, reflectivity, and absorbance. Microscopic analyses revealed that black gold comprises nanograins aggregated into a cauliflower-like structure. Positron annihilation spectroscopy reported a high concentration of pores in the black gold layer with a mean size of 1.1 nm. This highly porous structure allows high light absorbance in the visible wavelength range and strong electron plasma interactions in the infrared wavelength range. Preliminary results for gas sensing response to NO2, CH3CH2OH, NH3 and CO reveal the promising use of black gold coating for sensor applications.

Bi2O2Se belongs to a group of quasi-2D semiconductors that can replace silicon in future high-speed/low-power electronics. However, the correlation between crystal/band structure and other physical properties still eludes understanding: carrier mobility increases non-intuitively with carrier concentration; the observed T^2 temperature dependence of resistivity lacks explanation. Moreover, a very high relative out-of-plane permittivity of about 150 has been reported in the literature. A proper explanation for such a high permittivity is still lacking. We have performed infrared (IR) reflectivity and Raman scattering experiments on a large perfect single crystal with defined mosaicity, carrier concentration and mobility. Five of the eight phonons allowed by factor group theory have been observed and their symmetries determined. The IR spectra show that the permittivity measured in the tetragonal plane is as high as ε_r≈500, and this high value is due to a strong polar phonon with a low frequency of 34 cm^-1 ( 1 THz). Such an unusually high permittivity allows the screening of charge defects, leading to the observation of high electron mobility at low temperatures. It also allows effective modulation doping providing a platform for high performance 2D electronics. DFT calculations suggest the existence of a very low frequency acoustic phonon 14 cm^-1 ( 0.4 THz). Both the low frequency phonons cause anomalous phonon DOS, which is reflected in the unconventional temperature dependence of the heat capacity, c_M≈T^3.5. The temperature-dependent, two-component group velocity is proposed to explains the unusual temperature dependence of the thermal conductivity, κ≈T^1.5

We assessed four exchange-correlation functionals (LDA CA-PZ, GGA parametrized by PBE, PBEsol, and WC) in predicting the lattice parameters of SrTi1-xMnxO3 perovskites, assuming cubic structures. Predictions were verified using X-ray diffraction (XRD) for Mn content of x = 0.0, 0.1, 0.2, 0.3, 0.5, 1.0, confirming cubic symmetry and a linear decrease in lattice parameters with increasing Mn. PBEsol, and WC demonstrated the highest precision (deviations <0.20 %). Additionally, bulk moduli were calculated using the same functionals and verified with the experimental bulk modulus of SrTiO3 (183 +/- 2 GPa, Pulse-Echo method). The predicted bulk moduli exhibited a slow, linear increase with increasing Mn. The best correspondence with the experimental bulk modulus was achieved by PBEsol and WC (deviations <0.7 %). These findings highlight the reliability of PBEsol and WC functionals for accurately modeling structural properties of SrTi1-xMnxO3 perovskites, having better precision than commonly employed LDA and PBE functionals.

We report dielectric and magnetoelectric studies of single-crystalline CeMgAl11O19, a Kramers triangular magnet embedded in a polarizable hexaaluminate lattice. In zero magnetic field, the permittivity epsilon'(T ) follows the Barrett law of a quantum paraelectric down to <^>25 K, below which a broad minimum develops near 3 K without evidence of static ferroelectric or magnetic order. Application of magnetic fields up to 9 T shifts this minimum to higher temperatures and broadens it, evidencing a tunable magnetoelectric response. The magnetoelectric coupling was characterized using results from magnetization measurements. The anomaly temperature T*, extracted from the local minimum of epsilon'(T ), exhibits a linear dependence on the squared magnetization M2, consistent with the biquadratic magnetoelectric coupling allowed in centrosymmetric systems. This magnetoelectric effect, mediated by spin-orbit-entangled Kramers doublets interacting with a frustrated antipolar liquid, establishes CeMgAl11O19 as a prototype for exploring quantum magnetoelectricity in frustrated systems.

The correlation between static magnetoelectric coupling and magnetic structures was investigated in $TbMn_{0.98}Fe_{0.02}O_{3}$ with magnetic field up to 8 T and down to 2 K. Neutron diffraction experiments reveal a substantial increase in the temperature dependence of the incommensurate modulation vector of the antiferromagnetic phase, responsible for the significant increase in magnetoelectric coupling, i.e., stronger changes of ferroelectric polarization with applied magnetic field than in other multiferroic materials. This shows that even a small 2% substitution of $Mn^{3+}$ by $Fe^{3+}$ significantly enhances the destabilization of the incommensurably modulated magnetic cycloidal structure of $TbMnO_{3}$ in a magnetic field above 5 T.

We report on the microwave, terahertz (THz), infrared, and Raman spectroscopic studies of BiMn 7 O 12 ceramics, shedding more light onto the nature of two structural phase transitions and their possible relation with ferroelectricity in this compound. We observed a softening of one polar phonon in the THz range on cooling towards 460 and 300 K, i.e., temperatures at which BiMn 7 O 12 undergoes subsequent structural phase transitions from monoclinic I 2/ m to polar monoclinic Im and triclinic P 1 phases. The soft phonon causes dielectric anomalies typical for displacive ferroelectric phase transitions. Microwave measurements performed at 5.8 GHz up to 400 K qualitatively confirmed not only the dielectric anomaly at 300 K, but also revealed two other weak dielectric anomalies near the magnetic phase transitions at 60 and 28 K. This evidences the multiferroic nature of the low -temperature phases, although the relatively high conductivity in the kHz and Hz spectral range prevented us from directly measuring the permittivity and ferroelectric polarization. Some Raman modes sense the magnetic phase transitions occurring near 60 and 25 K, showing that spin -phonon coupling is relevant in this compound and in this temperature range. The deviation of the Mn-O stretching mode frequency from the anharmonic temperature behavior was successfully explained by the spin correlation function calculated from the magnetic contribution to the specific heat.

Microwave (MW) transmission, absorption, and reflection loss spectra of the ferrimagnetic U-type hexaferrite Sr4CoZnFe36O60 ceramics were studied from 100 MHz to 35 GHz at temperatures between 10 and 390 K. Nine MW magnetic excitations with anomalous behavior near ferrimagnetic phase transitions were revealed. They also change under the application of the weak bias magnetic field (0–700 Oe) at room temperature. Six pure magnetic modes are assigned to dynamics of the magnetic domain walls and inhomogeneous magnetic structure of the ceramics, to the natural ferromagnetic resonance (FMR), and to the higher-frequency magnons. Three modes are considered the magnetodielectric ones with the dominating influence of the magnetic properties on their temperature and field dependences. The presence of the natural FMR in all ferrimagnetic phases proves the existence of the non-zero internal magnetization and magnetocrystalline anisotropy. Splitting of the FMR into two components without magnetic bias was observed in the collinear phase and is attributed to a change in the magnetocrystalline anisotropy during phase transition. The high-frequency FMR component critically slows down to phase transition. At room temperature, FMR splitting and essential suppression of the higher-frequency modes were revealed under the weak bias field (300–700 Oe). The highly nonlinear MW response and FMR splitting are caused by the gradual evolution of the polydomain magnetic structure to a monodomain one. The high number of magnetic excitations observed in the MW region confirms the suitability of using hexaferrite Sr4CoZnFe36O60 ceramics as MW absorbers, shielding materials and highly tunable filters.

The possibility of inducing new polar and/or magnetic transient states through the pumping of optical phonons towards the non-linear regime has renewed the scientific interest in orthoferrites. Nonetheless, to perform these studies it is fundamental to have a deep knowledge of the lattice excitations at equilibrium conditions. In this work, we present a complete characterization of the optically-active zone-center phonons in NdFeO3 single crystals at room temperature by means of polarized Raman and infrared spectroscopies. The study is complemented with polarized infrared spectroscopy at 4 K and unpolarized Raman scattering at 10 K. The predicted polar phonons were successfully observed together with some of the crystal-field excitations. First-principles simulations further allow the eigenmode and symmetry assignments of the optical phonons. The calculated atomic motions of each mode are of significant interest, as they are common for all orthoferrites and to most of the large family of orthorhombic Pbnm perovskites.

The study of magnetic frustration in classical spin systems is motivated by the prediction and discovery of classical spin liquid states. These uncommon magnetic phases are characterized by a massive degeneracy of their ground state implying a finite magnetic entropy at zero temperature. While the classical spin liquid state is originally predicted in the Ising triangular lattice antiferromagnet in 1950, this state has never been experimentally observed in any triangular magnets. The discovery of an electric analogue of classical spin liquids on a triangular lattice of uniaxial electric dipoles in EuAl12O19 is reported here. This new type of frustrated antipolar phase is characterized by a highly-degenerate state at low temperature implying an absence of long-range antiferroelectric order, despite short-range antipolar correlations. Its dynamics are governed by a thermally activated process, slowing down upon cooling toward a complete freezing at zero temperature.