What Are Magnetic Multilayers?

Magnetic multilayers are thin-film structures consisting of alternating ferromagnetic and nonmagnetic metallic layers, each typically a few nanometers thick, that exhibit collective magnetic and electronic behaviors distinct from any single constituent layer. The term covers a wide class of artificially engineered materials in which the thickness and composition of each layer can be controlled at the atomic level during deposition. Because electrons traversing the stack experience both magnetic and electrostatic potentials from successive layers, these structures give rise to coupling and transport phenomena that have no analogue in bulk metals.

The field draws on condensed matter physics, surface science, and thin-film engineering. Its practical importance was established in 1988 when Albert Fert and Peter Grünberg independently discovered giant magnetoresistance (GMR) in Fe/Cr multilayer stacks, a finding recognized with the 2007 Nobel Prize in Physics, as described by NIST's account of the Nobel laureates' work.

Giant Magnetoresistance and Interlayer Exchange Coupling

The GMR effect arises from spin-dependent scattering of conduction electrons at the interfaces between ferromagnetic and nonmagnetic layers. When adjacent ferromagnetic layers are aligned in parallel, electrons whose spins match the magnetization direction pass with low scattering and the overall resistance is relatively low. When neighboring layers are aligned antiparallel, all electrons experience high scattering in at least one layer, and the resistance rises sharply. In Fert's original 30-layer Fe/Cr structure, the resistance dropped by 50 percent at low temperature when a saturating field forced parallel alignment.

The antiparallel ground state that enables this effect is maintained by interlayer exchange coupling, mediated through the nonmagnetic spacer via the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. This coupling oscillates between ferromagnetic and antiferromagnetic as a function of spacer thickness, with a characteristic period of roughly 1 to 4 nm, as detailed in NIST's analysis of exchange coupling in magnetic multilayers. Precise control of spacer thickness during deposition is therefore essential for tuning the coupling sign and magnitude.

Spin Valves and Thin-Film Fabrication

A particularly important variant is the spin valve, a three-layer structure consisting of a magnetically pinned layer, a thin nonmagnetic metallic spacer (typically copper), and a free layer whose magnetization can rotate in response to an applied field. The pinned layer is stabilized by exchange bias with an adjacent antiferromagnet. The free layer then acts as the sensing element: small applied fields tilt its magnetization relative to the pinned reference, producing a measurable resistance change proportional to the cosine of the angle between the two magnetizations.

Multilayer stacks are fabricated by molecular beam epitaxy or magnetron sputtering, both of which can maintain interface roughness below one atomic layer over wafer-scale areas. A broader survey of spintronics technology from GMR to spin-transfer-torque MRAM documents the progression from laboratory multilayers to manufacturable device structures. Coatings applied during fabrication serve both to protect interfaces from oxidation and to tune the magnetic anisotropy of the free layer.

Applications

Magnetic multilayers have applications in a wide range of fields, including:

• Hard disk drive read heads, where GMR spin valves enabled a tenfold increase in areal density during the late 1990s
• Magnetic field sensors in industrial and automotive settings
• Spin-transfer-torque magnetic random-access memory (STT-MRAM)
• Spintronic logic devices under investigation for post-CMOS computing
• Biomagnetic sensing for detecting weak fields from neural and cardiac sources