Haven’t tried loading a material specifically but you could have a look at StaticLoadObject(), e.g.
// Asset path in content browser - you could also dynamically build this string
FString path = TEXT("Material'/Game/Content/MaterialTest.MaterialTest'");
// Use StaticLoad to load the object
UMaterial* mat = Cast<UMaterial>(StaticLoadObject(UMaterial::StaticClass(), NULL, *path));
if (mat )
// Do something