arXiv subjects

R. D. Johnson

Publications and source records attributed to R. D. Johnson.

At least 55 records · Page 3Linked to original sources

arXiv2016

Modulated spin helicity stabilized by incommensurate orbital density waves in a quadruple perovskite manganite

Through a combination of neutron diffraction and Landau theory we describe the spin ordering in the ground state of the quadruple perovskite manganite CaMn7O12 - a magnetic multiferroic supporting an incommensurate orbital density wave that onsets above the magnetic ordering temperature, TN1 = 90 K. The multi-k magnetic structure in the ground state was found to be a nearly-constant-moment helix with modulated spin helicity, which oscillates in phase with the orbital occupancies on the Mn3+ sites via trilinear magneto-orbital coupling. Our phenomenological model also shows that, above TN2 = 48 K, the primary magnetic order parameter is locked into the orbital wave by an admixture of helical and collinear spin density wave structures. Furthermore, our model naturally explains the lack of a sharp dielectric anomaly at TN1 and the unusual temperature dependence of the electrical polarisation.

cond-mat.str-el↗

arXiv2016

Incommensurate Counterrotating Magnetic Order Stabilized by Kitaev Interactions in the Layered Honeycomb $α$-Li$_2$IrO$_3$

The layered honeycomb magnet $α$-Li$_2$IrO$_3$ has been theoretically proposed as a candidate to display novel magnetic behaviour associated with Kitaev interactions between spin-orbit entangled $j_{\rm eff}=1/2$ magnetic moments on a honeycomb lattice. Here we report single crystal magnetic resonant x-ray diffraction combined with powder magnetic neutron diffraction to reveal an incommensurate magnetic order in the honeycomb layers with Ir magnetic moments counter-rotating on nearest-neighbour sites. This type of magnetic structure has not been reported experimentally before in honeycomb magnets and cannot be explained by a spin Hamiltonian with dominant isotropic (Heisenberg) couplings. The magnetic structure shares many key features with the magnetic order in the structural polytypes $β$ and $γ$-Li$_2$IrO$_3$, understood theoretically to be stabilized by dominant Kitaev interactions between Ir moments located on the vertices of three-dimensional hyperhoneycomb and stripyhoneycomb lattices, respectively. Based on this analogy and a theoretical soft-spin analysis of magnetic ground states for candidate spin Hamiltonians, we propose that Kitaev interactions also dominate in $α$-Li$_2$IrO$_3$, indicative of universal Kitaev physics across all three members of the harmonic honeycomb family of Li$_2$IrO$_3$ polytypes.

cond-mat.str-el↗

arXiv2015

The monoclinic crystal structure of $α$-RuCl$_3$ and the zigzag antiferromagnetic ground state

The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic moments. Here we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently-assumed trigonal 3-layer stacking periodicity. We report electronic band structure calculations for the monoclinic structure, which find support for the applicability of the jeff=1/2 picture once spin orbit coupling and electron correlations are included. We propose that differences in the magnitude of anisotropic exchange along symmetry inequivalent bonds in the monoclinic cell could provide a natural mechanism to explain the spin gap observed in powder inelastic neutron scattering, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as neutron powder diffraction show a single magnetic transition at TN ~ 13K. The analysis of the neutron data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60T show a single transition around 8T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.

cond-mat.str-el↗

arXiv2015

The magnetic ground state of two isostructual polymeric quantum magnets, [Cu(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ and [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$, investigated with neutron powder diffraction

The magnetic ground state of two isostructural coordination polymers (i) the quasi two-dimensional S = 1/2 square-lattice antiferromagnet [Cu(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$; and (ii) a new compound [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$, were examined with neutron powder diffraction measurements. We find the ordered moments of the Heisenberg S = 1/2 Cu(II) ions in [Cu(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ are 0.6(1)$μ_{B}$, whilst the ordered moments for the Co(II) ions in [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ are 3.02(6)$μ_{B}$. For Cu(II), this reduced moment indicates the presence of quantum fluctuations below the ordering temperature. We show from heat capacity and electron spin resonance measurements, that due to the crystal electric field splitting of the S = 3/2 Co(II) ions in [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$, this isostructual polymer also behaves as an effective spin-half magnet at low temperatures. The Co moments in [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ show strong easy-axis anisotropy, neutron diffraction data which do not support the presence of quantum fluctuations in the ground state and heat capacity data which are consistent with 2D or close to 3D spatial exchange anisotropy.

cond-mat.str-el↗

arXiv2015

Non-ferroelectric nature of the conductance hysteresis in CH3NH3PbI3 perovskite-based photovoltaic devices

We present measurements of conductance hysteresis on CH3NH3PbI3 perovskite thin films, performed using the double-wave method, in order to investigate the possibility of a ferroelectric response. A strong frequency dependence of the hysteresis is observed in the range of 0.1 Hz to 150 Hz, with a hysteretic charge density in excess of 1000 μC/cm2 at frequencies below 0.4 Hz - a behaviour uncharacteristic of a ferroelectric response. We show that the observed hysteretic conductance, as well as the presence of a double arc in the impedance spectroscopy, can be fully explained by the migration of mobile ions under bias on a timescale of seconds. Our measurements place an upper limit of approximately 1 μC/cm2 on any intrinsic frequency-independent polarisation, ruling out ferroelectricity as the main cause of current-voltage hysteresis and providing further evidence of the importance of ionic migration in modifying the efficiency of CH3NH3PbI3 devices.

cond-mat.mtrl-sci↗

arXiv2014

Fermi surface of IrTe2 in the valence-bond state as determined by quantum oscillations

We report the observation of the de Haas-van Alphen effect in IrTe2 measured using torque magnetometry at low temperatures down to 0.4 K and in high magnetic fields up to 33T. IrTe2 undergoes a major structural transition around 283 K due to the formation of planes of Ir and Te dimers that cut diagonally through the lattice planes, with its electronic structure predicted to change significantly from a layered system with predominantly three-dimensional character to a tilted quasi-two dimensional Fermi surface. Quantum oscillations provide direct confirmation of this unusual tilted Fermi surface and also reveal very light quasiparticle masses (less than 1 me), with no significant enhancement due to electronic correlations. We find good agreement between the angular dependence of the observed and calculated de Haas-van Alphen frequencies, taking into account the contribution of different structural domains that form while cooling IrTe2.

cond-mat.supr-con↗

arXiv2014

Unconventional magnetic order on the hyperhoneycomb Kitaev lattice in $β$-Li2IrO3: full solution via magnetic resonant x-ray diffraction

The recently-synthesized iridate $β$-Li$_2$IrO$_3$ has been proposed as a candidate to display novel magnetic behavior stabilized by frustration effects from bond-dependent, anisotropic interactions (Kitaev model) on a three-dimensional "hyperhoneycomb" lattice. Here we report a combined study using neutron powder diffraction and magnetic resonant x-ray diffraction to solve the complete magnetic structure. We find a complex, incommensurate magnetic order with non-coplanar and counter-rotating Ir moments, which surprisingly shares many of its features with the related structural polytype "stripyhoneycomb" $γ$-Li$_2$IrO$_3$, where dominant Kitaev interactions have been invoked to explain the stability of the observed magnetic structure. The similarities of behavior between those two structural polytypes, which have different global lattice topologies but the same local connectivity, is strongly suggestive that the same magnetic interactions and the same underlying mechanism governs the stability of the magnetic order in both materials, indicating that both $β$- and $γ$-Li$_2$IrO$_3$ are strong candidates to realize dominant Kitaev interactions in a solid state material.

cond-mat.str-el↗

arXiv2014

Non-coplanar and counter-rotating incommensurate magnetic order stabilized by Kitaev interactions in $γ$-Li2IrO3

Materials that realize Kitaev spin models with bond-dependent anisotropic interactions have long been searched for, as the resulting frustration effects are predicted to stabilize novel forms of magnetic order or quantum spin liquids. Here we explore the magnetism of $γ$-Li$_2$IrO$_3$, which has the topology of a 3D Kitaev lattice of inter-connected Ir honeycombs. Using resonant magnetic x-ray diffraction we find a complex, yet highly-symmetric incommensurate magnetic structure with non-coplanar and counter-rotating Ir moments. We propose a minimal Kitaev-Heisenberg Hamiltonian that naturally accounts for all key features of the observed magnetic structure. Our results provide strong evidence that $γ$-Li$_2$IrO$_3$ realizes a spin Hamiltonian with dominant Kitaev interactions.

cond-mat.str-el↗

arXiv2014

First-principles study of multiferroic RbFe(MoO$_4$)$_2$

We have investigated the magnetic structure and ferroelectricity in RbFe(MoO$_4$)$_2$ via first-principles calculations. Phenomenological analyses have shown that ferroelectricity may arise due to both the triangular chirality of the magnetic structure, and through coupling between the magnetic helicity and the ferroaxial structural distortion. Indeed, it was recently proposed that the structural distortion plays a key role in stabilising the chiral magnetic structure itself. We have determined the relative contribution of the two mechanisms via \emph{ab-initio} calculations. Whilst the structural axiality does induce the magnetic helix by modulating the symmetric exchange interactions, the electric polarization is largely due to the in-plane spin triangular chirality, with both electronic and ionic contributions being of relativistic origin. At the microscopic level, we interpret the polarization as a secondary steric consequence of the inverse Dzyaloshinskii-Moriya mechanism and accordingly explain why the ferroaxial component of the electric polarization must be small.

cond-mat.mtrl-sci↗

arXiv2014

CaBaCo$_4$O$_7$ : A ferrimagnetic pyroelectric

Magneto-electric coupling in pyroelectric CaBaCo$_4$O$_7$ is investigated using ab-intio calculations and Landau theory. The former shows that exchange-striction is strong enough to produce a giant change in electric polarization upon ferrimagnetic ordering, comparable to the experimentally determined value of ~17 mC/m$^2$ . Furthermore, Landau theory demonstrates that magneto-elastic coupling in CaBaCo$_4$O$_7$ is responsible for the strong magneto-electric coupling appearing close to the magnetic phase transition.

cond-mat.str-el↗

arXiv2013

Magnetic fan structures in Ba$_{0.5}$Sr$_{1.5}$Zn$_2$Fe$_{12}$O$_{22}$ hexaferrite revealed by resonant soft X-ray diffraction

The hexaferrites are known to exhibit a wide range of magnetic structures, some of which are connected to important technological applications and display magnetoelectric properties. We present data on the low field magnetic structures stabilised in a Y-type hexaferrite as observed by resonant soft X-ray diffraction. The helical spin block arrangement that is present in zero applied magnetic field becomes fan-like as a field is applied in plane. The propagation vectors associated with each fan structure are studied as a function of field, and a new magnetic phase is reported. Mean field calculations indicate this phase should stabilise close to the boundary of the previously reported phases.

cond-mat.str-el↗

arXiv2013

MnSb2O6: A polar magnet with a chiral crystal structure

Structural and magnetic chiralities are found to coexist in a small group of materials in which they produce intriguing phenomenologies such as the recently discovered skyrmion phases. Here, we describe a previously unknown manifestation of this interplay in MnSb2O6, a trigonal oxide with a chiral crystal structure. Unlike all other known cases, the MnSb2O6 magnetic structure is based on co-rotating cycloids rather than helices. The coupling to the structural chirality is provided by a magnetic axial vector, related to the so-called vector chirality. We show that this unique arrangement is the magnetic ground state of the symmetric-exchange Hamiltonian, based on ab-initio theoretical calculations of the Heisenberg exchange interactions, and is stabilised by out-of-plane anisotropy. MnSb2O6 is predicted to be multiferroic with a unique ferroelectric switching mechanism.

cond-mat.str-el↗

arXiv2013

X-ray imaging and multiferroic coupling of cycloidal magnetic domains in ferroelectric monodomain BiFeO3

Magnetic domains at the surface of a ferroelectric monodomain BiFeO3 single crystal have been imaged by hard X-ray magnetic scattering. Magnetic domains up to several hundred microns in size have been observed, corresponding to cycloidal modulations of the magnetization along the wave-vector k=2π(δ,δ,0) and symmetry equivalent directions. The rotation direction of the magnetization in all magnetic domains, determined by diffraction of circularly polarized light, was found to be unique and in agreement with predictions of a combined approach based on a spin-model complemented by relativistic density-functional simulations. Imaging of the surface shows that the largest adjacent domains display a 120 degree vortex structure.

cond-mat.str-el↗

arXiv2012

An x-ray diffraction study of the temperature-induced structural phase transitions in SmVO3

Through powder x-ray diffraction we have investigated the structural behavior of SmVO3, in which orbital and magnetic degrees of freedom are believed to be closely coupled to the crystal lattice. We have found, contrary to previous reports, that SmVO3 exists in a single, monoclinic, phase below 200 K. The associated crystallographic distortion is then stabilized through the magnetostriction that occurs below 134 K. The crystal structure has been refined using synchrotron x-ray powder diffraction data measured throughout the structural phase diagram, showing a substantial Jahn-Teller distortion of the VO6 octahedra in the monoclinic phase, compatible with the expected G-type orbital order. Changes in the vanadium ion crystal field due to the structural and magnetic transitions have then been probed by resonant x-ray diffraction.

cond-mat.str-el↗

arXiv2012

Magneto-orbital helices and multiferroicity

Orbitally ordered states evolving coincidentally with structural distortions and magnetic ordering provide a novel route to controlling electronic and magnetic properties of materials through external fields. We report an unprecedented magneto-orbital helix in CaMn7O12, found to give rise to the largest magnetically induced ferroelectric polarisation measured to date. Characterisation of the structural modulation using x-ray diffraction, and analysis of magnetic exchange shows that orbital order is crucial in stabilising a chiral magnetic structure, thus allowing for electric polarisation. Additionally, the presence of a global structural rotation enables the coupling between this polarisation and magnetic helicity required for multiferrocitiy. These novel principles open up the possibility of discovering new high-temperature multiferroics.

cond-mat.str-el↗

arXiv2012

The Magnetic Structure of DyMn2O5 Determined by Resonant X-ray Scattering

Resonant magnetic x-ray scattering has been used to investigate the magnetic structure of the magnetoelectric multiferroic DyMn2O5. We have studied the magnetic structure in the ferroelectric phase of this material, which displays the strongest ferroelectric polarisation and magnetodielectric effect of the RMn2O5 (where R is a rare earth ion, Y or Bi) family. The magnetic structure observed is similar to that of the other members of the series, but differs in the direction of the ordered moments. In DyMn2O5 both the Dy and Mn moments lie close to the b-axis, whereas in other RMn2O5 they lie close to the a-axis.

cond-mat.str-el↗

arXiv2012

Electric field control of the magnetic chiralities in ferroaxial multiferroic RbFe(MoO4)2

The coupling of magnetic chiralities to the ferroelectric polarisation in multiferroic RbFe(MoO$_4$)$_2$ is investigated by neutron spherical polarimetry. Because of the axiality of the crystal structure below $T_\textrm{c}$ = 190 K, helicity and triangular chirality are symmetric-exchange coupled, explaining the onset of the ferroelectricity in this proper-screw magnetic structure - a mechanism that can be generalised to other systems with "ferroaxial" distortions in the crystal structure. With an applied electric field we demonstrate control of the chiralities in both structural domains simultaneously.

cond-mat.str-el↗

arXiv2012

Electric field control of chiral magnetic domains in the high-temperature multiferroic CuO

We have studied the high temperature multiferroic cupric oxide using polarized neutron diffraction as a function of temperature and applied electric field. We find that the chiral domain population can be varied using an external electric field applied along the b axis. Using representation analysis we derive the incommensurate magnetic structure in the multiferroic phase. The origin of the magnetoelectric coupling is consistent with models based on the inverse Dzyaloshinskii-Moriya interaction, but is different from the simple cycloidal mechanism.

cond-mat.str-el↗