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G. André

Publications and source records attributed to G. André.

8 recordsLinked to original sources
arXiv2014

Detailed neutron diffraction study of magnetic order in NdFe$_2$Al$_{10}$

The orthorhombic compound NdFe$_2$Al$_{10}$ has been studied by powder and single-crystal neutron diffraction. Below $T_N$ = 3.9 K, the Nd$^{3+}$ magnetic moments order in a double-$k$ [$\mathbf{k}_1 = (0, \frac{3}{4}, 0)$, $\mathbf{k}_2 = (0, \frac{1}{4}, 0)$] collinear magnetic structure, whose unit cell consists of four orthorhombic units in the $b$ direction.The refinements show that this structure consists of (0 1 0) ferromagnetic planes stacked along $b$, in which the moments are oriented parallel to $a$ (the easy anisotropy axis according to bulk magnetization measurements) and nearly equal in magnitude ($\approx 1.7-1.9\ μ_B$). The alternating 8-plane sequence providing the best agreement to the data turns out to be that which yields the lowest exchange energy if one assumes antiferromagnetic near-neighbor exchange interactions with $J_1 \gg J_2, J_3$. With increasing temperature, the single-crystal measurements indicate the suppression of the $\mathbf{k}_2$ component at $T = 2.7$ K, supporting the idea that the anomalies previously observed around 2--2.5 K result from a squaring transition. In a magnetic field applied along the $a$ axis, the magnetic Bragg satellites disappear at $H_c = 2.45$ T, in agreement with earlier measurements. Comparisons are made with related magnetic orders occurring in Ce$T_2$Al$_{10}$ ($T$: Ru, Os) and TbFe$_2$Al$_{10}$.

cond-mat.str-el↗
arXiv2014

Large Noncollinearity and Spin Reorientation in the Novel Mn2RhSn Heusler Magnet

Noncollinear magnets provide essential ingredients for the next generation memory technology. It is a new prospect for the Heusler materials, already well known due to the diverse range of other fundamental characteristics. Here, we present a combined experimental and theoretical study of novel noncollinear tetragonal Mn2RhSn Heusler material exhibiting unusually strong canting of its magnetic sublattices. It undergoes a spin-reorientation transition, induced by a temperature change and suppressed by an external magnetic field. Because of the presence of Dzyaloshinskii-Moriya exchange and magnetic anisotropy, Mn2RhSn is suggested to be a promising candidate for realizing the Skyrmion state in the Heusler family.

cond-mat.mtrl-sci↗
arXiv2014

Effect of Ising-type Tb$^{3+}$ ions on the low-temperature magnetism of La, Ca cobaltite

Crystal and magnetic structures of the $x=0.2$ member of La$_{\rm 0.8-x}$Tb$_{\rm x}$Ca$_{0.2}$CoO$_3$ perovskite series have been determined from the powder neutron diffraction. Enhancement of the diffraction peaks due to ferromagnetic or cluster glass ordering is observed below $T_C=55$ K. The moments evolve at first on Co sites, and ordering of Ising-type Tb$^{3+}$ moments is induced at lower temperatures by a molecular field due to Co ions. The final magnetic configuration is collinear F$_x$ for cobalt subsystem, while it is canted F$_x$C$_y$ for terbium ions. The rare-earth moments align along local Ising axes within \textit{ab}-plane of the orthorhombic $Pbnm$ structure. The behavior in external fields up to $70-90$ kOe has been probed by the magnetization and heat capacity measurements. The dilute terbium ions contribute to significant coercivity and remanence that both steeply increase with decreasing temperature. A remarkable manifestation of the Tb$^{3+}$ Ising character is the observation of a low-temperature region of anomalously large linear term of heat capacity and its field dependence. Similar behaviours are detected also for other terbium dopings $x=0.1$ and 0.3.

cond-mat.mtrl-sci↗
arXiv2011

Short-range spin order and frustrated magnetism in Mn2InSbO6 and Mn2ScSbO6

The complex metal oxides Mn2ASbO6 (A=In, Sc) with a corundum-related structure A3BO6 have been prepared as polycrystalline powders by a solid state reaction route. The crystal structure and magnetic properties have been investigated using a combination of X-ray and neutron powder diffraction, electron microscopy, calorimetric and magnetic measurements. At room temperature these compounds adopt a trigonal structure, space group R-3 with a = 8.9313(1) Å, c = 10.7071(2) Å (for In) and a = 8.8836(1) Å, c = 10.6168(2) Å (for Sc) which persists down to 1.6 K. The Mn and A cations were found to be randomly distributed over the A-sites. The overall behavior of the magnetization of Mn2InSbO6 and Mn2ScSbO6 is quite similar. In spite of the relatively large amount of Mn ions on the A-site, only short-ranged magnetism is observed. Neutron diffraction patterns of Mn2InSbO6 showed no evidence of a long-range magnetic ordering at 1.6 K, instead only a weak diffuse magnetic peak was observed at low temperatures. The factors governing the observed structural and magnetic properties of Mn2ASbO6 are discussed and compared with those of other Mn-containing complex metal oxides with a corundum-related structure. The influence of the A-cation sublattice on magnetic properties is also considered.

cond-mat.mtrl-sci↗
arXiv2011

Internal-strain mediated coupling between polar Bi and magnetic Mn ions in the defect-free quadruple-perovskite BiMn$_3$Mn$_4$O$_{12}$

By means of neutron powder diffraction, we investigated the effect of the polar Bi$^{3+}$ ion on the magnetic ordering of the Mn$^{3+}$ ions in BiMn$_3$Mn$_4$O$_{12}$, the counterpart with \textit{quadruple} perovskite structure of the \textit{simple} perovskite BiMnO$_3$. The data are consistent with a \textit{noncentrosymmetric} spacegroup $Im$ which contrasts the \textit{centrosymmetric} one $I2/m$ previously reported for the isovalent and isomorphic compound LaMn$_3$Mn$_4$O$_{12}$, which gives evidence of a Bi$^{3+}$-induced polarization of the lattice. At low temperature, the two Mn$^{3+}$ sublattices of the $A'$ and $B$ sites order antiferromagnetically (AFM) in an independent manner at 25 and 55 K, similarly to the case of LaMn$_3$Mn$_4$O$_{12}$. However, both magnetic structures of BiMn$_3$Mn$_4$O$_{12}$ radically differ from those of LaMn$_3$Mn$_4$O$_{12}$. In BiMn$_3$Mn$_4$O$_{12}$ the moments $\textbf{M}_{A'}$ of the $A'$ sites form an anti-body AFM structure, whilst the moments \textbf{M}$_{B}$ of the $B$ sites result from a large and \textit{uniform} modulation $\pm \textbf{M}_{B,b}$ along the b-axis of the moments \textbf{M}$_{B,ac}$ in the $ac$-plane. The modulation is strikingly correlated with the displacements of the Mn$^{3+}$ ions induced by the Bi$^{3+}$ ions. Our analysis unveils a strong magnetoelastic coupling between the internal strain created by the Bi$^{3+}$ ions and the moment of the Mn$^{3+}$ ions in the $B$ sites. This is ascribed to the high symmetry of the oxygen sites and to the absence of oxygen defects, two characteristics of quadruple perovskites not found in simple ones, which prevent the release of the Bi$^{3+}$-induced strain through distortions or disorder. This demonstrates the possibility of a large magnetoelectric coupling in proper ferroelectrics and suggests a novel concept of internal strain engineering for multiferroics design.

cond-mat.str-el↗
arXiv2008

Magnetic order in YbMnO$_3$ studied by Neutron Diffraction and Mössbauer Spectroscopy

The magnetic ordering of the hexagonal multiferroic compound YbMnO$_3$ has been studied between 100 K and 1.5 K by combining neutron powder diffraction, $^{170}$Yb Mössbauer spectroscopy and magnetization measurements. The Yb moments of the two crystallographic sites order at two different temperatures, the $4b$ site together with the Mn moments (at $T_N \simeq$85 K) and the $2a$ site well below (at 3.5 K). The temperature dependences of the Mn and Yb moments are explained within a molecular field model, showing that the $4b$ and $2a$ sites order via Yb-Mn and Yb-Yb interactions respectively. A simple picture taking into account the local Mn environment of the Rare earth R ($4b$) ion is proposed to couple R and Mn orders in hexagonal RMnO$_3$ manganites. The nature and symmetry of the R-Mn interactions yielding the R order are discussed.

cond-mat.str-el↗
arXiv2008

Incommensurate spin density modulation in a copper-oxide chain compound with commensurate charge order

Neutron diffraction has been used to determine the magnetic structure of Na$_8$Cu$_5$O$_{10}$, a stoichiometric compound containing chains based on edge-sharing CuO$_4$ plaquettes. The chains are doped with 2/5 hole per Cu site and exhibit long-range commensurate charge order with an onset well above room temperature. Below $T_N = 23$ K, the neutron data indicate long-range collinear magnetic order with a spin density modulation whose propagation vector is commensurate along and incommensurate perpendicular to the chains. Competing interchain exchange interactions are discussed as a possible origin of the incommensurate magnetic order.

cond-mat.str-el↗