Competing ferro- and antiferromagnetic exchange drives shape-selective $$\hbox{Co}_3\hbox{O}_4$$ nanomagnetism

Abstract

We have synthesized three different shapes of $$\hbox{Co}_{3}\hbox{O}_{4}$$ Co 3 O 4 nanoparticles to investigate the relationships between the surface Co$$^{2+}$$ 2 + and Co$$^{3+}$$ 3 + bonding quantified by exploiting the known exposed surface planes, terminations, and coordiations of $$\hbox{Co}_{3}\hbox{O}_{4}$$ Co 3 O 4 nanoparticle spheres, cubes and plates. Subsequently this information is related to the unusual behaviour observed in the magnetism. The competition of exchange interactions at the surface provides the mechanism for different behaviours in the shapes. The cubes display weakened antiferromagnetic interactions in the form of a spin-flop that occurs at the surface, while the plates show distinct ferromagnetic behaviour due to the strong competition between the interactions. We elucidate the spin properties which are highly sensitive to bonding and crystal field environments. This work provides a new window into the mechanisms behind surface magnetism.

Intervening Oxygen Enabled Magnetic Moment Modulation in Spinel Nanostructures

“…The Co L 3,2 -edge XAS spectrum of Co 3 O 4 is quite different from CoFe 2 O 4 , suggesting that the occupancy of Co is significantly different in normal and inverse spinels. We also presented the Co L 3,2 -edge XMCD of CoFe 2 O 4 (Figure b); on the other hand, XMCD was not observed for Co 3 O 4 because magnetism is driven by competing exchange interactions at the surface and the nanocrystallites studied here are of irregular shapes. Figure c,d shows the measured Fe L 3,2 -edge XAS and XMCD of Fe 3 O 4 and CoFe 2 O 4 , and the results are consistent with the spinel crystal structure. ,,, The Fe L 2,3 -edges are similar and in agreement with Mössbauer spectroscopy (see Figures S4 and S5) that Fe 3 O 4 and CoFe 2 O 4 are typical of ferrites.…”

Photothermal Catalytic Reduction of CO2 by Cobalt Silicate Heterojunction Constructed from Clay Minerals

“…Obviously, the binding energy of cobalt is closer to the former, indicating that the product is Co2SiO4, rather than Co3O4, which is consistent with the results of the XRD analysis. In addition, the energy separation between Co 2p1/2 and Co 2p3/2 of the product is 15.96 eV, which is close to Co2SiO4 (ΔCo 2p = 15.5 eV) [21]. Figure 4c shows the O 1s spectra of Co3O4, Co2SiO4, and Co2SiO4/Co3O4−x.…”

Probing spin waves in Co3O4 nanoparticles for magnonics applications

“…The strength of the interactions is rather similar because values of θ for nanoparticles are close 110 K, previously reported for the bulk Co 3 O 4 . 16,45…”

“…The strength of the interactions is rather similar because values of θ for nanoparticles are close 110 K, previously reported for the bulk Co 3 O 4 . 16,45 The magnetic properties of cubic and spherical nanoparticles are distinctively different at low temperatures. ZFC magnetization of the spherical nanoparticles peaks at ∼25 K, while FC magnetization gradually increases and saturates below 25 K. Other researchers have observed similar DC magnetization dependence in Co 3 O 4 nanoparticles, attributed to the coexistence of AFM-ordered core and FM-like surface shell of uncompensated spins with a freezing temperature of 25 K. [46][47][48] The alternative explanation of this transition is a spin glass-like phase of the surface magnetic moments.…”