Ground Kramers Research Articles

Magnetic properties of quasi-one-dimensional antiferromagnets (Y1−xNdx)2BaNiO5 (x=1, 0.15)

We present the magnetic properties of (Y1−xNdx)2BaNiO5 (x=1, 0.15) investigated by means of specific heat, magnetic susceptibility, and spectroscopic measurements. Magnetic ordering occurs at 47K and 13K in the compounds with x=1 and x=0.15, respectively. We estimate the magnetic contribution of the neodymium subsystem to magnetization and specific heat using temperature dependences of the splitting of ground Kramers doublet of Nd3+ ion obtained from spectroscopic experiment. We show that both Nd–Ni and Nd–Nd interactions should be taken into account. An origin of the observed spin-glass state below ∼5K and a contribution of the Ni chain breaks to the magnetization and specific heat are discussed.

Investigations of the g factors and crystal field parameters for Ce3+ in CaYAlO4 crystal

The EPR g factors g|| and g⊥ for Ce3+ at the tetragonal Y3+/Ca2+ site of CaYAlO4 crystal are calculated from a complete diagonalization (of energy matrix) method. In the method, the contributions to g factors of ground Kramers doublet due to all the rest doublets within the ground and excited multiplets 2F5/2 and 2F7/2 (rather than the doublets within only the ground multiplet 2F5/2 in the previous paper) are considered. The crystal field parameters Bkq in the energy matrix are obtained from the superposition model with the structural data of CaYAlO4 crystal. The calculated g factors are in much better agreement with the experimental values than those from the simple method in the previous paper. The calculated crystal field parameters Bkq are comparable with those of other trivalent rare earth ions in the isomorphous ABCO4 crystals and can be regarded as reasonable. The calculations suggest that the crystal field parameter B20 is positive in Ce3+:CaYAlO4 crystal. The opinion that the parameter B20 must be negative and the simple calculation method of g factors resulting in this opinion are unreliable. It appears that for the reasonable and exact calculations of g factors for 4f1 ion in crystals, the complete diagonalization (of energy matrix) method should be applied.

Investigations on the Optical Spectra and EPR Parameters for Tetragonal Sm3+ Center in SmBa2Cu3O7−δ

It is of interest to study the optical spectra and EPR parameter of the rare-earth ion in the typical high Tc superconductors. The proper interpretation for these values is helpful to understand the optical and magnetic characteristics of the superconductors. In this paper, we calculate the optical spectra and EPR parameters (g factors g∥,g⊥ and hyperfine structure constants A∥, A⊥) of tetragonal Sm3+ center in YBa2Cu3O7−δ from the perturbation formulas of EPR parameters for 4f5 ion in tetragonal symmetry. In these formulas, the crystal-field J-mixing of the first and second excited-state multiplets 6H7/2 and 6H9/2 into the ground state, 6H5/2, the mixtures among the states with the same J value via spin-orbit coupling interaction and the interactions between the ground Kramers doublet Γγ and the same irreducible representation as Γγ in other 11 Kramers doublets Γx within 6HJ (J=5/2, 7/2, 9/2) states via crystal field and orbital angular momentum (or hyperfine structure) are considered. The calculated results are in reasonable agreement with the observed values. The results are discussed.

Templating Odd Numbered Magnetic Rings: Oxovanadium Heptagons Sandwiched by β-Cyclodextrins

The oxovanadium heptagons sandwiched by beta-cyclodextrins showed stepped magnetization curves, which were characteristic of level crossings of spin sublevels. Doubly degenerate ground Kramers' doublets split into two spin states, and the splitting energy gaps depend upon ring distortions.

A Highly Anisotropic Cobalt(II)-Based Single-Chain Magnet: Exploration of Spin Canting in an Antiferromagnetic Array

In this article we report for the first time experimental details concerning the synthesis and full characterization (including the single-crystal X-ray structure) of the spin-canted zigzag-chain compound [Co(H2L)(H2O)]infinity [L = 4-Me-C6H4-CH2N(CPO3H2)2], which contains antiferromagnetically coupled, highly magnetically anisotropic Co(II) ions with unquenched orbital angular momenta, and we also propose a new model to explain the single-chain magnet behavior of this compound. The model takes into account (1) the tetragonal crystal field and the spin-orbit interaction acting on each Co(II) ion, (2) the antiferromagnetic Heisenberg exchange between neighboring Co(II) ions, and (3) the tilting of the tetragonal axes of the neighboring Co units in the zigzag structure. We show that the tilting of the anisotropy axes gives rise to spin canting and consequently to a nonvanishing magnetization for the compound. In the case of a strong tetragonal field that stabilizes the orbital doublet of Co(II), the effective pseudo-spin-1/2 Hamiltonian describing the interaction between the Co ions in their ground Kramers doublet states is shown to be of the Ising type. An analytical expression for the static magnetic susceptibility of the infinite spin-canted chain is obtained. The model provides an excellent fit to the experimental data on both the static and dynamic magnetic properties of the chain.

Structure, Magnetism, and Theoretical Study of a Mixed-Valence CoII3CoIII4 Heptanuclear Wheel: Lack of SMM Behavior despite Negative Magnetic Anisotropy

A mixed-valence Co(II)/Co(III) heptanuclear wheel [Co(II)3Co(III)4(L)6(MeO)6] (LH2 = 1,1,1-trifluoro-7-hydroxy-4-methyl-5-aza-hept-3-en-2-one) has been synthesized and its crystal structure determined using single-crystal X-ray diffraction. The valence state of each cobalt ion was established by bond valence sum calculations. Studies of the temperature dependence of the magnetic susceptibility and the field dependence of the magnetization evidence ferromagnetic interactions within the compound. In order to understand the magnetic properties of this Co7 wheel, we performed ab initio calculations for each cobalt fragment at the CASSCF/CASPT2 level, including spin-orbit coupling effects within the SO-RASSI approach. The four Co(III) ions were found to be diamagnetic and to give a significant temperature-independent paramagnetic contribution to the susceptibility. The spin-orbit coupling on the three Co(II) sites leads to separations of approximately 200 cm(-1) between the ground and excited Kramers doublets, placing the Co7 wheel into a weak-exchange limit in which the lowest electronic states are adequately described by the anisotropic exchange interaction between the lowest Kramers doublets on Co(II) sites. Simulation of the exchange interaction was done within the Lines model, keeping the fully ab initio treatment of magnetic anisotropy effects on individual cobalt fragments using a recently developed methodology. A good description of the susceptibility and magnetization was obtained for nearest-neighbor (J1) and next-nearest-neighbor (J2) exchange parameters (1.5 and 5.5 cm(-1), respectively). The strong ferromagnetic interaction between distant cobalt ions arises as a result of low electron-promotion energies in the exchange bridges containing Co(III) ions. The calculations showed a large value of the magnetization along the main magnetic axis (10.1 mu(B)), which is a combined effect of the ferromagnetic exchange interaction and negative magnetic anisotropy on the two marginal Co(II) sites. The lack of single-molecule magnet behavior in [Co(II)3Co(III)4(L)6(MeO)6] is explained by relatively large matrix elements of transverse magnetic moments between states of maximal magnetization of the ground Kramers doublet, evidenced by ab initio calculations, and the associated large tunneling rates between these states in the presence of dipolar transverse magnetic fields in the crystal.

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.

Magnetic anisotropy in the octanuclear [formula omitted] cluster exhibiting Single-Molecule Magnet behavior: Quantum-spin and classical-spin approaches

A theoretical model is developed for the explanation of the magnetic properties of the [MnCl] 4[Re(triphos)(CN) 3] 4 single molecule magnet, which represents a 3d–5d molecular cube compressed along one of the C 3 axes. In the model, the strong local trigonal crystal field on each Re(II) ion is assumed to split the ground cubic 2 T 2( t 2 5 ) term into the states 2 A 1 and 2 E in such a way that 2 E term proves to be the ground one. The ground Kramers doublet that appears under the spin–orbit splitting of the 2 E term is shown to be fully anisotropic ( g ⊥ = 0) and the magnetic superexchange (mediated by cyanide bridge) between Re and Mn ions is approximated by the Ising-like Hamiltonian. In this case, the magnetic anisotropy of the trigonally distorted cube represents a first order effect that is totally incorporated in the effective Ising-like Hamiltonian, including the pseudo-spin-1/2 operator for the Re(II) ion and the operator of the true spin-5/2 for the Mn(II) ion. The developed pseudo-spin-1/2 formalism is applied to the explanation of the DC magnetic susceptibility and magnetization data, and the set of the best fit parameters is found. The temperature dependence of the DC magnetic susceptibility and the field dependence of the magnetization calculated with this set of parameters are in accord with the experimental data being at the same time compatible with the existence of the barrier for magnetization reversal. Finally, the applicability of the quantum–classical spin approach for the calculation of the magnetic properties of large 3d–5d clusters is discussed.

Thermodynamic properties of NdFe 3(BO 3) 4

We present results on thermodynamic properties of single crystalline NdFe 3(BO 3) 4 studied by means of the specific heat c p ( T) and the magnetic susceptibility χ( T). At high temperatures, we observe no anomalies in c p ( T) and χ( T) obeys a Curie–Weiss law. The onset of long-range antiferromagnetic order at 29 K is accompanied by anomalies in both magnetization and specific heat. A Schottky anomaly is visible in c p at ∼4 K. At ∼6 K a low-temperature anomaly is also found in χ( T) if a magnetic field is applied perpendicular to the c-axis. Both phenomena are related to the depopulation of the upper component of the ground Kramers doublet of Nd 3+ split in the magnetically ordered state.

Nature of acceptor states in magnesium-doped gallium nitride

The most effective method of producing $p$-type gallium nitride is currently through incorporation of magnesium. However, such doping leads not only to the desired shallow acceptors but also to the creation of much deeper energy levels. Magnesium-related acceptors have been observed in many optically detected magnetic resonance experiments and their magnetic properties, as characterized by the $g$ values of the holes which they trap, have been found to vary significantly according to the growth conditions and doping levels. The purpose of the present paper is to present a model that accounts for these observations. The model assumes that, in the deep acceptors, the hole is located in an atomic orbital of $p$ character, presumed to be on a nitrogen atom. The orbital degeneracy is partly removed by the wurtzite crystal field and finally by a reduction in local symmetry associated with the relative positions of the magnesium dopant and the nitrogen atom upon which the hole is localized. Further changes in the local crystal field are caused by the presence of nearby defects or by strain. These changes in the crystal field are accompanied by changes in acceptor depth. The approach leads to the correct $g$ values and the correct correlation between the $g$ values and acceptor depth for reasonable choices of the parameters. In the limit that the low symmetry fields become small, the model evolves to one that is consistent with the correct forms of the ground and near-ground Kramers doublets that are observed by other workers in studies of shallow acceptors in material that is not doped with magnesium. Finally, the model is shown to be entirely consistent with a range of acceptor states of different depths being formed by simple substitution of a magnesium ion at a gallium site, rather than by the creation of more complicated defects. The conclusion also highlights the need for the GaN to be of high crystalline quality if effective $p$-type doping is to be achieved.

Isotropic Magnetic Exchange between Anisotropic Yb(III) Ions. Study of Cs3Yb2Cl9 and Cs3Yb2Br9 Crystals

Despite the prevalent belief about a strong anisotropy of the magnetic exchange in rare-earth compounds, Cs3Yb2Cl9 and Cs3Yb2Br9 crystals are found to exhibit fully isotropic exchange coupling between Yb3+ ions. In this article, we attempt to reveal the physical origin of this surprising feature. Our theoretical consideration is based on a model of the kinetic exchange between two octahedrally coordinated Yb3+ ions in their ground Kramers doublet states. It is shown that a mechanism of kinetic exchange involving intercenter electron hopping between 4f orbitals of two Yb3+ ions in a face-shared binuclear unit results in fully isotropic antiferromagnetic exchange coupling, while a mechanism in which the electron jumps from the 4f to the 5d orbital gives rise to a highly anisotropic interaction. Comparison of these results with the experimental data along with qualitative arguments regarding the relative significance of these two contributions to the overall exchange indicate that, in face-shared Yb3+ binuclear units, the 4f <--> 4f mechanism plays a dominant role.

Investigations of the EPR parameters for Sm 3+ ions in KY 3F 10 and LiYF 4 crystals

In this paper, we calculate the EPR parameters ( g factors g ‖, g ⊥ and hyperfine structure constants A ‖, A ⊥) of rare earth ion Sm 3+ in fluoride crystals KY 3F 10 and LiYF 4 from the perturbation formulas of EPR parameters for a 4f 5 ion in tetragonal symmetry. In these formulas, the crystal-field J-mixing of the first and second excited-state multiplets 6H 7/2 and 6H 9/2 into the ground state multiplet 6H 5/2, the mixtures among the states with the same J value via spin–orbit coupling interaction and the interactions between the ground Kramers doublet Γ γ and the same irreducible representation as Γ γ in other 11 Kramers doublets Γ x within 6H J ( J=5/2, 7/2, 9/2) states via crystal-field and orbital angular momentum (or hyperfine structure) are considered. The calculated results (which are in agreement with the observed values) are discussed.

Exchange Interaction via Crystal-Field Excited States and Its Importance in CsCoCl3

The low-energy effective Hamiltonian is derived for a typical quasi-one-dimensional Ising-like antiferromagnet CsCoCl 3 . In contrast to previous treatments, in which only the processes within the crystal-field ground Kramers doublet are considered, second-order processes via crystal-field excited states are also taken into account in the present paper. They result in a new term, which is essentially a next-nearest neighbor ferromagnetic exchange whose anisotropy is different from the nearest-neighbor exchange. By including the new term the magnetization curve and ESR in CsCoCl 3 are analyzed to show that those experiments can be explained in a way consistent with other experiments. Related theories on CsCoCl 3 are discussed in this connection.

EPR study of Dy 3 + ions in DyBa 2 Cu 3 O 6 + x

Concentrated polycrystalline DyBa2Cu3O6+x compounds are studied by X-band EPR spectroscopy. A broad resonance line due to the highly anisotropic EPR spectrum of Dy3+ ions is identified on several specimens at low temperatures. Powder simulation of the EPR spectra complies with the ground Kramers doublet predicted by crystal field analysis of Dy3+ ions. Calculations of the second and fourth moments of the resonance lines due to the dipole-dipole interactions of Dy3+ indicate the presence of substantial exchange narrowing of the dipolar-broadened EPR linewidth.

Paramagnetic NMR shifts in cyanoferricytochrome c: Investigation of thermal stability and deviations from Curie law behaviour

The paramagnetic shifts of 13C nuclei positioned α to the haem in cyanoferricytochrome c are reported and analysed in terms of molecular orbitals based on D 4h symmetry with a rhombic perturbation. The temperature dependence of the Fermi contact and dipolar shifts of the haem and axial histidine ligand show deviations from Curie Law behaviour which are explained by a Boltzmann distribution between partially filled 3e( π) molecular orbitals and the ground and first excited state Kramers doublets. The comprehensive explanation of the temperature dependence of the paramagnetic shifts leads to the conclusion that there is no detectable temperature dependence of the haem orientation or that of the His ligand orientation. This work also provides evidence for the role of the axial His ligand in determining the orientation of the magnetic z-axis.

Superexchange interaction between lanthanide ions. Spin-Hamiltonian calculations for the and superexchange

Exchange interaction between two lanthanide or actinide ions of configuration bridged by common diamagnetic ligands is theoretically studied using a modified version of the superexchange theory developed in this paper. Exchange spin Hamiltonians were calculated for the and dimers serving as models of the and - superexchange, respectively. Spin - orbit coupling and crystal field splitting of the configuration (resulting in the ground Kramers doublet and the effective spin of the metal ion), virtual transfers of electrons of the type - - via and valent orbitals of the bridging ligands, and exchange pathways in these dimers are considered in detail. The superexchange is found to be extremely anisotropic and very sensitive to the geometry of the dimer. The spin Hamiltonian of the dimer is , where the exchange parameters are rationalized in terms of and parameters referring, respectively, to the - and - pathways of the electron transfers, , and . The and values are analytically expressed through and overlap integrals, orbital energies and intra-ionic Slater parameters. Exchange interaction between ions in the dimer is described by an antiferromagnetic Ising Hamiltonian , where the z axis connects two metal ions. Unusual magnetic properties of (M = Li, Na, K and Rb) and oxides involving ions and distorted perovskite are discussed in the light of these theoretical results.

Magnetic phase transitions in the chain nickelates R2BaNiO5 (R = Sm, Eu, Tm) by optical spectroscopy

Abstract Magnetic ordering of Sm2BaNiO5 at 55 ± 4 K, of Eu2BaNiO5 at 23 ± 2 K and of Tm2BaNiO5 at 13 ±1 K was found by high resolution Fourier transform spectroscopy. The Sm3+ ground Kramers doublet is split by Δ=28 cm-1 in the magnetically ordered state of Sm2BaNiO5 while the first excited crystal field level lies at 25 cm-1. Low-dimensional magnetic correlations are present in all the compounds studied.

EPR of Co 2+ in Rb 2MgF 4 and anomalous g-values of pair and trimer spin systems

EPR experiments on Co 2+ spin in Rb 2Co 0.3Mg 0.7F 4 have been performed at 220, 370, 693.6 and 762.2 GHz in pulsed magnetic fields. Two absorption lines with smaller g-values than that of a single spin resonance have been observed and they are ascribed to the pair and trimer spin resonances. The g-value difference is considered to be due to the mixing of the ground Kramers doublet with the excited states with small g-values.

170Yb Mossbauer study of YbPt3 and Yb3Pt5

The intermetallic compounds YbPt3 and Yb3Pt5, which present magnetic ordering below 0.25 K and 0.95 K respectively, have been investigated by 170Yb Mossbauer spectroscopy at very low temperatures. In YbPt3, the electronic ground Kramers doublet of Yb3+ has been determined, as well as its exchange splitting and spin-spin relaxation rate; the lowest Yb3+ crystal field excitation has been estimated. The surprising finding that the crystal electric field has strong axial character in YbPt3, instead of the expected cubic symmetry, is discussed.

Effective Hamiltonian and ground state properties of rare-earth ions in iron garnets

An effective Hamiltonian for ground Kramers doublet and quasidoublet of non-Kramers rare-earth (RE) ions in garnets in the presence of exchange interactions with ferric sublattices is shown to reflect the anisotropy of both crystalline field and exchange interaction with different consequences for Kramers and non-Kramers ions. For ten ions of the RE series all possible crystal-field ground states which are to be considered in RE garnets are examined. Main attention is paid to the search for RE moments with Ising-like character imprinted by the crystal field. The exchange interaction of such moments with iron sublattices can then be described by a single-parameter Hamiltonian. This greatly simplifies general many-parameter theories of the low-temperature magnetic phenomena. It is found that in principle the Ising-like ground states may occur in Sm 3+-, Tb 3+-, Dy 3+- and Tm 3+-iron garnets. The results are compared with available experimental data.