Surface magnetism of Al-substituted Sr-M-type hexagonal ferrites

The surface and bulk magnetic structures of Sr-M-type single-crystal hexagonal ferrites (with the chemical formula SrFe_12?x Al_xO₁₉) have been directly compared by simultaneous gamma, x-ray, and electron Mössbauer spectroscopy. It was found, that when the magnetic lattice of Sr-M hexagonal ferrites is slightly diluted by diamagnetic Al ions, namely, for x=1.8 (SrFe_10.2Al_1.8O₁₉), a ～300-nm thick macroscopic anisotropic layer forms on the crystal surface, wherein iron-ion magnetic moments are oriented differently from those in the bulk of the sample. The reason for the onset of a noncollinear magnetic structure in the surface layer of SrFe_10.2Al_1.8O₁₉ crystals is the additional lowering of the exchange interaction energy caused by the presence of such a “defect” as the surface. Thus an anisotropic surface layer predicted theoretically by L. Néel in 1954 has been detected in 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.     

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.     

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)].     

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).     

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.     

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.     

Epitaxial films of Ba-M type hexagonal ferrites

Single-crystal films of the Ba_0.87Bi_0.13Al_3.96Fe_8.04O₁₉ hexagonal ferrite were prepared by liquid-phase epitaxy on single-crystal plates of the nonmagnetic material SrGa₁₂O₁₉, whose crystal structure is similar to that of the film grown on it. A Mössbauer study of the films revealed that the magnetic moments of the iron ions in the bulk of a film are perpendicular to the film growth plane, whereas the moments of the ions residing in a surface layer ～300-nm thick are canted away from the normal of the film by 30°±5°.     

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.     

Structural and magnetic properties of SrFe12O19 hexaferrite synthesized by a modified chemical co-precipitation method

M-type strontium hexaferrite (SrFe₁₂O₁₉) particles had been prepared by a modified chemical co-precipitation route. Structural and magnetic properties were systematically investigated. Rietveld refinement of X-ray powder diffraction results showed that the sample was single-phase with the space group of P6₃/mmc and cell parameter values of a=5.8751 Å and c=23.0395 Å. The results of field-emission scanning electronic microscopy showed that the grains were regular hexagonal platelets with sizes from 2 to 4 μm. The composition determined by energy dispersive spectroscopy is the stoichiometry of SrFe₁₂O₁₉. The ferrimagnetic to paramagnetic transition was sharp with Curie temperature T_C=737 K, which further confirmed that the samples were single phase. However, it was found that the coercivity, saturation magnetization and the squareness ratio of the synthesized SrFe₁₂O₁₉ samples were lower than the theoretical values, which could be explained by the multi-domain structure and the increase of the demagnetizing factor.     

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.     

Influence of surface on the effective magnetic fields in α-Fe2O3 and FeBO3

First measurements of the effective magnetic fields as a function of the depth at which the iron ions are in the surface layer of α-Fe₂O₃ and FeBO₃ single crystals are reported. The method used is the depth-selective conversion-electron Mössbauer spectroscopy. An analysis of experimental spectra revealed that the magnetic properties of the crystal surface vary smoothly from the bulk to surface characteristics within a layer ～100 nm thick. The layers lying below ～100 nm from the surface are similar in properties to the bulk of the crystal, and their spectra consist of narrow lines. The spectral linewidths increase smoothly as one approaches the crystal surface. The spectra obtained from a ～10-nm thick surface layer consist of broad lines indicating a broad distribution of effective magnetic fields. Calculations show that the field distribution width in this layer is δ=2.1(3) T, for an average value H _eff=32.2(4) T. It has been experimentally established that, at room temperature (291 K), the effective magnetic fields smoothly decrease as one approaches the crystal surface. The effective fields in a 2.4(9)-nm surface layer of α-Fe₂O₃ crystals are lower by 0.7(2)% than the fields at the ion nuclei in the bulk of the sample. In the case of FeBO₃, the effective fields decrease by 1.2(3)% in a surface layer 4.9(9) nm thick.     

Effect of pH value on the structural and magnetic properties of nanocrystalline strontium hexaferrite thin films

Strontium hexaferrite SrFe₁₂O₁₉ thin films have been synthesized at different pH, adjusted by NH₄OH, on the Si (1 0 0) substrate using a spin coating sol-gel process. Fourier transform infrared spectroscopy analysis and theoretical calculations were conducted for determination and controlling metal citrates in solution precursors. X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer were applied to evaluate the composition, microstructure, crystallite size and magnetic properties of the SrFe₁₂O₁₉ thin films. Using the solution with pH 7, the approximately single phase strontium hexaferrite thin films with optimum physical properties can be obtained at calcination temperature of 800 °C. The SrFe₁₂O₁₉ thin films derived from the solution with pH 7 after calcination at 800 °C exhibited crystallite size of 42 nm and magnetic properties of M_s=267 emu/cm³ (at 10 kOe), M_r=134 emu/cm³ and H_c=4290 Oe.     

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.     

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 states of the surface and bulk of ferrites near a phase transition at the Curie temperature

Investigations of the magnetic state of a surface layer ~200 nm thick and of the bulk in macroscopic ferrite crystals of the type Ba-M (BaFe₁₂O₁₉) are performed in the phase transition region around the Curie temperature (T _c). The method of simultaneous gamma, x-ray, and electron Mössbauer spectroscopy, which made it possible to compare directly the phase states of the surface and bulk of the sample, is used for the measurements. It is observed experimentally that in BaFe₁₂O₁₉ the transition of a surface layer ~200 nm thick to the paramagnetic state occurs at temperatures below T _c. It is established that the transition temperature T _c(L) of a thin layer localized at depth L from the surface of the crystal increases with distance from the surface and reaches the value T _c at the lower boundary of the “critical” surface layer. Therefore, near T _c a nonuniform state in which the crystal is magnetically ordered in the bulk but disordered at the surface is observed. A phase diagram of the states of the surface and of the bulk of macroscopic magnets near the Curie (or Néel) point is proposed on the basis of all the experimental results obtained in the present work as well as previously published results.     

On synthesis of BaFe<Subscript>12</Subscript>O<Subscript>19</Subscript>, SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>, and PbFe<Subscript>12</Subscript>O<Subscript>19</Subscript> hexagonal ferrite ceramics with multiferroid properties

We analyze the possibility of obtaining M-type hexagonal ferrites of barium, strontium, and lead with multiferroid properties with the help of ceramic technology. Using the modified ceramic technology (especially pure initial raw materials, admixture of B₂O₃, and sintering in the oxygen atmosphere), we obtained for the first time the BaFe₁₂O₁₉ and SrFe₁₂O₁₉ samples with intense multiferroid properties at room temperature. At the same time, the employed technology does not make it possible to obtain PbFe₁₂O₁₉ samples exhibiting ferroelectricity. The multiferroid characteristics of experimental samples are compared with the characteristics of classical high-temperature multiferroic BiFeO₃ and with the characteristics of BaFe₁₂O₁₉, SrFe₁₂O₁₉, and PbFe₁₂O₁₉ ferrite ceramics obtained in accordance with polymer precursor technology. We propose a mechanism explaining multiferroid properties of the hexagonal ferrite ceramic samples and note the importance of our results for applications.     

Zn and Zr co-doped M-type strontium hexaferrite: Synthesis,characterization and hyperthermia application

In the present study, hard ferromagnetic (M-type strontium hexaferrite) SrFe₁₂O₁₉ was co-doped by Zn and Zr for magnetic hyperthermia applications. As a result of the high concentration of single domain SrFe₁₂O₁₉ nanoparticles (suspended in the ferrofluid), they found a large hydrodynamic diameter, which caused a long-time Brownian relaxation under the AC magnetic field. On the other hand, increasing the Zn-Zr content (low concentration of SrFe₁₂O₁₉) led to a drop in anisotropy, which coincided with a short-time N´eel relaxation. All of the substituted samples with a multi-disperse state in ferrofluid exhibited an almost equal amount of the N´eel and Brownian effects. Consequently, the magnetic saturation (M_s) was considered as the dominant factor in the specific absorption rate (SAR) of the substituted samples. Transformation to the mono-disperse state was followed by the decrease of the Brownian relaxation time and hence the increase of the SAR. The interesting point in mono-disperse state was the heat generation of pure SrFe₁₂O₁₉ under the AC magnetic field as a result of the decrement of the Brownian relaxation time.