Antiferromagnetic order and domains in Sr3Ir2O7probed by x-ray resonant scattering

Abstract

We report on a detailed x-ray resonant scattering study of the bilayer iridate compound Sr 3 Ir 2 O 7 at the Ir L 2 and L 3 edges. Resonant scattering at the Ir L 3 edge has been used to determine that Sr 3 Ir 2 O 7 is a long-range ordered antiferromagnet below T N ≈ 230 K with an ordering wave vector q = ( 1 2 , 1 2 ,0). The energy resonance at the L 3 edge was found to be a factor of ∼30 times larger than that at the L 2 edge. This remarkable effect has been seen in the single-layer compound Sr 2 IrO 4 and has been linked to the observation of a J eff = 1 2 spin-orbit insulator. Our result shows that despite the modified electronic structure of the bilayer compound, caused by the larger bandwidth, the effect of strong spin-orbit coupling on the resonant magnetic scattering persists. Using the program SARAh, we have determined that the magnetic order consists of two domains with propagation vectors k 1 = ( 1 2 , 1 2 ,0) and k 2 = ( 1 2 , − 1 2 ,0), respectively. A raster measurement of a focused x-ray beam across the surface of the sample yielded images of domains of the order of 100 μm, with odd and even L components, respectively. Fully relativistic, monoelectronic calculations using the Green's function technique for a muffin-tin potential have been employed to calculate the relative intensities of the L 2,3 edge resonances, comparing the effects of including spin-orbit coupling and the Hubbard U term. A large L 3 to L 2 edge intensity ratio (∼5) was found for calculations including spin-orbit coupling. Adding the Hubbard U term had no significant effect on the calculated spectra.

Crystal growth and intrinsic magnetic behaviour of Sr2IrO4

“…For example, the sizable c-axis component of the uniform magnetic susceptibility is not understood and seems incompatible with the known magnetic structure of Sr 2 IrO 4 . In addition, the diamagnetism initially reported for Sr 3 Ir 2 O 7 is completely unexplained and is absent in samples grown under different conditions [27][28][29] . Therefore, we believe it is imperative to establish the intrinsic magnetic properties through growth of stoichiometric single crystals of the parent iridates before the doping-induced phenomena can be fully explored.…”

“…Moreover, we note that previous studies on the parent Sr 2 IrO 4 and its sister compounds have shown significant variations in the magnetic properties even without doping, likely reflecting different defect concentrations 2,4,18,[22][23][24][25][26][27][28][29] . For example, the sizable c-axis component of the uniform magnetic susceptibility is not understood and seems incompatible with the known magnetic structure of Sr 2 IrO 4 .…”

On the magnetic structure of Sr3Ir2O7: an x-ray resonant scattering study

“…In fact, similar to Sr 2 IrO 4 , Sr 3 Ir 2 O 7 is a G-type antiferromagnet at low temperature [5,6,7]. However, there remains some controversy over the details of the precise magnetic structure reported in the recent literature, and explanations of certain aspects of the bilayered system that are present in the experimental data [5,6,7,8]. This article reports an investigation of the antiferromagnetic structure with an in-depth study of the azimuthal and polarisation dependent resonant scattering signal with particular consideration of the domain behaviour, resulting as a consequence of the degeneracy of two magnetic propagation vectors k 1 =( 1 2 , 1 2 ,0) and The intensity of the ( 1 2 , 1 2 ,24) reflection, corresponding to the k 1 =( 1 2 , 1 2 ,0) magnetic propagation vector, has been measured as a function of x and y sample position in a temperature sequence T=90 K, 300 K, 90 K. The bright yellow regions are areas of high intensity and the dark areas are close to zero intensity.…”

“…Bulk magnetisation data in the 5 K to 300 K temperature interval reveals three distinct transitions at ∼280 K, ∼230 K and ∼50 K [10,12,5], although it should be noted that the precise values of these transitions vary by as much as 10 K from study to study. Our previous x-ray scattering investigation of this compound [5], combined with calculations performed using the SARAh program [13], which calculates irreducible representations based on symmetry analysis and group theory, indicated that Sr 3 Ir 2 O 7 displays a two-domain Gtype antiferromagnetic structure with wave vectors k 1 =( 1 2 , 1 2 ,0) and k 2 =( 1 2 ,-1 2 ,0). These two modulation vectors are represented by symmetry-related structures, denoted A and B in Figure 6 of Boseggia et al [5], and result in magnetic Bragg peaks at ( 1 2 , 1 2 ,L=even) and ( 1 2 , 1 2 ,L=odd) positions, respectively.…”

“…Our previous x-ray scattering investigation of this compound [5], combined with calculations performed using the SARAh program [13], which calculates irreducible representations based on symmetry analysis and group theory, indicated that Sr 3 Ir 2 O 7 displays a two-domain Gtype antiferromagnetic structure with wave vectors k 1 =( 1 2 , 1 2 ,0) and k 2 =( 1 2 ,-1 2 ,0). These two modulation vectors are represented by symmetry-related structures, denoted A and B in Figure 6 of Boseggia et al [5], and result in magnetic Bragg peaks at ( 1 2 , 1 2 ,L=even) and ( 1 2 , 1 2 ,L=odd) positions, respectively. By using a highly focussed x-ray beam and rastering over the surface of the sample, we were able to 'image' domains, relating to these two wave vectors.…”