J.C. Slonczewski

Current-driven excitation of magnetic multilayers

Abstract A new mechanism is proposed for exciting the magnetic state of a ferromagnet. Assuming ballistic conditions and using WKB wave functions, we predict that a transfer of vectorial spin accompanies an electric current flowing perpendicular to two parallel magnetic films connected by a normal metallic spacer. This spin transfer drives motions of the two magnetization vectors within their instantaneously common plane. Consequent new mesoscopic precession and switching phenomena with potential applications are predicted.

Excitation of spin waves by an electric current

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.

Measurement of the spin-transfer-torque vector in magnetic tunnel junctions

The transfer of spin angular momentum from a spin-polarized current to a ferromagnet can generate sufficient torque to reorient the magnet’s moment. This torque could enable the development of efficient electrically actuated magnetic memories and nanoscale microwave oscillators. Yet difficulties in making quantitative measurements of the spin-torque vector have hampered understanding. Here we present direct measurements of both the magnitude and direction of the spin torque in magnetic tunnel junctions, the type of device of primary interest for applications. At low bias V, the differential torque dτ/dV lies in the plane defined by the electrode magnetizations, and its magnitude is in excellent agreement with recent predictions for near-perfect spin-polarized tunnelling. We find that the strength of the in-plane differential torque remains almost constant with increasing bias, despite a substantial decrease in the device magnetoresistance, and that with bias the torque vector also rotates out of the plane.

Currents and torques in metallic magnetic multilayers

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.

Overview of interlayer exchange theory

Abstract Basic mechanisms of interlayer exchange coupling between two ferromagnets separated by a non-magnetic spacer are surveyed. Simple generic theoretical models yielding closed formulae are treated. First, intrinsic exchange between itinerant ferromagnets is treated in the free-electron band approximation, including corrections to the elementary sinusoidal dependences on metallic-spacer thickness and on angle θ included between the magnetic moments. Properties of metallic and insulating spacers are compared. Then three special mechanisms of non-cos θ coupling are described: (1) fluctuations of spacer thickness, (2) loose spins, and (3) a novel phenomenological coupling through a non-normal spacer, possibly acting in spacers composed of chromium or manganese.

Micromagnetics of laminated Permalloy films

The edge curling wall (ECW) is observed in an optical Kerr microscope. Conditions are derived energetically favoring single transverse domains with ECWs over other multiple closure domains or single longitudinal domains that are undesirable because of their low permeability. Computed figures illustrate how the maximum permissible spacer thickness depends on Permalloy sublayer thickness, uniaxial anisotropy, width of the magnetic strip, mismatch of Permalloy thickness, and perpendicular anisotropy. The case with strip width W=100 mu m and vanishing magnetostriction or stress should require only a few sublayers. However, if W is very small, or if stress-induced or some other form of perpendicular anisotropy is excessive, then attainment of single transversely magnetized domains requires many sublayers. Some of these conclusions are supported by microscopic observations. >

Origin of biquadratic exchange in magnetic multilayers (invited)

We postulate localized‐electron states with unpaired spin located within or at the interfaces of an otherwise nonmagnetic metallic spacer layer. Loose exchange coupling of these spins to two ferromagnets mediates a non‐Heisenberg exchange coupling between them which includes a biquadratic term. A particular version of the model assumes that each interfacial layer of magnetic atoms is weakly exchange coupled to the remainder of the ferromagnets. This model permits interpretation of the biquadratic‐coupling data of Gutierrez et al. for Fe/Al/Fe trilayers and of Fuss et al. for Fe/Au/Fe. According to this interpretation, the observed biquadratic coupling is intrinsic to the ideal multilayer structure rather than due to impurities or structural defects.

Mechanism of interlayer exchange in magnetic multilayers

Abstract The spin-current method is used to calculate the oscillatory exchange energy that couples two semi-infinite ferromagnets with exchange-split parabolic bands which are joined by a nonmagnetic metallic spacer. A closed asymptotic formula extends the previous RKKY-type formula to the case in which the ferromagnets and spacer have different Fermi vectors. The predicted amplitude of oscillatory coupling increases steeply with Fermi vector or electron density in the spacer, as do the experimental trends reported by Parkin. Numerical computations relevant to iron support this closed formula and show that the amplitude of the biquadratic (J2 cos2θ) and higher-order corrections to the conventional -J1 cos θ form of energy is less than 2%.

Magnetostatic mechanism for field-sensitivity of magnetoresistance in discontinous magnetic multilayers

Abstract We derive an expression for the inter-layer magnetostatic coupling which governs the field-sensitivity of the giant magnetoresistance in the thin discontinous magnetic multilayers of composition (Ni80Fe20)/Ag recently discovered by T.L. Hylton and coworkers. This expression provides estimates of the width G of the magnetic discontinuity needed to account for the external fields used to obtain the published experimental magnetoresistance data. The results are G = 10 A after the anneal which maximizes the saturation magnetoresistance and G ≈ 6 A after the anneal which maximizes its field-sensitivity.

The use of bubble lattices for information storage

A new approach to bubble memories, called a bubble lattice file (BLF), is described. This approach employs a periodic bubble lattice to define bit positions while the information is contained in the wall structure of the bubbles. The principal advantage of the BLF is the increased storage density, sixteenfold when the resolution capability of the fabrication process is the limiting factor. The device functions required for the BLF, including accessing and write/read operation, are described. Significant differences between BLF and the T‐I bar bubble memories are discussed.