Thickness-dependent angular dependent magnetoresistance in single-crystalline Co film and Co/Pt heterostructures

doi:10.1016/j.jmmm.2020.166863

Journal of Magnetism and Magnetic Materials

Volume 508, 15 August 2020, 166863

https://doi.org/10.1016/j.jmmm.2020.166863 Get rights and content

Highlights

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The zero anisotropic magnetoresistance was discovered for the 4.4 nm Co film.
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Transport properties of the Co layer in Co/Pt are influenced by the Pt underlayer.
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The spin Hall magnetoresistance was systematically studied in the Pt/Co bilayer.

Abstract

We report an unusual angular dependent magnetoresistance (ADMR) in single-crystalline Co films grown on Al₂O₃(0 0 0 1). The Co film with a thickness of 4.4 nm shows nearly zero anisotropic magnetoresistance for arbitrary magnetization orientations, and the sign of the ADMR reverses with changing Co thickness. By systematically measuring the Co-thickness-dependent anisotropic conductance in single-crystal Co/Pt, Pt/Co, and Pt/Co/Pt, we reveal that the ADMR of the Co layer grown on the Pt underlayer is significantly different from that of the Co layer grown on Al₂O₃(0 0 0 1). Our results suggest that the transport properties of the Co layer as a function of Pt thickness must be considered in order to better understand the spin Hall magnetoresistance in the Co/Pt bilayer.

Introduction

Magnetoresistance (MR) is a fundamental spin-dependent transport property that is essential to understand electron transport properties in magnetic materials and the associated technological applications. Besides conventional anisotropic magnetoresistance (AMR) in ferromagnetic (FM) materials stemming from anisotropic s-d scattering and the spin-orbit coupling effect [1], spin Hall magnetoresistance (SMR) in a heavy metal (HM) layer in contact with a ferromagnetic insulator (FMI) has recently attracted great attention [2], [3]. The SMR originates from concerted actions of the conversion from charge current to spin current by spin Hall effect and the conversion from the backflow transverse spin current to the charge current by the inverse spin Hall effect [2], [3]. The HM/ferromagnetic metal (FMM) system may contain richer transport phenomena, because both charge current and spin current can pass through the HM and FMM layers, and many new types of MR phenomena in HM/FMM heterostructures, such as SMR [4], Rashba-Edelstein MR [5], hybrid MR [6], [7], and anisotropic interface magnetoresistance (AIMR) [8], [9] have been reported. Thus, understanding the origin of MR effects in the HM/FMM system will certainly contribute to spintronics from both fundamental research and application points of view.

Recently, MR in Pt/Co heterostructures has been intensively studied, revealing many interesting spin-dependent transport phenomena such as spin-orbit torque switching [10], unidirectional MR [11], and SMR [12], [13]. The angular dependent magnetoresistance (ADMR) has also been investigated in Co/Pt [12], [13] and Pt/Co/Pt [8], [9] systems. ADMR that decreases with increasing Co layer thickness has been reported in Pt/Co/Pt trilayers, attributed to the anisotropic diffusive scattering probability of the electrons at the Pt/Co interface [8], [9]. In Pt/Co/Pt, ADMR shows an unusual behavior of ${R_{‖} > R}_{P} > R_{⊥}$ , where $R_{‖}$ ( $R_{⊥}$ ) is the longitudinal resistance with the in-plane magnetization M parallel (perpendicular) to the current, and $R_{P}$ is the resistance with M normal to the film surface. However, it is not clear whether the top and bottom Pt layers in Pt/Co/Pt contribute equally to its ADMR. For single-crystal Co/Pt/MgO(1 1 1), Xiao et al. [12] reported the unusual ADMR behavior of $R_{P} > R_{‖}$ , which is not present in other FM systems [6], [14], [15]. So, the ADMR properties in Co/Pt and Pt/Co/Pt seems to be different with the SMR in FM/FMI systems, which is expected to have the relation of $R_{‖} \approx R_{P} > R_{⊥}$ [2], [3]. Moreover, a recent study on SMR in Co/Pt [13] shows an unexpected large effective spin Hall angle of up to 0.35, which also increases with the Co thickness. Thus, to understand SMR in Co/Pt heterostructures, the ADMR of the Co layer must be carefully studied. Further investigations of ADMR in HM/FMM systems will certainly contribute to our understanding of spin-dependent transport and spintronics.

In this paper, we first investigate ADMR in a single-crystal Co film grown on Al₂O₃(0 0 0 1) at room temperature (RT). An unconventional ADMR behavior of $R_{P} > R_{⊥} > R_{‖}$ appears for Co thicknesses of d_Co < 4.4 nm, and the ADMR for thicker Co films shows the ordinary $R_{P} < R_{⊥} < R_{‖}$ relation. The Co film with d_Co ~ 4.4 nm shows a nearly vanishing ADMR with the $R_{P} ~ R_{‖} ~ R_{⊥}$ relation. By analyzing Co-thickness-dependent anisotropic conductance in Co, Co/Pt, Pt/Co, and Pt/Co/Pt, we find that the spin-dependent transport properties of the Co layer are significantly different for a Co layer grown on the substrate and that grown on the Pt underlayer. To clarify the origin of the unusual ADMR properties in Co/Pt and Pt/Co/Pt, the intrinsic transport properties of the Co layer should be carefully investigated.

Section snippets

Experimental details

We prepared single-crystal Co, Co/Pt, Pt/Co, and Pt/Co/Pt films by molecular beam epitaxy in an ultrahigh vacuum (UHV) system with a base pressure of 2 × 10⁻¹⁰ Torr [16]. The Al₂O₃(0 0 0 1) single-crystal substrates were prepared by annealing at 750 °C for one hour in the UHV system. The Co films were deposited at RT by thermal evaporation, while the Pt layers were grown by pulsed laser deposition at RT [12]. The film thickness was determined from the growth rate, measured using a calibrated

Results and discussion

We first performed thickness-dependent ADMR measurements on single-layer Co films. Here, as shown in Fig. 2(a), we define the anisotropic resistances as $Δ R_{xy} = R_{‖} - R_{⊥}$ in the $α_{xy}$ scan, $Δ R_{yz} = R_{P} - R_{⊥}$ in the $β_{yz}$ scan, and $Δ R_{xz} = R_{‖} - R_{P}$ in the $γ_{xz}$ scan. Fig. 2(b) shows the d_Co-dependent MR ratios $Δ R / R$ in the $α_{xy}$ , $β_{yz}$ , and $γ_{xz}$ scans, indicating that the single-layer Co exhibits interesting thickness-dependent ADMR properties. In our study, all the MR ratios $Δ R / R$ are calculated by $Δ R$ divided by the average

Conclusions

In summary, we report the unusual sign reversal of ADMR as a function of film thickness in a Co layer epitaxially deposited on Al₂O₃(0 0 0 1), with zero AMR at a Co thickness of 4.4 nm. Based on anisotropic conductance analysis, we reveal that the Co layers grown on the Pt underlayer or directly on the Al₂O₃ substrate show very different ADMR. Thus, the transport properties of the Co layer in the Co/Pt bilayer grown on certain substrates could gradually change with the Pt underlayer thickness.

CRediT authorship contribution statement

Mengwen Jia: Investigation, Resources, Data curation, Writing - original draft. Fanlong Zeng: Investigation. Xia Xiao: Investigation. Chao Zhou: Investigation. Xiaofeng Hu: Investigation. Yizheng Wu: Supervision, Writing - review & editing, Funding acquisition.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (Grant No. 2016YFA0300703), the National Natural Science Foundation of China (Grants Nos. 11734006, 11974079, 11434003, 11604132 and 61574071), and the Program of Shanghai Academic Research Leader (No. 17XD1400400).

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Research articlesThickness-dependent angular dependent magnetoresistance in single-crystalline Co film and Co/Pt heterostructures

Highlights

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusions

CRediT authorship contribution statement

Acknowledgements

Surf. Sci.

Surf. Sci.

IEEE Trans. Magn.

Phys. Rev. Lett.

Phys. Rev. B

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev. B

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev. B

Nature

Nat. Phys.

Appl. Phys. Lett.

Appl. Phys. Lett.

Phys. Rev. B

Phys. Rev. B

Research articles
Thickness-dependent angular dependent magnetoresistance in single-crystalline Co film and Co/Pt heterostructures