Thickness dependence of exchange coupling in epitaxialFe3O4/CoFe2O4

Abstract

Epitaxial magnetic heterostructures of (soft-)Fe 3 O 4 /(hard-)CoFe 2 O 4 (001) have been fabricated with a varying thicknesses of soft ferrite from 5 to 25 nm. We report a change in the regime of magnetic interaction between the layers from rigid-coupling to exchange-spring behavior, above a critical thickness of the soft magnetic Fe 3 O 4 layer. We show that the symmetry and epitaxial matching between the spinel structures of CoFe 2 O 4 and Fe 3 O 4 at the interface stabilize the Verwey transition close to the bulk value even for 5-nm-thick Fe 3 O 4. The large interface exchange-coupling constant estimated from low-temperature M(H) data confirmed the good quality of the ferrite-ferrite interface and the major role played by the interface in the magnetization dynamics. The results presented here constitute a model system for understanding the magnetic behavior of interfaces in core/shell nanoparticles and magnetic oxide-based spintronic devices.

Exchange-spring behavior below the exchange length in hard-soft bilayers in multidomain configurations

“…The critical thicknesses observed in experimental systems based on uniaxially anisotropic hard phases such as FePt, SmCo 5 , and Nd 2 Fe 14 B are in reasonably good agreement with theoretical models [2,15,16]. Even systems based in biaxial CoFe 2 O 4 (CFO) with Fe 3 O 4 as soft layer accurately follow the predictions [17]. Nonetheless, the complexity of real systems is extremely hard to fully model and experimental critical thicknesses are often slightly overestimated by the models [16,18].…”

“…This reversal behavior demonstrates nonrigid coupling as hard and soft layers do not reverse simultaneously. Nonrigidity is observed when H n < H sw [17], a condition that is fulfilled for all soft thicknesses as shown in Fig. 2(c).…”

“…Figure 2(a) shows the magnetization curves of FC2, FC4, and the reference CFO sample. CFO presents a saturation magnetization of M s (CFO) = 3.41 × 10 5 A m −1 , slightly smaller than the usual bulk value, presumably due to the strain from the substrate [17]. The squareness ratio is M r /M s = 0.46, indicating that the hard phase breaks into a multidomain pattern.…”

“…At higher fields the gradual irreversible rotation in CFO occurs. The onset of soft layer reversal at low negative fields is defined as the nucleation field H n , while the switching field H sw corresponds to the irreversible magnetization reversal of the hard layer [17,27]. H n and H sw are extracted from the magnetization curves as the inflection points in the soft and hard reversal parts of the demagnetization curve, respectively [17], as indicated with arrows in Fig.…”

Internal Structure and Magnetic Properties in Cobalt Ferrite Nanoparticles: Influence of the Synthesis Method

“…Such observation is in agreement with both the slight increase in the lattice parameter of SV NPs and the broader distribution of anisotropies evaluated by PDI MAG . The increased structural coherence of TD samples is also reflected in the larger remanence ratios, which are much closer to the theoretical M R / M S predicted for cubic anisotropy ( M R / M S = 0.83) than to the uniaxial case ( M R / M S = 0.5), while the larger density of defects observed in SV samples can favor uniaxial anisotropy because of local lattice strain . In this line, the assumption of uniaxial anisotropy may not be valid for TD samples, and therefore the K EFF obtained in such case cannot be taken as an estimation of the K1 magnetocrystalline anisotropy of Co‐ferrite.…”