Local-Field-Induced Effective Magnetic Hysteresis of Molecular

Oct 5, 2009 - Phone: +81-3-5452-6306. ... to a π-conjugated system with local-field-induced magnetic hysteresis in the visible region at room tempera...
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J. Phys. Chem. C 2009, 113, 18897–18901

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Local-Field-Induced Effective Magnetic Hysteresis of Molecular Magneto-Optical Effects in the Visible Region at Room Temperature: Phthalocyanine Thin Films on Ferromagnetic Inorganic Substrates Kazuyuki Ishii* and Kazutaka Ozawa Contribution from the Institute of Industrial Science, The UniVersity of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan ReceiVed: August 3, 2009; ReVised Manuscript ReceiVed: September 15, 2009

We report the first observation of molecular magneto-optical effects due to a π-conjugated system with localfield-induced magnetic hysteresis in the visible region at room temperature. Thin films were prepared on a ferromagnetic substrate (Ni and SrO · 6Fe2O3) using ditrihexylsiloxy(tetra-tert-butylphthalocyanato)silicon (SiPc(OTHS)2) since its surrounding substituents would prevent both aggregation and direct interaction with the ferromagnetic substrates. The magneto-optical effects of the diamagnetic SiPc(OTHS)2 on the Ni and SrO · 6Fe2O3 substrates show the magnetic field dependences that are similar to the corresponding substrates. In the case of the SrO · 6Fe2O3 substrate, this is the first observation of magnetic hysteresis of the magnetooptical effects due to the π-conjugated system in the visible region at room temperature. The magnetic field dependences of the molecular magneto-optical effects can be explained by the interfacial magnetic field of the ferromagnetic inorganic substrates. Molecular magneto-optical effects coupled with ferromagnetic materials have several advantageous features such as a remanent magneto-optical effect at room temperature, facile preparation of thin films by the cast method, and a sharp signal with changeable wavelength and intensity. Introduction Molecular magnetism has recently been utilized not only for achieving new functions coupled with photochemistry but also for improving luminescent materials. For example, photoinduced magnetization observed in Prussian blue analogues originates from both the charge transfer and the spin crossover between the high spin and the low spin states, and therefore, such magnetization has potential for developing photochemically controllable magnetic materials.1 In the case of electroluminescent materials, strong spin-orbit coupling in heavy metal complexes can increase the phosphorescent rate from the excited triplet state, which is efficiently formed from the electron-hole pair.2 Thus, the utilization of molecular magnetism with photofunctions is vital for developing novel scientific and technological applications. In molecular magnetism, it is fundamental to realize novel memories or switches on the basis of molecular architectures and fine electronic controls.3 However, in the bulky molecular structures showing weak magnetic interactions, it is difficult to achieve molecular magnetic hysteresis at room temperature. Thus, hybridizations of functional molecules with ferromagnetic inorganic substrates have been investigated in order to combine molecular properties with inorganic ferromagnetic properties. Suzuki et al. employed spin-polarized metastable deexcitation spectroscopy to show the spin-polarizations of several molecules, such as phthalocyanines (Pcs), benzene, and carbon monoxide, which are induced by the magnetic substrate.4 Scheybal et al. presented the first clear evidence of exchange coupling between a ferromagnetic substrate and manganese(III) tetraphenylporphyrin chloride by X-ray magnetic circular dichroism (XMCD) spectroscopy.5 Wende et al. reported the experimental XMCD * Corresponding author. Fax: +81-3-5452-6306. Phone: +81-3-54526306. E-mail: [email protected].

and computational studies that show the ferromagnetic coupling of paramagnetic iron porphyrins with a magnetic substrate via the superexchange interaction.6 In these studies, the X-ray and vacuum UV were utilized for measuring molecular electronic states influenced by the magnetic substrates, while the visible light will be appropriate for the practical uses. Moreover, only a few layers near the magnetic substrate surface can reflect the spin-polarizations due to the magnetic substrates. Thus, it is important to both develop novel detection systems in the visible region and investigate magnetic influences able to reach a lot of layers that are far from the magnetic substrate surface. Recently, we showed that the magnetic circular dichroism (MCD) signal of aromatic Pcs in the visible region is considerably more intense than those of magnetic iron oxide nanoparticles,7 which originates from the large orbital angular momentum due to the π-conjugated system.8-11 If ferromagnetic properties due to inorganic materials can be added to this intense magneto-optical signal, the molecular magneto-optical effects in the visible region will be useful photofunctions not only for investigating surface properties of inorganic magnetic materials but also for preparing novel magneto-optical materials, in which the sharp signal can be changed in terms of wavelength and intensity by varying the π-conjugated system. Herein, we report the first observation of molecular magnetooptical effects due to the π-conjugated system with local-fieldinduced magnetic hysteresis in the visible region at room temperature. We prepared thin films on ferromagnetic substrates (Ni and SrO · 6Fe2O3) using ditrihexylsiloxy(tetra-tert-butylphthalocyanato)silicon (SiPc(OTHS)2, Figure 1), since its surrounding substituents prevent both the aggregation and the direct interaction with the ferromagnetic substrates. In this study, we employed both the molecular magneto-optical effects of the large orbital angular momentum of the π-conjugated system and the interfacial magnetic field of the ferromagnetic inorganic sub-

10.1021/jp907461k CCC: $40.75  2009 American Chemical Society Published on Web 10/05/2009

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Figure 1. Molecular structure of SiPc(OTHS)2 (left): optimized structure of SiPc(OTHS)2 on the ferromagnetic inorganic substrate (right).

strates. Molecular magneto-optical effects, such as MCD and magnetic optical rotation dichroism (MORD), had been utilized mainly for spectroscopically analyzing molecular electronic structures in metal complexes or aromatic compounds, in contrast to the practical uses of the magneto-optical effects based on inorganic materials. This originates from a difficulty of molecular ferromagnetism at room temperature, because the bulky molecular structures decrease spin-spin interactions. In this study, we succeeded in observing molecular magneto-optical effects coupled with magnetic hysteresis attributed to the ferromagnetic substrates, which is reasonably interpreted by magnetic field of the ferromagnetic substrate surface. The magnetic field effect reaches up to a few tens of nanometers from the surface, which shows the usefulness of molecular magneto-optical effects. Experimental Section SiPc(OTHS)2 was prepared from SiPc(OH)2 by the method previously reported.12 Analytical data are summarized in Supporting Information. Ni and SrO · 6Fe2O3 substrates were purchased from Aldrich Chemical Co. and Neomag Co. Ltd., respectively. The thin films were prepared by the cast method (Supporting Information). Diffuse reflection and transmission spectra were measured using a JASCO U570 spectrophotometer by employing an integral sphere accessory.13 The diffuse reflection spectra were obtained in the %R mode without regular reflection and were converted using the Kubelka-Munk function. The diffuse transmission spectra were measured in the absorbance mode with regular reflection. Faraday-ellipticity, Kerr-ellipticity, and Kerr-rotation measurements were performed using a JASCO E-250 equipped with a JASCO electromagnet (+1.35 ∼ -1.35 T).7 Results and Interpretations Magneto-Optical Spectra. Figure 2a,b shows diffuse transmission and MCD () Faraday-ellipticity) spectra, respectively, of a SiPc(OTHS)2 film on a glass substrate in the Q-band region (600-750 nm). The transmission spectrum on the glass substrate exhibits an intense, sharp Q absorption band, which originates from the (a1ueg) electronic configuration (a1u and eg denote

HOMO (π) and degenerate LUMOs (π*), respectively).14 The Faraday-ellipticity spectrum of the SiPc(OTHS)2 film on the glass substrate shows a dispersion type signal corresponding to the Q-band, called the Faraday A term, which indicates that the S1 states are degenerate because of the degeneracy of the LUMOs.8-11 These spectroscopic properties, such as the sharp Q absorption band and intense magneto-optical signal, are similar to those of SiPc(OTHS)2 in an organic solution,14 while Pcs in the solid state generally tend to show a very broad Q-band and negligible magneto-optical intensity due to the strong intermolecular π-π interactions.13 Thus, SiPc(OTHS)2 exhibits desirable monomeric photophysical properties due to the bulky substituents, that is, four t-butyl groups and two trihexylsiloxy axial ligands, even when forming the solid film. A diffuse reflection spectrum of the SiPc(OTHS)2 film on the Ni substrate was similar to the transmission spectrum of the SiPc(OTHS)2 film (Figure 2d). In the case of SiPc(OTHS)2 on the Ni substrate, we succeeded in observing molecular Kerrellipticity in the Q-band region (Figure 2e). It should be noted that, in contrast to the similarity in the absorption spectra, the Kerr-ellipticity spectrum of the SiPc(OTHS)2 film observed in the reflection mode is obviously different from the Faradayellipticity spectrum observed in the transmission mode. That is, the Kerr-ellipticity spectrum of SiPc(OTHS)2 on the Ni substrate shows an integral-type spectral shape in the Q-band, while the peak of Kerr-ellipticity (680 nm) shows a deviation from the absorption peak (677 nm). In contrast to the many studies on molecular Faraday effects,8-11 there are a few reports of molecular Kerr effects,15,16 and therefore, it is essential to elucidate relationships between the transmission and the reflection modes. Consequently, we analyzed the differences between the Faraday and the Kerr effects from the viewpoint of an offdiagonal element of permittivity, xy () xy′ + i xy′′).17 In the case of the transmission mode generally used in solution, the Faraday-ellipticity spectra (∝ (n xy′ + κ xy′′)/(n2 + κ2), where n and κ denote the refractive index and optical quenching coefficient, respectively) can be transformed into xy′ because the κ value is negligible. Thus, the dispersion-type Faraday A term can be observed in the Q-band when the excited states are degenerate. On the other hand, xy′′ can contribute to the Kerr-

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Figure 2. Diffuse transmission (a), diffuse reflection (d), Faraday-ellipticity (b), Kerr-ellipticity (e), and Kerr-rotation (f) spectra of SiPc(OTHS)2 films on glass (a,b) and Ni (d-f) substrates. The Faraday and Kerr spectra were measured in the transmission (b) and reflection (e,f) modes, respectively. Dispersion-type spectral pattern (c) was reproduced by the linear combination of the Kerr-ellipticity (e) and Kerr-rotation (f) spectra.

Figure 3. Kerr-ellipticity spectra (a,d) and magnetic field dependences of Kerr-ellipticity (c: 680 nm, f: 680 nm) for the SiPc(OTHS)2 films on Ni (red lines: a,c) and SrO · 6Fe2O3 (red lines: d,f) substrates. The blue lines (a,b,d,e) show Kerr-ellipticity spectra (a,d) and magnetic field dependences of Kerr-ellipticity (b: 400 nm, e: 480 nm) for the Ni (a,b) and SrO · 6Fe2O3 (d,e) substrates. The magnetic field dependences of Kerr-ellipticity (c,f) due to SiPc(OTHS)2 were obtained by deducting the magnetic field dependences (680 nm) of Kerr-ellipticity due to the corresponding substrates, respectively.

ellipticity spectrum in the reflection mode () R xy′′ + β xy′, R ) n0n(n02 - n2 + 3κ2)/{(n2 + κ2)((n02 - n2 - κ2)2 + 4 n02 κ2)}, β ) n0κ(n02 - 3n2 + κ2)/{(n2 + κ2)((n02 - n2 - κ2)2 + 4 n02 κ2)}), which results in the difference between the Faraday and the Kerr effects. This explanation is supported by a linear combination of the Kerr-ellipticity and Kerr-rotation spectra () β xy′′ - R xy′), which reproduces the dispersion-type spectral shape (Figure 2c), similar to the Faraday-ellipticity spectrum observed in the transmission mode. Magnetic Field Dependences. Figure 3a,d shows Kerrellipticity spectra of SiPc(OTHS)2 films on Ni and SrO · 6Fe2O3

substrates, respectively, in the visible region (400-750 nm). In addition to the Kerr-ellipticity spectra due to the substrates, the integral-type magneto-optical signals due to SiPc(OTHS)2 are clearly seen around the Q-band region. To investigate the magnetic interactions between the ferromagnetic substrate and the SiPc(OTHS)2, the magnetic field dependences of Kerrellipticity were measured for the Ni and SrO · 6Fe2O3 substrates (Figure 3b,c,e,f). For the SrO · 6Fe2O3 substrate, the magnetic field dependence of Kerr-ellipticity (480 nm) shows both remanence and coercivity (Figure 3e). In contrast, the Kerrellipticity of SiPc(OTHS)2 on the Al substrate increases in

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Figure 4. Interfacial magnetic field calculated for Ni substrate. The horizontal axis shows the distance from the Ni surface.

proportion to the external magnetic field strength, which is characteristic of diamagnetic materials (Supporting Information). In the case of SiPc(OTHS)2 on the SrO · 6Fe2O3 substrate, the magneto-optical signal (680 nm) due to SiPc(OTHS)2 exhibits remanence and coercivity but does not exhibit saturation even at |1| T (Figure 3f). The coercivity reflects the ferromagnetic character of SrO · 6Fe2O3, while the observed unsaturation is similar to that of SiPc(OTHS)2 on the Al substrate. Thus, we succeeded in admixing the magneto-optical effects of the diamagnetic Pcs with the magnetic hysteresis due to the ferromagnetic substrate. To the best of our knowledge, this is the first observation of the magnetic hysteresis loop of magnetooptical effects based on π-conjugated system in the visible region at room temperature. The magneto-optical intensity (680 nm) due to SiPc(OTHS)2 on the Ni substrate increases in proportion to the external magnetic field strength (|0.6| T; Figure 3c). The magnetic field dependence of Kerr-ellipticity due to the Ni substrate (400 nm) reveals both negligible coercivity and saturation at ∼|0.6| T (Figure 3b), because of soft magnetism. The bending point (∼|0.6| T) observed in SiPc(OTHS)2 is similar to the saturation (∼|0.6| T) observed in the Ni substrate; this strongly indicates that this magnetic field dependence of SiPc(OTHS)2 is attributed to the magnetic influences of the Ni substrate.

By using Ni and SrO · 6Fe2O3 substrates, we succeeded in observing the magnetic field dependences of Kerr-ellipticity due to the Pcs that reflected the ferromagnetic properties of inorganic substrates. From here, we try to clarify the origin of the magnetic influence. In the case of ferromagnetic materials, the magnetooptical signal is strongly correlated with the magnetization, M. For metal porphyrins on a magnetic substrate, magnetic hysteresis loops of the XMCD due to the paramagnetic porphyrin complexes have been shown to behave ferromagnetically; this occurs when the central metal in the porphyrin complex interacts with the ferromagnetic substrate via spin polarizations.5,6 The spin polarizations rapidly decrease within a few layers from the magnetic substrate,5 since the spin polarizations originate from the orbital interactions between the molecules and the magnetic substrate, such as superexchange interactions. However, this spin polarization mechanism is not applicable to our system because of the following reasons. (1) The bulky substituents of SiPc would prevent the π orbitals of SiPc from electronically interacting with the ferromagnetic substrate. (2) The thickness of our cast films (∼20 and ∼32 nm for the Ni and SrO · 6Fe2O3 substrates, respectively) was considerably larger than the distance at which the spin polarizations could be reflected efficiently. (3) Our equipment could not selectively observe a few layers from the viewpoint of sensitivity. Therefore, the magneto-optical signal of the diamagnetic Pc film should be proportional to the magnetic field. In addition to the external magnetic field, Hex, we consider the interfacial magnetic field due to the ferromagnetic substrate, H’(M), which is strongly correlated with M of the ferromagnetic substrate. As a result, the similarity in the magnetic field dependence between the ferromagnetic substrate and SiPc(OTHS)2 should be explained by H’(M). To evaluate quantitatively, the interfacial magnetic field was calculated by considering the magnetic dipole moments due to Ni atoms within a hemisphere of radius of 300 nm.18,19 Figure 4 shows a distance dependence of the value of H’(Msat) calculated from the interface, when the magnetization is saturated. The H’(Msat) value at 1 nm is 0.21 T and decreases gradually with increasing distance from

Figure 5. Concept for novel molecular magneto-optical memory. Tetraazaporphyrin (TAP, Q-band at 600 nm), Pc (Q-band at 700 nm), and naphthalocyanine (Nc, Q-band at 800 nm) complexes are painted on each magnetic domain of the magnetic substrate,21 which provides magnetooptical signals (1,1,1) by magnetization. For example, when TAP is selectively irradiated by a 600 nm laser, both the magnetization of the magnetic domain painted by TAP and the magneto-optical effect due to TAP disappear because of the thermomagnetic effect (the magneto-optical signal becomes (0,1,1)). Thus, by employing N kinds of dyes, 2N information can be memorized.

Phthalocyanine Thin Films the interface; this behavior is in contrast to the spin polarization mechanism. The calculated H’(Msat) value at 30 nm (0.19 T) is sufficient to cause the magneto-optical effects of SiPc in our system, although the energy is considerably lesser than that of the spin polarization mechanism (∼60 T).5 In the case of the Ni substrate, when |Hex| < 0.6 T, M increases in proportion to the external magnetic field. Thereafter, M shows saturation (|Hex| > 0.6 T). Thus, the magneto-optical signal of SiPc(OTHS)2 is enhanced by Hex and H’(M) (|Hex| < 0.6 T), but it depends only on the Hex value after the saturation of M (|Hex| > 0.6 T). Similarly, the remanence and coercivity of the Kerr-ellipticity due to SiPc(OTHS)2 on SrO · 6Fe2O3 can be reasonably attributed to the remanent magnetism and coercivity of the SrO · 6Fe2O3 substrate, respectively.19 This is the first time that the interfacial magnetic field due to an inorganic magnetic substrate, which influences molecular magnetism, has been elucidated. Conclusions In this study, we reported the molecular magneto-optical effects of Pcs on a ferromagnetic substrate and succeeded in admixing the magneto-optical effects of diamagnetic Pcs with the magnetic hysteresis due to ferromagnetic materials. The magnetic field dependence of the molecular magneto-optical effects could be explained by the interfacial magnetic field of the ferromagnetic materials. Interfacial magnetic fields can be also useful for various molecular radical reactions, whose reaction yields can be controlled by magnetic field effects.20 In addition, the molecular magneto-optical effects coupled with ferromagnetic materials have several advantageous features such as a remanent magneto-optical signal at room temperature, facile preparation of thin films by the cast method, and sharp signals with changeable wavelength and intensity. On the basis of these features, a novel molecular magneto-optical memory can be proposed as shown in Figure 5. Therefore, our results are expected to be used for making scientific and technological advances related to both molecular magnetism and organicinorganic hybridization. Acknowledgment. This work was supported by a Grant-inAid for Scientific Research (Category B No. 19350028), the Global COE Program for Chemistry Innovation, and the Iketani Science and Technology Foundation. Supporting Information Available: Analytical data of SiPc(OTHS)2. Electronic absorption and magneto-optical spectra of SiPc(OTHS)2 in toluene solution, a SiPc(OTHS)2 film on a glass substrate, a SiPc(OTHS)2 film on an Al substrate, and a SiPc(OTHS)2 film on a Ni substrate. Magnetic field dependences of Kerr-ellipticity for SiPc(OTHS)2 films on Ni and SrO · 6Fe2O3 substrates in addition to magnetic field dependences of Kerrellipticity for the Ni and SrO · 6Fe2O3 substrates. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 272, 704. (b) Ohkoshi, S.; Yorozu, S.; Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Appl. Phys. Lett. 1997, 70, 1040. (2) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403, 750.

J. Phys. Chem. C, Vol. 113, No. 43, 2009 18901 (3) (a) Kahn, O. Molecular Magnetism; VCH: Weinheim, 1994. (b) Mannini, M.; Pineider, F.; Sainctavit, P.; Danieli, C.; Otero, E.; Sciancalepore, C.; Talarico, A. M.; Arrio, M.; Cornia, A.; Gatteschi, D.; Sessoli, R. Nat. Mater. 2009, 8, 194. (c) McInnes, E. J. L.; Pidcock, E.; Oganesyan, V. S.; Cheesman, M. R.; Powell, A. K.; Thomson, A. J. J. Am. Chem. Soc. 2002, 124, 9219. (4) (a) Suzuki, T.; Kurahashi, M.; Yamauchi, Y. J. Phys. Chem. B 2002, 106, 7643. (b) Suzuki, T.; Kurahashi, M.; Ju, X.; Yamauchi, Y. J. Phys. Chem. B 2002, 106, 11553. (c) Sun, X.; Fo¨rster, S.; Li, Q. X.; Kurahashi, M.; Suzuki, T.; Zhang, J. W.; Yamauchi, Y.; Baum, G.; Steidl, H. Phys. ReV. B 2007, 75, 035419. (d) Sun, X.; Yamauchi, Y.; Kurahashi, M.; Suzuki, T.; Wang, Z. P.; Entani, S. J. Phys. Chem. C 2007, 111, 15289. (5) Scheybal, A.; Ramsvik, T.; Bertschinger, R.; Putero, M.; Nolting, F.; Jung, T. A. Chem. Phys. Lett. 2005, 411, 214. (6) (a) Wende, H.; Bernien, M.; Luo, J.; Sorg, C.; Ponpandian, N.; Kurde, J.; Miguel, J.; Piantek, M.; Xu, X.; Eckhold, Ph.; Kuch, W.; Baberschke, K.; Panchmatia, P. M.; Sanyal, B.; Oppeneer, P. M.; Eriksson, O. Nat. Mater. 2007, 6, 516. (b) Bernien, M.; Xu, X.; Miguel, J.; Piantek, M.; Eckhold, Ph.; Luo, J.; Kurde, J.; Kuch, W.; Baberschke, K.; Wende, H.; Srivastava, P. Phys. ReV. B 2007, 76, 214406. (c) Gatteschi, D. Nat. Mater. 2007, 6, 471. (7) Ozawa, K.; Ishii, K. Phys. Chem. Chem. Phys. 2009, 11, 1019. (8) Michl, J. J. Am. Chem. Soc. 1978, 100, 6801. (9) (a) Stillman, M. J.; Nyokong, T. In Phthalocyanines Properties and Applications, Vol. I; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publishers: New York, 1989; Chapter 3. (b) Stillman, M. J. In Phthalocyanines Properties and Applications, Vol. III; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publishers: New York, 1993; Chapter 5. (10) Ceulemans, A.; Oldenhof, W.; Gorller-Walrand, C.; Vanquickenborne, L. G. J. Am. Chem. Soc. 1986, 108, 1155. (11) Miwa, H.; Ishii, K.; Kobayashi, N. Chem.sEur. J. 2004, 10, 4422. (12) Wheeler, B. L.; Nagasubramanian, G.; Bard, A. J.; Schechtman, L. A.; Kenney, M. E. J. Am. Chem. Soc. 1984, 106, 7404. (13) Ishii, K.; Kikukawa, Y.; Shiine, M.; Kobayashi, N.; Tsuru, T.; Sakai, Y.; Sakoda, A. Eur. J. Inorg. Chem. 2008, 2975. (14) Ishii, K.; Kobayashi, N. In The Porphyrin Handbook, Vol. 16; Kadish, K., Smith, R. M., Guilard, R., Eds.; Academic Press: San Diego, 2003; pp 1-42. (15) An enhancement of Kerr rotation angle has been investigated by using organic-dyes Co hybrid double layered film. Here, merocyanine and Rhodamine B were employed as organic dyes. Kitaguchi, T.; Katayama, T.; Suzuki, Y.; Tsukane, N.; Koshizuka, N. Jpn. J. Appl. Phys. 1991, 30, 3377. (16) Very recently, phthalocyanine thin films have been studied in terms of magneto-optical Kerr effect spectroscopy. Here, diamagnetic substrates were used, which are contrary to our ferromagnetic substrates. In addition, phthalocyanines strongly interacting each other in the thin film result in the broad spectra, which are different from our monomeric spectra obtained by introducing the bulky substituents. Fronk, M.; Bra¨uer, B.; Kortus, J.; Schmidt, O. G.; Zahn, D. R. T.; Salvan, G. Phys. ReV. B 2009, 79, 235305. (17) Light and magnetism; Sato, K.; Asakura-Shoten: Tokyo, 1988; in Japanese. (18) (a) Connolly, J. W. D. Phys. ReV. B 1967, 159, 415. (b) von Batcheld, F. W.; Raeuchle, R. F. Acta Crystallogr. 1954, 7, 464. (19) In the magnetic hysteresis of SiPc(OTHS)2 on SrO · 6Fe2O3 (Figure 3f), the remanent magneto-optical signal (∼0.015°) was one fourth of the magneto-optical signal (∼0.06°) at 1 T, by which the H′(Mrem) value was roughly evaluated as ∼0.3 T. This suggests that ∼0.3 T is sufficient to cause the magneto-optical effects of SiPc in our system. This evaluation is supported by a fact that the H′(Mrem) value evaluated for the SrO · 6Fe2O3 substrate is comparable not only to the surface inductive flux (∼0.1 T, Neomag Co. Ltd.) but also to the interfacial magnetic field (∼0.2 T) calculated for the Ni substrate. On the other hand, in the case of Figure 3c, the interfacial magnetic field was overestimated as ∼2 T from the difference in the slopes between the low (|0.6| T) external magnetic fields, since the standard deviation was very large, and since the experimental errors must occur by deducting the relatively large magnetic field dependences (680 nm) of the Ni substrate (Supporting Information, Figure S2). (20) (a) Okazaki, M.; Shiga, T. Nature 1986, 323, 240. (b) Harkins, T. T.; Grissom, C. B. Science 1994, 263, 958. (21) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661.

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