Low Temperature Phase Of Magnetite Research Articles

Intrinsic and extrinsic x-ray absorption effects in soft x-ray diffraction from the superstructure in magnetite

We studied the ($00\frac{1}{2}$) diffraction peak in the low-temperature phase of magnetite (${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$) using resonant soft x-ray diffraction at the $\mathrm{Fe}\ensuremath{-}{L}_{2,3}$ and $\mathrm{O}\ensuremath{-}K$ resonances. We studied both molecular-beam-epitaxy grown thin films and in situ cleaved single crystals. From the comparison, we have been able to determine quantitatively the contribution of intrinsic absorption effects, thereby arriving at a consistent result for the ($00\frac{1}{2}$) diffraction peak spectrum. Our data also allow for the identification of extrinsic effects, e.g., for a detailed modeling of the spectra in case a dead surface layer is present that is only absorbing photons but does not contribute to the scattering signal.

Low-temperature structure of magnetite studied using resonant x-ray scattering

We propose a model for the Fe atomic displacements in the low-temperature phase of magnetite $({\text{Fe}}_{3}{\text{O}}_{4})$, based on the analysis of the photon energy dependence of the scattered intensity of selected reflections in a resonant x-ray scattering experiment. The symmetry of the displacement pattern is forced to be consistent with the $Cc$ space group, long time claimed to be the actual symmetry of the low-temperature phase. Fe positions at octahedral sites and the corresponding charges are accounted for by a fitting procedure comparing simulations and experiment. We found a pattern of small distortions in the $a\text{\ensuremath{-}}b$ plane. An independent sensitivity to the charge occupancy permits to refine the model of charge ordering previously proposed. Finally we have computed the electric moment of the combined charge displacements to be $1.5\text{ }\ensuremath{\mu}\text{C}/{\text{cm}}^{2}$.

Twinning control of magnetic properties of multidomain magnetite below the Verwey transition revealed by measurements on individual particles

To elucidate mechanisms controlling the magnetic properties of multidomain magnetite below the Verwey transition, we have conducted magnetization measurements between 5 and 300 K as a function of temperature and of pre-treatment magnetic field strength, ranging from zero to 5 T, on two individual magnetite grains of ∼75 μm (grain 1 thereafter) and ∼120 × 80 × 40 μm (grain 2) in size. Both grains show a well-defined Verwey transition, with the transition temperatures (TV) of (123.33 ± 0.02) K on cooling versus (123.53 ± 0.02) K on heating for grain 1 and (116.33 ± 0.06) K versus (116.69 ± 0.08) K for grain 2, indicative of nearly stoichiometric magnetite. The principal experimental observations can be summarized as follows: (1)At 10 K both the coercive force and saturation remanence (SIRM) are higher after cooling through the Verwey transition temperature in zero magnetic field (ZFC), compared to cooling in a strong field (FC). This indicates that during ZFC—as opposed to FC—the grains are subdivided into regions with different easy magnetization axis directions (transformation twins).(2)Cooling in fields not exceeding certain critical value Hl (∼60 mT for grain 1 and ∼20 mT for grain 2) leads to ZFC-like LT SIRM with a SIRM maximum around Hl. In contrast, cooling in higher fields produces FC-like SIRM. The transition into FC regime is quite sharp in grain 1, but gradual in grain 2.(3)Below TV more numerous and larger jumps occur in magnetization versus temperature curves than above TV. This suggests that the pinning sites are formed on cooling through the transition and must, therefore, be caused by twinning. The observed diminishing of jumps with increasing cooling field also fits into this picture.(4)The fully stable FC state only forms after cooling in a strong field down to 60 K in grain 1, and down to 90 K in grain 2. Switching off the field at higher temperatures (but below TV) apparently still leads to twinning, probably of magnetostrictive origin. At 10 K both the coercive force and saturation remanence (SIRM) are higher after cooling through the Verwey transition temperature in zero magnetic field (ZFC), compared to cooling in a strong field (FC). This indicates that during ZFC—as opposed to FC—the grains are subdivided into regions with different easy magnetization axis directions (transformation twins). Cooling in fields not exceeding certain critical value Hl (∼60 mT for grain 1 and ∼20 mT for grain 2) leads to ZFC-like LT SIRM with a SIRM maximum around Hl. In contrast, cooling in higher fields produces FC-like SIRM. The transition into FC regime is quite sharp in grain 1, but gradual in grain 2. Below TV more numerous and larger jumps occur in magnetization versus temperature curves than above TV. This suggests that the pinning sites are formed on cooling through the transition and must, therefore, be caused by twinning. The observed diminishing of jumps with increasing cooling field also fits into this picture. The fully stable FC state only forms after cooling in a strong field down to 60 K in grain 1, and down to 90 K in grain 2. Switching off the field at higher temperatures (but below TV) apparently still leads to twinning, probably of magnetostrictive origin. The above experimental evidence strongly suggests that two different mechanisms of twin formation act in the low-temperature phase of magnetite: (i) transformation twinning immediately related to cooling through TV and (ii) magnetostrictive deformation twinning below TV.

Resonant X-Ray Diffraction Studies on the Charge Ordering in Magnetite

Here we show that the low temperature phase of magnetite is associated with an effective, although fractional, ordering of the charge. Evidence and a quantitative evaluation of the atomic charges are achieved by using resonant x-ray diffraction (RXD) experiments whose results are further analyzed with the help of ab initio calculations of the scattering factors involved. By confirming the results obtained from x-ray crystallography we have shown that RXD is able to probe quantitatively the electronic structure in very complex oxides, whose importance covers a wide domain of applications.

Domains in the low-temperature phase of magnetite from synchrotron-radiation x-ray topographs

Domains in the low-temperature, ferrimagnetic, and ferroelectric, phase of magnetite ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ were investigated in the bulk, via their ferroelastic distortion, by means of synchrotron radiation x-ray topography, complemented by Nomarski optical microscopy on the surface. After cooling through the Verwey transition under a magnetic field parallel to [001], the images show a hierarchy of domains. Domains associated with a small difference in lattice distortion provide a clear indication that the symmetry is triclinic. They appear as a substructure of monoclinic domains separated by walls creating no long-range stress. The domain structure deduced from the observation, under the experimentally self-tested assumption that the walls involved are stress free, is favorable from the elastic, magnetostatic, and electrostatic points of view.

Effect of uniaxial KU term in magnetocrystalline anisotropy energy on magnetoelectric polarization in low temperature phase of magnetite (Fe 3O 4)

Phenomenological theory of the magnetoelectric effect is applied to the monoclinic magnetic anisotropy energy, which procedure makes clear the effect of the K u term in the energy on magnetoelectric polarization. Based on this elucidation, a possible interpretation is proposed for the anomaly observed below 10 K in the temperature dependence of magnetoelectric polarization of magnetite.

Magnetocrystalline anisotropy of Fe 3(1−δ)O 4, 0 ⩽ δ < 0.01

The magnetocrystalline anisotropy of magnetite with varying metal to oxygen ratio was investigated. Homogeneous single-crystal specimens of Fe 3(1−δ)O 4 with 0 ⩽ δ < 0.0093 were prepared. The magnetization curves were measured along the principal crystal axes both above and below the Verwey transition. Anisotropy constants were determined by comparing calculated magnetization curves with experimental data. The magnetocrystalline anisotropy energy for the low temperature phase of magnetite was found to vanish at the δ value where the Verwey transition disappears.

Abrupt Change of NMR Line Shape in the Low Temperature Phase of Fe3O4

Temperature dependences of the NMR line shape were measured in the low temperature phase of magnetite. For the sample which was cooled to 4.2 K in an external magnetic field, we found an abrupt change of line shape at about 26 K. This phenomenon must be caused by the changing of an electron ordering structure in the magnetic domain walls and may be explained by use of a pinning plane model. That is, in the magnetically stabilized structure, a kind of plane in which the electron ordering structure is “abnormal” pins the major part of magnetic domain walls. The plane structure changes at 26 K and an ordinary structure of the magnetic domain is reconstructed drastically by the release of the magnetic domain walls from the plane.

Magnetocrystalline anisotropy of magnetite

Magnetocrystalline anisotropy of the low temperature phase of magnetite has been explained by considering the additional energy change due to the magnetoelectric effect.

Evolution of research in magnetism in Japan

After describing historical achievements by J. A. Ewing, K. Honda, S. Kaya and their coworkers, important inventions, research, and improvement of various magnetic materials which have been made in Japan are introduced. Then physical studies of various magnetic phenomena, such as domain observations, atomic order and induced anisotropies, the Invar Problems, the low temperature phase of magnetite, and studies related to other magnetic compounds are described. Finally achievements on magnetic theories mainly by T. Nagamiya and his disciples are briefly introduced.

Investigation of phonon softening in the low-temperature phase of magnetite

(1980). Investigation of phonon softening in the low-temperature phase of magnetite. Philosophical Magazine B: Vol. 42, No. 3, pp. 427-427.

Magnetoelectric Effect of Fe3O4 at 77 K. I. Crystal Symmetry

Magnetoelectric effect of the low temperature phase of magnetite was measured at 77 K. It was confirmed that the magnetic crystal symmetry at 77 K is triclinic but the breaking of the mirror symmetry parallel to the (1\bar10) plane is very small. Dispersion of the magnetoelectric effect was observed with the relaxation time of approximately 2 µs. Magnitude of magnetoelectric susceptibility depends on the direction of the applied electric field. Both phenomena were attributed to the dispersion and the anisotropy of the electric susceptibility. The structure of the low temperature phase was discussed briefly in connection with the model proposed by Mizoguchi (J. Phys. Soc. Jpn. 44 (1978) 1512).

Ionic ordering in the low-temperature phase of magnetite

Ionic ordering in the low-temperature phase of magnetite

Specific Heat and Electrical Conductivity of Low temperature Phase of Magnetite

It was observed for Fe 3 O 4 that the presence of residual stresses lowers the so-called Verwey temperature, T v , considerably, broadens the temperature-width of the anomalous specific heat at T v , produces another additional peak and decreases the discontinuous conductivity change at T v . The entropy change at the phase transition was determined to be 5.4 J /mole·deg but when the effect of short range order is included it becomes 9.6 which is much larger than the value expected for electron hopping under the Anderson's restriction. The anisotropic conductivity was measured below T v along the monoclinic axes a , b and c . The magnitude of conductivity was in the order of σ c >σ b >σ a in the range of T v > T >60 K, while it changed to σ b >σ a >σ c below 60 K. The conductivity along [\bar111], or the longest <111>, was also measured by using a squeezed crystal in which a monoclinic single domain is realized. This direction was least conductive among four <111>'s in the whole temperature range.

X-Ray Diffraction Study on the Low Temperature Phase of Magnetite

X-ray diffraction experiments of a single crystal of magnetite at 84 K were carried out to investigate the crystal symmetry below the transition temperature. A new lattice deformation which makes the crystal symmetry lower than rhom-bolaedral was clarified. The extra reflections which appear in the low temperature phase were observed for the first time with X-ray diffraction. The extinction of (4, 4, \(\pm\frac{1}{2}\)) reflections, which was found by Iizumi and Shirane by means of neutron diffraction, was also confirmed. All the experimental results can be explained reasonably by assuming that the low temperature phase of magnetite has monoclinic symmetry, and the space group is either C s 4 - C c or C 2 h 6 - C 2/ c .

Magnetocrystalline Anisotropy of Low Temperature Phase of Magnetite

The magnetocrystalline anisotropy of low temperature phase of magnetite (Fe 3 O 4 ) was measured for a monoclinic single phase specimen by using computerized fully-automatic torque magnetometer. The anisotropy is expressed by \begin{aligned} E_{\text{a}}=K_{a}\alpha^{2}_{a}+K_{b}\alpha^{2}_{b}+K_{aa}\alpha^{4}_{a}+K_{bb}\alpha^{4}_{b}+K_{ab}\alpha^{2}_{a}\alpha^{2}_{b}-K_{u}\alpha^{2}_{111}, \end{aligned} where α a , α b and α 111 are direction cosines of the magnetization with respect to the monoclinic a -, b - and cubic [111] axes, respectively, the last of which coincides with the longest cube diagonal. The values of the anisotropy constants at 4.2 K are: K a =25.5, K b =3.7, K u =2.1, K a a =1.8, K b b =2.4 and K a b =7.0 in 10 5 erg/cm 3 . It was found that K a , K b and K u exhibit the temperature dependence of an activation type with the activation energy of about 0.02 eV. It was also found that the constants K a a , K b b and K a b are well expressed in terms of cubic K 1 . The mechanism of the an...

Detailed Mössbauer Hyperfine Parameters of the Low Temperature Phase of Fe3O4

Mossbauer spectra of low temperature phase of Magnetite (Fe 3 O 4 ) has been studied by changing θ(the cooling-field angle measured from [001] to [1\bar10] within the (110) plane). Data were analyzed for θ=0° and 40° cases with a computer and new assignments have been obtained.

Neutron Scattering from Low-Temperature Phase of Magnetite

A high resolution neutron scattering study has been carried out on the charge ordering of magnetite below 123 K. Magnetic components were separated out by the application of magnetic field perpendicular to the c -axis which had been aligned by field cooling. Non-zero magnetic scattering was observed at (2, 0, l +1/2) referred to the cubic lattice, but not at (0, 0, 2), in contradiction to the previous report by Hamilton. This magnetic intensity distribution suggests the alternate Fe 2+ and Fe 3+ in the a b plane, in contrast to the Verwey model of the single modulation along the c -axis. Substantial disagreement remains between observed magnetic intensities and simple model calculations and further experiments are suggested to resolve this difficulty.

Crystal symmetry of the low temperature phase of magnetite

A neutron diffraction study of magnetite below the Verwey transition (123 K) reveals that the superlattice reflections are observed for ( hhl), with l = integers +1 2 but not for ( hhl). This determines a (110) glide plane in the direction of [100] and implies that only one of the doubly degenerate modes, proposed recently by Yamada, condenses at the transition.