Variation Of Magnetic Properties Research Articles

Influence of synthesis methods on the structural, dielectric and magnetic properties of cobalt ferrites

A systematic study comprising structural, magnetic and dielectric properties of the cobalt ferrite (CoFe2O4) powders at first is carried out by way of synthesizing the materials using different preparatory methods, such as conventional solid-state method, wet chemical sol-gel autocombustion method using citric acid as well as glycine, separately as fuels. X-ray diffraction (XRD) studies reveal the formation of spinel phase with clear variations of peak intensities in all the samples. Morphology studies using scanning electron microscopy (SEM) reveal well grown grains in solid state synthesized sample compared to sol-gel autocombustion samples. The variations observed in the grain growth as well as densities (porosity) were attributed to typical temperature treatment conducted on the samples. Dielectric tests provide the evidence of space charge polarization in all the samples. Magnetic studies using vibrating sample magnetometry (VSM) provide the values of saturation magnetization and coercivity of the samples explored. The sol-gel autocombustion sample using glycine matrix (Cogly) shows higher saturation magnetization (Ms = 59.2 emu/gm) comparing to other samples whereas, sample with citric acid matrix results with lower Ms value as 36.5 emu/gm. The typical reasons for the observed variations in structural, microstructural, magnetic, dielectric and impedance properties of these materials were well addressed in terms of theoretical aspects and presented in this paper.

Impact of Zn addition on structural, morphological, and magnetic properties of thermally annealed Li–Zn nanoferrite

The effect of Zn addition on cationic distribution, structural and magnetic properties of nanocrystalline Li-Zn ferrites are reported. X-ray diffraction confirms the formation of spinel nanoferrites with an average grain diameter of 34 − 82 nm. The addition of Zn results in a linear increase of lattice parameter and X-ray density. Magnetism in the studied samples is governed by the Yafet-Kittel three-sub-lattice model, shown by a non-zero canting angle. The variation in magnetic properties is correlated with the calculated cationic distribution. A new antistructural modeling explains the changes in the concentration of donor and acceptor surface active centers.

Analyzing the variations in electrical, structural and magnetic properties of zinc-doped MnFe2O4 ferrite obtained via co-precipitation

Analyzing the variations in electrical, structural and magnetic properties of zinc-doped MnFe2O4 ferrite obtained via co-precipitation

Effects of inserted ultrathin Pt buffer on perpendicular magnetic properties of Ta/TbFeCo/Ta layered structures

We investigated effects of inserted ultrathin Pt buffer on the perpendicular magnetic properties of Ta/TbFeCo/Ta layered structures. For Tb-rich TbFeCo films buffered with Ta and Ta/Pt a compensation thickness is observed at ∼ 6 nm, where the net saturation magnetization vanishes. The perpendicular magnetic anisotropy properties of two structures are found to be governed by a competition of a perpendicular bulk anisotropy and an in-plane interface anisotropy. Compared with Ta-buffered TbFeCo films, the counterparts with insertion of Pt buffer exhibits a larger negative interface anisotropy. A large variation of saturation magnetization and perpendicular magnetic anisotropy with increasing inserted thickness of Pt buffer is observed in the Ta/Pt/TbFeCo/Ta structure. This can be ascribed to the interface-driven variation of exchange interaction between Tb and FeCo at the interface, confirmed by thermomagnetic measurements. A clear correlation between the interface-driven variation of exchange interaction and perpendicular magnetic properties of the TbFeCo films is identified. Our results show the interface plays an important role in the perpendicular magnetic properties of TbFeCo films, which is critical for the application in spintronic devices.

Compositional dependence of magnetostrictive effect in Ta/CoFe(B)/MgO stacks with perpendicular magnetic anisotropy

Magnetostrictive effect in Ta/Co x Fe100−x (B)[CoFe(B)]/MgO for alloy compositions spanning Fe-rich (x = 20) to Co-rich (x = 80) stacks has been studied to investigate the relation between magnetostrictive effect and the onset of perpendicular magnetic anisotropy at Ta/CoFe(B)/MgO interfaces. Interestingly, for each Co composition, a t-dependent crossover between in-plane (ip) and out-of-plane (op) magnetic anisotropy is found at a different CoFe(B) thickness (t cro)–denoted as crossover thickness, which means compositional variations of magnetic properties at the interfaces. By considering the Ta/CoFe(B) and CoFe(B)/MgO interfaces as atomistic volumes with ip and op orbital magnetizations, respectively, the relative ratio of ip to op orbital magnetization in the atomistic volumes is found to be closely related to the dependence of magnetostriction constant (λ s) on the Co composition. The findings suggest that the composition dependence of ip and op orbital magnetization at the Ta/CoFe(B)/MgO interfaces plays an important role in controlling its interfacial anisotropy and magnetostrictive effect of the stacks with the interfaces.

Phase Formation and Magnetic Properties of (Y1−xSmx)Co5 Melt-Spun Ribbons

Using X-ray diffraction (XRD) and a vibrating sample magnetometer (VSM), the effects of Sm substitution, wheel speed, and annealing temperature on the phase formation and magnetic properties of (Y1−xSmx)Co5 (x = 0.2, 0.3, 0.4, 0.5) melt-spun ribbons were investigated. The results indicate the following: (1) With the increase in Sm substitution, it was found that (Y1−xSmx)Co5 ribbons are entirely composed of the (Y-Sm)Co5 phase with a CaCu5-type structure. Additionally, the coercivity gradually increases, while the remanence and saturation magnetization gradually decrease. (2) As the wheel speed increases, the (Y1−xSmx)Co5 ribbons exhibit an increasing proportion of (Y-Sm)Co5 phase until reaching a speed of 40 m/s, where they are entirely composed of the (Y-Sm)Co5 phase. Magnetic measurements show that the coercivity (Hcj) and remanence (Br) of (Y0.5Sm0.5)Co5 ribbons increase gradually with increasing wheel speed, while saturation magnetization decreases. The variation in magnetic properties is mainly attributed to the formation of nucleation centers for reversed magnetic domain (2:7 and 2:17 phases); (3) (Y0.5Sm0.5)Co5 ribbons are composed of the (Y-Sm)Co5 phase and a small amount of the Sm2Co7 phase after annealing at 550 °C, 600 °C, and 650 °C. Temperature elevation promotes crystallization of the amorphous phase, resulting in a gradual decrease in coercivity, while the remanence and saturation magnetization exhibit an overall increasing trend. Through continuous optimization of the process, favorable magnetic properties were achieved under the conditions of a 0.5 Sm substitution level, a wheel speed of 40 m/s, and an annealing temperature of 550 °C, with a coercivity of 7.98 kOe, remanence of 444 kA/m, and saturation magnetization of 508 kA/m.

In situ high-temperature transmission electron microscopy investigation of dynamic element migration for Sm-Co-Fe-Cu-Zr sintered magnets

The investigation delved into the dynamic migration of elements and the evolution of interface structures within the Sm(CobalFe0.09Cu0.09Zr0.03)7.2 magnet as temperature increased, employing high-temperature transmission electron microscopy. Notably, the Cu distribution exhibited temperature susceptibility. Elevating the temperature from room temperature (RT) to 400 °C led to a notable expansion in the distribution width of Cu, ranging from 28.5 to 50.8 nm. Simultaneously, the average Cu content at the cell boundaries decreased from 14.4 to 11.6 at.%. According to the high-temperature in-situ experimental results of the transmission electron microscopy thin-film samples, the derived diffusion coefficients, D300°Cand D400°C, were determined as 0.0525 nm2/s and 1.414 nm2/s, respectively. The significantly higher D400°C compared to D300°C underscores the considerable influence of temperature on Cu element diffusion behavior. The simulation results demonstrate that the trend of magnetic properties variation at different temperatures closely aligns with the observed trend. Subsequent calculation of the temperature coefficient of intrinsic coercivity (β) reveals that |β300–400°C| is notably higher than |βRT-300°C|. This suggests a substantial impact of Cu diffusion-induced deterioration on the β. Moreover, micromagnetic simulation results delineated the reverse magnetization process for magnets at varying temperatures, revealing that the presence of the Cu diffusion region diminished the pinning effect of the cell boundary phase. Furthermore, a wider Cu diffusion region facilitated easier magnetic moment reversal at the cell boundary phase, resulting in a marked deterioration of intrinsic coercivity. This elucidates that the formation of the Cu diffusion region stands as one of the primary reasons for the sharp decline in magnetic properties of Sm2Co17-type magnets under high-temperature conditions.

Compositional-driven variations in magnetic, conductivity, and ferroelectric properties of multiferroic BiFeO3–CoFe2O4 composite system

Compositional-driven variations in magnetic, conductivity, and ferroelectric properties of multiferroic BiFeO3–CoFe2O4 composite system

Mechanical and Magnetic Properties Variation in Non-Oriented Electrical Steels with Different Cutting Technology: A Review.

The problem of energy consumption reduction establishes important challenges for electric motor producers in the framework of new international regulations regarding the conditions that must be accomplished by motors in the near future. One of the most important topics is related to the core loss decrease directly linked to the effect of electrical steel degradation induced by the cutting technology. Understanding exactly how this phenomenon occurs by analyzing the chemical, mechanical, crystallographic, magnetic domain, and magnetic properties is of utmost importance when manufacturing processes must be changed and adapted to a new market characterized by high-efficiency motors. Today, mechanical and laser cutting technologies are the most used because of their reduced price and high-speed process. Still, unfortunately, these methods are not the best due to the fact that they lead, in most cases, to a high value of magnetic core losses, low electromagnetic torque, and hence reduced efficiency. This review paper shows that non-conventional technologies such as water jetting and electroerosion could be applied if proper modifications are added. This paper's main idea is to present a comprehensive study regarding the impact of cutting technologies on microhardness and residual stresses, crystallographic texture, magnetic domain structure, and magnetic properties of some non-oriented electrical steels used in motor production. It provides a detailed analysis of the abovementioned aspects by including the authors' research and findings in the wider context of other research group contributions. It also offers a general idea of the mechanisms present at the macro- and microscopic levels. The readers can find some of the most used analytical models, including the cutting process's damaged effect on the magnetic properties' variation based on a simple mathematical approach and examples of finite element modeling performed on real motor designs implemented in various programs. Last but not least, some practical implementations of the cutting procedure's influence on motor working conditions are presented in the last section of the paper. It provides an up-to-date analysis regarding how the cutting method should be included in high-efficiency motor production by emphasizing the importance of the topic and identifying where supplementary research must be undertaken. From the investigated literature, by analyzing specific sample geometries associated with different characterization methods, it can be concluded that all the cutting technologies have an important contribution to the mechanical and magnetic quantities. When the magnetic core of an electric motor is produced through non-conventional methods, the overall influence of the cutting procedure has a low percentage in the motor efficiency, as presented in this paper.

Variation of magnetic properties with current in ferrimagnetic semiconductor Mn3Si2Te6

Orbital currents play a fundamental role in a wide range of transport phenomena. Recently, the discovery of a novel chiral orbital current state in the ferrimagnetic nodal-line semiconductor Mn3Si2Te6 has attracted significant interest, supported by anomalous I–V characteristics and time-dependent bistable switching. However, the direct experimental verifications, combining electrical transport and magnetic measurement, that detect the variation of the magnetic properties vs the current are still rare. Here, we investigate the transport properties of Mn3Si2Te6 and track the current-induced dynamics of the magnetic moment. Reflective magnetic circular dichroism reveals that significant alterations in Mn3Si2Te6 magnetoresistance in response to an electric field are necessarily coupled with a magnetic phase transition, establishing a rare correlation. Our findings indicate the predominance of magnetic chiral orbital currents in the colossal angular magnetoresistance effect, offering a unique platform for advanced studies in orbital magnetism.

Composition dependent variation in structural, morphological, optical and magnetic properties of biogenic CuO/NiO mixed oxides nanoparticles

A series of CuO/NiO nanoparticles with different Cu and Ni precursor concentrations represented as N12 (1:2), N22 (1:1) and N21 (2:1) (Cu2+:Ni2+) were produced by modified green solution combustion route using Moringa oleifera leaves extract as an organic fuel. The prepared CuO/NiO nanoparticles were analytically characterized using UV-Vis diffuse reflectance spectroscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller analysis, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and vibrating sample magnetometry. Morphological studies showed uniform and nearly monodisperse nanoparticles having average grain size of 34 nm for N22 nanoparticles while the N12 and N21 compositions showed uniform nanoparticles with the grain size of 12 nm and 22 nm respectively. The existence of CuO and NiO phases in nanoparticles was verified by XPS analysis. N12, N22 and N21 had band gaps in the range of 2.19–2.49 eV and specific surface area in the range of 4.92–11.2 m2/g. The susceptibility vs. temperature plots for mixed oxides nanoparticles showed distinct magnetic behaviour for different ratio. The coercivity and saturation magnetization reached a maximum value for N22 (130 Oe and 0.133 emu/g). These biogenic nanoparticles could be a strong choice for biological applications due to their soft magnetic behaviour and biocompatibility.

Variations in perpendicular magnetic and electrical transport properties of ferrimagnetic (0 0 1) NiCo2O4 films

We fabricated epitaxial NiCo2O4 films on (001) MgAl2O4 substrates by varying the pulsed-laser energy density from 1.31 to 2.46 J/cm2, while keeping all other parameters fixed. X-ray diffraction measurements revealed that the increase in energy density increased the film deposition rate from 0.022 to 0.078 nm/s, while resulting in the decrease of the full-width at half-maximum of the rocking curve and out-of-plane lattice constant. At room temperature, the films grown at an intermediate energy density range of 1.97–2.13 J/cm2 exhibited more distinctive perpendicular magnetic anisotropy and magnetic domain structure compared to those grown at high or low energy densities. On the other hand, the lower the laser energy density, the more pronounced the perpendicular magnetic anisotropy becomes at low temperatures. In addition, the ferrimagnetic-to-paramagnetic transition temperature of the film increased with the energy density, reaching a maximum of approximately 390 K at energy densities above 2.13 J/cm2. Furthermore, temperature-dependent resistivity indicated that the metallic behavior in the films grown above 2.13 J/cm2 was retained up to ∼ 500 K, which is much higher than the ferrimagnetic transition temperature. These results suggest that pulsed-laser energy density can serve as an alternative parameter for controlling the perpendicular magnetic properties and enhancing the metallic behavior of the (001) NiCo2O4 film.

VARIATIONS IN MAGNETIC PROPERTIES PROPAGATION IN STEEL SHELL STRUCTURE CAUSED BY WELDING (PART 2)

The first stage of the research involves analyzing primary and secondary acoustic wave propagation behavior. The results of the first stage suggest that the properties of shell structures used at every stage of the experiment are distributed unevenly along the generator lines and across the width of the test pipes. In order to conduct a comparative study of properties, the authors of the research introduce the heterogeneity parameter. During the experiment, the parameter is changing locally within two orders of magnitude, even in case of test pipes in their initial state. Equal sensitivity to heterogeneities across steel products is found in its magnetic properties. As numerous studies prove, analyzing variations in magnetic properties makes it possible to detect flaws and areas of their concentration. This method can also be used to estimate the remaining service life. This is a crucial aspect to consider when assessing properties and using the findings to develop digital twins of shell structures. The authors focus on identifying acoustic and magnetic signals coming from flawed shell structures and weld joint areas in particular. For this purpose, magnetic memory method and X-ray analysis are used. It is found that both normal and tangential magnetic field components are sensitive to structural heterogeneities across a steel product. The combination of magnetic memory method and X-ray analysis make it possible to determine how typical weld joint flaws occurring during welding affect the magnetic properties. Sufficient measurements are required for statistical analysis of the findings. It is also possible to introduce further criteria in order to import metal properties correctly into the modelling program using numerical methods.

Effects of drawing and tension stress annealing on the structure and magnetic properties of Co-based amorphous wire

Cold drawing significantly improves the size uniformity of the amorphous wire, but leads to wire curling. Tensile stress annealing (TSA) is the key to regulating the straightness and magnetic properties of Co-based amorphous wires, but their intrinsic correlation is still unclear. Herein, the effects of TSA on the morphology, microstructure, soft magnetic properties and longitudinal driven giant magneto-impedance (LD-GMI) effect of cold drawn Co-based amorphous wires have been studied in detail. It was found that increasing tensile stress can effectively straighten the drawn amorphous wire, yet at the cost of decreasing fracture strength, increasing coercivity and deteriorating GMI effect. Furthermore, appropriate annealing can restore the magnetic properties of the TSA amorphous wire. The variation of magnetic properties is relative to the change of circular anisotropic field and magnetic domain structure.

Gallium substitution influence on microstructure and magnetic properties of Y-type Ba0.5Sr1.5Zn2Fe12O22 hexaferrites

Y-type hexagonal ferrites of Ba0.5Sr1.5Zn2Fe(12-x)GaxO22 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1) are synthesized by chemical co-precipitation method. X-ray diffraction (XRD) analysis reveals that the samples are single-phase and the space group is R3¯m. The field-emission scanning electronic microscope (FE-SEM) results indicate that the grains are regular fibrous shape with a width of about 2–5 µm and a thickness of 1–3 µm. In the M-T diagram, it can be observed that the temperature at which the transition from helical to ferrimagnetic (TS) increases first and then decreases with increasing substitution amount x. From σ-H, the values of coercive field (HC) experience alternating decreases and increases, and reaches a maximum value of 567.34 Oe at x = 0.8. While the specific saturation magnetization (σS) values exhibit a monotonic decrease with the increasing x. The variations in magnetic properties can tentatively be ascribed to the effect of Ga3+ substitution. These results above might be used as promising candidates in electric devices.

Remarkable variation in microstructural, thermoelectric, and magnetic properties of CaMnO3 through Ce doping

This work reports several alterations in crystal structure, grain size, electrical resistivity, power factor, and magnetic properties of Ce-substituted Ca1-xCexMnO3 samples (x = 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10). XRD analysis has showed that all samples present an orthorhombic perovskite structure with Pnma space group. Microstructure of the samples consists of randomly oriented polygonal-shaped grains whose size increases from 6.06 μm to 6.46 μm with Ce content up to 0.04. Electrical resistivity has considerably decreased with Ce doping due to the enhancement of Mn3+/Mn+4 proportions. Power factor of the samples has been noticeably improved by Ce, and the largest value was obtained in x = 0.06 sample (0.35 mW/K2m at 500 °C). A reduction of Néel temperature from 115 to 96 K was determined from magnetization versus temperature measurements which can indicate the antiferromagnetism suppression with increasing Ce-content. In a related manner, magnetocaloric effect properties of the samples have been enhanced in Ce-doped samples.

Investigation on bio-synthesized Ni- and Al-doped cobalt ferrite using lemon juice as eco-fuel

Pure and doped cobalt ferrite (CoFe2O4) nanoparticles were prepared using sol-gel auto-combustion route with the aid of lemon juice as eco-fuel. In this study, Ni2+ and Al3+ ions were chosen as the dopants. The cubic spinel crystal structure of the synthesized ferrite samples was elucidated from the powder X-ray diffraction analysis. The crystallographic parameters associated with the samples were obtained from the Rietveld profile refinement of the diffraction patterns. The precise lattice parameter obtained from the Nelson-Riley extrapolation method is found nearly matches with the lattice parameter obtained from the Rietveld refinement of respective samples. The average crystallite size and microstrain accompanying with the samples were estimated from various methods such as via the Debye-Scherrer method, Williamson-Hall analysis and size-strain plot. By employing Tauc’s plot method, the optical parameters such as the direct bandgap, sub-bandgap and the Urbach energy of all the samples were calculated by analyzing the UV-Visible reflectance spectra. The room temperature magnetic properties of all the samples were estimated from the vibrating sample magnetometry. The nature of magnetic hysteresis revealed the semi-hard ferrimagnetism of the prepared cobalt ferrite samples. Also, the variation of magnetic properties with doping was analyzed. The weakening of interaction among the ions in tetrahedral and octahedral sites causes a decrease in the values of remanent magnetization, saturation magnetization and coercivity of doped samples.

Archaeomagnetic analyses on fumiers burned under controlled experimental conditions

The improvement of the archaeomagnetic dating method requires compiling new and older data of the Earth's magnetic field (EMF) variations for the last millennia. Combustion events from fumier sequences have been proposed as good directional EMF recorders. However, they are subjected to diverse taphonomical processes and how these affect the archaeomagnetic record has not yet been studied. In order to evaluate it, here we report the first archaeomagnetic and rock-magnetic results on samples from experimentally recreated fumiers since 2014 under controlled conditions. A facies description with unprecedent resolution was used to study the variation of magnetic properties in depth. Rock-magnetic analyses indicate a homogenous magnetic mineralogy dominated by pseudo-single domain magnetite as main carrier in all facies, with not very high and similar contribution of the finest (superparamagnetic) grains. The low values of anisotropy of magnetic susceptibility (AMS) indicate that the studied sample set is mainly isotropic. The directional behaviour in well-preserved burned facies (here described as G, LM and DGB), are jointly characterized by highly reversible thermomagnetic curves, high Koenigberger (Qn) ratio values and intense, univectorial and normal polarity orthogonal NRM demagnetization diagrams. On the contrary, specimens affected by mechanical alteration processes are less magnetic and show anomalous directional behaviours. The high thermomagnetic reversibility of ashes indicates that they reached ca. 600–700 °C, in line with the thermocouples’ data. Temperatures of 460 °C were obtained for the DGB facies (subyacent black carbonaceous facies). Sampling of ashes located on the top of these combustion events should be avoided for archaeomagnetism. Being just beneath the last stabling episode they are the most prone to undergo mechanical alterations and do not preserve well the Earth's magnetic field direction. Despite their unlithified nature and the multiple taphonomic processes that fumier sequences may undergo, under certain quality criteria, they are valid geomagnetic field recorders providing both information of archaeological and geophysical interest.

Revival of Ferromagnetic-Metallic Phase and Emergence of Griffiths Phase via Ni and Cr Substitution in Charged-Ordered Pr0.75Na0.25MnO3 Manganite

Polycrystalline samples of Pr0.75Na0.25Mn1-xAxO3 (A = Ni, Cr; x = 0, 0.03) were prepared using conventional solid-state method and their structural and magnetic properties were investigated. Monovalent-doped Pr0.75Na0.25MnO3 exhibits insulating behavior which is related to the strong localization of charge carriers due to the presence of charge ordering (CO). The substitution of different types of magnetic ions, A, which are Ni2+ and Cr3+, at a concentration of x = 0.03 at the Mn site in the CO Pr0.75Na0.25Mn1-xAxO3 manganite, has been found to dramatically modify its structural and magnetic properties. X-ray diffraction patterns showed that all samples were present in single phase and crystallized in orthorhombic structure with Pnma space group. The Rietveld refinement analysis showed that the unit cell volume increased due to Ni2+ substitution, while it decreased due to Cr3+ substitution, which may be attributed to the different ionic radii of both ions. The suppression of the CO state and the induction of the ferromagnetic-metallic state in the Ni-substituted and Cr-substituted samples are suggested to be due to the induction of double-exchange interaction involving Ni2+–O–Mn4+ and Mn3+–O–Cr3+, respectively. The variation in magnetic properties between the Ni-substituted and Cr-substituted samples is suggested to be due to the existence of different strengths of ferromagnetic interaction between Ni2+–O–Mn4+ and Mn3+–O–Cr3+. In addition, the deviation of the temperature dependence of the inverse magnetic susceptibility curves from the Curie-Weiss law suggests the existence of Griffith’s phase-like behaviour for both substitution samples. The Ni-substituted sample induced a greater GP effect than the Cr-substituted sample due to the larger difference of TC and TG value (TC-TG).

Abrupt Northwest Atlantic deep-sea oxygenation decline preceded the Palaeocene-Eocene Thermal Maximum

Reconstruction of oceanic redox conditions across the Palaeocene-Eocene Thermal Maximum (PETM, ∼55.93 Ma) provides crucial information on potential PETM triggers and biotic turnover. Widespread oceanic deoxygenation has been documented to occur during the PETM, but the exact timing and regional variability is poorly understood. Here we document well-preserved magnetofossils with large variations in magnetic properties and crystal morphologies at International Ocean Discovery Program (IODP) Sites U1409 (∼2913 m palaeodepth) and U1403 (∼4374 m palaeodepth) across the PETM, and use these proxies to trace Northwest Atlantic palaeoredox changes. Our records suggest that deep-sea oxygenation at these sites decreased gradually, starting several hundred thousand years before the PETM, reaching a minimum ∼50 ky before the PETM, followed by a broad increase. This pattern in palaeoredox conditions likely reflects changes in deep-sea circulation and ocean temperature. We thus provide evidence for the existence of precursors to PETM warming, and suggest that ocean circulation changes might have played an important role in triggering carbon release to the ocean-atmosphere system at the PETM onset.