F. Ernult

Spin-dependent tunneling and Coulomb blockade in ferromagnetic nanoparticles

Abstract In this paper we review studies on spin-dependent transport in systems containing ferromagnetic nanoparticles. In a tunnel junction with a nanometer-scale-island, the charging effect leads to an electric current blockade phenomenon in which a single electron charge plays a significant role in electron transport, resulting in single-electron tunneling (SET) properties such as Coulomb blockade and Coulomb staircase. In a tunnel junction with a ferromagnetic nano-island and electrode, it was expected that the interplay of spin-dependent tunneling (SDT) and SET, i.e., spin-dependent single-electron tunneling (SD-SET), would give rise to remarkable tunnel magnetoresistance (TMR) phenomena. We investigated magnetotransport properties in both sequential tunneling and cotunneling regimes of SET and found the enhancement and oscillation of TMR. The self-assembled ferromagnetic nanoparticles we have employed in this study consisted of a Co–Al–O granular film with cobalt nanoparticles embedded in an Al–O insulating matrix. A Co 36 Al 22 O 42 film prepared by a reactive sputtering method produced a TMR ratio reaching 10% and superparamagnetic behavior at room temperature. The TMR ratio exhibited an anomalous increase at low temperatures but no indication of change with bias voltage. In Section 4, we show that the anomalous increase of the MR provided evidence for higher-order tunneling (cotunneling) between large granules through intervening small granules. We emphasize that the existence of higher-order tunneling is a natural consequence of the granular structure, since broad distribution of granule size is an intrinsic property of granular systems. In Section 5, we concentrate on SD-SET properties in sequential tunneling regimes. We fabricated two types of device structures with Co–Al–O film using focused ion-beam milling or electron-beam lithography techniques. One had a granular nanobridge structure: point-shaped electrodes separated by a very narrow lateral gap filled with the Co–Al–O granular film. The other had a current-perpendicular-to-plane (CPP) geometry structure: a thin Co–Al–O granular film sandwiched by ferromagnetic electrodes with the current flowing in the direction perpendicular to the film plane through a few Co particles. We found the enhancement and oscillation of TMR due to spin-dependent SET in sequential tunneling regimes. In Section 6, we report experimental evidence of a spin accumulation effect in Co nanoparticles leading to the oscillation of TMR with alternate sign changes. Furthermore, we discovered that the spin relaxation time in the nanoparticles is unprecedentedly enhanced up to the order of more than hundreds of nanoseconds, compared to that evaluated from the spin-diffusion length of ferromagnetic layers in previous CPP-GMR studies, i.e., the order of tens of picoseconds.

Spin-dependent single-electron-tunneling effects in epitaxial Fe nanoparticles

Fe/MgO/Fe nanoparticles/MgO/Co double tunnel junctions were prepared by molecular beam epitaxy for current-perpendicular-to-plane transport measurements on submicrometer-sized pillars. Microstructural observations indicate that the samples exhibit a fully epitaxial layered structure with sharp and flat interfaces including well-defined separated Fe nanoparticles between the barriers. The introduction of asymmetric MgO tunnel barriers, i.e., with different thicknesses, in the double junction leads to a clear observation of Coulomb staircase and associated tunnel magnetoresistance oscillations. An estimation of the capacitance of the system indicates that these transport phenomena are due to charging effects of the magnetic particles.

Spin accumulation in metallic nanoparticles

The rapid development of spin electronics has emphasized the importance of spin accumulation to transport properties and spin manipulation for quantum computing. The latter requires sufficiently long spin-relaxation times, that may be achieved in nanometric particles. Spin accumulation is then expected to take place and strongly affect the transport properties of the particles. The main transport phenomena occurring in nanoparticles will be reviewed, as well as their interplay with spin accumulation. Recent experimental results confirming the theoretical predictions will also be presented. They show clear evidence of the enhancement of the spin-relaxation time in nanometric particles and are promising for the development of spin-electronics devices.

Preparation and magnetotransport properties of MgO-barrier-based magnetic double tunnel junctions including nonmagnetic nanoparticles

MgO-barrier-based magnetic double tunnel junctions including Au or Cr nanoparticles were prepared by molecular beam epitaxy, and their magnetotransport properties were investigated. A double junction sample including Au nanoparticles showed the Coulomb blockade effect and clear magnetoresistive hysteresis loops. The observed bias voltage dependence of the resistance and magnetoresistance (MR) suggested that the MR effects of 1?2% at high bias voltages were caused by spin accumulation in the Au nanoparticles. In the case of Cr nanoparticles, a double junction with relatively low sample resistance was obtained, showing a clear Coulomb threshold.

Preparation of nanometer-scale iron dots on insulating layer

Abstract We studied the preparation of nanometer-sized magnetic dots on MgO tunnel barriers. The samples were formed by depositing Fe thin layers, which exhibited a 3D Volmer–Weber growth on MgO (001) substrates. The current–voltage characteristics were then measured using a scanning tunneling microscope. By comparing different preparation techniques such as the oxidation of a metallic Mg layer and direct deposition of MgO by electron beam evaporation, we found that direct deposition was the best way to achieve flat MgO surfaces and assemblies of dots with uniform heights. The growth of the Fe dots seems to be mainly governed by the density of surface defects of the buffer, which act as nucleation sites. The density of these defects appears to be modified by the insertion of an iron electrode prior to the deposition of the MgO layer.

Current-induced tunnel magnetoresistance due to spin accumulation in Au nanoparticles

Spin-dependent single electron tunneling was investigated in a magnetic double tunnel junction including Au nanoparticles as a center electrode. Tunnel magnetoresistance (TMR) clearly emerged with increasing spin-polarized current injected into Au nanoparticles and reached a maximum value of about 12% at 4.2K. The observation indicates that spin accumulation occurs in Au nanoparticles and causes TMR. The spin relaxation time in Au nanoparticles, as estimated from the critical current for the appearance of TMR, is of the order of 10ns, which is much longer than that in the bulk state.

Self-alignment of Fe nanoparticles on a tunnel barrier

Nanometric metallic particles were prepared on top of a thin epitaxial oxide layer. Samples with the following structure: Fe electrode∕MgO∕Fe particles were fabricated and the arrangement of the Fe particles could be tuned from random to self-aligned by simply varying the thickness of the Fe electrode. Under appropriate deposition conditions, the particles were found to be self-aligned along the ⟨110⟩ directions of the underlying Fe electrode. Scanning tunneling microscope (STM) showed that their mean diameter and size distribution were then significantly reduced compared to randomly organized particles. Transmission electron microscope (TEM) images indicated that the self-alignment process originates from the strain relaxation of the Fe electrode which favors faceting of its surface and the formation of pyramidal structures. These self-aligned particles may be straightly used for applications based on a thin oxide tunnel barrier such as single-electron tunneling devices.

Self-assembled metallic nanoparticles for spin dependent single electron tunneling

The study of spin-dependent single electron tunneling requires the preparation of particles with a nanometric size. These particles need not be ferromagnetic if the electrodes are. In order to fulfill the requirements of single electron tunneling effect, the preparation conditions of nanoparticles were studied, mainly for Fe and Au particles. The structure of these particles was then studied by scanning probe microscopy and transmission electron microscopy, and compared to the results of transport measurements. Particles with a diameter of the order of a nanometer were prepared and their lateral arrangement was controlled by self-organization phenomena.