Growth and magnetic properties of ultrathin Ni1 + xFe2 − xO4 films for spin filter junctions

doi:10.1016/j.tsf.2011.03.104

Thin Solid Films

Volume 519, Issue 23, 30 September 2011, Pages 8239-8242

https://doi.org/10.1016/j.tsf.2011.03.104 Get rights and content

Abstract

We investigated the spin filter effect in tunnel junctions with a Ni_1 + xFe_2 − xO₄ ferrimagnetic tunnel barrier. For higher T_c above room temperature, Ni_1 + xFe_2 − xO₄ film should be thicker than 4.0 nm and grown above 400 °C. The spin filter junctions (SFJs) of Fe₃O₄(001)/Ni_1 + xFe_2 − xO₄(001)/Al₂O₃/Fe/Au grown on MgO(001) substrates exhibited an inverse magnetoresistance effect at room temperature, which is consistent with the band calculation of NiFe₂O₄.

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 T_c to utilize the spin filter for spintronic devices [5], [6], [7], [8], [9]. Spinel ferrites, which have very high T_c 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/La_0.7Sr_0.3MnO₃, although they observed no TMR at room temperature due to the low T_c 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 (T_sub) of 300 °C in an N₂ atmosphere,

Results and discussion

Fig. 1(a), (b), and (c) show the RHEED patterns of Ni_1 + xFe_2 − 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 Ni_1 + xFe_2 − 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 Ni_1 + xFe_2 − xO₄ barrier for room temperature operation of spin filter junctions. We obtained ferrimagnetic characteristics at room temperature in the Ni_1 + xFe_2 − 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/Ni_1 + xFe_2 − xO₄/Al₂O₃/Fe spin filter junctions exhibited a small inverse TMR effect at room temperature

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Growth and magnetic properties of ultrathin Ni1 + xFe2 − xO4 films for spin filter junctions

Abstract

Introduction

Section snippets

Experiments

Results and discussion

Summary

Phys. Rev. Lett.

J. Phys. Condens. Matter

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev. B

Appl. Phys. Lett.

Phys. Rev. B

Appl. Phys. Lett.

Appl. Phys. Lett.

Growth and magnetic properties of ultrathin Ni_1 + xFe_2 − xO₄ films for spin filter junctions