Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics

[1] In environmental magnetism, rock and mineral magnetic techniques are used to investigate the formation, transportation, deposition, and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes. All materials respond in some way to an applied magnetic field, and iron-bearing minerals are sensitive to a range of environmental processes, which makes magnetic measurements extremely useful for detecting signals associated with environmental processes. Environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics, chemistry, and biology and environmental science, including research on climate change, pollution, iron biomineralization, and depositional and diagenetic processes in sediments to name a few applications. Magnetic parameters are used to routinely scan sediments, but interpretation is often difficult and requires understanding of the underlying physics and chemistry. Thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities, complexities, and limitations to interpretations. In this review, we evaluate environmental magnetic parameters based on theory and empirical results. We describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations. We also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology, chemistry, and physics. Finally, we discuss directions in which environmental magnetism is likely to develop in the future.

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Environmental magnetism: Principles and applications

The dielectric parameters are investigated as a function of composition, frequency, and temperature in copper containing ferrites. An “abnormal” behaviour of the dielectric constant is found and also the loss factor as a function of frequency and temperature in comparison with the “normal” behaviour of spinel ferrites. The origin of this abnormal behaviour is attributed to the presence of Cu1+ ions which determine the appearance of p-carriers in these ferrites.



On a analyse le comportement des parametres dielectriques en fonction de la composition, la frequence et la temperature dans les ferrites qui contiennent du cuivre. On a trouve un comportement „anormal” de la constante dielectrique et du facteur de pertes dielectriques en fonction de la frequence et la temperature par rapport au comportement „normal” pour les ferrites de type spinelle. L'origine de ce comportement anormal est attribue a la presence des ions Cu1+ qui determinent l'apparition des porteurs de type p dans ces ferrites.

Dielectric properties of copper containing ferrites

SUMMARY

Saturation isothermal remanent magnetization (SIRM) and AC susceptibility have been measured as a function of temperature between 5 K and room temperature for one multidomain and three pseudo-single-domain magnetite samples after cooling in a zero (ZFC) and in a strong magnetic field (FC), and also after three partial field coolings (PFC) when a magnetic field had been turned on in 300–150, 150–80 and 80–5 K ranges, respectively. For the multidomain sample, SIRM(5 K) after ZFC is about twice as high as after FC, while the low-field susceptibility is higher after FC. SIRM and susceptibility curves measured after PFC(300–150 K) and PFC(150–80 K) coincide with those measured, respectively, after ZFC and FC. PFC(80–5 K) curves are intermediate between the two extremes. This behaviour can be fairly well understood within the framework of a simple model, introduced back in the 1950s, which assumes that on cooling through the Verwey transition in a strong magnetic field easy magnetization axes in the low-temperature phase are set along the [001] directions of the high-temperature (cubic) phase closest to the field direction. If, on the other hand, the Verwey transition is passed in a zero field and a strong magnetic field is applied below the transition temperature, some of easy axes, initially set at random, can still be switched into the field direction, explaining observed SIRM and susceptibility versus temperature curves measured after PFC(80–5 K). Pseudo-single-domain grains show a more complex behaviour, which depends strongly on sample stoichiometry. In two samples with a relatively small non-stoichiometry (Verwey temperatures are 122 and 110 K, respectively) SIRM(5 K) is higher by 5–7 per cent after FC. After both PFC(150–80 K) and PFC(80–5 K), SIRMs are nearly equal in magnitude to SIRM acquired after FC, but are thermally demagnetized at a different rate below the Verwey transition. Low-field susceptibilities also show different temperature dependences below TV, dependent on a preceding thermomagnetic treatment. A strongly non-stoichiometric sample (TV= 95 K) shows very large, over 70 per cent, difference between SIRMs after FC and ZFC, respectively, and a 35 per cent difference between susceptibilities measured under the same conditions. These results suggest that in magnetite grains several microns in size, easy magnetization axes setting on first passing the Verwey transition from above and switching of easy axes on subsequent cooling below the transition occur, qualitatively, in the same way as in grains of larger size, but switching of easy axes is considerably facilitated. The latter process is subject to further enhancement in strongly non-stoichiometric magnetite, where it seems to be possible even in zero field, resulting in SIRM, susceptibility and magnetic hysteresis properties fairly different from more stoichiometric samples. Strongly depressed, compared with stoichiometric magnetite, magnetocrystalline anisotropy of the low-temperature phase could be a physical mechanism for this behaviour.

Low‐temperature magnetization and AC susceptibility of magnetite: effect of thermomagnetic history

Abnormal dielectric behaviour of copper containing ferrites

A model for the distribution of cations in the spinel lattice of manganese ferrites MnFe2O4+γ
was elaborated on the basis of the experimental studies of the basic magnetic quantities, electrical conductivity and magnetic relaxation of the given ferrites, taking into consideration their crystallographic properties. The conclusions following from this model are in good agreement with the experimental results obtained by us and by other authors both for stoichiometric manganese ferrite and for a ferrite where γ≠0.

The distribution of ions and their valencies in manganese ferrites I. MnFe 2 O 4+γ ferrites

A study was made of magnetic after-effect of the Richter type in samples of non-stoichiometric manganese ferrites differing by their excess of manganese and having a different content of oxygen.

Richter-type magnetic after-effect in manganese ferrite

The initial permeability disaccommodation in ferritesMn x Fe3−xO4+γ, 0·5≦x≦1, was studied in a temperature range around −200°C to +180°C. Four separate bands were found in the relaxation spectrum of these ferrites.

Magnetic relaxation spectrum of ferrites Mn x Fe3−x O4+γ

The behaviour of the initial permeabilityμ of the single crystal magnetite and of the Mn0·5Fe2·5O4 ferrite (Mn-ferrite) is studied by two methods in the 10 to 300 K temperature range at three audio frequencies. Two anomalies in the temperature dependence ofμ were found in magnetite. The first, occurring near the Verwey transition,Tv=121 K, appeared to be independent of the measuring frequency. The other, near 40 K, was found to be frequency dependent and confirmed thus the presence of an electronic type of the relaxation process.

Initial permeability of single crystal magnetite and Mn-ferrite

The magnetic after-effect caused by the diffusion of electrons was studied in detail in MnMg ferrites of the series Mg x Mn1.15−xFe1.85O4+γ; the comparative measurements were also carried out on a sample of MgFe2O4+γ(γ<0). By combining the two methods, (a) investigation of the disaccommodation of initial permeability at different temperatures and (b) measurement of the displacement of the maximum of the temperature dependence tanδ with the frequency, it was possible to study the relaxation processes whose time constants lay between 0.5 sec and several hours, or between 10−4 to 10−7 sec. It was found that the processes taking place at low temperatures and studied by method (a) differ from those observed at high temperatures by method (b), particularly in the lower activation energies and greater dispersion of the time constants. In addition to these two main relaxation processes a weak relaxation superposed over the main disaccommodation was found in ferrites with non-zero content of manganese. An analysis of the intensity of different relaxation effects showed the participation of Mn ions in these relaxation effects and it was found that both main relaxations are probably equivalent as regards the final steady state, to the creation of which they lead; they differ however in the mechanism of electron diffusion by which this state is realized. The main features of the different diffusion mechanisms are discussed.

Study of magnetic after-effect in magnesium manganese ferrites

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