The new era of spintronics

Albert Fert, Jean-Marie George, Henri Jaffrès, Richard Mattana, Pierre Seneor
2003 EurophysicsNews  
FEATURES 227 go beyond the 100 Gbit/in 2 of today's prototypes with conventional spin valves. A further increase of read head sensitivity and information density in hard discs will be probably achieved with other spintronic devices, magnetic twmel junctions [2] (see next paragraph), patterned multilayers for GMR in the non-conventional geometry where the current is perpendicular to the layers [4] or magnetic nanocontacts presenting a ballistic version of GMR known as BMR [5] . 111" High
more » ... ce stat! .~Tunnel Ferromagnetlc --__. __ electrodes --barners ,"0" low resistance state .. Fig. 1 : Top: Memory cells of an MRAM (Magnetic Random Access Memory). Each cell is a submicronic MTJ and the states "0" and"'" of the cell corresponds respectively to the parallel and antiparallel configurations ofthe magnetic moments of electrodes ofthe MTJ (Iow and high resistance states). Central sketch: schematic of an MRAM constructed of MTJ connected together in a point contact array. Conducting wires,"bit lines" and "word lines" permit voltage measurements to read the stored information.They also enable manipulation ofthe magnetization ofthe elements (writing) by the magnetic field created by currents in the Iines.The main advantage of MRAM over semiconductor-based DRAM or SRAM is their permanent character (for similar densi,ty and speed), Magnetic tunnel junctions and MRAM The magnetic tunnel junction (MTJ) [3] is the second type of spintronic device that will have soon important applications. A MTJ (Fig. la) is a structure in which two ferromagnetic layers (electrodes) are separated by a very thin insulating layer, commonly aluminum oxide. The electrons can tunnel through the insulating layer and, because the probability of tunneling from a ferromagnetic electrode depends on the spin direction, the resistance of the MTJ is different for the parallel and antiparallel orientations of the magnetic moments of the electrodes [3] . For electrodes of conventional ferromagnetic alloys, the relative change of resistance (Tunnel Magnetoresistance or TMR) can reach 70% at room temperature. MTJ ofvery small size, below the micron range, can be fabricated by lithographic techniques and an important application of these small size MTJ will be for a new type of computer memory, the MRAM (Magnetic Random Access Memory). As illustrated in the upper part of Fig.1 , each junction can store one bit of data, say "0" for the parallel configuration of the magnetic moments of the electrodes, "1" for the antiparallel configuration. The sketch below is a schematic representation of a MRAM with MTJ connected together in a point contact array. The MRAM presently in development are expected to reach similar densities and access times as the current DRAM or SRAM, but their main advantage on these volatile semiconductor-based memories is that they retain data after the power is S pintronics, at the interface between magnetism and electronics, is a new field ofresearch in considerable expansion [1]. The basic concept of spintronics is the manipulation of spin currents, in contrast to mainstream electronics in which the spin of the electron is ignored. Adding the spin degree of freedom provides new effects, new capabilities and new functionalities. Beyond today's applications to hard discs and memories, the potential of spintronics is very promising for new advances and an important impact on science and technology of the 21st century. The influence ofthe spin on the electronic transport properties of ferromagnetic materials has been known for a long time. The existence of spin polarized currents in ferromagnetic metals, first suggested by Mott, has been experimentally demonstrated and theoretically described about thirty years ago, mainly by European researchers (at Orsay, Strasbourg, Eindhoven). In metals like iron or cobalt, the "majority spin" and "minority spin" electrons carry currents that can be different by more than a factor of ten. This spin dependent conduction comes from the splitting of the "majority spin" and "minority spin" energy bands and also from the spin dependent cross section of some defects or impurities. Everybody has already a spintronic device on their desktop The first step on the road of the utilization of the spin degree of freedom was the discovery of the Giant Magnetoresistance of magnetic multilayers in 1988 [2] . A magnetic multilayer is a film composed of alternate ferromagnetic and nonmagnetic layers, Fe and er for example. The resistance of such a multilayer is lowest when the magnetic moments of ferromagnetic layers are aligned, and highest when they are antiparallel. As the relative change of resistance can be as high as 200%, this effect has been called Giant Magnetoresistance (GMR). The condition required for GMR is a much better conduction through a ferromagnetic layer by the electrons of one ofthe spin directions, say the majority spin direction for example. When the magnetic moments of all the layers are aligned, half of the electrons are majority spin electrons in all the magnetic layers, and the short circuit effect by this high conduction channel results in a low resistance. In the antiparallel configuration, each electron is alternately a majority and a minority spin electron, the short circuit effect does not exist and the resistance is much higher. In specially designed multilayers, known as spin valves, the magnetic configuration can be switched between parallel and antiparallel configuration by a field of only a few Oersted, so that a large change ofresistance can be induced by a very small field. The first spintronic devices have been based on spin valves. Today everybody has already a spintronic device on their desktop, since all modern computers use spin valves for the read heads of the hard disc. Because they can detect very small fields and very small magnetic bits, the spin valve-based read heads have led to an increase of the density of stored information by almost two orders of magnitude. It might however be hard to europhysics news
doi:10.1051/epn:2003609 fatcat:56ebrxnw6rderh5wp4aoa63uom