Recent theories of spin-current-induced magnetization reversal are formulated in terms of a spin-mixing conductance G mix . We evaluate G mix from first principles for a number of ͑dis͒ordered interfaces between magnetic and nonmagnetic materials. We predict that the magnetization direction of a ferromagnetic insulator or of one side of a tunnel junction in a multiterminal device can be switched even though a negligible charge current is passed. DOI: 10.1103/PhysRevB.65.220401 PACS number͑s͒: 72.25.Ϫb, 71.15.Ap, 73.21.Ac ''Giant magnetoresistance'' refers to the large change of resistance brought about by applying an external magnetic field to change the angle between the magnetization directions of magnetic films separated by nonmagnetic spacers. 1Since a spin injected into a magnetic material experiences a torque, it has been argued that passage of a current through adjacent magnetic layers should lead to the transfer of spinangular momentum from one layer to the other 2,3 with possible reorientation of the magnetizations for sufficiently large currents.2 Interestingly, the sign of the corresponding torque should be reversed on changing the current direction leading to the possibility of making an electronically accessible nonvolatile magnetic memory whose performance on downscaling compares favorably with other alternatives.4 Promising proof-of-principle experiments on current-induced magnetization reversal ͑''spin transfer''͒ have been carried out 5 but the large current densities required underline the need to optimize the effect.Theoretical discussion of the giant magnetoresistance and tunneling magnetoresistance ͑TMR͒ effects in collinear spin systems is greatly simplified when the spin-flip scattering is so weak that it is possible to consider the spin-up and spindown conduction channels separately.1 When studying current-induced magnetization reversal it is necessary to consider what happens when the spin-quantization axis rotates on going from one material to another; the current operator then has to be represented in a 2ϫ2 spin space even when spin-flip scattering is neglected entirely. Slonczewski 2 used a model of free electrons incident on a spin-dependent potential barrier to discuss the qualitative aspects of spin transfer. Free-electron models are known to miss an important contribution to spin transport in layered magnetic materials coming from the mismatch of the complex d bands responsible for itinerant ferromagnetism, 6,7 and it is important to take this into account. A more general framework, suitable for treating complex band structures and for including the effects of disorder, is provided by the scattering theoretical formalism of Waintal et al. 8 The ''circuit theory'' of Brataas et al. 9is nearly equivalent, but is more transparent and flexible, making the treatment of many-terminal devices straightforward. In this circuit theory, the torque and current are formulated in terms of ͑real͒ spin-up and spin-down conductances G ↑ and G ↓ , and a new spin-mixing conductance G mix which i...

The spin Hall angle (SHA) is a measure of the efficiency with which a transverse spin current is generated from a charge current by the spin-orbit coupling and disorder in the spin Hall effect (SHE). In a study of the SHE for a Pt|Py (Py=Ni_{80}Fe_{20}) bilayer using a first-principles scattering approach, we find a SHA that increases monotonically with temperature and is proportional to the resistivity for bulk Pt. By decomposing the room temperature SHE and inverse SHE currents into bulk and interface terms, we discover a giant interface SHA that dominates the total inverse SHE current with potentially major consequences for applications.

A systematic, quantitative study of the effect of interface roughness and disorder on the magnetoresistance of FeCo|vacuum|FeCo magnetic tunnel junctions is presented based upon parameter-free electronic structure calculations. Surface roughness is found to have a very strong effect on the spinpolarized transport while that of disorder in the leads (leads consisting of a substitutional alloy) is weaker but still sufficient to suppress the huge tunneling magneto-resistance (TMR) predicted for ideal systems.Tunneling magnetoresistance (TMR) refers to the dependence of the resistance of a FM 1 |I|FM 2 (ferromagnet|insulator|ferromagnet) magnetic tunnel junction (MTJ) on the relative orientation of the magnetization directions of the ferromagnetic electrodes when these are changed from being antiparallel (AP) to parallel (P):Since the discovery of large values of TMR in MTJs based upon ultrathin layers of amorphous Al 2 O 3 as insulator, 1 a considerable effort has been devoted to exploiting the effect in sensors and as the basis for non-volatile memory elements. Understanding TMR has been complicated by the difficulty of experimentally characterizing FM|I interfaces. The chemical composition of the interface has been shown 2 to have a strong influence on the magnitude and polarization of the TMR and knowledge of the interface structure is a necessary preliminary to analyzing MTJs theoretically. In the absence of detailed structural models of the junctions and the materials-specific electronic structures which could be calculated with such models, the effect was interpreted in terms of electrode conduction-electron spin polarizations P i , using a model suggested by Julliere 3 in which the TMR = 2P 1 P 2 /(1 − P 1 P 2 ). A great deal of discussion has focussed on the factors contributing to the quantity 4 P but the use of amorphous oxide as barrier material made impossible a detailed theoretical study with which to confront experiment. 6,7 The situation changed quite drastically with the recent observation of large values of TMR at room temperature in FeCo|MgO|FeCo MTJs in which the MgO tunnel barrier was mono-8,9 or poly-crystalline. 10 This work was motivated in part by the prediction 11,12 by materialsspecific transport calculations of huge TMR values for ideal Fe|MgO|Fe structures. This new development lends fresh urgency to the need to understand the factors governing the sign and magnitude of TMR because the largest observed value of 353% at low temperature, 9 is still well below the ab-initio predicted values of order 10,000% for the relevant thicknesses of MgO. 11 Some effort has been devoted to explaining the discrepancy in terms of interface relaxation 13 or the formation of a layer of FeO at the interface 14,15 but the role of interface disorder has only been speculated upon. 16

PACS 72.25.Mk -Spin transport through interfaces PACS 72.10.-d -Theory of electronic transport; scattering mechanisms PACS 85.75.-d -Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fieldsAbstract -Recent experimental and theoretical studies focus on spin-mediated heat currents at interfaces between normal metals and magnetic insulators. We resolve conflicting estimates for the order of magnitude of the spin transfer torque by first-principles calculations. The spin mixing conductance G ↑↓ of the interface between silver and the insulating ferrimagnet Yttrium Iron Garnet (YIG) is dominated by its real part and of the order of 10 14 Ω −1 m −2 , i.e. close to the value for intermetallic interface, which can be explained by a local spin model.

Details are presented of an efficient formalism for calculating transmission and reflection matrices from first principles in layered materials. Within the framework of spin density functional theory and using tight-binding muffin-tin orbitals, scattering matrices are determined by matching the wave functions at the boundaries between leads which support well-defined scattering states, and the scattering region. The calculation scales linearly with the number of principal layers N in the scattering region and as the cube of the number of atoms H in the lateral supercell. For metallic systems for which the required Brillouin zone sampling decreases as H increases, the final scaling goes as H 2 N. In practice, the efficient basis set allows scattering regions for which H 2 N ϳ 10 6 to be handled. The method is illustrated for Co/ Cu multilayers and single interfaces using large lateral supercells ͑up to 20ϫ 20͒ to model interface disorder. Because the scattering states are explicitly found, "channel decomposition" of the interface scattering for clean and disordered interfaces can be performed.

Recent years have witnessed a rapidly growing interest in exploring the use of spin waves for information transmission and computation toward establishing a spin-wave-based technology that is not only significantly more energy efficient than the CMOS technology, but may also cause a major departure from the von-Neumann architecture by enabling memory-in-logic and logic-in-memory architectures. A major bottleneck of advancing this technology is the excitation of spin waves with short wavelengths, which is a must because the wavelength dictates device scalability. Here, we report the discovery of an approach for the excitation of nm-wavelength spin waves. The demonstration uses ferromagnetic nanowires grown on a 20-nm-thick Y3Fe5O12 film strip. The propagation of spin waves with a wavelength down to 50 nm over a distance of 60,000 nm is measured. The measurements yield a spin-wave group velocity as high as 2600 m s−1, which is faster than both domain wall and skyrmion motions.

We study the effect of interface disorder on the spin-dependent interface resistances of Co/Cu, Fe/Cr, and Au/Ag multilayers using a newly developed method for calculating transmission matrices from first-principles. The efficient implementation using tight-binding linear-muffin-tin orbitals allows us to model interface disorder using large lateral supercells whereby specular and diffuse scattering are treated on an equal footing. Without introducing any free parameters, quantitative agreement with experiment is obtained. We predict that disorder reduces the majority-spin interface resistance of Fe/Cr͑100͒ multilayers by a factor 3. When two layers of magnetic material are separated by a non-magnetic spacer layer, the electrical resistance of the system depends strongly on whether the magnetization directions are aligned parallel or antiparallel. This effect is known as giant magnetoresistance ͑GMR͒. 1 The huge interest 2-4 in the physics of GMR is largely driven by the wide application potential of the effect, which has already been realized in magnetic recording heads.GMR can be observed in a number of different measuring configurations. The current-in-plane ͑CIP͒ configuration is experimentally the simplest and is what is used at present in applications. However, for gaining a better understanding of the underlying physics, the current-perpendicular-to-theplane ͑CPP͒ configuration 3,5-9 is preferred because of its higher symmetry, which should make it easier to understand, and because of higher MR ratios.The factors usually considered in theoretical treatments of GMR are the potential steps encountered by electrons passing from one material to another, impurity scattering in the bulk of the layers, and defect scattering at the interfaces. [2][3][4] There has been a great deal of discussion about the relative importance of these ingredients and their spin dependence, which cannot be resolved solely on the basis of model calculations which include these effects in parametrized form. Once the question has been suitably posed, however, detailed electronic structure calculations can be used to resolve the issue quantitatively. For example, the effect of potential steps and their microscopic origin could be established in this way. 10,11 In this paper we wish to address the relative role of specular and diffuse interface scattering. This has been studied by a large number of authors but so far only using simple models which do not allow for detailed quantitative analysis of specific materials. 12,13 We focus on the interface resistance of the resistor model which describes the observed thickness and layer dependence of CPP-GMR remarkably well. 3 Because it turns out to be strongly spin dependent and dominates the magnetoresistance for layer thicknesses which are not too large, the key to understanding CPP magnetoresistance lies in understanding the origin of the interface resistance. The methodology which we have developed allows us to include specular and diffuse scattering on an equal footing without introducing ...

We report a first principles formalism and its numerical implementation for treating quantum transport properties of nanoelectronic devices with atomistic disorder. We develop a nonequilibrium vertex correction (NVC) theory to handle the configurational average of random disorder at the density matrix level so that disorder effects to nonlinear and nonequilibrium quantum transport can be calculated from atomic first principles in a self-consistent and efficient manner. We implement the NVC into a Keldysh nonequilibrium Green's function (NEGF) -based density functional theory (DFT) and apply the NEGF-DFT-NVC formalism to Fe/vacuum/Fe magnetic tunnel junctions with interface roughness disorder. Our results show that disorder has dramatic effects on the nonlinear spin injection and tunnel magnetoresistance ratio.