Correlation of interfacial perpendicular magnetic anisotropy and interlayer exchange coupling in CoFe/W/CoFe structures

Abstract

As core device of spin-transfer torque magnetic random-access memory (STT-MRAM) and some other spintronics devices [1], the magnetic tunnel junction (MTJ) has been widely studied to improve performances such as endurance, scalability, read reliability and power consumption. The interfacial perpendicular magnetic anisotropy (PMA) in the CoFeB/MgO/CoFeB structure with a tunnel magnetoresistance (TMR) ratio of over 120% [2] proves that PMA can be used to build high-performance out-of-plane MTJ for STT-MRAM cells. Latter research found that the heavy metal (HM)/ferromagnet interface made great contribution to PMA [3]. First-principles investigations agree that both the MgO/CoFe interface and the CoFe/HM interface attribute to magnetic anisotropy energy (MAE) in the MgO/CoFe/HM structures [4, 5]. Moreover, the MgO/CoFeB/Ta/CoFeB/MgO double-interface structure was demonstrated to improve thermal stability by a factor of 1.9 while keeping comparable critical switching current with the single-interface structure Ta/CoFeB/MgO [6]. The MgO/CoFeB/W/CoFeB/MgO structure was demonstrated to possess strong PMA and large TMR ratio (249%) after annealing at a high temperature [7]. To investigate the MAE in the W-based structure, it is necessary to research the interaction within CoFeB/W/CoFeB layers on MAE. CoFeB/W/CoFeB sandwich may form ferromagnetic or antiferromagnetic coupling (FMC or AFMC), which is expressed as interlayer exchange coupling (IEC). However, it remains unclear about the interactions between MAE and IEC in the perpendicularly magnetized CoFeB/W/CoFeB structure, making the way to enhance PMA in double-interface structure not straight forward.In this work, we first investigated the MAE and IEC of the MgO/CoFe/W/CoFe/MgO structures as a function of W thickness with first-principles calculations as shown in Fig. 1 [8]. Clear oscillation of IEC can be observed in Fig. 1(a). The long and short period of IEC oscillations are 4.8 monolayers (MLs) and 3.2 MLs, respectively, according to simulation.Figure 1(b) shows the MAEs of MgO/CoFe/W/CoFe/MgO structures for FMC and for the arrangement corresponding to IEC are calculated for all thicknesses of W layers. Moreover, for the structure with W thickness of 7 MLs, a strong FMC as well as a strong PMA can be obtained simultaneously, which is promising for high-density STT-MRAM applications with strong thermal stability. Also, we observe a clear oscillation of MAE in ferromagnetically coupled MgO/CoFe/W/CoFe/MgO structure related to the thickness of W layer.To further study the effect of interface interactions on MAE, we calculate the k-space-resolved MAE according to force theorem [8]. Figure 1(c-f) shows the k-resolved MAEs of the ferromagnetic coupled CoFe/W/CoFe structures with the thickness of W layer varies from 1 ML to 4 MLs. The spin-orbit coupling (SOC) breaks the symmetry of 2D-Brillouin-zone slightly making MAE at the points around [kx,ky]= [0.457, 0.257] possessing the largest PMA in Fig. 2(c-f). These points that make huge contributions to MAE ferromagnetically coupled are called critical points. The electron states near the Fermi surface have a large influence on MAE. Therefore, we consider spin-resolved bands at critical points. The bands near Fermi energy for spin-up and spin-down states are different, which can be explained by exchange splitting.To explore the properties of the specific electron states, the band-decomposed charge densities of these specific electron states near Fermi energy are shown in Fig. 2(a-d). We can see that only spin-up electron states occupy W layers and that the alternating oscillation of the charge densities at center of W layers: antinode at 1 ML and 3 MLs W layer, while node at 2 MLs and 4 MLs W layers. It is reported that the electron wavefunction confined in thin film is modulated by an envelope function in reference [8]. For a given energy, as the thickness of the W layer in CoFe/W/CoFe structure varies, the center of film layers within the quantum well shows the alternation of node and antinode, shown in Fig. 2(a-d), indicating the presence of QWSs in W layers. The oscillation of IEC in CoFe/W/CoFe originates from QWSs [8]. Thus, these results clearly show that the QWSs correlate the oscillation of IEC and MAE in W-based double-interface structures.In summary, we investigated the MAE and IEC of the MgO/CoFe/W/CoFe/MgO structures as a function of W thickness with first-principles calculations. The oscillations of both IEC and MAE with the change of W thickness were observed in this structure. QWSs in the W layer are demonstrated to be critical for the oscillations of MAE, leading to significant enhancement of PMA by properly tuning W thickness. **