Explanation:

Ferrimagnetism:
- Ferrimagnetic materials have two different magnetic sublattices that are antiparallel in nature, but the magnetic moments of one sublattice are larger than the magnetic moments of another sublattice.
- Fe3O4 is an example of ferrimagnetic material where Fe2+ ions occupy tetrahedral sites and Fe3+ ions occupy octahedral sites.
- Ferrimagnetism arises due to the magnetic moments of tetrahedral and octahedral sites not being completely canceled out.

Changes in Magnetism with Temperature:
- When a magnetic material is heated, its thermal energy increases, and the thermal vibrations of atoms/molecules/magnetic moments increase.
- The magnetic moment of a magnetic material decreases with the increase in temperature because the thermal vibration causes a randomization of the magnetic moments.
- With a further increase in temperature, the magnetic moments align themselves in the direction of the applied magnetic field, leading to the material becoming paramagnetic.

Fe3O4 at Room Temperature:
- At room temperature, Fe3O4 is ferrimagnetic, with the magnetic moments of the tetrahedral and octahedral sites not being completely canceled out.
- The magnetic moments of Fe2+ and Fe3+ ions are aligned antiparallel to each other, with the magnetic moment of Fe2+ being larger than that of Fe3+.

Fe3O4 at 850 K:
- At 850 K, the thermal energy of Fe3O4 is high enough to randomize the magnetic moments of Fe2+ and Fe3+ ions, leading to the material becoming paramagnetic.
- The magnetic moments cease to be aligned antiparallel, and the material no longer exhibits ferrimagnetism.

Conclusion:
- Fe3O4 is ferrimagnetic at room temperature, but it becomes paramagnetic at 850 K due to the randomization of magnetic moments caused by thermal energy.