Magnetic Hysteresis of Molecular Faraday Effects of Phthalocyanine

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Magnetic Hysteresis of Molecular Faraday Effects of PhthalocyanineBased Thin Films on Bi, Al-Substituted DyIG Substrates at Room Temperature and Demagnetization of the Ferrimagnetic Substrates by Photothermal Effects of Phthalocyanines Masanobu Karasawa, and Kazuyuki Ishii J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07316 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 11, 2016

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The Journal of Physical Chemistry

Magnetic Hysteresis of Molecular Faraday Effects of Phthalocyanine-Based Thin Films on Bi, AlSubstituted DyIG Substrates at Room Temperature and

Demagnetization

Substrates

by

of

the

Photothermal

Ferrimagnetic Effects

of

Phthalocyanines Masanobu Karasawa and Kazuyuki Ishii* Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8505, Japan *(K. I.) E-mail: [email protected]. Telephone: +81-3-5452-6306. Fax: +81-3-5452-6306

ACS Paragon Plus Environment

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Abstract

We report the switching of magneto-optical signals of a ferrimagnetic substrate by a selective laser irradiation of a phthalocyanine-based thin film on the substrate followed by application of an external magnetic field. We prepared thin films based on µ-oxobis[hydroxyl{2,9(or 10),16(or 17),23(or 24)-tetra-tert-buthylphthalocyanato}silicon], (SiPc)2, onto a ferrimagnetic rare earth garnet Bi0.8Dy2.2Fe4.3Al0.7O12 substrate, whose Curie temperature is appropriate for demagnetization by laser irradiation. The magnetic hysteresis of the magnetic circular dichroism (MCD) signals of (SiPc)2 was observed, which reflects the ferrimagnetic properties of the substrate. To the best of our knowledge, this is the first observation of the magnetic hysteresis of molecular Faraday effects at room temperature. After the selective pulsed laser excitation of (SiPc)2, the MCD intensity of the substrate decreased, which reflects the demagnetization of the substrate, and indicates that the photothermal energy produced in the (SiPc)2-based thin films was transferred to the substrate. To the best of our knowledge, this is the first demonstration of the demagnetization of inorganic magnetic substrates based on the photoirradiation of deposited molecular thin films. The decreased MCD signal could be recovered by applying the external magnetic field, i.e., the magneto-optical effects are switchable. If various chromophores that show magneto-optical signals at different wavelengths can be independently deposited into the spot size of the focused laser beam, a large number of bits can be introduced into the diffraction limit of light. Therefore, this study provides an insight into novel molecular magneto-optical memory that would overcome the memory density limitation related to the diffraction limit of light.


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Introduction Recently, high-density optical data storage has been strongly desired due to the rapid growth of digital information, such as big data. In the case of optical memory, the storage capacity is strictly limited by the spot size of the focused laser beam. The spot size in the air cannot be smaller than approximately two-thirds of the wavelength of the laser beam, which is called the diffraction limit of light. To overcome this limit, several methods have been proposed and intensively investigated. The first method uses light with a shorter wavelength as well as a high numerical aperture.1,2 The second is three-dimensional recording methods, such as holographic memory3-8 and two-photon absorption-based multi-layered optical memory.9-13 The third is wavelength multiplex recording methods, in which the ON/OFF properties of various wavelengths are multiplexed into the spot size of light.3-6 As one of the wavelength multiplex recording methods, persistent spectral hole burning is promising, and has been continuously investigated after pioneering works in 1974.14,15 To date, room temperature hole burning materials,

i.e.,

samarium-doped

mixed

crystals16-19

and

dye-doped

polystyrene

microparticles,20,21 have been reported. In the case of samarium-doped mixed crystals, the hole burning is caused by photoionization, which is reversible even at room temperature. However, the bandwidth of f-f transitions in rare earth elements is extremely narrow; therefore, the system is inappropriate for the wavelength multiplex recording method due to the difficulty of making several holes in the narrow band. Dye-doped polystyrene microparticles show relatively broad bandwidths based on the inhomogeneous environment, and thus, some holes could be generated in the broad band. However, as hole burning is caused by the decomposition of the dye, this system does not have a desirable function, i.e., reversiblity. Even though the use of photochromic


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materials has also been proposed,9,22 it is difficult to prepare several photochromic molecules whose photochromic wavelengths cover a wide spectral range of wavelength. Recently, we proposed a novel concept for wavelength multiplex recording, as shown in Figure 1.23 When molecules A (absorption band at 600 nm), B (absorption band at 700 nm), and C (absorption band at 800 nm) are deposited onto each magnetic domain of an appropriate magnetic substrate, the magneto-optical signal (111) is observed after magnetization of the substrate. Here, if molecule A is selectively irradiated by a 600-nm laser, and if the thermodemagnetization of the substrate occurs based on the transfer of photothermal energy from the molecule A-based thin film to the substrate, the magneto-optical signal only due to molecule A should be turned off, and thus, a (011) signal can appear. As a result, 2N information can be memorized by integrating N kinds of dyes. This concept is based on our first observation of the magnetic hysteresis of molecular Kerr effects of SiPc(OTHS)2-based thin films on a ferromagnetic SrO ･ 6Fe2O3 substrate, which was realized because of not only the intense molecular Kerr effects owing to the large π-conjugated systems of phthalocyanine (Pc),24-27 but also the SiPc(OTHS)2-based Kerr effects further intensified by the interfacial magnetic field of the ferromagnetic inorganic substrate. However, it had not been shown whether the demagnetization of the magnetic substrate could be achieved in practical systems by the selective photoexcitation of molecular thin films on magnetic substrates. Thus, in order to realize the proposed recording system, it is necessary to demonstrate the demagnetization of the magnetic substrate by selective photoexcitation of molecular thin films on the basis of the photothermal energy transfer from the photoexcited molecular thin film to the magnetic substrate.


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Herein, we report the magnetic hysteresis of molecular Faraday effects of Pc-based thin films on ferrimagnetic Bi, Al-substituted dysprosium iron garnet (DyIG) substrates at room temperature, and the demagnetization of ferrimagnetic substrates by selective pulsed laser irradiation of the Pc-based thin films. We prepared ferrimagnetic Bi, Al-substituted DyIG thin films on quartz glass substrates, followed by the preparation of cast thin films consisting of µoxo-bis[hydroxyl{2,9(or

10),16(or

17),23(or

24)-tetra-tert-buthylphthalocyanato}silicon]

((SiPc)2) and poly(vinylidene fluoride) (PVDF). Here, (SiPc)2 was employed because of the following reasons: (1) As eight bulky tert-butyl groups that surround two SiPc units prevent the aggregation, the Q absorption band should be sharp even in thin films. (2) (SiPc)2 shows intense magnetic circular dichroism (MCD) signals at around 640 nm because of the large π-conjugated systems.24-27 (3) (SiPc)2 can efficiently convert photoexcitation energy to thermal energy (~99 %).28 In addition, it is also important to demonstrate the molecular magnetic hysteresis of magneto-optical effects at wavelengths different from the previous SiPc(OTHS)2-SrO･6Fe2O3 system (~680 nm).23 Further, the ferrimagnetic Bi, Al-substituted DyIG substrates, which have been intensively studied as magneto-optical materials such as recording media,29-33 were chosen based on the following characteristics: (1) for the substrates, a relatively low Curie temperature and thin thickness are necessary for demagnetization based on photothermal effects. (2) An interfacial magnetic field due to the magnetization of the substrate should be sufficient to intensify the molecular magneto-optical effects of the (SiPc)2-based thin films. (3) The Curie temperature and the magnetization can be controlled by changing its composition. By using the optimized ferrimagnetic substrate, Bi0.8Dy2.2Fe4.3Al0.7O12, we succeeded in inducing the magnetic hysteresis of the molecular magneto-optical effects of the (SiPc)2-based thin film by the ferrimagnetic substrate (MCD ON). Also, the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate could be


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demagnetized by the selective pulsed laser irradiation of the (SiPc)2-based thin film, which was interpreted as photothermal energy transfer from the photoexcited molecular thin film to the magnetic substrate (MCD OFF). Furthermore, the decreased MCD signal was recovered by applying an external magnetic field (MCD ON). The switching properties are discussed in terms of the magnetic hysteresis of molecular magneto-optical effects, photothermal energy transferbased demagnetization, and the stability of the Pc-based thin films towards the pulsed laser irradiation.

Figure 1. A concept for novel wavelength multiplex recording.


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Experimental Section Bi0.8Dy2.2Fe4.3Al0.7O12 substrates were prepared by the pyrolysis method as previously reported.34,35 Bi(NO)3 ･ 5H2O, Dy(NO)3 ･ 5H2O, Fe(NO)3 ･ 9H2O, and Al(NO)3 ･ 9H2O were dissolved in acetylacetone in stoichiometric proportions. The concentrations of Bi3+, Dy3+, Fe3+, and Al3+ in acetylacetone were 0.08, 0.22, 0.43, and 0.07 M, respectively. The solution was dropped onto a quartz glass substrate and dried on a hot plate at 130 ˚C for 5 min, followed by heating at 370 ˚C for 5 min. This process was repeated to achieve desired magnetizations, and it was finally annealed in an electric furnace at 670 ˚C for 3 h. (SiPc)2 was prepared by the previously reported method.36-38 To prepare the (SiPc)2 thin film, 20 µL of ethanol solution containing 0.21 mM of (SiPc)2 were dropped onto an area (5 × 10 mm2 determined by masking tapes) of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate. (SiPc)2 and poly(vinylidene fluoride) (PVDF) composite film ((SiPc)2-PVDF film) was prepared in the same procedure using the solution of 0.17 mM of (SiPc)2 and 0.6 gL-1 of PVDF in the mixed solvent (ethanol : N,N-dimethylformamide = 4 : 1). Diffuse transmission spectra were measured using a JASCO U570 spectrophotometer by employing an integral sphere accessory.39 The diffuse transmission spectra were measured in the absorbance mode with regular reflection. The MCD (= Faraday-ellipticity) measurements were performed using a JASCO E-250 equipped with a JASCO electromagnet (+1.35 T ~ -1.35 T).23 For the light irradiation experiment, a Xe lamp (C2577, Hamamatsu Photonics), a diode laser (LDX-2615-650-T03, S/N Q1963-1, 650 nm, Yamaki), and a dye laser (Sirah CSTRLG532-TRI-T; 640 or 650 nm) pumped with an Nd:YAG laser (Spectra Physics INDI 40, 10 Hz) were used.


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Results and Interpretations Preparation of ferrimagnetic substrates

In our system, magnetic substrates should have not

only a sufficient magnetization that can induce the magnetic hysteresis of molecular magnetooptical effects but also a Curie temperature that is lower than the temperature achievable by the photothermal energy transfer from molecular thin films to the magnetic substrate. Furthermore, the magnitude of the magnetization of the substrate is also dependent on the size of magnetic materials. Thus, in order to design magnetic substrates, the ferrimagnetic Bi, Al-substituted DyIG thin substrates were chosen because the relatively low Curie temperature and sufficient magnetization can be controlled by changing its composition. In Bi, Al-substituted DyIG, the saturated magnetization decreases with increasing the amounts of both Bi and Al components. However, the Curie temperature should be increased by the increase in the amount of the Al component, although it should be decreased by the increase in the amount of the Bi component. In order to find desirable magnetic substrates that can induce magnetic hysteresis of magnetooptical effects of (SiPc)2-based thin film, more than ten substrates, whose compositions and thicknesses were different, were prepared by the pyrolysis method, which is appropriate for precisely controlling the chemical composition of substrates.34,35 Thus, we found that a Bi0.8Dy2.2Fe4.3Al0.7O12 substrate, whose thickness was calculated to be approximately 1.2 µm, was appropriate for our systems and shown to demagnetize at 210 ± 10 ˚C.

Electronic absorption and Magneto-optical spectra

Figures 2a and 2b show the diffuse

transmission and MCD spectra of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate, respectively. In the diffuse transmission spectrum, absorbance gradually increases from 500 nm to 400 nm with


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decreasing wavelength, while the absorbance is very weak in the red light region (> 600 nm). In the MCD spectrum, an intense, broad signal is seen at around 450 nm, which is explained by the charge transfer transition between Fe ions in the octahedral and tetrahedral sites and oxygen ions.40-44 The diffuse transmission spectrum of (SiPc)2 film on glass substrate (Figure 2c) shows an intense and sharp Q absorption band at around 640 nm, which is similar to that of (SiPc)2 in conventional organic solvents.38 In the case of porphyrinic compounds, the Q absorption band is reasonably explained by the Gouterman’s four orbital model, which is the configuration interactions between transitions between four frontier orbitals, i.e., HOMO, nearby HOMO−1, and degenerate LUMOs (in D4h symmetry, these are a1u, a2u, and eg orbitals, respectively), and thus, the Q absorption band (~680 nm) of the SiPc monomer mainly originates from the single (a1ueg) configuration because of relatively small configuration interactions.24,45 Further, the blueshift of the Q band (640 nm) of (SiPc)2 is due to the exciton interaction among the component SiPc units.38 In order to investigate magneto-optical properties, the MCD measurements were performed. Here, the MCD spectral patterns are classified to three Faraday terms, A, B and C: the A term shows a dispersion type spectral pattern, which arises from the Zeeman splitting of degenerate excited states. The B term shows an integral type spectral pattern, which arises from the magnetic field induced mixing between non-degenerate excited states. The C term which depends on temperature can be seen when the ground states are degenerate. The MCD spectrum of (SiPc)2 film on glass substrate (Figure 2d) exhibits a dispersion type spectral pattern in the Qband region, i.e., the Faraday A term, which indicates the degeneracy of the excited states due to the degenerate LUMOs. Also, in the case of Pc derivatives, the MCD is intensified by the orbital


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angular momentum of aromatic π-conjugated system.26 Although Pcs in solid state generally show very broad Q-bands and negligible magneto-optical signals due to strong intermolecular ππ interaction,39 the spectroscopic properties of the (SiPc)2 film on glass substrate, such as the sharp Q absorption band and intense magneto-optical signal, are similar to those of (SiPc)2 in organic solvents.38 Thus, (SiPc)2 exhibits desirable photophysical properties without strong intermolecular π-π interaction even in solid state, because of eight bulky tert-butyl substituents, in a similar way of ditrihexylsiloxy(tetra-tert-butylphthalocyanato)silicon (SiPc(OTHS)2) in the previous report.23 In the diffuse transmission spectrum of the (SiPc)2 film on Bi0.8Dy2.2Fe4.3Al0.7O12 substrate (Figure 2e), the sharp Q absorption band of (SiPc)2 is seen in addition to the absorption band due to the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate at around 450 nm. Further, the MCD spectra of the (SiPc)2 film on the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate resemble the sum of the MCD spectra of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate and the (SiPc)2 film (Figure 2f), which exhibit the Faraday A term of the Q band at around 640 nm as well as the MCD signal due to the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate at around 450 nm.


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Figure 2. Diffuse transmission (a, c, e) and MCD (b, d, f) spectra of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate (a, b), the (SiPc)2 film on a glass substrate (c, d), and the (SiPc)2 film on the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate (e, f), respectively.

Magnetic Field Dependence of MCD

Magnetic field dependences of the MCD signals of

(SiPc)2 on ferrimagnetic substrate were measured to investigate the magnetic field effects of the ferrimagnetic substrate on the (SiPc)2 film. Figure 3b shows magnetic field dependences of the positive and negative MCD signals at 635 nm and 650 nm, respectively. To extract the magnetic


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field dependence of the MCD signal due to the Q band of (SiPc)2, the half-difference of the magnetic field dependence between the positive (635 nm) and negative (650 nm) signals was calculated (Figure 3c), by which the magnetic hysteresis owing to the substrate was removed.27 The half-difference of the magnetic field dependence exhibits both remanence and coercivity, which is similar to the magnetic field dependence of the MCD signal of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate at 450 nm (Figure 3a), although the MCD intensity of the halfdifference linearly increases when the external magnetic field is more than |0.3| T. In fact, a dispersion-type MCD spectral pattern due to (SiPc)2 was seen at around the Q-band region even at zero magnetic field. This magnetic field dependence of the magneto-optical signal due to (SiPc)2 is reasonably explained by the interfacial magnetic field of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate in a similar manner of the previous study on SiPc(OTHS)2-SrO･6Fe2O3 system.23 Thus, we succeeded in observing the molecular Faraday effect of (SiPc)2 with the magnetic hysteresis on Bi0.8Dy2.2Fe4.3Al0.7O12 substrate by controlling the magnetic properties of the substrate. To the best of our knowledge, this is the first observation of magnetic hysteresis of molecular Faraday effects on a magnetic substrate, which is different from the molecular Kerr effects previously observed in the SiPc(OTHS)2-SrO ･ 6Fe2O3 system. In comparison with the present Bi0.8Dy2.2Fe4.3Al0.7O12 substrate, when the Curie temperature or thickness was decreased, the magnetic hysteresis of the molecular magneto-optical signal was not observed because of the weak interfacial magnetic field strength of the substrate. This indicates that the interfacial magnetic field strength of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate is near the limit for observing the magnetic hysteresis in our equipment.


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Figure 3. Magnetic field dependences of the MCD signals for the (SiPc)2 film on the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate at 450 nm (a), 635 nm (b, blue line), 650 nm (b, red line), as well as half-difference between the positive (635 nm) and negative (650 nm) signals (c).

Switching MCD signals by pulsed laser irradiation and application of magnetic fields

To

investigate whether the demagnetization of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate can be caused by the selective excitation of (SiPc)2, the light irradiation experiments were conducted. First, a continuous wave light source, such as a Xe lamp (3.2 mW/mm2, > 560 nm) or a diode laser (10


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mW/mm2, 650 nm), was employed as the excitation source. However, the demagnetization of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate could not be achieved. Next, the pulsed laser irradiation effects were investigated (0.8 mJ/mm2, 10 Hz, 650 nm), but the decomposition of (SiPc)2 was observed before the substrate was demagnetized, in the case of the (SiPc)2 film on Bi0.8Dy2.2Fe4.3Al0.7O12 substrate. Finally, we tried to increase the stability by blending the PVDF polymer into the (SiPc)2 film, although, the blend of the PVDF polymer provides two disadvantages because of increases in the distance between (SiPc)2 and the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate: (1) the magnetic field effect of the substrate should decrease toward (SiPc)2, (2) the photothermal energy transfer from the photoexcited (SiPc)2 to the substrate might be inefficient. When the (SiPc)2-PVDF film whose ratio of (SiPc)2 to PVDF was approximately 1:2 was prepared, the (SiPc)2-PVDF film was relatively stable under pulsed laser irradiation. This indicates that the addition of the PVDF polymer prevented the photoexcited (SiPc)2 from decomposition based on the reaction with molecular oxygen. The diffuse transmission and MCD spectra of the (SiPc)2PVDF film on Bi0.8Dy2.2Fe4.3Al0.7O12 substrate are shown in Figures 4a and 4b, respectively, which are almost entirely similar to those of the (SiPc)2 film on the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate. Further, the (SiPc)2-PVDF film on the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate was found to show weak but distinguishable magnetic hysteresis (Figure 4c). To investigate whether the demagnetization of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate can be caused by the selective pulsed laser irradiation of the adjacently deposited (SiPc)2 film at 640 nm or 650 nm, for the (SiPc)2 film on the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate, the MCD signal due to the substrate at 450 nm and 0 mT was monitored before and after the pulsed laser irradiation. After the pulsed laser irradiation (0.8 mJ mm-2 per pulse, 10 Hz, 640 nm or 650 nm) for 10 min, the decrease in the MCD intensity, which reflects the demagnetization of the substrate, was observed, as typically shown in Figure


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5a. On the other hand, the MCD intensity of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate without the (SiPc)2-PVDF film was not changed before and after the pulsed laser irradiation at 640 nm. These results strikingly indicate that pulsed laser irradiation at 640 nm is appropriate only for the excitation of (SiPc)2, and that the demagnetization of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate originates from the temperature rise caused by photothermal energy transfer from the (SiPc)2PVDF film to the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate. Thus, we succeeded in the demagnetization of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate based on the selective excitation of (SiPc)2 by the combined use of pulsed laser and PVDF coating. Furthermore, even after the pulsed laser irradiation of (SiPc)2-PVDF film on Bi0.8Dy2.2Fe4.3Al0.7O12 substrate, the MCD signals due to (SiPc)2 (635 nm) and Bi0.8Dy2.2Fe4.3Al0.7O12 (450 nm) could be successfully recovered by applying an external magnetic field, which indicates that the addition of PVDF prevents (SiPc)2 from decomposition by pulsed laser irradiation. The switching properties were repeatable, as shown in Figure 5b.


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Figure 4. Diffuse transmission (a) and MCD (b) spectra of the (SiPc)2-PVDF film on Bi0.8Dy2.2Fe4.3Al0.7O12 substrate, as well as the half-difference of the magnetic field dependence of MCD signals between the positive (635 nm) and negative (650 nm) signals for the (SiPc)2PVDF film on Bi0.8Dy2.2Fe4.3Al0.7O12 substrate (c).

Figure 5. (a) MCD spectra of the (SiPc)2-PVDF film on Bi0.8Dy2.2Fe4.3Al0.7O12 substrate at 0 mT before laser irradiation (black solid line) and after laser irradiation (red broken line), as well as that after laser irradiation followed by applying the external magnetic field (blue broken line). (b) The repeatability of the ON/OFF switching of the MCD at 450 nm.


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Discussion In Figure 2f, the MCD signal owing to the (SiPc)2 film on Bi0.8Dy2.2Fe4.3Al0.7O12 substrate exhibited the dispersion-type spectral shapes. On the other hand, our group reported that the Kerr-ellipticity spectra of SiPc(OTHS)2 on Ni or SrO·6Fe2O3 substrate showed an integral-type spectral shape in the Q-band.23 This difference in spectral shapes between the Faraday effect (transmission mode) and Kerr effect (reflection mode) can be explained from the viewpoint of an off-diagonal element of permittivity, εxy (= εxy’ +i εxy”).23 In the case of the transmission mode, the Faraday-ellipticity spectra (∝ (n εxy’ + κ εxy”)/(n2 + κ2), where n and κ denote the refractive index and optical quenching coefficient, respectively) can be transformed into εxy’ because the κ value is negligible. Thus, the dispersion-type Faraday A term can be observed in the Q band, since the excited states are degenerate. On the other hand, εxy” can contribute to the Kerr-ellipticity spectrum in the reflection mode, which results in the difference between the Faraday and Kerr effects. In (SiPc)2 film or (SiPc)2-PVDF film on Bi0.8Dy2.2Fe4.3Al0.7O12 substrate, we succeeded in observing the molecular magneto-optical effect of (SiPc)2 with the magnetic hysteresis even at room temperature, which can be explained by the interfacial magnetic field of the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate. These observations correspond to the second example of molecular magneto-optical effect with magnetic hysteresis due to the interfacial magnetic field of inorganic substrate. Also, this is a further demonstration that the magneto-optical effects of molecular thin films can be induced by the magnetization of magnetic substrate. In the (SiPc)2-PVDF film on Bi0.8Dy2.2Fe4.3Al0.7O12 substrate, the demagnetization of the substrate indicating the randomly oriented magnetizations of each magnetic domain above the


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Curie temperature occurred after the pulsed laser irradiation, which was monitored by the MCD intensity of the magnetic substrate at 0 mT. To the best of our knowledge, this is the first example of the demagnetization of inorganic magnetic substrates based on the photoexcitation of the deposited molecular thin film. This demagnetization of the magnetic substrate results from its high temperature raised by the heat transfer from the photoexcited molecular thin film to the magnetic substrate (Figure 6), which is obviously different from the direct photoirradiation of the magnetic layer in conventional magneto-optical recording media.44 It is noteworthy that although the Bi0.8Dy2.2Fe4.3Al0.7O12 substrate was demagnetized only by the pulsed laser irradiation of the deposited molecular thin film at 650 nm, the demagnetization did not occur by the CW diode laser even when the power (10 mW/mm2) of it is more than that (8 mW/mm2) of the pulsed laser. Thus, the temperature of magnetic substrate could be raised above the Curie temperature only by the pulsed laser irradiation of the molecular thin film. This originates from the high energy peak of nanosecond pulsed laser (7 ns, 10 Hz), which is 107 times higher than that of the CW diode laser if the power and wavelength are similar between them. In this condition, after the pulsed laser irradiation of the Pc-based thin film, the MCD intensity decreased and became 30 % of the initial intensity. That is, 70 % of the magnetic substrate, whose thickness was approximately evaluated to be 1.2 µm, was demagnetized; thus, an efficient depth of the heat transfer from the photoexcited molecular thin film to the magnetic substrate was roughly estimated to be about 0.8 µm.


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Figure 6. A concept of the photoswitching of molecular Faraday effects. The MCD of Pc-based film on a ferrimagnetic substrate without an external magnetic field (a, Signal ON), demagnetization of the ferrimagnetic substrate by photothermal effects of Pcs (b), no MCD of Pc-based films (c, Signal OFF), and magnetization of the ferrimagnetic substrate by applying the external magnetic field (d).


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Conclusions

In this study, we demonstrated the magnetic hysteresis of the MCD of Pc-based thin films on ferrimagnetic Bi0.8Dy2.2Fe4.3Al0.7O12 substrate at room temperature. This corresponds to the first observation of the magnetic hysteresis of molecular Faraday effects on a magnetic substrate. Further, we successfully achieved the demagnetization of ferrimagnetic substrates by selective pulsed-laser irradiation of the Pc-based thin films, which is reasonably interpreted by the photothermal energy transfer from the photoexcited molecular thin film to the ferrimagnetic substrate. To the best of our knowledge, this is the first demonstration of the demagnetization of inorganic magnetic substrates based on the photoirradiation of deposited molecular thin films. Even after the pulsed laser irradiation, the MCD signals were successfully recovered by applying an external magnetic field. This switching property is repeatable; therefore, it should be a promising, key technology for the novel wavelength multiplex recording, which would overcome the memory density limitation related to the diffraction limit of light.


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Acknowledgements

This work was supported by a Grants-in-Aid for Scientific Research (Category B No. 16H04128) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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Magnetic Hysteresis of Molecular Faraday Effects of Phthalocyanine

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