Session L13: Focus Session: Low-Dimensional and Molecular Magnetism - 1D Magnetism/Single-Chain Magnets - Experiment

Magnetic Order in the Spin Chain Antiferromagnet Ca3Co2O6

We have used powder neutron diffraction to investigate the magnetic structure of the Ising spin chain compound Ca$_{3}$Co$_{2}$O$_{6}$. Our investigation focuses on the low-temperature regime (T $<$ 14 K $<<$ $T_{N}$ = 25 K) where previous neutron diffraction studies have shown that there is an increasing instability in the spin density wave (SDW) order within this material. The results of this work reveal that there is an order-order transition from the SDW structure to a new commensurate antiferromagnetic phase. The extraordinary time dependence of the magnetic reflections demonstrates that this transition occurs via a very slow transformation process. As the temperature is reduced the characteristic time of the transition process increases rapidly and at low temperatures the magnetic states become frozen. We have also investigated the stability of the low-temperature commensurate phase in an applied magnetic field.   

Density functional calculation of the electronic and magnetic properties of $\alpha$-CoV$_2$O$_6$

In this work, the magnetic properties of the low dimensional $\alpha$-CoV$_2$O$_6$ system have been investigated using density-functional calculations. This system is constituted of CoO$_6$ octahedra connected by the edges and forming one dimensional linear chains. The experimental magnetization curves recorded at very low temperature show a surprising magnetization plateau at one-third of the saturation magnetization and a strong anisotropy. The estimated Co magnetic moment is large reaching a value of 4.5 $\mu_B$ suggesting a large orbital contribution. Our calculations show that three different magnetic configurations for the Co are possible, the lowest energy one being a high spin configuration in agreement with the S=3/2 character of the Co$\sp{+2}$ ion observed in this compound. Spin-orbit interactions have been included in order to calculate the magnetic anisotropy and the orbital contribution to the magnetic moment. The results are discussed in terms of crystal field splitting of the $3d$ orbital and a tight-binding Hamiltonian. Using a broken-symmetry formalism we have evaluated the effective exchange interactions of the Heisenberg Hamiltonian. They allow us to propose the magnetic structures corresponding to the ground state and to the observed magnetization plateaus.   

Heat capacity and magnetization of CoNb$_2$O$_6$ near quantum critical point

CoNb$_2$O$_6$ is a quasi-1D quantum magnet in which magnetic Co$^{2+}$ ions are ferromagnetically arranged into nearly isolated chains along the c axis with the magnetic moment confined in the ac-plane. By applying transverse magnetic field along b-axis, quantum phase transition from magnetically ordered phase to paramagnetic phase occurs. Evidence for emergent E$_8$ symmetry was recently obtained by neutron scattering near the quantum critical point (QCP) in an applied transverse magnetic field of 5.5 T We will report on experiments to investigate the behavior of the heat capacity and torque magnetization in the vicinity of the QCP and discuss their implications.   

Terahertz excitations 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 optically active low energy excitations in CoNb$_2$O$_6$. We take advantage of the polarization sensitivity of this technique to characterize both electric and magnetic dipole active excitations in this compound. A connection is made from the $q=0$ response observed here to the excitations observed by neutron scattering. In addition, we will show preliminary data on the terahertz spectra of this material as it undergoes the magnetic field-tuned quantum phase transition.   

Thermal Transport in CuSb$_2$O$_6$ single crystals

CuSb$_2$O$_6$ behaves as a uniform, one-dimensional (1D) $S=1/2$ Heisenberg spin chain with long-range, antiferromagnetic ordering below $T_N\simeq 8.5$~K.\footnote{A. Nakua {\it et al.}, J. Solid State Chem. {\bf 91}, 105 (1991); B. J. Gibson {\it et al.}, J. Magn. Magn. Mater. {\bf 272-276}, 927 (2004).} Unusual for cuprates, the Cu$^{2+}$ ions lie within quite regular CuO$_6$ octahedra and 1D magnetism appears to arise from orbital ordering driven by correlation effects.\footnote{Deepa Kasinathan, Klaus Koepernik, and Helge Rosner, Phys. Rev. Lett. {\bf 100}, 237202 (2008).} We will report the results of thermal conductivity measurements on single crystals over the temperature range $5{\rm K}\leq {\rm T}\leq 330 {\rm K}$.   

Static and dynamic properties of Single-Chain Magnets with broad domain walls

It is well-known that long-range order cannot occur in 1d magnetic systems with short-range interactions. Remanent magnetization may, however, be observed in anisotropic spin chains due to slow dynamics. The physics of such systems -- called Single-Chain Magnets (SCMs) -- is mainly dictated by the temperature dependence of the relaxation time ($\tau$) and the correlation length ($\xi$). The behavior of $\tau$ and $\xi$ is, in turn, determined by domain-wall (DW) excitations. Both statics and dynamics are dependent on whether DWs extend over more than one lattice distance (\textit{broad}) or not (\textit{sharp}). The transition from one regime to the other is controlled by the strength of the magnetic anisotropy energy with respect to the exchange interaction. For broad domain walls, we found that the interplay between localized excitations and spin waves turns crucial at finite temperatures. Moreover, all the relevant quantities display universal behaviour, provided that temperature is measured in units of DW energy and distance in units of DW widths. These facts allowed us to explain the experimental behavior of a class of Mn-based SCMs with broad DWs and may also be relevant to the study of metallic nanowires.   

Light induced kickoff of magnetic domain walls in Ising chains

Controlling the speed at which systems evolve is a challenge shared by all disciplines, and otherwise unrelated areas use common theoretical frameworks towards this goal. A particularly widespread model is Glauber dynamics, which describes the time evolution of the Ising model and can be applied to any binary system. Here we show, using molecular nanowires under irradiation, that Glauber dynamics can be controlled by a novel domain-wall kickoff mechanism. Contrary to known processes, the kickoff has unambiguous fingerprints, slowing down the spin-flip attempt rate by several orders of magnitude, and following a scaling law. The required irradiation power is very low, a substantial improvement over present methods of magnetooptical switching: in our experimental demonstration we switched molecular nanowires with light, using powers thousands of times lower than in previous optical switching methods. This manipulation of stochastic dynamic processes is extremely clean, leading to fingerprint signatures and scaling laws. These observations can be used, in material science, to better study domain-wall displacements and solitons in discrete lattices. These results provide a new way to control and study stochastic dynamic processes. Being general for Glauber dynamics, they can be extended to different kinds of magnetic nanowires and to a myriad of fields, ranging from social evolution to neural networks and chemical reactivity. For nanoelectronics and molecular spintronics the kickoff affords external control of molecular spin-valves and a magnetic fingerprint in single molecule measurements. It can also be applied to the dynamics of mechanical switches and the related study of phasons and order-disorder transitions.   

On-Surface Design and Characterization of Magnetic Macromolecules

The formation of molecular chains from basic magnetic molecular building blocks is addressed on various surfaces. Via a surface catalyzed reaction multi-spin macromolecules are synthesized in ultra-high vacuum and then investigated by scanning tunneling microscopy (STM) at low temperatures. In my presentation, I will discuss the impact of the surface on the catalytic step and present a systematic comparison between Cu(111), Cu(100), NaCl/Cu(111), Co/Cu(111), and Au(111) surfaces. Depending on the substrate system, either unwanted sideproducts or spin-active macromolecules are created. In the latter case, scanning tunneling spectroscopy measurements are performed revealing an intermolecular spin-spin interaction through the analysis of the Kondo resonance. Our approach shades new light on molecular magnetism where magnetically coupled complexes will be synthesized from basic units on surfaces for future spintronic applications.   

NMR Study of the Spin-Peierls transition in TiPO$_4$

We investigated the magnetic and structural properties of the quasi-one dimensional 3$d^1$-quantum chain system TiPO$_4$ ($J \sim$ 965 K) by NMR measurements. TiPO$_4$ undergoes two magnetostructural phase transitions, one at 111 K and the other at 74 K. Below 74 K, NMR detects two different $^{31}$P signals and the magnetic susceptibility vanishes, while DFT calculations evidence a bond alternation of the Ti\ldots Ti distances within each chain. Thus, the 74~K phase transition is a spin-Peierls transition which evolves from an incommensurate phase existing between 111~K and 74~K.   

Magnetic exchange studies of one-dimensional Co(II) molecular chains

We present a detailed experimental and theoretical study of a 1D spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ Co-based molecular chain, trans-[CoCl$_{2}$(3,5-Br$_{2}$py)$_{2}$]. Our results show distinct features that are associated to both intra-molecular interactions (along the chain) and inter-molecular interactions (perpendicular to the direction of chains). The hysteresis observed at low temperature (230mK), indicate presence of 3D ordering attributed to the exchange interactions between the chains. Measurements done at different angles from the chains axial direction (c-axis), restricted within the a-c and b-c planes, reveal uniaxial anisotropy along the c-axis. The experimental data are explained using the mean field approximation, focusing on the behavior of inter-chain interactions and g-tensor anisotropy at the Co sites in the presence of a static magnetic field. A transition between two different relaxation regimes encountered for variable sweep rates will also be discussed.   

Magnetic structure and stability of 1D Co/Fe wires

As device scales continue to decrease low dimensional structures are needed to take the place of wires and thick films. Particularly in the field of spintronics, it is important to develop 1D structures that can transmit information via the spin. This work realizes one method of spin transport via mixed Co/Fe 1D chains on Ir(001). Pure chains and mixed chains were examined by spin-polarized scanning tunneling microscopy (SPSTM) and spectroscopy. Both Fe and Co self-assemble into bi-atomic chains on the Ir(001)-(5x1) surface reconstruction. The Fe deposits as a single stacking chain while the Co exhibits two different stackings. When co-deposited, the materials can be differentiated from one another by spectroscopy due to differences in the local density of states. Spin-resolved measurements of pure Fe chains show a periodic spin-spiral along the entire length of the chain which is stabilized in an applied field but fluctuates at a speed greater than the time resolution of the STM in zero applied field. The Co, however, exhibits a ferromagnetic ground state that is stable at 8K. When the materials are co-deposited on the surface, the Co stabilizes the Fe spin-spiral and information about the magnetic state of the Co can be transmitted via the Fe spin-spiral.   

Magnetic diamond chains of $Cu_3O_4$

We present magnetization measurements on a satellite phase of $YBa_2Cu_3O_x$ namely $BaCu_3O_4$. This two-dimensional material is composed of alternate layers of Ba and $Cu_3O_4$. In the latters, the Cu and O atoms are one-dimensionaly ordered in diamond-shaped chains of formula unit $Cu_3O_4$. We will show that this material encounters a magnetic phase transition at around 336 K which is due to intra-ordering through superexchange coupling of the spins born by the Cu atoms in these diamond chains. We will discuss the possibility of either ferrimagnetic or antiferromagnetic inter-ordering of these weakly coupled one-dimensional objects and the possible role of Dzyaloshinskii-Moriya interactions.   

Competing $^1$H Spin Relaxation Mechanisms in Low-Dimensional $Per_2Pt[mnt]_2$

$Per_2[Pt(mnt)_2]$ is a low-dimensional organic conductor consisting of parallel conducting ($perylene$) and magnetic chains ($Pt[mnt]_2$) which undergo a charge density wave (CDW) and spin-Peierls (SP)-transition, respectively. The conducting chain has been studied extensively, however fundamental questions about the spin-dynamics of the magnetic chain in the SP-state remained. By using $^1$H NMR, we discovered the low temperature nuclear relaxation rates (T$_1^{-1}$) display an anomalous upturn at the SP-transition which differs from classical SP-systems. This ``bump'' is suppressed by magnetic field and coincides with the Curie tail, seen in susceptibility measurements. The field-dependent activation energies, extracted from NMR and susceptibility, reveal two distinct behaviors evidencing coexisting spin systems. At low fields, the spin relaxation mechanism derived from a paramagnetic contribution, possibly unpaired Pt spins, is dominant, but is suppressed above 10T. Hence, the intrinsic SP behavior is recovered for high fields. Furthermore, spectra in the field induced (FICDW) state, up to 33T, reveal an increase in the electronic spin polarization. \textit{DMR-NSF-1005293}