The spin echo modulated small‐angle neutron scattering technique has been implemented using two superconducting magnetic Wollaston prisms at a reactor neutron source. The density autocorrelation function measured for a test sample of colloidal silica in a suspension agrees with that obtained previously by other neutron scattering methods on an identically prepared sample. The reported apparatus has a number of advantages over competing technologies: it should allow larger length scales (up to several micrometres) to be probed; it has very small parasitic neutron scattering and attenuation; the magnetic fields within the device are highly uniform; and the neutron spin transport across the device boundaries is very efficient. To understand quantitatively the results of the reported experiment and to guide future instrument development, Monte Carlo simulations are presented, in which the evolution of the neutron polarization through the apparatus is based on magnetic field integrals obtained from finite‐element simulations of the various magnetic components. The Monte Carlo simulations indicate that the polarization losses observed in the experiments are a result of instrumental artifacts that can be easily corrected in future experiments.
The recent discovery of intrinsic two-dimensional (2D) ferromagnetism has sparked growing interests in the search for new 2D magnets with diverse and tunable properties for both fundamental scientific advances and novel spintronic applications. Here we report on the synthesis of layered chromium sulfide (Cr2S3) nanoplates via a facile sulfurization approach and the studies of their highly tunable Raman and (magneto-)transport properties. Depending on the specific growth conditions, we have achieved both epitaxial (and hence strained) and non-epitaxial nanoplates of Cr2S3 on the c-cut sapphire substrates. Via Raman scattering and density functional theory (DFT) calculations, we determined both types of nanoplates to be a rhombohedral R3 phase whose bulk counterpart exhibits weak ferromagnetism below a metal–insulator transition (MIT) temperature of ~120 K. Compressive strain from the lattice-mismatched substrate yields a red-shift of up to 8 cm−1 in Raman peaks in comparison to the strain-free nanoplates obtained from the non-epitaxial growth. The strain-free nanoplate shows a variable-range-hopping type of insulating behavior, while the strained nanoplates exhibit an enhanced MIT up to ~275 K in comparison to 120 K in bulk samples. The room temperature resistivity values of the two types of nanoplates differ by 2 to 3 orders of magnitude. The distinct transport properties can be understood qualitatively based on the electronic band structures calculated by DFT.
The neutron Larmor diffraction technique has been implemented using superconducting magnetic Wollaston prisms in both single-arm and double-arm configurations. Successful measurements of the coefficient of thermal expansion of a single-crystal copper sample demonstrates that the method works as expected. The experiment involves a new method of tuning by varying the magnetic field configurations in the device and the tuning results agree well with previous measurements. The difference between single-arm and double-arm configurations has been investigated experimentally. We conclude that this measurement benchmarks the applications of magnetic Wollaston prisms in Larmor diffraction and shows in principle that the setup can be used for inelastic phonon line-width measurements. The achievable resolution for Larmor diffraction is comparable to that using Neutron Resonance Spin Echo (NRSE) coils. The use of superconducting materials in the prisms allows high neutron polarization and transmission efficiency to be achieved.The ability of conventional neutron diffraction to measure precise values of the d-spacings of crystalline materials is limited by factors such as the strength of the available neutron source and the practical length of neutron flight paths. The current limit is around Δd/d of 10 −3 . At reactor neutron sources however, high resolution measurements of Δd/d ~ 10 −6 have been achieved using the Larmor diffraction (LD) technique 1 first introduced by Rekveldt 2 . Like the neutron spin echo (NSE) technique proposed by Mezei 3 for energy encoding, the LD method makes use of Larmor precession of neutron spins in well-defined magnetic fields. The method allows the lattice spacing of the diffracting crystal to be encoded into the Larmor phase of the neutron spin by making this phase depend only on the scattering vector of the diffracting Bragg peak, a quantity that is independent of the monochromaticity and collimation of the neutron beam. This enables small changes of the lattice spacing to be measured through the change of the neutron Larmor phase instead of by measuring the change in the diffraction angle.The original Rekveldt proposal for LD involved magnetic fields before and after the sample. When the field boundaries of these two magnetic fields are aligned parallel to the crystal diffraction plane, all the diffracted neutrons will yield the same Larmor phase regardless their incident angle on the sample. Therefore, the Larmor phase of the diffracted neutrons will only depend on the geometry and intensities of the magnetic fields before and after the sample.The LD method has been used in a number of experiments and its recent applications have been summarized by Rekveldt 4 including absolute lattice spacing determination 5 and temperature induced lattice variations 6 . Up to now, LD has been implemented and routinely operated on the beamlines of TRISP (FRM II, MLZ) 1 , FLEXX (BER-II, HZB) 7 and ZETA (ILL) 8 , with a relative resolution of Δd/d ~10 −6 . Instead of using two static magnetic fields, these...
Pyrochlore iridates A 2 Ir 2 O 7 (A = rare earth elements, Y or Bi) hold great promise for realizing novel electronic and magnetic states owing to the interplay of spin-orbit coupling, electron correlation and geometrical frustration. A prominent example is the formation of all-in/all-out (AIAO) antiferromagnetic order in the Ir 4+ sublattice that comprises of corner-sharing tetrahedra. Here we report on an unusual magnetic phenomenon, namely a cooling-field induced shift of magnetic hysteresis loop along magnetization axis, and its possible origin in pyrochlore iridates with nonmagnetic Ir defects (e.g. Ir 3+ ). In a simple model, we attribute the magnetic hysteresis loop to the formation of ferromagnetic droplets in the AIAO antiferromagnetic background. The weak ferromagnetism originates from canted antiferromagnetic order of the Ir 4+ moments surrounding each non-magnetic Ir defect. The shift of hysteresis loop can be understood quantitatively based on an exchange-bias like effect in which the moments at the shell of the FM droplets are pinned by the AIAO AFM background via mainly the Heisenberg (J) and Dzyaloshinsky-Moriya (D) interactions. The magnetic pinning is stable and robust against the sweeping cycle and sweeping field up to 35 T, which is possibly related to the magnetic octupolar nature of the AIAO order.
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