Which of the following materials possesses anti-parallel spins of unequal magnitude, resulting in a spontaneous magnetic moment below a critical temperature?
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The classic example of a ferrimagnetic material is Magnetite ($\text{Fe}_3\text{O}_4$).
It contains both $\text{Fe}^{2+}$ and $\text{Fe}^{3+}$ ions, which occupy different lattice sites with opposing, unequal magnetic moments.