Electric field gradient calculations in rare-earth iron garnets

The electric field gradient (EFG) tensor at the three cation sites in RE₃Fe₅O₁₂ garnets (RE = Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu) has been calculated using a monopole-point-dipole model. The monopole contribution to the EFG tensor can explain the signs and the changes with the RE radius of the experimental quadrupole coupling constants at the octahedral and the tetrahedral sites. The monopole and dipole contributions together cannot account for the magnitude of the experimental tensors at these sites for any value of oxygen dipole polarizability α. The dipole contribution is important in garnets even for very small values of α. Presently available calculations of the overlap contribution to the EFG tensor combined with the lattice contribution also do not agree with the experimental data for these two sites. A possible cause of the disagreement is given. It is suggested that accurate experimental data of the quadrupole coupling constants at the dodecahedral sites in a RE₃Fe₅O₁₂ series, especially in Gd₃Fe₅O₁₂, could resolve the importance of various contributions at all three sites.     

Magnetization measurements on some rare-earth iron garnets

The magnetic properties of Fe-based singlecrystal garnets have been studied. A magnetic field up to 55 kOe in the temperature range from 4.5 to 300 K has been used. The compensation temperature (T _comp) of the studied garnets has been determined. The compensation temperature increases with the increase of the rare-earth ions (Ho³⁺ or Gd³⁺) substituting Y³⁺ ions in thec sublattice of R₃Fe₅O₁₂. AboveT _comp, the magnetization was found to be linearly field dependent. The results are in good agreement with magnetization measurements performed on polycrystaline samples, and with calculations based on the crystal field parameters of the isostructural Holmium Gallium Garnet (HoGG).     

Heat capacity of rare-earth cuprates,orthovanadates, and aluminum garnets,gallium garnets,and iron garnets

Physics of the Solid State - The correlation between the heat capacities of rare-earth cuprates, orthovanadates, and garnets with ionic radius R 3+ has been analyzed. It has been shown that the...     

Dissolution forms of rare-earth gallium-aluminium garnets

Acta Physica Hungarica - Single crystal spheres of rare-earth gallium-aluminium garnet crystals (R3Al5?x Ga x O12, whereR=rare-earths and 0≤x≤5) have been dissolved in H3PO4 and...     

Magnetooptics of magnetic-dipole transitions in the rare-earth paramagnetic garnets

The Faraday effect (FE) of the rare-earth (RE) paramagnetic garnet crystals associated with magnetic-dipole (MD) transitions has been investigated theoretically over a wide temperature range 80 to 300 K. Over this temperature range, the ??gyromagnetic?? Verdet V _gm ^(m) of the Eu₃Ga₅O₁₂ (EuGG) is not a linear function of the magnetic susceptibility ?? measured along the [111] axis. We find that the nontrivial character of the temperature dependence of the MD Verdet constant V _gm ^(m) is associated with the presence of a contribution to the Faraday rotation of the Van Vleck ??mixing?? by an external magnetic field of the Eu³⁺ electronic states. The optical absorption and magnetic circular dichroism (MCD) of Eu³⁺ ion in EuGG at 85 K for the MD transition ⁷ F ₀ ?? ⁵ D ₁ were measured along the [111] axis. We show that the appearance of the MCD feature observed for the absorption line near 526.6 nm for the ⁷ F ₀ ?? ⁵ D ₁ MD absorption band may be well-explained by the contribution of Van Vleck ??mixing?? between the Stark singlet states from the ⁷ F ₀ and ⁷ F ₁ multiplet manifolds mixed by the external magnetic field H.     

Morin transition in annealed iron implanted garnets

Y₃Fe₅O₁₂, Y₃Al₅O₁₂ and Gd₃Ga₅O₁₂ single crystal granets were implanted with 10¹⁷iron.cm⁻². Complementary techniques: CEMS, TEM and XRD at glancing angles have allowed to follow the behavior of implanted iron during thermal annealings. For Y₃Fe₅O₁₂ large α-Fe₂O₃ particles are formed after annealings and at 1200°C occurs a low temperature regime of the Morin transition at room temperature. For Y₃Al₅O₁₂, due to aluminium substitution in the precipitates, the Morin transition is only detected after an annealing at 1300°C whereas in Gd₃Ga₅O₁₂ no Morin transition is observed.     

Magnetoelectric interactions in rare-earth ferroborates

Magnetic-field-induced phase transitions in single crystals of the rare-earth ferroborates GdFe₃(BO₃)₄ and NdFe₃(BO₃)₄ have been studied by measuring magnetoelectric dependences and torque curves. These phase transitions can serve as one of the possible mechanisms for magnetic control of electric polarization. Magnetic phase transitions in GdFe₃(BO₃)₄ are analyzed theoretically.     

NMR of57Fe in erbium-yttrium iron garnets

NMR of 57Fe was studied by the two pulse spin echo method in systems Er(x)Y(3−x)Fe5012. For small (x<0.3) or large (x>2.7) concentrations of Er the spectra of 57Fe exhibit resolved satellite lines. The splitting between the satellite and the parent line reflects the change of the dipolar field on the Fe nuclei (diamagnetic yttrium is replaced by magnetic erbium ion) and the change which the substitution causes in the electronic structure of the ferric ion. Relative importance of these effects is discussed.     

Site occupancy of ferrous ions in iron garnets

Fe²⁺ site occupancy in iron garnets is investigated by means of optical absorption and magnetic circular dichroism. In particular measurements are carried out on bulk single crystals of yttrium iron garnet (YIG) doped with Si⁴⁺, Nb⁵⁺, Sn⁴⁺, Zr⁴⁺, Ti⁴⁺ and Th⁴⁺. The results are discussed in the light of the recent discovery of tetrahedral Fe²⁺ in Nb-doped YIG. We find that, in general, Fe²⁺ occupies both octahedral and tetrahedral sites, the actual distribution being related to site occupancy of the donor centers. This correlation may be relaxed by photoinduced charge transfer between iron sites, at low temperatures. Contrary to recent attributions, no dodecahedral component is found in the infrared spectra above 1 μm wavelength.     

Interplay of iron and rare-earth magnetic order in rare-earth iron pnictide superconductors under magnetic field

The magnetic properties of iron pnictide superconductors with magnetic rare-earth ions under strong magnetic field are investigated based on the cluster self-consistent field method. Starting from an effective Heisenberg model, we present the evolution of magnetic structures on magnetic field in RFeAsO(R = Ce, Pr, Nd, Sm, Gd, and Tb) and RFe_2As_2(R =Eu) compounds. It is found that spin-flop transition occurs in both rare-earth and iron layers under magnetic field, in good agreement with the experimental results. The interplay between rare-earth and iron spins plays a key role in the magneticfield-driven magnetic phase transition, which suggests that the rare-earth layers can modulate the magnetic behaviors of iron layers. In addition, the factors that affect the critical magnetic field for spin-flop transition are also discussed.     

Spectroscopic investigation of some rare earth iron garnets

Optical absorption spectra of Dysprosiumirongarnet (DyIG), Holmiumirongarnet (HoIG) and Erbiumirongarnet (ErlG) have been investigated in the near infrared region at temperatures of 4.2°K and 62°K. Exchange field splittings of the crystal field levels have been observed. — For the three garnets the excited levels of the groundterm up to 110 cm^?1 could be determined. The exchange fields acting on the rare earth ions have been deduced as (305±50)·10³ Oe for DyIG and (284±30)·10³ Oe for ErIG.     

Thermal conductivity of yttrium iron garnets at low temperatures

Thermal conductivity of YIG can be analysed by using the low temperature phonon- and magnon-conductivities. They are obtained by solving the nonlinear Boltzmann equation as previously done by Kumar.     

Optical spectroscopy of low-dimensional rare-earth iron borates

The family of RFe₃(BO₃)₄ borates (R=Pr, Nd, Eu-Er, Y) was studied by high-resolution optical absorption and Raman spectroscopies. Structural and magnetic phase transitions were detected and the types of magnetic structure were determined. Energy of crystal-field (CF) levels and exchange splitting of the ground state of the R³⁺ ion were obtained from the analysis of optical spectra. CF calculations were carried out. Effective magnetic field at the R³⁺ site was found, using the calculated value of the magnetic g-factor. Some peculiarities of modeling the paramagnetic susceptibility of NdFe₃(BO₃)₄ are discussed.     

Exchange interactions in polynuclear complexes

We present detailed optical and magneto-optical data on Cs₃Cr₂Cl₉ as a single crystal containing only pairs of Cr³⁺ ions. Measurements have been carried out in a wide range of temperatures down to pumped helium and in the whole spectral region covering all singly and several doubly excited states. MCD helped considerably in resolving the absorption spectra into their numerous components, and proved especially useful in demonstrating the occurrence of a double excitation to ⁴ T ₂. It provides also parallel g factors for several excited terms.

The main conclusions of our work are: (i) all transitions are electric dipole allowed; (ii) transitions to singly excited doublet states are essentially promoted via a single ion mechanism; (iii) vibronic interaction must be invoked to understand the absorption data on sharp lines; (iv) the trigonal plus spin-orbit interaction competes favourably with exchange interaction to produce excited states splittings.

We present also a new non-phenomenological model to describe the splitting of the ⁴ A ₂Θ² E manifold under the combined effects of exchange, trigonal and spin-orbit interaction. This model includes no adjustable parameter. It leads to λ₀ ～ 180 cm^-1 and offers a good starting basis for a coherent interpretation of most of our experimental data.     

Magnetic circular and linear dichroism of iron garnets thin films

Low temperature absorption, MCD and MLD spectra are reported for pure and substituted Y₃Fe₅O₁₂ thin films in the 13000–33000 cm^?1 spectral range. The materials were obtained by liquid phase epitaxial growth on a Gd₃Ga₅O₁₂ substrate. Above 19000 cm^?1 we demonstrate that our transmission experiments on garnet films provide the same information as do reflexion measurements on bulk crystals. Below 19000 cm^?1 however, the former are much more reliable and accurate. All our data indicate that the films are of excellent optical quality. MCD and MLD spectra show a considerably increased resolution as compared to the absorption spectrum and help in sorting out most of its numerous overlapping components. Finally we prove unambiguously that Clogston's theoretical interpretation is qualitatively correct in the 13000–23000 cm^?1 region and even probably at higher wavenumbers.