A palaeomagnetic study has been carried out on Vesuvian lava flows emplaced since 79 AD. This involved both palaeodirection and palaeointensity investigations of samples from sites on the W, S and SE slopes of the volcano. Thermal demagnetization of 3 component IRMs, susceptibility measurements and coercivity analyses have been carried out on one pilot specimen per site in order to identify the magnetic carriers and to estimate the magnetic grain size. The identificatìon of the primary direction of TRM was carried out following very stringent criteria (Incoronato, 1996). Palaeointensities were evaluated using both a modification of the Modified Thellier & Thellier method (McClelland et al., 1996) and the innovatìve microwave technique (Shaw et al., 1999). This study has shown that establishing whether or not different exposures or flows are contemporaneous can be established and, in most, but not all, cases can be undertaken successfully using magnetic information recorded by Vesuvian lavas to define the geomagnetic field direction and intensity at the time of their eruption. It is shown that numerous lava flows, outcropping on the W to S slopes of the volcano, must be associated to a large eruption in AD 1631, confirming some previous studies. A new age for a lava flow, ascribed in literature to the 1697 event, is suggested on the basis of both palaeodirection and palaeointensity investigations. Significantly different properties have been found between microwave and thermal experiments although they showed an exceptional level of agreement for the AD 1631 lava flow. In general the trend the palaeointensity variations was similar to that obtained by thermal processes for the last few centuries from other European, Mediterranean and Near East regions (Aitken et al., 1989).

No description available.

This study examines the tectonic evolution of the Mersin ophiolite of the central Tauride of Turkey, using palaeomagnetic, structural and geochemical analyses. This ophiolite represents one of the best examples of Tethyan-type ophiolites formed by supra-subduction zone spreading within the northern Neotethyan Ocean basin during the Late Cretaceous. It exposes a 3.0 km section of lower crustal, cumulate rocks, and tectonically separated exposures of the underlying mantle sequence and metamorphic sole, both of which are cut by basaltic dykes. Stepwise thermal and alternating field demagnetization of ultramafic and gabbroic cumulates from 18 sites mostly identified single components of remanent magnetization characterized by ENE-directed, moderately upwards inclined directions in geographic coordinates with high coercivities/high unblocking temperatures. The slight increase in scattering in remanence directions after tilt correction has been interpreted to be related to local variations in orientation of cumulates layering within the magma chamber. Rock magnetic investigations showed that magnetite is the main magnetic mineral in the majority of ultramafic, gabbro and dyke rock samples, and rock magnetic and demagnetization characteristics suggest that the samples carry thermoremanent magnetizations acquired during crustal accretion. Net tectonic rotation analyses show that all the units of the Mersin ophiolite, including lower crustal cumulates, dykes in the mantle sequence and dykes in the metamorphic sole have experienced large clockwise rotations around NE- trending, moderately plunging to sub-horizontal axes. Correcting anisotropy of magnetic susceptibility data for the effects of these rotations suggests that magmatic flow in the cumulates had an initial NNE-SSW orientation, which if assumed to relate to seafloor spreading suggests that the Mersin spreading axis was oriented WNW-ESE. This is consistent with regional palaeogeographic reconstructions. The net tectonic rotation data show that dykes in the metamorphic sole are rotated by c. 45°, significantly less than the c. 115° rotations seen in the mantle sequence and in the cumulate sequences of the overlying thrust sheets. These results therefore document an initial stage of intra-oceanic clockwise rotation of the ophiolite that occurred after initial detachment but prior to emplacement of dykes cutting the metamorphic sole. Subsequent additional clockwise rotation (of all units) may be attributed to further intra-oceanic rotation (preferred interpretation) or to later emplacement of the ophiolite onto the Tauride continental margin. Finally, some new, preliminary data are presented from the Lizard ophiolite of Cornwall in Appendix A, forming the results of a training project undertaken while awaiting permission for fieldwork in Turkey.

Magnetic minerals are abundant within our Earth's crust and can retain, through one of a number of processes, a remanent magnetisation induced by the Earth's magnetic field. Analyses of palaeomagnetic samples have been used for the past fifty years to improve our understanding of many of the Earth's major processes. Recent studies utilising newly developed imaging techniques, namely holographic transmission electron microscopy, have for the first time allowed direct observations of the magnetic structure in palaeomagnetic samples on a nanoscale. It is commonly observed that igneous rocks contain closely packed magnetic lamellae with a non-magnetic matrix, a result of the chemical process of exsolution. However, the results of current micromagnetic models, generated to predict the magnetic structure within such samples, are not in agreement with these direct observations. The results do, however, show strong similarities to the direct observations. The discrepancies between the direct observations and micromagnetic models indicate a lack of understanding of the magnetic interactions within such samples. To examine this two distinct hypotheses have been tested. Firstly, the geometry of the system has been altered to examine the effect of this on the magnetic structure of the grains. Secondly, a multiphase model has been produced. This multiphase model allows the simulation of more complicated systems that include more than one magnetic material in direct contact. This multiphase model has allowed us to examine the effect of varying the exchange in these multiphase structures and its effect on the modelled magnetic structure. Further, this multiphase model has allowed us to examine theoretical systems involving combinations of magnetic materials commonly found in palaeomagnetic samples.

Geomagnetic excursions, brief deviations in geomagnetic field behaviour from that expected during 'normal' secular variation, remain some of the most enigmatic features of geomagnetic field behaviour. This thesis presents high-resolution records of geomagnetic excursions recorded at the Blake-Bahama Outer Ridge in the Western North Atlantic. The highest resolution record yet of the Blake geomagnetic excursion (~125 ka) is measured in three cores from Ocean Drilling Program Site 1062 (ODP Leg 172). These cores have sufficiently high sedimentation rates (>10 cm ka^-1) to allow detailed reconstruction of the field behaviour at these sites during the excursions. Previous reconstructions of geomagnetic field behaviour during excursions from marine cores have been limited by low-resolution age models. This thesis discusses a new approach, whereby measurements of excess ²³⁰Th (²³⁰Th_xs) are used to constrain relative variations in sedimentation rate. Modifications are suggested to the methods previously used to calculate the concentration of ²³⁰Th_xs and a new MATLAB® program is developed and described that allows rapid and flexible calculation of ²³⁰Th_xs. Using this new approach, the duration (6.5±1.3 kyr) and age (129-122 ka) of the Blake excursion are accurately constrained. A palaeomagnetic study is also conducted on two ODP Sites, 1061 and 1062 on the Blake-Bahama Outer Ridge to obtain a high-resolution record of the Laschamp geomagnetic excursion (~41 ka). The Blake excursion is found to be of 'long' duration (6.5±1.3 kyr) whilst the Laschamp excursion is relatively short (<400 years) showing that excursions do not have a characteristic duration, linked to the conductivity of the inner core, but instead occupy a continuous range of durations. The records of both the Blake excursion and the Laschamp excursion from the Blake-Bahama Ridge sites also show rapid transitions to excursional geomagnetic pole positions (less than 500 years), much faster than often quoted for full geomagnetic reversals. Based on current estimates for reversal durations, this would imply that excursions and reversals are controlled by different processes.

In this thesis, I use multiple techniques to investigate the active faulting and deformation of the Altay Mountains, Western Mongolia. The Altay are an intracontinental transpressional mountain range, which are deforming in the far-field of the India-Asia collision. An anastomosing network of dextral faults strikes NNW-SSE, and accommodates NE-SW oriented shortening by rotating anticlockwise about vertical axes. I begin by characterising the Altay faults, and add to what is already known about their surface expression with new observations of active faulting and three previously undescribed ancient earthquake ruptures. I use ¹⁰Be cosmogenic dating and uranium-series dating on pedogenic carbonate to estimate the average Quaternary rate of slip for two of the major fault zones in the Altay. The slip rate on the Ölgiy fault is constrained to 0.3-2.1 mm/yr^-1. Results from the Hovd fault are ambiguous, demonstrating the complications encountered with application of Quaternary dating techniques. I measure palaeomagnetic directions from Cretaceous to Pliocene-aged sediments in the eastern Altay to constrain the degree of anticlockwise rotation. Results from thermal demagnetisation of specimens indicate that the eastern Altay has not undergone significant rotation, in contrast with previous studies from the Siberian Altay that reveal almost 40 degrees of anticlockwise rotation. This suggests that the eastern-most Altay fault is too young to have experienced significant rotation, or is kinematically different from the Siberian Altay. I apply apatite fission track (AFT) dating and track length modeling to the central Altay. Results from AFT dating show rapid cooling in the late Cretaceous due to the distal assembly of Central Asia, suggesting that there was pre-existing topography at the start of the Late Cenozoic phase of deformation, the timing of which is constrained to have initiated at least 20 Myr ago. My work demonstrates that combining results from techniques that cover a variety of time scales quantifies the evolution of active faulting and deformation in the region.

Das Magnetfeld der Erde wird durch Konvektionsströmungen im elektrisch leitfähigen, flüssigen eisenreichen äußeren Erdkern erzeugt. Eine drastische Ausprägung der dynamischen Prozesse im äußeren Erdkern sind sowohl Polaritätswechsel über geologische Zeiträume, als auch geomagnetische Feldexkursionen (kurze Umpolungen). Letztere sind in geologischen Archiven häufig unzureichend dokumentiert. Für ein verbessertes Verständnis über die Entwicklung des Erdmagnetfeldes in geologischer Vergangenheit benötigen wir Informationen über die Geometrie des gesamten Vektorfeldes, wofür neben der Bestimmung der Feldrichtungen auch die Bestimmung der absoluten Paläointensität und des Alters notwendig ist. Insbesondere Vulkanite bieten die Möglichkeit, Daten über die Richtung und vor allem auch die Intensität des Erdmagnetfeldes zur Zeit ihrer Platznahme zu gewinnen. Bisweilen ist eine genaue Charakterisierung der Entwicklung des Erdmagnetfeldes in Zeit und Raum schwer möglich, was sich in erster Linie auf den generellen Mangel an Paläointensitätsdaten zurückführen lässt. Ein Grund hierfür ist, dass die meisten Methoden zur absoluten Paläointensitätsbestimmung, auf Modifikationen der Thellier Methode basieren, welche nur auf magnetische Minerale im Einbereichs-Domänenzustand anwendbar ist und zudem hohe Ausschussraten liefert. Eine alternative Methode zur Bestimmung der absoluten Paläointensität ist die kürzlich entwickelte „multispecimen parallel differential pTRM“ (MS) Methode, welche im Vergleich zur Thellier Methode den Vorteil hat, dass sie theoretisch unabhängig ist vom Domänenzustand der magnetischen Minerale und somit auf alle Vulkanite anwendbar ist. Ein Schwerpunkt dieser Arbeit lag darauf, neue Informationen über das Auftreten und gegebenfalls die globale Gültigkeit von geomagnetischen Feldexkursionen zu gewinnen. Hierfür wurden etwa 75 Lavaflüsse des Transmexikanischen Vulkangürtels für paläomagnetische Studien beprobt. Eine Korrelation der mittleren Paläorichtungen von 56 mexikanischen Laven mit einer um Feldexkursionen ergänzten geomagnetischen Polaritätszeitskala, lieferte Hinweise auf 4 Exkursionen. Ein bedeutendes Ergebnis dieser Arbeit sind annährend komplett inversen Richtungen zweier Laven der Brunhes Chron. Dies gibt einen Hinweis darauf, dass diese Exkursionen kurze Zeitintervalle inverser Polarität mit globaler Gültigkeit repräsentieren könnten. Ein weiterer Schwerpunkt der vorliegenden Arbeit war, die neue MS Methode auf ihre Anwendbarkeit und Genauigkeit hin zu testen. Hierfür wurden Paläointensitätsexperimente an 11 historischen Laven aus Mexiko und Island durchgeführt. Ein Vergleich der Paläointensitäten mit Daten von magnetischen Observatorien ergab, dass die MS Methode einen generellen Trend zur Überschätzung der Paläointensität aufweisst, welcher anhand von komplementierenden gesteinsmagnetischen Daten mit magnetischen Mineralen im Mehrbereichsteilchen-Zustand in Verbindung gebracht werden konnte. Diese Beobachtung liefert demnach einen ersten Beweis dafür, dass die MS Methode möglicherweise nicht wie ursprünglich angenommen unabhängig vom Domänenzustand der Trägerminerale ist. Im weiteren wurde eine Komplementierung der Richtungsdaten mexikanischer Laven durch absolute Paläointensitätsbestimmungen angestrebt. Hierfür wurde die MS Methode herangezogen und zum ersten Mal in großem Umfang auf Vulkanite mit Altern von bis zu 3,5 Millionen Jahre angewendet. Ein Vergleich mit Rekonstruktionen des Dipol-Momentes, welche auf den Daten der gegenwärtigen globalen Paläointensitätsdatanbasis basieren, ergaben, dass diese MS Daten mit hoher statistischer Wahrscheinlichkeit im Mittel etwa 30% höher sind. Die generell zu hohen Paläonintensitäten nach der MS Methode bekräftigen daher die Ergebnisse von historischen Laven dieser Arbeit, sowie anderer experimenteller Studien an synthetischen Proben, bei denen Überschätzungen von MS Paläointensitäten von bis zu 30% festgestellt wurden. Der Process, aus dem diese Überschätzung der Paläointensität resultiert ist eine Asymetrie des Entmagnetisierungs- und Remagnetisierungsprozesses heisst, dass ein effektives Entmagnetisieren während der Remagnetisierung im angelegten Laborfeld erfolgt. Diese Asymetrie scheint besonders bei pseudo-Einbereichsteilchen ausgeprägt zu sein. Es wird allerdings davon ausgegangen, dass diese Überschätzung nicht größer ist, als was man bei einem Thellier Experiment an Proben mit ähnlicher magnetischer Korngröße erwarten würde. / The Earth’s magnetic field (EMF) is generated by convections in the electrically conducting liquid iron-rich outer core, modified by the Earth’s rotation. A drastic manifestation of the dynamics of this fluid body is the occurrence of geomagnetic field reversals in the Earth’s history but also geomagnetic excursions, which are more frequent features of otherwise stable polarity chrons, but often poorly constrained in the geological record. To better understand the origin of the field, we need to know how the field has varied on different geological timescales. This includes not only information about changes in the ancient field’s direction but also about the absolute intensity (palaeointensity) and the age. This palaeointensity record is needed for compiling a full-vector description of the field. A palaeomagnetic and palaeointensity study on lava flows allows gaining insights about the evolution of the EMF through time and space. However, constraining the EMF evolution over different geological timescales remains a difficult objective due to the paucity of available palaeointensity data. One new alternative approach in palaeointensity studies is the recently proposed multispecimen parallel differential pTRM (MS) method, which has potentially several advantages over the commonly used Thellier method, because it is in theory independent of magnetic domain state, less prone to biasing effects, such as thermal alteration and significantly faster to perform in the laboratory. A study of highly active volcanic regions, such as the Trans-Mexican Volcanic Belt, seems promising when attempting a full-vector reconstruction or when looking for field excursions. One aim of this thesis was to gain new information about the occurrence and global validity of geomagnetic excursions from the Brunhes- or Matuyama Chron. For this purpose some 75 lava flows from within the Trans-Mexican Volcanic Belt were sampled for palaeomagnetic analyses. The scatter of virtual geomagnetic poles from lavas younger than 1.7 Ma was used for estimating palaeosecular variation and was found to be consistent with latitude dependent Model G and other high quality palaeomagnetic data from Mexico. The palaeomagnetic mean-vectors of 56 lavas were correlated to the Geomagnetic Polarity Timescale supplemented with information on geomagnetic excursions. On the grounds of their associated radioisotopic ages, four lavas were tentatively correlated with known excursions from marine records. Two lava flows dating of Brunhes Chron were associated with the Big Lost and Delts/Stage 17 excursions, respectively. From further two flows dating of Matuyama Chron, one flow was associated with either the Santa Rosa- or Kamikatsura excursions, while the other could have been emplaced during the Gilsa excursion. The most significant outcome was the finding that both Brunhes excursional flows display nearly fully reversed directions that deviate almost 180°C from the expected normal polarity direction. This observation could indicate that in particular the Big Lost and Delta/Stage17 excursions may represent other short periods during which the field completed a full reversal for a short time, such as was previously found for other older cryptochrons or tiny wiggles. Another focus of this thesis was set on estimating the feasibility of the new MS method for routine palaeointensity determination. This was accomplished by applying the MS method to samples from 11 historical lava flows from Mexico and Iceland from which the actual field intensity was either known from contemporary observatory data, or deduced from magnetic field models. Comparing observed with expected intensity values allowed to test the accuracy of the MS method. It a was found that the majority of palaeointensity estimates after the MS method yielded results that were very close or indistinguishable within the range of uncertainty from the expected values. However, a general trend towards an overestimate in the palaeointensity was also observed, which, on the grounds of corroborating rock magnetic analyses, was associated with multidomain material. This observation was taken as first evidence that the MS method is not entirely independent of magnetic domain state, as was originally claimed. However, a second experiment in which a modification of the most widely used Thellier method was applied to sister samples from 5 Icelandic flows revealed that, in comparison to the MS method, the latter produced more accurate and statistically better defined palaeointensities. Thus, from these first results, the MS method appeared as a viable alternative for future palaeointensity studies. Subsequently it was attempted to corroborate the directional record from Mexican lavas with palaeointensity data. It was possible to acquire palaeointensity estimates for 32 out of 51 investigated lava flows. These new results revealed that the new MS palaeointensities for Mexico are, with a high degree of statistical significance, around 30% higher than expected. The generally high palaeointensities seem to corroborate the results obtained from historical lava flows in this study and other previous studies on synthetic samples where domain state effects were found to cause overestimates in the palaeointensity of up to 30 per cent in the MS method. The primary process that leads to this overestimate is assigned to an asymmetry in the demagnetisation and remagnetisation process. Yet, this overestimate is expected to be no larger than what might be expected from Thellier experiments performed on samples with a given degree of multidomain behaviour.

If a rock is to retain a geologically meaningful magnetic record of its history, it is essential that it contains magnetic minerals which are capable of carrying stable magnetic remanence. Of the natural occurring magnetic minerals, magnetite is the most important because of its abundance and strong magnetic signature. The stability, i.e., the resistance to demagnetisation or reorientation, of magnetic remanence is related to grain size; in smaller grains the magnetic moments align to have single domain (SD) structures, in larger grains complex magnetic patterns are formed (multidomain (MD)). “Classical” domain theory predicts that SD remanence is stable, whilst MD remanence is not. However experimental evidence has shown that both SD and MD grains can have stable remanences. In this thesis the origin of stable MD remanence is examined. There are two opposing theories; one suggests that the stability is due to independent SD-like structures, the other postulates that the stability is due to metastable MD structure. A series of experiments were designed to examine the stability using a selection of characterised synthetic and natural samples. Low-stress hydrothermal recrystallised samples where grown for this study. For the first time, the stability of thermoremanence induced in hydrothermal crystals to cooling was examined. The results agree with previous observations for crushed and natural magnetites, and support kinematic models. The behaviour of SIRM and thermoremanences in MD magnetite to low-temperature cooling to below the crystallographic Verwey transition at 120-124 K (T_v) and the cubic magnetocrystalline anisotropy isotropic point (T_k) at 130 K was investigated. On cooling through T_v, SIRM was observed to decrease and demagnetise, however thermoremanence was found to display a large increase in the magnetisation at T_v, which was partially re- versible on warming. The size of the anomaly is shown to be dependent on the temperature at which the thermoremanence is acquired, internal stress and grain size. The anomaly is attributed to the large increase in the magnetocrystalline anisotropy which occurs on cooling through T_v . It is postulated that low-temperature cycling demagnetisation is due to kinematic processes which occur on cooling between room temperature and T_k. Characterisation of low-temperature treated remanence and partially alternating field demagnetised remanence, suggest that the stable remanence is multidomain. Low-temperature cooling of remanence in single sub-micron crystals was simulated using micromagnetic models. The models predict the observed anomaly for thermoremanence on cooling through T_v, and also the relative behaviour of SIRM and thermoremanence. The single domain threshold was calculated for the low-temperature phase of magnetite, and was found to be 0.14 microns, compared to 0.07 microns at room temperature.

<p>Novel computational paradigms based on non-von Neumann architectures are being extensively explored for modern data-intensive applications and big-data problems. One direction in this context is to harness the intrinsic physics of spintronics devices for the implementation of nanoscale and low-power building blocks of such emerging computational systems. For example, a Probabilistic Spin Logic (PSL) that consists of networks of p-bits has been proposed for neuromorphic computing, Bayesian networks, and for solving optimization problems. In my work, I will discuss two types of device-components required for PSL: (i) p-bits mimicking binary stochastic neurons (BSN) and (ii) compound synapses for implementing weighted interconnects between p-bits. Furthermore, I will also show how the integration of recently discovered van der Waals ferromagnets in spintronics devices can reduce the current densities required by orders of magnitude, paving the way for future low-power spintronics devices.</p> <p>First, a spin-device with input-output isolation and stable magnets capable of generating tunable random numbers, similar to a BSN, was demonstrated. In this device, spin-orbit torque pulses are used to initialize a nano-magnet with perpendicular magnetic anisotropy (PMA) along its hard axis. After removal of each pulse, the nano-magnet can relax back to either of its two stable states, generating a stream of binary random numbers. By applying a small Oersted field using the input terminal of the device, the probability of obtaining 0 or 1 in binary random numbers (P) can be tuned electrically. Furthermore, our work shows that in the case when two stochastic devices are connected in series, “P” of the second device is a function of “P” of the first p-bit and the weight of the interconnection between them. Such control over correlated probabilities of stochastic devices using interconnecting weights is the working principle of PSL.</p> <p>Next my work focused on compact and energy efficient implementations of p-bits and interconnecting weights using modified spin-devices. It was shown that unstable in-plane magnetic tunneling junctions (MTJs), i.e. MTJs with a low energy barrier, naturally fluctuate between two states (parallel and anti-parallel) without any external excitation, in this way generating binary random numbers. Furthermore, spin-orbit torque of tantalum is used to control the time spent by the in-plane MTJ in either of its two states i.e. “P” of the device. In this device, the READ and WRITE paths are separated since the MTJ state is read by passing a current through the MTJ (READ path) while “P” is controlled by passing a current through the tantalum bar (WRITE path). Hence, a BSN/p-bit is implemented without energy-consuming hard axis initialization of the magnet and Oersted fields. Next, probabilistic switching of stable magnets was utilized to implement a novel compound synapse, which can be used for weighted interconnects between p-bits. In this experiment, an ensemble of nano-magnets was subjected to spin-orbit torque pulses such that each nano-magnet has a finite probability of switching. Hence, when a series of pulses are applied, the total magnetization of the ensemble gradually increases with the number of pulses</p> <p>applied similar to the potentiation and depression curves of synapses. Furthermore, it was shown that a modified pulse scheme can improve the linearity of the synaptic behavior, which is desired for neuromorphic computing. By implementing both neuronal and synaptic devices using simple nano-magnets, we have shown that PSL can be realized using a modified Magnetic Random Access Memory (MRAM) technology. Note that MRAM technology exists in many current foundries.</p> <p>To further reduce the current densities required for spin-torque devices, we have fabricated heterostructures consisting of a 2-dimensional semiconducting ferromagnet (Cr₂Ge₂Te₆) and a metal with spin-orbit coupling metal (tantalum). Because of properties such as clean interfaces, perfect crystalline nanomagnet structure and sustained magnetic moments down to the mono-layer limit and low current shunting, 2D ferromagnets require orders of magnitude lower current densities for spin-orbit torque switching than conventional metallic ferromagnets such as CoFeB.</p>

Interaction between minerals and bacteria represents an abundant natural phenomenon depictingnature's complexity and how abiotic and biotic components are intertwined. This interaction is evidentin modern-day ecosystems, and it significantly shaped the early stage of life on Earth by influencinggeochemical processes. Evidence of this interaction includes microbialites. In the first part, this master's thesis explores the impact of iron-redox bacteria on the magneticproperties of synthetic and natural magnetic materials, with significant implications for understandingearly Earth conditions and paleoenvironments. In the second part, the magnetic signal recorded in thesediment core from the Baltic Sea is examined to identify the ferrimagnetic minerals responsible for itand their origin (biotic versus abiotic). A better understanding of the origin of ferrimagnetic mineralsallows for a more conclusive interpretation of palaeomagnetism and palaeoenvironmental history of theBaltic Sea. In both parts, the change in magnetic susceptibility was used as the main method to depictmechanisms of mineral-bacteria interaction. Experiment with iron-oxidising bacteria (Leptothrix mobilis) showed a decrease in magneticsusceptibility over time, consistent with the oxidation of solid iron/magnetic materials. However, asmall difference between bacteria culture and control samples points out that the decrease is caused byabiotic oxidation rather than bacterial. Supporting evidence is the absence of viable cells in all bacterialsamples, suggesting that L. mobilis did not grow in these experiments. In experiments with iron-reducing bacteria (Geobacter sulfurreducens), magnetic susceptibility increased by 7%. Controlsamples with the same reducing media did not show a change in magnetic susceptibility, indicating thatthe susceptibility change is caused by bacterial reduction of iron oxides. Magnetic susceptibility signal obtained in the sediment core from the Baltic Sea indicates rapidlyoxidising, ferrimagnetic nanoparticles in two organic-rich sapropels. The pattern of the signal isconsistent with the presence of bacterial greigite (magnetofossils). Contrary to previous research, nomagnetic enhancement is observed in these layers.  Magnetic susceptibility, as a non-destructive and relatively simple method, may serve as a significantindicator of mineral-bacterial interactions. Combining it with other techniques and methods can providedeeper insights into the mechanisms behind these interactions. This approach can reveal the importanceof these interactions on early Earth, enhance our understanding of palaeomagnetism, and unveil possibleconditions of ancient environments.