Ferromagnetic resonance and magnetization in permalloy films with nanostructured antidot arrays of variable size

We present the results of structural and magnetic characterization of Permalloy (Fe₂₀Ni₈₀) films deposited by sputtering on self-organized nanoporous alumina previously treated with phosphoric acid to vary the pore diameters. SEM and AFM images of the top film surfaces show a triangular array of pores with diameters similar to the untreated porous alumina. However, the underlying pore enlargement is evidenced by the magnetic study. Indeed magnetization measurements reveal a decrease in the easy-plane anisotropy energy with elapsing time of chemical etching. Consistent with these results, ferromagnetic resonance measurements perpendicular to the film indicate a systematic reduction of the resonance field which can be directly related to an increase in the pore diameter. The effect of lateral confinement (due to the pores) in the spin-wave resonance is evidenced in multiple absorption lines when the applied field is in the film plane and perpendicular to it. This contrasts with the results for the continuous test film.     

The influence of long range interparticle correlations on the magnetically induced optical anisotropy in magnetic colloids

In this paper we develop a theoretical model for the magnetically induced optical anisotropy in dense magnetic colloids made of spherical and un-aggregated magnetic monodomain nanoparticles. Both dipole-field and dipole-dipole magnetic and electric interactions between the magnetic monodomain particles are taken into account in the Hamiltonian of the system. Using the pair correlation function in a colloidal suspension of magnetic nanoparticles developed by Ivanov and Kuznetsova (2001) [11], the complex dielectric constant of a magnetic colloid is modeled as a function of the light polarization direction, the magnetic field intensity and magnetic particle concentration and diameter. The two main features of the model are that, on the one hand, it predicts the possibility of magnetically induced optical anisotropy in dense magnetic colloids made of spherical and un-aggregated monodomain nanoparticles, and on the other hand, unlike the existing models for diluted samples, it predicts a non-linear dependence of dichroism and birefringence on magnetic particle concentration.     

Extinction of polarized light in ferrofluids with different magnetic particle concentrations

The magnetic field intensity and nanoparticle concentration dependence of the polarized light extinction in a ferrofluid made of magnetite particles stabilized with technical grade oleic acid dispersed in transformer oil was experimentally investigated. The magnetically induced optical anisotropy, i.e. the dichroism divided by concentration, was found to decrease with increasing sample concentration from 2% to 8%. The magnetically induced change in the optical extinction of light polarized at 54.74° with respect to the magnetic field direction was found to be positive for the less concentrated sample (2%) and negative for the samples with 4% and 8% magnetic nanoparticle concentrations, the more negative the higher the concentration and field intensity. Based on the theoretically proven fact [11] that the particle orientation mechanism has no effect on the extinction of light polarized at 54.74° with respect to the field direction, we analyzed the experimental findings in the frames of the agglomeration and long-range pair correlations theories for the magnetically induced optical anisotropy in ferrofluids. We developed a theoretical model in the approximation of single scattering for the optical extinction coefficient of a ferrofluid with magnetically induced particle agglomeration. The model predicts the existence of a polarization independent component of the optical extinction coefficient that is experimentally measurable at 54.74° polarization angle. The change in the optical extinction of light polarized at 54.74° is positive if only the formation of straight n-particle chains is considered and may become negative in the hypothesis that the longer chains degenerate to more isotropic structures (polymer-like coils, globules or bundles of chains). The model for the influence on the light absorption of the long-range pair correlations, published elsewhere, predicts that the change in the optical extinction of light polarized at 54.74° is always negative, the more negative the higher the magnetic field intensity and particle concentration.     

Structural and magnetic study of the Ti-doped barium hexaferrite ceramic samples: Theoretical and experimental results

We present a theoretical and experimental study of the structural and magnetic properties of M-type Ti⁴⁺-doped barium hexaferrite BaFe_{(12−(4/3)x)}Ti_xO₁₉ with x=0, 0.1, 0.2, 0.3, 0.5, 0.7 and 1.3. The XRD patterns and magnetic measurements show appreciable variations in the values of the saturation magnetization and the magnetic anisotropy field, H_an, with increasing Ti⁴⁺ content. We did not observe significant changes in the Lotgering factor along the (0 0 l) direction and in the texture coefficient, C_ex, which was estimated from the torque curves. The magnetic properties of these materials are explained by the combined effect of the coherent rotation of the magnetic domains and the replacements of Fe³⁺ by Ti⁴⁺ ions in the octahedral and tetrahedral sites. The influence of the Ti⁴⁺ content on the samples was studied theoretically by using a statistical phenomenological model. The main purpose of the model is to make preliminary predictions of the distribution of any dopant cation in the Fe³⁺ sites. As a result, we are able to analyze both structural and magnetic features of M-type barium hexaferrite.     

EPR study of the diluted magnetic semiconductor CuGa1−xMnxTe2

Electron paramagnetic resonance (EPR) experiments were made in the diluted magnetic semiconductor CuGa_1−xMn_xTe₂, in the temperature range 70<T<300 K. The samples were synthesized by direct fusion of stoichiometric mixtures of the elements, with Mn composition from x=0.0 to 0.25. The EPR spectra were measured as function of temperature, Mn composition, and field orientation. The temperature variation of the resonance field shows a critical point at about 235 K, and is associated with a transition from the ferromagnetic to the superparamagnetic state. The resonance field was also measured as a function of the field angle, and displays a well-defined uniaxial symmetry. This uniaxial field depends on the Mn concentration and is due to tetragonal distortions induced by Mn²⁺ at Ga sites, and the demagnetizing effects due to formation of ferromagnetism (FM) Mn-clusters.     

风冷冷冻柜中毛细管特性的试验研究

以某风冷冷冻柜为研究对象,分别从毛细管长度、制冷剂充注量、频率变化三个方面试验分析毛细管对制冷系统性能的影响。试验得出最佳毛细管长度为650mm;最佳充注量为1060g;系统高能效比的频率范围在20Hz～60Hz之间,对于在稳定工况下运行的机组,变频并不能起到很好的节能效果。通过标准毛细管和使用一定时间后的被测毛细管的对比分析,改变电流模拟分析系统启停,得到以库中央温度和冷凝器中央温度为代表的影响系统性能的变化曲线。实际运行中可通过监测库中央温度、冷凝器中央温度、压缩机吸气和排气温度、压缩机上下表面温度,与标准毛细管对比就可准确判断出系统运行状态是否正常。     

On the comparison of the polarisation behaviour of exchange-biased AF/F NiMn/Fe37Co48Hf15 bi-layer and multi-layer films with increased ferromagnetic cut-off frequencies

Antiferromagnetic/ferromagnetic (AF/F) NiMn/Fe₃₇Co₄₈Hf₁₅ films were investigated with respect to their exchange bias, in-plane unidirectional anisotropy, polarisation and high frequency behaviour. After deposition, carried out by r.f. magnetron sputtering, the films were post-annealed for 4 h at 300 °C in a static magnetic field, in order to induce exchange-bias, which results in a unidirectional anisotropy. Dependent on the presence of a bi-layer or multi-layer sandwich structure the films show a different exchange-bias field-ferromagnetic inter-layer thickness behaviour with exchange-bias fields μ₀?H_eb between 2 and 10 mT. The in-plane uniaxial (single film) or unidirectional anisotropy fields μ₀*H_UF were between 4 and 18 mT. This results in a significant increase of the cut-off frequency in the GHz range in comparison to a single Fe₃₇Co₄₈Hf₁₅ film, which is shown by frequency-dependent permeability plots. High damping in the imaginary part of the permeability, i.e., high resonance line broadening could be observed for films with high coercivity μ₀*H_c of around 7 mT in the easy axis of magnetisation.     

An experimental study of the properties of magnetoinductive waves in the presence of retardation

Magnetoinductive (MI) waves owe their existence to the magnetic coupling between metamaterial elements. First experiments confirming the existence of MI waves were carried out on capacitively loaded loops and Swiss Rolls about three orders of magnitude smaller than the operating wavelengths (5–15 m) so that the radiation effects did not play any significant role. In the present paper MI waves are studied experimentally on various types of split ring resonators of about 1 cm diameter operating in the microwave region between 1 and 2 GHz. Our results prove that retardation has a significant effect upon the propagation of MI waves.     

Effective anisotropy field variation of magnetite nanoparticles with size reduction

Size effect on the internal magnetic structure has been investigated on weakly interacting magnetite (Fe₃O₄) nanoparticles by ferromagnetic resonance experiments at 9.5 GHz as a function of temperature (4–300 K). A set of three samples with mean particle size of 2.5 nm, 5.0 nm and 13.0 nm, respectively, were prepared by chemical route with narrow size distribution (σ < 0.27). To minimize the dipolar interaction, the particles were dispersed in a liquid and a solid polymer matrix at ∼0.6% in mass. By freezing the liquid suspension with an applied external field, a textured was obtained. Thus, both random and textured suspensions were studied and compared. The ferromagnetic resonance experiments in zero-field-cooled and field-cooled conditions were carried out to study the size effect on the effective anisotropy field. The dc magnetization measurements clearly show that the internal magnetic structure was strongly affected by the particle size.     

Neutron depolarization study on the magnetic correlation length of nickel ferrite with different packing densities

The magnetic correlation length of a mixed nickel ferrite powder was studied by a newly commissioned depolarized neutron beamline at the W3 port of Tsing Hua Open Pool Reactor (THOR). In this work, Ni ferrite powder samples with different packing densities were studied. The magnetic correlation lengths of the sample were observed to be 2 μm at virgin state and about 3.1 μm at remanent state from the packing density of 20–60%. This magnetic domain size is smaller than particle size. No significant change of domain size at this packing density implies the domain wall motion is hindered by the porosity effectively up to at least 60% of packing density.     

First-principles study on the electronic structure, magnetic properties and phase stability of alloyed cementite with Cr or Mn

Using the first-principles technique, the electronic structures, magnetic properties and phase stability of alloyed cementite with Cr or Mn were investigated. The calculations show that the chemical and mechanical stability of alloyed cementite can be strengthened by the use of Cr/Mn-doped method. The Magnetic Moments (Ms) of Mn₁Fe₂C, Mn₂Fe₁C, Cr₁Fe₂C and Cr₂Fe₁C are 5.274, 0.941, 1.864 and 0.736 μ_B/f.u, respectively. The Ms of Cr in Fe₂CrC (−1.374 μ_B) and Cr₂FeC (−0.032 μ_B) are different due to replacing different sites Fe atoms. The magnetic behaviors of Mn are different from Cr in alloyed cementite. The Ms of Mn in Fe₂MnC and Mn₂FeC are 2.300 μ_B and −0.147 μ_B, respectively.     

Theoretical study on the electronic structures and phosphorescent properties of four Ir(III) complexes with different substituents on the ancillary ligand

The geometry structures, electronic structures, absorption and phosphorescent properties of four Ir(III) complexes {[(F₂-ppy)₂Ir(pta-X)], where F₂-ppy = (2,4-difluoro)phenylpyridine; pta = pyridine-1,2,4-triazole; X = –CF₃; –H; –CH₃; –N(CH₃)₂}, are investigated using the density functional method. The results reveal that the electron-accepting group –CF₃ has no obvious effect on absorption and emission properties, while the substitutive group –N(CH₃)₂ with strong electron-donating ability has obvious effect on the emission properties. The mobility of hole and electron were studied computationally based on the Marcus–Hush theory. Calculations of ionisation potential and electron affinity were used to evaluate the injection abilities of holes and electrons into these complexes. We hope that this theoretical work can provide a suitable guide to the future design and synthesis of novel phosphorescent materials for use in the organic light-emitting diodes.