Strain control of a bandwidth-driven spin reorientation in Ca3Ru2O7.

The layered-ruthenate family of materials possess an intricate interplay of structural, electronic and magnetic degrees of freedom that yields a plethora of delicately balanced ground states. This is exemplified by Ca3Ru2O7, which hosts a coupled transition in which the lattice parameters jump, the Fermi surface partially gaps and the spins undergo a 90∘ in-plane reorientation. Here, we show how the transition is driven by a lattice strain that tunes the electronic bandwidth. We apply uniaxial stress to single crystals of Ca3Ru2O7, using neutron and resonant x-ray scattering to simultaneously probe the structural and magnetic responses. These measurements demonstrate that the transition can be driven by externally induced strain, stimulating the development of a theoretical model in which an internal strain is generated self-consistently to lower the electronic energy. We understand the strain to act by modifying tilts and rotations of the RuO6 octahedra, which directly influences the nearest-neighbour hopping. Our results offer a blueprint for uncovering the driving force behind coupled phase transitions, as well as a route to controlling them.

Fragmentation in spin ice from magnetic charge injection.

The complexity embedded in condensed matter fertilizes the discovery of new states of matter, enriched by ingredients like frustration. Illustrating examples in magnetic systems are Kitaev spin liquids, skyrmions phases, or spin ices. These unconventional ground states support exotic excitations, for example the magnetic charges in spin ices, also called monopoles. Here, we propose a mechanism to inject monopoles in a spin ice at equilibrium through a staggered magnetic field. We show theoretically, and demonstrate experimentally in the Ho2Ir2O7 pyrochlore iridate, that it results in the stabilization of a monopole crystal, which exhibits magnetic fragmentation. In this new state of matter, the magnetic moment fragments into an ordered part and a persistently fluctuating one. Compared to conventional spin ices, the different nature of the excitations in this fragmented state opens the way to tunable field-induced and dynamical behaviors.Exploring unconventional magnetism facilities both fundamental understanding of materials and their real applications. Here the authors demonstrate that a magnetic monopole crystal is stabilized by a staggered magnetic field in the pyrochlore iridate Ho2Ir2O7, leading to a fragmented magnetization.

Phase transitions and interface phenomena in the cryogenic temperature domain of a niobate nanostructured ceramic.

Powder neutron diffraction and dielectric spectroscopy were used to investigate both crystallographic and dielectric permittivity properties of a Sr2KNb5O15 single phase ferroelectric oxide with nanosized grains ranging from 35 nm to 90 nm. Measurements were carried out in the temperature range from 10 K (cryogenic) to 550 K. All neutron diffraction data were indexed on the basis of a tetragonal double unit cell. From 10 K to room temperature the space group of the Sr2KNb5O15 ferroelectric phase was considered to be P4bm. The refinement of the paraelectric phase (at 550 K) was determined in the centrosymmetric space group P4/mbm. Dielectric spectroscopy measurements were performed in a thermal cycle. A set of four phase transitions non-related to symmetry changing was detected from Rietveld analysis of neutron powder diffraction data. During a thermal cycle, in the cryogenic temperature domain, strong thermal hysteresis is developed. Both phase transition and thermal hysteresis were correlated. These phenomena are associated with Nb-cation atomic displacements in the NbO6 octahedra along the c-axis direction and of the domain with different frequencies involving grains as well as an excess of interfaces ascribed to the grain boundary. The bulk/grain boundary interfaces in nanostructured ceramics are correlated with the thermal stability phenomenon.

Revealing the properties of Mn2Au for antiferromagnetic spintronics.

The continuous reduction in size of spintronic devices requires the development of structures, which are insensitive to parasitic external magnetic fields, while preserving the magnetoresistive signals of existing systems based on giant or tunnel magnetoresistance. This could be obtained in tunnel anisotropic magnetoresistance structures incorporating an antiferromagnetic, instead of a ferromagnetic, material. To turn this promising concept into real devices, new magnetic materials with large spin-orbit effects must be identified. Here we demonstrate that Mn2Au is not a Pauli paramagnet as hitherto believed but an antiferromagnet with Mn moments of ~4 µB. The particularly large strength of the exchange interactions leads to an extrapolated Néel temperature well above 1,000 K, so that ground-state magnetic properties are essentially preserved up to room temperature and above. Combined with the existence of a significant in-plane anisotropy, this makes Mn2Au the most promising material for antiferromagnetic spintronics identified so far.