We theoretically study the Josephson effect in a superconductor/normal metal/superconductor ({it S}/{it N}/{it S}) Josephson junction composed of $s$-wave {it S}s with {it N} which is sandwiched by two ferromagnetic insulators ({it F}s), forming a spin valve, in the vertical direction of the junction. We show that the 0-$pi$ transition of the Josephson critical current occurs with increasing the thickness of {it N} along the junction. This transition is due to the magnetic proximity effect (MPE) which induces ferromagnetic magnetization in the {it N}. Moreover, we find that, even for fixed thickness of {it N}, the proposed Josephson junction with the spin valve can be switched from $pi$ to 0 states and vice versa by varying the magnetization configuration (parallel or antiparallel) of two {it F}s. We also examine the effect of spin-orbit scattering on the Josephson critical current and argue that the 0-$pi$ transition found here can be experimentally observed within the current nanofabrication techniques, thus indicating a promising potential of this junction as a 0-$pi$ switching device operated reversibly with varying the magnetic configuration in the spin valve by, e.g., applying an external magnetic field. %with the magnetization configuration in the spin valve. Our results not only provide possible applications in superconducting electronics but also suggest the importance of a fundamental concept of MPE in nanostructures of multilayer {it N}/{it F} systems.
We introduce a block Lanczos (BL) recursive technique to construct quasi-one-dimensional models, suitable for density-matrix renormalization group (DMRG) calculations, from single- as well as multiple-impurity Anderson models in any spatial dimensions. This new scheme, named BL-DMRG method, allows us to calculate not only local but also spatially dependent static and dynamical quantities of the ground state for general Anderson impurity models without losing elaborate geometrical information of the lattice. We show that the BL-DMRG method can be easily extended to treat a multi-orbital Anderson impurity model. We also show that the symmetry adapted BL bases can be utilized, when it is appropriate, to reduce the computational cost. As a demonstration, we apply the BL-DMRG method to three different models for graphene: (i) a single adatom on the honeycomb lattice, (ii) a substitutional impurity in the honeycomb lattice, and (iii) an effective model for a single carbon vacancy in graphene. Our analysis reveals that, for the particle-hole symmetric case at half filling of electron density, the ground state of model (i) behaves as an isolated magnetic impurity with no Kondo screening while the ground state of the other two models forms a spin singlet state. We also calculate the real-space dependence of the spin-spin correlation functions between the impurity site and the conduction sites for these three models. Our results clearly show that, reflecting the presence of absence of unscreened magnetic moment at the impurity site, the spin-spin correlation functions decay as $r^{-3}$, differently from the non-interacting limit ($r^{-2}$), for model (i) and as $ r^{-4}$, exactly the same as the non-interacting limit, for models (ii) and (iii) in the asymptotic $r$, where $r$ is the distance between the impurity site and the conduction site.
The two-orbital double-exchange model is employed for the study of the magnetic and orbital orders in ($R$MnO$_3$)$_n$/($A$MnO$_3$)$_{2n}$ ($R$: rare earths; $A$: alkaline earths) superlattices. The A-type antiferromagnetic order is observed in a broad region of parameter space for the case of SrTiO$_3$ as substrate, in agreement with recent experiments and first-principles calculations using these superlattices. In addition, also a C-type antiferromagnetic state is predicted to be stabilized when using substrates like LaAlO$_3$ with smaller lattice constants than SrTiO$_3$, again in agreement with first principles results. The physical mechanism for the stabilization of the A- and C- magnetic transitions is driven by the orbital splitting of the $x^2-y^2$ and $3z^2-r^2$ orbitals. This splitting is induced by the $Q_3$ mode of Jahn-Teller distortions created by the strain induced by the substrates. In addition to the special example of (LaMnO$_3$)$_n$/(SrMnO$_3$)$_{2n}$, our phase diagrams can be valuable for the case where the superlattices are prepared employing narrow bandwidth manganites. In particular, several non-homogenous magnetic profiles are predicted to occur in narrow bandwidth superlattices, highlighting the importance of carrying out investigations in this mostly unexplored area of research.
In a recent publication (S. Dong et al., Phys. Rev. Lett.103, 127201 (2009)), two (related) mechanisms were proposed to understand the intrinsic exchange bias present in oxides heterostructures involving G-type antiferromagnetic perovskites. The first mechanism is driven by the Dzyaloshinskii-Moriya interaction, which is a spin-orbit coupling effect. The second is induced by the ferroelectric polarization, and it is only active in heterostructures involving multiferroics. Using the SrRuO$_3$/SrMnO$_3$ superlattice as a model system, density-functional calculations are here performed to verify the two proposals. This proof-of-principle calculation provides convincing evidence that qualitatively supports both proposals.
Based on the spin-pumping theory and first-principles calculations, the spin-mixing conductance (SMC) is theoretically studied for Pt/Permalloy (Ni$_{81}$Fe$_{19}$, Py) junctions. We evaluate the SMC for ideally clean Pt/Py junctions and examine the effects of interface randomness. We find that the SMC is generally enhanced in the presence of interface roughness as compared to the ideally clean junctions. Our estimated SMC is in good quantitative agreement with the recent experiment for Pt/Py junctions. We propose possible routes to increase the SMC in Pt/Py junctions by depositing a foreign magnetic metal layer in Pt, offering guidelines for designing the future spintronic devices.
The modulation of charge density and spin order in (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ ($n$=1-4) superlattices is studied via Monte Carlo simulations of the double-exchange model. G-type antiferromagnetic barriers in the SrMnO$_{3}$ regions with low charge density are found to separate ferromagnetic LaMnO$_{3}$ layers with high charge density. The recently experimentally observed metal-insulator transition with increasing $n$ is reproduced in our studies, and $n=3$ is found to be the critical value.
The origin of the spiral spin-order in perovskite multiferroic manganites $R$MnO$_{3}$ ($RE=$ Tb or Dy) is here investigated using a two $e_{rm g}$-orbitals double-exchange model. Our main result is that the experimentally observed spiral phase can be stabilized by introducing a relatively weak next-nearest-neighbor superexchange coupling ($sim10%$ of the nearest-neighbor superexchange). Moreover, the Jahn-Teller lattice distortion is also shown to be essential to obtain a realistic spiral period. Supporting our conclusions, the generic phase diagram of undoped perovskite manganites is obtained using Monte Carlo simulations, showing phase transitions from the A-type antiferromagnet, to the spiral phase, and finally to the E-type antiferromagnet, with decreasing size of the $R$ ions. These results are qualitatively explained by the enhanced relative intensity of the superexchanges.
Most previous investigations have shown that the surface of a ferromagnetic material may have antiferromagnetic tendencies. However, experimentally the opposite effect has been recently observed: ferromagnetism appears in some nano-sized manganites with a composition such that the antiferromagnetic charge-ordered CE state is observed in the bulk. A possible origin is the development of ferromagnetic correlations at the surface of these small systems. To clarify these puzzling experimental observations, we have studied the two-orbital double-exchange model near half-doping n=0.5, using open boundary conditions to simulate the surface of either bulk or nano-sized manganites. Considering the enhancement of surface charge density due to a possible AO termination (A = trivalent/divalent ion composite, O = oxygen), an unexpected surface phase-separated state emerges when the model is studied using Monte Carlo techniques on small clusters. This tendency suppresses the CE charge ordering and produces a weak ferromagnetic signal that could explain the experimental observations.
