Approaching the Ground State of Frustrated A-site Spinels: A Combined Magnetization and Polarized Neutron Scattering Study

We re-investigate the magnetically frustrated, {\it diamond-lattice-antiferromagnet} spinels FeAl$_2$O$_4$ and MnAl$_2$O$_4$ using magnetization measurements and diffuse scattering of polarized neutrons. In FeAl$_2$O$_4$, macroscopic measurements evidence a "cusp" in zero field-cooled susceptibility around 13~K. Dynamic magnetic susceptibility and {\it memory effect} experiments provide results that do not conform with a canonical spin-glass scenario in this material. Through polarized neutron scattering studies, absence of long-range magnetic order down to 4~K is confirmed in FeAl$_2$O$_4$. By modeling the powder averaged differential magnetic neutron scattering cross-section, we estimate that the spin-spin correlations in this compound extend up to the third nearest-neighbour shell. The estimated value of the Land\'{e} $g$ factor points towards orbital contributions from Fe$^{2+}$. This is also supported by a Curie-Weiss analysis of the magnetic susceptibility. MnAl$_2$O$_4$, on the contrary, undergoes a magnetic phase transition into a long-range ordered state below $\approx$ 40~K, which is confirmed by macroscopic measurements and polarized neutron diffraction. However, the polarized neutron studies reveal the existence of prominent spin-fluctuations co-existing with long-range antiferromagnetic order. The magnetic diffuse intensity suggests a similar short range order as in FeAl$_2$O$_4$. Results of the present work supports the importance of spin-spin correlations in understanding magnetic response of frustrated magnets like $A$-site spinels which have predominant short-range spin correlations reminiscent of the "spin liquid" state.Comment: 10 pages, 10 figures, double-column, accepted in Phys. Rev. B, 201

XYZ-polarisation analysis of diffuse magnetic neutron scattering from single crystals

Studies of diffuse magnetic scattering largely benefit from the use of a multi-detector covering wide scattering angles. Therefore, the different contributions to the diffuse scattering that originate from magnetic, nuclear coherent, and nuclear spin-incoherent scattering can be separated by the so-called XYZ-polarization analysis. In the past this method has been successfully applied to the analysis of diffuse scattering by polycrystalline samples of magnetic disordered materials. Single crystal studies that exploit the vector properties of spin correlations are of particular interest for furthering our understanding of frustration effects in magnetism. Based on the symmetry properties of polarised scattering a suitable extension of the conventional XYZ method has been derived, which allows for the complete separation and the analysis of features of diffuse magnetic scattering from single crystals.Comment: 6 pages 2 figures, revised as published, one Eq. removed, minor corrections, typos correcte

Inelastic neutron scattering study of crystal field excitations of Nd³⁺ in NdFeAsO

Inelastic neutron scattering experiments were performed to investigate the crystalline electric field (CEF) excitations of Nd3+ (J = 9/2) in the iron pnictide NdFeAsO. The crystal field level structures for both the high-temperature paramagnetic phase and the low-temperature antiferromagnetic phase of NdFeAsO are constructed. The variation of CEF excitations of Nd3+ reflects not only the change of local symmetry but also the change of magnetic ordered state of the Fe sublattice. By analyzing the crystal field interaction with a crystal field Hamiltonian, the crystal field parameters are obtained. It was found that the sign of the fourth and sixth-order crystal field parameters change upon the magnetic phase transition at 140 K, which may be due to the variation of exchange interactions between the 4f and conduction electrons.Comment: 5 pages, 4 figure

Magnetisation distribution in the tetragonal phase of BaFe2As2

We have determined the spatial distribution of the magnetisation induced by a field of 9 T in the tetragonal phase of BaFe2As2 using polarised neutron diffraction. Magnetic structure factors derived from the polarisation dependence of the intensities of Bragg reflections were used to make a maximum entropy reconstruction of the distribution projected on the 110 plane. The reconstruction shows clearly that the magnetisation is confined to the region around the iron atoms and that there is no significant magnetisation associated with either the As or Ba atoms. The distribution of magnetisation around the Fe atom is significantly non-spherical with a shape which is extended in the directions in the projection. These results show that the electrons which give rise to the paramagnetic susceptibility are confined to the Fe atoms their distribution suggests that they occupy 3d t_2g type orbitals with about 60% in those of xy symmetry

Magnetization distribution and orbital moment in the non-Superconducting Chalcogenide Compound K0.8Fe1.6Se2

We have used polarized and unpolarized neutron diffraction to determine the spatial distribution of the magnetization density induced by a magnetic field of 9 T in the tetragonal phase of K0.8Fe1.6Se2. The maximum entropy reconstruction shows clearly that most of the magnetization is confined to the region around the iron atoms whereas there is no significant magnetization associated with either Se or K atoms. The distribution of magnetization around the Fe atom is slightly nonspherical with a shape which is extended along the [0 0 1] direction in the projection. Multipolar refinement results show that the electrons which give rise to the paramagnetic susceptibility are confined to the Fe atoms and their distribution suggests that they occupy 3d t2g-type orbitals with around 66% in those of xz/yz symmetry. Detail modeling of the magnetic form factor indicates the presence of an orbital moment to the total paramagnetic moment of Fe2+Comment: 7 pages, accepted for publication in Physical Review

Magnetic structure of the Eu2+ moments in superconducting EuFe2(As1-xPx)2 with x = 0.19

The magnetic structure of the Eu2+ moments in the superconducting EuFe2(As1-xPx)2 sample with x = 0.19 has been determined using neutron scattering. We conclude that the Eu2+ moments are aligned along the c direction below T_C = 19.0(1) K with an ordered moment of 6.6(2) mu_B in the superconducting state. An impurity phase similar to the underdoped phase exists within the bulk sample which orders antiferromagnetically below T_N = 17.0(2) K. We found no indication of iron magnetic order, nor any incommensurate magnetic order of the Eu2+ moments in the sample.Comment: Accepted for publication in Phys. Rev. B (regular article

Structural and magnetic properties of [\lbrackErTb]\rbrackmultilayers

Abstract.: We have investigated the structural and magnetic properties of $\lbrack$ Er|Tb $\rbrack$ multilayers by different scattering methods. Diffuse X-ray scattering under grazing incidence reveals the interface structure in $\lbrack$ Er|Tb $\rbrack$ bilayers and trilayers, indicating vertically correlated roughness between the Er and Tb interfaces. The magnetic properties of $\lbrack$ ErnEr|TbnTb $\rbrack$ superlattices have been studied as a function of the superlattice composition (indices denote the number of atomic layers). Coupled ferromagnetic structures exist in all investigated samples. The phase transition temperature varies with the Tb layer thickness. Modulated magnetic order is short range for all samples beside the $\lbrack$ Er20|Tb5 $\rbrack$ superlattice, the sample with the smallest Tb layer thickness. We observe dipolar antiferromagnetic coupling between single ferromagnetic Tb layers in all samples, with the onset of this ordering depending on the Tb layer thickness. Due to competing interactions, exchange coupling is limited to the interface near region. Therefore long range modulated magnetic order is observed in the $\lbrack$ Er20|Tb5 $\rbrack$ superlattice only, where the interface regions overlap. The distinct differences to the magnetic structure of an Er0.8Tb0.2 alloy film are explained by a highly anisotropic arrangement of neighbouring atoms due to the correlated roughnes

Magnetic and structural transitions in La0.4_{0.4}Na0.6_{0.6}Fe2_2As2_2 single crystals

La$_{0.4}$Na$_{0.6}$Fe$_2$As$_2$ single crystals have been grown out of an NaAs flux in an alumina crucible and characterized by measuring magnetic susceptibility, electrical resistivity, specific heat, as well as single crystal x-ray and neutron diffraction. La$_{0.4}$Na$_{0.6}$Fe$_2$As$_2$ single crystals show a structural phase transition from a high temperature tetragonal phase to a low-temperature orthorhombic phase at T$_s$\,=\,125\,K. This structural transition is accompanied by an anomaly in the temperature dependence of electrical resistivity, anisotropic magnetic susceptibility, and specific heat. Concomitant with the structural phase transition, the Fe moments order along the \emph{a} direction with an ordered moment of 0.7(1)\,$\mu_{\textup{B}}$ at \emph{T}\,=\,5 K. The low temperature stripe antiferromagnetic structure is the same as that in other \emph{A}Fe$_{2}$As$_{2}$ (\emph{A}\,=\,Ca, Sr, Ba) compounds. La$_{0.5-x}$Na$_{0.5+x}$Fe$_2$As$_2$ provides a new material platform for the study of iron-based superconductors where the electron-hole asymmetry could be studied by simply varying La/Na ratio.Comment: 9 pages, 7 figures, to appear in Physical Review

Approaching the Ground State of Frustrated A-site Spinels: A Combined Magnetization and Polarized Neutron Scattering Study

We re-investigate the magnetically frustrated, {\it diamond-lattice-antiferromagnet} spinels FeAl$_2$O$_4$ and MnAl$_2$O$_4$ using magnetization measurements and diffuse scattering of polarized neutrons. In FeAl$_2$O$_4$, macroscopic measurements evidence a "cusp" in zero field-cooled susceptibility around 13~K. Dynamic magnetic susceptibility and {\it memory effect} experiments provide results that do not conform with a canonical spin-glass scenario in this material. Through polarized neutron scattering studies, absence of long-range magnetic order down to 4~K is confirmed in FeAl$_2$O$_4$. By modeling the powder averaged differential magnetic neutron scattering cross-section, we estimate that the spin-spin correlations in this compound extend up to the third nearest-neighbour shell. The estimated value of the Land\'{e} $g$ factor points towards orbital contributions from Fe$^{2+}$. This is also supported by a Curie-Weiss analysis of the magnetic susceptibility. MnAl$_2$O$_4$, on the contrary, undergoes a magnetic phase transition into a long-range ordered state below $\approx$ 40~K, which is confirmed by macroscopic measurements and polarized neutron diffraction. However, the polarized neutron studies reveal the existence of prominent spin-fluctuations co-existing with long-range antiferromagnetic order. The magnetic diffuse intensity suggests a similar short range order as in FeAl$_2$O$_4$. Results of the present work supports the importance of spin-spin correlations in understanding magnetic response of frustrated magnets like $A$-site spinels which have predominant short-range spin correlations reminiscent of the "spin liquid" state.Comment: 10 pages, 10 figures, double-column, accepted in Phys. Rev. B, 201

Phase diagram of Eu magnetic ordering in Sn-flux-grown Eu(Fe1−x_{1-x}Cox_{x})2_{2}As2_{2} single crystals

The magnetic ground state of the Eu$^{2+}$ moments in a series of Eu(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals grown from the Sn flux has been investigated in detail by neutron diffraction measurements. Combined with the results from the macroscopic properties (resistivity, magnetic susceptibility and specific heat) measurements, a phase diagram describing how the Eu magnetic order evolves with Co doping in Eu(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ is established. The ground-state magnetic structure of the Eu$^{2+}$ spins is found to develop from the A-type antiferromagnetic (AFM) order in the parent compound, via the A-type canted AFM structure with some net ferromagnetic (FM) moment component along the crystallographic $\mathit{c}$ direction at intermediate Co doping levels, finally to the pure FM order at relatively high Co doping levels. The ordering temperature of Eu declines linearly at first, reaches the minimum value of 16.5(2) K around $\mathit{x}$ = 0.100(4), and then reverses upwards with further Co doping. The doping-induced modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu$^{2+}$ moments, which is mediated by the conduction $\mathit{d}$ electrons on the (Fe,Co)As layers, as well as the change of the strength of the direct interaction between the Eu$^{2+}$ and Fe$^{2+}$ moments, might be responsible for the change of the magnetic ground state and the ordering temperature of the Eu sublattice. In addition, for Eu(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals with 0.10 $\leqslant$ $\mathit{x}$ $\leqslant$ 0.18, strong ferromagnetism from the Eu sublattice is well developed in the superconducting state, where a spontaneous vortex state is expected to account for the compromise between the two competing phenomena.Comment: 10 pages, 9 figure