Exchange coupling in nanostructured CoO/NiFe networks
Exchange coupling in nanostructured CoO/NiFe networks
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DOI：
10.1103/physrevb.64.184430
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发表时间：
2001-10
影响因子：
3.7
通讯作者：
Li Sun;Yang Ding;C. Chien;P. Searson
中科院分区：
文献类型：
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作者：
Li Sun;Yang Ding;C. Chien;P. Searson
当反铁磁~AF！/铁磁~FM！当双层被冷却到低于AF Ne ́el温度（TN）时，在大于铁磁体饱和场的磁场存在下，材料之间的界面相互作用将诱导单向交换各向异性，从而使铁磁体的磁滞回线从零场轴偏移。1-3位移的大小通常被称为交换偏置HE，其可以被定义为2（H左1H右）/2，其中H左和H右是零磁化时的两个外部场。除了磁滞回线的偏移之外，还发现耦合层中的矫顽力HC@2（H左2 H右）/2）]与单个FM层相比总是增强的。在本文中，我们表明，显着的增强交换偏置可以实现在纳米结构的AF CoO/NiFe双层膜通过沉积到多孔氧化铝模板。与均匀的交换耦合薄膜相比，沉积态薄膜具有相互连接的网络结构，并且在HE和HC方面都表现出显著的增强。不同FM层厚度的薄膜的交换场随温度的升高呈准线性下降，当FM层厚度大于40 nm时，交换场与FM层厚度成反比。与连续薄膜相比，HC和HE中的T增强归因于FM和AF网络结构中的畴限制和钉扎。为了研究网络结构中交换层的厚度依赖性，并允许比较连续膜和非耦合NiFe网络，制备了三种样品：~i！001！Si / NiFe楔形~4-78 nm！/ CoO ~30 nm！/ Cu ~ 2nm！，~ii！Al 2 O3（d5200 nm）/ NiFe楔形~4-78 nm！/ CoO ~30 nm！/ Cu ~ 2nm！，和~iii！Al 2 O3（d5200 nm）/NiFe楔形~4-80 nm！/ Cu ~ 2nm！NiFe和CoO层分别用直流和射频溅射法沉积。溅射2-nm Cu层以覆盖FM/AF双层。连续变化的NiFe层厚度使我们能够研究在相同沉积条件下生长的样品，其中厚度是唯一可变的。用轮廓仪和小角X射线衍射仪测定了不同材料的生长速率。氧化铝模板~Whatman！具有200 nm的平均孔径，具有伪六方有序。表面处的平均壁厚为40 nm。图1示出了扫描电子显微镜的平面图。
oO a When an antiferromagnetic ~AF!/ferromagnetic~FM! bilayer is cooled below the AF Ne ́el temperature ( TN) in the presence of a magnetic field larger than the saturation fiel the ferromagnet, the interfacial interactions between the materials will induce a unidirectional exchange anisotro which shifts the magnetic hysteresis loop of the ferromag from the zero-field axis. 1–3 The magnitude of the displace ment is usually referred to as the exchange bias HE , which can be defined as 2(H left1H right)/2, whereH left and H right are the two external fields at zero magnetization. In addit to the shift of the hysteresis loop, the coercivi HC@2(H left2H right)/2)] in the coupled layers is alway found to be enhanced compared to a single FM layer. In this paper, we show that significant enhancements the exchange bias can be achieved in nanostructured AF CoO/NiFe bilayers fabricated by deposition onto porous a mina templates. The as-deposited films have an interc nected network structure, and exhibit significant enhan ments in bothHE and HC compared to uniform exchang coupled thin films. The coercivity of films with different FM layer thicknesses decreases quasilinearly with increa temperature, and the exchange field is inversely proportio to the FM layer thickness when it is larger than 40 nm. T enhancements inHC and HE compared to the continuou films are attributed to domain confinement and pinning in FM and AF network structures. To study the thickness dependence of the exchange pling in the network structure, and allow a comparison continuous films and uncoupled NiFe networks, three ty of samples, were prepared: ~i! ~001! Si / NiFe wedge~4–78 nm!/CoO ~30 nm!/Cu ~2 nm!, ~ii ! Al2O3(d5200 nm)/ NiFe wedge ~4–78 nm!/CoO ~30 nm!/Cu ~2 nm!, and ~iii ! Al2O3(d5200 nm)/NiFe wedge~4–80 nm!/Cu ~2 nm!. The NiFe and CoO layers were deposited by dc and rf sputter respectively. The 2-nm Cu layer was sputtered to cap FM/AF bilayer. The continuously varying NiFe layer thick ness enabled us to study samples grown under iden deposition conditions, with the thickness being the only va able. The growth rates of the different materials were c brated by profilometer as well as small angle x-ray diffra tion. The alumina templates ~Whatman! have an average pore diameter of 200 nm with pseudohexagonal order The average wall thickness at the surface was 40 nm. Figure 1 shows a plan view scanning electron microsc