Anomalous Magnetothermopower in a Metallic Frustrated Antiferromagnet

Abstract

We report the temperature T and magnetic field H dependence of the thermopower S of an itinerant triangular antiferromagnet PdCrO 2 in high magnetic fields up to 32 T. In the paramagnetic phase, the zerofield thermopower is positive with a value typical of good metals with a high carrier density. In marked contrast to typical metals, however, S decreases rapidly with increasing magnetic field, approaching zero at the maximum field scale for T > 70 K. We argue here that this profound change in the thermoelectric response derives from the strong interaction of the 4d correlated electrons of the Pd ions with the shortrange spin correlations of the Cr 3þ spins that persist beyond the Néel ordering temperature due to the combined effects of geometrical frustration and low dimensionality. DOI: 10.1103/PhysRevLett.116.087202 The interplay between itinerant electrons and even simple magnetic structures can lead to spectacular effects, the giant magnetoresistance seen in magnetic multilayers being arguably the most prominent example [1]. In geometrically frustrated magnets, complex spin textures that couple to the conduction electrons create an altogether different landscape, where short-range correlations are expected to play a major role. Moreover, since magnetism in metals can be destabilized much more readily than in insulators, magnetic frustration in metallic systems offers a rich playground to search for the emergence of novel transport phenomena. Notable recent examples include the unconventional anomalous Hall effect (AHE) observed in magnetic pyrochlores [2,3] and the suppression of thermopower in a longitudinal magnetic field in the layered Curie-Weiss metal Na x CoO 2 [4].Despite their obvious potential for new physics, metallic frustrated magnets have been noticeably less studied than their insulating counterparts, largely due to the fact that such materials are rare. Of particular interest are materials in which the conduction electrons and magnetic moments arise from different subsystems. In this context, the quasi-two-dimensional (quasi-2D) antiferromagnet PdCrO 2 [4-6] is somewhat unique. PdCrO 2 has a delafossite crystal structure with layers of Pd ions arranged in a triangular lattice stacked between magnetic edgesharing CrO 6 octahedra. The latter contains Cr 3þ ions with localized (Mott insulating) 3=2 spins which order in the 120°antiferromagnetic (AFM) structure below T N ¼ 37.5 K [7,8]. The frustration parameter f, defined as an absolute ratio of the Weiss temperature Θ W and the ordering temperature T N , is around 13 for PdCrO 2 , indicating a high level of frustration [7][8][9]. According to band structure calculations [10], angle-resolved photoemission [10], and quantum oscillation (QO) studies [11,12],

Solution‐Processable Epitaxial Metallic Delafossite Oxide Films

“…[ 2,15–17 ] Moreover, as the isostructural compounds, PtCoO 2 has the lowest room temperature resistivity amongst the oxide system known, [ 1 ] and PdCrO 2 shows strong coupling between the antiferromagnetism and the conduction electrons. [ 18,19 ] Growing PtCoO 2 and PdCrO 2 thin films will expand the fundamental investigations and further potential applications of metallic delafossite oxides. In fact, thin films of PtCoO 2 and PdCrO 2 have been fabricated by rf‐sputtering about 40 years ago, [ 20 ] but the room temperature conductivity is around one order of magnitude inferior than recent reported single‐crystals.…”

Simultaneous loss of interlayer coherence and long-range magnetism in quasi-two-dimensional PdCrO2

“…1a, PdCrO 2 has a delafossite structure, consisting of stacked layers of highly conducting Pd layers sandwiched between planes of Mott-insulating CrO 2 (refs 7, 8). The Cr ions have a localized spin of 3/2 and are highly frustrated, forming a non-coplanar and non-collinear antiferromagnetic 120° helical structure at T N =37.5 K (refs 7, 9, 10, 11, 12). Quantum oscillation813 and angle-resolved photoemission studies14 have mapped out the Fermi surface (FS) of PdCrO 2 .…”

“…A recent magnetothermopower study also revealed a strong interaction between the 4 d 9 electrons and the short-range spin correlations persisting well above T N (ref. 10). …”

Unconventional aspects of electronic transport in delafossite oxides

“…Older polycrystalline samples [25] may have been further from stoichiometry. There are also discrepancies between results on PdCrO 2 single crystals [58,61]. so far not been possible to measure the out-of-plane Seebeck coefficient, however the measured in-plane values are in good agreement with the calculations [50].…”

“…However, Ref. [61] reports a Seebeck coefficient with a rather different temperature dependence. It also shows a strong suppression under intense magnetic fields, which is not seen in the moderate applied fields used in Ref.…”