Noise properties of thin film granular weak link SQUIDs

A study of the magnetic flux noise in thin film NbN SQUIDS with variable thickness weak links has been initiated using the fractional step rise parameter, a , to characterize the noise. A series of devices have been fabricated and measured which exhibit Kurkijarvi magnetic flux noise, each over a restricted temperature range below their T . The T \´ s for the devices ranged from below 5K to as (high as 15.7K. The variable thickness weak links are nominally 1 micrometer in length, between 50 microaeters to submicrometer in width and less than 10 MI. in thickness. For these thicknesses, the weak link region is granular ("inhomogeneous") in nature and the temperature dependence of the critical current obeys either the Josephson or the mean field theories depending on the strength of the intergranular coupling. The deduced magnetic flux noise obeys Kurki-jarvi theory for critical current values greater than 2@ /L but lessthanapproximately 61p /L for most deGices tested. The dependence of the \´measured magnetic flux noise on the degree of granularity of the weak link and the temperature dependence of the critical current is discussed.