This work reports the first study of current-driven magnetization noise in a single, nanometerscale, ferromagnetic (Co) particle, attached to normal metal leads by high-resistance tunneling junctions. As the tunnel current increases at low temperature, the magnetic switching field decreases, its probability distribution widens, while the temperature of the environment remains nearly constant. These observations demonstrate nonequilibrium magnetization noise. A classical model of the noise is provided, where the spin-orbit interaction plays a central role in driving magnetic tunneling transitions.

It is found that the magnetoresistance of a nanometer-scale Al particle, attached between a ferromagnetic and an Al lead, has strong asymmetry with respect to the bias voltage. The asymmetric magnetoresistance is explained in terms of the injection of a spin-polarized current from the ferromagnetic lead and the spin accumulation in the particle. The magnetic moment in the particle is parallel to the magnetic field, which is not collinear with the magnetization of the ferromagnetic lead. The field direction changes either discontinuously as the magnetization switches, or continuously as an external magnetic field is being varied, explaining the magnetoresistance.

We present measurements of spin-polarized electron tunneling through a nanometer-scale Al particle in contact with two ferromagnets as a function of the direction of the applied magnetic field at 4.2 K. We find that if the magnetizations of the ferromagnets are aligned parallel, the tunnel current has a weak dependence on magnetic-field direction I ↑↑ Ϸ const, while if the magnetizations of the ferromagnets are aligned antiparallel, the current has significant dependence on magnetic-field direction I ↑↓ ͑␣͒ϷI ↑↑,0 sin 2 ͑␣͒ + I ↑↓,0 cos 2 ͑␣͒, where ␣ is the angle between the magnetic field and the magnetizations. Those dependencies are in agreement with the model of spin accumulation by incoherent electron transport via Zeeman-split energy levels of the particle. They demonstrate that the electron spin in the particle at finite current accumulates along the direction of the magnetic field rather than the magnetization direction. In zero magnetic field, the spin-accumulation direction is set by the field of the environment making it possible to study inhomogeneous spin dephasing. A lower limit of 8 ns is found for the inhomogeneous spin-dephasing time.

We utilize single-electron tunneling spectroscopy to measure the discrete energy levels in a nanometer-scale cobalt particle at T=60mK, and find effective single-electron spin g-factors ≈ 7.3. These large g-factors do not result from the typical orbital contribution to g-factors, since the orbital angular momentum is quenched. Instead, they are due to non-trivial many-body excitations. A kink in the plot of conductance vs. voltage and magnetic field is a signature of degenerate total spin on the particle. Spin-Orbit interactions cause the new particle eigenstates to have 'spin' that is an admixture of pure spin states. Fluctuations in the discrete energy level spacing allow for the total change in 'spin' on the particle during a single-electron tunneling event to be ∆S ′ = 3/2, leading to a g-factor around 6.

We study electron tunneling through Co nanoparticles in the presence of repeated microwave pulses at 4.2K. While individual pulses are too weak to affect the magnetic switching field, repeated microwave pulses start to reduce the magnetic switching field at 10{\mu}s spacing. We use I-V curve as a thermometer to show that the microwave pulses do not heat the sample, showing that magnetization in Co nanoparticles is directly excited by microwave pulses, and the relaxation time of the excitation energy is in the range of microsecond.Comment: 10 pages, 5 figure