| Magnetic tunneling junction?MTJ?,a sandwich structure composed of two magnetic metal layer and one insulator layer between them,has a wide range of application in spintronics and information technology,especially as the core structure of read heads in hard disk drives and magnetic random access memories?MRAM?.With the large tunneling magnetoresistance?TMR?,the MgO based MTJ is now popularly used in MRAM application.In order to obtain the advantages of long-life,low energy consumption,high density and high speed for MRAM,some requests for the magnetic anisotropy of MTJ were proposed,including perpendicular magnetic anisotropy?PMA?,large magnetic anisotropy energy?MAE?and large voltage-controlled magnetic anisotropy?VCMA?coefficient.In this paper,using First-principles calculation,we study the magnetic anisotropy and its voltage control in MgO based MTJ.The main results are as follows:?1?We studied the influence of HfO2 interlayers on MAE of Fe/HfO2/MgO junction.We found that with one unit of HfO2inserting,the Fe-O bond length decreased from 2.20? to 1.77?.The hybridization of Fe-O bond is crucial for the PMA.Through the comparative analysis,we concluded that the decreased bond length redistributed the orbital states of the second and third closest Fe monolayer?ML?to Fe|MgO interface.And then their absolute value of layer-resolved MAE contribution increased.However,the MAE contribution at the closest Fe ML to Fe|MgO interface was barely influenced.The MAE of Fe/HfO2/MgO supercell with 5ML Fe increased and up to 1.95 m J/m2.?2?We studied the origin of the large VCMA in a Cr/Fe/MgO junction with an ultrathin Fe layer.We developed a k-resolved VCMA calculation method combined with the second-order perturbation theory to investigate the relation between Quantum-well?QW?states and VCMA.We found the VCMA coefficient reaches-297 fJ/Vm for 3 ML Fe and oscillates strongly with increasing the number of Fe ML,matching well with the previous experiment observed by Nozaki et al.In theory,firstly,the screening charges induced VCMA was verified.Secondly,the QW states of?1 electron at?point provide large?no?contribution to the VCMA with odd?even?MLs,resulting the strong oscillation of coefficient.Moreover,the change of the orbital-resolved Fermi surface at the interfacial Fe atom as well as the QW states produce the largest VCMA for 3 ML Fe.At last,the screening charges fill in the delocalized state at the interface,such as QW states,resulting in the PDOS redistribution inside the bulk Fe region and then the bulk VCMA contribution.?3?We designed Cr/HM?heavy metal?/Fe/MgO junctions to satisfy the all aspects of requests of MRAM technology,including large TMR,PMA,large MAE and large VCMA.As we thought,the QW states in Cr/HM/Fe/MgO junctions would be tuned to Fermi level by adjusting the type and thickness of HM layer.And then the large bulk VCMA and MAE contribution in HM layer would be obtained through the large spin-orbit coupling of HM cooperating with the QW states.Many types of HM with different thickness had been tested,and three satisfactory targeted junctions were found,namely,Cr/Pt?2 ML?/Fe?4 ML?/MgO,Cr/Ir?2 ML?/Fe?4 ML?/MgO and Cr/W?1 ML?/Fe?3 ML?/MgO.The MAE of these three junctions are 5.50 mJ/m2,2.57mJ/m2,and 2.00 mJ/m2,respectively.And the VCMA coefficients of the junctions are 287 fJ/Vm,435 fJ/Vm and 630 fJ/Vm,respectively. |
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