arXiv subjects

Fabio Orlandi

Publications and source records attributed to Fabio Orlandi.

30 records · Page 2Linked to original sources

arXiv2021

Heisenberg spins on an anisotropic triangular lattice: PdCrO2 under uniaxial stress

When Heisenberg spins interact antiferromagnetically on a triangular lattice and nearest-neighbor interactions dominate, the ground state is 120$^{\circ}$ antiferromagnetism. In this work, we probe the response of this state to lifting the triangular symmetry, through investigation of the triangular antiferromagnet PdCrO$_2$ under uniaxial stress by neutron diffraction and resistivity measurements. The periodicity of the magnetic order is found to change rapidly with applied stress; the rate of change indicates that the magnetic anisotropy is roughly forty times the stress-induced bond length anisotropy. At low stress, the incommensuration period becomes extremely long, on the order of 1000 lattice spacings; no locking of the magnetism to commensurate periodicity is detected. Separately, the magnetic structure is found to undergo a first-order transition at a compressive stress of $\sim$0.4 GPa, at which the interlayer ordering switches from a double- to a single-q structure.

cond-mat.str-el↗

arXiv2020

Emergent helical texture of electric dipoles

Long-range ordering of magnetic dipoles in bulk materials gives rise to a broad range of magnetic structures, from simple collinear ferromagnets and antiferromagnets, to complex magnetic helicoidal textures stabilized by competing exchange interactions. In contrast, in the context of dipolar order in dielectric crystals, only parallel (ferroelectric) and antiparallel (antiferroelectric) collinear alignments of electric dipoles are typically considered. Here, we report an observation of incommensurate helical ordering of electric dipoles by light hole-doping of the quadruple perovskite BiMn7O12. In analogy with magnetism, the electric dipole helicoidal texture is also stabilized by competing instabilities. Specifically, orbital ordering and lone electron pair stereochemical activity compete, giving rise to phase transitions from a non-chiral cubic structure, to an incommensurate electric dipole and orbital helix, via an intermediate density wave.

cond-mat.str-el↗

arXiv2020

Neutron diffraction and symmetry analysis of the martensitic transformation in Co-doped Ni$_2$MnGa

Martensitic transformations are strain driven displacive transitions governing the mechanical and physical properties in intermetallic materials. This is the case in Ni$_2$MnGa, where the martensite transition is at the heart of the striking magnetic shape memory and magneto-caloric properties. Interestingly, the martensitic transformation is preceded by a pre-martensite phase, and the role of this precursor and its influence on the martensitic transition and properties is still a matter of debate. In this work, we report on the influence of Co doping (Ni$_{50-x}$Co$_x$Mn$_{25}$Ga$_{25}$ with x = 3 and 5) on the martensitic transformation path in stoichiometric Ni$_2$MnGa by neutron diffraction. The use of the superspace formalism to describe the crystal structure of the modulated martensitic phases, joined with a group theoretical analysis allows unfolding the different distortions featuring the structural transitions. Finally, a general Landau thermodynamic potential of the martensitic transformation, based on the symmetry analysis is outlined. The combined use of phenomenological and crystallographic studies highlights the close relationship between the lattice distortions at the core of the Ni$_2$MnGa physical properties and, more in general, on the properties of the martensitic transformations in the Ni-Mn based Heusler systems.

cond-mat.mtrl-sci↗

arXiv2019

Giant spontaneous exchange bias in an antiperovskite structure driven by a canted triangular magnetic structure

Exchange bias (EB) refers to a shift of the hysteresis loop along the field axis in materials consisting of ferromagnetic (FM) and antiferromagnetic (AFM) layers, generally after a cooling procedure in high magnetic field. This effect is highly desirable for technological applications ranging from spintronics to magnetic recording. Achieving giant EB effect near room temperature in a small cooling field is thus an on-going technologically relevant challenge for the materials science community. In this work, we present the experimental realization of such a fundamental goal by demonstrating the existence of giant EB after a zero field cooled (ZFC) procedure in antiperovskite Mn3.5Co0.5N below 256 K. We found that it exhibits an EB field of 0.28 T at 50 K after a ZFC procedure accompanied by a large vertical magnetization shift (VMS). Interestingly, this EB field can be elevated up to 1.2 T after a cooling procedure with a small applied field of just 500 Oe. Mn3:5Co0:5N bears the first intermetallic material showing a strong correlation between EB and VMS.We attribute the observed EB effect to a completely new canted triangular magnetic structure determined by neutron diffraction experiment. Finally, we discuss the striking effect of Co substitution on the physical properties of antiperovskite materials and put forward a new strategy for antiperovskite lattice to exploit new single phase materials showing large EB effect at room temperature.

cond-mat.str-el↗

arXiv2019

Colossal magnetoresistance in the insulating ferromagnetic double perovskites Tl$_2$NiMnO$_6$: A neutron diffraction study

In the family of double perovskites, colossal magnetoresistance (CMR) has been so far observed only in half-metallic ferrimagnets such as the known case Sr$_2$FeMoO$_6$ where it has been assigned to the tunneling MR at grain boundaries due to the half-metallic nature. Here we report a new material-Tl$_2$NiMnO$_6$, a relatively ordered double perovskite stablized by the high pressure and high temperature synthesis-showing CMR in the vicinity of its Curie temperature. We explain the origin of such effect with neutron diffraction experiment and electronic structure calculations that reveal the material is a ferromagnetic insulator. Hence the ordered Tl$_2$NiMnO$_6$ (~70% of Ni$^{2+}$/Mn$^{4+}$ cation ordering) represents the first realization of a ferromagnetic insulating double perovskite, showing CMR. The study of the relationship between structure and magnetic properties allows us to clarify the nature of spin glass behaviour in the disordered Tl$_2$NiMnO$_6$ (~31% of cation ordering), which is related to the clustering of antisite defects and associated with the short-range spin correlations. Our results highlight the key role of the cation ordering in establishing the long range magnetic ground state and lay out new avenues to exploit advanced magnetic materials in double perovskites.

cond-mat.str-el↗

arXiv2019

Highly tunable magnetic spirals and electric polarization in Gd$_{0.5}$Dy$_{0.5}$MnO$_3$

Recent progress in the field of multiferroics led to the discovery of many new materials in which ferroelectricity is induced by cycloidal spiral orders. The direction of the electric polarization is typically constrained by spin anisotropies and magnetic field. Here, we report that the mixed rare-earth manganite, Gd$_{0.5}$Dy$_{0.5}$MnO$_3$, exhibits a spontaneous electric polarization along a general direction in the crystallographic ac-plane, which is suppressed below 10 K but re-emerges in an applied magnetic field. Neutron diffraction measurements show that the polarization direction results from a large tilt of the spiral plane with respect to the crystallographic axes and that the suppression of ferroelectricity is caused by the transformation of a cycloidal spiral into a helical one, a unique property of this rare-earth manganite. The freedom in the orientation of the spiral plane allows for a fine magnetic control of ferroelectricity, i.e. a rotation as well as a strong enhancement of the polarization depending on the magnetic field direction. We show that this unusual behavior originates from the coupling between the transition metal and rare-earth magnetic subsystems.

cond-mat.str-el↗

arXiv2019

Intermultiplet transitions and magnetic long-range order in Sm-based pyrochlores

We present bulk and neutron scattering measurements performed on the isotopically enriched $^{154}\mathrm{Sm_2Ti_2O_7}$ and $^{154}\mathrm{Sm_2Sn_2O_7}$ samples. Both compounds display sharp heat capacity anomalies, at 350 mK and 440 mK, respectively. Inelastic neutron scattering measurements are employed to determine the crystalline electric field (CEF) level scheme, which includes transitions between the ground-state and first excited $J$ multiplets of the $\mathrm{Sm}^{3+}$ ion. To further validate those results, the single-ion magnetic susceptibility of the compounds is calculated and compared with the experimental DC-susceptibility measured in low applied magnetic fields. It is demonstrated that the inclusion of intermultiplet transitions in the CEF analysis is fundamental to the understanding of the intermediate and, more importantly, low temperature magnetic behaviour of the Sm-based pyrochlores. Finally, the heat capacity anomaly is shown to correspond to the onset of an all-in-all-out long-range order in the stannate sample, while in the titanate a dipolar long-range order can be only indirectly inferred.

cond-mat.str-el↗

arXiv2018

Unravelling the complex magnetic structure of multiferroic pyroxene NaFeGe2O6: A combined experimental and theoretical study

Magnetic order and the underlying magnetic model of the multiferroic pyroxene NaFeGe2O6 are systematically investigated by neutron powder diffraction, thermodynamic measurements, density-functional bandstructure calculations, and Monte-Carlo simulations. Upon cooling, NaFeGe2O6 first reveals one-dimensional spin-spin correlations in the paramagnetic state below about 50 K, revealed by magnetic diffuse scattering. The sinusoidal spin-density wave with spins along the a-direction sets in at 13 K, followed by the cycloidal configuration with spins lying in the (ac) plane below 11.6 K. Microscopically, the strongest magnetic coupling runs along the structural chains, J1 ' 12 K, which is likely related to the one-dimensional spin-spin correlations. The interchain couplings J2 ' 3:8K and J3 ' 2:1K are energetically well balanced and compete, thus giving rise to the incommensurate order in sharp contrast to other transition-metal pyroxenes, where one type of the interchain couplings prevails. The magnetic model of NaFeGe2O6 is further completed by the weak single-ion anisotropy along the a-direction. Our results resolve the earlier controversies regarding the magnetic order in NaFeGe2O6 and establish relevant symmetries of the magnetic structures. These results, combined with symmetry analysis, enable us to identify the possible mechanisms of the magnetoelectric coupling in this compound. We also elucidate microscopic conditions for the formation of incommensurate magnetic order in pyroxenes.

cond-mat.str-el↗

arXiv2018

Incommensurate atomic and magnetic modulations in the spin-frustrated β-NaMnO2 triangular lattice

The layered β-NaMnO2, a promising Na-ion energy-storage material has been investigated for its triangular lattice capability to promote complex magnetic configurations that may release symmetry restrictions for the coexistence of ferroelectric and magnetic orders. The complexity of the neutron powder diffraction patterns underlines that the routinely adopted commensurate structural models are inadequate. Instead, a single-phase superspace symmetry description is necessary, demonstrating that the material crystallizes in a compositionally modulated q= (0.077(1), 0, 0) structure. Here, Mn3+ Jahn-Teller distorted MnO6 octahedra form corrugated layer stacking sequences of the β-NaMnO2 type, which are interrupted by flat sheets of the α-like oxygen topology. Spontaneous long-range collinear antiferromagnetic order, defined by the propagation vector k= (1/2, 1/2, 1/2), appears below TN1= 200 K. Moreover, a second transition into a spatially modulated proper-screw magnetic state (k+-q) is established at TN2= 95 K, with an antiferromagnetic order parameter resembling that of a two-dimensional (2D) system. The evolution of 23Na NMR spin-lattice relaxation identifies a magnetically inhomogene-ous state in the intermediate T-region (TN2 <T< TN1), while its strong suppression below TN2 indicates that a spin-gap opens in the excitation spectrum. High-resolution neutron inelastic scattering confirms that the magnetic dynamics are indeed gapped (Δ~5 meV) in the low-temperature magnetic phase, while simulations on the basis of the single-mode approximation suggest that Mn-spins residing on ad-jacent antiferromagnetic chains, establish sizable 2D correlations. Our analysis points that novel struc-tural degrees of freedom promote, cooperative magnetism and emerging dielectric properties in this non-perovskite-type of manganite.

cond-mat.mtrl-sci↗

arXiv2018

Coupling between Spin and Charge Order Driven by Magnetic Field in Triangular Ising System LuFe2O4+δ

We present a study of the magnetic-field effect on spin correlations in the charge ordered triangular Ising system LuFe2O4+δ through single crystal neutron diffraction. In the absence of a magnetic field, the strong diffuse neutron scattering observed below the Neel temperature (TN = 240 K) indicates that LuFe2O4+δ shows short-range, two-dimensional (2D) correlations in the FeO5 triangular layers, characterized by the development of a magnetic scattering rod along the 1/3 1/3 L direction, persisting down to 5 K. We also found that on top of the 2D correlations, a long range ferromagnetic component associated with the propagation vector k1 = 0 sets in at around 240 K. On the other hand, an external magnetic field applied along the c-axis effectively favours a three-dimensional (3D) spin correlation between the FeO5 bilayers evidenced by the increase of the intensity of satellite reflections with propagation vector k2 = (1/3, 1/3, 3/2). This magnetic modulation is identical to the charge ordered superstructure, highlighting the field-promoted coupling between the spin and charge degrees of freedom. Formation of the 3D spin correlations suppresses both the rod-type diffuse scattering and the k1 component. Simple symmetry-based arguments provide a natural explanation of the observed phenomenon and put forward a possible charge redistribution in the applied magnetic field.

cond-mat.str-el↗

arXiv2017

Tunable Weyl and Dirac states in the nonsymmorphic compound $\rm\mathbf{CeSbTe}$

Recent interest in topological semimetals has lead to the proposal of many new topological phases that can be realized in real materials. Next to Dirac and Weyl systems, these include more exotic phases based on manifold band degeneracies in the bulk electronic structure. The exotic states in topological semimetals are usually protected by some sort of crystal symmetry and the introduction of magnetic order can influence these states by breaking time reversal symmetry. Here we show that we can realize a rich variety of different topological semimetal states in a single material, $\rm CeSbTe$. This compound can exhibit different types of magnetic order that can be accessed easily by applying a small field. It allows, therefore, for tuning the electronic structure and can drive it through a manifold of topologically distinct phases, such as the first nonsymmorphic magnetic topological material with an eight-fold band crossing at a high symmetry point. Our experimental results are backed by a full magnetic group theory analysis and ab initio calculations. This discovery introduces a realistic and promising platform for studying the interplay of magnetism and topology.

cond-mat.mtrl-sci↗

arXiv2017

Unusual magnetic structure of high-pressure synthesized perovskites ACrO3(A=Sc, In, Tl)

Magnetic structures of metastable perovskites ScCrO3, InCrO3 and TlCrO3, stabilized under high-pressure and high-temperature conditions, have been studied by neutron powder diffraction. Similar to the other orthochromites LnCrO3 (Ln=lanthanide or Y), these materials crystallize into the orthorhombic structure with Pnma10 symmetry. The spin configuration of the metastable perovskites has been found to be C-type, contrasting with the G-type structure usually observed in LnCrO3. First-principles calculations demonstrate that the Ctype structure found in ScCrO3 and InCrO3 is attributed to a ferromagnetic (FM) nearest-neighbor interaction, while in TlCrO3, this type of magnetic ordering is stabilized by a strong next-nearest-neighbor antiferromagnetic (AFM) exchange. The spins in the C-type magnetic structure line up along the orthorhombic b-axis, yielding the Pnma magnetic symmetry. The dominant mechanism controlling this spin direction has been concluded to be the single ion anisotropy imposed by a uniaxial distortion of CrO6 octahedra.

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