Proximity effects in superconductor/insulating-ferromagnet NbN/GdN multilayers.

Proximity effects between superconducting and magnetic layers have been some of the most actively studied subjects in the past decades. It has been well established that magnetic moments and magnetic order are highly effective in destroying Cooper pairs and strongly depressing superconductivity. Thus in superconductor/ferromagnet (S/F) multilayers, one observes that the superconducting transition temperature sTcsd decreases monotonically and rapidly as the magnetic layer thickness is increased [1– 5]. Recently, the existence of the so-called p-phase state, in which the two adjacent superconducting layers have an opposite phase, has been theoretically predicted in superconductor/ferromagnet multilayers with suitable ferromagnetic layer thicknesses sdmd [6,7]. A damped oscillatory behavior in Tcs as a function of dm has been predicted, and recently observed in Nb/Gd multilayers [8]. However, in all previous cases, the ferromagnetic layers (e.g., Fe and Gd) in the multilayer systems are always metallic. The fundamental aspects of the low conductivity of the ferromagnetic but insulating layer have never been experimentally addressed. Because of the energy gap in the insulator, the proximity effects and the pair-breaking effects of the ferromagnet layers are likely to be strongly modified. To explore these important aspects, we have studied superconductor/insulating ferromagnet NbN/GdN multilayers. Unusual behaviors in superconducting properties and critical fields, very different from those of previous superconductor/metallic ferromagnet multilayers, have been observed. Most of the common ferromagnets are elemental metals and alloys. All previous studies of S/F bilayers and multilayers have used those elemental metals and alloys. Nonmetallic ferromagnetic thin films (e.g., oxides, ferrites, and garnets) cannot be readily incorporated into multilayered thin films because of the requirement of high-temperature processing, which is incompatible with the fabrication of high-quality multilayers. An exception is GdN, which is a ferromagnetic insulator with a Curie temperature sTcmd of as much as 60 K, depending on stoichiometry [9]. GdN has a resilient fcc NaCl structure (a ø 0.5 nm), which is retained even for offstoichiometric samples [10]. Thin films of GdN can be readily achieved using reactive sputtering with room temperature substrates. For the superconducting layers, we