Alexander Konovalenko

Surface spin-valve effect.

We report an observation of spin-valve-like hysteresis within a few atomic layers at a ferromagnetic interface. We use phonon spectroscopy of nanometer-sized point contacts as an in situ probe to study the mechanism of the effect. Distinctive energy phonon peaks for contacts with dissimilar nonmagnetic outer electrodes allow localizing the observed spin switching to the top or bottom interfaces for nanometer thin ferromagnetic layers. The mechanism consistent with our data is energetically distinct atomically thin surface spin layers that can form current- or field-driven surface spin-valves within a single ferromagnetic film.

Spectroscopy of phonons and spin torques in magnetic point contacts.

Phonon spectroscopy is used to investigate the mechanism of current-induced spin torques in nonmagnetic/ferromagnetic (N/F) point contacts. Magnetization excitations observed in the magneto-conductance of the point contacts are pronounced for diffusive and thermal contacts, where the electrons experience significant scattering in the contact region. We find no magnetic excitations in highly ballistic contacts. Our results show that impurity scattering at the N/F interface is the origin of the new single-interface spin torque effect.

Spin dynamics in point contacts to single ferromagnetic films

The first part of this thesis is an experimental study of the spin-dependent transport in magnetic point contacts. Nano-contacts are produced micromechanically, by bringing a sharpened non-magnetic (N) tip into contact with a ferromagnetic (F) film. The magnetic and magneto-transport properties of such N/F nanocontacts are studied using transport spectroscopy, spanning the ballistic, diffusive, and thermal transport regimes. Single N/F interfaces can exhibit current driven magnetic excitations, which are often manifest as peaks in the differential resistance of a point contact defining the N/F interface. Our experiments show that such surface magnetization excitations, and thus the single-interface spin torques, are observed for diffusive and thermal transport regimes where the conduction electrons experience strong scattering near the N/F interface, and are absent for purely ballistic contacts. We conclude that the single-interface spin torque effect is due to impurity scattering at N/F interfaces. Single N/F interfaces can also exhibit hysteretic conductivity, which is qualitatively similar to the spin-valve effect found in F/N/F trilayers. Based on our measurements of N/F point contacts in the size range of 1-30 nm, we propose two mechanisms of the observed hysteresis. The first mechanism relies on a non-uniform spin distribution near the contact core and is magnetoelastic in origin. This interpretation is in good agreement with some of our experiments on larger point contacts as well as with a numerical micromagnetic model we have developed, where a stress-induced anisotropy creates a non-uniform, domain-wall-like spin distribution in the contact core. The second mechanism we propose is a surface effect which relies on a difference between the surface and interior spins in the ferromagnet in terms of their exchange and anisotropy properties. The surface spin-valve mechanism is in good agreement with the hysteretic magnetoresistance observed for our smallest contacts (~1 nm) and for contacts to nanometer thin ferromagnetic films. This interpretation means that the surface magnetization can be reduced and weakly coupled to the interior spins in the ferromagnet. We find that this surface spin layer can be affected by both external fields and the spin torque of a transport current. The surface magnetization can even form nano-sized spin vorticies at the interface. The nature of the magnetic excitations induced by by nominally unpolarized currents through single N/F interfaces was probed directly using microwave irradiation. We observed two characteristic high-frequency effects: a resonant stimulation of spin-wave modes by microwaves, and a rectification of off-resonant microwave currents by spin-wave nonlinearities in the point contact conductance. These experiments demonstrate that the effects observed are spin-dynamic in nature. In the second part of the thesis we study the spin-dynamics in spin-flop tunnel junctions used in toggle magnetic random access memory. Current pulses in the range of 100 ps used to excite the magnetic moments of the two coupled Py free layers into an oscillatory state, in both the antiparallel and scissor states of the cell. These oscillations are detected directly by measuring the junction resistance in real time with a 6 GHz measurement bandwidth. The junctions had the shape of an ellipse, with lateral size ranging from 350x420 to 400x560 nm. The optical and acoustical precession modes of the the spin-flop trilayer are observed in experiment, as expected from single-domain model. The experimental spectra contain additional features, which are explained using numerical micromagnetic simulations, as originating from magnetic state transitions between different magnetization states with non-uniform spin distributions.

Micromagnetics of Spin-Flop Bilayers: S, C, and Vortex Spin States

Spin-flop tunnel junctions subjected to strong gigahertz excitations are found to exhibit highly stable resistance states, intermediate between the two spin-uniform ground states of high and low resistance. The associated spin distributions are necessarily nonuniform and differ significantly from the ground-state anti-parallel spin configuration in their static and dynamic properties. Detailed micromagnetic modeling reveals that inplane spin vortices in dipole-coupled thin-film bilayers are stable spin configurations, where the orientation of the vortex cores and the vortex chirality play an important role in the response of the system to external magnetic fields.

Spin-torque driven excitations and hysteresis in magnetic point contacts

Magnetic multilayers are known to exhibit magnetization excitations, which are due to exchange fields produced by densely distributed spin-polarized electron currents. We report an observation of such excitations as well as hysteretic spin states produced by unpolarized currents in point contacts to single ferromagnetic layers. By measuring diffusive and ballistic contacts for various material combinations, we investigate the microscopic mechanism of the single interface spin-torque effect and discuss the possible origin of the observed hysteresis.

Current driven tri-stable resistance states in magnetic point contacts

Point contacts between normal and ferromagnetic metals are investigated using magnetoresistance and transport spectroscopy measurements combined with micromagnetic simulations. Pronounced hysteresis in the point contact resistance versus both bias current and external magnetic field are observed. It is found that such hysteretic resistance can exhibit, in addition to bi-stable resistance states found in ordinary spin valves, tri-stable resistance states with a middle resistance level. We interpret these observations in terms of surface spin valve and spin vortex states, originating from a substantially modified spin structure at the ferromagnetic interface in the contact core. We argue that these surface spin states, subject to a weakened exchange interaction, dominate the effects of spin transfer torques on the nanometer scale.

Nonstationary magnetization dynamics of point contacts with a single ferromagnetic film

The electric conductivity of point nanocontacts between 5, 10, and 100nm thick ferromagnetic (F) cobalt films and a nonmagnetic (N) metal (copper or silver) needle is investigated. Two fundamentally different mechanisms for the formation of the response signal to microwave irradiation have been observed for the first time. One mechanism is due to the effect of the rectification of high frequency ac current on the nonlinearity of the current-voltage characteristics due to the precession of the magnetization under the action of a constant transport current flowing through the contact. The second one is associated with the resonance excitation of the precession of the magnetization vector at the fundamental frequency and its harmonic by an external high-frequency field. The experimental results support a previous contention that a “surface spin valve” whose static and dynamic properties are similar to conventional F1–N–F2 spin valve [Nano Letters 7, 927 (2007)] is formed in the experimental F–N nanocontacts.

Magnetic unipolar features in conductivity of point contacts between normal and ferromagnetic D-metals (Co, Ni, Fe)

In nanocontacts between normal and ferromagnetic metals (N-F) abrupt changes of the order of 1 % are detected in di erential resistance, dV/dI(V), versus bias voltage, V, on achieving of high current densities, 10 9 A/cm 2 . These features in dV/dI(V) are observed when the electron flow is directed from the nonmagnetic metal into the ferromagnet and connected with magnetization excitations in the ferromagnet induced by the current. Applying an external magnetic field leads to a shift of the observed features to higher biasing current, confirming the magnetic nature of the e ect. Such e ects are observed for the non-ballistic (not spectral) regime of current flow in the nanocontacts. Thus, the current induced magneto-conductance e ects in multilayered N-F struc- tures (nanopillars) extensively studied in the recent literature have much more general character and can be stimulated by elastic electron scattering at single N-F interfaces.

On the mechanism of hysteresis in conductance of point contacts to single ferromagnetic films

Single nonmagnetic/ferromagnetic interfaces can exhibit magnetic excitations and hysteretic switching, provided that the current density traversing the interface is sufficiently high (greater than ...

Vortex-like state observed in ferromagnetic contacts

Point-contacts (PC) offer a simple way to create high current densities, 109 A/cm2 and beyond, without substantial Joule heating. We have shown recently (Nano Letters, 7 (2007) 927) that conductivity of nanosized PCs between a normal and ferromagnetic metals exhibits bi-stable hysteretic states versus both bias current and external magnetic field – the effect typical for spin-valve structures. Here we report that apart from the bi-stable state a third intermediate-resistance state is occasionally observed. We interpret this state as due to a spin-vortex in the PC, nucleated either by Oersted field of the bias current and/or by the circular geometry of PC. The observed three-level-states in the PC conductivity testify that the interface spins are both weakly coupled to the spins in the bulk and have depressed exchange interaction within the surface layer.