Role of defects on the magnetic behaviour of the geometrically frustrated spinel ZnFe2O4

Abstract

Defective zinc ferrites ZnFe 2 O 4-δ obtained by vacuum annealing bulk Zn-ferrite at different temperatures T Ann were investigated by using XANES at Zn-K, Fe-K and XMCD at Fe-L 2,3 edges, static and dynamic magnetic measurements. The cation inversion that confers to the starting ferrite a weak ferromagnetic response is progressively lost when T Ann surpasses 250 ºC. Consequently, there is a detriment of the magnetic response that reaches its lower value for T Ann around 370 ºC, being the latter coincident with a negligible non-equilibrium occupancy. Above T Ann ∼ 450 ºC the ferrite progressively enhances its magnetic response. In such case, even though Zn and Fe occupy non-equilibrium sites at surface regions, the deficiency of oxygen predominates over inversion and the former triggers an increment of the magnetic coupling between Fe 3+ at octahedral spinel sites. We discuss the magnetic results that account for spin-glass and/or cluster-glass behavior of this geometrically frustrated compound as due to the randomness and frustration introduced by local defects.

Ab initio calculation of structural, electronic and magnetic properties and hyperfine parameters at the Fe sites of pristine ZnFe2O4

“…Depending on the method and growth conditions of the sample [7,38,39,Ref. 5 and references therein], or when the characteristic lengths of the sample are reduced to the nanoscale [32], meta-stable phases can be formed with a disordered arrangement of Zn and Fe in sites A and B.…”

“…Assuming that µ(O) is the half of the total energy of an oxygen molecule (calculated with the same precision that the energy of stoichiometric and reduced ZFO), the vacancy formation energy resulted to be +7.8eV, indicating that O-vacancy in ZFO cannot be formed spontaneously. However, depending on the method and growth conditions of the sample[5,7,38,39, and references therein], physical conditions can induce the formation of reduced ZFO.At this point, we study the possibility to flip one spin but now in presence of an oxygen vacancy. To do this, total energies calculations were performed flipping the spin of one Fe atom of the SC from positive to negative (or negative to positive) for the different spin configurations.…”

On the deviation from a Curie–Weiss behavior of the ZnFe2O4 susceptibility: A combined ab-initio and Monte-Carlo approach

“…The ZFO sample was prepared by solid state reaction from its precursors oxides. This sample presents a slight inversion, which was detected from its Fe L 3 -edge x-ray magnetic circular dichroism (XMCD) spectrum [30]. Treatments under vacuum at moderate temperatures (up to 450 ∘ C) diminish the cation inversion preserving the spinel structure.…”

“…Treatments under vacuum at moderate temperatures (up to 450 ∘ C) diminish the cation inversion preserving the spinel structure. The inversion level of sample annealed at 370 ∘ C (ZFO-370) is below the XMCD detection limit [30], i.e. the magnetic cations are mainly at B -sites.…”

“…In order to discuss a possible origin of the discrepancy between experimental data and the above simulations results, in the following we will perform a careful inspection of experimental DC susceptibility curves ( χ = Mfalse/H, μ0H=50 mT) of ZnFe 2 O 4 polycrystalline samples (ZFO and ZFO-370) reported in ref. [30]. The ZFO sample was prepared by solid state reaction from its precursors oxides.…”