Magnetic Switching of a Single Molecular Magnet due to Spin-Polarized Current

Magnetic switching of a single molecular magnet (SMM) due to spin-polarized current flowing between ferromagnetic metallic electrodes is investigated theoretically. Magnetic moments of the electrodes are assumed to be collinear and parallel to the magnetic easy axis of the molecule. Electrons tunneling through a barrier between magnetic leads are coupled to the SMM via exchange interaction. The current flowing through the system as well as the spin relaxation times of the SMM are calculated from the Fermi golden rule. It is shown that spin of the SMM can be reversed by applying a voltage between the two magnetic electrodes. Moreover, the switching is reflected in the corresponding current-voltage characteristics.Comment: 5 pages, 4 figures, final version as publishe

Effect of magnetic anisotropy on spin-dependent thermoelectric effects in nanoscopic systems

Conventional and spin-related thermoelectric effects in electronic transport through a nanoscopic system exhibiting magnetic anisotropy $-$with both uniaxial and transverse components$-$ are studied theoretically in the linear response regime. In particular, a magnetic tunnel junction with a large-spin impurity $-$either a magnetic atom or a magnetic molecule$-$ embedded in the barrier is considered as an example. Owing to magnetic interaction with the impurity, conduction electrons traversing the junction can scatter on the impurity, which effectively can lead to angular momentum and energy exchange between the electrons and the impurity. As we show, such processes have a profound effect on the thermoelectric response of the system. Specifically, we present a detailed analysis of charge, spin and thermal conductance, together with the Seebeck and spin Seebeck coefficients (thermopowers). Since the scattering mechanism also involves processes when electrons are inelastically scattered back to the same electrode, one can expect the flow of spin and energy also in the absence of charge transport through the junction. This, in turn, results in a finite spin thermopower, and the magnetic anisotropy plays a key role for this effect to occur.Comment: 23 pages with 16 figures; version as publishe

Spin-dependent thermoelectric effects in transport through a nanoscopic junction involving spin impurity

Conventional and spin-related thermoelectric effects in transport through a magnetic tunnel junction with a large-spin impurity, such as a magnetic molecule or atom, embedded into the corresponding barrier are studied theoretically in the linear-response regime. The impurity is described by the giant spin Hamiltonian, with both uniaxial and transverse magnetic anisotropy taken into account. Owing to the presence of transverse component of magnetic anisotropy, spin of a tunneling electron can be reversed during scattering on the impurity, even in the low temperature regime. This reversal appears due to exchange interaction of tunneling electrons with the magnetic impurity. We calculate Seebeck and spin Seebeck coefficients, and analyze their dependence on various parameters of the spin impurity and tunnel junction. In addition, conventional and spin figures of merit, as well as the electronic contribution to heat conductance are considered. We also show that pure spin current can be driven by a spin bias applied to the junction with spin impurity, even if no electron transfer between the electrodes can take place. The underlying mechanism employs single-electrode tunneling processes (electrode-spin exchange interaction) and the impurity as an intermediate reservoir of angular momentum.Comment: 24 pages with 7 figures, version as publishe

Effects of Transverse Magnetic Anisotropy on Current-Induced Spin Switching

Spin-polarized transport through bistable magnetic adatoms or single-molecule magnets (SMMs), which exhibit both uniaxial and transverse magnetic anisotropy, is considered theoretically. The main focus is on the impact of transverse anisotropy on transport characteristics and the adatom's/SMM's spin. In particular, we analyze the role of quantum tunneling of magnetization (QTM) in the mechanism of the current-induced spin switching, and show that the QTM phenomenon becomes revealed as resonant peaks in the average values of the molecule's spin and in the charge current. These features appear at some resonant fields and are observable when at least one of the electrodes is ferromagnetic. We also show that the conductance generally depends on the relative orientation of the average adatom's/SMM's spin and electrode's magnetic moment. This spin-valve like magnetoresistance effect can be used to control spin switching of the adatom's/SMM's spin.Comment: 5 pages, 3 figures (submitted

Transverse anisotropy effects on spin-resolved transport through large-spin molecules

The transport properties of a large-spin molecule strongly coupled to ferromagnetic leads in the presence of transverse magnetic anisotropy are studied theoretically. The relevant spectral functions, linear-response conductance and the tunnel magnetoresistance are calculated by means of the numerical renormalization group method. We study the dependence of transport characteristics on orbital level position, uniaxial and transverse anisotropies, external magnetic field and temperature. It is shown that while uniaxial magnetic anisotropy leads to the suppression of the Kondo effect, finite transverse anisotropy can restore the Kondo resonance. The effect of Kondo peak restoration strongly depends on the magnetic configuration of the device and leads to nontrivial behavior of the tunnel magnetoresistance. We show that the temperature dependence of the conductance at points where the restoration of the Kondo effect occurs is universal and shows a scaling typical for usual spin-one-half Kondo effect.Comment: 15 pages, 14 figures (submitted to Phys. Rev. B

Spin-resolved dynamical conductance of correlated large-spin magnetic molecules

The finite-frequency transport properties of a large-spin molecule attached to ferromagnetic contacts are studied theoretically in the Kondo regime. The focus is on the behavior of the dynamical conductance in the linear response regime, which is determined by using the numerical renormalization group method. It is shown that the dynamical conductance depends greatly on the magnetic configuration of the device and intrinsic parameters of the molecule. In particular, conductance exhibits characteristic features for frequencies corresponding to the dipolar and quadrupolar exchange fields resulting from the presence of spin-dependent tunneling. Moreover, a dynamical spin accumulation in the molecule, associated with the off-diagonal-in-spin component of the conductance, is predicted. This spin accumulation becomes enhanced with increasing the spin polarization of the leads, and it results in a nonmonotonic dependence of the conductance on frequency, with local maxima occurring for characteristic energy scales

Vibration-induced modulation of magnetic anisotropy in a magnetic molecule

We theoretically analyze the spectrum of a magnetic molecule when its charge and spin can couple to the molecular vibrations. More specifically, we show that the interplay between charge-vibron and spin-vibron coupling leads to a renormalization of the magnetic anisotropy parameters of the molecule. This effect is discussed for a model device consisting of an individual magnetic molecule embedded in a junction. We study the transport properties of the device and illustrate how the differential conductance is affected by the vibrationally induced renormalization of the magnetic anisotropy. Depending on the total molecular spin and the bare (intrinsic) magnetic anisotropy, the induced modulation can lead to visible shifts and crossings in the spectrum, and it can even be the cause of a transport blockade. It is therefore of particular interest to use mechanically controllable break junctions, since in such a case, the relevant coupling between the molecular spin and vibrations can be controlled via deformations of the molecule when stretching or compressing the junction.Comment: 18 pages, 7 figures, version as publishe

Shaping charge excitations in chiral edge states with a time-dependent gate voltage

We study a coherent conductor supporting a single edge channel in which alternating current pulses are created by local time-dependent gating and sent on a beam-splitter realized by a quantum point contact. The current response to the gate voltage in this setup is intrinsically linear. Based on a fully self-consistent treatment employing a Floquet scattering theory, we analyze the effect of different voltage shapes and frequencies, as well as the role of the gate geometry on the injected signal. In particular, we highlight the impact of frequency-dependent screening on the process of shaping the current signal. The feasibility of creating true single-particle excitations with this method is confirmed by investigating the suppression of excess noise, which is otherwise created by additional electron-hole pair excitations in the current signal