T. A. Ho

Y-doped La0.7Ca0.3MnO3 manganites exhibiting a large magnetocaloric effect and the crossover of first-order and second-order phase transitions

We prepared orthorhombic La0.7−xYxCa0.3MnO3 samples (x = 0, 0.04, 0.06, and 0.08) by conventional solid-state reaction and then studied their magnetic properties and magnetocaloric (MC) effect based on magnetization versus temperature and magnetic-field measurements, M(T, H). The experimental results revealed that an x increase in La0.7−xYxCa0.3MnO3 reduced the ferromagnetic-paramagnetic transition temperature (TC) from 260 K (for x = 0) to ∼126 K (for x = 0.08). Around the TC, maximum magnetic-entropy changes for a magnetic-field variation interval H = 50 kOe are about 10.7, 8.5, 7.4, and 5.8 J·kg−1·K−1 for x = 0, 0.04, 0.06, and 0.08, respectively, corresponding to refrigerant capacities RC = 250–280 J·kg−1. These values are comparable to those of some conventional MC materials, revealing the applicability of La0.7−xYxCa0.3MnO3 in magnetic refrigeration. Using the Arrott method and scaling hypothesis as analyzing high-field M(H, T) data, and the universal-curve construction of the magnetic entropy chang...

Coexistence of short- and long-range ferromagnetic order in La0.7Sr0.3Mn1−xCoxO3 compounds

Polycrystalline La0.7Sr0.3Mn1−xCoxO3 (x = 0.0–1.0) samples were synthesized by solid-state reaction. Structural and magnetization studies reveal the replacement of Co for Mn in La0.7Sr0.3Mn1−xCoxO3, and the decrease of the Curie temperature (TC) from 360 K (for x = 0) to 224 K (for x = 1). Positive slopes observed in the H/M versus M2 curves prove all the samples undergo a second-order magnetic phase transition. By analysis of the M(H) data at temperatures around TC using the Kouvel-Fisher method, we obtained the values of critical parameters (TC, β, γ, and δ). The results suggest an existence of short-range FM order in the sample x = 0 with β = 0.377. Meanwhile, for the case of Co-doped samples, their β values in the range of 0.403–0.457 indicate a coexistence of short and long-range FM order. This means that Co-doping favors establishing FM long-range order in La0.7Sr0.3Mn1−xCoxO3. From M(H) data, we have also determined the magnetic entropy change (ΔSm) for the samples. We have found that the ΔSm(T) cu...

Magnetic properties and magnetocaloric effect in Fe-doped La0.6Ca0.4MnO3 with short-range ferromagnetic order

The magnetic properties and magnetocaloric effect of La0.6Ca0.4Mn1-xFexO3 (x = 0–0.04) compounds fabricated by solid-state reaction have been studied. Magnetization measurements versus temperature revealed a decrease of the ferromagnetic-paramagnetic phase transition temperature (TC) with increasing Fe-doping content. The TC values determined for the samples with x = 0, 0.02 and 0.04 are about 260, 254 and 236 K, respectively. Based on magnetic-field dependences of magnetization, M(H), the magnetic entropy change (ΔSm) of the samples were calculated. Under an applied field change ΔH = 30 kOe, the maximum |ΔSmax| value decreases from 5.74 Jkg−1 K−1 for x = 0 to about 2.62 Jkg−1 K−1 for x = 0.04. These values correspond to relative cooling powers 140–180 J/kg, which are comparable to those of other manganites. Analyzing magnetic-field dependences of |ΔSm| for the samples indicates their power-law relation. Based on Banerjees criteria and Francos universal curves related to the magnetic-entropy change, we ...

Critical behavior and magnetocaloric effect of Pr1−xCaxMnO3

The critical behavior of Pr1−xCaxMnO3 samples with x = 0.25, 0.27, and 0.29 has been investigated. Detailed analyses of magnetic-field dependences of magnetization at temperatures around the paramagnetic-ferromagnetic transition, M(H, T), reveal that the samples undergo a second-order magnetic phase transition. The Arrott plot method predicts the values of critical parameters to be TC  ≈ 118 K, β = 0.351 ± 0.003, γ = 1.372 ± 0.002, and δ = 4.90 ± 0.02 for x = 0.25; TC  ≈ 116 K, β = 0.362 ± 0.002, γ = 1.132 ± 0.004, and δ = 4.09 ± 0.03 for x = 0.27; and TC  ≈ 110 K, β = 0.521 ± 0.002, γ = 0.912 ± 0.005, and δ = 2.71 ± 0.02 for x = 0.29. The values of β = 0.351 (for x = 0.25) and β = 0.362 (for x = 0.27) are close to the value β = 0.365 expected for the 3D Heisenberg model, proving an existence of short-range ferromagnetic interactions in these samples. A slight increase in Ca-doping content (x = 0.29) leads to the shift of the β value (=0.521) towards that of the mean-field theory (with β = 0.5) characteri...

Ferromagnetism in Zn 1– x Mn x O Nanoparticles Prepared by Ball Milling

Previous studies pointed out that ferromagnetism in Mn-doped ZnO was related to exchange interactions between Mn ions mediated by lattice defects. This means that it is possible to modify a Mn-doped ZnO paramagnet to a ferromagnet by creating lattice defects in it. The present work starts from a paramagnetic Zn0.98Mn0.02O sample prepared by solid-state reaction and then creates more defects upon mechanical milling. By changing the milling time (tm) from 0.5 to 20 h, we produced nanocrystalline (NC) samples with average crystallite sizes (d) ranging from 30 to 157 nm. The d decrease generated lattice strain and defects. This broadens and blurs the lines of Raman scattering and electron spin resonance (ESR) spectra. Interestingly, magnetization studies versus magnetic field revealed the samples with d ≤ 150 nm exhibiting room-temperature ferromagnetic (FM) order. The FM order became largest as d = 72 nm, corresponding to a saturation magnetization of Ms ≈ 0.006 emu/g. Apart from this d value, Ms would be gradually decreased. X-ray absorption fine structure (XAFS) spectra revealed a coexistence of Mn2+ and Mn3+ ions in the samples. Their concentration ratio was slightly changed with decreasing d, due to the slight shift of the absorption edge. With the features of Fourier-transformed XAFS and ESR spectra, we believe that ferromagnetism in the NC samples is related to oxygen vacancies residing on the surface of nanoparticles. Local lattice distortions can lead to zinc interstitials for the samples d <; 72 nm, which decreases Ms.

Ferromagnetism in Zn 1−x Mn x O nanoparticles prepared by mechanical milling

In this paper, we have prepared a Zn0.98Mn0.02O paramagnet by solid-state reaction, and then created defects by mechanical milling. Changing the milling time (tm) from 0.5 to 20 h (used the grinding zirconia medium), we obtained nanoparticles (NPs) with different crystallite sizes (d) of 30-157 nm (calculated from XRD patterns and the Williamson-Hall method). A decrease of d increases the lattice strain (ε), and density of lattice defects. This broadens and blurs the spectral lines of Raman scattering and electron spin resonance (ESR), not shown. Interestingly, magnetization studies versus magnetic field at 300 K reveal the samples d <; 150 nm exhibiting FM order. The FM order becomes largest for d = 72 nm, corresponding to saturation magnetization Ms ≈ 6×10-3 emu/g. Apart from this d value, Ms will be gradually decreased. To find out the origin/nature of the observed phenomenon, we recorded X-ray absorption fine structure (XAFS) spectra. It appears that there is the coexistence of Mn2+ and Mn3+ ions in NPs. Their concentration ratio is slightly changed with decreasing d, due to a slight shift of absorption edge. Based on the features of Fourier-transformed XAFS and ESR spectra, we suggest that ferromagnetism in NPs is mainly related to oxygen vacancies. Lattice distortions can lead to the presence of zinc interstitials for the samples d <; 72 nm, which decrease Ms.

Critical Behavior and Magnetocaloric Effect in La 2 NiMnO 6 Nanocrystals

In this paper, we present a detailed study on the magnetocaloric effect (MCE) and the critical behavior of La₂NiMnO₆ (LNMO) nanocrystals. Cubic-structured LNMO samples with different crystallite sizes (d = 200 to 26.4 nm) were prepared by a combination of solid-state reaction and mechanical milling with milling time (t_m) changing from 0 to 30 min. The results show that all the samples undergo the second-order phase transition. Using the modified Arrott plot and critical isotherm analysis methods, we have analyzed the critical behavior for nanocrystals to obtain the optimal values of the critical parameters (β, γ, δ, and T_C). The results indicate that shortand long-range ferromagnetic interactions coexist in the samples. In addition, we also assess the MCE of the samples via the magnetic entropy change (ΔS_M). Based on the obtained ΔS_M(T, H) data, we have also described the universal master curve for ΔS_M, which as a function of ΔS_M(T)/ΔS_M^P versus rescaled temperature θ = (T - TC)/(Tr - TC), where Tr is the reference temperature. Interestingly, all the ΔS_M (T, H) data points are collapsed into a universal curve in the whole temperature range.

Influence of Crystallite Size on Magnetocaloric Effect and Critical Behavior La 0.7 Sr 0.3 Mn 0.92 Co 0.08 O 3 Nanoparticles

Four samples of La_0.7Sr_0.3Mn_0.92Co_0.08O₃ (LSMCO) with different crystallite sizes were prepared by the combination of solid-state reaction and mechanical milling methods. Based on isothermal magnetization data, M(H), temperature dependences of magnetic entropy change, ΔS_mT, of the samples under a magnetic field change of 10 kOe were calculated. The maximum values of magnetic entropy change (|ΔS_max|) at room temperature are in the range of 0.9-1.4 J · kg^-1 · K^-1, corresponding to ferromagnetic (FM)-paramagnetic phase transition. In addition, M² versus H/M curves at temperatures around TC prove the samples exhibiting a second-order magnetic phase transition. The critical exponents β, γ, and δ were determined using the modified Arrott plot method and critical isotherm analysis. Here, these exponent values are located in between those expected for the mean-field theory and 3-D Heisenberg model. It means the coexistence of short-range and long-range FM interactions in LSMCO nanoparticles.

Large Magnetocaloric Effect Around Room Temperature in Double-Exchange Ferromagnets Pr 1– x Sr x MnO 3 With Short-Range Interactions

We present a detailed study on a large magnetocaloric effect at room temperature in polycrystalline ceramic samples of Pr_1-xSr_xMnO₃ (x = 0.3, 0.4, and 0.45), exhibiting a second-order phase transition. The experimental results demonstrate the replacement of Sr²⁺ for Pr³⁺ in Pr_1-xSr_xMnO₃, which leads to the Curie temperature (TC) increasing from 258 K for x = 0.3 to 301 K for x = 0.45. In particular, the maximum values of magnetic entropy change (|ΔS_max|) in the field H = 10 kOe are about 2.36-3.33 J·kg ^-1·K ^-1, corresponding to the refrigerant capacity RC = 50 - 53 J·kg ^-1 for x = 0.3-0.45. Based on the modified Arrott-plot method and the isothermal magnetization data around TC, we obtained the values of the critical exponents (β, γ, and δ) and TC for all the samples. Here, the values β = 0.370-0.397 are quite close to those expected for the 3-D Heisenberg model. This proves that ferromagnetic short-range interactions are present in Pr_1-xSr_xMnO₃ compounds. In addition, magnetic-field dependences of |ΔS_max| obey the power law, |ΔS_max| ~ Hn, where the field exponent n ≈ 0.6 is close to the value calculated from the relation n = 1 + (β - 1)/(β + γ).

Influence of the Change in Oxidation Number of Mn on Magnetic Properties of

We prepared BaTi_1-xMn_xO₃ samples (x = 0 and 0.05) by standard solid-state reaction at 700°C and 900°C in an Ar ambient to change oxidation number of Mn dopants. The fabricated samples were then studied structural characterization and electronic structures by means of X-ray diffraction and absorption, and Raman scattering and electron-spin-resonance spectrometers. We found oxidation numbers 2+ and 3+ of Mn ions coexisting in BaTi_1-xMn_xO₃ with x = 0.05, where the Mn²⁺/Mn³⁺ ratio is about one for the sample annealed at 700°C. These Mn ions prefer locating at the Ti site in the tetragonal BaTiO₃ host lattice. In particular, there is a change in the oxidation number of Mn²⁺ → Mn³⁺ when the annealing temperature changes from 700°C to 900°C. Their magnetic properties are accordingly changed. Meanwhile, annealing pure BaTiO₃ in an Ar ambient at 700°C and 900°C does not lead to ferromagnetic (FM) order; they are almost diamagnetic. With the obtained results, we believe that FM order in Mn-doped BaTiO₃ annealed in an Ar ambient is associated with exchange interactions of Mn²⁺ ions mediated by oxygen vacancies rather than associated with Mn³⁺ ions.