[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

Polarization-dependent x-ray absorption measurements were performed on crystalline ZnO three-dimensional arrays consisting of highly oriented microrods as well as on particulate thin films consisting of monodisperse spherical nanoparticles Strong anisotropic effects have been observed for the highly oriented ZnO rods, but not for the isotropic spherical nanoparticles Full-potential calculations of the orbital-resolved x-ray absorption of a ZnO wurtzite periodic crystal, including Zn 3d among the valence states, show very good agreement with the experimental findings Comprehensive fundamental knowledge of the electronic structure of ZnO is obtained by probing and demonstrating the orbital symmetry of oxygen and its contribution to the conduction band of this important II–VI semiconductor

https://iopscience.iop.org/article/10.1088/0953-8984/14/28/308/pdf

Polarization-dependent soft-x-ray absorption of highly oriented ZnO microrod arrays

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

Electrical permittivity theory has been applied to the interpretation of data describing the anomalously high Faraday rotation and ellipticity in the Y3−xBixFe5O12 ferrimagnetic system. By the use of exact forms of the basic phenomenological equations for the off‐diagonal tensor element e1 = e’1 + ie‘1, the measured Bi3+ influence on the separate component e’1 and e‘1 spectra from 1 to 5 eV has been closely fitted to theory by the superposition of three Fe3+ (diamagnetic) electric dipole transitions occurring at 2.6, 3.15, and 3.9 eV, respectively. The strong transition at 3.15 eV that is believed to originate from Fe‐O6 molecular complexes of the octahedral sublattice may also have a smaller companion peak in the vicinity of 5 eV. Transition bandwidths and excited‐state splittings determined from the matching of theory to experiment both indicate that strong Bi covalent interactions exist with the Fe‐O6 and Fe‐O4 complexes. These results also confirm that the enhanced magneto‐optical effects in the longe...

Spectral origins of giant Faraday rotation and ellipticity in Bi-substituted magnetic garnets

We have clarified the origin of a magnetically dead interface layer formed in yttrium iron garnet (YIG) films grown at above 700 \ifmmode^\circ\else\textdegree\fi{}C onto a gadolinium gallium garnet (GGG) substrate by means of laser molecular beam epitaxy. The diffusion-assisted formation of a Ga-rich region at the YIG/GGG interface is demonstrated by means of composition depth profiling performed by x-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and x-ray and neutron reflectometry. Our finding is in sharp contrast to the earlier expressed assumption that Gd acts as a migrant element in the YIG/GGG system. We further correlate the presence of a Ga-rich transition layer with considerable quenching of ferromagnetic resonance and spin wave propagation in thin YIG films. Finally, we clarify the origin of the enigmatic low-density overlayer that is often observed in neutron and x-ray reflectometry studies of the YIG/GGG epitaxial system.

/pdf/role-of-gallium-diffusion-in-the-formation-of-a-magnetically-528bv6ubwi.pdf

Role of gallium diffusion in the formation of a magnetically dead layer at the Y 3 Fe 5 O 12 /Gd 3 Ga 5 O 12 epitaxial interface

Analysis of cobalt contributions to the Faraday rotation spectra in the series of (YCa)3(FeCoGe)5O12 garnet films enabled us to distinguish between single contributions of Co2+ and Co3 ions in tetrahedral sites. It appeared that even the high counterdopes of calcium do not suppress the occurrence of tetrahedral Co2+ ions, although their concentration is greatly reduced in comparison with the original samples not containing any Ca. Noteworthy is also a rather high content of tetrahedral Co3+ in the samples not containing calcium even in cases when Ge content exceeds that of Co ions.

Optical and magnetooptical properties of Co-doped YIG films

The main results of the present paper can be summarized as follows:

a)

By choosing suitable PbO-B2O3-Bi2O4 melts combined with appropriategrowth conditions it is possible to reduce significantly the content of lead atoms in garnet films leading to the incorporation of approx. 0·1 Pb per one Bi atom (y/x≈ ≈ 0·1).




b)

Account of the interference of the respective YL γ2,3, BiM α1 and PbM α1, PbM β1 X-ray lines in the EPMA method enables sufficiently exact determinations of Bi and Pb contents in garnet films.




c)

Determination of Bi contents in thin garnet films (1–2 μm) is made possible by measuring the FR near 500 nm. The magnitude of the minumum of the specific Faraday coefficient determines the Bi content and it has been shown that this value is not influenced by the presence of Pb ions.

Correlation between Faraday rotation and bismuth and lead contents in LPE YIG films

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 influence of Bi and Pb substitution on the Faraday rotations of the yttrium iron garnet LPE films was studied in the visible part of the spectral region. By an adequate choice of the flux compositions and the growth conditions the three series of samples differing in the Pb/Bi ratios were prepared. The differences in the Faraday rotation coefficients of the properly chosen compositions enabled a determination of the individual contributions of Bi and Pb ions to the spectral behaviour of the Faraday rotation of pure YIG. The fitting procedures based on the phenomenological description of the Faraday rotation in terms of the paramagnetic and diamagnetic transitions appeared to be a reliable description of the behaviour of all the samples studied.

Contributions of Bi and Pb ions to Faraday rotation of YIG

Polar Kerr rotation spectra in the Mn- and Ti-ferrite systems

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