Figure 1: RHEED patterns along the [1-100] direction showing a CoFe2O4(111) (5 nm) single layer on α-Al2O3(0001) as well as CoFe2O4 (5 nm)/Fe3O4(15 nm) and CoFe2O4 (5 nm) α-Al2O3 (1.5 nm)/Co(15 nm) bilayer.
Currently promising in the field of spintronics is the spin filter effect, which generates a highly spin polarized current from a non-magnetic source via transport across a magnetic tunnel barrier. Spin filtering was first discovered in EuS tunnel barriers. Since the discovery of this spin filtering, studies have demonstrated this effect in a number of oxide barriers such as EuO, BiMnO3 and NiFe2O4. However, none of these materials have shown spin polarized transport at room temperature. As a part of the illustrated scientific context, we propose to study the innovative oxide cobalt ferrite: CoFe2O4, whose high Curie temperature (Tc= 793 K) and high coercive field (Hc= 2500 Oe at 300 K) make it a very promising material for a tunnelling barrier in spin filter devices.
This project relies on the growth of high quality CoFe2O4 epitaxial thin films by molecular beam epitaxy (MBE) as well as the demonstration of its spin filtering capacities, the final goal being to integrate this ferrite into magnetic tunnelling junctions (MTJs). Special attention is given to the chemical and structural properties of the magnetic barrier and their interfaces with magnetic electrodes, as they play a determinant role in their performance as spin filters. We have studied the epitaxial growth and the magnetic properties of CoFe2O4(111) single layers as well as CoFe2O4(111)/Ferromagnetic (FM) bilayers that may potentially be integrated into fully epitaxial spin filters MTJs. We have chosen to study Co and Fe3O4 as two possible ferromagnetic electrodes in the CoFe2O4(111)/FM bilayers.
At first, we have realized the epitaxial growth of CoFe2O4(111) single layers by MBE on epitaxial Pt(111) underlayers or α-Al2O3(0001) substrates. This growth has been performed as a function of numerous parameters (film thickness, flux, partial oxygen pressure). In situ reflection high energy electron diffraction (RHEED) (see figure 1) and ex situ X-ray diffraction have confirmed respectively the two dimensional growth mode and the high structural quality of the CoFe2O4(111) thin films. In situ X-ray photoemission spectroscopy (XPS) have also been performed in order to check the iron and cobalt oxidation state and to quantify the Co/Fe/O ratio in thin films within the accuracy limit of the method. The magnetism in CoFe2O4(111) thin films with thickness appropriate for tunnel barriers was studied by VSM magnetometry. The 3 nm thick films on Pt(111) clearly show a ferromagnetic behaviour with a net magnetic moment of 2.7 mB at high field and a coercive field of 220 Oe at room temperature. Finally the electrical measurements as a function of temperature have demonstrated a typical insulating behaviour with a resistivity at room temperature around 100 W.cm.
Figure 2: Cross-sectional HRTEM image of CoFe2O4 (5 nm)/Fe3O4(15 nm) bilayer viewed along the [11-2] zone axis. The EELS chemical map of the Co concentration in another CoFe2O4 (15 nm)/Fe3O4(15 nm) bilayer is shown in insert (Coll. C. Gatel, P. Bayle-Guillemaud DRFMC-SP2M, B. Warot-Fonrose, E. Snoeck, CEMES-CNRS).
Figure 3: Normalized room temperature magnetization curves of a CoFe2O4 (5 nm) α-Al2O3 (1.5 nm)/Co(3 nm) bilayer with the minor loop in inset.
The central issue that must be carefully considered in spin filter tunnel junctions is that the magnetic barrier (CoFe2O4) and electrode (Co or Fe3O4) are in contact with each other, increasing the probability that the two layers do not switch independently. Therefore magnetic hysteresis loops of the two bilayers were measured by VSM and SQUID magnetometry. The magnetization curves for the CoFe2O4 (5 nm)/Co bilayers did not exhibit independent switching but one single hysteresis loop with a coercive field of 200 Oe (not shown here) greater than that of a Co single layer (20 Oe) indicating an exchange coupling at the interface. The insertion of the thin α-Al2O3 spacer in between the CoFe2O4 and Co layers clearly destroys the magnetic coupling and each film recovers its individual coercive fields (see figure 3).
The magnetic switching behavior of the CoFe2O4(5 nm)/Fe3O4(15 nm) bilayers is more surprising as the loops systematically contain two inflection points around 200 Oe and 3000 Oe suggesting two independent switching events. The analysis of these measurements (magnetization height ratio, minor loops) and polarized neutron reflectometry experiments suggests that the CoFe2O4 layer switches first. The second switching field, corresponding to the Fe3O4, is ten times greater than that of a single layer. This result may be explained by the presence of an exchange coupling at the oxide/oxide interface. This new exchange phenomenon at the interface leads to a magnetic system displaying two magnetic states without the insertion of a non-magnetic spacer. The potential interest of CoFe3O4/Fe3O4 bilayers in spin filtering devices is therefore confirmed.