Giant ferromagnetic $\ensuremath{\pi}\ensuremath{-}d$ interaction in a phthalocyanine molecule

Giant ferromagnetic $π - d$ interaction in a phthalocyanine molecule

H. Murakawa, A. Kanda, M. Ikeda, M. Matsuda, and N. Hanasaki

Phys. Rev. B 92, 054429 – Published 21 August 2015

Abstract

We experimentally demonstrate that the ferromagnetic intramolecular $π - d$ interaction works between an itinerant $π$ -electron spin and a localized $d$ -electron's magnetic moment in the iron-phthalocyanine (Pc) molecular compound. The evaluation of the hidden $π - d$ coupling is achieved by preparing the isolated ${Fe (Pc) (CN)}_{2}$ molecular solution with unpaired $π$ - and $d$ -electron spins, which is generated through the oxidization by iodine bromide (IBr). The monotonic increase of the magnetization with IBr addition and the saturation value of the Curie constant indicate the ferromagnetic $π - d$ coupling. Furthermore, through the magnetization measurements of the single crystals of neutral $π$ radical ${Fe (Pc) (CN)}_{2} \cdot 2 {CHCl}_{3}$ , we reveal that the on-site $π - d$ interaction in ${Fe (Pc) (CN)}_{2}$ is extremely large ( $J_{π d} / k_{B} > 500 K$ ) among those in other molecular materials.

Received 26 January 2015
Revised 27 July 2015

DOI:https://doi.org/10.1103/PhysRevB.92.054429

Authors & Affiliations

H. Murakawa¹, A. Kanda¹, M. Ikeda¹, M. Matsuda², and N. Hanasaki¹

¹Department of Physics, Graduate School of Science, Osaka University, Osaka 560-0043, Japan
²Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan

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Vol. 92, Iss. 5 — 1 August 2015

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Figure 1
(a) Schematic image of the intramolecular magnetic interaction ( $π - d$ interaction) in the phthalocyanine (Pc) molecule. Here, the arrow and central (itinerant) sphere denote the spin and transition metal ion ( $π$ electron), respectively. (b) Giant magnetoresistance in $TPP {[{Fe (Pc) (CN)}_{2}]}_{2}$ quoted from Ref. [17]. (c) The one-dimensional stacking structure of the Pc ring in $TPP {[{Fe (Pc) (CN)}_{2}]}_{2}$ . (d) and (e) Possible ground states realized by an FM (d) and AFM (e) $π - d$ interaction in $TPP {[{Fe (Pc) (CN)}_{2}]}_{2}$ . The diameter of the sphere reflects the existence probability in the charge disproportionation state stabilized by effective on-site and nearest-neighbor Coulomb interactions. (f) Magnetic-field-induced conducting state, where the AFM order of the localized Fe moment is broken.
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Figure 2
(a) ${Fe (Pc) (CN)}_{2} / DMF$ solvent with various IBr concentrations $x$ . The light yellow one is the IBr/DMF solvent without ${Fe (Pc) (CN)}_{2}$ . (b) and (c) Electronic structure of (b) ${Fe}^{2 +} ({Pc}^{2 -}) {({CN}^{-})}_{2}$ and (c) oxidized ${Fe}^{3 +} ({Pc}^{-}) {({CN}^{-})}_{2}$ . (d) Absorption spectra of the ${Fe (Pc) (CN)}_{2} + x IBr / DMF$ solvent with $x = 0, 2$ , and 8 at room temperature.
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Figure 3
(a) Typical example of the magnetization of the ${Fe (Pc) (CN)}_{2} + 6.3 IBr / DMF$ and its background contribution (IBr/DMF). (b) Temperature dependence of the net magnetic susceptibility of ${Fe (Pc) (CN)}_{2}$ with various values of $x$ after subtraction of the background (B.G.) terms. (c) Curie constant determined as the intercept of the extrapolation of the linear fitting line of $χ T$ (including diamagnetic term) as shown in the inset. The constants obtained with ${Br}_{2}$ oxidant at $x = 8$ are within the longitudinal blue bar. The theoretical values of $C$ in the cases of the FM ( $J_{π d} / k_{B} > 100 K$ ), PM ( $J_{π d} / k_{B} = 0 K$ ), and AFM ( $J_{π d} / k_{B} < - 100 K$ ) $π - d$ interactions are marked as horizontal bars on the right ordinate.
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Figure 4
(a) Magnetic anisotropy in the ${Fe}^{3 +} ({Pc}^{-}) {(CN)}_{2}$ molecular unit. The arrow indicates the net magnetic moment pinned along the $z$ direction. (b) Comparison of $Δ χ T$ between the experimental data ( $x = 8.5$ ) and calculation curves with $J_{π d} / k_{B} = 50, 100,$ and $150 K$ with arbitrary offsets. (c) Temperature dependence of $χ T$ of single crystals of the neutral $π$ radical ${Fe (Pc) (CN)}_{2} \cdot 2 {CHCl}_{3}$ with random orientation, which is well reproduced by $1.34 exp (- J_{AFM} / k_{B} T) - 6 \times 10^{- 4} T$ with $J_{AFM} / k_{B} = 19.5 K$ (red line). (d) Comparison of $χ T$ between the experimental data in (c) and calculation curves with $J_{AFM} / k_{B} = 19.5 K, J_{π d} / k_{B} = 200, 300, 500$ , and $1000 K$ . (e) Relations between $| J_{d d} |$ and $| J_{π d} |$ in representative $π - d$ systems [8].
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Physical Review B

covering condensed matter and materials physics

Giant ferromagnetic $π - d$ interaction in a phthalocyanine molecule

H. Murakawa, A. Kanda, M. Ikeda, M. Matsuda, and N. Hanasaki

Phys. Rev. B 92, 054429 – Published 21 August 2015

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Physical Review B

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Giant ferromagnetic π−d interaction in a phthalocyanine molecule

H. Murakawa, A. Kanda, M. Ikeda, M. Matsuda, and N. Hanasaki

Phys. Rev. B 92, 054429 – Published 21 August 2015

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Giant ferromagnetic $π - d$ interaction in a phthalocyanine molecule