Presence Of Local Magnetic Field Research Articles

Role of magnetoelastic coupling and magnetic anisotropy in MnTiO3

We report the thermodynamic properties studied by thermal expansion, magnetostriction, magnetisation, and specific heat measurements as well as the low-energy magnetic excitations of \mto\ and investigate how magneto-elastic coupling and magnetic anisotropy affect the evolution of long-range order and the magnetic phase diagram. Specifically, we utilise high-resolution capacitance dilatometry and antiferromagnetic resonance (AFMR) studies by means of high-frequency electron spin resonance (HF-ESR) spectroscopy. The role of anisotropy is reflected by spin-reorientation at $B_{\rm{SF}}$~$\simeq 6$~T and a corresponding sign change in $\partial T_{\rm N}/\partial B$. Analysis of the AFMR modes enables us to establish the zero-field excitation gap $\Delta$ as well as its temperature dependence. We derive the effective anisotropy field $B_\mathrm{A} = 0.16(1)$~T which predominately originates from out-of-plane nearest-neighbour dipole-dipole interactions. Despite the nearly fully quenched orbital moment, our data show pronounced thermal expansion and magnetostriction anomalies at $T_{\rm N}$ and $B_{\rm{SF}}$ , respectively, which allows the experimental determination of sizable uniaxial pressure dependencies, i.e., $\partial B_{\rm SF}/\partial p_{\rm c} = -0.20(2)$~T/GPa, $\partial T_{\rm N}/ \partial p_b = 0.69(12)$~K/GPa, and $\partial T_{\rm N}/ \partial p_c = -2.0(4)$~K/GPa. Notably, short-range magnetic order appears up to at least 3$T_{\rm N}$ , as indicated by anisotropic lattice distortion, the violation of a constant Gr\"uneisen behavior, and the presence of local magnetic fields detected by HF-ESR.

Dynamics of initially correlated open quantum systems: Theory and applications

We show that the dynamics of any open quantum system that is initially correlated with its environment can be described by a set of (or less) completely positive maps, where d is the dimension of the system. Only one such map is required for the special case of no initial correlations. The same maps describe the dynamics of any system-environment state obtained from the initial state by a local operation on the system. The reduction of the system dynamics to a set of completely positive maps allows known numerical and analytic tools for uncorrelated initial states to be applied to the general case of initially correlated states, which we exemplify by solving the qubit dephasing model for such states, and provides a natural approach to quantum Markovianity for this case. We show that this set of completely positive maps can be experimentally characterised using only local operations on the system, via a generalisation of noise spectroscopy protocols. As further applications, we first consider the problem of retrodicting the dynamics of an open quantum system which is in an arbitrary state when it becomes accessible to the experimenter, and explore the conditions under which retrodiction is possible. We also introduce a related one-sided or limited-access tomography protocol for determining an arbitrary bipartite state, evolving under a sufficiently rich Hamiltonian, via local operations and measurements on just one component. We simulate this protocol for a physical model of particular relevance to nitrogen-vacancy centres, and in particular show how to reconstruct the density matrix of a set of three qubits, interacting via dipolar coupling and in the presence of local magnetic fields, by measuring and controlling only one of them.

Ionic Hydrogels with Biomimetic 4D‐Printed Mechanical Gradients: Models for Soft‐Bodied Aquatic Organisms

AbstractDirect‐ink writing (DIW), a rapidly growing and advancing form of additive manufacturing, provides capacities for on‐demand tailoring of materials to meet specific requirements for final designs. The penultimate challenge faced with the increasing demand of customization is to extend beyond modification of shape to create 4D structures, dynamic 3D structures that can respond to stimuli in the local environment. Patterning material gradients is foundational for assembly of 4D structures, however, there remains a general need for useful materials chemistries to generate gray scale gradients via DIW. Here, presented is a simple materials assembly paradigm using DIW to pattern ionotropic gradients in hydrogels. Using structures that architecturally mimic sea‐jelly organisms, the capabilities of spatial patterning are highlighted as exemplified by selectively programming the valency of the ion‐binding agents. Spatial gradients, when combined with geometry, allow for programming the flexibility and movement of iron oxide nanoparticle–loaded ionotropic hydrogels to generate 4D‐printed structures that actuate in the presence of local magnetic fields. This work highlights approaches to 4D design complexity that exploits 3D‐printed gray‐scale/gradient mechanics.

Vortex configuration in the presence of local magnetic field and locally applied stress

Vortex configuration in the presence of local magnetic field and locally applied stress

Vortex configuration in the presence of local magnetic field and locally applied stress

Vortex configuration is determined by the repulsive interaction, which becomes dominant with increasing vortex density, by the pinning potential, and by other considerations such as the local magnetic fields, currents flowing in the sample, or as we showed recently, by local stress applied on the sample. In this work we describe different ways to control vortex configuration using scanning SQUID microscopy.

Anomalous Tunneling of Spin Wave in Heisenberg Ferromagnet

The ferromagnetic spin wave (FSW) in classical Heisenberg chain exhibits the perfect transmission in the long-wavelength limit in the transmission-reflection problem with an inhomogeneity of exchange integral. In the presence of local magnetic field, on the other hand, FSW undergoes the perfect reflection in the long-wavelength limit. This difference in the long-wavelength limit is attributed to the symmetry property of the scatterers; it is crucial whether the potential preserves or breaks the spin rotation symmetry. Our result implies that the anomalous tunneling (i.e., perfect transmission in the low-energy limit) found both in scalar and spinor BECs is not specific to gapless modes in superfluids but is a common property shared with generic Nambu-Goldstone modes in the presence of a symmetry-preserving potential scatterer.

Spin Filter Effect in Organic Polymers in the Presence ofLocal Magnetic Field

Using a nonadiabatic evolution method, we investigate the spin filtereffect in organic polymers in the presence of a local magnetic field.Through a spin-dependent magnetic field, polarons (charge carrier) withdifferent spins will feel repulsive or attractive force determined bytheir spins. Our simulations show that in a single-site magnetic field(affecting electrons at a single site), for example V150 = 0.35 eV,or V150 = 0.45 eV, a spin-up polaron accelerated to saturated velocityby an electric field can pass through the field while the spin-down polaronis trapped. When the local field extends over several sites(Vn~exp[−(n−nc)2/n2w]), similar behaviour is alsofound. Simultaneously we find that it is more likely to realize the spinfilter effect in a comparatively large field since the polaron which feelsattractive force is easily trapped by a local magnetic field.

High-resolution1H NMR spectroscopy of absorbed molecules in the presence of strong magnetic fields

1H NMR spectra of molecules sorbed in porous materials may be strongly influenced by the presence of local magnetic fields due to susceptibility broadening which is a result of the polarization of the adsorbent grains in particular in strong magnetic fields. The linewidth can be essentially reduced by the application of magic angle spinning (MAS) techniques but is also determined by the translational motion of the adsorbed species over mean distances which are comparable to the dimensions of the grains. The lineshape analysis was performed on the basis of a modified Anderson–Weiss theory as proposed by Fenzke etal. and compared with 1H NMR spectra measured for butene molecules sorbed in NaX-type zeolites at various external fields B (up to 17.6. T) and different MAS frequencies. A considerable gain in resolution of the chemical shifts by using high magnetic fields is established. Copyright © 2000 John Wiley & Sons, Ltd.