Magnetic phase transition in YbNi 4 Si. Point-contact spectroscopy of CEF in PrB 6 and NdB /2007 Mariana Vasiľová Outlook: -magnetic phase transition in YbNi 4 Si: 1. introduction 2. results 3. conclusions -point-contact spectroscopy of CEF in PrB 6 and NdB 6 :1. introduction 2. preliminary results

Activities – 2006/2007: 1. conference - Strongly correlated electron systems ’07 in Houston, USA – “Magnetic phase transition in YbNi 4 Si” - poster Czech and Slovak Conference on Magnetism CSMAG'07 - “Point-contact spectroscopy of CEF in PrB 6 and NdB 6 “ – poster – following 2. papers - “Pressure and Field Effects on Spin Fluctuations in Ce 0.8 RE 0.2 Ni 5 (RE = Pr, Nd)” ( Marián REIFFERS, Martin Della MEA, Ernst BAUER, Gabriel Pristáš and Mariana VASIĽOVÁ ) – accepted in press – JPSJsuppl “Magnetic phase transition in YbNi 4 Si” ( Mariana Vasiľová, Marián Reiffers, Andrzej Kowalczyk, Michał Falkowski, Tomasz Toliński, Milan Timko, Josef Šebek, Eva Šantavá) – accepted – - Physica B “Point-contact spectroscopy of CEF in PrB 6 and NdB 6 ” ( M. Vasiľová, G. Pristáš, M. Reiffers, K.Flachbart and N.Shitsevalova) – - submitted - (CSMAG'07) 3. participation in experiment in GHMFL Grenoble – point-contact spectra measurements of RB 6 in high magnetic fields 4. home experiments – Point – contact measurements on TmB 4, PrB 6, NdB 6

Magnetic phase transition in YbNi 4 Si INTRODUCTION * the YbNi 4 Si compounds crystallize in the hexagonal CaCu 5 - type of structure, space group P6/mmm * previous results: * transport and heat properties studies of the polycrystalline sample YbNi 4 Si showed no sign of magnetic transition in the temperature range 4 – 300 K [1, 2] – fig. A, B * the temperature dependence of the electrical resistivity ρ(T) shows standard Fermi-liquid behavior with T 2 dependence up to about 60 K [1] * the Yb 2+ and Yb 3+ peaks observed by XPS in the valence band region confirm the domination of the Yb 3+ valence state [1] – fig.C * the standard fit of the temperature dependence of the specific heat C(T) in zero magnetic field yielded γ = 25 mJ.mol -1.K -2 [1] * substantial difference in Debye temperature Θ D, determined from both properties fig. A. Temperature dependence of the heat capacity C(T) of YbNi 4 Si in the temperature range K [1,2] fig. B. Temperature dependence of the electrical resistivity ρ(T) of YbNi 4 Si. Inset: quadratic dependence on temperature in the low temperature range (4-60 K) [1] fig. C. The X-ray photoemission valence band of YbNi 4 Si.Inset: the derivative of the intensity [1] [1] A. Kowalczyk et al., Mater. Res. Bull. DOI: /j.materresbull and references therein [2] A. Kowalczyk et al., Solid State Commun 139 (2006) 5

EXPERIMENTAL * sample of YbNi 4 Si was prepared by the induction melting of stoichiometric amounts of the constituent elements in a water-cooled boat under an argon atmosphere, the ingots were inverted and remelted for several times to ensure homogeneity [2] * the crystal structure was established by a powder X-ray diffraction technique. The lattice constants are a = Å and c = Å * heat capacity measurements were performed by commercial device PPMS (Quantum Design) using the two-t model of the relaxation method in zero field and in applied magnetic fields up to 9 T. * AC and magnetization measurements were performed by commercial device MPMS (Quantum Design) we present: * first observation of magnetic phase transition determined by low temperature study of C(T) of YbNi 4 Si in applied magnetic fields up to 9 T * magnetic part of heat capacity of YbNi 4 Si obtained by subtracting the heat capacity of isomorphous compounds YNi 4 Si [to be published] with no magnetic contribution using the method in [4] * magnetic entropy using the method in [4] and determined magnetic part of heat capacity * AC susceptibility measurements of phase transition [2] A. Kowalczyk et al., Solid State Commun 139 (2006) 5 [4] S. Kunii et al., J. Sol. St. Chem. 154 (2000), p. 275

fig.1 - low temperature part (up to 12 K) of the heat capacity of YbNi 4 Si in the applied magnetic fields of up to 9 T - first observation of sharp peak with a maximum at 2.7 K in zero field – ascribed to the transition into a magnetically ordered phase - strong influence of magnetic field - decreasing maximum intensity with increasing magnetic field (at 9 T – only small shoulder) - AF character of the transition – pointed by the small shift of the maximum to the lower temperature at least for the B = 0.1T - splitting of peak and shift of the new peak maximum to higher temperatures with increasing field fig.2 - temperature dependence of the ac susceptibility χ(T) of YbNi 4 Si - presence of strong maximum of AF character at the same temperature 2.7 K - inset - magnetization curves for fields up to 8 T and temperatures from 2 K to 30 K - nonlinear curvature is observed - increasingly pronounced at lowest temperatures with a tendency to the saturation to the value of 2  B - the saturation moment value of 2  B ( reduced from the saturation moment 4.54  B expected for free Yb +3 ) - result of the splitting of the 2F 7/2 multiplet by the crystal field - below 0.1 T M(H) dependence is linear

fig.3 - the temperature dependence of magnetic contribution C mag (T) to the heat capacity of YbNi 4 Si determined by subtraction of the heat capacity of YNi 4 Si (nonmagnetic and isomorphous compound containing only the electronic and phonon contribution) - tendency to heavy fermion-like behavior - γ = 352 mJ/mol.K -2 (when taking into account the lowest temperatures of C/T(T 2 )) fig.4 - temperature dependence of the magnetic entropy S(T)/R determined from the magnetic contribution C mag (T) - at high temperatures the ratio S(T)/R is reaching a value of 1.9 (agreement with theoretically expected value ln 8 (J = 7/2))

CONCLUSIONS * heat capacity maximum is associated with the antiferromagnetic ordering at T N = 2.7 K * T N is suppressed by applying magnetic fields * the degeneracy of the ground-state doublet is lifted by Zeeman splitting * the separation between remaining singlets rises with growing field yielding a shift of the associated Schottky anomaly to higher temperatures * tendency to heavy fermion-like behavior - γ = 352 mJ/mol.K -2 (when taking into account the lowest temperatures of C/T(T 2 ) )

Point-contact spectroscopy of CEF in PrB 6 and NdB 6 INTODUCTION: * PrB 6 - the multiplet J = 4 splits into 4 crystalline electric field (CEF) levels – two triplets Γ 5 and Γ 4, a non-magnetic doublet Γ 3 and singlet Γ 1 [1] - AF ordering at T N 1 = 7 K and T N 2 = 4.3 K [2, 3] - ground state is triplet Γ 5 * NdB 6 - complicated magnetic transport properties, namely the anomalous large variation of the Hall coefficient in the neighborhood of the critical temperature [4] - A-type collinear antiferromagnetic structure below T N  8 K [5] - ground state is quartet [1] * till now - only inelastic neutron scattering measurements - the information about CEF levels scheme above T N * now - study of crystal field effects above and below T N using PC spectroscopy (energy spectrum of quasiparticles like phonons, magnons, CEF excitations by injection of conduction electrons through a metallic PC) EXPERIMENTAL: * PC spectroscopy experiments have been performed on the cubic intermetallic compounds PrB 6 and NdB 6 - single crystals with crystallographic orientation [110] * samples were prepared by floating zone method * point contacts were made at liquid helium temperatures by bringing a Cu or Pt needle (heterocontacts arrangements) in touch with the surface of the PrB 6 or NdB 6 sample. Derivatives d 2 V/dI 2 (eV) and dV/dI(eV) of the I-V characteristics were measured using a standard PC technique [5] in the temperature range 1.5 – 10 K. Magnetic field was applied along PC axis. * measurements in the magnetic fields up to 20 T were performed in GHMFL Grenoble

PrB 6 :T N 1 = 7 K T N 2 = 4.3 K NdB 6 : T N  8 K Figures – left column – point – contact spectra of NdB 6 and PrB 6 - right column – crystal field splitting On a provisional basis observed energy peaks are in agreement with theoretical calculations. Behaviour in magnetic field is under treatment.

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