Stress-induced magnetic ordering in single crystal praseodymium

HAL Id: jpa-00218881

https://hal.archives-ouvertes.fr/jpa-00218881

Submitted on 1 Jan 1979

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Stress-induced magnetic ordering in single crystal praseodymium

K. Mcewen, C. Vettier, W. Stirling

To cite this version:

K. Mcewen, C. Vettier, W. Stirling. Stress-induced magnetic ordering in single crystal praseodymium.

Journal de Physique Colloques, 1979, 40 (C5), pp.C5-26-C5-27. �10.1051/jphyscol:1979509�. �jpa-

00218881�

JOURNAL DE PHYSIQUE Colloque C5, supplkment au no 5, Tome 40, Mai 1979, page C5-26

Stress-induced magnetic ordering in single crystal praseodymium

K. A. McEwen (I), C. Vettier (2) ( 3 ) and W. G . Stirling (2)

( I )

Department of Physics, University of Salford, Salford M5 4WT, UK

(') Institut Laue-Langevin, 38042 Grenoble, France

(3) Laboratoire Louis-Ntel, CNRS, 38042 Grenoble, France

RBsume. - L'application de contrainte uniaxiale le long de l'axe a induit un ordre magnttique du praskodyme monocristallin. A 800 bar, on observe une structure modulee parallklement a I'axe b au-dessous de TN=7,5 K.

Les excitations magnktiques de plus basse energie manifestent un comportement du type mode mou lorsqu'on applique une contrainte.

Abstract. - Magnetic ordering in single crystal Pr was induced by the application of uniaxial stress along an a-direction. At 800 bar a longitudinally modulated structure along the b-direction was detected below T N = 7.5 K.

The lowest energy magnetic exciton exhibits a clear soft-mode behaviour with increasing stress.

Dhcp Pr is a particularly interesting singlet ground state system since the coupling between the ions on the hexagonal sites is close to the critical value for long-range magnetic ordering' [l]. Previous neutron diffraction experiments [2, 31 have yielded very different results for polycrystals and single crystals.

Whilst polycrystalline Pr orders antiferromagnetically (on the hexagonal sites) below 25 K [2], there is no evidence of magnetic ordering in single crystal Pr down to 0.4 K [3].

The magnetic excitations propagating on the hexa- gonal sites have been investigated by Houmann et al. [4], who found that the lowest energy mode lies along the Brillouin zone direction T M and has a pronounced minimum at a wavevector of 0:25 A-'.

We have examined the stress dependence of these excitations, and investigated Jensen's prediction [5]

that magnetic ordering might be induced in single crystal Pr by the application of a uniaxial stress.

A crystal of size 8 x 8 x 5 mm was prepared from distilled Pr and heid with an a-direction [1Z10]

vertical in a uniaxial stress assembly, mounted on the triple axis spectrometer IN2 at the Institut Laue- Langevin.

Using neutrons with k, = 1.55 A-l, high-resolu- tion elastic scattering scans parallel to the b-direction [lo101 were made through the (0003) reciprocal lattice point at various temperatures and pressures.

Some of the results obtained at 5.2 K are shown in figure 1. At zero applied stress, very weak, broad scattering was observed, as found earlier [3]. On applying a stress of 600 bar, intense magnetic satel- lites appeared around (+ Q, 0, T Q, 3) with Q = 0.127 (0.252 A - I ) . Satellites were also found around the (0001) reciprocal lattice point but not around (1070). These observations imply that the

WAVEVECTOR f

Fig. 1. - Stress dependence of the ( ~ 0 s 3 ) satellite reflection observed in Pr. The zero stress data have been multiplied x 10 for display. The direction of the elastic scans is also indicated.

p = 0, A p = 600, p = 800 bar.

ordered magnetic moments are confined to the basal plane and form a sinusoidally modulated structure along the b-direction. All these satellites disappeared when the stress on the sample was removed. Increasing the stress to 800 bar produced a doubling of the satellite intensity at 5.2 K (see figure 1). From the temperature dependence of the intensity of this reflection, the Ntel temperature was deduced [6] as T, = 7.5 K at 800 bar : the saturation moment was 0.5 p,/ion.

Neutron inelastic scattering studies revealed the dispersion of the magnetic excitations in the ordered

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979509

STRESS-INDUCED MAGNETIC ORDERING IN SINGLE CRYSTAL Pr C5-27

phase at 5.2 K. As indicated by the zero stress results [4] shown in figure 2, the dispersion relation has four branches at a general wavevector along T M . In the present experiment, we have studied the stress dependence of the two lower modes. These correspond to transverse optic (TO) and longitu- dinal optic (LO) magnetic excitons propagating on the hexagonal sites. With the progressive application of stress, the energy of the LO mode was substan- tially reduced whilst the energy of the TO mode increased. The softening of the LO mode with stress is greatest near the ordering wavevector. It is not

WAVEVECTOR M A

Fig. 2. - Dispersion of the magnetic excitons propagating on the hexagonal sites in Pr at5.2 K : p = 800 bar, k , = 2.662 A- ' ^;

p = 800 bar, k , = 1.55 A - ' ; p = 0, k , = 2.662 k l . The symbol * denotes the satellite position. The dashed lines represent p = 0 data at 6.4 K of Houmann et al. [4].

yet clear whether this mode becomes totally soft at T N . In high-resolution scans below TN an excitation at 0.08 f 0.05 THz could just be resolved from the tail of the satellite peak about zero energy transfer.

The dispersion of the excitations was examined again after removal of the stress. Results in good agreement with those of Houmann et al. [4] were obtained.

The magnetic field dependence of the crystal field levels of the hexagonal site ions has recently been considered by Jensen [5] in a calculation including a coupling between the spin and lattice systems. The dominant coupling is to the y-strains (describing a basal plane distortion of the type Ex, - En) : from the field dependence of the low temperature elastic constants the coupling parameter was deduced as B,, = 20 meV [7]. Using this value to calculate the variation of the crystal field levels with applied uniaxial stress, a N6el temperature of 7.8 K is cal- culated for a stress of 800 bar along an a-direction, in good agreement with our experimental measure- ments.

The stresses of 600 and 800 bar produced a-direction strains of 730 x l o u 6 and 970 x respectively.

The thermal contraction of Pr is extremely aniso- tropic : ( a l l c - ale) is

K- ' at 300 K [8] and increases to - K - ' at 20 K [9]. Cooling poly- crystalline Pr therefore generates large internal strains which may be expected to induce magnetic ordering at a characteristic temperature.

References

[I] For a recent review, see Ch. 6 in Handbook of the Physics and Chemistry of Rare Earths, ed. K . A. Gschneidner (North- Holland), 1979.

[2] CABLE, J. W., MOON, R. M., KOEHLER, W. C. and WOLLAN, E. O., Phys. Rev. Lett. 12 (1964) 553.

[3] LEBECH, B., MCEWEN, K. A. and LINDG~RD, P. A., J. Phys. C : Solid State Phys. 8 (1975) 1684.

[4] HOUMANN, J. G., CHAPPELIER, M., MACKINTOSH, A. R., BAK, P., MCMASTERS, 0. D. and GSCHNEIDNER, K. A., Phys.

Rev. Lett. 34 (1975) 587.

[5] JENSEN, J., J. Phys. C : Solid State Phys. 9 (1976) 111 and private communications.

[6] MCEWEN, K. A., STIRLING, W. G. and VETTIER, C., Phys. Rev.

Lett. 41 (1978) 343.

[7] JENSEN, J. and PALMER, S. B., J. Phys. C : Solid State Phys.

11 (1978) 2465.

[8] SPEDDING, F. H., HANAK, J. J. and DAANE, A. H., J . Less- Common Metals 3 (1961) 110.

[9] O n , H. R., Solid State Commun. 16 (1975) 1355.