Doping the high- T(c) superconductor YBa2Cu3O6.7 with 1.5% of nonmagnetic Zn impurities in CuO2 planes is shown to produce a considerable broadening of 63Cu NMR spectra, as well as an increase of low-energy magnetic fluctuations detected in 63Cu spin-lattice relaxation measurements. A model-independent analysis demonstrates that these effects are due to the development of staggered magnetic moments on many Cu sites around each Zn and that the Zn-induced moment in the bulk susceptibility might be explained by this staggered magnetization. Several implications of these enhanced antiferromagnetic correlations are discussed.

Hydroxylated C60 molecules, also called fullerols, are a class of water‐soluble fullerenes. Here we report the synthesis in acidic conditions of a highly derivatized fullerol (up to 36 carbons per C60 are oxidized). Spectroscopic investigations (X‐ray photoelectron spectroscopy and infrared absorption) highlight the coexistence of both acidic and basic forms for the hydroxyl addends of derivatized C60. pH titrimetry reveals that, at millimolar concentrations, only ten protons per fullerol molecule are labile. Such a low value, as compared to 36 hydroxyl groups, is explained by the formation of clusters. A UV‐vis absorption study performed over a large range of concentrations also points to the aggregation phenomenon. Moreover, this study shows that the clusters of fullerols appear at relatively low (micromolar) concentrations. An electron spin resonance (ESR) study, based on the attack of singlet oxygen (1Δg) on 2,2,6,6‐tetramethyl‐4‐piperidinol (TMP‐OH), has proved the potential of hydroxylated C60 for performing efficient generation of singlet oxygen in aqueous solution. ESR measurements, which allow detection and quantification of 1Δg, have also revealed the generation of reactive oxygen species (ROS). The yield of generation of 1Δg and ROS is strongly correlated to the concentration of fullerol, thus also pointing to the aggregation of fullerol molecules. Exposing glioblastoma cells to oxidative stress in the presence of hydroxylated C60 and visible light has also been performed. Atomic force microscopy is used to monitor the relevant diminishment of the Young's modulus values for cells exposed to the oxidative stress. These results point to a possible application field of fullerols for performing bio‐oxidations.

We studied spin excitations of the non-centrosymmetric Ba2CoGe2O7 in high magnetic fields up to 33 T. In the electron spin resonance and far infrared absorption spectra we found several spin excitations beyond the two conventional magnon modes expected for such a two-sublattice antiferromagnet. We show that a multi-boson spin-wave theory describes these unconventional modes, including spin-stretching modes, characterized by oscillating magnetic dipole and quadrupole moment. The lack of the inversion symmetry allows each mode to become electric dipole active. We expect that the spin-stretching modes can be generally observed in inelastic neutron scattering and light absorption experiments in a broad class of ordered S > 1/2 spin systems with strong single-ion anisotropy and/or non-centrosymmetric lattice structure. Magnons are collective spin excitations in crystals with long-range magnetic order, often investigated by electromagnetic absorption and neutron scattering experiments. Both classical and quantum spin-wave theory of S = 1/2 systems predict one magnon branch in the spin-excitation spectrum for each spin in the magnetic unit cell [1]. This rule about the number of magnon branches is generally accepted and experimentally verified for S > 1/2 spin systems as long as the conventional spin-wave theory applies, requiring that the lengths (i.e., the absolute values of the expectation values) of the spins are preserved in the excited states and only their orientations change relative to the ground-state configuration [2]. However, the picture of one magnon mode per spin in the magnetic unit cell needed to be surpassed in several f -electron compounds with complicated quadrupolar ordering, such as CeB 6 [3] and UO 2 [4].Recently, additional spin-wave modes have been observed by far infrared (FIR) spectroscopy [5] and inelastic neutron scattering (INS) [6] in Ba 2 CoGe 2 O 7 , a simple two-sublattice easy-plane antiferromagnet (AF) with S = 3/2 spins [7, 8]. This material has attracted much interest owing to its multiferroic ground state where delicate magnetic control of the ferroelectric polarization [8, 9] and chirality [10] were realized. Moreover, spin waves in Ba 2 CoGe 2 O 7 exhibits giant directional dichroism and natural optical activity at THz frequencies due to the large ac magnetoelectric effect [5, 10]. A recent numerical diagonalization study on finite spin clusters found, besides the two conventional AF modes, additional spin resonances with peculiar optical properties [10, 11]. Nevertheless, the understanding of the unconventional magnon modes and the coupled dynamics of spins and electronic polarization on a fundamental level remained an open issue.In this Letter, we investigate the spin-wave excitations in Ba 2 CoGe 2 O 7 over a broad photon energy range combining electron spin resonance (ESR) and high-resolution FIR spectroscopy. The largest magnetic field, 33 T, applied in this study drastically changes the antiferromagnetic spin configuration for any field direction, in contrast to former e...

Broadband microwave spectroscopy has been performed on single-crystalline GaV4S8, which exhibits a complex magnetic phase diagram including cycloidal, Néel-type skyrmion lattice, as well as field-polarized ferromagnetic phases below 13 K. At zero and small magnetic fields two collective modes are found at 5 and 15 GHz, which are characteristic of the cycloidal state in this easy-axis magnet. In finite fields, entering the skyrmion lattice phase, the spectrum transforms into a multimode pattern with absorption peaks near 4, 8, and 15 GHz. The spin excitation spectra in GaV4S8 and their field dependencies are found to be in close relation to those observed in materials with Bloch-type skyrmions. Distinct differences arise from the strong uniaxial magnetic anisotropy of GaV4S8 not present in so-far known skyrmion hosts. The occurence of nontrivial topology in the spin pattern of magnets has gained considerable interest in condensed matter physics. Recent research focuses on magnetic skyrmions which are thermodynamically stabilized in compounds with noncentrosymmetric crystal structures, in a limited region of the magnetic field versus temperature phase diagram [1][2][3]. Skyrmions are whirllike objects of spins which can crystallize in skyrmion lattices (SkLs) with typical lattice constants from ten to hundred nanometers and give rise to emergent electrodynamics, like the topological Hall effect [4][5][6] or magnetic monopoles [7]. Individual skyrmions have been proposed as building blocks for novel nanomagnetic storage devices [8,9]. The SkL has raised high interest for microwavetechnology applications after collective spin excitations predicted in the GHz range [10] were evidenced in the insulating chiral magnet Cu 2 OSeO 3 [11][12][13][14][15][16]. Later it was found that different metallic, semiconducting, and insulating chiral magnets support the same set of characteristic excitations, i.e., three SkL modes characterized as clockwise (CW), counterclockwise (CCW) and breathing (BR) modes, that all follow a universal behavior [17].Besides the Bloch-type skyrmions reported in the aforementioned works, a Néel-type SkL has recently been discovered in GaV 4 S 8 [18], where the spins rotate radially towards the vortex core. In this semiconductor characterized by V 4 S 4 clusters with spin S = 1 2 [19], a structural Jahn-Teller transition [20] at 44 K is followed by the onset of magnetic order at the Curie temperature T C = 13 K. At the structural transition the lattice is stretched along one of the four body diagonals, resulting in a strongly anisotropic easy-axis magnet. The magnetic multi-domain structure strongly depends on the orientation and strength of the applied magnetic field and gives rise to complex magnetic phase diagrams [see Figures 1(a), 1(c) and 2(a)] including cycloidal (Cyc), SkL, and ferromagnetic (FM) regions. Specifically, the skyrmions do not follow the external magnetic field but are confined to the magnetic easy axes. The phases have been interpreted in terms of a competition of symmetric and ...

We observed the conduction electron spin resonance (CESR) in fine powders of MgB2 both in the superconducting and normal states. The Pauli susceptibility is chi(s) = 2.0 x 10(-5) emu/mole in the temperature range of 450 to 600 K. The spin relaxation rate has an anomalous temperature dependence. The CESR measured below T(c) at several frequencies suggests that MgB2 is a strongly anisotropic superconductor with the upper critical field, H(c2), ranging between 2 and 16 T. The high-field reversible magnetization data of a randomly oriented powder sample are well described assuming that MgB2 is an anisotropic superconductor with H(ab)(c2)/H(c)(c2) approximately 6-9.

C59N magnetic fullerenes were formed inside single-wall carbon nanotubes by vacuum annealing functionalized C59N molecules encapsulated inside the tubes. A hindered, anisotropic rotation of C59N was deduced from the temperature dependence of the electron spin resonance spectra near room temperature. Shortening of spin-lattice relaxation time, T1, of C59N indicates a reversible charge transfer toward the host nanotubes above ∼ 350 K. Bound C59N-C60 heterodimers are formed at lower temperatures when C60 is co-encapsulated with the functionalized C59N. In the 10-300 K range, T1 of the heterodimer shows a relaxation dominated by the conduction electrons on the nanotubes.Single-wall carbon nanotubes (SWCNTs) [1, 2] exhibit a variety of unusual physical phenomena related to their one-dimensional and strongly correlated electronic properties. These include excitonic effects [3,4], superconductivity [5], the Tomonaga-Luttinger liquid state [6], and the Peierls transition [7]. Magnetic resonance is a powerful method to study strong correlations in low dimensional systems. However, for SWCNTs both nuclear magnetic resonance (NMR) and electron spin resonance (ESR) are severely limited by NMR active 13 C nuclei and ESR active electron spins in residual magnetic catalytic particles and other carbon phases. Synthesis of 13 C isotope engineered SWCNTs solved the problem for NMR [8,9]. To enable ESR spectroscopy of SWCNTs, a local probe, specifically attached to SWCNTs, is required. The N@C 60 [10] and C 59 N [11] magnetic fullerenes are ideal candidates for such studies. In fullerene doped SWCNTs, fullerenes occupy preferentially the interior of the tubes and form "peapods" (C 60 @SWCNT) [12]. Fullerenes adhesing to the outside can be removed [13] in contrast to e.g. filling with iron [14]. ESR on encapsulated magnetic fullerenes could yield information on the electronic state of the tubes and it could also enable to study the fullerene rotational dynamics in a confined environment. In addition, magnetic fullerene peapods could exploit the combination of the SWCNT strength and the magnetic moment of molecules in magnetic scanning probe tips and they could enable a bottom-up design for magnetic storage devices or for building elements of quantum computers [15].Typical spin concentrations in (N@C 60 :C 60 )@SWCNT are low, ∼1 spin/tube, and the N spins are insensitive to SWCNT properties [16]. The C 59 N monomer radical is a better local probe candidate as the unpaired electron is on the cage. C 59 N can be chemically prepared but it forms spinless dimers (C 59 N) 2 or monomer adducts [11].The magnetic C 59 N monomer radical can be stabilized as C 59 N:C 60 , a dilute solid solution of C 59 N in C 60 [17].Here, we report on the first ESR study of SWCNT properties and peapod rotational dynamics using a paramagnetic local probe: C 59 N monomer radicals encapsulated inside SWCNTs. SWCNTs were first filled with chemically inert C 59 N derivatives. A heat treatment in vacuum removes the side-group and the monomer radical is left behin...

We have studied the electron spin resonance ESR of single-walled carbon nanotubes SWNTs both in their pristine state and after irradiation with fast electrons in order to detect the signal of conduction electrons. No metallic Pauli contribution was observed in the global spin susceptibility, the ESR signal of the conduction electrons is undetectable because it is broadened by magnetic impurities. We measured a paramagnetic contribution from localized states, with an effective Curie constant decreasing when the temperature increases, following a deactivation law of the type A-B exp-Ea /kBT. This behavior is supposed to be characteristic of semiconducting SWNTs interacting with metallic impurities from the catalyst

Antiferromagnetic resonance has been observed in powders of the conducting alkali fulleride linear polymers, RbC 60 and CsC 60 , at high frequencies (75, 150, and 225 GHz). This is proof for an antiferromagnetically ordered ground state and shows that these systems are not spin glasses. The sublattice magnetization is independent of applied magnetic field up to at least 8 T. Magnetic fluctuations are observed between 35 and 50 K. Comparison with the spin-density-wave system ͑TMTSeF͒ 2 PF 6 clearly shows that these polymers are also quasi-1D spin-density-wave systems with 3D ordering at low temperatures, as suggested previously.[S0031-9007 (97)04146-X] PACS numbers: 71.30. + h, 75.30.Fv, 76.50. + gRecently Chauvet et al. [1] suggested that the conducting alkali fulleride linear chain polymers, orthorhombic RbC 60 and CsC 60 , are quasi-one-dimensional conductors, and the phase transition below 50 K is the threedimensional (3D) ordering of spin-density waves (SDW). At present, both the dimensionality of the electronic structure and the nature of the low temperature phase remain open questions. The polymers have a relatively simple crystal structure [2] with parallel running chains of covalently bonded C 60 molecules. The electronic structure calculated for a single chain [3] has a very narrow conduction electron band for which correlations must be important. Mele et al. [4] proposed, however, that interchain overlap may be important, and the electronic structure may be 3D with large bandwidths both along and perpendicular to the chains. On the other hand, the physical properties are suggestive of a quasi-1D conductor. Above 50 K the polymers are metals with a large, weakly temperature dependent spin susceptibility and low frequency conductivity [5]. Magnetic fluctuations measured by NMR spin lattice relaxation [6] and the observation of a narrow ESR line in the metallic phase [3] support the idea of a quasi-1D metal. Above the transition, precursor effects are observed: Magnetic fluctuations increase dramatically [6], and a gradual development of a gap [5] has been inferred from the microwave and far infrared conductivities. Both NMR [6] and mSR (muon spin rotation) [7-9] studies confirm the development of static magnetic moments on the fullerene chains at low temperatures. Cristofolini et al.[7] and MacFarlene et al. [8] interpreted mSR data as indicative of a strongly disordered magnetic state developing gradually as the temperature is lowered, while Uemura et al. [9] believe that the mSR data do not exclude an antiferromagnetic or SDW order. On the other hand, Brouet et al. [6] interpreted the 133 Cs and 13 C NMR as indicative of a spin-flop transition. Both molecular and crystalline orientational disorder broaden the NMR, but a spin-flop transition is only possible in an antiferromagnetically ordered system.In this paper we present evidence for an antiferromagnetically ordered state below 35 and 30 K for the Rb and Cs polymers, respectively. We report on the antiferromagnetic resonance below the ordering transi...