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The characteristics of 360° domain walls have been investigated by magnetic force microscopy (MFM) in glass/Ta(10 nm)/NiFe(7.2 nm)/FeMn(8.0 nm)Ta(5 nm). The sample was prepared by radio-frequency-magnetron sputtering and characterized with a vibrating sample magnetometer. Both closed and line types of 360° domain walls were observed. The closed type (loop) walls include circular, ellipsoidal, and irregular shapes of domain wall, the long axis of which is generally perpendicular to the overall easy direction (pinned direction) of the exchange biased NiFe film. 360° wall loops were also classified by their chirality. Many 360° domain walls were observed near defects and the measured wall thickness ranged from 1.5 to 3 μm. The magnetic structure of the 360° loop was verified by simulations using a simplified model of the MFM imaging process. The formation of a 360° wall loops appears to originate from the rotational nature of the magnetization in the exchange-biased film, which comes from the distribution of the unidirectional anisotropy.

Noise properties of submicron scale tunneling magnetoresistive (TMR) sensors were investigated at frequencies up to 3 GHz. Noise spectral density was measured as a function of frequency, applied field, and bias current. Noise spectral density versus frequency dependence exhibits a pronounced peak, tunable over a wide frequency range. This peak appears to originate from current-driven precession of magnetization. The peak center frequency can be as low as 200 MHz and has a strong dependence on applied field and bias current. The damping constant α of the main precession mode in the TMR sensor free layer was found to be in the range of 0.05–0.18. It is shown that the magnetic state of a magnetoresistive sensor depends on the bias current and may be characterized by noise properties. The magnetoresistive element can operate as a source of high-frequency radiation with 1 nW emitting power from a 0.1 μm2 junction and signal to noise ratio of 10 dB.

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A circuit with two long parallel micron size wires was fabricated by photolithography to calibrate magnetic force microscopy (MFM) tips. The tip phase shift increased as the tip scan height decreased. With the tip scan height kept constant, a linear relationship was found between the current amplitude and the phase shift of the tip at the center position of the two wires separation. The estimated magnetic moment of the tip was one order larger than its nominal value. The results imply that with better control over the fabrication process, the micron size straight wires can be used to calibrate MFM tips.