R. Ballou

Electrically driven nuclear spin resonance in single-molecule magnets

Electrical control of nuclear spin qubits Quantum bits of information (qubits) that are based on spins of atomic nuclei are an attractive option for quantum information processing. It can sometimes be tricky to manipulate these qubits using magnetic fields directly. Thiele et al. developed a technique for electrically controlling a nuclear spin qubit in the single-molecule magnet TbPc2. When they hit the qubit with a microwave pulse, the microwaves electric field generated effective magnetic fields much larger than those available previously. Science, this issue p. 1135 The hyperfine interaction in a terbium bisphthalocyanine complex is modulated using electric fields to manipulate nuclear spin. Recent advances in addressing isolated nuclear spins have opened up a path toward using nuclear-spin–based quantum bits. Local magnetic fields are normally used to coherently manipulate the state of the nuclear spin; however, electrical manipulation would allow for fast switching and spatially confined spin control. Here, we propose and demonstrate coherent single nuclear spin manipulation using electric fields only. Because there is no direct coupling between the spin and the electric field, we make use of the hyperfine Stark effect as a magnetic field transducer at the atomic level. This quantum-mechanical process is present in all nuclear spin systems, such as phosphorus or bismuth atoms in silicon, and offers a general route toward the electrical control of nuclear-spin–based devices.

Analytical Determination of the {Ln–Aminoxyl Radical} Exchange Interaction Taking into Account Both the Ligand‐Field Effect and the Spin–Orbit Coupling of the Lanthanide Ion (Ln=DyIII and HoIII)

Numerous compounds in which a paramagnetic LnIII ion is in an exchange interaction with a second spin carrier, such as a transition metal ion or an organic radical, have been described. However, except for GdIII, very little has been reported about the magnitude of the interactions. Indeed, for these ions both the ligand-field effects and the exchange interactions between the magnetic centers become relevant in the same temperature range; this makes the analysis of the magnetic behavior of such compounds more difficult. In this study, quantitative analyses of the thermal variations of the static isothermal initial magnetic susceptibility measured on powdered samples of the [Ln(NO3)3-[organic radical]2] (Ln = DyIII and HoIII) compounds were performed. The ligand-field effects on the Ln ions were taken into account, and the exchange interactions within a molecule were treated exactly within an appropriate Racah formalism. Values of the intramolecular [Ln-aminoxyl radical] exchange parameter have thus been rigorously deduced for both the Dy Kramers and Ho non-Kramers ion-based compounds. Ferromagnetic [Ln-radical] interactions are found for both the Dy and Ho derivatives with J = 8 cm(-1) and J = 4.5 cm(-1), respectively.

Helimagnetism in the cubic Laves phase YMn2

Abstract Neutron diffraction experiments on YMn 2 using a wave length of λ N = 2.483 A show a splitting of the magnetic peaks. The magnetic structure is helimagnetic consistent with an angle modulation of the previously reported antiferromagnetic structure. The NMR spectrum can be explained as arising from a perturbation of the helix by the magnetocrystalline anisotropy. Below T N the observed frustration of the negative Mn interactions is inherent from the topology of the crystallographic structure. Above T N , it creates short range ordering whose thermal decrease may explain the increase in the paramagnetic neutron scattering as the temperature is increased.

Results from the OSQAR photon-regeneration experiment : No light shining through a wall

A new method to amplify the photon-axion conversions in a magnetic field is proposed using a buffer gas at a specific pressure in a photon-regeneration experiment. As a first result, new bounds for mass and coupling constant for laboratory experiments aiming to detect any hypothetical scalars and pseudoscalars, which can couple to photons were obtained, excluding with 95% confidence level, the recently withdrawn PVLAS result.

Mn moment instability in the TbMn2 intermetallic compound

Neutron diffraction experiments have been performed on a TbMn2 single crystal and on Tb(Mn0.96Fe0.04)2 powder samples. The magnetic structure of TbMn2 is metastable poised between two structures, S1 with propagation vector (2/3 2/3 0) and S2 with propagation vector (1/2 1/2 1/2). A transition from S1 to S2 can be induced either by an applied field of 4.5 T at 25 K or by chemical pressure induced by substitution of Mn by Fe. The S2 structure has been studied in Tb(Mn0.96Fe0.04)2. The transition to this structure is accompanied by a huge rhombohedral distortion and the structure itself is notable for the coexistence of magnetic and non-magnetic manganese atoms. This peculiar feature is attributed to instability of the Mn moment combined with frustration of the Mn itinerant antiferromagnetism.

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.

Magnetoelectric MnPS3 thiophosphate as a new candidate for ferrotoroidicity

We have revisited the magnetic structure of manganese phosphorus trisulfide MnPS3 using neutron diffrac- tion and polarimetry. MnPS3 undergoes a transition toward a collinear antiferromagnetic order at 78 K. The resulting magnetic point-group breaks both the time reversal and the space inversion thus allowing a linear magnetoelectric coupling. Neutron polarimetry was subsequently used to prove that this coupling provides a way to manipulate the antiferromagnetic domains simply by cooling the sample under crossed magnetic and electrical fields, in agreement with the nondiagonal form of the magnetoelectric tensor. In addition, this tensor has, in principle, an antisymmetric part that results in a toroidic moment and provides with a pure ferrotoroidic compound.

The Pyrrhotite 32 K Magnetic Transition

Pyrrhotite [1] is a common ferrimagnetic mineral in terrestrial rocks and has been identified recently as the major remanence carrier in Martian rocks, as in SNC (Shergotty-Nakhla-Chassigny) type meteorites. This compound undergoes a low temperature magnetic transition around 32 K with a change in the natural magnetic remanence [2]. This transition is going to be of growing importance in paleomagnetism and rock magnetism. To determine the structure change at 32K neutron diffraction was performed on two crystal samples at 50K and 20K (D10-ILL). Based on the magnetic torque measurements, performed on the same samples, a model of low-temperature triclinic structure was build. This model, explains previous Mössbauer experiments [4], the torque measurements, the magneto-crystalline anisotropy decreasing.

Large metal clusters and lattices with analogues to biology

The magnetic properties of two types of material derived from the brucite lattice, M(OH)2 are described. The structure-directing effects of simple templates on the brucite lattice parallels the processes seen in nature in the formation of biominerals. The first type exemplified by Fe17/Fe19 aggregates models the structural features of the iron storage protein ferritin. The magnetic behaviour also reveals some interesting parallels with the magnetic phenomena reported previously on ferritins. We have used a combination of experimental techniques including DC, AC and RF susceptibility measurements on powders and a microSQUID on single crystals. The second type is based on an extended-defect brucite structure with stoichiometry M2(OH)2 (ox) for M = Fe2+, Co2+. They show long-range ordering to antiferromagnetic phases and then, at much lower temperatures, undergo a phase transition to canted antiferromagnetic states. Symmetry arguments are used to predict the spin configurations in these extended materials.

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.