Antiferromagnetic Propagation Vector Research Articles

Electronic and magnetic properties of stoichiometric CeAuBi2.

We report the electronic and magnetic properties of stoichiometric CeAuBi2 single crystals. At ambient pressure, CeAuBi2 orders antiferromagnetically below a Néel temperature (TN) of 19 K. Neutron diffraction experiments revealed an antiferromagnetic propagation vector , which doubles the paramagnetic unit cell along the c axis. At low temperatures several metamagnetic transitions are induced by the application of fields parallel to the c axis, suggesting that the magnetic structure of CeAuBi2 changes as a function of field. At low temperatures, a linear positive magnetoresistance may indicate the presence of band crossings near the Fermi level. Finally, the application of external pressure favors the antiferromagnetic state, indicating that the 4f electrons become more localized.

NMR studies of the helical antiferromagnetic compound EuCo2P2

In EuCo2P2, 4f electron spins of Eu2+ ions order antiferromagnetically below a Néel temperature TN=66.5K. The magnetic structure below TN was reported to be helical with the helix axis along the c-axis from the neutron diffraction study. We report the results of 153Eu, 59Co and 31P nuclear magnetic resonance (NMR) measurements on EuCo2P2 using a single crystal and a powdered sample. In the antiferromagnetic (AFM) state, we succeeded in observing 153Eu, 59Co and 31P NMR spectra in zero magnetic field. The sharp 153Eu zero field NMR (ZF NMR) lines indicate homogeneous Eu ordered moment. The 59Co and 31P ZF NMR spectra showed an asymmetric spectral shape, indicating a distribution of the internal magnetic induction at each nuclear position. The AFM propagation vector k characterizing the helical AFM state can be determined from the internal magnetic induction at Co site. We have determined the model-independent value of the AFM propagation vector k distributed from (0, 0, 0.86)2π/c to (0, 0, 0.73)2π/c, where c is the lattice parameter.

NMR studies of the incommensurate helical antiferromagnet EuCo2P2 : Determination of antiferromagnetic propagation vector

Recently Ding et al. [Phys. Rev. B 95, 184404 (2017)] reported that their nuclear magnetic resonance (NMR) study on EuCo$_2$As$_2$ successfully characterized the antiferromagnetic (AFM) propagation vector of the incommensurate helix AFM state, showing that NMR is a unique tool for determination of the spin structures in incommensurate helical AFMs. Motivated by this work, we have carried out $^{153}$Eu, $^{31}$P and $^{59}$Co NMR measurements on the helical antiferromagnet EuCo$_2$P$_2$ with an AFM ordering temperature $T_{\rm N}$ = 66.5 K. An incommensurate helical AFM structure was clearly confirmed by $^{153}$Eu and $^{31}$P NMR spectra on single crystalline EuCo$_2$P$_2$ in zero magnetic field at 1.6 K and its external magnetic field dependence. Furthermore, based on $^{59}$Co NMR data in both the paramagnetic and the incommensurate AFM states, we have determined the model-independent value of the AFM propagation vector k = (0, 0, 0.73 $\pm$ 0.09)2$\pi$/$c$ where $c$ is the $c$-axis lattice parameter. The temperature dependence of k is also discussed.

NMR determination of an incommensurate helical antiferromagnetic structure in EuCo2As2

We report $^{153}$Eu, $^{75}$As and $^{59}$Co nuclear magnetic resonance (NMR) results on EuCo$_2$As$_2$ single crystal. Observations of $^{153}$Eu and $^{75}$As NMR spectra in zero magnetic field at 4.3 K below an antiferromagnetic (AFM) ordering temperature $T_{\rm N}$ = 45 K and its external magnetic field dependence clearly evidence an incommensurate helical AFM structure in EuCo$_2$As$_2$. Furthermore, based on $^{59}$Co NMR data in both the paramagnetic and the incommensurate AFM states, we have determined the model-independent value of the AFM propagation vector ${\bf k}$ = (0, 0, 0.73 $\pm$ 0.07)2$\pi$/$c$ where $c$ is the $c$ lattice parameter. Thus the incommensurate helical AFM state was characterized by only NMR data with model-independent analyses, showing NMR to be a unique tool for determination of the spin structure in incommensurate helical AFMs.

Importance of local structural distortions in magnetocaloric effect in Mn based antiperovskites

Mn3GaC and Mn3SnC are two antiperovskites which exhibit a large magnetocaloric effect associated with a magnetostructural transformation. Although structurally similar, the magnetic transition and the nature of magnetocaloric effect in the two compounds is completely different. While Mn3GaC transforms from a ferromagnetic to an antiferromagnetic state at Tt = 178K with the antiferromagnetic propagation vector along K = ½, ½, ½, in Mn3SnC the transformation occurs at Tt = 280K from a paramagnetic state to an antiferromagnetic state with K = ½, ½, 0. Mn K EXAFS has been employed to show that these differences in magnetic properties are related to local structural distortions in Mn6C octahedra which are quite different in the two compounds.

Spin polarization of Ru in superconducting Ba(Fe0.795Ru0.205)2As2studied by x-ray resonant magnetic scattering

We have employed the x-ray resonant magnetic scattering (XRMS) technique at the Ru ${L}_{2}$ edge of the Ba(Fe${}_{1\ensuremath{-}x}$Ru${}_{x}$)${}_{2}$As${}_{2}$ ($x=0.205$) superconductor. We show that pronounced resonance enhancements at the Ru ${L}_{2}$ edge are observed at the wave vector which is consistent with the antiferromagnetic propagation vector of the Fe in the undoped BaFe${}_{2}$As${}_{2}$. We also demonstrate that the XRMS signals at the Ru ${L}_{2}$ edge follow the magnetic ordering of the Fe with a long correlation length, ${\ensuremath{\xi}}_{ab}>2850\ifmmode\pm\else\textpm\fi{}400$ \AA{}. Our experimental observation shows that the Ru is spin polarized in Ba(Fe${}_{1\ensuremath{-}x}$Ru${}_{x}$)${}_{2}$As${}_{2}$ compounds.

Incommensurate Spin-Density Wave Order in Electron-DopedBaFe2As2Superconductors

Neutron diffraction studies of Ba(Fe(1-x)Co(x))(2)As)(2) reveal that commensurate antiferromagnetic order gives way to incommensurate magnetic order for Co compositions between 0.056 < x < 0.06. The incommensurability has the form of a small transverse splitting (0, ± ε, 0) from the commensurate antiferromagnetic propagation vector Q(AFM) = (1,0,1) (in orthorhombic notation) where ε ≈ 0.02-0.03 and is composition dependent. The results are consistent with the formation of a spin-density wave driven by Fermi surface nesting of electron and hole pockets and confirm the itinerant nature of magnetism in the iron arsenide superconductors.

Thermal expansion and magnetostriction of GdAg 2, and relations to the magnetoelastic paradox

The antiferromagnet GdAg 2 has been shown to be a good model system for the magnetoelastic paradox (MEP), because it exhibits large symmetry conserving magnetoelastic strains and the antiferromagnetic propagation vector breaks the tetragonal lattice symmetry (therefore a large symmetry breaking magnetoelastic strain can be expected in a single q magnetic structure). As in many similar Gd based compounds no symmetry breaking strain has been found in the experiment. In order to investigate this MEP further, we have measured magnetostriction and magnetization on a textured polycrystal. The behaviour closely resembles that of GdNi 2B 2C, the prototype system for the magnetoelastic paradox (MEP). Our forced magnetostriction data indicate that the crystal distorts in applied magnetic field and gives further evidence that the MEP is a low field effect. The observed phase transitions are in agreement with available specific heat and neutron diffraction data. Moreover, the saturation magnetic field was measured in high pulsed magnetic fields and agrees well with the value calculated from the Standard Model of Rare Earth Magnetism (SMREM).

NdRhSn: A ferromagnet with an antiferromagnetic precursor

NdRhSn has a ferromagnetic ground state [${T}_{t}=7.6(1)$ K] with a magnetic moment equal to 2/3 of the free Nd${}^{3+}$-ion moment. Upon cooling from the paramagnetic state, the ferromagnetism is preceded by an incommensurate antiferromagnetic state between ${T}_{N}=9.8(1)$ K and ${T}_{t}$. In both magnetic phases the magnetic moments are locked along the $c$ axis, which is the easy-magnetization axis of the system. We have investigated this unusual situation by measuring anomalies of several bulk physical properties characteristic for the magnetic phase transitions at ${T}_{N}$ and ${T}_{t}$ and the response of these properties to an applied magnetic field. Neutron-diffraction experiments have been also performed at low temperatures. Furthermore, in order to understand better the physical properties of NdRhSn, single crystals have been studied under external hydrostatic and uniaxial pressure and in high magnetic fields. An antiferromagnetic phase with a propagation vector (0 0 1/11) has been found between ${T}_{N}$ and ${T}_{t}$. The magnetic-ordering temperatures ${T}_{N}$ and ${T}_{t}$ are sensitive to both hydrostatic and uniaxial pressures, but in a different manner. Application of hydrostatic pressure leads to a reduction of the ordering temperatures at rates $\ensuremath{\Delta}{T}_{N}/\ensuremath{\Delta}p=\ensuremath{-}0.76(5)$ K/GPa and $\ensuremath{\Delta}{T}_{t}/\ensuremath{\Delta}p=\ensuremath{-}0.9(2)$ K/GPa while uniaxial pressure applied along the $c$ axis leads to an increase of the ordering temperatures at rates $\ensuremath{\Delta}{T}_{N}/\ensuremath{\Delta}p=+1.2(5)$ K/GPa and $\ensuremath{\Delta}{T}_{t}/\ensuremath{\Delta}p=+2.7(4)$ K/GPa. No considerable influence of pressure on the antiferromagnetic propagation vector has been found. The peculiar evolution of magnetism in NdRhSn with temperature indicates a complex hierarchy of exchange interactions.

Magnetic and Fermi surface properties of UNiGa5

In order to investigate the magnetic structure and Fermi surface properties of UNiGa5, we have performed elastic neutron scattering and de Haas-van Alphen(dHvA) experiments. In the neutron scattering experiment using a single crystalline sample, we determined the antiferromagnetic propagation vector Q=(1/2,1/2,1/2) in UNiGa5. In the dHvA experiment, we observed four branches. Two branches indicate cylindrical Fermi surface and the other two branches correspond to ellipsoidal ones. The cyclotron mass was not large, ranging from 1.3 to 3.1m0.

Geometrically frustrated magnetic structures of the heavy-fermion compound CePdAl studied by powder neutron diffraction

The heavy-fermion compound CePdAl with ZrNiAl-type crystal structure (hexagonal space group ) was investigated by powder neutron diffraction. The triangular coordination symmetry of magnetic Ce atoms on site 3f gives rise to geometrical frustration. CePdAl orders below with an incommensurate antiferromagnetic propagation vector , and a longitudinal sine-wave (LSW) modulated spin arrangement. Magnetically ordered moments at Ce(1) and Ce(3) coexist with frustrated disordered moments at Ce(2). The experimentally determined magnetic structure is in agreement with group theoretical symmetry analysis considerations, calculated by the program MODY, which confirm that for Ce(2) an ordered magnetic moment parallel to the magnetically easy c-axis is forbidden by symmetry. Further low-temperature experiments give evidence for a second magnetic phase transition in CePdAl between 0.6 and 1.3 K. Magnetic structures of CePdAl are compared with those of the isostructural compound TbNiAl, where a non-zero ordered magnetic moment for the geometrically frustrated Tb(2) atoms is allowed by symmetry.

The magnetic phases of pseudobinary Ce(Fe1-xMx)2 intermetallic compounds; M=Al, Co, Ru

Powder neutron diffraction measurements of the magnetic and structural properties of a range of pseudobinary Ce(Fe1-xMx)2 intermetallic compounds, with M=Al, Co, Ru, are reported. In each of these pseudobinaries addition of a small amount of the metallic impurity (M) to CeFe2 reduces the ferromagnetic ordering temperature and enhances the small antiferromagnetic component which appears at low temperature. At higher impurity concentrations antiferromagnetism is achieved. This antiferromagnetism is accompanied by a cubic to rhombohedral distortion of the crystalline lattice. The antiferromagnetic component of the ordered moment in these compounds is oriented at approximately=18.5 degrees to the antiferromagnetic propagation vector ( tau =(111)). An ordered cerium moment is seen in the ferromagnetic phase of CeFe2 and Ce(Fe1-xRux)2. This moment is coupled ferrimagnetically to the iron-sublattice moment and mu Ce/ mu Fe approximately=-0.3.

Magnetoresistance and the spin-flop transition in single-crystal La2CuO4+y.

Measurements are reported of the magnetoresistance (MR) for fields up to 23 T in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ single crystals, which order antiferromagnetically at ${T}_{N}$\ensuremath{\sim}240 K, and in which the conductivity at low temperature is characterized by hopping in localized states. Using the MR, the phase diagram of the spin-flop transition, observed when the magnetic field is applied parallel to the zero-field staggered magnetization, is mapped out. Two transitions of the background ${\mathrm{Cu}}^{2+}$ spins are observed, which are governed by the symmetric and antisymmetric anisotropic components of the superexchange tensor. The antiferromagnetic propagation vector changes from \ensuremath{\tau}\ensuremath{\parallel}a at zero field to \ensuremath{\tau}\ensuremath{\parallel}c at the highest fields. This subtle change in the ordering of the ${\mathrm{Cu}}^{2+}$ spins is accompanied by a large enhancement of the interlayer hopping conductivity up to a factor 2. We show that the magnetoconductance is proportional to the three-dimensional staggered moment with \ensuremath{\tau}\ensuremath{\parallel}c direction. In an appendix we discuss the possible relevance of these results to the behavior of superconducting ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$(Sr,Ba${)}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$.

Magnetic excitations in TmCu

The magnetic excitations have been studied in the rare-earth intermetallic compound TmCu by means of inelastic neutron scattering. Dispersion curves have been followed through the cubic Brillouin zone, in particular an intense and dispersive transverse branch: this latter presents a lifting of the degeneracy along particular directions. Within a generalized susceptibility formalism, this is related to the coupling of the excitations at q and q + Q , where Q = (π/ a, π/ a, 0) is the antiferromagn etic propagation vector in TmCu. The inter-ionic isotropic bilinear exchange parameters are deduced and compared with those of isomorphous compounds.