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.     

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.     

Reduced magnetic hyperfine field at the surface of α-Fe2O3 and FeBO3 single crystals

The surface magnetic ordering of an α-⁵⁷Fe₂O₃ and ⁵⁷FeBO₃ single crystal was studied by Depth Selective Conversion Electron Mössbauer Spectroscopy (DCEMS). The depth selective Mössbauer data of ⁵⁷FeBO₃ up to the Néel temperature (T _N = 348.35 K) reveal the coexistence of the canted antiferromagnetic bulk phase and a magnetic surface phase that shows time relaxation. Both phases are separated by a wall (interim layer) and the thickness of the magnetic surface layer increases as the crystal is heated to T _N.     

Surface and volume phase states of Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>x</Subscript>BO<Subscript>3</Subscript> crystals in the vicinity of the Néel point

The behavior of the magnetic system of a surface layer of macroscopic Fe_1?xGa_xBO₃ crystal (x=0, 0.3) in the vicinity of the Néel temperature T_N was studied. The studies were made by a method involving simultaneous gamma, x-ray, and electron Mössbauer spectroscopy that made it possible to obtain information simultaneously from surface layers and from the bulk of a macroscopic crystal. It was found that the temperature T_N(L) at which a thin layer at a depth L from the surface switches to a disordered state is lower than T_N for the bulk and is lower the closer this layer is to the surface. In the vicinity of T_N, a nonuniform state is observed in which the bulk of the crystal is magnetically ordered and the surface layer is disordered. The transition temperature T_N(L) decreases from T_N to its surface value within a surface layer of a “critical” thickness.     

Magnetic-resonance and thermophysical studies of the magnetic phase diagram for a GdFe<Subscript>2.1</Subscript>Ga<Subscript>0.9</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystal

The antiferromagnetic resonance, heat capacity, magnetic properties, and magnetic phase diagram of a GdFe₃(BO₃)₄ crystal in which some of the iron ions were substituted by diamagnetic gallium ions have been investigated. It has been found that the Neél temperature upon diamagnetic substitution decreased to 17 K compared to 38 K in the unsubstituted crystal. The effective exchange and anisotropy fields for GdFe_2.1Ga_0.9(BO₃)₄ have been estimated from the field dependences of magnetization and resonance measurements. The magnetic phase diagram of the crystal has been constructed from magnetic and resonance measurements. In GdFe_2.1Ga_0.9(BO₃)₄, there is no spontaneous reorientation and, in the absence of a magnetic field, the crystal remains an easy-axis one in the entire domain of magnetic ordering. The critical field of the reorientation transition to an induced easy-plane state in a magnetic field along the trigonal axis has been found to increase compared to that in the unsubstituted crystal.     

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.     

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.     

Electret states and the phase transition in a surface layer of the TlGaSe2 ferroelectric semiconductor

The electret polarization is investigated in the TlGaSe₂ ferroelectric semiconductor. It is proved for the first time that stable internal electric fields associated with residual electret polarization are induced in crystals of the TlGaSe₂ ferroelectric semiconductor at temperatures T < 200 K. It is experimentally established that the peak of the pyroelectric current measured in the vicinity of the phase transition to the ferroelectric polar phase depends substantially on the temperature at which the external electric field is switched off when the TlGaSe₂ ferroelectric crystal under investigation is preliminarily cooled from room temperature. The results obtained are discussed in the framework of a model according to which internal electret fields are induced by charges localized at different levels in the bulk and on the surface of the TlGaSe₂ ferroelectric crystal. These fields drastically change at temperatures in a narrow range near 135 K. The inference is made that a phase transition occurs in the surface layer of the TlGaSe₂ crystal at a temperature close to ～135 K.     

Lowering of magnetic field intensity at the surfaces of α-Fe2O3 and FeBO3 single crystals

Depth-selective conversion electron Mössbauer spectroscopy was used to study magnetic properties of the thin surface layers of the α-Fe₂O₃ and FeBO₃ single crystals. An analysis of the experimental spectra indicates that the magnetic properties of the layers at a depth of more than ～100 nm from the surface are similar to the properties of crystal bulk, and the corresponding spectra consist of narrow lines. The lines gradually broaden as the crystal surface is approached. The spectra of the ～10-nm-thick surface layers consist of broad lines, indicating a wide distribution δ=2.1 T of the effective magnetic field about its mean value of 32.2(4) T. The experimental spectra were used to determine the effective magnetic fields (H _eff) for the iron ions situated in the surface layers of thickness ～100 nm. The effective fields in these layers were found to gradually decrease at room temperature (291 K) as the crystal surface was approached. The H _eff values in the 2.4(9)-nm-thick surface layer of the α-Fe₂O₃ crystal and 4.9(9)-nm layer of FeBO₃ are 0.7(2) and 1.2(3)%, respectively, smaller than for the nuclei of the ions in the bulk of these crystals.     

Coexistence of superconductivity and long-range antiferromagnetic order in Gd1.2Mo6Se8

Neutron diffraction experiments confirm that the ternary rare earth compound Gd_1.2Mo₆Se₈ orders antiferromagnetically in the superconducting state at a Neél temperature T_N=0.75 K.     

A study of the structural phase transition in strontium titanate single crystal by coherent and incoherent second optical harmonic generation

The surface phase transition in a SrTiO₃ crystal was studied by second optical harmonic generation. Nonlinear optical response singularities were observed at temperature T*=145 K, which was 40 K higher than the T_c structural phase transition temperature in the crystal volume. Nonlinear critical opalescence in the crystal volume caused by the presence of point defects was studied. The second harmonic field and the intensity of critical opalescence were calculated based on the phenomenological model of nonlinear optical processes with the use of the Landau theory of phase transitions.     

Mössbauer studies of the surface and bulk properties of substituted iron-garnet films in the vicinity of the Curie point

The magnetic properties of the surface layer and bulk of iron-garnet films of composition Y_2.6Sm_0.4Fe_3.7Ga_1.3O₁₂, grown by liquid-phase epitaxy on substrates of a gadolinium-gallium garnet single crystal, are investigated. The method of simultaneous gamma-ray, x-ray, and electron Mössbauer spectroscopy is used for the investigations. It is found that the temperature T _c(L) of the transition to the paramagnetic state of a thin layer localized at a depth L below the surface within ～300 nm decreases smoothly with decreasing distance from the surface. The causes of the difference between the temperature of the transition to the paramagnetic state in the surface layer and that bulk of the film are: 1) variation of the degree of substitution of gallium for iron in the surface layer; 2) weakening of exchange interactions at the surface of the sample.     

Two-dimensional superconductive state near the intermediate plane

The superconductive layer near a plane surface arises due to the electronic states (Tamm levels) localized on the surface at temperature T₀, higher than the bulk critical temperature.     

氧化铝与Al(100)基体间界面原子结构研究

本文介绍了用MeV离子散射和沟道效应研究单晶铝表面无定型氧化层与基体之间界面原子结构的方法。报道了Al₂O₃/Al(100)界面原子结构的实验结果。实验表明,在纯氧气氛围中400℃下生成的氧化铝膜,铝和氧原子浓度比例严格为2与3之比;Al₂O₃膜和Al(100)基体之间的界面极其陡峭,氧化铝膜下Al(100)基体表面的再构层不大于一个原子层。由实验测量与用Monte Carlo方法计算结果比较,得到再构层原子离开原来晶 关键词：     

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.     

Spin-reorientation phase transition on the surface and in the bulk of α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> single crystals

Direct comparison of the properties of a thin surface layer and the bulk of macroscopic hematite (α-Fe₂O₃) crystals was used to study the magnetic structure of the surface layer and the bulk and the processes attendant on spin-reorientation phase transition (SRT). The investigation tool was simultaneous γ-ray, X-ray, and electronic Mössbauer spectroscopy, which enabled us to study the bulk and surface properties of macroscopic samples simultaneously and to compare them directly. Direct evidence of the existence of a surface “transition layer” on hematite crystals is obtained. The existence of this layer was suggested and described by Krinchik and Zubov [JETP 69, 707 (1975)]. The study in the SRT region showed that (1) the Morin SRT in the crystal bulk occurs in a jump (as a first-order phase transition), whereas in the surface layer of about 200 nm thick, some smoothness appears in the mechanism of magnetic-moment reorientation; (2) SRT in the surface layer, as in the bulk, involves an intermediate state in which low-and high-temperature phases coexist; and (3) SRT in the surface layer occurs at a temperature several degrees higher than in the bulk. Our experimental evidence on the SRT mechanism in the surface layer correlates with the inferences from phenomenological theory developed by Kaganov [JETP 79, 1544 (1980)].     

Experimental studies of the magnetic excitations in the 1D Ising-antiferromagnet CsCoCl3

The electronic Raman and infrared absorption spectrum within the ⁴T_1g ground term of the one dimensional antiferromagnet CsCoCl₃ is investigated. At 2K three magnon lines are observed whereas above the magnetic phase transition at 8K the number of lines is increased to twelve. No drastic change of the magnon spectrum is observed in passing the Neél temperature at 21.5K. The magnon spectrum could still be observed at 150K. Theoretically the one-exciton spectrum is described within the molecular field approximation, using an exchange coupling between nearest neighbours along the spin chains only. Satisfactory agreement is obtained between measured and calculated energies.     

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.     

The critical properties of magnetic films

Within the framework of the transverse Ising model and by using the effective field theory with a probability distribution technique that accounts for the self spin correlations, we have studied the critical properties of an L-layer film of simple cubic symmetry in which the exchanges strength are assumed to be different from the bulk values in N_S surface layers. We derive and illustrate the expressions for the phase diagrams, order parameter profiles and susceptibility. In such films, the critical temperature can shift to either lower or higher temperature compared with the corresponding bulk value. We calculate also some magnetic properties of the film, such as the layer magnetizations, their averages and their profiles and the longitudinal susceptibility of the film. The film longitudinal susceptibility still diverges at the film critical temperature as does the bulk longitudinal susceptibility.     

Critical surface wave velocity near phase transitions

The temperature dependent Rayleigh wave velocity is discussed for crystals with a surface layer of depth equal to the correlation length, ξ, with special reference to a critical region near phase transitions. For finite qξ the Rayleigh wave velocity reflects the specific critical properties of the surface layer. Under such conditions the temperature dependence of Rayleigh waves cannot be predicted on the temperature dependence of constants. A phenomenological analysis is made for qξ ≦? 1. Experimental results on SrTiO₃ show bulk-dominated critical Rayleigh wave velocities due to the short correlation length in this material.