Mossbauer Investigations in Hematite Nanoparticles

Hematite nanoparticles of average size 20 nm were synthesized using the sol-gel method, and the structural characterizations were conducted using XRD and TEM. The XRD profile revealed a small fraction of the maghemite phase and the main hematite phase. Mössbauer spectroscopy was used to study the magnetic structure of the particles and revealed a third but very slight non-magnetic phase. Mössbauer spectrum shows 35% of the nanoparticles exhibiting superparamagnetism. The weighted average Morin transition temperature for the particles determined by Mössbauer is 262 K, which is remarkably similar to the bulk value and higher than the Morin transition determined by VSM (about 250 K). The reported findings on the hematite nanoparticles will help understand the enhanced ferromagnetic behavior of the hematite nanoparticles at room temperature, which is crucial for potential applications.

Enhanced anomalous magnetization in carbonyl iron by Ni+ ion beam irradiation

We investigate the magnetic properties in carbonyl iron (CI) particles before and after Ni$$^{+}$$ + and H$$^{+}$$ + ion beam irradiation. Upon increasing temperatures, the saturation magnetization ($$M_{\text {s}}$$ M s ) in hysteresis loops exhibits an anomalous increase at a high temperature for the unirradiated and the Ni$$^{+}$$ + -beam-irradiated samples, unlike in H$$^{+}$$ + -beam-irradiated sample. Moreover, the magnetization values at low and high temperatures are more intense after Ni$$^{+}$$ + beam irradiation, whereas after H$$^{+}$$ + beam irradiation those are remarkably suppressed. Hematite ($$\alpha $$ α -Fe$$_{2}$$ 2 O$$_{3}$$ 3 ) phase introduced on the surface of our CI particles undergoes the Morin transition that was observed in our magnetization-temperature curves. The Morin transition causing canted antiferromagnetism above the Morin temperature was found in the unirradiated and Ni$$^{+}$$ + -beam-irradiated samples, but not in H$$^{+}$$ + -beam-irradiated sample. It is thus revealed that the CI particles undergoing the Morin transition cause the anomalous increase in $$M_{\text {s}}$$ M s . We may suggest that Ni$$^{+}$$ + ion beam increases uncompensated surface spins on the CI particles resulting in a more steep Morin transition and the intensified $$M_{\text {s}}$$ M s . Ion-beam irradiation may thus be a good tool for controlling the magnetic properties of CI particles, tailoring our work for future applications.

Field-dependent Morin Transition and Temperature-Dependent Spin-flop in Synthetic Hematite Nanoparticles

Background: In nano-size α-Fe2O3 particles, Morin transition temperature was reported to be suppressed. This suppression of the TM in nano-size α-Fe2O3 was suggested to be due to high internal strain and to the enhanced role of surface spins because of the enhanced surface to volume ratio. It was reported that for nanoparticles of diameters less than 20 nm, no Morin transition was observed and the antiferromagnetic phase disappears. In addition, annealing of samples was reported to result in both an increase of TM and a sharper transition which were attributed to reduction in de¬fects, crystal growth, or both. Objective: In this work we investigated the role of applied magnetic field on TM, the extent of the Morin transition, thermal hysteresis, and the spin-flop field in synthetic α-Fe2O3 nanoparticles of diameter around 20 nm. Methods: Hematite nanoparticles were synthesized using sol-gel method. Morphology and structural studies of the particles were done using TEM, and XRD, respectively. The XRD patterns confirm that the particles are hematite with a very small maghemite phase. The average size of the nanoparticles is estimated from both TEM images and XRD patterns to be around 20 nm. The magnetization versus temperature measurements were conducted upon heating from 5 K to 400 K and cooling down back to 5 K at several applied fields between 50 Oe and 500 Oe. Magnetization versus magnetic field measurements between -5 T and +5 T were conducted at several temperatures in the temperature range of 2-300 K. Results: We report three significant findings in these hematite nanoparticles. First, we report the occurrence of Morin transition in hematite nanoparticles of such size. Second, we report the slight field dependence of Morin transition temperature. Third, we report the strong temperature dependence of the spin-flop. Zero-field-cooled magnetization versus temperature measurements were conducted at several applied magnetic fields. Conclusion: From the magnetization versus temperature curves, Morin transition was observed to occur at all applied fields at Morin transition temperature, TM which is around 250 K with slight field dependence. From the magnetization versus magnetic field curves, spin-flop in the antiferromagnetic state was observed and found to be strongly temperature dependent. The results are discussed in terms of three components of magnetic phase in our sample. These are, the paramagnetic, soft ferromagnetic, and hard ferromagnetic components.

Magnetic characterisation of magnetite and hematite from the Blötberget apatite – iron oxide deposits (Bergslagen), south-central Sweden

Rock magnetic measurements were carried out on drill core material and hand specimens from the Blötberget apatite – iron oxide deposit in the Bergslagen ore province, south-central Sweden, to characterise their magnetic properties. Measurements included several kinds of magnetic susceptibility and hysteresis parameters. Petrographic and scanning electron microscopy were used to independently identify and quantify the amount and type of magnetite and hematite. Two hematite-rich samples were studied with laser ablation inductively coupled plasma mass spectrometry to quantify the trace element chemistry in hematite and investigate the potential influence of trace elements on magnetic properties. Three aspects of this study are noteworthy. (1) Hematite-rich samples display strong anisotropy of magnetic susceptibility, which is likely to affect the appearance and modelling of magnetic anomalies. (2) The magnitude-drop in susceptibility across Curie and Néel temperature transitions shows significant correlation with the respective weight percentage of magnetite and hematite. Temperature-dependent magnetic susceptibility measurements can therefore be used to infer the amounts of both magnetite and hematite. (3) Observations of a strongly depressed Morin transition at approximately −60 to −70 °C (200 to 210 K) are made during low-temperature susceptibility measurements. This anomalous Morin transition is most likely related to trace amounts of V and Ti that substitute for Fe in the hematite. When taken together, these magnetic observations improve the understanding of the magnetic anomaly signature of the Blötberget apatite – iron oxide deposits and may potentially be utilised in a broader context when assessing similar (Paleoproterozoic) apatite – iron oxide systems.