The magneto-electric (ME) coupling on spin-wave resonances in single-crystal Cu2OSeO3 was studied by a novel technique using electron spin resonance combined with electric field modulation. An external electric field E induces a magnetic field component mu_0 H^i = gamma E along the applied magnetic field H with gamma=0.7(1) mu T/(V/mm) at 10 K. We found that ME coupling strength gamma is temperature dependent and highly anisotropic. gamma(T) nearly follows that of spin susceptibility J(T) and rapidly decreases above the Curie temperature Tc. The ratio gamma/J monotonically decreases with increasing temperature without an anomaly at Tc.
The recent discovery of skyrmions in Cu$_2$OSeO$_3$ has established a new platform to create and manipulate skyrmionic spin textures. We use high-field electron spin resonance (ESR) spectroscopy combining a terahertz free electron laser and pulsed magnetic fields up to 64 T to probe and quantify its microscopic spin-spin interactions. Besides providing direct access to the long-wavelength Goldstone mode, this technique probes also the high-energy part of the excitation spectrum which is inaccessible by standard low-frequency ESR. Fitting the behavior of the observed modes in magnetic field to a theoretical framework establishes experimentally that the fundamental magnetic building blocks of this skyrmionic magnet are rigid, highly entangled and weakly coupled tetrahedra.
We report results of 75As nuclear magnetic resonance (NMR) experiments on a self-flux grown high-quality single crystal of SrFe2As2. The NMR spectra clearly show sharp first-order antiferromagnetic (AF) and structural transitions occurring simultaneously. The behavior in the vicinity of the transition is compared with our previous study on BaFe2As2. No significant difference was observed in the temperature dependence of the static quantities such as the AF splitting and electric quadrupole splitting. However, the results of the NMR relaxation rate revealed difference in the dynamical spin fluctuations. The stripe-type AF fluctuations in the paramagnetic state appear to be more anisotropic in BaFe2As2 than in SrFe2As2.
Muon spin rotation measurements have been performed on a powder sample of a-RuCl3, a layered material which previously has been proposed to be a quantum magnet on a honeycomb lattice close to a quantum spin liquid ground state. Our data reveal two distinct phase transitions at 11 K and 14 K which we interpret as originating from the onset of three-dimensional order and in-plane magnetic order, respectively. We identify, with the help of density functional theory calculations, likely muon stopping sites and combine these with dipolar field calculations to show that the two measured muon rotation frequencies are consistent with two inequivalent muon sites within a zig-zag antiferromagnetic structure proposed previously.
Our magnetic, electrical, and thermal measurements on single-crystals of the novel Mott insulator, Sr2IrO4, reveal a novel giant magneto-electric effect (GME) arising from a frustrated magnetic/ferroelectric state whose signatures are: (1) a strongly enhanced electric permittivity that peaks near a newly observed magnetic anomaly at 100 K, (2) a large (~100%) magneto-dielectric shift that occurs near a metamagnetic transition, and (3) magnetic and electric polarization hysteresis. The GME and electric polarization hinge on a spin-orbit gapping of 5d-bands, rather than the magnitude and spatial dependence of magnetization, as traditionally accepted.
Electron states in a inhomogeneous Ge/Si quantum dot array with groups of closely spaced quantum dots were studied by conventional continuous wave ($cw$) ESR and spin-echo methods. We find that the existence of quantum dot groups allows to increase the spin relaxation time in the system. Created structures allow us to change an effective localization radius of electrons by external magnetic field. With the localization radius close to the size of a quantum dot group, we obtain fourfold increasing spin relaxation time $T_1$, as compared to conventional homogeneous quantum dot arrays. This effect is attributed to averaging of local magnetic fields related to nuclear spins $^{29}$Si and stabilization of $S_z$-polarization during electron back-and-forth motion within a quantum dot group.
A. Maisuradze
,A. Shengelaya
,H. Berger
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(2012)
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"Magneto-Electric Coupling in Single Crystal Cu2OSeO3 Studied by a Novel Electron Spin Resonance Technique"
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Alexander Maisuradze
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