Nanostructured spinel-type ferrites have attracted significant interest from both the aspects of fundamental research and industrial application owing to their outstanding electronic and magnetic properties. Understanding the factors governing their magnetic properties is important for designing advanced materials with tailored features. In this work, we have conducted a systematic investigation on the structural, electronic, and magnetic properties of high-energy ball-milled CoFe2O4 nanoparticles, which possess a cubic spinel structure and average crystallite sizes (D) ranging from 334 nm for the initial bulk material to 11.7 nm after 120 min of milling. Our observations indicated that the high-energy milling leads to the migration of Co2+ ions from the octahedral B site to the tetrahedral A site, and vice versa for Fe3+ ions. The concentration of Co2+ ions at the A site increases linearly versus the 1/D value. Unlike previous studies on CFO, we have found that the spin canting of Fe3+ ions occurred at both the A and B sites within the inner core, not only on the surface of nanoparticles. The spin canting angles phi A and phi B increase with grain-size reduction. Although the milling-induced changes in the cation distribution and spin canting are expected to significantly increase the saturation magnetization, this effect is overshadowed by the decrease caused by surface spin disorder. In other words, the surface effects play the primary role affecting the magnetic properties of the milled CFO nanoparticles.

This study presents a novel cobalt-based catalyst, iron-doped calcium cobalt oxide (Ca3Co2−xFeₓO6, with x ranging from 0 to 0.3) for the reduction of 4-nitrophenol with NaBH4 in aqueous solutions. The catalyst was synthesized using a sol–gel method, and then examined for the structure and catalytic performance. Based on FTIR analysis, the optimal synthesis conditions were an annealing temperature of 1000 °C for 10 h, resulting in high purity and crystallinity. Scanning electron microscopy (SEM) analysis showed increased particle sizes with higher iron content. X-ray diffraction (XRD) analysis confirmed the phase purity of the samples and an average crystallite size of 21.6 ± 0.2 nm. Brunauer–Emmett–Teller (BET) equation analysis of nitrogen physisorption at 77 K revealed that iron doping significantly influenced the surface area and porosity of Ca3Co2−xFeₓO6, with an optimal doping level (x = 0.1) maximizing these properties, while higher concentrations led to a decline due to potential pore blockage or densification. These nitrogen physisorption results correlated well with the catalytic activity, as the Ca3Co1.9Fe0.1O6 composition exhibited the highest reaction rate for reducing 4-nitrophenol, with optimal performance achieved at a pH of 8. Reusability tests demonstrated that the catalyst remained relatively stable over 5 reuse cycles. This research provides valuable insights into the synthesis, structure, and catalytic performance of iron-doped calcium cobalt oxide materials, which have potential applications in environmental remediation and energy-related processes.

We present here a detailed study on the magnetic and magnetocaloric (MC) behaviors of a perovskite/hausmannite composite material of LYCMO/Mn3O4, where LYCMO (La0.5Y0.1Ca0.4MnO3) is a primary phase of 95 wt %. The analysis of M(T) data indicates a coexistence of ferromagnetic-paramagnetic transitions associated with LYCMO and Mn3O4 at about 56 and 43 K, respectively. Critical-behavior analyses have proved the composite exhibiting a second-order phase transition at magnetic fields H <= 10 kOe, with critical exponents beta= 0.347 and gamma = 1.167 characteristic of 3D-Heisenberg and 3D-Ising ferromagnets, respectively. At higher fields, it tends to exhibit crossover behaviors of first-/second-order transitions. As analyzing the MC effect upon isothermal M(H) data, we have found the maximum magnetic-entropy change of -3.1 J/kg & sdot;K, and the relative refrigerant capacity (RCP) of -150 J/kg for H = 30 kOe, which are higher than those obtained for other oxides in the same temperature and applied-magnetic ranges. With the absence of hysteresis loop and large RCP value, this material can be used in magnetic-cooling devices working at temperatures T = 40-85 K to liquefy nitrogen.

Impacts of hydrogen annealing on crystallographic characterization, electronic structure, and optical, photocatalytic, and magnetic properties of polycrystalline Zn1−xCoxO (x = 0.01–0.06) samples have been considered. Structural analyses based on powder X-ray diffraction, Rietveld refinement, and Raman spectroscopy prove all materials having the P63mc wurtzite-type structure. The Co-doping and hydrogenation changed the concentration of Zn– and O–related defects whose energy levels occupy the band gap. This also enhanced photocatalytic performance of hydrogenated samples with x > 0.02. X-ray and UV–Vis absorption analyses indicate the substitution of Co2+ for Zn2+ in the wurtzite-type ZnO lattice, leading to irregularly changed the unit-cell parameters. While all the as-prepared samples are paramagnetic, the hydrogenated ones exhibit weak ferromagnetism. Ferromagnetic (FM) ordering increases when x increases, particularly for x ≥ 0.02. According to the results achieved from studying crystalline and electronic structures, we believe that oxygen-vacancies-mediated interactions between Co2+ ions and H–Co–H exchange dimers enhanced FM ordering in hydrogenated Zn1−xCoxO. Computational investigations have also indicated that the magnetization value of H–Zn1−xCoxO is influenced by the positioning of the H dopant, meaning that the couplings between Co and H play an essential role in establishing FM order in H–Zn1−xCoxO.

This work has used thermal decomposition and high-energy grinding to fabricate ZnO nanoparticles. The grinding time (tm) for up to 120 min reduced the average crystallite size (d) from 97 nm of the initial material to 28 nm. Apart from Zn and O, no impurity related to the grinding was found in the fabricated nanoparticles. X-ray diffraction and Raman scattering data showed a single phase of the fabricated samples, crystallizing in the wurtzite hexagonal structure. With decreasing d, the lattice parameters gradually increased while more Zn- and O-related defects and structural distortions were created in the ZnO host lattice, which enhanced the lattice strain. These phenomena rapidly reduced excitonic emissions, enhanced the relative intensity of visible photoluminescence, widened asymmetrically and red-shifted the Raman modes, and stimulated vibration modes at wavenumbers of 500 620 cm−1. Using the spatial correlation model, the spatial correlation length characteristic for structural disorders decreased as d reduced. In particular, the study of the photocatalytic degradation of rhodamine-B under UV-light proved that lattice defects generated by the grinding reduced the degradation efficiency.

The dependence between stimulated-emission cross-sections and polarizability volumes has been studied for laser transitions of Nd3+, Ho3+, Er3+, and Tm3+ ions doped in dielectric crystals: complex oxides, simple and complex fluorides. The overall polarizability of each crystal containing Ln3+ ions has been calculated by inclusion of the dopant polarizability in respective concentration. The applied method has revealed distinctly different positions of Nd3+ ions in the mentioned dependence in a series of crystals, e.g. Y3Al5O12, YAlO3, LiYF4, compared to the other Ln3+ ions doped in the same lattices.

Double perovskite alloys are of significant interest owing to their intriguing magnetic properties and potential practical applications. In this study, we provided a detailed report on the structural, electrical and magnetic features of the double perovskite LaCaMnFeO _6 , prepared via the conventional solid-state reaction technique. Neutron diffraction analysis showed that the sample is single-phase adopting the Pnma orthorhombic structure with random distribution of La ^3+ /Ca ^2+ and Mn ^4+ /Fe ^3+ ions on A- and B-sites, respectively. A long-range G-type antiferromagnetic order was formed below T _N = 250 K. Magnetic measurements unveiled a cluster glassy behavior at low temperatures with a complex distribution of cluster magnetic anisotropy energy. Ferromagnetic clusters consisting of two Mn ^4+ and one Fe ^3+ were found to exist in the paramagnetic phase. The complex magnetic properties of LaCaMnFeO _6 are attributed to the cation disorder effect combined with the competition between magnetic interactions. Unusual electrical behavior with a large positive magnetoresistive effect was observed and correlated to magnetic phase transitions.

It has been known that a La2/3Ca1/3MnO3 (LMO) bulk sample has the maximum magnetic entropy change (|ΔSmax|) larger than |ΔSmax| of Gd – a conventional magnetocaloric (MC) material. However, such large change just takes place in a narrow range of temperature because of its first-order character. This influences the working temperature range (ΔT) and relative cooling power (RCP) of LMO. Previous works have revealed that the fabrication of LMO nanoparticles with the second-order character would improve the magnitude of ΔT and RCP, and reduce magnetic hysteresis losses. In this work, we suggest that the combination of LMO nanoparticles (NPs) with Gd powder as nanocomposites (NCPs), termed (100-x)LMO + xGd with x = 50 and 75 wt%, further enhances ΔT from 60 to ∼94 K (in the range T = 220–314 K) for applied fields H = 5–20 kOe. These values are larger than those of initial materials Gd and LMO NPs (ΔT ≈ 40 K), reported composites (ΔT < 50 K), and even the composites fabricated from a LMO bulk and Gd powder (ΔT < 60 K) in the same fields. Additionally, all NCPs exhibit the second-order character, and RCP of optimal NCPs is nearly comparable to that of Gd. These features demonstrate application potentials of NCPs for conventional refrigerators operating in a large temperature range.

In this paper, we discuss the magnetic behavior and magnetocaloric effect of lithium erbium tetraphosphate (LiErP4O12) single crystal in the temperature range 2-85 K under magnetic fields up to 50 kOe. Detailed investigation of the temperature-dependent magnetization M(T) proves an existence of magnetic ordering below T-m, which has been estimated as a minimum in the dM/dT vs. T dependence. Above T-m, LiErP4O12 possesses paramagnetic behavior and its magnetic susceptibility follows the Curie-Weiss law giving a higher value of the effective magnetic moment of Er3+ compared to the theoretical one. This could be due to the Kramers spin degeneracy and the Stark structure of multiplets. Under an applied field H = 50 kOe, the maximum value of the magnetic-entropy change at 2 K reaches about 15 J/kgK, corresponding to a relative-cooling-power value of similar to 343 J/kg. Assessments of the magnetic ordering exponent n and N(T, H) data prove LiErP4O12 exhibiting short-range magnetic order. Due to the absence of magnetic hysteresis and large magnetocaloric response, we believe that LiErP4O12 could be a promising candidate for magnetic-cooling applications in liquefying hydrogen and He-3 isotope.

We synthesized ZnFe2O4 and CoFe2O4 nanoparticles and investigated their magnetic and electromagnetic behaviors at room temperature. X-ray diffraction analyses indicated that these materials having average crystallite sizes of 22–31 nm, and their crystal structure belongs to the cubic-spinel class. While ZnFe2O4 exhibits soft magnetic behavior, with Ms = 88 emu/g and Hc = 130 Oe, CoFe2O4 exhibits hard magnetic behavior, with Ms = 65 emu/g and Hc = 620 Oe. Investigations into the complex permittivity and permeability dependent on the frequency (f) and thickness (t) revealed remarkable changes in their values at frequencies f = 12–16 GHz for both samples. In the range f = 13.5–13.8 GHz, reflection loss magnitudes of devices based on ZnFe2O4– and CoFe2O4– with t = 2.25–2.75 mm were about 11.3–14.2 dB, corresponding to electromagnetic wave absorption of 90–95

We have used X-ray diffraction and absorption, and Raman spectroscopy to study impacts of the milling time t m (particle size d ) on the structural characterization, electronic structure, and magnetic properties of CoFe 2 O 4 nanoparticles.

Polycrystalline BaFe12-xMnxO19 (x = 0.5, 1 and 2) hexaferrites have fabricated by using normal solid-state reactions. Rietveld refinement of powder X-ray diffraction patterns reveals the specimens having the single-phase hexagonal structure. The analysis of X-ray absorption spectra indicates a mixed oxidation state of Fe2+,3+ and Mn2+,3+ ions. There are chemical shifts of Fe2+ → Fe3+ and Mn2+ → Mn3+, and a decrease of Ms from ~31 to 24 emu/g when x increases from 0.5 to 2, respectively. Large M(H) hysteresis loops with Hc increasing from 3.2 kOe for x = 0.5 to ~4.1 kOe for x = 2 prove the hard-magnetic behavior of all the specimens. The ionic-radius difference between Mn and Fe, and Mn3+-related Jahn-Teller effect at the octahedral sites have enhanced the unit-cell volume when x increases. Having studied microwave absorption, we have found strong changes in values of the permittivity and permeability at frequencies f = 10~15 GHz. For a thickness t = 2.4 mm, reflection loss magnitudes are about 10~11 dB (i.e., above 90% microwave being absorbed), with an absorption bandwidth of ~3 GHz. Assessments of loss tangents and Cole-Cole curves prove electrical energy dissipation due to interfacial/dipolar polarizations and conductive loss playing a dominant role in microwave shielding of BaFe12-xMnxO19.

We have investigated the hydrogen-annealing influence on the crystalline and electronic structures, and magnetic properties of La2/3Ca1/3MnO3 (LCMO). The results have indicated that the annealing at 700 and 900 degrees C (labeled as LCMO-700 and LCMO-900, respectively) readily reduced single-phase LCMO, resulting in a complex phase composition of several isostructural oxygen-deficient perovskite-type phases coexisting with the Ruddlesden-Popper (La/Ca)2MnO4-type phase. While a Mn3+/Mn4+ mixed valence is present in LCMO, the hydrogen annealed samples mainly have Mn2+ and Mn3+ ions. Under such circumstance, large changes in magnetic parameters have been recorded, such as remarkable decreases in values of the magnetization, the Curie temperature (from 251 K for the as-prepared LCMO through 240 K for LCMO700 to 3221 K for LCMO-900), and the magnetic-entropy change. Particularly, the crystal and electronic-structure changes also enhance the magnetic inhomogeneity, resulting in a strong development of the Griffiths phase, and cause the first-to-second-order phase transformation. These results reflect the instability of the LCMO perovskite-type manganite versus hydrogenation. [doi:10.2320/matertrans.MT-MG2022003]

M-type hexaferrites doped with transition metals are important materials used widely in modern electronic devices. Although many research works considered their magnetic properties, the relation between their electronic structure and electromagnetic behaviors have been less taken into account, particularly Mn-doped M-type hexaferrites. This work presents a detailed study on the electronic structure, and dielectric, magnetic and reflection-loss behaviors of BaFe12-xMnxO19 (x = 0.5, 1 and 2) hexaferrite specimens prepared by conventional solid-state reactions. The analysis of X-ray absorption spectra proves a mixed oxidation state of Fe-2+,Fe-3+ and Mn-2+,Mn-3+ ions, and chemical shifts of Fe2+ -> Fe3+ and Mn2+ -> Mn3+ when x increases from 0.5 to 2. These factors change the unit-cell volume, and cause Mn3+-related Jahn-Teller distortions. Concurrently, the saturation magnetization M-s varies in the range from 24 to similar to 31 emu/g, which is associated with interaction competitions between Fe-2+,Fe-3+-Mn-2+,Mn-3+ pairs and Fe2+-related spin canting. Having studied electromagnetic behaviors, we have found strong changes in values of the permittivity and permeability at frequencies f = 10 similar to 15 GHz. For a thickness t = 2.4 mm, reflection-loss magnitudes of microwave are about 10 similar to 11 dB (corresponding to above 90% microwave being absorbed), with an absorption bandwidth of similar to 3 GHz. Assessments of the magnetic and dielectric loss tangents, and Cole-Cole curves indicate electrical energy dissipation associated with interfacial/dipolar polarizations and conductive loss playing a dominant role in microwave absorption of BaFe12-xMnxO19.

We report a detailed study on the magnetic behaviors and magnetocaloric (MC) effect of a single crystal of lithium samarium tetraphosphate, LiSm(PO3)4. The analyses of temperature-dependent magnetization data have revealed magnetic ordering established with decreasing temperature below Tp, where Tp is the minimum of a dM/dT vs. T curve and varies as a linear function of the applied field H. The Curie temperature has been extrapolated from Tp(H) data, as H → 0, to be about 0.51 K. The establishment of magnetic-ordering causes a substantial change in the heat capacity Cp. Above Tp, the crystal exhibits paramagnetic behavior. Using the Curie-Weiss (CW) law and Arrott plots, we have found the crystal to have a CW temperature θCW ≈ -36 K, and short-range magnetic order associated with a coexistence of antiferromagnetic and ferromagnetic interactions ascribed to the couplings of magnetic dipoles and octupoles at the Γ7 and Γ8 states. An assessment of the MC effect has shown increases in value of the absolute magnetic-entropy change (|ΔSm|) and adiabatic-temperature change (ΔTad) when lowering the temperature to 2 K, and increasing the magnetic-field H magnitude. Around 2 K, the maximum value of |ΔSm| is about 3.6 J kg-1 K-1 for the field H = 50 kOe, and ΔTad is about 5.8 K for H = 20 kOe, with the relative cooling power (RCP) of ∼82.5 J kg-1. In spite of a low MC effect in comparison to Li(Gd,Tb,Ho)(PO3)4, the absence of magnetic hysteresis reflects that LiSm(PO3)4 is also a candidate for low-temperature MC applications below 25 K.

The structural characterization, and the electronic, magnetic and magnetocaloric properties of polycrystalline samples of La0.7Ca0.3Mn1_xCuxO3 (x = 0, 0.04, 0.06, 0.08) have been investigated. X-ray powder diffraction analysis indicates all samples having an orthorhombic structure, belonging to the Pbnm space group. X-ray absorption fine structure spectra reveal that Mn is in the mixed state of Mn3+ and Mn4+ while Cu has divalent state (Cu2+). With the substitution of Cu2+ for Mn, the Curie temperature, TC, decreases monotonically from 248 K for x = 0 to 156 K for x = 0.08, which is due to weakened exchange interactions. The downturn in the temperature dependencies of the inverse magnetic susceptibility, & drangbrac;_1(T), curves observed above TC for x = 0 and 0.08 is characteristic of the Griffiths-like phase. The analysis of isothermal magnetization data M(T, H) based on the Banerjee's criteria has indicated x = 0, 0.04, and 0.06 samples undergoing a first-order magnetic phase transition. However, the x = 0.08 sample, the coexistences of second-order magnetic phase transition at low magnetic fields below 8 kOe and first-order magnetic phase transition at high magnetic fields were observed. The maximum magnetic entropy change measured at a magnetic field span of 50 kOe occurring near the TC decreases from 10.3 to 4.8 J/kg.K with increasing x from 0 to 0.08. However, the relative cooling power (RCP) tends to increase, in which a maximum RCP of 360 J/kg for x = 0.08 that is about 1.3 times greater than that observed for the parent sample (x = 0). [doi:10.2320/matertrans.MT-M2023005]

We present a detailed study on the structural characterization, electronic structure, and magnetic/electricpolarization properties of Ba1-xYxTi1-xFexO3 (BYxTFxO, x = 0-0.1) ceramic samples. These samples mainly crystalized in a tetragonal structure, and a small amount of a YFeO3-related secondary phase constituted as x & GE; 0.08. With increasing x, the lattice constant a increases while c decreases. Such structural changes reduce rapidly the intensity of characteristic Raman modes. Magnetization data have proved all BYxTFxO samples exhibiting ferromagnetism at room temperature. Both the saturation magnetization and coercivity tend to increase with increasing x. This is ascribed to an enhanced exchange interaction between Fe3+ ions, and the YFeO3-phase formation. Herein, the Fe3+ presence has been confirmed upon analyzing X-ray absorption spectra. Though all the samples exhibit ferroelectricity, the electric-polarization slightly decreases with increasing x. This is associated with an increased symmetry due to the changes in the lattice parameters towards the cubic structure.

It has been known that bulk La0.6Ca0.4MnO3 is an intermediate material of the first- and second-order characters with the tricritical-point exponents, and the doping of a metal ion in it usually causes a continuous second-order transition. The present work reports the re-entrance of a discontinuous first-order transition in orthorhombic La0.6-xYxCa0.4MnO3 (x = 0.03-0.09) compounds. This enhances the magnetocaloric effect. For the field H = 30 kOe, the maximum magnetic-entropy change (vertical bar Delta S-max vertical bar) and relative cooling power (RCP) have been evaluated being about 5.45-6.3 J/kg.K and 130-185 J/kg, respectively. If combining these compounds as refrigerant blocks in a rotary ring model, a magnetic cooling device can operate at temperatures T = 85-280 K, with (vertical bar Delta S-max vertical bar) approximate to 5.5 J/kg.K and RCP approximate to N 1073 J/kg. Aside from the re-entranced first-order phase transition, the magnetization and structural analyses have proved the enhanced magnetocaloric effect in La0.6-xYxCa0.4MnO3 related to a Griffiths singularity, and local Jahn-Teller distortions of the pemvskite structure (since the Mn3+/Mn4+ ratio and orthorhombic structural phase are unchanged vs. x).

We have systematically investigated the crystalline and electronic structures, magnetic and magnetocaloric properties of polycrystalline SrRu1_xFexO3 (x = 0 and 0.1) samples fabricated by the solid-state reaction method. The X-ray diffraction analyses of the samples indicated single phase orthorhombic perovskite structure. A thorough analysis of X-ray-absorption-based electronic structure revealed that both Fe2+ and Fe3+ ions are present in x = 0.1, in which concentration of Fe3+ ions is higher than that of Fe2+ ions. More careful analyses on the isothermal magnetization data derived from the Banerjee's criterion demonstrated that the ferromagnetic-paramagnetic phase transition in all samples belongs to the second-order phase transition type. These results were also confirmed by a recently proposed quantitative criterion, which considered an exponent n from the magnetic field and temperature dependences of the magnetic entropy change (ASm). Especially, around TC, we have found that the ASm reaches the maximum values of 1.65 and 1.32 J.kg_1.K_1 for x = 0 and 0.1, respectively, for a field change of AH= 50 kOe. Magnetic -field dependences of the maximum magnetic-entropy change (ASmax) obey a power law of ASmax(H) pound Hn, where the values of n = 0.92-0.94 are far from the mean -field-theory value (2/3), indicating short-range magnetic order existing in the samples.