Session P33: Focus Session: Spin Dependent Physics in Organic-Based Materials III

Comparison of Magnetization Tunneling in the Giant-Spin and Multi-Spin Descriptions of Single-Molecule Magnets

We perform a mapping of the spectrum obtained for a triangular Mn$_{3}$ single-molecule magnet (SMM) with idealized $C_{3}$ symmetry via exact diagonalization of a multi-spin (MS) Hamiltonian onto that of a giant-spin (GS) model which assumes strong ferromagnetic coupling and a spin $S$~=~6 ground state. Magnetic hysteresis measurements on this Mn$_{3}$ SMM reveal clear evidence that the steps in magnetization due to magnetization tunneling obey the expected quantum mechanical selection rules [J. Henderson \textit{et al.}, Phys. Rev. Lett. \textbf{103}, 017202 (2009)]. High-frequency EPR and magnetization data are first fit to the MS model. The tunnel splittings obtained via the two models are then compared in order to find a relationship between the sixth order transverse anisotropy term $B_6^6 $ in GS model and the exchange constant $J$ coupling the Mn$^{III}$ ions in the MS model. We also find that the fourth order transverse term $B_4^3 $\textbf{ }in the GS model is related to the orientation of JahnTeller axes of Mn$^{III}$ ions, as well as $J$   

Relieving Frustration: the Case of Antiferromagnetic Triangular Mn$_{3}$ Complexes

Recently, various trianglular [Mn$^{III}$]$_{3}$ complexes have been extensively studied due to the fact that one can modulate the magnitude and the sign of the inter-ion exchange, thereby giving rise to very simple molecular clusters that constitute some of the cleanest and best examples of so-called single-molecule magnets (SMMs) [Inorg. Chem. \textbf{48}, 3480; Phys. Rev. Lett. \textbf{103}, 017202; Dalton Trans. \textbf{2009}, 9157]. However, magnetic and electron paramagnetic resonance (EPR) characterizations of low-spin antiferromagnetic [Mn$^{III}$]$_{3}$ complexes have been problematic due to the significant spin frustration that exists for this topology. We show that this frustration is relieved in the highly distorted [NEt$_{4}$]$_{3}$[Mn$_{3}$Zn$_{2}$(salox)$_{3}$O(N$_{3})_{8}$]$\cdot$MeOH molecule: susceptibility data suggest a well isolated $S$ = 2 ground state; EPR measurements support this conclusion and further indicate the presence of a very significant zero-field-splitting (ZFS) separating the lowest-lying $m_{S}$~=~$\pm$2 states from excited levels. Remarkably, this ZFS is sufficient to give rise to magnetic bistability, as evidenced through the observation of low-temperature hysteresis.   

Susceptibility of single molecule magnet Mn$_{12}$-acetate single crystals as a function of temperature and transverse field

The longitudinal susceptibility of Mn$_{12}$-acetate single crystals has been measured in a magnetic field applied transverse to the Ising axis using micro-Hall effect magnetometry in a He$^3$ cryostat with a 3D vector superconducting magnet. We have investigated the blocking temperature as a function of longitudinal-field-sweep-rate and as a function of the magnitude of the transverse field. We find that the transverse field accelerates the relaxation to equilibrium and lowers the blocking temperature, as expected based on the Mn$_{12}$-ac spin-Hamiltonian and a classical model of single domain uniaxial nanomagnets. The susceptibility is found to obey a Curie-Weiss law, indicating a low temperature transition to a ferromagnetic phase due to dipolar interactions. We discuss these experiments as well as experiments in which an array of Hall-bars is used to spatially resolve the longitudinal susceptibility above the blocking temperature.   

The role of ligand disorder in the long range dipolar ordering of Mn$_{12}$-ac

The temperature dependence of the inverse magnetic susceptibility of both Mn$_{12}$-ac and Mn$_{12}$-ac-MeOH is found to give finite temperature intercepts corresponding to a Curie temperature $T_c$. This indicates a transition to a ferromagnetic phase at low temperature due to dipolar interactions. A transverse magnetic field suppresses $T_c$ to a $T=0$ quantum critical point. While the decrease of $T_c$ with transverse field, $H_\perp$, in Mn$_{12}$-ac-MeOH is consistent with mean field theory, the suppression of the ordering temperature of Mn$_{12}$-ac is more pronounced. We suggest that the different behavior is due to the ligand disorder which is known to cause a random distribution of molecular easy axis tilts in Mn$_{12}$-ac. Our experimental results can be explained in terms of random field Ising ferromagnetism (RFIFM) in a transverse field.   

The effect of sample aspect ratio on Curie temperature of Mn$_{12}$-ac

The inverse magnetic susceptibility measured in a SQUID-based Quantum Design MPMS magnetometer was investigated for different samples of Mn$_{12}$-ac as a function of the aspect ratio $(c/a$) of the crystal, where $c$ is the length and $a$ is the width of the sample. The Curie temperature $T_c$ inferred from the positive intercepts of the Curie-Weiss law increases from $0.3$ K for a sample of aspect ratio around $1$, and approaches saturation to a value $\sim 0.75$ K for crystals with aspect ratio above $5$. Our results can be explained by taking into account the effect of demagnetization field.   

Inelastic neutron scattering studies of S=12 and S=4 Mn$_{6}$ single molecule magnets

The magnetic properties of three Mn$_{6}$ molecular magnets have been investigated by means of inelastic neutron scattering. By comparing the obtained data to results from frequency domain magnetic resonance studies it was possible to determine the isotropic exchange and zero-field splitting parameters. The findings enable detailed quantification of the effect of small structural distortions of the molecular geometry on the magnetic properties. We show that the giant spin model completely fails to describe the spin level structure of the ground spin multiplets. A theoretical analysis of the corresponding spin Hamiltonian reveals that the excited S multiplets play a key role in determining the effective energy barrier for magnetization reversal for all three compounds.   

Magnetic Properties of Porous Metal-Organic Frameworks: Ni$_{2}$(BODC)$_{2}$(TED) and Ni$_{2}$(BDC)$_{2}$(TED)

Results of \textit{$\chi $}($T)$, $M(H)$, and heat capacity $C(T)$ measurements on two Ni dimer based porous materials Ni$_{2}$(BODC)$_{2}$(TED) and Ni$_{2}$(BDC)$_{2}$(TED) are reported. These materials form a tetragonal crystal structure of space group P4/ncc with $a=b$ = 14.9 {\AA} and $c$ = 19.4 {\AA} and Ni-Ni separation of 2.61{\AA} within the dimer. Magnetic data of Ni$_{2}$(BODC)$_{2}$(TED) revealed a ferromagnetic-like transition at about 17 K with \textit{$\theta $} = 8 K, and a coercivity field of 1700 G was observed in the hysteresis curve. Though isostructural to Ni$_{2}$(BODC)$_{2}$(TED), \textit{$\chi $}($T)$ and $M(H)$ results of Ni$_{2}$(BDC)$_{2}$(TED) showed an antiferromagnetic transition at 10 K with \textit{$\theta $} = - 132 K, and no hysteresis was observed. Although specific heat data $C(T)$ showed no clear transition in both compounds, nonlinear behavior is clearly seen in $C/T$ vs. $T$ plots, and a fit to the electron and phonon contributions to $C(T)$ gives a large heavy-fermion-like $\gamma $ in both cases. A model for the magnetic interactions is proposed and a comparison to the Cu and Co analogues is also made.   

Magnetic Properties of Three Metal-organic Coordination Networks M(N$_{3})_{2}$(4,4'-bpy), M = Ni, Co, and Cu

All three newly synthesized metal-organic coordination networks M(N$_{3})_{2}$(4,4'-bpy) (M = Ni, Co, and Cu) crystallize in orthorhombic crystal system of the space group \textit{Cmmm} (No. 65). The M ions sites have octahedral geometries with slight distortions. Results of M(H) and $\chi $(T) for Ni(N$_{3})_{2}$(4,4'-bpy) and Co(N$_{3})_{2}$(4,4'-bpy) showed antiferromagnetic behavior, characterized by a cusp at T$_{N}$ = 3.5 K for Ni and T$_{N}$ = 4.0 K for Co in the $\chi $(T) data curves. Curie-Weiss fittings yielded $\mu _{eff}$ = 2.73 $\mu _{B}$ for Ni and $\mu _{eff}$ = 5.55 $\mu _{B}$ for Co. Hysteresis was detected in the M(H) data of Ni and Co. Results of M(H) and $\chi $(T) for Cu did not show any ordering or hysteresis. No sizable anomaly was observed in C(T) data for all compounds. Fisher classical spin linear chain model fit to the $\chi $(T) data yielded $\frac{\vert J\vert }{k_B }$ = 2.49 for Ni, and $\frac{\vert J\vert }{k_B }$ = 5.23 for Co. Both Quantum Statistical with spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ and High Spin Fisher Semiclassical Fit applied to spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ were performed to fit the $\chi $(T) data of Cu, and the latter had a better statistical results.   

Quantum control of molecular antiferromagnets: an approach based on electric fields

Single molecule magnets show clear signatures of coherent behavior, and have a wide variety of effective low-energy spin Hamiltonian suitable for encoding qubits and spin-based quantum information processing. At the nanoscale, the prefered mechanism for control of quantum systems involves application of electric fields, which can be locally applied, and rapidly switched. In this work, we provide the tools for the search for single molecule magnets suitable for electric control. We analyze the mechanisms that leads to spin-electric coupling in the molecules with the shape of regular polygons. We find that the spin-electric coupling in triangular molecules is governed by the modification of the exchange interaction, while in pentagonal molecules the spin-electric coupling proceeds via spin-orbit interaction. We apply a Hubbard model to single-molecule magnet to find a connection between the spin-electric coupling and the properties of the chemical bonds in a molecule. We study the experimental signatures of spin-electric coupling in nuclear magnetic resonance, electron spin resonance, magnetization, electric polarization, and specific heat of the molecules.   

DFT studies of spin-electric effects in single-molecule magnets without inversion symmetry

Recently [1] it has been proposed that in single-molecule magnets (SMMs) lacking inversion symmetry, a mechanism based on the interplay among quantum exchange, spin orbit interaction and chirality of the underlying spin texture, allows for an effective coupling between the spin states and an external electric field. This effect could represent a very efficient and fast way of manipulating the magnetic states of the SMM via a localized electric field generated, e.g., by a nearby STM tip, with possible applications in quantum information processing. In this paper we use DFT encoded in the NRLMOL program to evaluate microscopically the electric dipole moment of a SMM, which is the key quantity controlling the strength of the spin-electric coupling. We present calculations for the triangular antiferromagnetic S= $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ Cu$_3$ SMM originally considered in Ref. [1], and comment on the case of Mn$_{12}$ acetate SMM, which also lacks inversion symmetry, but has spin S=10 and a large magnetic anisotropy. \\[4pt] [1] M. Trif et al., Phys. Rev. Lett. 101, 217201 (2008).   

Unzipping and Making Graphitic Ferromagnets: Ab Initio Study

We study the origins of high-temperature ferromagnetic behavior in graphite by means of unbiased ab-initio calculations and compare them with our data. The experimental results show that oxygen/sulfur-induced edges of graphitic fragments (via unzipping effect) play an essential role in this phenomenon, and that the finite magnetic moment appears if edges in a graphitic ribbon are occupied asymmetrically by either oxygen or sulphur. In particular, our ab-initio calculations performed within the local spin density approximation showed that in the case of pure graphene ribbon, its zig-zag edge carbon atoms carry large magnetic moment (~0.7 $\mu_{B}$/C). In an oxidized graphene, however, the magnetic moment at the edge with absorbed oxygen atoms gets considerably reduced, leading to effective ferromagnetic (more precisely, ferri-magnetic) behavior of the sample. The estimates show that the Curie temperature may be very high owing to peculiar band structure of the edge states. The results suggest ways of controlling magnetic behavior of graphitic systems at room temperature.   

Magnetic Properties of Prussian Blue Analog Films

The study of magnetization in thin film geometries of Prussian blue analogs, Rb$_{j}$M$^{\prime}_{k}$ [M(CN)$_{6}$]$_{l}\cdot n$H$_{2}$O, has shown emergent phenomena compared to bulk-like precursors. One example is the photoinduced decrease in magnetization for M$^{\prime}$=Co, M=Fe materials when the plane of the films is perpendicular to the applied magnetic field, as opposed to the usual increase in magnetization observed in the powder material [1,2]. By studying additional compounds without the photoinduced bistability, such as the M$^{\prime}$=Ni(S=1), M=Cr(S=3/2) ferromagnet with a T$_{C}$~$\sim $~75~K and the M$^{\prime}$=Cu(S=1/2), M=Fe(S=1/2) ferromagnet with a T$_{C}$~$\sim $~20~K, we have gained insight into the magnetic anisotropy present in the general class of Prussian blue analog thin films. Samples have been characterized by SQUID magnetometry, ESR, AFM, SEM, EDS and FT-IR. \\[4pt] [1] J.-H. Park, \emph{et al.}, Appl. Phys. Lett. \textbf{85}, 3797 (2004). \newline [2] F. A. Frye \emph{et al.}, Chem. Mater. \textbf{20}, 5706 (2008).