Tunable anomalous hall effect induced by interfacial catalyst in perpendicular multilayers

Accepted Manuscript Title: Tunable anomalous hall effect induced by interfacial catalyst in perpendicular multilayers Authors: J.Y. Zhang, W.L. Peng, Q.Y. Sun, Y.W. Liu, B.W. Dong, X.Q. Zheng, G.H. Yu, C. Wang, Y.C. Zhao, S.G. Wang PII: DOI: Reference:

S0169-4332(17)33396-2 https://doi.org/10.1016/j.apsusc.2017.11.114 APSUSC 37696

To appear in:

APSUSC

Received date: Revised date: Accepted date:

9-8-2017 19-10-2017 14-11-2017

Please cite this article as: Zhang JY, Peng WL, Sun QY, Liu YW, Dong BW, Zheng XQ, Yu GH, Wang C, Zhao YC, Wang SG, Tunable anomalous hall effect induced by interfacial catalyst in perpendicular multilayers, Applied Surface Science (2010), https://doi.org/10.1016/j.apsusc.2017.11.114 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Tunable anomalous Hall effect induced by interfacial catalyst in perpendicular multilayers

J. Y. Zhang,1 W. L. Peng,1 Q. Y. Sun,1 Y. W. Liu,1 B. W. Dong,1 X. Q. Zheng,1

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G. H. Yu,1* C. Wang,2 Y. C. Zhao2 and S. G. Wang1,2*

Department of Materials Physics ＆ Chemistry, University of Science and Technology Beijing, Beijing 100083, China

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Corresponding authors E-mail: [email protected] and [email protected]

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State Key Laboratory of Magnetism, Beijing National Laboratory for Condensed Matter Physics Institute of Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

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Graphical abstract

Highlights 

Anomalous Hall effect can be tuned by ultrathin functional layer (the thickness ~ 0.1 nm) at interfaces, which is in form of sub-nano clusters, similar to the behavior of nano-particles in catalytic engineering. 1

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The saturation anomalous Hall resistance (RAH) is increased by 16.5% with 0.1 nm Hf insertion compared with the reference sample without insertion. However, the RAH value is decreased by 9.0% with 0.1 nm Pt insertion.

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Interfacial structural analysis shows that the opposite behavior about the RAH originates from the different oxygen behavior due to various interfacial

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insertion.

Abstract

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The interfacial structures, playing a critical role on the transport properties and the perpendicular magnetic anisotropy in thin films and multilayers, can be modified by inserting an ultrathin functional layer at the various interfaces. The anomalous Hall effect (AHE) in the multilayers with core structure of Ta/CoFeB/X/MgO/Ta (X: Hf or Pt) is tuned by interfacial catalytic engineering. The saturation anomalous Hall resistance (RAH) is increased by 16.5% with 0.1 nm Hf insertion compared with the reference sample without insertion. However, the RAH value is decreased by 9.0% with 0.1 nm Pt insertion. The interfacial states were characterized by the X-ray photoelectron spectroscopy (XPS). The XPS results indicate that a strong bonding between Hf and O for Hf insertion, but no bonding between Pt and O for Pt insertion. The bonding between metal and oxygen leads to various oxygen migration behavior at the interfaces. Therefore, the opposite behavior about the RAH originates from the different oxygen behavior due to various interfacial insertion. This work provides a new approach to manipulate spin transport property for the potential applications.

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Keywords: Anomalous Hall effect, interfacial catalyst, oxygen migration, perpendicular multilayers.

Introduction Spintronic devices are blooming, which has promoted the development of information industry and social organization in past 2

decades.1-3 Extensive attention has been paid to the multilayered materials with perpendicular magnetic anisotropy (PMA), a promising candidate in non-volatile storage.4-8 The spin-dependent transport property in the perpendicular multilayers depends strongly on the interfacial structure. It was demonstrated experimentally and theoretically that interfacial structure can be successfully tuned by electric control, lattice strain and

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growth details.9-11 Therefore, interfacial modification has been used as an

effectively way not only to improve the performance of the spintronic devices,12-14 but also to enhance the catalytic properties in nano-scale

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materials and catalyst engineering.15-17 For another example, the catalytic

performance of nanocrystal tandem catalyst composed of Pt/CeO2 bilayers with metal/oxide (Pt-O) interface was enhanced due to sequential

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reactions.18 Very recently, high catalyst property has been achieved in

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single atom Pt catalyst adsorbed at nitrogen doping graphene, resulting

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from efficient electronic interaction in single Pt atom.19 Unfortunately, little work has been carried out to investigate the relationship between

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interfacial catalyst and spin-dependent transport property in multilayers. In this paper, anomalous Hall effect (AHE) in the multilayers with

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structure of Ta/CoFeB/X/MgO/Ta (X: Hf or Pt) was investigated by inserting Hf and Pt at the CoFeB/MgO interface. The AHE can be well

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tuned by the ultrathin insertion, where the saturation anomalous Hall resistance (RAH) is enhanced by 16.5% with 0.1 nm Hf insertion

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compared with Ta/CoFeB/MgO multilayers. However, the RAH value is decreases by 9.0% with 0.1 nm Pt insertion. The different interfacial structures due to Hf and Pt insertion was confirmed by X-ray

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photoelectron spectroscopy (XPS). The opposite behavior on the RAH in Ta/CoFeB/MgO perpendicular multilayers comes from the different interfacial oxygen migration behavior due to Hf and Pt insertion.

Sample fabrication Multilayers with structure of Ta(2)/CoFeB(1)/X(t)/MgO(2)/Ta(2) (X: Hf or Pt, t = 0 ~ 0.8 nm, in nm) were deposited on thermally oxidized Si 3

wafers at room temperature (RT) by magnetron sputtering. The CoFeB, Ta, Hf, and Pt layers were deposited by DC sputtering, and the MgO layer was deposited by RF sputtering, respectively. The atomic ratio of CoFeB target was 40: 40: 20, and the purity of all targets was 99.99%. The base pressure was 1.0×10-5 Pa, and the Ar pressure was kept at 0.2 Pa during growth. The nominal thickness of each layer was controlled by the

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deposition time based on the calibrated growth rate. After growth, the

vacuum annealing were carried out at 300℃ with 30 minutes. The

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samples were finally patterned into Hall bar by lithography with ion milling for transport measurements in a physical property measurement system by the standard four-probe technique. To eliminate the thermal

effect, 1 mA was applied in the transport measurement to obtain the

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saturation Hall resistance (RAH) and longitudinal resistance (Rxx). The

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interfacial states were investigated by X-ray Photoelectron Spectroscopy

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(XPS, ESCALAB 250Xi). The XPS peak area and peak decomposition were calculated by the software provided by XPS system based on

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Results and Discussion

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Gaussian (80%)-Lorentzian (20%) method.

The multilayered sample with core structure of Ta(2)/CoFeB(1)

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/MgO(2)/Ta(2) (in nm) was used as the reference, exhibiting PMA after annealing at 300 °C. Two samples with Hf and Pt layer inserted at

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CoFeB/MgO interface (called Hf-sample and Pt-sample), respectively. Figure 1 (a) presents the saturation anomalous Hall resistance (RAH) as a function of the inserted layer thickness for the Hf-sample (red curve) and the Pt-sample (blue curve), respectively. The RAH value for the reference

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sample is 8.46 Ω. For Hf-sample, RAH shows a nonlinear change with increasing Hf thickness in Fig. 1 (a). It reaches the maximum of 9.86 Ω as tHf = 0.1 nm, which is 16.5% larger than that of the reference sample. However, RAH decreases with further increasing Hf layer thickness up to 0.8 nm. For Pt-sample, RAH shows a monotonous decrease with increasing Pt thickness (0 ~ 0.8 nm). For example, RAH is only 7.69 Ω 4

with tPt = 0.1 nm, 9.0% lower than that in the reference sample. In brief, for 0.1 nm insertion, RAH increases for Hf, but decreases for Pt. However, when tHf,Pt is larger than 0.1 nm, the value of RAH for both Hf and Pt samples shows a continuous decrease with increasing tHf,Pt. It is worthy to emphasize that the effect of ultrathin inserted layer on AHE is significant, an obvious indication of interfacial behavior. Fig. 1 (b) shows the Hall

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loops for two samples with 0.1 nm insertion (Hf: red circle, Pt: blue

triangle) and the reference sample (black square) at RT, respectively. The

reference sample shows great PMA, in agreement with the previous

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study.20 Good squareness and PMA was observed for the sample with 0.1 nm Hf insertion. However, Hall loop for the sample with 0.1 nm Pt

insertion shows hard axis behavior, indicating in-plane magnetic

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anisotropy for the Pt-sample. Furthermore, PMA can be well maintained

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with increasing tHf from 0.1 nm to 0.8 nm, shown in Fig. 1(c).

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To further investigate the physical mechanism of different AHE

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behavior due to 0.1 nm Hf and Pt insertion, X-ray photoelectron spectroscopy measurements (XPS) were carried out. XPS technique has

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been proven to be a very powerful tool to detect non-destructive interfacial state in the multilayers.21-22 More details can be found in

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elsewhere.23-24 The sample was cleaned with Ar+ ion to remove surface adsorption before measurements. Fig. 2 (a) ~ (c) presents Fe 2p high

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resolution XPS spectra for Ta/CoFeB/MgO/Ta (reference sample), Ta /CoFeB/Hf(0.1 nm)/MgO/Ta (Hf-sample) and Ta/CoFeB/Pt(0.1 nm)/MgO /Ta (Pt-sample), respectively. For reference sample shown in Fig. 2 (a), peak A and peak B located at 706.6 eV and 708.6 eV corresponds to

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metallic Fe 2p and Fe 2p in FeOx (1
Similar behavior is observed for the sample with 0.1 nm thick Pt layer shown in Fig. 2 (c). However, the value of ε for Hf and Pt insertion is greatly different, where it is 0.49/1 and 0.79/1, respectively. It obviously indicates that such ultrathin insertion layer plays significant role on interfacial oxygen environment.

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Fig. 3 (a) presents Mg 2p high resolution XPS spectra for the above samples. For the reference sample (black curve), Mg 2p peak locates at 50.2 eV, whereas Mg 2p is at 50.4 eV in ideally stoichiometric MgO, a

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shift of 0.2 eV to lower binding energy. For Hf-sample (red curve), Mg 2p peak locates at 49.6 eV, a shift of 0.6 eV compared to the reference sample. For Pt-sample (blue curve), Mg 2p peak is at 50.0 eV. High

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resolution Hf 4f XPS spectra for the Hf sample was presented in Fig. 3

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(b). Due to ultrathin Hf layer (0.1 nm here), the intensity of Hf 4f XPS spectra is quite weak, together with small relative sensitivity factor (RSF)

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for Hf element. Three peaks for Hf 4f spectra approximately located at

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14.6 eV, 16.5 eV and 17.9 eV can be seen, which will be discussed in detail by fitting based on Gaussian (80%)-Lorentzian (20%) method.

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Finally, Fig. 3 (c) presents high resolution Pt 4f XPS spectra for the Pt sample. Although, Pt layer is ultrathin (0.1 nm as well), the Pt 4f intensity

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is larger than Hf 4f, mainly due to higher RSF. It can be seen that Pt 4f peak locates at 71 eV, a shift of 0.2 eV to higher binding energy from Pt

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4f in bulk Pt (70.8 eV). Generally, it is difficult for an ultrathin layer grown by sputtering

(the nominal thickness of 0.1 nm) to form a continuous film. In our study,

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0.1 nm thick metallic layer (Hf and Pt) at interfaces should be in form of sub-nano clusters, similar to the behavior of nano-particles in catalytic engineering. Therefore, it is easy for ultrathin metallic layer (Hf or Pt clusters) to produce electronic exchange interaction with adjacent atomic layers due to large specific surface area. According to XPS data, the value of ε describing the relative Fe-O 6

content is 0.62/1, 0.49/1, and 0.79/1, for reference sample, Hf-sample and Pt-sample, respectively. By Hf insertion, ε decreases by 21%, suggesting much less Fe-O at interface. However, by Pt insertion, ε increases by 27%, indicating more Fe-O at interface. Therefore, the effect of Hf/Pt insertion at CoFeB/MgO interface on Fe-O bonding is opposite. For Mg 2p XPS data, Mg 2p peak for the Hf sample shows a shift of 0.6 eV to lower

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binding energy compared with the reference sample, indicating more O vacancies in the MgO layer. Similar Mg 2p behavior can be found in the

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sample with 0.1 nm Pt insertion. This indicates that O2- ions migrate from MgO layer for both Hf and Pt insertion. Interfacial Hf state should also be concerned. Peaks located at 14.6 eV and 16.3 eV corresponds to Hf 4f 5/2

and Hf 4f7/2 peaks in metallic Hf, respectively. Peaks located at 16.5 eV

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and 17.9 eV corresponds to Hf 4f5/2 and Hf 4f7/2 peaks in Hf oxidation,

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respectively. It is reasonable to conclude there is Hf-O bonding mainly at

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interface. Therefore, the insertion (0.1 nm Hf and Pt) plays different role in tuning the orbital hybridization between Fe and O at interfaces, which

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will affect significantly the interfacial oxygen migration behavior. Schematically, oxygen environment at CoFeB/MgO interface in the

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reference sample, Hf-sample, and Pt-sample was shown in Fig. 4 (b) ~ (d), respectively. For Hf-sample shown in Fig. 4 (c), two kinds of oxygen

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migration take place. Firstly, oxygen migrates from MgO layer to Hf clusters. Secondly, oxygen bonding with Fe in CoFeB layer moves to Hf

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clusters. For Pt-sample, only one kind of oxygen migration exists at interface shown in Fig. 4 (d), where oxygen of MgO layer migrates through Pt clusters to CoFeB layer. Different oxygen migration results in

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various interfacial oxygen environments, which should have different effect on electron spin-dependent scattering. Fig. 5 (a) shows the longitudinal resistivity ρxx as a function of temperature (ρxx-T) for the three samples. For three samples, ρxx decreases monotonically with increasing T, indicating the insulating behavior (d ρxx/d T < 0). The value of ρxx for the reference sample is 332 7

μΩ•cm at 300 K. It only shows a slight increase with 0.1 nm Pt and Hf, and is 346 μΩ•cm and 360 μΩ•cm, respectively. Generally, saturation anomalous Hall resistivity (ρxy) is proportional to longitudinal resistivity ρxx. For our samples, ρxx for the Hf and Pt samples increases simultaneously, but exhibiting opposite AHE behavior. Therefore, it is reasonable to conclude that the opposite AHE behavior for the Hf and Pt

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sample with 0.1 nm insertion is not due to the increment of ρxx. Three

mechanisms including skew scattering (SS), side jump scattering (SJ) and

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Berry phase mechanism (BP) are widely used to explain the AHE in

ferromagnetic films. BP originates from spin-orbit coupling in materials, which is independent of interfacial structures. Therefore, BP can be neglected because only interfacial structures is changed in our samples.

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ρxy can be expressed as ρxy = a×ρxx + b×ρxx2, where a and b is SS and SJ

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parameter, respectively. The experimental data were fitted by this

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equation, shown in Fig. 5 (b). The linear fit can be observed for three samples. The intercept at Y axis and the slope for fitted line corresponds

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the parameter a and b, respectively. Fig. 5 (c) and (d) presents the value of a and b as a function of Hf/Pt thickness, respectively. The focus will be

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given on the sample with 0.1 nm Hf and Pt insertion. The value of a and b is -4.86×10-2 and 185 S/cm for the reference sample, respectively. The

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opposite sign of a and b indicates the competitive relationship between SS and SJ mechanism in the reference sample. For Hf-sample, the value

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of a and b as a function of the inserted thickness was shown in Fig. 5 (c). A complex change of a and b value was observed with tHf increasing. The value of a increases from -4.86×10-2 for the reference sample to -5.12×

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10-2 for 0.1 nm Hf insertion, meanwhile b decreases from 185 S/cm to 178 S/cm. It is worthy to mention that Hf insertion does not change the sign of a and b, where a and b is still opposite. The value of a increases but b decreases at the same time, leading to an enhancement of AHE. For Pt-sample, the value of a and b as a function of tPt was shown in Fig. 5 (d). The value of a and b decreases simultaneously to -4.1×10-2 and 151 S/cm 8

for the Pt sample with 0.1 insertion, respectively. It indicates the decrease of AHE comes from the reduced SS and SJ. Therefore, 0.1 nm Hf and Pt insertion plays an opposite role on the electron spin-dependent scattering, and the same effect for thicker insertion. According to XPS results and physical mechanism mentioned above, such thin layer (normal thickness is 0.1 nm) can not prevent completely

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the oxidation of CoFeB layer during sputtering. It is worthy to emphasize

that oxygen ions can migrate from CoFeB and MgO layer to Hf clusters

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when 0.1 nm Hf is deposited, forming Hf-O bonding at interface. However, oxygen migration takes place from MgO to CoFeB through Pt

clusters, with more Fe-O bonding at CoFeB layer. Therefore, different

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interfacial structure can be obtained in the sample with various functional layer. Generally, the spin-dependent scattering, especially SS mechanism,

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plays a crucial role in AHE behavior, which is largely related to the

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interfacial structure. The SS scattering parameter a has a significant

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increase for the Hf-sample with Hf-O bonding at interface, which comes from the impurity in materials. So the HfO2 cluster at interface can be

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considered as scattering centre, which will provide more spin-dependent scattering. The electron scattering diagram at interface for the sample

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with 0.1 nm Hf insertion was schematically shown in Fig. 6 (a). It is good agreement with the study of AHE behavior in Co/Pt multilayers

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sandwiched by HfO2 layer.25 However, the Pt cluster at interface shows different role in the SS mechanism compared with HfO2 cluster. For Pt-sample, Pt 4f peak shifts to higher binding energy, suggesting electronic potential wells exist at Pt clusters. The conduction electron will

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be trapped into potential well when it flows near Pt cluster. The electronic localization26 results in lower skew scattering in the Pt-sample as shown in Fig. 6 (b). The AHE behavior induced by SS mechanism has different change for the Hf-sample and Pt-sample due to various interfacial oxygen migration. The decrease of the b value indicates the SJ mechanism is weaken simultaneously with Hf and Pt insertion. Therefore, it is 9

reasonable different electron scattering is induced by Pt and Hf insertion, leading to opposite AHE behavior.

Conclusion In summary, tuning anomalous Hall effect by interfacial catalytic

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clusters was investigated. The RAH value increases by 16.5% for the sample with 0.1 nm Hf insertion, whereas it decrease by 9.0% for the sample with 0.1 nm Pt insertion. XPS results show that interfacial oxygen

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migration behavior can be tuned by various interfacial insertion, leading

to opposite AHE behavior induced electron spin-dependent scattering. This study would provide a new approach toward interfacial catalyst

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tuning AHE for the further magnetic storage device.

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Acknowledgments

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The work was supported by the Natural Science Foundation of China (Grant Nos. 51331002, 51431009, 51471183, 11504019 and Universities

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51625101), by the Fundamental Research Funds for the Central Grant

FRF-TP-15-005A1,

by

Postdoctoral

Science

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Foundation of China (Grant No. 2016M590043), by the Beijing Laboratory

of

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Materials

and

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for

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Transportation.

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Figure Caption

Fig. 1 (Color online) (a) Saturation Hall resistance RAH as a function of the inserted layer thickness for the sample Ta/CoFeB/Hf(t)/MgO/Ta (red curve) and Ta/CoFeB/Pt(t)/MgO/Ta (blue curve), respectively. (b) Hall

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loops at room temperature for the three samples Ta/CoFeB/MgO/Ta

(black loop), Ta/CoFeB/Hf(0.1 nm)/MgO/Ta (red loop) and Ta/CoFeB

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/Pt(0.1 nm)/MgO/Ta (blue curve), respectively. (c) Hall loops for the sample Ta/CoFeB/Hf(t)/MgO/Ta with several Hf thickness.

Fig. 2 (Color online) High resolution Fe 2p XPS spectra for the sample (a)

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/CoFeB/Pt(0.1 nm)/MgO/Ta, respectively.

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Ta/CoFeB/MgO/Ta, (b) Ta/CoFeB/Hf(0.1 nm)/MgO/Ta, and (c) Ta

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Fig. 3 (Color online) (a) Mg 2p XPS spectra for the three samples (b) Hf 4f XPS spectra for the sample Ta/CoFeB/Hf(0.1 nm)/MgO/Ta (c) Pt 4f

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XPS spectra for the sample Ta/CoFeB/Pt(0.1 nm)/MgO/Ta.

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Fig. 4 (Color online) (a) Structural schematic diagram of the multilayers Ta/CoFeB/MgO/Ta with perpendicular magnetic anisotropy. Interfacial oxygen migration behavior at interfaces for the multilayers (b) Ta/CoFeB

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/MgO/Ta, (c) Ta/CoFeB/Hf(0.1 nm)/MgO/Ta and (d) Ta/CoFeB/Pt(0.1

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nm)/MgO/Ta, respectively. Fig. 5 (Color online) (a) Longitude resistivity ρxx as a function of the temperature T. (b) ρxy/ρxx - ρxx curve for the sample Ta/CoFeB/MgO/Ta (black curve), Ta/CoFeB/Hf(0.1 nm)/MgO/Ta (red curve) and Ta/CoFeB

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/Pt(0.1 nm)/MgO/Ta (blue curve), respectively. The parameter a and b as a function of the inserted layer thickness for (c) the sample Ta/CoFeB/Hf /MgO/Ta and (d) Ta/CoFeB/Pt/MgO/Ta, respectively. Fig. 6 (Color online) The schematic electron scattering diagram at interface for the sample with (a) 0.1 nm Hf insertion and (b) 0.1 nm Pt 13

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Fig. 5 by J. Y. Zhang et al.

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Fig. 6 by J. Y. Zhang et al.

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