Is it possible to purchase larger NdFeB rare earth magnets or other magnets that are stronger?
R -Fe-B magnets are still the strongest handy and small permanent magnets I guess. Once a Japanese collaborator of mine gave me a few surprisingly strong small such magnets. These would definitely serve your purpose. I would also gladly give you a few. These are so strong that I was afraid to keep it near my desk because they could earse important data.
You can increase the flux density of your NdFeB magnets by arranging them in an array, such as a Halbach array. I have two published patents that further increase the flux density above what can be achieved via the Halbach array. The first is a 3 dimensional magnet array, United States Patent 8405479. The second is a hyper Halbach array, United States Patent 8009001 B1. This method can be used to augment the flux field of any type of magnet including the NdFeB magnets.
R-Fe-B magnets are currently the most strongest magnetic materials available, however, with the problem of temperature and water sensitivity. Therefore, many applications use the Co-Sm magnets instead. But of course, the strength of a magnet can be increased effectively by arranging them into Halbach arrays, as Mark has pointed out already.
To separate colloidally stable magnetic nanoparticles from solution, you have to create strong forces at sufficient distances. A high value of the magnetic field is of interest, but also a high value of the spatial variation of this field, so-called "gradient of the field" (really the gradient of B at the power two). Conveniently shaped and sized R-Fe-B magnets allow a high value of the "field gradient". For example a 2D array of parallel, alternatively magnetised very thin sheets of magnets, create a force varying as the inverse of the width of magnets. But the extension of this force varies as the width itself : you have to take in account the time of migration, and the rate of capture, you will for the colloidal particles...
The role of "field gradient" is emphasized in the HGMS devices (high gradient magnetic separators), particularly those using superconducting coils for the high field, and thin ferromagnetic wires for the high field gradient near the wires.
A particular device using large permanent magnets (Nd Fe B) for the field and ferromagnetic collectors for high gradient has been realised and studied. See on this subject the PhD thesis of SIDO Nana Mariama, Lorraine Polytechnic National Institute (Lorraine University)
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