Unusual Temperature Dependence of Zero-Field Ferromagnetic Resonance in Millimeter Wave Region on Al-Substituted ε -Fe 2 O 3

© 2013 Ohkoshi et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Unusual Temperature Dependence of Zero-Field Ferromagnetic Resonance in Millimeter Wave Region on Al-Substituted ε-Fe2O3


Introduction
Insulating magnetic materials absorb electromagnetic waves. This absorption property is one of the important functions of magnetic materials, which is widely applied in our daily life as electromagnetic wave absorbers to avoid electromagnetic interference problems [1][2][3][4][5]. For example, spinel ferrites are used as absorbers for the present Wi-Fi communication, which uses 2.4 GHz and 5 GHz frequency waves. With the development of information technology, the demand is rising for sending heavy data such as high-resolution images at high speed. Recently, high-frequency electromagnetic waves in the frequency range of 30-300 GHz, called millimeter waves, are drawing attention as a promising carrier for the next generation wireless communication. For example, 76 GHz is an important frequency, which is beginning to be used for vehicle radars. There are also new audio products coming to use, applying millimeter wave communication in the 60 GHz region [6,7]. However, there had been no magnetic material that could absorb millimeter waves above 80 GHz before our report on ε-Fe2O3.
In this chapter, we first introduce the synthesis, crystal structure, magnetic properties, and the formation mechanism of the original ε-Fe2O3 [8][9][10]. Then we report the physical properties of Al-substituted ε-Fe2O3, mainly focusing on its millimeter wave absorption properties due to zero-field ferromagnetic resonance. The resonance frequency was widely controlled from 112-182 GHz by changing the aluminum substitution ratio [23]. Furthermore, from a scientific point of view, temperature dependence of zero-field ferromagnetic resonance was investigated and was found to show an anomalous behavior caused by the spin reorientation phenomenon [28].

ε-Fe2O3
This section introduces the synthesis, crystal structure, magnetic properties, and the formation mechanism of ε-Fe2O3. ε-Fe2O3 had only been known as impurity in iron oxide materials, and its properties were clarified for the first time after our success in the synthesis of single-phase ε-Fe2O3 in 2004 [8].

Synthesis, crystal structure, and magnetic properties of ε-Fe2O3
Single-phase ε-Fe2O3 nanoparticles are synthesized by a chemical method, combining reversemicelle and sol-gel techniques ( Figure 1) [8][9][10]16]. In the reverse-micelle step, two reversemicelle systems, A and B, are formed by cetyl trimethyl ammonium bromide (CTAB) and 1butanol in n-octane. Reverse-micelle A contains aqueous solution of Fe(NO3)3 and Ba(NO3)2, and reverse-micelle B contains NH3 aqueous solution. These two microemulsion systems are mixed under rapid stirring. Tetraethoxysilane (C2H5O)4Si is then added to this solution, which forms SiO2 matrix around the Fe(OH)3 nanoparticles through 20 hours of stirring. The precipitation is separated by centrifugation and sintered in air at 1000˚C for 4 hours. The SiO2 matrix is removed by stirring in NaOH solution at 60˚C for 24 hours. With this synthesis method, rod-shaped ε-Fe2O3 is obtained due to the effect of Ba 2+ ions, which adsorb on particular planes of ε-Fe2O3, inducing growth towards one direction. Spherical ε-Fe2O3 nanoparticles can also be synthesized by a different method without Ba 2+ ions, which is an impregnation method using mesoporous silica nanoparticles [17,29,30]. Methanol and water solution containing Fe(NO3)3 is immersed into mesoporous silica and heated in air at 1200°C for 4 hours. The etching process is the same as above.
The crystal structure of ε-Fe2O3 is shown in Figure 2a. It has an orthorhombic crystal structure (space group Pna21) with four non-equivalent Fe sites, A, B, C, and D sites. A, B, and C sites are six-coordinated octahedral sites, and D site is a four-coordinated tetrahedral site. ε-Fe2O3 exhibits spontaneous magnetization at a Curie temperature (TC) of 500 K. Figure  2b presents magnetization versus external magnetic field curve at 300 K, which shows a huge Hc value of 20 kOe. Before this finding, the largest Hc value among metal oxide was 6 kOe of barium ferrite, BaFe12O19 [31], which indicates that the Hc of ε-Fe2O3 is over three times larger. The magnetic structure has been investigated using molecular field theory, which indicated that B and C sites have positive sublattice magnetizations, and A and D sites have negative sublattice magnetizations [32]. This result was consistent with the experimental results from neutron diffraction measurements, Mössbauer spectroscopy measurements, etc. [13,14], and was also consistent with first-principles calculation results [33].

Formation mechanism of ε-Fe2O3
Here we discuss the formation mechanism of ε-Fe2O3 from the viewpoint of phase transformation. By changing the sintering temperature in the present synthesis, a phase transformation of γ-Fe2O3 → ε-Fe2O3 → α-Fe2O3 was observed accompanied by an increase of particle size. γ-and α-Fe2O3 are very common phases of Fe2O3, and it has been well known that γ-Fe2O3 transforms directly into α-Fe2O3 in a bulk form. In the present case, it is considered that ε-Fe2O3 appeared as a stable phase at an intermediate size region due to the large surface energy effect. Free energy of each i-phase (Gi, i = γ, ε, or α) is expressed as a sum of chemical potential (μi) and surface energy (Aiσi): where Ai is molar surface area and σi is surface free energy of a particle. Since, Ai is equal to 6Vm,i/d, where Vm,i and d represent the molar volume and particle diameter, respectively, the free energy per molar volume is expressed as , , This equation indicates that the contribution of the surface energy increases with the decrease of particle diameter. When the parameters satisfy the following three conditions, , the free energy curve for each phase, Gγ/Vm,γ, Gε/Vm,ε, and Gα/Vm,α intersect to form ε-Fe2O3 as the most stable phase at an intermediate d value (Figure 3). Such nanosize effect has also been reported for other metal oxide materials, e.g. Al2O3 [34,35] and Ti3O5 [36].

Al-substituted ε-Fe2O3
In this section, synthesis, crystal structure, and various physical properties of Al-substituted ε-Fe2O3, ε-AlxFe2-xO3, is discussed. Especially, the millimeter wave absorption property by zero-field ferromagnetic resonance is focused.

Synthesis of Al-substituted ε-Fe2O3
ε-AlxFe2-xO3 samples (x = 0.06, 0.09, 0.21, 0.30, 0.40) were synthesized by the same method as the original ε-Fe2O3, using the combination of reverse-micelle and sol-gel techniques. Reverse-micelle A contained aqueous solution of Fe(NO3)3 and Al(NO3)3, and the mixing ratio was adjusted to obtain the different samples, x = 0.06, 0.09, 0.21, 0.30, and 0.40. The sintering temperature was 1050°C for x = 0.06, 0.09, 0.30, and 0.40, and 1025°C for x = 0.21. Only the sample for x = 0 was prepared by an impregnation method using mesoporous silica nanoparticles. The SiO2 matrices for all samples were etched by NaOH solution. The morphology and size of the obtained samples were examined using transmission electron microscopy (TEM), which showed spherical nanoparticles with an average particle size between 20-50 nm ( Figure 4).

Al-substitution effect in crystal structure and magnetic properties
X-ray diffraction (XRD) patterns indicated the samples to have the same orthorhombic crystal structure as the original ε-Fe2O3. The Rietveld analyses of the XRD patterns showed a constant decrease in the lattice constants with the degree of Al-substitution. The analysis results also indicated that the Al 3+ ions introduced in the samples have site selectivity in the substitution. For example, in the x = 0.21 sample, the Al 3+ substitution ratio of each Fe site was 0%, 3%, 8%, and 30% for A, B, C, and D site, respectively. This tendency for the Al 3+ ion to prefer D site was consistent with all of the Al-substituted samples ( Figure 5). This site selectivity can be understood by the smaller ion radius of Al 3+ (0.535 Å) compared to Fe 3+ (0.645 Å) [37]. The Al 3+ ions prefer to occupy the smaller tetrahedral D site than the octahedral A, B, and C sites. The magnetic properties of the samples are shown in Table 1. The field-cooled magnetization curves under an external magnetic field of 10 Oe showed that the TC value decreased from 500 K to 448 K with the increase of Al-substitution ( Figure 6, upper right). From the magnetization versus external magnetic field measurements, gradual change of the hysteresis loops was also observed. The obtained hysteresis loops of x = 0, 0.21, and 0.40 samples are shown in Figure 6. With Al-substitution, the Hc value decreased from 22.5 kOe to 10.2 kOe, and saturation magnetization (Ms) value increased. These changes in the magnetic properties can be explained by the metal replacement of Fe 3+ magnetic ions (3d 5 , S = 5/2) by non-magnetic Al 3+ ions (3d 0 , S = 0). As mentioned previously, ε-Fe2O3 is a ferrimagnet with positive sublattice magnetizations at B and C sites and negative sublattice magnetizations at A and D sites. With the substitution of D site Fe 3+ ions with non-magnetic Al 3+ , the total magnetization increases, leading to the increase of Ms value. In addition, the non-magnetic Al 3+ ions reduce the superexchange interaction between the magnetic sites, resulting in a decrease of TC [32]. In this way, the magnetic properties can be widely controlled by Al-substitution. Unusual Temperature Dependence of Zero-Field Ferromagnetic Resonance in Millimeter Wave Region on Al-Substituted ε-Fe2O3 201

Electromagnetic wave absorption of Al-substituted ε-Fe2O3 by zero-field ferromagnetic resonance
Zero-field ferromagnetic resonance is a resonance phenomenon caused by the gyromagnetic effect induced by an electromagnetic wave irradiation under no magnetic field (Figure 7). This phenomenon is observed in ferromagnetic materials with magnetic anisotropy. When the magnetization is tilted away from the easy-axis by the magnetic component of the electromagnetic wave, precession of the magnetization occurs around the easy-axis due to gyromagnetic effect. Resonance is observed when this precession frequency coincides with the electromagnetic wave frequency, resulting in electromagnetic wave absorption at the particular frequency [38]. This resonance frequency (fr) is proportional to the magnetocrystalline anisotropy (Ha) and can be expressed as where ν is the gyromagnetic ratio. If the sample is consisted of randomly oriented particles with uniaxial magnetic anisotropy, the Ha value is proportional to Hc. Therefore, electromagnetic wave absorption at high frequencies is expected with insulating materials exhibiting large coercivity, which is the case for ε-Fe2O3 based magnets.
With the general electromagnetic wave absorption measurement using free space absorption measurement system, the absorption frequencies of the present ε-AlxFe2-xO3 samples exceeded the measurement range, where the maximum is 110 GHz. Therefore, the absorption measurements were conducted using terahertz time domain spectroscopy (THz-TDS) at room temperature. The THz-TDS measurement system is shown in Figure 8. A mode-locked Ti:sapphire femtosecond pulse laser with a time duration of 20 fs at a repetition rate of 76 MHz was used. The output was divided into a pump and probe beam for the time-domain system. For THz wave emitter and detector, dipole type and bowtie type low-temperature-grown GaAs photoconductive antennas were used, respectively. The sample was set on a sample holder, which was inserted between a set of paraboloidal mirrors concentrating the THz wave at the location of the sample. The temporal waveforms of the electric component of the transmitted THz pulse waves were obtained by changing the delay time between the pump and probe pulses. The temporal waves were Fourier transferred to obtain the frequency dependence, and the absorption spectra were calculated using the following equation: where ( ) t  is the complex amplitude transmittance. An absorption of 20 dB indicates 99% absorption.
The electromagnetic wave absorption spectra are shown in Figure 9. GHz (x = 0.06), and 182 GHz (x = 0). The fr value decreased with Al-substitution, consistent with the behavior of the Hc value ( Figure 6, lower right). The observed electromagnetic wave absorption due to zero-field ferromagnetic resonance at exceptional high frequencies was achieved by the large Ha value of this series with large coercivity.

Temperature dependence of zero-field ferromagnetic resonance in Alsubstituted ε-Fe2O3
Among the ε-AlxFe2-xO3 samples discussed in the previous section, we focused on the x = 0.06 sample and measured the temperature dependence of zero-field ferromagnetic resonance. The Al 3+ substitution ratios of each Fe site in ε-Al0.06Fe1.94O3 are 3%, 0%, 0%, and 11% for A, B, C, and D site, respectively. The magnetic properties of ε-Al0.06Fe1.94O3 are shown in Figure 10. The field-cooled magnetization curve under 10 Oe external magnetic field showed a TC value of 496 K and a cusp at 131 K (= Tp). The cusp in the magnetization is due to the spin reorientation phenomenon, which is known to occur in this temperature region [11,12]. The magnetization versus external magnetic field curve exhibited an Hc value of 19.1 kOe at 300 K. For the THz-TDS measurement, ε-Al0.06Fe1.94O3 powder sample was pressed into a pelletform. The absorption spectra at different temperatures are shown in Figure 11a. These absorption spectra versus frequency were obtained by calibration of the background noise. They were also fitted by Lorentz function. At 301 K, the fr value was 172 GHz, consistent with the result in the previous section. With the decrease of temperature, the fr value gradually increased to 186 GHz at 204 K, and turned to an abrupt decrease down to 147 GHz at 77 K. The fr value continued to decrease with lowering the temperature, and at 21 K, the fr value was 133 GHz (Figure 11b). Temperature dependence was also observed in the linewidth of the absorption spectra. The full width at half maximum (Δf) value increased from 5 GHz at 301 K to 19 GHz at 77 K with decreasing temperature, and then, decreased to 16 GHz at 21 K (Figure 11c).
Temperature dependencies of magnetic hysteresis loop and ac magnetic susceptibility was studied in order to understand the anomalous temperature dependencies of fr and Δf. As mentioned in Figure 10a, the field-cooled magnetization curve shows an increase below TC, but a cusp appears at Tp = 131 K, where the magnetization turns to a decrease. The Hc value increased from 19.4 kOe at 300 K to 22.6 kOe at 200 K, and then decreased to 4.5 kOe at 70 K with the decrease of temperature. The Hc versus temperature plot indicates a sigmoid decrease in a wide temperature range of 200-60 K centered at Tp (i.e., ±70 K from the center temperature, Tp = 131 K) (Figure 12a). In other words, the beginning and ending temperatures of the spin reorientation are about 200 K and 60 K, respectively, with decreasing temperature. The temperature region of the sigmoid decrease of fr almost corresponds to the temperature range of the spin reorientation. The sigmoid increase of Δf was also observed in the spin reorientation temperature region. Figure 12b is the ac magnetic susceptibility versus temperature with frequency of 10 Hz under field amplitude of 1 Oe. As the temperature decreased, the real part of the ac magnetic susceptibility (χ') gradually increased to a maximum value of 3.8 × 10 -4 emu/g·Oe at 60 K and then decreased. The imaginary part (χ'') showed similar temperature dependence with a maximum around 70 K. These temperature dependencies of ac magnetic susceptibility correspond to that of Δf [39,40]. As mentioned previously, the fr value is proportional to the Ha value, and in this case with randomly oriented samples, fr is also related to the Hc value. Therefore, the observed anomalous temperature dependence of fr in ε-Al0.06Fe1.94O3 was understood by the temperature dependence of Hc. The sigmoid decrease centered at Tp originates from the disappearance of magnetic anisotropy due to the spin reorientation phenomenon [11][12][13].

Conclusion
In this chapter, a rare phase of diiron trioxide, ε-Fe2O3, and its Al-substituted series were introduced. The synthesis, crystal structure, and its exceptional physical properties were discussed, especially its huge magnetic anisotropy exhibiting a gigantic coercive field, which enables electromagnetic wave absorption due to zero-field ferromagnetic resonance at high frequencies in the millimeter wave region. Al-substitution effect was observed in the ε-AlxFe2-xO3 series, widely controlling the magnetic properties and the zero-field ferromagnetic resonance frequency: ε-AlxFe2-xO3 absorbed millimeter waves from 112-182 GHz at room temperature. Temperature dependence of zero-field ferromagnetic resonance was also investigated for ε-Al0.06Fe1.94O3 sample, and an anomalous behavior was observed due to spin reorientation phenomenon.
Since ε-AlxFe2-xO3 is composed of very common and low costing elements, it is friendly to the environment and can be economically produced. Its chemical stability is also an