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Nanomagnetism at NIST: Magnetic Ultrathin Films
When magnetic layers are made several nanometers thick ("ultrathin") a variety of interesting effects occur including, for example, giant magnetoresistance. NIST has developed a number of techniques to measure the properties of these systems and use them to the determine the behavior of such systems. One such technique is neutron reflectometry. When ultrathin magnetic films are stacked with other ultrathin films, it can be very difficult to measure the properties of the buried layers. Neutrons are one of the few probes that interact weakly enough to penetrate to the buried layers yet strongly enough to reflect and reveal the properties of the layers. Alternatively, the behavior of the buried layers can be inferred from their magnetotransport properties or from surface sensitive measurements, like Scanning Electron Microscopy with Polarization Analysis (SEMPA). In some cases it is possible to sputter away the layers and measure the depth profile with a surface sensitive measurement like SEMPA. NIST has carried out a comparison of the depth profile as revealed from sputtering and SEMPA in comparison to neutron reflectometry. These measurements answered one of the puzzles about samples used to study the giant magnetoresistance effect.
Interlayer exchange coupling is the interaction between two ultrathin magnetic films that are separated by a nanometer thick non-magnetic layer. NIST has been at the forefront in research on interlayer exchange coupling. Early measurements used neutron reflectometry to make some of the first observations of this coupling. Some of the iconic measurements were made using SEMPA. These measurements were precise enough to establish the underlying mechanism for the coupling through comparison with a model for that mechanism and first principles calculations.
In most applications of magnetic ultrathin films, one of the magnetic layers has its magnetization fixed by coupling it to an antiferromagnet through an effect called exchange bias. An effort combining ferromagnetic resonance with calculations based on simple models has uncovered and described a variety of behaviors associated with instabilities in the antiferromagnetic layer. Since exchange bias naturally occurs at a buried interface, neutron reflectometry has been used to determine the behavior of the spins at the interface, yielding some surprising results. A related system that has also been studied with neutron reflectometry is a hard ferromagnet coupled to a soft ferromagnet, a so-called "exchange spring" system. These systems have been proposed as a way to achieve the high moments found in many soft magnetic materials with the high coercivities found in hard magnetic materials.