SUPPLEMENTAL SECTION

Electrostatic doping as a source for robust ferromagnetism at the interface between antiferromagnetic cobalt oxides

Zi-An Li,1 N. Fontaíña-Troitiño,2 A. Kovács,3 S. Liébana-Viñas,1,2 M. Spasova,1

R. E. Dunin-Borkowski,3 M. Müller,4 D. Doennig,4 R. Pentcheva,1,4 M. Farle1 and V. Salgueiriño2

1Faculty of Physics and Center for Nanointegration (CENIDE),
University Duisburg-Essen 48047, Duisburg (Germany)

2Departamento de Física Aplicada, Universidade de Vigo 36310, Vigo (Spain)

3Ernst Ruska-Centre and Peter Grünberg Institute, Research Centre Jülich, 52425 Jülich, (Germany)

4Department of Earth and Environmental Sciences, Section Crystallography, LMU Munich,
Theresienstr. 41, 80333 Munich (Germany)

Table 1 Magnetic properties of Sample S-75nm

Temperature
[K] / Ms+
[Am2/kg] / Ms-
[Am2/kg] / Mr+
[Am2/kg] / Mr-
[Am2/kg] / µ0Hc+
[mT] / µ0Hc-
[mT]
ZFC / 5 / 1.750 / -1.759 / 0.313 / -0.329 / 50.5 / -57.7
100 / 1.577 / -1.567 / 0.346 / -0.364 / 57.0 / -57.4
300 / 1.580 / -1.573 / 0.261 / -0.261 / 35.7 / -35.4
FC / 5 / 1.796 / -1.720 / 0.517 / -0.339 / 50.7 / -84.7
100 / 1.603 / -1.535 / 0.422 / -0.320 / 49.4 / -69.0
300 / 1.565 / -1.565 / 0.262 / -0.262 / 35.6 / -35.7

Table2 Magnetic properties of Sample S-40nm

Temperature
[K] / Ms+
[Am2/kg] / Ms-
[Am2/kg] / Mr+
[Am2/kg] / Mr-
[Am2/kg] / µ0Hc+
[mT] / µ0Hc-
[mT]
ZFC / 5 / 2.826 / -2.829 / 0.451 / -0.456 / 42.4 / -42.7
100 / 2.591 / -2.585 / 0.421 / -0.424 / 42.9 / -42.6
300 / 2.601 / -2.597 / 0.327 / -0.328 / 28.7 / -28.7
FC / 5 / 2.849 / -2.800 / 0.574 / -0.443 / 40.6 / -57.2
100 / 2.622 / -2.587 / 0.448 / -0.406 / 40.3 / -44.9
300 / 2.580 / -2.583 / 0.326 / -0.326 / 28.5 / -28.4

Figure S3:
STEM-EELS mapping of the CoO/Co3O4 interface of a single cobalt oxide octahedron.

a, STEM-EELS spectrum image (SI) acquired with 22x16 pixels, at 80 kV using a probe size of 1 nm and magnified area near the crystal surface.

b, Background-subtracted typical EEL spectra at the O K-edge and Co L edges obtained from the core (A) and shell (B) regions indicated in a. The peaks a, b, and c in the O- K edge spectrum are commonly used as fingerprint for identifying the type of Co oxide. In the shell region B a high intensity of peak a indicates a Co3O4 phase. In the core region (A) peak “a” is much lower, that is typical for CoO.

c, Map of the ratio of the L edge intensities Co I(L3)/I(L2) determined from B. A ratio of 4.5 (red) and 2.4 (blue) is typical for Co in CoO and Co3O4. These values were adapted from J. Appl. Phys. Vol 108, 063704 by Y. Zhao et al.

Figure S2 | Temperature dependence of coercivity HC and exchange bias field (HEB) of a, Sample S-75nm. b, Sample S-40nm. The applied cooling field is 2 Tesla. The HEB is related to the presence of a ferromagnetic (FM)-antiferromagnetic (AFM) interface and vanishes above the Néel temperature (TN = 291 K) of the AFM. The non-vanishing coercivity reveals the presence of strong ferromagnetism.

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