doesn't the baud rate determine how fast the values are coming in and at what frequency the EM wave is that will be probed?
You are fundamentally limited by two factors, the speed of response of the hall effect device and the speed of the A/D converter. From the latter you can only expect to be able to cope with frequencies of about 20KHz and that is very very low for an EM wave. It is down in the atmospheric whoopers range and is nothing you would generate, not even if you want to communicate with submarines.