NiFe₂O₄: A Versatile Spinel Material Brings New Opportunities for Spintronics

Abstract

The search for novel functional materials is an extremely active field and to this purpose transition metal oxides have been under scrutiny for more than twenty years now. Their properties mainly arise from the interaction between the transition metal and oxygen ions and their sensitivity to bond length and angles.[1] Consequently, a large number of materials showing different physical effects can be found within one structural family. Perovskites are the most popular, as this crystal structure is shared by high superconducting temperature (T C ) superconductors, ferroelectrics, half-metallic ferromagnets, etc. This structural compatibility allows one to combine thin layers of very different materials to design multifunctional epitaxial heterostructures. Furthermore, the strong sensitivity of the physical properties to structural modifications often reveals unexpected behavior in strained thin films [2,3] or at the interface between two adjacent layers.[4]Besides perovskites, spinel oxides are also very attractive as they can be half-metallic (such as Fe 3 O 4 [5] ), ferrimagnetic insulators (like most spinel ferrites [6] ), transparent conductors (such as Cd 2 SnO 4[7] ), superconductors (LiTi 2 O 4[8]), or heavyfermion compounds (LiV 2 O 4 [9] ). Furthermore, spinel is a complex crystal structure, with many degrees of freedom available to engineer physical properties. Yet, their study in thin-film form has not been so intensive, [10] and much remains to be learnt concerning their properties in low dimensions and their potential in heteroepitaxial architectures.In this paper, we report on the growth and properties of ferrimagnetic NiFe 2 O 4 thin (3-12 nm) films onto perovskite SrTiO 3 (STO) substrates, layers of the ferromagnetic metallic oxide La 2/3 Sr 1/3 MnO 3 (LSMO), or LSMO/STO bilayers. These NiFe 2 O 4 nanometer-sized films have a considerably enhanced magnetic moment compared to the bulk, and their electronic properties can be tuned from conductive to insulating by changing the growth conditions. We have integrated these layers into epitaxial heterostructures to probe their potential for spintronics. We find that both types of NiFe 2 O 4 produce spin-polarized currents, but via two different mechanisms that might be combined to fabricate novel spintronics devices.NiFe 2 O 4 (NFO) films have been grown by target-facing-target radio frequency (RF) sputtering onto either STO(001) substrates or onto LSMO/STO or LSMO templates (see Experimental). The growth atmosphere consisted either of pure Ar or of a Ar/O 2 mixture. After growth, the films were cooled to room temperature in the growth atmosphere. Reflection high-energy electron diffraction (RHEED) patterns recorded before and after film deposition (Fig. 1a) indicate epitaxial growth of the spinel material onto the perovskite for both Ar and Ar/O 2 conditions. The pattern consists of organized spots for films grown in pure Ar and of streaks for Ar/O 2 . This indicates that the growth mode is modified from three-dimensional to two-dimensional when O 2 is i...

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

“…In addition, a detailed understanding of the growth mechanisms of these ferrimagnetic spinel films is fundamental for their utilization, because the growth conditions may introduce new degrees of freedom in tailoring the film properties [1]. It is quite remarkable that the magnetic response of thin films, in general, is very different from the bulk behaviour.…”

“…Moreover, spinel ferrites are nowadays integrated in complex heterostructures. Examples include multilayers formed by oxides with spinel (AB 2 O 4 ) and perovskite (ABO 3 ) structures, used in spintronic devices, such as spin filters [1,2], or multiferroic nanocomposites [3]. Dissimilar structures lead to structural mismatch and thus strained regions are formed at interfaces.…”

Exchange-biased NiFe2O4/NiO nanocomposites derived from NiFe-layered double hydroxides as a single precursor

“…Interest in nanosized ferro/ferrimagnetic (FM) particles has increased greatly in recent years because of their wide range of applications in various fields [23][24][25][26][27]. In the case of information storage, achieving ultrahigh densities requires the preparation of materials with very small particle size.…”

The investigation of giant magnetic moment in ultrathin Fe3O4 films

“…About the origin of the giant M s , our data suggest that the earlier proposition of the non-compensation and cation inversion was an oversimplification. 18,20 So we consider the most likely reason may be due to the spin of Fe ions in the tetrahedron site is much easier to switch parallel to the Fe ions in the octahedron site at the surface, interface, and grain boundaries. A schematic of this spin configuration is depicted in Fig.…”

“…This effect was ascribed to the cation inversion. 20 From the previous reports, the film thickness has great influence a Author to whom correspondence should be addressed. In this study, the epitaxial polycrystalline Fe 3 O 4 films were grown by Laser Molecular Beam Epitaxy (LMBE) using a KrF excimer laser (λ = 248 nm) with a repetition rate of 10 Hz and energy of 300 mJ on a ceramic target of Fe 3 O 4 .…”