Search Results

This chapter describes the basics of spin injection and spin transport in magnetic nanohybrid structures. Particular emphasis is placed on the nonlocal spin transport in a spin injection and detection device of F1/N/F2 structure, where F1 is a spin injector, N is a nonmagnetic metal or semiconductor, and F2 is a spin detector. The spin-dependent transport equations for the electrochemical potentials of up and down spins are solved, and the efficient spin injection, spin accumulation, spin current, spin transfer, and spin detection are examined in the structure of arbitrary junction resistance, its source ranging from a metallic contact to a tunneling regime. It is demonstrated that discussion concerning the nonlocal spin-injection devices provide opportunities to relate novel phenomena, such as pure spin-current injection and manipulation, spin injection into superconductors or semiconductors, and spin Hall effect in nanohybrid stuctures.

This chapter discusses the spin-transfer effect, which is described as the transfer of the spin angular momentum between the conduction electrons and the magnetisation of the ferromagnet that occurs due to the conservation of the spin angular momentum. L. Berger, who introduced the concept in 1984, considered the exchange interaction between the conduction electron and the localised magnetic moment, and predicted that a magnetic domain wall can be moved by flowing the spin current. The spin-transfer effect was brought into the limelight by the progress in microfabrication techniques and the discovery of the giant magnetoresistance effect in magnetic multilayers. Berger, at the same time, separately studied the spin-transfer torque in a system similar to Slonczewski's magnetic multilayered system and predicted spontaneous magnetisation precession.

This chapter discusses different phases of magnon superfluidity, including those in a magnetic trap; and signatures of magnon superfluidity: (i) spin supercurrent, which transports the magnetization for a macroscopic distance (up to 1 cm); (ii) spin current Josephson effect, which shows the interference between two condensates; (iii) spin current vortex — a topological defect which is an analog of a quantized vortex in superfluids, of an Abrikosov vortex in superconductors, and cosmic strings in relativistic theories; and (iv) Goldstone modes related to broken U(1) symmetry — phonons in the spin-superfluid magnon gas. The chapter also considers the topic of spin supercurrent in general, including spin-Hall and intrinsic quantum spin-Hall effects.

This chapter discusses the effects of a spin current injected into a uniformly magnetised ferromagnetic cell. The junction consists of two ferromagnetic layers separated by a nonmagnetic metal interlayer or insulating barrier layer. With a nonmagnetic metal interlayer, the junction is called a giant magnetoresistive nanopillar, and with an insulating barrier layer a magnetic tunnel junction. When charge current is passed through this device, the electrons are first spin polarised by the fixed layer and spin-polarised current is then injected into the free layer through the nonmagnetic interlayer. This spin current interacts with the spins in the host material by an exchange interaction and exerts a torque. If the exerted torque is large enough, magnetisation in the free layer is reversed or continuous precession is excited.

This chapter discusses another type of equilibrium spin current similar to the exchange spin current – the topological spin current. Topological spin currents are driven by topological band structure and classified into bulk and surface topological spin currents. The former is confined onto electron-band manifolds, sometimes affecting their motions. This confinement is addressed through the standard method of combining the equations of motion and the Boltzmann equation for semi-classical electrons in a band. The latter class, on the other hand, is a surface spin current, which is limited near surfaces of a three-dimensional system and flows along these surfaces. This type is known to appear in topological insulators, where the bulk is insulating but the surface or edge is electrically conducting due to the surface or edge state.

This chapter delves into the physics of multiferroics, the recent developments of which are discussed here from the viewpoint of the spin current and ‘emergent electromagnetism’ for constrained systems. It presents the three sources of U(1) gauge fields, namely, the Berry phase associated with the noncollinear spin structure, the spin-orbit interaction (SOI), and the usual electromagnetic field. The chapter reviews multiferroic phenomena in noncollinear magnets from this viewpoint and discusses theories of multiferroic behavior of cycloidal helimagnets in terms of the spin current or vector spin chirality. Relativistic SOI leads to a coupling between the spin current and the electric polarisation, and hence the ferroelectric and dielectric responses are a new and important probe for the spin states and their dynamical properties. Microscopic theories of the ground state polarisation for various electronic configurations, collective modes including the electromagnon, and some predictions including photoinduced chirality switching are discussed with comparison to experimental results.

This chapter is an introduction to the concept of spin current, the detailed formulation of which is not simple by any means and is still a challenging undertaking. However, it is a useful and versatile concept that has given birth to a number of phenomena in condensed matter science and spintronics. There exist certain types of flow, carried by electrons, in condensed matter. This flow of electron charge or electric current has been developed and is now a vital contributor to how electronics is understood today. Since an electron carries both charge and spin, the existence of an electric current naturally implies the existence of a flow of spin. This flow is called a spin current.

This chapter discusses the effects of a spin current injected into a uniformly magnetized ferromagnetic cell. The junction consists of two ferromagnetic layers separated by a nonmagnetic metal interlayer or insulating barrier layer. With a nonmagnetic metal interlayer, the junction is called a giant magnetoresistive nanopillar, and with an insulating barrier layer a magnetic-tunnel junction. When charge current is passed through this device, the electrons are first spin polarized by the fixed layer and spin-polarized current is then injected into the free layer through the nonmagnetic interlayer. This spin current interacts with the spins in the host material by an exchange interaction and exerts a torque. If the exerted torque is large enough, magnetization in the free layer is reversed or continuous precession is excited.

This chapter introduces the concept of exchange spin current, which derives from rewriting the exchange interaction in magnets and formulating a spin-wave spin current. States of matter can be classified into several types in terms of magnetic properties. In paramagnetic and diamagnetic states, matter has no magnetic order and exhibits zero magnetisation in the absence of external magnetic fields. In ferromagnetic states, the permanent magnetic moments of atoms or ions align parallel to a certain direction, and the matter exhibits finite magnetisation even in the absence of external magnetic fields. In ferrimagnets, the moments align antiparallel but the cancellation is not perfect and net magnetisation appears. This interaction that aligns spins is called the exchange interaction.

This chapter describes an experiment on the inverse spin Hall effect (ISHE) induced by spin pumping. Spin pumping is the generation of spin currents as a result of magnetisation M(t) precession; in a ferromagnetic/paramagnetic bilayer system, a conduction-electron spin current is pumped out of the ferromagnetic layer into the paramagnetic conduction layer in a ferromagnetic resonance condition. The sample used in the experiment is a Ni₈₁Fe₁₉/Pt bilayer film comprising a 10-nm-thick ferromagnetic Ni₈₁Fe₁₉ layer and a 10-nm-thick paramagnetic Pt layer. For the measurement, the sample system is placed near the centre of a TE₀₁₁ microwave cavity at which the magnetic-field component of the microwave mode is maximised while the electric-field component is minimised.

This chapter introduces the concept of incoherent spin current. A diffusive spin current can be driven by spatial inhomogeneous spin density. Such spin flow is formulated using the spin diffusion equation with spin-dependent electrochemical potential. The chapter also proposes a solution to the problem known as the conductivity mismatch problem of spin injection into a semiconductor. A way to overcome the problem is by using a ferromagnetic semiconductor as a spin source; another is to insert a spin-dependent interface resistance at a metal–semiconductor interface.

This chapter discusses a number of methods that can experimentally detect pure spin currents – spin currents without accompanying charge currents. One such method is the utilisation of the inverse spin Hall effect, a method that was demonstrated first by spin pumping and nonlocal techniques. In semiconductors, an optical method was also demonstrated. Lastly, as an alternative method, one can infer spin-current generation indirectly by measuring spin accumulation. The chapter also proposes a solution to the problem known as the conductivity mismatch problem of spin injection into a semiconductor. A way to overcome the problem is by using a ferromagnetic semiconductor as a spin source; another is to insert a spin-dependent interface resistance at a metal–semiconductor interface.

This chapter discusses and presents a schematic illustration of nonlocal spin injection. In this case, the spin-polarized electrons are injected from the ferromagnet and are extracted from the left-hand side of the nonmagnet. This results in the accumulation of nonequilibrium spins in the vicinity of the F/N junctions. Since the electrochemical potential on the left-hand side is lower than that underneath the F/N junction, the electron flows by the electric field. On the right-hand side, although there is no electric field, the diffusion process from the nonequilibrium into the equilibrium state induces the motion of the electrons. Since the excess up-spin electrons exist underneath the F/N junction, the up-spin electrons diffuse into the right-hand side. On the other hand, the deficiency of the down-spin electrons induces the incoming flow of the down-spin electrons opposite to the motion of the up-spin electron.

This chapter delves into the physics of multiferroics, the recent developments of which are discussed here from the viewpoint of the spin current and “emergent electromagnetism” for constrained systems. It presents the three sources of U(1) gauge fields, namely, the Berry phase associated with the noncollinear spin structure, the spin-orbit interaction (SOI), and the usual electromagnetic field. The chapter reviews multiferroic phenomena in noncollinear magnets from this viewpoint and discusses theories of multiferroic behavior of cycloidal helimagnets in terms of the spin current or vector spin chirality. Relativistic SOI leads to a coupling between the spin current and the electric polarization, and hence the ferroelectric and dielectric responses are a new and important probe for the spin states and their dynamical properties. Microscopic theories of the ground state polarization for various electronic configurations, collective modes including the electromagnon, and some predictions including photoinduced chirality switching are discussed with comparison to experimental results.

This chapter discusses another type of equilibrium-spin current similar to the exchange-spin current—the topological spin current. Topological spin currents are driven by topological-band structure and classified into bulk and surface topological spin currents. The former is confined onto electron-band manifolds, sometimes affecting their motions. This confinement is addressed through the standard method of combining the equations of motion and the Boltzmann equation for semi-classical electrons in a band. The latter class, on the other hand, is a surface-spin current, which is limited near surfaces of a three-dimensional system and flows along these surfaces. This type is known to appear in topological insulators, where the bulk is insulating but the surface or edge is electrically conducting due to the surface or edge state.

This chapter describes an experiment on the inverse spin Hall effect (ISHE) induced by spin pumping. Spin pumping is the generation of spin currents as a result of magnetization M(t) precession; in a ferromagnetic/paramagnetic bilayer system, a conduction-electron spin current is pumped out of the ferromagnetic layer into the paramagnetic conduction layer in a ferromagnetic resonance condition. The sample used in the experiment is a Ni₈₁Fe₁₉/Pt bilayer film comprising a 10-nm-thick ferromagnetic Ni₈₁Fe₁₉layer and a 10-nm-thick paramagnetic Pt layer. For the measurement, the sample system is placed near the centre of a TE₀₁₁ microwave cavity at which the magnetic-field component of the microwave mode is maximized while the electric-field component is minimized.

This chapter is an introduction to the concept of spin current, the detailed formulation of which is not simple by any means and is still a challenging undertaking. However, it is a useful and versatile concept that has given birth to a number of phenomena in condensed matter science and spintronics. There exist certain types of flow, carried by electrons, in condensed matter. This flow of electron charge or electric current has been developed and is now a vital contributor to how electronics is understood today. Since an electron carries both charge and spin, the existence of an electric current naturally implies the existence of a flow of spin. This flow is called a spin current.