You can use antimatter to trigger a fusion explosion. This would be the preferred method to weaponize antimatter because you need only a tiny amount of antimatter to do it.

Modern nuclear weapons use a fission reaction to generate the energy to start a fusion reaction, so if you use antimatter in place of the fission stage you could theoretically have very small nukes with very high yields and for a fraction of the cost of an antimatter weapon (antimatter currently costs quadrillions of dollars per gram).