Investigations of the magnetic structure of the surface and volume of aluminum-substituted Sr-M type hexaferrites

The magnetic structure of the surface layer of single crystals of hexagonal ferrites of the type Sr-M (SrFe₁₂O₁₉) in which some iron ions are replaced by diamagnetic Al ions is investigated, in direct comparison with the magnetic structure in the bulk of the sample, by the method of simultaneous gamma, x-ray, and electron Mössbauer spectroscopy. It is found that under conditions of diamagnetic dilution of the magnetic lattice of hexagonal ferrites of the type Sr-M by Al ions, a layer ~200 nm thick in which the orientation of the magnetic moments is not collinear with the direction of the moments in the bulk of the sample is observed on the surface of SrFe_10.2Al_1.8O₁₉ crystals. Thus a “transitional” surface layer has been observed on macroscopic ferromagnetic crystals.     

Simultaneous gamma, x-ray, and electron Mössbauer spectroscopy of the volume and surface magnetic structure of hexagonal M-ferrites

Direct comparative studies are made between the magnetic structures of a surface layer of thickness ～40 nm and the bulk magnetic structure of ferromagnetic single crystals of hexagonal M ferrites (BaFe₁₂O₁₉, SrFe₁₂O₁₉, PbFe₁₂O₁₉) with a magneto-plumbite structure. Measurements are made by simultaneous gamma, x-ray, and electron Mössbauer spectroscopy in order to investigate the properties of the surface layer and the bulk crystal simultaneously. Experimental data obtained with a depth resolution of ～ 10 nm show that the orientation of the magnetic moments of the iron ions (along the crystallographic c axis) does not change on approaching the surface from the crystal volume. Thus, to within an experimental error of ～ 10 nm, single crystals of the hexagonal ferrites BaFe₁₂O₁₉, SrFe₁₂O₁₉, and PbFe₁₂O₁₉ with a ferromagnetic structure do not have a “ transition” surface layer whose magnetic structure differs from that of the bulk crystal such as that which exists, with a depth of several hundred nm, in antiferromagnetic materials with weak ferromagnetism.     

Mössbauer studies of the surface and bulk magnetic structure of scandium-substituted Ba-M-type hexaferrites

A direct comparison of the magnetic structures of a surface layer and of the bulk of Ba-M-type hexagonal ferrites with iron ions partially replaced by Sc diamagnetic ions (BaFe_12?x Sc_xO₁₉) has been made by simultaneous Mössbauer spectroscopy with detection of gamma rays, characteristic x-ray emission, and electrons. It has been found that, if the magnetic lattice of a Ba-M-type hexagonal ferrite is weakly diluted by Sc diamagnetic ions, a ～300-nm thick macroscopic layer forms on the surface of a BaFe_11.4Sc_0.6O₁₉ crystal, in which the iron-ion magnetic moments are noncollinear with the moments in the bulk. The noncollinear magnetic structure forms in the near-surface layer of BaFe_12?x Sc_xO₁₉ crystals because the exchange interaction energy is additionally reduced by the presence of such a “defect” as the surface. This is the first observation in ferromagnetic crystals of an anisotropic surface layer whose magnetic properties, as predicted by Néel, differ from those of the bulk.     

Studies of the surface magnetic state of the Sr-M hexagonal ferrites near the phase transition at the Curie temperature

A study is reported of the temperature dependences of the hyperfine (HF) interaction parameters in a ～200-nm thick surface layer and in the bulk of macroscopic hexagonal ferrite crystals of the Sr-M type (SrFe₁₂O₁₉ and SrFe_10.2Al_1.8O₁₉). The method used for the measurements is Mössbauer spectroscopy with simultaneous detection of gamma quanta, characteristic x-ray emission, and electrons, which permits direct comparison of the HF parameters in the bulk and the near-surface layers of a sample. As follows from the experimentally determined temperature dependences of the effective magnetic fields, the fields at the nuclei of the iron ions located in a ～200-nm thick near-surface layer decrease with increasing temperature faster than those of the ions in the bulk. The transition to paramagnetic state in a ～200-nm thick surface layer was found to occur 3° below the bulk Curie temperature. This offers the first experimental evidence for the transition to paramagnetic state in a surface layer of macroscopic ferromagnets to take place below the Curie temperature T _c for the bulk of the crystal. It has been established that the transition temperature T _c (L) of a thin layer at a depth L from the surface of a crystal increases as one moves away from the surface to reach T _c at the inner boundary of the surface layer called critical. In the vicinity of T _c one observes a nonuniform state, with the crystal being magnetically ordered in the bulk but disordered on the surface. The experimental data obtained were used to construct a phase diagram of surface and bulk states for macroscopic magnets near the Curie (or Néel) temperature.     

Mössbauer investigations of the surface state of hexagonal ferrites Sr-M near the curie point

The temperature dependence of the parameters of the hyperfine interaction in the surface layers and in the bulk of macroscopic crystals of hexagonal ferrites of the type Sr-M (SrFe₁₂O₁₉) is investigated by the method of simultaneous gamma-, x-ray, and electron Mössbauer spectroscopy. It is shown experimentally that the transition of an ≈ 200 nm thick surface layer of macroscopic ferromagnets to the paramagnetic state occurs at a temperature 3° below the Curie point (T _c) for the bulk of the crystal. It was established that the transition temperatureT _c(L) of a thin layer localized at a depthL from the surface of the crystal increases away from the surface and reaches the valueT _c at the lower (away from the surface) boundary of the so-called “critical” surface layer. A nonuniform state in which the bulk region of the crystal is magnetically ordered while the surface region is disordered is observed nearT _N.     

Effect of Magnetic Pulse Processing on the Structure and Magnetic Properties of Ferrites

The effect a pulsed magnetic field has on the crystal structure and macroscopic magnetic parameters of hexagonal ferrites BaFe₁₂O₁₉ and SrFe₁₂O₁₉ are studied. It is shown that changes in the physical properties of ferrites are due to the ordering of cation vacancies on the boundaries of hexagonal and spinel blocks that minimize local distortion of the oxygen polyhedrons. Violation of the collinear ordering of the magnetic moments of iron ions in the nonequivalent positions of SrFe₁₂O₁₉ ferrite is observed, due to the selective localization of such vacancies (and thus violations of the magnetic relationships in Fe–O–Fe).     

Surface magnetism of Sc-substituted Ba-M hexaferrites

A technique of simultaneous gamma-ray, x-ray, and electron Mössbauer spectroscopy is used to study the magnetic structure of the surface layer with direct comparison to the magnetic structure inside single crystal samples of hexagonal Ba-M ferrites, in which part of the iron ions have been replaced by diamagnetic Sc ions (chemical formula BaFe_12?δ Sc_δO₉). It is found that when the diamagnetic Sc ions are introduced into the crystal lattice of BaFe_12?δ Sc_δO₁₉ at concentrations (x=0.4 and 0.6) far below the level at which the collinear magnetic structure inside the sample is destroyed, a macroscopic layer of thickness ～300 nm develops on the surface, in which the magnetic moments of the iron ions are oriented noncollinearly with respect to the moments inside the sample. The deviation 〈θ〉 of the magnetic moments in BaFe_11.6Sc_0.4O₁₉ was 10° ± 62° for x=0.4, and when the Sc concentration was raised to 0.6, the angle 〈θ〉 increased to 17° ± 62°. The noncollinear magnetic structure in the surface layer in these crystals develops because of further reduction in the energy of the exchange interactions owing to the presence of a “defect,” such as the surface. For the first time, therefore, an anisotropic surface layer whose magnetic properties differ from those in the interior of a sample has been observed experimentally in ferromagnetic crystals, as predicted by Néel [L. Néel, Phys. Radium. 15, 225 (1954)].     

Mössbauer study of the mixed ferrimagnetic-spin glass phase in SrFe12−x CrxO19 hexagonal ferrites

It is shown from zero magnetic field Mössbauer spectroscopy that the mixed ferrimagnetic-spin glass phase observed in SrFe_12–x Cr_xO₁₉ hexagonal ferrites is characterized by longitudinal spin components contributing to the ferrimagnetic long range magnetic ordering and transverse spin components contributing to the spin glass ordering. It is argued as well that different sublattices could behave in a different way owing to their different degree of magnetic frustration.     

Hard magnetic properties of nanosized Sr(Fe,Al)12O19 hexaferrites obtained by Pechini method

In this work, we report the magnetic properties of isotropic M-type SrFe_12−xAl_xO₁₉ (x=0.0,1.5) hexaferrites synthesized by means of Pechini method. A polycrystalline distribution of fine grains was verified by Transmission Electron Microscopy for both compositions, with average sizes below 60 nm. Remarkable coercivity values within the range 500–850 kA/m were attained as a consequence of a combined effect of grain size refinement together with an enhancement of the anisotropy field afforded by the incorporation of the Al³⁺ cations into the hexagonal crystal structure.     

Formation of submicron-sized SrFe12−xAlxO19 with very high coercivity

Submicron-sized SrFe_12−xAl_xO₁₉ (x=1.3) was formed in glass-ceramic matrix using controlled thermocrystallization of the SrO–Fe₂O₃–Al₂O₃–B₂O₃ glass and the hexaferrite powder was obtained by removing the matrix phases. The samples were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray (EDX) analysis and magnetization measurements. The glass-ceramic material exhibits very high coercivity value up to 10.18 kOe which approaches a theoretically estimated maximum value for the compound. The hexaferrite powder consists of well faceted single crystals, which adopt the shape of a truncated hexagonal bipyramid. The powder saturation magnetization value is close to the theoretically estimated one for bulk material. Crystal structure of the powder was refined by Rietveld method and distribution of Al atoms on Fe sites was determined. Al atoms occupy 41% of 2a sites, 14% of 12k sites and 5% of 4e(1/2) sites, while 4f sites are not affected.     

Magnetic and Mössbauer spectral studies of x (NiFe2O4) + (1−x)(SrFe12O19), x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0 nanocomposites

Nano size composite of x(NiFe₂O₄)+(1?x)(SrFe₁₂O₁₉) were prepared using sol gel and aerosol route. The percentage of the components of NiFe₂O₄ and SrFe₁₂O₁₉ calculated from X-ray diffraction pattern using Rietveld analysis. The hysteresis loop for the as obtained samples exhibits no hysteresis, which may be attributed to super paramagnetic relaxation. The saturation magnetization do not show a significant change with the increase of strontium ferrites, however, the coercivity increased from 115 to 6,000 Oe. The Mössbauer spectra of these nano composites were discussed along with the magnetic moment and X-ray results.     

Microstructure characterization and magnetic behavior of NiAlxFe2−xO4 and Ni1−yMnyAl0.2Fe1.8O4 ferrites

NiAl_xFe_2−xO₄ and Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements. The single spinel phase was confirmed for all prepared samples. A proper explanation of data is possible if the Al³⁺ ions are assumed to replace Fe³⁺ ions in the A and B sites simultaneously for NiAl_xFe_2−xO₄ ferrites, and if the Mn²⁺ ions are assumed to replace Ni²⁺ ions in the B sites for Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites. Microstructural factors play an important role in the magnetic behavior of Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites with large Mn²⁺ content.     

Depth-selective conversion-electron Mössbauer spectroscopy of the surface of Ba-M hexaferrite single crystals

The magnetic properties of a 50-to 2-nm-thick surface layer in hexagonal ferrite BaFe₁₂O₁₉ single crystals are investigated for the first time. Measurements are made on a conversion-electron Mössbauer spectrometer constructed on the basis of a unique ultrahigh-vacuum magnetostatic electron analyzer of the “orange” type. An analysis of the experimental Mössbauer spectra obtained from the surface layer of BaFe₁₂O₁₉ 50-to 2-nm-thick single crystals reveals that (1) the spectral line widths are close to the natural widths of Mössbauer lines and neither changes nor a set of the values of effective magnetic fields, as well as a paramagnetic state of iron ions, could appear due to a defect such as the “surface,” and (2) the experimental spectra are best described only under the assumption that the surface of a hexaferrite BaFe₁₂O₁₉ single crystal contains a 2-nm-thick layer, in which the magnetic moments of iron ions are deflected through ～20° from the crystallographic C axis along which the magnetic moments of ions located in the bulk of the crystal are oriented.     

Influence of magnetic ordering on the structural properties of dilute copper ferrites

The x-ray structural properties of samples in the CuGa_xAl_xFe_2?2x O₄ (x = 0?0.7) and CuGa_xAl_2x Fe_2?3x O₄ (x = 0?0.5) systems are studied. It is found that magnetic ordering in dilute copper ferrites affects their structural properties. It is concluded that the frustration of magnetic coupling leads to suppression of the cooperative Jahn-Teller effect in dilute copper ferrites with a frustrated magnetic structure.     

Modifications in magnetic anisotropy of M—type strontium hexaferrite crystals by swift heavy ion irradiation.

Using vibrating sample magnetometery (VSM) 50 MeV Li³⁺ ion irradiation effects on magnetic properties of single crystals of SrGa_xIn_yFe_12−(x+y)O₁₉ (where x=0, 5, 7, 9; y=0, 0.8, 1.3, 1.0), are reported. The substitution of Ga and In in strontium hexaferrite crystals decreases the value of magnetization sharply, which is attributed to shifting of collinear magnetic order to a non-collinear one. Reduction of magnetization is also explained to be as a result of the occupation of the crystallographic sites of Fe³⁺ by Ga³⁺ and In³⁺. The Li³⁺ ion irradiation decreases the value of magnetization, irrespective of whether the crystals are Ga–In substituted or unsubstituted crystals of SrFe₁₂O₁₉. The result is interpreted in terms of the occurrence of a paramagnetic doublet in crystals replacing magnetic sextuplet as a result of irradiation. Substitution of Ga–In in Strontium hexaferrite decreases the value of anisotropy constant. Irradiation with Li³⁺ ions increases the values of anisotropy field for both substituted as well as unsubstituted crystals. Substitution with Ga–In also decreases the Curie temperature (T_c) but the irradiation with Li³⁺ ions does not affect the curie temperature of either Ga–In substituted or pure SrFe₁₂O₁₉ crystals.     

Physical and magnetic properties of highly aluminum doped strontium ferrite nanoparticles prepared by auto-combustion route

Highly Al³⁺ ion doped nanocrystalline SrFe_12−xAl_xO₁₉ (0≤x≤12), were prepared by the auto-combustion method and heat treated in air at 1100 °C for 12 h. The phase identification of the powders performed using x-ray diffraction show presence of high-purity hexaferrite phase and absence of any secondary phases. With Al³⁺ doping, the lattice parameters decrease due to smaller Al³⁺ ion replacing Fe³⁺ ions. Morphological analysis performed using transmission electron microscope show growth of needle shaped ferrites with high aspect ratio at Al³⁺ ion content exceeding x≥2. Al³⁺ substitution modifies saturation magnetization (M_S) and coercivity (H_C). The room temperature M_S values continuously reduced while H_C value increased to a maximum value of 18,100 Oe at x=4, which is an unprecedented increase (∼321%) in the coercivity as compared to pure Sr-Ferrite. However, at higher Al³⁺ content x>4, a decline in magnetization and coercivity has been observed. The magnetic results indicate that the best results for applications of this ferrite will be obtained with an iron deficiency in the stoichiometric formulation.     

The effect of Pt on Ni3Al surface oxidation at low-pressures

The fully-oxidized surface that forms on (1 1 1) oriented Ni₃Al single crystals, with and without Pt addition, at 300-900 K under oxygen pressures of ca. 10⁻⁷ Torr was studied using XPS, AES, and LEIS. Two main types of surfaces form, depending upon oxidation temperature. At low-temperature, the predominant oxide is NiO, capped by a thin layer of aluminum oxide, which we refer to generically as Al_xO_y. At high-temperature (i.e., 700-800 K), NiO is replaced by a thick layer of Al_xO_y. By comparing samples that contain 0, 10 and 20 at.% Pt in the bulk, we find that the effect of Pt is to: (1) reduce the maximum amount of both NiO and Al_xO_y; and (2) shift the establishment of the thick Al_xO_y layer to lower temperatures. Platinum also decreases the adsorption probability of oxygen on the clean surface.     

Synthesis and magnetic properties of La-substituted M-type Sr hexaferrites

Single-phase M-type hexagonal ferrites Sr_1−xLa_xFe₁₂O₁₉ (0≤x≤1) were prepared by a ceramic route. The stability limits of the ferrite phases were determined with a combination of various microscopy techniques, electron-probe micro-analysis, powder X-ray diffraction and thermal analysis. SrFe₁₂O₁₉ (x=0) is stable up to 1420 °C, whereas LaFe₁₂O₁₉ (x=1) exists between 1360 and 1400 °C only. The lattice parameters of Sr_1−xLa_xFe₁₂O₁₉ exhibit a linear variation with x, i.e. a₀ slightly increases and c₀ decreases with x, leading to a decrease of the unit cell volume with x. The saturation magnetization at T=5 K decreases with increasing La concentration. Room temperature Mössbauer analysis shows that the Fe³⁺/Fe²⁺ valence change occurs in the 2a sites for the whole composition range.     

BaCO3 mediated modifications in structural and magnetic properties of natural nanoferrites

Preparing M-type barium hexaferrite and improving the magnetic response of natural ferrites by incorporating barium carbonate (BaCO₃) is ever-demanding. Series of barium carbonate doped ferrites with composition (100−x)Fe₃O₄·xBaCO₃ (x=0, 10, 20, 30 wt%) are prepared through solid state reaction method and sintered gradually at temperatures of 800 and 1000 °C. Nanoparticles of natural ferrite and commercial BaCO₃ are used as raw materials. Impacts of BaCO3 on structural and magnetic properties of these synthesized ferrites are inspected. The obtained ferrites are characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) at room temperature. Uniform barium hexaferrite particles in terms of both morphology and size are not achieved. The average crystallite size of BaFe₁₂O₁₉ is observed to be within 30–600 nm. The sintering process results phase transformation from Fe₃O₄ (magnetite) to α-Fe₂O₃ (hematite) and the formation of hexagonal barium ferrite crystals. The occurrence of barium crystal is found to enhance with the increase of BaCO₃ concentrations up to 20 wt% and suddenly drop at 30 wt%. Saturation and remanent magnetization of the doped ferrites are significantly augmented up to 16.37 and 8.92 emu g⁻¹, respectively compared to their pure counterpart. Furthermore, the coercivity field is slightly decreased as BaCO₃ concentrations are increased. BaCO₃ mediated improvements in the magnetic response of natural ferrites are demonstrated.