Anne Bernand-Mantel

Nanospintronics: when spintronics meets single electron physics

As spintronics goes nano, new phenomena are predicted resulting from the interplay between spin dependent transport and single electron physics. The long term goal of manipulating spins one by one would open a promising path to quantum computing. Towards this end, there is an ever-growing effort to connect spin tanks (i.e. ferromagnetic leads) to smaller and smaller objects in order to study spintronics in reduced dimensions. As the dimensions are reduced, spin dependent transport is predicted to interplay with quantum and/or single electron charging effects. We review experiments and theories on the interplay between Coulomb blockade and spin properties (namely magneto-Coulomb effects) in structures where a single nano-object is connected to ferromagnetic leads. We then discuss briefly future directions in the emerging field of nanospintronics towards quantum dots, carbon nanotubes and single molecule magnets.

Evidence for spin injection in a single metallic nanoparticle: A step towards nanospintronics

We have fabricated nanometer-sized magnetic tunnel junctions using a conductive tip nanoindentation technique in order to study the transport properties of a single metallic nanoparticle. Coulomb blockade effects show clear evidence for single-electron tunneling through a single 2.5nm Au cluster. The observed magnetoresistance is the signature of spin conservation during the transport process through a nonmagnetic cluster.

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.

Electric-field effect on coercivity distributions in FePt magneto-electric devices

We have investigated the contribution of stochastic thermally activated processes to the electric-field effects on coercivity in FePt. Coercive field distributions were measured under different gate voltages in solid-state field-effect structures. For low voltages, a shift in the coercive field distribution can be observed; however, it is not larger than the width of the distribution. Higher voltages are needed to obtain the splitting from the negative (zero) voltage distribution allowing for the unambiguous characterization of the electric-field effect. A virtual unipolarity in the electric-field effect has been identified as a feature introduced by the dielectric layer that disappears upon annealing.

Electric-field assisted depinning and nucleation of magnetic domain walls in FePt/Al2O3/liquid gate structures

We present a magneto-optical Kerr effect study of the magnetization reversal in a FePt/Al2O3 structure under electric (E) fields generated in a liquid electrolyte environment. The FePt film was partially covered with a thick Al2O3 layer that allowed for the study of a pinned domain wall between two regions of different coercive field. Depinning of the trapped domain wall into the region of higher coercivity was achieved by applying positive gate voltages during the magnetic field ramp and prevented in the presence of negative gate voltages. Moving from positive to negative gate voltages produced, in addition, an increase (decrease) in the number (size) of reverse domains in the high anisotropy region. This effect has been associated to an E-field induced decrease of the saturation field. Using a liquid gate to assist domain wall depinning as presented here can be used for the control of multiple pinning structures in parallel.

Ferroelectric control of magnetic domains in ultra-thin cobalt layers

Non-volatile ferroelectric control of magnetic domains has been demonstrated in ultra-thin cobalt layers at room temperature. The sensitivity of magnetic anisotropy energy to the electronic structure in a few atomic layers adjacent to the interface allows for ferroelectric control of coercivity and magnetic domain dynamics. These effects have been monitored and quantified using magneto-optical Kerr effect. In particular, the regimes, where the ferroelectric domains enhance/inhibit the magnetic domain nucleation or increase/reduce domain wall velocity, have been explored. Thus, non-destructive and reversible ferroelectric domain writing provides a tool to define the magnetic domain paths, create nucleation sites, or control domain movement.

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.

Switching probability sub-distributions and asymmetric magnetization reversal in FePt nanostructures

The coercive field statistics in FePt nanostructures reveals the existence of multiple switching probability sub-distributions that can be asymmetric with respect to the field orientation. Each sub-distribution is correlated with an individual magnetization reversal path whose selection cannot happen at the magnetization reversal in negative (positive) field but rather at the moment of applying the initial positive (negative) magnetic field. This serves to determine the reference magnetic state from which reversal in negative (positive) field will develop. The disappearance of the asymmetric sub-distributions upon increasing the initial magnetic field μ0Hmax supports this model. However, the sub-distributions remaining at high μ0Hmax are not necessarily those characterized by the highest coercive field. This is attributed to the fact that the initial magnetization state hierarchy and the coercive field hierarchy are essentially decorrelated.

Large voltage tuning of Dzyalonshinskii-Moriya interaction: towards a chirality switch?

Electric field can tune interfacial magnetism, thus paving the way towards new low power devices using gate voltage. In particular, its effect on skyrmions, which are promising to code information bits, is a critical issue. Here, we first address the effect of electric field on interfacial anisotropy in Pt/Co/AlOx ultrathin trilayers and the possibility to control skyrmion nucleation and annihilation with gate voltage. Then we report the effect of electric field on the interfacial interaction responsible for skyrmions, namely Dzyaloshinskii-Moriya Interaction (DMI). We demonstrate an unprecedented large electric field effect on DMI (βDMI = 600 fJV-1m-1) in Ta/FeCoB/TaOx ultrathin trilayers through Brillouin Light Scattering spectroscopy. Additional Kerr effect observations lead us to propose that electric field could ultimately reverse the sign of DMI, resulting in chirality switch.

Voltage control of magnetism in ferromagnetic structures

Until now, spintronics devices have relied on polarized currents, which still generate relatively high dissipation, particularly for nanodevices based on DW motion. A novel solution to further reduce power consumption is emerging, based on electric field (E) gating to control the magnetic state. Here, we will describe the state of the art and our recent experiments on voltage induced changes in the magnetic properties of ferromagnetic metals. A thorough description of the advances in terms of control of intrinsic properties such as magnetic anisotropy and ferromagnetic transition temperature as well as in intrinsic properties like coercive field and domain wall motion will be presented. Additionally, a section will be dedicated to the summary of the key aspects concerning the fabrication and performance of magneto-electric field-effect devices.