Phys. Status Solidi B 250, No. 3, 522–524 (2013) / DOI 10.1002/pssb.201200846 p s sb

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Part of SQuantum Criticality and

basic solid state physics

Crystalline electric field splitting in

YbNi4P2 measured by inelastic neutron scattering

Zita Huesges*,1, Oliver Stockert1, Michael Marek Koza2, Cornelius Krellner1, Christoph Geibel1,and Frank Steglich1

1 Max Planck Institute for Chemical Physics of Solids, Nothnitzer Str. 40, 01187 Dresden, Germany2 Institut Laue Langevin, 6 rue Jules Horowitz, 38042 Grenoble, France

Received 12 October 2012, accepted 15 November 2012

Published online 21 January 2013

Keywords crystalline electric field, inelastic neutron scattering, YbNi4P2

* Corresponding author: e-mail zita.huesges@cpfs.mpg.de, Phone: þ49 351 4646 3219, Fax: þ49 351 4646 3232

The heavy fermion (HF) compound YbNi4P2 is a promising

new system for the study of ferromagnetic quantum criticality.

Its particular crystal structure in which the Yb ions form chains

along the c-axis might cause quasi-1D magnetic interactions. In

thermodynamic and transport measurements, highly aniso-

tropic behaviour has been observed. To clarify the role of the

crystalline electric field (CEF) on this anisotropy, we have

measured its level scheme by inelastic neutron scattering. We

observed a broad magnetic excitation feature between 7 and

13 meV that is composed of two or three CEF transitions. We

discuss the probability of an additional CEF level at either very

high or very low energy.

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1 Introduction Heavy fermion (HF) compounds con-tinue to be model systems for the study of quantumcriticality: Due to the Kondo screening, the 4f or 5f magneticmoments in such systems order only at very low tempera-tures. This continuous phase transition into magnetic ordercan be tuned to occur at T¼ 0, thus reaching a quantumcritical point (QCP) in the phase diagram, by variation of anon-thermal tuning parameter such as pressure, dopingor magnetic field. So far, mostly antiferromagnetic QCPshave been studied, due to the rareness of ferromagneticcompounds that can be tuned to quantum criticality [1, 2].Recently, Krellner et al. reported that YbNi4P2 has atransition into ferromagnetism at only 170 mK, which canbe suppressed to zero by arsenic doping on the phosphorussite [3]. The compound contains magnetic Yb3þ ions, whileNi is non-magnetic, and shows Kondo characteristics withTK� 8 K. Thus, it is one of the few 4f-based intermetalliccompounds that allow to study ferromagnetic quantumcriticality.

YbNi4P2 crystallises in the tetragonal ZrFe4Si2 structurethat has several remarkable features: the lattice parameter c isonly half as large as the lattice parameter a so that quasi 1Dchains of Yb ions are formed along the c-axis. Neighbouringchains are shifted by c/2, which causes Yb ions in one chainto experience a crystalline electric field (CEF) that is rotated

by 908with respect to the next chain. This structure leads to astrong anisotropy of the magnetic properties, as studies onYbNi4P2 single crystals have shown [4, 5].

Here, we present the measurement of electronictransitions between the CEF levels by inelastic neutronscattering. The knowledge of the CEF splitting shouldfacilitate the interpretation of the high-temperature magneticproperties of YbNi4P2. Furthermore, if a fit to thethermodynamic data allows to deduce the CEF parameters,the symmetry of the ground state can be identified, whichalso has implications for the ordered state [6].

2 Experimental details Measurements were per-formed at the thermal neutron time-of-flight spectrometerIN4 at ILL, Grenoble, with an incident neutron energy ofEi¼ 36 meV (li¼ 1.5 A). The measurable range of wave-vector transfer was about Q¼ 1.0–7.3 A�1 at the elasticline. Our 5.4 g powder sample was placed in a 3� 4 cmrectangular sample holder, whose signal was subtracted fromall data. We accounted for the self-absorption of the samplewith a Paalman–Pings correction [7] and determined theefficiency of each 3He detector by measuring a vanadiumreference. All scans were normalised to the monitor that wasplaced in the beam before the sample. An orange cryostatallowed cooling to a base temperature of 1.6 K. For the

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Phys. Status Solidi B 250, No. 3 (2013) 523

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subtraction of the phonon contribution, we have measuredthe non-magnetic reference LuCo4Ge2 that has the samecrystal structure as YbNi4P2.

Polycrystalline samples of YbNi4P2 may contain grainsof Ni and Ni3P due to the flux growth. In our sample, thecontent of this phase was about 3%, as determined bymagnetisation measurements on part of the powder sample.We believe that this should not affect the low-temperaturemagnetic signal since Ni3P is a Pauli paramagnet [8] and Niis a ferromagnet that orders already well above roomtemperature.

3 Results3.1 Magnetic excitations and phonons Figure 1

shows the measured scattering intensity as a function ofenergy and wavevector transfer at 1.6 K. Two main featurescan be distinguished in the inelastic regime: intensitymaxima due to magnetic excitations can be found atE� 5–15 meV, while intensity maxima due to phononexcitations are at E� 12–22 meV. The distinction can bemade on the basis of the Q dependence: The intensity of themagnetic excitations follows the magnetic form factor andtherefore decreases with increasing momentum transfer,while the phonons show an I/Q2 characteristic.

For a quantitative analysis of the magnetic signal asubtraction of the phonon contribution was done accordingto the procedure of Murani [9] with the non-magneticreference compound LuCo4Ge2. In Murani’s method,spectra at high momentum transfer (where only phononshave significant intensity) are subtracted from the lowmomentum transfer spectra (where magnetic excitations aredominant). The high-Q-spectra need to be scaled with thephonon intensity ratio Int(low-Q)/Int(high-Q) that is deter-mined with the non-magnetic reference. This procedureshould be reliable despite the large differences in element

Figure 1 (online colour at: www.pss-b.com) Colour plot of themeasured neutron scattering intensity of YbNi4P2 at 1.6 K as func-tion of energy and wavevector transfer. Blue indicates low intensityand red high intensity; the intensity in the elastic line is above thecutoff limit.

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masses between YbNi4P2 and LuCo4Ge2 since their spectraare not directly subtracted.

3.2 CEF levels The Yb3þ ion has the electronconfiguration [Xe]4f13 and a total angular momentum ofJ¼ 7/2 in the ground state according to Hund’s rules. Themeasured effective moment at high temperatures suggeststhat this model fits well in case of YbNi4P2 [3]. The eight-fold multiplet is expected to split into four Kramer’s doubletsdue to the low-symmetric crystalline environment around theytterbium ions. All Yb ions are located on the samecrystallographic position, even if the main in-plane axis isrotated by 908 between the corner and the body centred sites.Therefore one expects the same CEF scheme for all Yb ions,and accordingly three CEF transitions, provided they havemeasurable intensity.

For the evaluation of the CEF levels, all spectra withQ¼ 2.5� 0.5 A�1 are summed up. This choice allows both agood ratio of magnetic to phonon intensity and a large rangeof energy transfers. Subtraction of phonon intensity, asoutlined above, then yields the magnetic intensity. Thesubtraction is not meaningful at small energy transfers tillabout 4 meV due to the varying intensity along the elasticline. Figure 2 shows the data at the lowest measuredtemperature. It is now apparent that the broad feature atE� 5–15 meV is composed of at least two overlappingpeaks. However, it is difficult to decide whether there are twoor three transitions. A fit with two gaussians yields transitionenergies of 8.5� 0.5 and 12.5� 0.5 meV and an energywidth of 5.0 meV at 1.6 K. The width exceeds the energyresolution of the spectrometer by far, which might be a resultof Kondo broadening.

In the inset of Fig. 2, the temperature development of theCEF feature is shown. The overall intensity decreases withtemperature, as expected for transitions from the ground statedue to its thermal depopulation. Furthermore, there seems to

Figure 2 (online colour at: www.pss-b.com) Magnetic intensity ofYbNi4P2 at Q¼ 2.5� 0.5 A�1 and T¼ 1.6 K. A fit with two gaus-sians is also shown. In the inset, the same data are plotted togetherwith spectra taken at higher temperatures.

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be a slight shift to lower energies when the temperature isincreased from 1.6 to 20 K. Since the CEF level schemeshould be temperature independent, such an apparent shiftmight indicate a further transition at slightly lower energies,originating not from the ground state but from a low lyingexcited CEF level. Thermodynamic and transport dataindeed suggest that the lowest Kramer’s doublet is at 1–3 meV [4]. At 20 K, this level would already be sufficientlythermally populated to give rise to additional transitions. At100 K, it would be nearly equally populated as the groundstate, which could explain the plateau-like feature weobserve. Unfortunately, our setup does not allow a directobservation of a transition below 3 meV, as it cannot bedistinguished from elastic scattering.

We also confirmed in a measurement with an incidentneutron wave length of l¼ 1.1 A that no further transitionscan be observed up to 60 meV. Krellner and Geibel [4]suggest that the highest CEF level is below 40 meV.Therefore, it seems unlikely that a transition occurs atE> 60 meV.

4 Summary We have measured electronic excitationsof the ytterbium ion in the CEF of YbNi4P2. Two transitionscould be observed at approximately 8.5 and 12.5 meV, whilea third transition in the same energy range cannot beexcluded. However, in consideration of the thermodynamicdata it seems more likely that the third transition lies at

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1–3 meV, for which we find indirect evidence in an apparentshift of the high-energy transitions to lower energies.

Acknowledgements This work was supported by theDeutsche Forschungsgemeinschaft through the research unit 960‘Quantum phase transitions’.

References

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