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Rapid Faraday Rotation on ε‑Iron Oxide Magnetic Nanoparticles by Visible and Terahertz Pulsed Light Shin-ichi Ohkoshi,*,† Kenta Imoto,† Asuka Namai,† Marie Yoshikiyo,† Seiji Miyashita,‡ Hongsong Qiu,§ Shodai Kimoto,§ Kosaku Kato,§ and Makoto Nakajima§ †

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Department of Physics, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan § Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan Downloaded via UNIV OF NEW ENGLAND on January 15, 2019 at 20:07:13 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Light- or electromagnetic wave-responsive magnetism is an attractive issue in spin chemistry and optical materials science. Herein we show the magnetization reversal induced by visible-light pulsed laser and the ultrafast dynamic magnetooptical effect caused by terahertz (THz) pulsed laser irradiation onto chemically synthesized magnetic films based on gallium−titanium−cobalt-substituted ε-Fe2O3 (GTC-ε-Fe2O3) and ε-Fe2O3 nanoparticles. Visible-light pulsed laser irradiation switches the sign of the Faraday effect in GTC-ε-Fe2O3 films. On the other hand, irradiating the ε-Fe2O3 film with pulsed THz light induces an ultrafast Faraday rotation in an extremely short time of 400 fs. The time evolution dynamics of these ultrafast magnetooptical effects are theoretically demonstrated by stochastic Landau−Lifshitz−Gilbert calculations of a nanoparticle model that considers all motions of the individual spins. These ε-iron oxide magnetic nanomaterials are expected to contribute to high-density magnetic memory media or high-speed operation circuit magnetic devices.

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INTRODUCTION

Research on optical functional nanomaterials is essential to develop high-density recording systems in the big-data era.1−27 Some of the results have been utilized in industrial applications, including optical recording materials such as digital versatile discs (DVDs) and Blu-ray discs.6,7 Research on magnetic materials is attractive because they can be used in magnetic recording systems such as hard-discs and magnetic tapes. High coercive field magnetic nanoparticles are especially important for magnetic recording media.28−32 As per this demand, ε-iron oxide (ε-Fe2O3) has drawn attention as a magnetic filler for next-generation magnetic recording33 because it exhibits a large coercive field34−38 and its size can be reduced to less than 8 nm.39 Currently, the recording density of the magnetic writing technique is reaching its upper limit, and several next-generation writing methods are starting to be investigated40,41 in the field of magnetic recordings. To increase the recording density of the magnetic storage media, light- or electromagnetic wave-assisted magnetic recordings are potential solutions.42,43 In the work described in the present paper, we observed the ultrafast dynamic magnetooptical effects caused by visible-light pulsed laser irradiation or terahertz (THz) pulsed laser irradiation onto chemically synthesized magnetic films based on gallium−titanium− cobalt-substituted ε-Fe2O3 and ε-Fe2O3 nanoparticles. © XXXX American Chemical Society

RESULTS AND DISCUSSION

The target magnetic nanomaterial of formula εGa0.27Ti0.05Co0.07Fe1.61O3 (GTC-ε-Fe2O3) was synthesized by partially arranging the ferrihydrite seed sol−gel method.39 Elemental analysis using inductively coupled plasma mass spectrometry (ICP-MS) indicated that the formula was εGa0.27Ti0.05Co0.07Fe1.61O3. A transmission electron microscopy (TEM) image showed that the particle size was 19 ± 4 nm. The X-ray diffraction (XRD) pattern showed that the sample had an orthorhombic crystal structure in the Pna21 space group (a = 5.0923(6) Å, b = 8.7741(11) Å, c = 9.4611(11) Å) (Figure 1a(i), Figure S1, Table S1). ε-Fe2O3 was also prepared by the ferrihydrite seed sol−gel method.39 TEM images showed that the particle size was 17 ± 5 nm. The XRD pattern indicated an orthorhombic crystal structure in the Pna21 space group (a = 5.0882(8) Å, b = 8.792(1) Å, c = 9.4772(8) Å). Drying a dispersion of magnetic nanoparticles under an applied external magnetic field yields crystallographically oriented magnetic films in a polymer composed of urethane and vinyl chloride, which are resins used in magnetic recording tapes (Figure 1a(ii), see Experimental Section). The orientation degree was evaluated on the basis of the peak intensities of the XRD pattern (Figure 1b, Figure S2, Table S2). From the XRD peak intensities of the oriented sample, the Lotgering factor Received: December 2, 2018

A

DOI: 10.1021/jacs.8b12910 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 2. Density of states, charge density maps, and optical transition probability obtained by first-principles periodic structure calculations. (a) Center is the density of states (DOS) of εGa0.25Ti0.125Co0.125Fe1.5O3. Black line shows the total DOS. Red, green, blue, and light blue lines indicate the partial DOS for Fe 3d, Ti 3d, Co 3d, and O 2p, respectively. Images on the left and right sides show the charge density maps of the energy states of the initial and final states of transitions I* and II*, respectively. Yellow surfaces denote the charge densities with an isosurface of 0.002 a0−3. Enlarged figures show that transitions I* and II* are both metal-to-metal charge transfer from hybridized orbitals of Co2+ and O2− to empty Ti4+ orbitals. (b) Faraday ellipticity spectrum calculated by first-principles calculations where the up-spin transitions and down-spin transitions are summed as positive and negative values, respectively.

Figure 1. Crystallographically oriented magnetic film based on ε-iron oxide and the Faraday spectrum. (a) Crystal structure (i, left), bar graph indicating the occupation of the FeA−FeD sites (i, center), and TEM image (i, right) of GTC-ε-Fe2O3. Scheme to prepare the crystallographically oriented magnetic film (ii). (b) XRD pattern and Rietveld analysis of a GTC-ε-Fe2O3 magnetic film. Red crosses, black line, gray line, and green bars are the observed pattern, calculated pattern, residual pattern, and calculated Bragg reflection, respectively. Inset is the simulated 3-D distribution showing the crystallographic aaxis direction of GTC-ε-Fe2O3. (c) Faraday ellipticity spectrum of the GTC-ε-Fe2O3 magnetic film. Inset is the optical absorption spectrum of the GTC-ε-Fe2O3 in the UV−vis region.

band is attributed to the up-spin state of Co2+ and the bottom of the conduction band is attributed to the down-spin state of Fe3+. The optical transition with the lowest energy gap is the electronic transition between the down-spin states with an energy gap of 2.1 eV (600 nm, I*), which corresponds to transition I in Figure 1c. This is a transition from the valence state of the hybridized orbitals of Co2+ and O2− to the conduction state of the empty Ti4+ orbitals. The transition corresponding to II is an electronic transition from Co2+ to Ti4+ of the up-spin state (2.2 eV, 570 nm, II*). Therefore, transitions I and II both originate from a metal-to-metal charge-transfer (MMCT) transition from Co2+ to Ti4+. Additionally, as optical transitions with higher energies, transition III* (2.7 eV, 460 nm) and transition IV* (3.0 eV, 420 nm) correspond to transitions III and IV in Figure 1c. These peaks are assigned to typical charge-transfer (CT) transitions from O2− to Fe3+ (Figure S5). Figure 2b shows the estimated Faraday ellipticity spectrum obtained by the convolution of all the electronic transitions, where the up-

(F), which is an indicator of the orientation degree, has a high value of 0.84. This implies that the crystallographic a-axis is well oriented perpendicular to the film. The ε-Fe2O3 magnetic film, which was fabricated in the same way, has an F value of 0.87 (Figure S3, Figure S4, Table S3). The Faraday effect was measured by a magnetooptical spectrometer at room temperature. The Faraday ellipticity spectrum shows a negative peak near 600 nm (denoted as I), a positive peak at 520 nm (II), a negative peak at 460 nm (III), and a negative peak at 400 nm (IV) (Figure 1c). This spectrum corresponds to the optical absorption spectrum (inset in Figure 1c). To understand the observed Faraday spectra, firstprinciples calculations of the electronic structure of GTC-εFe2O3 were performed using the Vienna ab initio simulation package (see Experimental Section). Figure 2a shows the density of states (DOS). In this system, the top of the valence B

DOI: 10.1021/jacs.8b12910 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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using 800 nm femtosecond laser light as the probe light (Experimental Section and Movie S2). The peak amplitude of the THz pulse corresponds to a magnetic field of 1.5 kOe. By THz pulsed light irradiation, a rapid change in the Faraday rotation angle is observed. The Faraday rotation shows a positive angle when irradiating with a THz pulse from the Npole direction of the film (Figure 3b(ii)), whereas a negative Faraday rotation peak is observed when irradiating with a THz pulse from the S-pole direction (i.e., when the film is inverted) (Figure 3b(iii), Movie S3). Additionally, the magnitude of the response on the Faraday rotation angle increases as the THz light intensity increases (Figure S6). The observed response on the Faraday rotation is ultrafast. The full width at halfmaximum of the time evolution of the Faraday rotation response is 400 fs. The rise and decay of the Faraday rotation angle occur in the same time range as the width of the fewcycle THz pulse. Such an ultrafast response of the magnetization rotation has yet to be reported. To understand the observed behaviors in response to visiblelight pulsed laser or THz pulsed laser light irradiation, time evolution spin dynamics simulations were carried out, based on a stochastic Landau−Lifshitz−Gilbert (s-LLG) model44−47 for a nanomagnet model composed of 515 spins, considering the motion of the individual spins. The present model uses a spin Hamiltonian (/ ) that considers the magnetic anisotropy (DA) and the external magnetic field (h) on each spin as well as the exchange interaction (J) between the spins (see Experimental Section). In the visible laser light irradiation experiment on the GTCε-Fe2O3 film, the magnetization of the sample was aligned toward the +z-axis, but the external magnetic field was applied along the −z-axis direction. Because the CoII→TiIV MMCT band absorbs 532 nm light, and the absorbed light is converted into the phonon energy of the system, the system is expected to be warmed. Thus, in the theoretical calculation using the sLLG model, we assumed that the temperature of the system instantly increases just after ns-laser irradiation. The calculation results demonstrate that the magnetization reversal is accompanied by precession of the spins. This well reproduces the experimental results (Figure 4a, Movie S4). On the other hand, in the THz light-induced ultrafast Faraday effect by pulsed THz light, the s-LLG model calculation was carried out under the appropriate conditions. In the present optical setup, the magnetization direction of the sample pointed toward the +z-axis direction, and few-cycle intense THz light was irradiated onto the ε-Fe2O3 nanoparticle film in the −z-axis direction (i.e., the x-polarized magnetic field component of the THz pulse) without an external magnetic field. The THz input pulse waveform of the experiment was used in the calculation. The calculation results show that the magnetization along the z-axis instantly decreases near the magnetic field component of the THz pulsed wave, precesses in the xy-plane, and returns to the original status after the THz light passes (Figure 4b, Movie S5). The ultrafast response on the Faraday rotation angle is well reproduced by the s-LLG simulation. Furthermore, the magnitude of the response on the Faraday rotation angle increases as the magnetic component of the THz light increases, which well reproduces the experimental data (Figure S7).

spin transitions and the down-spin transitions are summed as positive and negative values, respectively. The estimated Faraday ellipticity spectrum agrees well with the experimentally observed one (Figure 1c). A 532 nm yttrium−aluminum−garnet (YAG) laser with a pulse width of 10 ns was irradiated onto the GTC-ε-Fe2O3 film under an external magnetic field (H0) of 2.5 kOe inside the Faraday spectrometer (Figure 3a(i)). In the hysteresis loop of

Figure 3. Visible-light laser irradiation and THz light irradiation experiments. (a) Visible-light-laser-irradiation effect observed with the Faraday effect. (i) Schematic illustration of the experiment indicating before (upper) and after (lower) irradiating the GTC-ε-Fe2O3 magnetic film with a 532 nm nanosecond pulsed laser light. (ii) Hysteresis loop of the Faraday ellipticity at 390 nm before light irradiation (gray) and after light irradiation (dark and light green). Intensity of irradiating light is 12.7 mJ pulse−1. (b) THz light irradiation effect observed with the Faraday rotation angle. (i) Schematic illustration of the experiment indicating the irradiation of intense pulsed THz light onto the ε-Fe2O3 magnetic film. Faraday rotation angle versus time when the few-cycle THz light is irradiated onto the sample from the N-pole direction (ii) and the S-pole direction (iii).

the Faraday ellipticity at 390 nm, the coercive field (Hc) value is 4.0 kOe. Before light irradiation, a positive ellipticity signal is observed at 2.5 kOe (Figure 3a(ii), gray line). However, the sign of the Faraday ellipticity value is inverted after pulsed-light irradiation. That is, the magnetization is reversed. Upon remeasuring, the original hysteresis loop of the Faraday ellipticity is restored (Figure 3a(ii), green lines). These results imply that 10 ns pulsed laser light can generate a light-assisted magnetization reversal on ε-iron oxide (Movie S1). Next, a few-cycle THz pulsed light was irradiated onto the εFe2O3 film, where the crystallographic a-axes of the ε-Fe2O3 nanoparticles were directed perpendicular to the film (Figure 3b(i)). The response was measured by the Faraday rotation

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CONCLUSION In summary, we observed ultrafast dynamic Faraday responses on iron oxide magnetic films by visible-light pulsed laser C

DOI: 10.1021/jacs.8b12910 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Article

EXPERIMENTAL SECTION

Preparation of Crystallographically Oriented Magnetic Thin Films. A vehicle solution48 was initially prepared as follows. A urethane resin was mixed with acetylacetone, butyl stearate, and cyclohexanone until it became a homogeneous solution. A vinyl chloride polymer was then added, and the mixture was stirred at 1000 rpm until agglomerations disappeared. The solution was subsequently stirred overnight at 200 rpm to remove the bubbles, followed by dilution with cyclohexanone to obtain the vehicle solution. Magnetic fillers of GTC-ε-Fe2O3 or ε-Fe2O3 were added to the vehicle solution. A mixed solution of toluene and 2-butanone was then added, and the solution was shaken with 0.3 mm diameter zirconia balls. The dispersed solution was dropped onto a polyester film supported on a glass substrate and dried under an external magnetic field inside the superconducting magnet (Figure 1a(ii)). The thickness of the prepared film was 18 μm. First-Principles Calculation of the Electronic Structure and Optical Spectrum. Electronic structure calculations of εGa0.25Ti0.125Co0.125Fe1.5O3 were carried out using the Vienna ab initio simulation package (VASP) program.49 Spin-polarized density functional theory was used as the basis. The exchange-correlation functional was approximated using the generalized gradient approximation parametrized by Perdew−Burke−Ernzerhof. The Hubbard U term was taken into account to describe the Coulomb repulsion of the Fe 3d orbitals, and a U−J value of 5 eV was selected. The basis set was regulated by a cutoff energy of 520 eV, and the kmesh was set to 7×5×5 by the Gaussian method for integration. The magnetooptical transition probabilities of the up-spin and down-spin were obtained from the calculated optical matrix elements, while the sum was calculated using the Gaussian waveform for the transitions between the up-spin states (or between the down-spin states). The structural information, DOS, and band structure of εGa0.25Ti0.125Co0.125Fe1.5O3 are shown in Figure S8, Figure S9, and Table S4. Pulsed THz-Light Irradiation Measurement. The laser pulse for THz generation and the Faraday probe was provided by a femtosecond Ti:sapphire laser source (1 kHz, repetition regenerative amplifier, λ = 800 nm, and 100 fs pulse width). The incident THz pulse was generated from the LiNbO3 crystal pumped with a pulse front-tilted femtosecond laser.50 The transmitted probe pulse passed through a λ/2 waveplate and a Wollaston prism. The polarization change was detected by a pair of photodiodes (Figure S10). Model of the Stochastic LLG Equation. The LLG equation is written as

Figure 4. Mechanism of rapid Faraday rotation by the stochastic-LLG (s-LLG) model calculation. (a) (Upper left) Schematic illustration of visible-light irradiation to the GTC-ε-Fe2O3 magnetic film in the direction of the applied external magnetic field. (Lower left) Calculated time evolution of the z-component of the magnetization by light irradiation. (Right) 3-D trajectory of the time evolution of the total magnetization and snapshots of the spin dynamics before irradiation (I), at the reversal point (II), and after irradiation (III). Small red arrows indicate the individual spins, and large red arrows show the center site spin. (b) (Upper left) Schematic illustration of the THz-light irradiation to the GTC-ε-Fe2O3 magnetic film under a zero magnetic field in the direction of the magnetization of the nanoparticles. (Lower left) Graphs of the time evolution of the total magnetization when the THz pulse is irradiated onto the sample from the N-pole direction. Red, blue, and green indicate the z-, x-, and ycomponents of the magnetization, respectively. (Upper right) Snapshot of the calculated spherical nanoparticle at the time of 0.3 ps. (Lower right) Enlarged snapshots of the calculated spin dynamics at −5 ps (left), 0.3 ps (center), and 4.9 ps (right).

α dM i d M i = − γM i × Hieff + i M i dt Mi dt where γ is the gyromagnetic constant, Mi (≡ |Mi|) and αi are the magnetic moment and the damping constant at the i site, respectively, and Heff i

(=−

∂ /(M1, ∂M i

)

..., MN , t ) is the effective field at the i site.

To introduce a stochastic term, a Langevin noise formalism is adopted into the LLG equation.45,46 Furthermore, random noise is added to eff the effective field, Heff i → Hi + ξi, to give

irradiation or THz pulsed laser irradiation and theoretically demonstrated these phenomena by s-LLG simulations. The observation of a visible-light-assisted Faraday effect reversal indicates that it is possible to record on nanoparticles with a large coercive field by assisting the magnetization reversal by light irradiation. This finding is attractive, since one of the main obstacles for magnetic recording tapes is the difficulty in increasing the writing magnetic field. In addition, the rapid response to THz pulsed light enables writing and erasing times of less than 1 ps. Therefore, ultrafast operations may be realized with a repetition frequency in the THz range. By digitally controlling the power intensity and the phase shift of the THz light, multinary operators to a quantum number (l) of n (l = −n, −(n − 1), ..., 0, ..., n−1, n) are possible.

γ d Mi = − M i × (Hieff + ξi) dt 1 + αi2 αiγ − M i × [M i × (Hieff + ξi)] (1 + αi2)Mi (called the stochastic LLG equation), where ξi is the white Gaussian noise applied at the i site. Since the present systems are magnetic nanoparticles, the calculation model, which includes 515 spins, considers a spherical nanoparticle with 10 paramagnetic atoms aligned in its diameter. The parameters used for the simulation were set on the basis of the exchange interaction parameter (J) as follows. In the simulation of the Faraday effect by visible-light pulsed laser light, the uniaxial magnetic anisotropy constant, the initial temperature, and the maximum value of temperature were 0.027J, 0.56J, and 0.96J, D

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Journal of the American Chemical Society respectively. The values of α, γ, and the effective g-factor were 0.2, 1, and 7, respectively. The external magnetic field was set to 4 kOe. In the present calculation, 1.4J corresponded to the Curie temperature. The initial spin configuration for the simulation was along the magnetic easy axis (a-axis) since the samples were crystallographically oriented along the a-axis. In the simulation of the ultrafast Faraday effect by few-cycle THz light, the uniaxial magnetic anisotropy constant and simulation temperature were set as 0.102J and 0.42J, respectively, while the values of α, γ, and the effective g-factor were 0.5, 1, and 30, respectively. THz magnetic field of the experiment was used for the calculation.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b12910. Detailed procedure and characterization, including Figures S1−S10 and Tables S1−S4 (PDF) Movie S1, visible-light laser irradiation-induced Faraday effect (MPG) Movie S2, optical setup of the THz light-induced Faraday rotation measurement (MPG) Movie S3, rapid Faraday response by THz pulsed light irradiation (MPG) Movie S4, spin dynamics calculated by s-LLG simulations (visible-light laser irradiation) (MPG) Movie S5, spin dynamics calculated by s-LLG simulations (THz pulsed light irradiation) (MPG)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Shin-ichi Ohkoshi: 0000-0001-9359-5928 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present research was supported in part by a JSPS Grant-inAid for specially promoted Research, Grant No. 15H05697, and the “Advanced Research Program for Energy and Environmental Technologies” project commissioned by NEDO of METI. We also recognize the Cryogenic Research Center, The University of Tokyo, Nanotechnology Platform, which are supported by MEXT, and Elements Strategy Initiative Center for Magnetic Materials (ESICMM), National Institute for Materials Science. We are grateful to Prof. H. Tokoro for helpful discussions.

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REFERENCES

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DOI: 10.1021/jacs.8b12910 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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DOI: 10.1021/jacs.8b12910 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX