Supplementary Information

Highly efficient voltage control of spin and enhanced interfacial perpendicular magnetic anisotropy in iridium-doped Fe/MgO magnetic tunnel junctions

Takayuki Nozaki1†, Anna Kozioł-Rachwał1,2†, Masahito Tsujikawa3†, 4, Yoichi Shiota1, Xiandong Xu5,

Tadakatsu Ohkubo5, Takuya Tsukahara6, Shinji Miwa6,7, Motohiro Suzuki8, Shingo Tamaru1, Hitoshi

Kubota1, Akio Fukushima1, Kazuhiro Hono5, Masafumi Shirai3, 4, Yoshishige Suzuki1,6,7, and Shinji

Yuasa1

1 National Institute of Advanced Industrial Science and Technology, Spintronics Research Center,

Tsukuba, Ibaraki, 305-8568, Japan2 AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Al.

Mickiewicza 30, 30-059 Kraków, Poland3 Research Institute of Electrical Communication, Tohoku University, Sendai, Miyagi 980-8577,

Japan4Center for Spintronics Research Center, Tohoku University, Sendai, Miyagi, 980-8577, Japan

5 National Institute for Materials Science, Research Center for Magnetic and Spintronic Materials,

Tsukuba, Ibaraki 305-0047, Japan6 Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka

560-8531, Japan7 Japan Synchrotron Radiation Research Institute (JASRI), Sayo, Hyogo 679-5198, Japan

† These authors contributed equally to this work

* e-mail: nozaki-t@aist.go.jp

S1. Scalability issue of the VCMA effect for memory applications For memory applications driven by VCMA effect, both the required PMA energy, which relates to thermal stability, and VCMA coefficient depend on the element size. For simplicity’s sake, if we assume a free layer whose PMA is dominated by the interface magnetic anisotropy at the interface with the dielectric layer, the effective PMA energy is expressed as

Keff=K i(V )

t−1

2μ0 M

S2 (1),Here, t and MS are the thickness and saturation magnetization values of the free layer.

Ki(V) can be controlled by electric field, E with the VCMA coefficient, Then,K i(V )=K i (V =0 )−ηE (2).

The thermal stability factor of the free layer under the electric-field application can be expressed by

Δ (V )=

K eff⋅A⋅tk BT

=Δ0−ηAkB T

E (3),

Here, A, kBT and 0 are the element area, thermal energy at temperature T and thermal stability under the application of zero-strength electric field, respectively.Pulse voltage-induced dynamic switching requires that 0 be made null by the VCMA effect. Therefore, the required VCMA coefficient is expressed as,

η=kB TΔ0

AESW (4).

Where ESW is the amplitude of electric-field required for switching. Figure S1 summarizes the element diameter dependence of the required Keff t for various 0 values and VCMA coefficient required for nulling them under the assumption of t=1 nm and ESW=1 Vnm-1. For example, in giga-bit class cache memory applications, Keff t values in the range from 0.2 mJm-2 to 0.5 mJm-2 can serve as the target specifications. Then, the VCMA coefficient should exceed 200 fJV-1m-1 to 500 fJV-1m-1. For main memory and storage-class memory, even higher values of Keff t and the VCMA coefficient are demanded to demonstrate scalability. A similar discussion can be found in Ref. S1.

Figure S1 Scalability issue of the VCMA effect. MTJ diameter, d dependence of the required Keff tfree

for various 0 and required VCMA coefficients for voltage-induced dynamic magnetization switching.

S2. Details of experimental procedure of STEM observations and additional information

Thin lamellae for scanning transmission electron microscope (STEM) observations were prepared by the focused-ion-beam (FIB) lift-out technique using a FIB scanning electron microscope (Helios Nanolab 650, FEI, Hillsboro, Oregon). STEM observations was performed using a transmission electron microscope (Titan G2 80–200 TEM, FEI, Hillsboro, Oregon) with a probe-forming aberration corrector (DCOR, Corrected Electron Optical Systems GmbH, Heidelberg, Germany), which offers an unprecedented opportunity to probe structures with sub-Å resolutions. A probe current of 250 pA was used for the STEM imaging. The collection angle for the high-angle annular dark-field (HAADF) images was 70–200 mrad. Nanobeam electron-diffraction (NBED) patterns were taken using a nearly parallel coherent electron beam produced by a small condenser aperture with a diameter of 10 μm; the nanoprobe having a semi-angle of 1.12 mrad, was produced by a three-condenser system. Chemical analyses were carried out by means of energy-dispersive X-ray spectroscopy (EDX) using an in-column symmetrically distributed array of four silicon-drift X-ray detectors that enabled high detection efficiency. Figure S2-1(a) shows an additional cross-sectional HAADF-STEM image and several nanobeam electron diffractions taken from the Cr/Fe (1.0 nm)/Ir (0.05 nm)/MgO (2.5 nm)/Cr cap structure. Atomic-scale Z-contrast imaging made Ir atoms observable as bright spots. Figure S2-1(b) exhibit raw, refined, contrast-inverted STEM and inverted FFT images for the same sample. Local coherency with long-range mismatch dislocations were confirmed at the FeIr/MgO interface.

Figure S2-2 shows EDS mapping of Cr, Fe, Mg, Ir for the junction with thicker Ir doping layer of 0.15 nm, that is, Cr(30 nm)/Fe(1.0 nm)/Ir(0.15 nm)/MgO(2.5 nm)/Cr cap structure. As is the case with tIr=0.05 nm, diffusion of Ir in the Fe layer is identified from the elemental mapping. However, the intensity profile shown in Fig. S2-2(b) exhibits a slight shift in the peak positions between Fe and Ir signals. This behavior may indicate that a higher density of Ir is concentrated close to the interface compared with the case for tIr=0.05 nm, which probably leads to the reduction in the Ki,0 value for thicker tIr.

Figure S2-1 HAADF-STEM observations. (a) Atom-scale Z-contrast HAADF image and nanobeam electron diffractions. (b) Raw, refined, contrast-inverted STEM and inverse FFT images for the Cr(30 nm)/Fe(1.0 nm)/Ir(0.05 nm)/MgO(2.5 nm)/Cr(30 nm).

Figure S2-2 Structural analysis for tIr=0.15 nm. Cross-sectional EDS mapping (a) and intensity profile (b) of Cr, Mg, Fe, and Ir elements for the Cr(30 nm)/Fe(1.0 nm)/Ir(0.15

ba

IrFeMgCrIr=0.15 nm

1 nm

a

b

nm)/MgO(2.5 nm)/Cr cap structure. The intensities for each element were adjusted to focus on the profile shapes.S3. Longitudinal-MOKE measurement in the thin Ir-doped Fe thickness region

Figure S3 shows an example of longitudinal-MOKE measurement under in-plane magnetic fields for nominal Fe thicknesses of tFe= 0.4, 0.5, 0.6 nm with the Ir doping layer thickness of tIr=0.05 nm. We could confirm clear hysteresis curves with coercivity even for the thinnest Fe layer of 0.4 nm. Therefore, the zero-remanence loops observed under the polar configuration originate from the in-plane anisotropy, not from the paramagnetism. Similar tendencies were also confirmed for heavier Ir doping conditions for tIr=0.1 and 0.15 nm.

S4. Evaluation process for magnetic dead layer

To evaluate effective thickness and saturation magnetization value of Ir-doped Fe layer, we performed nominal Fe thickness dependence of magnetic moment measurement by SQUID. Fig. S5 shows an evaluation example for the case of tIr=0.05 nm; nominal Fe thickness dependence of (a) measured magnetization and (b) saturation magnetization value. Magnetic dead layer, td was estimated to be about 0.09 nm from the intercept of horizontal axis in Fig. S5(a). The magnetic dead layer mainly comes from the intermixing at the Cr/Ir-doped Fe interface. Slight reduction in the saturation magnetization was observed in the thinner Fe thickness ranges, however, relatively high value was still kept even at tFe=0.5 nm. These results also support the discussion in S3, i.e. the observed change in hysteresis curves at around SRT1 shown in Fig. 2 originates

Figure S3 Examples of longitudinal-MOKE hysteresis curves measured under in-plane magnetic fields for tFe=0.4, 0.5 and 0.6 nm with tIr=0.05 nm.

from the transition of magnetic easy axis between out-of-plane and in-plane, not from the transition between ferromagnetic and paramagnetic states. Through these analyses, not only the influence of Ir-doping, the reduction in saturation magnetization due to the intermixing or interdiffusion of Cr into the ultrathin Fe layer is took into account, which can have influence for the evaluation of perpendicular magnetic anisotropy energy and VCMA coefficient.

S5. X-ray absorption spectroscopy at the Ir L2,3 edge. We performed X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements at the Ir L edges to investigate the induced magnetic moment of Ir atoms diffused into the ultrathin Fe layer. The multilayer, consisting of Cr(30 nm)/Fe(1.0 nm)/Ir(tIr)/MgO(2.5 nm)/ITO cap (30 nm) with tIr=0.05 nm and tIr=0.15nm, depicted in Fig. S5-1(a) and (b), was employed for the measurements. Figures S3-1 displays the polarization-averaged XAS (upper) and XMCD (lower) spectra, which were determined by using a silicon drift detector to monitor the X-ray fluorescence yield from the samples. We observed clear XMCD signals, indicating that the 5d electrons of Ir are polarized and have magnetic moments.

The XAS and XMCD measurements were performed at BL39XU of the SPring-8, a synchrotron radiation facility. The measurements were conducted under a perpendicular magnetic field of 1.2 T. The polarization-averaged XAS is given by (μ+ + μ−)/2, where μ+

and μ− are the spectra taken with right and left helicities, respectively. The XMCD

Figure S4 Example of nominal Fe thickness dependence of magnetic moment performed for the quantitative evaluation of magnetic dead layer and saturation magnetization value.

signal is defined as the difference of the absorption coefficients, that is, μ+ − μ−. All measurements were conducted at room temperature in a normal atmosphere. The XAS spectra were normalized by the correction ratio of the L3 to the L2; 2.19. The sum-rule analysis was employed to evaluate the magnetic moments and hole number.S2, S3

Characterization of the white-line intensity of the Ir L edge in XAS was obtained by subtracting the XAS spectrum of Au, where the 5d band was nearly full, from the XAS spectrum of Ir.S4 We assumed that the hole number was proportional to the white-line intensity of L2,3 edges and that the hole number of bulk Ir was 2.7. The XAS of Au and Ir foils were measured by the transmission method as a reference.

Figure S5 Study of the magnetic moments of Ir. XAS and XMCD spectra were measured at the Ir L3 and L2 edges for the structure of Cr(30 nm)/Fe(1.0 nm)/Ir(tIr)/MgO(2.5 nm)/ITO (30 nm) cap with (a) tIr=0.05 nm and (b) tIr=0.15nm.

Table S5 summarizes the obtained values of hole number n for the 5d orbital in Ir, orbital magnetic moment mL, effective spin magnetic moment mS−7mT, and the ratio mL/(mS−7mT). Here, mS and mT are the spin magnetic and magnetic dipole moments, respectively. The negative XMCD at Ir L3 edge in Fig. S3-1 indicates that the spin magnetic moment of the Ir exhibits ferromagnetic coupling with Fe. Therefore, if mL is positive, it indicates parallel alignment of the spin and orbital angular momenta as expected for atoms with more than half-filled 5d shells.

When tIr=0.15 nm, large mS−7mT (~0.7B) and positive mL (+0.028) were confirmed. The large effective spin magnetic moment is similar to results reported for dilute FeIr alloy bulk specimensS4, S5 rather than for Fe/Ir multilayersS6, S7. This results is consistent with the STEM-EDS observations. However, the positive mL in the Ir was reported in the Fe/Ir multilayer structureS6, S7, and negative mL was reported for the case of a dilute

FeIr alloy, in which Ir atom should have isotropic environmentS4, S5. These results indicate that our sample has an alloy-like rather than a multilayer-like structure, however the electronic and magnetic states of Ir are not completely identical to the states in an isotropic dilute FeIr alloy.

When tIr=0.05 nm, even larger values of mS−7mT (~1μB) and mL values approaching zero were obtained. The effective spin magnetic moment (~1μB) is comparable to results reported for a dilute FeIr alloyS4. This result is consistent with the fact that the Ir atoms seem to be dispersed more uniformly in the ultrathin Fe layer when tIr=0.05 nm, judged from the intensity profile of the EDS mapping. However, the orbital magnetic moment was not identical for the dilute FeIr alloy. For instance, the value of mL/(mS−7mT) for the Fe97Ir3 alloy was reported to be −6% in ref. S6. Such discrepancies in the orbital magnetic moment between our sample and FeIr diluted alloy bulk specimen indicates that the electronic and magnetic structures of Ir should also differ from those of the isotropic dilute FeIr.

In summary, our Ir-doped ultrathin Fe/MgO system has an alloy-like structure rather than multilayer-like one, as indicated by the system’s large effective spin magnetic moment. This finding is consistent with the STEM-EDS results. However, the systemic electronic and magnetic states of Ir are not identical with the states of an isotropic dilute FeIr alloy, as indicated by orbital magnetic momenta. These unique magnetic properties should correlate with the high interfacial PMA observed in this study. We attribute these results to one of the following factors: low dimensionality in the Ir-doped ultrathin Fe film and formation of the interface with MgO. More detailed explorations of the angle-resolved XMCD would be helpful for studying the anisotropic portions of the electronic and magnetic structure that contribute to the PMA in such materials.

We also conducted XAS/XMCD measurements at the Fe L edges with similar multilayers, but significant changes in the magnetic moment of Fe, especially in the enhancement of the orbital magnetic moments by Ir doping, was not confirmed. Therefore, the observed high interfacial PMA in our Ir-doped Fe/MgO system should be attributed to the induced magnetism in Ir.

Table S5 Magnetic moments of Ir evaluated by XMCD. Inserted Ir thickness dependence of hole number n, orbital magnetic moment mL, effective spin magnetic moment mS−7mT, and mL/(mS−7mT) obtained in this study and reference data from Ref. S4 and S6.

THIS STUDY REF. S4 REF.S6

Ir 0.05 nm Ir 0.15 nm Fe97Ir3 alloy Fe(1nm)/Ir(0.5nm)

N 2.73 2.57 2.7 2.7

ML −0.003 μB +0.028 μB -0.064 μB +0.016 μB

MS−7MT +1.01 μB +0.72 μB +1.00 μB +0.17 μB

ML/(MS−7MT) −0.3% +3.9% -6.4% +9.4%

S6. Magnetic anisotropy at the Cr/Ir-doped Fe interface

To confirm the perpendicular magnetic anisotropy at the Cr/Ir-doped Fe interface, we performed SQUID measurement for Cr (30 nm)/ Ir-doped Fe (ttotal=0.82 nm)/Cr(10 nm) sandwich structure. Figure S6 shows the hysteresis curves measured under out-of-plane and in-plane external magnetic fields. Saturation field in the out-of-plane direction reaches about 20 kOe, indicating

Figure S6 Magnetic hysteresis curves for the Cr (30 nm)/Ir-doped Fe(ttotal=0.82 nm)/Cr (10 nm) sandwich structure measured under out-of-plane and in-plane magnetic fields by SQUID.

almost no PMA exists at the Cr/Ir-doped Fe interface. Therefore, the observed large PMA in the Cr/Ir-doped Fe/MgO structure mainly comes from the top Ir-doped Fe/MgO single interface. These results also prove that the observed PMA does not originate from bulk type PMA observed in FeCr alloy film through phase segregationS8. S7. Details of first principles calculation

The first-principles calculations were performed using a plane-wave-basis projector-augmented wave method as implemented in the Vienna Ab initio Simulation Package (VASP) codeS9. The generalized gradient approximation within the Perdew-Burke-Ernzerhof parameterization was used for the exchange-correlation potentialS10. An energy cutoff of 500eV was selected for the plane-wave expansion. The Ir-doped bcc-Fe(5ML)/MgO(5ML) multilayer was modeled with a supercell having 4 x 4 unit cell, as shown in Fig. 4(a). The in-plane lattice constant was fixed to the 11.44 Å, corresponding to an Fe-bulk lattice constant of 2.86 Å. We used the force theorem to estimate the magnetic anisotropy energy. The 6×6×2 k-points mesh was used for the first Brillouin-Zone integration.

The electric-field induced magnetic moment and magnetic anisotropy energy is estimated for the vacuum(1nm)/Cu(5ML)/Ir-doped Fe(5ML)/MgO(5ML)/vaccum(1nm) film as shown in Fig. S7-1. We apply periodic saw-tooth potential to simulate the external electric field.

Figure S6 Magnetic hysteresis curves for the Cr (30 nm)/Ir-doped Fe(ttotal=0.82 nm)/Cr (10 nm) sandwich structure measured under out-of-plane and in-plane magnetic fields by SQUID.

Figure S7-2 shows the projected density of states for the (a) interfacial and (b) sub-interfacial Ir atoms in the Ir-doped Fe/MgO. We can see the clear splitting of Ir-5d minority and majority spin states around the Fermi level induced by the hybridization between Fe-3d and Ir-5d orbitals.

S8. Temperature dependence of magnetic moment

Figure S8 shows an example of temperature dependence of magnetic moment of Ir-doped ultrathin Fe layer ttotal = 0.46 nm. We observed no drastic reduction in magnetic moment close to room temperature, indicating that the Curie temperature is still far above room temperature. Therefore, influence of voltage-induced change in Curie

Figure S7-2 Projected density of states on d orbitals for the (a) interfacial and (b) sub-interfacial Ir atoms in the Ir-doped Fe/MgO.

Figure S7-1 First principles calculations of electric-field modulation of magnetic anisotropy energy (MAE) in Ir-doped Fe/MgO system.

temperature can be neglected.

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Figure S8 Example of temperature dependence of magnetic moment for the Ir-doped Fe layer (ttotal=0.46 nm)

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