Showing papers by "Seong-Cho Yu published in 2015"

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...

Effect of Crystallite Size on the Thickness of Nonmagnetic Shell and Magnetic Properties of La0.7Ca0.3MnO3

Abstract: La0.7Ca0.3MnO3 compounds with the crystallite size of D = 45-200 nm were synthesized by utilizing the solid-state reaction and mechanical ball milling methods. The reduction of the crystallite size (D) resulted in the decrease of both T C and magnetization, broadening of the phase transition region, and appearance of Griffiths phase features in the materials. The temperature dependence of saturation magnetization (M S), the variations of the coercivity (H C), and the deviation of magnetization from expected Bloch’s law T 3/2 for these samples were also investigated. Based on the core-shell model, the core diameter and the nonmagnetic shell thickness for the samples were determined.

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

Abstract: 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 Banerjee's criteria and Franco's universal curves related to the magnetic-entropy change, we ...

Critical Behavior in Double-Exchange Ferromagnets of Pr 0.6 Sr 0.4 MnO 3 Nanoparticles

Abstract: In recent years, perovskite-type manganites (REMnO 3 with RE = rare-earth elements) have attracted considerable interest due to their complex magnetic and transport properties. Though REMnO 3 is an antiferromagnetic (AFM) insulator, substituting the RE site by a divalent alkali earth element (A) in order to form RE 1−x A x MnO 3 compounds makes these materials exhibiting the ferromagnetic (FM)-paramagnetic (PM) phase transition at the Curie temperature (T C ), and a FM metallic state below T C . It has been found that RE 1−x A x MnO 3 compounds usually exhibit unusual magneto-electro effects, such as colossal magnetoresistance (CMR) and magnetocaloric (MC) effects [1, 2]. Basically, a close interplay between magnetic and transport properties in CMR and MC materials is ascribed to the competition between Mn3+-Mn4+ FM double-exchange interactions and AFM super-exchange interactions of Mn3+-Mn3+ and Mn4+-Mn4+ pairs. Particularly, when the particle size of manganites is reduced to the nanometer scale, a number of outstanding physical properties (such as low-field magnetoresistance, surface spin-glass behavior, exchange bias effect, etc.) would appear. The inter-particle interaction has been also found to be strong, modifying the magnetic response of nanoparticles [3]. To further understand the magnetic properties in manganite nanoparticles, it is necessary to consider the influence of the crystallite size on the nature of their magnetic phase transition and FM interactions.

Magnetic Properties and Large Magnetocaloric Effect in Amorphous Fe-Ag-Ni-Zr for Room-Temperature Magnetic Refrigeration

Abstract: This paper reveals that a partial replacement of Fe by Ni (5%) and/or Ag (2%) in amorphous Fe85– x Ag x Ni5Zr10( $\boldsymbol {x} = 0$ and 2) alloys leads to the shift of $\boldsymbol {T}_{\boldsymbol {C}}$ toward room temperature (RT) ( $\boldsymbol {T}_{\boldsymbol {C}} =308$ and 303 K for $\boldsymbol {x} = 0$ and 2, respectively). Basing on isothermal magnetization data, the magnetic entropy change ( $\Delta \boldsymbol {S}_{\boldsymbol {m}})$ of the amorphous alloys was calculated. Their maximum $\Delta \boldsymbol {S}_{\boldsymbol {m}}$ values ( $|\Delta \boldsymbol {S}_{\mathrm {\mathbf {max}}}|$ ) achieve just around RT, corresponding to the ferromagnetic-paramagnetic (FM-PM) phase transition of the amorphous phase. Under a magnetic field change of 10 kOe, $|\Delta \boldsymbol {S}_{\mathrm {\mathbf {max}}}|$ values are $\sim 1$ J $\cdot ~\mathrm{kg}^{\mathrm {\mathbf {-1}}}~ \cdot ~\mathrm{K}^{\mathrm {\mathbf {-1}}}$ . We have also used different methods, such as the modified Arrott plots, the Kouvel–Fisher method, and critical isotherm analysis to study their critical property around the FM–PM phase transition. Our results suggest that the amorphous Fe85– x Ag x Ni5Zr10 alloys exhibit a second-order magnetic phase transition with the critical exponents of $\boldsymbol {\beta }=0.389$ –0.396 and $\boldsymbol {\gamma }= 1.279$ –1.334 close to those expected for the 3-D Heisenberg model. This proves the existence of short-range FM interactions in these alloys.

Magnetocaloric Effect and Critical Behavior in a Disordered Ferromagnet La 0.7 Sr 0.3 Mn 0.9 Ti 0.1 O 3

Abstract: The critical phenomena around the Curie temperature ( $T_{C}$ ) of a polycrystalline La0.7Sr0.3Mn0.9Ti0.1O3 were studied based on magnetic field and temperature dependence of magnetization. Using the modified Arrott plot method, we determined the critical parameters ${\rm T}_{C} \approx 234$ K, $\beta =0.372 \pm 0.012$ , $\gamma =1.171 \pm 0.112$ , and $\delta = 4.15 \pm 0.01$ . With these critical values, the M(H) data of the sample fall onto two universal branches of the scaling function $M(H,\varepsilon ) =\vert \varepsilon \vert ^{\beta }f_{\pm }(H/\vert \varepsilon \vert ^{\beta +\gamma })$ , where $\varepsilon = (T-T_{C})/T_{C} $ , and $f_{+}$ for $T > T_{C}$ and $f_{-}$ for $T . This pointed to the existence of a short-range ferromagnetic (FM) order in La0.7Sr0.3Mn0.9Ti0.1O3, which is attributed to FM clusters persisting in a wide temperature range, even above ${\rm T}_C$ . Electron-spin-resonance spectra affirmed the presence of the FM clusters at temperatures as high as $T = T_{\mathrm {min}}(\approx 1.3 ~T_{C})$ , above which the sample is completely paramagnetic. Based on the M(H) data, we also calculated the magnetic-entropy change ( $\Delta S_{m})$ , which reached a maximum ( $\vert \Delta S_{\mathrm {max}}\vert ) \approx$ 4.3 J/kg $\cdot$ K) around the ${\rm T}_{C}$ , corresponding to the relative cooling power of $\sim 200$ J/kg for a field change $\Delta H = 50 ~{\textrm {kOe}}$ . A linear relationship $\vert \Delta S_{\mathrm {max}}\vert \propto H^{n}$ is achieved with a local exponent $n = 0.59$ determined from $n = 1+ (\beta - 1)/(\beta + \gamma)$ , indicative of a short-range FM system.

Effect of Annealing Temperature to Oxidation State of Sr 2 FeMoO 6 Double Perovskite Film

Abstract: We report our investigation on structural and magnetic properties of double perovskite Sr2FeMoO6 (SFMO) films fabricated at different annealing temperatures. Structural properties have been investigated by means of X-ray absorption near-edge spectra and extended X-ray fine structure, where a change of oxidation state has been observed depending on the annealing temperature. It is found that the oxidation state becomes more deficient for a lower annealing temperature, indicating a lower number of unoccupied Fe states with decreasing Mo coordination number. Ordering degree of Fe and Mo ions in SFMO films is found to strongly correlate to the oxidation state, resulting in the subsequent magnetic property change.

Magnetic Properties and Magnetocaloric Effect in Fe90−xSnxZr10 Alloy Ribbons

Abstract: This work points out the possibility of tuning the magnetocaloric (MC) effect in Fe90-x Sn x Zr10 alloy ribbons in the temperature range from 235 to 315 K by changing the Sn-doping content (x). Under an applied field change from 0 to 50 kOe, the maximum magnetic-entropy changes around the ferromagnetic-paramagnetic phase transition are about 3.6, 4.1 and 3.3 J kg−1·K−1 for x = 0, 2 and 4, respectively, which correspond to relative cooling powers of 280 ∼ 410 J·kg−1. Studying the magnetic properties of the alloy ribbons based on Banerjee’s criteria and assessing the magneticordering parameter n = dLn|ΔS m |/dLnH (where ΔS m and H are the magnetic-entropy change and the magnetic field, respectively) reveals that the alloys undergo a second-order phase transition and exhibit a short-range ferromagnetic order. The nature of these phenomena is further analyzed by means of the results obtained from the analyses of the crystal structure, the Curie-Weiss law, and the Griffith phase.

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

Abstract: 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 $(t_{m})$ 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 $d \leq 150$ nm exhibiting room-temperature ferromagnetic (FM) order. The FM order became largest as $d = 72~{\rm nm}$ , corresponding to a saturation magnetization of $M_{s} \approx 0.006$ emu/g. Apart from this $d$ value, $M_s$ 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 , which decreases $M_s$ .

Critical behavior and magnetocaloric effect of Pr1−xCaxMnO3

Abstract: 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...

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

Abstract: Four samples of La0.7Sr0.3Mn0.92Co0.08O3 (LSMCO) with different crystallite sizes were prepared by the combination of solid-state reaction and mechanical milling methods. Based on isothermal magnetization data, ${M}$ ( $\boldsymbol {H}$ ), temperature dependences of magnetic entropy change, $\Delta \boldsymbol {S} _{m} {T}$ , of the samples under a magnetic field change of 10 kOe were calculated. The maximum values of magnetic entropy change (| $\Delta {S} _{\mathrm {max}}$ |) at room temperature are in the range of 0.9–1.4 J $\cdot $ $\mathrm{kg}^{{\mathbf {-1}}}\cdot $ $\mathrm{K}^{{\mathbf {-1}}}$ , corresponding to ferromagnetic (FM)–paramagnetic phase transition. In addition, ${M} ^{{\mathbf {2}}}$ versus ${H}/{M}$ curves at temperatures around ${T} _{C}$ prove the samples exhibiting a second-order magnetic phase transition. The critical exponents $\beta $ , $\gamma $ , and $\delta $ 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.

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

Abstract: Currently, dilute magnetic semiconductors (DMSs) are still attracting much interest of the solid-state physics community because of integrating the electrical, optical and magnetic properties. This can lead to the birth of multi-functional electronic/spintronic devices, employing both the charge and spin behaviors of electrons. In experiment, to fabricate DMSs, nonmagnetic semiconductors are usually doped with a small amount of a transition metal (TM). It is expected to fabricate DMSs with ferromagnetic (FM) order around room temperature.

Deviation from Bloch’s T3/2 Law and Spin-Glass-Like Behavior in La0.7Ca0.3MnO3 Nanoparticles

Abstract: Three samples of La 0.7Ca 0.3MnO 3 nanoparticles were prepared by a reactive milling method with the milling times t m = 8, 12, and 16 h. The studies based on X-ray diffraction and electron microscopes reveal that the mean particle size of nanoparticles is about 7 nm. Saturation magnetization values determined from fitting the Langevin function to the magnetization curve at 5 K decrease from 36.8 to 14.7 emu/g with increasing t m from 8 to 16 h. Particularly, temperature dependences of saturation magnetization for these samples do not follow Bloch’s T 3/2 law, but follow a T e law. The results obtained from investigating the DC magnetization and AC susceptibility indicate an existence of the spin-glass-like behavior in the samples, which is ascribed to the competition of ferromagnetic and anti-ferromagnetic interactions.

Effect of annealing to oxidation state of Sr 2 FeMoO 6 films with double perovskite structure

Abstract: Among the group of materials with the double perovskite structure, a half-metallic Sr 2 FeMoO 6 (SFMO) system has attracted increasing attention. Recently, numerous studies have been carried out to understand the structural, magnetic, and magnetocaloric properties of SFMO [1-3] since the SFMO materials have T C about the room temperature, high low-field magnetic resistance (MR), and magnetocaloric effect. With these achievable properties, SFMO with double perovskite structure has become one of promising materials for spintronic applications such as spin-valve device, novel magnetic recording media, and spin-polarized current source material [4,5].

Critical Behavior and Magnetocaloric Effect in La 2 NiMnO 6 Nanocrystals

Abstract: 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 ( $\beta, \gamma, \delta$ , and ${T_{C}}$ ). The results indicate that short- and long-range ferromagnetic interactions coexist in the samples. In addition, we also assess the MCE of the samples via the magnetic entropy change ( ${\Delta {S_{M}}}$ ). Based on the obtained ${\Delta {S_{m}} (T, H)}$ data, we have also described the universal master curve for ${\Delta {S_{M}}}$ which as a function of ${\Delta {S_{M}} (T)/ \Delta {{S_{M}}^{P}}}$ versus rescaled temperature ${\theta = (T - {T_{C}})/({T_{r}} - {T_{C}})}$ where ${T_{r}}$ is the reference temperature. Interestingly, all the ${\Delta {S_{M}} (T, H)^{H}}$ data points are collapsed into a universal curve in the whole temperature range.

A first principles study of chalcopyrite (AlGaMn)P 2 alloys

Abstract: A recent strategy to achieve further control over the spin degree of freedom is based on dilute ferromagnetic semiconductors, prepared by substituting magnetic ions such as V, Cr, Mn, Fe, Co, and Ni into non-magnetic semiconductor hosts. Ferromagnetism has been reported in various semiconductor groups including II-VI, III-V, etc [1]. However, to date, the low solubility of magnetic ions in non-magnetic semiconductor hosts has limited the opportunities. When ferromagnetic metals are used as spin injectors, the polarization in the semiconductor tends to be quickly lost via spin-flip scattering. It is one of the primary challenges to create the ferromagnetic semiconductors due to the difficulty in the spin-injection into the semiconductors to form diluted magnetic semiconductor (DMS) at room temperature or above room temperature.

Critical behaviour and magneticaloric effect in La 2 NiMnO 6 nanoparticles

Abstract: It has been known that A 2 B'B"O 6 typed compounds (A is an alkaline-earth or rare-earth element, B' and B" are different transition-metal elements) belong to the double perovskite family, which have attacted considerable interes due to their complex electrical-magnetic properties. Among these, La 2 NiMnO 6 is a ferromagnetic (FM) semiconductor, and considered as one of the most promising materials for applications in spintronic devices [1]. Though the crystal structure, electromagnetic, and magnetic properties of this material have been widely studied, the nature of FM interactions and magnetocaloric effect (MCE) has not been fully understood yet. Within this context, it is essential to investigate the correlation between the MCE and critical behavior near the FM-para-magnetic (PM) transition in this material.