Thermomagnetic Data Research Articles

Petrophysical cobalt exploration indicators in Kuusamo, northern Finland

The increasing global demand for cobalt, a critical green transition metal, encourages the development of improved exploration workflows, as Co typically occurs as a by-product of other metals, and the exploration and processing methods are rarely optimised for it. This paper investigates the possibility of extracting geophysical cobalt exploration indicators from petrophysical properties in Kuusamo, one of the most prominent cobalt-enriched areas in the largest European Co producer, Finland. The study region hosts 20 Au–Co deposits and occurrences classified as orogenic gold deposits with atypical metal association. Two separate mineralization stages have been distinguished: a pyrrhotite-dominated Co-stage and a pyrite-dominated Au–Co stage. The assumption in this study was that Co-enriched samples could be distinguished from Au-enriched samples and host rocks in petrophysical data due to the mineral associations. The investigation used an extensive previously acquired petrophysical and geochemical drill hole dataset from six targets complemented with new thermomagnetic data for magnetic mineral phase detection. The samples were divided into ore classes for the petrophysical data analysis. The results indicate that cobalt’s association with monoclinic pyrrhotite is a key factor for geophysical applications: the high remanent magnetisation characteristic of monoclinic pyrrhotite suggests that Co-enriched ores should typically associate with magnetic anomaly sources. Co enrichment is also related to higher electrical conductivities in comparison to Au. The workflow applied in this study illustrates how key petrophysical parameters can be connected with ore-bearing minerals and rocks, and the results indicate how cobalt could be explored more directly and effectively in the study area.

Microstructural and Magnetic Property Investigations into Antiferromagnetic Mn2FeSi Heusler Alloy for Spintronics Applications

Magnetic materials, possessing low magnetic moment and high spin-polarization capability, are much in demand for spintronics applications these days. In this work, a ternary Mn2FeSi Heusler alloy was prepared by vacuum arc remelting technique followed by annealing it in vacuum at 773 K for 120 h. The crystal structure, chemical composition, and magnetic properties of the alloy annealed at optimum conditions were studied and proved using relevant experimental techniques. The X-ray diffraction analysis revealed the formation of the Heusler phase with a simple cubic structure with a = 5.63 Å in the alloy. This value matches well with that theoretically predicted. The chemical composition analysis of the sample shows that it is highly homogeneous and being closer to nominal. The Curie-Weiss equation was used to fit the thermomagnetic data and the paramagnetic Curie temperature (θ P) calculated, point to the occurrence in the alloy of the antiferromagnetic ordering of spins at low temperature. The magnetic measurements of the alloy showed its antiferromagnetic transformation (TN) at 64.5 K. This study demonstrates that Mn2FeSi Heusler phase may be explored as a low magnetic moment material for spintronics applications.

Thermal Magnetic Properties Variation of Rock During In-Situ Combustion Process

Summary In-situ combustion (ISC) has been proven as a promising technique for the extraction of heavy oils. It has been used in oil fields since the 1920s; however, it is still not as widely used as steam injection. One of the difficulties limiting its wide application is monitoring and controlling the movement of the combustion front. This work is aimed at studying the change in the properties of rock during the ISC process, which is expected to be used for developing an effective monitoring method of the combustion front movement. Rock samples before and after the ISC process were obtained from the Xinjiang Oil field (China) where an ISC industrial pilot has been implemented. In the temperature range of lower than 500℃, the minerals may only alter slightly. Therefore, it is difficult to determine whether the rock was heated or not during the ISC processes using general mineralogical or geochemical methods, for example, X-ray diffraction. This work takes a comprehensive approach to study the variation of rock properties. Magnetic analysis was chosen as the primary method since a very tiny change in the mineral composition during heating leads to profound changes in the magnetic properties. We analyzed magnetic susceptibility (MS), natural remanent magnetization (NRM), hysteresis parameters and thermomagnetic data. In addition, we performed differential thermomagnetic analysis (DTMA) for tracing magnetic minerals based on their Curie temperatures as well as for monitoring transformations in magnetic minerals during heating. Simultaneously, X-ray diffractometer (XRD), optical microscope for thin-sections, and organic content measurements were used as assistive methods to get a comprehensive evaluation on the variation of rock. We found that there is a big difference in magnetic minerals between the initial samples (not subjected to the ISC process) and burned samples from different wells and depths in the ISC pilot. Several magnetic clusters with different coercive force and domain structure were found in these samples. Based on the difference in magnetic properties, we found that the burned samples were heated to different temperatures during the ISC process. In addition, for some rock samples, the heating temperature during the ISC process was determined, and an analysis was made of the propagation of the combustion front. The thermal magnetic properties variation of rock during the ISC process is obvious, which makes it promising to be used for monitoring the propagation direction of the combustion front. Theoretical calculations of magnetic anomalies that occur due to changes in the magnetic properties of rocks during the ISC process indicate the possibility of the detection of such anomalies from the Earth’s surface through high-precision magnetic surveys. The findings in this work provide a theoretical base and direction for developing combustion front monitoring technologies.

Large magnetocaloric effect in DO3-type Fe3Al alloys

We have synthesized DO3-structured Fe3Al binary alloys and studied the magneto-caloric properties. The samples are prepared using a vacuum arc melting furnace and annealed in a vacuum. X-ray diffraction studies confirm ordered cubic DO3 crystallographic phase formation. We have employed Rietveld analysis, to obtain the refined lattiace parameter. The magnetic properties are determined from thermo-magnetic data. The sample exhibited ferromagnetic nature below 879 K marking the transition temperature (Tc). We have derived the change in the magnetic entropy (ΔSM) and calculated the refrigeration capacity (RC) of Fe3Al alloys. This is the first report of relatively large ΔSM recorded over a wide temperature range in binary alloys.

PyTherNal: A python program for analyzing curie temperature from thermomagnetic data

Thermomagnetic analysis is performed by bringing subject materials into its cooled and heated state, followed by analyzing the magnetic moment change. Performing these would result in obtaining the Curie Temperature of the materials, which is essential in estimating magnetic minerals contained in material samples. PyTherNal (Python Thermomagnetic Analyzer) is a thermomagnetic analysis tool in Python environment meant to assist in analyzing thermomagnetic data. The advantages of Python in its functionality and flexibility of being used in any operating system (OS) became the main reason for the program to be written in Python. PyTherNal is designed to assist in estimating Curie temperature of materials through thermomagnetic method, by locating the maximum curvature of the highest value of second (2nd) derivative of both cooling and heating data. To facilitate these, PyTherNal generates three figures, which are the curves for the thermomagnetic data, its 1st derivative, and its 2nd derivative. An advantage of the program is that it performs smoothing to increase the accuracy in estimating the Curie temperature as doing so would significantly minimize the variability of the derivative curve. Since the program is written in Python, it is open-source and therefore free to use. It is also capable of cross-platforming.

The Thermal Stability of Quenched and Partitioned Steel Microstructures

The quenching and partitioning (Q&P) process is a heat treatment process, aiming at the creation of steel microstructures composed of martensite and retained austenite. Herein, the thermal stability of the microstructure is investigated upon reheating steel microstructures, created with different Q&P settings, in different thermal routes, using dilatometry and thermomagnetometry to quantitatively monitor phase fractions. Analysis of the derivative of dilatometry curves and thermomagnetic data reveals that upon reheating the retained austenite decomposes. The decomposition occurs in two stages when the heating rate is relatively low. The retained austenite completely transforms to ferrite and cementite upon reheating to 550 °C, followed by isothermal holding for 1800 s. Increasing the partitioning time from 50 to 300 s at 400 °C after quenching to 260 °C significantly increases the thermal stability of retained austenite. In all conditions, both carbon‐depleted martensite (formed in the initial quenching step) and fresh martensite (formed in final Q&P quenching) are found to be partially tempered during the reheating experiments.

Observation of Cluster Glass and Griffiths‐like Phase in Fe3O4Nanostructures

In the present investigation, the rare occurrence of Griffiths‐like phase (GP) in Fe3O4(magnetite) nanostructures is observed for the first time. Initially, Fe3O4monodispersed nanoparticles and an intercalated ensemble of nanoparticles and nanorods are successfully synthesized by a facile one‐step synthesis. The magnetization, as a function of the applied field, confirms the absence of superparamagnetism in both samples. The thermomagnetic data at cryogenic temperatures show glassy nature, and the alternating current (AC) susceptibility measurements confirm the existence of cluster glass. The low‐field direct current (DC) magnetization measurements of Fe3O4nanostructures show an anomalous magnetic behavior, which indicates the presence of GP above its Curie temperature. This unusual behavior is attributed to the presence of short‐range magnetic correlations and ferromagnetic clusters present in the system due to the size reduction.

Correction to “A procedure to obtain the parameters of curie temperature distribution from thermomagnetic and magnetocaloric data” orginally published as J. non-cryst. solids 520, 119,460 (2019)

• A new method to determine the tc distribution parameters is shown. • Inflexion point from saturation magnetization: T i n f = T C ¯ − 0.73 Δ . . T C • Peak temperature of magnetic entropy change: T p k M C E = T C ¯ − 0.66 Δ . . T C • Peak temperature of paramagnetic susceptibility: T p k χ = T C ¯ + 0.49 Δ T C − 0.0034 . . Δ T C 2 • Temperature dependent mössbauer experiments agree with model predictions.

Magnetic Mineral Populations in Lower Oceanic Crustal Gabbros (Atlantis Bank, SW Indian Ridge): Implications for Marine Magnetic Anomalies

AbstractTo learn more about magnetic properties of the lower ocean crust and its contributions to marine magnetic anomalies, gabbro samples were collected from International Ocean Discovery Program Hole U1473A at Atlantis Bank on the Southwest Indian Ridge. Detailed magnetic property work links certain magnetic behaviors and domain states to specific magnetic mineral populations. Measurements on whole rocks and mineral separates included magnetic hysteresis, first‐order reversal curves, low‐temperature remanence measurements, thermomagnetic analysis, and magnetic force microscopy. Characteristics of the thermomagnetic data indicate that the upper ~500 m of the hole has undergone hydrothermal alteration. The thermomagnetic and natural remanent magnetization data are consistent with earlier observations from Hole 735B that show remanence arises from low‐Ti magnetite and that natural remanent magnetizations are up to 25 A m−1 in evolved Fe‐Ti oxide gabbros, but are mostly <1 A m−1. Magnetite is present in at least three forms. Primary magnetite is associated with coarse‐grained oxides that are more frequent in the upper part of the hole. This magnetic population is linked to dominantly “pseudo‐single‐domain” behavior that arises from fine‐scale lamellar intergrowths within the large oxides. Deeper in the hole the magnetic signal is more commonly dominated by an interacting single‐domain assemblage most likely found along crystal discontinuities in olivine and/or pyroxene. A third contribution is from noninteracting single‐domain inclusions within plagioclase. Because the concentration of the highly magnetic, oxide‐rich gabbros is greatest toward the surface, the signal from coarse oxides will likely dominate the near‐bottom magnetic anomaly signal at Atlantis Bank.

Magnetic fabrics of the neoproterozoic piquiri syenite massif (Southernmost Brazil): Implications for 3D geometry and emplacement

The study of magnetic fabrics and rock magnetic properties, together with geological and structural mapping, was carried out in a syenite pluton to investigate its shape and emplacement history. The Piquiri Syenite Massif (PSM) is an alkaline pluton which exhibits S >> L magmatic fabric and is interpreted to be part of the last Neoproterozoic post-collisional magmatic episodes in southernmost Brazil. Thermomagnetic curves, hysteresis data and coercivity spectra obtained from representative samples of different facies in the massif reveal that magnetic susceptibility is dominated by ferromagnetic minerals, especially magnetite. Magnetic fabric data were determined by using Anisotropy of Magnetic Susceptibility (AMS) and Anisotropy of Anhysteretic Remanence (AARM). Both fabrics are coaxial, and the parallelism of AMS and AARM tensors in more than 84 % of the sampled sites rules out the possibility of significant effects of Single Domain (SD) crystals. The magnetic foliation is concordant with the magmatic foliation field measurements, both parallel to pluton contacts, with high, inward dip angles. The magnetic lineation shows distinct but related behaviour from one facies to another. It is dominantly subvertical in the marginal facies rocks and plunges at moderate to shallow angles in the main facies. It is sub-horizontal in the quartz-syenites and plunges at shallow angles in the granitic rocks. Oxidizing conditions determined from the study of magnetic mineralogy leads to challenge former interpretation of in situ differentiation and crystallization and points to the multi-intrusive character of the pluton. Field relations such as fragments of marginal facies rocks found within the main facies rocks, which are in turn intruded by quartz-syenites, together with the general absence of contact metamorphism except near the marginal facies, lead to interpret that a sequence of magmatic pulses have built up the pluton. Thus, a first magmatic pulse may have heated the host rocks and resulted in the marginal facies which was followed by the next pulses to form the main facies and the quartz-rich varieties, therefore constructing the pluton from outside inwards.

Absolute Paleointensity Study of Miocene Tiva Canyon Tuff, Yucca Mountain, Nevada: Role of Fine‐Particle Grain‐Size Variations

AbstractFine‐grained, Ti‐poor titanomagnetite in the ~12.7 Ma Tiva Canyon (TC) Tuff systematically increases in grain size from superparamagnetic (SP) at the flow base to single domain (SD) at a few meters height. This allows us to examine the role of grain‐size variation on paleointensity, within the transition from SP to stable SD. We present magnetic properties from two previously unreported sections of the TC Tuff, as well as Thellier‐type paleointensity estimates from the lowermost ~7.0 m of the flow. Magnetic hysteresis, frequency‐dependent susceptibility, and thermomagnetic data show that sample grain‐size distribution is dominated by SP in the lower ~3.6 m, transitioning upwards to mostly stable SD. Paleointensity results are closely tied to stratigraphic height and to magnetic properties linked to domain state. SD samples have consistent absolute paleointensity values of 28.5 ± 1.94 μT (VADM of 51.3 ZAm2) and behaved ideally during paleointensity experiments. The samples including a significant SP fraction have consistently higher paleointensities and less ideal behavior but would likely pass many traditional quality‐control tests. We interpret the SD remanence to be a primary thermal remanent magnetization but discuss the possibility of a partial thermal‐chemical remanent magnetization if microcrystal growth continued at T < Tc and/or the section is affected by post‐emplacement vapor‐phase alteration. The link between paleointensity and domain state is stronger than correlations with water content or other evidence of alteration and suggests that the presence of a significant SP population may adversely impact paleointensity results, even in the presence of a stable SD fraction.

A procedure to obtain the parameters of Curie temperature distribution from thermomagnetic and magnetocaloric data

We propose a procedure for the determination of the parameters of the Curie temperature distribution (TCD) in a compositionally inhomogeneous ferromagnetic material. Assuming a Gaussian TCD and using a mean field approach based on the Brillouin function, we report that with respect to the average value of the distribution: a) both inflection point of magnetization, Tinf, and temperature at maximum magnetic entropy change curves, TpkMCE, shift to lower temperatures and b) temperature at maximum paramagnetic susceptibility, Tpkχ, shifts to higher temperatures. Using these evolutions as a function of the TCD broadening and fitting them to a second order polynomial function, a self-consistent procedure to determine the parameters of the distribution is supplied. These predictions have been experimentally tested for a ball milled Fe70Zr30 amorphous alloy using thermomagnetic and magnetocaloric measurements. The obtained parameters using the proposed procedure agree with those directly measured using Mössbauer spectrometry.

The history and state-of-the-art of the studies of native iron in terrestrial and extraterrestrial rocks

A brief review on the history of studying native iron in terrestrial rocks and meteorites is presented. The synthesis and systematization of extensive data on the distribution, composition, and grain size of metal iron particles in sediments, other terrestrial rocks, and meteorites obtained by thermomagnetic analysis with heating to 800 °C combined with microprobe analysis are described. The studies show that ironparticles in all the studied sediments typically have extraterrestrial origin. Based on the statistical analysis of the thermomagnetic data, a number of indications (statistical regularities) are proposed testifying to the overwhelmingly extraterrestrial origin of iron particles in sediments. The similarity of the formation conditions of terrestrial and extraterrestrial metal iron particles makes it possible to suggest a petromagnetic scheme for the structure of planets based on the distribution of metal iron in them.

Sign reversal of magnetization in Sm2CrMnO6 perovskites

Abstract A sign reversal of magnetization is observed in Sm2CrMnO6 synthesized via the citrate-gel method. The material crystallizes in monoclinic crystal structure with P121/n1 space group. The presence of Cr3+ and Mn3+ cations is confirmed by X-ray photoelectron spectroscopy analysis. The transition temperature (TC) is found to be at 50 K from the thermomagnetic data. The magnetization reverses its sign below a particular temperature under field cooled condition and the effect is reduced and disappeared at higher applied magnetic fields. The combined interaction between different magnetic components due to the presence of Sm3+, Cr3+, and Mn3+ leads to the sign reversal of magnetization.

Relative paleointensity (RPI) and age control in Quaternary sediment drifts off the Antarctic Peninsula

Lack of foraminiferal carbonate in marine sediments deposited at high latitudes results in traditional oxygen isotope stratigraphy not playing a central role in Quaternary age control for a large portion of the globe. This limitation has affected the interpretation of Quaternary sediment drifts off the Antarctic Peninsula in a region critical for documenting past instability of the West Antarctic Ice Sheet (WAIS) and Antarctic Peninsula Ice Sheet (APIS). Here we use piston cores recovered from these sediment drifts in 2015 during cruise JR298 of the RRS James Clark Ross to test the usefulness for age control of relative paleointensity (RPI) data augmented by scant δ18O data. Thermomagnetic and magnetic hysteresis data, as well as isothermal remanent magnetization (IRM) acquisition curves, indicate the presence of prevalent magnetite and subordinate oxidized magnetite (“maghemite”) in the cored sediments. The magnetite is likely detrital. Maghemite is an authigenic mineral, associated with surface oxidation of magnetite grains, which occurs preferentially in the oxic zone of the uppermost sediments, and buried oxic zones deposited during prior interglacial climate stages. Low concentrations of labile organic matter apparently led to arrested pore-water sulfate reduction explaining oxic zone burial and downcore survival of the reactive maghemite coatings. At some sites, maghemitization has a debilitating effect on RPI proxies whereas at other sites maghemite is less evident and RPI proxies can be adequately matched to the RPI reference template. Published RPI data at ODP Site 1101, located on Drift 4, can be adequately correlated to contemporary RPI templates, probably as a result of disappearance (dissolution) of maghemite at sediment depths >∼10 m.

Magnetization sign reversal, exchange bias, and Griffiths-like phase in orthorhombic perovskite Pr2FeMnO6

Abstract The perovskite Pr2FeMnO6 has been synthesized by citrate-gel combustion method. The material has an orthorhombic crystal structure with Pbnm space group. The presence of Fe3+ and Mn3+ cations in the material are confirmed by XPS analysis. A magnetization sign reversal is observed for the material at low-temperatures in low applied magnetic fields. When the applied field or temperature is increased, the negative magnetization got diminished. Such a sign reversal of magnetization is observed for the material as a result of the interaction between components of paramagnetic Pr3+ and Mn3+ moments opposite to the component of the ferromagnetic Fe3+ moment or the anti-aligned Fe-rich regions to that of Mn-rich regions containing with Pr3+. Negative exchange bias is also exhibited by Pr2FeMnO6 below 200 K which can be ascribed to the Dzyaloshinskii-Moriya interaction and magnetic exchange anisotropy. A high-temperature magnetic transition at TC = 576 K is identified for the material from the thermomagnetic data. From the detailed analysis of the thermomagnetic data, the presence of ferromagnetic short-range correlations is observed far above the TC, which suggests a Griffiths-like phase with Griffiths temperature at TG = 681 K.

Magnetic and magnetocaloric properties of [formula omitted

We investigate the crystal, magnetic and magnetocaloric properties of the Fe5−xMnxSn3 series (0≤x≤5) from X-ray diffraction and DC magnetization experiments. The alloys crystallize with the lacunar Ni2In type of structure (P 63/mmc). The diffraction patterns of the richer Fe alloys (x≤2) contain tiny superstructure lines due to ordering of T atoms and vacancies within the partially filed 2d position. The Curie temperatures decrease in a non-monotonous manner with the Mn content from 544K in Fe5Sn3 down to 190K in Fe1Mn4Sn5, before increasing again up to 219K in Mn5Sn3. While most of the alloys behave as simple ferromagnets, low-field thermomagnetization data show that the Fe5−xMnxSn3 alloys with intermediate compositions (x=2.5 and 3) undergo two close ferromagnetic-like transitions. The magnetocaloric effect is of moderate magnitude (~1Jkg−1K−1 for μ0ΔH=2T). The results are compared with previously published data and briefly discussed in view of potential high-temperature magnetocaloric applications.

Mechanical activation on aluminothermic reduction and magnetic properties of NiO powders

We report the mechanically activated aluminothermic reduction of NiO [NiO–Al(x wt.%) with x = 0, 20, 40] into NiO–Ni–Al2O3 nanocomposites using high-energy planetary ball milling under dry milling and the resulting structural and magnetic properties. Structural studies reveal that both NiO and NiO–Al powders exhibit a face centered cubic structure with large crystal size reduction. However, the NiO–Al milled powders unveil the process of aluminothermic reaction kinetics, which changes from gradual reaction as a function of milling time for x = 20 powders to self-propagating combustion reaction for x = 40. This allows us to achieve a maximum NiO reduction of 40% and 90% for x = 20 and 40, respectively. The process of NiO reduction by Al is further confirmed through thermal studies. Pure NiO shows an antiferromagnetic (AFM) nature, which transforms into a ferromagnetic (FM) one with the moderate magnetization of about 1 emu g−1 with decreasing crystal size. The formation of FM Ni from AFM NiO matrix in milled NiO–Al powders could be precisely monitored by the change in the magnetization, which increases up to 4 emu g−1 and 28 emu g−1 for the gradual and combustion reactions, respectively. This results in a considerable exchange bias and its magnitude strongly depends on the relative fractions of NiO and Ni phases. Thermomagnetization data confirm the presence of mixed magnetic phases and the component of induced FM phase fades out due to the formation of Ni from the reduction of NiO. The changes in the structural and magnetic properties of milled NiO–Al powders are discussed on the basis of milling time-dependent mechanically activated reduction reaction of NiO into NiO–Ni–Al2O3 nanocomposites. The process of mechanical activation on the aluminothermic reduction allows for a controlled reduction of NiO; thus, it is suitable for the applications in catalysis and the ore reduction process.

Microstructures and magnetic fabrics of the Ngaoundéré granite pluton (Cameroon): Implications to the late-Pan-African evolution of Central Cameroon Shear Zone

The Ngaoundéré granite pluton, in Central-North Cameroon, located near the Central Cameroon Shear zone (CCSZ), and previously studied for its petrography and geochemistry, is characterized by the absence of macroscopic markers of deformation. In this study, we report microstructures and magnetic fabrics (AMS) of this pluton and discuss the relationship with the Pan-African evolution of the CCSZ. The pluton consists of a porphyritic Hbl-Bt-monzogranite at its rim and a porphyritic biotite-granite at its core, a petrographic distribution denoting a normal zoning pattern, i.e. more silicic toward the centre. As expected, magnetic susceptibilities values also exhibit a zoning pattern in agreement with petrographic zonation. Thermomagnetic data indicate that this pluton is dominantly ferromagnetic in behaviour. As indicated by its microstructures, the pluton has suffered a continuum of deformation from the magmatic state to the high temperature solid-state during magma crystallization and solidification. The magnetic foliations dominantly strike NE-SW and dip moderately to steeply and the lineations mostly plunge shallowly to the NE or SW, roughly parallel to NE-to ENE-trending Central Cameroun Shear Zone (CCSZ). The foliation poles define a girdle pattern with a zone axis (52°/11°) rather close to the best line of the lineations (44°/21°). These fabrics correlate with the structures of the country rocks ascribed by several workers to a regional transpression. Toward the margins of the pluton, particularly the northern one, the lineations tend to rotate from NE to N in azimuth. This change is interpreted as due to strain partitioning, simple shearing with NE-SW extension being relayed by compression toward the northern pluton border. This new magnetic fabric study suggests that the Ngaoundéré pluton (poorly dated at c. 575 Ma) was emplaced during the late stages of the CCSZ dextral transpressive movement. It also provides some more constraints on the correlation between the CCSZ system and the shear zone system of NE-Brazil.

Phase transformations of siderite ore by the thermomagnetic analysis data

Thermal decomposition of Bakal siderite ore (that consists of magnesium siderite and ankerite traces) was investigated by thermomagnetic analysis. Thermomagnetic analysis was carried-out using laboratory-built facility that allows automatic registration of sample magnetization with the temperature (heating/cooling rate was 65°/min, maximum temperature 650°C) at low- and high-oxygen content. Curie temperature gradually decreases with each next cycles of heating/cooling at low-oxygen content. Curie temperature decrease after 2nd cycle of heating/cooling at high-oxygen content and do not change with next cycles. Final Curie temperature for both modes was ~320°C. Saturation magnetization of obtained samples increases up to 20Am2/kg. The final product of phase transformation at both modes was magnesioferrite. It was shown that intermediate phase of thermal decomposition of Bakal siderite ore was magnesiowustite.