well, according to the datasheet the A1302 should work fine for your application.
5V only though, but it's linear 1.3 mV/G so for every 1.3mV on its output you have 1 gauss... if you used a 4V precision Vref (some pics have them onboard) then you could actually make your bot detect and measure the EM field.
Best yet, it cheep £0.99 each...

Here is the farnell link... http://uk.farnell.com/allegro-micros...sip/dp/1791388

Also, ontop of this... you don't have to use an AC circuit to drive the wire... you can run DC through it... thus saving even more time and money...