I n 1996, Luc Berger and John Slonczewski [1] conceived of a way to use an electrical current to reverse the magnetization of a thin metal film. The trick was to apply a current of electrons whose spins were aligned in the same direction: Upon passing through the film, this “spin current” would exert a large enough torque on the film’s magnetization to flip it around. Experimentalists soon demonstrated the predicted switching effect [2], which led, among other things, to racetrack memories—devices that use a spin current to move and position information encoded in tiny magnetic domains [3]. Seokhwan Choi of the Korea Advanced Institute of Science and Technology and coworkers have now uncovered a new type of spin-current switching effect, this time in an iron-based superconductor [4]. The researchers show that the current can be used to modify two phenomena—magnetism and superconductivity—that coexist in the material. The effect could, like the one Berger and Slonczewski predicted, lead to new types of devices and experiments.

The magnetic hysteresis of a film consisting of a single magnetic domain is analyzed. A method of designing logical computer elements employing thin films is presented and illustrated. It is concluded that recently discovered films approaching single domain behavior offer two possible advantages in applications: 1) As a storage element in a memory array a film can be switched faster than presently available polycrystalline cores. 2) A film can be used to perform a variety of logical operations more simply, in principle, than it is possible with polycrystalline cores.

We expose the theory of quantized spin-polarized electron transport perpendicular to the plane of a magnetic multilayer with non-collinear magnetization vectors. The dependence of resistance and current-driven torque on relative angle between 2 magnetic moments of a multilayer pillar are derived. Spacers of both metallic and insulating tunnel-barrier types are considered. The classical Landau-Lifshitz equation describes the dynamics of the magnetization created by spintransfer torque.

A theoretical study is presented concerning the relationship between spin transfer torque and absolute current-perpendicular-to-plane magnetoresistance in metallic spin-valves (SV) and magnetic tunnel junctions. In a first step, using Valet and Fert Boltzmann-like theory extended to any metallic magnetic multilayers with non-collinear magnetizations, linear relationships between spin torque and absolute current-perpendicular-to-plane giant magnetoresistance are derived numerically, when varying the parameters of the structure one by one. The obtained results are compared with an extension of J.C. Slonczewski's circuit theory. The latter model gives an analytical expression of this linear dependence. In a second step, using an out-of-equilibrium perturbation formalism (Keldysh technique), we study the tunnel magnetoresistance (TMR) dependence of the spin torque amplitude in magnetic tunnel junctions when varying the parameters one by one in the junction. Once again, linear dependences are obtained, with different characteristics when the TMR vanishes. It is shown that these two equivalent behaviours are associated with different origins of the spin torque.

We apply known relations for voltage-driven current and torque to magnetic tunneling junctions (MTJs) symmetric in composition. Postulated are effects of possibly asymmetric microstructures of the two interfaces separating the two electrodes from a barrier composed of (0 0 1)MgO. Our model takes into account effects of (i) dependence of state density on energy, (ii) possible asymmetry of the elastic transfer Hamiltonian, and tunneling enabled by inelastic mechanisms (iii) within the barrier or (iv) within the electrodes. It leads to distinct observable signatures for these effects in the voltage dependences of current and torque. A parameter-free relation connecting torque with conductance makes possible direct experimental tests of the model. Additionally, a derived formula illustrates how coulomb-correlation may contribute to these effects at high voltage.

Extensions of an existing circuit theory for current perpendicular to plane magnetoresistance and current-driven torque in noncollinear magnetic-multilayer pillar devices are presented. Our expressions for monodomain critical-current threshold ${J}_{c}$ and giant magnetoresistance $\mathrm{\ensuremath{\Delta}}R$ are firstly derived in terms of assumed spin-channel resistances for each of the two ferromagnets. Spinflips are thus neglected. We find a class of closed linear relationships connecting ${J}_{c}^{\ensuremath{-}1}$ and $\mathrm{\ensuremath{\Delta}}R$. We then derive more general expressions for these quantities which take into account spin-flip relaxation. In this case, we assume analytically calculable linear $2\ifmmode\times\else\texttimes\fi{}2$ current-voltage matrices for the separate two-channel ferromagnets. These expressions again lead to a class of closed linear relationships connecting ${J}_{c}^{\ensuremath{-}1}$ and $\mathrm{\ensuremath{\Delta}}R$. The latter generalization gives a simple theoretical framework to take into account bulk and interfacial spin flip and more complicated multilayer structures often used in experiments.

Bardeen's transfer-Hamiltonian method is applied to magnetic tunnel junctions having a general degree of atomic disorder. The results reveal a close relationship between magnetoconduction and voltage-driven pseudotorque, and also provide a means of predicting the thickness dependence of tunnel-polarization factors. Among the results: (i) The torque generally varies with moment direction as $\mathrm{sin}\phantom{\rule{0.2em}{0ex}}\ensuremath{\theta}$ at constant applied voltage. (ii) Whenever polarization factors are well defined, the voltage-driven torque on each moment is uniquely proportional to the polarization factor of the other magnet. (iii) At finite applied voltage, this relation implies significant voltage-asymmetry in the torque. For one sign of voltage the torque remains substantial even if the magnetoconductance is greatly diminished. (iv) A broadly defined junction model, called ideal middle, allows for atomic disorder within the magnets and $\mathrm{F}∕\mathrm{I}$ interface regions. In this model, the spin-$(\ensuremath{\sigma})$ dependence of a basis-state weighting factor proportional to the sum over general state index $p$ of ${(\ensuremath{\int}\ensuremath{\int}dydz{\ensuremath{\Psi}}_{p,\ensuremath{\sigma}})}^{2}$ evaluated within the (e.g., vacuum) barrier generalizes the local state density in previous theories of the tunnel-polarization factor. (v) For small applied voltage, tunnel-polarization factors remain legitimate up to first order in the inverse thickness of the ideal middle. An algebraic formula describes the first-order corrections to polarization factors in terms of newly defined lateral autocorrellation scales.

A theory is given for electron transport across a very thin non-magnetic metallic spacer joining two ferromagnetic metals whose moment vectors include a general angle. An assumed condition of negligible interfacial reflection for majority-spin electrons is approached by certain multilayer compositions including the experimentally important composition Co/Cu. Analytic formulas based on a non-spin-diagonal density operator inside the spacer connect the spin-channel currents and electro-chemical voltages across the spacer. The quantum-mechanically derived torques on sublayer moments agree with a previous macroscopic relation. The single additional parameter of the resulting macroscopic magnetoelectronic transport formulation depends only on bulk electron structure of the spacer. Illustrative application of the new connection formulas to the special case of two equal thin ferromagnets predicts closely related expressions for angular dependences of magnetoresistance and current-driven torque. A simple relation between magnetoresistance and asymmetry of exciting currents holds for this case.

We examine the sweep-rate dependence of magnetic switching field, Hs, in submicron magnetic tunnel junctions where shape anisotropy is dominant. Experimental data support the use of a single-domain thermal activation model for description of activated magnetic reversal in junctions 0.2 by 0.5 μm or less in size. A scaling law is obtained for the thermal activation energy which varies as the cube of junction size.

The excitation of spin waves in an unbounded ferromagnetic film by a direct spin-polarized current distributed over a small area is treated macroscopically. The derived critical threshold current for excitation has two additive terms: The first arises from radiation of spin waves and is constant. The second arises from local viscous dissipation and varies in proportion to damping coefficient, external field, and area. An application of these predictions modifies the existing interpretation of experiments by Tsoi and collaborators employing currents flowing through point contacts.

Understanding the magnetic switching behavior in micron and submicron scale specimens is important for a number of applications. In this study, magnetic-tunnel junctions of various sizes and shapes were fabricated and their switching behavior was studied in detail. Using exchange bias to offset the magnetic response of one electrode, the response of the other (free) electrode was determined from measurements of junction resistance. Switching threshold curves were measured by sweeping magnetic fields in both easy and hard direction. Single domain like switching was observed in some of our smallest submicron junctions. The observed behavior was compared with predictions from the Stoner–Wohlfarth rotational model and from numerical calculations.

This lecture is based on papers by the author and describes different aspects of the magnetic tunneling valve effect. In the section 1 [1] the role of interface in magnetic tunneling is discussed. Section 2 [2] deals with the current-driven excitation of magnetic multilayers. Some questions of a theory for interlayer exchange energy is presented in section 3 [3].

We represent the state of a circa 10-nm thick submicron dimensioned magnetic film with a superposition of two-dimensional (2D) magnetic pseudovortices. The effective intervortex exchange-stiffness potential and local magnetization angle at a complex distance z=x+iy from the kth vortex center are given by the real and imaginary parts respectively, of the function −wklnz. Each of the four corners of the rectangle has a stationary quarter vortex with negative winding (wk=−1). Two mobile semivortices with winding wk=+1 and N and S magnetic poles lie at general positions X1 and X2, respectively, along edges of the rectangle. The approximate boundary condition of vanishing M-component normal to each edge is satisfied by repeated reflections which generate a periodic extension of this vortex array to a lattice filling all of a 2D space. The internal energy V(X1,X2) is principally composed of the inter vortex exchange stiffness. Given this function, numerical evaluation of the integrated moment m=m(X1,X2) provides the functional dependence of V on m as parametrized by (X1,X2). The function V(X1,X2) has four equal minima representing states with the N and S vortices located at diagonally opposite corners, in agreement with direct numerical simulations. Therefore, the predicted hysteresis behavior of our vortex model has significantly more complex transitions than those of a uniaxial single-domain particle having only two minima. Our employment of but two variables, rather than the continuum of straightforward micromagnetics, makes possible a more insightful analysis of the smallscale structures used in storage and memory.