I have no idea why the DE fad is so strong, it does NOT kill mites!
Clean the coop and treat the coop and the chickens with permethrin. Most people use a poultry or horse dust with permethrin in it. I greatly prefer using a spray (usually marked for kittens and dogs etc.), but it is all the same drug.
It works great. You do need to treat twice, 3 times is best if you have a true infestation.