Spin-up conversion, exchange-interactions, and tailored magnetic properties in core-shell La2NiMnO6 of small crystallites.

La2NiMnO6- a ferromagnetic (FM) insulator offers tunable charge carriers and spins useful to devise its multiple properties and applications. In this view, we studied a core-shell La2NiMnO6(2-3 nm shell on 65-80 nm core) of a Ni2+/Ni3+(d7) to Mn4+/Mn3+(d4) spin-up conversion - revived a new FM phase-2, raising a spin-density σs= 0.7 s/a over the Ni2+/Mn4+species (phase-1), σs= 0.5 s/a, i. e. 2.12 μB/f.u. larger spin moment. HRTEM images studied with X-ray diffraction characterizing core-shell structure that plays a crucial role in tuning the high spin FM phase-2 of profound properties. Below 110 K, the dc magnetization and ac magnetic susceptibility χ(ω, T) reveal a metastable magnetic behaviour on an antiferromagnetic canting of a spin-glass nature. The results follow a Vogel-Fulcher type relaxation with a relaxation time τ0∼ 10-13s, confirming a spin-glass freezing behavior. Uniquely, FM field of phase-1 controls magnetics of phase 2 of a coupled magnet, modulating joint features with small thermal-magnetic hysteresis on heating-cooling cycles.