AF Phase Research Articles

Magnetic and Transport Properties of PrRu2Si2Single Crystal under High Pressure

We report on magnetization and electrical resistivity with respect to temperature, magnetic field and hydrostatic pressure ( p ) up to 0.9 GPa. Moderate dependence of T C on pressure d T C /d p = -1.8 K/GPa was observed whereas the value of T N remains almost intact up to highest applied pressure. In the AF phase region ( T C < T < T N ) a metamagnetic transition occurs with rather low value of the critical metamagnetic field (µ 0 H C ∼50 mT at 15 K). The µ 0 H C value increases nearly twice when applying pressure of 0.8 GPa (the d H C /d p is almost constant yielding 4 ×10 -2 T/GPa).

Antiferromagnetism and Superconductivity in Layered Organic Conductors: Variational Cluster Approach

The kappa-(ET)2X layered conductors (where ET stands for BEDT-TTF) are studied within the dimer model as a function of the diagonal hopping t' and Hubbard repulsion U. Antiferromagnetism and d-wave superconductivity are investigated at zero temperature using variational cluster perturbation theory (VCPT). For large U, Néel antiferromagnetism exists for t' < t(c2)', with t(c2)' approximately 0.9. For fixed t', as U is decreased (or pressure increased), a d(x2-y2) superconducting phase appears. When U is decreased further, then a d(xy) order takes over. There is a critical value of t(c1)' approximately 0.8 of t' beyond which the AF and dSC phases are separated by the Mott disordered phase.

Role of heterostructures and multiple magnetic phases in the low-field magnetization of Fe–Cr giant magnetoresistive multilayers

Zero-field-cooled (ZFC) and field-cooled (FC) magnetizations along with ac magnetization versus temperature and M-H loop measurements are reported for two series of ion beam sputtered Fe–Cr giant magnetoresistive (GMR) (maximum of 33%) multilayers where the interface roughness is different. The ZFC and FC magnetization data follow different curves below an irreversibility temperature (Tirr). The FC data show a T3∕2 thermal demagnetization behavior at lower temperatures with a very small spin-wave stiffness constant but they go as 1∕T at higher temperatures (above Tirr). The ZFC data show maxima at Tm(H), from which we find the glass transition temperature Tg using the de Almeida and Thouless plot. ac magnetization versus T also shows broad peaks roughly near the same Tg. This behavior is interpreted in terms of the coexistence of spin-glass∕superparamagnetic, FM and AF phases. Most of the above exotic low-field features are absent in molecular beam epitaxy grown Fe–Cr multilayers (GMR &amp;gt;50%) with very smooth interfaces. The relatively rough interfaces in the present sputtered samples give rise to frustrations of Fe and Cr moments, leading to spin-glass-like behavior.

Influence of the genesis on the structural and hyperfine properties of Cr-substituted hematites

Several series of Cr-substituted hematites with Cr:Fe molar ratio up to 0.112 were prepared by forced hydrolysis of Fe 3+ solutions and by thermal dehydroxylation of Cr substituted goethites. Powder X-ray diffraction was used in order to assess the structural characteristics of the whole series. Rietveld refinement of XRD data for samples obtained by forced hydrolysis indicated that the incorporation of Cr causes anisotropic changes in the a-lattice parameter and stacking faults. In hematite samples obtained from Cr-goethites, the cell-parameters decrease with the increase in Cr-content. In all cases, the particles were multidomainic, with sizes that varied between 0.13 ± 0.02 and 1.07 ± 0.05 μm. The Morin transition was detected in substituted samples with low Cr-content, but it was no longer present in samples containing μ Cr ≥ 4.3 ± 0.2 ( μ Cr = 100 × [Cr] / [Cr] + [Fe] mol mol − 1 ). Mössbauer spectroscopy suggested that the AF phase of Cr-hematites obtained by forced hydrolysis of Fe(III) salts, did not present parallel spin alignment along the [111] direction. These solids presented an unusually low value for T M which was ascribed to the interplay of anisotropic lattice changes, small grain sizes, metal vacancy sites and structurally bound water. All samples presented spin orientation in the (111) basal plane in the WF phase. Néel temperature variation was also explained by the concomitant contributions coming from Cr-for-Fe substitution and anisotropic lattice variations.

Life in the Universe: from the Miller Experiment to the Search for Life on Other Worlds

Life in the Universe: from the Miller Experiment to the Search for Life on Other Worlds

A zero-field [formula omitted]SR study of glassy spin dynamics in [formula omitted]-MnCu

We have performed zero-field muon spin relaxation ( μ SR) measurements of a number of γ - Mn x Cu 1 - x alloys around the critical concentration ( x = 0.72 ) in order to investigate the dynamical properties of the system as it moves from short-range to long-range magnetic order. Above the critical concentration the muon spin relaxation is exponential in form, whilst for Mn concentrations ⩽ 71 at % the relaxation is stretched-exponential, with the “stretch” exponent β decreasing from unity at high temperatures (i.e. a simple monopolar Lorentzian relaxation regime) to around 1 3 at the transition temperature—indicated by a divergence in the muon relaxation rate, λ . This behaviour is indicative of a concentrated spin-glass ground state. For a Mn concentration of 73 at % we find a re-entrant transition, from a cluster-glass phase at T g ≈ 260 K to an AF phase at T N ≈ 140 K .

Fragility of theA-type AF and CE phases of manganites: Insulator-to-metal transition induced by quenched disorder

Using Monte Carlo simulations and the two eg-orbital model for manganites, the stability of the CE and A-type antiferromagnetic insulating states is analyzed when quenched disorder in the superexchange JAF between the t2g localized spins and in the on-site energies is introduced. At vanishing or small values of the electron-(Jahn-Teller)phonon coupling, the previously hinted "fragility" of these insulating states is studied in detail, focusing on their charge transport properties. This fragility is here found to induce a rapid transition from the insulator to a (poor) metallic state upon the introduction of disorder. A possible qualitative explanation is presented based on the close proximity in energy of ferromagnetic metallic phases, and also on percolative ideas valid at large disorder strength. The scenario is compared with previously discussed insulator-to-metal transitions in other contexts. It is argued that the effect unveiled here has unique properties that may define a new class of giant effects in complex oxides. This particularly severe effect of disorder must be present in other materials as well, in cases involving phases that arise as a compromise between very different tendencies, as it occurs with striped states in the cuprates.

Magnetic excitations in FCC Mn (37%Fe, 3%Cu) alloy

Within the program of our systematic studies of magnetic excitations of manganese alloys the inelastic neutron scattering in FCC Mn(37%Fe,3%Cu) was measured for the temperature range 1.7–600 K. This alloy has the FCC crystal structure both in paramagnetic and AF phase and its composition is close to the range where the enhanced itinerant behaviour was predicted by the theory. The measured spin-wave velocity and the energy gap in the spin wave spectrum were reported earlier to reach the maximum values in this composition range. Our data did not confirm these earlier results. The expected anisotropy of the inelastic neutron scattering was not found neither in the antiferromagnetic phase nor in the paramagnetic one. The q-dependence of the spin-wave damping is quadratic rather than linear and the spin-wave damping increase with temperature, as usually predicted for damping due to magnon scattering on magnons and on other excitations. This result is in contrast with the data for the FCT Mn(10%Fe,3%Cu) where the q-dependence of the spin-wave damping is linear and the damping parameter was found to be temperature independent excepting the region close to TN. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Double exchange model in cubic vanadates

AbstractWe investigate the role of the double exchange mechanism for stability of the metallic C‐type antiferromagnetic (C‐AF) phase, which was observed experimentally in hole doped La1–x Srx VO3. The double exchange model treats localized electrons in xy orbitals as classical S = 1/2 spins, which interact by Hund's exchange J H with yz /zx electrons in partly filled t 2g orbitals. Including strong on‐site Coulomb repulsion U between t 2g electrons, and using slave boson method we demonstrate that C ‐AF and metallic phase can be stabilized due to the specific features of the hopping in degenerate and partly filled t 2g orbitals. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Infrared spectroscopy under multiextreme conditions: Direct observation of pseudogap formation and collapse in CeSb

Infrared reflectivity measurements of CeSb under multiextreme conditions (low temperatures, high pressures, and high magnetic fields) were performed. A pseudogap structure, which originates from the magnetic band folding effect, responsible for the large enhancement in the electrical resistivity in the single-layered antiferromagnetic structure (AF-1 phase) was found at a pressure of 4 GPa and at temperatures of 35--50 K. The optical spectrum of the pseudogap changes to that of a metallic structure with increasing magnetic field strength and increasing temperature. This change is the result of the magnetic phase transition from the AF-1 phase to other phases as a function of the magnetic field strength and temperature. This result is an important optical observation of the formation and collapse of a pseudogap under multiextreme conditions.

Chemical Pressure Effect for the SDW Phase in Heavy Fermion Ce(Ru0.85Rh0.15)2Si2Compound

Susceptibility measurements have been performed on polycrystalline samples Ce 1- y R y (Ru 0.85 Rh 0.15 ) 2 Si 2 ( R = Sc, La) in order to clarify the chemical pressure effect for the SDW in the heavy fermion Ce(Ru 0.85 Rh 0.15 ) 2 Si 2 . Lattice parameters a and c both decrease with increasing Sc concentration and increase with increasing La concentration. For Ce 1- y Sc y (Ru 0.85 Rh 0.15 ) 2 Si 2 system, the Néel temperature T N decreases with increasing Sc concentration and disappears for y ≥0.2. We observed the Curie–Weiss law in the susceptibility at high temperature and a tendency of the saturation in the low temperature susceptibility. The tendency of the saturation and the Weiss temperature Θ determined from the high temperature susceptibility increase with increasing Sc concentration. These facts can be explained through the increase of the c – f hybridization and itinerancy of the f electron. For Ce 1- y La y (Ru 0.85 Rh 0.15 ) 2 Si 2 system, the T N slightly decreases with increasing La concentration and the tendency of the saturation becomes weaker with increasing y and is no more observed down to T N for y > 0.15. The Θ rapidly decreases with increasing La concentration and becomes almost zero for y > 0.15. These facts suggest that the c – f hybridization becomes weak with increasing y and that the itinerant AF phase for y < 0.15 is replaced by the AF phase with the localized moment for y > 0.15. The Kondo temperature T K estimated from Θ is scaled by a single proportional function of the lattice parameter c in both cases for the Sc and La substitution.

DHvA effect in the lowest field phase of CeSb

We have successfully grown high-quality single crystals of CeSb and have observed the dHvA effect in the lowest-field AF phase for the first time. We report the Fermi properties in the AF phase and discuss them in terms of the p–f mixing model.

Phase Transition and Collective Fluctuation of a Confined Antiferroelectric Liquid Crystal: Dielectric Spectroscopy of the Frustrated Ferroelectric Smectic-C* Phase

The dielectric and switching behaviors have been investigated using cells of varying thickness of an antiferroelectric liquid crystal, (S)-4-(1-methylheptyloxycarbonyl) phenyl 4'-octylbiphenyl-4-carboxylate (S-MHPBC) which shows antiferroelectric and ferrielectric subphases, designated as the AF and Sm-Cγ* phases, respectively, in a temperature region above the Sm-CA* phase. While the tristable switching with a characteristic dc threshold and hysteresis is clearly observed in the AF phase in a cell thicker than about 8 µm with homogeneous alignment, a thresholdless bistable switching is confirmed in a cell thinner than 3.9 µm. The emergence of thresholdless bistable switching means that confinement of the antiferroelectric liquid crystal to the thin cell induces the phase transformation from the AF and Sm-Cγ* phases to the frustrated ferroelectric Sm-C* phase. In addition, a novel dielectric relaxation appears in the temperature region of the AF and Sm-Cγ* phases in the liquid crystal confined to a cell 3.9 µm thick. We concluded that the novel dielectric relaxation appearing in the confined antiferroelectric liquid crystal is associated with a Goldstone mode of the frustrated ferroelectric Sm-C* phase transformed from the AF and Sm-Cγ* phases.

Changes in the structural and magnetic properties of Ni?substituted hematite prepared from metal oxinates

Samples prepared by the novel method based on the thermal decomposition at 700 °C in air of mixed Fe and Ni oxinates were characterized by thermogravimetric analysis, X-ray diffraction, magnetization measurements, and variable-temperature Mossbauer spectroscopy. It is found that the combustion treatment produces Ni-hematite and trevorite, the fraction of the latter increasing with the increment of Ni in the metal oxinates. Results indicate that the substitution of Ni2+ for Fe3+ in the hematite structure, which was found to be less than 5.3 mol%, is accompanied by the presence of oxygen and structural vacancy sites. Both the metal replacement and the presence of defects cause the a and c cell hematite parameters to decrease. The Neel temperature and the difference between the saturation fields for the antiferromagnetic AF and the weakly ferromagnetic WF phases are also found to decrease with nickel content. These effects are due to the fact that the magnetic behavior of hematite depends on the presence of Ni, vacancy sites, and lattice distortion. The large decrease in the Neel temperature with Ni doping suggests that structural vacancies are also present On the contrary, Ni incorporation does not appreciably affect the Morin temperature and the temperature range in which both AF and WF phases coexist. It is suggested that these effects are probably due to the differing effects that Ni and defects can produce on the magnetic dipolar and the single ion anisotropies.

Macroscopically inhomogeneous state at the boundary between the superconducting, antiferromagnetic, and metallic phases in quasi-one-dimensional(TMTSF)2PF6

We report on experimental studies of the character of phase transitions in the quasi-1D organic compound (TMTSF)2PF6 in the close vicinity of the borders between the paramagnetic metal PM, antiferromagnetic insulator AF, and superconducting SC states. In order to drive the system through the phase border P_0(T_0), the sample was maintained at fixed temperature T and pressure P, whereas the critical pressure P_0 was tuned by applying the magnetic field B. In this approach, the magnetic field was used (i) for tuning (P-P_0), and (ii) for identifying the phase composition (due to qualitatively different magnetoresistance behavior in different phases). Experimentally, we measured R(B) and its temperature dependence R(B,T) in the pressure range (0 - 1)GPa. Our studies focus on the features of the magnetoresistance at the phase transition between the PM and AF phases, in the close vicinity to the superconducting transition at T~1K. We found pronounced history effects arising when the AF/PM phase border is crossed by sweeping the magnetic field: the resistance depends on a trajectory which the system arrives at a given point of the P-B-T phase space. In the transition from the PM to AF phase, the features of the PM phase extends well into the AF phase. At the opposite transition from the AF to PM phase, the features of the AF phase are observed in the PM phase. These results evidence for a macroscopically inhomogeneous state, which contains macroscopic inclusions of the minority phase. When the system is driven away from the transition, the homogeneous state is restored; upon a return motion to the phase boundary, no signatures of the minority phase are observed up to the very phase boundary.

Thermal hysteresis of spin reorientation at Morin transition in alkoxide derived hematite nanoparticles

We present results on the structural and magnetic properties of highly crystalline α-Fe2O3 nanoparticles of average size ∼200 nm, synthesized from a novel sol-gel method using metal alkoxide precursor. These particles are multi-domain, showing the weak ferromagnetic–antiferromagnetic (WF-AF) transition (i.e., the Morin transition) at TM=256(2) K. Mössbauer measurements revealed a jump in hyperfine parameters (HP’s) at T∼TM, which also displays thermal hysteresis upon cooling or heating the sample. The analysis of HP’s as a function of temperature allowed us to discard temperature gradients as well as the coexistence of WF/AF phases as possible origins of this hysteretic behaviour. Instead, the hysteresis can be qualitatively explained by the small size and high-crystallinity of the particles, which hinder the nucleation of the WF or AF phases yielding metastable states beyond TM.

Dielectric relaxation behavior of a liquid crystal showing an unusual type of antiferroelectric-ferroelectric-antiferroelectric phase sequence.

The temperature and frequency dependent dielectric relaxation behavior of a liquid crystalline (S)-(+)-1-methylheptyl 4-[2-(4-alkoxyphenyl) thiophene-5-carbonylthiooxy] benzoate system is reported. Interesting successive antiferroelectric-ferroelectric-antiferroelectric (AF-FE-AF) phase transitions are observed in this system resembling the successive phase transitions observed in crystalline Rochelle salt. The smectic-C* (SmC*) to AF1 phase transition (around 103.0 degrees C) is first order in nature, predicted from the use of Orihara and Ishibashi theory. It is also found that a contribution of the ferroelectric SmC* phase ordering penetrates even in the antiferroelectric AF1 (SmC(*)(A)) and AF2 (SmC(*)(A)) phases very close to the SmC*-AF1 and SmC*-AF2 phase boundaries (critical regions). It is suggested that this type of mixing of AF and FE phases might cause surface induced ferroelectric- or ferroelectric-type ordering near the AF-FE phase transitions. A soft mode with Debye-type dispersion was observed in the SmA phase. The thermal behaviors of dielectric dispersion, absorption, and dielectric strength in different phases are also reported and discussed.

Effect of pressure on the electrical resistivity and magnetism in updsn

The electrical resistivity of a UPdSn single crystal exerted to various hydrostatic pressures was measured as a function of temperature and magnetic field. Clear anomalies in the temperature dependence of resistivity along the c-axis mark the magnetic phase transitions between paramagnetic and antiferromagnetic (AF) state at T N and the AF1↔AF2 transition at T 1. Large negative magnetoresistance effects have been observed not only in the AF state as a result of the metamagnetic transition to canted structure at B c , but also at temperatures far above T N . The latter result is attributed to the existence of AF correlations or short range AF orderings in the paramagnetic range. The value of T N increases with increasing applied pressure, whereas T 1 simultaneously decreases. It is also found that B c decreases with increasing pressure. As a consequence, the stability range of the AF-1 phase expands with applied pressure partially on account of the ground-state AF-2 phase.

Brave new world of unconventional density waves

Recently many people discuss unconventional density wave (i.e. UCDW and USDW). Unlike in conventional density waves, the quasiparticle excitations in these systems are gapless. The appearence of these systems suggests paradigm shift from quasi ID system to quasi 2D and 3D systems. Here we limit ourselves to the angular dependent magnetoresistance (ADMR) observed in the low temperature phase (LTP) of α-(BEDT-TTF)2KHg(SCN)4. Here we show that UCDW describes successfully many features of ADMR as manifestation of the Landau quantization of the quasiparticle spectrum in magnetic field. Indeed ADMR will provide a unique window to access UDW like the AF phase in URu2Si2, the pseudogap phase in high Tc cuprates and the glassy phase in organic superconductor κ-(ET)2 salts.

Unconventional density wave in the pseudogap phase of high Tc cuprates

We propose here that the pseudogap phase of high Tc superconductors is probably unconventional spin density wave (USDW). We present our analysis on the angular dependent magnetoresistance (ADMR) in the pseudogap phase which should be accessible experimentally. Also this result applies also to the AF phase of URu2Si2. We show that this model is consistent with the optical dichroism seen by ARPES.