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Functional Inorganic Materials and Devices

Heteroepitaxy of Co-based Heusler compound/muscovite for flexible spintronics Yi-Cheng Chen, Min Yen, Yu-Hong Lai, Anastasios Markou, Liguo Zhang, YiYing Chin, Hong-Ji Lin, Chien-Te Chen, Claudia Felser, and Ying-Hao Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12219 • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

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ACS Applied Materials & Interfaces

Heteroepitaxy of Co-based Heusler compound/muscovite for flexible spintronics

Yi-Cheng Chen1,2, Min Yen1, Yu-Hong Lai1, Anastasios Markou2, Liguo Zhang2, Yi-Ying Chin3, Hong-Ji Lin4, Chien-Te Chen4, Claudia Felser2, Ying-Hao Chu1,5,6*

1Department

of Materials Science and Engineering, National Chiao Tung

University, Hsinchu 30010, Taiwan

2Max

Planck Institute for Chemical Physics of Solids, Dresden 01187, Germany

3Department

4National

of Physics, National Chung Cheng University, Chiayi 62102, Taiwan

Synchrotron Radiation Research Center, Hsinchu 30010, Taiwan

1 ACS Paragon Plus Environment

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5Center

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for Emergent Functional Matter Science, National Chiao Tung University,

Hsinchu 30010, Taiwan

6Institue

of Physics, Academia Sinica, Taipei 11529, Taiwan

Keywords: Heusler compound, muscovite, flexible, magnetism, spintronics

Abstract Materials with high spin-polarization play an important role in the development of spintronics. Co-based Heusler compound is a promising candidate for practical applications due to its high Curie temperature and tunable halfmetallicity. However, it is a challenge to integrate Heusler compound into thin film heterostructure due to the lack of control on crystallinity and chemical disorder, critical factors of novel behaviors. Here, muscovite is introduced as a growth substrate to fabricate epitaxial Co2MnGa films with mechanical flexibility. The feature of heteroepitaxy is evidenced by the results of X-ray diffraction and transmission electron microscopy. Moreover, high chemical ordering with superior 2 ACS Paragon Plus Environment

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properties is delivered according to the observation of large Hall conductivity (680 Ω-1cm-1) and highly saturated magnetic moment (~3.93 μB/f.u.), matching well with bulk crystal. Furthermore, the excellence of magnetic and electrical properties retains under the various mechanical bending conditions. Such a result suggests that the development of Co2MnGa/muscovite heteroepitaxy provides not only a pathway to thin film heterostructure based on high-quality Heusler compound but also a new aspect of spintronic applications on flexible substrate.

1. Introduction Scientists have developed various high-performance devices by the utilization of spin feature named “Spintronics”. In this research field, spin injection, spin transportation and spin detection are three fundamental tasks for the realization of spintronic devices1. Half metal is the materials with the band structure of high spin polarization, meaning the density of state (DOS) is continuous in majority spin while DOS vanishes around the Fermi energy in minority spin. In a prototype heterostructure of half-metallicity ferromagnet on semiconductor, it can 3 ACS Paragon Plus Environment


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be used for injecting spin current to the semiconductor for spintronic devices. Among various materials, CrO2 and La0.7Sr0.3MnO3 were reported near 100% spinpolarized half metals, however the Curie temperatures (Tc) of these materials are 390K2 and 369K3 respectively, setting a limit of practical applications. In these two decades, Heusler compounds have become attracting in the research field of spintronics because of the prediction of its charming half-metallicity4, high Curie temperature and tunable band structure. More attentions have been paid to Cobase Heusler compound due to the lattice constant close to those of commercial III-V semiconductors, such as GaAs5, suggesting a possible integration with current semiconductor industry.

The magnetic and related spintronic properties of Heusler compound couple to its crystallinity and chemical ordering5. For example, several studies have pointed out a strong connection of half-metallicity and saturated magnetization (Ms)6. Thus, promising features were demonstrated in the form of single crystals due to its superior crystallinity and chemical homogeneity. However, it is hard for


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conventional single crystals to be effectively applied into spintronic devices. Therefore, it is important to develop thin film heterostructures based on Heusler compound. It was reported that epitaxial Co2MnGa (CMG) thin film shows prior properties with the saturation magnetization of 3.5 μB/f.u.5 better than those reported on polycrystalline one (2.5 μB/f.u.)5. Thus, it is important to fabricate epitaxial Heusler compound thin films to acquire high crystallinity and chemical ordering.

However, in the development of spintronics, pursuing high performance is not the only task, but also extending the aspect of applications. The requirement of flexible spintronics become an important issue due to the era of Industry 4.0 and Internet of Things (IoT). To conserve both performance and flexibility, substrate plays an important role. Muscovite (monoclinic, a=5.25 Å, b=9.02 Å, c=10.14 Å, α=90°, β=100°, γ=90°) is a substrate with high crystallinity and excellence of mechanical

flexibility.

In

1964,

the

first

metal/muscovite

heteroepitaxy

demonstrated was Au/muscovite7 due to the excellent electrical properties. After



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that various metals were studied for practical applications, including Pt8, Ag9, Cu10, Ti11. From 2016, many functional oxides, such as MoO212, ITO13, AZO13, Fe3O414 and YSZ15 have been demonstrated epitaxially on muscovite for different applications. According to these results, there is a possibility of integrating Heusler compound on muscovite epitaxially to open a new avenue to flexible spintronics. In this study, we developed a flexible CMG/muscovite heteroepitaxy with excellence of magnetic and electrical properties. In addition, the intrinsic properties retain with superior mechanical flexibility. Taken together, our study delivers a novel platform for Heusler heteroepitaxy and reveals a new aspect of spintronic applications.

2. Results and discussions

40 nm CMG films were prepared by co-sputtering technique on top of muscovite substrate after a substrate cleaning process. In order to promote the growth of CMG film on muscovite substrate, a seeding layer of 10 nm Cr was inserted. Without it, only polycrystalline CMG film can be obtained. The details on 6 ACS Paragon Plus Environment

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the growth conditions can be found in the method section. In order to verify the heteropeitaxy of CMG/muscovite, structural information of the heterostructure was studied by 4-circle high resolution X-ray diffractometer(XRD). Through the θ-2θ scan as shown in Figure 1(a), the presence of only (202) diffraction peak indicates the growth of CMG (202) along muscovite (00L) without any impure phase, indicating the feature of single phase. The lattice parameter (a) of CMG film extracted from the X-ray normal scan is ~5.732Å. Compared to the bulk value, a=5.77 Å,16 the difference is ~0.66%, suggesting a smaller measured out-of-plane lattice parameter than the bulk, in the XRD geometry. This implies an out-of-plane compression, and hence an in-plane tension. The value is similar to the one grown on rigid MgO substrate (~0.64 %)17. To further characterize the in-plane orientation relationship, sample azimuth(φ) scan measurements were performed. CMG (004) and muscovite (202) were aligned parallel to X-ray beam and the sample was rotated along the z-axis. The result shows (Figure 1(b)) there are six peaks with 60-degree intervals of CMG (004), exhibiting a multi-domain feature with three kinds of domains in the heterostructure. In addition, the peak of CMG (004) shows 7 ACS Paragon Plus Environment


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a good alignment with the one of muscovite (202). On the basis of the XRD results, the epitaxial relationship can be determined as (202)CMG||(001)muscovite and [001]CMG||[200]muscovite. In presence of only 00L reflection, there is no observation of superlattice peaks such as (111) or (113) during the asymmetric scans, suggesting an existence of the B2 type structure18. Further, the quality of the film was characterized by a ω-scan. The full width of half maxima (FWHM) is 1.22° as shown in Figure 1(c). This value is comparable to the one (1.47°)19 of Co-based Heusler thin film on the rigid substrate with Cr seeding layer. The crystallinity is restricted to the Cr seeding layer due to the low growth temperature (10000 Oe). The change of the slope is due to the saturation of the intrinsic magnetic moment in ferromagnetic thin film34. The anomalous Hall conductivity (AHC) value is 680 Ω-1cm-1 at 5K, which is lower than the bulk value (~1500 Ω-1cm1) 21

and the epitaxial film on MgO substrate (~1138 Ω-1cm-1)17. The lower value

may come due to the B2-type structure and mosaicity (FWHM=1.22°). Compared to other Co-based Heusler compound such as Co2MnGe (228 Ω-1cm-1)33, Co2MnSn (174 Ω-1cm-1)35, Co2MnSi (193 Ω-1cm-1 )35, the relatively high values should be sufficient for practical applications. According to the theoretical prediction, the large AHC might relate to Weyl points36 or nodal lines37. The Weyl


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fermion was predicted in the standard model; however, it has not been investigated or

observed

experimentally.

In

the

measurement

of

longitudinal

magnetoresistance (Figure 3(c)), the resistance gradually increases at low-field, and decreases at high-field, in each temperature. The field of downturn point increases while cooling which implies the interaction between current and magnetic moment. The positive magnetoresistance can be attributed from an orientation change of magnetic moment. The magnetic moment rotates from the in-plane easy axis to the out-of-plane hard axis with an increase of perpendicular magnetic field. The rotation of the magnetic moment induces different local fields, which results in the spin-dependent scattering30. This interaction between current and the direction of magnetic moment is similar to the effect of anisotropic magnetoresistance (AMR), which gives rise to the peak until the moment saturated around 10000 Oe (ρxx⊥ > ρxx||). The system shows negative magnetoresistance even when all the magnetic moments have already been aligned in the hard axis which is the typical ferromagnet behavior. The interaction mainly occurs in the lowfield regime and it has been declined after the saturation of the magnetization. In 15 ACS Paragon Plus Environment


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order to find out the origin of this behavior, a comparison of the longitudinal magnetoresistance to the transverse magnetoresistance at 300K is shown in Figure 3(d). One can find the turning point at the similar field, indicating the mechanism should correlate to the reorientation of magnetic moment. The establishment of magneto-transport properties not only gives the possibility in unveiling the fundamental physics but also shows a great potential in magnetic devices.

One key feature of films on muscovite is the mechanical flexibility. The combination of spin-dependent transport and mechanical flexibility can be pushed for the application of flexible spintronics. In order to extend such a possibility, a series of bending test was employed to the CMG/muscovite heterostructure to study the bending effects on the physical properties under external stress. In Figure 4(a), the measured resistance remains the same under the bending radii of 5 mm in the flex-in mode and 8 mm in the flex-out mode, respectively, suggesting the CMG/muscovite heterostructure is robust against the external stress. The


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mechanical stability and cyclability of the heterostructure were investigated under further bending tests. The change of resistance under external strain for long period (>106 s) with constant current is less than 1% (Figure 4(b)). The CMG/muscovite also exhibits a stable resistance (Figure 4(c)) after 1000 bending cycles under the bending radii of 5 mm in compressive mode and 10 mm in tensile mode.

We further study the effect of the external stress on the magnetic properties of the CMG/muscovite. In Figure 4(d) & 4(e), we show the magnetization hysteresis curves with the magnetic field applied in-plane under compressive and tensile modes with various bending radii. The shape of M-H loops illustrates near identical under compressive and tensile stress even when the bending radius is smaller than 10 mm, delivering the critical information of stable magnetic feature against mechanical bending. Similar results were observed when the magnetic field was applied perpendicular to the film plane (Figure 4(d) insert), the magnetic anisotropy did not vary with the bending radius, suggesting no rotation of the



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magnetic moment from the in-plane to out-of-plane directions. The result differs from the previous study on Fe3O4/muscovite by Wu et al14. The rotation of easyaxis to the out of plane in Fe3O4/muscovite is believed due to the enlargement of magnetic domain14. The reason may arise due to the relaxation of external stress. During the bending process, the dislocations in CMG might slip, resulting in the relaxation of external strain, a critical feature in alloy systems. Hence, it causes the opposite result in the two half-metal/muscovite heteroepitaxial systems. In addition, the spin dependent transports, an important function in spintronic devices, were tested under mechanical bending. The magneto- and Hall resistances were characterized under the critical bending radius of the CMG/muscovite heterostructure in both flex-in and flex-out modes. The change of the magneto- and Hall resistance is negligible (Figure 4(f)). These results demonstrate the CMG/muscovite heteroepitaxy can maintain the inherent nature even under mechanical bending, delivering a robust spintronic system for flexible electronics.


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3. Conclusion In summary, high-quality epitaxial Co-based Heusler films have been fabricated on flexible muscovite substrate. The crystallinity of the heterostructure and the epitaxial relationship between the film and substrate have been investigated by XRD and HR-TEM. The novel magnetic and transport properties have been detailed established by SQUID, XAS-MCD and PPMS. Moreover, the heterostructure retains inherent performance under the mechanical flexibility, cyclability and retention tests. In addition, the high AHC of CMG/muscovite provides not only the fundamental research for particle physics but also the possibility in application of IoT, such as current sensor in smart grid. The demonstration of CMG/muscovite heteroepitaxy provides a new perspective on developing the spintronic devices with a huge potential for practical applications.

4. Method



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A 1cm*1cm muscovite substrate was used in this study. The substrate was mounted on a holder made of Molybdenum and then transferred into the UHV chamber (Bestec) at the base pressure lower than 2E-8 mbar. CMG thin films were deposited in the Argon (Ar) atmosphere of 3E-3mbar with 15 sccm flow rate by DC magnetron sputtering equipped with three independent 2” targets (Co, Mn, and MnGa) aiming at the center of substrate. The power for the deposition of Co, Mn and MnGa was set to be 32W, 5W and 20W respectively. The substrate was kept 350 oC during the deposition. After the growth, the sample was kept at the same temperature for 20 mins to increase the chemical ordering. To prevent thin film from oxidation, Aluminum was capped at room temperature. In addition, a Cr seeding layer was deposited at 600 oC prior to the CMG deposition for promoting the crystallization. The deposition rate of the CMG and Cr was 0.38 Å /s and 0.12 Å/s, respectively. The four-circle XRD (Panalytical XPERT3 MRD) with CuKα1(λ=1.5406Å) radiation was applied to confirm the crystal structure and epitaxial relationship.


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TEM specimen was prepared by focused-ion-beam technique and examined by JEOL JEM ARM 200F microscope.

Magnetic hysteresis loops were measured in the Quantum Design MPMS magnetometer at 50K and 300K. The sample was cut into 3*3 mm2 and mounted in the plastic straw. For further investigation of magnetism, XAS-MCD was conducted at Dragon beamline at the National Synchrotron Radiation Research Center.

The temperature-dependent resistance, magneto-resistance and Hall measurement were performed in Quantum Design PPMS. The sample was mounted on the standard PPMS puck. Longitudinal resistance was measured by four-probe method and the Hall measurement was conducted with standard Hall bar geometry.

A homemade Teflon stage was applied for magnetic hysteresis loops, magnetoresistance, and Hall measurement by sticking the sample with double side



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tape on the stage. The edges of the sample were sealed with glue to ensure the external stress applied on the whole system.

ASSOCIATED CONTENT

Supporting Information is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION

Corresponding Author * Correspondence to: [email protected]

ORCID Ying-Hao Chu: 0000-0002-3435-9084 Notes The authors declare no competing financial interest.

Author Contributions


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Y.C.C., M.Y. and Y.H.C. designed the experiments Y.C.C. prepared all the samples in this study, performed XRD, PPMS and bending measurement, and designed all schematics. A. M. were responsible for SQUID measurement and analysis. L.Z. carried out the AHC analysis. Y.H.L. performed the TEM measurement and analysis. Y.Y.C, H.J.L. and C.T.C. carried out the XAS-MCD measurement and analysis. Y.C.C., A.M., C.F., and Y.H.C. designed the study and wrote the manuscript with the help of all authors. All authors discussed the results, and Y.H.C. directed the study.

ACKNOWLEDGMENT This work is supported by the Ministry of Science and Technology, Taiwan (Grant no. MOST 106-2119-M-009-011-MY3), ERC Advanced Grant No. 742068 “TOPMAT”, and the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. This work is also supported by the Sandwich Scholarship 23 ACS Paragon Plus Environment


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Programme of Ministry of Science and Technologay, Taiwan and Deutscher Akademischer Austauschdienst, Germany (Grant no. MOST-106-2911-I-009-512)

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(20) Sagar, J.; Yu, C. N. T.; Lari, L.; Hirohata, A. Growth of Polycrystalline Heusler Alloys for Spintronic Devices. J. Phys. D-Appl. Phys. 2014, 47 (26), 4, DOI: 10.1088/00223727/47/26/265002. (21) Manna, K.; Sun, Y.; Muechler, L.; Kubler, J.; Felser, C. Heusler, Weyl and Berry. Nat. Rev. Mater. 2018, 3 (8), 244-256, DOI: 10.1038/s41578-018-0036-5. (22) Jorge, E. A. Band Structure of Heusler Compounds Studied by Photoemission and Tunneling Spectroscopy. Doktor Johannes Gutenberg-Universitaet in Mainz, 2011. (23) Kallmayer, M.; Elmers, H. J.; Balke, B.; Wurmehl, S.; Emmerling, F.; Fecher, G. H.; Felser, C. Magnetic Properties of Co2Mn1-xFexSi Heusler Alloys. J. Phys. D-Appl. Phys. 2006, 39 (5), 786-792, DOI: 10.1088/0022-3727/39/5/s03. (24) Klaer, P.; Kallmayer, M.; Blum, C. G. F.; Graf, T.; Barth, J.; Balke, B.; Fecher, G. H.; Felser, C.; Elmers, H. J. Tailoring the Electronic Structure of Half-Metallic Heusler Alloys. Phys. Rev. B 2009, 80 (14), 13, DOI: 10.1103/PhysRevB.80.144405. (25) Elmers, H. J.; Fecher, G. H.; Valdaitsev, D.; Nepijko, S. A.; Gloskovskii, A.; Jakob, G.; Schonhense, G.; Wurmehl, S.; Block, T.; Felser, C.; Hsu, P. C.; Tsai, W. L.; Cramm, S. Element-Specific Magnetic Moments from Core-Absorption Magnetic Circular Dichroism of The Doped Heusler Alloy Co2Cr0.6Fe0.4Al. Phys. Rev. B 2003, 67 (10), 8, DOI: 10.1103/PhysRevB.67.104412. (26) Fecher, G. H.; Ebke, D.; Ouardi, S.; Agrestini, S.; Kuo, C. Y.; Hollmann, N.; Hu, Z. W.; Gloskovskii, A.; Yakhou, F.; Brookes, N. B.; Felser, C. State of Co and Mn in HalfMetallic Ferromagnet Co2MnSi Explored by Magnetic Circular Dichroism in Hard X-Ray Photoelectron Emission and Soft X-Ray Absorption Spectroscopies. Spin 2014, 4 (4), 16, DOI: 10.1142/s2010324714400177. (27) Chen, C. T.; Idzerda, Y. U.; Lin, H. J.; Smith, N. V.; Meigs, G.; Chaban, E.; Ho, G. H.; Pellegrin, E.; Sette, F. Experimental Confirmation of the X-Ray Magnetic Circular Dichroism Sum Rules for Iron and Cobalt. Phys. Rev. Lett. 1995, 75 (1), 152-155, DOI: 10.1103/PhysRevLett.75.152. (28) Gabor, M. S.; Belmeguenai, M.; Petrisor, T.; Ulhaq-Bouillet, C.; Colis, S.; Tiusan, C. Correlations Between Structural, Electronic Transport, and Magnetic Properties of Co2FeAl0.5Si0.5 Heusler Alloy Epitaxial Thin Films. Phys. Rev. B 2015, 92 (5), 12, DOI: 10.1103/PhysRevB.92.054433. (29) Vidal, E. V.; Schneider, H.; Jakob, G. Influence of Disorder On Anomalous Hall Effect for Heusler Compounds. Phys. Rev. B 2011, 83 (17), 6, DOI: 10.1103/PhysRevB.83.174410. (30) Zhu, L. J.; Zhao, J. H. Anomalous Resistivity Upturn in Epitaxial L21-Co2MnAl Films. Sci Rep 2017, 7, 7, DOI: 10.1038/srep42931.


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Figure 1 (a) θ-2θ scan of Co2MnGa/Cr/muscovite heterostructure (b) φ-scans of Co2MnGa (004) and muscovite (202) (c) Rocking curve of Co2MnGa (202) with 1.22 degree of FWHM (d) Schematic of the Co2MnGa/Cr/muscovite heteroepitaxy (e) The TEM images of cross-section & the corresponding Fast Fourier Transform patterns


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Figure 2 (a) In-plane magnetization loops at 50k and 300k. (b) The XAS-XMCD spectra of Co (c) The XAS-XMCD spectra of Mn (d) XMCD hysteresis loops of Co and Mn



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Figure 3 (a) Temperature dependent longitudinal resistance (b) The Hall conductivity under different temperatures (c) Field dependent MR ratio at different temperatures (d) A combination of Hall resistance and MR ratio


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Figure 4 (a) the longitudinal resistance difference under various bending radii (b) the longitudinal resistance change as the function of bending duration (c) the longitudinal resistance variation as the function of bending cycles (d) In-plane and out-of plane (insert)hysteresis loops under various bending radii with flexin mode (e) In-plane hysteresis loops under various bending radii with flex-out mode (f) the results of MR and the Hall measurement in a flat state and under various bending radius with flex-in and flex-out mode



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muscovite for flexible

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