Crystal Electric Field Effects Research Articles

Vertical Floating Zone Crystal Growth of R<sub>2</sub>PdSi<sub>3</sub> Intermetallic Compounds (R=Pr and Nd)

The class ofR2PdSi3single crystals (R= rare earth element) with hexagonal AlB2-type crystallographic structure reveals the systematic dependence of anisotropic magnetic properties governed by the interplay of crystal-electric field effects and magnetic two-ion interactions. Here we compare the floating zone (FZ) crystal growth with radiation heating of compounds Pr2PdSi3and Nd2PdSi3. The congruent melting behavior enabled moderate growth velocities of 3 to 5 mmh-1. The preferred growth directions are close to the basal plane of the hexagonal unit cell. The composition of the crystals, is slightly Pd-depleted with respect to the nominal composition 16.7 at.% Pd. The Pr2PdSi3compound exhibit antiferromagnetic order below the Néel temperatures TN: 2.17 K and Nd2PdSi3compound order ferromagnetically below the Curie temperatureTC= 15.1 K.

Inelastic neutron scattering study on the noncentrosymmetric compounds PrCuAl3and NdCuAl3

We present the structural, magnetic, thermal, and transport properties of two noncentrosymmetric compounds PrCuAl${}_{3}$ and NdCuAl${}_{3}$ from the magnetic susceptibility $\ensuremath{\chi}(T)$, isothermal magnetization $M(H)$, specific heat ${C}_{\mathrm{p}}(T)$, neutron diffraction, and inelastic neutron scattering (INS) investigations. Our x-ray diffraction and neutron diffraction data confirm that both PrCuAl${}_{3}$ and NdCuAl${}_{3}$ crystallize in BaNiSn${}_{3}$-type noncentrosymmetric tetragonal crystal structure. While PrCuAl${}_{3}$ remains paramagnetic down to 1.8 K, a clear evidence for an antiferromagnetic ordering below 2.35 K in NdCuAl${}_{3}$ is inferred from the $\ensuremath{\chi}(T)$ and ${C}_{\mathrm{p}}(T)$. The absence of magnetic order in PrCuAl${}_{3}$ can be attributed to crystal electric field (CEF) effect. The ${C}_{\mathrm{p}}(T)$ data suggest the CEF ground state to be a nonmagnetic singlet in PrCuAl${}_{3}$ and a magnetic doublet in NdCuAl${}_{3}$, which are further confirmed by the INS study. The INS spectra reveal two sharp CEF excitations near 2 and 9 meV in PrCuAl${}_{3}$, and four CEF excitations near 1.4, 3.2, 8.9, and 10.9 meV in NdCuAl${}_{3}$. An analysis of the INS data of both the compounds is presented based on the CEF model, which accounts for the observed position and intensity of all the observed CEF excitations. The ground state wave functions have been evaluated from the analysis of the INS data.

Ferromagnetic ordering in CeIr2B2: Transport, magnetization, specific heat, and NMR studies

We present a complete characterization of the ferromagnetic system CeIr${}_{2}$B${}_{2}$ using powder x-ray diffraction (XRD), magnetic susceptibility $\ensuremath{\chi}(T)$, isothermal magnetization $M(H)$, specific heat $C(T)$, electrical resistivity $\ensuremath{\rho}(T,H)$, and thermoelectric power $S(T)$ measurements. Furthermore, ${}^{11}$B NMR study was performed to probe the magnetism on a microscopic scale. Rietveld refinement of powder XRD data confirms that CeIr${}_{2}$B${}_{2}$ crystallizes in CaRh${}_{2}$B${}_{2}$-type orthorhombic structure (space group fddd). The $\ensuremath{\chi}(T)$, $C(T)$, and $\ensuremath{\rho}$(T) data confirm bulk ferromagnetic ordering with ${T}_{c}=5.1$ K. Ce ions in CeIr${}_{2}$B${}_{2}$ are in a stable trivalent state. Our low-temperature $C(T)$ data measured down to 0.4 K yield a Sommerfeld coefficient $\ensuremath{\gamma}$ = 73(4) mJ/mol K${}^{2}$, which is much smaller than the previously reported value of $\ensuremath{\gamma}$ = 180 mJ/mol K${}^{2}$ deduced from the specific heat measurement down to 2.5 K. For LaIr${}_{2}$B${}_{2}$, $\ensuremath{\gamma}$ = 6(1) mJ/mol K${}^{2}$, which implies the density of states at the Fermi level $\mathcal{D}({E}_{F})=2.54$ states/(eV f.u.) for both spin directions. The renormalization factor for quasiparticle density of states and hence for quasiparticle mass due to $4\phantom{\rule{-0.16em}{0ex}}f$ correlations in CeIr${}_{2}$B${}_{2}$ is $\ensuremath{\approx}$12. The Kondo temperature ${T}_{\mathrm{K}}\ensuremath{\sim}4$ K is estimated from the jump in specific heat of CeIr${}_{2}$B${}_{2}$ at ${T}_{c}$. Both $C(T)$ and $\ensuremath{\rho}(T)$ data exhibit a gapped-magnon behavior in the magnetically ordered state with an energy gap ${E}_{g}\ensuremath{\sim}3.5$ K. The $\ensuremath{\rho}$ data as a function of magnetic field $H$ indicate a large negative magnetoresistance (MR) which is highest for $T=5$ K. While at 5 K the negative MR keeps on increasing up to 10 T, at 2 K an upturn is observed near $H=3.5$ T. On the other hand, the thermoelectric power data have small absolute values ($S\ensuremath{\sim}7\ensuremath{\mu}$V/K), indicating a weak Kondo interaction. A shoulder in $S(T)$ at about 30 K, followed by a minimum at $\ensuremath{\sim}$10 K, is attributed to crystal electric field effects and the onset of magnetic ordering. ${}^{11}$B NMR line broadening provides strong evidence of ferromagnetic correlations below 40 K.

Crystal electric field effects in Pr0.5Sr0.5CoO3

The energy structure and temperature evolution of the magnetic excitation spectra of Pr0.5Sr0.5CoO3 are studied by inelastic neutron scattering. The measurements are performed on a polycrystalline sample of Pr0.5Sr0.5CoO3 and its non-magnetic analogue La0.5Sr0.5CoO3 on the high intensity time-of-flight spectrometer IN4 (ILL, Grenoble) in the temperature range 10 K < T < 300 K. The crystal electric field parameters in Pr0.5Sr0.5CoO3 are calculated and the splitting scheme of the 4f ground multiplet of Pr3+ ions is determined based on the experimental data.

Unusual magnetic properties induced by local structure in a quasi-one-dimensional Ising chain system:α-CoV2O6

We have investigated the origins of large orbital magnetic moment (OM) and unique magnetic anisotropy in the quasi-1D magnetic cobaltate, $\ensuremath{\alpha}$-CoV${}_{2}$O${}_{6}$, employing both the ab initio band structure method and the microscopic cluster model calculations. We have found that the peculiar crystal electric field effect in $\ensuremath{\alpha}$-CoV${}_{2}$O${}_{6}$ combined with the strong spin-orbit coupling induces the unusually large OM and other intriguing magnetic properties of $\ensuremath{\alpha}$-CoV${}_{2}$O${}_{6}$. The observed 1/3 magnetization plateau in the $M$-$H$ curve is explained by the spin-flip mechanism based on the MC simulation. Anomalous magnetic entropy behavior is attributed to the strong uniaxial magnetic anisotropy in the quasi-1D Ising chain system.

Magnetic and transport properties of Pr2Pt3Si5

We have investigated the magnetic and transport properties of a polycrystalline Pr2Pt3Si5 sample through the dc and ac magnetic susceptibilities, electrical resistivity, and specific heat measurements. The Rietveld refinement of the powder X-ray diffraction data reveals that Pr2Pt3Si5 crystallizes in the U2Co3Si5-type orthorhombic structure (space group Ibam). Both the dc and ac magnetic susceptibility data measured at low fields exhibit sharp anomaly near 15K. In contrast, the specific heat data exhibit only a broad anomaly implying no long range magnetic order down to 2K. The broad Schottky-type anomaly in low temperature specific heat data is interpreted in terms of crystal electric field (CEF) effect, and a CEF-split singlet ground state is inferred. The absence of the long range order is attributed to the presence of nonmagnetic singlet ground state of the Pr3+ ion. The electrical resistivity data exhibit metallic behavior and are well described by the Bloch–Grüniesen–Mott relation.

Magnetism and superconductivity in the Heusler alloy Pd2YbPb

We report the temperature dependence (1.8–300 K) of the heat capacity (Cp) and electrical resistivity (ρ) for the Heusler alloy Pd2YbPb and its isomorphous nonmagnetic counterpart, Pd2YPb. Both compounds appear to be the same MnCu2Al structure with space group Fm3¯m. From the resistivity data, the superconducting transition for Pd2YbPb and Pd2YPb are observed at T = 2.85 K and T = 4.75 K, respectively. At low temperatures, the Cp/T versus T data exhibits a small jump at around 4 K for Pd2YPb, while, for Pd2YbPb, this superconducting transition is hidden by an upturn feature observed below T = 3 K, presumably due to nuclear spin polarization. From low temperature analysis of the Cp/T data, the electronic contribution to Cp and Debye temperature were determined to be γ = 4.5 mJ/mole-K and ΘD = 140 K for Pd2YbPb and γ = 1.6 mJ/mole-K and ΘD = 173 K for Pd2YPb. The magnetic contribution of the Yb+3 ions to the heat capacity and resistivity were calculated by subtracting the Pd2YPb data from that for Pd2YbPb. The scheme of the energy levels created by the crystal electric field (CEF) splitting for cubic symmetry was determined from the Schottky contribution to Cp. In addition, the effect of CEF on the resistivity has been investigated. Both analyses lead us to suggest that the ground state of Yb ions is a quartet Γ8. The first excited level between the ground state Γ8 and a Γ7 doublet, Δ1 = 125 K, and the second energy splitting between Γ8 and Γ6, Δ2 = 268 K, were deduced from the heat capacity data, while values of Δ1 = 155 K and Δ2 = 355 K were obtained from resistivity data. Furthermore, at higher temperatures, we observed that Cp for Pd2YbPb exceeds the Dulong-Petit limit, leading us to include a small anharmonic correction in the fitting analysis.

Magnetic Field–Temperature Phase Diagram of the Ferro-quadrupolar State and Crystal Electric Field Effect in UCu2Sn

The ternary uranium compound UCu 2 Sn shows a ferro-quadrupolar transition at T Q = 16 K originating from the ordering of the non-Kramers doublet Γ 5 ground state. To investigate the magnetic field–temperature phase diagram of the ferro-quadrupolar state in UCu 2 Sn, we measured elastic modulus C 66 at magnetic fields of up to 14 T. We found an anisotropic field dependence of T Q ( H ) in H ∥[\bar12\bar10], [0001], and [01\bar10]. We optimized the crystal electric field level scheme using C 66 under fields, magnetic susceptibility, and electronic entropy. The quadrupolar phase diagram is well reproduced by the obtained level scheme.

Field-induced phases in UPt2Si2

The tetragonal compound UPt2Si2 has been characterised as a moderately mass enhanced system with an antiferromagnetic ground state below T_N = 32 K. Here, we present an extensive study of the behavior in high magnetic fields. We have performed pulsed field magnetization and static field resistivity measurements on single crystalline samples UPt2Si2. Along the crystallographic a axis, at low temperatures, we find a metamagnetic-like transition in fields of the order 40 T, possibly indicating a first order transition. Along the crystallographic c axis, in magnetic fields of B>= ~24 T, we find distinct anomalies in both properties. From our analysis of the data we can distinguish new high field phases above the AFM ground state. We discuss the emergence of these new phases in the context of Fermi surface effects and the possible occurrence of a Lifshitz or electronic topological transition, this in contrast to previous modellings of UPt2Si2 based on crystal electric field effects.

High pressure resistivity of UPd3

Electrical resistivity of UPd3, the only known binary U-based intermetallic with localized 5f states, was studied under high pressures up to 10 GPa, testing a possible incipient delocalization due to the lattice compression. The data obtained revealed no tendency to any delocalization. The shoulder related to crystal-electric field effects between 10 and 15 K remains visible, being only shifted slightly towards higher temperatures with the increase in pressure.

Magnetic phase diagram of CeAu2Ge2: High magnetic anisotropy due to crystal electric field

CeAu2Ge2 single crystals (tetragonal ThCr2Si2 structure) have been grown in Au-Ge flux (AGF) as well as in Sn flux (SF). X-ray powder-diffraction and EDX measurements indicate that in the latter case Sn atoms from the flux are incorporated in the samples, leading to a decrease of the lattice constants by ~ 0.3% compared to AGF samples. For both types of samples, a strong anisotropy of the magnetization M for the magnetic field B parallel and perpendicular to the c direction is observed with M||/M^{\bot} ~ 6 - 7 in low fields just above 10 K. This anisotropy is preserved to high fields and temperatures and can be quantitatively explained by crystal electric field effects. Antiferromagnetic ordering sets in around 10 K as previously found for polycrystalline samples. From the magnetization data of our single crystals we obtain the phase diagrams for the AGF and SF samples. The magnetic properties depend strongly on the flux employed. While the AGF samples exhibit a complex behavior indicative of several magnetic transitions, the SF samples adopt a simpler antiferromagnetic structure with a single spin-flop transition. This effect of a more ordered state induced by disorder in form of Sn impurities is qualitatively explained within the ANNNI model, which assumes ferromagnetic and antiferromagnetic interactions in agreement with the magnetic structure previously inferred from neutron-scattering experiments on polycrystalline CeAu2Ge2 by Loidl et al. [Phys. Rev. B 46, 9341, (1992)].

Crystal growth of the intermetallic compound Nd2PdSi3

AbstractNd2PdSi3 single crystals were grown by a vertical floating zone method with radiation heating at a zone traveling rate of 3 mm/h. The compound exhibits congruent melting behavior at a liquidus temperature of about 1790 °C. The actual crystal composition (35.3 ± 0.5) at.% Nd, (16.2 ± 0.5) at.% Pd, and (48.5 ± 0.5) at.% Si is slightly depleted in Pd and Si with respect to the nominal stoichiometry. Therefore, the gradual accumulation of these elements in the traveling zone led to a decrease of the operating temperature during the growth process. Single crystalline samples exhibit a large anisotropy due to the crystal electric field effect and order ferromagnetically below the Curie temperature TC = 15.1 K. The [001] orientation was identified as the magnetic easy axis at low temperatures. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Floating zone crystal growth of selected R2PdSi3 ternary silicides

Substitution of various rare earths R within the class of R2PdSi3 single crystals with hexagonal AlB2-type crystallographic structure reveals the systematic dependence of anisotropic magnetic properties governed by the interplay of crystal-electric field effects and magnetic two-ion interactions. Here we compare the floating zone (FZ) crystal growth with radiation heating of compounds with R=Tb, Tm, Pr and Gd. The congruent melting behavior enabled moderate growth velocities of 3–5mmh−1. The composition of the crystals, except of Tb2PdSi3, is slightly Pd-depleted with respect to the nominal concentration 16.7at% Pd. Thin precipitates of RSi secondary phases were detected in the crystal matrix. Their phase fraction can be diminished by growth from appropriate feed rod compositions and/or annealing treatments. The compounds exhibit antiferromagnetic order below the Néel temperatures TN: 23.6K (Tb2PdSi3), 1.8K (Tm2PdSi3), 2.17K (Pr2PdSi3) and 22K (Gd2PdSi3) with different grades of magnetic anisotropy.

Magnetic structures of the anisotropic intermetallic compoundsEr2CoGa8andTm2CoGa8

Two members of the isostructural ${R}_{2}{\text{CoGa}}_{8}$ intermetallic series, ${\text{Er}}_{2}{\text{CoGa}}_{8}$ and ${\text{Tm}}_{2}{\text{CoGa}}_{8}$, have been studied by powder neutron diffraction. Antiferromagnetic ordering of the rare-earth sublattices was confirmed to occur at 3.0 K and 2.0 K, respectively. Furthermore, determination of the critical exponent showed ${\text{Er}}_{2}{\text{CoGa}}_{8}$ to adopt a three-dimensional universality class. In spite of a common magnetic easy axis and similar structural characteristics, the antiferromagnetic structures were found to be different for the erbium- and thulium-based compounds. The corresponding magnetic space groups were determined to be ${P}_{2a}mm{m}^{\ensuremath{'}}$ and ${P}_{C}mmm$. The difference in magnetic structures is discussed based on crystal electric field effects that are known to be prevalent in such materials.

Crystal-electric-field effects and quadrupole fluctuations inCe3Au3Sb4detected by Sb NQR

We report $^{121,123}$Sb NQR studies on single crystals of the narrow gap semiconductor Ce3Au3Sb4. The temperature dependence of the nuclear quadrupole frequency ($\nu_Q$), as well as the magnetic susceptibility, is well explained by crystal electric field effects. The nuclear spin-lattice relaxation rate ($T_1^{-1}$) of both $^{121}$Sb and $^{123}$Sb increases rapidly with decreasing temperature. The ratio of $T_1^{-1}$ for the two Sb isotopes is constant at high temperature but it decreases at low temperatures, indicating the important role of quadrupole fluctuations of the Ce ions. We propose that quadrupole fluctuations could be the origin of the large specific heat at low temperature via phonon-quadrupole moment coupling.

Anomalous magnetism and 209Bi nuclear spin relaxation in Bi4Ge3O12 crystals

The quadrupole 209Bi spin–spin and spin–lattice relaxation were studied within 4.2–300 K for pure and doped Bi4Ge3O12 single crystals which exhibit, as was previously found, anomalous magnetic properties. The results revealed an unexpectedly strong influence of minor amounts of paramagnetic dopants (0.015–0.5 mol.%) on the relaxation processes. Various mechanisms (quadrupole, crystal electric field, electron spin fluctuations) govern the spin–lattice relaxation time T 1 in pure and doped samples. Unlike T 1, the spin–spin relaxation time T 2 for pure and Nd-doped samples was weakly dependent on temperature within 4.2–300 K. Doping Bi4Ge3O12 with paramagnetic atoms strongly elongated T 2. The elongation, although not so strong, was also observed for pure and doped crystals under the influence of weak (∼30 Oe) external magnetic fields. To confirm the conclusion about strong influence of crystal field effects on the temperature dependence of T 1 in the temperature range 4.2–77 K, the magnetization vs. temperature and magnetic field was measured for Nd- and Gd-doped Bi4Ge3O12 crystals using a SQUID magnetometer. The temperature behavior of magnetic susceptibility for the Nd-doped crystal was consistent with the presence of the crystal electric field effects. For the Gd-doped crystal, the Brillouin formula perfectly fitted the curve of magnetization vs. magnetic field, which pointed to the absence of the crystal electric field contribution into the spin–lattice relaxation process in this sample.

Crystal growth of the Pr2PdSi3 intermetallic compound

Abstract Pr 2 PdSi 3 single crystals were grown by a vertical floating zone method with radiation heating at a zone traveling rate of 3 mm/h. The compound exhibits congruent melting behavior at a liquidus temperature of about 1770 °C. The slightly Pd-depleted composition of the crystal, with respect to the nominal Pr 2 PdSi 3 stoichiometry, led to gradual accumulation of Pd in the traveling zone and to a decreasing operating temperature during the growth process. Thin platelet-like precipitates of a PrSi phase were detected in the crystal matrix. Single crystalline samples exhibit a huge anisotropy due to the crystal electric field effect and order antiferromagnetically below the Neel temperature T N =2.17 K. The [1 1 0] orientation was identified as the magnetic easy axis at room temperature. At lower temperature (∼20 K) magnetic easy and hard axes interchange with each other. Two additional magnetic phase transitions are observed at temperatures below 1 K.

Nuclear quadrupole spin-lattice relaxation in Bi4Ge3O12 single crystals doped with atoms of d or f elements. Crystal field effects in compounds exhibiting anomalous magnetic properties

The nuclear quadrupole spin-lattice relaxation was studied in the range 4.2–300 K for single crystals of Bi4Ge3O12 doped with minor amounts (the tenth fractions of mol%) of paramagnetic atoms of Cr, Nd, and Gd. Unusual spin dynamic features were recently found for these crystals at room temperature: a dramatic (up to 8-fold) increase in the effective nuclear quadrupole spin-spin relaxation time T2* occurred upon doping the pure Bi4Ge3O12 sample. Unlike T2*, the effective spin-lattice relaxation time T1* at room temperature differs insignificantly for both doped and pure samples. But at lower temperatures, the samples exhibit considerably different behavior of the spin-lattice relaxation with temperature, which is caused by different contributions to the relaxation process of the dopant paramagnetic atoms. The distinctive maximum in the temperature dependence of the spin-lattice relaxation time for the Nd-doped crystal is shown to result from the crystal electric field effects.

Magnetic properties ofRFe2Zn20andRCo2Zn20(R=Y,Nd,Sm,Gd–Lu)

Magnetization, resistivity, and specific heat measurements were performed on solution-grown single crystals of $R{\text{Fe}}_{2}{\text{Zn}}_{20}$ and $R{\text{Co}}_{2}{\text{Zn}}_{20}$ $(R=\text{Y},\text{Nd},\text{Sm},\text{Gd-Lu})$. Whereas ${\text{LuCo}}_{2}{\text{Zn}}_{20}$ and ${\text{YCo}}_{2}{\text{Zn}}_{20}$ manifest unremarkable metallic behavior, ${\text{LuFe}}_{2}{\text{Zn}}_{20}$ and ${\text{YFe}}_{2}{\text{Zn}}_{20}$ display behaviors such as characteristic of nearly ferromagnetic Fermi liquids. When the well-defined $4f$ local moments $({\text{Gd}}^{3+}{\text{-Tm}}^{3+})$ are embedded into this strongly polarizable host, they manifest enhanced ferromagnetic ordering and the values of ${T}_{\text{C}}$ for $R{\text{Fe}}_{2}{\text{Zn}}_{20}$ $(R=\text{Gd-Tm})$ scale with the de Gennes factor. In addition, data on the $R{\text{Fe}}_{2}{\text{Zn}}_{20}$ compounds indicate a small crystal electric field (CEF) effect compared with the interaction energy scale. On the other hand, the local moment bearing members of $R{\text{Co}}_{2}{\text{Zn}}_{20}$ $(R=\text{Nd},\text{Sm},\text{Gd-Tm})$ manifest weak magnetic interactions and the magnetic properties for $R=\text{Dy-Tm}$ members are strongly influenced by the CEF effect on the $R$ ions. The magnetic anisotropy and specific heat data for the Co series were used to determine the CEF coefficient of $R$ ion with its cubic point symmetry. These CEF coefficients, determined for the Co series, are consistent with the magnetic anisotropy and specific heat data for the Fe series, which indicates similar CEF effects for the Fe and Co series. Such analysis, combined with specific heat and resistivity data, indicates that for $R=\text{Tb-Ho}$, the CEF splitting scale is smaller than their ${T}_{\text{C}}$ values, whereas for ${\text{ErFe}}_{2}{\text{Zn}}_{20}$ and ${\text{TmFe}}_{2}{\text{Zn}}_{20}$ the $4f$ electrons lose part of their full Hund's rule ground state degeneracy above ${T}_{\text{C}}$. ${\text{YbFe}}_{2}{\text{Zn}}_{20}$ and ${\text{YbCo}}_{2}{\text{Zn}}_{20}$ manifest typical but distinct heavy fermion behaviors associated with different Kondo temperatures.

Antiferromagnetic rare earth coupling in Pr 1− xTb xNi and Nd 1− xTb xNi single crystals

Magnetization and specific heat measurements were carried out on pseudo-binary Pr 1− x Tb x Ni ( x = 0.0, 0.1) and Nd 1− x Tb x Ni ( x = 0.0, 0.05, 0.1, 0.15) single crystals. The magnetic transition temperatures for Pr 1− x Tb x Ni increased with x, whereas those for Nd 1− x Tb x Ni decreased slightly as x increased. The effective magnetic moments, deduced from the temperature dependence of susceptibility, increased with x for both systems. The saturation magnetizations observed at the magnetically easy directions for all samples suggest that the magnetic coupling between the Pr (or Nd) and Tb moments is antiferromagnetic. A noticeable increase in magnetization in higher external field, onset at 5 T, is observed for Pr 0.9Tb 0.1Ni and indicates that there is a rearrangement of the magnetic moments. Calculations of the crystal electric field effect of Pr and Tb in Pr 1− x Tb x Ni were carried out using a simple point-charge model.