def find_mdf(df):
    """
    Finds the median destructive field for AF demag data
    Parameters
    __________
        df : dataframe of measurements
    Returns
    ______
        mdf : median destructive field
    """
    mdf_df=df[df.meas_norm<=0.5]
    mdf_high=mdf_df.treat_ac_field_mT.values[0]
    mdf_df=df[df.meas_norm>=0.5]
    mdf_low=mdf_df.treat_ac_field_mT.values[-1]
    mdf=int(0.5*(mdf_high+mdf_low))
    return mdf