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We theoretically investigate the effective exchange interaction, $J_\mathrm{eff}$, mediated by conductive electrons within a nonmagnetic metal spacer, in the presence of a bias voltage, sandwiched by two ferromagnetic insulators. On the basis of the tight-binding model, we show the voltage and spacer thickness dependences of $J_\mathrm{eff}$, and its contorollability is demonstrated. We also propose a new magnonic device with the functions of both field effect transistor and non-volatile memory.

The anomalous low-temperature behaviour of the spontaneous magnetization of ordered arrays of quantum dots is discussed. It is shown that the experimental results can be well understood, within a mean field approximation, assuming collective response through rather strong spin tunneling processes between neighbouring dots.

We theoretically investigate the spin--wave spin current induced by the spin Seebeck effect in magnonic spin tunneling junctions (MSTJs) for arbitrary magnetization directions. We show that the MSTJ functions as a \textit{tunable} spin Seebeck diode in which the tunneling spin current can be turned on and off with high efficiency by controlling the magnetization direction., 3 pages, 1 figure

arXiv:1509.07291v1, We report characterization and magnetic studies of mixtures of micrometer-size ribbons of Mn12 acetate and micrometer-size particles of YBaCuO superconductor. Extremely narrow zero-field spin-tunneling resonance has been observed in the mixtures, pointing to the absence of the inhomogeneous dipolar broadening. It is attributed to the screening of the internal magnetic fields in the magnetic particles by Meissner currents flowing between superconducting grains surrounding the particles., The work at the University of Barcelona has been supported by the Spanish Government Project No. MAT2011-23698. A.G.-S. acknowledges support from Universitat de Barcelona. I.I. and J.E. thank the MINECO for the Ramon y Cajal contract and the FPI fellowship, respectively. The work of EMC at CUNY Lehman College has been supported by the Office of Science of the U.S. Department of Energy through grant No. DE-FG02-93ER45487.

Magnetic properties of a mixed spin- 1 2 and spin- 3 2 Ising model on honeycomb lattice are investigated within the framework of an exact star-triangle mapping transformation. The particular attention is focused on the effect of uniaxial and biaxial crystal-field potentials that basically influence a magnetic behavior of the spin- 3 2 atoms. Our results for basic thermodynamic quantities as well as dynamical time-dependent autocorrelation function indicate a spin tunneling between | ± 3 2 〉 and | ∓ 1 2 〉 states in two different magnetically ordered phases OP1 and OP2, respectively.

The magnetoresistance across interfaces in the itinerant ferromagnetic oxide ${\mathrm{SrRuO}}_{3}$ have been studied. To define appropriately the interfaces, epitaxial thin films have been grown on bicrystalline and laser-patterned ${\mathrm{SrTiO}}_{3}$ substrates. Comparison is made with results obtained on similar experiments using the double-exchange ferromagnetic oxide ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_{3}.$ It is found that in ${\mathrm{SrRuO}}_{3},$ interfaces induce a substantial negative magnetoresistance, although no traces of the low-field spin tunneling magnetoresistance are found. We discuss these results on the basis of the distinct degree of spin polarization in ruthenates and manganites and the different nature of the surface magnetic layer formed at interfaces.

A criterion is derived for the determination of parameter domains of first-order phase transitions in quantum-mechanical tunneling models. The criterion is tested by application to various models, in particular to some that have been used recently to explore spin tunneling in macroscopic particles. In each case agreement is found with previously heuristically determined domains.

We prove that the spin-boson system is ergodic, for arbitrary strengths of the coupling between the spin and the boson bath, provided the spin tunneling matrix element is small enough.

We present a theory whose goal is to account for the recent exciting hysteresis experiments on the spin cluster crystal Mn12-acetate that indicate the occurrence of spin tunneling. We have found the tunneling probability P for the tunneling of spin from one spin state into another at a spin level crossing in response to a swept applied magnetic field H. It is shown that P is a function of only one parameter, the dimensionless sweep rate ρ given by ρ ≡ 2/Δ20, where 2 is the rate at which the two energy levels approach each other as the field is swept (being proportional to the sweep rate dH/dt). Δ0 is the minimum energy gap at the anti-crossing, Δ0/ being the tunneling rate thereat. Numerical integration leads to P(ρ) = 1 − exp [ − [(π)/(2ρ)]]. We find that, contrary to widespread belief, the experiments cannot be understood in terms of tunneling of individual spin clusters in the presence of a static applied transverse field. Rather, the required transverse field must have dynamic nuclear spins and other dynamic spin clusters as its source.

The laser-induced ultrafast demagnetization of CoFeB/MgO/CoFeB magnetic tunneling junction is exploited by time-resolved magneto-optical Kerr effect (TRMOKE) for both the parallel state (P state) and the antiparallel state (AP state) of the magnetizations between two magnetic layers. It was observed that the demagnetization time is shorter and the magnitude of demagnetization is larger in the AP state than those in the P state. These behaviors are attributed to the ultrafast spin transfer between two CoFeB layers via the tunneling of hot electrons through the MgO barrier. Our observation indicates that ultrafast demagnetization can be engineered by the hot electrons tunneling current. It opens the door to manipulate the ultrafast spin current in magnetic tunneling junctions.

Spin tunneling in molecular magnets controlled by dipole-dipole interactions (DDI) in the disordered state has been considered numerically on the basis of the microscopic model using the quantum mean-field approximation. In the actual case of a strong DDI spin coherence is completely lost and there is a slow relaxation of magnetization, described by t^{3/4} at short times. Fast precessing nuclear spins, included in the model microscopically, only moderately speed up the relaxation., Comment: 10 pages, 9 figures, to be published in EPJB

Combination of the thermal effet in magnetic deflagration with resonance spin tunneling controlled by the dipole-dipole interaction in molecular magnets leads to the increase of the deflagration speed in the dipolar window near tunneling resonances., 4 pages, 4 figures

We report here on anisotropy of spin polarization obtained in lateral all-semiconductor all-electrical spin injection devices, employing $p^{+}-$(Ga,Mn)As/$n^{+}-$GaAs Esaki diode structures as spin aligning contacts, resulting from the dependence of the efficiency of spin tunneling on the orientation of spins with respect to different crystallographic directions. We observed an in-plane anisotropy of $~8%$ in case of spins oriented either along $[1\bar{1}0]$ or $[110]$ directions and $~25%$ anisotropy between in-plane and perpendicular-to-plane orientation of spins., 9 pages, 3 figures

We study the quantum dynamics of a system consisting of a magnetic molecule placed on a microcantilever. The amplitude and frequencies of the coupled magneto-mechanical oscillations are computed. Parameter-free theory shows that the existing experimental techniques permit observation of the driven coupled oscillations of the spin and the cantilever, as well as of the splitting of the mechanical modes of the cantilever caused by spin tunneling., 4 pages, 3 figures

We study spin-rotation effects in a magnetic molecule bridged between two conducting leads. Dynamics of the total angular momentum couples spin tunneling to the mechanical rotations. Landau-Zener spin transition produced by the time-dependent magnetic field generates a unique pattern of mechanical oscillations that can be detected by measuring the electronic tunneling current through the molecule., Comment: 5 pages, 2 figures

We study low temperature resonant spin tunneling in molecular magnets induced by a field sweep with account of dipole-dipole interactions. Numerical simulations uncovered formation of self-organized patterns of the magnetization and of the ensuing dipolar field that provide resonant condition inside a finite volume of the crystal. This effect is robust with respect to disorder and should be relevant to the dynamics of the magnetization steps observed in molecular magnets., 4 Phys. Rev. pages, 5 figures

We have shown that the size of the magnetization step due to resonant spin tunneling in a molecular magnet can be strongly affected by sound. The transverse acoustic wave can also generate macroscopic quantum beats of the magnetization during the field sweep., Comment: 4 pages, 6 figures

Spin tunneling in the particular case of the magnetic molecular cluster octanuclear iron(III), Fe8, is treated by an effective Hamiltonian that allows for an angle-based description of the process. The presence of an external magnetic field along the easy axis is also taken into account in this description. Analytic expressions for the energy levels and barriers are obtained from a harmonic approximation of the potential function which give results in good agreement with the experimental results. The energy splittings due to spin tunneling is treated in an adapted WKB approach and it is shown that the present description can give results to a reliable degree of accuracy., Comment: 17 pages, 2 figures, preprint submitted to Physica A

A storage mechanism in ferromagnetic-insulator-ferromagnetic spin-polarized tunneling junctions has been found by us. Based on this mechanism, we designed and fabricated a 2/spl times/2 bit Spin Tunneling Random Access Memory (STram). These junctions provide an excellent means of storing a binary data in the hard components, and sensing its remanent state by switching the soft component in such a way that the magnetic state of the hard component remains unaltered.

Magnetic properties of a mixed spin-$\frac{1}{2}$ and spin-1 Ising model on honeycomb lattice are exactly investigated within the framework of generalized star-triangle mapping transformation. The particular attention is focused on the effect of uniaxial and biaxial crystal-field anisotropies that basically influence the magnetic behavior of the spin-1 atoms. Our results for the basic thermodynamic quantities, as well as the dynamical time-dependent autocorrelation function indicate the spin tunneling between the $\ensuremath{\mid}+1⟩$ and $\ensuremath{\mid}\ensuremath{-}1⟩$ states in the magnetically ordered phase.

In this paper, we present a method for reducing the three dimensional Schrodinger equation to study confined metallic states, such as quantum well states, in a multilayer film geometry. While discussing some approximations that are employed when dealing with the three dimensionality of the problem, we derive a one dimensional equation suitable for studying such states using an envelope function approach. Some applications to the Cu/Co multilayer system with regard to spin tunneling/rotations and angle resolved photoemission are discussed., Comment: 14 pages, 1 figure

The level splittings in a dimer with the antiferromagnetic coupling between two single-molecule magnets are calculated perturbatively for arbitrary spin. It is found that the exchange interaction between two single-molecule magnets plays an important role in the level splitting. The results are discussed in comparison with the recent experiment., 12 pages, to be published in Phys. Rev. B

The question addressed in this paper is that of the influence of the density of dislocations on the spin tunneling in ${\mathrm{Mn}}_{12}$ clusters. We have determined the variation in the mosaicity of fresh and thermally treated single crystals of ${\mathrm{Mn}}_{12}$ by analyzing the widening of low angle x-ray diffraction peaks. It has also been well established from both isothermal magnetization and relaxation experiments that there is a broad distribution of tunneling rates which is shifted to higher rates when the density of dislocations increases.

We show that dislocations should be the main source of spin tunneling in ${\mathrm{Mn}}_{12}$ crystals. Long range strains caused by dislocations produce broad distribution of relaxation times that has been seen in many experiments. When the external magnetic field is applied along the $c$ axis of the crystal, local rotations of the magnetic anisotropy axis due to dislocations result in the effective local transverse magnetic field that unfreezes odd tunneling resonances. Scaling law is derived that provides universal description of spin tunneling for all resonances.

Spin tunneling in molecular magnets has attracted much attention, however theoretical considerations of this phenomenon up to now have not taken into account the many-spin nature of molecular magnets. We present, to our knowledge, the first successful attempt of a realistic calculation of tunneling splittings for Mn$_{12}$ molecules, thus achieving a quantitatively accurate many-spin description of a real molecular magnet in the energy interval ranging from about 100 K down to 10$^{-12}$ K. Comparison with the results of the standard single-spin model shows that many-spin effects affect the tunneling splittings considerably. The values of ground state splitting given by single-spin and many-spin models differ from each other by a factor of five., 3REVTeX pages, 2 figures

The significance of the Wess--Zumino term in spin tunneling is explored, and a formula is established for the splitting of energy levels of a particle with large fermionic spin as an applied magnetic field is switched on., Comment: Latex, 7 pages

The tunneling splitting of the energy levels of a ferromagnetic particle in the presence of an applied magnetic field - previously derived only for the ground state with the path integral method - is obtained in a simple way from Schroedinger theory. The origin of the factors entering the result is clearly understood, in particular the effect of the asymmetry of the barriers of the potential. The method should appeal particularly to experimentalists searching for evidence of macroscopic spin tunneling., 10 pages, LaTex, 4 figures included using graphicx

The level splitting formulae much discussed in the study of spin tunneling in macroscopic ferromagnetic particles and previously derived only by complicated pseudoparticle methods for the ground state, are derived from those of eigenvalues of periodic equations and extended to excited states., 8 pages, LaTeX, no figures

We report the thermal spin injection and accumulation in crystalline CoFe/MgO tunnel contacts to n-type Si through Seebeck spin tunneling (SST). With the Joule heating (laser heating) of Si (CoFe), the thermally induced spin accumulation is detected by means of the Hanle effect for both polarities of the temperature gradient across the tunnel contact. The magnitude of the thermal spin signal scales linearly with the heating power and its sign is reversed as we invert the temperature gradient, demonstrating the major features of SST and thermal spin accumulation. We observe that, for the Si (CoFe) heating, the thermal spin signal induced by SST corresponds to the majority (minority) spin accumulation in the Si. Based on a quantitative comparison of thermal and electrical spin signals, it is noted that the thermal spin injection through SST can be a viable approach for the efficient injection of spin accumulation, 15 pages, 5 figures

We investigate the non-Abelian Josephson effect in F=2 spinor Bose-Einstein condensates with double optical traps. We propose a real physical system which incorporates the non-Abelian Josephson effect and has very different density and spin tunneling characteristics compared with the Abelian case. We calculate the frequencies of the pseudo Goldstone modes in different phases between two traps, which are the crucial feature of the non-Abelian Josephson effect. We also give an experimental protocol to observe this novel effect in future experiments.

We provide compelling evidence to establish that, contrary to one's elementary guess, the tunneling spin polarization (TSP) of amorphous CoFeB is larger than that of highly textured fcc CoFeB. First principles atomic and electronic structure calculations reveal striking agreement between the measured TSP and the predicted s-electron spin polarization. Given the disordered structure of the ternary alloy, not only do these results strongly endorse our communal understanding of tunneling through AlOx, but they also portray the key concepts that demand primary consideration in such complex systems., Comment: To be published in Physical Review Letters. 5 double-column pages with 3 figures

Recently, Bellessa et al in a Physical Review Letter entitled "Phonon-Assisted Tunneling in High-Spin Molecules: Experimental Evidence", reported observation of a peak in the imaginary part of the ac-susceptibility of Mn-12 molecular nanomagnet, that decreases strongly below 0.1 K down to 0.02 K. They explained this effect by a "phonon-induced tunneling process". The purpose of this Comment is to show that the explanation of Bellessa et al is logically inconsistent and physically impossible. All formulas in the Letter supporting that explanation are incorrect.