arXiv subjects

Bachir Ouladdiaf

Publications and source records attributed to Bachir Ouladdiaf.

14 recordsLinked to original sources

arXiv2026

Magnetic structure in the two-dimensional van der Waals ferromagnet Fe$_3$GaTe$_2$

High-quality single crystals of the two-dimensional van der Waals ferromagnet Fe$_3$GaTe$_2$ (FGaT) were successfully grown using the chemical vapour transport method, which effectively reduced surface impurities compared with conventional self-flux growth. Structural and magnetic characterizations were performed using single-crystal X-ray and neutron diffraction. The results confirm that FGaT crystallizes in the hexagonal $P6_3/mmc$ structure, with Fe occupying two inequivalent sites (Fe$^{i}$ and Fe$^{ii}$), where the magnetic moment of Fe$^{i}$ [1.9(2) $μ_B$] is larger than that of Fe$^{ii}$ [1.4(6) $μ_B$]. The magnetic easy axis is oriented along the $c$ axis and the Curie temperature ($T_C$) is approximately 355-360 K. Compared with Fe$_3$GeTe$_2$ (FGT), FGaT exhibits a slightly expanded $a$ axis and a contracted $c$ axis, resulting in a reduction in the Fe$^{i}$-Fe$^{ii}$ interatomic distance along the $c$ axis. This pronounced contraction could strengthen the Fe$-$Fe exchange interaction, which is believed to be the key factor responsible for the significantly higher $T_C$ in FGaT relative to FGT.

cond-mat.str-el↗

arXiv2026

Long-range magnetic interactions in Nd$_2$PdSi$_3$ and the formation of skyrmion phases in centrosymmetric metals

We present an extensive X-ray and neutron scattering study of the structure and magnetic excitations of Nd$_2$PdSi$_3$, a sister compound of Gd$_2$PdSi$_3$ which was recently found to host a skyrmion lattice phase despite its centrosymmetric crystal structure. Dispersive magnetic excitations were measured throughout the Brillouin zone and modeled to determine the magnetic interactions between Nd ions. Our analysis reveals that the magnetic interactions in this system extend over large distances and are significantly affected by a crystallographic superstructure formed by ordering of the Pd and Si atoms. The results suggest that the mechanism for the skyrmion phase formation in this family of materials, specifically Gd$_2$PdSi$_3$, is through the long-range RKKY interactions rather than short-range triangular-lattice frustration.

cond-mat.str-el↗

arXiv2023

Nuclear and magnetic structure of an epitaxial La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ film using diffraction methods

We use a combination of transmission electron microscopy, X-Ray and neutron diffraction, SQUID magnetometry and symmetry analysis to determine the nuclear and magnetic structure of an epitaxial LSMO film, deposited on a Si substrate. The film undergoes a magnetic ordering transition at 260K. At 300K, in the paramagnetic phase, the manganite film has a $I4/m$ space group. In the magnetic phase, SQUID and neutron diffraction results lead us to assign a ferromagnetic spin-order to the film, associated to magnetic moments with a slightly out-of-plane component, namely $(3.5, 0, 0.3)\,μ_B$. Symmetry analysis further shows that only the $P\bar 1'$ group is compatible with such a magnetic ordering.

cond-mat.str-el↗

arXiv2022

The magnetic structure of the topological semimetal Co$_3$Sn$_2$S$_2$

Co$_3$Sn$_2$S$_2$ has recently been predicted to be a Weyl semimetal in which magnetic order is key to its behavior as a topological material. Here we report unpolarized neutron diffraction and spherical neutron polarimetry measurements, supported by magnetization and transport data, which probe the magnetic order in Co$_3$Sn$_2$S$_2$ below $T_\textrm{C} = 177$ K. The results are fully consistent with ferromagnetic order in which the spins on the Co atoms point along the crystal $c$ axis, although we cannot rule out some canting of the spins. We find no evidence for a type of long-ranged $(\textbf{k}=\textbf{0})$ in-plane 120$^\circ$ antiferromagnetic order which had previously been considered as a secondary phase present at temperatures between $\sim$90 K and $T_\textrm{C}$. A discontinuous change in bulk properties and neutron polarization observed at $T = 125$ K when samples are cooled in a field and measured on warming is found to be due to a sudden reduction in ferromagnetic domain size. Our results lend support to the theoretical predictions that Co$_3$Sn$_2$S$_2$ is a magnetic Weyl semimetal.

cond-mat.str-el↗

arXiv2021

Magnetic and electronic structure of the topological semimetal YbMnSb$_2$

The antiferromagnetic (AFM) semimetal YbMnSb$_2$ has recently been identified as a candidate topological material, driven by time-reversal symmetry breaking. Depending on the ordered arrangement of Mn spins below the Néel temperature, $T_\mathrm{N}$ = 345 K, the electronic bands near the Fermi energy can ether have a Dirac node, a Weyl node or a nodal line. We have investigated the ground state magnetic structure of YbMnSb$_2$ using unpolarized and polarized single crystal neutron diffraction. We find that the Mn moments lie along the $c$ axis of the $P4/nmm$ space group and are arranged in a C-type AFM structure, which implies the existence of gapped Dirac nodes near the Fermi level. The results highlight how different magnetic structures can critically affect the topological nature of fermions in semimetals.

cond-mat.str-el↗

arXiv2019

Magnetic structure and excitations of the topological semimetal YbMnBi$_2$

We investigated the magnetic structure and dynamics of YbMnBi$_2$, with elastic and inelastic neutron scattering, to shed light on the topological nature of the charge carriers in the antiferromagnetic phase. We confirm C-type antiferromagnetic ordering of the Mn spins below $T_{\rm N} = 290$ K, and determine that the spins point along the $c$-axis to within about $3^\circ$. The observed magnon spectrum can be described very well by the same effective spin Hamiltonian as was used previously to model the magnon spectrum of CaMnBi$_2$. Our results show conclusively that the creation of Weyl nodes in YbMnBi$_2$ by the time-reversal-symmetry breaking mechanism can be excluded in the bulk.

cond-mat.str-el↗

arXiv2017

Neutron Diffraction Study on Single-crystalline UAu${_2}$Si$_2$

Magnetic structure of tetragonal UAu$_2$Si$_2$ was investigated by single-crystal neutron diffraction experiments. Below $T_{\rm N}$ = 20 K it orders antiferromagnetically with a propagation vector of $k = (2/3, 0, 0)$ and magnetic moments of uranium ions pointing along the tetragonal $c$-axis. Weak signs of the presence of a ferromagnetic component of magnetic moment were traced out.Taking into account a group theory calculation and experimental results of magnetization and $^{29}$Si-NMR, the magnetic structure is determined to be a squared-up antiferromagnetic structure, with a stacking sequence ($+ + -$) of the ferromagnetic $ac$-plane sheets along the $a$-axis. This result highlights similar magnetic correlations in UAu$_2$Si$_2$ and isostructural URu$_2$Si$_2$.

cond-mat.str-el↗

arXiv2016

Field Induced Magnetic States in Holmium Tetraboride

A study of the zero field and field induced magnetic states of the frustrated rare earth tetraboride, HoB$_4$, has been carried out using single crystal neutron diffraction complemented by magnetisation measurements. In zero field, HoB$_4$ shows magnetic phase transitions at $T_{\mathrm{N1}}$ = 7.1 K to an incommensurate state with a propagation vector ($δ$, $δ$, $δ'$), where $δ$ = 0.02 and $δ'$ = 0.43 and at $T_{\mathrm{N2}}$ = 5.7 K to a non-collinear commensurate antiferromagnetic structure. Polarised neutron diffraction measurements in zero field have revealed that the incommensurate reflections, albeit much reduced in intensity, persist down to 1.5 K despite antiferromagnetic ordering at 5.7 K. At lower temperatures, application of a magnetic field along the $c$-axis initially re-establishes the incommensurate phase as the dominant magnetic state in a narrow field range, just prior to HoB$_4$ ordering with an up-up-down ferrimagnetic structure characterised by the $(h,k,\frac{1}{3})$-type reflections between 18 and 24 kOe. This field range is marked by the previously reported $M/M_{\mathrm{sat}}$= $\frac{1}{3}$ magnetisation plateau, which we also see in our magnetisation measurements. The region between 21 and 33 kOe is characterised by the increase in the intensity of the antiferromagnetic reflections, such as (100), the maximum of which coincides with the appearance of the narrow magnetisation plateau with $M/M_{\mathrm{sat}}\approx$ $\frac{3}{5}$. Further increase of the magnetic field results in the stabilisation of a polarised state above 33 kOe, while the incommensurate reflections are clearly present in all fields up to 59 kOe. We propose the $H-T$ phase diagram of HoB$_4$ for the $H \parallel c$ containing both stationary and transitionary magnetic phases which overlap and show significant history dependence.

cond-mat.str-el↗

arXiv2015

Magnetic structures in the rich magnetic phase diagram of Ho$_2$RhIn$_8$

The magnetic phase diagram of the tetragonal Ho$_2$RhIn$_8$ compound has similar features to many related systems, revealing a zero magnetic field AF1 and a field-induced AF2 phases. Details of the magnetic order in the AF2 phase were not reported yet for any of the related compounds. In addition, only the Ho$_2$RhIn$_8$ phase diagram contains a small region of the incommensurate zero-field AF3 phase. We have performed a number of neutron diffraction experiments on single crystals of Ho$_2$RhIn$_8$ using several diffractometers including experiments in both horizontal and vertical magnetic fields up to 4 T. We present details of the magnetic structures in all magnetic phases of the rich phase diagram of Ho$_2$RhIn$_8$. The Ho magnetic moments point along the tetragonal $c$ axis in every phase. The ground-state AF1 phase is characterized by propagation vector $\textbf{k}$ = (1/2, 0, 0). The more complex ferrimagnetic AF2 phase is described by four propagation vectors $\textbf{k}_{0}$ = (0, 0, 0), $\textbf{k}_{1}$ = (1/2, 0, 0), $\textbf{k}_{2}$ = (0, 1/2, 1/2), $\textbf{k}_{3}$ = (1/2, 1/2, 1/2). The magnetic structure in the AF3 phase is incommensurate with $\textbf{k}_{AF3}$ = (0.5, $δ$, 0). Our results are consistent with theoretical calculations based on crystal field theory.

cond-mat.str-el↗

arXiv2014

Magnetic structures of non-cerium analogues of heavy-fermion Ce2RhIn8: case of Nd2RhIn8, Dy2RhIn8 and Er2RhIn8

R2RhIn8 compounds (space group P4/mmm, R is a rare-earth element) belong to a large group of structurally related tetragonal materials which involves several heavy-fermion superconductors based on Ce. We have succeeded to grow single crystals of compounds with Nd, Dy and Er and following our previous bulk measurements, we performed neutron-diffraction studies to determine their magnetic structures. The Laue diffraction experiment showed that the antiferromagnetic order below the Néel temperature is in all three compounds characterized by the propagation vector k = (1/2, 1/2, 1/2). The amplitude and direction of the magnetic moments, as well as the invariance symmetry of the magnetic structure, were determined by subsequent experiments using two- and four-circle diffractometers. The critical exponents were determined from the temperature dependence of the intensities below TN.

cond-mat.supr-con↗

arXiv2012

Magnetoelastic effects in multiferroic YMnO$_3$

We have investigated magnetoelastic effects in multiferroic YMnO$_3$ below the antiferromagnetic phase transition, $T_N \approx 70$ K, using neutron powder diffraction. The $a$ lattice parameter of the hexagonal unit cell of YMnO$_3$ decreases normally above $T_N$, but decreases anomalously below T$_N$, whereas the $c$ lattice parameter increases with decreasing temperature and then increases anomalously below T$_N$. The unit cell volume also undergoes an anomalous contraction below $T_N$. By fitting the background thermal expansion for a non-magnetic lattice with the Einstein-Grüneisen equation, we determined the lattice strains $Δa$, $Δc$ and $ΔV$ due to the magnetoelastic effects as a function of temperature. We have also determined the temperature variation of the ordered magnetic moment of the Mn ion by fitting the measured Bragg intensities of the nuclear and magnetic reflections with the known crystal and magnetic structure models and have established that the lattice strain due to the magnetoelastic effect in YMnO$_3$ couples with the square of the ordered magnetic moment or the square of the order parameter of the antiferromagnetic phase transition.

cond-mat.str-el↗

arXiv2008

Formation of collective spins in frustrated clusters

Using magnetization, specific heat and neutron scattering measurements, as well as exact calculations on realistic models, the magnetic properties of the \lacuvo compound are characterized on a wide temperature range. At high temperature, this oxide is well described by strongly correlated atomic $S$=1/2 spins while decreasing the temperature it switches to a set of weakly interacting and randomly distributed entangled pseudo spins $\tilde S=1/2$ and $\tilde S=0$. These pseudo-spins are built over frustrated clusters, similar to the kagomé building block, at the vertices of a triangular superlattice, the geometrical frustration intervening then at different scales.

cond-mat.str-el↗

arXiv2007

Magnetic superelasticity and inverse magnetocaloric effect in Ni-Mn-In

Applying a magnetic field to a ferromagnetic Ni$_{50}$Mn$_{34}$In$_{16}$ alloy in the martensitic state induces a structural phase transition to the austenitic state. This is accompanied by a strain which recovers on removing the magnetic field giving the system a magnetically superelastic character. A further property of this alloy is that it also shows the inverse magnetocaloric effect. The magnetic superelasticity and the inverse magnetocaloric effect in Ni-Mn-In and their association with the first order structural transition is studied by magnetization, strain, and neutron diffraction studies under magnetic field.

cond-mat.mtrl-sci↗

arXiv2006

Hierarchical geometric frustration in La3Cu2VO9

The crystallographic structure and magnetic properties of the La3Cu2VO9 were investigated by powder neutron diffraction and magnetization measurements. The compound materializes geometric frustration at two spatial scales, within clusters and between clusters, and at different temperature scales. It is shown by exactly solving the hamiltonian spectrum that collective spins are formed on each clusters at low temperature before inter-clusters coupling operates.

cond-mat.str-el↗