Cecile Grezes

Ultra-low switching energy and scaling in electric-field-controlled nanoscale magnetic tunnel junctions with high resistance-area product

We report electric-field-induced switching with write energies down to 6 fJ/bit for switching times of 0.5 ns, in nanoscale perpendicular magnetic tunnel junctions (MTJs) with high resistance-area product and diameters down to 50 nm. The ultra-low switching energy is made possible by a thick MgO barrier that ensures negligible spin-transfer torque contributions, along with a reduction of the Ohmic dissipation. We find that the switching voltage and time are insensitive to the junction diameter for high-resistance MTJs, a result accounted for by a macrospin model of purely voltage-induced switching. The measured performance enables integration with same-size CMOS transistors in compact memory and logic integrated circuits.

Flux qubits with long coherence times for hybrid quantum circuits.

We present measurements of superconducting flux qubits embedded in a three dimensional copper cavity. The qubits are fabricated on a sapphire substrate and are measured by coupling them inductively to an on-chip superconducting resonator located in the middle of the cavity. At their flux-insensitive point, all measured qubits reach an intrinsic energy relaxation time in the 6-20  μs range and a pure dephasing time comprised between 3 and 10  μs. This significant improvement over previous works opens the way to the coherent coupling of a flux qubit to individual spins.

Quantum memory for microwave photons in an inhomogeneously broadened spin ensemble.

We propose a multimode quantum memory protocol able to store the quantum state of the field in a microwave resonator into an ensemble of electronic spins. The stored information is protected against inhomogeneous broadening of the spin ensemble by spin-echo techniques resulting in memory times orders of magnitude longer than previously achieved. By calculating the evolution of the first and second moments of the spin-cavity system variables for current experimental parameters, we show that a memory based on nitrogen vacancy center spins in diamond can store a qubit encoded on the |0> and |1> Fock states of the field with 80% fidelity and outperform classical memory strategies for storage times ≤69  μs.

Electric-Field-Controlled Magnetoelectric RAM: Progress, Challenges, and Scaling

We review the recent progress in the development of magnetoelectric RAM (MeRAM) based on electric-field-controlled writing in magnetic tunnel junctions (MTJs). MeRAM uses the tunneling magnetoresistance effect for readout in a two-terminal memory element, similar to other types of magnetic RAM. However, the writing of information is performed by voltage control of magnetic anisotropy (VCMA) at the interface of an MgO tunnel barrier and the CoFeB-based free layer, as opposed to current-controlled (e.g., spin-transfer torque or spin-orbit torque) mechanisms. We present results on voltage-induced switching of MTJs in both resonant (precessional) and thermally activated regimes, which demonstrate fast (<;1 ns) and ultralow-power (<;40 fJ/bit) write operations at voltages ~1.5-2 V. We also discuss the implications of the VCMA-based write mechanism on memory array design, highlighting the possibility of crossbar implementation for high bit density. Results are presented from a 1 kbit MeRAM test array. Endurance and voltage scaling data are presented. The scaling behavior is analyzed, and material-level requirements are discussed for the translation of MeRAM into mainstream memory applications.

Giant interfacial perpendicular magnetic anisotropy in MgO/CoFe/capping layer structures

Magnetic tunnel junction (MTJ) based on CoFeB/MgO/CoFeB structures is of great interest due to its application in the spin-transfer-torque magnetic random access memory (STT-MRAM). Large interfacial perpendicular magnetic anisotropy (PMA) is required to achieve high thermal stability. Here we use first-principles calculations to investigate the magnetic anisotropy energy (MAE) of MgO/CoFe/capping layer structures, where the capping materials include 5d metals Hf, Ta, Re, Os, Ir, Pt, Au and 6p metals Tl, Pb, Bi. We demonstrate that it is feasible to enhance PMA by using proper capping materials. Relatively large PMA is found in the structures with capping materials of Hf, Ta, Os, Ir and Pb. More importantly, the MgO/CoFe/Bi structure gives rise to giant PMA (6.09 mJ/m2), which is about three times larger than that of the MgO/CoFe/Ta structure. The origin of the MAE is elucidated by examining the contributions to MAE from each atomic layer and orbital. These findings provide a comprehensive understanding of the PMA and point towards the possibility to achieve advanced-node STT-MRAM with high thermal stability.

Storage and retrieval of microwave fields at the single-photon level in a spin ensemble

We report the storage of microwave pulses at the single-photon level in a spin-ensemble memory consisting of

Enhancement of voltage-controlled magnetic anisotropy through precise control of Mg insertion thickness at CoFeB|MgO interface

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Perpendicular magnetic tunnel junction with W seed and capping layers

nitrogen-vacancy centers in a diamond crystal coupled to a superconducting

In-plane magnetic field effect on switching voltage and thermal stability in electric-field-controlled perpendicular magnetic tunnel junctions

LC

Towards a spin-ensemble quantum memory for superconducting qubits

resonator. The energy of the signal, retrieved