Large-spin Molecules Research Articles

Giant superconducting proximity effect on spintronic anisotropy

We investigate theoretically the interplay of proximity effects due to the presence of a superconductor and normal ferromagnetic leads on the formation of the spintronic quadrupolar exchange field in a large-spin molecule. We show that the spintronic anisotropy can be enhanced by a few orders of magnitude, and tuned by changing the strength of coupling to the superconductor. Especially large anisotropy is generated in the vicinity of the charge parity changing transition of the molecule. We also provide predictions of measurable spectral properties being the hallmarks of these phenomena.

Magnetoresistive properties of a double magnetic molecule spin valve in different geometrical arrangements

Abstract Spin-dependent transport through a spin valve consisting of two large-spin molecules, such as single molecular magnets, weakly coupled to external ferromagnetic electrodes is theoretically studied. The main focus is on the dependence of the tunneling current, its spin polarization, differential conductance and tunnel magnetoresistance on the arrangement of molecules embedded in the tunnel junction. These quantities are calculated by using the real-time diagrammatic technique in the lowest-order perturbation theory with respect to the coupling strength. When the molecules’ geometry is parallel, both an enhanced as well as inverse tunnel magnetoresistance can develop depending on the molecule’s occupation. On the other hand, if the two molecules form a serial geometry, a strong current-suppression occurs due to the Pauli spin blockade effect, resulting in negative differential conductance. In this transport regime we also find large tunnel magnetoresistance, which now exhibits strong asymmetry with respect to the bias voltage reversal. In addition, we show that an enhanced magnetoresistance and negative differential conductance can develop when a T-shaped molecular geometry is taken under consideration. These effects are explained by invoking particular occupation of molecular states and nonequilibrium spin accumulation that builds up in the case of the antiparallel magnetic configuration of the device.

Spin-resolved dynamical conductance of a correlated large-spin magnetic molecule

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.

Signatures of Transverse Magnetic Anisotropy in Transport through a Large-Spin Molecule in the Kondo Regime

Signatures of Transverse Magnetic Anisotropy in Transport through a Large-Spin Molecule in the Kondo Regime

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.

Chirality tunneling and quantum dynamics for domain walls in mesoscopic ferromagnets

We studied the quantum dynamics of ferromagnetic domain walls (topological kink-type solitons) in one-dimensional ferromagnetic spin chains. We show that the tunneling probability does not depend on the number of spins in a domain wall; thus, this probability can be large even for a domain wall containing a large number of spins. We also predict that there is a strong interplay between the tunneling of a wall from one lattice site to another (tunneling of the kink coordinate) and the tunneling of the kink topological charge (so-called chirality). Both of these elementary processes are suppressed for kinks in one-dimensional ferromagnets with half-integer spin. The dispersion law (i.e., the domain wall energy versus momentum) is essentially different for chains with either integer or half-integer spins. The predicted quantum effects could be observed for mesoscopic magnetic structures, e.g., chains of magnetic clusters, large-spin molecules, or nanosize magnetic dots.

Dynamics of large-spin molecules in a time-dependent magnetic field

A numerically exact solution of the time-dependent Schr\odinger equation for noninteracting large-spin $(S=10)$ molecules in the presence of crystal-field anisotropy and time-dependent longitudinal magnetic field is performed. Crystal-field parameters and a field sweeping rate to fit experiment on ${\mathrm{Fe}}_{8}$ and ${\mathrm{Mn}}_{12}$ molecules are used. Magnetization steps are found for values of the sweeping field at which anticrossing of two energy levels occurs. The size of the step depends crucially on the field sweeping rate and on the presence of a transverse field. An efficient approximation is proposed in order to reduce the solution of the original $(2S+1)$-level problem to a sequence of two-level problems. This approach allows us to solve the time-dependent Schr\odinger equation with the experimental field sweeping rate, which is prevented in the original $(2S+1)$-level problem because of the huge computing time.

A Rational Route for Large Spin Molecules Based on Cyanometalate Complexes

Polynuclear molecules with large-spin state and high anisotropy are discrete entities situated at the frontier between quantum and classical systems. These well defined complexes displaying uniform shape, magnetic moment and anisotropy can behave as single-molecule magnets. Following a bottom-up approach of nanomaterials, we present a rational synthetic route of such molecules based on hexacyanometalates [M(CN)6]3−cores. Thus, were built and fully characterised (mass spectroscopy, crystallographic structure, magnetic susceptibility, …) polynuclear species such as [M(CNM'L)6]9± or [M(CN)4(CNM'L)2]± (with M= CrIII, CoIII, FeIII; M′= NiII,CoII,CuII,…; L =polydentate ligand) showing large ground spin states.

Quantum fluctuations in many-spin magnetic molecules

A new type of mesoscopic quantum effect in large-spin molecules possessing easy-axis anisotropy, such as Mn12, is predicted. The response of such a system to an external field applied perpendicular to the easy axis is considered. It is shown that the susceptibility of this system exhibits a peculiar peak of purely quantum origin. This effect arises from very general properties of quantum fluctuations in spin systems. We demonstrate that the effect is entirely accessible for contemporary experimental techniques. Our studies show that the many-spin nature of the Mn12 clusters is important for a correct description of this quantum peak.