While the magnetite particles are much above single-domain limit, they indeed form spike-like structure under strong magnetic field. Would not that be predominantly caused by aligning of particles physically rather than realignment of their magnetic domain, due to the not very high temperature and magnetite being ferrimagnetic? If that magnetic orientation is locked by congealing the "spikes" by cement or wax, would that arrangement have a net arranged magnetic field upon withdrawal of external magnetic field, provided that the particles can move no longer? If this is not possible, then why? If possible, then what would be the strength of that magnetic field compared to a bar magnet? (for a comparable distance and geometry)
Will the slurry be fluid enough to allow the magenit spikes to form like in gas or vacuum?(relation between % solid ,% magnetic particles allowed and time to set as well as maximum residual magnetic field of the arrangement are asked; or at least hint to the model). Another point to mention, melting point of wax would be below 100 degrees C to not affect magnetite magnetism, but what would be role of Typical cement chemicals (e.g. portland, plaster of paris) that may chemically degrade the magnetite?
@Sumit: The magnetism will be retained in cement. There is no reason or a chemical reaction in an alkaline medium, such as cement, and so the magnetite will remain stable. But the aggregation of the magnetite particles depends on the treatment before gelation of the cement.
Sumit Bhowmick - I second to Roland Fabis - the aggregation (getting rid of it) is important, as is perfect mixing and viscous character. You may want to look into technologies for bonded magnets:
https://magneticsmag.com/bonded-magnets-a-versatile-class-of-permanent-magnets/
You might have problems getting the cement slurry liquid enough for the particles to move and align within the magnetic field. If you actually try it, I would recommend using a superplasticizer otherwise you will need a huge amount of water and your product will be very porous. If you use a white cement you could test if spikes did form within by slicing the hardened product. You could be able to see from the magnetite particle distribution if it worked.
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