Anomalous Magnetic Susceptibility Research Articles

Magnetoelastic coupling, negative thermal expansion, and two-dimensional magnetic excitations in FeAs

We present a temperature-dependent investigation of the local structure and magnetic dynamics of the FeAs binary. The magnetic susceptibility $\ensuremath{\chi}(T)$ result shows an anomalous broad feature up to 550 K, with \ensuremath{\chi} continuing to increase above the N\'eel antiferromagnetic ordering temperature (${T}_{\mathrm{N}}=70\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), peaking at \ensuremath{\sim}250 K, then decreasing gently above. It is remarkable that this peak susceptibility temperature corresponds to the onset of anisotropic negative thermal expansion in both the $a$ and $c$ axes, suggesting that magnetic interactions are affecting the structure even well above the N\'eel point. A systematic investigation into local bonding correlations, from time-of-flight neutron pair distribution function analyses, shows the octahedral volume around each Fe site growing monotonically while adjacent octahedra tilt toward one another before relaxing away past this peak in the anomalous magnetic susceptibility. We use inelastic-neutron scattering to map spin-wave excitations in FeAs at temperatures above and below the ${T}_{\mathrm{N}}$. We find magnetic excitations near ${T}_{\mathrm{N}}$ to be very different from the excitations in the ground state at 1.5 K. Spin waves measured at 1.5 K are three dimensional (3D), however, in the vicinity of the magnetic transition, the magnetic fluctuations clearly indicate two-dimensional (2D) character in this intrinsically 3D crystal structure. Unlike the undoped 2D parents of iron-arsenide superconductors, where the magnetic correlations are considerably weaker along the $c$ axis than in the $\mathit{ab}$ plane, inelastic neutron scattering here shows that the spin fluctuations in the 3D FeAs binary are nearly 2D in the $\mathit{bc}$ plane at 90 K. These results demonstrate the importance of short-range correlations in understanding the magnetic properties of transition-metal binaries, and suggest how 2D excitations, even in a 3D structure, can potentially become a breeding ground for unconventional superconductivity.

Emergence of Griffiths phase and exploiting magnetic ordering state in the intermetallic LaCeCo7

We have studied the evolution of Griffiths phase and the magnetic ordering state in the intermetallic compound LaCeCo7. A ferromagnetic-like ordering phase is built up at 260 K but an anomalous magnetic susceptibility related to the Griffiths phase occurs in 300 K. The studies of critical behavior confirm that LaCeCo7 exhibits a second-order paramagnetic-ferromagnetic phase transitions. The critical exponents of β = 0.665, γ = 1.151, and δ = 2.728 determined by the magnetic field dependence on magnetic entropy change method deviate a little from the theoretical estimated long-range mean-field model (β = 0.5, γ = 1.0, and δ = 3.0). This result implies that the disorder effect in the intermetallic LaCeCo7 possibly plays a key role on the anomalous critical behavior and the establishment of magnetic ordering phase.

Sources of paleomagnetic signal in iron-bearing marine sedimentary deposits

This paper presents the results of interpreting the change in the magnetic signal of ferruginous marine sedimentary rocks using the example of the Cretaceous-Paleogene Bakchar deposit (Western Siberia). Anomalous magnetic susceptibility (MS) values are determined by two factors: (a) a diagenetic process accompanied by the formation of pyrrhotite, greigite and siderite in ironstones, (b) an increase detrital input with deposition of magnetite and ilmenite in the siltstones on the background of climate moistening and intense weathering. It is shown that increased MS values in iron-bearing marine sedimentary deposits can be associated with the formation of ferrimagnetic sulfides, which indicates to the methane emission through ancient marine bottom sediments.

Neutron Laue diffraction study of the complex low-temperature magnetic behaviour of brownmillerite-type Ca2Fe2O5

The atomic and magnetic structure of brownmillerite Ca2Fe2O5has been refined against single-crystal neutron Laue diffraction data collected at 300, 100 and 10 K under zero-field and low-magnetic field (35 Oe = 35 × 103/4π A m−1) conditions. Ca2Fe2O5is a canted G-type antiferromagnet withPcm′n′ symmetry, the magnetic moments on Fe being directed approximately along the crystallographiccaxis at room temperature. The refinement results show clearly that this magnetic structure persists down toT= 10 K, despite a previous suggestion that an anomalous magnetic susceptibility enhancement observed in Ca2Fe2O5single crystals between 40 and 140 K might signify a reorientation of the antiferromagnetic easy axis fromctoabelow 40 K. Alternative explanations for this susceptibility anomaly are considered in terms of the evidence for partial or short-range loss of order in the anomalous regime, possibly due to the presence of multiple competing sublattice interactions.

The Blagodatnoe gold-sulfide deposit (Yenisei Ridge, Russia): the nature of geophysical anomalies and the succession and causes of formation of petrophysical zoning

Data on geophysical fields and petrophysical heterogeneity, parageneses and the thermodynamic conditions and age of their formation, and fluid inclusions were used for the genetic reconstruction of petrophysical zoning at the Blagodatnoe gold-sulfide deposit. Petrophysical associations of the preore and ore stages of the deposit formation are clearly reflected in anomalies of the magnetic and natural electric fields and the aureoles of radioactive elements. At the early preore stage (752Ma), reduced solutions with high activity of K, enriched in U, Th, and, probably, Au, were supplied to intensely foliated tectonic zones. Their interaction with initial metasedimentary rocks gave rise to contiguous zones of quartz–muscovite and chlorite metasomatic rocks. Accompanying graphitization led to a high electrochemical activity of the metasomatic rocks, which generated anomalies of up to –300mV in the natural electric field; the most intensely carbonized zones became enriched with U (up to 6.5×10–4%) and, probably, Au. The quartz–muscovite metasomatic rocks accumulated Th and K (up to 29×10–4% and 4%, respectively), whereas the chlorite metasomatic rocks accumulated rock-forming elements (particularly Fe), which led to the compaction of these rocks and the acceleration due to gravity in local positive anomalies. The nonmagnetic character of the fresh pre-ore metasomatic rocks suggests the predominantly pyritic composition of early sulfides. At the ore stage (698Ma), the minerals were deposited from H2O–CO2–As–S solutions at 560 to 315°C. The activity of these solutions caused a redistribution of radioactive elements and a high petrophysical differentiation of the ore-bearing structure. The amplitudes of the anomalies above this structure vary from 500 to 80 nT in the magnetic field and from –130 to +10mV in the natural electric field. It has been found that the hydrothermal fluid hardly affected the polarization properties of graphitized rocks at the maximum temperatures but caused an intense removal of U and the development of magnetic pyrrhotite after pyrite. The temperature decrease in the mineral-forming system was favorable for the formation of siderite. Carbonaceous schists which experienced carbonatization lost their electrochemical activity. The binding of carbon dioxide in the solid phase influenced the migration capability of trace elements and their zonal distribution. With this evolution of the solution, Th accumulated at the lower levels of the mineralized zone, whereas the upper levels of the deposit became enriched with U. Productive gold–arsenopyrite–pyrite–pyrrhotite paragenesis with anomalous magnetic susceptibility evolved at the ore stage. The late galena–sphalerite–chalcopyrite paragenesis (365Ma) was of strictly local occurrence and reduced the magnetic susceptibility of ores.

Anomalous magnetic susceptibility in Nd0.5Sr0.5MnO3 manganite single crystals

Chaotic oscillations of the low-frequency magnetic susceptibility χ in a Nd0.5Sr0.5MnO3 (NSMO) manganite single crystal have been observed in the temperature interval of the coexistence of the ferro- and antiferromagnetic phases of the given composition. It is established that the amplitude of χ oscillations depends on the constant bias magnetic field. The appearance of susceptibility oscillations in NSMO single crystals is attributed to the formation of magnetic transition regions, which can exist at the boundaries between ferro- and antiferromagnetic phases.

Anomalous magnetic susceptibility and inhomogeneous magnetic state in lanthanum strontium manganites

Anomalous magnetic susceptibility χ has been observed in mono-and polycrystalline (ceramic) samples of lanthanum strontium manganites. The oscillations of χ observed for single crystal samples in the vicinity of the Curie temperature (and in the paramagnetic region) are explained by the existence of magnetic clusters. The appearance of susceptibility oscillations in ceramic samples is attributed to the formation of magnetic clusters, which may occur both in grains (at the interface between ferro-and antiferromagnetic phases) and at the grain boundaries.

Anomalous magnetic susceptibility values and traces of subsurface microbial activity in carbonate banks on San Salvador Island, Bahamas

Pure limestones beneath the paleosols on San Salvador Island, Bahamas, contain strong positive magnetic susceptibility anomalies, although the iron content is generally very low. These magnetic phenomena differ from those associated with disconformities, which are marked by accumulation of paramagnetic airborne dust deposits with relatively high iron content. The strength and characters of the magnetic response in these subsurface zones correspond to the presence of magnetite, particularly small single-domain magnetite crystals of microbial origin. These crystals are not present elsewhere in the intergranular rock pores or microvugs. They are preferentially concentrated in capillary microborings, which developed concurrently with formation of calcite cements that have soil-related C and O isotope compositions. These magnetic zones occur several meters below the overlying soil horizons. Very thin and long linear microborings may be attributable to cyanobacterial microborers. The single-domain magnetites in these micrometer-size tunnels plugged by calcite appear to result from later occupation of these tiny holes by magnetotactic bacteria. Inorganic origin of the magnetite seems unlikely. Numerous traces that suggest subsurface microbial activity provide evidence that may be used to develop possible scenarios for subsequent biological studies of the precise bacteria involved.

Mössbauer study of Y 2Fe 3Si 5 and Sc 2Fe 3Si 5

Fe-containing Y 2Fe 3Si 5 and Sc 2Fe 3Si 5 have unusual superconducting and related properties. Relevant to this study is their anomalous magnetic susceptibility, which undergoes a maximum near 100 K. Among the various explanations yet to be verified is spin-glass formation. However, Mössbauer measurements rule out this possibility, since paramagnetic behavior remains from 295 down to 4.2 K as revealed by a simple quadrupole splitting in the spectra in these ternary compounds.

The Effects of Iron Addition on the Glass-Forming Ability and Properties of Zr–Ti–Cu–Ni–Be–Fe Bulk Metallic Glass

Zr–Ti–Cu–Ni–Be–Fe bulk metallic glasses and metallic glassy matrix composites were produced by water quenching method. The effect of Fe addition on glass forming ability, hardness, magnetic susceptibility and thermal stability of the alloys was investigated. It was found that the glass forming ability and the properties of the alloys were sensitive to Fe content. A single amorphous phase was obtained in the alloys up to 8 at%Fe addition. These bulk metallic glasses exhibited high thermal stability, wide super-cooled liquid region, and anomalous magnetic susceptibility χ versus temperature in the supercooled liquid region. Metallic glassy composites consisting of nanocrystalline FeZr2 particles were obtained in the alloys with more than 10 at%Fe addition.

Inverse to normal magnetic fabric transition in an Upper Miocene Marly Sequence from Tuscany, Italy

The magnetic susceptibility fabric of an upper Miocene marly sequence exposed along the Morra creek (Tuscany, Italy) changes from inverse (maximum susceptibility axis sub‐perpendicular to bedding) to normal from the bottom to the top of the section. The change is gradual, with the presence of anomalous magnetic susceptibility fabrics at some levels. The anhysteretic remanence anisotropy is always normal throughout the sequence. The changes in the magnetic fabric seem to be controlled by sedimentological variations, with the inverse susceptibility fabric related to the abundance of iron‐bearing carbonates in the lower part of the section. In the upper part of the section subtle changes in the detrital inputs in the basin, and particularly a larger contribution of clayey and ferrimagnetic particles, turned the susceptibility fabric to a normal type.

ChemInform Abstract: Dynamics of Spin Equilibria in Metal Complexes

Publisher Summary This chapter examines the dynamics of spin-equilibrium processes, principally from studies in solutions. The properties of the complexes that are relevant to the dynamics studies are reviewed. The techniques used to observe these rapid processes are also described in the chapter. The chapter presents some aspects of solid-state dynamics and describes some implications for the description of intersystem crossing processes in excited states and for spin equilibria in heme proteins. Many of the complexes that occur in spin equilibrium possess ligands with complicated structures. Trivial abbreviations are used, with structural formulas presented in the table. Generally the complexes are of low symmetry, but in the description of their electronic structure idealized symmetries are assumed and the appropriate term symbols are used accordingly. Some of the concepts, which are used to describe intramolecular spin equilibria, can be extended to the description of these coordination-spin equilibria. Examples include equilibria among four-, five-, and six-coordinate nickel (II) complexes and equilibria involving coordination number changes in iron porphyrin complexes and in heme proteins. Anomalous magnetic susceptibility is the characteristic feature of spin-equilibrium complexes. The Evans nuclear magnetic resonance (NMR) method gives excellent results provided adequate care is taken. There have been very few reports of the Raman spectra of spin-equilibrium complexes.

Critical analysis of a reported anomalous low-temperature susceptibility of solid 3He at high molar volume.

The observation of an anomalous magnetic susceptibility of solid $^{3}\mathrm{He}$ for bcc samples of large molar volume was a new challenge for the understanding of the solid phase. We review the experimental facts, comment on their interpretation, and show that the melting of the sample should be considered as a possible explanation of the anomaly.

Structure, soft modes, and superconductivity in Cl-doped cadmium sulfide.

Previous measurements on pressure-quenched CdS have revealed anomalous magnetic susceptibility indicative of high-temperature superconductivity, but only over a narrow critical range of chlorine doping. X-ray-diffraction studies as a function of chlorine doping now show that at the critical chlorine doping the material undergoes a phase transition, leaving it as wurzite. Photoacoustic spectroscopy shows that a soft lattice mode appears just beyond critical doping, over the same range as the anomalous magnetism. Lastly, we find by analysis that superparacurrent is induced by a magnetic field in wurzite, providing the lattice mode associated with the wurzite zinc-blende phase transition is very soft.

Effect of 4He on the magnetic susceptibility of 3He adsorbed on sintered silver powder.

The magnetic susceptibilities of $^{3}\mathrm{He}$ films on $^{4}\mathrm{coated}$ sintered silver powder were measured with a superconducting quantum interference device magnetometer in the temperature range between about 1 and 10 mK. Even a monolayer of $^{4}\mathrm{He}$ was found to have a dramatic effect on the susceptibilities of $^{3}\mathrm{He}$ adsorbed on top of it. Two layers of $^{4}\mathrm{He}$ completely extinguished the anomalous magnetic susceptibility of $^{3}\mathrm{He}$, leaving only the Fermi-degenerated temperature-independent susceptibility.

Anomalous magnetic susceptibility and relaxation effects in the one-dimensional antiferromagnet K2FeF5above the Neel temperature

Mossbauer measurements of induced hyperfine fields have been used to obtain information on the magnetic susceptibility of single crystals of K2FeF5. At temperatures well above the Neel temperature the behaviour corresponds to the normal isotropic paramagnetic susceptibility of the Fe3+ ions. At temperatures just above the Neel temperature the induced fields observed indicate that the susceptibility is both field dependent and slightly anisotropic. In addition, for fields applied along the magnetic easy axis, the Mossbauer spectra show considerable line broadening effects at temperatures just above the Neel temperature. This behaviour can be related to the correlation of the spins in one dimension above the Neel temperature and to spin relaxation effects associated with the formation of three-dimensionally ordered clusters as the magnetic transition is approached.

Anomalous magnetic susceptibility behavior of some Yb compounds: Thermally excited interconfiguration crossover

The magnetic susceptibility of the intermetallic compounds YbCu, YbAg, YbAu, and YbZn has been measured in the temperature range 4.2-1000 K. The susceptibility of all the compounds is constant at low temperature and is not highly exchange enhanced. In the case of YbCu and YbZn, the susceptibility increases as the temperature is raised, while for YbAg and YbAu it remains constant until the crystallographic FeB\ensuremath{\rightarrow}CsCl phase transition at which there is a sharp jump. The susceptibility of YbAg stays independent of temperature after the phase transition, while that of YbAu increases with temperature. The phase transition in YbAg is reversible, whereas that of YbAu is not. The temperature behavior of the susceptibility of all the compounds can be successfully explained according to the recent ionic model of interconfiguration fluctuation. The susceptibility increase with temperature is due to thermal excitation of the nonmagnetic ground-state ${\mathrm{Yb}}^{2+}$ ion into the excited magnetic ${\mathrm{Yb}}^{3+}$ configuration. The applicability of the resonant-scattering theory based on the virtual bound state can not, however, be unequivocally ruled out for the case of YbCu and YbAu (CsCl phase).

Many-impurity effects in a weakly interacting Kondo system: Comparison of theory with experiment

The theory of the many-impurity Kondo system developed in a previous paper is coupled with a statistical model, using a phenomenological relationship between the magnetic susceptibility χ and the effective Kondo temperature, to obtain the magnetic susceptibility of a very dilute Kondo system well below the single-impurity Kondo temperature. We find that the magnetic susceptibility can be represented as arising from a single-impurity effect proportional to the impurity concentration c, another term proportional to c 2, as well as a much smaller c 3 contribution. The coefficient of the c 2 term in χ has an approximate Curie law behavior at sufficiently low temperatures. The detailed temperature and concentration dependence of χ is in very good quantitative agreement with experiment, and is the first theoretical explanation of the anomalous magnetic susceptibility for Cu-Fe below T K. The modification of the susceptibility arises in the theory from the inhibition of the spin-compensated state by the impurity-impurity interaction. The latter renormalizes the ln T term even in the region of temperature where the system is magnetically completely disordered.

Magnetic behavior of the two-dimensional antiferromagnet RbFeF 4

RbFeF 4 shows anomalous magnetic susceptibility behavior rather similar to that of KFeF 4, in which the existence of two-dimensional antiferromagnetic ordering has recently been clearly demonstrated. A neutron diffraction study of powder samples reveals that RbFeF 4 orders antiferromagnetically in three dimensions at T N = 134 K. In addition around T N , the diffraction patterns indicate a magnetic ridge, thus giving, in accordance with KFeF 4, the evidence of the two-dimensional nature of the magnetism in RbFeF 4.

Magnetic Transition of Wurster's Blue Perchlorate. III. The Nature of the Transition

The anomalous magnetic susceptibility of the low-temperature phase of Wurster's blue perchlorate can be explained on the model of the inequivalent ion pair. Quantitative agreement between the calculated and experimental susceptibilities, between the calculated and experimental heat of transition, as well as between the calculated and experimental integrated difference in enthalpy content of the high- and low-temperature phase has been obtained. The statistics to be used depend on an assumption about the way in which triplets can be formed in this system. In general, a system in which any pair of ions can form a triplet is favored.