Magnetoresistance and magnetic ordering in praseodymium and neodymium hexaborides

The magnetoresistance Δρ/ρ of single-crystal samples of praseodymium and neodymium hexaborides (PrB₆ and NdB₆) has been measured at temperatures ranging from 2 to 20 K in a magnetic field of up to 80 kOe. The results obtained have revealed a crossover of the regime from a small negative magnetoresistance in the paramagnetic state to a large positive magnetoresistive effect in magnetically ordered phases of the PrB₆ and NdB₆ compounds. An analysis of the dependences Δρ(H)/ρ has made it possible to separate three contributions to the magnetoresistance for the compounds under investigation. In addition to the main negative contribution, which is quadratic in the magnetic field (−Δρ/ρ ∝ H ²), a linear positive contribution (Δρ/ρ ∝ H) and a nonlinear ferromagnetic contribution have been found. Upon transition to a magnetically ordered state, the linear positive component in the magnetoresistance of the PrB₆ and NdB₆ compounds becomes dominant, whereas the quadratic contribution to the negative magnetoresistance is completely suppressed in the commensurate magnetic phase of these compounds. The presence of several components in the magnetoresistance has been explained by assuming that, in the antiferromagnetic phases of PrB₆ and NdB₆, ferromagnetic nanoregions (ferrons) are formed in the 5d band in the vicinity of the rareearth ions. The origin of the quadratic contribution to the negative magnetoresistance is interpreted in terms of the Yosida model, which takes into account scattering of conduction electrons by localized magnetic moments of rare-earth ions. Within the approach used, the local magnetic susceptibility χ_loc has been estimated. It has been demonstrated that, in the temperature range T _N < T < 20 K, the behavior of the local magnetic susceptibility χ_loc for the compounds under investigation can be described with good accuracy by the Curie-Weiss dependence χ_loc ∝ (T − Θ _p )⁻¹.     

Low temperature thermoelectric power of LaB6, PrB6 and NdB6

We have measured the thermoelectric power of LaB₆, PrB₆ and NdB₆ in the temperature range 2–20K. PrB₆ and NdB₆ order antiferromagnetically at T_N = 6.99K and 7.74K respectively, whereas LaB₆ is non-magnetic. The thermoelectric power data have a negative maximum near 5.5K in all three hexaborides. This negative peak is thought to be mainly due to phonon drag in LaB₆. In PrB₆ and NdB₆ there is an additional contribution due to magnon drag. It is expected that the high temperature thermoelectric power is mainly due to an electron diffusion term and a contribution due to spin disorder scattering. The thermoelectric power and the resistivity have been compared in the vicinity of T_N. A qualitative similarity in the temperature derivatives of these two quantities has been found for PrB₆ and NdB₆ near T_N.     

Multiple phase transitions in rare earth tetraborides at low temperature

We report the temperature dependence of the magnetic susceptibility of single crystals of PrB₄, GdB₄, TbB₄, HoB₄ and TmB₄, both parallel and perpendicular to the tetragonal c-axis. We also present low temperature resistance measurements on crytals of GdB₄ through TmB₄. Two magnetic phase transitions are found for TbB₄, DyB₄, HoB₄ and TmB₄. For the latter two compounds, the lower transitions appear to be first order. For HoB₄, we have measured the low temperature specific heat. The lower transition in TbB₄ and HoB₄ is rapidly depressed upon dilution with YB₄.     

de Haas-van Alphen effect in the superconducting state of YNi2B2C

To examine the Fermi surfaces of the recently discovered quaternery compounds RENi₂B₂C measurements of the de Haas-van Alphen (dHvA) effect were made on YNi₂B₂C single crystals in magnetic fields up to 12 T. ForBc we observe two dHvA frequenciesF ₁=0.499 kT andF ₂=6.933 kT which corresponds to approximately 1,5% and 21% of the Brillouin zone cross section area. Both frequencies could be observed deep in the vortex state of the type-II superconductor, the lower dHvA frequency down to 2 T corresponding to roughly 1/5 of the upper critical fieldB _c2 which was found to be 10.6 T in resistivity measurements. The field dependent quasiparticle damping in the superconducting state is very weak, since the amplitude of the dHvA oscillations seems to be unaffected by the phase transition atB _c2.     

De Haas-van Alphen effect in the organic superconductor κ-(BEDT-TTF)2I3

In the two-dimensional organic superconductor -(BEDT-TTF)₂I₃ de Haas-van Alphen oscillations were observed at magnetic fields above 5 T and temperatures between 0.4 and 2 K. We found two dHvA frequencies at 3.846 kT and 0.570 kT, which correspond to the cross-sectional areas of the Fermi surface expected from a tight-binding calculation. From the temperature dependence of the oscillation amplitudes the effective mass belonging to the larger orbit was found to be 3.80m_o. Precise measurements of the angular dependence of the dHvA frequency show no deviation from that expected for a cylindrical Fermi surface. The angular dependence of the amplitude including spin splitting zeroes can essentially be described by a two-dimensional Fermi surface. Certain systematical deviations, however, hint for a slight corrugation.     

de Haas-van Alphen effect and the Fermi surface of PrNi5

The Fermi surface of PrNi₅ has been studied by the measurements of the de Haas-van Alphen (dHvA) effect at temperatures between 0.3 and in magnetic fields up to 12 T. Two dHvA frequencies have been obtained. The electronic structure of PrNi₅ was calculated using the full potential linearized augmented plane wave method. Five sheets of the Fermi surface and the multiple extremal cross sections were determined. First and second sheet have a hole-like structure. The agreement between theory and experiment is obtained by a small downward shift ( 0.1 eV) of the Fermi energy which is probably due to an underestimation of the role of 4 f electrons. Received 9 May 2000 and Received in final form 20 September 2000     

Quantum oscillations and the Sermi surface of LuB12

To examine the Fermi surface of LuB₁₂, measurements of the de Haas-van Alphen (dHvA) effect were made at temperatures between 0.35 and 2 K in magnetic fields up to 12 Tesla. Oscillations in the susceptibility occurred above 5 Tesla in any field direction relative to the single crystal sample. From the Fourier transform of the data obtained, we conclude the Fermi surface of both conduction bands to have multiple extremal cross sections. For some of these orbits, the temperature dependence of the dHvA signal was investigated to determine the corresponding cyclotron mass. For a better understanding, a Full Potential Linearized Augmented Plane Wave-(FLAPW-) band structure calculation was carried out and the shapes of the Fermi surfaces were determined. In addition, we investigated the transverse magnetoresistance as a function of the field and the field direction. Its anisotropy, as well as the Shubnikov-de Haas (SdH) oscillations occurring in certain geometries, are in agreement with the results of the dHvA measurements.     

Fermi surface measurement of ZrB2 by the de Haas-van Alphen effect

The de Haas-van Alphen (dHvA) effect in ZrB₂ single crystal has been studied using the field modulation technique. Observed dHvA frequencies in the (10$\text{1}$0), (11$\text{2}$0) and (0001) planes range from the order of 10⁶-10⁷ G. The dHvA frequencies, together with a recent APW band structure calculation, show that the Fermi surface of ZrB₂ consists of a set of ring-like electron surfaces around the K points on which the nearly ellipsoidal arms are joined together, and a wrinkled dumbbell-like hole surface at the A point. Estimation of the carrier concentration indicates that ZrB₂ is a semi-metal with 0.04 electrons (holes) per unit cell.     

Crystalline-electric-field level scheme of NdB6

We have determined the crystalline-electric-field (CEF) level scheme of NdB₆ by means of Raman spectroscopy. We could resolve two CEF-transitions at 92 cm^–1 and 98 cm^–1 due to the spectral resolution of 2 cm^–1 (=0.25 meV). The temperature dependent Raman scattering reveals the two transitions to be on top of one another, in agreement with fits of the temperature dependence of the magnetic susceptibility. The spectroscopically determined CEF level scheme yields a good fit of the elastic constants and hence the mean field prediction of a ferroquadrupolar ordering near 6 K. This result sheds new light on the phase transition in NdB₆ nearT _N =8.6 K which has so far been interpreted only in terms of antiferromagnetic ordering.Work supported by Deutsche Forschungsgemeinschaft, SFB 125Work supported by Deutsche Forschungsgemeinschaft, SFB 126     

Electronic structure and magnetism in transition metal compounds: VNi3, MnPd3 and MoPd3

The electronic structures of VNi₃, MnPd₃ and MoPd₃ are studied in the paramagnetic phase using the method of orthogonalized linear combination of atomic orbitals. The Stoner criterion is applied to predict magnetic ordering in these compounds. VNi₃ has a very low density of states at the Fermi level and therefore does not satisfy the Stoner criterion for magnetic ordering. This is in agreement with the magnetic susceptibility data. Magnetic ordering predicted for MnPd₃ is in agreement with the observed antiferromagnetic phase transition in this compound. The Stoner criterion is not satisfied for MoPd₃ and no experimental data are available for the ordered phase.     

Specific features of the formation of the ground state in PrB6

The temperature dependences of the thermal conductivity and magnetization of a PrB₆ single crystal in magnetic fields of 0–14 T have been measured. An analysis of the data amassed has revealed that, apart from the well-known magnetic phase transitions at T ～ 7 and ～4 K, there appears a spontaneous magnetization with a relatively small magnetic moment in PrB₆ at temperatures below 20 K, which is initiated by the splitting of the ground state, apparently, due to dynamic structure distortions.     

Detailed study of the de Haas-van Alphen effect in the Shubnikov state of LuNi2B2C

We present de Haas-van Alphen (dHvA) measurements in the normal and in the superconducting state of LuNi₂B₂C. Inside the superconducting state, we observe quantum oscillations of a spherical Fermi-surface sheet in all crystallographic directions. Apart from the field region close to the phase transition where a strong peak effect hampers the analysis of the dHvA signal, the additional damping of the quantum oscillations inside the superconducting state is much smaller than expected from theory. For the magnetic field aligned along the [100] direction, three different dHvA frequencies are visible in the superconducting state. In particular, the orbit related to a cushion-like Fermi surface does not show any additional damping at and below the upper critical field contrary to theoretical expectations of simple effective one-band theories. Consequently, the superconducting gap on this Fermi-surface sheet can only evolve at lower fields than the observed bulk critical field, B _c2 ≈ 8 T, which clearly points to a Fermi-surface-sheet-dependent gap opening in LuNi₂B₂C.     

Thermionic emission properties of hexaborides

Thermionic emission properties of the single crystal hexaborides LaB₆, CeB₆, PrB₆, NdB₆, SmB₆, EuB₆, (La, Sr)B₆, (La, Ba)B₆, (La, Ce)B₆, (La, Pr)B₆, (La, Sm)B₆, and (La, Dy)B₆ are measured in the temperature range between 1250 and 1700°C. Of these, LaB₆ is shown to have the highest emission current density in the temperature range investigated. The LaB₆-based mixed hexaborides, (La, M)B₆, show current densities similar to LaB₆, but a little lower. Analyses by Auger electron spectroscopy indicate that the surface composition of (La, M)B₆ approaches that of LaB₆ at elevated temperatures and that the thickness of the surface layer whose composition is different from that of the bulk is typically several atomic layers. The formation of the surface layer is considered to be caused by a relatively slow evaporation rate of La compared to that of the other metal.     

Doping-dependent electronic structure of cuprates studied using angle-scanned photoemission

Full k -maps of the electronic structure near the Fermi level of differently doped cuprates measured with angle-scanned photoelectron spectroscopy are presented. The valence band maximum of the antiferromagnetic insulator Sr₂CuO₂Cl₂, which is taken as a representative of an undoped cuprate, and the Fermi surfaces of overdoped, optimally doped and underdoped Bi₂Sr₂CaCu₂O_8+δ high-temperature superconductors are mapped in the normal state. The results confirm the existence of large Luttinger Fermi surfaces at high doping with a Fermi surface volume proportional to (1+x), where x is the hole concentration. At very low doping, however, we find that this assumption based on Luttinger's theorem is not fulfilled. This implies a change in the topology of the Fermi surface. Furthermore the intensity of the shadow bands observed on the Fermi surface of Bi₂Sr₂CaCu₂O_8+δ as a function of the doping is discussed. Received 12 October 1999 and Received in final form 12 April 2000     

Low temperature phase transitions and magnetic structure of PrB6

Specific heat, thermal expansion and electrical resistivity measurements on PrB₆ single crystals show that there are two low temperature phase transitions at 6.9 K and 4.2 K, respectively, the latter temperature varying somewhat among different crystals. Neutron diffraction measurements were made on both single and polycrystalline samples of PrB₆. The neutron data indicate a spontaneous incommensurate magnetic ordering at 6.9 K with Q = (0.23, 0.23, 0.5) 2π/a_o. At 4.2 K a commensurate magnetic phase is seen with Q = (0.25, 0.25, 0.5) 2π/a_o coexisting with the incommensurate phase At 1.74 K, only the commensurate phase remains. A model is proposed for the commensurate antiferromagnetic structure and a profile analysis based on that model yields a magnetic moment of 1.77 Bohr magnetons per praseodymium ion at 1.74 K.     

Magnetic structure of NdB6

Neutron diffraction measurements were made on a polycrystalline sample of NdB₆ at 78 and 4.2 K. This material orders antiferromagnetically with a Neel temperature of 8.6 K. The magnetic structure inferred from the diffraction data indicates a simple antiferromagnetic doubling of the unit cell in one direction. Based on this structure a profile analysis of the data gives a magnetic moment at 4.2 K of 1.74 Bohr magnetons per neodymium ion.     

Magnetic phase transitions in layered intermetallic compounds

Magnetic, magnetoelastic, and magnetotransport properties have been studied for the RMn₂Si₂ and RMn₆Sn₆ (R is a rare earth metal) intermetallic compounds with natural layered structure. The compounds exhibit wide variety of magnetic structures and magnetic phase transitions. Substitution of different R atoms allows us to modify the interatomic distances and interlayer exchange interactions thus providing the transition from antiferromagnetic to ferromagnetic state. Near the boundary of this transition the magnetic structures are very sensitive to the external field, temperature and pressure. The field-induced transitions are accompanied by considerable change in the sample size and resistivity. It has been shown that various magnetic structures and magnetic phase transitions observed in the layered compounds arise as a result of competition of the Mn–Mn and Mn–R exchange interactions.     

Nearly antiferromagnetic Fermi liquids: a progress report

I describe recent theoretical and experimental progress in understanding the physical properties of the two dimensional nearly antiferromagnetic Fermi liquids (NAFL's) found in the normal state of the cuprate superconductors. In such NAFL's, the magnetic interaction between planar quasiparticles is strong and peaked at or near the commensurate wave vector, Q ≡ (fy fy). For the optimally doped and underdoped systems, the resulting strong antiferromagnetic correlations produce three distinct magnetic phases in the normal state: mean field above T_cr, pseudoscaling between T_cr and T_*, and pseudogap below T_*. I present arguments which suggest that the physical origin of the pseudogap found in the quasiparticle spectrum below Tcr is the formation of a precursor to a spin-densitywave- state, describe the calculations based on this scenario of the dynamical spin susceptibility, Fermi surface evolution, transport, and Hall effect, and summarize the experimental evidence in its support.     

A DISCRETE VARIATIONAL METHOD STUDY ON THE ELECTRONIC STRUCTURES OF CaMnO3 AND LaMnO3

The electronic structures of the perovskite oxides, LaMnO₃ and CaMnO₃, with various magnetic structures are studied using the first-principles discrete variational (DV) cluster method based on ab initio local-spin-density approximation (LSDA). The ground states of different magnetic phases (including ferromagnetic (FM), A-type antiferromagnetic (A-AFM), and G-type antiferromagnetic (G-AFM)) have been described in this work. The cubic CaMnO₃ with observed G-AFM magnetic order is found to have a 0.1 eV calculated gap. Both FM CaMnO₃ and LaMnO₃ have "half-metallic" character, which is in agreement with other works. LaMnO₃ with both A-type and G-type antiferromagnetic order have metallic band structures. Part of Jahn-Teller (JT) distortion (Q₂ type) has been taken into consideration for A-AFM LaMnO₃. Under Q₂ type JT distortion, the occupied and unoccupied states of O 2p and Mn 3d states move farther away from the Fermi energy. It is also found that the distortion can further stabilize the structure. The density of states and the binding energy of the distorted A-AFM LaMnO₃ are given in this paper.     

Negative chemical pressure effects induced by Y substitution for Ca on the ‘exotic’ magnetic behavior of the spin-chain compound, Ca3Co2O6

The magnetic behavior of a solid solution, Ca_{3 x} Y_x CO₂ O₆, based on the ‘exotic’ spin-chain compound, Ca₃Co₂O₆, crystallizing in K₄CdCl₆-derived rhombohedral structure is investigated. Among the compositions investigated(x = 0.0, 0.3, 0.5, 0.75 and 1.0), single-phase formation persists up tox = 0.75, with the elongation of the c-axis. The present investigations reveal that the temperature at which the ‘so-called’ ‘partially disordered antiferromagnetic structure’ sets in (which occurs at 24 K for the parent compound,x = 0.0) undergoes gradual reduction with the substitution of Y for Ca, attaining the value of about 2.2 K for the nominalx = 1.0. The trend observed in this characteristic temperature is opposite to that reported under external pressure, thereby establishing that Y substitution exerts negative chemical pressure. Anomalous steps observed in the isothermal magnetization at very low temperatures (around 2 K) forx = 0.0, which have been proposed to arise from ‘quantum tunneling effects’ are found to vanish by a small substitution (x = 0.3) of Y for Ca. Systematics in AC and DC magnetic susceptibility behavior with Y substitution for Ca have also been probed. We believe that the present results involving the expansion of chain length without disrupting the magnetic chain may be useful to the overall understanding of the novel magnetism of the parent compound.