Professor Paolo G. Radaelli OSI

Abstract:

The magnetostructural phase diagram of multiferroic TbMn2O 5 was investigated as a function of temperature and magnetic field by neutron diffraction. It was observed that dielectric and magnetic anomalies were associated with steps in the magnetic propagation vector, and in the structural parameters. The geometrically frustrated magnetic structure were found to be stabilized by "canted antiferroelectric" displacements of the Mn³⁺ ions. It was found that the Tb moments order ferromagnetically at low temperatures in an applied field, while the Mn magnetic structure is largely unchanged.

Abstract:

The experimental determination of the magnitude and momentum dependence of electron-phonon coupling (EPC) is an outstanding problem in condensed matter physics. The intensity of phonon peaks in Resonant Inelastic X-ray Scattering (RIXS) spectra can be related to the underlying EPC strength under significant approximations whose validity deserves careful verification. We measured the Cu L3 RIXS phonon intensity as a function of incident photon energy and momentum transfer in several layered cuprates. For CaCuO2, La2−xSrxCuO4+δ, and YBa2Cu3O6, using a generally accepted theoretical model, we quantitatively estimate the EPC for the bond-stretching mode along the high-symmetry directions (ζ,0) and (ζ,ζ), and as a function of the azimuthal angle φ at fixed q∥. We compare our results with theoretical predictions and find that the q∥-dependence of the phonon RIXS intensity can be largely ascribed to the phonon symmetry. However, a more satisfactory prediction of the experimental results requires an accurate description of the electronic structure close to the Fermi level. Our extensive investigation indicates that Cu L3 RIXS can reliably determine the momentum dependence of EPC for the bond-stretching modes of cuprates. Moreover, the large experimental basis provided here constitutes a stringent test for advanced theoretical predictions on the EPC.

Abstract:

Antiferromagnetic (AFM) materials provide a platform to couple altermagnetic (AM) spin-splitting with the magneto-optical Kerr effect (MOKE), offering potential for next-generation quantum technologies. In this work, first-principles calculations, symmetry analysis, and k·p modeling are employed to show that interlayer sliding in AFM multiferroic bilayers enables control of electronic, magnetic, and magneto-optical properties. This study reveals an intriguing dimension-driven AM crossover: the 2D paraelectric (PE) bilayer exhibits spin-degenerate bands protected by the [C₂∥M_c] spin-space symmetry, whereas the 3D counterpart manifests AM spin-splitting along k_z ≠ 0 paths. Furthermore, interlayer sliding breaks this M_c symmetry and stabilizes a ferroelectric (FE) state with compensated ferrimagnetism, where the Zeeman-like field is responsible for the nonrelativistic spin-splitting. In the FE phase, spin–orbit coupling (SOC) lifts accidental degeneracies and produces “alternating” spin-polarized bands through the interplay of Zeeman and Rashba effects. Crucially, spin polarization, ferrovalley polarization (ΔE_V), and the Kerr angle (θ_k) can all be reversed by switching either sliding ferroelectricity or the Néel vector. Our findings reveal the rich coupling among electronic, magnetic, and optical orders in sliding multiferroics, illustrating new prospects for ultralow-power spintronic and optoelectronic devices.

Abstract:

Antiferromagnetic materials are promising platforms for the development of ultrafast spintronics and magnonics due to their robust magnetism, high-frequency relativistic dynamics, low-loss transport, and the ability to support topological textures. However, achieving deterministic control over antiferromagnetic order in thin films is a major challenge due to the formation of multidomain states stabilized by competing magnetic and destressing interactions. Thus, the successful implementation of antiferromagnetic materials necessitates careful engineering of their anisotropy. Here, we demonstrate strain-based, robust control over multiple antiferromagnetic anisotropies and nanoscale domains in the promising spintronic candidate α-Fe2O3 at room temperature. By applying isotropic and anisotropic in-plane strains across a broad temperature–strain phase space, we systematically tune the interplay between magneto-crystalline and magneto-elastic interactions. We observe that strain-driven control steers the system toward an aligned antiferromagnetic state, while preserving topological spin textures, such as merons, antimerons, and bimerons. We directly map the nanoscale antiferromagnetic order using linear dichroic scanning transmission X-ray microscopy integrated with in situ strain and temperature control. A Landau model and micromagnetic simulations reveal how strain reshapes the magnetic energy landscape. These findings suggest that strain could serve as a versatile control mechanism to reconfigure equilibrium or dynamic antiferromagnetic states on demand in α-Fe2O3 for implementation in next-generation spintronic and magnonic devices.