Orbital domain state and finite size scaling in ferromagnetic insulating manganites

55Mn and 139La NMR measurements on a high quality single crystal of ferromagnetic (FM) La0.80Ca20MnO3 demonstrate the formation of localized Mn(3+,4+) states below 70 K, accompanied with strong anomalous increase of certain FM neutron Bragg peaks. (55,139)(1/T1) spin-lattice relaxation rates diverge on approaching this temperature from below, signalling a genuine phase transition at T(tr) approx. 70 K. The increased local magnetic anisotropy of the low temperature phase, the cooling-rate dependence of the Bragg peaks, and the observed finite size scaling of T(tr) with Ca (hole) doping, are suggestive of freezing into an orbital domain state, precursor to a phase transition into an inhomogeneous orbitally ordered state embodying hole-rich walls.Comment: 4 pages, 4 figure

Ferrimagnetism of the magnetoelectric compound Cu2_2OSeO3_3 probed by 77^{77}Se NMR

We present a thorough $^{77}$Se NMR study of a single crystal of the magnetoelectric compound Cu$_2$OSeO$_3$. The temperature dependence of the local electronic moments extracted from the NMR data is fully consistent with a magnetic phase transition from the high-T paramagnetic phase to a low-T ferrimagnetic state with 3/4 of the Cu$^{2+}$ ions aligned parallel and 1/4 aligned antiparallel to the applied field of 14.09 T. The transition to this 3up-1down magnetic state is not accompanied by any splitting of the NMR lines or any abrupt modification in their broadening, hence there is no observable reduction of the crystalline symmetry from its high-T cubic \textit{P}2$_1$3 space group. These results are in agreement with high resolution x-ray diffraction and magnetization data on powder samples reported previously by Bos {\it et al.} [Phys. Rev. B, {\bf 78}, 094416 (2008)]. We also develop a mean field theory description of the problem based on a microscopic spin Hamiltonian with one antiferromagnetic ($J_\text{afm}\simeq 68$ K) and one ferromagnetic ($J_\text{fm}\simeq -50$ K) nearest-neighbor exchange interaction

Establishing the fundamental magnetic interactions in the chiral skyrmionic Mott insulator Cu2OSeO3 by terahertz electron spin resonance

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.Comment: 5 pages, 3 Figure

The quantum origins of skyrmions and half-skyrmions in Cu2OSeO3

The Skyrme-particle, the $skyrmion$, was introduced over half a century ago and used to construct field theories for dense nuclear matter. But with skyrmions being mathematical objects - special types of topological solitons - they can emerge in much broader contexts. Recently skyrmions were observed in helimagnets, forming nanoscale spin-textures that hold promise as information carriers. Extending over length-scales much larger than the inter-atomic spacing, these skyrmions behave as large, classical objects, yet deep inside they are of quantum origin. Penetrating into their microscopic roots requires a multi-scale approach, spanning the full quantum to classical domain. By exploiting a natural separation of exchange energy scales, we achieve this for the first time in the skyrmionic Mott insulator Cu$_2$OSeO$_3$. Atomistic ab initio calculations reveal that its magnetic building blocks are strongly fluctuating Cu$_4$ tetrahedra. These spawn a continuum theory with a skyrmionic texture that agrees well with reported experiments. It also brings to light a decay of skyrmions into half-skyrmions in a specific temperature and magnetic field range. The theoretical multiscale approach explains the strong renormalization of the local moments and predicts further fingerprints of the quantum origin of magnetic skyrmions that can be observed in Cu$_2$OSeO$_3$, like weakly dispersive high-energy excitations associated with the Cu$_4$ tetrahedra, a weak antiferromagnetic modulation of the primary ferrimagnetic order, and a fractionalized skyrmion phase.Comment: 5 pages, 3 figure

Spin-polarized oxygen hole states in cation deficient La(1-x)CaxMnO(3+delta)

When holes are doped into a Mott-Hubbard type insulator, like lightly doped manganites of the La(1-x)CaxMnO3 family, the cooperative Jahn-Teller distortions and the appearance of orbital ordering require an arrangement of Mn(3+)/Mn(4+) for the establishment of the insulating canted antiferromagnetic (for x<=0.1), or of the insulating ferromagnetic (for 0.1<x<= 0.2) ground state. In the present work we provide NMR evidence about a novel and at the same time puzzling effect in La(1-x)CaxMnO(3+delta) systems with cation deficience. We show that in the low Ca-doping regime, these systems exhibit a very strong hyperfine field at certain La nuclear sites, which is not present in the stoichiometric compounds. Comparison of our NMR results with recent x-ray absorption data at the Mn K edge, suggests the formation of a spin-polarized hole arrangement on the 2p oxygen orbitals as the origin of this effect.Comment: 10 pages, 4 Figures, submitted to PR

Ferrimagnetism of the magnetoelectric compound Cu2OSeO3 probed by 77Se NMR

We present a thorough 77Se nuclear-magnetic-resonance (NMR) study of a single crystal of the magnetoelectric compound Cu 2OSeO3. The temperature dependence of the local electronic moments extracted from the NMR data is fully consistent with a magnetic phase transition from the high- T paramagnetic phase to a low- T ferrimagnetic state with 3/4 of the Cu2+ ions aligned parallel and 1/4 aligned antiparallel to the applied field of 14.09 T. The transition to this 3up-1down magnetic state is not accompanied by any splitting of the NMR lines or any abrupt modification in their broadening, hence there is no observable reduction in the crystal symmetry from its high- T cubic P 213 space group. These results are in agreement with high-resolution x-ray diffraction and magnetization data on powder samples reported previously by Bos [Phys. Rev. B 78, 094416 (2008)]10.1103/PhysRevB.78.094416. We also develop a mean-field theory description of the problem based on a microscopic spin Hamiltonian with one antiferromagnetic (Jafm ≃68 K) and one ferromagnetic (J fm ≃-50 K) nearest-neighbor exchange interaction. © 2010 The American Physical Society