Hi, I write here for the first time and i hope so far i am doing everything properly I am writing about Electronic Article Surveillance (EAS) for my college (University of Latvia), it's popular science homework. This is not very common subject in latvian books so I had to read everything in english and I managed to do it. Except that I couldn't understand how excatly in acousto-magnetic systems bias magnet makes material (highly magnetostrictive) in tags to oscillate at frequency F (not 2F) which is frequency of used electromagnetic field as I understood. Thanks!!!!