Session A31: Focus Session: Molecular Magnets

Photocontrol of Magnetism above 77~K in Nanoscaled Heterostructures of Cyanometallate Coordination Networks: Mechanism and Limits

Using nanometer-sized heterostructures of cyanometallate coordination networks, specifially core@shell nanoparticles of CoFe@CrCr-PBA (PBA = Prussian blue analogues), irradiation by white light at 80~K modifies the magnetic response, and these changes remain intact and persist without continued irradiation to nominally 125~K.\footnote{O.~N.~Risset, T.~V.~Brinzari, M.~W.~Meisel, D.~R.~Talham, submitted.} Preliminary pressure studies indicate the photoinduced changes can be maintained up to 200~K, the transition temperature of the ferromagnetic CrCr-PBA component. The effect, which we first reported up to 70~K,\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.} arises from thermally induced interface strain, which is relaxed by irradiation of the photactive consituent, CoFe-PBA. The ferromagnetic domains in the strained interface region are affected and generate the persistent changes of the magnetism. Our understanding of this photo-magnetostructural mechanism enabled us to extend the phenomenon to include photoactive spin-crossover systems\footnote{C.~R.~Gros, M.~K.~Peprah, B.~D.~Hosterman, T.~V.~Brinzari, P.~A.~Quintero, M.~Sendova, M.~W.~Meisel, D.~R.~Talham, \emph{J.~Am.~Chem.~Soc.~}{\bf 136} (2014) 9846.} and other ferromagnetic PBAs.\footnote{O.~N.~Risset, P.~A.~Quintero, T.~V.~Brinzari, M.~J.~Andrus, M.~W.~Lufaso, M.~W.~Meisel, D.~R.~Talham, \emph{J.~Am.~Chem.~Soc.~}{\bf 136} (2014) 15660.} The potential path to higher temperatures will be sketched.   

Single Crystal Neutron Diffraction Study of Organic Multiferroic (ND$_{4}$)$_{2}$[FeCl$_{5}$(D$_{2}$O)]

(NH$_{\mathrm{4}})_{\mathrm{2}}$[FeCl$_{\mathrm{5}}$(H$_{\mathrm{2}}$O)] is a new organic multiferroic that exhibits intriguing magnetic/multiferroic behavior as a function of temperature and applied magnetic field. Unlike its counterpart compounds where NH$_{\mathrm{4}}$ group is replaced by K, Cs, and Rb, (NH$_{\mathrm{4}})_{\mathrm{2}}$[FeCl$_{\mathrm{5}}$(H$_{\mathrm{2}}$O)] is the only system that undergoes two successive magnetic transitions accompanied by pronounced spontaneous electric polarization. Large deuterated (ND$_{\mathrm{4}})_{\mathrm{2}}$[FeCl$_{\mathrm{5}}$(D$_{\mathrm{2}}$O)] single crystals were grown and characterized by specific heat and magnetization measurements showing no significant effects due to deuteration. Here we report single crystal neutron diffraction results that reveal an incommensurate antiferromagnetic order below T$_{\mathrm{N}}$ $\sim$ 7.3 K with a propagation vector of (0 0 $\sim$ 0.77). Higher harmonic Bragg peaks were observed indicate ``squaring up'' behavior upon further cooling. At 1.5 K, a field induced incommensurate-to-commensurate transition was also observed by applying magnetic field along the $a$-axis.   

Slow relaxation of the magnetization in an Isostructural series of Zinc-lanthanide complexes: an integrated EPR and AC susceptibility study

Lanthanide based molecular complexes have shown potential to behave as single molecule magnets proficient to function above cryogenic temperatures. In this work we explore the dynamics of one such family, [Zn(LH)$_{2}$Ln](NO$_{3}$)$_{3}$$\cdot$6H$_{2}$O - (Ln $=$ Nd$^{3+}$, Dy$^{3+}$, Tb$^{3+}$, Ho$^{3+}$, Er$^{3+}$, Yb$^{3+})$. The series has a single lanthanide ion as a magnetic center in a low symmetry environment; the dynamics and energy landscape of the series is explored using X-band EPR, AC and DC susceptibility over a range of temperature, field and frequency. DC magnetic data show $\chi $T value consistent with expected behavior. EPR spectra for Er$^{3+}$ and Yb$^{3+}$ complexes shows EPR spectra typical for easy-plane and quasi-isotropic systems respectively, thus explaining the lack of out of phase susceptibility even in an external applied filed. However, Dy$^{3+}$ derivative show slow relaxation of the magnetization in zero field up to 15 K and is, accordingly EPR silent.   

Controlled Under Pressure: Understanding Spin Orbit Coupling and Exchange Anisotropy in Organic Magnets

The application of high pressure in the study of molecule-based materials has gained considerable interest, in part due to their high compressibilities, but also because the relevant electronic/magnetic degrees of freedom are often very sensitive to pressure. For example, small changes in the coordination environment around a magnetic transition metal ion can produce quite dramatic variations in both the on-site spin-orbit anisotropy as well as the exchange interactions between such ions when assembled into clusters or 3D networks. This has spurred the development of sophisticated spectroscopic tools that can be integrated with high-pressure instrumentation. The study of magnetic structure/property relations requires not only precise crystallographic data, but also detailed spectroscopic information concerning the unpaired electrons that give rise to the magnetic properties. This invited talk will begin with a brief description of the development and application of methods enabling EPR studies of oriented single-crystal samples subjected to hydrostatic pressures of up to 3.5 GPa. After an introductory example,\footnote{Prescimone et al., Angew. Chem. Int. Ed. {\bf 51}, 7490 (2012).} the remainder of the talk will focus on a family of heavy atom organic radical ferromagnets (containing S and Se heteroatoms) that hold records for both the highest transition temperature and coercivity (for organic magnets). The latter is the result of an unexpectedly high magnetic anisotropy, attributable to spin-orbit-mediated exchange (hopping) processes.\footnote{Winter et al., J. Am. Chem. Soc. {\bf 133}, 8126 (2011).}$^,$\footnote{Winter et al., Phys. Rev. B {\bf 85}, 094430 (2012).} Ferromagnetic resonance (FMR) measurements reveal a continuous increase in the magnetic anisotropy with increasing pressure in the all Se compound, in excellent agreement with ab initio calculations based on the known pressure-dependence of its structure.\footnote{Thirunavukkuarasu et al., submitted (2014).} The large value of anisotropic exchange terms in this heavy atom organic ferromagnet emphasizes the important role of spin-orbit coupling in a wide range of organics where this effect is usually considered to be small.   

Realization of Long-Term Air Stability in the Organic Magnet Vanadium Tetracyanoethylene

The organic ferrimagnet vanadium tetracyanoethylene (V[TCNE]x) has potential uses in both microwave electronics and spintronics due to the combination of high temperature magnetic ordering (TC \textgreater 600 K), extremely sharp ferromagnetic resonance (peak to peak linewidth of 1 G), and low-temperature conformal deposition via chemical vapor deposition (CVD; deposition temperature of 50 C), but air-sensitivity leads to the complete degradation of the films within 2 hours under ambient conditions. We have encapsulated thin films of V[TCNE]x using a UV-cured epoxy that increases film lifetime to over 700 hours as measured by the remanent magnetization. The saturation magnetization and Curie temperature decay more slowly than the remanence, and the coercivity is unchanged after 340 hours of air exposure. Fourier transform infrared spectroscopy (FTIR) shows that the epoxy does not react with the film, and magnetometry measurements show that the epoxy does not impact bulk magnetic properties. This encapsulation strategy enables experimental protocols and investigations that were not previously possible for air-sensitive samples and points the way toward the development of practical applications for this promising organic-based magnetic material.   

Photomagnetic and structural studies of Prussian blue analogue CoFe@CoCr core@shell heterostructures

The photomagnetic and structural properties of core@shell nanostructures of Prussian blue analogues, Rb$_{0.24}$Co[Fe(CN)$_6$]$_{0.74}$@K$_{0.10}$Co[Cr(CN)$_6$]$_{0.70} \cdot n$H$_2$O, with different shell thicknesses have been studied as a function of temperature and under white light irradiation.\footnote{O. N. Risset \emph{et al}., J. Am. Chem. Soc. (2014) DOI:10.1021/ja5084283.} The nature of the charge transfer induced spin transition (CTIST) of the core was affected by the presence of the shell. Specifically, while a continuous and hysteretic CTIST was observed in the bare cores, a discontinuous and non-hysteretic behavior was observed for the core@shell systems. In addition, the core@shell nanoparticles show light-induced magnetization changes radically different from the bare cores. These changes were modeled as a combination of the expected light-induced magnetism change in the cores and a modification of the magnetism in a region of the shell close to the interface, where the depth of the modified region was found to be about 25~nm for all shell thicknesses investigated.   

Separation of a molecular electronic configuration transition from the spin-crossover transition

We have investigated the unoccupied electronic structure of several molecular spin crossover systems including [Fe(H$_{2}$B(pz)$_{2})_{2}$(bipy)] [Fe(H$_{2}$B(pz)$_{2})_{2}$phen], [Fe(PM-AzA)$_{2}$(NCS)$_{2}$] and [Fe(phen)$_{2}$(NCS)$_{2}$] by inverse photoemission (IPES) and X-ray absorption spectroscopy (XAS). The XAS clearly shows the change of iron L2 edge spectra, typically associated with thermal induced spin crossover, occurring at temperatures well below the temperatures of the spin crossover transition. This suggests a change in the electronic structure configuration occurring separately from the spin ordering from a low spin to high spin state. These results may be significant to understand the observations that indicate that the spin crossover transition, and certainly the unoccupied electronic structure, is influenced by electric field. In some respects, these results for the molecular spin crossover transition resemble the separation of the charge ordering transition from the ferromagnetic transition in the manganates.   

Changes in the unoccupied electronic structure of [Fe(H$_{2}$B(pz)$_{2})_{2}$(bipy)] thin films

We have investigated the unoccupied electronic structure of ultra thin films of the spin crossover [Fe(H$_{2}$B(pz)$_{2})_{2}$(bipy)] complex (with H$_{2}$B(pz)$_{2}=$bis(hydrido)bis(1H-pyrazol-1-yl)borate and bipy $=$ 2,2'-bipyridine) deposited on Au(111) by inverse photoemission (IPES) and X-ray absorption spectroscopy (XAS). The XAS clearly shows the changes of iron L edge spectra typically associated with thermal induced spin crossover and found to be very consistent with the changed of the lowest unoccupied molecular orbitals seen in inverse photoemission with temperature. A band gap of 2-3 eV is deduced from combined UPS and IPES measurements of the films on Au substrates. The shift of the unoccupied density of states seen in inverse photoemission and XAS, with temperature, differs little from the molecular powder suggesting little influence of the substrate. We suggested that the ordering of this spin-crossover molecule on the gold substrate, may lead to a small anisotropy energy, for the molecular high spin state.   

Spin-lattice interactions as revealed by the pressure-temperature phase diagram of Co[N(CN)2]2

We combined diamond anvil cell techniques, synchrotron-based infrared and Raman spectroscopies, and complementary lattice dynamics calculations to investigate spin-lattice coupling and the magnetic crossover mechanism in the molecule-based quantum magnet Co[N(CN)2]2. These findings along with prior magnetic properties work were brought together to create a pressure-temperature phase diagram in which the second-order structural boundaries converge on key areas of activity involving the spin state, exposing how the pressure-induced local lattice distortions trigger the ferromagnetic to antiferromagnetic crossover transition. Similar triggering events may take place in other materials.   

Observation of the interplay between Photoinduced Magnetisation and Pressure Induced Electron Transfer in Cobalt Hexacyanoferrate Prussian Blue Analogue

The Prussian blue analogue K${}_{i}$Co[Fe(CN)${}_{6}$]${}_{j}$$\cdot$nH${}_{2}$O (CoFe-PBA) has been studied in both its light and dark states as a function of pressure up to 2.23~GPa. The material is known to undergo photoinduced magnetization (PIM) where irradiation leads to an increase in magnetization.\footnote{O. Sato \textit{et al.} Science \textbf{272} (1996) 704.} On the other hand, application of pressure results in a pressure induced electron transfer (PIET) where a decrease in magnetization is observed.\footnote{V. Ksenofontov \textit{et al.} Phys. Rev. B \textbf{68} (2003) 024415.} Both phenomena involve a spin transition between the low spin $(S=0)$ and the high spin $(S=2)$ states, and our work encompasses the application of both light and pressure to study the interplay between these two external stimuli. Our magnetic results indicate the suppression of the PIM at 2.23~GPa, but below this pressure, the coexistence of PIET and PIM is observed. Furthermore, in the high temperature region, we observe a increase in the temperature at which the charge transfer induced spin transition (CTIST) occurs.\footnote{N. Shimamoto \textit{et al.} Chem. Lett. \textbf{31} (2002) 486.}   

A fully first-principles approach to the Molecular Kondo problem

There has been a great effort in recent years to understand the emerging Kondo-like resonances in different magnetic molecules such as MnPc. Theoretical approaches based on atomic models have proven to be very useful for the study of this phenomenon when the magnetic moment is essentially localized on a magnetic atom [1,2]. Nevertheless the Kondo effect can arise in pure carbon-based systems as has been demonstrated experimentally in fullerenes and carbon nanotubes [3,4]. In this communication we present a multiorbital Anderson model where the orbitals are not atomic but molecular orbitals. This model is fully obtained from Density Functional Theory calculation in combination with Green's functions methodologies [5,6]. Standard impurity solver techniques are used to solve the model which is applied to fullerenes and other nanographene structures [7].\\[4pt] [1] A. Str\'{o}zecka et. al. Phys. Rev. Lett. 109, 147202 (2012);\\[0pt] [2] D. Jacob et. al. Phys. Rev. B 88, 134417 (2013);\\[0pt] [3] J. J. Parks et. al. Phys. Rev. Lett. 99, 026601 (2007);\\[0pt] [4] P. Jarillo-Herrero et. al. Nature 434, 484. (2005);\\[0pt] [5] ANT.G03. www.alacant.dfa.ua.es;\\[0pt] [6] D. Jacob et. al. Phys. Rev. B. 82, 195115 (2010);\\[0pt] [7] J Fernandez-Rossier et. al. Phys. Rev. Lett. 99, 177204 (2007).