Three-dimensional Ising critical behavior in R0.6Sr0.4MnO3(R=Pr,Nd) manganites

Magnetic as well as calorimetric measurements have been performed on single crystal samples of Pr0.6Sr0.4MnO3 and Nd0.6Sr0.4MnO3 to develop a complete critical behavior study of the paramagnetic to ferromagnetic transition in both manganites. The critical exponents α,β,γ , and δ have been independently obtained. For Pr0.6Sr0.4MnO3, these are α = 0.09, β = 0.312, γ = 1.106, and δ = 4.545, while for Nd0.6Sr0.4MnO3 they are α = 0.11, β = 0.308, γ = 1.172, and δ = 4.75. All these values agree with the three-dimensional (3D)-Ising universality class (α = 0.11, β = 0.3265, γ = 1.237, and δ = 4.79) and are very far away from any other known universality class. This suggests the presence of magnetocrystalline anisotropies in the system that must be taken into account to fully describe the magnetism of these manganites, which deviates from a simple double exchange model

Single crystal growth, structure and magnetic properties of Pr2Hf2O7 pyrochlore

Large single crystals of the pyrochlore Pr2Hf2O7 have been successfully grown by the floating zone technique using an optical furnace equipped with high power Xenon arc lamps. Structural investigations have been carried out by both synchrotron X-ray and neutron powder diffraction to establish the crystallographic structure of the materials produced. The magnetic properties of the single crystals have been determined for magnetic fields applied along different crystallographic axes. The results reveal that Pr2Hf2O7 is an interesting material for further investigations as a frustrated magnet. The high quality of the crystals produced make them ideal for detailed investigations, especially those using neutron scattering techniques.Comment: Accepted for publication in J. Phys.: Condens. Matte

Candidate quantum spin ice in the pyrochlore Pr2_2Hf2_2O7_7

We report the low temperature magnetic properties of the pyrochlore Pr$_2$Hf$_2$O$_7$. Polycrystalline and single-crystal samples are investigated using time-of-flight neutron spectroscopy and macroscopic measurements, respectively. The crystal-field splitting produces a non-Kramers doublet ground state for Pr$^{3+}$, with Ising-like anisotropy. Below 0.5 K ferromagnetic correlations develop, which suggests that the system enters a spin ice-like state associated with the metamagnetic behavior observed at $\mu_0H_c\sim2.4$~T. In this regime, the development of a discrete inelastic excitation in the neutron spectra indicates the appearance of spin dynamics which are likely related to cooperative quantum fluctuations.Comment: 7 pages, 5 figures, 1 tabl

Fragile ground state and rigid field-induced structures in the zigzag ladder compound BaDy2O4

We report on a sequence of field-induced transitions in the zigzag ladder compound BaDy2O4 studied with powder neutron diffraction and magnetization measurements. In agreement with the previously published results, the low-temperature zero-field structure is characterized by two half-integer propagation vectors k1=[1/2 0 1/2] and k2=[1/2 1/2 1/2]. However, on application of an external magnetic field, the Bragg peaks corresponding to the zero-field structure lose their intensity rather rapidly and disappear completely in a field of 2.5 kOe. In the intermediate fields, 2.5 to 22.5 kOe, new peaks are observed characterized by the propagation vector k′=[0 0 1/3] corresponding to an up-up-down (uud) structure as well as the k= 0 ferromagnetic peaks. This regime of fields corresponds to a pronounced plateau in the magnetization curve. Remarkably, the uud structure survives heating to at least 1.4 K, three times higher temperature than the TN of 0.48 K for the zero-field structure. Above 22.5 kOe, the k' peaks disappear while the k=0 peaks gain significant intensity indicating an increase in the polarization of the system. The analysis of the intensities of the field-induced reflections allows for a clear identification of the magnetic structures in both the intermediate- and high-field regimes

Surface analysis of the PrB6 (001) cleavage plane by scanning tunneling microscopy and spectroscopy

Scanning tunneling microscopy and spectroscopy were performed on the (001) cleavage plane of praseodymium hexaboride (PrB6). We found three different ordered morphologies, namely, a chainlike (2 × 1) reconstruction and two uniform terminations. The chainlike (2 × 1) reconstruction is rationalized as parallel Pr rows on top of a complete B6 network. The two uniform terminations are identified as complete Pr or B6 layers. Although the uniform terminations could be expected to be simply (1 × 1) reconstructed, one of them shows a rather stripelike atomic corrugation for close tip-sample distances. All morphologies share two spectral features at −0.2 and +0.2 eV around EF. In addition, one uniform termination shows an additional peak in the differential conductance at −0.7 eV. Similarly, the chainlike (2 × 1) reconstruction reveals a feature in the differential conductance at −1.1 eV when moving the tip closer to the surface. The distance dependency points towards rather localized electronic states, which we tentatively attribute to a 4 f -related feature

Surface resonance of the (2×1) reconstructed lanthanum hexaboride (001)-cleavage plane : a combined STM and DFT study

We performed a combined study of the (001)-cleavage plane of lanthanum hexaboride (LaB6) using scanning tunneling microscopy and density-functional theory (DFT). Experimentally, we found a (2×1) reconstructed surface on a local scale. The reconstruction is only short-range ordered and tends to order perpendicularly to step edges. At larger distances from surface steps, the reconstruction evolves to a labyrinthlike pattern. These findings are supported by low-energy electron diffraction experiments. Slab calculations within the framework of DFT show that the atomic structure consists of parallel lanthanum chains on top of boron octahedra. Scanning tunneling spectroscopy shows a prominent spectral feature at −0.6eV. Using DFT, we identify this structure as a surface resonance of the (2×1) reconstructed LaB6 (100) surface which is dominated by boron dangling bond states and lanthanum d states

Crystal growth by the floating zone method of Ce-substituted crystals of the topological Kondo insulator SmB6

SmB6 is a mixed valence topological Kondo insulator. To investigate the effect of substituting Sm with magnetic Ce ions on the physical properties of samarium hexaboride, Ce-substituted SmB6 crystals were grown by the floating zone method for the first time as large, good quality single crystal boules. The crystal growth conditions are reported. Structural, magnetic and transport properties of single crystals of Sm

Magnetic properties and crystal field in Pr2Zr2O7

In this work, we revisit the crystal field acting on the non-Kramers Pr3+ ion (4f2) in the quantum spin ice candidate Pr2Zr2O7 using both a standard calculation restricted to the ground spin-orbit multiplet and intermediate coupling states in the full basis of the f 2 configuration. Analysis of the thermal variation of the polycrystal magnetic susceptibility and of the local susceptibilities χ⊥ and χ// determined by means of polarized neutron diffraction experiments reveals that the effective antiferromagnetic exchange is strongly depleted at low temperature with respect to its high-temperature value.We then discuss the influence of crystal field imperfections arising from residual strains, which are especially important for a non-Kramers ion. We find that they are an essential ingredient to account for the very low temperature M(H) magnetization curves, showing that the saturation is not achieved even at 8 T. Furthermore, as possible candidates to qualitatively understand the Curie-like behavior observed below 0.5 K, we discuss the influence of the magnetic hyperfine interaction

Optical floating zone crystal growth of rare-earth disilicates, R2Si2O7 (R = Er, Ho, and Tm)

The wealth of structural phases seen in the rare-earth disilicate compounds promises an equally rich range of interesting magnetic properties. We report on the crystal growth by the optical floating zone method of members of the rare-earth disilicate family, R2Si2O7 (with R = Er, Ho, and Tm). Through a systematic study, we have optimized the growth conditions for Er2Si2O7. We have grown, for the first time using the floating zone method, crystal boules of Ho2Si2O7 and Tm2Si2O7 compounds. We show that the difficulties encountered in the synthesis of polycrystalline and single crystal samples are due to the similar thermal stability ranges of different rareearth silicate compounds in the temperature−composition phase diagrams of the R-Si-O systems. The addition of a small amount of SiO2 excess allowed the amount of impurity phases present in the powder samples to be minimized. The phase composition analysis of the powder X-ray diffraction data collected on the as-grown boules revealed that they were of single phase, except in the case of thulium disilicate, which was comprised of two phases. All growths resulted in multigrain boules, from which sizable single crystals could be isolated. The optimum conditions used for the synthesis and crystal growth of polycrystalline and single crystal R2Si2O7 materials are reported. Specific heat measurements of erbium and thulium disilicate compounds confirm an antiferromagnetic phase transition below TN = 1.8 K for D-type Er2Si2O7 and a Schottky anomaly centered around 3.5 K in C-type Tm2Si2O7, with an upturn in C(T)/T below 1 K suggesting the onset of short-range magnetic correlations. Magnetic susceptibility data of E-type Ho2Si2O7 reveals an antiferromagnetic ordering of the Ho spins below TN = 2.3 K