Effect of tungsten doping on perpendicular magnetic anisotropy and its voltage effect in single crystal Fe/MgO(0 0 1) interfaces

Perpendicular magnetic anisotropy (PMA) in magnetic heterostructures, in particular magnetic tunnel junction-based layered structures, has attracted growing interest in fundamental physics in magnetism and practical application to nonvolatile magnetic memory devices [1–10]. More recently, the phenomenon of voltage controlled magnetic anisotropy (VCMA) in magnetic tunnel junctions (MTJs) is expected to be a useful means of electrical magnetization switching with a high energy efficiency for magnetic random access memories (MRAMs) [11–20]. Understanding PMA and VCMA is considered to be indispensable for developing MRAM technologies in this context.

Since the magnetic anisotropy originates from the spin–orbit interaction, it is believed that in general heavy metal elements play a significant role for the occurrence of PMA in magnetic heterostructures. In fact, Pt-containing ordered alloy films and heterostructures exhibit particularly large PMA. On the other hand, significant PMA can also arise at specific interfaces even without heavy metal elements. For example, the CoFeB/MgO heterostructure is a well-known system that shows useful interface PMA. First-principles calculations explain the mechanism of the PMA based on the minority spin's interface electronic state modification due to hybridization of the Fe-3d and O-2p states [21, 22], in which the PMA mechanism proposed by Bruno works well as an anisotropy of orbital magnetic moments [23]. Interface PMA energy densities larger than those in poly-crystal CoFeB/MgO heterostructures can be obtained in single crystal Fe/MgO(0 0 1) and Fe/MgAl₂O₄(0 0 1) heterostructures that may be suitable for direct comparisons with the theoretical calculations [6, 24, 25].

The effects of heavy metal doping on the mechanism of hybridization of the Fe-3d and O-2p states have been little understood because of their complexities in doping sites and hybridizations. A noteworthy study was done for this issue by Nozaki et al, in which enhanced PMA was successfully demonstrated for poly-crystal FeB/MgO heterostructures with interface insertion of a very thin W or Ir layer [26]. The enhancement of PMA by doping W is more significant than that by doping Ir, while Ir doping is more effective for VCMA than W doping. An interesting prediction on giant interface PMA has also been made based on the first-principles calculations by Masuda and Miura [25]: a double magnitude of interface PMA can be obtained by inserting W atomic layers into Fe/MgO(0 0 1) or Fe/MgAl₂O₄(0 0 1) interfaces. In this study, we examined PMA and related properties in single crystal Fe/MgO(0 0 1) heterostructures in which a very thin W interface layer was nominally inserted as doping, although it was unlikely that atomically controlled W interface layer grew as the theoretical calculation assumed. The results obtained were compared with the previous study using poly-crystal FeB/MgO.

Heterostructure film samples with the stacks of MgO(0 0 1) seed/30 nm Cr buffer/0.7 nm Fe/0, 0.05, or 0.15 nm W/2 nm MgO/1 nm Ru cap were prepared on a single crystal MgO(0 0 1) substrate by molecular beam epitaxy (figure 1(a)), except for W insertion and Ru capping layers that were deposited by radio frequency and direct current sputtering, respectively. The Cr, Fe, and MgO layers were in situ post-annealed at 800 °C, and 250 °C and 400 °C, respectively. Epitaxial growth was confirmed with good streak patterns in reflection high-energy electron diffraction (RHEED) (figure 1(a)). It is noted that the sputter-deposited W layer is presumably mixed with the underneath Fe layer. Namely, W-doped Fe layer is expected to be formed in the heterostructure films, which has been preliminarily confirmed by microstructure analysis and magnetization measurement studies (not shown). PMA was determined by evaluating the area surrounded by the in-plane and perpendicular magnetization curves measured at room temperature with a vibrating sample magnetometer. X-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements were performed with an external magnetic field of  ±1 T at the Photon Factory, High-energy Accelerator Research Organization (KEK-PF), BL-7A. The XMCD analysis was made by using the magnetooptical sum rules to evaluate the element-specific spin and orbital magnetic moments for each element [27]. When the magnetization can be sufficiently saturated even in the hard magnetization direction, both parallel and perpendicular components of orbital magnetic moment per atom $m_{{\rm orb}}^{\parallel }$ and $m_{{\rm orb}}^{\bot }$ were determined from the experimental data for the normal (NI) and 60° grazing (GI) incidence measurement setups, respectively. The orbital moment anisotropy was defined as Δm_orb  =  $m_{{\rm orb}}^{\bot }$  −  $m_{{\rm orb}}^{\parallel }$. The coefficients of VCMA, i.e. ratios of changes in the PMA energy density and the applied electric field, were evaluated by using MTJ samples, in which a 10 nm thick Fe layer was deposited as a counter electrode on the top MgO layer. The details of the VCMA measurements are basically in the same manner described in [17].

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3.1. PMA

3.2. XMCD

Figure 2 shows XAS and XMCD for t_W  =  0 and 0.15 nm. XMCD signals are clearly observed, and the results shown in figures 2(c) and (d) indicate the finite orbital magnetic moments of Fe. The analyzed data using magneto-optical sum rules are summarized in table 1. In the present study, the XMCD data for t_W  =  0 and 0.05 nm in the GI geometry could not be obtained because of the insufficient applied magnetic field to saturate the magnetization; however, previous studies showed that there exists Δm_orb of the order of 0.01–0.1 µ_B for single crystal Fe/MgO and poly-crystal CoFeB/MgO [29, 30]. On the other hand, for the W-doped Fe/MgO heterostructure exhibiting almost no PMA (t_W  =  0.15 nm), at least Δm_orb is one order of magnitude smaller than that in non-doped Fe/MgO, as the PMA decreases with decreasing Δm_orb. This behaviour agrees with the expectation in Bruno's model to explain PMA in highly exchange-split ferromagnetic materials like Fe-based alloy films as an anisotropy of orbital moments. The angular dependence of m_spin shows a negligible contribution of additional spin dipole term [31]. The reduction in PMA for W-doped Fe/MgO(0 0 1) single crystal heterostructures has been consistently examined both by macroscopic and microscopic measurements, i.e. magnetization curves and XMCD.

Table 1. Spin (m_spin) and orbital (m_orb) magnetic moments of Fe for Fe/MgO heterostructures with W layer insertion (t_W  =  0, 0.05, and 0.15 nm) determined by XMCD measurements. Anisotropy of orbital magnetic moments Δm_orb is also shown for t_W  =  0.15 nm. The values estimated may include errors of approximately 20%.

tW (nm)	0	0.05	0.15
	NI	NI	NI	GI
mspin (µB)	1.66	1.57	1.47	1.45
morb (µB)	0.095	0.073	0.093	0.093
Δmorb (µB)	...	...	<0.01

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3.3. VCMA

Figure 3 shows areal PMA energy density, i.e. K_eff t_Fe, as a function of applied electric field for an Fe/0.05 nm W/MgO/Fe MTJ, where K_eff is the effective PMA energy constant and t_Fe is the thickness of Fe layer. Only in the negative electric field range, a large VCMA of ~190 fJ Vm⁻¹ is obtained, which is comparable or a little smaller than the previously reported result for a Fe/MgO/Fe MTJ without W insertion [17, 32]. In the positive electric field range, the VCMA is close to zero, and a change in slope happens around 100 mV nm⁻¹. This behaviour is also similar with the previous result [17].

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It can be naturally understood that slightly smaller VCMA is observed for the W-doped Fe/MgO/Fe MTJ exhibiting a little smaller PMA. In fact, however, there is no physical reason for such a simple correlation between PMA and VCMA. In the study by Nozaki et al, W doping is more effective to enhance the PMA in FeB/MgO heterostructures than Ir doping, whereas in the case of VCMA, Ir doping leads to larger effect than W doping [26]. It is suggested that VCMA gives another insight into the understanding of interface PMA in single crystal W-doped Fe/MgO heterostructures.

PMA, XMCD and VCMA were investigated for single crystal Fe/MgO(0 0 1) heterostructures in which a very thin W interface layer was nominally inserted. The PMA energy density was reduced with increasing the W layer thickness, being different from the expectation from the results in a previous study for W-doped poly-crystal FeB/MgO heterostructures. The XMCD measurements showed that the present PMA can be understood as the effect of anisotropy of orbital magnetic moments in Fe. The VCMA coefficient for the Fe/MgO heterostructure with 0.05 nm thick W interface insertion was determined to be ~190 fJ Vm⁻¹. Further studies are required to obtain the giant PMA proposed theoretically for Fe/MgO heterostructures with W interface insertion.

The authors acknowledge X Xu and K Hono for microstructure analyses and Y Miura, K Masuda, S Kasai and M Al-Mahdawi for discussion. This work was partly supported by the ImPACT Program of the Council for Science, Technology and innovation (Cabinet Office, Government of Japan) and JSPS KAKENHI Grant No. 16H06332. Synchrotron radiation experiments were performed under the approval of the Photon Factory Program Advisory Committee, KEK (No. 2017G060). Y I acknowledges the National Institute for Materials Science for the provision of the NIMS Graduate Research Assistantship.