Cerium containing rare-earth compounds are especially interesting because of their variable electronic structure, due to the ground state of Ce3+, which is a Kramers doublet. The energy of the inner 4f level is nearly the same as that of the outer (valence) electrons, and only small amounts of energy are required to change the relative occupancy of these electronic levels. This gives rise to “dual valency states”, which can be apparent when a volume change of about 10 percent occurs upon the application of high pressures or low temperatures. It is not surprising, therefore, that CeP, a monopnictide compound (CeX, where X=P, As, Sb, Bi) with the NaCl-type structure, exhibits unusual transport and magnetic properties. When cooled to temperatures below 10 K in a magnetic field lower than 0.5 Tesla, CeP shows a simple anti-ferromagnetic phase (AFM), with ferromagnetic planes coupled antiferromagnetically along the cubic axis (see Figure 118a). At higher magnetic fields the ordering of the Ce magnetic moments changes dramatically, with a long period magnetic structure in which ferromagnetically coupled Ce-ion double layers with a large magnetic moment of about 2 µB co-exist with weakly coupled magnetic Ce layers, as shown in Figure 118a.

Fig. 118: a) Proposed magnetic ordering as function of temperature and magnetic field. b) Magnetic phase diagram of CeP determined by resonant X-ray diffraction (square dots).


These magnetic modulations, coupled with interlayer distance variations, are strongly sensitive to the magnetic field and high pressure conditions. This has been determined in numerous neutron and X-ray scattering experiments, carried out to find the magnetic and structural modulations as a function of the magnetic field and high pressure [1-2]. The unusual magnetic properties have been ascribed to the strong magnetic polaron effect, produced by the combination of localisation of the low-density carriers and mixing effects between Ce ions and the p state of the neighbouring pnictogens (p-f mixing).

High magnetic field resonant magnetic X-ray diffraction experiments were performed at ID20 using the new superconducting magnet station to investigate the CeP low temperature magnetic phase diagram up to 10 Tesla.

Using the Ce L3 edge resonance, the magnetic interlayer ordering of the Ce can be studied in great detail, as illustrated in Figure 118b. Different phases are represented in different colours. Phase I (yellow) and phase II (green) are characterised by two-sublattice magnetisations, where two adjacent Ce ferromagnetic (F) layers carry the largest magnetic moment (8 ground state). The (F) layers are separated by a sequence of weak antiferromagnetic (AF) Ce planes (7 ground state) with propagation vector 2/11 and 2/9, respectively. The sample can therefore be regarded as an exchange bias system, but at the nanoscale. The odd number of weak AF layers is due to the interlayer coupling between F and weak-AF planes. At higher temperatures and higher magnetic field these weak-AF planes became paramagnetic, and the so-called ferro-paramagnetic phase (FP) occurs. The FP phase consists of a stacking of F planes aligned along the magnetic field and paramagnetic layers (phase III).

Fig. 119: Observation of phase boundary crossings. The phase II (=4/9) present at low temperature disappears after a short coexistence with the phase III (=4/8). Further increasing of the temperature makes more favourable a modulation =4/9 (T~15 K) and finally of (=4/10) around 25 K.


Direct microscopic proof of the existence of these different FP phases was needed above 5T. New phase boundaries (full squares) were found by scanning the reciprocal space as a function of both temperature and magnetic field. They correspond to the magnetic phase transition determined previously by magnetoresistance measurements [3], and they appear in quite a systematic way as a function of applied magnetic field (2/10, 2/9, 2/8, …). In other words, the F and P atomic planes distances can be continuously modulated by a magnetic field.

Figure 119 illustrates the smooth change in the modulation of the Ce planes as a function of applied magnetic field, which reduces the number of planes separating the ferromagnetic ones. It would be interesting to follow this progressive reduction up to the full ferromagnetic ordering of the Ce moments. To achieve this, high fields up to 60 Tesla will be needed.



[1] M. Kohgi, K. Iwasa, K. Kuwahara, A. Hannan, D. Kawana, Y. Noda, T. Shobu, K. Katsumata, Y. Narumi and Y. Tabata, Physica B 345, 55 (2004).
[2] A. Hannan, D. Kawana, K. Kuwahara, M. Kohgi, Y. Narumi, Y. Tabata, S. Shimomura, Y. Tanaka and K. Katsumata, J. Phys. Soc. Jpn. 74, 2301 (2005).
[3]T. Terashima, S. Uji, H. Aoki, J.A.A.J. Perenboom, Y. Haga, A. Uesawa, T. Suzuki, S. Hill and J.S. Brooks, Phys. Rev. B 58, 309 (1998).


V. Scagnoli (a), L. Paolasini (a), M. Kohgi (b), K. Kuwahara (c), K. Iwasa (d).
(a) ESRF
(b) Tokyo University of Fisheries (Japan)
(c) Tokyo Metropolitan University (Japan)
(d) University of Tohoku, Sendai (Japan)