Materials for spin-transfer-torque magnetoresistive random-access memory
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Introduction A magnetic tunnel junction (MTJ), which consists of an ultrathin insulating layer (a tunnel barrier) sandwiched between two ferromagnetic metal layers (electrodes), shows a magnetoresistance effect. As illustrated in Figure 1a, the tunneling resistance of an MTJ depends on the relative magnetic alignment (parallel or antiparallel) of the ferromagnetic electrodes. Usually, the resistance for the parallel magnetic state, RP, (Figure 1a [left]) is lower than that for the antiparallel state, RAP (Figure 1a [right]). This phenomenon, called the tunnel magnetoresistance (TMR) effect, is the core technology in several spintronic devices. The size of the TMR effect is measured by the fractional difference in resistance, (RAP – RP)/RP × 100 (%), which is called the magnetoresistance (MR) ratio. The TMR effect originates from spin-dependent electron tunneling between the ferromagnetic electrodes. The MR ratio of MTJs can be phenomenologically expressed as: MR = 2 PP 1 2 /(1 − PP 1 2 ),
(1)
where P1 and P2 are the spin polarizations of tunneling electrons at the Fermi energy (EF) of the two electrodes. Because the
density of states (DOS) in ferromagnets is spin-polarized at EF, the tunneling resistance changes depending on the relative magnetic alignment.1 Figure 1b shows the typical structure of a practical MTJ. The upper ferromagnetic electrode is called the free layer and the lower ferromagnetic electrode is called the reference layer. The magnetization of the reference layer is pinned to one direction by exchange bias from the antiferromagnetic (AF) layer (such as face-centered-cubic [fcc] Ir-Mn(111)), as part of a synthetic antiferromagnet (SAF) structure. The SAF structure consists of two ferromagnetic layers aligned antiparallel to each other through a SAF spacer layer. When the free-layer magnetization is configured such that it can rotate continuously with an external magnetic field, the MTJ can act as a magnetic sensor device, such as the read head of a hard disk drive (HDD). When the free-layer magnetization is configured to have two remnant states, the MTJ can store 1 bit of information in the form of the magnetic alignment (pointing right or left) and therefore can function as a nonvolatile memory bit in a magnetoresistive random-access memory (MRAM) (Figure 1c). The stored information can be read out by the resistance change due to the TMR effect. For writing of
Shinji Yuasa, National Institute of Advanced Industrial Science and Technology, Spintronics Research Center, Japan; [email protected] Kazuhiro Hono, Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science, Japan; [email protected] Guohan Hu, IBM T.J. Watson Research Center, USA; [email protected] Daniel C. Worledge, IBM T.J. Watson Research Center, USA; [email protected] doi:10.1557/mrs.2018.93
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