The tiny magnetic particles are produced in bacteria
A team at the University of Edinburgh has developed a method of making the nanomagnets stronger, opening the way for their use in cancer treatment.
The bacteria-produced magnets are better than man-made versions because of their uniform size and shape, the Nature Nanotechnology study reported.
It is hoped one day the magnets could be guided to tumour sites and then activated to destroy cancerous cells.
The bacteria take up iron from their surroundings and turn it into a string of magnetic particles.
They use the chains of particles like a needle of a compass to orientate themselves and search for oxygen-rich environments.
There has been a lot of interest in their potential application in medicine, but how useful they could be will depend on the strength of the magnets.
Scientists at Edinburgh University grew the bacteria in a mixture that contained more cobalt than iron.
The addition of cobalt in the nanomagnets made them 36-45% stronger.
This meant they stayed magnetised longer when taken out of a magnetic field.
The ability of the nanomagnets to remain magnetised opens the way for their use in killing tumour cells, the researchers said.
They could be guided to the site of a tumour magnetically.
Once there, applying an opposite magnetic field would cause the nanomagnets to heat up, destroying cells in the process.
They could also potentially be used to carry drugs directly to the cancerous tissue.
Study leader, Dr Sarah Staniland, a research fellow at the University of Edinburgh, said: "For nanoparticles to be used in medicine you need them to be a very uniform size and shape and bacteria are very good for that.
"This increases the scope for their use in cancer.
"You would move them with a normal magnetic field and then heat them with the opposing field."
Liz Baker, Cancer Research UK's science information officer, said: "Targeting treatments specifically to cancer cells is an exciting area of research, but in this case work is still at a very early stage.
"It will be interesting to see if further research into nanomagnets will provide us with a new and effective anti-cancer therapy."
The research was carried out alongside scientists at Daresbury Laboratory in the UK and the Institut Laue-Lengevin in Grenoble, France.