C. Baraduc

The Skyrmion Switch: Turning Magnetic Skyrmion Bubbles on and off with an Electric Field

Nanoscale magnetic skyrmions are considered as potential information carriers for future spintronics memory and logic devices. Such applications will require the control of their local creation and annihilation, which involves so far solutions that are either energy consuming or difficult to integrate. Here we demonstrate the control of skyrmion bubbles nucleation and annihilation using electric field gating, an easily integrable and potentially energetically efficient solution. We present a detailed stability diagram of the skyrmion bubbles in a Pt/Co/oxide trilayer and show that their stability can be controlled via an applied electric field. An analytical bubble model with the Dzyaloshinskii-Moriya interaction imbedded in the domain wall energy accounts for the observed electrical skyrmion switching effect. This allows us to unveil the origin of the electrical control of skyrmions stability and to show that both magnetic dipolar interaction and the Dzyaloshinskii-Moriya interaction play an important role in the skyrmion bubble stabilization.

Electrical study of ferromagnet-oxide-semiconductor diode for a magnetic memory device integrated on silicon

This work focuses on electrical characterization of NiFe∕SiO2∕Si tunnel diodes that can be used for spin injection into silicon in future spintronic devices. Capacitance-voltage characteristics show a large increase of the Si∕SiO2 interfacial state density compared to similar Al∕SiO2∕Si diodes. This result suggests that nickel and/or iron may have diffused across the SiO2 layer. Consistently the current-voltage experimental characteristics have been modeled by using trap assisted electron transport mechanism. These traps may be attributed to ferromagnet atoms in the oxide bulk.

Correlation between write endurance and electrical low frequency noise in MgO based magnetic tunnel junctions

The write endurance and the 1/f noise of electrical origin were characterized in CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ) for spin transfer torque or thermally assisted magnetic random access memories. A statistical study carried out on a set of 60 nominally identical patterned junctions of 200 nm diameter revealed a correlation trend between the electrical 1/f noise power in the unexercised MTJs and the number of write cycles that these MTJs can withstand before electrical breakdown. The junctions showing the largest 1/f noise power before the write endurance test (successive 30 ns pulses of 1.73 V) have the lowest endurance. In contrast, MTJs initially exhibiting lower 1/f noise tend to have a better electrical reliability, i.e., much longer write endurance. This correlation is explained by the presence of electron trapping sites in the MgO barrier and the role of electron trapping/detrapping phenomena in both MTJ reliability and its 1/f electrical noise power. These results suggest that 1/f noise could be used as a predictive characterization of the MTJ endurance.

Double barrier magnetic tunnel junctions with write/read mode select layer

In this study, special STT-RAM were designed, built and tested, allowing to read and write at similar voltages. This is achieved by maximizing the Spin-Transfer-Torque (STT) efficiency on the storage layer magnetization during write and minimizing it during read. In order to achieve this STT tuning, double barrier magnetic tunnel junctions were prepared wherein the storage layer is sandwiched between two polarizing layers. Each polarizing layer is separated from the storage layer by a tunnel barrier. The magnetization of one of the polarizing layer is always pinned in a fixed direction whereas the other one, called mode select layer, can be switched parallel or antiparallel to the first one depending whether the magnetic tunnel junction (MTJ) is in read or respectively write mode. In the parallel configuration of the polarizing layers, the STT efficiency is minimized allowing to read at relatively high voltage leading to fast readout without risk of write disturb during read. In the antiparallel configuration of the polarizing layer, the STT efficiency is maximized allowing writing at lower current density. In this system, the magnetization of the storage layer is switched by STT whereas the magnetization of the mode select layer is switched by field. Switching from read mode to write mode and vice versa is achieved by sharing a single pulse of magnetic field for all bits of the same word.

Spin-current vortices in current-perpendicular-to-plane nanoconstricted spin valves

The charge and spin diffusion equations, taking into account spin-flip and spin-transfer torque, were numerically solved using a finite element method in complex noncollinear geometry with strongly inhomogeneous current flow. As an illustration, spin-dependent transport through a nonmagnetic nanoconstriction, separating two magnetic layers was investigated. Unexpected results such as vortices of spin-currents in the vicinity of the nanoconstriction were obtained. The angular variations of magnetoresistance and spin-transfer torque are strongly influenced by the structure geometry.

Modulation of spin transfer torque amplitude in double barrier magnetic tunnel junctions

Magnetization switching induced by spin transfer torque is used to write magnetic memories (Magnetic Random Access Memory, MRAM) but can be detrimental to the reading process. It would be quite convenient therefore to modulate the efficiency of spin transfer torque. A solution is adding an extra degree of freedom by using double barrier magnetic tunnel junctions with two spin-polarizers, with controllable relative magnetic alignment. We demonstrate, for these structures, that the amplitude of in-plane spin transfer torque on the middle free layer can be efficiently tuned via the magnetic configuration of the electrodes. Using the proposed design could thus pave the way towards more reliable read/write schemes for MRAM. Moreover, our results suggest an intriguing effect associated with the out-of-plane (field-like) spin transfer torque, which has to be further investigated.

Large-Voltage Tuning of Dzyaloshinskii–Moriya Interactions: A Route toward Dynamic Control of Skyrmion Chirality

Electric control of magnetism is a prerequisite for efficient and low-power spintronic devices. More specifically, in heavy metal-ferromagnet-insulator heterostructures, voltage gating has been shown to locally and dynamically tune magnetic properties such as interface anisotropy and saturation magnetization. However, its effect on interfacial Dzyaloshinskii-Moriya Interaction (DMI), which is crucial for the stability of magnetic skyrmions, has been challenging to achieve and has not been reported yet for ultrathin films. Here, we demonstrate a 130% variation of DMI with electric field in Ta/FeCoB/TaO x trilayer through Brillouin Light Spectroscopy (BLS). Using polar magneto-optical Kerr-effect microscopy, we further show a monotonic variation of DMI and skyrmionic bubble size with electric field with an unprecedented efficiency. We anticipate through our observations that a sign reversal of DMI with an electric field is possible, leading to a chirality switch. This dynamic manipulation of DMI establishes an additional degree of control to engineer programmable skyrmion-based memory or logic devices.

High sensitivity magnetic field sensor for spatial applications

A high sensitivity 1D magnetic field sensor is developed for spatial applications, in order to replace the heavy search-coils currently used. This new sensor combines a flux concentrator, biasing coils for field modulation and magnetic tunnel junctions. These three elements are fabricated and independently characterized. Finally, the expected performance of a sensor combining these three elements can be estimated.

Diffusive model of current-in-plane-tunneling in double magnetic tunnel junctions

We propose a model that describes current-in-plane tunneling transport in double barrier magnetic tunnel junctions in diffusive regime. Our study shows that specific features appear in double junctions that are described by introducing two typical length scales. The model may be used to measure the magnetoresistance and the resistance area product of both barriers in unpatterned stacks of double barrier magnetic tunnel junctions.

Noise study of magnetic field sensors based on magnetic tunnel junctions

Low frequency noise has been studied for two types of magnetic field sensors based on magnetic tunnel junctions (MTJ). The first structure, composed of a few large MTJs, is designed for low noise applications; the second one, composed of hundreds of small MTJs, is designed for general purposes. At low frequency, both structures exhibit 1/f noise, but with very different amplitudes. The sensors for general purposes show a much higher noise level compared to the low-noise sensors. However, the sensitivity of the low noise sensors is much smaller compared to the other ones. Thus, the limit of detection, defined as the ratio of noise and sensitivity, turns out to be roughly the same for both technologies. Using the advantages of each sensor could help to design a sensor with an improved limit of detection.