A magnetic tunnel junction (MTJ) consists of an ultrathin insulating layer (tunnel barrier) sandwiched between two ferromagnetic (FM) metal layers (electrodes), as shown in Figure 11.1a. že resistance of MTJ depends on the relative magnetic alignment (parallel or antiparallel) of the electrodes. že tunnel resistance, R, of MTJ is lower when the magnetizations are parallel than it is when the magnetizations are antiparallel, as shown in Figure 11.1b. žat is, RP < RAP. žis change in resistance with the relative orientation of the two magnetic layers, called the tunnel magnetoresistance (TMR) e¥ect, is one of the most important phenomena in spintronics. že size of this e¥ect is measured by the fractional change in resistance (RAP − RP)/ RP × 100 (%), which is called the magnetoresistance (MR) ratio. že MR ratio at room temperature (RT) and a low magnetic ¢eld (typically <10 mT) represent a performance index for industrial applications.