Can we use a magnetic encoder?！

Yes, although we don't recommend it. 

Magnetic encoders suffer from poor linearity which will lead to a rougher motion control than with optical encoders. 
This linearity error is made worse by minor missalignments between the magnet and the sensor, or with interfering magnetic fields. A problem when you mount one directly to a dense electromagnet!

If you are still keen to give a magnet encoder a try, then try to mount the magnet in a plastic holder if possible. Also place a magnetic shield between the motor and the encoder. A low-carbon steel shim (>0.5mm) should suffice. 

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