Spin Density Wave Transition Research Articles

Magnetic properties and spin dynamics in Ba(Fe1−xMnx)2As2 compounds studied by 57Fe Mössbauer spectroscopy

Ba(Fe1−xMnx)2As2 compounds with x = 0.016 and 0.064 have been studied by 57Fe Mössbauer spectroscopy in the temperature range from 30 to 300 K. The unusual magnetic splitting spectra at lower temperatures have been analyzed using the distribution of hyperfine field. It is found that the influence of Mn dopant spreads beyond the nearest Fe magnetic moments, and the Fe 3d electrons behave more localized compared with those in the electron-doped compounds. This reduces the hyperfine interactions between iron nucleus and the sounding electrons. The shape of the spin density wave is near-rectangular at 6.4%-Mn doping, indicating quite different interband interactions compared with electron-doped compounds. A distinct broadening of the spectral linewidth around the spin density wave transition temperature has been observed and the spin correlation time is deduced according to the linewidth. The correlation time is further related to the spin–lattice relaxation rate by a simple model and the magnetic fluctuations can be explored effectively. It is found that the magnetic fluctuations of iron spins in Mn-doped compounds can be described well using a phenomenological two-component model and the resulting Curie–Weiss temperature is far from the quantum critical point at the present doping levels.

Tunable interplay between3dand4felectrons in Co-doped iron pnictides

We study the interplay of $3d$ and $4f$ electrons in the iron pnictides CeFe${}_{1\ensuremath{-}x}$Co${}_{x}$AsO and GdFe${}_{1\ensuremath{-}y}$Co${}_{y}$AsO, which correspond to two very different cases of $4f$-magnetic moment. Both CeFeAsO and GdFeAsO undergo a spin-density-wave (SDW) transition associated with Fe $3d$ electrons at high temperatures, which is rapidly suppressed by Fe/Co substitution. Superconductivity appears in a narrow doping range: $0.05<x<0.2$ for CeFe${}_{1\ensuremath{-}x}$Co${}_{x}$AsO and $0.05<y<0.25$ for GdFe${}_{1\ensuremath{-}y}$Co${}_{y}$AsO, showing a maximum transition temperature ${T}_{\mathrm{sc}}$ of about 13.5 K for Ce and 19 K for Gd. In both compounds, the $4f$ electrons form an antiferromagnetic (AFM) order at low temperatures over the entire doping range and Co $3d$ electrons are ferromagnetically ordered on the Co-rich side; the Curie temperature reaches ${T}_{\mathrm{C}}^{\mathrm{Co}}\ensuremath{\approx}$ 75 K at $x=1$ and $y=1$. In the Ce compounds, the N\'eel temperature ${T}_{\mathrm{N}}^{\mathrm{Ce}}$ increases upon suppressing the SDW transition of Fe and then remains nearly unchanged with further increasing Co concentration up to $x\ensuremath{\simeq}0.8$ (${T}_{\mathrm{N}}^{\mathrm{Ce}}\ensuremath{\approx}$ 4 K). Furthermore, evidence of Co-induced polarization on Ce moments is observed on the Co-rich side. In the Gd compounds, the two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic behavior in the magnetic susceptibility on the Co-rich side. For $0.7\ensuremath{\le}y<1.0$, the system undergoes a possible magnetic reorientation below the N\'eel temperature of Gd (${T}_{\mathrm{N}}^{\mathrm{Gd}}$). Our results suggest that the effects of both electron hybridizations and magnetic exchange coupling between the $3d$-$4f$ electrons give rise to a rich phase diagram in the rare-earth iron pnictides.

Ultrafast quasiparticle relaxation dynamics in superconducting iron pnictide Ca(Fe0.944Co0.056)2As2

Nonequilibrium quasiparticle relaxation dynamics is reported in superconducting Ca(Fe0.944Co0.056)2As2 single crystals by measuring transient reflectivity changes using femtosecond time-resolved pump-probe spectroscopy. Large changes in the temperature-dependent differential reflectivity values in the vicinity of the spin density wave (TSDW) and superconducting (TSC) transition temperatures of the sample have been inferred to have charge gap opening at those temperatures. We have estimated the zero-temperature charge gap value in the superconducting state to be ∼1.8kBTSC and an electron–phonon coupling constant λ of ∼0.1 in the normal state that signifies the weak coupling in iron pnictides. From the peculiar temperature-dependence of the quasiparticle dynamics in the intermediate temperature region between TSC and TSDW we infer a temperature scale where the charge gap associated with the spin ordered phase is maximum and closes on either side while approaching the two phase transition temperatures.

Anomalous magnetoresistance and hidden spin canting in (DIETSe)2MCl4(M=Fe, Ga)

The quasi-one-dimensional (Q1D) molecular conductors (DIETSe)${}_{2}$$M$Cl${}_{4}$ [$M=\text{Fe}$, Ga] undergo a spin density wave (SDW) transition below 12 K. The SDW ground state is suppressed by applying high pressure, recovering the Q1D Fermi surface which is confirmed by the appearance of Lebed resonance in the angle-dependent magnetoresistance (MR). Above the critical pressure of SDW, MR shows kink structures at high magnetic fields, reminiscent of field-induced spin density wave (FISDW) transition in both salts. The $\ensuremath{\pi}$-$d$ hybrid (DIETSe)${}_{2}$FeCl${}_{4}$ also exhibits an antiferromagnetic (AF) transition of $d$-electron spins at 2.5 K, below which the spin-flop-induced positive large MR are observed. The change in the interlayer MR reaches 130$%$ at 10.5 kbar. The resistance anomalies associated with spin flop are also observed in the angle-dependent MR at low magnetic fields below 5 T, associated with clear hysteresis. A polar plot of these anomalies reveals the presence of hidden spin canting. Two magnetic easy axes of $d$-electron spins are found to be tilted $\ifmmode\pm\else\textpm\fi{}$16 degrees from the $b$ axis towards the $c$ axis. The interplay between the SDW instability of Q1D $\ensuremath{\pi}$ electrons and the local moments of AF $d$-electron spins is considered as the origin of the anomalous transport behaviors.

Dual role ofdelectrons in iron pnictides

In the stoichiometric Fe-pnictides the same d-electrons form the local moments and participate in the metallic conductivity. In the manuscript the emergence of the magnetic moments on the Fe-sites, their interaction with the itinerant electrons are studied in the paramagnetic phase at the different relations between the onsite Coulomb repulsion and the intraatomic Hund's exchange. The energy of the local center that is due to its interaction with the conduction electrons is calculated exactly. It is found that the spin density wave transition is controlled by the RKKY interactions between the centers rather than by the nesting mechanism. In the paramagnetic phase the electrons scatter on the randomly oriented moments. The calculated inverse mean free time provides the interpretation for few available experimental data on the conductivity. The multi-band structure of the energy spectrum in iron pnictides is taken into account.

Selective Raman Scattering Detection of the Dirac Node and the Anti-node of the Spin Density Wave Gap and Magnetic Excitations in BaFe2As2

The electronic and magnetic excitations at the spin density wave (SDW) transition are investigated by Raman scattering. The multi-orbital electronic states induce the Dirac nodes in the SDW gap. The excitations near the nodes and anti-nodes are separately detected in accordance with the two-orbital tight-binding model. The exchange interactions are found to be given by the second derivative of the total energy with respect to the angle of the moment from two-magnon scattering. The two-magnon peak has the large spectral weight above twice the maximum energy of magnon. It is interpreted by the magnetic self-energy of the electron spectral function in the localized spin model or particle-hole excitations in the itinerant spin model.

Acoustic and optical phonon dynamics from femtosecond time-resolved optical spectroscopy of the superconducting iron pnictide Ca(Fe0.944Co0.056)2As2

We report the temperature evolution of coherently excited acoustic and optical phonon dynamics in the superconducting iron pnictide single crystal Ca(Fe0.944Co0.056)2As2 across the spin density wave transition at TSDW ∼ 85 K and the superconducting transition at TSC ∼20 K. The strain pulse propagation model applied to the generation of the acoustic phonons yields the temperature dependence of the optical constants, and longitudinal and transverse sound velocities in the temperature range from 3.1 K to 300 K. The frequency and dephasing times of the phonons show anomalous temperature dependence below TSC indicating a coupling of these low-energy excitations with the Cooper-pair quasiparticles. A maximum in the amplitude of the acoustic modes at T ∼ 170 is seen, attributed to spin fluctuations and strong spin-lattice coupling before TSDW.

Evolution of Eu2+spin dynamics in Ba1−xEuxFe2As2

Single crystals of Ba${}_{1\ensuremath{-}x}$Eu${}_{x}$Fe${}_{2}$As${}_{2}$ were studied by magnetic susceptibility, heat capacity, resistivity, and electron spin resonance (ESR) measurements. Spin-density wave (at ${T}_{\text{SDW}}$) and antiferromagnetic (at ${T}_{N}$) phase transitions were mapped as a function of $x$. For $x\ensuremath{\ge}0.2$, we found a single Eu${}^{2+}$ ESR Dysonian line that presents an isotropic linear increase (Korringa) of its linewidth ($\ensuremath{\Delta}H$) above ${T}_{\text{SDW}}$ which systematically decreases with decreasing $x$. In contrast, for a critical concentration ${x}_{c}$ ($0.10l{x}_{c}l0.20$), $\ensuremath{\Delta}H$ decreases with increasing $T$, suggesting a distinct relaxation process that we associate with a Eu${}^{2+}$ Kondo single impurity regime. The Korringa rate suppression towards the Ba-rich compounds is claimed to be due to the reduction of the $q$-dependent exchange interaction between the Eu${}^{2+}$ $f$ electrons and the conduction electrons, which is likely associated with an increasing of localization of Fe $d$ electrons. This result may help the understanding of the SDW phase suppression (that can lead to superconductivity) in this class of materials.

Appearance and disappearance of superconductivity in SmFe1−xNixAsO (x = 0.0–1.0)

Bulk polycrystalline Ni-substituted SmFe1−xNixAsO (x = 0.0–1.0) samples are synthesized by solid state reaction route in an evacuated sealed quartz tube. The cell volume decreases with increase of Ni content in SmFe1−xNixAsO, thus indicating successful substitution of smaller ion Ni at Fe site. The resistivity measurements showed that the spin density wave (SDW) transition is suppressed drastically with Ni doping and subsequently superconductivity is achieved in a narrow range of x from 0.04 to 0.10 with maximum Tc of 9K at x = 0.06. For higher content of Ni (x ≥ 0.10), the system becomes metallic and superconductivity is not observed down to 2K. The magneto-transport [R(T)H] measurements exhibited the upper critical field [Hc2(0)] of up to 300 kOe. The flux flow activation energy (U/kB) is estimated ∼98.37K for 0.1T field. Magnetic susceptibility measurements also confirm bulk superconductivity for x = 0.04, 0.06 and 0.08 samples. The lower critical field (Hc1) is around 100 Oe at 2K for x = 0.06 sample. Heat capacity CP(T) measurements exhibited a hump like transition pertaining to SDW in Fe planes at around 150K and an AFM ordering of Sm spins below temperature of 5.4K for ordered Sm spins [TN(Sm)]. Though, the SDW hump for Fe spins disappears for Ni doped samples, the TN (Sm) remains unaltered but with a reduced transition height, i.e., decreased entropy. In conclusion, complete phase diagram of SmFe1−xNixAsO (x = 0.0–1.0) is studied in terms of its structural, electrical, magnetic and thermal properties.

Ba2Ti2Fe2As4O: A New Superconductor Containing Fe2As2 Layers and Ti2O Sheets

We have synthesized a new oxypnictide, Ba2Ti2Fe2As4O, via a solid-state reaction under a vacuum. The compound crystallizes in a body-centered tetragonal lattice, which can be viewed as an intergrowth of BaFe2As2 and BaTi2As2O, thus containing Fe2As2 layers and Ti2O sheets. Bulk superconductivity at 21 K is observed after annealing the as-prepared sample at 773 K for 40 h. In addition, an anomaly in resistivity and magnetic susceptibility around 125 K is revealed, suggesting a charge- or spin-density wave transition in the Ti sublattice.

Effect of Ni doping on magnetism and superconductivity in Eu0.5K0.5Fe2As2

The effect of Ni-doping on the magnetism and superconductivity in Eu0.5K0.5Fe2As2 has been studied through a systematic investigation of magnetic and superconducting properties of Eu0.5K0.5(Fe1-xNix)2As2 (x = 0, 0.03, 0.05, 0.08 and 0.12) compounds by means of dc and ac magnetic susceptibilities, electrical resistivity and specific heat measurements. Eu0.5K0.5Fe2As2 is known to exhibit superconductivity with superconducting transition temperature Tc as high as 33 K. The Ni-doping leads to a rapid decrease in Tc; Tc is reduced to 23 K with 3% Ni-doping, and 8% Ni-doping suppresses the superconductivity to below 1.8 K. In 3% Ni-doped sample Eu0.5K0.5(Fe0.97Ni0.03)2As2 superconductivity coexists with short range ordering of Eu2+ magnetic moments at Tm ~ 6 K. The suppression of superconductivity with Ni-doping is accompanied with the emergence of a long range antiferromagnetic ordering with TN = 8.5 K and 7 K for Eu0.5K0.5(Fe0.92Ni0.08)2As2 and Eu0.5K0.5(Fe0.88Ni0.12)2As2, respectively. The temperature and field dependent magnetic measurements for x = 0.08 and 0.12 samples reflect the possibility of a helical magnetic ordering of Eu2 moments. We suspect that the helimagnetism of Eu spins could be responsible for the destruction of superconductivity as has been observed in Co-doped EuFe2As2. The most striking feature seen in the resistivity data for x = 0.08 is the reappearance of the anomaly presumably due to spin density wave transition at around 60 K. This could be attributed to the compensation of holes (K-doping at Eu-site) by the electrons (Ni-doping at Fe site). The anomaly associated with spin density wave further shifts to 200 K for x = 0.12 for which the electron doping has almost compensated the holes in the system.

Electronic Correlations and Unconventional Spectral Weight Transfer in the High-Temperature PnictideBaFe2−xCoxAs2Superconductor Using Infrared Spectroscopy

We report an infrared optical study of the pnictide high-temperature superconductor BaFe(1.84)Co(0.16)As(2) and its parent compound BaFe(2)As(2). We demonstrate that electronic correlations are moderately strong and do not change across the spin-density wave transition or with doping. By examining the energy scale and direction of spectral weight transfer, we argue that Hund's coupling J is the primary mechanism that gives rise to correlations.

Magnetic Interactions on Single-Crystal EuFe2As2Studied by57Fe Mössbauer Spectroscopy

Magnetic interactions on single-crystal AFe 2 As 2 (A: Eu and Sr) were studied by 57 Fe Mossbauer spectroscopy. An abrupt appearance of the effective magnetic hyperfine field ( B eff ) is observed at the spin density wave transition temperature (∼190 K for Eu and ∼200 K for Sr) in both samples. The value of B eff in EuFe 2 As 2 shows a small but significant increase around the Neel temperature T N Eu (=19 K) of the Eu 2+ sublattice, while B eff in SrFe 2 As 2 almost saturates at lower temperatures. Consistent with this, the isomer shift of EuFe 2 As 2 indicates an anomalous temperature dependence below approximately T N Eu . These results provide direct evidence of the coupling between the localized Eu 2+ moments and the conduction electrons from the FeAs layers and suggest that this coupling may affect the observed superconductivity in EuFe 2 As 2 under pressure.

Phase diagram as a function of doping level and pressure in the Eu1−xLaxFe2As2system

We establish the phase diagram of Eu$_{1-x}$La$_x$Fe$_2$As$_2$ system as a function of doping level x and the pressure by measuring the resistivity and magnetic susceptibility. The pressure can suppress the spin density wave (SDW) and structural transition very efficiently, while enhance the antiferromagnetic (AFM) transition temperature T$_N$ of Eu$^{2+}$. The superconductivity coexists with SDW order at the low pressure, while always coexists with the Eu$^{2+}$ AFM order. The results suggests that Eu$^{2+}$ spin dynamics is disentangeld with superconducting (SC) pairing taken place in the two-dimensional \emph{Fe-As} plane, but it can strongly affect superconducting coherence along c-axis.

High field (14 T) magneto transport of Sm/PrFeAsO

We report high field magneto transport of Sm/PrFeAsO. Below spin density wave transition (TSDW), the magneto-resistance (MR) of Sm/PrFeAsO is positive and increasing with decreasing temperature. The MR of SmFeAsO is found to be 16%, whereas it is 21.5% in the case of PrFeAsO, at 2.5 K under applied magnetic field of 14 Tesla (T). In the case of SmFeAsO, the variation of isothermal MR with field below 20 K is nonlinear at lower magnetic fields (<2 T) and it is linear at moderately higher magnetic fields (H ≥ 3.5 T). On the other hand, PrFeAsO shows almost linear MR at all temperatures below 20 K. The anomalous behavior of MR being exhibited in PrFeAsO is originated from Dirac cone states. The stronger interplay of Fe and Pr ordered moments is responsible for this distinct behavior. PrFeAsO also shows a hump in resistivity (R-T) with a possible conduction band (FeAs) mediated ordering of Pr moments at around 12 K. However, the same is absent in SmFeAsO even down to 2 K. Our results of high field magneto-transport of up to 14 T brings about clear distinction between ground states of SmFeAsO and PrFeAsO.

Spin-Density-Wave Gap with Dirac Nodes and Two-Magnon Raman Scattering in BaFe2As2

Raman selection rules for electronic and magnetic excitations in BaFe 2 As 2 were theoretically investigated and applied them to the separate detection of the nodal and anti-nodal gap excitations at the spin density wave (SDW) transition and the separate detection of the nearest and the next nearest neighbor exchange interaction energies. Raman spectra are composed of magnetic excitations with gradually decreasing intensity toward far above the SDW transition temperature ( T SDW ) and electronic excitations induced by the Brillouin zone folding below T SDW . The SDW gap has Dirac nodes, because many orbitals participate in the electronic states near the Fermi energy. Using a two-orbital band model the electronic excitations near the Dirac node and the anti-node are found to have different symmetries. Applying the symmetry difference to Raman scattering the nodal and anti-nodal electronic excitations are separately obtained. The low-energy spectra from the anti-nodal region have critical fluctuation just above T SDW and change into the gap structure by the first order transition at T SDW , while those from the nodal region gradually change into the SDW state. Magnetic excitations are observed as a very broad peak in all polarization configurations. The selection rule for two-magnon scattering from the stripe spin structure was obtained. Applying it to the two-magnon Raman spectra it is found that the magnetic exchange interaction energies are not presented by the short-range superexchange model, but the second derivative of the total energy of the stripe spin structure with respect to the moment directions. The selection rule and the peak energy are expressed by the two-magnon scattering process in an insulator, but the large spectral weight above twice the maximum spin wave energy is difficult to explain by the decayed spin wave. It may be explained by the electronic scattering of itinerant carriers with the magnetic self-energy in the localized spin picture or the particle–hole excitation model in the itinerant spin picture. The magnetic scattering spectra are compared to the insulating and metallic cuprate superconductors whose spins are believed to be localized.

Magnetic properties of single crystal EuCo2As2

Magnetization measurements at ambient pressure have been performed on single crystal EuCo2As2, a compound having a similar crystalline structure as the pressure-induced EuFe2As2 superconductor. Similar to the magnetic characteristics of EuFe2As2 at ambient pressure, the effective paramagnetic moment at high temperatures arises primarily from the Eu2+ spins, with the Eu2+ spins ordering antiferromagnetically below 39 K. Also, a strong anisotropic behavior is found below this ordering temperature in both magnetic field and temperature dependences. While the magnetization at the lowest temperatures increases nearly linearly with field for fields parallel to the c-axis, the magnetization for fields parallel to the ab-plane exhibits a step-like behavior before further increasing linearly to the highest applied field of 5.0 T. This behavior can be explained in terms of a spin-reorientation of the Eu2+ spins being in an A-type antiferromagnetic structure. However, unlike for EuFe2As2, no evidence is found for any spin density wave transition at higher temperatures.

Complex evolution of the electronic structure of Cr with temperature

Employing state-of-the-art high resolution photoemission spectroscopy, we studied the electronic structure evolution of Cr with temperature. Experimental results reveal signature of a pseudogap much below the spin density wave transition temperature. A sharp peak appears near the Fermi level at low temperatures presumably related to the orbital Kondo effect. These results provide possible origin of the complex electronic properties observed in this system.

Doping dependent evolution of magnetism and superconductivity in Eu1−xKxFe2As2 (x = 0–1) and temperature dependence of the lower critical field Hc1

We have synthesized polycrystalline samples of Eu1−xKxFe2As2 (x = 0–1) and carried out systematic characterization using x-ray diffraction, ac and dc magnetic susceptibility, and electrical resistivity measurements. A clear signature of the coexistence of a superconducting transition (Tc = 5.5 K) with spin density wave (SDW) ordering is observed in our underdoped sample with x = 0.15. The SDW transition disappears completely for the x = 0.3 sample and superconductivity arises below 20 K. The superconducting transition temperature Tc increases with increase in the K content and a maximum Tc = 33 K is reached for x = 0.5, beyond which it decreases again. The doping dependent T(x) phase diagram is extracted from the magnetic and electrical transport data. It is found that magnetic ordering of Eu moments coexists with the superconductivity up to x = 0.6. The isothermal magnetization data taken at 2 K for the doped samples suggest the 2+ valence state of the Eu ions. We also present the temperature dependence of the lower critical field Hc1 of the superconducting polycrystalline samples. The values of Hc1(0) obtained for x = 0.3, 0.5, and 0.7 after taking the demagnetization factor into account are 202, 330, and 212 Oe, respectively. The London penetration depth λ(T) calculated from the lower critical field does not show exponential dependence at low temperature, as would be expected for a fully gapped clean s-wave superconductor. In contrast, it shows a T2 power law feature up to T = 0.3Tc, as observed in Ba1−xKxFe2As2 and BaFe2−xCoxAs2.

Quantum magnetoresistance of the PrFeAsO oxypnictide

We report the observation of an unusual B dependence of transverse magnetoresistance (MR) in the PrFeAsO, one of the parent compound of pnictide superconductors. Below the spin density wave transition, MR is large, positive and increases with decreasing temperature. At low temperatures, MR increases linearly with B up to 14 T. For T≥40 K, MR versus B curve develops a weak curvature in the low-field region which indicates a crossover from B linear to B2 dependence as B→0. The B linear MR originates from the Dirac cone states and has been explained by the quantum mechanical model proposed by Abrikosov.