Scott Franzen

Real-time direct measurement of field rise time and dynamic instability of perpendicular writers

A method is presented to measure the dynamic write field of a perpendicular recording head directly in the time domain using a tunneling magnetoresistive read sensor. The method is used to measure the magnetic field rise time of two different writer designs and real-time measurements of the write field without averaging are demonstrated, enabling investigation of transient switching behavior that would not be observed with stroboscopic or frequency-domain techniques. A dynamic instability is observed and characterized in a particular writer design and is attributed to the insertion of an antiferromagnetic coupling layer in the writer pole.

Characterization and analysis of the training effect of exchange bias in coupled NiFe/IrMn bilayers

The training effect of the so-called exchange bias in coupled polycrystalline NiFe∕IrMn thin films has been experimentally and theoretically investigated. A different formula is used to describe the exchange bias dependence on the measurement cycle number. It can be understood to be a consequence of the antiferromagnetic domain dynamics based on the Kolmogorov-Avrami [Izv. Akad. Nauk, Ser. Math. 3, 355 (1937); J. Chem. Phys. 8, 212 (1940)] model, which describes the nucleation and growth of antiferromagnetic domains in the film structures.

Thermal fluctuation of magnetization and random telegraph noise in magnetoresistive nanostructures

The thermally activated magnetization reversals in magnetic nanodevices have been theoretically investigated. This kind of thermal fluctuations is found to be the dominant cause of intrinsic write error in magnetoresistive random access memory elements, on which our study is aimed. Calculations are performed in the Stoner–Wohlfarth model assuming a single-domain free layer magnetization. The thermal fluctuations as a form of random telegraph noise in output voltage level are systematically characterized in both the time and frequency domains. The relationship of the thermal fluctuations with the free layer magnetic property, temperature and external magnetic field has been established.