On the nature of the magnetic fields generated during the electroweak phase transition

In this Letter we reanalyse the question of the origin of magnetic fields during the electroweak phase transition. We show that their formation is intimately connected to some semiclassical configurations of the gauge fields, such as electroweak Z-strings and W-condensates. We describe the formation of these semiclassical configurations during a first order phase transition and argue that they might be generated also in the case of a second order phase transition. We suggest that the instability of electroweak strings does not imply the disappearance of the embedded magnetic field.