Distinguishing Interface Magnetoresistance and Bulk

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Surfaces, Interfaces, and Applications

Distinguishing interface magnetoresistance and bulk magnetoresistance through rectification of Schottky heterojunctions Qikun Huang, Jing Wang, Shiyang Lu, Yanxue Chen, Lihui Bai, You-yong Dai, Yufeng Tian, and Shi-shen Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06929 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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ACS Applied Materials & Interfaces

Distinguishing

interface

magnetoresistance

and

bulk

magnetoresistance through rectification of Schottky heterojunctions Qikun Huang,† Jing Wang,† Shiyang Lu,† Yanxue Chen,† Lihui Bai,† Youyong Dai,† Yufeng Tian,*,† and Shishen Yan*,†,‡ †

School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan,

250100, P. R. China ‡

School of Physics & Electronic Engineering, Kashgar University, Kashi 844006, P. R. China

Abstract High performance of many spintronics devices strongly depends on the spin-polarized electrical

transport,

especially

the

magnetoresistance

(MR)

in

magnetic

heterojunctions. But it has been a great challenge to distinguish the bulk MR and interface MR by transport measurements because the bulk resistance and interface resistance formed a series circuit in magnetic heterojunctions. Here, a unique interface sensitive rectification MR method is proposed to distinguish the interface MR and bulk MR of non-magnetic In/GeOx/n-Ge and magnetic Co/GeOx/n-Ge diode-like heterojunctions. It is demonstrated that the low field“butterfly” hysteresis loop observed only in the conventional MR curve originates from the anisotropic MR of ferromagnetic bulk Co layer, while the orbit-related large non-saturating positive MR contains contributions from both the Schottky interface and bulk Ge substrate. This rectification MR method could be extended to magnetic heterojunctions with asymmetric potential barriers to realize a deeper understanding of the fundamental interface-related functionalities. Keywords:

magnetoresistance,

rectification,

Schottky,

heterojunctions 1

ACS Paragon Plus Environment

interface,

magnetic

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction High performance of many spintronics devices strongly depends on the spin-polarized electrical transport, especially the magnetoresistance (MR) in magnetic heterojunctions.1 But it has been a great challenge to distinguish the bulk MR and interface MR by transport measurements because the bulk resistance and interface resistance formed a series circuit in magnetic heterojunctions. Though the interface and bulk effects usually are phenomenologically inseparable,2-4 the urgent demands of utilizing the interfacial functionality make it necessary to overcome this difficulty, especially after the discovery of emergent interfacial phenomena, 5 - 10 such as two-dimensional electron gas, interfacial magnetism, etc. For instance, large magnetoresistance (MR) has been reported in the polar-nonpolar LaAlO3/SrTiO3 interface.11-13 Still, Adrian et al. showed that during the growth of SrTiO3/LaAlO3 bilayers, the surface-passivation effect induced high-mobility carriers in the bulk SrTiO3 substrate also resulting a colossal positive MR as high as 30000%.9 This similarity highlights both the importance and the necessity of in-depth exploration of the difference between the interfacial and bulk properties. Especially, in the functional heterojunctions, such as p-n junctions,14,15 n-n junctions,16 Schottky heterojunctions and magnetic diodes,17-19 where using the extraordinary interface properties rather than the bulk properties is crucial to advance the interface-based magnetoelectric and spintronic devices with high density and low energy consumption.20 As a matter of fact, in these heterojunctions, the resistance of host bulk materials and the interfacial junction resistance formed a series circuit, which makes it difficult to distinguish the interface MR from bulk MR by the conventional transport measurements. In the particular case of magnetic diodes, this leads to the suspicious that whether the spin dependent behavior comes from the spin accumulation at the interface or from the 2


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bulk property of the magnetic sublayers. Because depending on the relative strength of the spin, orbit and exchange coupling effect of individual materials, magnetic sublayer itself such as transition metal doped oxides could show different magnetotransport

properties,

including

both

positive

and

negative

magnetoresistance.21-24 And up to date, the contributions from host bulk materials is usually neglected if the resistance of bulk materials is relatively small compared to the interfacial junction resistance. Hence, an interface sensitive transport measurements is highly desired to realize a deeper understanding of the fundamental magnetotransport properties of the interface. In other words, distinguishing the interface MR and bulk MR could first help verify whether interface has spin-related transport properties as expected, and then introduce or enhance the spin-related interface effects for realistic applications by optimizing the design of existing devices. In this work, we propose a unique interface sensitive rectification MR method to distinguish the interface MR and bulk MR of non-magnetic In/GeOx/n-Ge and magnetic Co/GeOx/n-Ge diode-like heterojunctions. It is demonstrated that the low field “butterfly” hysteresis loop observed only in the conventional MR curve originates from the anisotropic MR of ferromagnetic bulk Co layer, while the orbit-related large non-saturating positive MR contains contributions from both the bulk Ge substrate and Schottky interface. This method could be adapted to other heterojunctions with asymmetric potential barriers, where fresh and insightful understanding of interfacial properties are worthy of expectation.

Experimental section The as-prepared Schottky heterojunctions have a core structure of In/GeOx/n-Ge and Co/GeOx/n-Ge, where 50 nm nonmagnetic In layer or ferromagnetic Co layer is 3



deposited on the oriented n-type Ge substrate (resistivity 10 ~ 30 Ωcm) with a native oxide layer (GeOx) by using magnetron sputtering. Then, top electrode is prepared by using soldering iron. Actually, by controlling the temperature and the time of soldering operation, both Ohmic and Schottky In/Ge top electrode can be prepared. It is worthy to mention that to avoid Co oxidation, a 30 nm Au capping layer as top electrode is deposited before the samples are exposed to the atmosphere. In order to get an Ohmic Ge/In connection as the bottom electrode, Gallium-Indium-Tin eutectic (Ga:In:Sn = 62:22:16 wt%) is used to react with the Ge substrates for 30 minutes before the In electrode is prepared by using soldering iron. The as-prepared devices have an effective junction area of 3 mm × 3 mm. The electrical transport properties are measured by using two points method unless specially mentioned. And all the electrical measurements of the heterojunctions are performed under top-bottom configuration and the external magnetic field is applied perpendicular to the current flowing across the junctions, i.e., magnetic field parallel to the interface. The current flowing from top Co layer (or In layer) to the Ge substrates is defined as the positive direction. For the current versus voltage (I-V) curves and conventional MR measurements, Keithley 2400 source meter and Keithley 2182A nanovoltmeter are used. While for the rectification MR measurements, a sinusoidal alternating current with a frequency of 1 kHz is supplied by Keithley 6221 source meter and the magnetic field dependent rectifying voltage is detected by Keithley 2182A nanovoltmeter. For the rectification MR measurements, we define the nominal resistance as the rectifying voltage divided by the amplitude of the alternating current, as compared with the resistance of conventional MR measurements by direct current.

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Results and discussion We first briefly introduce the method how to distinguish the interface MR and bulk MR through the rectification of Schottky heterojunctions. Figure 1a is the equivalent circuit of In/GeOx/n-Ge nonmagnetic heterojunctions with Ohmic contacts, where the resistance of In, In/GeOx Ohmic contact interface, and GeOx/n-Ge substrate is in series. The conventional MR curves are obtained by measuring the magnetic field dependence of the direct current voltage at a fixed direct current. In this case, the resistance of In, In/GeOx interface, and GeOx/n-Ge substrate all contributes to the conventional MR, and therefore it is hard to distinguish their individual contributions from the conventional MR curves. The linear I-V curves in Figure 1b confirm that the In/GeOx/Ge electrode is Ohmic contact without any rectification at 100K. And a large non-saturating positive MR without any hysteresis is observed in the conventional MR curve, as shown in Figure 1c. However, since the MR of In and the Ohmic contacts is negligibly small, the observed large non-saturating positive MR without any hysteresis is mainly from the bulk contribution of the GeOx/Ge substrate. The fundamental origin of the observed positive non-saturating MR of the GeOx/Ge substrate has been attributed to the quantization of the carrier motion by the magnetic field and consequent modification of the impurity and/or interfacial states energy band,19,25 which is related to the orbit degree of freedom of electrons rather than the spin degree of freedom of electrons. On the other hand, the rectification MR curves are obtained by measuring the magnetic field dependence of the rectification voltage when an alternating current with fixed frequency and amplitude is applied.19,25 In Figure 1a, all resistance of In, In/GeOx Ohmic interface, and GeOx/Ge substrate is in series, and has no contribution to rectification MR, as shown in Figure 1d. 5



Figure 1e is the equivalent circuit of an independent In/GeOx/n-Ge heterojunctions with Schottky interface, where the resistance of In, diode of In/GeOx Schottky interface, and resistance of GeOx/Ge substrate is in series. The nonlinear and asymmetric I-V curves in Figure 1f measured at 100K confirm the formation of interfacial Schottky heterojunction and the existence of significant rectification. Here, the most important feature is a similar large non-saturating positive MR without any hysteresis in both the conventional MR (Figure 1g) and rectification MR (Figure 1h) curves. More importantly, the resistance of In and GeOx/Ge substrate has no contribution to rectification MR as demonstrated in Figure 1d, and only In/GeOx Schottky interface can contribute to the rectification MR, as shown in Figure 1h. Therefore, it is nature to believe that the positive non-saturating conventional MR observed in Figure 1g includes the contribution of not only the In/GeOx Schottky heterojunctions but also the GeOx/Ge substrate. Moreover, as shown in another reference sample (Figure S1), once the top electrode changes from Ohmic connection at high temperature into Schottky connection at low temperature, rectification MR appears accordingly. Hence, rectification MR is a direct and simple method for distinguishing interface MR and bulk MR.19,25 The simplicity of measurements is a great advantage of this rectification MR method, comparing to the surface-modification techniques that enable the qualitative distinction of bulk and interface spin scattering in the spin valve devices.26,27 It is relatively easy to distinguish the interface MR and bulk MR in the nonmagnetic heterojunctions, and here we extend this method to the ferromagnetic heterojunctions with spin-polarized transport. Figure 2 shows the transport properties of the as-prepared Co/GeOx/n-Ge magnetic Schottky heterojunctions measured at 6


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100K, where the electrical current passed through the ferromagnetic Co layer is spin-polarized and may induce spin accumulation at the Co/GeOx Schottky interface. In Figure 2a, the asymmetric I-V curves indicate the formation of Co/GeOx interfacial Schottky heterojunction and the existence of significant rectification as well. Moreover, a clear “butterfly” hysteresis loop is observed in the low magnetic field range of the conventional MR curve in Figure 2b, which was regarded as the feature of magnetic Schottky heterojunctions by other researchers.17,18, 28 According to previous reports, the appearance of the hysteresis transport behavior could be a reflection of spin dependent scattering due to spin accumulation/injection at the interfaces. However, the rectification MR curve in Figure 2c shows no sign of any hysteresis behavior, which is quite strange because rectification MR originates from the simultaneous implementation of the rectification and MR effect of the Schottky interface.19,25 Since the rectification MR is directly related to the Schottky interface, it is expected to show a hysteresis loop in the rectification MR curve if spin accumulation/injection indeed exists at the Schottky interface. So in our case it is certain that the low field“butterfly” hysteresis loop of the conventional MR curve cannot come from the Schottky interface. Figure 3a shows the magnetic hysteresis loop of the Co/GeOx/n-Ge Schottky heterojunctions with applied in-plane magnetic field. It is clear that the coercivity and saturation magnetization are consistent with the magnetic properties of bulk Co film. Figures 3b and 3c show the anisotropic MR of the Co sublayer measured at 100K by Van der Pauw configuration. During the measurements, the in-plane current is either parallel (longitudinal configuration) or perpendicular (transvers configuration) to the applied in-plane magnetic field. At low temperature, the interfacial potential of the Co/GeOx Schottky junction becomes lager, which blocks the current path from the Co 7



film to the substrate.29 As a result, the positive MR from Ge substrate is almost completely suppressed and Co layer dominates the detected MR (Figure S2). Remarkable anisotropic MR is observed in a low magnetic field range, i.e., negative MR is found in the transverse configuration, while positive MR is detected in the longitudinal configuration. It is clear that the coercivity fields in Figure 3b and 3c (~580 Oe) are consistent with that in the magnetic hysteresis loop in Figure 3a (~600 Oe). This anisotropic MR effect arises from the joint interaction of spin-orbit interaction and magnetization of Co layer. 30 , 31 Therefore, we can conclude by combining Figure 2 and Figure 3 that the low field “butterfly” hysteresis loop of the conventional MR curve is due to the anisotropic MR of ferromagnetic Co layer, rather than from the Co/GeOx Schottky interface.

Conclusions To summarize, the interface MR and bulk MR of non-magnetic In/GeOx/n-Ge and magnetic Co/GeOx/n-Ge diode-like heterojunctions have been successfully distinguished by rectification MR measurements. It is demonstrated that the low field “butterfly” hysteresis loop observed only in the conventional MR curve originates from the anisotropic MR of ferromagnetic bulk Co layer, while the orbit-related large non-saturating positive MR contains contributions from both the bulk Ge substrate and Schottky interface. Furthermore, this simple interface sensitive rectification MR measurements can be adapted to other heterojunctions, especially systems with pure resistance and rectifying components formed an in series circuit. Hopefully with the help of this rectification MR techniques, a deeper understanding of the fundamental properties of the interface can be achieved, which could advance the development of interfaced-based spintronics devices through both material selection and device 8


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structure design.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Transport properties of a reference In/GeOx/n-Ge heterojunctions measured at different temperature, Figure S1. Transverse and longitudinal magnetoresistance of Co/GeOx/n-Ge heterojunctions, Figure S2.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], *E-mail: [email protected], Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by the National Science Foundation of China (Grant Nos. 11434006 and 11774199), the National Basic Research Program of China (Grant No. 2015CB921502), the 111 Project B13029, and the Taishan Scholar Program of Shandong Province.

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Figure 1. (a) The equivalent circuit of In/GeOx/n-Ge non-magnetic heterojunctions with Ohmic contacts. R1, R2 and R3 are respectively the resistance of In layer, In/GeOx interface, and GeOx/n-Ge substrate. (b)-(d) The I-V curves, conventional MR, and rectification MR of In/GeOx/n-Ge non-magnetic heterojunctions with Ohmic contacts measured at 100 K. (e)-(h) are the corresponding results of an independent In/GeOx/n-Ge heterojunctions with In/GeOx Schottky contact. Inset of (f) shows the schematic experimental configuration.

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Figure 2. (a), (b) and (c) respectively show the I-V curves under different magnetic field, conventional MR and rectification MR of Co/GeOx/n-Ge magnetic diode-like heterojunctions measured at 100K. Insets are the enlarged view of low field MR data.

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Figure 3. (a) Magnetic hysteresis loops of the Co/GeOx/n-Ge magnetic heterojunctions measured at 100K. (b) The transverse MR (I⊥H) of Co/GeOx/n-Ge, where the current is perpendicular to the in-plane magnetic field. (c) The longitudinal MR (I∥H) of Co/GeOx/n-Ge, where the current is parallel to the in-plane magnetic field.

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ToC Figure

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Distinguishing Interface Magnetoresistance and Bulk

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