3d Sublattice Research Articles

Interplay of magnetic and electric coupling across the spin density wave to conical magnetic ordering in a BaHoFeO4 spin-cluster chain compound

In this study, we performed a detailed analysis of the magnetoelectric properties of a polycrystalline BaHoFeO4 system. Rietveld refinement of the synchrotron x-ray powder diffraction pattern (SXRD) with space group Pnma confirmed that the orthorhombic crystal structure was preserved down to the lowest temperature (T) = 15 K. The DC magnetization indicated complex magnetic behavior with multiple magnetic transitions at TN1 = 49 K, TN2 = 36 K, and TN3 = 7 K. The complex interplay of 3d and 4f magnetic sublattices, and the competing interactions of two different Fe-sites can explain the field (H)-induced metamagnetic behavior in BaHoFeO4 at T < TN3 and TN3< T < TN2, respectively. A strong anomaly in the dielectric constant (ε′) was observed near TN2 and TN3, which exemplified the coupling between magnetism and dielectric properties. The magnetodielectric (MD) coupling measurements via ε′ (T, H) under different H confirmed the unprecedented magnetoelectric (ME) nature with MD sign changes at the onset of TN1, TN2, and TN3 magnetic transitions; subsequently, the ME H-T phase diagram was constructed. Furthermore, the thermal variation in the lattice parameters exhibited minute changes at TN2. Spin-spiral calculations demonstrated that the spin moments of the Fe atoms are modulated in the ab-plane, whereas the spin moments of the Ho atoms can point in any direction. Our results demonstrate that the interplay between the complex magnetic modulation of Ho and Fe magnetic sublattices may play a vital role in the ME properties of BaHoFeO4.

Простая спин-переориентационная модель редкоземельных ортоферритов и ортохромитов

A simple theory of spontaneous spin reorientation in rare-earth orthoferrites RFeO3 and orthochromites RCrO3 induced by 4f-3d interaction is presented. It is shown that both the temperature and the nature of the spin reorientation are the result of the competition between the second- and fourth-order spin anisotropy of the 3d sublattice, the crystal field for 4f-ions, and also the 4f-3d interaction. A case of the substitution of the rare-earth ions with a non-magnetic impurity is also considered.

Simple Realistic Model of Spin Reorientation in 4f-3d Compounds

This is a simple but realistic microscopic theory of spontaneous spin reorientation in rare-earth perovskites, orthoferrites RFeO3 and orthochromites RCrO3, induced by the 4f-3d interaction, namely, the interaction of the well-isolated ground-state Kramers doublet or non-Kramers quasi-doublet of the 4f ion with an effective magnetic field induced by 3d sublattice. Both the temperature and the nature of the spin-reorientation transition are the result of competition between the second- and fourth-order spin anisotropy of the 3d sublattice, the crystal field for 4f ions, and 4f-3d interaction.

Influence of Fe doping on the Yb valence state and the magnetic structures in HoFe6Sn6-type YbMn6−yFeySn6 alloys (y ≤ 1.00)

We determine the magnetic structures of HoFe6Sn6-type (Immm) YbMn6−yFeySn6 alloys (y=0.25, 0.50, 0.75 and 1.00) from powder neutron diffraction experiments. The valence state and local magnetism of Yb are investigated using X-ray Near Edge Spectroscopy (XANES) and X-ray Magnetic Circular Dichroism (XMCD) at the Yb L3 edge. The mean Yb valence decreases upon Fe doping (from υ~2.66 down to υ~2.46) and there is no 4f Yb magnetism throughout the series. The Fe for Mn substitution leads to change in the spins arrangement of the 3d sublattice by reinforcing antiferromagnetic exchange interactions without significant change in T=Mn/Fe moment magnitude (m~2.35 µB). The ground state of the alloy with y=0.25 is ferromagnetic whilst Fe richer alloys are helimagnets with wave-vector k=0, qy, 0. The turn-angle increase upon Fe doping manifests by a mitigation of the Yb L3 dichroic effect, solely due to hybridization between its 5d states and the self-polarized 3d states of T.

Synthesis and magnetic properties in quaternary Ca1−x Pr x Co2As2 single crystals

Series of Ca1−x Pr x Co2As2 (x = 0, 0.10, 0.25, 0.4, 0.6, 0.75, 0.85, 1) single crystals have been synthesized in order to clarify the variation of magnetic order from antiferromagnetic (AFM) in CaCo2As2 to ferromagnetic (FM) in PrCo2As2. It is found that the lattice constant of c-axis are contracted with the introduction of Pr into Ca sites in CaCo2As2. Electronic transport measurements reveal the metallicity in this system. Systematic magnetic measurements and analysis show that substituting only 10% of Pr for Ca changes the magnetic ground state from A-type AFM ordering of Co magnetic moment in CaCo2As2 to FM ordering in Ca1−x Pr x Co2As2 (0.1 ⩽ x ⩽ 1). Most importantly, the abrupt drop of low temperature magnetic susceptibility below T FiM with x ⩾ 0.25 and the observed magnetic pole reversal with x ⩾ 0.4 suggests an AFM coupling between Co 3d and Pr 4f magnetic sublattice. Finally, a detailed magnetic phase diagram in this system has been obtained.

Magnetic properties of the non-stoichiometric TbCo2Mnx and TbCo2Nix alloys

The crystal structure, magnetic and magnetothermal properties of non-stoichiometric TbCo2Mnx and TbCo2Nix alloys were studied. It was shown that single-phase compounds with the MgCu2-type structure exist up to a certain content of x (x < 0.4 for compounds with Mn and x ≤ 0.1 for compounds with Ni). The concentration changes of the lattice parameter, Curie temperature, magnetic moment of the 3d sublattice in TbCo2Mnx and TbCo2Nix alloys were compared. The magnetocaloric effect in alloys were estimated using data of magnetic and heat capacity measurements.

Features of Electronic Structure of Intermetallic Compounds CeNi4M (M = Fe, Co, Ni, Cu)

The evolution of the electronic structure of CeNi4M (M = Fe, Co, Ni, Cu) intermetallics depending on the type of nickel substitutional impurity is explored. We have calculated band structures of these compounds and considered options of substituting one atom in nickel 3d sublattice in both types of crystallographic positions: 2c and 3g. The analysis of total energy self-consistent calculations has shown that positions of 2c type are more energetically advantageous for single iron and cobalt impurities, whereas a position of 3g type is better for a copper impurity. The Cu substitutional impurity does not change either the nonmagnetic state of ions or the total density at the Fermi level states. Fe and Co impurities, on the contrary, due to their considerable magnetic moments, induce magnetization of 3d states of nickel and cause significant changes in the electronic state density at the Fermi level.

Magnetocaloric Effect in RFe10X2( X=Mo, V)

We present a mean-field study on the magnetocaloric effect (MCE) in RFe10X2, where X=Mo, V, and R=Gd, Tb, Ho, Tm, Dy, Er, Nd for X=V. For X=Mo, R=Dy, Gd, and Nd. The two-sublattice model, involving the 4f (rare earth) and 3d(Fe) sublattices, is used. For both systems, magnetization, magnetic heat capacity, magnetic entropy and isothermal entropy change ∆Sm are calculated for different magnetic fields in the 0-5T range and the temperature range from 0 to 700K. Direct and inverse MCEs are shown to take place in these ferromagnetic/ferrimagnetic compounds. For a field change ∆H=5T, the maximum isothermal magnetic entropy change has been calculated for ferromagnetic NdFe10Mo2 compound to be 6.6 J/K mol at Tc=441 K. Both direct, and inverse MCEs have been found in ferrimagnetic compounds, e.g., for TmFe10V2, with maximum -∆Sm= J/K mol at Tc=521K and ∆Sm= J/K mol at TN=127 K. Mean-field analysis is suitable for handling the systems we report on. Further study on the lattice and electronic contribution to entropy is planned.

Effect of manganese doping on the electronic structure and optical properties of Ce2Fe17-x Mn x (x = 0, 1, 2)

In this paper we investigate the electronic structure and optical properties of the Ce2Fe17-x Mn x (x = 0, 1, 2) intermetallic compounds and their change with the Mn-concentration growth within theoretical ab initio calculations. From these results we plot and compare experimental and theoretical optical conductivity utilizing interband transitions to interpret the experimental curve. To estimate the influence of manganese substitutions on the 3d sublattice, exchange Fe/Mn interaction parameters were computed and analysed. It was found that for the Mn substitutional impurity in the dumbbell positions the dominating ferromagnetic exchange interaction becomes the antiferromagnetic one favoring noncollinear magnetic ordering which was found in Ce2Fe17-x Mn x at low temperatures.

The effect of adding aluminum and iron to Tb–Dy–Ho–Co multicomponent alloys on their structure and magnetic and magnetocaloric properties

A comprehensive analysis of the structure, phase composition, surface topology features, and magnetic and magnetocaloric properties of Tb0.3Dy0.35Ho0.35Co1.75T0.25 (T = Al, Fe) multicomponent alloys has been performed. The specifics of variations in the structure and functional properties induced by the partial substitution of cobalt atoms in the 3d sublattice of RCo2 with aluminum or iron atoms have been determined.

Low temperature giant magnetocaloric effect in multiferroic GdMnO3 single crystals

Magnetocaloric (MC) properties of GdMnO3 single crystals are investigated using magnetic and magneto-thermal measurements. GdMnO3 exhibits a giant MC effect (isothermal change in magnetic entropy (−ΔSM) ~ 31 J (kg K)−1 at 7 K and adiabatic change in temperature ~10 K at 19 K for magnetic field variation 0–80 kOe). Complex interactions between 3d and 4f magnetic sublattices influence MC properties. The rare-earth antiferromagnetic ordering induces an inverse MC effect (positive ΔSM) along ‘a’ and ‘c’ axes whereas it’s not seen along the ‘b’ axis, revealing complex anisotropic magnetic ordering. The antiferromagnetic ordering possibly changes to ferromagnetic ordering at higher fields.

Calculation of the electronic structure of the intermetallic compounds ErNi5 − x Al x (x = 0, 1, 2)

The evolution of the electronic structure of the intermetallic compounds ErNi5 − x Al x (x = 0, 1, 2) with different positions of the substitution of aluminum atoms for nickel atoms has been investigated. For this purpose, spin-polarized calculations of the energy band spectra of these compounds have been performed using the LSDA + U method, which in the local spin density approximation takes into account strong electron correlations in the 4f shell of the erbium ion. Variants of the substitution of aluminum atoms for nickel atoms in different crystallographic positions in the 3d sublattice have been considered. An analysis of the band structure has demonstrated that substitutional aluminum impurities lead to the formation of nonmagnetic nickel 3d states and to a significant decrease in the electron density of states at the Fermi level. A comparison of the total energies of the substitutional configurations has revealed that the occupation of the 3g positions by aluminum atoms is more energetically favorable.

Competing anisotropies on 3d sub-lattice of YNi4–xCoxB compounds

The magnetic anisotropy of 3d sub-lattices has an important rule on the overall magnetic properties of hard magnets. Intermetallics alloys with boron (R-Co/Ni-B, for instance) belong to those hard magnets family and are useful objects to help to understand the magnetic behavior of 3d sub-lattice, specially when the rare earth ions R do not have magnetic nature, like YCo4B ferromagnetic material. Interestingly, YNi4B is a paramagnetic material and Ni ions do not contribute to the magnetic anisotropy. We focused therefore our attention to YNi4–xCoxB series, with x = 0, 1, 2, 3, and 4. The magnetic anisotropy of these compounds is deeper described using statistical and preferential models of Co occupation among the possible Wyckoff positions into the CeCo4B type hexagonal structure. We found that the preferential model is the most suitable to explain the magnetization experimental data.

Cobalt-related features of spectral and magnetic properties of RNi4Co (R=Ho, Er)

In this paper we investigate spectral and magnetic properties of RNi4Co (R=Ho, Er) intermetallics to reveal the features associated with the presence of Co substituting ion in comparison with the parent HoNi5 and ErNi5 compounds. Our ab initio LSDA+U calculations show that in the electronic structure Co states are strongly manifested near the Fermi energy for the case of Co in 2c position, while Co in 3g position results only in weak changes of the total DOS curve. Theoretical optical conductivity calculated assuming Co equally redistributed among 2c or 3g positions agrees well with the experimental optical conductivity. Cobalt ion in RNi4Co (R=Ho, Er) is characterized by magnetic moment of 1μB that increases exchange interaction in the 3d sublattice by several times that contributes to the growth of Curie temperature in comparison with the parent compounds.

Tunable spin reorientation transition and magnetocaloric effect in Sm0.7−xLaxSr0.3MnO3 series

We report electrical resistivity, magnetic, and magnetocaloric properties in Sm0.7−xLaxSr0.3MnO3 series for x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.65, and 0.7. All the compounds show second order paramagnetic to ferromagnetic (FM) transition at T = Tc, which is tunable anywhere between 83 K and 373 K with a proper choice of the doping level (x). The insulating ferromagnet x = 0 transforms to a ferromagnetic metal below Tc for x = 0.1, and the insulator-metal transition temperature shifts up with increasing x. The magnetization (M) exhibits an interesting behavior as a function of temperature and doping level. The field-cooled M(T) of all but x = 0.7 compounds show a cusp at a temperature T* much below Tc. While the Tc increases monotonically with increasing x, T* increases gradually, attains a maximum value (T* = 137 K) for x = 0.6 and decreases rapidly thereafter. It is suggested that the decrease of M(T) below T* is due to ferrimagnetic interaction between Sm(4f) and Mn(3d) sublattices that promotes spin-reorientation transition of the Mn-sublattice. The observed anomalous feature in M(T) does not have impact on the dc resistivity. Magnetic entropy change (ΔSm) was estimated from magnetization isotherms. The sign of ΔSm is found to change from negative above T* to positive below T* indicating the coexistence of normal and inverse magnetocaloric effects. ΔSm is nearly composition independent (−ΔSm = 1.2 ± 0.2 J/Kg K for μ0ΔH = 1 Tesla) and refrigeration capacity lies between 40 and 50 J/kg K for 0.1 ≤ x ≤ 0.6. We show scaling of magnetic entropy change under different magnetic fields and analysis of critical exponents associated with the phase transition in x = 0.6 compound. The tunability of Curie temperature with nearly constant ΔSm value along with high refrigeration capacity makes this series of compounds interesting for magnetic refrigeration over a wide temperature range.

Magnetoelastic and elastocaloric effects in rare-earth metals, their alloys and compounds in the region of magnetic phase transitions

This article discusses experimental data and their theoretical interpretation concerning the volume magnetostriction, spontaneous magnetostriction, variation of magnetization under the action of pressure, and elastocaloric effects in rare-earth metals, as well as their alloys and compounds. Particular attention is paid to the region of phase transitions. The volume magnetostriction ω of true magnetization was investigated near the Curie temperature Θ as a function of magnetization and determined from the change of magnetostriction under the action of pressure. From these data we obtained the dependence of the exchange integrals on the unit cell volume. Giant volume magnetostriction and magnetoelastic elastocaloric effects were discovered in the rare-earth metals and alloys in the region of their magnetic phase transitions. It was established that giant volume magnetostriction in RCo2 compounds is caused by a critical increase of the magnetic moment of the 3d sublattice of cobalt in magnetic fields that exceeds the critical field at T > Θ. Giant volume magnetostriction in R2Fe17 compounds near the temperature Θ is shown to occur due to strong deformational dependences of exchange interaction and the value of the 3d electron bandwidth.

Magnetic properties and exchange interactions in TbNi5−xMx (M=Co and Fe) compounds: Ab initio calculations

In order to investigate magnetic structure and the mechanism of the Curie temperature (TC) enhancement in TbNi4Co and TbNi4Fe compounds in comparison with TbNi5, electronic structure calculations were performed within the LSDA + U method (local spin density approximation with Hubbard U-correction). Magnetic moments for Ni, Fe, and Tb ions resulted from the LSDA + U calculations for TbNi5 and TbNi4Fe compounds were found to be in very good agreement with the ones refined from the neutron data. Experimental magnetization measurements demonstrate a substantial increase of TC in TbNi4Co (TC ∼ 60 K) and especially in TbNi4Fe (TC ∼ 280 K) in comparison with TbNi5 (TC ∼ 23 K) that correlates with the growth of the exchange–interaction parameter in the 3d sublattice calculated in the present work.

Kinetic studies on chromium-glycinato complexes in acidic and alkaline media

Two solid complexes, fac–[Cr(gly)3] and [Cr(gly)2(OH)]2, (where gly is glycinato ligand) were prepared and their acid-catalysed aquation products were identified. The structure of [Cr(gly)3] was solved by X-ray diffraction, revealing a cationic 3D sublattice with perchlorate anions inside its cavities. Acid-catalysed aquation of [Cr(gly)3] and [Cr(gly)2(OH)]2 leads to the same inert product, [Cr(gly)2(H2O)2]+, in a two-stages process. At the first stage, intermediate complexes, [Cr(gly)2(O–glyH)(H2O)]+ and [Cr(gly)2(H2O)–OH–Cr(gly)2(H2O)]+, are formed respectively. Kinetics of the first aquation stage of [Cr(gly)3] were studied in HClO4 solutions. The dependencies of the pseudo first-order rate constants on [H+] are as follows: k obs1H = k 0 + k 1 K p1[H+], where k 0 and k 1 are rate constants for the chelate-ring opening via spontaneous and acid-catalysed reaction paths, respectively, and K p1 is the protonation constant. The proposed mechanism assumes formation of the reactive intermediate as a result of proton addition to the coordinated carboxylate group of the didentate ligand. Some kinetic studies on the second reaction stage, the one-end bonded glycine liberation, were also done. The obtained results were analogous to those for stage I. In this case, the proposed reactive species are intermediates, protonated at the carboxylate group of the monodentate glycine. Base hydrolysis of two complexes, [Cr(gly)2(O–gly)(OH)]− and [Cr(gly)2(OH)2]−, was studied in 0.2–1.0 M NaOH. The pseudo first-order rate constants, k obsOH, were [OH−] independent in the case of [Cr(gly)2(O–gly)(OH)]−, whereas those for [Cr(gly)2(OH)2]− linearly depended on [OH−]. The reaction mechanisms were proposed, where the OH− -catalysed reaction path was rationalized in terms of formation of the reactive conjugate base, [Cr(gly)2(OH)(O)]2−, as a result of OH− ligand deprotonation. Activation parameters were determined and discussed.

Crossover behavior from decoupled criticality

We study the thermodynamic phase transition of a spin Hamiltonian comprising two 3D magnetic sublattices. Each sublattice contains XY spins coupled by the usual bilinear exchange, while spins in different sublattices only interact via biquadratic exchange. This Hamiltonian is an effective model for XY magnets on certain frustrated lattices such as body centered tetragonal. By performing a cluster Monte Carlo simulation, we investigate the crossover from the 3D-XY fixed point (decoupled sublattices) and find a systematic flow toward a first-order transition without a separatrix or a new fixed point. This strongly suggests that the correct asymptotic behavior is a first-order transition.

Pressure effect on magnetic properties of [formula omitted

The experimental study of the pressure effect on magnetization M S and compensation temperature T C o m p of ferrimagnetic GdCo 2Fe 2B in a magnetic field up to 7 T under hydrostatic pressures up to 10 kbar is presented. Due to the mutual antiparallel arrangement of Gd and 3d sublattices and the negligible pressure effect on localized Gd magnetic moment, the observed increase of M s with pressure ( d M s ( 5 K ) / d p = + 0.007 μ B kbar − 1 ) can be attributed to the pressure induced decrease of 3d magnetic moments, predominately to the decrease of Fe moment, d m Fe / d p = − 2.8.1 0 − 3 μ B / at . kbar . The pressure induced increase of M s is also responsible for the remarkable increase of T C o m p under pressure ( d T C o m p / d p = + 0.90 K / kbar ) . This opens the field to tunablibilty of the T C o m p .