Partitioning the two-leg spin ladder in Ba2Cu1– xZnxTeO6 : from magnetic order through spin-freezing to paramagnetism

E.J.C., O.M., and C.P. acknowledge financial support from the Leverhulme Trust Research Project Grant No. RPG-2017-109. O.M. is grateful for funding via the Leverhulme Trust Early Career Fellowship ECF-2021-170. A.S.G. acknowledges funding through an EPSRC Early Career Fellowship EP/ T011130/1. A.S.G. and H.T. acknowledge funding through the Humboldt Foundation and the Max Planck Institute for Solid State Research. The authors thank the Science and Technology Facilities Council for beamtime allocated at ISIS through proposal RB1990046 (DOI: 10. 5286/ISIS.E.RB1990046) and the Swiss Muon Source at the Paul Scherrer Institute through proposal numbers 20150959 and 20211440. The authors are grateful for access to the MPMS3 instrument at The Royce Discovery Centre at the University of Sheffield (EPSRC grant no. EP/R00661X/1) and the PPMS instrument at the University of St. Andrews (EPSRC grant no. EP/T031441/1).Ba2CuTeO6 has attracted significant attention as it contains a two-leg spin ladder of Cu2+ cations that lies in close proximity to a quantum critical point. Recently, Ba2CuTeO6 has been shown to accommodate chemical substitutions, which can significantly tune its magnetic behavior. Here, we investigate the effects of substitution for non-magnetic Zn2+ impurities at the Cu2+ site, partitioning the spin ladders. Results from bulk thermodynamic and local muon magnetic characterization on the Ba2Cu1 – xZnxTeO6 solid solution (0 ≤ x ≤ 0.6) indicate that Zn2+ partitions the Cu2+ spin ladders into clusters and can be considered using the percolation theory. As the average cluster size decreases with increasing Zn2+ substitution, there is an evolving transition from long-range order to spin-freezing as the critical cluster size is reached between x = 0.1 to x = 0.2, beyond which the behavior became paramagnetic. This demonstrates well-controlled tuning of the magnetic disorder, which is highly topical across a range of low-dimensional Cu2+-based materials. However, in many of these cases, the chemical disorder is also relatively strong in contrast to Ba2CuTeO6 and its derivatives. Therefore, Ba2Cu1 – xZnxTeO6 provides an ideal model system for isolating the effect of defects and segmentation in low-dimensional quantum magnets.Publisher PDFPeer reviewe

Complex magnetic ordering behavior in the frustrated perovskite Ba2MnMoO6

New and exotic ground states of magnetic materials are highly sought after and are extensively studied for the insights they provide into the thermodynamics of disorder and fundamental magnetic interactions. By controlling the crystal structure of an appropriate magnetic lattice, it is possible to cause the strong magnetic exchange interactions to sum to zero and so be frustrated. Due to the presence of this frustration, the lowest energy configuration that results may be crucially dependent on the tiniest of energy differences between a multitude of states that have (almost) the same energy. The keen interest in these materials arises from the fact that these finely balanced systems offer a way of probing classical or quantum mechanical interactions that are of fundamental importance but are too weak to be observed in non-frustrated systems. Here, we combine local and crystallographic probes of the cation-ordered double perovskite Ba2MnMoO6 that contains a face-centered cubic lattice of S = 5/2 Mn2+ cations. Neutron diffraction measurements below 9.27(7) K indicate that a fourfold degenerate non-collinear antiferromagnetic state exists with almost complete ordering of the Mn2+ spins. Muon spin relaxation measurements provide a local probe of the magnetic fields inside this material over the t1/2 = 2.2 µs lifetime of a muon, indicating a slightly lower Néel transition temperature of 7.9(1) K. The dc susceptibility data do not show the loss of magnetization that should accompany the onset of the antiferromagnetic order; they indicate that a strongly antiferromagnetically coupled paramagnetic state [θ = −73(3) K] persists down to 4 K, at which temperature a weak transition occurs. The behavior of this material differs considerably from the closely related compositions Ba2MnMO6 (M = W, Te), which show collinear ordering arrangements and well defined antiferromagnetic transitions in the bulk susceptibility. This suggests that the Mo6+ cation leads to a fine balance between the nearest and next-nearest neighbor superexchange in these frustrated double perovskite structures

Studies of Helium Droplet Mass Spectrometry and Magnetic Nanoparticles

As the title suggests, this MPhil thesis is separated into two research topics. The first topic is based in the field of superfluid helium droplet science, and explores mass spectrometry of conjugated molecules in superfluid helium droplets. Recent observations have suggested conjugated molecules behave very differently to other molecules upon electron ionization in the helium droplet environment. The helium droplet mass spectrum of p-benzoquinone was recorded and compared to the gas phase mass spectrum. To try to explain the fragmentation process, density functional theory calculations were performed on the initial fragmentation pathways. By combining the experimental data and the theoretical calculations, a model to explain the reduced parent ion signal in the helium droplet mass spectrum of p-benzoquinone was developed. The model suggests the high energy of the proton loss pathway and the potential of a large energy barrier to parent ion ejection, forces p-benzoquinone to almost completely fragment upon ionisation inside the helium droplet. Further studies with other conjugated molecules could reveal more information about the influence of the helium matrix on dopant species. The second research topic investigates new methods to produce high moment iron oxide/iron nanoparticles. Iron oxide nanoparticles have been the focus of great research interest due to their various applications. Here, a modified approach to make iron oxide based on the co-precipitation technique was explored as a simple route to increase the nanoparticle magnetic moment. By applying a weak magnetic field to the reaction vessel it was found the magnetic saturation of the nanoparticles could be increased by a few emu/g. A novel method of producing pure iron nanoparticles based on liquid plasma nanosynthesis has also been proposed. The design for the liquid plasma reactor could allow iron nanoparticles, which have even higher moments than iron oxide, to be produced on a large scale

Investigation into the magnetic properties of CoFeNiCryCux alloys

The search for cheap, corrosion-resistant, thermally-mechanically stable functional magnetic materials, including soft magnetic and magneto-caloric materials has led to research focused on high entropy alloys (HEAs). Previous research shows that alloying elements with negative enthalpies of mixing can facilitate a second-order phase transition. On the other side of the spectrum, compositional segregation cause by positive enthalpy of mixing alloying additions (such as Cu) may also be used to tune magnetic properties. This paper studies the structural, magnetic and magneto-caloric effect of the FCC alloys CoFeNiCr y Cu x (x = 0.0, 0.5, 1.0 and 1.5, y = 0.0, 0.8 and 1.0) to tune these properties with Cu and Cr alloying. Scanning electron microscopy of the compositions show nanoparticles forming within the grains as the Cu concentration increases. Cr addition to CoFeNiCu1.0 has a larger effect on the magnetic and magneto-caloric properties compared to the Cu addition to CoFeNiCr1.0. The addition of Cu (x = 0.5) to CoFeNiCr1.0 improved both the saturation magnetisation and Curie temperature; addition of Cr (y = 1.0) to CoFeNiCu1.0 decreased the Curie temperature by 900 K. All alloys were determined to have a second-order phase transition around their Curie temperature. The refrigerant capacity at 2 T was found to be similar to existing HEAs, although the Curie temperatures were lower than room temperature. Based on this data the CoFeNiCr0.8Cu composition was fabricated to increase the Curie temperature towards 300 K to explore these HEAs as new candidates for room temperature magneto-caloric applications. The fabricated composition showed Curie temperature, saturation magnetisation, and refrigerant capacity increasing with the small reduction in Cr content.This work was supported by the Royal Society Mid-Career Leverhulme Trust Fellowship scheme (SRF\R1\180020) and the Leverhulme Trust (RPG-2018-324). The samples have been prepared within the FP7 European project AccMet NMP4-LA-2011-263206. We wish to acknowledge the Henry Royce Institute for Advanced Materials, funded through EPSRC grants EP/R00661X/1, EP/S019367/1, EP/P02470X/1 and EP/P025285/1 for access to the MPMS-3 SQUID within the Royce at the University of Sheffield. ZYL would like to thank Dr Pratik Desai for his useful discussions on Cu transition states and magnetisation

Investigation into the magnetic properties of CoFeNiCryCux alloys

International audiencehe search for cheap, corrosion-resistant, thermally-mechanically stable functional magnetic materials, including soft magnetic and magneto-caloric materials has led to research focused on high entropy alloys (HEAs). Previous research shows that alloying elements with negative enthalpies of mixing can facilitate a second-order phase transition. On the other side of the spectrum, compositional segregation cause by positive enthalpy of mixing alloying additions (such as Cu) may also be used to tune magnetic properties. This paper studies the structural, magnetic and magneto-caloric effect of the FCC alloys CoFeNiCryCux (x = 0.0, 0.5, 1.0 and 1.5, y = 0.0, 0.8 and 1.0) to tune these properties with Cu and Cr alloying. Scanning electron microscopy of the compositions show nanoparticles forming within the grains as the Cu concentration increases. Cr addition to CoFeNiCu1.0 has a larger effect on the magnetic and magneto-caloric properties compared to the Cu addition to CoFeNiCr1.0. The addition of Cu (x = 0.5) to CoFeNiCr1.0 improved both the saturation magnetisation and Curie temperature; addition of Cr (y = 1.0) to CoFeNiCu1.0 decreased the Curie temperature by 900 K. All alloys were determined to have a second-order phase transition around their Curie temperature. The refrigerant capacity at 2 T was found to be similar to existing HEAs, although the Curie temperatures were lower than room temperature. Based on this data the CoFeNiCr0.8Cu composition was fabricated to increase the Curie temperature towards 300 K to explore these HEAs as new candidates for room temperature magneto-caloric applications. The fabricated composition showed Curie temperature, saturation magnetisation, and refrigerant capacity increasing with the small reduction in Cr content

Site-Selective d10/d0d^{10}/d^0 Substitution in an S = 12\frac{1} {2} Spin Ladder Ba2_2CuTe1–x_{1–x}Wx_xO6_6 (0 ≤ x ≤ 0.3)

Isovalent nonmagnetic d$^{10}$ and d$^0$ B″ cations have proven to be a powerful tool for tuning the magnetic interactions between magnetic B′ cations in A$_2$B′B″O$_6$ double perovskites. Tuning is facilitated by the changes in orbital hybridization that favor different superexchange pathways. This can produce alternative magnetic structures when B″ is d$^{10}$ or d$^0$. Furthermore, the competition generated by introducing mixtures of d$^{10}$ and d$^0$ cations can drive the material into the realms of exotic quantum magnetism. Here, Te$^{6+}$ d$^{10}$ was substituted by Wu$^{6+}$ d$^0$ in the hexagonal perovskite Ba$_2$CuTeO$_6$, which possesses a spin ladder geometry of Cu$^{2+}$ cations, creating a Ba$_2$CuTe$_{1–x}$W$_x$O$_6$ solid solution (x = 0–0.3). We find W$^{6+}$ is almost exclusively substituted for Te$^{6+}$ on the corner-sharing site within the spin ladder, in preference to the face-sharing site between ladders. The site-selective doping directly tunes the intraladder, J$_{rung}$ and J$_{leg}$, interactions. Modeling the magnetic susceptibility data shows the d$^0$ orbitals modify the relative intraladder interaction strength (J$_{rung}$/J$_{leg}$) so the system changes from a spin ladder to isolated spin chains as W$^{6+}$ increases. This further demonstrates the utility of d$^{10}$ and d$^0$ dopants as a tool for tuning magnetic interactions in a wide range of perovskites and perovskite-derived structures

Site-Selective d10/d0 Substitution in an S = 1/2 Spin Ladder Ba2CuTe1-xWxO6 (0 ≤ x ≤ 0.3).

Isovalent nonmagnetic d10 and d0 B″ cations have proven to be a powerful tool for tuning the magnetic interactions between magnetic B' cations in A2B'B″O6 double perovskites. Tuning is facilitated by the changes in orbital hybridization that favor different superexchange pathways. This can produce alternative magnetic structures when B″ is d10 or d0. Furthermore, the competition generated by introducing mixtures of d10 and d0 cations can drive the material into the realms of exotic quantum magnetism. Here, Te6+ d10 was substituted by W6+ d0 in the hexagonal perovskite Ba2CuTeO6, which possesses a spin ladder geometry of Cu2+ cations, creating a Ba2CuTe1-xWxO6 solid solution (x = 0-0.3). We find W6+ is almost exclusively substituted for Te6+ on the corner-sharing site within the spin ladder, in preference to the face-sharing site between ladders. The site-selective doping directly tunes the intraladder, Jrung and Jleg, interactions. Modeling the magnetic susceptibility data shows the d0 orbitals modify the relative intraladder interaction strength (Jrung/Jleg) so the system changes from a spin ladder to isolated spin chains as W6+ increases. This further demonstrates the utility of d10 and d0 dopants as a tool for tuning magnetic interactions in a wide range of perovskites and perovskite-derived structures.E.J.C., O.H.J.M., and C.P. acknowledge financial support from Leverhulme Trust Research Project Grant RPG-2017-109. O.H.J.M. is grateful for funding via Leverhulme Trust Early Career Fellowship ECF-2021-170. A.S.G. acknowledges funding through EPSRC Early Career Fellowship EP/T011130/1. The authors thank the Science and Technology Facilities Council for beam time allocated at ISIS. The authors are grateful for access to the MPMS3 instrument at the Materials Characterisation Laboratory at ISIS. The authors acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA III beamline P02.1. Components of this research utilized the HADES/MIDAS facility at the University of Sheffield established with financial support from EPSRC and BEIS, under Grant EP/T011424/1. (62) Use of the National Synchrotron Light Source II, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886 and beam time proposal number 303200. The authors are grateful to Bruce Ravel for assistance with acquisition of W L3 XAS data. Heat capacity measurements were performed using the Advanced Materials Characterisation Suite, funded by EPSRC Strategic Equipment Grant EP/M000524/1. S.E.D. acknowledges funding from the Winton Programme for the Physics of Sustainability (Cambridge) and EPSRC (EP/T028580/1)

CSD 2128596 - 2128600: Experimental Crystal Structure Determination

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