Microscopic insights of magnetism in Sm$ _{2} $NiMnO$ _{6} $ double perovskite

TL;DR

This study investigates the microscopic magnetic interactions in Sm$_{2}$NiMnO$_{6}$ double perovskite, revealing how anti-site disorders influence magnetic ordering and phase competition through neutron scattering and computational analysis.

Contribution

It provides detailed insight into the local and global magnetic structures of SNMO, highlighting the role of anti-site disorders in magnetic phase competition and ordering mechanisms.

Findings

Ni-Mn sublattice exhibits long-range ferromagnetic order below 160 K.

Antiparallel polarization of Sm moments observed near 35 K.

Anti-site disorders induce competing ferromagnetic and antiferromagnetic phases.

Abstract

The functional characteristics of double perovskites with unique ferromagnetic-insulator ground state have been controversial due to the unavoidable presence of anti-site disorders (ASDs). Here, we aim to investigate the origin of magnetic ordering on local and global scales in Sm$_{2}$NiMnO$_{6}$ (SNMO) double perovskite system. Different calcination routes are exploited to generate different cation arrangements in SNMO and the corresponding magnetic configurations are examined using the high energy (E $ \sim $0.3 eV) `hot neutrons', which has helped to overcome Sm absorption as well as to record total (Bragg's+diffuse) scattering profiles with high momentum transfer (Q$ _{max} \sim $24 angstrom$ ^{-1} $). We have observed that the Ni-Mn sublattice adopts long range collinear ferromagnetic $ F_{x}F_{z} $ structure with commensurate $k$=(0, 0, 0) propagation vector, below ordering temperature T $ \lesssim $ 160 K, irrespective of variable ASD concentrations. In addition, the signatures indicating the antiparallel polarization of Sm paramagnetic moments with respect to Ni-Mn network, are noticed in the vicinity of anomalous magnetic transitions at T $ \lesssim $ 35 K. The real space pair distribution function calculations have provided a direct visualization of ASDs by means of broadening in Ni/Mn-Mn/Ni linkage. Employing the Reverse Monte Carlo approach on diffuse magnetic scattering profiles, we have observed the negative spin-spin correlation function which suggests the Ni-Ni antiferromagnetic exchange interactions ranging up to first nearest neighbor distance. These results confirm that the existence of ASDs in cation ordered host matrix leads to competing ferromagnetic-antiferromagnetic phases in a broad temperature range, which quantitatively governs the temperature dependent bulk magnetic observables of SNMO system.