Van Vleck Paramagnetism Research Articles

The nonlinear Zeeman and parastriction effects in luminescence spectra of [formula omitted] crystals

Splitting of the first excited Γ 34 1 ( H 6 3 ) doublet state of Tm 3 + ions in the LiY 1 - x Tm x F 4 crystals in external magnetic fields up to 5.5 T has been studied using measurements of luminescence spectra at low temperatures. In the magnetic field perpendicular to the S 4 -symmetry axis of the crystal lattice, the splitting of the non-Kramers doublet is caused by the joint action of the nonlinear Zeeman effect and the piezospectroscopic effect related to parastriction. Experimental dependences of the doublet splitting on the magnetic field strength and direction are in good correspondence with the theoretical estimations obtained in the framework of the semiphenomenological crystal field model. It is shown that spectroscopic data present direct information on the forced magnetostriction in the basal plane of uniaxial Van Vleck paramagnets that contain non-Kramers rare earth ions. The measured concentration dependence of the doublet splitting can be explained by taking into account variations of the elastic constants with the concentration of Tm 3 + ions.

Synthesis, Structural and Magnetic Characterization of a New Scheelite Related Compound: Eu2Mo3O12.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Zeeman effect of single-molecule lines

The spectral line shifts induced by a magnetic field were measured for single terrylene molecules in the Shpol’skiĭ matrix n-hexadecane at liquid-helium temperatures. Quadratic red shifts with magnitudes between about 0 and −8 MHz/T 2 were observed. The red shift is tentatively ascribed to a predominant contribution of van Vleck paramagnetism. The broad variation between the molecules is due to the random molecular orientations in different samples. The magnitude of the shift is much smaller than in porphyrins, since terrylene does not have two closely spaced excited states.

Synthesis, Structural and Magnetic Characterization of a New Scheelite Related Compound: Eu2Mo3O12

AbstractPolycrystalline Eu2Mo3O12 has been synthesized and characterized by X‐ray and electron diffraction, high resolution electron microscopy and magnetic measurements. This oxide shows a complex scheelite superstructure whose essential feature is the existence of isolated Mo tetrahedra. Each Mo atom has four oxygen neighbors at distances ranging from 1.75 to 1.91 Å, whereas each Eu atom has eight oxygen neighbors at 2.43 Å. The temperature dependence of the inverse magnetic susceptibility shows that Eu2Mo3O12 behaves as a Curie–Weiss paramagnet at high temperature (160–300 K), whereas in the range 100–20 K it shows Van Vleck paramagnetism. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Structure and magnetic property of Ce xEu 1− xCoO 3 prepared by means of the thermal decomposition of Ce xEu 1− x[Co(CN) 6]· nH 2O

Eu[Co(CN) 6]·4H 2O and Ce x Eu 1− x [Co(CN) 6]·4H 2O ( x = 0.1, 0.15, 0.2, 0.3 and 0.4) were synthesized as the precursors to obtain the homogeneous perovskite-type oxides, EuCoO 3 and Ce x Eu 1− x CoO 3. The oxides EuCoO 3 and Ce x Eu 1− x CoO 3 were obtained by the thermal decompositions of Eu[Co(CN) 6]·4H 2O and Ce x Eu 1− x [Co(CN) 6]·4H 2O, respectively. In the ranges of x ≤ 0.5, the lattice parameters of these perovskite-type oxides, a and b, linearly lengthen with the increase of the value of x, but those of c are nearly constant. These findings indicated that Ce x Eu 1− x CoO 3 was formed in the range of x ≤ 0.5. The structures of EuCoO 3 and Ce x Eu 1− x CoO 3 were determined by means of the Rietveld method, and these oxides were orthorhombic and Pnma. The magnetic susceptibility of EuCoO 3 showed the paramagnetic property, which includes Van Vleck's paramagnetism. The Co III in EuCoO 3 were in the low ground spin state ( t 2 g 6 e g 0 , S = 0 ), and the susceptibility of EuCoO 3 was dependent on the Eu III in observed temperature ranges. The valence combination of Ce 0.1Eu 0.9CoO 3 was considered to be C e 0.1 III E u 0.9 III Co O 3 from the temperature dependency of the magnetic susceptibility of Ce 0.1Eu 0.9CoO 3.

Dzyaloshinsky-Moriya spin canting in the low-temperature tetragonal phase ofLa2−x−yEuySrxCuO4

The Cu spin magnetism in La2-x-yEuySrxCuO4 (x<=0.17; y<=0.2) has been studied by means of magnetization measurements up to 14 T. Our results clearly show that in the antiferromagnetic phase Dzyaloshinsky-Moriya (DM)superexchange causes Cu spin canting not only in the LTO phase but also in the LTLO and LTT phases. In La1.8Eu0.2CuO4 the canted DM-moment is about 50% larger than in pure La2CuO4 which we attribute to the larger octahedral tilt angle. We also find clear evidence that the size of the DM-moment does not change significantly at the structural transition at T_LT from LTO to LTLO and LTT. The most important change induced by the transition is a significant reduction of the magnetic coupling between the CuO2 planes. As a consequence, the spin-flip transition of the canted Cu spins which is observed in the LTO phase for magnetic field perpendicular to the CuO2 planes disappears in the LTT phase. The shape of the magnetization curves changes from the well known spin-flip type to a weak-ferromagnet type. However, no spontaneous weak ferromagnetism is observed even at very low temperatures, which seems to indicate that the interlayer decoupling in our samples is not perfect. Nonetheless, a small fraction (<15%) of the DM-moments can be remanently magnetized throughout the entire antiferromagnetically ordered LTT/LTLO phase, i.e. for T<T_LT and x<0.02. It appears that the remanent DM-moment is perpendicular to the CuO2 planes. For magnetic field parallel to the CuO2 planes we find that the critical field of the spin-flop transition decreases in the LTLO phase, which might indicate a competition between different in-plane anisotropies. To study the Cu spin magnetism in La2-x-yEuySrxCuO4, a careful analysis of the Van Vleck paramagnetism of the Eu3+ ions was performed.

Magnetic and Transport Properties of PrPtSn

Physical behavior of PrPtSn has been studied by means of X-ray powder diffraction, magnetization, dc magnetic susceptibility and electrical resistivity measurements. PrPtSn was found to crystallize in the orthorhombic structure with space group Pnma. No indication of long-range magnetic ordering down to 0.4 K has been observed most probably due to the singlet ground state of the Pr3+ ion yielding van Vleck paramagnetism below 20 K. The high-temperature magnetic susceptibility of PrPtSn can be described by a Curie-Weiss behavior with the paramagnetic Curie temperatures θ a =-12.8(1); K, θ b =9.1(4); K, θ c =-139(4); K with the b-axis being the easy-magnetization direction. Deviations from the Curie-Weiss law below 60 K can be ascribed to crystal-field interaction which is also responsible for the overall anisotropy of magnetic properties.

Ultrahigh-frequency NMR of Tm 3+ ions in single crystals of thulium ethylsulfate at high magnetic fields

Resonant transitions predicted earlier between low-lying electron–nuclear sublevels of the Tm 3+ ground state were observed at frequencies up to 700 MHz in a dielectric Van Vleck paramagnet—thulium ethylsulfate (TmES) single crystal. It is shown that, due to the distortion of the 4f-electron shell of rare-earth ion in an applied magnetic field, the parameters of electron–nuclear interaction become field dependent. Experimental results of second moment of Tm 3+ enhanced NMR line in TmES are presented. The possible mechanisms of line broadening are considered, particularly as a result of the first excited doublet splitting.

Magnetism of the singlet-singlet system PrNi 5− xCu x

The temperature dependence of the specific heat of the paramagnetic and ferromagnetic compounds PrNi 5− x Cu x ( x≤1.2) has been studied. It was found that the lowest singlet levels of the Pr ions in the crystal field of the compounds approach each other as Cu substitutes for Ni. The lowering of the crystal field symmetry of the quasibinary compounds PrNi 5− x Cu x as compared to the terminal binary Van Vleck paramagnets is mainly responsible for their ferromagnetic ordering.

Investigation of magnetic properties in strong and ultrastrong magnetic fields using the single-turn coil technique: new phenomena in intermetallic compounds and rare-earth zircons

Abstract Magnetic anomalies in rare-earth zircons RXO 4 (X=V, As, P; R-rare-earth ion), which are caused by the crossing of the lowest-lying energy levels of the van Vleck paramagnets, have been studied at different field sweep rates and different cryogenic temperatures. These anomalies occur at about 50 T for PrVO 4 and at about 30 T for TmPO 4 . Recent magnetization measurements of the intermetallic compounds RMn 2 Ge 2 (R=Gd, Tb, Dy, Ho, Er, Y) reveal new magnetic phase transitions for some R in strong magnetic fields up to 150 T.

Magnetic order in PrB2C2

Abstract Tetragonal REB 2 C 2 compounds are usually characterized by competition between quadrupole and magnetic interactions. An exception of this rule is PrB 2 C 2 , being a van Vleck paramagnet down to low temperatures. We present wide temperature range measurements of magnetization and specific heat of this compound in external magnetic fields up to 14 T. The energies of the crystal field levels are evaluated from the specific heat data above 2 K. An increase of the specific heat observed below 1 K we ascribe to the tail of the Schottky anomaly.

Orbital magnetism in axially deformed sodium clusters

Low-energy orbital magnetic dipole excitations, known as scissors mode (SM), are studied in alkali metal clusters. Subsequent dynamic and static effects are explored. The treatment is based on a self-consistent microscopic approach using the jellium approximation for the ionic background and the Kohn-Sham mean field for the electrons. The microscopic origin of SM and its main features (structure of the mode in light and medium clusters, separation into low- and high-energy plasmons, coupling high-energy M1 scissors and E2 quadrupole plasmons, contributions of shape isomers, etc) are discussed. The scissors M1 strength acquires large values with increasing cluster size. The mode is responsible for the van Vleck paramagnetism of spin-saturated clusters. Quantum shell effects induce a fragile interplay between Langevin diamagnetism and van Vleck paramagnetism and lead to a remarkable dia-para anisotropy in magnetic susceptibility of particular light clusters. Finally, several routes for observing the SM experimentally are discussed.

Anisotropic magnetization, specific heat and resistivity of RFe 2Ge 2 single crystals

We have grown RFe 2Ge 2 single crystals for R=Y and ten members of the lanthanide series (Pr, Nd, Sm, Gd–Tm, Lu) using Sn flux as the solvent. The method yields clean, high quality crystal plates as evidenced by residual resistivities and RRR values in the range of 3–12 μΩ cm and 20–90, respectively. The crystals are also virtually free of magnetic impurities or secondary phases, allowing the study of the intrinsic anisotropic magnetic behavior of each compound. Characterization was made with X-Ray diffraction, and temperature and field-dependent magnetization, specific heat and resistivity. Very strong anisotropies arising mostly from CEF effects were observed for all magnetic rare earths except Gd. Antiferromagnetic ordering occurred at temperatures between 16.5 K (Nd) and 1.1 K (Ho) that roughly scale with the de Gennes factor for the heavy rare earths. For some members there is also a lower temperature transition associated with changes in the magnetic structure. Tm did not order down to 0.4 K, and appears to be a van Vleck paramagnet. All members which ordered above 2 K showed a metamagnetic transition at 2 K for fields below 70 kOe. The calculated effective moments per rare earth atom are close to the expected free ion values of R 3+ except for Sm which displays anomalous behavior in the paramagnetic state. The non-magnetic members of this series (Y, Lu) are characterized by an unusually large electronic specific heat coefficient ( γ∼60 mJ/mol K 2 ) and temperature-independent susceptibility term ( χ 0∼0.003 emu/mol ), indicative of a relatively large density of states at the Fermi surface.

Pressure-induced Magnetic Transition in the Van Vleck Paramagnet PrCu2

We report the pressure effects on lattice constants, magnetic susceptibility, specific heat and electrical resistivity in the orthorhombic PrCu 2 . Anisotropic compression is realized to be κ a ≈κ c <κ b even under hydrostatic pressure. Above 1.2 GPa, susceptibility shows a remarkable maximum at T max . This fact implies that the pressure-induced magnetic ordering occurs above 1.2 GPa. Also the results of specific heat and electrical resistivity reveal the anomaly at P =1.2 GPa to be due to an antiferromagnetic ordering. The temperature–pressure phase diagram is presented up to 2 GPa.

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.

Verification of the 2( N + 1) 2 rule for fullerenes

In general, a neutral or charged fullerene with 2( N + 1) 2 π-electrons exhibits a large negative NICS (nucleus-independent chemical shift) at the center of the carbon cage, where N is an arbitrarily chosen positive integer. This rule was verified on the basis of the fact that a fullerene π-system can be viewed as an electron gas bound on the spherical surface. Diamagnetic susceptibility was derived for this electron gas. Van Vleck paramagnetism is presumably minimized for fullerenes with 2( N + 1) 2 π-electrons. It turned out that the NICS value at the cage center is not related to the global or local aromaticity in the cage.

59Co, 23Na NMR and electric field gradient calculations in the layered cobalt oxides NaCoO 2 and HCoO 2

59Co and 23Na NMR has been applied to the layered cobalt oxides NaCoO 2 and HCoO 2 at three different magnetic field strengths (4.7, 7.1 and 11.7 T). The 59Co and 23Na quadrupole and anisotropic shift tensors have been determined by iterative fitting of the NMR line shapes at the three magnetic field strengths. Due to the large 59Co quadrupole interaction in NaCoO 2, a frequency-swept irradiation procedure was used to alleviate the limited bandwidth of the excitation. While the 59Co and 23Na shift and quadrupole coupling tensors in NaCoO 2 are found to be coincident and axially symmetric in agreement with the crystal symmetry requirements, the fits of the 59Co NMR spectra clearly show the presence of structural disorder in HCoO 2. The 23Na chemical shift anisotropy can be reproduced by shift tensor calculations using a point dipole model and considering that the magnetic susceptibility in NaCoO 2 is due to Van Vleck paramagnetism for Co 3+. Electric field gradient calculations using either the empirical point charge model or the ab initio full potential-linearized augmented plane wave method are compared with the experimental NMR data.

Magnetic properties of rare-earth metal tritelluridesRTe3(R=Ce,Pr,Nd,Gd,Dy)

The magnetic properties of $R{\mathrm{Te}}_{3}$ $(R=\mathrm{Ce},$ Pr, Nd, Gd, Dy), which has a structure consisting of the alternate stacking of one $R\mathrm{Te}$ layer and two Te layers, were investigated using magnetization and electrical resistivity measurements. The magnetic transition temperatures were found at 3.0 K for $R=\mathrm{Ce},$ 3.2 K for Nd, 11.5 K for Gd, and 4.5 K for Dy, respectively, and the Pr compound showed van Vleck paramagnetism. The ordered magnetic moment of $R{\mathrm{Te}}_{3}$ lies within the layer. In contrast, the ordered magnetic moments of $R{\mathrm{Te}}_{2},$ having alternate stacking of one $R\mathrm{Te}$ and one Te layer, point to the layer-stacking direction, although the point symmetry at the R site and the coordination of the Te atoms to the R atoms are almost the same. The difference of the magnetic anisotropy in $R{\mathrm{Te}}_{2}$ and $R{\mathrm{Te}}_{3}$ was discussed in terms of the effective charge of the Te atoms in the Te layer with an electron charge transferred from the $R\mathrm{Te}$ layer. We propose magnetic compounds designed by charge introduction based on this result.

Van Vleck paramagnetic susceptibility in single-crystal Tm 3Al 5O 12

Single crystals of garnet-type Tm 3Al 5O 12 have been grown by a flux method. The magnetic susceptibility measurement from 2.0 to 300 K reveals the typical Van Vleck paramagnetism. The crystal electric field analysis of the magnetic susceptibility indicates that the ground state is singlet Γ 2 and the excitation energy to the first excited state, triplet Γ 5 (2), is 68 K .

Effect of Heat Treatment on the Magnetic Properties of Hydrothermally Grown ZnO Crystals

The magnetic susceptibility of ZnO single crystals annealed in vacuum, in an oxygen atmosphere, and in the presence of Li2CO3 is measured from 4.2 to 300 K. The susceptibility versus temperature curves of the crystals are similar in shape and differ by a roughly constant value, which is interpreted in terms of Van Vleck (temperature-independent) paramagnetism. The anomalies found below 30 K are due to paramagnetic defects such as LiZn and VO, whose concentration depends on the growth and annealing conditions. Some other features of the susceptibility data are attributable to exchange interactions in Xi VZn (X = Zn, Si), LiZnZni , and VOLiZn donor–acceptor pairs, which lead to magnetic saturation in the corresponding defect subsystem. The effect of annealing on the 300-K susceptibility of the crystals is attributable to Van Vleck paramagnetism, which results from the mixing of the ground and excited electronic states caused by defect-induced local strain fields.