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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%. Considering loss tangents, we found that both dielectric and magnetic losses contributed to the electromagnetic dissipation in the investigated materials. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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°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 hydrogenannealed 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. [ABSTRACT FROM AUTHOR]

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 248K for x = 0 to 156K for x = 0.08, which is due to weakened exchange interactions. The downturn in the temperature dependencies of the inverse magnetic susceptibility, »-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). [ABSTRACT FROM AUTHOR]

We present a study on the magnetic and magnetocaloric characters of Sm1-xSrxCoO3 (x = 0.4–0.6) synthesized by solid-state reactions. Recorded M(T) data at H = 100 Oe show an increase of the Curie temperature (TC) from 143 K for x = 0.4 to 153 K for x = 0.5, but a higher x value (x = 0.6) reduces TC to ~ 92 K. A singularity region sandwiched between the Griffiths and pure paramagnetic phases has been observed in x = 0.4, but it becomes invisible at x = 0.5 and 0.6. These properties are assigned to a competition between ferromagnetic and anti-ferromagnetic interactions induced by Co3+ and Co4+ ions. The Co3+/Co4+ mixed valence has been confirmed upon the analysis of Co K-edge x-ray absorption spectra. Using Maxwell's relations and M(H) data, we have calculated the absolute magnetic entropy change (|∆Sm|). Around TC, the maximum magnetic entropy changes (|∆Smax|) of x = 0.4, 0.5 and 0.6 are about 1.19, 1.22 and 0.81 J/kg K for H = 50 kOe, respectively. The values of the relative cooling power are in the range of 58 ~ 93 J/kg. Our analyses also indicate all |∆Sm (T, H)| data obeying the universal master curve as constructing the |∆Sm|/|∆Smax| vs. θ plots, where θ is defined as the reduced temperature. When considering exponential parameters N(T, H) and n associated with |∆Sm(T, H)| and |∆Smax(H)|, respectively, we have found their values are different from the mean-field theory value n = 2/3. These results prove the Sm1-xSrxCoO3 samples have the second-order nature and short-range magnetic order. [ABSTRACT FROM AUTHOR]

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 (ΔSm). Especially, around TC, we have found that the ΔSm 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 ΔH = 50 kOe. Magnetic-field dependences of the maximum magnetic-entropy change (ΔSmax) obey a power law of ΔSmax(H) ∝ 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. [ABSTRACT FROM AUTHOR]

We have studied the magnetic and magnetocaloric behaviors of a single crystal of LiPr(PO3)4 through DC magnetization (M) and heat-capacity (Cp) measurements. The analysis of M data indicates an establishment of magnetic order at low temperatures that results in the departure of magnetic-susceptibility behavior from the Curie–Weiss law. Under the application of high magnetic fields, the antiferromagnetic ground state is unstable, leading to a first-order metamagnetic transition to a ferromagnetic phase. Because the critical field of this transition increases with increasing temperature above the Néel temperature, it is thought to be related to the spin-flop process, spin populations on multiplets, and jj-coupling. In particular, after the transition, the maximum magnetic-entropy change (ΔSmax) increases according to a power law y ∝ Hn, with n = 1.54. Under an applied field H = 50 kOe, the |ΔSmax| value at 2 K is about 9.8 J/kg K. Using thermodynamic relations and Cp(T, H) data, the largest adiabatic-temperature change of LiPr(PO3)4 calculated at 2 K is about 2.8 K for the field of H = 20 kOe, which is attainable using a permanent magnet. [ABSTRACT FROM AUTHOR]