Quentin FAURE – Université Paris-Saclay, CNRS, CEA, Laboratoire Léon Brillouin
Materials with strong spin-orbit coupling are recently receiving a great deal of attention thanks to their potential realization of novel electronic and magnetic phases . Compounds based on 4d transition metals –e.g. ruthenium- are playing an important role in this field, since they allow exploring fundamental concepts such as multiband superconductivity in Sr2RuO4 or quantum spin liquids achieving Kitaev’s physics in the honeycomb system α-RuCl3. In this context, the member of the so-called Ruddlesden–Popper family Ca3Ru2O7 aroused a lot of interest due to the simultaneous presence of a magnetic long-range order and of a polar crystal structure, allowing for the formation of non-trivial magnetic textures.
Ca3Ru2O7 belongs to the Bb21m space group and consists in bilayers of RuO6 octahedra, with can rotate and tilt around the  and  axes thanks to the relatively small size of the surrounding calcium atoms . This leads to a rich temperature-dependent physics [3,4] : Ca3Ru2O7 becomes antiferromagnetic with spins aligned along the anisotropy axis a (AFMa) below TN = 60 K but remains metallic. Upon further cooling, Ca3Ru2O7 undergoes a spin reorientation transition around TMI = 48 K, during which the anisotropy changes progressively from the a– to the b-axis; this transition being concomitant with a metal-insulator transition (MIT).
Figure 1 – Temperature evolution of the magnetic structure of Ca3Ru2O7 in zero-field around the spin reorientation transition: AFMa below TN (right), incommensurate cycloid (middle) and AFMb below TMI (left).
In a previous study, we have shown using resonant X-ray diffraction that an incommensurate cycloid is actually mediating this reorientation in a very narrow temperature range, i.e. between 49 and 47 K . Interestingly, this incommensurate magnetic structure was previously observed under an applied magnetic field or through chemical substitution of the magnetic ions [6,7] but never in pure samples and in zero-field, showing the extreme sensitivity of the electronic and magnetic properties to tiny changes of stoichiometry. In addition, it was shown recently that single crystals systematically suffer from twinning problems that can lead to ambiguities in the interpretation of experimental data .
Figure 2 – Field-temperature phase diagram of a detwinned Ca3Ru2O7 sample, with a magnetic field applied along the b-axis.
In our last paper*, we have therefore entirely revisited the phase diagram of Ca3Ru2O7 using neutron diffraction, magnetometry and magnetoresistance under a magnetic field applied along the b-axis on previously detwinned single crystals. We have then been able to confirm the cycloidal nature of the incommensurate magnetic structure, which is stabilized even at zero-field. These observations improve our understanding of the scenario leading to the spin reorientation transition in this fascinating material, although its microscopic mechanisms remain elusive to this day.
*Original paper : Q. Faure, C.D. Dashwood, C.V. Colin, R.D. Johnson, E. Ressouche, G.B.G. Stenning, J. Spratt, D.F. McMorrow and R.S. Perry, Phys. Rev. Research 5, 013040 (2023)
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 C. D. Dashwood et al., Phys. Rev. B 102, 180410(R) (2020)
 D. A. Sokolov et al., Nat. Phys. 15, 671 (2019)
 X. Ke et al., Phys. Rev. B 89, 220407(R) (2014)