The divalent Eu-chalcogenides EuX (X = O, S, Se, Te) are well-known model substances for Heisenberg magnetism because of the spin-only J = S = 7/2 4f-moment of the Eu2+-ions and their simple NaCl structure. The variation of the magnetic ordering temperatures within the chemical series is described by the ferromagnetic J1 exchange between neighbouring Eu-ions and the mostly antiferromagnetic J2 exchange via the chalcogen ligands. High pressure studies of the EuX series with 151Eu-Mössbauer spectroscopy and other methods [1,2] have contributed important information on the magnetic interactions of EuX in their NaCl phases. Here we report on the first study of the magnetic properties of EuS and EuSe in the CsCl-type high-pressure phase at pressures up to 120 GPa using the 151Eu nuclear forward scattering (NFS) technique developed at the ESRF [3].

Fig. 10: 151Eu-NFS spectra of EuS at 120 GPa below and above TM = 295 K (note intensity in log scale).

Microgram samples of EuS and EuSe were pressurised in a special diamond anvil cell designed to fit into the cryomagnet at ID22N [3]; the NaCl to CsCl phase transitions occur around 20 and 15 GPa, respectively. For both samples we measured at each pressure a variety of NFS spectra at various temperatures (see Figure 10), from which we determined the magnetic ordering temperature TM and the saturation magnetic hyperfine field Bhf (T 0 K). Above TM we measured the isomer shift SIS using an additional reference absorber [3]. Additional NFS-studies in external magnetic fields revealed the ferromagnetic nature of TM as well as the sign of Bhf, similar to a previous study of EuTe at pressures up to 22 GPa [3]. The p-V relationship for EuS and EuSe was measured with energy-dispersive X-ray diffraction [4] to derive the interatomic distances used in Figure 11.

Fig. 11: Magnetic ordering temperature TM, magnetic hyperfine field Bhf and isomer shift SIS of EuS and EuSe (this study) together with EuTe [3] as a function of the Eu-Eu distance in the CsCl- and NaCl-structure.

The present data on TM, Bhf and SIS for EuS and EuSe in the CsCl-phase are compiled in Figure 11 together with the corresponding data in the NaCl phase. Within the NaCl structure a strong increase of the magnetic interactions is observed with decreasing Eu-Eu distance, reflected by the strong increase of TM and Bhf. In the CsCl structure, we observe a further dramatic increase of TM up to 295 K for EuS at 120 GPa and up to 300 K for EuSe at 77 GPa. The corresponding hyperfine fields Bhf exhibit a markedly different behaviour in the CsCl phase. One observes first much smaller and initially decreasing values of Bhf. We attribute this to an alteration of the J2 exchange, which contributes now, due to the different coordination and exchange paths, ferromagnetically (and therefore positively) to the negative values of Bhf[4].

Above 50 GPa for EuS and 60 GPa for EuSe, marked by vertical lines in Figure 11, a drastic change in the pressure dependent slope of SIS and Bhf was observed. The strong increase of SIS towards trivalent Eu indicate the onset of a mixed valence, more pronounced for EuS than for EuSe. The concomitantly observed increase of Bhf is attributed to admixtures of higher magnetic states with J = 1, 2 of the Eu3+ 4f6 (7FJ=0,1,2) multiplet to the wavefunction of the mixed-valent Eu-ions. Details of this mechanism are described in [4]. From the additional strong increase of the isomer shift we estimate the valence v’ of the Eu-ions in EuS at 120 GPa as v’ = 2.5(1) and in EuSe at 77 GPa as v’ = 2.2(1) [4]. Most interestingly, the variation of TM, Bhf and SIS seems to saturate for EuS at that high pressures, while for EuSe a further strong increase of TM well above 300 K is expected from the slopes of TM, Bhf and SIS.

This first study of CsCl-type EuS and EuSe yielded a wealth of information about the magnetic and electronic properties at these ultrahigh pressures. To the best of our knowledge there are no comparable studies of rare-earth systems in the 100 GPa range. The observation of the coexistence of strong ferromagnetic order and mixed valence in EuS in the CsCl-phase is completely different to the behaviour of EuO in the NaCl-phase [1], where a strong decrease of T M in conjunction with the onset of a mixed valence was observed. Therefore similar high-pressure 151Eu-NFS studies of EuO in the CsCl-phase are asked for.

 

References

[1] M.M. Abd-Elmeguid, R.D. Taylor, Phys. Rev. B 42, 1048 (1990).
[2] R. Lübbers, K. Rupprecht, G. Wortmann, Hyperfine Interactions 128, 115 (2000) and references therein.
[3] O. Leupold et al., Europhys. Lett. 35, 671 (1996); O. Leupold, K. Rupprecht, G. Wortmann, Structural Chemistry 14, 97 (2003).
[4] K. Rupprecht, PhD thesis, Paderborn (2004); K. Rupprecht et al., submitted.

Authors

K. Rupprecht (a), U. Ponkratz (a,b), O. Leupold (c), G. Wortmann (a)
(a) Department Physik, Universität of Paderborn (Germany)
(b) ESRF
(c) ESRF, now at HASYLAB, DESY, Hamburg (Germany)