Page 24 - ILL Annual Report 2019

MAGNETISM
Elsa Lhotel. French
Institut Néel, CNRS, Univ. Grenoble Alpes, France
‘I am a senior scientist working on frustrated magnetic materials using very low temperature magnetometry and neutron scattering.’
Spin decoupling under
a staggered field in the
Gd Ir O pyrochlore 2 2 7
High-intensity powder diffractometer D1B, disordered materials diffractometer D4, diffuse scattering diffractometer D7 and time-of-flight spectrometer IN6-SHARP Powder diffractometer G41 (LLB)
The complexity rooted in condensed matter nurtures the discovery of new states of matter, enriched by ingredients such as frustration. This is exemplified in pyrochlore compounds of the formula R2M2O7, with R3+ a rare-earth ion and M4+ a metal ion, both occupying a lattice made of corner-sharing tetrahedra. Depending on the rare-earth ion, different anisotropies of the magnetic moments and different types of magnetic interactions between them can be present, giving rise to a considerable variety of magnetic behaviours. The presence of
a magnetic metal ion such as Ir further expands the possible phases compared with the case of non-magnetic M, which is what we have shown in Gd2Ir2O7 [1].
AUTHORS
E. Lhotel and V. Simonet (Néel Institute, CNRS and Grenoble University UGA, France)
L. Mangin-Thro and H. Fischer (ILL)
ARTICLE FROM
Phys. Rev. B (2019)—doi: 10.1103/PhysRevB.99.060401
REFERENCES
ANNUAL REPORT 2019
[1] [2]
E. Lefrançois, L. Mangin-Thro, E. Lhotel, J. Robert, S. Petit,
V. Cathelin, H. E. Fischer, C.V. Colin, F. Damay, J. Ollivier,
P. Lejay, L.C. Chapon, V. Simonet and R. Ballou, Phys. Rev.
B Rapid Com. 99 (2019) 060401
E. Lefrançois, V. Cathelin, E. Lhotel, J. Robert, P. Lejay, C.V. Colin, B. Canals, F. Damay, J. Ollivier, B. Fak, L.C. Chapon, R. Ballou and V. Simonet, Nat. Commun. 8 (2017) 209
Indeed, the Ir sublattice orders magnetically at a temperature in the order of 100 K and then produces
a local magnetic field, along which the rare-earth magnetic moments align in an exotic, non-collinear order all magnetic moments pointing inward or outward from each tetrahedron (all-in/all-out order) (figure 1a). New phenomena arise when, at a much lower temperature (in the order of 1 K), the rare-earth magnetic moments interact with each other in a manner that competes with the Ir local field. This has led, for Ising moments in Ho compounds,
to the fragmentation of the magnetic moments (evidenced thanks to neutron scattering experiments at the ILL and LLB) into two equal components: one contributing to an ordered phase, the other to a fluctuating phase [2].
In the present case we focused on the rare-earth element Gd, which is the textbook example of an isotropic ion whose magnetic moment should orient indifferently in
any spatial direction. Naively, one might therefore
expect the Gd2Ir2O7 compound to order in an all-in/ all-out arrangement along the local field of the Ir and to remain in this state down to the lowest temperature, since it is compatible with the antiferromagnetic interactions between isotropic Gd magnetic moments. Surprisingly, by combining different neutron techniques we have shown that this is not the case.
Neutron diffraction measurements (on D1B and D4) also using polarised neutrons (on D7, at very low temperature using a dilution insert) showed that the Gd magnetic moment ordering occurs in two steps in two distinct temperature ranges (figure 2): the first corresponds to the ordering below 50 K of the all-in/all-out component of the magnetic moment along the local directions joining the centre to the corners of the tetrahedra (figure 1b);