Duong Anh Tuan

Origin of ferromagnetism in BaTiO3 nanoparticles prepared by mechanical milling

Recent studies have pointed out an existence of the room-temperature ferromagnetism in nanostructured BaTiO3 materials. It has been thought that the ferromagnetism is due to intrinsic defects. To elucidate this issue, we have prepared BaTiO3 nanoparticles by mechanical milling, starting from diamagnetic/nonmagnetic BaTiO3 powders, and then investigated their magnetic properties. Our idea is motivated by the fact that the mechanical milling introduces more lattice defects to a ground material. If so, the ferromagnetic (FM) order will increase with increasing the density of defects. Here, the defect density can be changed upon the milling time (tm). Our magnetic measurements at 300 K have indicated that the FM order increases with increasing tm as expected. To understand the nature of ferromagnetism, we studied X-ray absorption fine structure (XAFS) spectra of the samples for the Ti K-edge. The shift of the absorption edge towards lower energies of Ti3+ with increasing tm, proving an increase in Ti3+ concen...

Influences of the first-to-second order magnetic phase transformation on the transport properties of La0.7Ca0.3−xBaxMnO3 compounds

We studied the influence of the magnetic phase transition on the transport properties of La0.7Ca0.3−xBaxMnO3 compounds with x = 0.05, 0.075, and 0.10. Our experimental results demonstrated the ferromagnetic-paramagnetic and metal-insulator transitions taking place at temperatures TC = 265–300 K and TMI = 280–310 K, respectively, which increase with increasing Ba-doping content. While the x = 0.05 sample undergoes a first-order magnetic phase transition (FOMT), x = 0.10 undergoes a second-order magnetic phase transition (SOMT). The other sample with x = 0.075 is considered as a threshold concentration of the FOMT-SOMT transformation. Further, ρ(T) data in different temperature regions were fitted to different models. The activation energy Ep and density of states at the Fermi level N(EF) were accordingly determined. Notably, N(EF) increases while Ep decreases in the case of applying an external field. We also have found that N(EF) increases when materials transfer from the FOMT to the SOMT, which becomes s...

Strain effects in epitaxial Mn2O3 thin film grown on MgO(100)

We report on the epitaxial growth and magnetic properties of Mn2O3 thin films grown on MgO(001) substrate by molecular beam epitaxy. We observed the reduction in binding energy of Mn valence states, the increase in satellite separation up to 12.7 eV, and the smaller band gap of 3.32 eV. In addition, the antiferromagnetic ordering below 90 K in bulk changed to ferrimagnetic up to 175 K. The results were possibly to be explained by a lattice mismatch strain on Mn2O3 film on MgO(001) substrate.

Formation and ferromagnetic properties of FeSi thin films

In this work, the growth and ferromagnetic properties of e-FeSi thin film on Si(100) substrate prepared by molecular beam epitaxy are reported. The inter-diffusion of Fe layer on Si(100) substrate at 600 °C results in polycrystalline e-FeSi layer. The determined activation energy was 0.044 eV. The modified magnetism from paramagnetic in bulk to ferromagnetic states in e-FeSi thin films was observed. The saturated magnetization and coercive field of e-FeSi film are 4.6 emu/cm3 and 29 Oe at 300 K, respectively.

Giant magnetocaloric effect of Mn0.92Ba0.08As thin film grown on Al2O3(0001) substrate

The epitaxial Mn0.92Ba0.08As thin film was grown on Al2O3(0001) substrate by molecular beam epitaxy. The Curie temperature (TC) around 350 K was enhanced with the addition of Ba, compared to that of bulk MnAs (TC ∼ 318 K). We have observed the linear resistivity versus the square of temperature and high negative magnetoresistance near Curie temperature. Moreover, the giant magnetocaloric effect was found with maximum magnetic entropy change, 65 J/kgK, around room temperature at 5 T.

Ferromagnetism and magneto-transport properties of Mn0.92Ca0.08As thin film grown on Al2O3(0001) substrate

The epitaxial Mn0.92Ca0.08As thin film was grown on Al2O3(0001) substrate by molecular beam epitaxy. The Curie temperature (TC) around 340 K was enhanced with the addition of Ca, compared to that of bulk MnAs (TC ∼ 318 K). The maxima magnetoresistance, ∼2.08% at 0.7 T, was observed near the critical magnetic transition temperature. Moreover, the giant magnetocaloric effect was found with the maximum magnetic entropy change, ∼200 J/kgK, around 330 K at 5 T.

Magnetism and transport properties of epitaxial Fe-Ga thin films on GaAs(001)

Epitaxial Fe–Ga thin films in disordered bcc α-Fe crystal structure (A2) have been grown on GaAs(001) by molecular beam epitaxy. The saturated magnetization (MS) decreased from 1371 to 1105 kA/m with increasing Ga concentration from 10.5 to 24.3 % at room temperature. The lattice parameter increased with the increase in Ga content because of the larger atomic radius of Ga atom than that of Fe. The increase in carrier density with Ga content caused in lower resistivity.

Magnetic and Magnetotransport Properties of

We have studied magnetic and magnetotransport properties of a polycrystalline ceramics of La_0.7Sr_0.2Ca_0.1MnO₃ (LSCM) prepared from nanoparticles. The Curie temperature (<i>T</i>_C) of the sample is about 340 K. Basing on the magnetic-field dependences of magnetization measured around <i>T</i>_C, maximum magnetic-entropy changes under the applied fields of 10, 20 and 30 kOe are about 1.73, 2.74, 3.52 J/kg.K, respectively, which are higher than those reported on a parent compound of La_0.7Sr_0.3MnO₃. To understand magnetic-interaction mechanisms taking place in the system, the critical behavior has been taken into account. By using the modified Arrott plot method, β and γ values associated with the spontaneous magnetization (<i>M</i>_s) and inverse initial susceptibility (χ₀^-1) have been obtained to be 0.472 ± 0.016 and 1.061 ± 0.024, respectively. These critical parameters are close to those expected for the mean-field theory (with β = 0.5 , and γ = 1.0 ). A small deviation in value between our results and those expected for the mean-field theory could be due to an inhomogeneity in magnetism and/or interaction competitions between antiferromagnetic pairs ( Mn³⁺-Mn³⁺ and Mn⁴⁺-Mn⁴⁺ ) and a ferromagnetic pair of Mn³⁺-Mn⁴⁺ in LSCM. Interestingly, in the temperature range of 180-300 K, magnetoresistance values keep quite stable at about 16 and 27% for the magnetic fields of 1 and 7 kOe, respectively. This makes LSCM more suitable to applications of magnetic sensitive sensors and reading heads working in a large temperature range.

{\rm La}_{0.7}{\rm Sr}_{0.2}{\rm Ca}_{0.1}{\rm MnO}_{3}

The Fe1−xGax thin films (x = 0.4, 0.5) have been grown on GaSb(100) substrate using molecular beam epitaxy. An epitaxial film with bcc α-Fe crystal structure (A2) is observed in Fe0.6Ga0.4 film, while an impure Fe3Ga phase with DO3 structure is appeared in Fe0.5Ga0.5 film. The saturated magnetizations at room temperature are observed to be 570 emu/cm3 and 180 emu/cm3 and the coercivities to be 170 and 364 Oe for Fe0.6Ga0.4 and Fe0.5Ga0.5, respectively. A hysteresis trend in Hall resistance vs. magnetic field is observed for Fe0.5Ga0.5 film. However, there is a weak hysteresis noticed in Fe0.4Ga0.6 thin film.

Prepared From Nanoparticles

Herein, we propose and provide evidence for the experimentally and theoretically promising route of exploring spintronics materials with high performance via a synthetic hybrid of half-metallic ferromagnet and diluted magnetic semiconductor. Crystalline and well-ordered [Co2FeGe/Ge(Mn)]n superlattice, which is free of secondary phase separation, were prepared by the hybridization of end members, Co2FeGe and Ge(Mn), using the molecular beam epitaxy technique. Besides comparable magnetic properties with respect to the Co2FeGe films, the superlattice sample exhibits superior properties in electric conductivity and spin polarization, thus enhancing in favor of the spin injection efficiency. These results demonstrate the high feasibility of spintronics materials with low saturation magnetization, small coercivity, high Curie temperature, and high spin injection efficiency through the proposed route in this work.