Thermomagnetic Curves Research Articles

Thermo-magneto-mechanical properties of near stoichiometric Ni2MnGa (Mn > Ga) thin films deposited by radio-frequency magnetron sputtering on Si substrate

Near stoichiometric annealed thin films (1.5 to 3 µm) of Ni2MnGa alloy and whose compositions are such that the Martensitic Transformation Temperatures TM(e/a) are in the range -60 °C to 200 °C have been deposited by radio-frequency magnetron sputtering on SiO2/Si substrate. Crystallographic structure characterization such as planar spacing d(i), Thermal Expansion Coefficient CTE and structural domain size L(i)*, has been realized by X-ray diffraction over a wide temperature range from 25 to 205 °C. The mechanical properties such as the Young's modulus (E) and hardness (HB), have been measured by ultra-nano-indentation tests carried out from ambient temperature to 175 °C. SQUID-VSM magnetometer measurements from -263 °C to 127 °C provide some magnetic characteristics: evolution of the magnetization M versus the temperature and the Curie temperature TC. Three diffraction peaks which can be attributed to the tetragonal (NM) or modulated (5 M or 7 M) martensitic structures of polycrystalline films have been identified. At ambient temperature Ta, the calculated planar spacing d(i) associated with at least two diffraction peaks present a linear decreasing with TM(e/a). As a function of the temperature and for the austenitic state only one value of the CTE has been determined. For the martensitic state, two values have been extracted and whose change is close to TM(e/a). No evident evolution has been observed at the vicinity of the Curie temperature. The Young’ modulus and particularly the hardness are very dependent on the film composition, the temperature and the state of the microstructure. At room temperature HB linearly decreases with TM(e/a). As a function of the increasing temperature, for the martensitic state, E slightly decreases whereas HB is almost constant. In the austenitic domain, E slightly increases and HB linearly increases with the temperature. A phenomenological model has been proposed. The whole of the annealed films show a ferromagnetic behavior and from the thermomagnetic curves it has been shown that four distinct states can be observed: para or ferromagnetic austenite and para or ferromagnetic martensite. A transition on the magnetization, certainly due to a change of the martensitic crystal structure, occurs at the neighborhood of TC. Mechanical (HB) and magnetic (M) properties are strongly linked and at Ta it has been shown that M is a linear decrease function of HB.

Anomalous Nernst Effect in Flexible Co-Based Amorphous Ribbons.

Fe3Co67Cr3Si15B12 ribbons with a high degree of flexibility and excellent corrosion stability were produced by rapid quenching technique. Their structural, magnetic, and thermomagnetic (Anomalous Nernst Effect) properties were studied both in an as-quenched (NR) state and after stress annealing during 1 h at the temperature of 350 °C and a specific load of 230 MPa (AR). X-ray diffraction was used to verify the structural characteristics of our ribbons. Static magnetic properties were explored by inductive technique and vibrating sample magnetometry. The thermomagnetic curves investigated through the Anomalous Nernst Effect are consistent with the obtained magnetization results, presenting a linear response in the thermomagnetic signal, an interesting feature for sensor applications. Additionally, Anomalous Nernst Effect coefficient SANE values of 2.66μV/K and 1.93μV/K were estimated for the as-quenched and annealed ribbons, respectively. The interplay of the low magnetostrictive properties, soft magnetic behavior, linearity of the thermomagnetic response, and flexibility of these ribbons place them as promising systems to probe curved surfaces and propose multifunctional devices, including magnetic field-specialized sensors.

Magnetic signatures of a creosote oil contaminated site: case study in São Paulo, Brazil

Soils and groundwater contamination modifies the physical–chemical conditions of the environment, altering natural biogeochemical processes of the ground. As a result, several mineral transformations occur, in which iron plays a decisive role. The presence of iron enables the study of magnetic properties, improving the understanding of the geophysical signatures of highly dynamic environments (e.g., biogeochemical hotspots and contamination plumes). In this work, we seek to identify creosote biodegradation related to the precipitation of magnetic minerals on sediments at a contaminated site in São Paulo, Brazil. Several rock magnetism analyses were carried out to provide the magnetic mineralogy of the samples in terms of their composition, size, and abundance. We conducted high-temperature thermomagnetic curves, frequency-dependent magnetic susceptibility, anesthetic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) data, superparamagnetic concentration and dipole moment (SPCDM), and scanning electron microscopy (SEM) analyses. The magnetic signatures of the contaminated samples suggest an increase of superparamagnetic grains in the water table fluctuation zone if compared to the magnetic signatures of the uncontaminated samples. Thermomagnetic curves of contaminated samples showed a lower heterogeneity of the magnetic mineral phases than the uncontaminated ones. This work contributes to the advancement of the understanding of how natural biogeochemical processes are impacted by human actions, such as soil contamination, and even by climate change, which should affect soil redox conditions in periods of drought and flooding.

Experimental and theoretical investigation of FeCrVAl and related compounds

We have carried out a combined theoretical and experimental investigation of FeCrVAl, and the effect of Mn and Co doping on its structural, magnetic, and electronic band properties. Our first principles calculations indicate that FeCrVAl, FeCr0.5Mn0.5VAl, and FeCr0.5Co0.5VAl exhibit nearly perfect spin polarization, which may be further enhanced by mechanical strain. At the same time, FeCrV0.5Mn0.5Al and FeCrV0.5Co0.5Al exhibit a relatively small value of spin polarization, making them less attractive for practical applications. Using arc melting and high vacuum annealing, we synthesized three compounds FeCrVAl, FeCr0.5Mn0.5VAl, and FeCr0.5Co0.5VAl, which are predicted to exhibit high spin polarization. The room temperature x-ray diffraction patterns of all samples are fitted with full B2 type disorder with a small amount of FeO2 secondary phase. All samples show very small saturation magnetizations at room temperature. The thermomagnetic curves M(T) of FeCrVAl and FeCr0.5Co0.5VAl are similar to that of a paramagnetic material, whereas that of FeCr0.5Mn0.5VAl indicates ferrimagnetic behavior with the Curie temperature of 135 K. Our findings may be of interest for researchers working on Heusler compounds for spin-based electronic applications.

Role of graphene doped Al[formula omitted]O[formula omitted] flexible sheets as the substrate for Anomalous Nernst Effect study

GnP doped Al2O3 flexible tapes were produced through Tape Casting technique and used as substrates to produce Co2FeAl/GnP doped Al2O3 heterostructures. The Co2FeAl layers were grown using Magnetron Sputtering with a thickness of 30 nm. The integration of Tape Casting and Magnetron sputtering techniques allowed us to explore the thermomagnetic properties of the heterostructure through the Anomalous Nernst Effect. The structural and morphological properties of the substrates and the magnetization curves support the thermomagnetic results. The Co2FeAl ferromagnetic films presented isotropic magnetic behavior, which was reflected in the thermomagnetic curves through the stability of the curve’s shape irrespectively of the thermal gradient and angle of the external magnetic field used during the measurements. Our findings open new possibilities to produce flexible heterostructures with multifunctional properties, where the versatilities of the Tape casting and magnetron sputtering techniques bring new routes to build nanostructured materials.

Ferrite materials with high saturation magnetic induction intensity and high permeability for magnetic field energy harvesting: Magnetization mechanism and Brillouin function temperature characteristics

Manganese-zinc (MnZn) ferrites play an indispensable role in energy transform and harvesting. It is notable to mention that the combined enhancement of initial permeability (μi), saturation magnetic induction (Bs) and Curie temperature (Tc) is conducive to improving the output power and safety of magnetic field energy harvesting devices. In this paper, high permeability MnZn ferrites were synthesized by a conventional oxide ceramic method. The thermomagnetic curves of MnZn ferrites were fitted according to the Néel molecular field theory (NMFT) and the Brillouin function. Then the molecular field coefficient γaa, γab, γbb, and Tc were calculated. Aiming at the magnetization mechanism of permeability, the contributions of domain wall motion and spin rotation were analyzed by fitting the complex permeability spectrum. Furthermore, the temperature characteristics of μi and Bs are analyzed, and we simulated and compared the effect of different magnetic properties of MnZn ferrites on the output of non-invasive magnetic field harvester. The MnZn ferrites with superior comprehensive performance own potential applications for non-invasive magnetic field harvester.

Magnetic and Resonance Properties of a Low-Dimensional Cobalt–Aluminum Oxide–Germanium Film Tunnel Junction Deposited by Magnetron Sputtering

The temperature behavior of saturation magnetization and the temperature behavior of the integral signal intensity in electron magnetic resonance spectra is experimentally studied comprehensively using a low-dimensional Al2O3/Ge/Al2O3/Co (aluminum oxide–cobalt–aluminum oxide–germanium) tunnel junction with different deposition velocities of a ferromagnetic metal (Co) thin layer and non-magnetic layers (Al2O3/Ge/Al2O3). The cobalt ferromagnetic layer was deposited on aluminum oxide in two ways: in one cycle of creating the structure and with atmospheric injection before deposition of the cobalt layer. The thermomagnetic curves revealed the appearance of minima observed at low temperatures on both sides of the cobalt layer. Possible sources of precession perturbations at low temperatures can be explained by: the influence of the Al2O3 layer structure on the Al2O3/Co interface; residual gases in the working chamber atmosphere and finely dispersed cobalt pellets distributed over the cobalt film thickness. The work offers information of great significance in terms of practical application, for both fundamental physics and potential applications of ultrathin films.

Magnetic fabric of the early-Ediacaran Itapetim monzogranitic pluton: magma flow during oblique extension along strike-slip shear zones (Eastern Brazil)

The Itapetim pluton is a sigmoid-shaped intrusion bounded by the NE-SW, sinistral Itapetim and Taperoá shear zones. The Anisotropy of Magnetic Susceptibility (AMS) and Anhysteretic Remanent Magnetization (AARM) techniques and microstructural analysis were employed in order to characterize the pluton's emplacement mechanism and its relationships with regional tectonics. Microstructures are mainly sub-magmatic/incipient solid-state. Magnetic susceptibilities are typically paramagnetic (k ≤ 0.5 mSI). Thermomagnetic curves and hysteresis loops further indicate that negligible ferrimagnetic susceptibilities may be the result of fine-grained, oxidized magnetite inclusions. AARM fabrics do not show direct correlations with AMS fabrics. A WNW to E-W trending, shallowly-plunging magnetic lineation is associated with an intermediate-to steep-dipping magnetic foliation. The magnetic lineation locally displays intermediate plunges that may represent possible feeder zones for magma ascent during the initial stages of emplacement. The proposed emplacement model for the Itapetim pluton combines bulk NNW-SSE shortening with NE-SW trending simple shear along the Itapetim and Taperoá shear zones, and a WNW-ESE/E-W dextral stretching in the splay between both shear zones. The crystallizing magma mush flowed laterally along the splay during bulk NNW-SSE transpression, which is consistent with continental collision and subsequent extrusion of the Central Domain of the Borborema Province during the Neoproterozoic.

From conventional to inverse magnetocaloric effect in GdMn1-x Cr x O3

In this work, a phenomenological model (PM) is used to simulate the magnetocaloric effect (MCE) of GdMn1-x Cr x O3 (0 ≤x ≤0.4) (GMCO) samples through the modelling of experimental isofield thermo-magnetization curves. The results showed that the MCE of GMCO samples depends strongly on Cr content, achieving a conventional MCE for low Cr content (level of doping x ≤ 0.3). However, the MCE of a higher Cr content sample has an inverse MCE. The behaviour of MCE in GMCO samples indicated that GMCO compounds are interesting magnetocaloric materials and can fruitfully be functioned as cryogenic magnetic refrigerants below 40 K especially in radiation detectors for outer space research.

Formation of laser-induced periodic surface structures on amorphous Fe- and Co-based alloys and its impact on magnetic properties

Laser-induced periodic surface structures, so-called LIPSS, were obtained using linearly polarized femtosecond laser pulses on metal glasses with compositions Fe73.5Nb3Cu1Si15.5B7 and Co73Fe1Mn3Mo1Si13B9. The effect of laser parameters on spatial period and regularity of LIPSS was observed. Highly regular and low spatial frequency LIPSS with a period in the range of 850–950 nm was revealed using FESEM studies. The backscattered X-ray diffraction method revealed no devitrification of the amorphous structure upon LIPSS forming. Magnetic properties including saturation magnetization, coercive force, thermomagnetic curves were measured for both materials before and after LIPSS generation. It was observed that LIPSS impacts the coercive force of the amorphous Fe- and Co-based alloys.

Anisotropic magnetostructural transition in epitaxial Mn–Ni–Co–Ti Heusler alloy thin film

Heusler alloys are distinctive functional materials related to the phase transitions due to the strong magnetic and structural coupling. By the epitaxial strain from the rigid substrates, anisotropic magnetostructural transition (MST) might be observed in different crystalline orientations, leading to novel properties and functions. In this work, 62 nm thick (001)-oriented Mn49.1Ni34.0Co9.6Ti7.3 films are epitaxially grown on (001) MgO substrates. Clear MST is observed accompanied by a broad transition hysteresis from the thermomagnetic (M–T) curves with an out-of-plane (OP) magnetic field, while the transition hysteresis is absent under an in-plane (IP) magnetic field. It is related to the main lattice distortion occurring in OP orientation during the martensitic transformation because of the kinetic arrest of the IP structure by the MgO substrate. The spin glass (SG) phase is observed in IP orientation, which is lacking in OP orientation and the bulk. Clear exchange bias (EB) is observed in both the IP and OP directions. OP EB originates from exchange coupling between the ferromagnetic (FM) and antiferromagnetic phases, while IP EB is partially contributed by the exchange coupling between the SG and FM phases. It is ascribed to the establishment of metastable spin configuration in IP orientation with sufficient surrounding spins, but stable spin configuration in OP orientation due to the limited number of surrounding spins, which has been confirmed by Monte-Carlo simulation results. Our results provide a novel strategy for the modification of the physical properties of Heusler alloys and the design of novel magnetic devices.

Magnetic fabric and sedimentary characterization of near-slope to basinal deposits from the Chicontepec Formation, central and southern region of the Tampico-Misantla basin

We report results of a study of the Chicontepec Formation magnetic fabric in the central and southern region of the Tampico-Misantla basin in the state border region between Veracruz and Hidalgo. Samples were collected at 16 sites corresponding to two main facies associations: channel-fill facies and channel overbank facies. The channel facies dominate the relief developing prominent geoforms adjacent to low hills developed in facies dominated by shales. Measurements were made on channel and overbank facies. Laboratory analyses include anisotropy of magnetic susceptibility (AMS) and remanence anisotropy (AARM), as well as isothermal remanence acquisition (IRM) and thermomagnetic curves to characterize magnetic mineralogy. The magnetic susceptibility values of the entire collection are of the order of 40 to 70 x10-6 SI, so the susceptibility fabric is controlled by the paramagnetic fraction. The IRM acquisition curves are near saturation with inductions < 0.3 T and can be modeled with a low coercivity component contributing ~ 90% and a high coercivity component contributing < 10%. The thermomagnetic curves are dominated by paramagnetic phases. In the AMS fabric, three types of behavior are observed, corresponding to a lesser extent to sedimentary fabrics (k3 ~ vertical), and mostly to incipient and well-developed tectonic fabrics. The sedimentary fabrics are characterized by the imbrication of the magnetic foliation perpendicular to the paleocurrent data obtained in the field, which are generally consistent from NW to SE. At sites with tectonic fabric, which generally correspond to sites closer to the deformation front, the magnetic lineaments are well clustered in the NW quadrant with low plunges. AMS and AARM fabrics may combine in complex patterns. AARM appears to record either maximum axis directions that correspond to alignment of elongated particles perpendicular to flow or the direction of thrust motion.

Large magnetoresistance and magnetostrain in the batch-processed (Mn2Sb)1-xBix crystals (x = 0.06, 0.08, 0.10, 0.12)

We investigate the effects of Bi substitution on the structure, magnetization, magnetoresistance, and magnetostrain in single crystals of (Mn2Sb)1-xBix (x = 0.06, 0.08, 0.1, and 0.12). It is found that the element Bi can be partially substituted for Sb in the tetragonal Mn2Sb main phase, and partially forms the MnBi and Bi-rich secondary phases. The introduction of Bi causes the simultaneous lattice shrinkages in a and c for the main phase, and also enhances the transition temperature of the first-order magnetoelastic ferrimagnetic-antiferromagnetic transition. Such an enhancement in transition temperature due to contraction of the unit-cell volume contraction is also experimentally proven based on the thermomagnetic curves under hydrostatic pressure for a crystal with × = 0.12. The volume contraction rate due to hydrostatic pressure is deduced to be −0.0386 Å3/GPa. Moreover, the highest values of in-plane magnetoresistance (∼-30%) and magnetostrain (∼−1448 ppm) with good reversibility are attained at 120 K under 30 kOe, which was proven to be the optimal magnetic field. Our experimental results show that Mn2Sb-based intermetallic compounds can be considered promising candidates for low-cost, multifunctional magnetic materials.

Cd-doping effects in Ni–Mn–Sn: experiment and ab-initio study

Martensitic transformation (MT), magnetic properties, and magnetocaloric effect (MCE) in Heusler-type Ni47Mn40Sn13−x Cd x (x= 0, 0.75, 1, 1.25 at. %) metamagnetic shape memory alloys (MetaMSMAs) are investigated, both experimentally and theoretically, as a function of doping with Cd. Ab-initio computations reveal that the ferromagnetic (FM) configuration is energetically more favorable in the cubic phase than the antiferromagnetic (AFM) state in undoped and doped alloys as well. Moreover, it is revealed that the alloys in the ground state exhibit a tetragonal structure confirming the existence of MT, in agreement with the experiments. It was indicated, both in theory and practice, that a reduction of the unit cell volume and an increase of the MT temperature as a function of the Cd doping. Indirect estimations of MCE in the vicinity of MT were carried out by using thermomagnetization curves measured under different magnetic fields up to 5 T. The results demonstrated that the doped alloys exhibit enhanced values of the inverse MCE comparable with those of Ni-Mn-based MetaMSMAs. Maximum magnetic entropy change in a field change of 2 T increases from 3.0 for the undoped alloy to 3.4 and 5.0 for the alloys doped with 0.75 and 1 at.% of Cd, respectively. The inverse and conventional MCE were explored by direct measurements of the adiabatic temperature change under the magnetic field change of 1.96 T. The Cd doping increased the maximum of inverse MCE by nearly 78% from 0.9 K to 1.6 K for the undoped and doped alloys, respectively. The results depicted that Cd doping can effectively tailor the structural, magnetic, and MCE properties of the Ni–Mn–Sn MetaMSMAs.

Production of thick Gd freestanding films for energy conversion applications

The use of magnetocaloric material in microelectromechanical energy conversion devices requires freestanding films with smooth surfaces to allow fast and efficient heat transfer. We present and discuss a mechanical method to produce freestanding thick Gadolinium films showing excellent magnetic properties, and high-quality surfaces on both sides. Polycrystalline Gd films have been prepared by sputtering on a silicon substrate with a thermally oxidized layer with different thicknesses. Films have been grown at 543 K. Tantalum and Tungsten have been used as buffer layers because of their corrosion resistance and of their thermal conductivity. The magnetic and surface properties of the freestanding films are presented, with special focus on the role of using different capping layers. Full characterizations of the three freestanding Gd films, including AFM measured surface roughness, XRD patterns, Curie temperature, thermo-magnetic curves, and maximum entropy change, are presented and discussed.

Integration of chemical fractionation, Mössbauer spectrometry, and magnetic methods for identification of Fe phases bonding heavy metals in street dust

Street dust is one of the most important carriers of heavy metals (HMs) originating from natural and anthropogenic sources. The main purpose of the work was to identify which of Fe-bearing phases bind HMs in street dust. Magnetic parameters of the Fe-bearing components, mainly magnetically strong iron oxides, are used to assess the level of HM pollution. Chemical sequential extraction combined with magnetic methods (magnetic susceptibility, magnetization, remanent magnetization) allowed determining the metal-bearing fractions and identifying the iron forms that are mostly associated with traffic-related HMs. The use of Mössbauer spectrometry (MS) supplemented by magnetic methods (thermomagnetic curves and psarameters of hysteresis loops) enabled precise identification and characterization of iron-containing minerals. The classification of HMs into five chemical fractions differing in mobility and bioaccessibility revealed that iron is most abundant (over 95%) in the residual fraction followed by the reducible fraction. HMs were present in reducible fraction in the following order: Pb>Zn>Mn>Cr>Ni>Fe>Cu, while they bound to the residual fraction in the following order: Fe>Ni>Cr>Mn>Pb>Cu>Zn. The signature of the anthropogenic origin of street dust is the presence of strongly nonstoichiometric and defected grains of magnetite and their porous surface. Magnetite also occurs as an admixture with maghemite, and with a significant proportion of hematite. A distinctive feature of street dust is the presence of metallic iron and iron carbides. Magnetic methods are efficient in the screening test to determine the level of HM pollution, while MS helps to identify the iron-bearing minerals through the detection of iron.

Tectonics, cooling rates and temperatures during emplacement of the Rajmahal traps, India

Paleomagnetism is used to decipher the emplacement setting of the Rajmahal traps. Sampling was restricted to flows with known age. Three magnetic minerals are identified, titanomaghemite, Ti-poor magnetite, and possibly Mg–Al-rich titanomagnetite. Detailed analysis of thermomagnetic curves revealed that the emplacement temperature for Rajmahal traps was lower than 600 °C. Two magnetization components are distinguished. The low coercivity component, probably chemical remanent magnetization, is erased in fields up to 3.1 mT. In contrast, the high coercivity thermal remanent magnetization is more stable, sometimes throughout the demagnetization. The paleomagnetic results show that the sites have suffered up to 11° of clockwise and 17° of counter-clockwise rotation. As there is no field evidence of faulting and pervasive deformation, we propose that this rotation is probably due to penecontemporaneous joints or faults. These structures could have acted as conduits for fluids that increased the cooling rate.

An integrated approach combining magnetic, geochemical and particle-based techniques to assess metal(loid) loadings in urban venues frequented by children

Urban venues frequented by children, such as playgrounds, are potentially important sources of exposure to anthropogenic metal(loids). Environmental quality of outdoor playgrounds is mainly assessed through direct geochemical monitoring, which is time-consuming and expensive. In this study we adapted a multidisciplinary approach combining magnetic measurements, geochemical analyses, particle-based techniques and bioaccessibility data so as to evaluate the applicability of magnetic methods as a low-cost and easy-to-use technology to monitor pollution level in public playgrounds. Playground sands were collected and their magnetic characteristics were studied in detail aiming to gain helpful additional details in relation to the type, concentration and particle-size distribution of the sand-bound magnetic particles. The obtained χlf values indicated an enhanced level of sand-bound magnetic components, while the dominant control of SSD grains on the magnetic load of playground-PG sands was revealed. Hysteresis parameters and thermomagnetic curves indicated low-coercivity ferrimagnetic minerals, such as magnetite and/or maghemite, as the predominant magnetic carriers. Ratios of χARM/χlf and χARM/SIRM indicated the dominance of coarser anthropogenic magnetic grains in the sampled PG sands compared to other recreational areas. Correlation analysis among magnetic variables and reported metal(loid) contents designated χARM as a more effective indicator for the detection of anthropogenic load in PG sand samples than χlf or SIRM. Simultaneously, through geochemical analyses in magnetic extracts separated from PG sands, metal(loid) contents were notably enriched in the magnetic fraction validating their strong affinity with sand-bound magnetic particles. Finally, bioaccessibility tests revealed lower UBM-extracted fractions for the magnetic extracts of PG sands compared to bulk samples. However, arsenic (As) was more bioaccessible in the sand-bound magnetic particles raising serious concerns for the children exposed to playground sands.

High‐Latitude Paleointensities During the Cretaceous Normal Superchron From the Okhotsk–Chukotka Volcanic Belt

AbstractWe conducted an absolute paleointensity survey on 74 lava flows from the Okhotsk–Chukotka Volcanic Belt (NE Russia), emplaced 90–83 Ma toward the end of the Cretaceous Normal Superchron (CNS, 121–84 Ma). Relying on preliminary results, we restricted our analysis to eight lava flows (140 Thellier–Coe experiments), two of which also yielded successful Wilson determinations. A detailed analysis of the magneto‐mineralogy—based on X‐ray structural analysis, reflected‐light microscopy, and thermomagnetic curves—indicates that the determinations from solely two lava flows can be fully trusted, with remanence carriers (a) dominated by low‐titanium titanomagnetite, (b) showing unambiguous traces of high‐temperature oxidation, and (c) yielding partial thermoremanent magnetization (pTRM) tails representative of pseudo‐single‐domain grains. Recalculated in terms of geomagnetic dipole strength, our two successful flow‐mean determinations yield virtual dipole moments of 4.76 ± 0.26 × 1022 Am2 (N = 7) and 9.07 ± 0.84 × 1022 Am2 (N = 8). Using an updated version of the paleointensity database for the Cretaceous epoch, we stress that determinations based on nonglassy whole rocks, submarine basaltic glasses, and single crystals are mutually inconsistent, suggesting a separate analysis of their distributions is more appropriate. Despite statistically indistinguishable estimates of dipole strength before and after the onset of the CNS—in accord with recent studies refuting the existence of a strict correlation between chron duration and dipole moment—we found that the distribution of dipole moments during the CNS is slightly bimodal with a leptokurtic dominant mode, thus more inclined to produce outliers and suggesting distinct geomagnetic field behavior during the CNS.

Alloying-driven transition between ferromagnetism and antiferromagnetism in UTGe compounds: UCo1−xIrxGe

The evolution of magnetic properties of isostructural and isoelectronic solid solutions of the superconducting itinerant $5f$-electron ferromagnet (FM) UCoGe with antiferromagnet (AFM) UIrGe was studied by magnetization, AC susceptibility, specific heat, and electrical resistivity measurements of a series of $\mathrm{U}{\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x}\mathrm{Ge}$ compounds in polycrystalline and single-crystalline form at various temperatures and magnetic fields. Both the weak FM and superconductivity in UCoGe were found to have vanished already for very low Ir substitution for Co $(x=0.02)$. The AFM of UIrGe is gradually suppressed. This is documented by a rapid decrease in both N\'eel temperature and the critical field of the metamagnetic transition with decreasing Ir concentration, which both tend to vanish just above $x=0.8$. The section of the $T\text{\ensuremath{-}}x$ phase diagram in the range $0.02\ensuremath{\le}x\ensuremath{\le}0.8$ is dominated by a correlated paramagnetic phase (CPM) exhibiting very broad bumps in temperature dependencies of $b$-axis magnetization and specific heat developing with increasing $x$. For $x\ensuremath{\ge}0.24$, wide peaks appear in the $c$-axis thermomagnetic curves due to AFM correlations which may eventually lead to frozen incoherent spin configurations at low temperatures. The CPM is also accompanied by specific electrical resistivity anomalies. The $T\text{\ensuremath{-}}x$ phase diagram determined for the $\mathrm{U}{\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x}\mathrm{Ge}$ compounds contrasts with the behavior of the related $\mathrm{U}{\mathrm{Rh}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x}\mathrm{Ge}$ system, which was reported to exhibit an extended concentration range of stable FM in Rh-rich compounds and a discontinuous transformation between the FM and AFM phases at a critical Rh-Ir concentration. The striking difference is tentatively attributed to (a) the instability of tiny U moment in the weak itinerant FM UCoGe induced by substitutional disorder at already very low Ir doping, (b) the rather stable U moment in URhGe formed by much less delocalized $5f$ electrons assisted by weakly varying lattice parameters of $\mathrm{U}{\mathrm{Rh}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x}\mathrm{Ge}$ compounds.