We report on the structural, electrical, magnetic and magnetocaloric properties of polycrystalline and nanocrystalline La0.7-xHoxBa0.3MnO3 (x = 0.03, 0.05, 0.1, 0.15) compounds. Bulk compounds were prepared by the conventional solid-state method. Nano-scale compounds were obtained by the sol-gel method. Resistivity measurements done on all polycrystalline samples revealed Colossal Magnetoresistive behavior with large negative magnetoreristivity. FM/PM second-order phase transitions were established for all samples. Bulk compounds with x = 0.03 and x = 0.05 exhibit near-room temperature transition T-c with 298 K and 272 K, respectively. Application of Modified Arrott Plot method and Kouvel - Fisher method confirmed mean-field model critical behavior for the nano-scale compounds and established a peculiar case for the bulk samples where the gamma exponent lies within tri-critical mean-field model range and beta exponent belongs to 3D Heisenberg model range. Entropy change -dSM was estimated from isothermal measurements with the highest values around Tc. Universal curves based on -dSM were constructed to confirm critical exponents, second order phase transition and universality class of the samples. Bulk sample La0.67Ho0.03Ba0.3MnO3 shows a fairly large magnetic entropy change of 5.36 J/ KgK in mu(o)dH = 4 T, and sample La0.65Ho0.05Ba0.3MnO3 shows the highest value in mu(o)dH =1 T at 2.11 J/Kgk. All samples with lower substitution levels exhibit high values of temperature-averaged entropy change (TEC) and all, including nanocrystalline samples, show large relative cooling power.

The influence of Cd2+ content and heat treatment on the morphology, structure and magnetic behavior of CdxZn1-xFe2O4 (x = 0.0; 0.2; 0.4; 0.6; 0.8; 1.0) nanoparticles encapsulated in SiO2 produced by the sol-gel route was studied. At 800 degrees C, Fe2SiO4, Fe2O3 and CdO accompanied the poorly crystallized ferrites, while at 1200 degrees C the well crystallized ferrite was convoyed by Zn2SiO4 and SiO2. Encapsulation of nano-sized CdxZn1-xFe2O4 in inert SiO2 allowed the particle size control, minimized agglomeration, and improved the magnetic behavior. The low heat treatment temperature produced well-individualized nanoparticles of similar to 40 nm, with the particle diameter being larger than the ferrite crystallites due to the SiO2 coating. Increasing the Cd2+ content resulted in small particle size, whereas increasing the heat treatment temperature led to larger particle size, resulting in submicron clusters. The increase of Cd2+ content and heat treatment temperature also determined the increase in lattice constant, density and hopping length and the decrease in crystallite size and porosity. The nanoparticles were found to be mesoporous with a narrow pore size distribution. The magnetic features increased with heat treatment temperature and decreased with increasing Cd2+ content until paramagnetic-like behavior was reached for CdFe2O4.

The current work reports the characterization of bismuth-doped cobalt ferrites with the general formula CoBi phi Fe2-phi O4 (phi = 0.2-1.0) incorporated in SiO2, prepared by the sol-gel method and heat-treated in air at different temperatures (400-1300 degrees C). The correlation between the trivalent ion content, heat treatment temperature, morphology, structure, thermal and magnetic properties was evaluated. Thermal analysis confirmed the formation of Co, Bi and Fe succinates at 141-210 degrees C and their decomposition to ferrites at 287-318 degrees C. The development and decomposition of succinate precursors, followed by the formation of spinel ferrite and SiO2 was confirmed by infrared spectroscopy. The formation of Co-Bi ferrites with crystallite sizes of 41.2-107.5 nm was studied by X-ray diffraction. Crystalline ferrites were accompanied by the SiO2 crystalline phases at high heat treatment temperatures, while poorly crystalline ferrite was attended by silicates at low heat treatment temperatures. Atomic force microscopy images revealed rounded nanoparticles composed of a ferrite core covered by a thin SiO2 layer at low heat treatment temperature and rounded cube-shaped ferrite cores covered by a thin SiO2 layer at high heat treatment temperatures. The heat treatment temperature and the Bi3+ doping increase caused the increase of the particle size and the change of the ferrite core shape. The nanocomposite phi = 0.2 had the highest coercivity, saturation and remanent magnetizations, and the magnetic properties decreased with increasing Bi3+ doping.

Co-doped ferrites are promising functional materials for many practical applications and their physical properties can be tailored by changing their composition. This study assesses the structure, morphology and magnetic properties of CaxCo1-xFe2O4/SiO2 (x = 0.0-0.5) nanocomposites (NCs) obtained by sol-gel method and annealed at different temperatures (400, 800 and 1200 degree celsius). The effect of Ca2+ doping on the properties of nano-structured Ca-Co ferrite embedded in the SiO2 matrix was investigated by thermogravimetry and differential thermal analysis, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy, Brunauer-Emmett-Teller surface area, inductively coupled plasma optical emission spectrometry, atomic force microscopy (AFM) and magnetic measurements. The thermal analysis showed the formation of metal succinates up to 200 degree celsius and of ferrite above 280 degree celsius. The presence of poorly crystalline ferrite accompanied by silicates at low annealing temperatures and of highly crystalline ferrite accompanied by SiO2 and Ca silicates at high annealing temperatures was identified by XRD. The increased Ca2+ doping led to a decrease in the structural parameters estimated by XRD and the surface area. The AFM images revealed that the NCs have a crystalline core covered by a layer of amorphous SiO2. The hysteresis loop shape indicated the superparamagnetic-like and ferromagnetic behavior of the obtained NCs. The magnetic properties improved with the increase in annealing temperature and depended on the particle size and dopant content. The NCs with low Ca2+ content annealed at 800 degree celsius still show appreciable hysteresis, but doping with high Ca2+ contents drastically affects the magnetic properties.

Here we report investigations of structural, electrical and magnetic properties of bulk and nano-sized Pr0.65-xNdxSr0.35MnO3 compounds (x <= 0.35). Polycrystalline compounds were produced by solid - state reaction and nanocrystalline samples were obtained by sol-gel method. Analysis of x-ray diffraction patterns revealed a change in lattice structure from 'Pbnm' to 'R3c' symmetry, with diminishing cell volume upon increasing Nd ions substitution for all samples. Optical microscopy was used for bulk surface morphology and transmission electron microscopy was implemented for nano-sized samples. Iodometric titration showed oxygen deficiency for bulk compounds and oxygen excess for nano-sized particles. Measurements of resistivity of bulk samples revealed a single peak at temperatures associated with grain boundary conditions and with ferromagnetic/paramagnetic transition and negative magnetoresistivity. Critical magnetic behavior analysis disclosed that the polycrystalline samples are governed by a tricritical mean field and by 3D Heisenberg models while nanocrystalline samples are governed by a mean field model. Curie temperature T-c values remain in near room temperature range for all bulk compounds; they lower with increasing Nd ions substitution from 295 K for the parent bulk compound to 268 K for x = 0.35. T-c is lowered from 255 K for the parent nano-sized compound in small temperature intervals. All compounds exhibit second-order magnetic phase transitions and relatively high magnetic entropy change, with the highest value of 5.74 J/kgK for x = 0.25 bulk sample in mu(o)Delta H = 4 T. Strong magnetocaloric effect, stability and the possibility of fine-tuning T-c by Nd ions substitution make the investigated bulk polycrystalline compounds promising for application in magnetic refrigeration. Nano-sized samples possess lower magnetic entropy changes of maximum 2.4 J/kgK in mu(o)Delta H = 4 T for x = 0.15, but wider effective entropy change temperature (delta T-fwhm) and relative cooling power on par with other manganites, bringing them into the conversation as a viable option in cooling materials.

Polycrystalline Pr0.65-xGdxSr0.35MnO3 (x = 0.01, 0.05, 0.1, 0.2, 0.3) manganites were prepared by conventional solid-state method. Structural analysis has revealed a change in structure with x = 0.2, from Rhombohedral (R3c) to Orthorhombic (Pbnm) symmetry. Small oxygen deficiency was found in the samples. The electrical measurements revealed typical colossal magnetoresistance (CMR) behavior, with single metal-insulator peak at Tp for all investigated samples. The small polaron hopping (SPH) and variable-range hopping (VRH) models were used to describe the electrical conduction above Tp. Magnetic measurements show an increase in magnetization below the Curie temperature, TC. Values of Tc diminish with increasing Gd3+ content. Isothermal measurements around critical points suggest a possible interaction between the 4f-ions and Mn-sublattice The magnetic critical behavior analysis established the tri-critical mean-field model to be the governing model for the series. Large magnetic entropy change Delta SM at near room temperature (278 K) was exhibited by the x = 0.01 sample at 2.2 J/ kgK in mu 0 Delta H = 1 T and 5.36 J/kgK in mu 0 Delta H = 4 T. The largest | Delta SM| value was shown by the x = 0.1 sample. The values of the relative cooling power (RCP) and the temperature-averaged entropy change (TEC) increased with substitution qualifying these compounds as magnetocaloric materials.

Structural tuning of CoFe2O4 nanoparticles by doping with rare earth (La3+) ions stands out as a novel technique to tune their physical properties and to provide new nanocomposites for various applications. CoLaxFe2_xO4 (x = 0.0; 0.3; 0.6; 0.9; 1.2; 1.5) nanoparticles embedded in SiO2 (40 wt%) matrix were synthesized by sol-gel method and annealed at 400, 800 and 1200 degrees C. The effect of Fe3+ substitution by La3+ on the structure, morphology and magnetic properties of nano-structured Co-La ferrite embedded in the SiO2 matrix was investigated by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET) surface area, atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and magnetic measurements. The annealing at low temperatures led to poorly crystalline ferrites, while the annealing at high temperatures directed highly crystalline ferrites accompanied by secondary phases. The crystallite size, dislocation density and porosity increased, while the lattice parameter, unit cell volume and hopping lengths decreased with the increase in La3+ content. By substituting Fe3+ ion with La3+ ion, the specific surface area decreased from 244 m2/g to 111 m2/g at 400 degrees C, reaching values below 0.5 m2/g at 800 and 1200 degrees C. The AFM revealed a spherical-shaped crystalline core covered by an amorphous SiO2 layer. The TEM images revealed spherical shape nanoparticles of size increasing with La3+ content and annealing temperature. The average particle size measured by AFM ranged between 14 nm and 72 nm, which is in good agreement with the XRD. SEM indicated that the ferrite nanoparticles were nearly spherical and had varying agglomeration degrees. The saturation magnetization, remanent magnetization, coercivity, squareness, magnetic moment per formula unit and anisotropy constant decreased with the increase in La3+ content and increased with the increase in crystallite size. By incorporating La3+ in the ferrite system, the gradual change from hard (CoFe2O4 @SiO2) to soft (CoLa1.5Fe0.5O4 @SiO2) magnetic ferrite occured. The magnetic properties of low La3+ content ferrites recommend them for a wide range of applications.

Here we report the synthesis and investigation of bulk and nano-sized La0.7Ba0.3−xCaxMnO3 (x = 0, 0.15, 0.2 and 0.25) compounds that are promising candidates for magnetic refrigeration applications. We compare the structural and magnetic properties of bulk and nano-scale polycrystalline La0.7Ba0.3−xCaxMnO3 for potential use in magnetic cooling systems. Solid-state reactions were implemented for bulk materials, while the sol–gel method was used for nano-sized particles. Structurally and morphologically, the samples were investigated by X-ray diffraction (XRD), optical microscopy and transmission electron microscopy (TEM). Oxygen stoichiometry was investigated by iodometry. Bulk compounds exhibit oxygen deficiency, while nano-sized particles show excess oxygen. Critical magnetic behavior was revealed for all samples using the modified Arrott plot (MAP) method and confirmed by the Kouvel–Fisher (KF) method. The bulk polycrystalline compound behavior was better described by the tricritical field model, while the nanocrystalline samples were governed by the mean-field model. Resistivity in bulk material showed a peak at a temperature Tp1 attributed to grain boundary conditions and at Tp2 associated with a Curie temperature of Tc. Parent polycrystalline sample La0.7Ba0.3MnO3 has Tc at 340 K. Substitution of x = 0.15 of Ca brings Tc to 308 K, and x = 0.2 brings it to 279 K. Nanocrystalline samples exhibit a very wide effective temperature range in the magnetocaloric effect, up to 100 K. Bulk compounds exhibit a high and sharp peak in magnetic entropy change, up to 7 J/kgK at 4 T at Tc for x = 0.25. To compare the magnetocaloric performances of the studied compounds, both relative cooling power (RCP) and temperature-averaged entropy change (TEC) figures of merit were used. RCP is comparable for bulk polycrystalline and nano-sized samples of the same substitution level, while TEC shows a large difference between the two systems. The combination of bulk and nanocrystalline materials can contribute to the effectiveness and improvement of magnetocaloric materials.

This work presents the effect of monovalent (Ag+, Na+), divalent (Ca2+, Cd2+), and trivalent (La3+) metal ion doping and annealing temperature (500, 800, and 1200 °C) on the structure, morphology, and magnetic properties of MnFe2O4/SiO2 ceramic nanocomposites synthesized via sol–gel method. Fourier-transform infrared spectroscopy confirms the embedding of undoped and doped MnFe2O4 nanoparticles in the SiO2 matrix at all annealing temperatures. In all cases, the X-ray diffraction (XRD) confirms the formation of MnFe2O4. In the case of undoped, di-, and trivalent metal-ion-doped gels annealed at 1200 °C, three crystalline phases (cristobalite, quartz, and tridymite) belonging to the SiO2 matrix are observed. Doping with mono- and trivalent ions enhances the nanocomposite’s structure by forming single-phase MnFe2O4 at low annealing temperatures (500 and 800 °C), while doping with divalent ions and high annealing temperature (1200 °C) results in additional crystalline phases. Atomic force microscopy (AFM) reveals spherical ferrite particles coated by an amorphous layer. The AFM images showed spherical particles formed due to the thermal treatment. The structural parameters calculated by XRD (crystallite size, crystallinity, lattice constant, unit cell volume, hopping length, density, and porosity) and AFM (particle size, powder surface area, and thickness of coating layer), as well as the magnetic parameters (saturation magnetization, remanent magnetization, coercivity, and anisotropy constant), are contingent on the doping ion and annealing temperature. By doping, the saturation magnetization and magnetocrystalline anisotropy decrease for gels annealed at 800 °C, but increase for gels annealed at 1200 °C, while the remanent magnetization and coercivity decrease by doping at both annealing temperatures (800 and 1200 °C).

CoFe2O4 is a promising functional material for various applications. The impact of doping with different cations (Ag+, Na+, Ca2+, Cd2+, and La3+) on the structural, thermal, kinetics, morphological, surface, and magnetic properties of CoFe2O4 nanoparticles synthesized via the sol-gel method and calcined at 400, 700 and 1000 °C is investigated. The thermal behavior of reactants during the synthesis process reveals the formation of metallic succinates up to 200 °C and their decomposition into metal oxides that further react and form the ferrites. The rate constant of succinates’ decomposition into ferrites calculated using the isotherms at 150, 200, 250, and 300 °C decrease with increasing temperature and depend on the doping cation. By calcination at low temperatures, single-phase ferrites with low crystallinity were observed, while at 1000 °C, the well-crystallized ferrites were accompanied by crystalline phases of the silica matrix (cristobalite and quartz). The atomic force microscopy images reveal spherical ferrite particles covered by an amorphous phase, the particle size, powder surface area, and coating thickness contingent on the doping ion and calcination temperature. The structural parameters estimated via X-ray diffraction (crystallite size, relative crystallinity, lattice parameter, unit cell volume, hopping length, density) and the magnetic parameters (saturation magnetization, remanent magnetization, magnetic moment per formula unit, coercivity, and anisotropy constant) depend on the doping ion and calcination temperature.

This study presents the effect of Ni2+ substitution by Co2+ on the structure, morphology and magnetic properties of Co alpha Ni0.9-alpha Zn0.1Fe2O4 nanoparticles calcined at different temperatures. Fourier-transform in-frared spectroscopy revealed the presence of Fe-O, Co-O, Zn-O and Ni-O bonds in samples calcined at 200 degrees C, while the presence of the characteristic bonds of the SiO2 confirmed the embedding of Co alpha Ni0.9-alpha Zn0.1Fe2O4 at all calcination temperatures. Single phase poorly crystalline ferrite for samples calcined at 400 degrees C and ferrite accompanied by traces of Fe2SiO4 and SiO2 for samples calcined at high temperatures (800 and 1200 degrees C) were identified. The dependence of X-ray diffraction parameters on the Co2+ content of nanoparticles was also investigated. The increase in Co2+ content and calcination temperature led to crystallite growth from 4 to 43 nm, while the porosity decreased with the increase in Co2+ content and calcination temperature. The magnetic properties of the rectangular shape nanoparticles evolved pro-portionally with the particle size. The magnetization, coercivity and magnetocrystalline anisotropy in-creased with the increase in Co2+ content and calcination temperature. The shape of hysteresis loops indicated a superparamagnetic-like behavior of the nanoparticles calcined at 400 degrees C and a ferrimagnetic-like behavior at higher calcination temperatures (800 and 1200 degrees C). These features make the obtained nanoparticles attractive candidates for various technical applications.(c) 2023 Elsevier B.V. All rights reserved.

(Co0.4Zn0.4Ni0.2Fe2O4)α(SiO2)(100−α) samples obtained by embedding Co0.4Zn0.4Ni0.2Fe2O4 nanoparticles in SiO2 in various proportions were synthesized by sol-gel process and characterized using thermal analysis, Fourier-transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, inductively coupled plasma optical emission spectrometry, and magnetic measurements. Poorly crystalline Co–Zn–Ni ferrite at low annealing temperatures (500 °C) and highly crystalline Co–Zn–Ni ferrite together with traces of crystalline Fe2SiO4 (800 °C) and SiO2 (tridymite and cristobalite) (1200 °C) were obtained. At 1200 °C, large spherical particles with size increasing with the ferrite content (36–120 nm) were obtained. Specific surface area increased with the SiO2 content and decreased with the annealing temperature above 500 °C. Magnetic properties were enhanced with the increase in ferrite content and annealing temperature.

Here we report investigations of bulk and nano-sized Pr0.65Sr(0.35−x)CaxMnO3 compounds (x ≤ 0.3). Solid-state reaction was implemented for polycrystalline compounds and a modified sol–gel method was used for nanocrystalline compounds. X-ray diffraction disclosed diminishing cell volume with increasing Ca substitution in Pbnm space group for all samples. Optical microscopy was used for bulk surface morphology and transmission electron microscopy was utilized for nano-sized samples. Iodometric titration showed oxygen deficiency for bulk compounds and oxygen excess for nano-sized particles. Measurements of resistivity of bulk samples revealed features at temperatures associated with grain boundary condition and with ferromagnetic (FM)/paramagnetic (PM) transition. All samples exhibited negative magnetoresistivity. Magnetic critical behavior analysis suggested the polycrystalline samples are governed by a tricritical mean field model while nanocrystalline samples are governed by a mean field model. Curie temperatures values lower with increasing Ca substitution from 295 K for the parent compound to 201 K for x = 0.2. Bulk compounds exhibit high entropy change, with the highest value of 9.21 J/kgK for x = 0.2. Magnetocaloric effect and the possibility of tuning the Curie temperature by Ca substitution of Sr make the investigated bulk polycrystalline compounds promising for application in magnetic refrigeration. Nano-sized samples possess wider effective entropy change temperature (ΔTfwhm) and lower entropy changes of around 4 J/kgK which, however, puts in doubt their straightforward potential for applications as magnetocaloric materials.

This paper presents the influence of Mn2+ substitution by Ni2+ on the structural, morphological and magnetic properties of Mn1−xNixFe2O4@SiO2 (x = 0, 0.25, 0.50, 0.75, 1.00) nanocomposites (NCs) obtained by a modified sol-gel method. The Fourier transform infrared spectra confirm the formation of a SiO2 matrix and ferrite, while the X-ray diffraction patterns show the presence of poorly crystalline ferrite at low annealing temperatures and highly crystalline mixed cubic spinel ferrite accompanied by secondary phases at high annealing temperatures. The lattice parameters gradually decrease, while the crystallite size, volume, and X-ray density of Mn1−xNixFe2O4@SiO2 NCs increase with increasing Ni content and follow Vegard’s law. The saturation magnetization, remanent magnetization, squareness, magnetic moment per formula unit, and anisotropy constant increase, while the coercivity decreases with increasing Ni content. These parameters are larger for the samples with the same chemical formula, annealed at higher temperatures. The NCs with high Ni content show superparamagnetic-like behavior, while the NCs with high Mn content display paramagnetic behavior.

Here, we report synthesis and investigations of bulk and nano-sized La(0.7−x)EuxBa0.3MnO3 (x ≤ 0.4) compounds. The study presents a comparison between the structural and magnetic properties of the nano- and polycrystalline manganites La(0.7−x)EuxBa0.3MnO3, which are potential magnetocaloric materials to be used in domestic magnetic refrigeration close to room temperature. The parent compound, La0.7Ba0.3MnO3, has Curie temperature TC = 340 K. The magnetocaloric effect is at its maximum around TC. To reduce this temperature below 300 K, we partially replaced the La ions with Eu ions. A solid-state reaction was used to prepare bulk polycrystalline materials, and a sol-gel method was used for the nanoparticles. X-ray diffraction was used for the structural characterization of the compounds. Transmission electron spectroscopy (TEM) evidenced nanoparticle sizes in the range of 40–80 nm. Iodometry and inductively coupled plasma optical emission spectrometry (ICP-OES) was used to investigate the oxygen content of the studied compounds. Critical exponents were calculated for all samples, with bulk samples being governed by tricritical mean field model and nanocrystalline samples governed by the 3D Heisenberg model. The bulk sample with x = 0.05 shows room temperature phase transition TC = 297 K, which decreases with increasing x for the other samples. All nano-sized compounds show lower TC values compared to the same bulk samples. The magnetocaloric effect in bulk samples revealed a greater magnetic entropy change in a relatively narrow temperature range, while nanoparticles show lower values, but in a temperature range several times larger. The relative cooling power for bulk and nano-sized samples exhibit approximately equal values for the same substitution level, and this fact can substantially contribute to applications in magnetic refrigeration near room temperature. By combining the magnetic properties of the nano- and polycrystalline manganites, better magnetocaloric materials can be obtained.

This paper presents the effect of Mn2+ substitution for Co2+, in CoFe2O4 embedded in SiO2 matrix, on the structural, surface, morphological and magnetic properties. X-ray diffraction (XRD) and Mossbauer spectroscopy indicate the presence of a nanocrystalline mixed cubic spinel. In all cases, for the nanocomposites (NCs) heat-treated at 200 degrees C, a single, low crystalline ferrite phase was remarked, while for the other heat-treatment temperatures up to 1200 degrees C and with increasing Mn content, the secondary phase of alpha-Fe2O3 appears, accompanied also by the secondary phase of SiO2 at 1200 degrees C. The Fourier transform infrared (FT-IR) spectroscopy confirms the consumption of starting metallic nitrates, the formation of Co-O, Mn-O, Fe-O bonds in ferrites@SiO2 matrix. The Mossbauer spectra show the characteristic magnetic patterns of Co and Mn spinels. According to the atomic force microscopy (AFM) analysis, the particle size increases from 15 to 80 nm with the increase of Mn content. The specific surface area varies in the range 150-450 m(2)/g due to the substitution of Co2+ ion with Mn2+ ion and decreases with increasing heat treatment temperature, reaching values below 1 m(2)/g at 1200 degrees C. All NCs have pores within the mesoporous range, with high dispersion of pores' sizes. Furthermore, the release of fine nanoparticles in aqueous environment is facilitated by the powders' mesoporous structure preserved at 200, 500 and 800 degrees C heat treatment temperatures. The porous network collapse after heat treatment at 1200 degrees C leads to releasing of bigger nanoparticles, in good agreement with AFM observation. Magnetization, coercivity and anisotropy evolve proportionally with the particle size for the NCs heat-treated at 800 degrees C (M-s = 18.9-36.3 emu/g; M-R = 3.05-14.1 emu/g, H-c = 31.83-53.2 kA/m, K= 0.378.10(-3) -1.21.10(-3) erg/cm(-1)) and inverse proportionally for those heat-treated at 1200 degrees C (M-s = 30.7-19.4 emu/g; M-R = 11.60 7.20 emu/g, H-c= 127.3-15.9 kA/m, K= 2.45.10(-3)-0.19.10(-3) erg/cm(-1)). The NCs with high Mn content heat-treated at 1200 degrees C show superparamagnetic behavior, while those with low Mn content display ferrimagnetic behavior. (C) 2021 Elsevier B.V. All rights reserved.

The structure, morphology and magnetic properties of (Ni0.6Mn0.4Fe2O4)α(SiO2)100−α (α = 0–100%) nanocomposites (NCs) produced by sol-gel synthesis were investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM) and vibrating sample magnetometry (VSM). At low calcination temperatures (300 °C), poorly crystallized Ni0.6Mn0.4Fe2O4, while at high calcination temperatures, well-crystallized Ni0.6Mn0.4Fe2O4 was obtained along with α-Fe2O3, quartz, cristobalite or iron silicate secondary phase, depending on the Ni0.6Mn0.4Fe2O4 content in the NCs. The average crystallite size increases from 2.6 to 74.5 nm with the increase of calcination temperature and ferrite content embedded in the SiO2 matrix. The saturation magnetization (Ms) enhances from 2.5 to 80.5 emu/g, the remanent magnetization (MR) from 0.68 to 12.6 emu/g and the coercive field (HC) from 126 to 260 Oe with increasing of Ni0.6Mn0.4Fe2O4 content in the NCs. The SiO2 matrix has a diamagnetic behavior with a minor ferromagnetic fraction, Ni0.6Mn0.4Fe2O4 embedded in SiO2 matrix displays superparamagnetic behavior, while unembedded Ni0.6Mn0.4Fe2O4 has a high-quality ferromagnetic behavior.

ZnxMn1-xFe2O4@SiO2 nanocomposites (NCs) (x = 0.00, 0.25, 0.50, 0.75, 1.00) were prepared by eco-friendly sol–gel synthesis followed by heat treatment at different temperatures and characterized. The X-ray diffraction shows poorly crystallized ferrite after the heat treatment at low temperatures and highly crystalline ferrite accompanied by several secondary phases at high temperatures. The crystallite size increases from 2.4 to 45.2 nm with the increase of heat treatment temperature. The specific surface decreases from 281 to 13 m2/g with the increase of the heat treatment temperature, reaching values below 1 m2/g at 1200 °C. All NCs have pores within the mesoporous range, with high dispersion of pores’ sizes. The NCs show ferrimagnetic behavior, close to the superparamagentic limit. The main magnetic parameters, saturation magnetization, remanence, coercivity and magnetic anisotropy constant of ZnxMn1-xFe2O4@SiO2 nanoparticles increase with the increase of particle size and heat treatment temperature and decrease with increase of Zn content. This behavior could be explained presuming that the Zn2+, Mn2+ and Fe3+ ions can simultaneously occupy both the tetrahedral and octahedral sites in the ZnxMn1-xFe2O4 ferrite.

Near room-temperature magnetic and magnetocaloric properties of the compounds Pr0.63-xHoxSr0.37MnO3 (x=0, 0.05) were investigated. The parent compound Pr0.63Sr0.37MnO3 has a large magnetocaloric effect around its Curie temperatures of about 300 K. Partially replacing of Pr3+ ions with Ho3+ ions in Pr0.63Sr0.37MnO3 gives rise to disorder which leads to the decrease of the Curie temperature without a significant reduction of the magnetocaloric effect. The samples were prepared by solid state reaction and were found to be single phase by x-ray diffraction. The magnetic measurements reveal paramagnetic to ferromagnetic second order phase transitions. For the sample with x = 0, the magnetic entropy change |ΔSM| was in the range from 1.91 J/kg٠K (RCP = 42 J/kg) for μ0 ΔH = 1 T to 4.86 J/kg٠K (RCP = 184 J/kg) for μ0 ΔH = 4 T. For the sample with x = 0.05, the magnetic entropy change |ΔSM| was in the range from 1.61 J/kg٠K (RCP = 57.9 J/kg) for μ0 ΔH = 1 T to 4.38 J/kg٠K (RCP = 236 J/kg) for μ0 ΔH = 4 T and Tc = 216 K. These values recommend these materials to be potential candidates to be used in near room-temperature refrigeration applications.

The effect of SiO2 embedding on the obtaining of single-phase ferrites, as well as on the structure, morphology and magnetic properties of (Zn0.6Mn0.4Fe2O4)δ(SiO2)100−δ (δ = 0–100%) nanoparticles (NPs) synthesized by sol-gel method was assessed. The phase composition and crystallite size were investigated by X-ray diffraction (XRD), the chemical transformations were monitored by Fourier transform infrared (FT-IR) spectroscopy, while the morphology of the NPs by transmission electron microscopy (TEM). The average crystallite size was 5.3–27.0 nm at 400 °C, 13.7–31.1 nm at 700 °C and 33.4–49.1 nm at 1100 °C. The evolution of the saturation magnetization, coercivity and magnetic anisotropy as a function of the crystallite sizes were studied by vibrating sample magnetometry (VSM) technique. As expected, the SiO2 matrix shows diamagnetic behavior accompanied by the accidentally contribution of a small percent of ferromagnetic impurities. The Zn0.6Mn0.4Fe2O4 embedded in SiO2 exhibits superparamagnetic-like behavior, whereas the unembedded Zn0.6Mn0.4Fe2O4 behaves like a high-quality ferrimagnet. The preparation route has a significant effect on the particle sizes, which strongly influences the magnetic behavior of the NPs.