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.2018 May 25;9(1):2058.
doi: 10.1038/s41467-018-04512-1.

Demonstration of Ru as the 4th ferromagnetic element at room temperature

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Demonstration of Ru as the 4th ferromagnetic element at room temperature

P Quarterman et al. Nat Commun..

Abstract

Development of novel magnetic materials is of interest for fundamental studies and applications such as spintronics, permanent magnetics, and sensors. We report on the first experimental realization of single element ferromagnetism, since Fe, Co, and Ni, in metastable tetragonal Ru, which has been predicted. Body-centered tetragonal Ru phase is realized by use of strain via seed layer engineering. X-ray diffraction and electron microscopy confirm the epitaxial mechanism to obtain tetragonal phase Ru. We observed a saturation magnetization of 148 and 160 emu cm-3 at room temperature and 10 K, respectively. Control samples ensure the ferromagnetism we report on is from tetragonal Ru and not from magnetic contamination. The effect of thickness on the magnetic properties is also studied, and it is observed that increasing thickness results in strain relaxation, and thus diluting the magnetization. Anomalous Hall measurements are used to confirm its ferromagnetic behavior.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1
Epitaxial relation for tetragonal Ru growth. The expected crystallographic structure and epitaxial relation is shown usinga a cartoon showing the epitaxial growth relation for (110) Al2O3//(110) Mo//011¯Ru, andb X-ray diffraction using grazing incidence, by placing the detector in the (110) plane of Al2O3 and rotating the sample over 360°.c Conventional XRD spectra with the orientation parallel to {110} Al2O3, andd X-ray reflectivity for 2.5, 6, and 12 nm Ru films; with the inset ofd showing the grazing incidence coupled scan when aligned to the short (black) and long (blue) edge of (110) Mo
Fig. 2
Fig. 2
Characterization of Ru tetragonal structure. Cross-section STEM images along the [001] zone axis of Al2O3.a The annular bright field (ABF) STEM images of the full sample stack. Scale bar is 10 nm.b High-angle annular dark field (HAADF) STEM at the Mo–Ru interface, with the inset showing the high-precision HADDF STEM for the [111] Ru zone axis using the non-rigid registration method. The lattice distortion is shown using the dashed black line. Scale bar is 2 nm.c A HAAADF STEM zoom in on a Ru grain boundary due to equivalent surfaces growth surfaces. Scale bar is 1 nm
Fig. 3
Fig. 3
Determination of Ru lattice parameters.a FFT of the 6 nm Ru film along the [111] zone axis, from indicated region of Fig. 2b, with the expected BCT grouping highlighted in yellow, and distortions in the tetragonal ordering are highlighted in red. Scale bar is 5 nm−1.b The expected [111] zone axis projection for BCT Ru, and by using the measured interatomic spacing and atomic plane angles, an estimate ofc/a for a BCT structure can be calculated. The dashed red line shows the orientation of the lattice distortion
Fig. 4
Fig. 4
Observation of ferromagnetism. Magnetization vs. field hysteresis curves with an in-plane field orientation for 2.5, 6, and 12 nm of Ru ata 10 K andb room temperature, with the inset for each showing a zoom in near the coercive field region
Fig. 5
Fig. 5
Hall resistance confirmation of ferromagnetism. Hall resistance (RHall) vs.H (applied perpendicular to surface) for textured (blue) and non-textured (black) Mo/Ru films. The substrate/Mo/Ru sample, which has no crystallographic texture, shows only the ordinary Hall effect, but the Ru sample with BCT texture shows the anomalous Hall effect in addition to the ordinary Hall effect. Given that the Ru samples do not have a perpendicular easy axis, the resistance will change once the field is strong enough to saturate the demagnetization field of 4πMs, corresponding to aMs of ~318 emu cm−3. The saturated regions are designated by black arrows
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