E. del Barco

Spin-transfer-induced precessional magnetization reversal

A magnetoelectronic device is proposed in which a spin-current pulse produces a rapid reversal of the magnetization of a thin film nanomagnet. A spin-transfer torque induces the reversal and the switching speed is determined by the precession frequency of the magnetization in a thin film element’s demagnetization field. Micromagnetic simulations show that this switching occurs above a threshold pulse current and can be faster than 50 ps. In contrast to present spin-transfer devices, the switching does not require an initial fluctuation or deviation of magnetic layers from collinear alignment and is far more energy efficient. This device operates at room temperature and can be realized with present-day magnetic nanostructure technology.

Tunable magnetocaloric effect in ceramic perovskites

A large entropy variation (magnetocaloric effect) has been discovered in ceramic perovskites with the formulas La0.65Ca0.35Ti1−xMnxO3−z and La0.5+x+yLi0.5−3yTi1−3xMn3xO3−z. Both Curie temperature and entropy change were studied from 4.2 to 400 K for different stoichiometric compositions and applied magnetic fields. Our conclusion is that these materials are excellent candidates for working materials in magnetic refrigeration and liquefaction devices in a wide temperature range.

Magnetic qubits as hardware for quantum computers

We propose two potential realizations for quantum bits based on nanometre-scale magnetic particles of large spin S and high-anisotropy molecular clusters. In case (1) the bit-value basis states |0 and |1 are the ground and first excited spin states Sz = S and S-1, separated by an energy gap given by the ferromagnetic resonance frequency. In case (2), when there is significant tunnelling through the anisotropy barrier, the qubit states correspond to the symmetric, |0, and antisymmetric, |1, combinations of the twofold degenerate ground state Sz = ±S. In each case the temperature of operation must be low compared to the energy gap, Δ, between the states |0 and |1. The gap Δ in case (2) can be controlled with an external magnetic field perpendicular to the easy axis of the molecular cluster. The states of different molecular clusters and magnetic particles may be entangled by connecting them by superconducting lines with Josephson switches, leading to the potential for quantum computing hardware.

Quantum coherence in Fe8 molecular nanomagnets

We report observation of coherent quantum oscillations in spin-10 Fe8 molecular clusters. The powder of magnetically oriented Fe8 crystallites was placed inside a resonator, in a dc magnetic field perpendicular to the magnetization axis. The field dependence of the ac-susceptibility was measured up to 5 T, at 680 MHz, down to 25 mK. Two peaks in the imaginary part of the susceptibility have been detected, whose positions coincide, without any fitting parameters, with the predicted two peaks corresponding to the quantum splitting of the ground state in the magnetic field parallel and perpendicular to the hard magnetization axis.

Symmetry of Magnetic Quantum Tunneling in Single Molecule Magnet Mn12-Acetate

The symmetry of magnetic quantum tunneling has been studied in the prototype single molecule magnet Mn12-acetate using a micro-Hall effect magnetometer and superconducting high field vector magnet system. An average crystal fourfold symmetry is shown to be due to local molecular environments of twofold symmetry that are rotated by 90 degrees with respect to one another, confirming that disorder which lowers the molecule symmetry is as important to magnetic quantum tunneling. We have studied a subset of these lower (twofold) site symmetry molecules and present evidence for a Berry phase effect consistent with a local twofold symmetry.

High-frequency resonant experiments in Fe 8 molecular clusters

Precise resonant experiments on

Quantum superposition of high spin states in the single molecule magnet Ni4

{\mathrm{Fe}}_{8}

Fabrication of nanogapped single-electron transistors for transport studies of individual single-molecule magnets

magnetic clusters have been conducted down to 1.2 K at various tranverse magnetic fields, using a cylindrical resonator cavity with 40 different frequencies between 37 and 110 GHz. All the observed resonances for both single crystal and oriented powder have been fitted by the eigenstates of the Hamiltonian

Magnetocaloric effect in La0.65Ca0.35Ti1 − xMnxO3 ceramic perovskites

\mathcal{H}=\ensuremath{-}{\mathrm{DS}}_{z}^{2}{+ES}_{x}^{2}\ensuremath{-}g{\ensuremath{\mu}}_{B}\mathbf{H}\ensuremath{\cdot}\mathbf{S}{+C(S}_{+}^{4}{+S}_{\ensuremath{-}}^{4}).

Dynamic spin injection into chemical vapor deposited graphene

We have identified the resonances corresponding to the coherent quantum oscillations for different orientations of spin