A famous example is high temperature superconductivity which is found in materials containing layers of Cu-O or Fe-As atoms. A fascinating layered material, from an electronic point of view, is strontium ruthenate, in which conducting layers of Ru-O are separated by Sr-O ‘spacers.’ By altering the balance of Ru-O to Sr-O layers, one can tune the material from superconductivity to ferromagnetism via metamagentism (a sudden rise of magnetisation over only a narrow range in field).
Sr3Ru2O7 is the bilayer and metamagnetic member of the strontium ruthenate family. It has been known for some time that at low temperatures (less than 1 K), the application of a large (8 T) magnetic field creates a new phase which only exists over a small field range. This phase shows a number of unusual properties, including anisotropic electrical resistance, but its exact microscopic nature has remained a mystery.
In an article published in Nature Materials, a team of researchers from Bristol, Birmingham and Kent Universities, UCL and ISIS have shown for the first time that the low temperature, high-field phase in Sr3Ru2O7 is a spin density wave. Using neutron diffraction on the WISH and LET instruments at the ISIS Facility, the team found a small magnetic moment which occurs over only a narrow range of fields near 8 T.
Pascal Manuel is the instrument scientist on WISH. He says, “Neutrons are a powerful tool to probe magnetic structures, especially when used in combination with high magnetic fields and ultra-low temperatures. The use of the new TS-2 instruments was crucial because they allow high resolution data to be taken and tiny magnetic moments to be observed, all this inside heavy duty sample environment."
By establishing that the low temperature phase of strontium ruthenate is a spin density wave, the team have solved a long-standing puzzle and have revealed possible applications for this material. For example, the state of the spin density wave could be controlled by rotating the sample with respect to the magnetic field, leading to large changes in the resistance of the material. The results have analogies with molecular systems, where phases with broken translation symmetry (e.g. liquid crystals, ice) exist over a range of pressure (instead of field) bounded by isotropic phases of higher symmetry.
Dr Lester adds, "While many of the gross features of the magnetically-ordered phase have been established with neutron measurements made at ISIS, more measurements are required to fully understand the physical origin of this state. Future experiments at ISIS will be designed to reveal the relevant interactions responsible for the field-induced magnetic order."
Pascal Manuel and Chris Lester
Research date: January 2015
Field-tunable spin-density-wave phases in Sr3Ru2O7, C. Lester, S. Ramos, R. S. Perry, T. P. Croft, R. I. Bewley, T. Guidi, P. Manuel, D. D. Khalyavin, E. M. Forgan & S. M. Hayden, Nature Materials (2015)