We report a combined neutron scattering and magnetization study on the multiferroic DyFeO3 which shows a very strong magnetoelectric effect. Applying magnetic field along the c-axis, the weak ferromagnetic order of the Fe ions is quickly recovered from a spin reorientation transition, and the long-range antiferromagnetic order of Dy becomes a short-range one. We found that the short-range order concurs with the multiferroic phase and is responsible for its sizable hysteresis. Our H-T phase diagram suggests that the strong magnetoelectric effect in DyFeO3 has to be understood with not only the weak ferromagnetism of Fe but also the short-range antiferromagnetic order of Dy.PACS numbers: 75.85.+t,75.30.Kz Multiferroic (MF) materials, which possess two or more (anti-)ferroic orders, are topical for both fundamental interest and their application potentials [1][2][3][4][5]. Since the discovery of spontaneous magnetoelectric (ME) coupling in TbMnO 3 [6], a lot of materials have been actively explored in hope to realize a mutual control of magnetization or electric polarization using electric or magnetic field. For Type II multiferroics, ferroelectricity has magnetic origin, and study of rare-earth iron oxides ReFeO 3 have become a fertile ground for exploring such a multiferroic effect. DyFeO 3 has been discovered to have one of the strongest ME couplings [7] and more recently piezomagnetoelectric effect [8]. Electric-field generation and reversal of ferromagnetic moments in a single-component bulk material have been realized in Tb and Gd doped DyFeO 3 [9]. Multiferroic effects have since been observed also in many similar orthoferrites [10-12].Research on rare-earth orthoferrites ReFeO 3 dates back to 1960s [13][14][15][16]. Taking advantage of strong coupling between the Re 3+ and F e 3+ magnetic moments, these materials have shown a vast range of nontrivial magnetism, including solitonic lattice [17], laser induced spin switch [18,19], and unconventional magnetization reversal [20,21]. For DyFeO 3 , both rare-earth and iron moments develop complex magnetic orders at zero magnetic field. At T F e N ≈ 600 K, the Fe magnetic moments develop a canted antiferromagnetic order with a weak ferromagnetic (WFM) c-axis component due to the Dzyaloshinskii-Moriya interaction and it is denoted as the G x A y F z state in Bertaut's notation [22]. The WFM state undergoes a spin reorientation through a Morintype transition into the A x G y C z state at T . The multiferroic effect takes place when a magnetic field is applied in the c-axis [7]. The field recovers the WFM state and suppresses the Morintype spin reorientation [24,25]. This recovery of WFM is currently believed to be crucial to the MF effect, from symmetry consideration as well as the proposed exchange striction mechanism [7,9,26,27]. The Dy AFM order is also one of the ingredients in the exchange striction mechanism, but is generally regarded as being field insensitive. So far, no direct experimental evidence exists for the finite-field magnetic phases proposed in ...

Sr2Cr3As2O2 is composed of alternating square-lattice CrO2 and Cr2As2 stacking layers, where CrO2 is isostructural to the CuO2 building-block of cuprate high-Tc superconductors and Cr2As2 to Fe2As2 of Fe-based superconductors. Current interest in this material is raised by theoretic prediction of possible superconductivity. In this neutron powder diffraction study, we discovered that magnetic moments of Cr(II) ions in the Cr2As2 sublattice develop a C-type antiferromagnetic structure below 590 K, and the moments of Cr(I) in the CrO2 sublattice form the La2CuO4-like antiferromagnetic order below 291 K. The staggered magnetic moment 2.19(4)µB /Cr(II) in the more itinerant Cr2As2 layer is smaller than 3.10(6)µB /Cr(I) in the more localized CrO2 layer. Different from previous expectation, a spin-flop transition of the Cr(II) magnetic order observed at 291 K indicates a strong coupling between the CrO2 and Cr2As2 magnetic subsystems.

We report a single crystal neutron and x-ray diffraction study of the hybrid improper multiferroic Ca3Mn1.9Ti0.1O7 (CMTO), a prototypical system where the electric polarization arises from the condensation of two lattice distortion modes. With increasing temperature (T ), the out-of-plane, antiphase tilt of MnO6 decreases in amplitude while the in-plane, inphase rotation remains robust and experiences abrupt changes across the first-order structural transition. Application of hydrostatic pressure (P ) to CMTO at room temperature shows a similar effect. The consistent behavior under both T and P reveals the softness of antiphase tilt and highlights the role of the partially occupied d orbital of the transition metal ions in determining the stability of the octahedral distortion. Polarized neutron analysis indicates the symmetry-allowed canted ferromagnetic moment is less than 0.04 µB/Mn site, despite a substantial out-of-plane tilt of the MnO6 octahedra.PACS numbers: 75.58.+t,81.40.Vw,61.05.F-Multiferroic compounds with spontaneous elastic, electrical, magnetic orders are considered as the key materials to achieve cross-control between magnetism and electricity in solids with small energy dissipation [1,2]. The functional properties including colossal magnetoelectric effect could be used in solid-state memories and sensors [3]. The desired multifunctional behavior requires common microscopic origin of the long-range order such that one order parameter is strongly coupled to the conjugate field of the other one. So far, the majority of attention has focused on exploring materials with magnetic origin, where the underlying microscopic mechanisms are primarily classified into three types: symmetric spin exchange interaction Σ ij (S i · S j ) [4, 5], antisymmetric spin-exchange interaction S i × S j [6, 7], and spindependent p − d hybridization due to spin-ligand interaction (e il · S i ) 2 e il [8]. The material-by-design efforts focusing on magnetic oxides has been productive; the ferroelectricity is induced either through epitaxial strain engineering or chemical substitution of stereochemical inactive ions with lone-pair-active cations [9,10].However, this approach requires a strong coupling between ferroelectricity and magnetism. The microscopic mechanism with spin origin also implicitly suggests a low operating temperature because of the magnetic frustration. On the other hand, perovskites in the form of ABO 3 and their derivatives are favorably chosen for functional materials due to their high susceptibility toward polar structural instability and the intimate coupling between the ferroelectric polarization and the magnetic, orbital, and electronic degrees of freedom. Recently, a novel mechanism termed as "hybrid improper ferroelectric" has been proposed to search for materials with spontaneous ferroelectricity. The central idea is that the polar mode is driven by the condensation of two nonpolar lattice modes, which represent oxygen octahedral rotation (X + 2 ) and tilt (X − 3 ), respectively [11][12][13]. It i...

The magnetic and iron vacancy orders in superconducting (Tl,Rb)2Fe4Se5 single-crystals were investigated using a high-pressure neutron diffraction technique. Similar to the temperature effect, the block antiferromagnetic order gradually decreases upon increasing pressure while the Fe vacancy superstructural order remains intact before its precipitous disappearance at the critical pressure Pc =8.3 GPa. Combined with previously determined Pc for superconductivity, our phase diagram under pressure reveals the concurrence of the block AFM order, the √ 5 × √ 5 iron vacancy order and superconductivity for the 245 superconductor. A synthesis of current experimental data in a coherent physical picture is attempted.PACS numbers: 74.62. Fj, 25.40.Dn, 74.25.Ha, The recently discovered metal-intercalated iron selenide superconductors A 2 Fe 4 Se 5 (A=K, Cs, Tl-K, Rb, Tl-Rb) (245) compounds, with T c ∼ 30 K, have attracted much interest [1,2].A high transitiontemperature (T N ≈ 470-560 K) and large magnetic moment (3.3µ B /Fe) block antiferromagnetic (AFM) order exists in the superconducting samples [3][4][5]. And magnetic order-parameter experiences an anomaly when T c is approached [4,5]. The superconductors crystallize with a highly ordered √ 5 × √ 5 superstructure, in which the Fe1 site of the I4/m structure is only a few percent occupied and the Fe2 site fully occupied [4,6]. The nonsuperconducting samples at low-T also crystallize in the I4/m structure, but both Fe sites are fractionally occupied [7,8], since the numbers of the Fe vacancies in the samples and the vacant sites in the √ 5 × √ 5 pattern are mismatched. The partially ordered √ 5 × √ 5 vacancy order becomes one of three competing phases for temperature below the room temperature up to ∼ 500 K, namely, these samples are phase-separated and in the miscibility gap at ambient condition [8,9].Close to the miscibility gap, it is not surprising that the nonstoichiometric 245 superconductors often contain several phases of different space-group symmetry. It has been a complex and controversial issue to determine the sample composition of the superconductors. The KFe 1.5 Se 2 (234) of the orthorhombic Fe vacancy order has been proposed as the parent compound [10]. However, this phase is not even the ground state for KFe 1.5 Se 2 , and a partially ordered √ 5 × √ 5 vacancy superlattice is more stable at low temperature [8]. The KFe 2 Se 2 (122) of I4/mmm symmetry has also been proposed as the superconducting phase [11]. But its existence in films grown by molecular beam epitaxy method likely requires charge transfer with the substrate, and there is no trace of its existence in bulk superconducting samples [4,12]. Detected in the 245 superconductors is the alkaline metal deficient A x Fe 2 Se 2 (x ∼ 0.3-0.6) phase embedded in √ 5 × √ 5 iron vacancy ordered superstructure [12][13][14], forming various microstructure patterns in plane [15,16] High pressure adds an additional dimension to the complex composition phase-diagram of 245 superconductors [8], offering a "clea...

We report an investigation of the structural, magnetic and electronic properties of Ba(Fe1−xVx)2As2 using x-ray, transport, magnetic susceptibility and neutron scattering measurements. The vanadium substitutions in Fe sites are possible up to ∼40%. Hall effect measurements indicate strong hole-doping effect through V doping, while no superconductivity is observed in all samples down to 2 K. The antiferromagnetic and structural transition temperature of BaFe2As2 is gradually suppressed to finite temperature, then vanishes at x = 0.245 with the emergence of spin glass behavior, suggesting an avoided quantum critical point (QCP). Our results demonstrate that the avoided QCP and spin glass state which were previously reported in the superconducting phase of Co/Ni-doped BaFe2As2 can also be realized in non-superconducting Ba(Fe1−xVx)2As2.

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An interplay of geometrical frustration and strong quantum fluctuations in a spin-1/2 triangular-lattice antiferromagnet (TAF) can lead to exotic quantum states. Here, we report the neutron-scattering, magnetization, specific heat, and magnetocaloric studies of the recently discovered spin-1/2 TAF Na 2 BaCo(PO 4 ) 2 , which can be described by a spin-1/2 easy axis XXZ model. The zero-field neutron diffraction experiment reveals an incommensurate antiferromagnetic ground state with a significantly reduced ordered moment of about 0.54(2) μ B /Co. Different magnetic phase diagrams with magnetic fields in the a b plane and along the easy c -axis were extracted based on the magnetic susceptibility, specific heat, and elastic neutron-scattering results. In addition, two-dimensional (2D) spin dispersion in the triangular plane was observed in the high-field polarized state, and microscopic exchange parameters of the spin Hamiltonian have been determined through the linear spin wave theory. Consistently, quantum critical behaviors with the universality class of d  = 2 and ν z = 1 were established in the vicinity of the saturation field, where a Bose–Einstein condensation (BEC) of diluted magnons occurs. The newly discovered quantum criticality and fractional magnetization phase in this ideal spin-1/2 TAF present exciting opportunities for exploring exotic quantum phenomena.