Perpendicular Magnetic Recording

PMR

PMR stands the bit’s magnetization up, perpendicular to the disk. It is the magnetic orientation inside a drive, not the track layout sold as CMR or SMR.

In one minute

PMR stores each bit with its magnetization perpendicular to the disk surface. Western Digital’s November 2007 white paper dates a 2005 HGST field test, a 2006 commercial introduction, and the first terabyte desktop drive, the Deskstar 7K1000, to January 2007. The paper describes trailing-shield writers and granular CoCrPt oxide media with a soft underlayer as the stack that made that density practical.

Longitudinal recording laid magnetization along the track. PMR stands it perpendicular to the disk. Western Digital’s November 2007 paper describes the soft underlayer and trailing-shield writers used to write those bits.

The same paper places a commercial PMR introduction in 2006 and names the Deskstar 7K1000, January 2007, as the first terabyte desktop drive. A 2005 date in that paper is an HGST field test, not a product launch. Later labels such as CMR and SMR describe whether tracks overlap, not whether the bits stand up.

Seagate’s path note says conventional PMR runs out of steam just over 1 terabit per square inch, and that HAMR media was completely reengineered. Those are Seagate’s claims about the limit of this stack, not a law that every drive hits on the same day.

Compared to neighbors

CMR and SMR name the track layout: tracks that do not overlap, or tracks that do. PMR names the direction of the magnetization. A shipping drive can be PMR and CMR, or PMR and SMR. Longitudinal recording is the older in-plane orientation that the 2007 Western Digital paper treats as the generation PMR replaced in volume products.

Strengths

Trailing-shield writers and a soft underlayer — Western Digital’s November 2007 paper describes these as the write-field and media changes behind PMR.
Terabyte-class desktop drives — that paper names the January 2007 Deskstar 7K1000 as the first terabyte desktop drive.
The vendor baseline Seagate still cites — Seagate’s “From PMR to SMR to HAMR” note treats PMR as the mode that shingling and HAMR extend.

Limits

Seagate’s density claim — that note says today’s PMR runs out of steam just over 1 terabit per square inch. That is a Seagate figure, not a measured cutoff for every model.
HAMR does not keep the same media — Seagate says HAMR media was completely reengineered, using FePt, so a later method is not the PMR stack with one extra part.
Not a host-visible mode — datasheets now say CMR or SMR for whether tracks overlap. PMR is the bit orientation under those labels, not a setting the host selects.

Typical fits

Reading a datasheet row that says the recording method is perpendicular
Separating that row from a CMR or SMR label, which describes track overlap
Placing Seagate’s stated PMR density limit next to the HAMR and SMR pages

Uncertainty notes

Years are taken from the Western Digital paper: 2005 is the HGST field test it describes, 2006 is the commercial introduction it describes, and January 2007 is the Deskstar 7K1000. This card does not treat any of those as the first ship date of every vendor.
The just-over-1-Tb/in² line is quoted from Seagate. It is not an independent measurement.

Sources

01Western Digital — Perpendicular Magnetic Recording Technology white paper (November 2007) · accessed 2026-10-07
02Seagate — From PMR to SMR to HAMR (technical insights) · accessed 2026-10-07