Antiferromagnetic Ordering Temperature TN Research Articles

Doping effect on structural, morphological and magnetic properties of YbMnO3 compound with non-magnetic Lutetium ion

The present study presents an analysis of the structural, morphological and magnetic properties of multiferroic YbMnO3 (YBO) due to Lu doping. YbMnO3 (YBO) and Yb0.85Lu0.15MnO3 (YBLu) were prepared using a conventional solid state reaction method. The investigation employed techniques such as X-ray diffraction (XRD), Field Effect Scanning Electron Microscope (FE-SEM), Raman spectroscopy and magnetic measurements. The X-ray diffraction (XRD) patterns of both samples confirmed the growth of these samples in a polycrystalline single hexagonal phase having space group P63cm with no detectable impurities. The evaluation of crystalline parameters, such as lattice parameters, cell volume, specific bond lengths etc. has been carried out. The Lu-doped sample shows a minor reduction in cell parameters and cell volume which can be attributed to the smaller ionic radius of Lu3+. Lu doping in YBO results in slight shift in Raman Lines towards slightly frequencies which is the result of a small change in the average bond length. The effective magnetic moment of YBLu is found to decrease ∼6 % and the antiferromagnetic ordering temperature TN nearly remains the same because Lu3+ has zero magnetic moment. Both samples show ferri/ferromagnetic behaviour below 5 K due to the ordering of the Yb3+ lattice.

Molecular-field-theory fits to magnetic susceptibilities of antiferromagnetic GdCu2Si2, CuO, LiCrO2, and [formula omitted]-CaCr2O4 single crystals below their Néel temperatures

A recently-developed molecular field theory (MFT) has been used to fit single-crystal magnetic susceptibility χ versus temperature T data below the respective antiferromagnetic ordering temperature TN for a variety of collinear and coplanar noncollinear Heisenberg antiferromagnets. The spins in the system are assumed to interact by Heisenberg exchange and to be identical and crystallographically equivalent. The fitting parameters for χ(T) of collinear antiferromagnets are measurable quantities: the Weiss temperature θp in the Curie–Weiss law, TN,χ(TN), and the spin S. For coplanar noncollinear helix and cycloid structures, an additional fitting parameter is the turn angle between layers of ferromagnetically-aligned spins. Here MFT fits to anisotropic χ(T) data from the literature for single crystals of the collinear antiferromagnets GdCu2Si2 and CuO and the noncollinear antiferromagnets LiCrO2 with a 120° cycloidal structure and α-CaCr2O4 with a 120° helical structure below their respective Néel temperatures are presented. The MFT fit to the anisotropic χ(T⩽TN) data for CuO is poor, whereas the fits to the data for GdCu2Si2, LiCrO2, and α-CaCr2O4 are quite good. The poor fit for CuO is attributed to the influence of strong quantum fluctuations associated with the small Cu spin and the quasi-one-dimensional magnetism that are not taken into account by the MFT. The magnetic contribution to the zero-field heat capacity of the collinear antiferromagnet GdNiGe3 at T⩽TN is also fitted by the MFT.

Tunable quantum critical point and detached superconductivity in Al-doped CrAs

The origin of unconventional superconductivity and its relationship to a T = 0 K quantum critical point (QCP), which is hidden inside the dome of a superconducting state, have long been an outstanding puzzle in strongly correlated superconductors. The observation and tuning of the hidden QCP, which is key to resolving the mystery, however, has been rarely reported. Here we report the controlling of a hidden QCP in the helical antiferromagnet CrAs and separation of the tuned QCP from the pressure-induced superconducting phase. The Al doping in CrAs increases the antiferromagnetic ordering temperature TN from 265 to 275 K, while it suppresses the QCP from 8 to 4.5 kbar. Pressure-induced superconductivity in the high-pressure regime is almost independent of Al doping, but superconductivity below 6 kbar is suppressed, revealing the clear separation between the tuned antiferromagnetic QCP and Tc maximum. These discoveries illustrate subtleties in the interplay between superconductivity and quantum criticality and warrant a deeper insight in understanding of unconventional superconductivity.

Novel multiferroicity in orthorhombic SmCrO3

This paper reports the ferroelectricity in antiferromagnetic SmCrO3 with a reasonably large value of spontaneous electric polarization (70 μC/cm2 mC cm−2 at 50 K) based on the measurement of pyroelectric current. The experimental results showed that the temperature of the onset of polar order was evidently higher (about 220 K) than the Neel temperature (TN =197 K) in the magnetodielectric studies. The calculated magnetodielectric effect (MDE) value was explicitly negative with maximum | MDE | values of about 3.6% around 230 K. The absence of a convincing signature in MDE at antiferromagnetic ordering temperature TN indicated the absence of magnetoelectric coupling around this temperature regime. The results further confirmed that the onset of polar order can be attributed to the structural distortion, rather than the canted noncollinear magnetic structure.

Magnetism and High-magnetic Field Magnetization in Alkali Superoxide CsO2

Alkali superoxide CsO2 is one of candidates for the spin-1/2 one-dimensional (1D) antiferromagnet, which may be sequentially caused by an ordering of the pi-orbital of O2- molecule below TS ~ 70 K. Here, we report on the magnetism in powder CsO2 and high-magnetic field magnetization measurements in pulsed-magnetic fields of up to 60 T. We obtained the low temperature phase diagram around the antiferromagnetic ordering temperature TN = 9.6 K under the magnetic field. At 1.3 K, remarkable up-turn curvature in the magnetization around a saturation field of ~ 60 T is found, which corresponds to the spin-1/2. We will compare it with the theoretical calculation.

Disentangling superconducting and magnetic orders in NaFe1−xNixAs using muon spin rotation

Muon spin rotation and relaxation studies have been performed on a "111" family of iron-based superconductors NaFe_1-xNi_xAs. Static magnetic order was characterized by obtaining the temperature and doping dependences of the local ordered magnetic moment size and the volume fraction of the magnetically ordered regions. For x = 0 and 0.4 %, a transition to a nearly-homogeneous long range magnetically ordered state is observed, while for higher x than 0.4 % magnetic order becomes more disordered and is completely suppressed for x = 1.5 %. The magnetic volume fraction continuously decreases with increasing x. The combination of magnetic and superconducting volumes implies that a spatially-overlapping coexistence of magnetism and superconductivity spans a large region of the T-x phase diagram for NaFe_1-xNi_xAs . A strong reduction of both the ordered moment size and the volume fraction is observed below the superconducting T_C for x = 0.6, 1.0, and 1.3 %, in contrast to other iron pnictides in which one of these two parameters exhibits a reduction below TC, but not both. The suppression of magnetic order is further enhanced with increased Ni doping, leading to a reentrant non-magnetic state below T_C for x = 1.3 %. The reentrant behavior indicates an interplay between antiferromagnetism and superconductivity involving competition for the same electrons. These observations are consistent with the sign-changing s-wave superconducting state, which is expected to appear on the verge of microscopic coexistence and phase separation with magnetism. We also present a universal linear relationship between the local ordered moment size and the antiferromagnetic ordering temperature TN across a variety of iron-based superconductors. We argue that this linear relationship is consistent with an itinerant-electron approach, in which Fermi surface nesting drives antiferromagnetic ordering.

Uniaxial pressure effects on the unusual antiferromagnetic transition in the Kondo semiconductor CeOs2Al10

We have studied the relation between the unusual antiferromagnetic (AFM) ordering and anisotropic c-f hybridization in CeOs2Al10 by measuring the magnetic susceptibility χ under uniaxial pressures P up to 0.45 GPa. For P // B // a and P // B // c, the AFM ordering temperature TN = 28.5 K hardly changes with increasing P, while the value of χ decreases and the maximal temperature of χ(T) shifts to higher temperatures. These findings suggest that the intensity of c-f hybridization is enhanced along the pressurized a and c axes. On the other hand, for P // b, TN increases up to 30 K at 0.15 GPa, while the value of χ remains unchanged. This observation for P // b suggests that the net intensity of c-f hybridization decreases, while the c-f hybridization along the b-axis hardly changes. We discuss these results in terms of the atomic displacements of Os, Al(3) and Al(4) under uniaxial pressures.

The origin of thermally stimulated depolarization currents in multiferroic CuCrO2

We have measured the thermally stimulated depolarization currents (TSDC) of multiferroic CuCrO2. We observe a sharp peak near the antiferromagnetic ordering temperature TN ∼ 24 K, below which the material becomes ferroelectric. In addition, we observe three other peaks above TN at ∼50, 120, and 150 K, when the poling is done at a higher temperature than TN. These peaks are not related to exotic kinds of ferroelectricity. Using the poling field dependence of TSDC, the origins of the first two peaks are ascribed to the relaxation of defect dipoles and to space charge relaxation due to the release of trapped charges, respectively. Upon polishing, the peaks observed at 120 and 150 K disappear, suggesting a surface defect origin. Moreover, using temperature and frequency dependent dielectric measurements, we find Maxwell–Wagner type dielectric relaxation. In connection with the mechanism of one of the TSDC peaks, we suggest a Schottky barrier formation to explain the dielectric relaxation.

Research Update: Magnetic phase diagram of EuTi1−xBxO3 (B = Zr, Nb)

We report the magnetic phase diagram of EuTi1−xBxO3 (B = Zr, Nb), determined from magnetization and heat capacity measurements. Upon Zr-doping, the antiferromagnetic ordering temperature TN of EuTi1−xZrxO3 gradually decreases from 5.6 K (x = 0) to 4.1 K (x = 1). Whereas a similar decrease in TN is observed for small amounts of Nb doping (x ≤ 0.05), ferromagnetism is induced in EuTi1−xNbxO3 with x > 0.05. The ferromagnetic interaction between localized Eu 4f spins mediated by itinerant electrons introduced by Nb doping results in the ferromagnetism in EuTi1−xNbxO3.

Pressure-induced ferromagnetism in antiferromagnetic Fe1.03Te

The magnetic properties of Fe1.03Te under hydrostatic pressure up to p ~ 5.7 GPa were investigated by means of muon spin rotation, dc magnetization, and neutron depolarization measurements. With increasing pressure the antiferromagnetic ordering temperature TN decreases continuously from 70 K at ambient pressure towards higher pressures. Surprisingly, the commensurate antiferromagnetic order of FeTe enters a region of incommensurate and dynamical magnetic order before at p ~ 1.7 GPa the system turns ferromagnetic. The ferromagnetic ordering temperature TC increases with increasing pressure.

Pressure-induced Suppression of the Antiferromagnetic Transition in YbNi3Al9 Single Crystal

We have measured the electrical resistivity ρ(T) of heavy fermion antiferromagnet YbNi3Al9 under hydrostatic pressure up to 8 GPa. We found that the antiferromagnetic ordering temperature TN is suppressed with increasing pressure, whereas the localized character of the trivalent Yb state becomes stronger inferred from the reduction of the Kondo scattering. We will discuss the possible origin of the suppression of TN under pressure.

Analysis of the magnetic structure of the manganese oxychalcogenides R2Mn2Se2O (R=LaO, BaF) by density functional calculations

We examined the magnetic structures of R2Mn2Se2O (R=LaO, BaF) by evaluating the spin exchange interactions on the basis of density functional theory (DFT) calculations, and compared how they differ from that of the Fe analog (BaF)2Fe2Se2O. In R2Mn2Se2O (R=LaO, BaF), the two-dimensional antiferromagnetic square lattice defined by J1 is spin-frustrated, and the exchange J1 is stronger for (LaO)2Mn2Se2O than for (BaF)2Mn2Se2O by a factor of 1.5 explaining why the magnetic susceptibility maximum occurs at a higher temperature for (LaO)2Mn2Se2O than for (BaF)2Mn2Se2O (360 vs. 210K). We identified two probable reasons why the specific heat anomaly at the long-range antiferromagnetic ordering temperature TN is significantly weaker for R2Mn2Se2O than for (BaF)2Fe2Se2O.

Lanthanum doped multiferroic DyFeO3: Structural and magnetic properties

Lanthanum doped multiferroic DyFeO3 was synthesized by solid state reaction. X-ray diffraction and refinement show that the lattice parameters of Dy1−xLaxFeO3 increase linearly with the La content. Raman spectroscopy reveals that the short-range force constant in Dy1−xLaxFeO3 is decreased by La3+ ion substitution. The spin reorientation phase transition temperature (TSRPT) is observed to decrease along with the doping level. The antiferromagnetic ordering temperature TN of Fe3+ ions is depressed with increasing doping level. Both decreasing TSRPT and decreasing TN indicate that Fe–Dy and Fe–Fe interactions are weakened by La substitution. It is found that the electron configuration of Fe3+ is high spin state and not affected by the La doping in all the samples above TN.

High Field ESR Study of the New Low Dimensional S=1/2 System: Cu(NO3)2⋅H2O

Cu(NO3)2⋅H2O is a new S=1/2 system based on transition metal nitrates and contains two-dimensional Cu2+ (S=1/2) layers. We studied single crystalline samples by means of high field (HF) ESR and NMR. Cu2+ HF-ESR and 1H-NMR data reveal pronounced spin-spin correlations at temperatures much higher than the antiferromagnetic ordering temperature TN∼3.6 K. These observations clearly suggest a two-dimensional character of magnetic interactions in Cu(NO3)2⋅H2O. We discuss the obtained results by considering the possible exchange paths between inequivalent Cu-sites in this compound.

Effect of pressure on the anomalous magnetoresistance and antiferromagnetism of single crystal GdB4

Electrical resistance of single-crystalline GdB4 has been measured at high pressures and high magnetic fields. The antiferromagnetic ordering temperature TN increases with pressure at a rate 0.3 K/GPa, but the hidden transition temperature Ta is almost pressure independent. The magnitude of magnetoresistance at 4.2 K (about 6000% at 8.5 T) is not affected by applying pressures below 3 GPa. This fact suggests similar origins of the hidden transition and of the extremely large magnetoresistance.

Electronic and magnetic properties of GdPdSb

Magnetic and resistivity measurements were used to examine the ternary rare earth compound GdPdSb. A second magnetic transition below the antiferromagnetic ordering temperature TN was found, indicating a possible ferromagnetic ordering. The electronic structure of GdPdSb was also examined using first principles electronic structure calculations. It was found that the antiferromagnetic configuration is more stable than the ferromagnetic configuration, but that the energy difference is very small between these configurations. For this reason, ferromagnetic behaviour could be expected at low temperature. According to band structure calculations, ferromagnetically ordered GdPdSb is a possible half metal, the first half metallic ferromagnet with the hexagonal LiGaGe structure type.

Low-energy spin fluctuations in the non-Fermi-liquid compound YbRh2Si2

We report on inelastic neutron scattering experiments on YbRh2Si2 powder to study the low-energy spin dynamics at temperatures between T = 0.8 and 22 K. The low-energy magnetic response is quasielastic. However, it exhibits an unusual form not modelled by a simple relaxation rate yielding a Lorentzian lineshape, but can satisfactorily be described by a phenomenological model involving a distribution of relaxation rates. The lower bound of the relaxation rates varies roughly linear with temperature indicating a pronounced slowing down of the critical modes above the antiferromagnetic ordering temperature TN ≈ 70 mK.

Effect of pressure on the magnetic properties of the series Cu1−xZnxCr2Se4

The spinel series Cu1−xZnxCr2Se4 has been investigated by AC susceptibility measurements under applied pressures of up to 6kbar. Applied pressure is found to cause a decrease in the antiferromagnetic ordering temperature TN (∼10% at 6kbar). We argue that this is evidence for TN being determined by the near-neighbour coupling which in these materials is ferromagnetic, despite the overall antiferromagnetic order.

Ordering phenomena and transport properties of Bi1/2Sr1/2MnO3 single crystals

Measurements of the electrical and thermal conductivities, thermopower, and paramagnetic susceptibility have been performed on single-crystal samples of Bi1/2Sr1/2MnO3, complemented with the x-ray powder diffraction data. A pronounced hysteretic behavior, observed below the cubic-to-orthorhombic transition at Tcrit=535 K, is related to the onset of long-range charge order (CO) at TCO=450 K and its further evolution down to about 330 K. The diffraction data suggest that the charge-ordered state is formed by Zener pairs, represented by Mn4+ dimers linked by one extra eg electron, and is possibly stabilized by cooperative Bi,Sr displacements. An extremely low thermal conductivity is observed down to the lowest temperatures, without any recovery at the antiferromagnetic ordering temperature TN=150 K. Such behavior points to a presence of strong scatterers of phonons. Their possible origin can be linked to “optical-like” oscillations that are associated with fluctuating charges within the Zener pairs.

Coexistence of superconductivity and antiferromagnetism in the La1−xPrxBa2Cu3O7 system

Structural, magnetic, and calorimetric properties are reported for the La1−xPrxBa2Cu3O7 solid solution. The structure remains orthorhombic with lattice parameter increases from a=0.3872, b=0.3927, and c=1.1709 nm for PrBa2Cu3O7 to a=0.3903, b=0.3944, and c=1.1801 nm for LaBa2Cu3O7. Substitution of Pr for La depresses Tc from 90 to 52 K for La0.5Pr0.5Ba2Cu3O7, which was reported to be a nonsuperconductor. The Pr antiferromagnetic ordering temperature TN decreases monotonically from 18 K for PrBa2Cu3O7 to 6 K for La0.8Pr0.2Ba2Cu3O7. Thus there is a large range of coexistence of superconductivity and antiferromagnetism for 0.2⩽x⩽0.5. The superconductivity and anomalous Pr ordering will be discussed with the R1−xPrxBa2Cu3O7 systems (R=Y, Nd–Lu).