Inelastic neutron scattering study in cubic NaNbO3

Inelastic neutron scattering measurements performed on NaNbO₃ close to the cubic to tetragonal phase transition show that the 2D fluctuations earlier revealed by X-ray diffuse scattering correspond to an overdamped type dynamics. The sharp decrease in the spectral width observed at T-T_c =10° is interpreted in terms of the increase in correlation lengths and the cross-over between 2D and 3D behaviour of the fluctuations.     

Dispersion of acoustical magnons and phonons in Mn1.7Fe1.3O4

The room-temperature spin wave spectrum has been investigated in Mn_1.7Fe_1.3O₄ by means of inelastic neutron scattering. The acoustical branch could be obtained in the symmetry directions. Values for the effective exchange parameters J_AR and J_RR have been derived. Some acoustical phonon branches could also be observed and the elastic constants C₄₄ and (C₁₁ – C₁₂) were estimated.     

Superspin glass state in MnFe2O4 nanoparticles

Nanosized MnFe₂O₄ ferrites were synthesized by a simple method, which is based on the solid state ball-milling and calcinations of nitrate precursors and citric acid. The samples were characterized by using different methods. The results indicate that the products mainly consist of MnFe₂O₄ nanoparticles. The effect of different annealing temperatures on particle sizes and crystallinity of the samples was also studied. By increasing the particle size, the coercivity and magnetization of the samples increase. The increase of magnetization by increasing the crystallite size could be attributed to the lower surface spin canting and surface spin disorder of the larger magnetic nanoparticles. Our analysis of ac susceptibility measurements shows that the interparticle magnetic interaction leads to the superspin glass-like behavior in these nanoparticle samples.     

Structural, microstructural and magnetic properties of NiFe2O4, CoFe2O4 and MnFe2O4 nanoferrite thin films

The structural, microstructural and magnetic properties of nanoferrite NiFe₂O₄ (NF), CoFe₂O₄ (CF) and MnFe₂O₄ (MF) thin films have been studied. The coating solution of these ferrite films was prepared by a chemical synthesis route called sol-gel combined metallo-organic decomposition method. The solution was coated on Si substrate by spin coating and annealed at 700 °C for 3 h. X-ray diffraction pattern has been used to analyze the phase structure and lattice parameters. The scanning electron microscopy (SEM) and atomic force microscopy (AFM) have been used to show the nanostructural behavior of these ferrites. The values of average grain's size from SEM are 44, 60 and 74 nm, and from AFM are 46, 61 and 75 nm, respectively, measured for NF, CF and MF ferrites. At room temperature, the values of saturation magnetization, M_s∼50.60, 33.52 and 5.40 emu/cc, and remanent magnetization, M_r∼14.33, 15.50 and 1.10 emu/cc, respectively, are observed for NF, CF and MF. At low temperature measurements of 10 K, the anisotropy of ferromagnetism is observed in these ferrite films. The superparamagnetic/paramagnetic behavior is also confirmed by χ′(T) curves of AC susceptibility by applying DC magnetizing field of 3 Oe. The temperature dependent magnetization measurements show the magnetic phase transition temperature.     

Inelastic neutron scattering study of the lattice dynamics of LaCoO3

The lattice dynamics of lanthanum cobaltite is investigated using the coherent inelastic neutron scattering technique in a temperature range from 10 to 536 K, in which two phase transitions are observed: a change in the Co spin state and a metal-insulator phase transition. The measurements were performed using an IN8 high flux thermal neutron triple-axis spectrometer (ILL, Grenoble). Temperature changes in the phonon dispersion curves caused by the rearrangement of the electron and spin subsystems are found.     

Inelastic neutron scattering from transverse acoustic modes in K2SnCl6

In cubic K₂SnCl₆ the dispersion curve für q q; [110] of [1$\text{1}$0] polarized acoustic phonons has been measured at three temperatures near the phase transition temperature T_c1. The acoustic branch shows no temperature dependence in the low wave vector region and stiffens slightly near the X-point of the fcc-Brillouin zone as the temperature approaches the phase transition T_c1= 262 K from above. The results support a previously developed model on acoustic anomalies in this compound.     

Inelastic neutron scattering study of the ferroelastic phase transition of lead phosphate (PO4)2Pb3

A soft phonon mode is observed above 395°C at zone boundary points of the high temperature β phase. A quasi-elastic scattering exists above the ferroelastic transition at 180°C and progressively vanishes when the temperature is raised. A cross over from 3d to 2d regime is observed in the same temperature range.     

Raman scattering from magnons in CoCl2 and FeCl2

One-magnon Raman scattering has been observed in the metamagnets CoCl₂ and FeCl₂. The k = 0 magnon energies are 16 ± 1 cm^-1 at 21 K and 16.4 ± 0.4 cm^-1 at 12 K, respectively and these values are in good agreement with previous AFMR and neutron scattering results. A search for two-magnon scattering in both compounds was unsuccessful, largely because of masking from nearby first-order phonons and a weak temperature dependent broad band at 140 cm^-1 in CoCl₂, which is assigned to two-phonon scattering from acoustic phonons.     

Synthesis and characterization of MnFe2O4-BiFeO3 multiferroic composites

The mixed spinel-perovskite composites of xMnFe₂O₄-(1-x)BiFeO₃ with x=0, 0.1, 0.2, 0.3 and 0.4 were prepared by solid state reaction method. The structure and grain size were examined by means of X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), respectively. The XRD results showed that the composites consisted of spinel MnFe₂O₄ and perovskite BiFeO₃ phases after being calcined at the temperature 950 °C for 2 h. The grain size ranged from 0.8 to 1 μm. Magnetization was found to increase with increasing concentration of ferrite content. The variation of dielectric constant and dielectric loss with frequency showed dispersion in the low frequency range. Magnetocapacitance was also observed in the prepared composites, which may be the sign of magnetoelectric coupling in the synthesized composites at room temperature.     

Inelastic neutron scattering study of multimagnon states in the 1-D-ferromagnet CsNiF3

Two theoretical propositions concerning scattering from higher-order-spin-wave processes have been tested by means of neutron scattering. The so-called “second magnon” scattering could not be observed as such, and an upper limit for its intensity relative to the single-spin-wave scattering is given. Experimental evidence for the scattering from the simultaneous creation of two spin waves has been observed. The results are compared with the corresponding theories.     

Inelastic neutron scattering study of magnetic excitations in the helimagnetic and antiferromagnetic phases of NiBr2

The spin wave dispersion in NiBr₂ has been studied by medium and long wavelength inelastic neutron scattering in the [1 1 0], [1 0 0] and [0 0 1] directions at 4.2 and 30 K, i.e. in the incommensurate helical and collinear antiferromagnetic phases. The values of the intralayer Heisenberg exchange constant J_ij and XY anisotropy constant D at 4.2(30) K are J₀₁ 0.379(1)(0.379(1)), J₀₂ 0.0036(50)(0.0036(50)), J₀₃ - 0.105(5) (?0.105(5)), J′ - 0.0423(50)(?0.389(50))D 0.0364(50)(0.0290(50)), where J′; is the interlayer exchange constant. In fitting the 4.2 K data account is taken of the co-existence of three equivalent domains and of intensity arising from $\text{ω(}\text{q}\text{)}$ and $\text{ω(}\text{q ± k}{}_0\text{)}$ where $\text{k}{}_0$ is the wavevector of the helix. In the low frequency region of the dispersion curve such peaks are resolved. The results reinforce the hypothesis that in zero-field the commensurate-incommensurate phase transition is driven by fluctuations.     

The effect of grinding on magnetic properties of agglomereted MnFe2O4 nanoparticles

The effects of grinding on interparticle magnetic interactions for an ensemble of agglomerated MnFe₂O₄ nanoparticles have been studied. Structural analyses showed that by grinding the samples, a small variation in size of crystallites and lattice strain will occur. ac Magnetic susceptibility measurements under different conditions and spin dynamics analysis suggest that freezing temperature is frequency dependent and it is in good agreement with critical slowing down model. This is an indication that these nanoparticles have superspin glass behavior. The estimated zυ and τ₀ parameters using critical slowing down model show that by increasing the grinding time the interaction between nanoparticles decreases. ac Susceptibility measurements in cooling and heating process show a thermal hysteresis. The thermal hysteresis decreased by increasing the grinding time. Also, the thermal hysteresis is frequency dependent and it increased as frequency decreased. These results showed that interparticle interactions such as dipole-dipole and exchange interactions between nanoparticles become weaker by grinding.     

MnFe204晶体的基态能级和零场分裂参量

由不可约张量理论构成一个3d^4／3d^6离子三角(C3v)对称的晶体场和自放—轨道相互作用哈密顿矩阵，由这个晶体场和自放—轨道相互作用哈密顿矩阵被完全对角化后能够求出MnFe204晶体中的Fe^2 离子的电子顺磁共振零场分裂参量D和F—a，计算了低自旋态(^3L态)对电子顺磁共银零场分裂参量(D，F—a)的贡献。结果显示低自放^3L态对电子顺磁共振的零场分裂参量的贡献是较强的。理论计算的结果与实验值是相符的。     

The phase transition of two-dimensional squaric acid H2C4O4 studied by neutron scattering

At T_c = 370 K squaric acid undergoes a phase transition, which was studied in detail by neutron scattering. The transition is essentially two-dimensional with β = 0.137 ± 0.010. Diffuse scattering above T_e is found to be anisotropic. From “diffuse crystallography” it is concluded that proton disorder is not the dominant source of diffuse scattering. Individual molecules retain their low-temperature shape in the high-temperature phase.     

Dependence of Néel temperature on the particle size of MnFe2O4

Mn-ferrite nanoparticles having diameter in the range 17-45 nm were synthesized by modified co-precipitation method using metal nitrate solutions. Different concentrations of NaOH were found to affect the growth of particle size. Néel temperature (T_N) was found to increase with increasing particle size. The obtained Néel temperature was higher than that of the bulk. The shift in the Néel temperature is described by the finite size-scaling theory [T_N(d)−T_N(bulk)]/T_N(bulk)=(d/d₀)^−1/v, where d is particle size, v=0.6±0.1 and d₀=1.7±0.1 nm.     

Synthesis and characterization of single-crystalline MnFe2O4 nanorods via a surfactant-free hydrothermal route

Uniform, high-quality, single-crystalline MnFe₂O₄ nanorods with diameter around 25 nm and length up to 500 nm, have been reproducibly synthesized via a surfactant-free hydrothermal route. The growth direction of the obtained nanowires was determined to be its [1 1 1] direction, resulting in the increase of saturation magnetization. Mn²⁺ is responsible for one-dimensional growth of the nanorods, and the effects of reaction time and solution concentration on the morphology and crystallization of the MnFe₂O₄ nanorods were investigated. Saturation magnetization of the nanorods is 74.0 emu/g, which is among the best value reported so far.     

Concerning the cation distribution in MnFe2O4 synthesized through the thermal decomposition of oxalates

A single phase manganese ferrite powder have been synthesized through the thermal decomposition reaction of MnC₂O₄·2H₂O-FeC₂O₄·2H₂O (1:2 mole ratio) mixture in air. DTA-TG, XRD, Mössbauer spectroscopy, FT-IR and SEM techniques were used to investigate the effect of calcination temperature on the mixture. Firing of the mixture in the range 300-500 °C produce ultra-fine particles of α-Fe₂O₃ having paramagnetic properties. XRD, Mössbauer spectroscopy as well as SEM experiments showed the progressive increase in the particle size of α-Fe₂O₃ up to 500 °C. DTA study reveals an exothermic phase transition at 550 °C attributed to the formation of a Fe₂O₃-Mn₂O₃ solid solution which persists to appear up to 1000 °C. At 1100 °C, the single phase MnFe₂O₄ with a cubic structure predominated. The Mössbauer effect spectrum of the produced ferrite exhibits normal Zeeman split sextets due to Fe³⁺ions at tetrahedral (A) and octahedral (B) sites. The obtained cation distribution from Mössbauer spectroscopy is (Fe_0.92Mn_0.08)[Fe_1.08Mn_0.92]O₄.     

Cation distribution dependence of magnetic properties of sol-gel prepared MnFe2O4 spinel ferrite nanoparticles

MnFe₂O₄ nanoparticles have been synthesized with a sol-gel method. Both differential thermal and thermo-gravimetric analyses indicate that MnFe₂O₄ nanoparticles form at 400 °C. Samples treated at 450 and 500 °C exhibit superparamagnetism at room temperature as implied from vibrating sample magnetometry. Mössbauer results indicate that as Mn²⁺ ions enter into the octahedral sites, Fe³⁺ ions transfer from octahedral to tetrahedral sites. When the calcination temperature increases from 450 to 700 °C, the occupation ratio of Fe³⁺ ions at the octahedral sites decreases from 43% to 39%. Susceptibility measurements versus magnetic field are reported for various temperatures (from 450 to 700 °C) and interpreted within the Stoner-Wohlfarth model.     

Effect of uniaxial stress cooling on the neutron magnetic scattering in a ZnCr2O4 single crystal

For the determination of the magnetic structure, elimination of the magnetic domains is indispensable and the appropriate stress will be effective when the crystal has a structural transformation at T_N. The effect of uniaxial stress cooling on the neutron magnetic scattering in a ZnCr₂O₄ single crystal was studied. The pressure axis was set parallel to the [1 0 0] or [0 0 1], parallel or perpendicular to the diffraction plane, (0 0 1). The [1 0 0] pressure cooling increases one half-integer spots along the [1 0 0], whereas decreases others. The [0 0 1] pressure cooling increases two half-integer spots in the (0 0 1) plane, whereas decreases one half-integer spots.     

Inelastic light scattering and photoluminescence in the mixed valence systems Eu3O4, Eu3S4 and Sm3S4

The thermally activated valence fluctuations of Eu₃S₄ and Sm₃S₄ have been studied using light scattering and photoluminescence techniques and are compared with measurements on the non-fluctuating compound Eu₃O₄, Eu₃S₄ exhibits an anomalous vibrational mode which is associated with the frequency factor of hopping, ν₂=1.3×10¹³ sec^?1. In Sm₃S₄ electronic Raman scattering is observed within the ⁷F₉ multiplet of the Sm²⁺ ion. An anomalous frequency shift of the5d-4f photoluminescence emission band in Eu₃S₄ is related to the temperature dependent fluctuation rate which passes through the reference time scale of the photoluminescence. Intra-4f photoluminescence has also been observed in Eu₃S₄ and Sm₃S₄.