新型磁性隧道结材料及其隧穿磁电阻效应

A Typical Magnetic Tunnel Junction Material and Effects of Tunnel Magneto-Resistance

典型的磁性隧道结是"三明治"结构,即由上下两个铁磁电极以及中间厚度为1 nm量级的绝缘势垒层构成。当外加磁场使两铁磁电极的磁矩由平行态向反平行态翻转时,隧穿电阻会发生低电阻态向高电阻态的转变。自从1995年发现室温隧穿磁电阻(TMR)以来,非晶势垒的AlOx磁性隧道结在磁性随机存储器(MRAM)和磁硬盘磁读头(Read Head)中得到了广泛的应用,2007年室温下其磁电阻比值可达到80%。下一代高速、低功耗、高性能的自旋电子学器件的发展,迫切需要更高的室温TMR比值和新型的调制结构。2001年通过第一性原理计算发现:由于MgO(001)势垒对不同对称性的自旋极化电子具有自旋过滤(Spin Filter)效应,单晶外延的Fe(001)/MgO(001)/Fe(001)磁性隧道结的TMR比值可超过1 000%,随后2004年在单晶或多晶的MgO磁性隧道结中获得室温约200%的TMR比值,2008年更是在赝自旋阀结构CoFeB/MgO/CoFeB磁性隧道结中获得高达604%的室温TMR比值。伴随着新势垒材料的不断发现和各种磁性隧道结结构的优化,共振隧穿和自旋依赖的库仑阻塞磁电阻等新效应以及磁性传感器、磁性随机存储器和自旋纳米振荡器及微波检测器等新器件逐渐成为科学和工业界所关注的研究与应用热点。对磁性隧道结(MTJ)材料及其器件应用研究和进展进行了简要介绍。

A typical magnetic tunnel junction （MTJ） consists of a thin insulating layer （a tunnel barrier） , sandwiched by two ferromagnetic electrode layers. When the relative magnetic configuration of both ferromagnetic electrode layers changes from parallel state to anti-parallel state with external magnetic field, the resistance of MTJ would become high from low, exhibiting tunnel magneto-resistance （TMR） due to spin-dependent electron tunnelling. Amorphous A1Ox barrier MTJs were extensively studied and have been used in magneto-resistance random access memory （MRAM） and read heads of hard disk drives, since the discovery of room-temperature TMR in 1995. However, the spin electronic devices develop- ment of the next-generation high-speed, low-power-consumption and high-performance need much higher TMR ratio and a novel structure. In 2001, the first-principle calculation predicted that the TMR ratio of epitaxial Fe （001）/MgO （001）/ Fe （001） MTJs would be over 1 000% , due to spin filter effect of MgO barrier for different symmetry spin polarized elec- tron. In 2004 TMR ratios of about 200% were obtained in MTJs with a single-crystal MgO （001） barrier or a textured MgO （001） barrier. In 2005, the TMR ratio of 604% has been reported in pseudo-spin-valve MTJs with core structure of CoFeB/MgO/CoFeB. Recently, Quantum well （Qw） resonances tunneling and spin-dependent Coulomb blockade mag- neto-resistance （CBMR） effect in MgO-barrier MTJs were proposed and demonstrated in theories and experiments. Mag- netic sensors, MRAM, spin nano-oscillator and microwave detector based on MTJs have attracted attention of electron sci- ence and devices. In the paper we briefly introduced the investigation and development of magnetic tunnel junction materi- al and its device applications.