Strain-induced stabilization of new magnetic spinel structures in epitaxial oxide heterostructures

Abstract

We report here on the growth of NiFe 2 O 4 epitaxial thin films of different thickness (3 nm ≤ t ≤ 32 nm) on single crystalline substrates having spinel (MgAl 2 O 4 ) or perovskite (SrTiO 3 ) structure. Ultrathin films, grown on any of those substrates, display a huge enhancement of the saturation magnetization: we will show that partial cationic inversion may account for this enhancement, although we will argue that suppression of antiparallel collinear spin alignment due to size-effects cannot be excluded. Besides, for thicker films, the magnetization of films on MAO is found to be similar to that of bulk ferrite; in contrast, the magnetization of films on STO is substantially lower than bulk. We discuss on the possible mechanisms leading to this remarkable difference of magnetization.

Distinct magnetism in ultrathin epitaxial NiFe2O4films on MgAl2OFoerster1, Rebled2, Estradé3 et al. 2011Phys. Rev. B

“…(ii) The observation of a smaller negative slope in MAO at low temperature suggests This last observation is in line with earlier reports. 13 However, the magnetization loop of NFO(24)//MAO in Fig. 1(c) displays a nonhomogeneous shape which may signal the presence of distinct contributions to M(H), recalling the observation [see Fig.…”

Probing optical band gaps at the nanoscale in NiFe2O4 and CoFe2O4 epitaxial films by high resolution electron energy loss spectroscopy

“…This is surprising as earlier report on high magnetization in NFO in the form of ultra-thin films has been ascribed to the possibility of a normal spinel structure. 27 However, as there is no direct evidence, the experimentally reported higher moments for NFO could be due to partial deviation from inverse spinel structure. This result is interesting because if one can engineer the A site cation mixing by controlling growth conditions, e.g., temperature, pressure, or the lattice parameter constraints, it may be possible to increase the total magnetization of the system.…”

Epitaxial strain effects in the spinel ferritesCoFe2O4andNiFe2O

“…Indeed, a reoriention of the magnetic easy axis under different conditions has been reported for CoFe 2 O 4 , 12-14 and a strong enhancement of magnetization and conductivity has been observed in NiFe 2 O 4 thin films. 10,15,16 In order to efficiently optimize the properties of thin film materials, it is important to clarify whether the observed deviations from bulk behavior are indeed due to the epitaxial strain or whether they are induced by other factors, such as for example defects, off-stoichiometry, or genuine interface effects. First principles calculations based on density functional theory (DFT), [17][18][19] can provide valuable insights in this respect by allowing to address each of these effects separately.…”