Unwinding of a cholesteric liquid crystal and bidirectional surface anchoring

We examine the influence of bidirectional anchoring on the unwinding of a planar cholesteric liquid crystal induced by the application of a magnetic field. We consider a liquid crystal layer confined between two plates with the helical axis perpendicular to the substrates. We fixed the director twist on one boundary and allow for bidirectional anchoring on the other by introducing a high-order surface potential. By minimizing the total free energy for the system, we investigate the untwisting of the cholesteric helix as the liquid crystal attempts to align with the magnetic field. The transitions between metastable states occur as a series of pitchjumps as the helix expels quarter or half-turn twists, depending on the relative sizes of the strength of the surface potential and the bidirectional anchoring. We show that secondary easy axis directions can play a significant role in the unwinding of the cholesteric in its transition towards a nematic, especially when the surface anchoring strength is large

Magnetic control of large room-temperature polarization

Numerous authors have referred to room-temperature magnetic switching of large electric polarizations as The Holy Grail of magnetoelectricity.We report this long-sought effect using a new physical process of coupling between magnetic and ferroelectric relaxor nano-regions. Here we report magnetic switching between the normal ferroelectric state and the ferroelectric relaxor state. This gives both a new room-temperature, single-phase, multiferroic magnetoelectric, PbZr0.46Ti0.34Fe0.13W0.07O3, with polarization, loss (<4%), and resistivity (typically 108 -109 ohm.cm) equal to or superior to BiFeO3, and also a new and very large magnetoelectric effect: switching not from +Pr to negative Pr with applied H, but from Pr to zero with applied H of less than a Tesla. This switching of the polarization occurs not because of a conventional magnetically induced phase transition, but because of dynamic effects: Increasing H lengthens the relaxation time by x500 from 100 ?s, and it couples strongly the polarization relaxation and spin relaxations. The diverging polarization relaxation time accurately fits a modified Vogel-Fulcher Equation in which the freezing temperature Tf is replaced by a critical freezing field Hf that is 0.92 positive/negative 0.07 Tesla. This field dependence and the critical field Hc are derived analytically from the spherical random bond random field (SRBRF) model with no adjustable parameters and an E2H2 coupling. This device permits 3-state logic (+Pr,0,negative Pr) and a condenser with >5000% magnetic field change in its capacitance.Comment: 20 pages, 5 figure

Magnetic bearings: Fifty years of progress

Magnetic bearings are just beginning to be flown in spacecraft systems, but their development spans more than 50 years. The promise of completely noncontacting, unlubricated rotating systems operating at speeds substantially beyond the range of conventional bearings, and with no wear and virtually no vibration, has provided the incentive to develop magnetic bearing technology for many diverse applications. Earnshaw theorized in 1842 that stable magnetic suspension is not possible in all three spatial directions unless the magnetic field is actively controlled. Since that time, researchers have attempted to successfully support spinning rotors in a stable manner. Development of magnetic suspension systems over the past fifty years has included progress on both passive (permanent magnet) and active (electromagnet) systems. The improvements in bearing load capacity, stiffness, and damping characteristics are traced. The trends in rotor size, rotational kinetic energy, and improvements in active control systems capabilities are also reviewed. Implications of superconductivity on suspension system design and performance are discussed

A rock- and paleomagnetic study of a Holocene lava flow in Central Mexico

Magnetic measurements of the Tres Cruces lava flow (ca. 8500 years BP, Central Mexico) show the presence of two remanence carriers, a Ti-rich titanomagnetite with a Curie temperature between 350 and 400 °C and a Ti-poor magnetite with a Curie temperature close to 580°C. Magnetic changes after heating indicate that the titanomagnetite exsolves into magnetite w and ilmenite when the sample is heated to 580 °C. Paleointensity estimates with the Thellier and Thellier method [Thellier, E., Thellier, O., 1959. Sur l'intensité du champ magnetique terrestre dans le passe historique et geologique. Ann. Geophysique., 15, 285-376] were only successful up to temperatures of 350 to 400 °C. This temperature corresponds with the Curie temperature of the titanomagnetite, which is probably pseudo-single or multi-domain. Therefore, the paleointensities should be interpreted with caution. The magnetic composition changes after 580 °C heating may explain the large w variations in previous paleointensity determinations for the Tres Cruces rocks [Gonzalez, S., Sherwood, G., Bohnel, H., Schnepp, E., 1997. Palaeosecular variation in Central Mexico over the last 30,000 years: the record from lavas. Geophys. J. Int., 130, 201-219] using the [Shaw method Shaw, J., 1974. A new method of determining the magnitude of the palaeomagnetic field: application to five historic lavas and five archaeological samples. Geophys. J. R. Astr. Soc., 39, 133-141]

Stability of anhysteretic remanent magnetization in fine and coarse magnetite and maghemite particles

Further experiments have been performed to investigate the biasing-field dependency of alternating field demagnetization curves of anhysteretic remanent magnetization as a simple test for the domain state of magnetite and maghemite particles. The biasing-field dependency in fine-grained particles was opposite to that in coarse-grained particles. The experiments were conducted on well sized synthetic specimens in the single domain, pseudo-single domain and multi-domain grain size ranges. A single domain-like biasing-field dependency was observed in equidimensional particles up to 0.2µ mean grain size and up to 0.4µ elongated grains. Either the single domain/pseudo-single domain boundary lies above at least 0.2µ grain size or this field dependency test does not distinguish between single domain and pseudo-single domain states. A multidomainlike trend was observed in very coarse magnetite. The test may possibly distinguish the change from pseudo-single domain to multi-domain states. If both fine and coarse fractions are present a confusing overlap of the demagnetization curves occurs

Tetrataenite in chondrites and experimental demonstration on formation of tetrataenite fine grains

Tetrataenite (Fe_Ni_γ" phase) is a unique metallic mineral in meteorites. A magnetic granulometry analysis of chondrites on the basis of magnetic hysteresis parameters at various temperatures has shown that some LL chondrites contain very fine grains of tetrataenite metal, the grain sizes of which are 10nm or smaller in diameter. The coalescence growth process of a joint cloud of fine smoke particles evaporated from Fe and that from Ni produces fine grains of tetrataenite (FeNiγ"), in addition to those of ordered kamacite (Fe_3Ni), awaruite (FeNi_3) as well as disordered taenites. An outline of the experimental procedures of coalescence growth formation of tetrataenite and the other ordered crystal grains of Fe-Ni alloy and main results of the experiments is described

Evidence for a dynamical ground state in the frustrated pyrohafnate Tb2Hf2O7

We report the physical properties of Tb2Hf2O7 based on ac magnetic susceptibility \chi_ac(T), dc magnetic susceptibility \chi(T), isothermal magnetization M(H), and heat capacity C_p(T) measurements combined with muon spin relaxation (\muSR) and neutron powder diffraction measurements. No evidence for long-range magnetic order is found down to 0.1 K. However, \chi_ac(T) data present a frequency-dependent broad peak (near 0.9 K at 16 Hz) indicating slow spin dynamics. The slow spin dynamics is further evidenced from the \muSR data (characterized by a stretched exponential behavior) which show persistent spin fluctuations down to 0.3 K. The neutron powder diffraction data collected at 0.1 K show a broad peak of magnetic origin (diffuse scattering) but no magnetic Bragg peaks. The analysis of the diffuse scattering data reveals a dominant antiferromagnetic interaction in agreement with the negative Weiss temperature. The absence of long-range magnetic order and the presence of slow spin dynamics and persistent spin fluctuations together reflect a dynamical ground state in Tb2Hf2O7.Comment: 11 pages and 8 figure