We synthesize and study single crystals of a new double-perovskite Sr2YIrO6. Despite two strongly unfavorable conditions for magnetic order, namely, pentavalent Ir5+(5d4) ions which are anticipated to have Jeff=0 singlet ground states in the strong spin-orbit coupling (SOC) limit and geometric frustration in a face-centered cubic structure formed by the Ir5+ ions, we observe this iridate to undergo a novel magnetic transition at temperatures below 1.3 K. We provide compelling experimental and theoretical evidence that the origin of magnetism is in an unusual interplay between strong noncubic crystal fields, local exchange interactions, and "intermediate-strength" SOC. Sr2YIrO6 provides a rare example of the failed dominance of SOC in the iridates.

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Our magnetic, electrical, and thermal measurements on single crystals of the J eff =1/ 2 Mott insulator, Sr 2 IrO 4 , reveal a giant magnetoelectric effect ͑GME͒ arising from a frustrated magnetic/ferroelectric state whose signatures are: ͑1͒ a strongly enhanced electric permittivity that peaks near an observed magnetic anomaly at 100 K, and ͑2͒ a large ͑ϳ100%͒ magnetodielectric shift that occurs near a metamagnetic transition. The GME hinges on a spin-orbit gapping of 5d bands rather than the magnitude and spatial dependence of magnetization, as traditionally accepted.

We report measurements of the ferromagnetic resonance (FMR) spectra of arrays of submicron size periodic particle arrays of permalloy produced by electron-beam lithography. In contrast to plane ferromagnetic films, the spectra of the arrays show a number of additional resonance peaks, whose position depends strongly on the orientation of the external magnetic field and the interparticle interaction. Time-dependent micromagnetic simulation of the ac response show that these peaks are associated with coupled exchange and dipolar spin wave modesComment: 4 pages, 4 figure