Observation of surface magnons and crystalline electric field shifts in superantiferromagneticNdCu2nanoparticles

Abstract

An ensemble of superantiferromagnetic ${\mathrm{NdCu}}_{2}$ nanoparticles has been produced to perform a detailed analysis of magnetic excitations using inelastic neutron scattering. Neutron diffraction measurements indicate a mean nanoparticle size of $\ensuremath{\langle}D\ensuremath{\rangle}\ensuremath{\approx}13$ nm, where the bulk commensurate antiferromagnetic structure is retained at the nanoparticle core. Magnetic measurements evidence the interaction among the magnetic moments located at the nanoparticle surface to be strong enough to establish a spin glass behavior. Specific heat analyses show a broad Schottky contribution, revealing the existence of a crystalline electric field. Inelastic neutron scattering analyses disclose that the splitting of the crystalline electric field levels associated with the ${\mathrm{Nd}}^{3+}$ ions, as well as the spin-wave excitations that emerged below the N\'eel transition $({T}_{\mathit{\text{N}}}\ensuremath{\approx}6 \mathrm{K})$ in polycrystalline ${\mathrm{NdCu}}_{2}$ are maintained in the nanoparticle state. We have been able to isolate the scattering contribution arising from the nanoparticle surface where both crystalline electric field splitting and the collective magnetic excitations are well-defined despite the symmetry breaking. Quantitative analyses of this surface scattering reveal that finite-size effects and microstrain lead to a partial inhibition of the transitions from the ground state to the first excited level, as well as a positive shift $(\ensuremath{\sim}15%)$ of the energy associated to collective magnon excitations.