Van Vleck Ions Research Articles

Long-range antiferromagnetic order of formally nonmagneticEu3+Van Vleck ions observed in multiferroicEu1−xYxMnO3

We report on resonant magnetic x-ray scattering and absorption spectroscopy studies of exchange-coupled antiferromagnetic ordering of Eu$^{3+}$ magnetic moments in multiferroic Eu$_{1-x}$Y$_{x}$MnO$_3$ in the absence of an external magnetic field. The observed resonant spectrum is characteristic of a magnetically ordered $^7F_1$ state that mirrors the Mn magnetic ordering, due to exchange coupling between the Eu $4f$ and Mn $3d$ spins. This is the first observation of long range magnetic order generated by exchange coupling of magnetic moments of formally non-magnetic Van Vleck ions, which is a step further towards the realization of exotic phases induced by exchange coupling in systems entirely composed of non-magnetic ions.

Effect of Cr doping on HgTe properties

AbstractElectrical and magnetic investigations of HgTe doped by Cr are presented. The low temperature features of Hall coefficient magnetoresistance, thermoelectric power and magnetic susceptibility were revealed. They point out to the creation of an impurity level by the Cr dopant and to the behavior of Cr2+(d4) as Van–Vleck ion. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the hyperfine interaction in rare-earth Van Vleck paramagnets at highmagnetic fields

The influence of high magnetic fields on the hyperfine interactionin rare-earth ions with a non-magnetic ground state (Van Vleckions) is investigated theoretically for the case of Tm3+ions in an axially symmetrical crystal electric field (in an ethylsulphate crystal). Itis shown that magnetic-field-induced distortions of the 4f-electron shell lead tocrucial changes in the hyperfine magnetic field at the nucleus. The proposedtheoretical model is in agreement with recent experimental data.

Electron paramagnetic resonance of Cu2+ ions in the tetragonal perovskite LaCuO3- delta.

Electron paramagnetic resonance (EPR) from ${\mathrm{Cu}}^{2+}$ ions is observed at very high temperatures in the tetragonally distorted perovskite ${\mathrm{LaCuO}}_{3\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, where for small \ensuremath{\delta} most of the copper ions are presumably in trivalent states. Upon lowering the temperature, linewidths rapidly increase and the spectra are shifted to lower fields. The temperature dependence of the EPR linewidth is similar to what has been predicted, but not yet observed, for high-temperature superconductors and related antiferromagnets. A nonzero but small magnetization at low temperatures and low magnetic fields points to a canted magnetism due to strong long-ranged antisymmetric exchange interactions between randomly distributed ${\mathrm{Cu}}^{2+}$ ions in a matrix of ${\mathrm{Cu}}^{3+}$ Van Vleck ions.

Structural and magnetic properties of stage-2 EuCl3-graphite intercalation compound.

Structural and magnetic properties of stage-2 ${\mathrm{EuCl}}_{3}$-graphite intercalation compounds have been studied by using x-ray diffraction, electron diffraction, and dc magnetic susceptibility. The repeat distance along the c axis is 13.28\ifmmode\pm\else\textpm\fi{}0.02 \AA{}. The ${\mathrm{EuCl}}_{3}$ intercalate layer forms a three-layer sandwich of Cl-Eu-Cl, where the distance between Eu and Cl layers is 1.67 \AA{}. The layer structure of the ${\mathrm{EuCl}}_{3}$ intercalate layer is similar to that of ${\mathrm{YCl}}_{3}$ type. The ${\mathrm{EuCl}}_{3}$ layer is hexagonal with two Eu ions per unit cell (lattice constant 7.38 \AA{}) and consists of domain structures generated by the rotation of the (3\ifmmode\times\else\texttimes\fi{}3) commensurate structure with respect to the graphite lattice by \ifmmode\pm\else\textpm\fi{}6.3\ifmmode^\circ\else\textdegree\fi{}, \ifmmode\pm\else\textpm\fi{}10\ifmmode^\circ\else\textdegree\fi{}, \ifmmode\pm\else\textpm\fi{}13.8\ifmmode^\circ\else\textdegree\fi{}, and \ifmmode\pm\else\textpm\fi{}17.5\ifmmode^\circ\else\textdegree\fi{}. The ${\mathrm{EuCl}}_{3}$ layer is modulated by the graphite layer. The dc magnetic susceptibility is found to be a sum of contributions from ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ ions. The concentrations of ${\mathrm{Eu}}^{3+}$ and ${\mathrm{Eu}}^{2+}$ in the ${\mathrm{EuCl}}_{3}$ intercalate layer are 0.994 and 0.006, respectively. The susceptibility contribution from ${\mathrm{Eu}}^{2+}$ ions obeys the Curie-Weiss law with a Curie-Weiss temperature FTHETA=10.1\ifmmode\pm\else\textpm\fi{}1.1 K. The susceptibility contribution from the van Vleck ion ${\mathrm{Eu}}^{3+}$ is almost independent of temperature, where the energy difference between $^{7}$${\mathit{F}}_{0}$ and $^{7}$${\mathit{F}}_{1}$ levels is given by ${\mathit{E}}_{1}$=538 K.

Temperature Dependence of EPR Linewidth of Impurities in Van‐Vleck Paramagnets

AbstractThe specific spin‐lattice relaxation mechanisms (“fluctuation” and “Waller” mechanisms) of impurity paramagnetic ions in rare‐earth Van‐Vleck paramagnets are shown to be the effective mechanisms of EPR line broadening too. On the basis of these mechanisms one can interpret qualitatively and, in a great extent, quantitatively all peculiarities of the temperature dependence of Gd3+ impurity ion EPR linewidth in rare‐earth zircons, observed by Mehran, Stevens et al. It is concluded, that in interpretation of their experimental results one can avoid the consideration of the hardly perceptible quasi Jahn‐Teller effect on excited doublets of the host Van Vleck ions.

3d Transition Metals in II-VI Semiconductors

ABSTRACTThe magnetic and electronic properties of some I-VI semiconductors with 3d transition metals other than Mn are presented. For example, the nonzero orbital moment in Fe2+ leads to more complex electronic energy levels than for Mnt ions. In (CdFe)Se, inelastic light scattering experiments demonstrate that the ground state is nonmagnetic (Van Vleck ion), and directly measures the energy spacing between the three lowest levels. For Sc2+ ions the donor level lies above the bottom of the conduction band in the Cd-based materials.

Magnetic properties of Co-R (R=Pr,Nd,Sm,Gd,Dy,Er) amorphous alloys

Magnetization and susceptibility have been measured in the temperature range 4.2–300 K for amorphous Co100−x Rx (R=Pr,Nd,Sm,Gd,Dy,Er; 50≤x≤70) alloys prepared by a liquid-quenching technique. All alloys show a normal ferromagnetic behavior at low temperatures and a Curie–Weiss-type temperature dependence of susceptibility in the paramagnetic regime except for the Co-Sm alloy system. In the case of the Co-Sm alloys, a field-cooling effect at lower temperatures and a temperature dependence of paramagnetic susceptibility similar to that of the Van Vleck ion are observed. From analysis of the average effective moment, it is concluded that the Co no longer sustains a moment above 60 at. % R.

A151Eu Mossbauer study of the magnetic hyperfine interactions in the metallic compound Eu2Co12P7containing trivalent europium

Magnetic susceptibility measurements of Eu2Co12P7 show a ferromagnetic phase transition at the Curie temperature of TC=151+or-0.5 K, which is attributed to the magnetic order of the cobalt sublattice. 151Eu Mossbauer spectra reveal an unusual magnetic hyperfine structure splitting of the Van Vleck ion Eu3+. The temperature dependence of the magnetic flux density at the 151Eu nuclei was determined and compared with theoretical values. The exchange field acting on the europium ions may be assumed to follow the Brillouin function, where the best agreement is achieved for J=1. An exchange constant of mu BBexch(0)/k=7.5+or-0.1 K was deduced from the saturation value of the magnetic flux density Bsat=19.0+or-0.1 T. The possible contributions of Fermi contact terms of the magnetic flux density are discussed.

Nuclear magnetic resonance of rare-earth Van Vleck paramagnets

Theoretical and experimental investigations of nuclear magnetic resonance in rare-earth Van Vleck paramagnetic materials are reviewed. The majority of the observed aspects of the phenomenon is explained by the interaction of nuclei with the residual electronic magnetic moment in the singlet ground state and the modulation of this interaction as a result of the coupling of the electronic moments among themselves and with the oscillations of the crystal lattice, and also by various spin-spin interactions involving enhanced nuclear moments. At liquid helium and higher temperatures the most significant effect is the modulation of the hyperfine interaction resulting from real thermal transitions of Van Vleck ions between the ground state and the nearest excited states. Within the framework of a single simple theory based on the randomly varying in time hyperfine interaction Hamiltonian, a description is given of the temperature dependence of the shifts and widths of NMR lines, of the spin-lattice relaxation times of nuclei within Van Vleck ions and within diamagnetic atoms, and also of the frequency and orientation dependences of these quantities. The theory is in agreement with experiment not only qualitatively, but to a significant extent also quantitatively without resorting to any adjustable parameters. Some special features of nuclear magnetism of Van Vleck paramagnetic materials at ultralow temperatures are also considered.

Collective excitations in the Van Vleck enhanced nuclear antiferromagnet HoVO4. I. Effective nuclear Hamiltonian

The problem of interacting Van Vleck ions at low temperatures is examined in some detail using both simple intuitive arguments and a high-order perturbation approach. Particular attention is paid to the Van Vleck enhanced nuclear antiferromagnet HoVO4. As a simple example, the problem of a neighbouring pair of Ho3+ ions in host YVO4 is solved exactly. It is shown that in addition to the expected dipole-dipole interactions between enhanced nuclear moments, there are subtle effects, due to the presence of dipole field-induced moments on the 4f shells, which strengthen the interaction between the two sites. Following a derivation of the many-body Hamiltonian for HoVO4, it is shown that the Hamiltonian can be expressed either in the form of a continued equation or as the sum of 'effective' two-body interactions, between enhanced nuclear moments, which take into account the role played by all the dipole field-induced moments in the lattice. Finally it is demonstrated that the mean field energy eigenvalues and eigenfunctions used by Bleaney (1980), in his discussion of the nuclear ordered state, can be used in conjunction with Green function methods of Haley and Erdos (1972) to compute the excitation energies epsilon (q) versus q in the nuclear ordered phase of HoVO4.

Induced Magnetism in a System Consisting of van Vleck Ions Coupled to an Electron Gas in the Narrow Band Limit

AbstractThe magnetic properties of a model consisting of an electron gas interacting by exchange with van Vleck ions, is studied under the action of a crystal field in the narrow band limit. Iso‐Tc curves (in the plane of the interaction parameters), ionic and electronic magnetizations and susceptibilities versus temperature, as well as the magnetic specific heat, are obtained for several values of exchange and crystal field parameters.

Magnetic Behavior of van Vleck Ions and an Electron Gas Interacting by Exchange

AbstractThe magnetic behavior of a model in which van Vleck ions, under the action of a crystal field, interacting by exchange with an electron gas is investigated. The condition of onset of ferromagnetism and the behavior of the critical temperature, band and ionic magnetizations (and susceptibilities) versus temperature, as a function of the band width, exchange interaction, and the crystal field splitting energy parameters are obtained within an approximation equivalent to a molecular field formulation.

Spin polarization of a magnetic electron gas induced by a van Vleck ion

AbstractThe mutual polarization of a magnetic electron gas and a van Vleck ion, interacting via exchange, are theoretically investigated using the double‐time Green function method. A pair of equations describing the dynamics of the electron gas and the ion are conveniently decoupled and an analytic expression for the electron gas polarization is obtained which depends on the square of the exchange parameter. Besides a RKKY‐like term, a new term associated to the process of formation of the magnetic moment of the ion appears.