O. Zaharko

Dipolar Antiferromagnetism and Quantum Criticality in LiErF4

Dropping a Dimension? In most magnetic materials, the exchange interaction causes the spins on the neighboring sites of a crystal lattice to align. In the absence of exchange interactions, dipolar interactions, which are highly orientation dependent, are also expected to be able to cause magnetism. Kraemer et al. (p. 1416) present evidence for antiferromagnetism in a dipolar-coupled material, LiErF4. Although a three-dimensional system, its critical behavior was more reminiscent of a two-dimensional material. Scattering experiments reveal that magnetic ordering can arise from dipolar interactions, not only from exchange. Magnetism has been predicted to occur in systems in which dipolar interactions dominate exchange. We present neutron scattering, specific heat, and magnetic susceptibility data for LiErF4, establishing it as a model dipolar-coupled antiferromagnet with planar spin-anisotropy and a quantum phase transition in applied field Hc|| = 4.0 ± 0.1 kilo-oersteds. We discovered non–mean-field critical scaling for the classical phase transition at the antiferromagnetic transition temperature that is consistent with the two-dimensional XY/h4 universality class; in accord with this, the quantum phase transition at Hc exhibits three-dimensional classical behavior. The effective dimensional reduction may be a consequence of the intrinsic frustrated nature of the dipolar interaction, which strengthens the role of fluctuations.

Spin Amplitude Modulation Driven Magnetoelectric Coupling in the New Multiferroic FeTe2O5Br

The magnetic and ferroelectric properties of the layered geometrically frustrated cluster compound FeTe2O5Br were investigated with single-crystal neutron diffraction and dielectric measurements. An incommensurate transverse amplitude modulated magnetic order with the wave vector q=(1/2,0.463,0) develops below T(N)=10.6(2) K. Simultaneously, a ferroelectric order due to exchange striction involving polarizable Te4+ lone-pair electrons develops perpendicular to q and to Fe3+ magnetic moments. The observed magnetoelectric coupling is proposed to originate from the temperature dependent phase difference between neighboring amplitude modulation waves.

Magnetic ordering in Ce3Cu4Sn4 and Ce3Cu4Ge4

Abstract Specific heat, magnetic susceptibility, magnetization and neutron diffraction measurements were carried out on Ce 3 Cu 4 Sn 4 and Ce 3 Cu 4 Ge 4 compounds (structure type Gd 3 Cu 4 Ge 4 , Immm, 2 Ce sites). The linear coefficient of specific heat is γ =330xa0mJ/mol Ce xa0K 2 for the stannide and γ =225xa0mJ/mol Ce xa0K 2 for the germanide, indicative for heavy fermion behaviour. The Ruderman–Kittel–Kasuya–Yosida exchange interactions in the presence of the magnetocrystalline anisotropy induce magnetic ordering, which is complex and proceeds in several steps. For both compounds below the Neel temperature T N =10.3xa0K incommensurate magnetic arrangements are favoured. They lock into a commensurate structure with the wave vector q 3 =(0 0 0) at T 2 ( T 2 =7.3xa0K for stannide and T ′ 2 =7.8xa0K for germanide). Due to a strong anisotropy the Ce magnetic moments are aligned along the x -axis in the T 2 – T 3 interval, but at T 3 = T ′ 3 =2.6xa0K a reorientation of the moments into the xy -plane occurs. The evolution of the magnetic order and the moment values differ for the two inequivalent crystallographic Ce sites. The ordered Ce magnetic moment is 2.05(2)xa0 μ B at the 2d site and 1.10(2)xa0 μ B at the 4e site for germanide. For stannide the corresponding values are 0.75(1)xa0 μ B and 1.94(2)xa0 μ B . Two different magnetic arrangements are possible for Ce 3 Cu 4 Sn 4 at low temperatures.

Incommensurate magnetic ordering in Cu2Te2O5X2 (X=Cl,Br) studied by single crystal neutron diffraction

Polarized and unpolarized neutron-diffraction studies have been carried out on single crystals of the coupled spin tetrahedra systems Cu2Te2O5X2 (X=Cl, Br). A model of the magnetic structure associated with the propagation vectors k() (Cl) approximate to (-0.150,0.422, (1)/(2)) and k(Br) approximate to (-0.172,0.356, (1)/(2)) and stable below T-N = 18 K for X= Cl and T-N = 11 K for X = Br is proposed. A feature of the model, common to both the bromide and chloride, is a canted coplanar motif for the four Cu2+ spins on each tetrahedron which rotates on a helix from cell to cell following the propagation vector. The Cu2+ magnetic moment determined for X = Br, 0.395(5)mu(B), is significantly less than for X= Cl, 0.88 (1)mu(B) at 2 K. The magnetic structure of the chloride associated with the wave vector k differs ;from that determined previously for the wave vector k approximate to (0.150,0.422, (1)/(2))

Evolution of short-range to long-range monoclinic order of MB type with decreasing temperature in 0.75[Pb(Mg1∕3Nb2∕3)O3]-0.25PbTiO3

Results of Rietveld analysis of powder neutron diffraction data on 0.75[Pb(Mg1∕3Nb2∕3)O3]-0.25PbTiO3 (PMN-0.25PT) at room temperature and 80K are presented. It is shown that the currently believed structural model based on average rhombohedral structure (space group R3m) with local ⟨110⟩ disordered Pb atoms cannot account for the structure of PMN-0.25PT at 80K. The low temperature structure of PMN-0.25PT is best refined using Cm space group of MB type. It is argued that with decreasing temperature the short-range monoclinic order at 300K grows into a truly long-range ordered monoclinic structure at 80K.

Incommensurate Magnetic Ordering in Cu2Te2O5X2 (X=Cl,Br) Studied by Neutron Diffraction

We present the results of the first neutron powder and single crystal diffraction studies of the coupled spin tetrahedra systems Cu2Te2O5X2 (X = Cl,Br). Incommensurate antiferromagnetic order with the propagation vectors kCl approximately [0.150,0.422,1/2], kBr approximately [0.158,0.354,1/2] sets in below TN = 18 K for X = Cl and 11 K for X = Br. No simple collinear antiferromagnetic or ferromagnetic spin arrangements within Cu2+ tetrahedra fit these observations. Fitting the diffraction data to more complex but physically reasonable models with multiple helices leads to a moment of 0.67(1)microB/Cu2+ at 1.5 K for the Cl compound. The reason for such a complex ground state may be geometrical frustration of the spins due to the intratetrahedral and intertetrahedral couplings having similar strengths. In neither compound has any evidence for a structural transition accompanying the magnetic ordering been found.

Persistent Spin Dynamics Intrinsic to Amplitude-Modulated Long-Range Magnetic Order

An incommensurate elliptical helical magnetic structure in the frustrated coupled-spin-chain system FeTe(2)O(5)Br is surprisingly found to persist down to 53(3) mK (T/T(N)~1/200), according to neutron scattering and muon spin relaxation. In this state, finite spin fluctuations at T→0 are evidenced by muon depolarization, which is in agreement with specific-heat data indicating the presence of both gapless and gapped excitations. We thus show that the amplitude-modulated magnetic order intrinsically accommodates contradictory persistent spin dynamics and long-range order and can serve as a model structure to investigate their coexistence.

The magnetic structure of multiferroic BaMnF4

The multiferroic material BaMnF(4) has been investigated with unpolarized and polarized neutron diffraction. The structure has been shown to be antiferromagnetic. The magnetic moments are aligned at 12° to the b direction in the bc plane, 3° different from the previously determined value. The ferromagnetic component that is indicative of the linear magnetoelectric effect was not observed.

Atomic motions in the layered copper pseudochalcogenide CuNCN indicative of a quantum spin-liquid scenario

We explore the thermodynamic properties of the layered copper(II) carbodiimide CuNCN by heat-capacity measurements and investigate the corresponding thermal atomic motions by means of neutron powder diffraction as well as inelastic neutron scattering. The experiments are complemented by a combination of density-functional calculations, phonon analysis and analytic theory. The existence of a soft flexural mode-bending of the layers, characteristic for the material structure-is established in the phonon spectrum of CuNCN by giving characteristic temperature-dependent contributions to the heat capacity and atomic displacement parameters. The agreement with the neutron data allows us to extract a residual-on top of the lattice-presumably spinon contribution to the heat capacity [Formula: see text], speaking in favor of the spin-liquid picture of the electronic phases of CuNCN.

Exchange anisotropy as mechanism for spin-stripe formation in frustrated spin chains

We investigate the spin-stripe mechanism responsible for the peculiar nanometer modulation of the incommensurate magnetic order that emerges between the vector-chiral and the spin-density-wave phase in the frustrated zigzag spin-1/2 chain compound