Magnetic phase transition in YbNi4Si. Point-contact spectroscopy of CEF in PrB6 and NdB6

2006/2007Mariana Vasiľová

Outlook:-magnetic phase transition in YbNi4Si: 1. introduction

2. results 3. conclusions

-point-contact spectroscopy of CEF in PrB6 and NdB6:1. introduction 2. preliminary results

Activities – 2006/2007: 1. conference - Strongly correlated electron systems ’07 in Houston, USA – “Magnetic phase transition in YbNi4Si” - poster

Czech and Slovak Conference on Magnetism CSMAG'07 - “Point-contact spectroscopy of CEF in PrB6 and NdB6“ – poster – following

2. papers - “Pressure and Field Effects on Spin Fluctuations in Ce0.8RE0.2Ni5 (RE = Pr,

Nd)” (Marián REIFFERS, Martin Della MEA, Ernst BAUER, Gabriel Pristáš and

Mariana VASIĽOVÁ ) – accepted in press – JPSJsuppl “Magnetic phase transition in YbNi4Si”

(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 PrB6 and NdB6”

(M. Vasiľová, G. Pristáš, M. Reiffers,

K.Flachbart and N.Shitsevalova) – - submitted - (CSMAG'07)

3. participation in experiment in GHMFL Grenoble – point-contact spectra measurementsof RB6 in high magnetic fields

4. home experiments – Point – contact measurements on TmB4, PrB6, NdB6

Magnetic phase transition in YbNi4Si

INTRODUCTION * the YbNi4Si compounds crystallize in the hexagonal CaCu5- type of structure, space group P6/mmm * previous results:

* transport and heat properties studies of the polycrystalline sample YbNi4Si 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 T2 dependence up to about 60 K [1]* the Yb2+ and Yb3+ peaks observed by XPS in the valence band region confirm the domination of the Yb3+ 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 ΘD, determined from both properties

fig. A. Temperature dependence of the heat capacity C(T) of YbNi4Si in the temperature range 4-300 K [1,2]

fig. B. Temperature dependence of the electrical resistivity ρ(T) of YbNi4Si. Inset: quadratic dependence on temperature in the low temperature range (4-60 K) [1]

fig. C. The X-ray photoemission valence band of YbNi4Si .Inset: the derivative of the intensity [1]

[1] A. Kowalczyk et al., Mater. Res. Bull. DOI:10.1016/j.materresbull.2007.02.041 and references therein [2] A. Kowalczyk et al., Solid State Commun 139 (2006) 5

EXPERIMENTAL * sample of YbNi4Si 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 = 4.820 Å and c = 3.996 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 YbNi4Si in applied magnetic fields up to 9 T * magnetic part of heat capacity of YbNi4Si obtained by subtracting the heat capacity of isomorphous compounds YNi4Si [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

0 2 4 6 8 10 120

2

4

6

B = 0 T B = 0,1 T B = 0,5 T B = 1 T B = 1,5 T B = 3 T B = 6 T B = 9 T

C(T

) [J

/mol

.K]

T [K]

fig.1 - low temperature part (up to 12 K) of the heat capacity of YbNi4Si 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

2 4 6 8 10 120,00

0,02

0,04

0,06

0,08

0 2 4 60,0

0,5

1,0

1,5

2,0 '

AC(T) [emu]

f = 100 Hz 0,25 mT

ac [

em

u]

T [K]

M [

eff/f

.u.]

H [T]

T=2K T=3.5K T=15K T=30K

fig.2 - temperature dependence of the ac susceptibility χ(T) of YbNi4Si - 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 2F7/2 multiplet by the crystal field - below 0.1 T M(H) dependence is linear

0 10 20 30 40 50 60 70

0

2

4

6

8

10

Cmag

(T)

Cm

ag(T

) [J

/mo

l.K

]

T [K]

0 50 100 150 200 250 3000,0

0,5

1,0

1,5

2,0

S/R ln 8

S(T

)/R

T(K)

fig.3 - the temperature dependence of magnetic contribution Cmag(T) to the heat capacity of YbNi4Si determined by subtraction of the heat capacity of YNi4Si (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(T2))

fig.4 - temperature dependence of the magnetic entropy S(T)/R determined from the magnetic contribution Cmag(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 TN = 2.7 K * TN 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(T2) )

Point-contact spectroscopy of CEF in PrB6 and NdB6

INTODUCTION:* PrB6 - the multiplet J = 4 splits into 4 crystalline electric field (CEF) levels – two triplets Γ5 and Γ4, a non-magnetic doublet Γ3 and

singlet Γ1 [1] - AF ordering at TN1

= 7 K and TN2 = 4.3 K [2, 3]

- ground state is triplet Γ5

* NdB6 - 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 TN 8 K [5] - ground state is quartet [1]* till now - only inelastic neutron scattering measurements - the information about CEF levels scheme above TN

* now - study of crystal field effects above and below TN 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 NdB6 - 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 PrB6 or NdB6 sample. Derivatives d2V/dI2(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

0 5 10 15 20

0

2

4

6

8

10

12

14

16

18

20

22

24

26

diffe

renc

e

temperature

0 5 10 15 20

0

5

10

15

20

25

30

35

40

diffe

renc

e

temperature

0,00 0,01 0,02 0,03 0,04-5

0

5

10

15

20

25

30

35

40

45

50

55

CEFCEF

Voltage [V]

d2V

/dI2

[a

.u.]

1,4 K 5,2 K 8,5 K

PrB6 - Pt, R =4,8

CEF phonons

0,00 0,02 0,040

20

40

60CEF

CEF

Voltage [V]

d2V

/dI2

[a

.u.]

NdB6 - Pt, R =6,3

1,5 K 4,2 K 9,1 K

CEF

phonons

PrB6 :TN1 = 7 K

TN2 = 4.3 K

NdB6 : TN 8 K

Figures – left column – point – contact spectra of NdB6 and PrB6

- 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.

thank you