Read heads

The MR effect is a change of electrical resistivity of a metal or alloy when it is in a magnetic field. This change in resistivity due to the magnetic field can be used to sense magnetic field (data) in the magnetic medium. An MR read head is schematically shown in Fig. 43. It can be seen that the MR sensor is placed between two shields in order to reduce the pulse width of the MR sensor. The most 

For Permalloy and Co—Fe alloys, the decrease in resistivity is about 2-6% of the resistivity in the absence of the magnetic field, at room temperature [126]. 

Enhanced magnetoresistive effect, called giant magnetoresistance (GMR) effect, was observed in magnetic layered structures consisting of magnetic/nonmagnetic metal multilayers [131,132], First, GMR multilayers were produced by vacuum deposition 

The GMR multilayers require a large saturation field ( 10000 Oe) to overcome the antiferromagnetic coupling of magnetic layers in order to display the large GMR ratios. These multilayers are potentially useful for application in magnetic read heads. 

Parkin etal. [136] reported saturation magnetoresistance values (AR/R) of more than 65% at room temperature (295 K) in magnetron-sputtered [137] antiferromagnetic Co/Cu multilayers. The magnitude of magnetoresistance is usually expressed by AR(H)/Ro, or simply A R/R, where R(H) is the resistance in field H, Ro = R(H = 0) is the sample resistance 

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Before reading this section, you should read Heading 3 of the manual, which deals with creating reactions data bases with the CHAOSBASE program. In addition, you will have to have created the data base shown there ("CHBASEl") in order to follow the exercise in this section. 

Magnetic random-access memory (MRAM) is a new type of computer memory. MRAMs retain their state of magnetization even with the power off, but unlike present forms of nonvolatile memory, they have switching and rewritability rates that challenge (are faster than) those of conventional RAM. In today s read heads as well as those of MRAMs, key features are made of ferromagnetic metallic alloys. Such metal-based devices make up the first—and most mature—of the various categories of spintronics. 

Figure 5.3 Diagram showing the belt onto which is mounted the image plates and the spinning mirror read-head within the Raxis IV++. IP, image plate PMT. (Courtesy of Dr Joseph Ferrara, Rigaku Americas Corporation.)...

In contrast to the discussion above with amorphous barriers, it is possible to use first-principles electron-structure calculations to describe TMR with crystalline tunnel barriers. In the Julliere model the TMR is dependent only on the polarization of the electrodes, and not on the properties of the barrier. In contrast, theoretical work by Butler and coworkers showed that the transmission probability for the tunneling electrons depends on the symmetry of the barrier, which has a dramatic influence on the calculated TMR values [20]. In the case of Fe(100)/Mg0(100)/Fe (100) the majority of electrons in the Fe are spin-up. They are derived from a band of delta-symmetry. In 2004 these theoretical predictions were experimentally confirmed by Parkin et al. and Yusha et al. [21, 22]. Remarkably, by 2005 TMR read heads were introduced into commercial hard disk drives. 

In hard disk read heads, one ferromagnetic layer has its spin orientation fixed by coupling to an antiferromagnetic layer (Figure 9.13). A second ferromagnetic layer separated from the first by a nonmagnetic metal, is free to change its spin orientation when a field is applied. 

FIGURE 9.13 Two ferromagnetic layers, as in a magnetic hard disk read head, illustrating pinning hy an antiferromagnetic layer. 

The electronic and magnetic properties of nanolayers are important in devices formed from electronic materials that are more conventional. We have already discussed quantum well lasers (see Chapter 8) and giant magnetoresistance (GMR) devices used for hard disk read heads (see Chapter 9). Quantum well lasers may be an important component of light-based computers. Other possibilities include magnets with unusual properties (Section 11.2). 

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