May I know what's requirement of Dikhr? Is it sufficient to recite a dhikr in the heart not necessarily reciting it in whisper or aloud?

There is a heart Thikr and a tongue Thikr. The heart Thikr is the observation of Allah almighty in all cases, while the tongue Thikr is the movement of the tongue with a certain Thikr. The benefit of a tongue Thikr depends on moving the tongue and lips with it, and the best of it is لا إله إلا الله, because it is a hidden Thikr, because it is not noticable.