  Squeezing ensemble of spins provides a way to surpass the standard quantum
limit (SQL) in quantum metrology and test the fundamental physics as well, and
therefore attracts broad interest. Here we propose an experimentally accessible
protocol to squeeze a giant ensemble of spins via the geometric phase control.
Using the cavity-assisted Raman transitions in a double $\Lambda$-type system,
we realize an effective Dicke model. Under the condition of vanishing effective
spin transition frequency, we find a particular evolution time where the cavity
decouples from the spins and the spin ensemble is squeezed considerably. Our
scheme has the potential to improve the sensitivity in quantum metrology with
spins by about two orders.
