Refining As-Cast Structures of Novel SixTiVCrZr High-Entropy Alloys Using Estimated Effective Solidification Temperature Obtained Using Chvorinov's Rule

High-entropy alloys (HEAs), i.e., multicomponent alloys where (typically five or more) elements are combined in equal, or roughly equal, quantities, are of great current interest, due to their formation of single, simple structured phases, and the unusual properties they can potentially exhibit. Phase presence may be predicted using semi-empirical methods, but deviations from predictions may be seen during the course of alloy synthesis, with the formation of unexpected phases. The generation of such phases may be controlled with knowledge of the effective solidification temperature; in this full article, Chvorinov’s rule for solidification time is used to estimate this temperature as part of the design of a new multiphase alloy system, TiVCrZr-Six. Further heat treatment of the TiVCrZr-Si system confirms the applicability of this approach. The new compositions demonstrate mechanical properties that suggest potential for optimization for high-temperature applications.</jats:p

Tuning the mechanical properties of molecular perovskites by controlling framework distortions via A-site substitution

Molecular perovskites are important materials in the area of barocalorics, improper ferroelectrics and ferroelastics, where the search for principles that link composition, structure and mechanical properties is a key challenge. Herein, we report the synthesis of a new series of dicyanamide-based molecular perovskites [A]Ni(C2N3)3, where the A-site cation (A+) is a range of alkylated piperidinium cations. We use this new family to explore how A+ cations determine their mechanical response by measuring the bulk modulus (B) – using high-pressure powder X-ray diffraction. Within the series, we find a positive correlation between the network distortions of the pseudocubic [Ni(C2N3)3]− network and B. Furthermore, we show that we can tune framework distortions, and therefore B, by synthesising A-site solid solutions. The applied methodology is a blueprint for linking framework distortions and mechanical properties in network materials and guides us toward principles for designing macroscopic properties via systematic compositional changes in molecular perovskites

Nitrogen vacancy defects in single-particle nanodiamonds sense paramagnetic transition metal spin noise from nanoparticles on a transmission electron microscopy grid

Spin-active nanomaterials play a vital role in current and upcoming quantum technologies, such as spintronics, data storage and computing. To advance the design and application of these materials, methods to link size, shape, structure, and chemical composition with functional magnetic properties at the nanoscale level are needed. In this work, we combine the power of two local probes, namely, Nitrogen Vacancy (NV) spin-active defects in diamond and an electron beam, within experimental platforms used in electron microscopy. Negatively charged NVs within fluorescent nanodiamond (FND) particles are used to sense the local paramagnetic environment of Rb0.5Co1.3[Fe(CN)6]·3.7H2O nanoparticles (NPs), a Prussian blue analogue (PBA), as a function of FND-PBA distance (order of 10 nm) and local PBA concentration. We demonstrate perturbation of NV spins by proximal electron spins of transition metals within NPs, as detected by changes in the photoluminescence (PL) of NVs. Workflows are reported and demonstrated that employ a Transmission Electron Microscope (TEM) finder grid to spatially correlate functional and structural features of the same unique NP studied using NV sensing, based on a combination of Optically Detected Magnetic Resonance (ODMR) and Magnetic Modulation (MM) of NV PL, within TEM imaging modalities. Significantly, spin–spin dipole interactions were detected between NVs in a single FND and paramagnetic metal centre spin fluctuations in NPs through a carbon film barrier of 13 nm thickness, evidenced by TEM tilt series imaging and Electron Energy-Loss Spectroscopy (EELS), opening new avenues to sense magnetic materials encapsulated in or between thin-layered nanostructures. The measurement strategies reported herein provide a pathway towards solid-state quantitative NV sensing with atomic-scale theoretical spatial resolution, critical to the development of quantum technologies, such as memory storage and molecular switching nanodevices

Controlling Noncollinear Ferromagnetism in van der Waals Metal–Organic Magnets

Van der Waals (vdW) magnets both allow exploration of fundamental 2D physics and offer a route toward exploiting magnetism in next generation information technology, but vdW magnets with complex, noncollinear spin textures are currently rare. We report here the syntheses, crystal structures, magnetic properties and magnetic ground states of four bulk vdW metal–organic magnets (MOMs): FeCl2(pym), FeCl2(btd), NiCl2(pym), and NiCl2(btd), pym = pyrimidine and btd = 2,1,3-benzothiadiazole. Using a combination of neutron diffraction and bulk magnetometry we show that these materials are noncollinear magnets. Although only NiCl2(btd) has a ferromagnetic ground state, we demonstrate that low-field hysteretic metamagnetic transitions produce states with net magnetization in zero-field and high coercivities for FeCl2(pym) and NiCl2(pym). By combining our bulk magnetic data with diffuse scattering analysis and broken-symmetry density-functional calculations, we probe the magnetic superexchange interactions, which when combined with symmetry analysis allow us to suggest design principles for future noncollinear vdW MOMs. These materials, if delaminated, would prove an interesting new family of 2D magnets

Bi2Se3 interlayer treatments affecting the Y3Fe5O12 (YIG) platinum spin Seebeck effect

In this work, we present a method to enhance the longitudinal spin Seebeck effect at platinum/yttrium iron garnet (Pt/YIG) interfaces. The introduction of a partial interlayer of bismuth selenide (Bi2Se3, 2.5% surface coverage) interfaces significantly increases (by ∼380%–690%) the spin Seebeck coefficient over equivalent Pt/YIG control devices. Optimal devices are prepared by transferring Bi2Se3 nanoribbons, prepared under anaerobic conditions, onto the YIG (111) chips followed by rapid over-coating with Pt. The deposited Pt/Bi2Se3 nanoribbon/YIG assembly is characterized by scanning electron microscope. The expected elemental compositions of Bi2Se3 and YIG are confirmed by energy dispersive x-ray analysis. A spin Seebeck coefficient of 0.34–0.62 μV/K for Pt/Bi2Se3/YIG is attained for our devices, compared to just 0.09 μV/K for Pt/YIG controls at a 12 K thermal gradient and a magnetic field swept from −50 to +50 mT. Superconducting quantum interference device magnetometer studies indicate that the magnetic moment of Pt/Bi2Se3/YIG treated chips is increased by ∼4% vs control Pt/YIG chips (i.e., a significant increase vs the ±0.06% chip mass reproducibility). Increased surface magnetization is also detected in magnetic force microscope studies of Pt/Bi2Se3/YIG, suggesting that the enhancement of spin injection is associated with the presence of Bi2Se3 nanoribbons

Spin-state dependent pressure responsiveness of Fe(<scp>ii</scp>)-based triazolate metal–organic frameworks

Fe(II)-containing Metal–Organic Frameworks (MOFs) that exhibit temperature-induced spin-crossover (SCO) are candidate materials in the field of sensing, barocalorics, and data storage. Their responsiveness towards pressure is therefore of practical importance and is related to their longevity and processibility. The impact of Fe(II) spin-state on the pressure responsiveness of MOFs is yet unexplored. Here we report the synthesis of two new Fe(II)-based MOFs, i.e. Fe(cta)2 ((cta)− = 1,4,5,6-tetrahydrocyclopenta[d][1,2,3]triazolate) and Fe(mta)2 ((mta)− = methyl[1,2,3]triazolate), which are both in high-spin at room temperature. Together with the isostructural MOF Fe(ta)2 ((ta)− = [1,2,3]triazolate), which is in its low-spin state at room temperature, we apply these as model systems to show how spin-state controls their mechanical properties. As a proxy, we use their bulk modulus, which was obtained via high-pressure powder X-ray diffraction experiments. We find that an interplay of spin-state, steric effects, void fraction, and absence of available distortion modes dictates their pressure-induced structural distortions. Our results show for the first time the role of spin-state on the pressure-induced structural deformations in MOFs and bring us a step closer to estimating the effect of pressure as a stimulus on MOFs a priori

Sensing the Spin State of Room-Temperature Switchable Cyanometallate Frameworks with Nitrogen-Vacancy Centers in Nanodiamonds

Room-temperature magnetically switchable materials play a vital role in current and upcoming quantum technologies, such as spintronics, molecular switches, and data storage devices. The increasing miniaturization of device architectures produces a need to develop analytical tools capable of precisely probing spin information at the single-particle level. In this work, we demonstrate a methodology using negatively charged nitrogen vacancies (NV–) in fluorescent nanodiamond (FND) particles to probe the magnetic switching of a spin crossover (SCO) metal–organic framework (MOF), [Fe(1,6-naphthyridine)2(Ag(CN)2)2] material (1), and a single-molecule photomagnet [X(18-crown-6)(H2O)3]Fe(CN)6·2H2O, where X = Eu and Dy (materials 2a and 2b, respectively), in response to heat, light, and electron beam exposure. We employ correlative light–electron microscopy using transmission electron microscopy (TEM) finder grids to accurately image and sense spin–spin interacting particles down to the single-particle level. We used surface-sensitive optically detected magnetic resonance (ODMR) and magnetic modulation (MM) of FND photoluminescence (PL) to sense spins to a distance of ca. 10–30 nm. We show that ODMR and MM sensing was not sensitive to the temperature-induced SCO of FeII in 1 as formation of paramagnetic FeIII through surface oxidation (detected by X-ray photoelectron spectroscopy) on heating obscured the signal of bulk SCO switching. We found that proximal FNDs could effectively sense the chemical transformations induced by the 200 keV electron beam in 1, namely, AgI → Ag0 and FeII → FeIII. However, transformations induced by the electron beam are irreversible as they substantially disrupt the structure of MOF particles. Finally, we demonstrate NV– sensing of reversible photomagnetic switching, FeIII + (18-crown-6) ⇆ FeII + (18-crown-6)+ •, triggered in 2a and 2b by 405 nm light. The photoredox process of 2a and 2b proved to be the best candidate for room-temperature single-particle magnetic switching utilizing FNDs as a sensor, which could have applications into next-generation quantum technologies

Tuning magnetic properties in layered metal-organic frameworks

This thesis explores the role of chemistry and structure in the magnetic properties of metal-organic framework materials. New magnetic materials with enhanced properties are promising for next-generation information technology with higher speeds, energy efficiency and data density. Realising these new technologies calls for programmable materials where the magnetic properties can be tuned by controlling the chemical composition and crystal structure. Metal-organic magnets (MOMs), assembled from metal nodes connected by organic molecular linkers into extended networks, offer these benefits. The modularity of their construction allows for the tuning of magnetic properties by substituting the nodes and linkers while retaining network topology. The chemical and structural complexity of these materials makes predicting their magnetic properties challenging. Developing this understanding requires deeper exploration of the influence chemistry and structure have on the magnetism in new MOMs. In this thesis I discuss six chemically distinct MOMs with closely related chemical connectivity and crystal structures, MCl2L with M = Cr2+, Fe2+ and Ni2+ ; L = pym (pyrimidine), btd (2,1,3–benzothiadiazole). In these compounds the M2+ are coordinated equatorially by four Cl-ligands and axially by two N atoms from the L ligands to form MCl4N2 octahedra. These octahedra form MCl2 chains along the crystallographic a direction, which are connected into layers by L in the b direction. These layers stack in the crystallographic c direction through van der Waals (vdW) interactions. I determined their structures through single crystal X-ray diffraction, powder X-ray diffraction and powder nuclear neutron diffraction. To characterise their bulk magnetic properties I carried out magnetisation measurements under variable temperature and field conditions. All of these compounds were found to magnetically order at low temperature. With the crystal structures and magnetic ordering temperatures established, I was able to determine their magnetic ground states through powder magnetic neutron diffraction. Finally, I was able to determine the magnetic interaction energies of the Cr analogues by characterising their excitations with inelastic neutron scattering. I show that substituting the metal centre for one with a different spin state and/or d-electron configuration will drastically alter the magnetic properties. Substituting the ligand can further tune magnetic properties by modulating the energy of magnetic interactions and the crystal structural geometry. In Chapter 3 I report the synthesis, crystal structure, bulk magnetisation behaviour and magnetic ground state of the Fe and Ni analogues. I show that ligand substitution in these compounds can be used to control spincanting, which can introduce or tune metamagnetism and weak ferromagnetism. This effect is caused by controlling tilt angle between the MCl4N2 octahedra and demonstrates how the structure of the organic ligands can be leveraged to tune for properties. In Chapter 4 I report the synthesis, crystal structure, bulk magnetisation, magnetic ground state and magnetic excitations of CrCl2(pym). I show that the pym ligand carries a weak ferromagnetic superexchange interaction, effectively magnetically separating the CrCl2 into quasi-one-dimensional spin chains and uncovering a potential chemical design route for the S = 2 Haldane phase. In Chapter 5 I report the synthesis, crystal structure, bulk magnetisation behaviour, magnetic ground state and magnetic excitations of CrCl2(btd). In contrast to pym, the btd ligand carries a strong antiferromagnetic superexchange, effectively magnetically connecting the CrCl2 chains, raising the magnetically dimensionality to quasi-two-dimensional. I show that in the Cr compounds the ligand can be substituted to modulate the exchange interactions

Tuning magnetic properties in layered metal-organic frameworks

This thesis explores the role of chemistry and structure in the magnetic properties of metal-organic framework materials. New magnetic materials with enhanced properties are promising for next-generation information technology with higher speeds, energy efficiency and data density. Realising these new technologies calls for programmable materials where the magnetic properties can be tuned by controlling the chemical composition and crystal structure. Metal-organic magnets (MOMs), assembled from metal nodes connected by organic molecular linkers into extended networks, offer these benefits. The modularity of their construction allows for the tuning of magnetic properties by substituting the nodes and linkers while retaining network topology. The chemical and structural complexity of these materials makes predicting their magnetic properties challenging. Developing this understanding requires deeper exploration of the influence chemistry and structure have on the magnetism in new MOMs. In this thesis I discuss six chemically distinct MOMs with closely related chemical connectivity and crystal structures, MCl2L with M = Cr2+, Fe2+ and Ni2+ ; L = pym (pyrimidine), btd (2,1,3–benzothiadiazole). In these compounds the M2+ are coordinated equatorially by four Cl-ligands and axially by two N atoms from the L ligands to form MCl4N2 octahedra. These octahedra form MCl2 chains along the crystallographic a direction, which are connected into layers by L in the b direction. These layers stack in the crystallographic c direction through van der Waals (vdW) interactions. I determined their structures through single crystal X-ray diffraction, powder X-ray diffraction and powder nuclear neutron diffraction. To characterise their bulk magnetic properties I carried out magnetisation measurements under variable temperature and field conditions. All of these compounds were found to magnetically order at low temperature. With the crystal structures and magnetic ordering temperatures established, I was able to determine their magnetic ground states through powder magnetic neutron diffraction. Finally, I was able to determine the magnetic interaction energies of the Cr analogues by characterising their excitations with inelastic neutron scattering. I show that substituting the metal centre for one with a different spin state and/or d-electron configuration will drastically alter the magnetic properties. Substituting the ligand can further tune magnetic properties by modulating the energy of magnetic interactions and the crystal structural geometry. In Chapter 3 I report the synthesis, crystal structure, bulk magnetisation behaviour and magnetic ground state of the Fe and Ni analogues. I show that ligand substitution in these compounds can be used to control spincanting, which can introduce or tune metamagnetism and weak ferromagnetism. This effect is caused by controlling tilt angle between the MCl4N2 octahedra and demonstrates how the structure of the organic ligands can be leveraged to tune for properties. In Chapter 4 I report the synthesis, crystal structure, bulk magnetisation, magnetic ground state and magnetic excitations of CrCl2(pym). I show that the pym ligand carries a weak ferromagnetic superexchange interaction, effectively magnetically separating the CrCl2 into quasi-one-dimensional spin chains and uncovering a potential chemical design route for the S = 2 Haldane phase. In Chapter 5 I report the synthesis, crystal structure, bulk magnetisation behaviour, magnetic ground state and magnetic excitations of CrCl2(btd). In contrast to pym, the btd ligand carries a strong antiferromagnetic superexchange, effectively magnetically connecting the CrCl2 chains, raising the magnetically dimensionality to quasi-two-dimensional. I show that in the Cr compounds the ligand can be substituted to modulate the exchange interactions

Low dimensional metal-organic magnets as a route towards the S=2 Haldane phase

Metal-organic magnets (MOMs), modular magnetic materials where metal atoms are connected by organic linkers, are promising candidates for next-generation quantum technologies. MOMs readily form low-dimensional structures, and so are ideal systems to realise physical examples of key quantum models, including the Haldane phase, where a topological excitation gap occurs in integer-spin antiferromagnetic (AFM) chains. Thus far the Haldane phase has only been identified for S = 1, with S ≥ 2 still unrealised because the larger spin imposes more stringent requirements on the magnetic interactions. Here, we report the structure and magnetic properties of CrCl2(pym) (pym=pyrimidine), a new quasi-1D S = 2 AFM MOM. We show, using X-ray and neutron diffraction, bulk property measurements, density-functional theory calculations and inelastic neutron spectroscopy (INS) that CrCl2(pym) consists of AFM CrCl2 spin chains (J1 = −1.13(4) meV) which are weakly ferromagnetically coupled through bridging pym (J2 = 0.10(2) meV), with easy-axis anisotropy (D = −0.11(1) meV). We find that although small compared to J1, these additional interactions are sufficient to pre- vent observation of the Haldane phase in this material. Nevertheless, the proximity to the Haldane phase together with the modularity of MOMs suggests that layered Cr(II) MOMs are a promising family to search for the elusive S = 2 Haldane phase