In endohedral fullerenes otherwise unstable atomic clusters can be stabilized. Here we investigate the electronic and magnetic properties of the dimetallic endohedral fullerene CeTi@C80, a molecular system encapsulating nominally trivalent cations of Ce and Ti within a C80 carbon cage. Using low temperature magnetometry and temperature-dependent X-ray Absorption Spectroscopy (XAS) at the Ce M45-edge, we explore the magnetism and the orientation of the Ce-Ti endohedral unit. The magnetization measurements indicate "tender" single-molecule magnetism with small hysteresis and a 3.3 mu B magnetic moment. The measured XA spectra of drop-cast molecules can be simulated with a ligand field of a didipole that consists of a C-Ti and an opposite Ce-C dipole. They confirm trivalent Ce with a Jz=5/2 ground state and random distribution of the Ce-Ti axes. Temperature-dependent XAS below room temperature shows minimal spectral change, indicating a thermally robust ground-state. These findings establish CeTi@C80 as a stable electric and magnetic didipole single molecule magnet.

The formation of plutonium (III–VI) complexes in nitric and hydrochloric acid solutions was simulated using DMol 3 and Relativistic Discrete-Variational (RDV) methods. Both explicit and explicit-plus-implicit approaches for the modeling of solution boundary conditions were used. For the explicit modeling of molecular environment of plutonium ions we used 22 and 32 water molecules and the counter ions NO 3 − or Cl − randomly distributed around actinide atom. For the additional implicit modeling of solvent environment, COSMO potential (Conductor-like Screening Model) for water ( ε = 78.54) was used. The original method for the calculation of interaction energies between selected parts of the large multi-atomic systems provides the quantitative comparison of the stability of plutonium complexes with various compositions and the estimation of the roles of NO 3 − , Cl − and water molecules of the nearest and next-nearest solution layers. We obtained that the average interaction energies between Pu Z+ ion and each nearest H 2 O molecule were slightly dependent on the size and composition of optimized plutonium complex.

X-ray absorption spectroscopy (XAS) has the highest sensitivity for chemical element detection on surfaces. With this approach, small amounts of lanthanide-containing endofullerene molecules (Ho3N@C80) have been measured by total electron yield at a low flux bending magnet beamline. The monolayer coverage is calibrated by extrapolating the signals of constant doses (3×1014 cm−2) of Ho ions implanted into SiO2 with energies between 2 and 115 keV. At room temperature, the Ho XAS spectra of the molecules and implanted ions indicate trivalent but not identical Ho ground states. Still, this approach demonstrates a way for calibration of small coverages of molecules containing open core-shell elements.

Using ab initio calculations we investigate the adsorption of Co atoms, dimers and small cobalt clusters of 5 and 13 atoms on pristine graphene and graphene with a double vacancy. We report the atomic, electronic, magnetic and energetic properties of these systems. Stable adsorption configurations tend to maximise the number of cobalt-carbon bonds. On graphene, the adsorption energy of the clusters is only about 0.4 to 1 eV, and the clusters are relatively mobile on graphene. Interestingly, for different adsorbed Co13 isomers on graphene it is found that they converge to the same atomic structure. On graphene with a divacancy, the Co clusters bind in the divacancy site and isomerisation also occurs for the Co5 cluster system as well as for Co13. Co atoms and clusters can be effectively immobilised on the divacancy with corresponding adsorption energy being significantly enhanced by about 5 to 7 eV. All clusters act as electron donors in the interaction with the graphene/divacancy systems, and the amount of electron charge transfer increases with cluster size. Finite magnetic moments occur for all systems, where upon adsorption, the magnetic moment of the isolated Co atom (31iB) is significantly reduced due to electron transfer and bonding, resulting in values varying from approximate to 0.9-2.2 1iB per Co atom. For the pristine graphene substrate, the total induced magnetic moments on the carbon atoms are negligible, while on the divacancy system, they are of the order of 0.1-0.3 1iB. The attractive physical properties of these hybrid systems could find applications in catalysis and materials science.

The molecular self‐assembly and the magnetic properties of two cyclooctatetraenide (COT)‐based single‐ion magnets (SIM) adsorbed on Ag(100) in the sub‐monolayer (ML) range are reported. Our study combines scanning‐tunneling microscopy, X‐ray photoemission spectroscopy and polarized X‐ray absorption spectroscopy to show that Cp*ErCOT (Cp* = 1,2,3,4,5‐pentamethylcyclopentadienide anion) SIMs self‐assemble as alternating compact parallel rows including standing‐up and lying‐down conformations, following the main crystallographic directions of the substrate. Conversely, K[Er(COT)2], obtained from subliming the [K(18‐c‐6)][Er(COT)2]·2THF salt, forms uniaxially ordered domains with the (COT)2− rings perpendicular to the substrate plane. The polarization‐dependent X‐ray absorption spectra reproduced by the multiX simulations suggest that the strong in‐plane magnetic anisotropy of K[Er(COT)2]/Ag(100) and the weak out‐of‐plane anisotropy of Cp*ErCOT/Ag(100) can be attributed to the strikingly different surface ordering of these two complexes. Compared to the bulk phase, surface‐supported K[Er(COT)2] exhibits a similarly large hysteresis opening, while the Cp*ErCOT shows a rather small opening. This result reveals that despite structural similarities, the two organometallic SMMs have strongly different magnetic properties when adsorbed on the metal substrate, attributed to the different orientations and the resulting interactions of the ligand rings with the surface.

Endofullerene DySc2N@C80 is a single-molecule magnet with a large magnetic anisotropy and high blocking temperature, which is promising for nanomagnetic applications. As the easy axis of magnetization coincides with the Dy-N bond direction, it is important to understand the structure of the DySc2N unit in the fullerene cage and to control the orientation of the molecules. Here we report on the experimental determination of Dy-N axis by x-ray absorption spectroscopy (XAS) with linear polarized light at the Dy-M-4,M-5 white lines. DySc2N@C80 molecules were adsorbed on a Pt(111) surface and XAS was performed as a function of temperature in the range between 35 and 300 K. The M-5/M-4 branching ratio shows a clear and reversible variation with temperature which can be explained, on the basis of a thermodynamic model, by a change of average orientation of the molecules with temperature. The XAS spectra are well reproduced by ligand field multiplet calculations. It is shown that the angle between the magnetization (Dy-N) axis and the surface plane can be directly inferred from the XAS spectra with in-plane polarization by comparison with calculated spectra. It is found that the endohedral unit is randomly oriented at room temperature but tends towards orientation parallel to the surface at low temperature, indicating a weak but non-negligible interaction between the endohedral units and the metal surface.

Van der Waals (vdW) magnets are an ideal platform for tailoring 2D magnetism with immense potential for spintronics applications and are intensively investigated. However, little is known about the microscopic origin of magnetic order in these antiferromagnetic systems. X‐ray photoemission electron microscopy is used to address the electronic and magnetic properties of the vdW antiferromagnet FePS 3 down to the monolayer. The experiments reveal a giant out‐of‐plane magnetic anisotropy of 22 meV per Fe ion, accompanied by unquenched magnetic orbital moments. Moreover, the calculations suggest that the Ising magnetism in FePS 3 is a visible manifestation of spin–orbit entanglement of the Fe 3 d electron system.

One-dimensional metal-organic chains often possess a complex magnetic structure susceptible to modification by alteration of their chemical composition. The possibility to tune their magnetic properties provides an interesting playground to explore quasi-particle interactions in low-dimensional systems. Despite the great effort invested so far, a detailed understanding of the interactions governing the electronic and magnetic properties of the low-dimensional systems is still incomplete. One of the reasons is the limited ability to characterize their magnetic properties at the atomic scale. Here, we provide a comprehensive study of the magnetic properties of metal-organic one-dimensional (1D) coordination polymers consisting of 2,5-diamino-1,4-benzoquinonediimine ligands coordinated with Co or Cr atoms synthesized under ultrahigh-vacuum conditions on a Au(111) surface. A combination of integral X-ray spectroscopy with local-probe inelastic electron tunneling spectroscopy corroborated by multiplet analysis, density functional theory, and inelastic electron tunneling simulations enables us to obtain essential information about their magnetic structures, including the spin magnitude and orientation at the magnetic atoms, as well as the magnetic anisotropy.

From macroscopic heavy-duty permanent magnets to nanodevices, the precise control of the magnetic properties in rare-earth metals is crucial for many applications used in our daily life. Therefore, a detailed understanding and manipulation of the 4f-metals' magnetic properties are key to further boosting the functionalization and efficiency of future applications. We present a proof-of-concept approach consisting of a dysprosium-iridium surface alloy in which graphene adsorption allows us to tailor its magnetic properties. By adsorbing graphene onto a long-range ordered two-dimensional dysprosium-iridium surface alloy, the magnetic 4f-metal atoms are selectively lifted from the surface alloy. This selective skyhook effect introduces a giant magnetic anisotropy in dysprosium atoms as a result of manipulating its geometrical structure within the surface alloy. Introducing and proving this concept by our combined theoretical and experimental approach provides an easy and unambiguous understanding of its underlying mechanism. Our study sets the ground for an alternative path on how to modify the crystal field around 4f-atoms and therefore their magnetic anisotropies.

This review provides an overview of the different methods and computer codes that are used to interpret 2p x-ray absorption spectra of 3d transition metal ions. We first introduce the basic parameters and give an overview of the methods used. We start with the semi-empirical multiplet codes and compare the different codes that are available. A special chapter is devoted to the user friendly interfaces that have been written on the basis of these codes. Next we discuss the first principle codes based on band structure, including a chapter on Density Functional theory based approaches. We also give an overview of the first-principle multiplet codes that start from a cluster calculation and we discuss the wavefunction based methods, including multi-reference methods. We end the review with a discussion of the link between theory and experiment and discuss the open issues in the spectral analysis.

Abstract Geometry optimization and the electronic structure calculations of Pu Z+ complexes (Z = 3–6) in water solution have been performed, within the framework of the DMol3 and Relativistic Discrete-Variational (RDV) methods. For the simulation of Pu Z+ molecular environment in aqueous solution we used 22 and 32 water molecules randomly distributed around cation. To model the effect of bulk solvent environment we used COSMO (Conductor-like Screening Model) potential for water (ε = 78.54). The obtained results showed that this approach allows the modeling of water dissociation and the formation of hydrolysis products. Our previously suggested scheme for the calculation of interaction energies between selected fragments of multi-molecular systems provides the quantitative estimation of the interaction strengths between plutonium in various oxidation states and each ligand in the first and second coordination shells in water solution.

Theoretical study of the formation of PuZ+ (Z = 3, 4, 5, 6) complexes with the two types of diamide molecules C8N2H10O2(CH3)(2) (dimethylbicyclicdiamide-DMDA) with bicyclic structure and C3N2H2O2(CH3)(4) (tetramethylmalonamide-TMMA) with acyclic structure was carried out using ab initio DFT based DMol(3) and relativistic discrete variational methods. The results of gas phase modeling showed that the appropriate positions of the oxygen atoms in DMDA in comparison with TMMA cannot explain the considerable difference in sorption affinity of these molecules. For the modeling of solutions, the implicit (COSMO) and explicit methods were used. In the latter approach, we included 40 water molecules, 16NO(3)(-) or 16Cl(-) and 16H(+) ions into the systems under investigation. The obtained results showed the principle role of solution in the weakening of actinide bonding with TMMA and DMDA molecules due to interaction with H2O and NO3- or Cl- ions.

Quantum spin liquids are materials that feature quantum entangled spin correlations and avoid magnetic long-range order at T = 0 K. Particularly interesting are two-dimensional honeycomb spin lattices where a plethora of exotic quantum spin liquids have been predicted. Here, we experimentally study an effective S = 1/2 Heisenberg honeycomb lattice with competing nearest and next-nearest-neighbour interactions. We demonstrate that YbBr3 avoids order down to at least T = 100 mK and features a dynamic spin–spin correlation function with broad continuum scattering typical of quantum spin liquids near a quantum critical point. The continuum in the spin spectrum is consistent with plaquette type fluctuations predicted by theory. Our study is the experimental demonstration that strong quantum fluctuations can exist on the honeycomb lattice even in the absence of Kitaev-type interactions, and opens a new perspective on quantum spin liquids.

Marco Fronzi, ∗ Simone Piccinin, Bernard Delley, Enrico Traversa, and Catherine Stampfl Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University, Japan CNR-IOM Democritos, c/o SISSA, via Bonomea 265, I-34136 Trieste, Italy Paul-Scherrer-Institut, CH-5232 Villigen PSI, Switzerland Physical Sciences and Engineering Division, King Abdullah University of Science and Technology , Kingdom of Saudi Arabia School of Physics, The University of Sydney, Sydney, New South Wales 2006, Australia (Dated: April 28, 2019)

perform DFT calculations within the COSMO framework to study the adsorption properties of the CaHPOx (x = 1,...,4) phosphate family, and of the related HPOx acids, on a paradigmatic anatase (001) surface. Of particular interest is CaHPO4, a precursor of hydroxyapatite, which forms during osseointegration of titanium implants. Our results, obtained within the COSMO framework, are based on total energy calculations, and are interpreted in terms of electronegativity differences between the relevant system components. They show that adsorption of HPOx and in particular of HPO4, is highly enhanced by the presence of Ca adatoms on the TiO2 substrate, a result which is in agreement with our previous work on the wettability of anatase surfaces. We also study the adsorption of CaHPOx molecules on TiO2 surfaces, and show that molecular adsorption is strongly favoured for x = 4, while for x = 1, 2 dissociative adsorption of HPOx molecules and Ca adatoms prevails.

The compounds BaDy2O4 and BaHo2O4 are part of a family of frustrated systems exhibiting interesting properties, including spin-liquid-type ground states, magnetic-field-induced phases, and the coexistence of short-and long-range magnetic order, with dominant one-dimensional correlations, which can be described as Ising J(1)-J(2) zigzag chains along the c axis. We have investigated polycrystalline samples of BaDy2O4 and BaHo2O4 with both neutron diffraction and neutron spectroscopy, coupled to detailed crystalline electric field calculations. The latter points to site-dependent anisotropic magnetism in both materials, which is corroborated by the magnetic structures we determined. The two systems show the coexistence of two different long-range orders: two double Neel up arrow up arrow down arrow down arrow orders in the ab plane with propagation vectors k(1) = (1/2,0,1/2) and k(2) = (1/2,1/2,1/2) for BaDy2O4, and two distinct arrangements of simple Neel up arrow down arrow up arrow down arrow orders along the c axis, both with the propagation vector k(0) = (0, 0, 0) for BaHo2O4. The order for both wave vectors in BaDy2O4 occurs at T-N = 0.48 K, while in BaHo2O4 the first order sets in at T-N similar to 1.3 K and the second one has a lower ordering temperature of 0.84 K. Both compounds show extensive diffuse scattering which we successfully modeled with a one-dimensional axial next-nearest neighbor Ising (ANNNI) model. In both materials, strong diffusive scattering persists to temperatures well below where the magnetic order is fully saturated.

The novel magnetic phase diagram of the $Cs_2CuCl_{4-x}Br_x$ mixed system is established by means of single crystal neutron diffraction in the lowest temperature region and zero magnetic field. Two long-range ordered magnetic phases exist in this mixed system depending on the Cl/Br concentration. In the rich Cl concentration range, the ordered magnetic state occurs below the ordering temperature $T_N = 0.51(1)K$ for $Cs_2CuCl_3Br_1$ and at $Cs_2CuCl_{2.6}Br_{1.4}$ below $T_N = 0.24(2)K$. Magnetic order with a temperature-independent position $(0, 0.573(1), 0)$ below the ordering temperature $T_N = 0.63(1)K$ appears in the rich Br concentration for $Cs_2CuCl_{0.6}Br_{3.4}$. Between the rich Cl and rich Br concentration ranges (two magnetic phases), there is a range of x without magnetic order down to $50mK$. A suggestion about the magnetic exchange paths in the $bc$-layer for different regimes is presented, which can be controlled depending on the preferred Br-occupation in the [CuX4] tetrahedra. The density functional theory (DFT) calculations of the exchange coupling constants J, J^' for some ordered compositions of the mixed system $Cs_2CuCl_{4-x}Br_x$ indicate that these are not frustrated.

We employed x-ray absorption spectroscopy and x-ray magnetic circular dichroism to study the magnetic properties of single rare-earth (RE) atoms (Nd, Tb, Dy, Ho, and Er) adsorbed on the graphene/Ir(111) surface. The interaction of RE atoms with graphene results for Tb in a trivalent state with 4f(n-1) occupancy, and in a divalent state with 4f(n) occupancy for all other studied RE atoms (n corresponds to the 4f occupancy of free atoms). Among the studied RE on graphene/Ir(111), Dy is the only one that shows magnetic hysteresis and remanence at 2.5 K. By comparing measured spectra and magnetization curves with multiplet calculations, we determine the energy diagram of the magnetic states and show for each element the magnetization reversal process that determines the timescale of its magnetic bistability.

Density functional theory (DFT) calculations have been performed on five models of periodic, polysynthetic twin interfaces in the ambient-temperature phase of KLiSO4, which has space group P6(3). The models represent the three merohedric twin laws (m parallel to z, 2 perpendicular to z and (1) over bar) with boundary plane (1 0 (1) over bar 0), also with boundary plane (0 0 0 1) in case of m, and with boundary plane (1 (2) over bar 1 0) in case of (1) over bar. The models satisfy stoichiometry at the boundary plane and maintain the fourfold coordination of the Li and S atoms and the twofold coordination of the oxygen atoms. Relaxed lattice parameters and atomic positions were determined by DFT, using the DMol(3) code with functional PBEsol. The energy difference between polysynthetic twin and single crystal per primitive cell of the twin is 0.0009 eV for m(0 0 0 1), 0.09 eV for (1) over bar (1 0 (1) over bar 0), 0.58 eV for m(1 0 (1) over bar 0) and 0.55 eV for 2(1 0 (1) over bar 0). In KLiSO4 crystals grown from aqueous solutions the first twin was frequently observed, similarly also the second twin in Cr-doped crystals, whereas the third twin appeared only rarely and the fourth was not observed. Not only for KLiSO4 but also for quartz, the energy of twins and the frequency of their occurrence are closely connected for crystals grown from aqueous solutions, whereas for the formation of transformation twins the availability of twin nuclei plays a major role.

We apply X-ray magnetic circular dichroism to study the internal magnetic structure of two very promising star shaped macrocyclic complexes with a CuII3TbIII core. These complexes are rare examples prepared with a macrocyclic ligand that show indications of SMM (Single Molecule Magnet) behavior, and they differ only in ring size: one has a propylene linked macrocycle, [CuII3TbIII(LPr)(NO3)2(MeOH)(H2O)2](NO3)·3H2O (nickname: Cu3Tb(LPr)), and the other has the butylene linked analogue, [CuII3TbIII(LBu)(NO3)2(MeOH)(H2O)](NO3)·3H2O (nickname: Cu3Tb(LBu)). We analyze the orbital and spin contributions to the Cu and Tb ions quantitatively by applying the spin and orbital sum rules concerning the L2 (M4)/L3 (M5) edges. In combination with appropriate ligand field simulations, we demonstrate that the Tb(iii) ions contribute with high orbital magnetic moments to the magnetic anisotropy, whereas the ligand field determines the easy axis of magnetization. Furthermore, we confirm that the Cu(ii) ions in both molecules are in a divalent valence state, the magnetic moments of the three Cu ions appear to be canted due to 3d-3d intramolecular magnetic interactions. For Cu3Tb(LPr), the corresponding element specific magnetization loops reflect that the Cu(ii) contribution to the overall magnetic picture becomes more important as the temperature is lowered. This implies a low value for the 3d-4f coupling.