Magnetic dipole fields at interstitial sites in disordered binary alloys of ferromagnetic crystals

The dipole fields at interstitial sites occupied by positive particles (μ ⁺,p) in f.c.c. and b.c.c. diluted binary alloys of ferromagnetic metals are investigated. The characteristic shapes of the distribution functions of the dipole fields is shown to depend on the interstitial site occupied by the positive particle, and they may give information on the formation of diatomic complexes of the light interstitials with substitutional impurity atoms as well as on the degree of short-range order of the alloy.     

Distribution of dipole fields in disordered crystalline and amorphous ferromagnetic alloys

By means of computer simulation we have calculated the distribution functions of dipole fields in disordered crystalline and amorphous ferromagnetic alloys A_1-xB_x. It is shown that for all cubic lattice sites in simple cubic, body-centred cubic and face-centred cubic materials as well as for amorphous materials the envelopes of the distribution functions may be obtained in a satisfactory approximation by considering only the nearest contributing atoms. Whereas in crystalline materials we have a complicated structure of the distribution functions for arbitrary values ofx, we obtain simple Gaussian distributions for the case of amorphous materials, using Heimendahl's model of the amorphous structure. The influence of isotropic and anisotropic short-range order is discussed in detail.     

Deformation induced texture in Mössbauer absorbers

Anomalies in line intensities of Mössbauer absorbers subjected to non-hydrostatic pressure are attributed to texture. The dependence of the texture function on the deformation of the sample and on the shape of the crystallites is calculated. From the texture function the asymmetry of a 1/2→3/2 quadrupole doublet is determined. The possibilities of avoiding texture effects in Mössbauer spectroscopy are discussed and a method is suggested for preparing texture-free absorbers.     

Influence of nickel on iron-deficient spinel ferrites

A detailed study of the electric and magnetic properties of ceramic nickel-ferrites, Ni _x Fe_3−x−ΔO₄ withX≧0.8 and Δ≦10⁻² is presented. The physical interpretation of the results, the x-ray structural analysis and the magnetic aftereffects spectra allow us to define the substitution mechanism Ni↔Fe and explain the progressive variation of the properties in the samples with increasing concentrations of nickel.     

Ferrimagnetic resonance spectra of magnetic bubble films at low microwave frequencies

Magnetic bubble films exhibit a number of ferrimagnetic resonance modes due to the spatial variation of the anisotropy. The resonance frequencies have been measured as a function of the applied bias fieldH ₀. In the lower field range the magnetization of the transient layer, which has negative anisotropy, is not yet parallel toH ₀. In this range the resonance frequencies are shifted to higher values due to pinning effects. In films grown by the vertical dipping method an additional layer on top of the transient layer is observed within which the magnetization rotates from the direction in the transient layer to that of the bulk of the film. In films grown by horizontal dipping no such layer could be detected. Each ferrimagnetic resonance mode excites transverse elastic waves in the film due to the magnetoelastic interaction and thus gives rise to elastic resonances of the whole crystal, film and substrate. These elastic resonances lead to a fine-structure of the ferrimagnetic resonances. The observed fine-structure vanishes periodically with frequency and from this behaviour the thickness of the magnetic film and of the transient layer has been determined.     

Determination of cation distribution in Ti4+ and Co2+ substituted barium ferrite by Mössbauer spectroscopy

In barium ferrite, BaFe₁₂O₁₉, the Fe³⁺ ions were gradually substituted by Ti⁴⁺ and Co²⁺. The cation distribution of the various lattice sites is determined as a function of the degree of replacement. The preferred site population is discussed by a substitution model.     

Effect of porosity on the magnetic behaviour of nickel ferrites

The magnetic behaviour of polycrystalline materials is closely related to their specific manufacturing process. Beside composition, the various factors that affect this behaviour and cause the particular microstructure of these materials include non-magnetic inclusions, grain size, pressing, temperature, sintering time, and cooling rate. A study has been done of magnetization curves and static hysteresis loops in a set of polycrystalline nickel ferrites. The experimental results were fitted by a second-order rational function, which easily allowed us to calculate their characteristic parameters, and analyze them according to the non-magnetic inclusions model.     

Incomplete magnetic ordering in Fe2(1−y)Mg1+y Ti y O4 spinels with intermediate compositiony

Static magnetization measurements on the ferrimagnetic spinels Fe_2(1?y)Mg_1+y Ti _y O₄ withy=0.3, 0.4, 0.5, and 0.6 show that these compounds have no well-defined orderdisorder transition temperature and that their ferrimagnetism may not be described in terms of the Néel theory. From the Mössbauer spectra we conclude that a temperature dependent number of the ferric ions does not participate in the ferrimagnetism of those compounds with compositiony≧0.4. The explanation of the observed magnetic and Mössbauer properties is based on the assumption that each ferric ion must have at least two magnetic linkages of the type Fe _A ³⁺ ?O^2??Fe _B ³⁺ in order to couple its magnetic moment to the neighbouring ones over the entire temperature interval between 0 K and the respective Néel temperature.     

Magnetic aftereffects in nickel ferrites

We have studied the magnetic aftereffects in the Ni _x Fe_3−x−ΔO₄ system, for 0≦x≦1 and 10⁻⁵≦Δ≦2×10⁻¹, between 80 and 500 K. The samples were obtained by sintering at 1400°C in an appropriate gas atmosphere. The measurements are based on the deviation from equilibrium that is produced in a Maxwell-Wien bridge when the self-induction of a coil with ferrite core varies because of the phenomenon of magnetic aftereffects. The numerical analysis of the results shows the presence of relaxation processes at 300 K (III), 330 K (IIa), and above 500 K (I). The Processes III and IIa are related to the concentration of nickel,x, and of vacancy, Δ. It is seen that the IIa peak can be attributed to a process of diffusion of Ni ions in the spinel lattice by means of vacancies on octahedral sites.     

Characterization of magnetic phases in the FexMn0.65−xAl0.35 disordered alloys

Melted alloys of the Fe_xMn_0.65−xAl_0.35 disordered system, 0.25?x?0.65, were experimentally studied by Mössbauer spectrometry, vibrating sample magnetometry and AC magnetic susceptibility. All the alloys exhibit the BCC structure with a nearly constant lattice parameter (2.92 Å). Mössbauer studies at room temperature (RT) show that in the 0.25 ?x?0.45 range the alloys are paramagnetic (P) while in the 0.50?x?0.65 range, they are ferromagnetic. At 77 K, Mössbauer studies show that the alloy with x=0.25$x=0.25$ presents weak magnetic character that is consistent with an antiferromagnetic (AF) behavior due to the high Mn content, while those with 0.30?x?0.40 are paramagnetic, and those in the 0.45?x  ?0.65 range are ferromagnetic (F) with a mean field increasing with the Fe content. Hysteresis cycles at RT prove the paramagnetic character of the alloys between x=0.25$x=0.25$ and 0.40 and the ferromagnetic character for x?0.45$x?0.45$. Complementary measurements using AC magnetic susceptibility permit a magnetic phase diagram to be proposed, with the P phase for high temperature and all the compositions, the AF phase for low Fe content and at low temperature, the F phase for high Fe content above RT and the spin glass phase for all the compositions and at temperatures lower than 46 K. In addition, the mean field renormalization group (MFRG) method, applied to a random competitive and site dilute Ising model with nearest-neighbor, gives rise to magnetic phase diagram, which fairly agrees with previous experimental one.     

The nature of the passivating oxide layer on iron powder

Passivated iron powder is studied by Mössbauer spectroscopy, x-ray diffraction and magnetization measurements in order to elucidate the nature of the oxide layer. A gradual transition from FeO at the metal/oxide interface via Fe₃O₄ to γ-Fe₂O₃ at the outside agrees with the experimental results and with the available data on chemical composition, calorimetry, in fared spectroscopy and chemical stability.     

Novel RCo5Ga7 intermetallic compound

The novel RCo₅Ga₇ (R=Y, Tb, Dy, Ho and Er) intermetallic compounds have been synthesized, and their crystallographic and magnetic properties have been studied using X-ray diffraction and magnetic measurement. RCo₅Ga₇ crystallizes in an orthorhombic structure with ScFe₆Ga₆ type. The space group is Immm, and Z=2. According to the structural refinement result, the 2a, 4e, 4f, 4g, 4h, and 8k crystal positions are occupied by 2R, 4Ga^I, 4(Ga^II, Co^I), 4Ga^III, 4(Ga^IV,Co^II), and 8(Co^III,Ga^V), respectively. The RCo₅Ga₇ intermetallic compound can be stabilized in the range of the radius ratio of R_Re/R_(Co,Ga)<1.36. The RCo₅Ga₇ compound exhibits a paramagnetic behavior. The magnetization at 5 K ranges from 28.93 to 40.62 emu/g.     

Magneto-structural correlations in Pr0.15Gd0.85Mn2Ge2

Magneto-structural correlations in Pr_0.15Gd_0.85Mn₂Ge₂ have been studied by synchrotron diffraction in the temperature range between 11 and 300 K. This compound crystallizes in the ThCr₂Si₂-type structure (space group ). The unit cell parameters a and c were determined by Rietveld refinements as a function of temperature. Anomalies in the temperature dependence of the unit cell parameters a and c, the c/a ratio and the unit cell volume V at about 240 and 140 K, which is close to the magnetic phase transition temperatures, indicate a pronounced magneto-structural correlation. Spontaneous volume change and linear magnetostrictions are derived as a function of temperature.