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Fundamentally an inertial measurement doesn't include the magnetic field. In practice the orientation of the sensor is estimated from both the magnetometer and the inertial sensors, so this message only includes a derived orientation estimate not the original magnetic field measurements.

There is the sensor_msgs/MagneticField message type but I'm not sure if all the IMU driver nodes support publishing this as well as the IMU message. This is probably what you're looking for but it isn't as commonly supported.