AF Transition Research Articles

Magnetic Field as an Important Tool in Exploring the Strongly Correlated Fermi Systems and Their Particle–Hole and Time-Reversal Asymmetries

In this review, we consider the impact of magnetic field on the properties of strongly correlated heavy-fermion compounds such as heavy-fermion metals and frustrated insulators with quantum spin liquid. Magnetic field B can be considered a universal tool, allowing the exploration of the physics controlling the remarkable properties of heavy-fermion compounds. These vivid properties are T/B scaling, exhibited under the application of magnetic field B and at fixed temperature T, and the emergence of Landau Fermi liquid behavior under the application of magnetic field. We analyze the influence of quasiparticle–hole asymmetry on the properties of heavy-fermion (HF) compounds such as the universal scaling behavior of the thermopower S/T exhibited under the application of magnetic field B. We show that universal scaling is demonstrated by different HF compounds such as β-YbAlB4, YbRh2Si2, and strongly correlated layered cobalt oxide [BiBa0.66K0.36O2]CoO2. Analyzing YbRh2Si2, we show that the T/B scaling behavior of S/T is violated at the antiferromagnetic phase (AF) transition. The residual resistivity ρ0 and the density of states N0 experience jumps at the AF transition, causing two jumps in the thermopower and its sign reversal. Our consideration is based on the flattening of the single-particle spectrum that strongly affects ρ0 and N0 and leads to the violation of particle–hole symmetry. The particle–hole asymmetry generates the asymmetrical part Δσd(V) of tunneling differential conductivity σd(V), Δσd(V)=σd(V)−σd(−V), where V is the voltage bias. We demonstrate that in the presence of magnetic field, the quasiparticle–hole asymmetry vanishes, the LFL behavior is restored, and the asymmetry disappears. Our calculations of the mentioned properties of HF compounds, based on the fermion condensation theory, are in good agreement with the experiment and support our conclusion that the fermion condensation theory is capable of describing the properties of HF compounds, including those exhibited under the application of magnetic field.

Structural and magnetic investigations of CaBaCo4−xFexO7 solid solutions

Solid solutions of CaBaCo4−xFexO7 (x=0, 0.05, 0.2, 1, 2) were synthesized by glycine-nitrate combustion process. Their structural and magnetic properties were characterized by means of x-ray diffraction and magnetization measurements. In the examined range of the solid solutions, their crystal lattice symmetry changes from orthorhombic with space group Pbn21 (0<x<0.5) to hexagonal with space group P63mc (x>0.5). The magnetic measurements revealed that even small doping (x=0.05) of the transition metal sublattice noticeably suppresses ferrimagnetism of the parent compound and induces an AF transition at 88K. An increase in the doping concentration lowers the frustration parameter |θCW|/TC from 17.2 (for x=0.05) to 13.7 (for x=2) and strengthens the antiferromagnetic interactions, which manifests itself in the Curie–Weiss temperature (θCW) growth. The samples with x=0.05 and x=0.2 also show an additional magnetic transition at temperatures Tg (55 and 70K respectively). The temperature irreversibility between the ZFC and FC magnetization curves may suggest the formation of a spin-glass state below that temperature.

Magnetic ordering in Gd5Ir2Bi and Gd5Ir2Sb

155Gd Mössbauer spectroscopy and neutron powder diffraction have been used to study magnetic ordering in Gd5Ir2Bi and Gd5Ir2Sb. Despite the hyperfine fields (Bhf) at the two Gd sites differing by more than a factor of two for both compounds, the moments derived from neutron diffraction are essentially equal in Gd5Ir2Bi. This implies an unusual departure from the commonly assumed scaling between BhfGd and μGd. Neutron powder diffraction shows that Gd5Ir2Bi is a c-axis ferromagnet at 3.6 K. We find no evidence for a FM → AF transition.

22pBE-7 κ-(BETS)_2Fe_ Ga_xBr_4の磁気転移近傍における臨界現象と電子状態

22pBE-7 κ-(BETS)_2Fe_ Ga_xBr_4の磁気転移近傍における臨界現象と電子状態

Magnetic short- and long-range order inPbFe0.5Ta0.5O3

PbFe$_{1/2}$Ta$_{1/2}$O$_{3}$ (PFT) belongs to the family of PbB$'_{x}$B$"_{1-x}$O$_{3}$ which have inherent chemical disorder at the B-site. Due to this disorder, a complex magnetic phase diagram is expected in the material. In this paper, we report experimental results of magnetic properties in PFT through macroscopic characterization, neutron scattering and M\"{o}ssbauer spectroscopy techniques. With these results we show for the first time that PbFe$_{1/2}$Ta$_{1/2}$O$_{3}$ behaves very similar to PbFe$_{1/2}$Nb$_{1/2}$O$_{3}$, i.e, it undergoes AF transition at 153 K and has a spinglass transition at 10 K, below which the antiferromagnetism coexists with spinglass. We suggest that the mechanism which is responsible for such a non-trivial ground state can be explained by a speromagnet-like spin arrangement similar to the one proposed for PbFe$_{1/2}$Nb$_{1/2}$O$_{3}$.

Cationic doping effect on the structural, magnetic and spectroscopic properties of delafossite oxides CuCr 1− x(Sc,Mg) xO 2

This article reports the effect of Sc and (Sc + Mg) substitution on the structural and physical properties of delafossite-type CuCrO 2 oxide. X-ray diffraction and Raman spectroscopy show that the structure is maintained for 10% Sc and (10% Sc + 4% Mg) substitutions for chromium. The incorporation of these elements generates very anisotropic microstrains in the structure. Strain also increases with oxygen intercalation, probably because a non-uniform distribution of intercaled oxygen. Thermal stability in oxygen was found to decrease with increasing the M 3+ cation size. The evolution of relative intensities of Raman bands points towards an increase in lattice distortion with substitution, in agreement with ionic radii considerations. Comparing the substitution effect of Mg 2+ to those of the nonmagnetic Sc 3+, magnetization around the AF transition does not seem to correlate with randomness and lattice parameter but with itinerancy of the doped hole.

Electron concentration effects on the Shastry-Sutherland phase stability in Ce2−xPd2+yIn1−zsolid solutions

The stability of a Shastry-Sutherland ShSu phase as a function of electron concentration is investigated through the field dependence of thermal and magnetic properties of the solid solution Ce_{2-x}Pd_{2+y}In_{1-z} in the antiferromagnetic branch. In these alloys the electronic (holes) variation is realized by increasing $Pd$ concentration. The AF transition T_M decreases from 3.5K to 2.8K as $Pd$ concentration increases from y=0.2 to y=0.4. By applying magnetic field, the ShSu phase is suppressed once the field induced ferromagnetic polarization takes over at a critical field B_{cr} which increases with $Pd$ content. A detailed analysis around the critical point reveals a structure in the maximum of the dM/dB derivative, which is related with incipient steps in the magnetization M(B) as predicted by the theory for the ShSu lattice. The crossing of M(B) isotherms, observed in ShSu prototype compounds, is also analyzed. The effect of $In$ substitution by $Pd$ is interpreted as an increase of the number of 'holes' in the conduction band and results in a unique parameter able to describe the variation of the magnetic properties along the studied range of concentration.

Staging model of the ordered stacking of vacancy layers and phase separation in layeredNaxCoO2(x≳0.71)single crystals

Phase diagram of ${\text{Na}}_{x}{\text{CoO}}_{2}(x\ensuremath{\gtrsim}0.71)$ has been reinvestigated using electrochemically fine-tuned single crystals. Both phase-separation and staging phenomena as a result of sodium multivacancy cluster ordering have been found. Phase-separation phenomenon is observed in the narrow ranges of $0.76\ensuremath{\lesssim}x\ensuremath{\lesssim}0.82$ and $0.83\ensuremath{\lesssim}x\ensuremath{\lesssim}0.86$. While $x=0.820$ shows $A$-type antiferromagnetic ($A$-AF) ordering below 22 K, $x=0.833$ is confirmed to have a magnetic ground state of $A$-AF ordering below $\ensuremath{\sim}8\text{ }\text{K}$ and is only reachable through slow cooling. In addition, $x=0.859$ is found to be responsible for the highest $A$-AF transition temperature at about 29 K. Staging model based on ordered stacking of multivacancy layers is proposed to explain the hysteretic behavior and $A$-AF correlation length for $x$ $\ensuremath{\sim}0.82--0.86$.

Resistivity of antiferromagnetic compounds Ce0.8La0.2(Ru1-xRhx)2Si2

We have measured the susceptibility χ and resistivity ρ on single-crystalline Ce0.8La0.2(Ru1-xRhx)2Si2 in directions parallel ∥ and perpendicular (⊥) to the c-axis. The χ∥ for each Rh concentration increases with decreasing temperature, and then rapidly decreases for x ≤ 0.2 and x ≥ 0.8, indicating the AF transition at the Néel temperature TN. The ρ∥ for x = 0 shows a cusp at around TN = 5.5 K, however, ρ⊥ indicates a broad maximum at higher temperature than TN and a rapid decrease at much lower temperature than TN, without evidence of a gap-type AF ordering. The ρ⊥ for x = 0.2 indicates a clear cusp at about TN. Such sharp anomaly in ρ⊥ once disappears for x = 0.6 and is again observed at TN for x ≥0.8 probably due to another type of the AF ordering.

Magnetization reversal in bulk and thin films of the ferrimagnetic ErCo 0.50Mn 0.50O 3 perovskite

We present herein a comparison of the magnetic properties of bulk ceramics and thin films of the ferrimagnetic ErCo 0.50Mn 0.50O 3 compound. Epitaxial thin films were deposited onto (1 0 0) SrTiO 3 substrates by pulsed-laser ablation while bulk ceramics were prepared by solid state reaction. When cooling under low applied fields, a spin reversal is observed in both thin film and bulk due to the competition between two magnetic sublattices (Co/Mn and Er) coupled by a negative exchange interaction. Original features are observed in the M( H) loops for bulk materials: abrupt jumps at 4 T due to a reorientation of domains, while in the low field region, the increasing and decreasing branches of the magnetization intersect each other. In the thin film, the ordering temperature increased from 69 to 75 K, and the ZFC anomaly (AF transition) became sharper, compared to the bulk specimen. The oxygen content and the microstructure are crucial to observe the intersection of the magnetization branches.

A low-temperature phase diagram for ilmenite-rich compositions in the system Fe2O3-FeTiO3

aB stract An approximate low-temperature, metastable phase diagram is drawn for the system (1 – X) Fe2O3-(X)FeTiO3. It is based on published and new magnetic data from nine synthetic samples with bulk compositions in the range 0.6 < X < 1.0. Fields are plotted for (1) the paramagnetic phase (PM); the Fe2O3-rich ferrimagnetic phase (FM); (2) the FeTiO3-rich antiferromagnetic phase (AF); and (3) a re-entrant spin-glass phase (RSG). In addition, two subfields are plotted: (1) FM′, a subfield of the FM-phase, which occurs below a characteristic temperature TK, at which the magnetic susceptibility drops sharply on cooling, and (2) PM′, a subfield of the PM-phase (traditionally called superparamagnetic) forms below a sharp rise in susceptibility at TS, and exhibits measurable dispersion in the magnetic susceptibility at T < TS. The diagram is drawn with a bicritical point, Tλλ′, at X ≈ 0.87, T ≈ 39 K, which is the intersection of second-order magnetic phase boundaries for the paramagnetic → ferrimagnetic [PM(PM′) → FM] transition, TC(X), and the PM(PM′) → AF transition, TN(X). In addition, the RSG phase is plotted as one of four stable phases at Tλλ′, a construction that is not required by the phase rule, but is strongly favored by the physics of competition between the incompatible magnetically ordered structures of the FM- and AF-phases. These phase relations are at such low temperature as to be of little consequence for terrestrial magnetism, however, they may well be essential for interpreting the magnetism of the Moon, Mars, and other cold planets. These phase relations are also essential for the characterization of fine natural and synthetic intergrowths, and for understanding magnetic materials for low-temperature technologi cal applications.

Magnetothermopower in Nd1−xEuxNiO3 compounds

We have measured magnetization M(T,H), thermal conductivity κ(T,H), and thermopower S(T,H) of polycrystalline samples of Nd1−xEuxNiO3, 0⩽x⩽0.35, as a function of temperature and external magnetic field. The data indicate a metal-insulator (MI) transition in a wide range of temperature (200&amp;lt;TMI&amp;lt;325K). The magnetic susceptibility χ(T) data, after the subtraction of the rare-earth contribution, exhibit a Curie-Weiss-like behavior at temperatures above TMI. Although a clear antiferromagnetic AF transition of the Ni sublattice is observed at TN⩽TMI, χ(T) still increases down to 5K, suggesting a heterogeneous ground state. The thermal conductivity of the NdNiO3 compound is not affected by an external magnetic field of 90kOe in a wide range of temperature, and its temperature dependence below 15K is approximately quadratic, strongly suggesting the presence of disorder. S(T) is negative above TMI and varies linearly with temperature. Below TMI, there is a minimum close to 120K, and S(T) changes its sign at T∼30K, indicating a competition between two types of charge carriers. A pronounced peak in S(T) at TS∼20K is observed and the peak remains unaltered under magnetic fields up to 90kOe. However, its magnitude is enhanced by ∼25% with applied magnetic field, exhibiting a clear magnetothermopower effect. The combined results indicate a coexistence of ordered and disordered phases below TN and that an applied magnetic field is suitable for enhancing the thermoelectric properties close to TS.

Chemical pressure effect in the heavy fermion compound Ce 1−yLa y(Ru 0.85Rh 0.15) 2Si 2

We have measured the susceptibility and resistivity on single-crystalline Ce 1− y La y (Ru 0.85Rh 0.15) 2Si 2 along the tetragonal c-axis. The susceptibility increases with decreasing temperature, and then rapidly decreases, indicating the AF transition in each La concentration y. The resistivity also shows an anomaly in each La concentration y at around the T N. For y⩽0.1, we observe a jump in resistivity just below T N as is seen for Ce(Ru 0.85Rh 0.15) 2Si 2. This is interpreted in terms of the gap opening on the Fermi-surface along the c-axis. However, for y⩾0.2 the resistivity shows a downward bend at T N. These experimental facts suggest that the crossover from the spin density wave (SDW) with the gap opening to the localized AF order occurs at around y=0.1.

Ferromagnetic–antiferromagnetic transition in double perovskites

We study the ordered double perovskite of the type Sr 2FeMO 6 using a tight-binding model with the renormalized perturbation expansion technique. In the ferromagnetic phase (F), we determine the behavior of the critical temperature as a function of the band filling and the Fe–M charge transfer energy. In the antiferromagnetic phase (AF) as in Sr 2FeWO 6 the bands reduce to three one-dimensional bands. The electronic energy remains larger than in the F state but the difference decreases with increasing charge transfer energy. Therefore, with increasing Fe–M charge transfer energy, the transition to the AF state can be easily driven by the superexchange interaction. We find that the transition occurs for values of the charge transfer energy such that the Fe and M valences obtained are compatible with the values 2 + and 6 + usually considered for Sr 2FeWO 6. Our approach thus provides a sensible scheme for the transition. We also discuss the F–AF transition in Sr 2 FeMo x W 1 - x O 6 as a function of doping.

Phase separation behaviour on κ-(BEDT-TTF)2Cu[N(CN)2]Br

The system of the deuterated κ-(BEDT-TTF-d[n,n]) 2 Cu[N(CN) 2 ]Br is situated in the critical region between the superconducting(SC) phase and the antiferromagnetic insulator (AFI) phase, and is just on the Mott type critical boundary of paramagnetic metal (PM) phase and paramagnetic insulator (PI) phase. We perform two single-crystal 13 C-NMR studies on the non and the fully deuterated κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br at low temperature with the applied field 9.4T. We found two new experimental results. One is that the phase separation is found in κ-(BEDT-TTF-d[0,0]) 2 Cu[N(CN) 2 ]Br salt, like partially deuterated κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br salt. The other is that the phase separation is observed at around T* with the applied field parallel to the conducting layer, unlike partially deuterated κ-(BEDT-TTF-d[n,n]) 2 Cu[N(CN) 2 ]Br. The former result suggests the field induces AF transition. the latter result suggests there is the field direction dependence of the M-I phase separation behavior in κ -(BEDT-TTF-d[4,4]) 2 Cu[N(CN) 2 ]Br.

An analysis of the process of adiabatic inducement of the F–AF transition in FeRh by pressure

By using the model S– T diagram of the FeRh alloy in the F–AF transition region and the experimental data, it is shown that for inducing the transition adiabatically over the whole alloy volume, it is necessary to apply a pressure equal to or larger than a limit pressure P ↑ l, which is well above the critical one at the same temperatures. Thus, for inducing the complete transition at the critical temperature 308 K, at the critical temperature of the reverse transition 315 K and at 348 K adiabatically, it is necessary to apply a pressure of ∼0.21, ∼0.35, and ∼1.0 GPa, respectively. At that, the expected values of the alloy temperature change are 11.7, 12.2, and 15.0 K, respectively. It is shown that the difference in the temperature of ∼27 K near room temperature between high and low heat reservoirs can be reached when organizing the heat transfer cycles based on adiabatic inducement of the AF↔F transition by pressure within the limits of 1.0 GPa.

Possible change of the superconducting symmetry in the vicinity of the SC–AF transition in NH3A3C60

We have studied vertex corrections to both the intraband and interband electron–phonon couplings in the doped C60. The corrections to the Migdal approximation was studied for the self-energy of the anomalous Green function. On the basis of our results, we suggest a possible change of the symmetry of the superconductivity in the vicinity of the phase boundary between the superconducting and the antiferromagnetic phases observed in the family of NH3A3C60 fullerides.

High-magnetic-field NMR studies ofLiVGe2O6:A quasi-one-dimensional spinS=1system

We report ${}^{7}\mathrm{Li}$ pulsed NMR measurements in polycrystalline and single-crystal samples of the quasi-one-dimensional $S=1$ antiferromagnet ${\mathrm{LiVGe}}_{2}{\mathrm{O}}_{6},$ whose AF transition temperature is ${T}_{\mathrm{N}}\ensuremath{\simeq}24.5 \mathrm{K}.$ The field ${(B}_{0})$ and temperature $(T)$ ranges covered were 9--44.5 T and 1.7--300 K, respectively. The measurements included NMR spectra, the spin-lattice relaxation rate ${(T}_{1}^{\ensuremath{-}1}),$ and the spin-phase relaxation rate ${(T}_{2}^{\ensuremath{-}1}),$ often as a function of the orientation of the field relative to the crystal axes. The spectra indicate an AF magnetic structure consistent with that obtained from neutron diffraction measurements, but with the moments aligned parallel to the c axis. The spectra also provide the T-dependence of the AF order parameter and show that the transition is either second-order or weakly first-order. Both the spectra and the ${T}_{1}^{\ensuremath{-}1}$ data show that ${B}_{0}$ has at most a small effect on the alignment of the AF moment. There is no spin-flop transition up to 44.5 T. These features indicate a very large magnetic anisotropy energy in ${\mathrm{LiVGe}}_{2}{\mathrm{O}}_{6},$ with orbital degrees of freedom playing an important role. Below 8 K, ${T}_{1}^{\ensuremath{-}1}$ varies substantially with the orientation of ${B}_{0}$ in the plane perpendicular to the c axis, suggesting a small energy gap for magnetic fluctuations that is very anisotropic.

Pressure dependence of the SDW transition in (TMTTF) 2Br

We investigated the pressure dependence of the resistivity in (TMTTF) 2 Br, applying with pressure up to 1.2GPa. At ambient pressure the resistance shows the kink structure at 18K where the spin susceptibility starts to decrease, despite the AF transition occurs at 14K. With increasing pressure, the peak of the derivative of the resistance appears 4-8K above the AF transition. It's suggested to be the new phase transition above the AF transition. Above 0.5GPa, the pressure dependence of T SDW is consistent with the prediction of the mean field theory.

Mechanism of spontaneous transition from typical atrial flutter to atrial fibrillation: role of ectopic atrial fibrillation foci.

Paroxysmal AF has been known to be initiated by ectopic beats, especially in the pulmonary veins (PVs), and radiofrequency catheter ablation could cure it. We considered that the spontaneous transition from typical atrial flutter to AF also could be initiated by ectopic beats. Twenty patients (18 men, mean age 66 +/- 14 years) with episodes of spontaneous transition from typical atrial flutter to AF were included in this study. They underwent detailed mapping of both atria. All the patients had spontaneous AF initiated by ectopic beats, and all of them had typical atrial flutter and spontaneous transition from typical atrial flutter (12 patients with counterclockwise atrial flutter and 8 patients with clockwise atrial flutter) to AF. The transition was initiated by ectopic beats from the PVs (17 foci, 85%), crista terminalis (2 foci, 10%), and superior vena cava (1 focus, 5%). After successful ablation of AF foci, typical atrial flutter was induced again, but no spontaneous transition was found after at least 10 minutes of observation. We concluded that paroxysmal AF and spontaneous transition from typical atrial flutter to AF were initiated by ectopic beats, and successful catheter ablation of the ectopic foci can eliminate paroxysmal AF and spontaneous transition from typical atrial flutter to AF.