The first observation of photoinduced tuning of AC magnetization for organic–inorganic hybrid materials composed of Mn12-acetate and azobenzene

New organic–inorganic hybrid material composed of typical single-molecule magnet [Mn₁₂(CH₃COO)₁₆(H₂O)₄O₁₂]·2CH₃COOH·4H₂O (Mn₁₂-acetate) and photochromic azobenzene in PMMA cast films was prepared to observe novel photomagnetic phenomenon. We characterized it by means of UV–visible absorption and IR spectra to confirm photoisomerization of azobenzene even at room temperature. We measured temperature and field dependence of DC susceptibility, and temperature and frequency dependence of AC susceptibility as comparing with bulk precipitates of Mn₁₂-acetate. We found some differences in hysteresis loops for them. After alternate illumination of UV or visible light for 3 min at 3 K, we could observe reversible decrease or increase of in-phase AC susceptibility (and little changes of DC susceptibility) for the hybrid materials accompanying cis–trans photoisomerization of azobenzene.     

The first detection of photomodulation by both DC and in-phase AC susceptibility for organic/inorganic hybrid materials containing cyano-bridged Gd–Cr complex and azobenzene

New organic/inorganic hybrid material containing a ferrimagnetic cyano-bridged binuclear complex, [Gd(DMF)₄(H₂O)₃Cr(CN)₆]·H₂O (DMF=N,N′-dimethylformamide), and photochromic azobenzene in poly(methylmethacrylate) (PMMA) cast films has been prepared. We characterized magnetic properties of bulk [Gd(DMF)₄(H₂O)₃Cr(CN)₆]·H₂O and confirmed that it was not changed by light illumination at all. For the first time, we could detect reversible changes of both DC and in-phase AC susceptibility for the hybrid materials accompanying with cis–trans photoisomerization of azobenzene after UV and visible light illumination for 3 min at 3 K. We also discuss the kind of detectable magnetic changes (e.g. DC, in-phase AC, and both DC and in-phase AC) and the nature of some photo-magnetic functional hybrid materials (cis–trans photoisomerization and increase/decrease of magnetization) known to estimate their mechanism and interactions.     

BiFeO3/Zn1−xMnxO bilayered thin films

BiFeO₃/Zn_1−xMn_xO (x = 0-0.08) bilayered thin films were deposited on the SrRuO₃/Pt/TiO₂/SiO₂/Si(1 0 0) substrates by radio frequency sputtering. A highly (1 1 0) orientation was induced for BiFeO₃/Zn_1−xMn_xO. BiFeO₃/Zn_1−xMn_xO thin films demonstrate diode-like and resistive hysteresis behavior. A remanent polarization in the range of 2P_r ∼ 121.0-130.6 μC/cm² was measured for BiFeO₃/Zn_1−xMn_xO. BiFeO₃/Zn_1−xMn_xO (x = 0.04) bilayer exhibits a highest M_s value of ∼15.2 emu/cm³, owing to the presence of the magnetic Zn_0.96Mn_0.04O layer with an enhanced M_s value.     

Magnetic properties of CoFeP films prepared by electroless deposition

Structure and magnetization of CoFeP films prepared by the electroless deposition were systematically investigated by varying the bath composition and deposition parameters to optimize soft magnetic properties. The cobalt content in the CoFeP films varies from 40.4 to 94.9 wt% by controlling the bath composition. Increase of the metallic ratio FeSO₄·7H₂O/(CoSO₄·7H₂O+FeSO₄·7H₂O) affects the films’ microstructure, which switches from amorphous to crystalline structure. The magnetic properties of CoFeP films reveal that the coercivity (H_c) values range from 80 up to 185 A/m and the saturation magnetization (M_s) from 82 to 580 eum/g depending on the bath composition, deposition parameters and heat-treatment conditions. Increase of M_s and remanent magnetization (M_r) as well as decrease of H_c are observed for the CoFeP films with bath pH, temperature and the metallic molar ratio increasing. It is also found that the H_c is enhanced with the increase of NaH₂PO₂·H₂O concentration. CoFeP films showing good soft magnetic properties with coercivities less than 140 A/m and M_s close to 600 emu/g can be obtained in high pH bath and thereafter heat treatment. The deposit is found to be suitable as soft magnetic materials for core materials.     

Magnetic characterization of Mn5SiB2 and Mn5Si3 phases

In this work the Mn₅Si₃ and Mn₅SiB₂ phases were produced via arc melting and heat treatment at 1000 °C for 50 h under argon. A detailed microstructure characterization indicated the formation of single-phase Mn₅Si₃ and near single-phase Mn₅SiB₂ microstructures. The magnetic behavior of the Mn₅Si₃ phase was investigated and the results are in agreement with previous data from the literature, which indicates the existence of two anti-ferromagnetic structures for temperatures below 98 K. The Mn₅SiB₂ phase shows a ferromagnetic behavior presenting a saturation magnetization M_s of about 5.35×10⁵ A/m (0.67 T) at room temperature and an estimated Curie temperature between 470 and 490 K. In addition, AC susceptibility data indicates no evidence of any other magnetic ordering in 4-300 K temperature range. The magnetization values are smaller than that calculated using the magnetic moment from previous literature NMR results. This result suggests a probable ferrimagnetic arrangement of the Mn moments.     

High-frequency properties of discontinuous FeCoSi/native-oxide multilayer films

Discontinuous [FeCoSi (d)/native-oxide]₅₀ multilayer films were fabricated by DC magnetron sputtering without any post-deposition treatment. The films exhibit good soft magnetic properties with initial permeability μ_i larger than 100, the saturation magnetization 4πM_s and the in-plane uniaxial anisotropy field H_k increase as the magnetic FeCoSi layer thickness d is increased from 5.5 to 20.5 Å. As a consequence, the ferromagnetic resonance frequencies f_r of the films increase from 2.0 to 3.9 GHz. The combination of high f_r and large μ_i makes these films potential candidates for magnetic devices applied in the high-frequency range. The origin of the excellent high-frequency properties in discontinuous FeCoSi/native-oxide multilayer films is discussed.     

Magnetization reversal in bulk and thin films of the ferrimagnetic ErCo0.50Mn0.50O3 perovskite

We present herein a comparison of the magnetic properties of bulk ceramics and thin films of the ferrimagnetic ErCo_0.50Mn_0.50O₃ compound. Epitaxial thin films were deposited onto (1 0 0) SrTiO₃ substrates by pulsed-laser ablation while bulk ceramics were prepared by solid state reaction. When cooling under low applied fields, a spin reversal is observed in both thin film and bulk due to the competition between two magnetic sublattices (Co/Mn and Er) coupled by a negative exchange interaction. Original features are observed in the M(H) loops for bulk materials: abrupt jumps at 4 T due to a reorientation of domains, while in the low field region, the increasing and decreasing branches of the magnetization intersect each other. In the thin film, the ordering temperature increased from 69 to 75 K, and the ZFC anomaly (AF transition) became sharper, compared to the bulk specimen. The oxygen content and the microstructure are crucial to observe the intersection of the magnetization branches.     

Polyol synthesis and magnetic study of Mn3O4 nanocrystals of tunable size

Nanosized manganese oxide particles were prepared by the so-called polyol process. The average diameter of the particles was controlled by the growth time. X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photon spectroscopy (XPS) show that the particles are well crystallized, pure, stoichiometric Mn₃O₄ single crystals of uniform size ranging from about 5 to 12 nm. The variation of their dc-magnetization, M, as a function of the magnetic field, H, and temperature, T, clearly corresponds to ferromagnetic ordering at low temperature, with a Curie temperature slightly higher than 40 K. The evidence for superparamagnetism in these particles, due to their very small size, has been discussed in the light of their M(H) and M(T) for zero-field-cooled (ZFC) and field-cooled (FC) plots.     

Influence of crystallization on the spectral features of nano-sized ferroelectric barium strontium titanate (Ba0.7Sr0.3Tio3) thin films

Ferroelectric barium strontium titanate (Ba_0.7Sr_0.3TiO₃)(BST) thin films have been prepared from barium 2-ethylhexanoate [Ba[CH₃(CH₂)₃CH(C₂H₅)CO₂]₂]_, strontium 2-ethylhexanoate [Sr[CH₃(CH₂)₃CH(C₂H₅)CO₂]₂] and titanium(IV) isopropoxide [TiOCH(CH₃)₂]₄ precursors using a modified sol-gel technique. The precursor except [TiOCH(CH₃)₂]₄ were synthesized in the laboratory. Transparent and crack-free films were fabricated on pre-cleaned quartz substrates by spin coating. The structural and optical properties of films annealed at different temperatures have been investigated. The as-fired films were found to be amorphous that crystallized to the tetragonal phase after annealing at 550 °C for 1 h in air. The lattice constants “a” and “c” were found to be 3.974 A and 3.990 A, respectively. The grain sizes of the films annealed at 450, 500 and 550 °C were found to be 30.8, 36.0 and 39.8 nm respectively. The amorphous film showed very high transparency (∼95%), which decreases slightly after crystallization (∼90%). The band gap and refractive index of the amorphous and crystalline films were estimated. The optical dispersion data are also analyzed in the light of the single oscillator model and are discussed.     

XRD and XPS characterization of mixed valence Mn3O4 hausmannite thin films prepared by chemical spray pyrolysis technique

Spray pyrolysis technique has been employed successfully for the synthesis of single phase mixed valence spinel hausmannite (Mn₃O₄) thin films using alcoholic start solution of manganese acetate (Mn(CH₃COO)₂·4H₂O) on pyrex glass substrates at atmospheric pressure using air as a carrier gas. Thermal decomposition of the precursor in the temperature range 320-490 °C led to the formation of Mn₃O₄ phase as revealed from the thermogravimetry analysis. Prepared samples are characterized by X-ray diffraction that shows spinel structure with space group I4₁/amd. Pure and well crystallized specimen is subjected to X-ray photoelectron spectroscopy for the surface chemistry investigation of these systems at a molecular level. Surface Mn/O ratio is compared to the bulk composition of the sample. Atomic force micrographs revealed that the morphology and the surface grains of the films largely influenced by the substrate temperature.     

Synthesis and magnetic properties of Zn1−xMnxO films prepared by the sol-gel method

We report on the ferromagnetic characteristics of Zn_1−xMn_xO films (x=0.1-0.3) prepared by the sol-gel method on silicon substrates using transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD) and superconducting quantum interference device (SQUID) magnetometry at various temperatures. Magnetic measurement show that the Curie temperature (T_C) and the coercive field (H_C) were ∼39 K and ∼2100 Oe for the film of x=0.2, respectively. EDS and TEM measurements indicate that Mn content at the interface is significantly higher than that at the center of the Zn_0.8Mn_0.2O film showing the ratio, Zn:Mn:O≅1:12:15. This experimental evidence suggests that ferromagnetic precipitates containing manganese oxide may be responsible for the observed ferromagnetic behavior of the film.     

Ferromagnetism modulation by phase change in Mn-doped GeTe chalcogenide magnetic materials

In this work, an effective method to modulate the ferromagnetic properties of Mn-doped GeTe chalcogenide-based phase change materials is presented. The microstructure of the phase change magnetic material Ge_1?x Mn _x Te thin films was studied. The X-ray diffraction results demonstrate that the as-deposited films are amorphous, and the crystalline films are formed after annealing at 350 °C for 10 min. Crystallographic structure investigation shows the existence of some secondary magnetic phases. The lattice parameters of Ge_1?x Mn _x Te (x = 0.04, 0.12 and 0.15) thin films are found to be slightly different with changes of Mn compositions. The structural analysis clearly indicates that all the films have a stable rhombohedral face-centered cubic polycrystalline structure. The magnetic properties of the amorphous and crystalline Ge_0.96Mn_0.04Te were investigated. The measurements of magnetization (M) as a function of the magnetic field (H) show that both amorphous and crystalline phases of Ge_0.96Mn_0.04Te thin film are ferromagnetic and there is drastic variation between amorphous and crystalline states. The temperature (T) dependence of magnetizations at zero field cooling (ZFC) and field cooling (FC) conditions of the crystalline Ge_0.96Mn_0.04Te thin film under different applied magnetic fields were performed. The measured data at 100 and 300 Oe applied magnetic fields show large bifurcations in the ZFC and FC curves while on the 5,000 Oe magnetic field there is no deviation.     

Chemical bath deposition of hausmannite Mn3O4 thin films

A low-temperature chemical bath deposition (CBD) technique has been used for the preparation of Mn₃O₄ thin films onto glass substrates. The kinetic behavior and the formation mechanism of the solid thin films from the aqueous solution have been investigated. Structure (X-ray diffraction and Raman), morphological (atom force microscope), and optical (UV-vis-NIR) characterizations of the deposited films are presented. The results indicated that the deposited Mn₃O₄ thin films of smooth surface with nanosized grains were well crystalline and the optical bandgap of the film was estimated to be 2.54 eV.     

Optical properties of Mn-activated Na2SnF6 and Cs2SnF6 red phosphors

Alkaline hexafluorostantanate red phosphors Na₂SnF₆:Mn⁴⁺ and Cs₂SnF₆:Mn⁴⁺ are synthesized by chemical reaction in HF/NaMnO₄ (CsMnO₄)/H₂O₂/H₂O mixed solutions immersed with tin metal. X-ray diffraction patterns suggest that the synthesized phosphors have a tetragonal symmetry with the space group D_4h¹⁴ (Na₂SnF₆:Mn⁴⁺) and a trigonal symmetry with the space group D_3d³ (Cs₂SnF₆:Mn⁴⁺). Photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and the Raman scattering techniques are used to investigate the optical properties of the phosphors. The Franck-Condon analysis of the PLE data yields the Mn⁴⁺-related optical transitions to occur at ∼2.39 and ∼2.38 eV (⁴A_2g→⁴T_2g) and at ∼2.83 and ∼2.76 eV (⁴A_2g→⁴T_1g) for Na₂SnF₆:Mn⁴⁺ and Cs₂SnF₆:Mn⁴⁺, respectively. The crystal field parameters (Dq) of the Mn⁴⁺ ions in the Na₂SnF₆ and Cs₂SnF₆ hosts are determined to be ∼1930 and ∼1920 cm⁻¹, respectively. Temperature-dependent PL measurements are performed from 20 to 440 K in steps of 10 K, and the obtained results are interpreted by taking into account the Bose-Einstein occupation factor. Comprehensive discussion is given on the phosphorescent properties of a family of Mn⁴⁺-activated alkaline hexafluoride salts.     

Ferromagnetism of epitaxially grown CaRu1−xMxO3 (M=Ti, Mn) films

Epitaxial thin films of CaRu_1−xM_xO₃ (M=Ti, Mn) were fabricated on a (0 0 1)-SrTiO₃ substrate by spin-coat method using organometallic solutions (metal alkoxides). Results of X-ray diffraction and transmission electron microscopy indicate that the epitaxial films were grown pseudomorphically so as to align the [0 0 l] axis of the CaRu_1−xM_xO₃ films perpendicular to the (0 0 1) plane of the SrTiO₃ substrate. Ferromagnetism and metal-insulator transition are induced by the substitution of transition metal ions. The occurrence of ferromagnetism was explained qualitatively assuming a TiRu₆ cluster model for CaRu_1−xTi_xO₃ film and a mixed valence model for CaRu_1−xMn_xO₃ film. Ferromagnetism was also observed for layered CaRuO₃/CaMnO₃ film and CaRuO₃/CaMnO₃/CaRuO₃/CaMnO₃ multilayer film and the magnetism was explained by an interfacial exchange interaction model with magnetic Mn³⁺, Mn⁴⁺, and Ru⁵⁺ ions.     

Pressure dependence of temperature variation of AC susceptibility for Fe50Ni37Mn13 and comparison to Fe68.1Ni31.9

Fe₅₀Ni₃₇Mn₁₃, which is a ferromagnetic alloy with FCC crystal structure, has been reported to show the Invar effect below the Curie temperature; however, this alloy shows a typical anti-Invar effect above the Curie temperature. In this paper, we discuss the pressure dependence of the temperature variation of the alternative current (AC) susceptibility at a frequency of 1 kHz for Fe₅₀Ni₃₇Mn₁₃ at various pressures up to 7.5 GPa above 77 K; we then compare the results with those for Fe_68.1Ni_31.9, which were obtained in a previous study. Fe₅₀Ni₃₇Mn₁₃ was in a ferromagnetic state throughout the entire pressure range measured. T_C decreases in inverse proportion to the increasing pressure; dT_C/dp and dln T_C/dp for Fe₅₀Ni₃₇Mn₁₃ are −26 K GPa⁻¹ and −0.07 GPa⁻¹, respectively. Further, the temperature variation of the shape of the χ′-T curve for different pressures indicates continuous combining of magnetic interactions occurs at high pressures. These results are similar to those obtained for Fe_68.1Ni_31.9.     

The magnetic susceptibility of hydrogen-doped partially crystalline (Zr76Ni24)1−xHx metallic glasses

The magnetizations of Zr₇₆Ni₂₄ metallic glass and hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses have been measured in the temperature range 10-300 K and magnetic fields up to 2 T for various dopant concentrations (x=0, 0.024, 0.043, 0.054). It is found that the samples are paramagnetic and magnetic susceptibility at room temperature, χ(300 K), shows a nonmonotonic behaviour upon hydrogenation. The values of χ(300 K) of the hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses are reduced with increase in hydrogen content up to x=0.043, whereas for x=0.054, an enhancement of χ(300 K) has been revealed. The magnetic susceptibility is weakly temperature dependent down to 110 K, below which an increase is observed. A shallow minimum exists between 90 and 120 K. The form and magnitude of the observed temperature dependence of the magnetic susceptibility are well accounted for by the sum of the quantum corrections to the magnetic susceptibility. Hydrogen reduces the electronic diffusion constant and influences strongly the quantum interference at defects, slowing down the spin diffusion and enhancing the magnetic susceptibility in the temperature range from 110 down to 10 K.     

Preparation of Nd-doped BiFeO3 films and their electrical properties

The Nd-doped BiFeO₃ thin films were prepared on SnO₂(FTO) substrates spin-coated by the sol–gel method using Nd(NO₃)₃·6H₂O, Fe(NO₃)₃·9H₂O and Bi(NO₃)₃·5H₂O as raw materials. The microstructure and electric properties of the BiFeO₃ thin films were characterized and tested. The results indicate that the diffraction peak of the Nd-doped BiFeO₃ films is shifted towards right as the doping amounts are increased. The structure is transformed from the rhombohedral to pseudotetragonal phase. The crystal grain is changed from an elliptical to irregular polyhedron. Structure transition occurring in the Bi_0.85Nd_0.15FeO₃ films gives rise to the largest Pr of 64 μC/cm². The leakage conductance of the Nd doped thin films is reduced. The dielectric constant and dielectric loss of Bi_0.85Nd_0.15FeO₃ thin film at 10 kHz are 190 and 0.017 respectively.     

The crystalline orientation and ferroelectric properties of Bi3.25La0.75Ti3O12 thin films mediated by the intermediate layer of LaNiO3

c-Axis-oriented and (1 1 7)-oriented Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films are successfully controlled by the intermediate layer of LaNiO₃ (LNO) with chemical solution deposition (CSD), respectively. X-ray diffraction (XRD) demonstrates that the structure and orientation of LNO thin films have a strong effect on the orientation of BLT thin films. Scanning electron microscopy suggests that BLT thin films on LNO electrode exhibit crack-free, uniform size grains and dense microstructure. A crystalline orientation dependent remanent polarization is observed in BLT thin films, and it is found that the remanent polarization (2P_r) of (1 1 7)-oriented films is larger than that of c-axis-oriented films. Our research directly demonstrates that the vector of the main spontaneous polarization in these layered perovskite materials (BLT) is along a-axis.