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Roger D. Johnson

Publications and source records attributed to Roger D. Johnson.

At least 19 recordsLinked to original sources
arXiv2026

Quasi two dimensional magnetic structure of the triclinic double perovskite Ca$_2$CuWO$_6$

We present the antiferromagnetic ground state of the triclinic double perovskite Ca$_2$CuWO$_6$solved by neutron powder diffraction, and analyze it in direct comparison with tetragonal Sr$_2$CuWO$_6$. Below $T_\mathrm{N} \simeq 32$ K, the magnetic Bragg reflections of Ca$_2$CuWO$_6$ are indexed by the commensurate propagation vector $\mathbf{k}=(\tfrac{1}{2},\tfrac{1}{2},0)$ in its native $P\bar{1}$ cell. Rietveld refinement yields a collinear structure with equal-magnitude, antiparallel moments on the crystallographically inequivalent Cu1 and Cu2 sites and an ordered moment of $0.66(3)~\mu_{\mathrm B}$ per Cu at 1.5 K. A direct comparison with Sr$_2$CuWO$_6$ is obscured by different crystallographic settings and orientations of the cooperative Jahn-Teller elongation axes. Hence, we introduce a common crystallographic supercell through which effects of symmetry lowering from tetragonal to triclinic in the double perovskite are explored: Apparently different magnetic propagation vectors map onto the same supercell wave vector, revealing a common magnetic structure stabilized by tungsten-mediated, second-neighbor interactions. Mean field calculations further show that tetragonal symmetry preserves the degeneracy of four magnetic $\mathbf{k}$-domains in Sr$_2$CuWO$_6$, whereas the triclinic splitting of symmetry-related exchange pathways in Ca$_2$CuWO$_6$, most strongly within the Cu2 network, selects a single $\mathbf{k}$-domain. These results establish the magnetic ground state of Ca$_2$CuWO$_6$ and show how symmetry breaking selects a given ordered state without changing the underlying magnetic motif of the Sr analogue.

cond-mat.str-el↗
arXiv2026

Antiferroquadrupolar Order in Altermagnetic CoF$_2$

Altermagnets host non-relativistic spin-split electronic states whose microscopic origin is theoretically linked to a hidden charge order, yet experimental studies of this charge order are limited. We therefore investigate charge order within CoF$_2$, a $d$-wave altermagnetic compound with a rutile crystal structure and $\Gamma$-point antiferromagnetism. Combining resonant elastic X-ray scattering, symmetry analysis and ab initio calculations, we directly observe charge ordering and identify it as antiferroquadrupolar in nature. Via electronic structure calculations, we show that the experimentally observed antiferroquadrupolar order gives rise to the characteristic altermagnetic spin-splitting, thereby establishing empirical evidence for the decomposition of the altermagnetic order parameter into magnetic and charge degrees of freedom. We hence demonstrate that antiferroquadrupolar order is the microscopic origin of altermagnetism in CoF$_2$, with implications to the wider family of rutile altermagnets. Furthermore, our approach is applicable to studying altermagnetism in general, having demonstrated that resonant elastic X-ray scattering can serve as a direct probe of the charge multipoles that underpin spin-split electronic states in these materials.

cond-mat.str-el↗
arXiv2026

Competing collinear and non-collinear spin textures imaged by spatially-resolved REXS in Eu(Al0.4Ga0.6)4

Here, we use resonant elastic x-ray scattering (REXS) to investigate the inhomogeneity of the zero-field magnetic spin texture of Eu(Al$_{0.4}$Ga$_{0.6}$)$_4$. By using spatially-resolved REXS, we show that the two magnetic transitions at T$_{N1}$ = 17 K and T$_{N2}$ = 14 K originate from two nearly degenerate, yet distinct, orthogonal pairs of q-vectors. The corresponding phases are segregated spatially, such that one fraction of the sample comprises coexisting orthogonal spin-density-wave domains, while another fraction hosts coexisting orthogonal helical domains. Additionally, the helical state forms inversion domains indicating that the inversion symmetry is not broken prior the magnetic transition. Our results suggest that the magnetic state is single-q and revealed a large variation of spin textures across a 0.8 - 1 mm area of the sample surface. These results demonstrate clear differences between the locally and globally probed magnetic textures, typically assumed to be representative of the system as a whole, highlighting the importance of spatially-resolved probes for accurately describing the magnetic behavior of this class of materials.

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arXiv2026

Inverse Melting of 3D Antiferromagnetic Order in Multi-sublattice Magnetic Perovskites

In conventional antiferromagnets a long-range ordered 3D ground state transitions to a disordered paramagnetic state on warming, often via lower dimensional spin correlations within the critical regime. Here we demonstrate a striking departure from this paradigm. Through analysis of neutron powder diffraction data, we show that the magnetic ground state of columnar-ordered quadruple perovskites, Na$R$Mn$_2$Ti$_4$O$_{12}$ ($R$ = Dy, Sm), lacks long-range order, hosting only 2D spin correlations. On warming, this disordered state transitions into a 3D long-range ordered antiferromagnetic structure prior to the phase transition to the paramagnetic state. Our results establish an unconventional order-by-heating mechanism in which intrinsic A-site chemical disorder is coupled to competing exchange interactions between the rare earth and Mn sub-lattices, leading to a novel type of magnetic phase transition.

cond-mat.str-el↗
arXiv2026

Modification of Charge and Spin Textures by Light Chemical Substitution in Eu(Al$_{1-x}$Ga$_{x}$)$_4$ ($x=0.1$)

We present the results of a resonant X-ray diffraction experiment, resolving both charge and spin textures in the intermetallic topological magnet Eu(Al$_{1-x}$Ga$_{x}$)$_4$, $x$ = 0.1. Below $\approx$ 75 K the system develops a charge density wave (CDW) with propagation vector kCDW ~ (0, 0, 0.18). The CDW order parameter grows monotonically on cooling until ~ 15 K, when a sudden decrease in the CDW amplitude occurs. Pairs of magnetic satellites of the (0, 0, 8) Bragg reflection corresponding to two distinct domains, k = ($\pm δ_\text{m}$, 0, 0), k2 = (0, $\pmδ_\text{m}$, 0), $δ_\text{m} = 0.2002(4)$ were studied at the Eu L3 edge, appearing below TN = 14.8 K. Our measurement of TN is exactly coincident with the sudden drop in the CDW amplitude, which suggests strong coupling between the charge and spin orders, as observed in other compounds of the Eu(Al$_{1-x}$Ga$_{x}$)$_4$ series. Azimuthal measurements revealed a single helical spin arrangement with an elliptical envelope of $μ_\text{Y}/μ_\text{Z}$ = 1.19(6) for the k2 domain, and a tilted helical (helicoidal) spin arrangement for the k1 domain, with $μ_\text{Y}/μ_\text{Z}$ = 1.14(4) and $μ_\text{X}/μ_\text{Z}$ = 0.20(2) that may be hidden for the k2 domain due to multiple subdomains. Temperature evolution of the magnetic satellite intensities in linear and circularly polarised light found the respective ratio to be invariant with temperature, suggesting a single magnetic phase below TN. This behaviour is unlike the x = 0 material, in which a spin density wave forms first, transitioning to a helical ground state on cooling through intermediate phases. Future theoretical work on the Eu electronic ground state, supported by related experiments, will help understand the effects of Ga substitution on the evolution of the magnetic structure.

cond-mat.str-el↗
arXiv2025

Symmetry-designed BiFeO3 single domain spin cycloid for efficient spintronics

Deterministic control of coupled ferroelectric and antiferromagnetic orders remains a central challenge in multiferroics, limiting their integration into functional magnetoelectrics and magnonic-devices. (111)pc BiFeO3 with a robust single spin cycloid, offers direct magnetoelectric-coupling and a platform for efficient spin transport, yet multi-magnetic domains and ferroelectric-fatigue have prevented reproducible control. Here, we show that anisotropic-compressive in-plane strain stabilizes a single antiferromagnetic domain with unique spin-cycloid vector, by breaking the symmetry of the (111)pc plane. Epitaxial BiFeO3 films grown on orthorhombic NdGaO3 (011)o [(111)pc] substrates impose the required anisotropic in-plane strain and stabilizes single antiferromagnetic domain, as confirmed through direct imaging with scanning NV microscopy and non-resonant-x-ray-magnetic-scattering. Remarkably, these engineered films exhibit deterministic and non-volatile 180° switching of ferroelectric and single antiferromagnetic domains over 1,000 cycles. The monodomain state also enables anisotropic and threefold enhanced magnon transport with reduced scattering. Thus, symmetry-designed (111)pc monodomain BiFeO3 offers a robust platform for advanced magnetoelectric and magnonic applications.

cond-mat.mtrl-sci↗
arXiv2025

Magnetic-field-induced ordering in a spin-1/2 chiral chain

We present neutron diffraction, muon spin rotation and pulsed-field magnetometry measurements on the Heisenberg quantum chiral chain [Cu(pym)(H2O)4]SiF6.H2O, which displays a four-fold-periodic rotation of the local environment around the Cu(II) S = 1/2 ions from site to site along the chain. Previous measurements on this material have shown the absence of magnetic order down to surprisingly low temperatures >= 20 mK, as well as the presence of an energy gap for magnetic excitations that grows linearly with magnetic field. Here we find evidence at dilution refrigerator temperatures for a field-induced transition to long-range magnetic order above an applied magnetic field of 3 T. From the polarization of magnetic moments observed in applied fields we can identify the static magnetic structure that best accounts for the data. The proposed model is supported microscopically by the presence of an alternating component of the g tensor, which produces an internal two-fold staggered field that dictates both the direction of the ordered moments and the effective coupling between adjacent chains. The observed magnetic structure is contrary to previous proposals for the departure of the magnitude and field dependence of the energy gap from the predictions of the sine-Gordon model.

cond-mat.str-el↗
arXiv2025

Elastic softness of low-symmetry frustrated $A$Ti$_2$O$_5$ ($A$ = Co, Fe)

Orthorhombic pseudobrookites CoTi$_2$O$_5$ and FeTi$_2$O$_5$ have a low-symmetry crystal structure comprising magnetic Co$^{2+}$/Fe$^{2+}$ ions and nonmagnetic Ti$^{4+}$ ions, where the orbital-nondegenerate Co$^{2+}$/Fe$^{2+}$ ions form one-dimensional chains running along the orthorhombic $a$ axis. These compounds undergo an antiferromagnetic phase transition at $T_N \sim$ 26 K for CoTi$_2$O$_5$ and $T_N \sim$ 40 K for FeTi$_2$O$_5$. Ultrasound velocity measurements on single crystals of CoTi$_2$O$_5$ and FeTi$_2$O$_5$ reveal that CoTi$_2$O$_5$ exhibits unusual elastic softness above $T_N$ in the symmetry-lowering elastic mode of $ac$-plane shear elastic modulus, inconsistent with the structural symmetry breaking caused by antiferromagnetic ordering at $T_N$. This suggests the presence of two distinct types of magnetostructural fluctuations above $T_N$ that should be a precursor to the symmetry-lowering lattice distortion at $T_N$. In contrast, FeTi$_2$O$_5$ exhibits either negligible or smaller elastic softness, indicating weaker spin-lattice coupling. These findings highlight CoTi$_2$O$_5$ and FeTi$_2$O$_5$ as unique spin-latticed-coupled frustrated systems with low crystal symmetry, where, while the exchange interactions are quasi-one-dimensional in nature, the frustration is released by further lowering the crystal symmetry through three-dimensional spin-lattice coupling, which is stronger in CoTi$_2$O$_5$ than in FeTi$_2$O$_5$.

cond-mat.str-el↗
arXiv2024

Origin of the insulating state in the Kitaev candidate Cu$_2$IrO$_3$

Through a combination of crystal symmetry analysis and density functional theory calculations we unveil a possible microscopic origin of the unexpected insulating behavior reported in the honeycomb Kitaev material Cu$_2$IrO$_3$. Our study suggests that this material hosts an instability towards charge ordering of the Ir ions, with alternating magnetic Ir$^{4+}$ and non-magnetic Ir$^{3+}$ ions arranged on the honeycomb lattice. In this case, the next-nearest-neighbor interactions that couple magnetic Ir$^{4+}$ ions form an enlarged triangular lattice, instead of the expected honeycomb lattice. The magnetic Cu$^{2+}$ ions located at the centre of the iridium honeycomb voids also form a triangular lattice, and additionally contribute to the magnetization of the system. Together, the interpenetrated Ir$^{4+}$ and Cu$^{2+}$ triangular lattices present a novel type of honeycomb Kitaev lattice composed of two types of magnetic ions.

cond-mat.str-el↗
arXiv2024

Compass-model physics on the hyperhoneycomb lattice in the extreme spin-orbit regime

The physics of spin-orbit entangled magnetic moments of $4d$ and $5d$ transition metal ions on a honeycomb lattice has been much explored in search for unconventional magnetic orders or quantum spin liquids expected for compass spin models, where different bonds in the lattice favour different orientations for the magnetic moments. Realizing such physics with rare-earth ions is a promising route to achieve exotic ground states in the extreme spin orbit limit, however this regime has remained experimentally largely unexplored due to major challenges in materials synthesis. Here we report successful synthesis of powders and single crystals of $β$-Na$_2$PrO$_3$, with $4f^{1}$ Pr$^{4+}$ $j_\mathrm{eff}\!=\!1/2$ magnetic moments arranged on a hyperhoneycomb lattice with the same threefold coordination as the planar honeycomb. We find a strongly noncollinear magnetic order with highly dispersive gapped excitations that we argue arise from frustration between bond-dependent, anisotropic off-diagonal exchanges, a compass quantum spin model not explored experimentally so far. Our results show that rare-earth ions on threefold coordinated lattices offer a platform for the exploration of quantum compass spin models in the extreme spin orbit regime, with qualitatively distinct physics from that of $4d$ and $5d$ Kitaev materials.

cond-mat.str-el↗
arXiv2024

Direct imaging and control of Berry curvature in noncollinear antiferromagnetic single-crystal thin films

The discovery of the intrinsic anomalous Hall effect (AHE) in noncollinear antiferromagnets where transverse Hall voltage emerges without magnetic field, has opened a plethora of promising opportunities in antiferromagnetic devices. The key challenges limiting their full potential are (1) high-quality epitaxial thin-film growth and (2) the understanding of Berry curvature domain physics. Here, we focus on a noncollinear antiperovskite antiferromagnet Mn3NiN as a model system, successfully grown as a single-crystal epitaxial thin film. Combining multiple experiments supported by theoretical calculations, we probe the Berry curvature associated with antiferromagnetic {\Gamma}4g domains in Mn3NiN and its strong connection to an AHE. We directly image the intrinsic Berry curvature with high-resolution Sagnac microscopy, controlling spatial distribution and dynamics by varying temperature and applied magnetic fields. We discover that the {\Gamma}4g domains are switchable near the N\'eel transition, but become frozen and unresponsive to external stimuli at low temperature. This behavior enables the tuning of Berry-curvature driven AHE and magneto-optic Kerr effect responses through controlled experimental conditions. Our findings provide critical advancement of the fundamental understanding and wide tunability of Berry curvature in noncollinear antiferromagnets important for realization in potential spintronic applications.

cond-mat.mtrl-sci↗
arXiv2023

Switching of ferrotoroidal domains via an intermediate mixed state in the multiferroic Y-type hexaferrite Ba$_{0.5}$Sr$_{1.5}$Mg$_2$Fe$_{12}$O$_{22}$

We report a detailed study of the magnetic field switching of ferrotoroidal/multiferroic domains in the Y-type hexaferrite compound Ba$_{0.5}$Sr$_{1.5}$Mg$_2$Fe$_{12}$O$_{22}$. By combining data from SQUID magnetometry, magneto-current measurements, and resonant X-ray scattering experiments, we arrive at a complete description of the deterministic switching, which involves the formation of a temperature-dependent mixed state in low magnetic fields. This mechanism is likely to be shared by other members of the hexaferrite family, and presents a challenge for the development of high-speed read-write memory devices based on these materials.

cond-mat.str-el↗
arXiv2020

Unusual effects of magnetic dilution in the ferrimagnetic columnar ordered $\mathrm{Sm_2MnMnMn_{4-x}Ti_xO_{12}}$ perovskites

Powder neutron diffraction experiments have been employed to establish the effects of site-selective magnetic dilution in the Sm2MnMnMn4-x Tix O12 A-site columnar ordered quadruple perovskite manganites (x = 1, x = 2 and x = 3). We show that in all three compositions the Mn ions adopt a collinear ferrimagnetic structure below 27 K, 62 K and 34 K, respectively. An unexpected increase in the ordering temperature was observed between the x = 1 and x = 2 samples, which indicates a considerable departure from mean field behaviour. This result is corroborated by large reductions in the theoretical ground state magnetic moments observed across the series, which indicate the presence of spin fluctuations and or disorder. We show that long range magnetic order in the x = 3 sample, which occurs below the percolation threshold for B-B exchange, can only be understood to arise if magnetic order in Sm2MnMnMn4-xTixO12 is mediated via both A-B and B-B exchange, hence confirming the importance of A-B exchange interactions in these materials. Finally we show that site-selective magnetic dilution enables the tuning of a ferrimagnetic compensation point and the introduction of temperature-induced magnetization reversal.

cond-mat.str-el↗
arXiv2020

Emergent helical texture of electric dipoles

Long-range ordering of magnetic dipoles in bulk materials gives rise to a broad range of magnetic structures, from simple collinear ferromagnets and antiferromagnets, to complex magnetic helicoidal textures stabilized by competing exchange interactions. In contrast, in the context of dipolar order in dielectric crystals, only parallel (ferroelectric) and antiparallel (antiferroelectric) collinear alignments of electric dipoles are typically considered. Here, we report an observation of incommensurate helical ordering of electric dipoles by light hole-doping of the quadruple perovskite BiMn7O12. In analogy with magnetism, the electric dipole helicoidal texture is also stabilized by competing instabilities. Specifically, orbital ordering and lone electron pair stereochemical activity compete, giving rise to phase transitions from a non-chiral cubic structure, to an incommensurate electric dipole and orbital helix, via an intermediate density wave.

cond-mat.str-el↗
arXiv2019

FeTi$_2$O$_5$: a spin Jahn-Teller transition enhanced by cation substitution

We have used muon-spin rotation, heat capacity and x-ray diffraction measurements in combination with density functional theory and dipole field calculations to investigate the crystal and magnetic structure of FeTi$_2$O$_5$. We observe a long range ordered state below $T_{\rm N}$=41.8(5) K with indications of significant correlations existing above this temperature. We determine candidate muon stopping sites in this compound, and find that our data are consistent with the spin Jahn-Teller driven antiferromagnetic ground state with $\boldsymbol{k}$=(1/2,1/2,0) reported for CoTi$_2$O$_5$ ($T_{\rm N}$=26 K). By comparing our data with calculated dipolar fields we can restrict the possible moment size and directions of the Fe$^{2+}$ ions.

cond-mat.str-el↗
arXiv2019

Determining the anisotropy and exchange parameters of polycrystalline spin-1 magnets

Although low-dimensional $S = 1$ antiferromagnets remain of great interest, difficulty in obtaining high-quality single crystals of the newest materials hinders experimental research in this area. Polycrystalline samples are more readily produced, but there are inherent problems in extracting the magnetic properties of anisotropic systems from powder data. Following a discussion of the effect of powder-averaging on various measurement techniques, we present a methodology to overcome this issue using thermodynamic measurements. In particular we focus on whether it is possible to characterise the magnetic properties of polycrystalline, anisotropic samples using readily available laboratory equipment. We test the efficacy of our method using the magnets [Ni(H$_{2}$O)$_{2}$(3,5-lutidine)$_{4}$](BF$_{4}$)$_{2}$ and Ni(H$_{2}$O)$_{2}$(acetate)$_{2}$(4-picoline)$_{2}$, which have negligible exchange interactions, as well as the antiferromagnet [Ni(H$_{2}$O)$_{2}$(pyrazine)$_{2}$](BF$_{4}$)$_{2}$, and show that we are able to extract the anisotropy parameters in each case. The results obtained from the thermodynamic measurements are checked against electron-spin resonance and neutron diffraction. We also present a density functional method, which incorporates spin-orbit coupling to estimate the size of the anisotropy in [Ni(H$_{2}$O)$_{2}$(pyrazine)$_{2}$](BF$_{4}$)$_{2}$.

cond-mat.str-el↗
arXiv2019

Gapless spin-liquid state in the structurally disorder-free triangular antiferromagnet NaYbO$_2$

We present the structural characterization and low-temperature magnetism of the triangular-lattice delafossite NaYbO$_2$. Synchrotron x-ray diffraction and neutron scattering exclude both structural disorder and crystal-electric-field randomness, whereas heat-capacity measurements and muon spectroscopy reveal the absence of magnetic order and persistent spin dynamics down to at least 70\,mK. Continuous magnetic excitations with the low-energy spectral weight accumulating at the $K$-point of the Brillouin zone indicate the formation of a novel spin-liquid phase in a triangular antiferromagnet. This phase is gapless and shows a non-trivial evolution of the low-temperature specific heat. Our work demonstrates that NaYbO$_2$ practically gives the most direct experimental access to the spin-liquid physics of triangular antiferromagnets.

cond-mat.str-el↗
arXiv2018

Spin Jahn-Teller antiferromagnetism in CoTi$_2$O$_5$

We have used neutron powder diffraction to solve the magnetic structure of orthorhombic CoTi$_2$O$_5$, showing that the long-range ordered state below 26 K identified in our muon-spin rotation experiments is antiferromagnetic with propagation vector ${\bf k}=(\pm \frac{1}{2}, \frac{1}{2}, 0)$ and moment of 2.72(1)$μ_{\rm B}$ per Co$^{2+}$ ion. This long range magnetic order is incompatible with the experimentally determined crystal structure because the imposed symmetry completely frustrates the exchange coupling. We conclude that the magnetic transition must therefore be associated with a spin Jahn-Teller effect which lowers the structural symmetry and thereby relieves the frustration. These results show that CoTi$_2$O$_5$ is a highly unusual low symmetry material exhibiting a purely spin-driven lattice distortion critical to the establishment of an ordered magnetic ground state.

cond-mat.str-el↗