Search

7 pags., 6 figs., The evolution of the magnetic anisotropy directions has been studied in a magnetite (FeO) thin film grown by infrared pulsed-laser deposition on SrTiO(100):Nb substrate. The magnetic easy axes at room temperature are found along the in-plane 〈100〉 film directions, which means a rotation of the easy axis by 45° with respect to the directions typically reported for bulk magnetite and films grown on single-crystal substrates. Moreover, when undergoing the Verwey transition temperature, T, the easy axis orientation evolves to the 〈110〉 film directions. This anomalous behavior has been demonstrated by measuring first the angular dependence of coercivity and remanence well above and below T by high-resolution vectorial magneto-optical Kerr effect (v-MOKE). Ferromagnetic resonance (FMR) measurements have additionally proven a well-defined fourfold magnetic anisotropy induced during growth with confirmed easy axis directions along 〈100〉 for T > T and 〈110〉 for T < T. These results provide a clear proof of the possibility of tuning magnetic anisotropy in FeO thin films by proper control on the growth parameters and substrate choice., This research was supported by the Regional Government of Madrid through NANOMAGCOST project (P2018/NMT-4321) and by the Spanish Ministry of Economy and Competitiveness (MINECO) through Project CTQ2016-75880-P. M. O. thanks CSIC for contract and E. R. thanks MINECO for the tenure of a Ramón y Cajal contract (No. RYC-2011-08069). The work in Lausanne was supported by the Sinergia grant “Mott physics beyond the Heisenberg model” of the Swiss NSF. The work in BUTE (Hungary) was supported by NKFIH Grant No. K119442 and 2017-1.2.1-NKP-2017-00001, and by the BME-Nanotechnology FIKP grant of EMMI (BME FIKP-NAT). IMDEA Nanoscience is supported by the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D, MINECO [grant number SEV-2016-0686]

We present the development and performance of a Fourier transformation (FT) based Raman spectrometer working with visible laser (532 nm) excitation. It is generally thought that FT-Raman spectrometers are not viable in the visible range where shot-noise limits the detector performance and therein they are outperformed by grating based, dispersive ones. We show that contrary to this common belief, the recent advances of high-performance interference filters makes the FT-Raman design a valid alternative to dispersive Raman spectrometers for samples which do not luminesce. We critically compare the performance of our spectrometer to two dispersive ones: a home-built single channel and a state-of-the-art CCD based instruments. We demonstrate a similar or even better sensitivity than the CCD based dispersive spectrometer particularly when the laser power density is considered. The instrument possesses all the known advantages of the FT principle of spectral accuracy, high throughput, and economic design. We also discuss the general considerations which helps the community reassess the utility of the different Raman spectrometer designs.

Low-energy magnetic excitations of the easy-axis antiferromagnet TbFe$_3$(BO$_3$)$_4$ are investigated by far-infrared absorption and reflection spectroscopy in high magnetic fields up to 30 T. The observed field dependence of the resonance frequencies and the magnetization are reproduced by a mean-field spin model for magnetic fields applied both along and perpendicular to the easy axis. Based on this model we determined the full set of magnetic interactions, including Fe-Fe and Fe-Tb exchange interactions, single-ion anisotropy for Tb ions and $g$-factors, which describe the ground-state spin texture and the low-energy spin excitations of TbFe$_3$(BO$_3$)$_4$. Compared to earlier studies we allow a small canting of the nearly Ising-like Tb moments to achieve a quantitative agreement with the magnetic susceptibility measurements. The additional high energy magnetic resonance lines observed, besides the two resonances expected for a two-sublattice antiferromagnet, suggest a more complex six-sublattice magnetic ground state for TbFe$_3$(BO$_3$)$_4$.

Black phosphorus has emerged as the next member in the graphene inspired two-dimensional materials family. Its electronic and magnetic properties are studied herein using electron and nuclear magnetic resonance techniques (ESR and NMR) and microwave conductivity measurement. The latter is a unique technique to study conductivity on air sensitive samples. The ESR study indicates the absence of free charge carriers and no sign of paramagnetic defects are found. $^{31}$P NMR shows the presence of a characteristic Pake doublet structure due to the interaction between $I = 1/2$ nuclei. Microwave conductivity shows, in accordance with the ESR results, that black phosphorus behaves as a semiconductor and we identify extrinsic and intrinsic charge carrier contributions to the conductivity and extracted the sizes of the gaps. ESR measurement also yields that bP might find applications as a microwave absorbent.

The $1/4$-filled organic compound, $\ensuremath{\delta}$-(EDT-TTF-${\mathrm{CONMe}}_{2}$)${}_{2}{\mathrm{AsF}}_{6}$ is a frustrated two-dimensional triangular magnetic system as shown by high-frequency (111.2 and 222.4 GHz) electron spin resonance (ESR) and structural data in the literature. The material gradually orders antiferromagnetically below 40 K, but some magnetically disordered domains persist down to 4 K. We propose that in defect free regions frustration prevents true magnetic order down to at least 4 K in spite of the large first- and second-neighbor exchange interactions along chains and between chains, respectively. The antiferromagnetic (AFM) order gradually developing below 40 K nucleates around structural defects that locally cancel frustration. Two antiferromagnetic resonance modes mapped in the principal planes at 4 K are assigned to the very weakly interacting one-dimensional molecular chains in antiferromagnetic regions.

Broadband microwave spectroscopy has been performed on single-crystalline ${\mathrm{GaV}}_{4}{\mathrm{S}}_{8}$, which exhibits a complex magnetic phase diagram including cycloidal and N\'eel-type skyrmion lattice phases below 13 K. At small magnetic fields two collective modes with large frequency separation are found that reflect the cycloid state subject to a large uniaxial anisotropy. In the skyrmion lattice phase three modes are resolved. Analyzing the spin-precessional motion microscopically, we explain the low frequency of the breathing mode in these N\'eel skyrmions by the hard-axis orientation of relevant spins.

Gd3N@C80 endohedral fullerene, a starting material for a potential magnetic resonance imaging contrast agent, is investigated by high-frequency electron spin resonance (ESR) and SQUID magnetometry. The magnetic moments of the three Gd ions of the endohedral Gd3N molecule are ferromagnetically aligned at low temperatures and are uncorrelated at high temperatures. The 4 T broad 210 and 315 GHz ESR spectra measured at 2 K are well-described by a single transition between the lowest Zeeman levels of static molecules shifted by fine structure effects. At higher temperatures there is a gradual transition to a rotating state. At ambient temperatures the rotation frequency is much larger than the fine structure broadening, and a single ESR line is observed at a gyromagnetic ratio of g = 1.995.

We describe a quasi-optical multifrequency ESR spectrometer operating in the 75-225 GHz range and optimized at 210 GHz for general use in condensed matter physics, chemistry and biology. The quasi-optical bridge detects the change of mm wave polarization at the ESR. A controllable reference arm maintains a mm wave bias at the detector. The attained sensitivity of 2x10^10 spin/G/(Hz)1/2, measured on a dilute Mn:MgO sample in a non-resonant probe head at 222.4 GHz and 300 K, is comparable to commercial high sensitive X band spectrometers. The spectrometer has a Fabry-Perot resonator based probe head to measure aqueous solutions, and a probe head to measure magnetic field angular dependence of single crystals. The spectrometer is robust and easy to use and may be operated by undergraduate students. Its performance is demonstrated by examples from various fields of condensed matter physics.

In organic conductors the overlap integral between layers is small In-plane momentum scattering is rapid and transport perpendicular to the layers is expected to be blocked We present a high frequency conduction electron spin resonance (CESR) study in the layered organic metals kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]X, X = Cl Br which verifies that the inter-layer spin hopping is effectively blocked The method relies on resolving the CESR lines of adjacent layers in which the orientation of the g-factor tensors differs We find that at ambient pressure and in the metallic phase the electron spin diffusion is two dimensional in both the X = Cl and Br compounds i e electrons diffuse longer than the spin lifetime within a single molecular layer without inter-layer hopping Application of pressure at 250 K Increases rapidly the inter-layer hopping rate of spins (C) 2009 Elsevier B V All rights reserved

Controlling magnetic anisotropy is an important objective towards engineering novel magnetic device concepts in oxide electronics. In thin film manganites, magnetic anisotropy is weak and it is primarily determined by the substrate, through induced structural distortions resulting from epitaxial mismatch strain. On the other hand, in cobaltites, with a stronger spin orbit interaction, magnetic anisotropy is typically much stronger. In this paper, we show that interfacing LaSrMnO (LSMO) with an ultrathin LaCoO (LCO) layer drastically modifies the magnetic anisotropy of the manganite, making it independent of the substrate and closer to the magnetic isotropy characterizing its rhombohedral structure. Ferromagnetic resonance measurements evidence a tendency of manganite magnetic moments to point out-of-plane suggesting non collinear magnetic interactions at the interface. These results may be of interest for the design of oxide interfaces with tailored magnetic structures for new oxide devices., Work at UCM supported by Spanish MINECO through Grant Nos. MAT2014-52405-C02-01 and MAT2014-52405-C02-02 and by CAM through Grant No. CAM S2013/MIT-2740. M.V. and M.C. acknowledge support from Fundación BBVA and from MINECO through Grant No. MAT2015-066888-C3-3-R. J.S. thanks the University Paris-Saclay (D’Alembert program) and CNRS for financing his stay at CNRS/Thales. T.F. acknowledges support from the Hungarian Research Fund OTKA No. K107228.

We have measured the spin susceptibility, chi(s), of the CuO(2) planes in the underdoped high T(c) superconductor, YBa(2)Cu(4)O(8) by Gd(3+) electron spin resonance (ESR) in single crystals and aligned powders in fields up to 15.4 T. At low temperatures and high fields, chi(s) is enhanced slightly in the B parallel c orientation with respect to the B perpendicular c orientation. The enhancement at 15.4 T (approximately equal to 0.15H(c2)) at 16 K (0.2 T(c)) is small: approximately 10% of chi(s)(T(c)), suggesting that the second critical field of superconductivity, H(c2) approximately equal to 100 T, would not suppress the pseudogap. This work demonstrates the potential of high field ESR in single crystals for studying high T(c) superconductors.

Using an original modulation technique, the longitudinal (spin-lattice) relaxation of the impurity ${\mathrm{Gd}}^{3+}$ ions has been investigated in the superconducting state of ${\mathrm{Y}}_{0.99}{\mathrm{Gd}}_{0.01}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ magnetically aligned powders. The results agree well with the linewidth analysis performed on a single crystal. At low temperatures $(Tl60\mathrm{K})$ the relaxation rate ${T}_{1}^{\ensuremath{-}1}$ is found to be proportional to ${T}^{3},$ suggesting d-wave superconducting pairing. The data are consistent with previous Knight shift ${(}^{\mathrm{Gd}}K)$ measurements that revealed a linear temperature dependence of the uniform spin susceptibility. The Korringa relation between ${T}_{1}$ and ${}^{\mathrm{Gd}}K$ is confirmed indicating the validity of the Fermi-liquid model.

We report on the magnetic resonance of NH_3K_3C_60 powders in the frequency range of 9 to 225 GHz. The observation of an antiferromagnetic resonance below the phase transition at 40 K is evidence for an antiferromagnetically ordered ground state. In the normal state, above 40 K, the temperature dependence of the spin-susceptibilty measured by ESR agrees with previous static measurements and is too weak to be explained by interacting localized spins in an insulator. The magnetic resonance line width has an unusual magnetic-field dependence which is large and temperature independent in the magnetically ordered state and decreases rapidly above the transition. These observations agree with the suggestion that NH_3K_3C_60 is a metal in the normal state and undergoes a Mott-Hubbard metal to insulator transition at 40 K.

The microscopic parameters of the Gd-Gd pair interaction are measured in single crystals of the ${\mathrm{Gd}}_{0.01}{\mathrm{Y}}_{0.99}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6}$ cuprate using high-frequency (225 GHz) ${\mathrm{Gd}}^{3+}$ electron-spin resonance. In addition to the fine-structure spectrum of single ${\mathrm{Gd}}^{3+}$ ions, a series of low intensity resonance transitions are resolved at high magnetic fields and low temperatures which arise from Gd-Gd pairs. From the resonance line positions of nearest-neighbor Gd-Gd pairs we determine an isotropic exchange energy of $J=156\mathrm{mK}$ and an anisotropic (mainly dipolar) interaction energy of $D=51.7\mathrm{mK}.$ The anisotropic interaction parameters of second, third, and fourth neighbors correspond to the values expected for dipolar interaction only.

The magnetism of La0.7Ca0.3MnO3 (LCMO) epitaxial thin films grown on SrTiO3 (STO) and BaTiO3 (BTO) substrates is studied using polarized neutron reflectometry (PNR) and ferromagnetic resonance (FMR) techniques. In LCMO/BTO, PNR reveals a strongly suppressed magnetization of 300 kA/m, equivalent to a magnetic moment of 2 μB/Mn, throughout the LCMO layer, amounting to half the expected value. The largest suppression occurs near the interface with BTO, with magnetization values of 50 kA/m, equivalent to 0.3 μB/Mn. FMR is observable at 8.9 GHz only around the [110] crystallographic direction in thin LCMO/BTO. The resonance barely shifts as the applied field is rotated away from [110]. The FMR results are analyzed in terms of magnetoelastic anisotropy and compared to LCMO/STO grown under the same conditions. A two-layer magnetization model is proposed, based on strong out-of-plane anisotropy near the BTO interface and shown to qualitatively explain the main characteristics of the FMR results.

In this paper, we explore the interfacial effects appearing in highly strained La0.7Ca0.3MnO3 (LCMO) ultra-thin films (10-12nm) grown on BaTiO3 (BTO) ferroelectric substrates. The strong tendency to phase separation of this optimally doped manganite contributes to the exotic phenomena observed in magnetism and transport experiments: the so-called Matteucci magnetic loops, magnetic granularity and a second metal insulator transition are observed between 50K and the LCMO Curie temperature, 180K. All these properties define the multiferroic character of these heterostructures, which in LCMO//BTO system is strongly linked to magnetoelastic coupling.

Using ferromagnetic resonance spectroscopy at 34 GHz we explored the magnetic anisotropy of single-crystalline GaV4S8 in the field-polarized magnetic state. We describe the data in terms of an easy-axis type uniaxial anisotropy with an anisotropy constant K-1 = 1.6 . 10(5) erg cm(-3) at 2 K, corresponding to a relative exchange anisotropy Delta J/J approximate to 5%, and about 1.0 . 10(5) erg cm(3) near 11 K, i.e. at temperatures where the skyrmion-lattice phase was recently discovered. The relatively large value of K-1 explains the confinement of the skyrmion tubes to the < 1 1 1 > easy axes. A distinct set of resonances in the spectra is attributed to the co-existence of different rhombohedral domains. Complementary broadband spectroscopy demonstrates that non-collinear spin states may sensitively be detected by electron spin resonance techniques.

We studied spin excitations in the magnetically ordered phase of the noncentrosymmetric Ba(2)CoGe(2)O(7) 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 multiboson spin-wave theory describes these unconventional modes, including spin-stretching modes, characterized by an oscillating magnetic dipole and quadrupole moment. The lack of 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 noncentrosymmetric lattice structure.

The interplane spin cross relaxation time Tx measured by high frequency ESR in X-ray irradiated κ-(BEDT-TTF)2Cu[N(CN)2]Cl is compared to the interplane resisitivity ρ⊥ and the in-plane resistivity ρII between 50 K and 250 K. The irradiation transforms the semiconductor behavior of the non-irradiated crystal into metallic. Irradiation decreases Tx, ρ⊥ and ρII but the ratio Tx/ρ⊥ and ρ⊥/ρII remain unchanged between 50 and 250 K. Models describing the unusual defect concentration dependence in κ-(BEDT-TTF)2Cu[N(CN)2]Cl are discussed.

A high frequency (111.2-420 GHz) electron spin resonance study of the interlayer spin diffusion is presented in the conducting phases of the layered organic compounds, kappa-( BEDT-TTF)(2)Cu[N(CN)(2)]X (kappa-ET2-X), X = Cl or Br. The interlayer spin cross relaxation time T-x and the intrinsic spin relaxation time T-2 of single layers are measured as a function of temperature and pressure. Spin diffusion is two dimensional in the high temperature bad-metal phase (i.e., electrons are confined to a single molecular layer for longer than T-2). The interlayer electron hopping frequency nu(perpendicular to) = 1/(2T(x)) decreases along the bad-metal to Mott insulator crossover and increases along the bad-metal to normal metal (or superconductor) crossover. The density of states (DOS) is determined from a comparison of T-x and the interlayer resistivity. In the bad-metal phase it is four to five times larger than the DOS calculated from the electronic structure neglecting electron correlations. In kappa-ET2-X the DOS increases with pressure along the bad-metal to normal metal crossover. Results are compared with predictions of the dynamical mean field theory.

7 páginas, 7 figuras.-- et al., We report on the magnetic anisotropy controlled modulation of the superconductivity in La0.7 Ca0.3 MnO3/ YBa 2 Cu3O7-δ / La0.7 Ca 0.3 MnO3 ferromagnet/superconductor/ferromagnet hybrids with biaxial easy axes. The magnetoresistance (MR) is determined by the local misalignment of the magnetizations in the two layers and exhibits a positive MR plateau for antiparallel alignment along the easy axes and negative MR peaks at the coercive field near the hard axes. This evidences the importance of spin-dependent interfacial scattering effects (as opposed to stray fields) in the MR behavior of superconducting oxide inverse spin switches. © 2010 The American Physical Society., We thank A. Goldman for fruitful discussions within the framework of the joint U.S.-Spain NSF Materials World Network under Grant No. 709584. This work was supported by the U.S. Department of Energy, Basic Energy Science, under Contracts No. DE-AC02-06CH11357 and No. DEAC02NA25396, by Spanish MICINN under Grant No. MAT2008-06517, Consolider Ingenio 2010 Grant CSD2009-00013 Imagine, and “Ramon y Cajal” program, by CAM under Grant S2009/Mat-1756 Phama , and by OTKA under Grants No. F61733, No. K68807, and No. PF63954, and the “Bolyai” program of the Hungarian Academy of Sciences.

3 páginas, 2 figuras.-- et al., We report a memory concept utilizing ferromagnet/superconductor/ferromagnet La0.7Ca0.3MnO3/YBa2Cu 3O7/La0.7Ca0.3MnO3 thin film hybrid structures. The orientation of the magnetic field with respect to the ferromagnetic easy axis has a strong effect on superconductivity as indicated by a strong variation in the magnetoresistance (MR). MR can be controlled by rotating a small magnetic field applied in the plane of the film in a way that is determined by the in-plane biaxial magnetic anisotropy. The proposed memory device has the advantages of superconducting detection elements (fast response and low dissipation), small (100-150 Oe) writing fields, and resistance read-out without need for applied field. © 2010 American Institute of Physics., Work was supported by the U.S. Department of Energy, Basic Energy Science under Contract Nos. DE-AC02-06CH11357 and DE-AC02NA25396, by Spanish MICINN under Contracts “Ramon y Cajal,” Grant Nos. MAT2008-06517 and CONSOLIDER INGENIO 2010 CSD2009-00013 IMAGINE , by CAM under PHAMA Grant No. S2009/Mat-1756, and by OTKA Grant Nos. K68807 and PF63954 and the “Bolyai” program of the Hungarian Academy of Sciences.

The radical cation salt, ET2MnCu[N(CN)(2)](4), [ET=bis(ethylenedithio)tetrathiafulvalene] with an unusual three-dimensional anionic polymeric network is studied by x-ray diffraction, static susceptibility measurements, and electron spin resonance (ESR) at frequencies between 9 and 420 GHz. The magnetic properties are determined by the alternating two-dimensional layers of the Mn2+ ions of the network and the partially charged ET molecules. At ambient temperature the overlap between Mn2+ ions and ET molecules is weak and an exchange integral vertical bar J(Mn-ET)vertical bar approximate to 4.10(-2) K is estimated from their resolved ESR lines. At lower temperatures, ET2MnCu[N(CN)(2)](4) is not a simple system of weakly interacting paramagnetic ions in spite of the isotropic, Curie-like static susceptibility. There are first-order phase transitions at 292 K and in the range of 120-180 K. One of the lattice constants shows anomalous temperature dependence below 292 K. Anisotropic ESR shifts appear below 150 K, which we explain by demagnetizing fields of the platelike crystals and an exchange-narrowed fine structure. The latter contributes significantly to the shift when the populations of Zeeman levels are altered in high magnetic fields at low temperatures. We estimated the exchange coupling between Mn2+ ions within a layer, J(Mn-Mn)approximate to-48 K and determined the fine structure parameters below 150 K, showing a distortion in the plane of the Mn2+ ions.

The layered organic compounds, $\ensuremath{\kappa}\mathrm{\text{\ensuremath{-}}}(\mathrm{BEDT}\mathrm{\text{\ensuremath{-}}}\mathrm{TTF}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]X$ $X=\mathrm{Cl}$, Br) are metals at ambient temperatures. At low temperatures, the Cl compound is a weakly ferromagnetic Mott insulator while the isostructural Br compound is a superconductor. We find by conduction electron spin resonance and antiferromagnetic resonance (AFMR) an extreme anisotropy of spin transport and magnetic interactions in these materials. In the metallic state spin diffusion is confined to single molecular layers within the spin lifetime of ${10}^{\ensuremath{-}9}\text{ }\text{ }\mathrm{s}$. Electrons diffuse several hundreds of nm without interlayer hopping. In the magnetically ordered insulating phase of the Cl compound we observe and calculate the four AFMR modes of the weakly coupled single molecular layers. The interplane exchange field is comparable or less than the typically 1 mT dipolar field and almost ${10}^{6}$ times less than the intralayer exchange field.

The layered organic compounds, kappa-(BEDT-TTF)2Cu[N(CN)2]X X=Cl, Br) are metals at ambient temperatures. At low temperatures, the Cl compound is a weakly ferromagnetic Mott insulator while the isostructural Br compound is a superconductor. We find by conduction electron spin resonance and antiferromagnetic resonance (AFMR) an extreme anisotropy of spin transport and magnetic interactions in these materials. In the metallic state spin diffusion is confined to single molecular layers within the spin lifetime of 10(-9) s. Electrons diffuse several hundreds of nm without interlayer hopping. In the magnetically ordered insulating phase of the Cl compound we observe and calculate the four AFMR modes of the weakly coupled single molecular layers. The interplane exchange field is comparable or less than the typically 1 mT dipolar field and almost 10(6) times less than the intralayer exchange field.

The magnetic properties of La0.7Ca0.3MnO3 (LCMO) manganite thin films were studied with magnetometry and ferromagnetic resonance as a function of film thickness. They maintain the colossal magnetoresistance behavior with a pronounced metal-insulator transition around 150-200 K, except for the very thinnest films studied (3 nm). Nevertheless, LCMO films as thin as 3 nm remain ferromagnetic, without a decrease in saturation magnetization, indicating an absence of dead-layers, although below approx. 6 nm the films remain insulating at low temperature. Magnetization hysteresis loops reveal that the magnetic easy axes lie in the plane of the film for thicknesses in the range of 4-15 nm. Ferromagnetic resonance studies confirm that the easy axes are in-plane, and find a biaxial symmetry in-plane with two, perpendicular easy axes. The directions of the easy axes with respect to the crystallographic directions of the cubic SrTiO3 substrate differ by 45 degrees in 4 nm and 15 nm thick LCMO films., Presented at Intermag conference (Madrid, 2008). Accepted for publication in IEEE Transactions on Magnetics

The spin-lattice relaxation time, ${T}_{1}$, of conduction electrons is measured as a function of temperature and magnetic field in $\mathrm{Mg}{\mathrm{B}}_{2}$ in the normal and superconducting states. The method is based on the detection of the $z$ component of the conduction electron magnetization under electron-spin-resonance conditions with amplitude-modulated microwave excitation. Measurement of ${T}_{1}$ below ${T}_{c}$ at $0.32\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ allows us to disentangle contributions from the two Fermi surfaces of $\mathrm{Mg}{\mathrm{B}}_{2}$, as this field restores the normal state on the part of the Fermi surface with $\ensuremath{\pi}$ symmetry only.

The finite phase transition temperature, T-N, of quasi two-dimensional (2D) Heisenberg antiferromagnetic (AF) crystals results from in-plane anisotropy or coupling between layers. It is usually not known which is the determining factor when both are weak. We show by an electron spin resonance experiment that in the quasi 2D antiferromagnetic crystal,kappa-(BEDT-TTF)(2)Cu[N(CN)(2)] Cl, TN is determined by in-plane anisotropy while interlayer coupling plays a minor role. The compound has a large isotropic Heisenberg exchange interaction between sites with S = 1/2 spins. The Dzyaloshinskii-Moriya (DM) interaction is the main source of anisotropy, while in-plane anisotropy and the interlayer coupling are very weak. The external-field field-induced static and fluctuating AF magnetizations are independent in adjacent layers above the (zero-field) ordering temperature.

The transition metal tellurium oxychloride, Ni-5(TeO3)(4)Cl-2, has been investigated by high-field electron-spin resonance for frequencies up to 3 THz, at temperatures well below the magnetic ordering at 23 K. At zero external field several resonance modes have been identified. When the applied magnetic field is perpendicular to both the a and b crystallographic directions, one of the magnetic-resonance modes softens, and a spin-flop transition occurs around 10 T. The results are discussed in terms of the crystal structure, and compared to other magnetically ordered materials with multiple magnetic sublattices, including orthoferrites and triangular antiferromagnets.

Williams and Kramer [Phys. Rev. B {\bf 64}, 104506 (2001)] have recently argued against the existence of staggered magnetic moments residing on several lattice sites around Zn impurities in YBCO superconductors. This claim, which is in line with an earlier publication by Williams, Tallon and Dupree [Phys. Rev. B {\bf 61}, 4319 (2000)], is however in contradiction with a large body of experimental data from different NMR groups. On the contrary, the authors argue in favor of a very localized spin and charge density on Cu sites first neighbors to Zn. We show that the conclusions of Williams and Kramer arise from erroneous interpretations of NMR and NQR data.

Electron-spin resonance has been studied in a stoichiometric single crystal of ${\mathrm{LaMnO}}_{3}$ in the temperature range of 4.2 K--250 K. ${\mathrm{LaMnO}}_{3}$ exhibits an extensively studied antiferromagnetic (AF) transition at ${T}_{N}=141\mathrm{K}.$ The spin resonance was detected both in a fixed-frequency microwave setup with various fixed frequencies (75 GHz, 150 GHz, and 225 GHz) in field-sweep mode or in a broad frequency band instrument at various fixed fields. The latter facility covers the far-infrared range with a lower cutoff frequency of 4 cm (120 GHz). Magnetic fields up to 14 T have been applied along the crystallographic b direction (the easy-axis direction in the AF state). The field dependence of the resonance is fully mapped and the results at low temperatures are well described by the AF resonance theory of Kittel and Keffer modified for the weak ferromagnetism. Deviations from the theory are evident at temperatures close to ${T}_{N}.$ In the paramagnetic state the spin resonance is broad, and in addition to the expected paramagnetic signal around $g=2,$ a temperature-dependent anomalous resonance has been also observed.

Hole induced changes in the antiferromagnetic structure of a lightly Ca doped Gd:Y 1 - x Ca x Ba 2 Cu 3 O 6 copper oxide single crystal with x 0.008 is investigated by Gd 3 + electron spin resonance. Holes do not localize toCa 2 + ions above 2.5 K since the charge distribution and spin susceptibility next to the Ca 2 + are independent of temperature. Both hole doped and pristine crystals are magnetically twinned with an external magnetic field dependent antiferromagnetic domain structure. Unlike the undoped crystal, where the easy magnetic axis is along [100] at all temperatures, the easy direction in the holc doped crystal is along the [110] diagonal at low temperatures and changes gradually to the [100] direction between 10 and 100 K. The transition is tentatively attributed to a magnetic anisotropy introduced by hole ordering.

Hole induced changes in the antiferromagnetic structure of a lightly Ca doped Gd:Y(1-x)CaxBa2Cu3O6 copper oxide single crystal with x approximately 0.008 is investigated by Gd3+ electron spin resonance. Holes do not localize to Ca2+ ions above 2.5 K since the charge distribution and spin susceptibility next to the Ca2+ are independent of temperature. Both hole doped and pristine crystals are magnetically twinned with an external magnetic field dependent antiferromagnetic domain structure. Unlike the undoped crystal, where the easy magnetic axis is along [100] at all temperatures, the easy direction in the hole doped crystal is along the [110] diagonal at low temperatures and changes gradually to the [100] direction between 10 and 100 K. The transition is tentatively attributed to a magnetic anisotropy introduced by hole ordering.

We observed the conduction electron spin resonance (CESR) in fine powders of ${\mathrm{MgB}}_{2}$ both in the superconducting and normal states. The Pauli susceptibility is ${\ensuremath{\chi}}_{s}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\mathrm{emu}/\mathrm{mole}$ in the temperature range of 450 to $600\mathrm{K}$. The spin relaxation rate has an anomalous temperature dependence. The CESR measured below ${T}_{c}$ at several frequencies suggests that ${\mathrm{MgB}}_{2}$ is a strongly anisotropic superconductor with the upper critical field, ${H}_{c2}$, ranging between 2 and $16\mathrm{T}$. The high-field reversible magnetization data of a randomly oriented powder sample are well described assuming that ${\mathrm{MgB}}_{2}$ is an anisotropic superconductor with ${H}_{c2}^{\mathrm{ab}}{/H}_{c2}^{c}\ensuremath{\approx}6--9$.

A Comment on the Letter by Yoichi Ando, A. N. Lavrov, and Kouji Segawa, Phys. Rev. Lett. 83, 2813 (1999). The authors of the Letter offer a Reply.