Spin waves and the anomalous Hall effect in ferromagnetic (Ga,Mn)As

discovered independently in 1988 by future Nobel Prize laureates, PeterGrünberg of the KFA research institute in Jülich and Albert Fert of theUniversity of Paris-Sud [11, 12]. Shortly after, the spin valve (Fig. 1.1a) exploitingthis effect was put into mass production in read heads of magnetichard disk drives by the spintronics guru Stuart Parkin and his colleaguesat Almaden Laboratory of IBM. 3 These first simple spin-based devices allowedto increase the data storage capacity about 10 000 times, whilst themanufacturing cost has dropped by 100. Ironically, the now venerable electromechanicaldisk drives are being supplanted by faster, nonvolatile andmore reliable solid-state disks, which are not the spintronics’ achievement.Then, one may think that spintronics has already reached its full potentialin this field. Nothing of the kind—it quests after what is called theholy grail of memory technology, MRAM (magnetoresistive random accessmemory) (Fig. 1.1c) [14, 15]. It combines the advantages of all memorytypes used in our computers (the density of DRAM, the speed of SRAMand the nonvolatility of flash or hard disk) with none of their shortcomings,and in the future it will replace them with a combo memory chip. 4MRAM, initially employing the GMR and later its close cousin exhibitinghigher magnetoresistance at room temperature, the TMR effect (tunnellingmagnetoresistance) [16], competes with other types of novel memories atthe stage of development, like the racetrack memory invented by Parkin(Fig. 1.1b) [17] or Spin Torque Transfer RAM [18, 19]. The described devicesemploy a spin-polarised current in metals, obtained by passing theelectrons through a ferromagnetic material. The metal-based spintronicsdevices, together with various types of magnetic sensors, utilise the spinin a passive way—for detecting and reading tiny magnetic fields associatedwith magnetically stored data. They are certainly one of the most successfultechnologies of the past decade.However, spintronics is projected to go beyond passive spin usage, andintroduce new applications (or even new technologies) based on the activecontrol of spins. In such devices, spins can either play a peripheral rolein storing, processing and transmitting information by electric charges orhandle these functions themselves, without involving charge at all. Theirquintessential examples are the spin-FET (spin field-effect transistor) andspin logic [20]. The theoretical proposal of spin-FET by Datta and Das in3 GMR sandwich structures in read heads of magnetic hard disk drives were commercialisedwithin ten years from the discovery of the fundamental physical effect, while atypical timeline from discovery to the marketplace is 20–30 years [13].4 In 2006 Freescale started selling the first commercial MRAM modules with 4Mbit ofmemory for 25US dollars a piece. Although still of low density and quite expensive, morethan a million chips were sold. They already proved themselves in critical programs anddata storage in extreme environments, and soon will be put to use in flight control computerson the next-generation Airbus aeroplanes. Various companies working on MRAMestimate a realistic timeframe for a cellphone, PDA or a Disk-On-Key with MRAM as2011, while for a personal computer as 2015.15