Formation of Nanoparticles of ε-Fe₂O₃ from Yttrium Iron Garnet in a Silica Matrix:  An Unusually Hard Magnet with a Morin-Like Transition below 150 K

Abstract

The elusive ε-Fe2O3 has been obtained as nanoparticles by vacuum heat treatment of yttrium iron garnet in a silica matrix at 300 °C followed by annealing at 1000 °C for up to 10 h in air and employing formamide as a gel modifier. Its nuclear structure is temperature independent as observed from the neutron powder diffraction patterns and has been modeled by the published structures on analogous MM‘O3 compounds. It displays complex magnetic properties that are characterized by two transitions: one at 480 K from a paramagnet (P) to canted antiferromagnet (CAF1) and the second at ca. 110 K from the canted antiferromagnet (CAF1) to another canted antiferromagnet (CAF2) that has a smaller resultant magnetic moment (i.e., smaller canting angle). The latter transition resembles that of Morin for α-Fe2O3 at 260 K. The magnetization shows unusual history dependence: it has a bifurcation below 100 K if the field is applied at low temperatures after zero-field-cooled, whereas the bifurcation is above 150 K if the field is applied at high temperatures. The magnetic hardness first increases slightly from 300 to 200 K, then it drastically decreases to zero at 100 K and follows a further increase down to 2 K. The coercive field reaches an unexpected and quite exceptional 22 kOe at 200 K. There appears to be a further ill-defined metamagnetic transition below 50 K, characterized by a doubling of the measured magnetization in 50 kOe. The AF1−AF2 transition is accompanied by sharp peaks in both the real and imaginary components of the ac-susceptibility due to the hard−soft effect, and their peak maxima shift to lower temperatures on increasing the frequency. Mössbauer spectra are characterized by a change in hyperfine field of the tetrahedral Fe by ca. 40% around the transition, suggesting a change of geometry.

Crystallization and magnetic property of iron oxide nanoparticles precipitated in silica glass matrix

“…γ-phase has a spinel structure and ferrimagnetic behavior with a Curie temperature (T C ) of 928 K 1,4 . Recently, an enormously large room temperature coercivity about 2 T was reported on ε-Fe 2 O 3 phase [5][6][7] , but the value is not reproducible 8,9 . It has an orthorhombic structure and is ferromagnetic with T C = 480 K 6,10 .…”

Unusual Temperature Dependence of Zero-Field Ferromagnetic Resonance in Millimeter Wave Region on Al-Substituted ε-Fe2O3

“…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.…”

“…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].…”

“…However, our research group first succeeded in preparing a pure phase of ε-Fe2O3, which is a rare phase of iron oxide Fe2O3 that is scarcely found in nature [8][9][10]. Since then, its physical properties have been actively studied, and one of the representative properties is the gigantic coercive field (Hc) of 20 kilo-oersted (kOe) at room temperature [11][12][13][14][15][16][17][18]. We have also reported metal-substituted ε-Fe2O3 (ε-MxFe2-xO3, M = In, Ga, Al, and Rh), and showed that this series absorb millimeter waves from GHz at room temperature due to zero-field ferromagnetic resonance (so called natural resonance) [19][20][21][22][23][24][25][26][27][28][29].…”

High-frequency Millimeter Wave Absorber Composed of a New Series of Iron Oxide Nanomagnets

“…In recent years, we firstly succeeded to obtain a single phase of ε-Fe 2 O 3 nanomagnet (Figure 1), and found that ε-Fe 2 O 3 nanomagnet exhibited an extremely large H c value of 20 kOe at room temperature, which is the highest H c value for insulating magnetic materials. [10][11][12][13][14][15][16][17][18][19] In this paper, we report a new millimeter wave absorber composed of ε-Ga x Fe 2-x O 3 (0.10 ≤ x ≤ 0.67) nanomagnets, which shows a natural resonance in the range of 35-147 GHz at room temperature. 20 This is the first example of a magnetic material which shows a natural resonance above 80 GHz.…”