What Are Magnetic Films?

Magnetic films are thin layers of magnetic material, typically ranging from a few nanometers to several micrometers in thickness, deposited on a substrate for use in data storage, sensing, and electronic devices. The term encompasses a broad family of materials, from soft ferromagnetic alloys such as permalloy (Ni-Fe) and CoFe to hard magnetic films used as recording media, to antiferromagnetic and ferrimagnetic layers employed in spintronic devices. Their physical properties differ substantially from those of bulk magnets because the reduced dimensionality constrains domain structure, modifies magnetization dynamics, and introduces interfacial effects that have no counterpart in thick materials.

The field grew from foundational work in magnetic recording during the 1950s and accelerated in the 1980s with the discovery that stacking alternating ferromagnetic and nonmagnetic layers could produce giant magnetoresistance (GMR), a finding that earned the 2007 Nobel Prize in Physics. Magnetic films now form the active layer in read heads for hard disk drives, the storage cells of magnetic random-access memory (MRAM), and thin-film inductors in power electronics.

Deposition and Structural Control

Magnetic films are produced by several physical and chemical deposition methods. Sputtering, which ejects atoms from a target material using an argon plasma, is the most widely used industrial process and gives precise control over composition, layer thickness, and crystallographic texture. Molecular beam epitaxy (MBE) grows films one atomic layer at a time and is used in research when extreme crystalline perfection is required. Electrodeposition, used for writing elements in early hard disk read/write heads, can coat non-planar geometries and is compatible with batch wafer processing. In all cases, the substrate temperature, deposition rate, and ambient gas composition influence grain size, residual stress, and magnetic anisotropy in the finished film.

Magnetic Properties and Characterization

The magnetic behavior of a thin film is determined by its composition, crystalline order, thickness, and the interfaces it shares with adjacent layers. Anisotropy, the preference for magnetization to align along a particular axis, can be controlled by applying a field during deposition or by exploiting the crystalline symmetry of the substrate. Key parameters measured during characterization include saturation magnetization, coercivity, remanence, and ferromagnetic resonance frequency. The technique of magnetic force microscopy for spintronic device characterization, which images the stray field above a film's surface with nanometer-scale resolution, is the standard tool for visualizing magnetic domains and verifying that a film's domain structure matches the design intent for a given device. The NIST metrology of magnetic materials program provides reference measurement services and standard reference materials for calibrating these characterization instruments. Magnetometry based on vibrating sample magnetometers (VSM) and SQUID instruments provides quantitative bulk magnetic moment data.

Spintronics and Multilayer Structures

The most technically significant property of magnetic multilayer films is their ability to exhibit GMR and tunneling magnetoresistance (TMR). In a GMR stack, resistance drops when adjacent ferromagnetic layers align parallel and rises when they align antiparallel, because conduction electrons experience less scattering in the parallel configuration. In a magnetic tunnel junction (MTJ), a thin insulating barrier replaces the nonmagnetic spacer; spin-polarized electrons tunnel through the barrier at a rate that depends on the relative orientation of the two magnetic layers. MTJs form the core storage element in MRAM, where the IEEE Magnetics Society roadmap identifies MRAM as a primary candidate for replacing conventional embedded flash memory due to its non-volatility, low operating voltage, and endurance. Research into synthetic antiferromagnets and perpendicular magnetic anisotropy materials continues to push bit densities and switching speeds.

Applications

Magnetic films have applications in a wide range of disciplines, including:

• Read and write heads in hard disk drives for computer data storage
• Magnetic random-access memory (MRAM) cells for embedded and standalone non-volatile memory
• Spin valves and magnetic tunnel junctions in magnetoresistive sensors
• Thin-film inductors and transformer cores in power electronics
• Magnetic shielding layers in precision instruments and medical devices