Observation of surface magnons and crystalline electric field shifts in superantiferromagneticNdCu2nanoparticles

Abstract

An ensemble of superantiferromagnetic 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 D ≈ 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 Nd 3+ ions, as well as the spin-wave excitations that emerged below the Néel transition (T N ≈ 6 K) in polycrystalline 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 (∼15%) of the energy associated to collective magnon excitations.

Magnetic order and disorder environments in superantiferromagnetic $$\hbox {NdCu}_{\mathbf{2}}$$ nanoparticles

“… 56 . For the MNPs, Rietveld refinements indicated a mean nanoparticle size of and 13.0(5) nm for 2h and 5h-milled MNPs; together with microstrain values of and , respectively 39 . These values agree with the ones obtained from the nuclear magnetic structure, where nm and 12.6(6) nm, respectively, as with the ones pointed from TEM characterization (see Supplementary Fig.…”

“…The nonmagnetic neutron diffraction (ND) (above ) and the X-ray diffraction patterns have been discussed previously (Ref. 39 ). This study showed that the bulk orthorhombic Imma structure is maintained also at the MNPs, with lattice parameters close to those of the bulk alloy reported in Ref.…”

“…The indicated values of the MNP sizes inserted in Fig. 1 c and e, 18 nm and 13 nm, respectively, have been obtained by means of Rietveld refinements of the pattern in the paramagnetic region at K 39 .…”

“…Generally, it is found that the symmetry environment and the coordination number of the magnetic moments located within the nanoparticle core remain essentially the same as the ones found in the bulk counterpart 36 . For the surface spins, however, the situation is significantly different due to the existence of a non-negligible lattice distortion (strain) and reduced coordination numbers 37 – 39 . As a result, the magnetic interactions among surface spins will be changed and even frustrated, yielding disordered spin arrangements in the vicinity of the surface.…”

“…At K a reorientation of the magnetic moments takes place, and an incommensurate structure is present up to . Concerning MNPs, these have evidenced a superantiferromagnetic (SAF) structure, where (at K) the core magnetic moments exhibit the commensurate square-up modulation of bulk 22 , 39 , 44 . By contrast, the magnetic moments located at the surface feature a collective freezing mechanism, following a SG dynamics.…”