Dimer Physics in the Frustrated Cairo Pentagonal Antiferromagnet Bi_{2}Fe_{4}O_{9}.

The research field of magnetic frustration is dominated by triangle-based lattices but exotic phenomena can also be observed in pentagonal networks. A peculiar noncollinear magnetic order is indeed known to be stabilized in Bi_{2}Fe_{4}O_{9} materializing a Cairo pentagonal lattice. We present the spin wave excitations in the magnetically ordered state, obtained by inelastic neutron scattering. They reveal an unconventional excited state related to local precession of pairs of spins. The magnetic excitations are then modeled to determine the superexchange interactions for which the frustration is indeed at the origin of the spin arrangement. This analysis unveils a hierarchy in the interactions, leading to a paramagnetic state (close to the Néel temperature) constituted of strongly coupled dimers separated by much less correlated spins. This produces two types of response to an applied magnetic field associated with the two nonequivalent Fe sites, as observed in the magnetization distributions obtained using polarized neutrons.

Operating Quantum States in Single Magnetic Molecules: Implementation of Grover's Quantum Algorithm.

Quantum algorithms use the principles of quantum mechanics, such as, for example, quantum superposition, in order to solve particular problems outperforming standard computation. They are developed for cryptography, searching, optimization, simulation, and solving large systems of linear equations. Here, we implement Grover's quantum algorithm, proposed to find an element in an unsorted list, using a single nuclear 3/2 spin carried by a Tb ion sitting in a single molecular magnet transistor. The coherent manipulation of this multilevel quantum system (qudit) is achieved by means of electric fields only. Grover's search algorithm is implemented by constructing a quantum database via a multilevel Hadamard gate. The Grover sequence then allows us to select each state. The presented method is of universal character and can be implemented in any multilevel quantum system with nonequal spaced energy levels, opening the way to novel quantum search algorithms.

The magnetic properties and structure of the quasi-two-dimensional antiferromagnet CoPS3.

The magnetic properties and magnetic structure are presented for CoPS3, a quasi-two-dimensional antiferromagnet on a honeycomb lattice with a Néel temperature of [Formula: see text] K. The compound is shown to have XY-like anisotropy in its susceptibility, and the anisotropy is analysed to extract crystal field parameters. For temperatures between 2 K and 300 K, no phase transitions were observed in the field-dependent magnetization up to 10 Tesla. Single-crystal neutron diffraction shows that the magnetic propagation vector is k = [Formula: see text] with the moments mostly along the [Formula: see text] axis and with a small component along the [Formula: see text] axis, which largely verifies the previously-published magnetic structure for this compound. The magnetic Bragg peak intensity decreases with increasing temperature as a power law with exponent [Formula: see text] for [Formula: see text].

Fragmentation in spin ice from magnetic charge injection.

The complexity embedded in condensed matter fertilizes the discovery of new states of matter, enriched by ingredients like frustration. Illustrating examples in magnetic systems are Kitaev spin liquids, skyrmions phases, or spin ices. These unconventional ground states support exotic excitations, for example the magnetic charges in spin ices, also called monopoles. Here, we propose a mechanism to inject monopoles in a spin ice at equilibrium through a staggered magnetic field. We show theoretically, and demonstrate experimentally in the Ho2Ir2O7 pyrochlore iridate, that it results in the stabilization of a monopole crystal, which exhibits magnetic fragmentation. In this new state of matter, the magnetic moment fragments into an ordered part and a persistently fluctuating one. Compared to conventional spin ices, the different nature of the excitations in this fragmented state opens the way to tunable field-induced and dynamical behaviors.Exploring unconventional magnetism facilities both fundamental understanding of materials and their real applications. Here the authors demonstrate that a magnetic monopole crystal is stabilized by a staggered magnetic field in the pyrochlore iridate Ho2Ir2O7, leading to a fragmented magnetization.

Anisotropy-Tuned Magnetic Order in Pyrochlore Iridates.

The magnetic behavior of polycrystalline samples of Er(2)Ir(2)O(7) and Tb(2)Ir(2)O(7) pyrochlores is studied by magnetization measurements and neutron diffraction. Both compounds undergo a magnetic transition at 140 and 130 K, respectively, associated with an ordering of the Ir sublattice, signaled by thermomagnetic hysteresis. In Tb(2)Ir(2)O(7), we show that the Ir molecular field leads the Tb magnetic moments to order below 40 K in the all-in-all-out magnetic arrangement. No sign of magnetic long-range order on the Er sublattice is evidenced in Er(2)Ir(2)O(7) down to 0.6 K where a spin freezing is detected. These contrasting behaviors result from the competition between the Ir molecular field and the different single-ion anisotropy of the rare-earth elements on which it is acting. Additionally, this strongly supports the all-in-all-out iridium magnetic order.

Magneto- to electroactive transmutation of spin waves in ErMnO3.

The low-energy dynamical properties of the multiferroic hexagonal perovskite ErMnO3 have been studied by inelastic neutron scattering as well as terahertz and far infrared spectroscopies on a synchrotron source. From these complementary techniques, we have determined the magnon and crystal field spectra and identified a zone center magnon excitable only by the electric field of an electromagnetic wave. Using a comparison with the isostructural YMnO3 compound and crystal field calculations, we propose that this dynamical magnetoelectric process is due to the hybridization of a magnon with an electroactive crystal field transition.

Symmetry-protected hidden order and magnetic neutron Bragg diffraction by URu2Si2.

We investigate how the order parameter of a continuous phase transition can be protected from view by symmetry in a magnetic crystal. The symmetry in question forbids atomic displacements and formation of magnetic dipoles, rendering the order parameter invisible in standard x-ray and magnetic neutron Bragg diffraction. Analysis of the allowed magnetic space-groups reveals exact properties of the hidden order parameter. We demonstrate that Bragg spots forbidden by the chemical structure can unveil magnetic hidden order. The method is applied to URu2Si2, which has been thoroughly investigated in the past few decades using all manner of experimental techniques. Starting from the established chemical structure of URu2Si2, we have performed a critical analysis of available data for magnetic neutron Bragg diffraction.

Quantum interference oscillations of the superparamagnetic blocking in an Fe8 molecular nanomagnet.

We show that the dynamic magnetic susceptibility and the superparamagnetic blocking temperature of an Fe(8) single molecule magnet oscillate as a function of the magnetic field H(x) applied along its hard magnetic axis. These oscillations are associated with quantum interferences, tuned by H(x), between different spin tunneling paths linking two excited magnetic states. The oscillation period is determined by the quantum mixing between the ground S=10 and excited multiplets. These experiments enable us to quantify such mixing. We find that the weight of excited multiplets in the magnetic ground state of Fe(8) amounts to approximately 11.6%.

THz magnetoelectric atomic rotations in the chiral compound Ba3NbFe3Si2O14.

We have determined the terahertz spectrum of the chiral langasite Ba3NbFe3Si2O14 by means of synchrotron-radiation measurements. Two excitations are revealed that are shown to have a different nature. The first one, purely magnetic, is observed at low temperature in the magnetically ordered phase and is assigned to a magnon. The second one persists far into the paramagnetic phase and exhibits both an electric and a magnetic activity at slightly different energies. This magnetoelectric excitation is interpreted in terms of atomic rotations and requires a helical electric polarization.

Hidden order in URu2Si2 unveiled.

We report on measurements, by polarized neutron elastic scattering, of the magnetization distribution induced in a single crystal of URu2Si2 under a magnetic field applied along the tetragonal c axis. A subtle change in this distribution, revealed by maximum entropy analysis of the data, is found when the temperature is decreased to the range of the hidden order. An analysis in terms of U(4+) ionic states reveals that this change is a fingerprint of a freezing of rank 5 multipoles, i.e., dotriacontapoles.

Magnetic dipolar ordering and quantum phase transition in an Fe8 molecular magnet.

We show that a crystal of mesoscopic Fe(8) single-molecule magnets is an experimental realization of the quantum Ising model in a transverse field, with dipolar interactions. Quantum annealing has enabled us to explore the quantum and classical phase transitions between the paramagnetic and ferromagnetic phases, at thermodynamical equilibrium. The phase diagram and critical exponents that we obtain agree with expectations for the mean-field universality class.

Parity-broken chiral spin dynamics in Ba3NbFe3Si2O14.

The spin-wave excitations emerging from the chiral helically modulated 120° magnetic order in a langasite Ba3NbFe3Si2O14 enantiopure crystal were investigated by unpolarized and polarized inelastic neutron scattering. A dynamical fingerprint of the chiral ground state is obtained, singularized by (i) spectral weight asymmetries answerable to the structural chirality and (ii) a full chirality of the spin correlations observed over the whole energy spectrum. The intrinsic chiral nature of the spin waves' elementary excitations is shown in the absence of macroscopic time-reversal symmetry breaking.

Role of antisymmetric exchange in selecting magnetic chirality in Ba3NbFe3Si2O14.

We present an electron spin resonance (ESR) investigation of the acentric Ba(3)NbFe(3)Si(2)O(14), featuring a unique single-domain double-chiral magnetic ground state. Combining simulations of the ESR linewidth anisotropy and the antiferromagnetic-resonance modes allows us to single out the Dzyaloshinsky-Moriya (DM) interaction as the leading magnetic anisotropy term. We demonstrate that the rather minute out-of-plane DM component d(c)=45 mK is responsible for selecting a unique ground state, which endures thermal fluctuations up to astonishingly high temperatures.

Domain-wall spin dynamics in kagome antiferromagnets.

We report magnetization and neutron scattering measurements down to 60 mK on a new family of Fe based kagome antiferromagnets, in which a strong local spin anisotropy combined with a low exchange path network connectivity lead to domain walls intersecting the kagome planes through strings of free spins. These produce unfamiliar slow spin dynamics in the ordered phase, evolving from exchange-released spin flips towards a cooperative behavior on decreasing the temperature, probably due to the onset of long-range dipolar interaction. A domain structure of independent magnetic grains is obtained that could be generic to other frustrated magnets.

Single domain magnetic helicity and triangular chirality in structurally enantiopure Ba3NbFe3Si2O14.

A novel doubly chiral magnetic order is found in the structurally chiral langasite compound Ba3NbFe3Si2O14. The magnetic moments are distributed over planar frustrated triangular lattices of triangle units. On each of these they form the same triangular configuration. This ferrochiral arrangement is helically modulated from plane to plane. Unpolarized neutron scattering on a single crystal associated with spherical neutron polarimetry proved that a single triangular chirality together with a single helicity is stabilized in an enantiopure crystal. A mean-field analysis allows us to discern the relevance on this selection of a twist in the plane to plane super-superexchange paths.

Propagation and ghosts in the classical kagome antiferromagnet.

We investigate the classical spin dynamics of the kagome antiferromagnet by combining Monte Carlo and spin dynamics simulations. We show that this model has two distinct low temperature dynamical regimes, both sustaining propagative modes. The expected gauge invariance type of the low energy, low temperature, out-of-plane excitations is also evidenced in the nonlinear regime. A detailed analysis of the excitations allows us to identify ghosts in the dynamical structure factor, i.e., propagating excitations with a strongly reduced spectral weight. We argue that these dynamical extinction rules are of geometrical origin.

Logarithmic Fermi-liquid breakdown in NbFe2.

The d-electron low temperature magnet NbFe2 is poised near the threshold of magnetism at ambient pressure, and can be tuned across the associated quantum critical point by adjusting the precise stoichiometry within the Nb1-yFe2+y homogeneity range. In a nearly critical single crystal (y= -0.01), we observe a T3/2 power-law dependence of the resistivity rho on temperature T and a logarithmic temperature dependence of the Sommerfeld coefficient gamma=C/T of the specific heat capacity C over nearly 2 orders of magnitude in temperature, extending down to 0.1 K.

Hidden magnetic frustration by quantum relaxation in anisotropic Nd langasite.

The static and dynamic magnetic properties of the Nd(3)Ga(5)SiO(14) compound, which appears as the first materialization of a rare-earth kagome-type lattice, were reexamined, owing to contradictory results in the previous studies. Neutron scattering, magnetization, and specific heat measurements were performed and analyzed, in particular, by fully taking account of the crystal electric field effects on the Nd(3+) ions. One of the novel findings is that the peculiar temperature independent spin dynamics observed below 10 K expresses single-ion quantum processes. This would short-circuit the frustration induced cooperative dynamics, which would emerge only at very low temperature.

Spin-liquid correlations in the Nd-langasite anisotropic kagomé antiferromagnet.

Dynamical magnetic correlations in the geometrically frustrated Nd(3)Ga(5)SiO(14) compound were probed by inelastic neutron scattering on a single crystal. A scattering signal with a ring shape distribution in reciprocal space and unprecedented dispersive features was discovered. Comparison with calculated static magnetic scattering from models of correlated spins suggests that the observed phase is a spin liquid inherent to an antiferromagnetic kagomé-like lattice of anisotropic Nd moments.

Electrochemical growth of iron and cobalt arborescences under a magnetic field.

Pattern formation in the electrochemical deposition of the magnetic Fe and Co metals from thin layers of Fe(SO4) or Co(SO4) aqueous solutions were investigated in circular geometry and under magnetic field. Sparse arborescences with few thick branches and dense arborescences with many thin branches can be generated when no magnetic field is applied. Unlike for nonmagnetic metals, no tendency towards growth spiraling or asymmetric branching is found out in magnetic field normal to the plane of the growth. The morphology of the deposits appears instead to become more sparse. Under in-plane magnetic field, the sparse arborescences get into a needle morphology, oriented along the field, while the dense arborescences show a circular to rectangular morphology symmetry breaking, one edge of the rectangle being parallel to the field. Unexpected in most instances, these magnetic field effects cannot be understood without invoking the magnetic dipolar interaction inside the magnetized growing aggregate together with its interaction with the applied field.