High-field State Research Articles

Kinetic arrest of the first order austenite to martensite phase transition inNi50Mn34In16: dc magnetization studies

We report results of dc magnetization studies focusing particularly on the austenite-martensite phase transition in ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{34}{\mathrm{In}}_{16}$. We show that the nature of this phase transition depends significantly on the temperature (T) and magnetic field (H) history of the sample. In the presence of high magnetic field, this austenite to martensite first order phase transition is kinetically arrested. The low-temperature and high-field magnetic state shows a typical nonergodic glasslike dynamical response. Comparisons are made with similar phenomena observed recently in various classes of magnetic materials including CMR manganites.

Importance of In-Plane Anisotropy in the Quasi-Two-Dimensional AntiferromagnetBaNi2V2O8

The phase diagram of the quasi-two-dimensional antiferromagnet BaNi(2)V(2)O(8) is studied by specific heat, thermal expansion, magnetostriction, and magnetization for magnetic fields applied perpendicular to c. At micro(o)H* approximately 1.5 T, a crossover to a high-field state, where T(N)(H) increases linearly, arises from a competition of intrinsic and field-induced in-plane anisotropies. The pressure dependences of T(N) and H* are interpreted using the picture of a pressure-induced in-plane anisotropy. Even at zero field and ambient pressure, in-plane anisotropy cannot be neglected, which implies deviations from pure Berezinskii-Kosterlitz-Thouless behavior.

Multimagnon bound states in the frustrated ferromagnetic one-dimensional chain

We study a one-dimensional Heisenberg chain with competing ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions in a magnetic field. Starting from the fully polarized high-field state, we calculate the dispersions of the lowest-lying $n$-magnon excitations and the saturation field $(n=2,3,4)$. We show that the lowest-lying excitations are always bound multimagnon states with a total momentum of $\ensuremath{\pi}$ except for a small parameter range. We argue that Bose condensation of the bound $n$ magnons leads to novel Tomonaga-Luttinger liquids with multipolar correlations; nematic- and triatic-ordered liquids correspond to $n=2$ and $n=3$.

A Linear AC Trap for Polar Molecules in Their Ground State

A linear AC trap for polar molecules in high-field seeking states has been devised and implemented, and its characteristics have been investigated both experimentally and theoretically. The trap is loaded with slow 15ND3 molecules in their ground state (para-ammonia) from a Stark decelerator. The trap's geometry offers optimal access as well as improved loading. We present measurements of the dependence of the trap's performance on the switching frequency, which exhibit a characteristic structure due to nonlinear resonance effects. The molecules are found to oscillate in the trap under the influence of the trapping forces, which were analyzed using 3D numerical simulations. On the basis of expansion measurements, molecules with a velocity and a position spread of 2.1 m/s and 0.4 mm, respectively, are still accepted by the trap. This corresponds to a temperature of 2.0 mK. From numerical simulations, we find the phase-space volume that can be confined by the trap (the acceptance) to be 50 mm3 (m/s)3.

Discontinuous spring magnet-type magnetostrictive Terfecohan/YFeCo multilayers: A novel nanostructured material principle for excellent magnetic softness

Novel physics and reversal mechanisms of the whole system switching (WS) and individual switching (IS) type are reported for hard/soft TbFeCo/YFeCo exchange-spring multilayers. The WS type usually occurs in multilayered systems, in which the magnetic anisotropy of hard TbFeCo layers is neglectable. For such a system, the ferrimagnetically coupled hard/soft multilayered state is recovered after removing applied fields from the magnetized state. At low negative fields, the magnetization switching occurs collectively for all magnetic moments in the whole system. In this case, the low-coercivity mechanism is discussed on the basis of a hard/soft interfacial point contact. This configuration is realized for TbFeCo/YFeCo discontinuous exchange-spring multilayers, in which the magnetic (Fe,Co) nanograins coexist with non-magnetic amorphous phase in the soft layers. In this state, a magnetic coercivity as small as 0.4 mT is achieved. It is considered as an excellent magnetic softness of rare-earth-based systems. Enhancing the magnetic anisotropy in the hard TbFeCo layers, the magnetization switching follows the IS type at low temperatures. Starting to decrease the applied magnetic field from the high-field state, one observes the first reversal of the magnetic moments in the soft high-magnetization YFeCo-layers in positive magnetic fields. This is the reason for the observation of the negative coercivity as well as negative-biasing phenomena.

Fermi-liquid effects in the Fulde-Ferrell-Larkin-Ovchinnikov state of two-dimensionald-wave superconductors

We study the effects of Fermi-liquid interactions on quasi-two-dimensional $d$-wave superconductors in a magnetic field. The phase diagram of the superconducting state, including the periodic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in high magnetic fields, is discussed for different strengths of quasiparticle many-body interactions within Landau's theory of Fermi liquids. Decreasing the Fermi-liquid parameter ${F}_{0}^{a}$ causes the magnetic spin susceptibility of itinerant electrons to increase, which in turn leads to a reduction of the FFLO phase. It is shown that a negative ${F}_{0}^{a}$ results in a first-order phase transition from the normal to the uniform superconducting state in a finite temperature interval. Finally, we discuss the thermodynamic implications of a first-order phase transition for $\mathrm{Ce}\mathrm{Co}{\mathrm{In}}_{5}$.

Mechanisms of the giant electrorheological effect

Electrorheology (ER) denotes the control of rheological characteristics through applied electric field. ER fluids constitute a class of colloids whose viscosity can increase under increasing electric field. Under very strong fields some ER fluids can turn into anisotropic solids, characterized by a yield stress. The recent discovery of the giant electrorheological (GER) effect in suspensions of nanoparticles has challenged the conventional wisdom on ER fluids, as the GER fluids can break the theoretical upper bound on the (high-field state) yield stress. Starting from experimental observations of the GER characteristics, we show that the model of aligned molecular dipole layers in the contact region of coated nanoparticles can yield predictions in excellent agreement with measured data. The statistical mechanics of the aligned dipole layers is studied through Monte Carlo simulations. We propose electrowetting between the particles and the suspending liquid, with hydrogen bonding as a contributing element, in inducing the aligned state.

Measurement of the three-dimensional velocity distribution of Stark-decelerated Rydberg atoms

The full three-dimensional velocity distributions of decelerated and accelerated particles in a Stark decelerator for Rydberg atoms and molecules have been measured. In the experiment, argon atoms in a supersonic beam are excited to low-field and high-field seeking Stark states with principal quantum number in the range n=15 to 25 and are decelerated in a 3 mm long decelerator consisting of four electrodes on which time-dependent voltages are applied. The time dependence of the resulting inhomogeneous electric field is chosen such that the decelerating force acting on the high-field seeking states is maximized at each point along the trajectories. The three-dimensional velocity distribution of the atoms before and after the deceleration is determined by measuring times of flight and two-dimensional images of the atomic cloud on the detector. Under optimal deceleration conditions, the decrease in kinetic energy in the longitudinal dimension amounts to 1.0×10-21 J and the increase in mean kinetic energy in the transverse dimensions is only 1.0×10-23 J. The corresponding temperatures of 100 mK and 300 mK in the two transverse dimensions are sufficiently low that trapping can be envisaged. The possibility of focusing a Rydberg atom beam is demonstrated experimentally.

Magnetic phase transitions in the two-dimensional frustrated quantum antiferromagnetCs2CuCl4

We report magnetization and specific heat measurements in the two-dimensional frustrated spin-$1∕2$ Heisenberg antiferromagnet ${\mathrm{Cs}}_{2}\mathrm{Cu}{\mathrm{Cl}}_{4}$ at temperatures down to $0.05\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and high magnetic fields up to $11.5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ applied along $a$, $b$, and $c$ axes. The low-field susceptibility $\ensuremath{\chi}(T)\ensuremath{\simeq}M∕B$ shows a broad maximum around $2.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ characteristic of short-range antiferromagnetic correlations and the overall temperature dependence is well described by high temperature series expansion calculations for the partially frustrated triangular lattice with $J=4.46\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${J}^{\ensuremath{'}}∕J=1∕3$. At much lower temperatures $(\ensuremath{\leqslant}0.4\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and in in-plane field (along $b$ and $c$ axes) several new intermediate-field ordered phases are observed in between the low-field incommensurate spiral and the high-field saturated ferromagnetic state. The ground state energy extracted from the magnetization curve shows strong zero-point quantum fluctuations in the ground state at low and intermediate fields.

Anisotropy in the electrical transport properties of the high-field heavy-fermion state in [formula omitted

The electrical resistivity ρ has been measured on the filled skutterudite PrFe 4 P 12 as functions of both magnetic field H and field angle θ , in magnetic fields rotating within the (1 1 ¯ 0) plane. At 0.4 K, we observed a clear anomaly in ρ ( H ) for H ∥ [ 1 1 1 ] around 8 T, reflecting the onset of the new high-field ordered state. A double-peak structure found in ρ ( θ ) in the vicinity of the [1 1 1] direction indicates that the high-field ordered state appears only in a narrow angular range.

Vortex correlations in the liquid states ofBi2Sr2CaCu2O8+ywith tilted columnar defects

The interlayer and intralayer vortex correlations are studied in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+y}$ with tilted and nontilted columnar defects (CDs) by the Josephson plasma-resonance experiments under various field angles. The interlayer correlation is enhanced at the recoupling field, near which we observe clear triple-resonance peaks. The recoupling field is determined only by the density of CDs in the superconducting layer, irrespective of CD angles from the $c$ axis. In the low-field decoupled liquid state, we observe an increase of the interlayer correlations due to increased pinning by enlarged defects. By contrast, scaling analysis shows little effect of the pinning strength on the interlayer and intralayer correlations in the high-field recoupled liquid state.

New high field state of flux line lattice in CeCoIn 5

We have measured the ultrasound velocity of quasi-two-dimensional superconductor CeCoIn 5 with extremely large Pauli paramagnetic susceptibility. The results indicate that the new high field superconducting phase, which is revealed by the recent heat capacity measurements, is characterized by the unusual softening of flux line lattice. The softening is most likely due to the collapse of the flux line lattice tilt modulus and transition to quasi-two-dimensional vortex state. These results provide a strong evidence that the high field phase is the Fulde–Ferrell–Larkin–Ovchinnikov phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicularly to the vortices.

On the electrostatic deceleration of argon atoms in high Rydberg states by time-dependent inhomogeneous electric fields

Argon atoms in a pulsed supersonic expansion are prepared in selected Stark components of Rydberg states with effective principal quantum number in the range n* = 15–25. When traversing regions of inhomogeneous electric fields, these atoms get accelerated or decelerated depending on whether the Stark states are low- or high-field seeking states. Using a compact electrode design, which enables the application of highly inhomogeneous and time-dependent electric fields, the Rydberg atoms experience kinetic energy changes of up to 1.2 × 10−21 J (i.e. 60 cm−1 in spectroscopic units) in a single acceleration/deceleration stage of 3 mm length. The resulting differences in the velocities of the low- and high-field seeking states are large enough that the corresponding distributions of times of flight to the Rydberg particle detector are fully separated. As a result, efficient spectral searches of the Rydberg states best suited for acceleration/deceleration experiments are possible. Numerical simulations of the particle trajectories are used to analyse the time-of-flight distributions and to optimize the time dependence of the inhomogeneous electric fields. The decay of the Rydberg states by fluorescence, collisions and transitions induced by black-body radiation takes place on a timescale long enough not to interfere significantly with the deceleration during the first ∼5 µs.

Controlling the motion of hydrogen molecules: design of a two-dipole Rydberg decelerator

Hydrogen molecules are excited by 1 + 1′ (VUV + UV) laser excitation to Rydberg states ( n = 15–20) in the inhomogeneous field generated by an electrostatic dipole. High-field or low-field seeking Stark states are selected spectroscopically and experience a force either towards or away from the dipole, respectively. Deflection of the molecular beam is observed if the dipole is oriented parallel to the beam direction, whereas acceleration or deceleration is observed for a perpendicular orientation. The measured deflections and acceleration/deceleration are consistent with trajectory simulations based on the known field and Stark map. A two-dipole device is proposed that creates a travelling potential minimum and would enable deceleration of an H 2 beam seeded in argon to a mean velocity of zero. Simulations are presented of its characteristics.

Ac Electric Trap for Ground-State Molecules

We here report on the realization of an electrodynamic trap, capable of trapping neutral atoms and molecules in both low-field and high-field seeking states. Confinement in three dimensions is achieved by switching between two electric field configurations that have a saddle point at the center of the trap, i.e., by alternating a focusing and a defocusing force in each direction. The ac trapping of 15ND(3) molecules is experimentally demonstrated, and the stability of the trap is studied as a function of the switching frequency. A 1 mK sample of 15ND(3) molecules in the high-field seeking component of the |J,K=|1,1 level, the ground state of para-ammonia, is trapped in a volume of about 1 mm(3).

Magnetic and mechanical buckling: Modified Landau theory approach to study phase transitions in micromagnetic disks and compressed rods

Using the rigid magnetic vortex model, we develop a substantially modified Landau theory approach for analytically studying phase transitions between different spin arrangements in circular submicron magnetic dots subject to an in-plane externally-applied magnetic field. We introduce a novel order parameter: the inverse distance between the center of the circular dot and the vortex core. This order parameter is suitable for describing closed spin configurations such as curved or bent-spin structures and magnetic vortices. Depending on the radius and thickness of the dot as well as the exchange coupling, there are five different regimes for the magnetization reversal process when decreasing the in-plane magnetic field. The magnetization-reversal regimes obtained here cover practically all possible magnetization reversal processes. Moreover, we have derived the change of the dynamical response of the spins near the phase transitions and obtained a ``critical slowing down'' at the second order phase transition from the high-field parallel-spin state to the curved (C-shaped) spin phase. We predict a transition between the vortex and the parallel-spin state by quickly changing the magnetic field---providing the possibility to control the magnetic state of dots by changing either the value of the external magnetic field and/or its sweep rate. We study an illuminating mechanical analog (buckling instability) of the transition between the parallel-spin state and the curved spin state (i.e., a magnetic buckling transition). In analogy to the magnetic-disk case, we also develop a modified Landau theory for studying mechanical buckling instabilities of a compressed elastic rod embedded in an elastic medium. We show that the transition to a buckled state can be either first or second order depending on the ratio of the elasticity of the rod and the elasticity of the external medium. We derive the critical slowing down for the second-order mechanical buckling transition.

New High-Field Ordered State in PrFe4P12

Dc magnetization and specific heat measurements on the filled skutterudite PrFe 4 P 12 reveal the presence of a new ordered state (B phase) in high magnetic fields applied along the [111] direction. The present data suggest that the B phase exists in a limited angular range around the [111] direction. A possible origin of the B phase is discussed within the framework of the Γ 1 –Γ 4 model.

Spin reorientation and in-plane magnetoresistance of lightly dopedLa2−xSrxCuO4in magnetic fields up to 55 T

The magnetoresistance (MR) in the in-plane resistivity is measured in magnetic fields up to 55 T in lightly doped La_{2-x}Sr_{x}CuO_{4} in the N\'eel state (x = 0.01) and in the spin-glass state (x = 0.03) using high-quality untwinned single crystals. In both cases, a large negative MR is observed to appear when the magnetic order is established. For x = 0.01, it is found that the MR is indicative of a one-step transition into a high-field weak-ferromagnetic state at around 20 T when the magnetic field is applied from the spin easy axis (b axis), which means that there is no spin-flop transition in the N\'eel state of this material; this is contrary to a previous report, but is natural in light of the peculiar in-plane magnetic susceptibility anisotropy recently found in this system. In the spin-glass state, we observe that the large (up to \sim20%) negative MR saturates at around 40 T, and this MR is found to be essentially isotropic when the magnetic field is rotated within the ab plane. Our data show that the large negative MR is inherent to LSCO in a magnetically ordered state, in which the weak-ferromagnetic (WF) moment becomes well-defined; we discuss that the observed MR is essentially due to the reorientation of the WF moments towards the magnetic field direction both in the N\'eel state and in the spin-glass state.

A quasi-analytic model of a linear Stark accelerator/decelerator for polar molecules

This article describes a quasi-analytic model of a linear Stark accelerator/decelerator for polar molecules in both their low- and high-field seeking states, and examines the dynamics of the acceleration/deceleration process and its phase stability. The requisite time-dependent inhomogeneous Stark fields, used in current experiments, are Fourier-analyzed and found to consist of a superposition of partial waves with well-defined phase velocities. The kinematics of the interaction of molecules with the partial waves is discussed and the notion of a phase of a molecule in a travelling field is introduced. Next, the net potential and the net force that act on the molecules are derived. A special case, the first-harmonic accelerator/decelerator, is introduced. This represents a model system many of whose properties can be obtained analytically. The first-harmonic accelerator/decelerator dynamics is presented and discussed along with that of the isomorphic biased-pendulum problem. Finally, the general properties of the velocity of the molecules in a phase-stable accelerator/decelerator are examined.

Directionally dependent magneto-optical response of(La0.4Pr0.6)1.2Sr1.8Mn2O7: Understanding the high-field ferromagnetic metallic state

We investigate the polarized magneto-optical response of ${({\mathrm{La}}_{0.4}{\mathrm{Pr}}_{0.6})}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ in order to reveal the microscopic nature of the states involved in the field-induced paramagnetic semiconductor to ferromagnetic metal transition. We find significant changes in the electronic structure of the title compound under magnetic fields due to the evolution of the Jahn-Teller-split ${\mathrm{Mn}}^{3+}\phantom{\rule{0.3em}{0ex}}{e}_{g}$ orbitals. Consequences of this field-induced modification of the $d$ orbitals include the substantial colossal magnetoresistence effect, unusual magnetic relaxation behavior, and a change in orbital occupation. Based upon the directional dependence of the magneto-optical spectra, we demonstrate that the high-field ferromagnetic state is homogeneous.