Detection of Cylindrical Magnetic Domains

Abstract

Cylindrical magnetic domains in thin platelets of anisotropic single-crystal materials have been detected using the Hall effect in silicon, the pseudo-Hall effect in Permalloy, flux change, and Faraday rotation. The silicon detector, active area ≈400 μ2, active thickness ≈1 μ, input and output resistance ≈1.5 kΩ, and 8-mA rated current, delivered 0.5-mV signals from orthoferrite domains. For 20-mA input current the Permalloy detector, input and output resistance ≈15 Ω, diameter ≈50 μ, and thickness ≈0.03 μ, yielded 0.9-mV signals from 60-μ-diam domains in Sm0.55Tb0.45FeO3 orthoferrite. Domains in Y2.25Tb0.75Ga0.9Fe4.1O12, diameter ≈7 μ, were read with a Permalloy detector approximately 7 μ in diameter. The output was 0.2 mV at 2-mA input current. An output of 1 mV μsec was obtained by detecting the flux change on collapse of a TmFeO3 domain previously expanded 40 times in surface area. A Faraday-rotation detection system consisting of a 5-μW He–Ne laser, sheet polarizers, and an avalanche photodiode has operated at 106 bits/sec with a 20-dB signal-to-noise ratio. Among these detection methods, the Permalloy device appears most promising at this time. It offers the simultaneous advantages of relatively simple fabrication, small active area, small initial cost, and low power consumption. Additional experimental results relating to these detection methods are presented. Their relative merits and limitations are discussed and compared to those of the magnetoresistor and the magnetodiode. The problems associated with the detection of small domains at high speed are also considered.