Okay, so I may be putting my hand up for a "kook of the day" badge, but I have a different hypothesis for the generation of planetary magnetic fields. The use of seismology has given us insight into the makeup of the Earth's core, being a reasonably solid crystallising iron with some nickel under immense pressure. We also know that when pressure is applied to these crystals, an electrical charge is produced - piezoelectrics. Magma is known to behave as a plasma, providing a conduit for the flow of an electrical charge, which thanks to Maxwell we know produces both electrical and magnetic fields.
On Earth, we have plate tectonics, which allows the movement of the charges (and fields) through to the surface. This may explain some of the lightning ubiquitous with active volcanic eruptions. Check out the research by Friedemann Freund if you would like to know more. The lack of tectonic plate movement on Venus may be due to less movement of magma deep in the planet which would otherwise facilitate the flow of charge (and hence fields) towards the surface of the planet, resulting in a vastly reduced manifestation of a magnetic field.
If I may briefly transition from a badge for being a kook to a proper branding, this piezo model can be extended to the Sun, whereby a solid core of crystalised iron produces a plasma of charged particles on the surface which we see as positively charged protons (hydrogen ions) and negatively charged electrons providing the spectrographic signature for hydrogen.
Another leap for the hypothesis, which to be honest is more speculation, is to make sense of observed magnetic fields which span not just galaxies, but the vast distances between them. The polarisation of light informs us of their presence, but I'm yet to hear of a plausible explanation for the electrical current producing them. It's important to keep in mind that thanks to old mate Maxwell, we know a magnetic field doesn't exist without an electrical charge or current. So what is the hypothesised generator of these magnetic fields measured in thousands of lightyears? Glad you asked...Piezo... Imagine the immense gravitational pressures toward the centres of galaxies. If we stretch our concept of stars to include iron and nickel etc crystalline cores, we have our mechanism. As for that being a stretch, to be honest I feel it takes more of a stretch to believe that a forming star would, under gravitational forces, see hydrogen collapsing into the centre to form the star, with heavy elements, such as iron, settling further out where planets are forming planets...
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