Observation of a Charge-Neutral Muon-Polaron Complex in Antiferromagnetic Cr2O3

Abstract

We report a comprehensive muon spin rotation (µSR) study of the prototypical magnetoelectric antiferromagnet Cr2O3. We find the positively charged muon (µ + ) occupies several distinct interstitial sites, and displays a rich dynamic behavior involving local hopping, thermally activated site transitions and the formation of a charge-neutral complex composed of a muon and an electron polaron. The discovery of such a complex has implications for the interpretation of µSR spectra in a wide range of magnetic oxides, and opens a route to study the dopant characteristics of interstitial hydrogen impurities in such materials. We address implications arising from implanting a µ + into a linear magnetoelectric, and discuss the challenges of observing a local magnetoelectric effect generated by the charge of the muon.

Local Electronic Structure and Dynamics of Muon-Polaron Complexes in Fe2O3

“…This was also observed in isomorphic Cr 2 O 3 [31] and has been proposed for Fe 2 O 3 [34][35][36]. In Cr 2 O 3 , the magnetic structure (↑↓↑↓) breaks the inversion symmetry (B(r) = −B(−r)) such that sufficiently fast local hopping leads to a complete cancellation of the internal field and subsequent loss of the oscillatory signal, while in Fe 2 O 3 (↓↑↑↓), B(r) = B(−r), and fast hopping only causes a cancellation of the radial in-plane component (⊥ĉ) [34], resulting in the drop in f 1 and the peak in λ 1 around 80 K. Our simulation of the muon polarization function [31,42] based on (1) a simple parametrization L(T ) of the T -dependence of the order parameter [43] [Fig. 1 (a), gray dashed line] and (2) assuming local hopping between adjacent sites with Arrhenius-like activation (using an activation energy E 1 = 55 meV, prefactor A 1 = 2 × 10 12 Hz and angle θ 1 = ∠(B 1 ,ĉ) = 6.1°) yields good agreement (solid red lines) with the step in f 1 and shape, position and magnitude of the peak in λ 1 .…”

“…Note that the energies of C 0 1 and C 0 2 are very close [Table I], en-abling the back-and -forth transitions postulated above, with the energy difference matching closely ∆E 2↔3 . In sharp contrast, the different complex configurations in Cr 2 O 3 are well separated in energy [31], and neither unusual dynamics, nor frequencies deviating from the order parameter are observed. The difference in energy separation (and consequently, dynamic behavior) is attributed to the strength of the polaron-induced Jahn-Teller (JT) distortion [55]; Cr 2+ with 3d 4 (high spin) is strongly JTactive, whereas Fe 2+ with 3d 6 (high spin) is only weakly JT-active.…”

“…In µSR studies of magnetic materials, usually only the positive charge state is considered. However, it is clear that in Fe 2 O 3 , as in Cr 2 O 3 [31], the single C + site can not explain the data and other charge states have to be taken into account. The neutral C 0 1 -C 0 4 can be characterized as muon-polaron complexes: the bound electron predominantly localizes on a nearby Fe ion to form a small polaron, occupying an empty minority spin t 2g orbital and changing the Fe valence from Fe 3+ (3d 5 ) to Fe 2+ (3d 6 ) [Fig.…”

“…Recent µSR studies reported polaronic Mu centers in non-magnetic TMOs such as SrTiO 3 and TiO 2 [28][29][30], in which an oxygen-bound, positive muon and a small polaron located on a neighboring TM ion form an overall charge-neutral complex, suggesting that isolated H defects form analogous H-polaron centers. Recently, muon-polaron complexes have been shown to exist in the antiferromagnet Cr 2 O 3 [31]. In a crucial distinction to non-magnetic TMOs however, the excess electron spin strongly couples to the unpaired d electrons of the Cr host, resulting in a µSR signal that is difficult to distinguish from the usual signal of the positive charge state (i.e.…”

Bulk-like dielectric and magnetic properties of sub 100 nm thick single crystal Cr2O3 films on an epitaxial oxide electrode

“…It is one of the few single-phase materials that demonstrates an uncompensated surface magnetization that is switchable by its intrinsic magnetoelectric effect at room-temperature 3,8 . This materials configuration opens a diverse set of ways to create energy efficient spintronic devices [8][9][10][11][12][13][14][15] .…”