Magnetization Process of Atacamite: A Case of Weakly Coupled S=1/2 Sawtooth Chains.

We present a combined experimental and theoretical study of the mineral atacamite Cu_{2}Cl(OH)_{3}. Density-functional theory yields a Hamiltonian describing anisotropic sawtooth chains with weak 3D connections. Experimentally, we fully characterize the antiferromagnetically ordered state. Magnetic order shows a complex evolution with the magnetic field, while, starting at 31.5 T, we observe a plateaulike magnetization at about M_{sat}/2. Based on complementary theoretical approaches, we show that the latter is unrelated to the known magnetization plateau of a sawtooth chain. Instead, we provide evidence that the magnetization process in atacamite is a field-driven canting of a 3D network of weakly coupled sawtooth chains that form giant moments.

Patterning of sodium ions and the control of electrons in sodium cobaltate.

Sodium cobaltate (Na(x)CoO2) has emerged as a material of exceptional scientific interest due to the potential for thermoelectric applications, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physics of the superconducting copper oxides. The density x of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on the triangular Co layers. Recent electron diffraction measurements reveal a kaleidoscope of Na+ ion patterns as a function of concentration. Here we use single-crystal neutron diffraction supported by numerical simulations to determine the long-range three-dimensional superstructures of these ions. We show that the sodium ordering and its associated distortion field are governed by pure electrostatics, and that the organizational principle is the stabilization of charge droplets that order long range at some simple fractional fillings. Our results provide a good starting point to understand the electronic properties in terms of a Hubbard hamiltonian that takes into account the electrostatic potential from the Na superstructures. The resulting depth of potential wells in the Co layer is greater than the single-particle hopping kinetic energy and as a consequence, holes preferentially occupy the lowest potential regions. Thus we conclude that the Na+ ion patterning has a decisive role in the transport and magnetic properties.

Magnetic frustration, phase competition, and the magnetoelectric effect in NdFe3(BO3)4.

We present an element selective resonant magnetic x-ray scattering study of NdFe3(BO3)4 as a function of temperature and applied magnetic field. Our measurements show that the magnetic order of the Nd sublattice is induced by the Fe spin order. When a magnetic field is applied parallel to the hexagonal basal plane, the helicoidal spin order is suppressed and a collinear ordering, where the moments are forced to align in a direction perpendicular to the applied magnetic field, is stabilized. This result excludes a noncollinear spin order as the origin of the magnetically induced electric polarization in this compound. Instead our data imply that magnetic frustration results in a phase competition, which is the origin of the magnetoelectric response.

Cycloidal order of 4f moments as a probe of chiral domains in DyMnO3.

Using soft x-ray diffraction at the Dy-M5 resonance, pronounced circular dichroism in the ferroelectric phase of DyMnO3 is observed in connection with sizable b and c components of the Dy-4f magnetic moments. This provides strong evidence for cycloidal order of the 4f moments, corroborating that inversion-symmetry breaking in this material is not accomplished by the Mn spins alone. The 4f circular dichroism allows us to image multiferroic domains that are imprinted on the surface of DyMnO3 using the local charging by the x-ray beam via the photoelectric effect.

First-principles bottom-up study of 1D to 3D magnetic transformation in the copper pyrazine dinitrate S = (1)/(2) antiferromagnetic crystal.

On the basis of magnetic susceptibility and heat capacity data, copper pyrazine dinitrate crystal [abbreviated CuPz(NO(3))(2)] has long been considered a good prototype for S = (1)/(2) antiferromagnetic (AFM) Heisenberg chain behavior down to 0.05 K. However, a recent muon-spin rotation experiment indicated the presence of a previously unnoticed 1D to 3D magnetic transition below 0.107 K. Our aim in this work is to perform a rigorous quantitative study of the mechanism of this 1D-3D magnetic transformation, by doing a first-principles bottom-up study of the CuPz(NO(3))(2) crystal at 158 K, where the magnetic properties are clearly 1D, and at 2 K, at which the neutron structure (reported in this work) is considered nearly identical with that below 0.1 K (due to small thermal effects). A change in the magnetic topology is found between these two structures: at 158 K, there are isolated AFM spin chains (J(intra) = -5.23 cm(-1)), while at 2 K, the magnetic chains (J(intra) = -5.96 cm(-1)) weakly interact (the largest of the J(inter) parameters is -0.09 cm(-1)). This change is caused by thermal contraction upon cooling (no crystallographic phase transition is detected down to 2 K, and one will not likely occur below that temperature). The computed and experimental magnetic susceptibility chi(T) curves are nearly identical. The calculated heat capacity C(p)(T) curve has a maximum at 6.92 K, close to the 5.20 K maximum found in the experimental curve at zero external field. In spite of the 3D magnetic topology of the crystal at low temperature, the magnetic susceptibility and heat capacity curves behave as a pure 1D AFM chain in all regions because of the large J(intra)/J(inter) ratio (66.2 in absolute value) and the effect of including the J(inter) interactions will not be easily appreciated in any of these experiments. The impact of the presence of odd- and even-membered regular AFM finite chains in the CuPz(NO(3))(2) crystal has also been evaluated. Odd-membered interacting chains produce an increase in both chi(T) and C(p)(T) curves when the temperature is very close to zero, in agreement with the experimental observations, while even-membered chains produce a small shoulder in the C(p)(T) curve between 0.8 and 5 K. No changes are seen in the remaining regions. Concerning the spin gap, odd-membered chains present a quasi-zero gap but the finite even-membered chains still have a sizable one. Finally, the effect of increasing the magnitude of J(inter) was investigated by fixing the value of J(intra) to that found for the 2 K CuPz(NO(3))(2) crystal. The magnetic susceptibility and heat capacity curves remain practically unchanged.

Spontaneous Magnetic Ordering in the Fullerene Charge-Transfer Salt (TDAE)C60.

The zero-field muon spin relaxation technique has been used in the direct observation of spontaneous magnetic order below a Curie temperature (T(c)) of approximately 16.1 kelvin in the fullerene charge-transfer salt (tetrakisdimethylaminoethylene)C(60) [(TDAE)C(60)]. Coherent ordering of the electronic magnetic moments leads to a local field of 68(1) gauss at the muon site at 3.2 kelvin (parentheses indicate the error in the last digit). Substantial spatially inhomogeneous effects are manifested in the distribution of the local fields, whose width amounts to 48(2) gauss at the same temperature. The temperature evolution of the internal magnetic field below the freezing temperature mirrors that of the saturation magnetization, closely following the behavior expected for collective spin wave (magnon) excitations. The transition to a ferromagnetic state with a T(c) higher than that of any other organic material is now authenticated.

Transverse magnetism in uniaxial antiferromagnet UNiGa.

UNiGa crystallizes in the hexagonal ZrNiAl structure and orders antiferromagnetically below T(N)=39.3 K with the U moments oriented along the c-axis (easy magnetization axis). There are four different antiferromagnetic phases in zero magnetic field and two field induced magnetic phases in UNiGa. For all of them, a strong uniaxial anisotropy is encountered. All the magnetic phases are described by propagation vectors of (0 0 q(i))-type, where q(i) describes the stacking of the ferromagnetic basal planes along the hexagonal c-axis. However, recently (0 0 L)-type Bragg reflections associated with the magnetic ordering have been observed by neutron diffraction. Based on unpolarized and polarized neutron diffraction and non-resonant and resonant synchrotron x-ray scattering experiments combined with polarization analysis we conclude that small amounts of magnetic moments oriented perpendicular to the c-axis exist in UNiGa. Whether these moments reside on Ni atoms outside the U-Ni planes or at interstitial regions could not be determined.

The magnetic order of GdMn2Ge2 studied by neutron diffraction and x-ray resonant magnetic scattering.

The magnetic structure of GdMn2Ge2 (tetragonal I4/mmm) has been studied by hot neutron powder diffraction and x-ray resonant magnetic scattering techniques. These measurements, along with the results of bulk experiments, confirm the collinear ferrimagnetic structure with moment direction parallel to the c-axis below T(C) = 96 K and the collinear antiferromagnetic phase in the temperature region T(C) < T < T(N) = 365 K. In the antiferromagnetic phase, x-ray resonant magnetic scattering has been detected at Mn K and Gd L2 absorption edges. The Gd contribution is a result of an induced Gd 5d electron polarization caused by the antiferromagnetic order of Mn-moments.

Enhanced ferroelectric polarization by induced Dy spin order in multiferroic DyMnO3.

Neutron powder diffraction and single crystal x-ray resonant magnetic scattering measurements suggest that Dy plays an active role in enhancing the ferroelectric polarization in multiferroic DyMnO3 above T(Dy)(N)=6.5 K. We observe the evolution of an incommensurate ordering of Dy moments with the same periodicity as the Mn spiral ordering. It closely tracks the evolution of the ferroelectric polarization. Below T(Dy)(N), where Dy spins order commensurately, the polarization decreases to values similar for those of TbMnO3. The higher P(s) found just above T(Dy)(N) arises from the contribution of Dy spins so as to effectively increase the amplitude of the Mn spin spiral.

Coupling of frustrated ising spins to the magnetic cycloid in multiferroic TbMnO(3).

We report on diffraction measurements on multiferroic TbMnO(3) which demonstrate that the Tb- and Mn-magnetic orders are coupled below the ferroelectric transition T(FE) = 28 K. For T

Electron spin resonance in sine-gordon spin chains in the perturbative spinon regime.

We report the low-temperature multifrequency ESR studies of copper pyrimidine dinitrate, a spin-1/2 antiferromagnetic chain with alternating g tensor and the Dzyaloshinskii-Moriya interaction, allowing us to test a new theoretical concept proposed recently by Oshikawa and Affleck [Phys. Rev. Lett. 82, 5136 (1999)]. Their theory, based on bosonization and self-energy formalism, can be applied for precise calculation of ESR parameters of S=1/2 antiferromagnetic chains in the perturbative spinon regime. Excellent quantitative agreement between the theoretical predictions and experiment is obtained.

Giant spin canting in the S=1/2 antiferromagnetic chain [CuPM(NO3)2(H2O)2]n observed by 13C-NMR.

We present a combined experimental and theoretical study on copper pyrimidine dinitrate [CuPM(NO3)2(H2O)2]n, a one-dimensional S=1/2 antiferromagnet with alternating local symmetry. From the local susceptibility measured by NMR at the three inequivalent carbon sites in the pyrimidine molecule we deduce a giant spin canting, i.e., an additional staggered magnetization perpendicular to the applied external field at low temperatures. The magnitude of the transverse magnetization, the spin canting of (52+/-4) degrees at 10 K and 9.3 T, and its temperature dependence are in excellent agreement with exact diagonalization calculations.

Excitation hierarchy of the quantum sine-gordon spin chain in a strong magnetic field.

The magnetic excitation spectrum of copper pyrimidine dinitrate, a material containing S = 1 / 2 antiferromagnetic chains with alternating g tensor and the Dzyaloshinskii-Moriya interaction and exhibiting a field-induced spin gap, is probed using submillimeter wave electron spin resonance spectroscopy. Ten excitation modes are resolved in the low-temperature spectrum, and their frequency-field diagram is systematically studied in magnetic fields up to 25 T. The experimental data are sufficiently detailed to make a very accurate comparison with predictions based on the quantum sine-Gordon field theory. Signatures of three breather branches and a soliton, as well as those of several multiparticle excitation modes, are identified.