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 σ
s = 0.7 s a−
1 over the Ni2+/Mn4+ species (phase-1), σ
s = 0.5 s a−
1, 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 behavior on an antiferromagnetic canting of a spin-glass nature. The results follow a Vogel–Fulcher type relaxation with a relaxation time τ
0 ∼ 10−13 s, 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.
La2NiMnO6 is a unique compound of multiferroics that exhibits two magnetic transitions of Curie points TC1 and TC2 below room temperature on a partial conversion of charge carriers, Ni2+ → Ni3+ and Mn4+→ Mn3+, stabilized with O2- vacancies in distorted octagons in a double perovskite of a spin cluster (glass), wherein the spins freezing at a critical Tg point at lower temperatures. The peak temperature and irreversibility temperature shifted to the lower temperature. The dc magnetization M (T) measurements indicate random ferromagnetic and antiferromagnetic interactions and multiple magnetic transitions. The critical slowing properties signify the spin-glass nature proved by the thermomagnetic relaxation measure.
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