Geomagnetic field strength 3.2 billion years ago recorded by single silicate crystals. 2007. JA Tarduno, et al. Nature 446: 657-660.
From the press release:
The Earth's magnetic field was nearly as strong 3.2 billion years ago as it is today. The findings, which are contrary to previous studies, suggest that even in its earliest stages the Earth was already well protected from the solar wind, which can strip away a planet's atmosphere and bathe its surface in lethal radiation.
Using a SQUID—a Superconducting Quantum Interface Device—researchers measured the magnetic fields of nano-meter sized magnetic inclusions in rocks that lock in a record of the Earth's magnetic field as they cool from molten magma to hard rock.
"The data suggest that the ancient magnetic field strength was at least 50 percent of the present-day field, which typically measures 40 to 60 microteslas," says Tarduno. "This means that a magnetosphere was definitely present, sheltering the Earth 3.2 billion years ago."