Resonant X-ray scattering and applications to spin-induced ferroelectrics in van der Waals materials

Yi Tseng^1*, Connor A. Occhialini¹, Qian Song¹, Paolo Barone², Sahaj Patel¹, Meghna Shankar¹, Raul Acevedo-Esteves³, Jiarui Li¹, Christie Nelson³, Silvia Picozzi^4,5, Ronny Sutarto⁶, Riccardo Comin¹

¹Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
²Consiglio Nazionale delle Ricerche (CNR-SPIN), Rome, Italy
³National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York, USA
⁴Consiglio Nazionale delle Ricerche (CNR-SPIN), Chieti, Italy
⁵Department of Materials Science, University of Milan-Bicocca, Milan, Italy
⁶Canadian Light Source, Saskatoon, Saskatchewan, Canada

* Presenter:Yi Tseng, email:ytseng@nycu.edu.tw

The recent discovery of intrinsic magnetic order in atomically thin van der Waals (vdW) materials has created new opportunities for the study of collective spin phenomena in free-standing two-dimensional (2D) systems and nanoscale devices. In past years, substantial efforts have been made to achieve direct electrical control and manipulation of magnetic properties in two dimensions, but the mechanisms remain unclear. A more promising avenue towards realizing electrical control of 2D magnetism may be found in vdW materials with intrinsic type-II multiferroicity. In type-II multiferroics, the direct coupling between the magnetic and ferroelectric order parameters is enabled by the presence of a spin configuration lacking inversion symmetry, resulting in a large and robust magnetoelectric response

In this vein, type-II multiferroicity from non-collinear spin order is recently explored in the van der Waals material NiI₂. Despite the importance for improper ferroelectricity, the microscopic mechanism of the helimagnetic order remains poorly understood. Here, the magneto-structural phases of NiI₂ are investigated using resonant magnetic X-ray scattering (RXS) and X-ray diffraction (XRD). Two competing magnetic phases are identified. Below 60 K, an incommensurate magnetic reflection (q ~ [0.143, 0, 1.49] reciprocal lattice units) is observed which exhibits finite circular dichroism in RXS, signaling the inversion symmetry-breaking helimagnetic ground state. At elevated temperature, in the non-polar phase (60 K < T < 75 K), a distinct q ~ [0.087, 0.087, 1.5] magnetic order is observed, attributed to a collinear incommensurate (CI) state. The first-order CI-helix transition is concomitant with a structural transition characterized by a significant interlayer shear, which drives the helimagnetic ground state as evidenced by a mean-field Heisenberg model including interlayer exchange and its coupling to the structural distortion. These findings identify interlayer magneto-structural coupling as the key driver behind multiferroicity in NiI₂.

Keywords: spin-spiral order, multiferroics, resonant X-ray scattering