The head can read only large magnetic domains. The domains could be closer if the head could distinguish them. When a rapidly spinning disk gets too close to the head, it touches and crashes. Even a particle of cigarette smoke is larger than that gap, so bringing the head and disk any closer together gets tricky. What the Livermore lab, IBM and University of California at Berkeley collaborators have done is deposit thin coats of synthetic diamond on the disk and the read-write head using a cathodic arc.
Crashes cause less damage because the disk and head are so much harder.
IBM's drives will store up to 2.6G of data per square inch without the diamond coating. To read such domains, the disk and head must be so close that they almost touch all the time. Current coats, between 12 nanometers and 15 nanometers thick, keep the disk and head too far apart. The diamond coating can be much thinner and harder, promising data densities as great as 10G per square inch at little increase in cost. Greater data density means more data on small drives, faster access and higher data transfer rates.