Eel Point Research Articles

Pressure-dependent magnetic properties of geometrically frustratedZnCr2O4

${\mathrm{ZnCr}}_{2}{\mathrm{O}}_{4}$ is a geometrically frustrated antiferromagnet with a first-order transition at ${T}_{N}=12.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, where it also undergoes a structural transition from cubic to tetragonal structures. Our pressure- and field-dependent magnetization measurements and our subsequent analysis using a quantum tetrahedral mean field theory are found to be consistent with theories based on spin-lattice coupling. We also found that there is a weak, but unquestionable, field-induced transition around 2.5 T, at which our magnetization shows a subtle anomaly and, at the same time, both the N\'eel point and the estimated gap energy exhibit a sharp increase. Another finding is that our high pressure data imply that there may well be a pressure-induced splitting in the ordering temperatures of the bond and spin order parameters.

A Mid-Upper Palaeolithic human humerus from Eel Point, South Wales, UK

We report here on a human humerus directly dated to 24,470 ± 110 BP, placing it within the Gravettian, or Mid-Upper Palaeolithic. The partial humerus is an isolated find and can be attributed (with some caution) to the Pleistocene ‘bone cave’ of Eel Point on Caldey Island, Wales (UK). The humerus is probably male, similar in robusticity to other Gravettian right humeri. The apparent absence of stone tools and presence of hyaena bone and coprolites suggest that the element may not derive from an intentional burial. After a maxilla from Kent's Cavern and the Gravettian Paviland 1, Eel Point represents the third oldest anatomically modern human known from Britain. Stable carbon and nitrogen isotope measurements do not support certain use of marine foods but highlight the need for more research on contemporary faunal remains in order to better interpret human values from this period.

Boats, Bones, and Biface Bias: The Early Holocene Mariners of Eel Point, San Clemente Island, California

By 8000 B.P., sea-mammal hunting and open-sea voyages were established at Eel Point, San Clemente Island, California. The early inhabitants of Eel Point depended heavily on sea-mammal hunting and shellfish collecting, rather than the intensive fishing that developed during the Late Holocene along the Southern California coast. Eel Point technological capabilities rivaled those of Late Holocene groups such as the Chumash Indians, including the ability to fabricate sophisticated watercraft. These data question traditional models of progressive maritime cultural development in coastal Southern California, and reveal the need for more empirical methods of assessing the seafaring capabilities of ancient maritime populations.

Surface magnetism at the Néel temperature of anFeBO3single crystal

In the near-surface region of an ${\mathrm{FeBO}}_{3}$ single crystal a new magnetic-phase transition was discovered at the bulk N\'eel point by depth selective M\"ossbauer spectroscopy. The results neither fit into the concept of an ordinary nor that of an extraordinary type of surface transition. The sublattice magnetization dynamics of the near-surface phase is changed with respect to that of the bulk and exhibits similarities to that of a superparamagnetic system. In the same way as the M\"ossbauer line shape reveals the critical slowing down in the time-correlation function of the local moment, the increase in thickness of the near-surface region mirrors the divergence of the correlation length in the system when approaching the N\'eel temperature. The results on the pure ${\mathrm{FeBO}}_{3}$ crystal are compared to those gained by replacing one-quarter of the Fe atoms by Ga. The phase transition in the ${\mathrm{FeBO}}_{3}$ single crystal was studied from 291 K up to the N\'eel temperature ${\mathrm{T}}_{N}=348.35\mathrm{K}$ by depth selective conversion electron M\"ossbauer spectroscopy in an ultrahigh vacuum of ${10}^{\ensuremath{-}9}\mathrm{mbar}.$

Separation of the magnetic phases at the Néel point in the diluted spin-Peierls magnetCuGeO3

The impurity-induced antiferromagnetic ordering of the doped spin-Peierls magnet ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}{\mathrm{GeO}}_{3}$ was studied by the electron-spin-resonance (ESR) technique. Crystals with $x<4%$ demonstrate the coexistence of paramagnetic and antiferromagnetic ESR modes. This coexistence indicates the separation of a macroscopically uniform sample in the paramagnetic and antiferromagnetic phases. In the presence of long-range spin-Peierls order (at $x=1.71%)$ the volume of the antiferromagnetic phase immediately below the N\'eel point ${T}_{N}$ is smaller than the volume of the paramagnetic phase. In the presence of the short-range spin-Peierls order $(x=2.88$ and 3.2%) there are comparable volumes of two phases at ${T=T}_{N}.$ The fraction of the antiferromagnetic phase increases with lowering temperature. In the absence of the spin-Peierls dimerization (at $x=4.57%)$ the whole sample exhibits a transition into an antiferromagnetic state, and there is no phase separation. These results are explained by a consideration of clusters of staggered magnetization appearing near impurities within the singlet spin-Peierls matrix. Overlapping clusters form the antiferromagnetic phase, and isolated clusters contribute to the paramagnetic resonance signal.

Induced anisotropy and positive exchange bias: A temperature, angular, and cooling field study by ferromagnetic resonance

Exchange-biased ${\mathrm{MnF}}_{2}/\mathrm{F}\mathrm{e}$ bilayers, examined by variable angle and temperature ferromagnetic resonance (FMR), exhibit a sudden onset of a unidirectional and fourfold anisotropy below the ${\mathrm{MnF}}_{2}$ N\'eel temperature. This unexpected fourfold symmetry arises from frustrated perpendicular coupling between the ${\mathrm{MnF}}_{2}$ and the Fe overlayer in the presence of twinning in the antiferromagnet layer. These data are consistent with earlier polarized-neutron-reflectometry results. The FMR data show a clear reversal in the direction of the unidirectional anisotropy as a function of cooling field, switching sign at ${H}_{\mathrm{FC}}=13\mathrm{kOe},$ which is consistent with the onset of positive exchange bias observed in conventional magnetometry experiments. The low-temperature FMR linewidth reflects the in-plane symmetry of the resonance itself, exhibiting surprising divergence in the hard directions. Temperature-dependent FMR measurements reveal a sharp reduction in the resonance field below the N\'eel point due to the ferromagnetic/antiferromagnetic coupling.

Magnetic and magnetotransport properties ofGdBaCo2O5+δ: A high magnetic-field study

Magnetic, transport, and optical measurements performed on ${\mathrm{GdBaCo}}_{2}{\mathrm{O}}_{5.5}$ under pulsed magnetic fields up to 35 T have allowed us to determine its complete ${\ensuremath{\mu}}_{0}H\ensuremath{-}T$ phase diagram. A first-order field-induced transition takes place below the N\'eel point ${(T}_{N}\ensuremath{\approx}255 \mathrm{K}),$ which implies the appearance of a ferromagnetic component and steep changes in the conductivity. The ferromagnetic component $(0.45\ifmmode\pm\else\textpm\fi{}0.1){\ensuremath{\mu}}_{B}/(\mathrm{Co}\mathrm{}\mathrm{ion})$ does not appreciably change below ${T}_{N}.$ The nature of the high-field ferromagnetic state is discussed in connection with the zero-field ferromagnetic (or ferrimagnetic) phase above ${T}_{N}.$

Influence of interfacial disorder and temperature on magnetization reversal in exchange-coupled bilayers

Polarized neutron reflectometry is used to measure the thermal response of the net-magnetization vector of polycrystalline ferromagnetic (F) Fe films exchange coupled to twinned $(110){\mathrm{MnF}}_{2}$ antiferromagnetic (AF) layers. We observe a strong correlation between the temperature dependencies of the net sample magnetization perpendicular to the applied field at coercivity and exchange bias. For cooling field and measurement conditions involving magnetization reversal via rotation, we find a range of temperature dependencies. For the smoothest F-AF interface, the temperature dependence of exchange bias compares well to a $S=\frac{5}{2}$ Brillouin function---an observation predicted by some theoretical models. This temperature dependence is expected for the sublattice magnetization and the square root of the anisotropy constant $\sqrt{{K}_{1}}$ of bulk ${\mathrm{MnF}}_{2}.$ In contrast, for a rough F-AF interface the magnetization reversal process (and exchange bias) showed little temperature dependence up to temperatures approaching the AF N\'eel point---a clear consequence of increasing interfacial disorder in a F-AF epitaxial system.

Infrared spectroscopic study of CuO: Signatures of strong spin-phonon interaction and structural distortion

Optical properties of single-crystal monoclinic CuO in the range $70--6000 {\mathrm{cm}}^{\ensuremath{-}1}$ were studied at temperatures from 7 to 300 K. Normal reflection spectra were obtained from the (001) and (010) crystal faces thus giving separate data for the ${A}_{u}$ and ${B}_{u}$ phonon modes excited in the purely transverse way (TO modes). Mode parameters, including polarizations of the ${B}_{u}$ modes not determined by the crystal symmetry, were extracted by the dispersion analysis of reflectivity curves as a function of temperature. Spectra of all the components of the optical conductivity tensor were obtained using the Kramers-Kronig method recently extended to the case of the low-symmetry crystals. The number of strong phonon modes is in agreement with the factor-group analysis for the crystal structure currently accepted for the CuO. However, several ``extra'' modes of minor intensity are detected; some of them are observed in the whole studied temperature range, while existence of others becomes evident at low temperatures. Comparison of frequencies of ``extra'' modes with the available phonon dispersion curves points to possible ``diagonal'' doubling of the unit cell ${\mathbf{a},\mathbf{b},\mathbf{c}\stackrel{\ensuremath{\rightarrow}}{}}{\mathbf{a}+\mathbf{c},\mathbf{b},\mathbf{a}\ensuremath{-}\mathbf{c}}$ and formation of the superlattice. The previously reported softening of the ${A}_{u}^{3}$ mode $(\ensuremath{\sim}400 {\mathrm{cm}}^{\ensuremath{-}1})$ with cooling at ${T}_{N}$ is found to be $\ensuremath{\sim}10%$ for the TO mode. The mode is very broad at high temperatures and strongly narrows in the antiferromagnetic phase. We attribute this effect to strong resonance coupling of this mode to optical or acoustic bimagnons and reconstruction of the magnetic excitations spectrum at the N\'eel point. A significant anisotropy of ${\ensuremath{\epsilon}}^{\ensuremath{\infty}}$ is observed: it was found to be 5.9 along the $\mathbf{b}$ axis, 6.2 along the [101] chains, and 7.8 along the $[101\ifmmode\bar\else\textasciimacron\fi{}]$ chains. The transverse effective charge ${e}_{\mathrm{T}}^{*}$ is more or less isotropic; its value is about two electrons.

Trans-Holocene Marine Mammal Exploitation on San Clemente Island, California: A Tragedy of the Commons Revisited

Occupied from ca. 7040 B.C. to A.D. 1400, the Eel Point Site (CA-SCLI-43) on San Clemente Island, California represents one of the longest sequences of near-continuous marine resource exploitation on the west coast of North America. Faunal remains suggest transitions from heavy exploitation of fur seals and sea lions during the early Holocene, to increased hunting of cetaceans at mid-Holocene, to a focus on sea otters and fish during the late Holocene. These trends are consistent with patterns of overexploitation and economic intensification on the California and Oregon mainland, but they also suggest watercraft-based hunting earlier on the island than elsewhere. Fur seal and sea lion bones mainly represent females and juveniles, indicating that exploitation of island rookeries was guided more by self-interest than by principles of game conservation. Two intervals of temporary site abandonment, ca. 6150–3970 B.C. and A.D. 1020–1400, were both followed by periods of increased marine mammal exploitation and may reflect intervals during which marine mammal populations rebounded. Broad-scale diachronic trends in the zooarchaeological remains do not correlate with flux in paleo-sea temperatures and are best interpreted as products of overhunting and increased use of watercraft over time.

Coexistence of Haldane-gap excitations and long-range antiferromagnetic order in mixed-spin nickelatesR2BaNiO5

The spin dynamics of the $S=1$ Ni chains in mixed-spin antiferromagnets ${\mathrm{Pr}}_{2}{\mathrm{BaNiO}}_{5}$ and ${\mathrm{Nd}}_{x}{\mathrm{Y}}_{2\ensuremath{-}x}{\mathrm{BaNiO}}_{5}$ is described in terms of a simple Ginzburg-Landau Lagrangian coupled to the sublattice of rare-earth ions. Within this framework we obtain a theoretical explanation for the experimentally observed coexistence of Haldane-gap excitations and long-range magnetic order, as well as for the increase of the Haldane-gap energy below the N\'eel point. We also predict that the degeneracy of the Haldane triplet is lifted in the magnetically ordered phase. The growth of both gaps are shown to follow from the magnon repulsion. The theoretical results are consistent with the available experimental data.

Effects of temperature and hydrostatic pressure on the magnetoelasticity of a commensurate spin-density-wave antiferromagnetic Cr-3.5 at. % Al alloy single crystal.

Measurements are reported of the temperature and hydrostatic-pressure dependences of the elastic constants and thermal expansion of a Cr--3.5 at. % Al alloy single crystal through the N\'eel transition. The magnetic contributions to the volume strain and bulk modulus have been found to fit the equation a+${\mathit{bT}}^{2}$+${\mathit{cT}}^{4}$, predicted by theory, rather well up to temperatures close to the N\'eel temperature (${\mathit{T}}_{\mathit{N}}$). The pressure derivatives of the elastic constants at different constant temperatures are used to calculate the acoustic-mode Gr\"uneisen parameters as a function of temperature through the N\'eel point. The mean long-wavelength acoustic Gr\"uneisen parameter, \ensuremath{\gamma}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}^{\mathit{e}\mathit{l}}$\ensuremath{\approxeq}-5, is negative in the commensurate spin-density-wave phase (T${\mathit{T}}_{\mathit{N}}$), it increases sharply to a positive value of \ensuremath{\gamma}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}^{\mathit{e}\mathit{l}}$\ensuremath{\approxeq}+35 just above ${\mathit{T}}_{\mathit{N}}$ and then decreases to a value \ensuremath{\gamma}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}^{\mathit{e}\mathit{l}}$\ensuremath{\approxeq}+2 at temperatures well into the paramagnetic phase. The spin density wave couples mainly with the longitudinal-mode acoustic phonons in the long-wavelength limit. Shear-mode anharmonicity is only weakly affected by the N\'eel transition.

Weak ferromagnetism and tricriticality in pure La2CuO4.

New data for the field dependent magnetization $M(H)$ of pure antiferromagnetic ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ are fitted by a theory which treats the interlayer coupling using mean-field theory (MFT), and includes the Dzyaloshinskii-Moriya coupling. The fits yield a novel measure of the two-dimensional (2D) staggered susceptibility ${\ensuremath{\chi}}_{2\mathrm{D}}^{\ifmmode\dagger\else\textdagger\fi{}}$, which deviates below 360 K from the 2D Heisenberg model. The fits also imply that the weak-ferromagnetic transition below the N\'eel point ${T}_{N}$ is weakly first order, with a tricritical point close to ${T}_{N}$. The MFT nicely reproduces the order parameter and the cusp in $\ensuremath{\chi}=\frac{\mathrm{dM}}{\mathrm{dH}}$.

Hopping conductivity in fully oxygenated PrBa2Cu3Oy, YBa2Cu2CoOy, and PrBa2Cu2CoOy.

We report results of a comparative study of three nonmetals related to the 1-2-3 cuprate superconductors: fully oxygenated ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$ (PBCO), ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{2}$${\mathrm{CoO}}_{\mathit{y}}$ (YBCCO), and ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{2}$${\mathrm{CoO}}_{\mathit{y}}$ (PBCCO). We measured the resistivity up to 300 K and the absolute thermopower (TEP) up to 400 K. The temperature dependence of the resistivity follows the Mott variable-range-hopping (VRH) law \ensuremath{\rho}=${\mathrm{\ensuremath{\rho}}}_{0}$exp(${\mathit{T}}_{0}$/T${)}^{1/4}$ in PBCO up to \ensuremath{\sim}100 K and in the other two systems at least up to room temperature. The results of the TEP measurements show that in YBCCO and in PBCCO the VRH probably takes place via energy states lying mainly below the chemical potential, while in PBCO this process utilizes states lying symmetrically on both sides of the chemical potential. In PBCO the TEP exhibits a cusp at low temperatures that is probably associated with the N\'eel point of the Pr sublattice. In PBCCO we found at low temperatures a sharp sign reversal of the TEP, from p type at high temperatures to n type at low temperatures, which needs further investigation.

Localized d-d excitations in NiO(100) and CoO(100).

The localized character of the 3d electrons in the antiferromagnetic oxides NiO(100) and CoO(100) has been studied with electron energy-loss spectroscopy at primary energies between 20 and 1200 eV. The spectra of both charge-transfer compounds exhibit weak but sharp loss structures within the insulating gap region due to crystal-field excitations of the 3${\mathit{d}}^{\mathit{n}}$ configuration (${\mathit{d}}^{\mathit{n}}$\ensuremath{\rightarrow}${\mathit{d}}^{\mathit{n}\mathrm{*}}$). These parity forbidden d-d excitations are strongly enhanced at low primary energies due to exchange scattering. Their observed loss intensity depends on the momentum transfer at the scattering process as is shown by angle-dependent measurements. At NiO(100) a surface d state is found that is due to the lower symmetry of the ligand field at the surface. The measurements demonstrate the existence of spin-forbidden exchange excitations $^{3}$${\mathit{A}}_{2\mathit{g}}$\ensuremath{\rightarrow}${(}^{1}$${\mathit{E}}_{\mathit{g}}$${,}^{1}$${\mathit{T}}_{1\mathit{g}}$) in NiO(100) and reveal their excitation energies. A resonant enhancement of all intra-atomic d-d transitions in NiO(100) is found at the Ni 3s excitation threshold. Temperature-dependent measurements up to the N\'eel point show only little influence of the antiferromagnetic ordering on the EELS spectra of NiO(100). The different shapes of the absorption edges due to interatomic d-d transitions across the insulating gaps of NiO(100) and CoO(100) are discussed.

Magnetoelastic and neutron-diffraction studies of Cr-Al-alloy single crystals.

Both magnetoelastic and neutron-diffraction studies have been performed on Cr-Al single crystals containing 1.2, 1.9, and 2.6 at. % Al. These concentrations lie in a controversial region of the magnetic phase diagram. For the crystals containing 1.2 and 1.9 at. % Al the transition at the N\'eel point is from a paramagnetic to an incommensurate spin-density-wave (ISDW) state while it is from the paramagnetic to a commensurate (C) SDW state for Cr+2.6 at. % Al. All magnetic transitions are observed as second-order phase changes within our experimental resolution. No ISDW-CSDW transition was observed in any of the crystals, and no evidence was found for the existence of mixed ISDW and CSDW phases in the alloys, as previously predicted. The temperature dependence of the elastic constants is described fairly well by existing thermodynamic models, while the magnetization ${\mathit{M}}^{2}$(T)/${\mathit{M}}^{2}$(0) of Cr + 1.9 at. % Al and Cr+2.6 at. % Al varies as [1-\ensuremath{\alpha}(T/${\mathit{T}}_{\mathit{N}}$${)}^{2}$+\ensuremath{\delta}(T/${\mathit{T}}_{\mathit{N}}$${)}^{4}$] at T${\mathit{T}}_{\mathit{N}}$. The values of \ensuremath{\alpha} and \ensuremath{\delta} compare fairly well with values obtained from magnetovolume data.

133Cs nuclear magnetic resonance study of one-dimensional fluctuations in CsH2PO4 and its ferroelectric and antiferroelectric transitions at high pressure.

Pressure and temperature effects on the one dimensional (1D) and higher-dimensionality correlations associated with the ferroelectric and antiferroelectric phase transitions in cesium dihydrogen phosphate were studied by means of the $^{133}\mathrm{Cs}$ nuclear magnetic resonance (NMR) spin-lattice relaxation time ${T}_{1}$. We measured ${T}_{1}$ at 6.5 MHz at temperatures down to the ferroelectric (FE) Curie point ${T}_{C}$ at 1 bar and at 1.5 and 3.0 kbar, down to the triple point ${T}_{t}$=124.6 K at 3.3 kbar, and down to the antiferroelectric (AFE) N\'eel point ${T}_{N}$ at 3.6 kbar. With decreasing temperature, ${T}_{1}$ first decreases exponentially due to 1D fluctuations associated with the ${J}_{b}$ interactions in disordered hydrogen-bonded chains running along b. As the temperature falls further, ${T}_{1}$ then decreases linearly as the ${J}_{c}$ interaction between these chains in hydrogen-bonded planes comes into play. From these results and the known pressure derivatives of ${T}_{C}$ and ${T}_{N}$, we calculated pressure dependences for ${J}_{b}$, ${J}_{c}$, and for the interplanar interaction ${J}_{a}$. At 3.3 kbar ${J}_{a}$ changes sign, so the plane stacking becomes AFE instead of FE. Above 8.9 kbar, where ${J}_{c}$ extrapolates to zero, a new AFE phase with a checkerboard arrangement of FE b chains is predicted.

Magnetic excitations in chromium. II.

Neutron scattering measurements on pure chromium metal have been performed under various conditions of experimental resolution, energy transfer, temperature, and magnetic field. The temperature and energy dependence of the commensurate-diffuse scattering surrounding the (0,0,1) point in reciprocal space has been followed from the spin-flip temperature (${T}_{\mathrm{sf}}$=122 K) to temperatures as high as 700 K, well above the N\'eel point (${T}_{N}$=312 K). Magnetic correlations extending over 11 bcc unit cells persist to these high temperatures. The spectral width of the magnetic scattering is found to increase rapidly with temperature above ${T}_{N}$. The importance of the commensurate-diffuse modes of excitation in the disappearance of the long-range-ordered spin-density-wave (SDW) state at ${T}_{N}$ is discussed. The magnetic field dependence of the excitations in the transversely polarized SDW phase has been investigated and found to be absent. Evidence is also presented for the absence of a spin-wave energy gap greater than 50 \ensuremath{\mu}eV. We have placed the scattering in the paramagnetic phase on an absolute scale by normalizing to the integrated intensities of selected phonons, and have estimated an effective magnetic moment per atom.

Sublattice susceptibilities of neodymium metal

The magnetic sublattice susceptibilities of atoms on the cubic and hexagonal sites of metallic Nd have been deduced from polarized-neutron-diffraction experiments over the temperature range from 1.7 to 100 K. Above 40 K both sites have the same susceptibility. Below the N\'eel point of 19.9 K the cubic site has a much larger susceptibility. Both susceptibilities have a cusp at the N\'eel point and a broad maximum near 8.5 K. The maximum covers the temperature region where anomalies have been reported in the specific-heat and neutron-diffraction data. These results are consistent with proposed models of the magnetic structure. The average of the two susceptibilities is in fair agreement with bulk susceptibility measurements.

Neutron-diffraction study of the magnetic structure of dysprosium aluminum garnet

Several of the magnetically ordered phases of dysprosium aluminum garnet have been studied using neutron-diffraction techniques. At temperatures below the N\'eel point, the intensities of the magnetic reflections in zero magnetic field exhibit unexpected hysteresis. The intensities obtained after cooling in zero field are significantly different from those obtained after quenching the field to zero at a constant temperature. This behavior is shown to be consistent with a model based on extinction effects which are dependent upon the size of the magnetic domains. The behavior with a magnetic field applied parallel to the [001] and [110] directions has also been studied. For fields along [001] we find at low temperatures a transition to a state in which the spins perpendicular to the field are aligned with a different arrangement than that found in zero field. With the field along [110] we find a low-temperature transition to a state in which the transverse spins appear to be ferromagnetically aligned. These results for the low-temperature\char22{}high-field ordered states are in good agreement with the predictions of previous workers.