New features of magnetic tunnel junctions by carbon insertion in the magnesium oxide (MgO) barrier

Scientists from IPCMS (CNRS/University of Strasbourg), the Institut Jean Lamour (CNRS/University of Lorraine), and the Institut de Chimie de Strasbourg (CNRS/University of Strasbourg) have reported in their paper, “MgO tunneling spintronics across capacitively-coupled atomic clusters“, the impact of carbon on spin-polarized electrical transport across microscale magnetic tunnel junctions

The research:
A magnetic tunnel junction (MTJ) is a spintronic device consisting of two ferromagnetic layers separated by a thin insulating barrier. It exploits the quantum tunneling effect: the electrical resistance of the device depends on the relative orientation of the magnetizations of the two layers. When the magnetizations are parallel, the resistance is low; when they are antiparallel, the resistance is higher. This principle, known as tunnel magnetoresistance (TMR), is used in magnetic memories such as MRAM and in magnetic field sensors.
In terms of past research, and industrial products on the market, magnetic tunnel junctions with a magnesium oxide (MgO) tunnel barrier constitute THE prototypical spintronic device. Despite this center stage, how the MgO barrier’s realworld properties affects the device performance has remained a rather niche interest. The IPCMS and IJL team has spent over 15 years detecting and understanding certain “tiny defects” called oxygen vacancies in the MgO material. These are locations where oxygen atoms are missing. These vacancies can occur individually, in pairs, or in clusters, and they can significantly influence the material’s behavior and performance. In early 2025, they published a study in NPJ Spintronics that demonstrates how oxygen vacancy clusters are crucial to a technology-enabling effect called spin transfer torque. So far, however, since the devices simply work, this work has been perceived as mostly academic / ‘after-the-fact’.
Meanwhile, research had shown that the interface between a ferromagnetic metal and molecules could promote very high spintronic performance. Within ANR project ‘Spinapse’, upon inserting carbon at the interfaces of MgO spintronic devices, the teams at IPCMS and IJL made an unexpected discovery: one of their junctions was generating electrical power, even though this class of devices is inherently passive.
A consortium of three labs (IPCMS, IJL, IC) has reported in Nano Letters on the impact on (spin-polarized) electrical transport across microscale magnetic tunnel junctions upon inserting carbon atoms within the MgO tunnel barrier. The insertion of carbon atoms causes the appearance of nanoscale conduction hotspots according to conducting tip atomic force microscopy experiments. These hotspots are presumed to result from the paramagnetic states of carbon upon filling an oxygen vacancy. NV center microscopy is a measurement technique based on diamond defects called nitrogen-vacancy (NV) centers. These defects are highly sensitive to magnetic fields: when illuminated by a laser, they emit light whose properties vary depending on the local magnetic environment. This technique enables the measurement and mapping of magnetic fields at the nanoscale with high sensitivity, often at room temperature. Using this approach, the researchers demonstrated that the magnetic field observed near certain carbon atoms does not stem directly from the magnetic “dipole” effect of the FeCoB electrode. This indicates that the origin of the field is more complex than a simple classical magnetic effect.
To learn more:
Magnetotransport between FeCoB electrodes across a MgO barrier that has been carbon doped exhibit a range of unusual spintronic properties. Whereas without doping, the maximum tunnel magnetoresistance is found at/near V=0, here the TMR is absent until large conductance peaks are reached, at typically several hundreds of mV. In fact, the TMR bias dependence mimicks that of the junction conductance due to  a bias shift in these conductance peaks upon switching the relative orientation of electrode magnetization. The data thus elegantly confirm the theoretical prediction made by A. Fert nearly 30 years ago of spin accumulation upon spin-polarized Coulomb blockade.
Carbon doping also leads to a rich picture of transport noise. In particular, the authors show that reaching a conductance peak can lead to weighed transport at, but also between, two extremal current values. As the conductance peak is surpassed with increasing bias voltage, the original current branch gives way to the new current branch. This is the first report of such effects in a MgO MTJ.
The experimental data also reveal that these junctions possess a memristive behavior. Beyond a certain bias threshold, the junction jumps to another current-voltage response regime. This results in a rigid shift of the junction conductance peaks around V=0. The amplitude of the conductance gap, the amplitude of the shift, and the presence of electrical noise in one memristive regime corresponding to a conductance peak in another regime, led the authors to provide the following microscopic picture of the junction’s nanotransport path. This path contains one or several C atoms forming a nanodot. This transport nanodot is capacitively coupled to another nanodot that can be charged/discharged by a bias voltage. Similarly to mesoscopic physics experiments, the authors propose a charge/discharge cycle of both nanodots to explain the device’s memristive response. The ‘transport’ and ‘gating’ attributions of the nanodots was found to vary upon temperature cycling to T>200K.  In essence, this work describes the smallest gate ever operated — at the scale of a few atoms only.
Conclusion:
This work is generating intense research within ADAGE moonshot project of the Research SPIN Program on MTJ metrology criteria such as signal-to-noise ratios, thanks to a CEA-IJL-IPCMS collaboration. It also opens interesting prospects to position MgO tunneling spintronics onto the quantum technologies roadmap, whether for information/communication, or energy technologies, by harnessing the spin qubit properties of these carbon atoms within the peculiar quantum spintronic coupling that is generated by the magnetic tunnel junction itself.
Read the paper
Co-autors: Mathieu Lamblin,* Victor Da Costa, Loic Joly, Bhavishya Chowrira, Léo Petitdemange, Bertrand Vileno, Romain Bernard, Benoit Gobaut, Samy Boukari, Wolfgang Weber, Michel Hehn, Daniel Lacour, and Martin Bowen*

Co-écrit avec Martin Bowen

Margaux Monvoisin

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15 May 2024