Iron Subsystem Research Articles

Short notes: Inversion of exchange interaction sign of Nd3+ excited states and its manifestation in optical absorption spectra of NdFe3(BO3)4 crystal

Experimental studies and group-theoretical analysis of low-temperature optical absorption spectra of neodymium ferroborate crystal NdFe3(BO3)4 were carried out. It was shown that the sign of exchange interaction with the iron subsystem in an excited state of Nd3+ can be different from that in the ground state.

Spin dynamics and exchange interaction in orthoferrite TbFeO3 with non-Kramers rare-earth ion

The low-temperature spin dynamics of the orthorhombic TbFeO$_3$ perovskite has been studied. It has been found that the inelastic neutron scattering (INS) spectrum contains two modes corresponding to different sublattices in the compound. The iron subsystem orders antiferromagnetically at T$_N$ = 632 K and exhibits the high-energy magnon dispersion. Magnetic dynamics of this subsystem has been described using the linear spin wave theory and our solution yields sizable anisotropy between in-plane and out-of-plane exchange interactions. This approach was previously used to describe the magnon dispersion in the TmFeO$_3$ compound. Three non-dispersive crystal electric field levels corresponding to Tb$^{3+}$ ions have been established in the region below 40 meV at about 17, 26, and 35 meV. Study of diffuse scattering at different temperatures has elucidated the behavior of the magnetic correlation length. The behavior of the Tb$^{3+}$ ion subsystem has been numerically described in the framework of the point charge model. The numerical data agree satisfactorily with the experiment and with the general concept of the single-ion approximation applied to the rare-earth subsystem of orthorhombic perovskites.

Magnetic properties and structural anomalies observed in multiferroic NdFe3(BO3)4 by 57Fe Mossbauer spectroscopy

The results of studies of the NdFe3(BO3)4 by 57Fe Mossbauer spectroscopy in comparison with the data of single crystal X-ray diffraction measurements are presented. Scanning of the crystal cell parameters in a wide temperature range T = 15–500 K revealed a negative thermal expansion along the c axis and structural anomalies. The temperature dependences of the Mössbauer parameters of hyperfine interaction in the paramagnetic state of NdFe3(BO3)4 correlate well with the behavior of crystal cell parameters obtained by X-ray diffraction data. The temperature of the magnetic phase transition TN = 32.54(4) K is established, below which the iron ions form a 3D magnetic order of the Izing type. The magnetic transition of the iron subsystem from a commensurate to an incommensurate structure at a temperature of about T ≈ 15 K is discussed. The "Mössbauer" Debye temperature ΘM was estimated to be 485(2) K.

P.096 Efficacy of agomelatine on anxiety symptoms and functional impairment in adult patients with generalized anxiety disorder

We report on the high-resolution spectroscopic study of multiferroic ErFe3(BO3)4. The energies of all eight Kramers doublets of the ground 4I15/2 multiplet of the Er3+ ion were determined by the high-resolution 4I13/2 → 4I15/2 infrared luminescence spectra. The spectroscopically determined temperature dependence of the splitting of the ground Kramers doublet was used to calculate the contribution of the erbium subsystem into the specific heat and the magnetic susceptibility of erbium iron borate. The analysis of the thermodynamic properties based on these calculations allowed us to suggest the domain structure in the easy-plane antiferromagnetically ordered iron subsystem, with two magnetically nonequivalent erbium positions in each domain.

Magnetic structure of ErFe3(BO3)4: Spectroscopic and thermodynamic studies

We report on the high-resolution spectroscopic study of multiferroic ErFe3(BO3)4. The energies of all eight Kramers doublets of the ground 4I15/2 multiplet of the Er3+ ion were determined by the high-resolution 4I13/2 → 4I15/2 infrared luminescence spectra. The spectroscopically determined temperature dependence of the splitting of the ground Kramers doublet was used to calculate the contribution of the erbium subsystem into the specific heat and the magnetic susceptibility of erbium iron borate. The analysis of the thermodynamic properties based on these calculations allowed us to suggest the domain structure in the easy-plane antiferromagnetically ordered iron subsystem, with two magnetically nonequivalent erbium positions in each domain.

Self‐Consistent Four‐Particle Cluster Model of Fe3+ Heisenberg Chains: Spectral and Magnetic Properties of YFe3(BO3)4 Crystals

The magnetic properties of antiferromagnetic quasi‐1D YFe3(BO3)4 crystals are studied based on the analysis of the measured optical spectra of the Fe3+ ions in different rare‐earth (RE) iron borates and a self‐consistent four‐particle cluster approach to helical iron chains. The parameters of crystal fields affecting the Fe3+ ions are calculated in the framework of the exchange charge model. The parameters of the isotropic intrachain and interchain exchange interactions between the Fe3+ ions are determined from modeling the temperature dependences of magnetic susceptibilities, the phase transition temperature, and spontaneous magnetic moments. The magnetic easy‐plane anisotropy is explained as the result of dipolar interactions between the Fe3+ ions in the trigonal crystal lattice. The developed model can be used to analyze and predict the properties of multiferroic multifunctional RE iron borates and highlight contributions of the iron subsystem into the magnetoelectric and magnetoelastic effects in these compounds.

Origin of domain wall induced magnetoelectricity in rare-earth iron garnet single crystals and films

ABSTRACTIn this article, we discuss the mechanism of magnetoelectric effect in rare-earth iron garnets related to the low symmetry environment of rare-earth ions and magnetic inhomogeneity created by iron subsystem. Low symmetry environment of rare-earth ions leads to antiferroelectric arrangement of their electric dipole moments and favors releasing of iron garnets ferroelectricity. Magnetic domain walls act as the sources of non-uniform effective magnetic field breaking antiferroelectric ordering. We calculate electric polarization in a vicinity of Bloch domain walls in rare-earth iron garnet crystals and films, pick out their distinguished features, and touch upon experimental evidences of domain wall induced magnetoelectricity.

Transformation from an easy-plane to an easy-axis antiferromagnetic structure in the mixed rare-earth ferroborates PrxY1−xFe3(BO3)4: magnetic properties and crystal field calculations

The magnetic structure of the mixed rare-earth system PrxY1−xFe3(BO3)4 (x = 0.75, 0.67, 0.55, 0.45, 0.25) was studied via magnetic and resonance measurements. These data evidence the successive spin reorientation from the easy-axis antiferromagnetic structure formed in PrFe3(BO3)4 to the easy-plane one of YFe3(BO3)4 associated with the weakening of the magnetic anisotropy of the Pr subsystem due to its diamagnetic dilution by nonmagnetic Y. This reorientation occurs through the formation of an inclined magnetic structure, as was confirmed by our previous neutron research in the range of x = 0.67 ÷ 0.45. In the compounds with x = 0.75 and 0.67 whose magnetic structure is close to the easy-axis one, a two-step spin reorientation takes place in the magnetic field H||c. Such a peculiarity is explained by the formation of an interjacent inclined magnetic structure with magnetic moments of Fe ions located closer to the basal plane than in the initial state, with these intermediate states remaining stable in some ranges of the magnetic field.An approach based on a crystal field model for the Pr3+ ion and the molecular-field approximation is used to describe the magnetic characteristics of the system PrxY1−xFe3(BO3)4. With the parameters of the d–d and f–d exchange interactions, of the magnetic anisotropy of the iron subsystem and of the crystal field parameters of praseodymium thus determined, it is possible to achieve a good agreement between the experimental and calculated temperature and field dependences of the magnetization curves (up to 90 kOe) and magnetic susceptibilities (2–300 K).

First-principles study of the Fe | MgO(0 0 1) interface: magnetic anisotropy

We present a systematic first-principles study of Fe | MgO bilayer systems emphasizing the influence of the iron layer thickness on the geometry, the electronic structure and the magnetic properties. Our calculations ensure the unconstrained structural relaxation at scalar relativistic level for various numbers of iron layers placed on the magnesium oxide substrate. Our results show that due to the formation of the interface the electronic structure of the interface iron atoms is significantly modified involving charge transfer within the iron subsystem. In addition, we find that the magnetic anisotropy energy increases from 1.9 mJ m−2 for 3 Fe layers up to 3.0 mJ m−2 for 11 Fe layers.

Magnetoelectricity of domain walls of rare-earth iron garnets

We investigate a type of magnetoelectric effect in rare-earth iron garnets related to the presence of domain walls of the iron subsystem. The lack of space inversion in the dodecahedral rare-earth ion environments allows ferroelectric ordering in these materials. Electric polarization is absent in single-domain samples due to the antiferroelectricity of the electric-dipole structure. We show that the magnetic domain walls of the iron subsystem generate an effective inhomogeneous magnetic field arising from the rare-earth iron exchange interaction, which leads to emergence of electric polarization. A hallmark of this effect is that magnetoelectricity is independent of the type of magnetic domain walls. In contrast to the accepted concepts, we argue that the electric polarization in rare-earth iron garnets also occurs in the vicinity of Bloch magnetic walls. Magnetoelectric properties are inherent to rare-earth iron garnets in the presence of magnetic domain structure in any temperature range. Rare-earth iron garnets refer to multiferroics with $d$ and $f$ interacting subsystems.

Fe-spin reorientation in PrFeAsO: Evidences from resistivity and specific heat studies

We report the magnetic field dependence of resistivity (ρ) and specific heat (C) for the non-superconducting PrFeAsO compound. Our study shows a hitherto unobserved anomaly at TSR in the resistivity and specific heat data, which arises as a result of the interplay of antiferromagnetic (AFM) Pr and Fe sublattices. Below the AFM transition temperature (TNPr), Pr moment orders along the crystallographic c axis and its effect on the iron subsystem causes a reorientation of the ordered in-plane Fe moments in a direction out of the ab plane. Application of magnetic field introduces disorder in the AFM Pr sublattice, which, in turn, reduces the out-of-plane Pr-Fe exchange interaction responsible for Fe spin reorientation. Both in ρ(T) and d(C/T)/dT curves, the peak at TSR broadens with the increase of H due to the introduction of the disorder in the AFM Pr sublattice by magnetic field. In the ρ(T) curve, the peak shifts toward lower temperature with H and disappears above 6 T, while in the d(C/T)/dT curve, the peak remains visible up to 14 T. The broadening of the anomaly at TNPr in C(T) with increasing H further confirms that magnetic field induces disorder in the AFM Pr sublattice.

Interplay of rare earth and iron magnetism inRFeAsO(R=La, Ce, Pr, and Sm): Muon-spin relaxation study and symmetry analysis

We report zero-field muon spin relaxation $(\ensuremath{\mu}\text{SR})$ measurements on $R\text{FeAsO}$ with $R=\text{La}$, Ce, Pr, and Sm. We study the interaction of the FeAs and $R$ (rare-earth) electronic systems in the nonsuperconducting magnetically ordered parent compounds of $R{\text{FeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ superconductors via a detailed comparison of the local hyperfine fields at the muon site with available M\ossbauer spectroscopy and neutron-scattering data. These studies provide microscopic evidence of long-range commensurate magnetic Fe order with the Fe moments not varying by more than 15% within the series $R\text{FeAsO}$ with $R=\text{La}$, Ce, Pr, and Sm. At low temperatures, long-range $R$ magnetic order is also observed. Different combined Fe and $R$ magnetic structures are proposed for all compounds using the muon site in the crystal structure obtained by electronic potential calculations. Our data point to a strong effect of $R$ order on the iron subsystem in the case of different symmetry of Fe and $R$ order parameters resulting in a Fe spin reorientation in the $R$-ordered phase in PrFeAsO. Our symmetry analysis proves the absence of collinear $\text{Fe-}R$ Heisenberg interactions in $R\text{FeAsO}$. A strong Fe-Ce coupling due to non-Heisenberg anisotropic exchange is found in CeFeAsO which results in a large staggered Ce magnetization induced by the magnetically ordered Fe sublattice far above ${T}_{N}^{\text{Ce}}$. Finally, we argue that the magnetic $R\text{-Fe}$ interaction is probably not crucial for the observed enhanced superconductivity in $R{\text{FeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ with a magnetic $R$ ion.

Role of Er3+ ion in the formation of the ErFeO3 magnetic properties in the region of spin-reorientation phase transition

The 90° reorientation of Er3+ spins in ErFeO3 have been directly observed using optical spectroscopy. The peculiarities of the absorption spectrum of ErFeO3 in the region of the 4I15/2 → 4F9/2 transition of the Er3+ ion in the temperature range of ac-spin reorientation have been studied. It is shown that the spin reorientation phase transition (SRPT) is accompanied by splitting of the ground and excited states into Kramers doublets in the iron exchange field. This fact is a direct evidence of purely magnetic origin of SRPT. The experimental results were used to analyze the magnetic properties of ErFeO3. This analysis showed that the order of magnitude of the magnetic moment, its variation in the SRPT region, and the existence of Tcomp can be interpreted in terms of the single-ion model, taking into account the influence of the crystal field and the molecular field produced by the iron subsystem ions.

Magnetization and specific heat of DyFe3(BO3)4 single crystal

We present thermodynamic and magnetic studies of single crystalline DyFe3(BO3)4. The data indicate an easy axis antiferromagnetic order below TN~ 38 K which we attribute to the Fe subsystem. The Dy subsystem remains paramagnetic down to the lowest investigated temperatures of 2 K, but it is polarized by the Fe spins due to a f-d interaction. External magnetic field leads to a spin-flop transition in the iron subsystem as well as to superposed magnetization in the Dy subsystem. The repopulation of two low-lying Kramers doublets in Dy3+ ions results in well defined Schottky anomalies in specific heat and magnetization.

Antiferromagnetic resonance and magnetic anisotropy in single crystals of the YFe3(BO3)4-GdFe3(BO3)4 system

The antiferromagnetic resonance in single crystals of the YFe3(BO3)4-GdFe3(BO3)4 system is studied in the frequency range 25–140 GHz and the temperature range 4.2–50.0 K. It is established that the YFe3(BO3)4 crystal containing only the magnetic subsystem of Fe3+ ions is an antiferromagnet with an easy anisotropy plane. The temperature dependences of the gaps in the antiferromagnetic resonance spectra of GdFe3(BO3)4 and Y x Gd1 − x Fe3(BO3)4 are used to calculate the contributions of the Fe3+ and Gd3+ subsystems to the magnetic anisotropy of these crystals. The contributions are found to be close in magnitude and have opposite signs. This leads to a relatively weak uniaxial anisotropy field in the crystals under investigation. Since the exchange interaction between the Gd3+ and Fe3+ ions magnetizes the magnetic subsystem of gadolinium, both subsystems start to contribute simultaneously at the Neel temperature of the iron subsystem.

Magnetic and thermal properties of single-crystal NdFe3(BO3)4

The neodymium ferroborate NdFe3(BO3)4 undergoes an antiferromagnetic transition at T N = 30 K, which manifests itself as a λ-type anomaly in the temperature dependence of the specific heat C and as inflection points in the temperature dependences of the magnetic susceptibility χ measured at various directions of an applied magnetic field with respect to the crystallographic axes of the sample. Magnetic ordering occurs only in the subsystem of Fe3+ ions, whereas the subsystem of Nd3+ ions remains polarized by the magnetic field of the iron subsystem. A change in the population of the levels of the ground Kramers doublet of neodymium ions manifests itself as Schottky-type anomalies in the C(T) and χ(T) dependences at low temperatures. At low temperatures, the magnetic properties of single-crystal NdFe3(BO3)4 are substantially anisotropic, which is determined by the anisotropic contribution of the rare-earth subsystem to the magnetization. The experimental data obtained are used to propose a model for the magnetic structure of NdFe3(BO3)4.

Rare-earth ferroborates RFe3(BO3)4

A brief review of the physical properties of the rare-earth ferroborate family RFe3(BO3)4 is given. At high temperatures some of these compounds exhibit a first-order structural phase transition from the high-symmetry R32 to the low-symmetry P3121 modification. The temperature of this transition decreases systematically with increasing radius of the rare-earth ion. All compounds of this family have antiferromagnetic ordering of the iron subsystem at low temperatures. The rare-earth subsystem, while remaining paramagnetic to the lowest temperatures, is magnetically biased by the magnetic field of the ordered iron subsystem. Although the antiferromagnetic transition temperature TN=20–40K depends weakly on the species of rare-earth ion, the orientation of the magnetic moments of the Fe3+ ions in the ordered state is determined by the R3+ ion. For example, in compounds with Y, Nd, Er, and Tm the magnetic moments of the iron are oriented in the ab plane, while in compounds with Tb and Dy they are parallel to the c axis. In GdFe3(BO3)4 a first-order spin-reorientation phase transition is observed at TSR∼9K. It is found that GdFe3(BO3)4 and NdFe3(BO3)4 are multiferroics; this is manifested in magnetoelectric and magnetoelastic effects that are dependent on the orientation and magnitude of the external magnetic field.

Heat capacity of rare-earth ferroborates RFe 3(BO 3) 4

The heat capacity of ferroborates RFe 3(BO 3) 4 (R=Y, Y 0.5Gd 0.5, Gd, Er, Tb, Tm) were studied in the range 0.4–300 K. In TbFe 3(BO 3) 4 at 192 K and GdFe 3(BO 3) 4 at 156 K first-order structural phase transitions were observed. At 50% substitution of Gd for Y the transition temperature shifts to 234 K. An antiferromagnetic ordering of the Fe subsystem occurs at about 37 K in each member of this series, except TmFe 3(BO 3) 4 ( T N ∼ 2 4 K ). Besides, in GdFe 3(BO 3) 4 at 9 K a spin-reorientation first-order transition takes place. In the temperature range studied the rare-earth subsystem remains paramagnetic being fully polarized by the Fe subsystem. In a staggered field of the iron subsystem a Zeeman splitting of the ground state of rare-earth ions occurs, which leads to a Schottky-type anomaly and release of additional entropy.

Antiferromagnetic resonance and phase diagrams of gadolinium ferroborate GdFe3(BO3)4

Antiferromagnetic resonance in single crystals of rhombohedral gadolinium ferroborate GdFe3(BO3)4 was studied. The frequency-field dependences of antiferromagnetic resonance over the frequency range 26–70 GHz and the temperature dependences of resonance parameters for magnetic fields oriented along the crystal axis and in the basal plane were determined. It was found that the iron subsystem, which can be treated as a two-sublattice antiferromagnet with anisotropy of the easy-plane type, experienced ordering at T=38 K. At temperatures below 20 K, the gadolinium subsystem with the opposite anisotropy sign strongly influenced the anisotropic properties of the crystal. This resulted in a spontaneous spin-reorientation transition from the easy-plane to the easy-axis state at 10 K. Below 10 K, magnetic field-induced transitions between the states were observed. Experimental phase diagrams on the temperature-magnetic field plane were constructed for fields oriented along the crystal axis and in the basal plane. A simple model was used to calculate the critical transition fields. The results were in close agreement with the experimental values measured at T=4.2 K for both field orientations.

Dimorphism and magnetic properties in iron rare earth germanates

Polycrystalline samples of iron and lanthanide FeRGe2O7 germanates, R=Dy, Ho, Er, Yb have been prepared. They present the new II-FeRGe2O7 structure type, S.G. P21/m (No. 11) Z=4. Results of the refinements from neutron diffraction data for Dy and Yb compounds as well as X-ray data (Ho, Er) are given. Lattice dimensions are a/Å=9.6391(4), 9.635(1), 9.646(2) and 9.6554 (5); b/Å=8.4743(3), 8.475(1), 8.511(1) and 8.5125(4); c/Å=6.7113(3), 6.6701(9), 6.655(1) and 6.6804(4); and β/°=100.538(7), 100.612(7), 100.83(1) and 100.733(3), for R=Dy, Ho, Er and Yb, respectively. Magnetic susceptibility measurements between 350 and 1.7 K reveal the existence of two separated anomalies for each of the four compounds, which appear at T1 and T2 (T2<T1): 41 and 24 K, 38 and 17 K, 40 and 8 K, 38 and 4 K, respectively. T1 coincides with the setting up of magnetic ordering in Fe3+ and R3+ sublattices, as determined by neutron diffraction in Tb and Dy compounds, and correspond to the Néel temperatures. χ(T) maxima at T2 do not correspond to any phase transition but they are caused by the exchange interaction of R3+ magnetic moments with the ordered iron subsystem. The crystal structure and magnetic properties of these FeRGe2O7 compounds are compared with those of I-FeRGe2O7 (R=Pr, Nd and Gd) and other II- (R=Y, Tb) compounds.