Session M16: Focus Session: Ferromagnetic Chains/Nanostructures

Terahertz excitations near the quantum critical point in the 1D Ising chain quantum magnet CoNb$_2$O$_6$

The one-dimensional magnet CoNb$_2$O$_6$ was recently demonstrated to be an excellent realization of a one-dimensional quantum Ising spin chain. It has been shown to undergo a quantum phase transition in a magnetic field oriented transverse to its ferromagnetically aligned spin chains. Low energy spin-flip excitations in the chains were recently observed via inelastic neutron scattering.\footnote{R. Coldea, \textit{et al}, Science \textbf{327}, 177 (2010)} The energy spectrum of these excitations was shown to have a interesting energy scaling governed by symmetries of the E8 exceptional Lie group. Here, time-domain terahertz spectroscopy (TDTS) is used to investigate these optically active spin flip excitations in CoNb$_2$O$_6$ in an external magnetic field. For static magnetic fields oriented transverse to the spin chains, the terahertz excitations show evidence of the phase transitions that occur near the quantum critical magnetic field. Additional spin flip excitations are also observed for longitudinally oriented magnetic fields.   

Low Temperature 1D-Ising-like Behaviour of Cobalt Niobate

Cobalt niobate, CoNb$_{2}$O$_{6}$, is a material that exhibits 1D-Ising-like behaviour at low temperatures, based primarily on chains of spins of the Co$^{2+}$ atoms. Specific heat and magnetic susceptibility measurements on cobalt niobate have found magnetic transitions at 1.9 K and 2.9 K, in agreement with previous work. Specifically, we have performed specific heat measurements in zero field down to 330 mK and have mapped some of the field dependence of the specific heat above 2 K. The low temperature specific heat measurements show an increasingly long relaxation time, implying that the spins become increasingly decoupled from the lattice with decreasing temperature. We have also been the first group to examine the magnetic properties of this material with muon spin rotation ($\mu$SR). This work found that the cobalt moments remain largely dynamic on the microsecond timescale for temperatures well below 1.9 K, indicating that the ground state of CoNb$_{2}$O$_{6}$ is more complex than previously thought.   

Dispersion relations near quantum criticality in the quasi one-dimensional Ising chain CoNb$_2$O$_6$ in transverse magnetic field

The Ising chain in a transverse magnetic field is one of the canonical examples of a quantum phase transition. We have recently realized this model experimentally in the quasi-one-dimensional (1D) Ising-like ferromagnet CoNb$_2$O$_6$ [1]. Here, we present single-crystal inelastic neutron scattering measurements of the magnetic dispersion relations in the full three-dimensional (3D) Brillouin zone for magnetic fields near the critical point and in the high field paramagnetic phase. We explore the gap dependence as a function of field and quantify the cross-over to 3D physics at the lowest energies due to the finite interchain couplings. We parametrize the dispersion relations in the high-field paramagnetic phase to a spin wave model to quantify the sub-leading terms in the spin Hamiltonian beyond the dominant 1D Ising exchange. [1] R. Coldea, D.A. Tennant, E.M. Wheeler et al, Science 327 177-180 (2010).   

Electron-phonon and magnetoelastic interactions in ferromagnetic Co[N(CN)$_2$]$_2$

Many of the most attractive properties of multifunctional materials can be traced to the competition between charge, structure, and magnetism. The discovery that these interactions can be tuned with various physical stimuli has accelerated investigation of their behavior under extreme conditions. In this work, we combined Raman and infrared vibrational spectroscopies with complementary lattice dynamics calculations and magnetization measurements to highlight the signatures of two different coupling processes in the molecule-based magnet Co[N(CN)$_2$]$_2$. In addition to a large anisotropy, our work reveals electron-phonon coupling as a field-driven avoided crossings of the low-lying Co$^{2+}$ electronic excitation with the ligand phonons and a magnetoelastic effect that signals a flexible local CoN$_6$ environment. These findings broaden our understanding of charge-lattice-spin interactions under extreme conditions and demonstrate rich new aspects of multifunctionality in tunable molecular materials.   

Colossal reduction in Curie temperature due to finite-size effects in CoFe2O4 nanoparticles

In this talk I will show the tremendous size effect on the ordering transition temperature, $T_{O}$, in samples of CoFe$_{2}$O$_{4}$ nanoparticles with diameters ranging from 1 to 9 nm. Samples were characterized by HRTEM and XRD analyses and show a bimodal particle size distribution centered at 3 nm and around 6 nm for ``small'' and ``large'' particles, respectively. The results and their interpretation are derived from studies of the magnetization dependence of the samples on temperature at low and high magnetic fields and relaxation times using both DC and AC fields. The large particles show a typical superparamagnetic behavior with blocking temperatures, $T_{B}$, arround 100K and a Curie temperature, $T_{C}$, above room temperature. The small particles, however, show a colossal reduction of their magnetic ordering temperature and display paramagnetic behavior down to about 10K. At lower temperatures these small particles are blocked and show both exchange and anisotropy field values above 5T. The order of magnitude reduction in $T_{O}$ demonstrates a heretofore unreported magnetic behavior for ultrasmall nanoparticles of CoFe$_{2}$O$_{4}$, suggesting its further study as an advanced material.   

The effect of shape, spin, and grain boundary on the vibrational properties of iron nanoparticles

We have performed both spin-polarized and nonspin-polarized density functional theory (DFT) calculations of vibrational modes for Fe113 of either rectangular or spherical shape. We also have calculated them for a spherical nanoparticle with a single grain boundary ($\sum $3(111)) to understand the effect of grain boundary. We used both classical molecular dynamics and DFT to optimize the geometry of the Fe113 nanoparticles. Regarding the vibrational density of states (VDOS) of the nonspin-polarized Fe nanoparticles, the spherical shape exhibits a slightly enhanced VDOS in high frequency modes as compared to rectangular shape. The grain boundary brings about remarkable changes in the VDOS in all frequency ranges (as compared to the VDOS of Fe nanoparticles without a grain boundary: (1) enhanced VDOS in low frequency range (10-15 meV) (2) peak shift to higher frequency in middle range (20 -- 35 meV) (3) new peaks in high frequency range (40 - 55 meV). Most remarkable changes occur when spin is taken into account for Fe113 nanoparticle. The average magnetic moment (per atom) of the spherical Fe113 nanoparticle calculated by DFT is 2.7 Bohr magneton, which is already close to that of iron bulk (2.2 Bohr magneton). The spin-induced features in VDOS (as compared to non-spin cases) are: remarkable (1) increase in the low and middle frequency regions (7-30 meV) and (2) decrease in the high frequency regions. These spin effects are possibly correlated to spin-induced Fe-Fe bond softening (Fe-Fe bond length expansion). Work supported by DOE Grant No. DE-FG02-07ER46354.   

Controlling Magnetism by Light in Nanoscaled Heterostructures of Cyanometallate Coordination Networks: the role of increased complexity

Nanometer-sized heterostructures of the Prussian blue analogues A$_j$Co$_k$[Fe(CN)$_6$]$_{\ell} \cdot n$H$_2$O (Co-Fe PBA, with A = K, Rb) and Rb$_a$Ni$_b$[Cr(CN)$_6$]$_c \cdot m$H$_2$O (Ni-Cr PBA) have been investigated, and new phenomena, not observed for the constituent bulk phases, have been observed.\footnote{ D.M.~Pajerowski, M.J.~Andrus, J.E.~Gardner, E.S.~Knowles, M.W.~Meisel, D.R.~Talham, \emph{J.~Am.~Chem.~Soc.~}{\bf 132} (2010) 4058.}$^,$\footnote{ M.F.~Dumont, E.S.~Knowles, A.~Guiet, D.M.~Pajerowski, A.~Gomez, S.W.~Kycia, M.W.~Meisel, D.R.~Talham, \emph{\mbox{Inorg.}~Chem.~}{\bf 50} (2011) 4295.}$^,$\footnote{ D.M.~Pajerowski, J.E.~Gardner, M.J.~Andrus, S.~Datta, A.~Gomez, S.W.~Kycia, S.~Hill, D.R.~Talham, M.W.~Meisel, \emph{Phys.~Rev.~B} {\bf 82} (2010) 214405.}$^,$\footnote{ E.S.~Knowles, M.F.~Dumont, M.K.~Peprah, M.W.~Meisel, C.H.~Li, M.J.~Andrus, D.R.~Talham, arxiv:1207.2623 (2012).} A crucial aspect of the ability to photocontrol the persistent magnetism up to 70~K is the role of the strain coupling present at the interfaces between the nanoscaled regions of the constituents. Increasing the morphological complexity of the samples has the potential to provide materials possessing novel combinations of properties. In parallel, the interplay between long-range magnetic order and structural coherence is an important consideration in our attempts to design new systems. Open, unresolved issues will be discussed, and potential future paths will be sketched.   

Effects of Pressure on the Magnetic Properties of Prussian Blue Analogue Heterostuctures

Magnetic studies on the Prussian blue analogues (PBAs), Li$_x$Cu[Fe(CN)$_6$]$_y\cdot$$m$H$_2$O (CuFe-PBA) and Li$_k$Ni[Cr(CN)$_6$]$_l \cdot$$n$H$_2$O (NiCr-PBA), as well as CuFe@NiCr-PBA core-shell heterostructures, have been conducted under pressures ranging from ambient to $\approx$ 1.4~GPa and at temperatures of 2 - 90~K. Our results for the single phase CuFe-PBA indicate robust magnetic properties under the range of pressures studied: a \textit{T$_c$} of 20 K was observed at all pressures.\footnote{M. Verdaguer, G. S. Girolami, Magnetism: Molecules to Materials V, (Wiley 2005) p 303; M. Okubo \textit{et al.}, Angew. Chem. Int. Ed. \textbf{50} (2011) 6269.} However, our pressure studies of single phase NiCr-PBA are consistent with the results of Zentkov\'a \textit{et al.} up to 1~GPa.\footnote{ M. Zentkov\'a \textit{et al.}, J. Phys.: Condens. Matter \textbf{19} (2007) 266217.} At pressures above 1.0 GPa, the decrease in magnetization is accompanied by a decrease in the \textit{T$_c$}, an indication of changes in the superexchange value, an effect not reported by Zentkov\'a \textit{et al.} Lastly, our results on the effects of pressure on the magnetic properties of heterostructed PBAs, specifically CuFe@NiCr-PBA, will be presented.   

Strain-Mediated Photocontrol in Core-Shell Prussian Blue Analogue Particles

The Prussian blue analogue (PBA), A$_i$Ni[Cr(CN)$_6$]$_j\cdot n$H$_2$O (\textbf{A}), has been shown to exhibit a pressure-induced decrease in magnetization under both external isotropic pressure\,\footnote{M. Zentkov\'a \emph{et al.}, J. Phys.: Condens. Matter \textbf{19} (2007) 266217; \\ M.~K. Peprah \emph{et al.}, in preparation.} and internal photoinduced structural strain when layered with Rb$_i$Co[Fe(CN)$_6$]$_j\cdot n$H$_2$O (\textbf{B}).\footnote{M. F. Dumont \emph{et al.}, Inorg. Chem. \textbf{50} (2011) 4295; D. M. Pajerowski \emph{et al.}, J. Am. Chem. Soc. \textbf{132} (2010) 4058.} Current investigations of a series of core-shell PBAs, consisting of the photoactive ferrimagnetic \textbf{B} surrounded by ferromagnetic \textbf{A}, quantitatively model this photoinduced phenomenon, which is shown to affect both the magnetic moment and superexchange of roughly half the volume of the \textbf{A} shells. An accurate understanding of the mechanism of strain-mediated photocontrol in these heterostructures will allow the pursuit of rationally designed room temperature photocontrol systems by incorporating pressure-sensitive materials with higher magnetic ordering temperatures.   

First-Principles Modeling of Bonding and Magnetic Exchange in the Metal-TCNE Magnet Family

The chemical bond and its role as a mediator of magnetic exchange interaction remains a crucial aspect in the study of molecular magnetism. Within the M-TCNE (M$=$3$d$ metal; TCNE$=$tetracyanoethylene) class of organic-based magnets, only V[TCNE]$_{\mathrm{x}}$ (x$\sim $2) orders magnetically above room-temperature ($T_{\mathrm{c}}\sim $400 K), while structural factors underlying this exceptional behavior remain elusive. Conversely, Mn-TCNE complexes of diverse crystal structure, e.g., 1D-chain MnTPP[TCNE] ($T_{\mathrm{c}}\sim $10 K), 2D-layer [Mn(TCNE)(NCMe)$_{2}$]SbF$_{6}$ ($T_{\mathrm{c}}\sim $75 K), and 3D-network [Mn(TCNE)$_{1.5}$](I$_{3}$)$_{0.5}(T_{\mathrm{c}}\sim $170 K) have recently become available. Using this structural data, hybrid DFT simulations has been performed and the spin-polarized electronic structures resolved. The nature of bonding and non-bonding orbital interactions crucial for understanding magnetic behavior was revealed. Orbital ordering, hybridization, and trends in spin-density transfer (bonding/backbonding) as well as the formation of exchange/superexchange pathways have been identified and interpreted in terms of the dimensionality of magnetic interaction. The role of these and additional factors in establishing high-$T_{\mathrm{c}}$ magnetism in the broader M-TCNE class will be discussed.   

Neutron-Scattering Evidence for the Spin State of a Molecule-Based Magnet with Interpenetrating Sublattices

The molecule-based magnet [Ru$_2$(O$_2$CMe)$_4$]$_3$[Cr(CN)$_6$] contains two interpenetrating cubic sublattices. Each sublattice is magnetically frustrated by the easy-plane anisotropy of the spin-3/2 diruthenium (II/III) paddlewheel complexes, which lie at the middle of each cube edge and are antiferromagnetically coupled by the exchange interaction J$_c \sim $ 1.7 meV to two spin-3/2 Cr(III) ions at the cube corners. Symmetry considerations suggest that each cubic sublattice has a non-collinear spin state with net moment along one of the cubic diagonals. The moments of the two interpenetrating sublattices are antiferromagnetically coupled at small magnetic fields and become aligned above a critical field of about 1000 Oe $\sim $ K$_c$/$\mu_B$, where K$_c \sim $ 2 x 10$^{-3}$ meV is the weak dipolar coupling between sublattices. Powder neutron-diffraction measurements on a deuterated sample confirm that the sublattice moments lie along the cubic diagonals and provide indications for substantial quantum corrections to the spin state of each sublattice.   

Approach to criticality in disorder-tuned antiferromagnetic manganese thin films

Using a specialized high vacuum deposition/characterization chamber, we study the \textit{in situ} temperature-dependent conductivity $\sigma (T$,$R_{0})$~of thin magnetic films ( Gd, Cr {\&} Mn) prepared at different stages of disorder where disorder is characterized by the sheet resistance $R_{0}$ measured at $T$~=~5~K. The temperature dependence of normalized conductivity in these thin-films follows power-law dependence of the form, $\sigma(T$,$R_{0})$~=~$A$~+~$\textit{BT}^{P}$. The fitting parameters $A$, $B$ and $P$ vary systematically with increasing disorder. For Mn the parameter $A$ asymptotically approaches zero but always remains positive on the metallic side of a possible metal-insulator transition (MIT) for this material. In contrast, for Gd the parameter $A$ crosses from positive (metal) to negative (insulator) values at critical disorder ($A$ = 0) with a critical disorder strength $R_{0}$ = $R_{C}$ = 22.67~k$\Omega$ at the MIT. The behavior of Mn is strikingly different when compared with Gd, where the MIT occurs before granularity emerges. Most likely this difference of behavior occurs because the inelastic phase breaking length $L_{\phi }$ is not sufficiently high in antiferromagnet Mn to reach the 3D limit where $L_{\phi}$ is less than the film thickness $b$.   

Determination of ground state in potassium intercalated polyacenes

Intercalated compounds of polycyclic aromatic hydrocarbons have been drawing much attention from the view point of new type of organic superconductors. The mechanism of superconductivity in these materials is still unclear, and therefore the true ground states with various carrier concentrations must be understood. The antiferromagnetic ground states were reported particularly on K-doped pentacene, a typical polyacene. In the present study, we focus on the synthesis and the magnetic properties of K-intercalated polyacenes, such as anthracene, tetracene, and pentacene. The improved synthetic method based on the conventional solid state reaction was employed to obtain high quality bulk samples. The X-ray powder diffraction profiles of doped samples showed new stable phases. Interestingly, a pronounced hump at 150 K was observed in the temperature dependence of magnetic susceptibility of K$_1$anthracene. In ESR measurements the linewidth of the signals decreased significantly with a decrease in temperature below 150 K and no Pauli magnetic contribution was detected. These results clearly indicate that charge transfer occurs but the most stable ground state is still insulating via antiferromagnetic interactions. Further discussion will be made among these K-intercalated polyacenes.