Growth and magnetic properties of ultrathin Ni1 + xFe2 − xO4 films for spin filter junctions
Introduction
Spin filter junctions (SFJs) are tunnel junctions with a ferro- or ferrimagnetic tunnel barrier [1]. The magnetic tunnel barrier forms a spin-dependent barrier height in the junctions due to exchange splitting. Such junctions generate highly spin polarized tunnel current because the tunnel probability strongly depends on the barrier height. So far, Europium chalcogenide, e.g. EuS [2] or EuO [3], has been employed as a spin filter material. In 1988, Moodera et al. demonstrated the spin filter effect using tunnel junctions with a EuS barrier and superconducting Al electrodes. They successfully observed spin polarization of 80% in the tunnel current at 0.4 K. LeClair et al. demonstrated that the spin filter junctions combined with a ferromagnetic electrode showed a magnetoresistance effect of 100% [4]. However, these results were also restricted to low temperature because of the low Currie temperature for Europium chalcogenide. Recently, researchers have been investigating magnetic insulators having high Tc to utilize the spin filter for spintronic devices [5], [6], [7], [8], [9]. Spinel ferrites, which have very high Tc above 800 K [10], are the most promising candidate. Takahashi et al. fabricated SFJs out of Pt/CoFe2O4/MgO/FeCo by sputtering and observed a tunnel magnetoresistance (TMR) of 30% at 4 K [9]. Furthermore, they succeeded in observing a TMR effect of 3% even at room temperature, which corresponds to a spin filter efficiency of 4.3%. As for NiFe2O4, Lüder et al. reported a TMR of 52% at 4 K for Au/NiFe2O4/La0.7Sr0.3MnO3, although they observed no TMR at room temperature due to the low Tc of the LSMO electrode [6]. To achieve a large TMR ratio at room temperature, it is necessary to improve the quality of ultrathin spinel films, including factors such as defects, antiphase boundary, and roughness. In this study, we investigated ultrathin NiFe2O4 films fabricated by molecular beam epitaxy (MBE), and SFJs with a NiFe2O4 barrier.
Section snippets
Experiments
Samples were fabricated by an MBE system manufactured by AVC with a reflection high-energy electron diffraction (RHEED) system. This system achieves a base pressure in the 10− 8 Pa range. MgO(001) substrates were cleaned in acetone, ethanol, and water sequentially before being loaded into a growth chamber. To obtain a clean MgO(001) surface, the substrates were heated at 800 °C for 30 min. Following the deposition of a TiN buffer layer at a substrate temperature (Tsub) of 300 °C in an N2 atmosphere,
Results and discussion
Fig. 1(a), (b), and (c) show the RHEED patterns of Ni1 + xFe2 − xO4 (4 nm) grown on TiN (40 nm) buffer layers at 200, 300, and 400 °C, respectively. The patterns revealed that all the films grew epitaxially, however, the surfaces strongly depended on the growth temperature. The Ni1 + xFe2 − xO4 grown at 200 °C showed a streaky pattern, meaning that the surface was flat. On the other hand, the film grown at 400 °C showed a spotty pattern, which indicated that islands existed on the surface. We found that
Summary
We studied an ultrathin ferrimagnetic Ni1 + xFe2 − xO4 barrier for room temperature operation of spin filter junctions. We obtained ferrimagnetic characteristics at room temperature in the Ni1 + xFe2 − xO4 films thicker than 4.0 nm, which were annealed at a temperature higher than 400 °C. The surface roughness was suppressed by low temperature growth, which was confirmed by RHEED observations. The Fe3O4/Ni1 + xFe2 − xO4/Al2O3/Fe spin filter junctions exhibited a small inverse TMR effect at room temperature
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