arXiv subjects
Publications and source records attributed to M. G. Banks.
A new multiferroic material, CuBr2, is reported for the first time. CuBr2 has not only a high transition temperature (close to liquid nitrogen temperature) but also low dielectric loss and strong magnetoelectric coupling. These findings reveal the importance of anion effects in the search for the high temperature multiferroics materials among these low-dimensional spin systems.
We report a detailed examination the magnetic structure of anhydrous cupric chloride CuCl2 carried out by powder neutron diffraction, magnetic susceptibility and specific heat measurements on polycrystalline and single crystal samples as well as an evaluation of the spin exchange interactions by first principles density functional theory (DFT) calculations. Anhydrous CuCl2 shows one dimensional antiferromagnetic behavior and long range antiferromagnetic ordering below a Neel temperature of 23.9 K. Neutron powder and single crystal diffraction reveal that, below 23.9K, CuCl2 undergoes a phase transition into an incommensurate magnetic structure (propagation vector (1,0.2257,0.5) with a spin-spiral propagating along b and the moments confined in the bc crystallographic plane. Our DFT calculations show that the spin-spiral results from competing ferromagnetic nearest neighbor and antiferromagnetic next-nearest neighbor spin-exchange interaction along the spin chains. Implications for possible multiferroic behavior of CuCl2 are discussed.
TmGa$_{3}$ (AuCu$_3$ structure) undergoes two phase transitions, an antiferroquadrupolar transition at $\sim$ 4.29 K and long-range antiferromagnetic ordering at $\sim$ 4.26 K. Due to the close vicinity of the two phase transitions, TmGa$_3$ offers an interesting system to study the interplay of charge and magnetic degrees of freedom. In order to understand this interplay we have performed inelastic neutron scattering experiments on TmGa$_{3}$ in the paramagnetic regime ($T >$ 5 K) to redetermine the crystal electric field level scheme. By fitting our spectra at various temperatures we obtain a new crystal field level scheme with Lea, Leask and Wolf parameters of $x_{\rm LLW}$ = -0.44(2) and $W$ = -0.222(2) K. The total crystal field splitting at 5K amounts to $\sim$ 2.3 meV, about an order of magnitude less than found previously, but in good agreement with the splitting extrapolated from the related ErGa$_3$ system. Our analysis yields a $Γ_{2}$ singlet as the crystal field ground state followed closely by a (nonmagnetic) $Γ_{1}$ singlet at 0.009 meV. The next excited states are a $Γ_{5}^{(2)}$ triplet at $\sim$0.5 meV, which is almost degenerate to a $Γ_{4}$ doublet. This level scheme is adverse to previous findings. Subsequent analysis of the magnetisation along several crystallographic directions and the temperature dependant susceptibility as well as of the magnetic contribution to the specific heat are consistent with our new crystal field parameters. Implications for the antiferroquadrupolar and the antiferromagnetic transition are discussed.
An interesting twist of the Hirsch index is given, in terms of an index for topics and compounds. By comparing both the hb index and m for a number of compounds and topics, it can be used to differentiate between a new so-called hot topic with older topics. This quick method is shown to help new comers to identify how much interest and work has already been achieved in their chosen area of research.
We have investigated the high field magnetisation of the frustrated one dimensional compound LiCuVO4. In zero field, LiCuVO4 undergoes long range antiferromagnetic order at T_{N} ~ 2.5 K with a broad short range Schottky type anomaly due to one dimensional correlations in the specific heat at 32 K. Application of a magnetic field induces a rich phase diagram. An anomaly in the derivative of the magnetisation with respect to the applied magnetic field is seen at ~ 7.5 T with H ll c in the long range order phase. We investigated this in terms of a first experimental evidence of a middle field cusp singularity (MFCS). Our numerical DMRG results show that in the parameter range of LiCuVO4 as deduced by inelastic neutron scattering (INS), there exists no MFCS. The anomaly in the derivative of the magnetisation at ~ 7.5 T is therfore assigned to a change in the spin structure from the ab plane helix seen in zero field neutron diffraction.