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Shinjo 2000 Taylor&Francis_Book_Chapter_1 Experiments of Giant Magnetoresistance

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Spin dependent transport in magnetic nanostructures T. Shinjo November 1, 2000 Contents 1 INTRODUCTION 2 2 GMR IN MULTILAYERED SYSTEMS : EXPERIMENTAL ASPECTS 4 2.1 Before the discovery of GMR . . . . . . . . . . . . . . . . . . . 4 2.2 The GMR effect (Fe/Cr multilayers) . . . . . . . . . . . . . . . 6 3 Other coupled-type GMR systems and interlayer coupling 3.1 Non-coupled systems (multilayers) . . . . . . . . . . . . . . . . 3.2 Non-coupled sandwiches (Spin valve systems) . . . . . . . . . 3.
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  Spin dependent transport in magneticnanostructures T. ShinjoNovember 1, 2000 Contents 1 INTRODUCTION 22 GMR IN MULTILAYERED SYSTEMS : EXPERIMENTALASPECTS 4 2.1 Before the discovery of GMR . . . . . . . . . . . . . . . . . . . 42.2 The GMR effect (Fe/Cr multilayers) . . . . . . . . . . . . . . . 6 3 Other coupled-type GMR systems and interlayer coupling 8 3.1 Non-coupled systems (multilayers) . . . . . . . . . . . . . . . . . 103.2 Non-coupled sandwiches (Spin valve systems) . . . . . . . . . . 163.3 Extension of studies on GMR-related phenomena . . . . . . . . 173.4 Geometry of GMR . . . . . . . . . . . . . . . . . . . . . . . . . 203.5 Studies on nanomagnetic systems using GMR effect . . . . . . . 243.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271  1 INTRODUCTION The technical term, magnetoresistance (MR), has been used to express allkinds of electric conductivity change caused by applying a magnetic field andcovers a great variety of phenomena. For instance, the features of MR ef-fect in metallic systems are greatly different from those in semiconductors. Inferromagnetic metals and alloys, the difference of resistivity regarding to thedirection of magnetization has been known as the anisotropic magnetoresis-tance (AMR) from a long time ago. Normally the resistivity is smaller if theelectric current direction is perpendicular to the direction of magnetizationthan in the condition that those are parallel. The srcin of AMR is consideredto be the orbital angular momentum of magnetic ions and also the Lorentzforce acting on conduction electrons. The relative change of resistivity (MRratio) due to the AMR effect is fairly small; a few present for Ni 80 Fe 20  alloy(permalloy) at room temperature and somewhat larger at lower temperatures.Nevertheless, this phenomenon has a significant importance for technical ap-plications such as magnetic sensors. As illustrated in Fig. 1, if a magnet isattached on a rotating disk, an MR sensor can detect the number of rotationsor the speed of motion very easily from the resistance change of the MR sensor.It has also been attempted to apply an AMR sensor for a magnetic recordingtechnology. Using an MR read-out head, information recorded in a magneticstorage medium is directly converted into electric signals. For ultrahigh den-sity recording, a very high sensitivity of head is indispensable and thus theMR ratio is required as high as possible. However it was unlikely to find anymaterial having a large MR effect at room temperature. Some magnetic semi-conductors have been known to exhibit large MR ratios but only at low tem-peratures. Although a large MR effect at room temperature under a moderatemagnetic field is desirable from a technical viewpoint, no major success hasbeen achieved in the exploration of new MR materials, until the discovery of the giant magnetoresistance (GMR) effect.[Figure 1 about here.]The GMR effect has been observed in 1988 by Baibich  et al.  in the resistiv-ity measurements on Fe/Cr multilayers [1], as shown in Fig. 2. The discoveryof GMR was a great breakthrough in the field of thin film magnetism andmagneto-transport studies. At 4.2 K, the resistivity of [Fe 30˚A/Cr 9˚A] mul-tilayer was decreased by almost 50 % by applying an external field. Evenat 300 K, the decrease of resistivity reaches to 16 %, which is significantlylarger than MR changes caused by the AMR effect, and therefore the newphenomenon was named “giant”. The mechanism of GMR was promptly at-tributed to the change of the magnetic structure induced by an external field.At zero field, magnetizations in each Fe layer are aligned antiparallel, but areoriented to be parallel by applying an external field larger than 2 T. In the2  antiparallel magnetic structure, conduction electrons are much more scatteredthan in a parallel magnetic structure. Until the discovery of GMR, it was notrealized that the spin-dependent scattering can make such a large contributionto the resistivity.In the next section, the experimental aspects of GMR effectwill be described and studies done before the GMR discovery also are brieflysurveyed.[Figure 2 about here.]It is easily speculated that the resistivity of a magnetic material is influ-enced by spin structures. In a textbook on magnetism, we are able to find forinstance the temperature dependence of resistivity of pure Ni metal as shown inFig. 3, which was measured more than 50 years ago. At the Curie temperatureof631 K, the resistivity curve shows a change of the temperature coefficient.The difference of gradients in ferromagnetic and paramagnetic regions is aclear evidence that the resistivity depends on the magnetic structure and theferromagnetically ordered state has a smaller resistivity. In a paramagneticregion with randomly oriented (or fluctuating) spins, conduction electrons aremore scattered by magnetic origins. Indeed, to measure the temperature de-pendence of resistivity has been known as a method to determine the magnetictransition temperature. However if you measure the temperature dependenceof resistivity for the purpose to determine the transition temperature, youwill notice that this method is not sensitive and get an impression that thecontribution of magnetism on resistivity is fairly small, because a change of resistivity at the transition temperature is not remarkable. On the other hand,you can extend the resistivity curve in the paramagnetic region (higher than T  c ) down to lower temperatures without any theoretical guidance (the dottedline in the Fig. 3), Then, you will be aware how big is the contribution of themagnetic structure to the resistivity. At room temperature for instance, theextrapolated resistivity is twice as large as the real value. This comparisonis of course non-sense because a magnetically disordered structure at roomtemperature is fictitious, but is useful to draw an image on the size of spindependent scattering. From this classic result, one could have got an idea thata large MR effect may happen if the magnetic structure can be varied.[Figure 3 about here.]However, at lower temperatures than  T  c , it is generally impossible in normalferromagnetic materials to realize any kind of disordered spin structure, byapplying a moderate magnetic field. The GMR experiment has revealed thatthe magnetic structure in multilayers composed of ferromagnetic and non-magnetic layers can be varied by applying external fields. We have obtaineda hint here how to construct multilayers whose magnetic structures may bemodified by external magnetic fields.3  As is described in the next chapter, the GMR effect has evidenced thatthe spin dependent scattering can induce a significantly large MR change atroom temperature. Subsequently studies on various phenomena relating to theinterplay of magnetism and transport have been very much stimulated in fun-damental and also technological aspects, and the term “magnetoresistance” hasbecome a fashionable key word. Besides metallic multilayer systems, similarphenomena are found in granular systems where small ferromagnetic clustersare dispersed in non-magnetic matrices. Extremely large MR effect has beenfound in manganese perovskite oxides and is named to be the colossal MR(CMR) effect which means to be greater than giant. Latest achievements onperovskite oxides have been described in another volume edited by Tokura inthe same monograph series as this. Currently remarkable progress has beenmade also in the studies on MR effect using tunneling currents, which is oftenabbreviated to be TMR. Detailed description on TMR will be given in otherchapters. In order to express more comprehensively the GMR including re-lated phenomena such as CMR and TMR, we may denote as “XMR” (X=G,T, C), If necessary, X may include A (i.e., AMR) also. The term, XMR cancover all novel MR effects and in addition X sounds to express our further wishto find something new and exotic. 2 GMR IN MULTILAYERED SYSTEMS : EX-PERIMENTAL ASPECTS 2.1 Before the discovery of GMR Pioneering works to study the dependence of conductivity on the magneticstructure have been carried out for tunneling junctions much earlier than thediscovery of GMR. A tunneling current from one ferromagnetic metal to an-other ferromagnetic one through a thin potential barrier may depend on therelative orientations of the two magnets. This idea was verified for the junc-tions ofCo/Ge/Ni and Ni/NiO/Co respectively by Julliere [2], and byMaekawaand Gafvert [3]. However they have observed only small MR effect at low tem-peratures and the results did not gather intense attention at that time. In veryrecent years, there have been remarkable progresses in the studies of tunnel-ing current MR (TMR), experimentally and theoretically. Since it has beenconfirmed that a large MR ratio can be achieved at room temperature, the im-portance of TMR also for industrial sides is recognized, as will be mentionedin the forthcoming chapters.In ferromagnetic thin films, the magnetic structures become more or lessinhomogeneous in the process of magnetization reversal and disordered mag-netic fractions, domain walls for instance, will act as sources of resistance. In aresistivity measurement as a function of external field, it is common to observe4
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