Effective reduction of ${\mathrm{PdCoO}}_{2}$ thin films via hydrogenation and sign tunable anomalous Hall effect

Letter

Effective reduction of ${PdCoO}_{2}$ thin films via hydrogenation and sign tunable anomalous Hall effect

Gaurab Rimal, Caleb Schmidt, Hussein Hijazi, Leonard C. Feldman, Yiting Liu, Elizabeth Skoropata, Jason Lapano, Matthew Brahlek, Debangshu Mukherjee, Raymond R. Unocic, Matthew F. Chisholm, Yifei Sun, Haoming Yu, Shriram Ramanathan, Cheng-Jun Sun, Hua Zhou, and Seongshik Oh

Phys. Rev. Materials 5, L052001 – Published 14 May 2021

Abstract

${PdCoO}_{2}$ , belonging to a family of triangular oxides called delafossite, is one of the most conducting oxides. Its in-plane conductivity is comparable to those of the best metals and exhibits hydrodynamic electronic transport with an extremely long mean free path at cryogenic temperatures. Nonetheless, it is nonmagnetic despite the presence of the cobalt ion. Here, we show that a mild hydrogenation process reduces ${PdCoO}_{2}$ thin films to an atomically mixed alloy of PdCo with strong out-of-plane ferromagnetism and sign-tunable anomalous Hall effect. Considering that many other compounds remain little affected under a similar hydrogenation condition, this discovery may provide a route to creating novel spintronic heterostructures combining strong ferromagnetism, involving oxides and other functional materials.

Received 1 February 2021
Revised 4 April 2021
Accepted 27 April 2021

DOI:https://doi.org/10.1103/PhysRevMaterials.5.L052001

Physics Subject Headings (PhySH)

Anomalous Hall effect Ferromagnetism Magnetic anisotropy

Hydrogenated materials Thin films Transition metal oxides

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Gaurab Rimal ^*, Caleb Schmidt, Hussein Hijazi, and Leonard C. Feldman

Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA

Yiting Liu

Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA and State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China

Elizabeth Skoropata, Jason Lapano, and Matthew Brahlek

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

Debangshu Mukherjee, Raymond R. Unocic, and Matthew F. Chisholm

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

Yifei Sun, Haoming Yu , and Shriram Ramanathan

School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, USA

Cheng-Jun Sun and Hua Zhou

X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, USA

Seongshik Oh^†

Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA and Center for Quantum Materials Synthesis, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA

^*gr380@physics.rutgers.edu
^†ohsean@physics.rutgers.edu

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Vol. 5, Iss. 5 — May 2021

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Figure 1
Structure and room-temperature resistivity of ${PdCoO}_{2}$ films. (a) Annular dark-field scanning transmission electron microscopy image for a ${PdCoO}_{2}$ thin film with a zoomed region showing the atomic layering. Also overlaid is the unit cell aligned to proper atomic sites. (b) Comparison of room-temperature resistivity for thin films of various metals (open markers) and ${PdCoO}_{2}$ . The solid horizontal lines show the bulk value for the labeled oxides, taken from Refs. [9, 29]. For metals, the resistivity values for Au were taken from Ref. [30], the resistivity values for Cu and Ag were taken from Ref. [31], and the resistivity values for Al were taken from Ref. [32].
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Figure 2
Structural comparison of pristine and hydrogenated ${PdCoO}_{2}$ films. XRD for 9-nm-thick ${PdCoO}_{2}$ (a) before and (b) after hydrogenation. (c) RBS spectra for 100-nm thick films. The elements corresponding to features in spectra are marked. The inset shows the oxygen region corresponding to backscattering contributions from the film and the substrate. The shift in edges of the backscattering features for various elements as indicated by the arrows is due to the reduction of film thickness. The approximate overlap of the curves for the 5- and 15-h annealed curves indicates that the reduction process reached its limit of negligible oxygen content (see also Supplemental Material Fig. S1 [14]). (d) STEM image for a pristine ${PdCoO}_{2}$ film, showing atomic layering. (e) STEM for a 9-nm-thick film hydrogenated at $200^{\circ} C$ for 30 min. The lack of contrast between atoms and layers is due to intermixing of Pd and Co. (f) Schematic of ${PdCoO}_{2}$ and (g) hydrogenated ${PdCoO}_{2}$ .
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Figure 3
XAS comparisons between pristine ${PdCoO}_{2}$ , Pd, and hydrogenated ${PdCoO}_{2}$ with $T_{A} = 400^{\circ} C$ and $t_{A} = 30 \min$ . (a) Pd $K$ -edge XAS spectra in the near-edge region. The main edge for hydrogenated sample is redshifted from the pristine ${PdCoO}_{2}$ and overlaps with Pd metal. (b) PdCo model for fitting EXAFS employing the $R \bar{3} m$ structure. $d_{1}, d_{2}$ , and $d_{3}$ are the nearest- (Co), second-nearest- (Pd), and third-nearest- (Co) atomic distances to the absorbing Pd site. (c) $k^{2}$ weighted EXAFS showing the fine structure of interference due to nearest-neighbor scattering. (d) Magnitude of radial distance dependence of EXAFS. The best fit for hydrogenated ${PdCoO}_{2}$ at $400^{\circ} C$ using the model shown in (b) is also shown as the dashed line.
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Figure 4
Magnetic and transport properties of 9-nm-thick hydrogenated ${PdCoO}_{2}$ films. (a) Room-temperature magnetic hysteresis comparing the IP and OP directions. (b) AHE curves at room temperature and low temperature showing the sharp transition corresponding to strong OP anisotropy. (c) Temperature dependence of resistivity for various anneal durations ( $t_{A}$ ) at $T_{A} = 200^{\circ} C$ . (d) Temperature dependence of resistivity for various anneal temperatures ( $T_{A}$ ) with $t_{A} = 30 \min$ .
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Figure 5
AHE for hydrogenated films with various anneal parameters. (a)–(d) AHE for 9-nm-thick films at 2 K for $T_{A} = 200^{\circ} C$ for various anneal durations. (e)–(h) AHE for 9-nm-thick films at 295 K for $T_{A} = 200^{\circ} C$ for various anneal durations. (i)–(l) AHE for films annealed at various temperatures for $t_{A} = 30 \min$ .
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Physical Review Materials

Effective reduction of PdCoO2 thin films via hydrogenation and sign tunable anomalous Hall effect

Gaurab Rimal, Caleb Schmidt, Hussein Hijazi, Leonard C. Feldman, Yiting Liu, Elizabeth Skoropata, Jason Lapano, Matthew Brahlek, Debangshu Mukherjee, Raymond R. Unocic, Matthew F. Chisholm, Yifei Sun, Haoming Yu, Shriram Ramanathan, Cheng-Jun Sun, Hua Zhou, and Seongshik Oh

Phys. Rev. Materials 5, L052001 – Published 14 May 2021

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Effective reduction of ${PdCoO}_{2}$ thin films via hydrogenation and sign tunable anomalous Hall effect