Regensburg 2013 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 28: Correlated Electrons: Spin Systems, Itinerant Magnets 3
TT 28.2: Vortrag
Dienstag, 12. März 2013, 09:45–10:00, H19
Spin-charge-lattice coupling in the semimetallic ferromagnet EuB6 — •F. Schnelle1, R. S. Manna1, P. Das1, J. Müller1, Z. Fisk2, and M. Lang1 — 1Institute of Physics, Goethe-University Frankfurt, 60438 Frankfurt (M), Germany — 2Department of Physics, University of California, Irvine, California 92697, USA
The semimetallic EuB6 exhibits a complex sequence of electronic and magnetic phase transitions at ∼15.5 K (Tc1) and ∼12.5 K (Tc2). In the paramagnetic regime below ∼35 K, magnetic polarons (MPs) are expected to form since it is energetically favorable for the charge carriers to localize and thereby spin polarize the local Eu2+ moments over a finite distance. Thermal expansion measurements show pronounced lattice effects at the phase transition temperatures, the one occurring at Tc2 being much larger than that at Tc1. By applying a small magnetic field of less than 50 mT, the anomaly at Tc1 is fully suppressed, while the one at Tc2 shifts to higher temperature and broadens as the field is increased and finally fades out at a field B > 5 T. The complementary magnetostriction measurements for a set of temperatures from below Tc2 to above Tc1 highlight the extraordinarily large magnetoelastic effects in this material. This supports the notion that charge localization, due to the formation of isolated MPs at temperatures corresponding to the paramagnetic regime, gives rise to lattice expansion. By cooling or applying external magnetic fields, the MPs merge and percolation sets in, resulting in lattice contraction. We discuss the relation of this mechanism to the observed large negative magnetoresistance in this material.