Additional Magnetic Anomaly Research Articles

High-entropy effect at rare-earth site in DyNi

We report the structural and magnetic properties of RNi (R = Dy, Tb1/3Dy1/3Ho1/3, and Gd1/5Tb1/5Dy1/5Ho1/5Er1/5) to investigate the high-entropy effect at the rare-earth site. The lattice parameters are almost unchanged by the increase of configurational entropy, which is due to the successive partial substitution of Dy by a pair of rare earth elements located on both sides of Dy in the Periodic Table. All compounds exhibit ferromagnetic ground states. The replacement of Dy with Tb + Ho, which does not have magnetic interactions in competition with Dy, does not affect the magnetic ordering temperature. Although (Gd1/5Tb1/5Dy1/5Ho1/5Er1/5)Ni shows the Curie temperature close to that of DyNi, an additional magnetic anomaly, which would be a spin reorientation, is observed probably due to the introduction of competing magnetic interactions between R = Gd and Er compounds and R = Tb, Dy, and Ho ones. We have also assessed the magnetocaloric effect, and the configurational entropy dependence of the magnetic entropy change reflects that of the temperature derivative of the magnetic susceptibility. Our analysis suggests the possibility of enhancing magnetocaloric properties by designing the anisotropy of rare-earth magnetic moments in the high-entropy state.

Resistivity and magnetoresistance properties of R2NiSi3 (R = Gd, Dy, Ho, Er, Tm) compounds

The resistivity and magnetoresistance behaviour of the hexagonal intermetallic compounds R2NiSi3(R = Gd, Dy, Ho, Er, and Tm) are reported here. All the studied polycrystalline compounds exhibit metallic behaviour along with additional magnetic anomalies at low temperatures. A well-defined resistivity minima is observed in Gd2NiSi3and Dy2Ni0.87Si2.95at a temperature much higher than their respective magnetic transition temperatures. The anomaly has been ascribed to the charge carrier localization caused by magnetic precursor effect. Magnetic field induced crossover from positive to negative magnetoresistance (MR) behaviour associated with antiferromagnetic ground state is evidenced for Gd2NiSi3and Er2NiSi3in the low temperature region. Although Tm2Ni0.93Si2.93does not exhibit any long range magnetic order down to 2 K, a sudden drop in resistivity behaviour is observed below ~10 K. Presence of short range magnetic correlation observed in a wide temperature range, much beyond their respective magnetic ordering temperatures, has been argued to be responsible for achieving finite negative MR for all the compounds. A subtle resemblance between the observed transport anomalies and the magnetic properties of these systems have been discussed.

Antiferromagnetism and the emergence of frustration in the sawtooth lattice chalcogenide olivines Mn2SiS4−xSex ( x=0–4 )

The magnetism in the sawtooth lattice of Mn in the olivine chalcogenides, ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4\ensuremath{-}x}{\mathrm{Se}}_{x}$ ($x=1--4$), is studied in detail by analyzing their magnetization, specific heat, and thermal conductivity properties and complemented with density functional theory calculations. The air-stable chalcogenides are antiferromagnets and show a linear trend in the transition temperature ${T}_{N}$ as a function of Se content ($x$), which shows a decrease from ${T}_{N}\ensuremath{\approx}86\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4}$ to 66 K for ${\mathrm{Mn}}_{2}{\mathrm{SiSe}}_{4}$. Additional magnetic anomalies are revealed at low temperatures for all the compositions. Magnetization irreversibilities are observed as a function of $x$. The specific heat and the magnetic entropy indicate the presence of short-range spin fluctuations in ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4\ensuremath{-}x}{\mathrm{Se}}_{x}$. A spin-flop antiferromagnetic phase transition in the presence of applied magnetic field is present in ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4\ensuremath{-}x}{\mathrm{Se}}_{x}$, where the critical field for the spin flop increases from $x=0$ towards 4 in a nonlinear fashion. Density functional theory calculations show that an overall antiferromagnetic structure with ferromagnetic coupling of the spins in the $ab$ plane minimizes the total energy. The band structures calculated for ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4}$ and ${\mathrm{Mn}}_{2}{\mathrm{SiSe}}_{4}$ reveal features near the band edges similar to those reported for Fe-based olivines suggested as thermoelectrics; however the experimentally determined thermal transport data do not support superior thermoelectric features. The transition from long-range magnetic order in ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4}$ to short-range order and spin fluctuations in ${\mathrm{Mn}}_{2}{\mathrm{SiSe}}_{4}$ is explained using the variation of the Mn-Mn distances in the triangle units that constitutes the sawtooth lattice upon progressive replacement of sulfur with selenium. Overall, the results presented here point towards the role played by magnetic anisotropy and geometric frustration in the antiferromagnetic state of the sawtooth olivines.

Weak-ferromagnetism of CoF3 and FeF3

Cobalt trifluoride CoF3 and Iron trifluoride FeF3 are known as a G-type antiferromagnetic structure in the R-3c rhombohedral crystal structure. The neutron powder diffraction study had been reported that Co antiferromagnetic spin direction is parallel to 3-fold crystal axis in CoF3 whereas Fe antiferromagnetic spin direction is perpendicular to 3-fold crystal axis in FeF3. However, the resolution of early neutron powder diffractometer is not enough to distinguish between (003)h and (101)h rhombohedral magnetic peaks definitely.We employed high-resolution powder neutron diffractometer(SuperHRPD) at room temperature in MLF, J-PARC. The time-of-flight neutron powder diffractometer distinguishes clearly between (003)h and (101)h magnetic peaks. Then, we confirmed the proposed magnetic structural models of CoF3 and FeF3 at room temperature. In a magnetic susceptibility, CoF3 displays the additional magnetic anomaly at T* = 40 K. We discussed possible weak-ferromagnetism by the interplay of spin tilting and Dzyaloshinski-Moriya interactions.

On the edge of periodicity: Unconventional magnetism of Gd117Co56.4Sn114.3

Magnetization measurements reveal the onset of magnetic ordering at TC = 65 K followed by three additional magnetic anomalies at T1 = 47 K, T2 = 28 K, and T3 = 11 K in Gd117Co56.4Sn114.3 – a compound with a giant cubic unit cell that crystallizes in the Dy117Co56Sn112 structure type with space group Fm3¯m and lattice parameter a = 30.186 Å. The magnetic ordering temperature increases with applied magnetic field; however, the analysis of magnetic data indicates that antiferromagnetic interactions also play a role in the ground state. AC magnetic susceptibility confirms multiple magnetic anomalies and shows minor frequency dependence. The local magnetic ordering below 60 K is supported by the Mössbauer spectroscopy. A single broad anomaly is detected at T3 in the heat capacity; we suggest that magnetic domains form below this temperature. These data highlight unique features of magnetism in this and, potentially, other rare-earth intermetallics crystallizing with giant unit-cells where the exchange correlation lengths are much shorter when compared to the periodicity of the crystal lattice.

Insight into the magnetism of a distorted Kagome lattice, Dy3Ru4Al12, based on polycrystalline studies

The layered compound with distorted Kagome nets, Dy3Ru4Al12, was previously reported to undergo antiferromagnetic ordering below (TN=) 7 K, based on investigations on single crystals. Here, we report the results of our investigation of AC and DC magnetic susceptibility (χ), isothermal remnant magnetization (MIRM), heat-capacity, magnetocaloric effect and magnetoresistance measurements on polycrystals. The present results reveal that there is an additional magnetic anomaly around 20 K, as though the Néel order is preceded by the formation of ferromagnetic clusters. We attribute this feature to geometric frustration of magnetism. In view of the existence of this phase, the interpretation of the linear-term in the heat-capacity in terms of spin-fluctuations from the Ru 4d band needs to be revisited. Additionally, in the vicinity of TN, AC χ shows a prominent frequency dependence and, below TN, MIRM exhibits a slow decay with time. This raises a question whether the antiferromagnetic structure in this compound is characterized by spin-glass-like dynamics. In contrast to what was reported earlier, there is a change in the sign of the magnetoresistance (MR) at the metamagnetic transition. A butter-fly-shaped (isothermal) MR loop (interestingly spanning over all the four quadrants) is observed at 2 K with distinct evidence for the magnetic phase co-existence phenomenon in zero field after travelling through metamagnetic transition field. The results on polycrystals thus provide additional information about the magnetism of this compound, revealing that the magnetism of this compound is more complex than what is believed, due to geometric frustration intrinsic to Kagome net.

Magnetic behavior of Gd3Ru4Al12, a layered compound with distorted kagomé net

The magnetic behavior of the compound, Gd3Ru4Al12, which was reported about two decades ago to crystallize in a hexagonal structure (space group P63/mmc), has not been investigated in the past literature despite interesting structural features (that is, magnetic layers and triangular as well as kagomé-lattice features favoring frustrated magnetism) characterizing this compound. We report here the results of studies of magnetization, heat capacity and magnetoresistance in the temperature range T = 1.8–300 K. The results establish that there is a long-range magnetic order of antiferromagnetic type below (TN =) 18.5 K, despite a much larger value (~80 K) of paramagnetic Curie temperature with a positive sign characteristic of ferromagnetic interaction. We attribute this to geometric frustration. The most interesting finding is that there is an additional magnetic anomaly below ~55 K before the onset of long-range order in the magnetic susceptibility data. Concurrent with this observation, the sign of isothermal change in entropy, ΔS = S(0) − S(H), where H is the externally applied magnetic field, remains positive above TN, with a broad peak. This observation indicates the presence of ferromagnetic clusters before the onset of long-range magnetic order. Thus, this compound may serve as an example of a situation in which magnetic frustration due to geometrical reasons faces competition from such magnetic precursor effects. There is also a reversal of the sign of −ΔS in the curves for lower final fields (H < 30 kOe) on entering the magnetically ordered state consistent with the entrance to an antiferromagetic state. The magnetoresistance behavior is consistent with the above conclusions.

Structure, charge ordering and physical properties of Yb2Fe3O7

The structural and physical properties of the layered Yb2Fe3O7 have been extensively investigated. Transmission electron microscopy (TEM) observations at room temperature reveal the presence of diffuse zigzag-type streaks at 1/3(h h l) running along the c* axis direction, suggesting the presence of a charge ordered state with a shorter coherence length in comparison with that in Lu2Fe3O7. The measurements of magnetization demonstrate that the replacement of Lu3+ by the magnetic Yb3+ ion in this layered system could result in visible effects on the low-temperature magnetic properties: the ferrimagnetic phase transition temperature decreases and an additional magnetic anomaly possibly attributed to antiferromagnetic coupling between Yb and Fe layers appears at around 50 K. Analysis of the dielectric properties shows that the Yb2Fe3O7 material in general has a large dielectric constant of about 5000 at room temperature, and a broader relaxation time distribution in comparison with ErFe2O4.

Specific heat and AC magnetic susceptibility of the alkali-metal iron antimonides (Na,K)Fe4Sb12 and BaFe4Sb12

Abstract The newly synthesized alkali metal iron antimonides with a filled skutterudite structure NaFe 4 Sb 12 and KFe 4 Sb 12 together with the known BaFe 4 Sb 12 have been investigated by means of specific heat and AC susceptibility measurements. The alkali metal samples exhibit ferromagnetism below 85 K and also show additional magnetic anomaly at 40 K.

Ferromagnetic ordering in alkali-metal iron antimonides: NaFe4Sb12 and KFe4Sb12.

New alkali-metal compounds with the filled-skutterudite structure were synthesized and their chemical and physical properties investigated. X-ray diffraction, microprobe, and chemical analysis established the structure and the composition without defects on the cation site. Magnetization, ac susceptibility, specific heat, resistivity, and NMR or NQR demonstrated NaFe4Sb12 to be ferromagnetic below approximately 85 K and to exhibit an additional magnetic anomaly around 40 K. Band structure calculations find a large density of states at the Fermi energy and a ferromagnetic ground state. Similar behavior was observed for KFe4Sb12.

Late Tertiary tectonic evolution of the seafloor spreading system off the coast of California between the Mendocino and Murray Fracture Zones

A forward modeling program of two‐plate spreading on a sphere was utilized to reconstruct the late Tertiary tectonic evolutionary history of the seafloor spreading system off the coast of California between the Mendocino and Murray fracture zones. This analytical technique was applied to a unique set of magnetic anomaly data consisting principally of a new digital data set: the original Raff‐Mason data from one of the most significant marine geophysical surveys: the 1955–1956 USCGS Pioneer survey. All the data, critical for the development of seafloor spreading and plate tectonic theory, were digitized (32 ° to 52°N); however, only the southern half was analyzed in this study. Investigation of the Raff‐Mason data, supplemented by additional magnetic anomaly data, indicates that the primary mechanism for reorganization of this part of the Pacific‐Farallon ridge, as the spreading center migrated toward and arrived at the North American trench, was rift propagation. The model presented here, containing a total of 21 propagation episodes from 36 Ma to 19.8 Ma (initiation time of the last propagator), shows generally excellent agreement with the isochrons identified from the magnetic anomaly pattern, although minor exceptions exist locally in complex areas. A step‐by‐step evolution of the model is presented in a series of reconstructions documenting the history of spreading between the Pacific plate and five smaller plates created from the breakup of the large ancestral Farallon plate. These plates include the Vancouver plate north of the Pioneer, the Farallon plate existing between the Murray and Pioneer transforms prior to the major reorganization at chron 10 (30.33 Ma), and the Arguello, Monterey, and Reyes microplates with northwest‐southeast spreading existing after chron 10.

Experimental Low-Altitude Aeromagnetic Reconnaissance for Petroleum in Arctic National Wildlife Refuge, Alaska, Using Horizontal Gradients--A Progress Report: ABSTRACT

Variations in the earth's magnetic field arising from areally restricted increased amounts of shallow-buried magnetite over hydrocarbon deposits have been mapped in the Arctic National Wildlife Refuge and elsewhere is northern Alaska. The anomalies have been delineated with a low-flying (90 m; 295 ft) magnetic horizontal gradiometer mounted on a fixed-wing airplane. Limited data from stable carbon isotope and remanent magnetism measurements of rock cores form the Cape Simpson region strongly suggest that the magnetic anomalies result from the chemical reduction of iron oxides in the presence of seeping hydrocarbons. Relatively large magnetic contrast between typical sedimentary rocks and those locally enriched with this epigenetic magnetite results in distinctive high wav -number and low-amplitude total field anomalies. Magnetometers extended from each wingtip and in a tail stinger permit calculation of the resultant horizontal gradient vector relative to the flight path. This calculation provides data for the unmeasured area between adjacent flight lines spaced at 1.5 km (.9 mi), thereby allowing End_Page 451------------------------------ generation of accurate computer-enhanced images or maps. Problems related to diurnal variations and solar storms at high magnetic latitude are largely overcome because changes in the total magnetic field do not significantly affect the magnetic gradient. Analysis of an experimental survey, covering 4,418 line km (2,745 line mi), suggests that the Marsh Creek anticline in the Arctic National Wildlife Refuge is prospective for oil and/or gas. Additional magnetic anomalies were also identified. Although the effect of permafrost on epigenetic processes has not been investigated, the data suggest that special purpose aeromagnetic surveying may be a useful and relatively inexpensive way to explore for oil and gas in this hostile environment. End_of_Article - Last_Page 452------------

Cuvier Basin: A product of ocean crust formation by Early Cretaceous rifting off western Australia

Four high-quality seismic refraction profiles were recorded parallel to the structural grain in the Cuvier Basin and adjacent Wharton Basin to study the nature of the earth's crust in this area. The principal result of this experiment is that this area is generally floored with oceanic crust. No transitional velocity structure exists at the base of the continental slope. Departures from a standard oceanic crustal section are observed on an intermediate profile that are attributed to structural complications on the flank of an abandoned spreading ridge. Additional magnetic anomaly profiles in the eastern Cuvier Basin are used to correlate the lineations in that area with Early Cretaceous reversals M-5 to M-10. This correlation dates the onset of plate separation at 120–125 m.y., essentially contemporaneous with the opening of the Perth Basin to the south. However, it leaves a 220-km gap between M-4 and M-5 in the Cuvier Basin that suggests a ridge jump of that magnitude occurred nominally at 118 m.y. Early Cretaceous magnetic lineations northwest of the Exmouth Plateau suggest that spreading at the seaward edge of the Exmouth Plateau began 120 m.y. ago, while Late Jurassic marine sediments and fault structures landward of the Exmouth Plateau suggest rifting in that area at 155 m.y.