Magnetic effects in Co3−x O4 defect films from the data of emission Mössbauer spectroscopy

The emission Mössbauer spectra of Co_3?x O₄ defect films are measured in external magnetic fields at strengths of 0.6–3.5 T. It is shown that the memory effect is observed in the spectra of Co_3?x O₄ defect films after exposure to an external magnetic field. At temperatures above the Néel temperature T _N=26 K, the memory effect manifests itself in an increase in the relative contribution from the spectral line of the tetrahedral A sublattice in the spinel structure of Co_3?x O₄ crystallites ([Co _0.83 ²⁺ ]_tetr[Co _2.20 ³⁺ ]_octO₄ and [Co _0.95 ²⁺ ]_tetr[Co _2.10 ³⁺ ]_octO₄ prior to and after the magnetic field treatment, respectively). The isomer shifts δ and the quadrupole splitting ΔE of the spectral lines for both A and B sublattices also change from δ_A=?0.19 mm/s, δ_B=?0.31 mm/s, and ΔE _B =0.83 mm/s to the values δ_A=?0.24 mm/s, δ_B=?0.33 mm/s, and ΔE _B =0.60 mm/s, which are close to δ and ΔE for stoichiometric Co₃O₄ oxide. In the low-temperature spectra (T<T _N), the memory effect additionally shows itself as a decrease in the hyperfine magnetic field H _hf of the spectral component for the A sublattice as compared to that in the spectra measured prior to the magnetic field treatment. It is assumed that the concentration of cation vacancies decreases (and, correspondingly, the fraction of reduced Co²⁺ cations increases) in the Co_3?x O₄ defect films under the action of an external magnetic field. A possible mechanism of this process is proposed.     

Near infrared magneto-absorption in the laser material KZn1−xCoxF3

Infrared absorption (1.4–2μm) of KZn_1−xCo_xF₃ (0⩽ x<0.15) at 4.2 K has been studied in a magnetic field (B⩽7 T). Large field-induced splittings were observed for Co²⁺-ion lines. Details of the cobalt concentration dependence of the absorptivity at B = 0 and at field are given along with the dependences on field direction and strength. Assignments are made of the observed Co²⁺ single-ion and pair transitions. The data has enabled the determination of the infrared (IR) transition g values.     

Temperature dependence of magnetooptic effects in rare-earth magnetic semiconductor γ-Dy<Subscript>2</Subscript>S<Subscript>3</Subscript>

Temperature dependences of the Faraday effect (FE), which is linear in a magnetic field B; of the nonreciprocal linear birefringence (NB) associated with magnetic field-induced spatial dispersion; and of the Cotton-Mouton effect (CME), which is quadratic in a magnetic field B, have been studied in the transmission region of the γ-Dy₂S₃ cubic magnetic semiconductor (T _d symmetry class) at wavelength λ = 633 nm in the temperature range T = 25–294 K. As the temperature is lowered, the magnitudes of the FE and of the two main NB components, α₀₀₁ and α₀₁₁, increase in proportion to the magnetic susceptibility χ. This behavior implies that the magnitude of these effects is determined by the magnetic moment m of the Dy³⁺ ion induced by the magnetic field B. The CME component \(\beta _{001} (k||[1\bar 10],B||[001])\) grows in proportion to the magnetic susceptibility squared, χ²; i.e., β₀₀₁ ～ m². By contrast, the component \(\beta _{111} (k||[1\bar 10],B||[111]))\) exhibits a weaker temperature dependence, which indicates the manifestation of microscopic mechanisms in the CME component β₁₁₁ that differ from those for β₀₀₁.     

Crystal field effect on the magnetic moment's direction of rare earth ions with low point symmetry in r4co3 (r = Tb, Ho, Dy, Er, Tm) compounds

Neutron diffraction measurements, made on powder samples, show that Ho₄Co₃ and Er₄Co₃ intermetallic compounds are ferrimagnetic at 4.2 K. The magnetic moments of the 2 holmium sites are 8.7 and 2.1 μ_B and those of the erbium sites are equal to 8.7 and 8.1μ_B. The cobal⁺ magnetic moment is 0.2μ_B for both compounds. The easy magnetization direction lies on the hexagonal plane for Ho₄Co₃ while for Er₄Co₃ there are 2 anisotropy directions. Exchange interactions between rare-earth ions of both sites are very weak compared with the total crystal field splitting of the ground state multiplet J. The crystal field parameters are calculated and the magnitude and direction of the rare-earth magnetic moments in each site is determined.     

Untersuchung ferromagnetischen Materials mit polarisierter γ-Strahlung

Circularly polarized Co⁶⁰ γ-ray quanta have been used to measure the polarization-dependent part of the Compton effect on an iron-cobalt alloy. The asymmetryδ=(N ₊?N _?)/(N ₊+N _?) observed by inversion of the magnetic field direction was investigated as a function of the mean induction¯B between 1 and 23kI ^?. The experimental values of δ do not nearly drop as fast as¯B when going down from saturation to zero.     

Dependence of the coercive force on the magnetic field sweep rate in (PrDy)(FeCo)B alloys

The dependence between the coercive force and the average hysteresis loop record time was revealed in sintered (Pr_{1 – x} Dy _x )₁₃(Fe_{1 – y} Co _y )₇₉B₈ magnets. The coercive force was established to grow by 22% with an increase in the average hysteresis loop sweep rate within a range of 1.1 × 10²–3 × 10⁵ Oe/min, obeying a logarithmic dependence on the loop passage velocity with saturation at low rates. Some domain structure transformations produced by a magnetic field in the process of magnetization were established by magnetic force microscopy.     

Investigation of a Spin Transition in a LaCoO<Subscript>3</Subscript> Single Crystal by the Method of X-Ray Magnetic Circular Dichroism at the Cobalt <Emphasis Type="Italic">K</Emphasis>- and <Emphasis Type="Italic">L</Emphasis><Subscript>2,3</Subscript>-Edges

Spin transitions of cobalt ions in LaCoO₃ single crystals have been studied by the method of X-ray magnetic circular dichroism (XMCD) at the K- and L_2,3-edges of Co³⁺ ions. The orbital momentum of cobalt ions obtained for the K-edge at the 3d level in the region of the spin transition in the temperature range from 25 to 120 K increases by a factor of approximately 1.6, whereas the slope of the magnetization curve value in the same temperature range and magnetic field increases by a factor of more than 10. XMCD experiments at the cobalt L_2,3-edges demonstrate gradual growth of the ratio of the orbital momentum to the spin one L/S from 0.48 to 0.53 in the temperature range from 60 K to 120 K.     

Co-Si-B and Fe-Co-B amorphous alloys: Induced anisotropy and various magnetic properties

The dependence on the metalloid content of some magnetic properties of Co_100−x(Si_0.6B_0.4)_x (22.5 ⩽ x ⩽ 30) and Co₇₅Si_25−xB_x (10 ⩽ x ⩽ 25) amorphous alloys has been studied.Ribbons were subjected to different kinds of heating treatments: field annealing, stress annealing and stress-field annealing (tensile stress and longitudinal magnetic field applied simultaneously). While the anisotropies induced by simple field annealings are of the order of magnitude of 0.1 kJm^-3, the anisotropy induced by stress-field annealing can reach values up to 0.5 kJm^-3. The preferred axis is longitudinal for most of the annealing conditions. The temperature and composition dependence of the magnetostriction have been studied too.Stress, field and stress-field induced anisotropies have also been measured in Co₆₆Fe₉B₂₅ samples (λ_s > 0). In this case the preferred axis is transverse to the ribbon axis.     

Low-frequency behavior of optical spatial dispersion effects

A theoretical explanation is given for the frequency independence of the nonreciprocal birefringence of light, which was recently observed in the semiconductors Cd_1?x Mn_xTe, Zn_1?x Mn_xTe, and GaAs in the frequency range below the frequency corresponding to the interband absorption edge. It is shown that the symmetry of the effect becomes higher at such frequencies if the light-induced excitation energy ?ω_n(k) only slightly depends on the photon momentum k. In this case, the nonreciprocal birefringence is completely determined by the second-rank magnetoelectric tensor. It is shown that the nonreciprocal birefringence of light can be observed in magnetic media with a tensor order parameter.     

Some estimates for magnetic fluctuations in a turbulent medium

A new integral relationship between the fluctuations b(r, t) of a magnetic field and its mean B ₀(r, t) is derived for the steady-state magnetic field in a turbulent medium. This formula provides the estimate 〈b?curlb〉=?B ₀?curlB ₀. Simultaneously, the coefficient of amplification of the mean magnetic field α effect) is obtained: α=(η+β)B ₀? curlB ₀/B ₀ ² . The formula for α allows for a decrease in this coefficient owing to the back action of the magnetic field on the turbulent velocity field. It is shown that the Zel’dovich’s estimate 〈 b ²〉?β/η B ₀ ² for two-dimensional turbulence holds for magnetic fields at the instant the fluctuations 〈a ²〉 of the vector potential, rather than 〈b ²〉, reach a maximum. Here, η and β are the ohmic (molecular) and turbulent diffusion coefficients, respectively. This estimate is refined with allowance made for the fact that the condition for diffusion approximation itself relates the β, b, and B ₀ quantities to each other.     

A hydrogen atom in a superstrong magnetic field and the Zeldovich effect

We consider the problem of a hydrogen atom in a superstrong magnetic field, B? B _a =2.35×10⁹ G. The analytical formulas that describe the energy spectrum of this atom are derived for states with various quantum numbers n_ρ and m. A comparison with available calculations shows their high accuracy for B?B _a . We note that the derived formulas point to a manifestation of the Zeldovich effect, i.e., a rearrangement of the atomic spectrum under the influence of strong short-range Coulomb potential distortion. We discuss the relativistic corrections to level energies, which increase in importance with magnetic field and become significant for B?10¹⁴ G. We suggest the parameters in terms of which the Zeldovich effect has the simplest form. Analysis of our precision numerical calculations of the energy spectrum for a hydrogen atom in a constant magnetic field indicates that the Zeldovich effect is observed in the spectrum of atomic levels for superstrong fields, B?5×10¹¹ G. Magnetic fields of such strength exist in neutron stars and, possibly, in magnetic white dwarfs. We set lower limits for the fields B_min required for the manifestation of this effect. We discuss some of the properties of atomic states in a superstrong magnetic field, including their mean radii and quadrupole moments. We calculated the probabilities of electric dipole transitions between odd atomic levels and a deep ground level.     

Magnetoresistance and magnetic phase transitions in a Pr11B6 antiferromagnet

In isotopically pure praseodymium hexaboride (Pr¹¹B₆) single-crystal samples, the transverse magnetoresistance Δρ/ρ has been measured in a temperature range of 2–20 K in magnetic fields up to 80 kOe. The field and angular dependences Δρ/ρ(H, ?, T ₀) reveal a new magnetic phase in the AFM state of Pr¹¹B₆ which is observable only for the external magnetic field orientation in a narrow angular range near H ‖ 〈110〉. The data remove the previous contradictions in the Pr11B6 magnetic phase diagram representation and can be explained under the assumption that the spin-polarized regions (ferrons) are involved in the formation of the complex magnetic structure in the Pr¹¹B₆ AFM state in the 5d band in the vicinity of the rare-earth ions.     

Reciprocal and nonreciprocal magnetic linear birefringence in the γ-Dy<Subscript>2</Subscript>S<Subscript>3</Subscript> sesquisulfide

A study has been made of the spectral dependence of the Cotton-Mouton effect (CME) quadratic in magnetic field, nonreciprocal birefringence (NB) linear in magnetic field, and the Faraday effect (FE) in the cubic magnetic semiconductor γ-Dy₂S₃. Unlike the FE, the CME and the NB in this crystal are anisotropic, with the pattern of the anisotropy being dependent on the photon energy. The dependence of the CME and NB dispersion on the direction of the magnetic field B indicates contribution from a variety of electronic transitions and mechanisms to these phenomena. It is shown that the resonant contributions to the CME and NB in the transparency region originate from electronic transitions near E?3.4 eV (beyond the band edge E _g=2.8 eV), which are likely transitions from the localized ground state of the Dy³⁺ ion to states derived from mixing of the band and 4f ^N?1 5d states of the dysprosium ion. The character of the CME anisotropy in the transparency region and near the local electronic transition ⁶ H _15/2 → ⁶ F _3/2 connecting states of the unfilled 4f shell of the Dy³⁺ ion suggests the presence of a strong axial component of the crystal field acting on the rare earth ion.     

Mictomagnetic State in an EuBaCo<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>5.5–δ</Subscript> Single Crystal

The magnetic properties of an EuBaCo_1.9O_5.36 single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo₂O_5.5 composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 Å, b = 7.833 Å, and c = 7.551 Å. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below T_C = 242 K. At T < 300 K, all three spin states of the Co³⁺ ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co³⁺ ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co³⁺ spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo_1.9O_5.36 single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo_1.9O_5.36 is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.     

Magnetic domain structure in Fe78.8−xCoxCu0.6Nb2.6Si9B9 nanocrystalline alloys studied by Lorentz microscopy

The magnetic domain structures of Fe_78.8−xCo_xCu_0.6Nb_2.6Si₉B₉ (x=0, 20, 40, 60) alloys are investigated by Lorentz microscopy coupled with the focused ion beam method. The specimen prepared using the FIB method is found to have a considerably more uniform thickness compared to that prepared using the ion-milling method. In Fe_38.8Co₄₀Cu_0.6Nb_2.6Si₉B₉ and Fe_18.8Co₆₀Cu_0.6Nb_2.6Si₉B₉ alloys, 180° domain walls extending in the direction of the induced magnetic anisotropy are observed. Analysis with Lorentz microscopy reveals that the width of the magnetic domains decreases with an increase in the cobalt content or the induced magnetic anisotropy K_u, that is, the domain width d is proportional to the induced magnetic anisotropy (K_u)^−1/4. On the other hand, in the in situ Lorentz microscopy observation as a function of temperature, magnetic ripple structures are found to appear in a localized area due to the fluctuation of magnetization vectors from 423 K. It is observed that the induced magnetic anisotropy caused by the applied magnetic field at 803 K is not suppressed by the magnetic ripple structures observed at 423–443 K.     

The superparamagnetic response of a Co54Ga46 alloy to low d.c. fields

The low d.c. field susceptibilities and thermoremanence of a Co₅₄Ga₄₆ alloy are presented. A broad peak is observed in the zero field cooled susceptibility at a temperature T_B, which decreases with increasing magnetic field, while the thermoremanence is found to persist to temperatures greater than T_B. The behaviour is discussed in terms of the growth and subsequent blocking of superparamagnetic assemblies.     

The change in the character of phase transition with nickel in the Ho(Co,Ni)2 compounds

The series Ho(Co_1-cN_c)₂ has been investigated in the composition range c?0.15 by spin-echo NMR of ¹⁶⁵Ho and by magnetic measurements. From NMR data a value of (9.5±0.1)μ_B is estimated for holmium in HoCo₂. Magnetisation measurements on the same compound give (7.9±0.1)μ_B/fu. An assumption of 0.8μ_B for the cobalt moment aligned antiparallel to that of holmium leads to a value of 9.5μ_B for holmium in agreement with the value estimated from NMR. The substitution of nickel for cobalt causes a rapid fall in both the hyperfine field, and the ordering temperatures, while the molecular moment shows a simultaneous rise. These are attributed to a rapid fall in the cobalt moment which leads to a bootstrap process involving the strength of the exchange field and the magnitude of the cobalt moment. Nickel substitution also leads to a change in the character of the magnetic phase transition. Initially first order, this transition becomes second order on exceeding ≈ 9 at% Ni.     

Magnetic properties of (GdxY1−x)Co2B2 compounds

Magnetic measurements were performed on the (Gd_xY_1−x)Co₂B₂ compounds, in the temperature range 2–800 K and fields up to 70 kOe. YCo₂B₂ is a paramagnet. The (Gd_xY_1−x)Co₂B₂ compounds with x≥0.2 shows a ferromagnetic type ordering. The saturation magnetization at 2 K coincides only with the contribution of gadolinium. The Curie temperatures are nearly linearly dependent on the composition. Above the Curie points, the thermal variations of the magnetic susceptibility can be described as a superposition of a temperature independent term ϰ₀ on a Curie-Weiss behavior. The Curie constants are determined by the contribution of Gd³⁺ ions only. The ϰ₀ values increase when the gadolinium content is greater. The observed properties are discussed in the wider framework of the magnetic behavior of cobalt in GdCo_xB_y compounds.     

Valence and magnetic states of impurity iron ions in the La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Co<Subscript>0.98</Subscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript> perovskite

The local magnetic and valence states of impurity iron ions in the rhombohedral La_0.75Sr_0.25Co_0.98 ⁵⁷Fe_0.02O₃ perovskite were studied using Mössbauer spectroscopy in the temperature range 87–293 K. The Mössbauer spectra are described by a single doublet at 215–293 K. The spectra contained a paramagnetic and a ferromagnetic component at 180–212 K and only a broad ferromagnetic sextet at T < 180 K. The results of the studies showed that, over the temperature range 87–295 K, the iron ions are in a single (tetrahedral) state with a valence of +3. In the temperature range 180–212 K, two magnetic states of Fe³⁺ ions were observed, one of which is in magnetically ordered microregions and the other, in paramagnetic microregions; these states are due to atomic heterogeneity. In the magnetically ordered microregions in the temperature range 87–212 K, the magnetic state of the iron ions is described well by a single state with an average spin S = 1.4 ± 0.2 and a magnetic moment μ(Fe) = 2.6 ± 0.4μ _B .     

The hyperfine magnetic field at57Fe and181Ta in Hf(Fe1−x Co x )2 system

The hyperfine magnetic field at⁵⁷Fe in the ferromagnetic Laves compounds Hf(Fe_1?x Co _x )₂ was measured by the Mössbauer effect method. The substitution of Fe atoms by Co atoms induces a gradual change ofB _hf(Fe) and a decrease in the magnetic moment values in Fe?Co sublattice. The perturbed angular correlation measurements of¹⁸¹Ta were carried out forX=0,X=0.1,X=0.55 at 300 K, andX=0.4 atT=400°C. An abrupt change ofB _hf (Ta) in 0<X<0.1 was observed.