Spin splitting and Kondo effect in quantum dots coupled to noncollinear ferromagnetic leads

We study the Kondo effect in a quantum dot coupled to two noncollinear ferromagnetic leads. First, we study the spin splitting $\delta\epsilon=\epsilon_{\downarrow}-\epsilon_{\uparrow}$ of an energy level in the quantum dot by tunnel couplings to the ferromagnetic leads, using the Poor man's scaling method. The spin splitting takes place in an intermediate direction between magnetic moments in the two leads. $\delta\epsilon \propto p\sqrt{\cos^2(\theta/2)+v^2\sin^2(\theta/2)}$, where $p$ is the spin polarization in the leads, $\theta$ is the angle between the magnetic moments, and $v$ is an asymmetric factor of tunnel barriers ($-1<v<1$). Hence the spin splitting is always maximal in the parallel alignment of two ferromagnets ($\theta=0$) and minimal in the antiparallel alignment ($\theta=\pi$). Second, we calculate the Kondo temperature $T_{\mathrm{K}}$. The scaling calculation yields an analytical expression of $T_{\mathrm{K}}$ as a function of $\theta$ and $p$, $T_{\mathrm{K}}(\theta, p)$, when $\delta\epsilon \ll T_{\mathrm{K}}$. $T_{\mathrm{K}}(\theta, p)$ is a decreasing function with respect to $p\sqrt{\cos^2(\theta/2)+v^2\sin^2(\theta/2)}$. When $\delta\epsilon$ is relevant, we evaluate $T_{\mathrm{K}}(\delta\epsilon, \theta, p)$ using the slave-boson mean-field theory. The Kondo resonance is split into two by finite $\delta\epsilon$, which results in the spin accumulation in the quantum dot and suppression of the Kondo effect.Comment: 11 pages, 8 figures, revised versio

Current-driven Magnetization Reversal in a Ferromagnetic Semiconductor (Ga,Mn)As/GaAs/(Ga,Mn)As Tunnel Junction

Current-driven magnetization reversal in a ferromagnetic semiconductor based (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction is demonstrated at 30 K. Magnetoresistance measurements combined with current pulse application on a rectangular 1.5 x 0.3 um^2 device revealed that magnetization switching occurs at low critical current densities of 1.1 - 2.2 x 10^5 A/cm^2 despite the presence of spin-orbit interaction in the p-type semiconductor system. Possible mechanisms responsible for the effect are discussed.Comment: 16 pages, 4 figure

Inverse Tunneling Magnetoresistance in nanoscale Magnetic Tunnel Junctions

We report on our theoretical study of the inverse TMR effect in the spin polarized transport through a narrow channel. In the weak tunneling limit, we find the ordinary positive TMR. The TMR changes its sign as the transmission probability becomes large close to a unity. Our results might be relevant to the magnetic tunnel junction with a pinhole or a quantum point contact.Comment: 11 pages, 4 figures, To be published in Phys. Rev. B (in press

Voltage controlled spin injection in a (Ga,Mn)As/(Al,Ga)As Zener diode

The spin polarization of the electron current in a p-(Ga,Mn)As-n-(Al,Ga)As-Zener tunnel diode, which is embedded in a light-emitting diode, has been studied theoretically. A series of self-consistent simulations determines the charge distribution, the band bending, and the current-voltage characteristics for the entire structure. An empirical tight-binding model, together with the Landauer- Buttiker theory of coherent transport has been developed to study the current spin polarization. This dual approach allows to explain the experimentally observed high magnitude and strong bias dependence of the current spin polarization.Comment: Submitted to Phys. Rev. B Rapid Communication

Efficient nonlinear room-temperature spin injection from ferromagnets into semiconductors through a modified Schottky barrier

We suggest a consistent microscopic theory of spin injection from a ferromagnet (FM) into a semiconductor (S). It describes tunneling and emission of electrons through modified FM-S Schottky barrier with an ultrathin heavily doped interfacial S layer . We calculate nonlinear spin-selective properties of such a reverse-biased FM-S junction, its nonlinear I-V characteristic, current saturation, and spin accumulation in S. We show that the spin polarization of current, spin density, and penetration length increase with the total current until saturation. We find conditions for most efficient spin injection, which are opposite to the results of previous works, since the present theory suggests using a lightly doped resistive semiconductor. It is shown that the maximal spin polarizations of current and electrons (spin accumulation) can approach 100% at room temperatures and low current density in a nondegenerate high-resistance semiconductor.Comment: 7 pages, 2 figures; provides detailed comparison with earlier works on spin injectio

Correlation between tunneling magnetoresistance and magnetization in dipolar coupled nanoparticle arrays

The tunneling magnetoresistance (TMR) of a hexagonal array of dipolar coupled anisotropic magnetic nanoparticles is studied using a resistor network model and a realistic micromagnetic configuration obtained by Monte Carlo simulations. Analysis of the field-dependent TMR and the corresponding magnetization curve shows that dipolar interactions suppress the maximum TMR effect, increase or decrease the field-sensitivity depending on the direction of applied field and introduce strong dependence of the TMR on the direction of the applied magnetic field. For off-plane magnetic fields, maximum values in the TMR signal are associated with the critical field for irreversible rotation of the magnetization. This behavior is more pronounced in strongly interacting systems (magnetically soft), while for weakly interacting systems (magnetically hard) the maximum of TMR (Hmax) occurs below the coercive field (Hc), in contrast to the situation for non-interacting nanoparticles or in-plane fields (Hmax=Hc). The relation of our simulations to recent TMR measurements in self-assembled Co nanoparticle arrays is discussed.Comment: 21 pages, 8 figures, submitted to Physical Review

Tunneling magnetoresistance in diluted magnetic semiconductor tunnel junctions

Using the spin-polarized tunneling model and taking into account the basic physics of ferromagnetic semiconductors, we study the temperature dependence of the tunneling magnetoresistance (TMR) in the diluted magnetic semiconductor (DMS) trilayer heterostructure system (Ga,Mn)As/AlAs/(Ga,Mn)As. The experimentally observed TMR ratio is in reasonable agreement with our result based on the typical material parameters. It is also shown that the TMR ratio has a strong dependence on both the itinerant-carrier density and the magnetic ion density in the DMS electrodes. This can provide a potential way to achieve larger TMR ratio by optimally adjusting the material parameters.Comment: 5 pages (RevTex), 3 figures (eps), submitted to PR

A theoretical investigation of ferromagnetic tunnel junctions with 4-valued conductances

In considering a novel function in ferromagnetic tunnel junctions consisting of ferromagnet(FM)/barrier/FM junctions, we theoretically investigate multiple valued (or multi-level) cell property, which is in principle realized by sensing conductances of four states recorded with magnetization configurations of two FMs; that is, (up,up), (up,down), (down,up), (down,down). To obtain such 4-valued conductances, we propose FM1/spin-polarized barrier/FM2 junctions, where the FM1 and FM2 are different ferromagnets, and the barrier has spin dependence. The proposed idea is applied to the case of the barrier having localized spins. Assuming that all the localized spins are pinned parallel to magnetization axes of the FM1 and FM2, 4-valued conductances are explicitly obtained for the case of many localized spins. Furthermore, objectives for an ideal spin-polarized barrier are discussed.Comment: 9 pages, 3 figures, accepted for publication in J. Phys.: Condens. Matte

Spin-Polarized Transprot through Double Quantum Dots

We investigate spin-polarized transport phenomena through double quantum dots coupled to ferromagnetic leads in series. By means of the slave-boson mean-field approximation, we calculate the conductance in the Kondo regime for two different configurations of the leads: spin-polarization of two ferromagnetic leads is parallel or anti-parallel. It is found that transport shows some remarkable properties depending on the tunneling strength between two dots. These properties are explained in terms of the Kondo resonances in the local density of states.Comment: 8 pages, 11 figure

Spin Diode Based on Fe/MgO Double Tunnel Junction

We demonstrate a spin diode consisting of a semiconductor free nano-scale Fe/MgO-based double tunnel junction. The device exhibits a near perfect spin-valve effect combined with a strong diode effect. The mechanism consistent with our data is resonant tunneling through discrete states in the middle ferromagnetic layer sandwiched by tunnel barriers of different spin-dependent transparency. The observed magneto-resistance is record high, ~4000%, essentially making the structure an on/off spin-switch. This, combined with the strong diode effect, ~100, offers a new device that should be promising for such technologies as magnetic random access memory and re-programmable logic.Comment: 14 page