arXiv subjects

Nicola D. Kelly

Publications and source records attributed to Nicola D. Kelly.

7 recordsLinked to original sources

arXiv2026

Incommensurate structural and magnetic modulations in potassium-rich cryptomelane, K$_x$Mn$_8$O$_{16}$ ($x\approx1.45$)

Cryptomelane is a hollandite-like material consisting of K$^+$ cations in an $α$-MnO$_2$ tunnel-like crystallographic motif. Cryptomelane with stoichiometry K$_x$Mn$_8$O$_{16}$ ($x\approx1.45$) has been synthesized and its magnetic properties investigated using variable-temperature magnetic susceptibility, heat capacity, and neutron powder diffraction. Three distinct transitions at $T_1=184$\,K, $T_2=54.5$\,K, and $T_3=24$\,K are observed. At $T_1$ there is a subtle tetragonal$\rightarrow$monoclinic transition associated with emergence of a set of non-magnetic superstructure peaks indexable to a $\vec{k}_\mathrm{struc}\approx0.74\vec{c^*}$ incommensurate modulation parallel to the $α$-MnO$_2$ tunnels. Our findings are consistent with a relation previously reported in titanate hollandites, that $x\approx2|\vec{k}_\mathrm{struc}|$. Magnetic Bragg peaks emerge below $T_2=54.5$\,K, and their positions indicate an incommensurate modulated magnetic structure. The model consistent with the data is a dual-$\vec{k}_\mathrm{mag}$ structure with a ferromagnetic $|\vec{k}_\mathrm{mag}|=0$ component and an incommensurate $\vec{k}_\mathrm{mag}\approx0.37\vec{c^*}$, with the latter most likely to be helical. The period of oscillation of the incommensurate magnetic component is in line with predictions based on a Heisenberg spin Hamiltonian [Mandal \textit{et al}. Phys. Rev. B 90, 104420 (2014)]. Below $T_3=24$\,K, there is a magnetic transition, which gives rise to a different set of magnetic Bragg peaks indicative of a highly complex magnetic structure.

cond-mat.mtrl-sci↗

arXiv2026

Bulk magnetic properties of distorted square lattice compounds M'-LnTaO4 (Ln = Tb, Dy, Ho, Er)

We report bulk magnetic properties of the monoclinic lanthanide tantalates, M'-LnTaO4 (Ln = Tb, Dy, Ho, Er), where the magnetic Ln3+ ions are arranged on a distorted 2D square lattice. The heavier analogue M'-YbTaO4 has been investigated as a spin-orbit-coupled, quasi-two-dimensional frustrated magnet, and the properties of the other M'-LnTaO4 are expected to vary depending on the electronic configuration of the Ln ion, namely Kramers vs non-Kramers behaviour and different crystal electric field parameters. In this work, powder neutron diffraction is used to confirm the crystal structure for Ln = Tb, Ho, Er, and to determine the magnetic structure of M'-TbTaO4, which displays long-range antiferromagnetic (AFM) order below T_N = 2.1 K. The Tb3+ moments are aligned primarily along the c-axis with AFM nearest-neighbour interactions. Susceptibility data suggest that M'-DyTaO4 may display short-range ordering around 2.7 K, while M'-HoTaO4 and M'-ErTaO4 show AFM correlations but do not order above 1.8 K. Measurements of the magnetic specific heat provide evidence for a Kramers doublet ground state in M'-ErTaO4, similar to its heavier analogue M'-YbTaO4.

cond-mat.str-el↗

arXiv2026

High-pressure synthesis of quantum magnet M-YbTaO4 with a stretched diamond lattice

We report bulk magnetic properties of ytterbium tantalate in its monoclinic fergusonite modification, M-YbTaO4. The spin-1/2 Yb3+ ions in this phase are arranged on a geometrically frustrated "stretched diamond" lattice. M-YbTaO4 cannot be prepared at ambient pressure and was instead prepared in a belt-type apparatus at 6 GPa and 1800 C. Susceptibility and specific heat data show no long-range ordering down to 1.8 K and are consistent with a Jeff = 1/2 Kramers doublet which splits in an applied field. Furthermore, under high-pressure synthesis the entire solid solution YbNbxTa1-xO4 (0 < x < 1) can be stabilised in the M phase, in contrast to ambient-pressure synthesis which favours the competing M' phase for Ta-rich compositions. Subsequent annealing of the Nb-Ta mixed samples resulted in colour changes, suggesting oxygen deficiency in some of the as-prepared high pressure samples. There was little variation in the bulk magnetic properties upon varying either the Nb/Ta ratio or the annealing conditions.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic Properties of the Quasi-1D Magnesium Lanthanide Borates Mg$Ln$B$_5$O$_{10}$

Lanthanide borates are widely studied for their optical and magnetic properties. A wide variety of structures are known with 3, 2, 1 and 0 dimensional connectivity of lanthanide ions. Here, we explore Mg$Ln$B$_5$O$_{10}$, with a quasi-1D arrangement of the $Ln$ ions. Polycrystalline samples of Mg$Ln$B$_5$O$_{10}$ ($Ln$ = La, Pr, Nd, Sm--Er) were synthesised with high purity via a sol-gel method. Powder X-ray diffraction data confirmed the reported monoclinic space group ($P2_1/c$). The magnetic $Ln^{3+}$ ions in Mg$Ln$B$_5$O$_{10}$ form relatively isolated zig-zag chains parallel to the $b$ axis. Magnetic susceptibility and isothermal magnetisation were measured: all samples except $Ln=$ (Eu, Sm) fit the Curie-Weiss Law in isothermal magnetisation at high temperatures, in broad agreement with theoretical expectations. $Ln =$ (Nd, Tb, Dy, Ho) exhibit signatures characteristic of Ising spin saturation, implying single ion anisotropy, while Gd exhibits characteristics of Heisenberg spins. Estimation of magnetic interactions suggests that Mg$Ln$B$_5$O$_{10}$ are candidate materials for quasi-1D magnetism. The magnetocaloric entropy change was also calculated, with MgGdB$_5$O$_{10}$ showing promise for application to solid-state refrigeration at liquid helium temperatures.

cond-mat.mtrl-sci↗

arXiv2022

Magnetism on the stretched diamond lattice in lanthanide orthotantalates

The magnetic Ln$^{3+}$ ions in the fergusonite and scheelite crystal structures form a distorted or stretched diamond lattice which is predicted to host exotic magnetic ground states. In this study, polycrystalline samples of the fergusonite orthotantalates $M$-LnTaO$_4$ (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er) are synthesized and then characterized using powder diffraction and bulk magnetometry and heat capacity. TbTaO$_4$ orders antiferromagnetically at 2.25 K into a commensurate magnetic cell with $\vec{k}=0$, magnetic space group 14.77 ($P2_1$$'/c$) and Tb moments parallel to the $a$-axis. No magnetic order was observed in the other materials studied, leaving open the possibility of exotic magnetic states at $T<2$ K.

cond-mat.mtrl-sci↗

arXiv2022

Free-spin dominated magnetocaloric effect in dense Gd$^{3+}$ double perovskites

Frustrated lanthanide oxides with dense magnetic lattices are of fundamental interest for their potential in cryogenic refrigeration due to a large ground state entropy and suppressed ordering temperatures, but can often be limited by short-range correlations. Here, we present examples of frustrated fcc oxides, Ba$_2$GdSbO$_6$ and Sr$_2$GdSbO$_6$ and the new site-disordered analog Ca$_2$GdSbO$_6$ ([CaGd]$_A$[CaSb]$_B$O$_6$), in which the magnetocaloric effect is influenced by minimal superexchange ($J_1 \sim 10$ mK). We report on the crystal structures using powder x-ray diffraction and the bulk magnetic properties through low-field susceptibility and isothermal magnetization measurements. The Gd compounds exhibit a magnetic entropy change of up to -15.8 J/K/mol$_\textrm{Gd}$ in a field of 7 T at 2 K, a 20 % excess compared to the value of -13.0 J/K/mol$_\textrm{Gd}$ for a standard in magnetic refrigeration, Gd$_3$Ga$_5$O$_{12}$. Heat capacity measurements indicate a lack of magnetic ordering down to 0.4 K for Ba$_2$GdSbO$_6$ and Sr$_2$GdSbO$_6$, suggesting cooling down through the liquid 4-He regime. A mean-field model is used to elucidate the role of primarily free spin behavior in the magnetocaloric performance of these compounds in comparison to other top-performing Gd-based oxides. The chemical flexibility of the double perovskites raises the possibility of further enhancement of the magnetocaloric effect in the Gd$^{3+}$ fcc lattices.

cond-mat.str-el↗

arXiv2020

Magnetic properties of quasi-one-dimensional lanthanide calcium oxyborates Ca$_4$LnO(BO$_3$)$_3$

This study examines the lanthanide calcium oxyborates Ca$_4$LnO(BO$_3$)$_3$ (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb). The reported monoclinic structure (space group $Cm$) was confirmed using powder X-ray diffraction. The magnetic Ln$^{3+}$ ions are situated in well-separated chains parallel to the c axis in a quasi-one-dimensional array. Here we report the first bulk magnetic characterisation of Ca$_4$LnO(BO$_3$)$_3$ using magnetic susceptibility $χ$(T) and isothermal magnetisation M(H) measurements at T $\geq$ 2 K. With the sole exception of Ca$_4$TbO(BO$_3$)$_3$, which displays a transition at T = 3.6 K, no magnetic transitions occur above 2 K, and Curie-Weiss analysis indicates antiferromagnetic nearest-neighbour interactions for all samples. Calculation of the magnetic entropy change $ΔS_m$ indicates that Ca$_4$GdO(BO$_3$)$_3$ and Ca$_4$HoO(BO$_3$)$_3$ are viable magnetocaloric materials at liquid helium temperatures in the high-field and low-field regimes respectively.

cond-mat.str-el↗