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{\bf Hypothesis:} Introducing charged terminal groups to polymers that graft nanoparticles enables Coulombic control over their assembly by tuning pH and salinity of aqueous suspensions. {\bf Experiments:} Gold nanoparticles (AuNPs) are grafted with poly(ethylene glycol) (PEG) terminated with CH3 (charge neutral), COOH (negatively charged), or NH2 (positively charged) groups. The nanoparticles are characterized using dynamic light scattering, {\zeta}-potential, and thermal gravimetric analysis. Liquid surface X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS) techniques are employed to determine the density profile and in-plane structure of the AuNP assembly across and on the aqueous surface. {\bf Findings:} The assembly of PEG-AuNPs at the liquid/vapor interface can be tuned by adjusting pH or salinity, particularly for COOH terminals. However, the effect is less pronounced for NH2 terminals. These distinct assembly behaviors are attributed to the overall charge of PEG-AuNPs and the conformation of PEG. The COOH-PEG corona is the most compact, resulting in smaller superlattice constants. The net charge per particle depends not only on the PEG terminal groups but also on the cation sequestration of PEG and the intrinsic negative charge of the AuNP surface. NH2-PEG, due to its closeness to overall charge neutrality and the presence of hydrogen bonding, enables the assembly of NH2-PEG-AuNPs more readily., Comment: Submitted to the Journal of Colloid and Interface Science, and it's under review currently

The magnetic and electronic properties of the layered Zintl-phase compound EuAl$_2$Ge$_2$ crystallizing in the trigonal CaAl$_2$Si$_2$-type structure are reported. Our neutron-diffraction measurements show that EuAl$_2$Ge$_2$ undergoes A-type antiferromagnetic (AFM) ordering below $T_{\rm N} = 27.5(5)$~K, with the Eu moments (Eu$^{2+}$, $S = 7/2$) aligned ferromagnetically in the $ab$ plane. The $H = 0$ magnetic structure consists of trigonal AFM domains associated with $ab$-plane magnetic anisotropy and a field-induced reorientation of the Eu spins in the domains is evident at $T = 2$~K below the critical field $H_{c1} = 2.5(1)$ kOe. Electrical resistivity and ARPES measurements show that EuAl$_2$Ge$_2$ is metallic both above and below $T_{\rm N}$. In the AFM phase, we directly observe folded bands in ARPES due to the doubling of the magnetic unit cell along the $c$ axis with an enhancement of quasiparticle weight due to the complex change in the coupling between the magnetic moments and itinerant electrons on cooling below $T_{\rm N}$. The observed electronic structure is well reproduced by first-principle calculations, which also predict the presence of nontrivial electronic states near the Fermi level in the AFM phase with $Z_2$ topological numbers 1;(000)., 16 pages, 13 captioned figures, 53 references Updated several affiliations

Control of magnetization and electric polarization is attractive in relation to tailoring materials for data storage and devices such as sensors or antennae. In magnetoelectric materials, these degrees of freedom are closely coupled, allowing polarization to be controlled by a magnetic field, and magnetization by an electric field, but the magnitude of the effect remains a challenge in the case of single-phase magnetoelectrics for application. We demonstrate that the magnetoelectric properties of the mixed-anisotropy antiferromagnet LiNi$_{1-x}$Fe$_x$PO$_4$ are profoundly affected by replacing a fraction of the Ni$^{2+}$ ions with Fe$^{2+}$ on the transition metal site. This introduces random site-dependent single-ion anisotropy energies and causes a lowering of the magnetic symmetry of the system. In turn, magnetoelectric couplings that are symmetry-forbidden in the parent compounds, LiNiPO$_4$ and LiFePO$_4$, are unlocked and the dominant coupling is enhanced by two orders of magnitude. Our results demonstrate the potential of mixed-anisotropy magnets for tuning magnetoelectric properties.

We report neutron-diffraction results on single-crystal CaMn$_2$P$_2$ containing corrugated Mn honeycomb layers and determine its ground-state magnetic structure. The diffraction patterns consist of prominent (1/6, 1/6, $L$) reciprocal lattice unit (r.l.u.; $L$ = integer) magnetic Bragg reflections, whose temperature-dependent intensities are consistent with a first-order antiferromagnetic phase transition at the N\'eel temperature $T_{\rm N} = 70(1)$ K. Our analysis of the diffraction patterns reveals an in-plane $6\times6$ magnetic unit cell with ordered spins that in the principal-axis directions rotate by 60-degree steps between nearest neighbors on each sublattice that forms the honeycomb structure, consistent with the $P_Ac$ magnetic space group. We find that a few other magnetic subgroup symmetries ($P_A2/c$, $P_C2/m$, $P_S\bar{1}, P_C2, P_Cm, P_S1$) of the paramagnetic $P\bar{3}m11^\prime$ crystal symmetry are consistent with the observed diffraction pattern. We relate our findings to frustrated $J_1$-$J_2$-$J_3$ Heisenberg honeycomb antiferromagnets with single-ion anisotropy and the emergence of Potts nematicity

Eu-based Zintl-phase materials EuA$_2$Pn$_2$ (A = Mg, In, Cd, Zn; Pn = Bi, Sb, As, P) have generated significant recent interest owing to the complex interplay of magnetism and band topology. Here, we investigated the electronic, magnetic, and electronic properties of the layered Zintl-phase single crystals of EuMg$_2$Sb$_2$ with the trigonal CaAl$_2$Si$_2$ crystal structure (space group $P\bar{3}m1$). Electrical resistivity measurements complemented with angle-resolved photoemission spectroscopy (ARPES) studies find an activated behavior with the intrinsic conductivity at high temperatures indicating a semiconducting electronic ground state with a narrow energy gap of 370 meV. Magnetic susceptibility and zero-field heat-capacity measurements indicate that the compound undergoes antiferromagnetic (AFM) ordering at the Neel temperature $T_{\rm N}$ = 8.0(2) K. Zero-field neutron-diffraction measurements reveal that the AFM ordering is A-type where the Eu ordered moments (Eu$^{2+}$, S= 7/2) arranged in ab-plane layers are aligned ferromagnetically in the ab plane with the Eu moments in adjacent layers aligned antiferromagnetically. We also find that Eu-moment reorientation in the trigonal AFM domains within the ab planes occurs below $T_{\rm N}$ at low fields < 0.05 T due to very small in-plane anisotropy. Although isostructural semimetallic EuMg$_2$Bi$_2$ is reported to host Dirac surface states, the observation of narrow-gap semiconducting behavior in EuMg$_2$Sb$_2$ implies a strong role of spin-orbit coupling in tuning the electronic states of these materials., 15 pages, 15 captioned figures, 33 references

CaCo2As2 is a unique itinerant system having strong magnetic frustration. Here we report the effect of electron doping on the physical properties resulting from Ni substitutions for Co. The A-type antiferromagnetic transition temperature TN = 52 K for x = 0 decreases to 22 K with only 3 percent Ni substitution and is completely suppressed for x > 0.11. For 0.11 < x < 0.52 strong ferromagnetic (FM) fluctuations develop as revealed by magnetic susceptibility chi(T) measurements. Heat-capacity Cp(T) measurements reveal the presence of FM quantum spin fluctuations for 0.11 < x < 0.52. Our density-functional theory (DFT) calculations confirm that FM fluctuations are enhanced by Ni substitutions for Co. The Sommerfeld electronic heat-capacity coefficient is enhanced for x = 0, 0.21, and 0.42 by about a factor of two compared to DFT calculations of the bare density of states at the Fermi energy. The crystals with x > 0.52 do not exhibit FM spin fluctuations or magnetic order, which was found from the DFT calculations to arise from a Stoner transition. Neutron-diffraction studies of crystals with x = 0.11 and 0.16 found no evidence of A-type ordering as observed in CaCo2As2 or of other common magnetic structures., 19 pages, 18 captioned figures, 7 tables, 79 references

The trigonal compound EuSn2As2 was recently discovered to host Dirac surface states within the bulk band gap and orders antiferromagnetically below the Neel temperature TN = 24 K. Here the magnetic ground state of single-crystal EuSn2As2 and the evolution of its properties versus temperature T and applied magnetic field H are reported. Included are zero-field single-crystal neutron-diffraction measurements versus T, magnetization M(H,T), magnetic susceptibility chi(H,T) = M(T)/H, heat capacity Cp(H,T), and electrical resistivity rho(H,T) measurements. The neutron-diffraction and chi(T) measurements both indicate a collinear A-type antiferromagnetic (AFM) structure below TN =23.5(2) K, where the Eu{2+} spins S = 7/2 in a triangular ab-plane layer (hexagonal unit cell) are aligned ferromagnetically in the ab plane whereas the spins in adjacent Eu planes along the c axis are aligned antiferromagnetically. The chi(H{ab},T) and chi(H{c},T) data together indicate a smooth crossover between the collinear AFM alignment and an unknown magnetic structure at H ~ 0.15 T. Dynamic spin fluctuations up to 60 K are evident in the chi(T), Cp(T) and rho(H,T) measurements, a temperature that is more than twice TN. The rho(H,T) of the compound does not reflect a contribution of the topological state, but rather is consistent with a low-carrier-density metal with strong magnetic scattering. The magnetic phase diagrams for both H||c and H||ab in the H-T plane are constructed from the TN(H), chi(H,T), Cp(H,T), and rho(H,T) data., 15 pages, 19 captioned figures, 2 tables, 66 references

Neutron diffraction, magnetization and muon spin relaxation measurements, supplemented by density functional theory (DFT) calculations are employed to unravel temperature-driven magnetization reversal (MR) in inverse spinel Co$_2$VO$_4$. All measurements show a second-order magnetic phase transition at $T_{\rm C} = 168$\,K to a collinear ferrimagnetic phase. The DFT results suggest the moments in the ferrimagnetic phase are delocalized and undergo gradual localization as the temperature is lowered below $T_{\rm C}$. The delocalized-localized crossover gives rise to a maximum magnetization at $T_{\rm NC} = 138$\,K and the continuous decrease in magnetization produces sign-change at $T_{\rm MR} \sim 65$\,K. Muon spectroscopy results support the DFT, as a strong $T_1$-relaxation is observed around $T_{\rm NC}$, indicating highly delocalized spin-carriers gradually tend to localization upon cooling. The magnetization reversal determined at zero field is found to be highly sensitive to the applied magnetic field, such that above $B\sim 0.25$\,T instead of a reversal a broad minimum in the magnetization is apparent at $T_{\rm MR}$. Analysis of the neutron diffraction measurements shows two antiparallel magnetic sub-lattice-structure, each belonging to magnetic ions on two distinct crystal lattice sites. The relative balance of these two structural components in essence determines the magnetization. Indeed, the order parameter of the magnetic phase on one site develops moderately more than that on the other site. Unusual tipping of the magnetic balance, caused by such site-specific magnetic fluctuation, gives rise to a spontaneous flipping of the magnetization as the temperature is lowered., Comment: 10 pages and 8 figures

Muon spin spectroscopy ($\mu^+$SR) is now an established method to measure atomic scale diffusion coefficients of ions in oxides. This is achieved via the ion hopping rate, which causes periodic change in the local magnetic field at the muon site(s). We present here the first systematic study on a single crystalline sample. The highly anisotropic diffusion of Li-ions in the battery cathode material LiFePO$_4$, combined with the extensive investigation of this material with $\mu^+$SR and other techniques make it a perfect model compound for this study. With this experiment we can confirm that Li diffusion in the bulk LiFePO$_4$ is measurable with $\mu^+$SR. Hence, surface/interface effects, which might play a crucial role in case of powders/nano crystals, are less significant for macroscopic single crystals where bulk diffusion is in fact present. We observe that the internal magnetic field fluctuations caused by the diffusing Li-ions are different depending on the crystal orientation. This is not obviously expected based on theoretical considerations. Such fluctuation rates were used to estimate the diffusion coefficient, which agrees well with values estimated by first principle calculations considering anisotropic diffusion., Comment: 7 pages, 4 figures

Neutron diffraction and magnetic susceptibility studies show that orthorhombic single-crystals of topological semimetals ${\rm Sr(Mn_{0.9}Cu_{0.1})Sb_2}$ and ${\rm Sr(Mn_{0.9}Zn_{0.1})Sb_2}$ undergo three dimensional C-type antiferromagnetic (AFM) ordering of the Mn$^{2+}$ moments at $T_N = 200\pm10$ and $210\pm12$ K, respectively, significantly lower than that of the parent SrMnSb$_2$ with $T_N=297 \pm 3$ K. Magnetization versus applied magnetic field (perpendicular to MnSb planes) below $T_N$ exhibits slightly modified de Haas van Alphen oscillations for the Zn-doped crystal as compared to that of the parent compound. By contrast, the Cu-doped system does not show de Haas van Alphen magnetic oscillations, suggesting that either Cu substitution for Mn changes the electronic structure of the parent compound substantially, or that the Cu sites are strong scatterers of carriers that significantly shorten their mean free path thus diminishing the oscillations. Density functional theory (DFT) calculations including spin-orbit coupling predict the C-type AFM state for the parent, Cu-, and Zn-doped systems and identify the $a$-axis (i.e., perpendicular to the Mn layer) as the easy magnetization direction in the parent and 12.5% of Cu or Zn substitutions. In contrast, 25% of Cu content changes the easy magnetization to the $b$-axis (i.e., within the Mn layer). We find that the incorporation of Cu and Zn in SrMnSb$_2$ tunes electronic bands near the Fermi level resulting in different band topology and semi-metallicity. The parent and Zn-doped systems have coexistence of electron and hole pockets with opened Dirac cone around the Y-point whereas the Cu-doped system has dominant hole pockets around the Fermi level with a distorted Dirac cone. The tunable electronic structure may point out possibilities of rationalizing the experimentally observed de Haas van Alphen magnetic oscillations.

Employing elastic and inelastic neutron scattering (INS) techniques, we report on detailed microscopic properties of the ferromagnetism in he magnetic topological insulator (Bi$_{0.95}$Mn$_{0.05}$)$_{2}$Te$_{3}$. Neutron diffraction of polycrystalline samples show the ferromagnetic (FM) ordering is long-range within the basal plane, and mainly 2D in character with short-range correlations between layers below $T_{\mathrm{C}} \approx 13$ K. Despite the random distribution of the dliute Mn atoms, we find that the 2D-like magnetic peaks are commensurate with the chemical structure, and the absence of (00L) magnetic peaks denote that the Mn$^{2+}$ magnetic moments are normal to the basal planes. Surprisingly, we observed collective magnetic excitations, in this dilute magnetic system. Despite the dilute nature, the excitations are typical of quasi-2D FM systems, albeit are severely broadened at short wavelengths, likely due to the random spatial distribution of Mn atoms in the Bi planes. Detailed analysis of the INS provide energy scales of the exchange couplings and the single ion anisotropy.

Neutron diffraction and magnetic susceptibility studies of orthorhombic single crystal {\Ksub} confirm the three dimensional (3D) C-type antiferromagnetic (AFM) ordering of the Mn$^{2+}$ moments at $T_{\rm N}=305 \pm 3$ K which is slightly higher than that of the parent SrMnSb$_2$ with $T_{\rm N}=297 \pm 3$ K. Susceptibility measurements of the K-doped and parent crystals above $T_{\rm N}$ are characteristic of 2D AFM systems. This is consistent with high temperature neutron diffraction of the parent compound that display persisting 2D AFM correlations well above $T_{\rm N}$ to at least $\sim 560$ K with no evidence of a ferromagnetic phase. Analysis of the de Haas van Alphen magnetic oscillations of the K-doped crystal is consistent with hole doping.

Neutron powder diffraction (NPD) study of \textit{Ln}MnSbO (\textit{Ln }$=$ La or Ce) reveals differences between the magnetic ground state of the two compounds due to the strong Ce-Mn coupling compared to La-Mn. The two compounds adopt the \textit{P4/nmm} space group down to 2 K and whereas magnetization measurements do not show obvious anomaly at high temperatures, NPD reveals a C-type antiferromagnetic (AFM) order below $T_{\mathrm{N}} = 255 $ K for LaMnSbO and 240 K for CeMnSbO. While the magnetic structure of LaMnSbO is preserved to base temperature, a sharp transition at $T_{\mathrm{SR}} = 4.5 $K is observed in CeMnSbO due to a spin-reorientation (SR) transition of the Mn$^{\mathrm{2+}}$ magnetic moments from pointing along the $c$-axis to the \textit{ab}-plane. The SR transition in CeMnSbO is accompanied by a simultaneous long-range AFM ordering of the Ce moments which indicates that the Mn SR transition is driven by the Ce-Mn coupling. The ordered moments are found to be somewhat smaller than those expected for Mn$^{\mathrm{2+}}$ ($S = 5/2$) in insulators, but large enough to suggest that these compounds belong to the class of local-moment antiferromagnets. The lower $T_{\mathrm{N\thinspace }}$ found in these two compounds compared to the As-based counterparts ($T_{\mathrm{N}} = 317$ for LaMnAsO, $T_{\mathrm{N}} = 347$ K for CeMnAsO) indicates that the Mn-$Pn$ ($Pn=$ As or Sb) hybridization that mediates the superexchange Mn-$Pn$-Mn coupling is weaker for the Sb-based compounds.

Inelastic neutron scattering was employed to investigate the impact of electronic nematic order on the magnetic spectra of LaFeAsO and Ba(Fe$_{0.953}$Co$_{0.047}$)$_{2}$As$_{2}$. These materials are ideal to study the paramagnetic-nematic state, since the nematic order, signaled by the tetragonal-to-orthorhombic transition at $T_{{\rm S}}$, sets in well above the stripe antiferromagnetic ordering at $T_{{\rm N}}$. We find that the temperature-dependent dynamic susceptibility displays an anomaly at $T_{{\rm S}}$ followed by a sharp enhancement in the spin-spin correlation length, revealing a strong feedback effect of nematic order on the low-energy magnetic spectrum. Our findings can be consistently described by a model that attributes the structural/nematic transition to magnetic fluctuations, and unveils the key role played by nematic order in promoting the long-range stripe antiferromagnetic order in iron pnictides., Comment: 6 pages, 5 figures, 66 references, plus one Supplemental Material. Physical Review Letters, Accepted and in production

Inelastic neutron scattering (INS) experiments under applied magnetic field at low temperatures show detailed low lying magnetic excitations in the so called tridiminshed iron icosahedron magnetic molecule. The magnetic molecule consists of nine iron Fe$^{3+}$ ($s = 5/2$) and three phosphorous atoms that are situated on the twelve vertices of a nearly perfect icosahedron. The three phosphorous atoms form a plane that separates the iron cluster into two weakly coupled three- and six-ion fragments, \{Fe$_3$\} and \{Fe$_6$\}, respectively. The magnetic field INS results exhibit an $S=1/2$ ground state expected from a perfect equilateral triangle of the \{Fe$_3$\} triad with a powder averaged $g$-value $=2.00$. Two sets of triplet excitations whose temperature and magnetic field dependence indicate an $S=0$ ground state with two non-degenerate $S=1$ states are attributed to the \{Fe$_6$\} fragment. The splitting may result from a finite coupling between the two fragments, single-ion anisotropy, antisymmetric exchange couplings, or from magnetic frustration of its triangular building blocks.

We examine the applicability of various model profiles for the liquid/vapor interface by X-ray reflectivities on water and ethanol and their mixtures at room temperature. Analysis of the X-ray reflecivities using various density profiles shows an error-function like profile is the most adequate within experimental error. Our finding, together with recent observations from simulation studies on liquid surfaces, strongly suggest that the capillary-wave dynamics shapes the interfacial density profile in terms of the error function.

Surface-pressure isotherms, X-ray reflectivity and X-ray near-total reflection fluorescence were used to study the properties of 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C$_{61}$ (PCBM) that was pre-mixed with cesium carbonate and spread as a film at the air-water interface. The pre-mixed PCBM with cesium carbonate demonstrated a strikingly strong effect on the organization of the film. Whereas films formed from pure PCBM solution were rough due to strong inter-molecular interactions, the films formed from the mixture were much smoother. This indicates that the cesium carbonate moderates the inter-molecular interactions among PCBM molecules, hinting that the cesium diffusion observed in inverted organic photovoltaics and the likely ensuing ionic Cs-PCBM interaction decreases aggregation tendency of PCBM. This implies that the use of cesium salts affects the morphology of the organic layer and consequently improves the efficiency of these devices.

We report on the different roles of two orbital-active Fe$^{2+}$ at the A site and V$^{3+}$ at the B site in the magnetic excitations and on the anomalous spin wave broadening in FeV$_{2}$O$_{4}$. FeV$_{2}$O$_{4}$ exhibits three structural transitions and successive paramagnetic (PM)-collinear ferrimagnetic (CFI)-noncollinear ferrimagnetic (NCFI) transitions. The high-temperature tetragonal/PM -orthorhombic/CFI transition is accompanied by the appearance of an energy gap with a high magnitude in the magnetic excitations due to strong spin-orbit coupling induced anisotropy at the Fe$^{2+}$ site. While there is no measurable increase in the energy gap from the orbital ordering of V$^{3+}$ at the orthorhombic/CFI-tetragonal/NCFI transition, anomalous spin wave broadening is observed in the orthorhombic/CFI state due to V$^{3+}$ spin fluctuations at the B site. The spin wave broadening is also observed at the zone boundary without softening, which is discussed in terms of magnon-phonon coupling., Comment: 4 pages, 3 figures, including one supplemental material

Neutron and synchrotron resonant X-ray magnetic scattering (RXMS) complemented by heat capacity and resistivity measurements reveal the evolution of the magnetic structures of Fe and Ce sublattices in single crystal CeFeAsO. The RXMS of magnetic reflections at the Ce $L_{\rm II}$-edge shows a magnetic transition that is specific to the Ce antiferromagnetic long-range ordering at $T_\texttt{Ce}\approx$ 4 K with short-range Ce ordering above $T_\texttt{Ce}$, whereas neutron diffraction measurements of a few magnetic reflections indicate a transition at $T^{*}\approx$ 12 K with unusual order parameter. Detailed order parameter measurements on several magnetic reflections by neutrons show a weak anomaly at 4 K which we associate with the Ce ordering. The successive transitions at $T_\texttt{Ce}$ and $T^{*}$ can also be clearly identified by two anomalies in heat capacity and resistivity measurements. The higher transition temperature at $T^{*}\approx$ 12 K is mainly ascribed to Fe spin reorientation transition, below which Fe spins rotate uniformly and gradually in the \textit{ab} plane. The Fe spin reorientation transition and short-range Ce ordering above $T_\texttt{Ce}$ reflect the strong Fe-Ce couplings prior to long-range ordering of the Ce. The evolution of the intricate magnetic structures in CeFeAsO going through $T^{*}$ and $T_\texttt{Ce}$ is proposed., Comment: 10 pages, 11 figures, Physical Review B, in press

The interplay between structure, magnetism and superconductivity in single crystal Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ (x=0.047) has been studied using high-resolution X-ray diffraction by monitoring charge Bragg reflections in each twin domain separately. The emergence of the superconducting state is correlated with the suppression of the orthorhombic distortion around \emph{T}$_\texttt{C}$, exhibiting competition between orthorhombicity and superconductivity. Above \emph{T}$_\texttt{S}$, the in-plane charge correlation length increases with the decrease of temperature, possibly induced by nematic fluctuations in the paramagnetic tetragonal phase. Upon cooling, anomalies in the in-plane charge correlation lengths along $a$ ($\xi_{a}$) and $b$ axes ($\xi_{b}$) are observed at \emph{T}$_\texttt{S}$ and also at \emph{T}$_\texttt{N}$ indicative of strong magnetoelastic coupling. The in-plane charge correlation lengths are found to exhibit anisotropic behavior along and perpendicular to the in-plane component of stripe-type AFM wave vector (101)$_{\rm O}$ below around \emph{T}$_\texttt{N}$. The temperature dependence of the out-of-plane charge correlation length shows a single anomaly at \emph{T}$_\texttt{N}$, reflecting the connection between Fe-As distance and Fe local moment. The origin of the anisotropic in-plane charge correlation lengths $\xi_{a}$ and $\xi_{b}$ is discussed on the basis of the antiphase magnetic domains and their dynamic fluctuations., Comment: 7 pages, 6 figures, 34 references, submitted for publication in Physical Review B

The iron magnetic form factor in SrFe2As2 has been determined by neutron diffraction and by density functional theory (DFT). As noted previously, the magnitude of the calculated moment using DFT is sensitive to the Fe-As distance. However, the shape of the calculated form factor is practically insensitive to the Fe-As distance, and further we show that the form factor closely resembles that of bcc iron, and agrees well with experiment. The spin density exhibits some anisotropy due to geometry and As hybridization., 4 pages, 4 figures, accepted by PRB Rapid Communication

Elastic and inelastic neutron scattering studies of Li(Ni$_{1-x}$Fe$_{x}$)PO$_4$ single crystals reveal anomalous spin-wave dispersions along the crystallographic direction parallel to the characteristic wave vector of the magnetic incommensurate phase. The anomalous spin-wave dispersion ({\it magnetic soft mode}) indicates the instability of the Ising-like ground state that eventually evolves into the incommensurate phase as the temperature is raised. The pure LiNiPO$_4$ system ($x=0$), undergoes a first-order magnetic phase transition from a long-range incommensurate phase to an antiferromagnetic ground state at {\it T}$_N$ = 20.8 K. At 20% Fe concentrations, although the AFM ground state is to a large extent preserved as that of the pure system, the phase transition is second-order, and the incommensurate phase is completely suppressed. Analysis of the dispersion curves using a Heisenberg spin Hamiltonian that includes inter- and in-plane nearest and next-nearest neighbor couplings reveals frustration due to strong competing interactions between nearest- and a next-nearest neighbor site, consistent with the observed incommensurate structure. The Fe substitution only slightly lowers the extent of the frustration, sufficient to suppress the IC phase. An energy gap in the dispersion curves gradually decreases with the increase of Fe content from $\sim$2 meV for the pure system ($x=0$) to $\sim$0.9 meV for $x=0.2$.

X-ray reflectivity and fluorescence near total reflection experiments were performed to examine the affinities of divalent ions ($\mathrm{Ca^{2+}}$ and $\mathrm{Ba^{2+}}$) from aqueous solution to a charged phosphatidic-acid (PA) surface. A phospholipid (1,2-Dimyristoyl-sn-Glycero-3-Phosphate, DMPA), spread as a monolayer at the air/water interface, was used to form and control the charge density at the interface. We find that for solutions of the pure salts (i.e., $\mathrm{CaCl_{2}}$ and $\mathrm{BaCl_{2}}$), the number of bound ions per DMPA at the interface is saturated at concentrations that exceed $\mathrm{10^{-3}M}$. For a 1:1 $\mathrm{Ca^{2+}/Ba^{2+}}$ mixed solutions, we find that the bound $\mathrm{Ca^{2+}/Ba^{2+}}$ ratio at the interface is 4:1. If the only property determining charge accumulation near PA were the ionic charges, the concentration of mixed $\mathrm{Ca^{2+}/Ba^{2+}}$ at the interface would equal that of the bulk. Our results show a clear specific affinity of PA for Ca compared to Ba. We provide some discussion on this issues as well as some implications for biological systems. Although our results indicate an excess of counterion charge with respect to the surface charge, that is, charge inversion, the analysis of both reflectivity and fluorescence do not reveal excess of co-ions (namely, $\mathrm{Cl^{-}}$ or $\mathrm{I}^{-}$).

A detailed elastic neutron scattering study of the structural and magnetic phase transitions in single-crystal SrFe$_2$As$_2$ reveals that the orthorhombic (O)-tetragonal (T) and the antiferromagnetic transitions coincide at $T_\texttt{O}$ = $T_\texttt{N}$ = (201.5 $\pm$ 0.25) K. The observation of coexisting O-T phases over a finite temperature range at the transition and the sudden onset of the O distortion provide strong evidences that the structural transition is first order. The simultaneous appearance and disappearance within 0.5 K upon cooling and within 0.25 K upon warming, respectively, indicate that the magnetic and structural transitions are intimately coupled. We find that the hysteresis in the transition temperature extends over a 1-2 K range. Based on the observation of a remnant orthorhombic phase at temperatures higher than \emph{T}$_\texttt{O}$, we suggest that the T-O transition may be an order-disorder transition.

Inelastic neutron scattering (INS) experiments were performed to investigate the spin dynamics in magnetoelectric effect (ME) LiCoPO$_4$ single crystals. Weak dispersion was detected in the magnetic excitation spectra along the three principal crystallographic axes measured around the (0 1 0) magnetic reflection. Analysis of the data using linear spin-wave theory indicate that single-ion anisotropy in LiCoPO$_4$ is as important as the strongest nearest-neighbor exchange coupling. Our results suggest that Co$^{2+}$ single-ion anisotropy plays an important role in the spin dynamics of LiCoPO$_4$ and must be taken into account in understanding its physical properties. High resolution INS measurements reveal an anomalous low energy excitation that we hypothesize may be related to the magnetoelectric effect of LiCoPO$_4$., 6 pages, 5 figures, accepted for publication in Phys. Rev. B

Surface sensitive synchrotron x-ray scattering studies were performed to obtain the distribution of monovalent ions next to a highly charged interface. A lipid phosphate (dihexadecyl hydrogen-phosphate) was spread as a monolayer at the air-water interface to control surface charge density. Using anomalous reflectivity off and at the $L_3 $ Cs$^{+}$ resonance, we provide spatial counterion (Cs$^{+}$) distributions next to the negatively charged interfaces. Five decades in bulk concentrations are investigated, demonstrating that the interfacial distribution is strongly dependent on bulk concentration. We show that this is due to the strong binding constant of hydronium H$_3$O$^{+}$ to the phosphate group, leading to proton transfer back to the phosphate group and to a reduced surface charge. The increase of Cs$^+$ concentration modifies the contact value potential, thereby causing proton release. This process effectively modifies surface charge density and enables exploration of ion distributions as a function of effective surface charge-density. The experimentally obtained ion distributions are compared to distributions calculated by Poisson-Boltzmann theory accounting for the variation of surface charge density due to proton release and binding. We also discuss the accuracy of our experimental results in discriminating among possible deviations from Poisson-Boltzmann theory.

Spin-wave dispersions in the antiferromagnetic state of single crystal LiFePO$_4$ were determined by inelastic neutron scattering measurements. The dispersion curves measured from the (010) reflection along both {\it a}$^\ast$ and {\it b}$^\ast$ reciprocal-space directions reflect the anisotropic coupling of the layered Fe$^{2+}$ (S = 2) spin-system. The spin-wave dispersion curves were theoretically modeled using linear spin-wave theory by including in the spin-Hamiltonian in-plane nearest- and next-nearest-neighbor interactions ({\it J}$_1$ and {\it J}$_2$), inter-plane nearest-neighbor interactions ({\it J}$_\bot$) and a single-ion anisotropy ({\it D}). A weak (010) magnetic peak was observed in elastic neutron scattering studies of the same crystal indicating that the ground state of the staggered iron moments is not along (010) direction, as previously reported from polycrystalline samples studies, but slightly rotated away from this axis.