Kaveh Ahadi1 Susanne Stemmer1

1, University of California, Santa Barbara, Santa Barbara, California, United States

Topology, both in real and momentum space, is the source of some of the most interesting phenomena in condensed matter physics. For example, chiral spin textures can give rise to the topological Hall effect [1]. The underlying interactions giving rise to chiral spin textures and skyrmions are however less well understood. For example, Dzyaloshinskii-Moriya (DM) interaction may not be needed as helimagnetic phases are also reported in centrosymmetric structures. As will be shown here, electron doped EuTiO3 is an interesting material to study the influence of topology on a wide range of transport phenomena. Stoichiometric EuTiO3 is a quantum paraelectric with cubic perovskite structure at room temperature, similar to SrTiO3. Spins on the Eu site [4f7 (s=7/2)] order antiferromagnetically at the Neel temperature of ~5.5 K.
Here, we investigate high quality, single crystal EuTiO3 thin films grown by hybrid molecular beam epitaxy. We report the results of low temperature magnetotransport and magnetization measurements. While the stoichiometric EuTiO3 films are highly resistive, Sm-doped samples (nRT=1.2, 3.4, 6.5 and 8.7×1020 cm-3) show metallic behavior [2]. The topological Hall effect (THE) is observed in all samples, regardless of the carrier concertation. Furthermore, THE peak experiences a sign change with carrier concentration. The sign change is concurrent with a drastic change in the magnetic properties of doped EuTiO3 thin films. Dilute samples show a carrier-controlled itinerant metamagnetism with a very narrow magnetic hysteresis. Such metamagnetic transitions can be the source of very interesting phenomena in condensed matter physics including quantum critical fluctuations [3]. Here, the results are not conclusive whether there is a cross-over, or a first-order phase transition accompanied by a low temperature critical point. However, the resistivity data from 2 to 5 K under various magnetic fields (0-9 T) using the general expression R(T)=R0+ATn for transport, shows evidence of “n” dropping and “A” increasing. The temperature coefficient is directly proportional to carrier mass and an increase in “A” hints a carrier mass enhancement.

[1] N. Nagaosa and Y. Tokura, Nat. Nanotechnol. 8, 899 (2013).
[2] K. Ahadi, L. Galletti, and S. Stemmer, Appl. Phys. Lett. 111, 172403 (2017).
[3] S. A. Grigera, R. S. Perry, A. J. Schofield, M. Chiao, S. R. Julian, G. G. Lonzarich, S. I. Ikeda, Y. Maeno, A. J. Millis, and A. P. Mackenzie, Science 294, 329 (2001).