Magnetic super-structure and active surface role in the onset of magnetic excitons revealed in TbCu₂ nanoparticles

Antiferromagnetic (AFM) materials, once considered technologically irrelevant, are now of significant interest due to their potential in spintronics and information technology. Unlike ferromagnetic materials, AFM materials exhibit complex magnetic arrangements and excitations, particularly at the nanoscale. This complexity presents both challenges and opportunities for technological applications. Two key aspects require further investigation: first, understanding the preservation of complex AFM structures in nanoparticle ensembles where reduced dimensionality and symmetry breaking can alter magnetic properties, and second, elucidating the energy level schemes and magnetic excitations in AFM materials, especially in the nanoscale regime, which is less explored compared to bulk single-crystal studies. This study addresses these challenges using TbCu₂ as a model system, examining both its bulk and nanoparticle (8 nm) forms to unveil its static and dynamic magnetic structures.

Previous research on TbCu₂ has established that bulk and magnetic nanoparticles (MNPs) retain a similar antiferromagnetic commensurate arrangement within their core. However, surface magnetic moments lead to a spin-glass-like state, resulting in a global superantiferromagnetic (SAF) arrangement. Existing literature lacks investigations into more complex AFM structures beyond the collinear-commensurate arrangement in TbCu₂, particularly in nanoparticle ensembles. While inelastic neutron scattering (INS) is a standard technique for studying single crystals, investigations on polycrystalline materials and, especially, nanoparticles are scarce, particularly for 4f AFM materials. Understanding how long-range correlations within an AFM super-structure are affected in the nanoparticle regime, alongside the energy level schemes and magnetic excitations, remains a significant knowledge gap that this research aims to address.

This study employed small-angle neutron scattering (SANS) and inelastic neutron scattering (INS) to investigate the static and dynamic magnetic properties of both bulk and 8 nm TbCu₂ nanoparticles. SANS measurements were performed at the SANS2D instrument at ISIS, using a wavelength range between 1.75 Å and 16.5 Å, temperatures between 5 and 285 K, and applied fields between 0 and 6 T. The data were analyzed to detect magnetic super-structures extending beyond the magnetic unit cell. INS measurements were conducted on the IN4 and IN6 spectrometers at the ILL, using various incident neutron energies and Q values to determine the crystalline electric field (CEF) energy levels and magnetic excitations. Measurements were performed on both Tb0.1Y0.9Cu₂ (a non-magnetically ordered alloy used to determine the CEF schemes) and TbCu₂, both in bulk and nanoparticle forms. Transmission electron microscopy (TEM) was used to characterize the size and morphology of the TbCu₂ nanoparticles. Heat capacity measurements were also conducted using the relaxation method.

SANS measurements revealed a magnetic super-structure in TbCu₂, consistent with a helix-like configuration, present in both bulk and 8 nm nanoparticles. This super-structure is characterized by an incommensurate propagation vector (approximately 1/12 in reduced wave vector units) and a turn angle of approximately 30 degrees. The intensity of the SANS peak associated with the super-structure was observed to be more robust along the parallel direction compared to the perpendicular direction. The super-structure showed robustness against size reduction; however, increased disorder in the nanoparticles led to a broader peak and reduced robustness against applied magnetic fields. INS measurements on Tb0.1Y0.9Cu₂ allowed for the determination of the CEF energy level scheme. Comparing the bulk and nanoparticle samples, a slight downward shift in energy levels was observed in the nanoparticles, reflecting the effects of surface broken symmetry. In the magnetically ordered region (T < TN), INS measurements revealed magnetic excitations. The nanoparticles showed a lack of energy shift in the magnetic excitations, indicating that a surface molecular field counteracts the CEF softening typically observed in nanoparticles. This suggests that the surface moments contribute significantly to the onset of magnetic excitons. Furthermore, analysis of peak intensities revealed a partial inhibition of transitions in the nanoparticles, indicating modifications in the RKKY exchange interactions driven by the surface modifications. The heat capacity measurements indicated the presence of additional CEF levels at higher energy values, beyond the experimental range accessible by INS.

The discovery of the helix-like magnetic super-structure in TbCu₂ extends our understanding of complex magnetic order in AFM materials. Its persistence in nanoparticles despite surface effects highlights potential for incorporating such structures in nanoscale devices. The INS findings demonstrate the active role of the nanoparticle surface in modifying both CEF and magnetic excitations. The surface molecular field counteracts the effects of size reduction and disorder, indicating a complex interplay between core and surface magnetic dynamics. The observed modifications in transition probabilities reflect altered magnetic interactions, further emphasizing the importance of surface effects in nanoscale AFM systems. These results shed light on the dual-spin dynamics (core and surface) of magnetic nanoparticles and advance understanding of the interplay between size reduction and magnetic interactions in nanoparticle ensembles.

This research successfully identified a helix-like magnetic super-structure in TbCu₂, robust to nanoscale dimensions. The study also provided a comprehensive model of the CEF energy levels and magnetic excitations, illustrating the significance of surface effects in modifying magnetic dynamics in nanoparticles. Future research could focus on verifying whether the observed excitations are magnons by improving energy level and dispersion curve definition, potentially using single crystals and polarized inelastic neutron scattering. This work provides a valuable experimental foundation for advancing the field of complex magnetic materials at the nanoscale and further understanding the role of surface effects in nanoscale magnetic phenomena.

The polycrystalline nature of the samples used in this study might have limited the resolution in some of the measurements, particularly the INS experiments. This could affect the precision in determining the precise energy levels and dispersion curves. Further studies using single crystals would likely enhance the accuracy of these measurements. Also, the limited size range of nanoparticles investigated may limit the generalizability of the findings to other sizes.