T. A. Ho

Bio: T. A. Ho is an academic researcher from Chungbuk National University. The author has contributed to research in topics: Magnetization & Magnetic refrigeration. The author has an hindex of 9, co-authored 30 publications receiving 262 citations.

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

Abstract: 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...

Local geometric and electronic structures and origin of magnetism in Co-doped BaTiO3 multiferroics

Abstract: We have prepared polycrystalline samples BaTi1−xCoxO3 (x = 0–0.1) by solid-state reaction. X-ray diffraction and Raman-scattering studies reveal the phase separation in crystal structure as changing Co-doping content (x). The samples with x = 0–0.01 are single phase in a tetragonal structure. At higher doping contents (x > 0.01), there is the formation and development of a secondary hexagonal phase. Magnetization measurements at room temperature indicate a coexistence of paramagnetic and weak-ferromagnetic behaviors in BaTi1−xCoxO3 samples with x > 0, while pure BaTiO3 is diamagnetic. Both these properties increase with increasing x. Analyses of X-ray absorption spectra recorded from BaTi1−xCoxO3 for the Co and Ti K-edges indicate the presence of Co2+ and Co3+ ions. They locate in the Ti4+ site of the tetragonal and hexagonal BaTiO3 structures. Particularly, there is a shift of oxidation state from Co2+ to Co3+ when Co-doping content increases. We believe that the paramagnetic nature in BaTi1−xCoxO3 sampl...

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

Abstract: 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 colossal magnetoresistance of LaÑCaÖMnO3 materials doped with Fe

Abstract: Author(s): Ahn, KH; Wu, XW; Liu, K; Chien, CL | Abstract: The effect of Fe doping (l20%) on the Mn site in the ferromagnetic ((Formula presented)) and the antiferromagnetic ((Formula presented)) phases of (Formula presented) has been studied. The same ionic radii of (Formula presented) and (Formula presented) cause no structure change in either series, yet conduction and ferromagnetism have been consistently suppressed by Fe doping. Colossal magnetoresistance has been shifted to lower temperatures, and in some cases enhanced by Fe doping. Doping with Fe bypasses the usually dominant lattice effects, but depopulates the hopping electrons and thus weakens the double exchange. © 1996 The American Physical Society.

Review on Magnetocaloric Effect and Materials

Abstract: A detailed discussion of magnetocaloric properties of distinct materials is a vital aspect in magnetic refrigeration technology. This review paper deals with all kinds of magnetocaloric materials such as ferromagnetic perovskites, glass ceramics, oxide-based composites and spinel ferrites. The comparative study of magnetocaloric properties revealed that manganites have the potential applications in magnetorefrigeration technology.

Influence of Mn doping on the magnetic and optical properties of ZnO nanocrystalline particles

Abstract: The structural, optical and magnetic properties of Mn doped ZnO nanocrystalline particles, Zn1-xMnxO, with different percentages of Mn content have been studied. XRD and XPS measurements showed that all samples with Mn doping up to x = 0.1 possess typical wurtzite structure and have no other impurity phases. The incorporation of Mn ions into the ZnO lattice was also confirmed by FTIR and UV–Vis. spectroscopy results. Both XRD and SEM results indicated a slight decrease in the grain size with increasing the Mn doping level. The XPS results indicated an increase in the oxygen vacancies concentration with increasing the Mn doping level. The magnetization measurements revealed a weak ferromagnetic behavior at room temperature and a clear ferromagnetic behavior with relatively large coercive fields at low temperature. The ferromagnetic order is improved by increasing the Mn doping. In addition, we observed an increase in the concentration of oxygen vacancies, which is also induced by increasing the Mn doping level. A ferromagnetic coupling of the local moment of Mn dopants through the sp-d exchange interaction and oxygen vacancies, in addition to different magnetic contributions due to different forms of Mn ions that coexist in the Mn doped nanoparticles were presented in order to interpret the observed magnetic behavior. We observed a clear red shift in the direct band gap and an increase in the coercive field and saturation magnetization values with increasing the Mn doping level.