The Thin Film head, which debuted in 1979. Years later, the Giant Magneto Resistive head, invented by IBM, allowed for greater accuracy in positioning of data, which lead directly to improvements in areal density. The "giant" in GMR doesn't refer to the size of the head but the large changes in resistance that occur when magnetic fields are applied. (Different resistance levels can be tagged as ones and zeros of data.) You could stack up 250,000 of these heads and it would be less than an inch high.