Imagine an airplane flying one millimeter above the ground and circling the Earth once every 25 seconds while counting every blade of grass. Shrink all that down so that it fits in the palm of your hand, and you'd have something equivalent to a modern hard drive, an object that can likely hold more information than your local library. So how does it store so much information in such a small space? At the heart of every hard drive is a stack of high-speed spinning discs with a recording head flying over each surface. Each disc is coated with a film of microscopic magnetised metal grains, and your data doesn't live there in a form you can recognize. Instead, it is recorded as a magnetic pattern formed by groups of those tiny grains. In each group, also known as a bit, all of the grains have their magnetization's aligned in one of two possible states, which correspond to zeroes and ones. Data is written onto the disc by converting strings of bits into electrical current fed through an electromagnet. This magnet generates a field strong enough to change the direction of the metal grain's magnetization. Once this information is written onto the disc, the drive uses a magnetic reader to turn it back into a useful form, much like a phonograph needle translates a record's grooves into music. But how can you get so much information out of just zeroes and ones? Well, by putting lots of them together. For example, a letter is represented in one byte, or eight bits, and your average photo takes up several megabytes, each of which is 8 million bits. Because each bit must be written onto a physical area of the disc, we're always seeking to increase the disc's areal density, or how many bits can be squeezed into one square inch. The areal density of a modern hard drive is about 600 gigabits per square inch, 300 million times greater than that of IBM's first hard drive from 1957. This amazing advance in storage capacity wasn't just a matter of making everything smaller, but involved multiple innovations. A technique called the thin film lithography process allowed engineers to shrink the reader and writer. And despite its size, the reader became more sensitive by taking advantage of new discoveries in magnetic and quantum properties of matter. Bits could also be packed closer together thanks to mathematical algorithms that filter out noise from magnetic interference, and find the most likely bit sequences from each chunk of read-back signal. And thermal expansion control of the head, enabled by placing a heater under the magnetic writer, allowed it to fly less than five nanometers above the disc's surface, about the width of two strands of DNA. For the past several decades, the exponential growth in computer storage capacity and processing power has followed a pattern known as Moore's Law, which, in 1975, predicted that information density would double every two years. But at around 100 gigabits per square inch, shrinking the magnetic grains further or cramming them closer together posed a new risk called the superparamagnetic effect. When a magnetic grain volume is too small, its magnetization is easily disturbed by heat energy and can cause bits to switch unintentionally, leading to data loss. Scientists resolved this limitation in a remarkably simple way: by changing the direction of recording from longitudinal to perpendicular, allowing areal density to approach one terabit per square inch. Recently, the potential limit has been increased yet again through heat assisted magnetic recording. This uses an even more thermally stable recording medium, whose magnetic resistance is momentarily reduced by heating up a particular spot with a laser and allowing data to be written. And while those drives are currently in the prototype stage, scientists already have the next potential trick up their sleeves: bit-patterned media, where bit locations are arranged in separate, nano-sized structures, potentially allowing for areal densities of twenty terabits per square inch or more. So it's thanks to the combined efforts of generations of engineers, material scientists, and quantum physicists that this tool of incredible power and precision can spin in the palm of your hand.
Predstavljajte si letalo, ki leti en milimeter nad tlemi in vsakih 25 sekund obkroži Zemljo, pri tem pa prešteje vse travne bilke. Če to pomanjšate do te mere, da se prilega vaši dlani, dobite nekaj enakovrednega sodobnemu trdemu disku, predmetu, ki lahko vsebuje več informacij kot vaša lokalna knjižnica. Kako pa lahko hrani toliko informacij na tako majhnem prostoru? V središču vsakega trdega diska je skladovnica hitro vrtečih se plošč z bralno/pisalno glavo, ki leti nad vsako od površin. Vsaka plošča je prekrita s prevleko mikroskopskih magnetiziranih kovinskih zrn in vaši podatki tam ne živijo v obliki, ki bi jo prepoznali. Namesto tega so zapisani kot magnetni vzorec, ki ga tvorijo skupine teh drobnih zrn. V vsaki skupini, znani tudi kot bit, imajo vsa zrna magnetizacijo poravnano v eno od dveh možnih stanj, ki ustrezata ničlam in enicam. Podatki se na disk zapišejo s pretvorbo bitnih nizov v električni tok, ki ga dovaja elektromagnet. Ta magnet tvori polje, ki je dovolj močno, da spremeni smer magnetizacije kovinskih zrn. Ko so informacije enkrat zapisane na disk, pogon uporabi magnetni bralnik, da jih pretvori nazaj v uporabno obliko, zelo podobno gramofonski igli, ki utore na plošči prevaja v glasbo. Toda kako lahko dobimo toliko informacij iz samo ničel in enic? Tako, da jih damo mnogo skupaj. Na primer, črko predstavlja en bajt ali osem bitov, vaša povprečna slika pa zavzame več megabajtov, vsak od njih pa ima 8 milijonov bitov. Ker mora biti vsak bit zapisan na fizični del diska, venomer iščemo načine za povečanje gostote zapisa na disku oz. koliko bitov lahko stisnemo na kvadraten palec. Gostota zapisa modernih trdih diskov je okoli 600 gigabitov na kvadratni palec, 300-milijonkrat več kot pri IBM-ovemu prvemu trdemu disku iz leta 1957. Ta neverjetni napredek v kapaciteti shrambe ni bil rezultat le pomanjševanja vsega, ampak je vključevalo številne inovacije. Tehnika, imenovana proces litografije tankega filma, je inženirjem omogočila pomanjšanje bralno/pisalne glave. In navkljub svoji velikosti, je bralnik postal bolj občutljiv z izkoriščenjem novih odkritij na področju magnetnih in kvantnih lastnostih snovi. Biti so lahko bili tudi bolj zgoščeni zahvaljujoč matematičnim algoritmom, ki filtrirajo motnje magnetne interference in najdejo najbolj verjetne bitne nize iz vsakega kosa povratnega bralnega signala. Nadzor nad termičnim raztezanjem glave, ki ga omogočimo z grelnikom pod magnetnim zapisovalnikom, ji je omogočilo lebdenje na manj kot petih nanometrih nad površino diska, kar je približno širina dveh vej DNK. Zadnjih nekaj desetletij je eksponentna rast kapacitete računal- niškega pomnilnika in procesorske moči sledila vzorcu, ki ga poznamo kot Moorov zakon, ki je 1975 predvidel, da se bo gostota informacij podvojila vsaki dve leti. Vendar pa je pri okoli 100 gigabitih na kvadratni palec nadaljnje pomanjševanje magnetnih zrn, ali njihovo še večje tlačenje skupaj, predstavljalo novo tveganje imenovano super-paramagnetni učinek. Ko je prostornina magnetnega zrna premajhna, njegovo magnetizacijo zlahka zmoti toplotna energija in lahko povzroči, da se biti nenamenoma zamenjajo, kar vodi k izgubi podatkov. Znanstveniki so to omejitev rešili na presenetljivo enostaven način: s spremembo smeri zapisovanja iz vzdolžne v navpično, kar je omogočilo gostoto zapisa blizu enega terabita na kvadratni palec. Nedavno se je potencialna meja ponovno zvišala z uporabo toplotnega magnetnega zapisovanja (HAMR). Ta uporablja še bolj termično stabilen zapisovalni medij, čigar magnetno odpornost se za trenutek zniža s segrevanjem določene točke z laserjem in omogočenim zapisovanjem podatkov. In medtem ko so ti pogoni trenutno v fazi prototipov, imajo znanstveniki v rokavu že nov potencialni trik: medij bitnih vzorcev (BPM), na katerem so lokacije bitov razvrščene v ločene nano strukture, kar potencialno omogoča gostote zapisa 20 terabitov na kvadratni palec ali več. Zahvalimo se torej lahko skupnim prizadevanjem večih generacij inženirjev, znanstvenikov snovi in kvantnih fizikov, da se to orodje neverjetne moči in natančnosti lahko vrti v vaši dlani.