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Surfaces, Interfaces, and Applications
Switchable Magnetic Anisotropy of Ferromagnet by Dual-ion Manipulated Orbital Engineering Lei Wang, Chun Feng, Yukun Li, Fei Meng, Shiru Wang, Mingke Yao, Xiulan Xu, Feng Yang, Baohe Li, and Guanghua Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09342 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on August 1, 2019
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ACS Applied Materials & Interfaces
Switchable Magnetic Anisotropy of Ferromagnet by Dual-ion Manipulated Orbital Engineering Lei Wang1, Chun Feng1, *, Yukun Li1, Fei Meng1, Shiru Wang1, Mingke Yao1, Xiulan Xu1, Feng Yang2, Baohe Li3, Guanghua Yu1, * 1
Department of Materials Physics and Chemistry, University of Science and Technology Beijing,
Beijing 100083, China; Beijing Laboratory of Metallic Materials and Processing for Modern Transportation, University of Science and Technology Beijing, Beijing 100083, China. 2State
Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing, Beijing
102249, China. 3 Department
of Physics, School of Sciences, Beijing Technology and Business University, Beijing
100048, China.
ABSTRACT: Tailoring magnetic anisotropy of ferromagnetic films is a critical issue in constructing energy-efficient and high-density magnetic memory devices. Presently, the effective tunability was focus on a single-ion manipulated electronic structure evolution. Here, we reported a new strategy of dual-ion tuned orbital structure and magnetic anisotropy of ferromagnetic films. Ndoped Fe/MgO bilayer films were deposited on shape memory alloy substrates which can generate a significant lattice strain on the films. Before the N ions participate into the manipulation, the film shows an in-plane magnetic anisotropy, which may be due to an excessive Fe-O orbital hybridization. Interestingly, the N and O ions synergistically manipulate electronic coordination of the Fe layer, which can be further modified by the lattice strain through a charge transfer among NFe-O. Under such effect, the magnetic anisotropy of the film is switchable from in-plane to 1
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perpendicular magnetic anisotropy (PMA). The X-ray line dichroism (XLD) characterization reveals that the anisotropy regulation is related to Fe 3d orbital evolution: N-Fe orbital hybridization promotes the Fe dz2 orbital occupation effectively, which is beneficial to increase PMA by strengthening Fe-O orbital hybridization along out-of-plane direction. However, the compressive strain induces a N-Fe-O charge transfer and reduces the Fe dz2 electronic occupation, which weakens the PMA of films. These findings provide a new dimensionality for regulating orbital performance of ferromagnetic materials and developing strain-assisted memory devices.
KEYWORDS: Dual-ion manipulation, magnetic anisotropy, orbital occupancy, charge transfer, orbital hybridization
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1. INTRODUCTION Ferromagnetic films with perpendicular magnetic anisotropy (PMA) are core components for constructing magnetic storage and logic devices, such as magnetic random access memory, track memory, memristor, etc.
1-8.
The magnetic anisotropy of ferromagnetic materials determines a
variety of practical properties of the devices, including storage density, thermal stability, storage speed, and energy consumption. Therefore, a controllable magnetic anisotropy of ferromagnetic film is crucial for achieving energy-efficient and high-density memory devices. Ion manipulation is a universal method to tailor the magnetic anisotropy of ferromagnets by either chemical reaction or kinetic tunability. For example, in 3d ferromagnet/metallic oxide bilayer films, superior PMA was realized by a moderate orbital hybridization between oxygen anions (O2-) and ferromagnetic atoms, which can be elaborately controlled by annealing, electric field or lattice strain
9-20.
In addition,
injection of cations (such as He+, Ne+, Xe+) into ferromagnetic films can adjust microstructure of the films dynamically, resulting in an effective magnetic anisotropy modulation21-23. Besides, H+ ion was implanted into a Pd/Co/Pd multilayer film, where the H+ movement can be controlled by a voltage to toggle a 90º reversible transition of magnetic anisotropy of the film
24.
Presently, the
traditional works realize the anisotropy modification merely by a single ion intervention. In principle, the increment of adjustable ion number can increase the degree of freedom to achieve a broader property regulation and enrich the functionality for device application. However, the research work on multi-ion tuned magnetic anisotropy of ferromagnetic materials has scarcely been reported until now. Similar to the O atoms, Nitrogen atoms have a good affinity to electrons and thereby become an effective regulator of the band structure in various semiconductor materials 25-29. Inspired by the 3
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fact, this work attempts to modulate the magnetic anisotropy of ferromagnetic materials by a synergistic interaction of the N and O ions. In fact, the electronic affinity of ions is closely related to the local crystal environment which can be well tuned by a lattice strain. In order to adjust the crystal environment significantly to obtain diverse regulation behavior, a shape memory alloy (SMA) which is capable to generate large strain output
30-32
was used as the substrate of Fe/MgO bilayer
films, the core structure of a typical tunnel junction. N atoms were doped into the Fe layer during deposition, and the O ions in the subsequently deposited MgO layer induce an orbital hybridization with Fe atoms by interfacial chemical reaction. The N and O dual ions were found to be capable to change electronic coordination of the Fe layer effectively, which can be elaborately controlled by a charge transfer among N-Fe-O atoms under the large lattice strain. Surprisingly, the anisotropy of the films is switchable between in-plane magnetic anisotropy (IMA) and perpendicular magnetic anisotropy (PMA). The tunability effect is related to the change of Fe 3d orbital constituent: the introduction of N atoms leads to a prominent charge redistribution and an increased electronic occupation on Fe dz2 orbit, which promotes out-of-plane orbital hybridization of Fe-O for enhancing PMA. Furthermore, the compressive strain toggles a charge transfer among N-Fe-O atoms, thereby reducing the dz2 orbital occupation, which drives the anisotropy to restore from PMA to IMA.
2. EXPERIMENTAL SECTION Sample Preparation: First, the Ni45Ti45Nb10 substrate (thickness 0.5 mm) was subjected to a prestretching treatment to induce a martensite phase. Subsequently, the surface was polished until its surface roughness was close to 1 nm, as described in the previous work 32. Then, series of Cr(5 nm)/FeN(2 nm)/MgO(2 nm)/Cr(4 nm) heterostructures were deposited on the SMA substrates by magnetron sputtering at room temperature. The FeN layer was deposited by reactive sputtering with thoroughly 4
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Ar and N2 gas mixture of 3.5 mTorr. The N doping amount in Fe layer was controlled in the range of 0-36 at.% by changing gas flow ratio of Ar to N2. The other layers were deposited at an Ar gas pressure of 3.5 mTorr. All the samples were prepared in a vacuum chamber with the base pressure lower than 3×10-5 Pa. After the deposition, the samples were transferred to a vacuum furnace for a heat treatment at 200℃/2 minutes under the base pressure lower than 5×10-5 Pa, which induced an inverse martensitic transformation of the SMA substrate to generate shrinkage deformation and compress the adjacent films (Figure S1 in Supporting Information). The macroscopic shape variation of the substrate (ε) was controlled by changing the pre-stretching amount, where ε was selected to be 0%, 2.7%, 3.5%, 5.5%. Here, the 0% strain means the substrate was not pre-stretched. Sample Characterization: The N concentration in FeN layer (CN) was determined by a NonRutherford backscattering (RBS) measurement. The magnetic properties of the samples were measured using a physical property measurement system (PPMS) with in-plane or out-of-plane applied magnetic fields up to 8 kOe. The real FeN layer thickness was determined by fitting X-ray reflectivity spectra. The crystal structure in the samples was characterized by X-ray diffraction (XRD, D/max-TTR III). X-ray Photoelectron spectroscopy (XPS) measurements were conducted to reveal the electronic structure at the FeN/MgO interface. An Mg Kα source was used as the incident radiation source to provide X-rays of energy 1253.6 eV with the energy analyzer's passing energy 30 eV. The XPS spectra were collected after Ar+ etching for 60 seconds with an etching rate of 0.6 Å/s. X-ray absorption spectroscopy (XAS) and X-ray linear dichroism (XLD) spectra were measured at BL12B-a beamline of the National Synchrotron Radiation Laboratory (NSRL) in the total electron yield mode under a vacuum better than 5×10−8 Pa. The XAS spectra were detected with photon polarization parallel (E//a) and almost perpendicular (E//c) to the sample plane, respectively. The 5
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XLD is the difference between the normalized XAS along the two polarization directions.
3. RESULTS AND DISCUSSION First, the Cr(5 nm)/FeN(2 nm)/MgO(2 nm)/Cr(4 nm) heterostructure was prepared on the SMA substrate, as shown in Figure 1a. During deposition of the Fe layer, a certain amount of N atoms is introduced into the Fe layer by reactive sputtering in a nitrogen gas environment. Moreover, the O ions in the subsequently deposited MgO layer generate an orbital hybridization with Fe atoms by interfacial reaction, thereby constructing a material system with N and O dual-ion manipulation. Figures 1b-1d show hysteresis loops of the films with typical N doping amounts. When only O ions participate in regulation (CN =0 at.%), the film exhibits an apparent IMA due to a large out-of-plane saturation field up to 5 kOe. With the incorporation of N ions, the out-of-plane curve transfers from hard to easy magnetization feature, accompanying with a significant decrement of the saturation field down to 2 kOe. Accordingly, the in-plane saturation field increases from 1 to 5 kOe, suggesting that the magnetic anisotropy of heterostructure is effectively modified from IMA to PMA with the N intervention. The effective magnetic anisotropy energy (Keff) can be quantitatively calculated by using the area difference between the in-plane and out-of-plane magnetization curves, as shown in Figure 2a. When Keff is larger than zero, the film has PMA and otherwise has IMA. As the CN increases, the Keff value varies from -2.2×106 erg/cm3 (CN =0 at.%) to 1.2×106 erg/cm3 (CN=36 at.%), equal to a large increment of 3.4×106 erg/cm3. These results indicate that the synergistic regulation of N and O ions can promote a remarkable anisotropy transition from IMA to PMA. On this basis, we performed a rapid heat treatment of 200℃ on the sample, which causes an inverse martensitic transformation in the SMA substrate and generates a significant compressed lattice strain on the film, as shown in Figure 1h. The lattice strain is positively proportional to the 6
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substrate deformation amount (Figure S2 in Supporting Information). The hysteresis loops of the films under different strains are shown in Figures 1e-1g. As the substrate deformation increases, the out-of-plane saturation field increases from 2 to 6 kOe, meanwhile, the in-plane curve transfers to an easy magnetization feature. Accordingly, the Keff value decreases from 1.2×106 erg/cm3 (ε=0%) to -1.5×106 erg/cm3 (ε=5.5%). Actually, a similar tuning trend is also observed in other N-doped films (Figure S3 in Supporting Information), demonstrating that the lattice strain can change the tunability behavior of N and O dual ions and restore the system from PMA to IMA. In fact, the Keff of a film is mainly contributed by the bulk anisotropy energy (Kv) and the interface anisotropy energy (Ki), and their relationship is shown in formula 1 33: Keff×t = Kv × t + Ki
(1)
By preparing samples with different Fe thickness (t), the relationship between Keff×t and t is obtained to determine the Ki value and evaluate the interfacial contribution to Keff, as shown in Figure 2b. Before the strain application, a significant Ki increment from -0.25 to 0.35 erg/cm2 is observed with the increase of N doping amount. However, with the strain treatment, Ki decreases to -0.05 erg/cm2. The changing trend of Ki is consistent with that of Keff in the Fig. 2a, which indicates that N and O ions mainly modulate the interfacial magnetic anisotropy and further results in the substantial transformation of the overall magnetic anisotropy. In order to further analyze the reason for the interfacial anisotropy modulation, we studied the elemental chemical state at the FeN/MgO interface by XPS, as shown in Figure 3. Figure 3a shows evidence that the interfacial Fe element is composed of metallic Fe, FeOy(y
