Symmetry-breaking phenomena underpin various intriguing physical properties in condensed matter such as magnetism [1,2,3,4] and superconductivity [5,6]. Understanding such fundamental properties holds significant importance for the development of cutting-edge technologies, including spintronics, quantum computing, and advanced magnetic materials, as well as foster innovation in energy-related applications [7]. The crystalline solid NaCr₂O₄ represents an ideal example in this regard, as it exhibits fascinating low temperature electronic properties (i.e., dual itinerant and localized electronic nature, anomalous colossal magneto-resistance, exceptional coexistence of positive and negative charge transfer states, coexistence of antiferromagnetic (AFM) and ferromagnetic (FM) correlations in its magnetic ground state), but it also lends itself well as a study model for the development of Na-ion battery materials in a high temperature regime [8]. Therefore, NaCr₂O₄ is suitable for the realization of magnetoelectronic devices for data storage, magnetic sensors, as well as solid oxide fuel cells, and clarifying its magnetic structure and magnetic coupling mechanisms is the first step towards the optimization and employment of this material for such applications. Neutron diffraction is the only method that allows for direct determination of the spatial arrangement of magnetic moments and their orientations in the crystal lattice and, in this work [3], this technique has been employed to solve the complex magnetic structure of NaCr₂O₄. We identified a commensurate canted magnetic cell with unusually large value of the Cr moment, suggested to be originated by itinerant electrons, coexisting with an incommensurate cycloidal magnetic supercell, suggested to be originated by localized electrons. Indications of unconventional critical behavior for the commensurate magnetic phase transition in NaCr₂O₄ were also found. In particular, we suggest that the magnetic transition might be at the edge of a tricritical point with possible formation of a metamagnetic hidden phase.

Figure 1: a) Commensurate canted magnetic cell in NaCr₂O₄. b) Incommensurate cycloidal magnetic supercell in NaCr₂O₄. c) Low temperature neutron diffraction pattern of NaCr₂O₄. The magenta arrow indicates the magnetic Bragg peak. Figure adapted from ref. [3].

The neutron powder diffraction measurements were performed at the time-of-flight powder diffractometers iMATERIA and SPICA at the high intensity proton accelerator facility J-PARC, in Japan. The powder samples (m ≈ 0.72 g) were mounted into cylindrical vanadium cells with diameters 6 mm (for SPICA) and 5 mm (for iMATERIA). The cells, sealed using an aluminium cap, aluminium screws and an indium wire, were mounted on a closed cycle refrigerator to reach temperatures between 2 K and 300 K. SPICA and iMATERIA were designed to measure at a single scattering angle while reaching a wide range in reciprocal space using large position sensitive detector banks which could provide different d-ranges (Q-ranges) with gradually changing resolutions. More specifically, in SPICA the high angle detector bank provides a d-range from 0.3 Å up to 3.7 Å (resolution Dd/d = 0.12%), while the low angle (LA) detector bank has a d-range from 0.5 Åup to 11 Å. In iMATERIA the backscattering detector bank (BS), allows a d-range from 0.181 Å up to 5.09 Å (resolution Dd/d = 0.16%), while the LA detector banks (LA35 and LA15) allow to resolve d-ranges from 0.25 Å up to 40 Å. The high angle banks in SPICA and iMATERIA are therefore suitable for detailed structural characterization, while the LA banks were ideal for identifying any magnetic Bragg peak in the high-d (low-Q) range.

Now we aim to perform magnetic field dependent measurements to discern the effects of itinerant and localized electrons on the magnetic structure of NaCr₂O₄, and outline the magnetic H/T phase diagram in this material. The latter will allow to clarify the unconventional critical behavior of the system across the magnetic phase transition and unveal the putative hidden metamagnetic phase.

This work is part of the PhD thesis of E. Nocerino (KTH, SE). Material synthesis was carried out at the National Institute for Material Science (NIMS, JP). The experimental efforts were carried out in collaboration with Chalmers University of Technology (SE), Ibaraki University (JP), Babes-Bolyai University (RO), Linköping University (SE), TU Wien (AT), High Energy Accelerator Research Organization (JP), Comprehensive Research Organization for Science and Society (CROSS, JP).

Contact

Dr. Elisabetta Nocerino
Postdoctoral researcher
Stockholm University, Paul Scherrer Institute
elisabetta.nocerino@mmk.su.se
elisabetta.nocerino@psi.ch