Coexistence of High Magnetization and Anisotropy with Non-monotonic Particle Size Effect in Ferromagnetic PrMnO₃ Nanoparticles

Abstract

Instances of the coexistence of high ferromagnetic magnetization with large anisotropy are scarce in the rare-earth manganite family. In manganites, high magnetizations are compromised with small coercivity and vice versa. Using nonaqueous sol−gel techniques, the undoped PrMnO 3 nanoparticles with oxygen nonstoichiometry were rendered with exceptional ferromagnetic character. While ∼40 nm sized nanoparticles had magnetization of 84 emu/g and coercivity of 885 Oe with 50 kOe sweeping field, the bulk 2 μm sized particles showed a magnetization of 51 emu/g albeit with a higher coercivity of 2000 Oe. These parameters are so far the highest among manganite systems with similarly sized particles. The competition between the ferromagnetic and antiferromagnetic phases both at the particle core and at the grain boundaries resulted in a non-monotonous trend of magnetic properties between 20, 40, and 2 μm particles. The sudden increase of coercivity toward lower temperatures was a result of the freezing of random spins at the surface of the strongly interacting nanoparticles which also increased the magnetic anisotropy. These results are of prime significance since the coexistence of such a large magnetization with high coercivity was rarely observed in pristine or doped manganites.

Spin Disorder and Particle Size Effects in Cobalt Ferrite Nanoparticles with Unidirectional Anisotropy and Permanent Magnet-like Characteristics

“…The nonmonotonic changes in magnetic parameters with NP size (Figure S4) are common in NPs depending on the material under study. 65 In C-1, the highest decrease of H eb from 414 Oe under 2 T to 46 Oe under 4 T applied field results from a substantial drop in surface disordered spins at 4 T, which strongly supports the origin of exchange bias from SG−FiM interactions. Because of the large magnetocrystalline anisotropy of this system, the real estimation of H eb might be tricky, which reflects the need of even larger cooling fields, beyond the scope of our current measurements.…”

“…When the cooling field is increased to 4 T, the loop shifts decrease, probably due to a competing force against the interfacial spin coupling. The nonmonotonic changes in magnetic parameters with NP size (Figure S4) are common in NPs depending on the material under study . In C-1, the highest decrease of H eb from 414 Oe under 2 T to 46 Oe under 4 T applied field results from a substantial drop in surface disordered spins at 4 T, which strongly supports the origin of exchange bias from SG–FiM interactions.…”