We have investigated the magnetic structures of the Swedenborgite compound CaBaFe_44O_77 using ma... more We have investigated the magnetic structures of the Swedenborgite compound CaBaFe_44O_77 using magnetic susceptibility and neutron diffraction experiments on powder and single-crystal samples. Below T_{N1}TN1 = 274 K the system orders in a ferrimagnetic structure with spins along the c axis and an additional antiferromagnetic component within the kagome plane which obviously cannot satisfy all exchange interactions. Competing single-ion anisotropy and exchange interactions lead to a transition into a multi-\mathbf{q}𝐪 conical structure at T_{N2}TN2 = 202 K. The derivation of the complex ordering scheme below T_{N2}TN2 is an important step towards the understanding of the magnetoelectric effect under magnetic fields in this polar ferrimagnet.
We report an unusual in-field behaviour of SrEr_22O_44 for a magnetic field applied along two hig... more We report an unusual in-field behaviour of SrEr_22O_44 for a magnetic field applied along two high-symmetry directions, the a and c axes. This geometrically frustrated magnet hosts two crystallographically inequivalent Er ions, Er1 and Er2, that are both located on triangular zigzag ladders, but only one site, Er1, forms a long-range magnetic order at low temperatures in a zero field. We follow the sequence of peculiar field induced states in SrEr_22O_44 with detailed single-crystal magnetisation and neutron diffraction experiments. On application of an external field along the cc axis, the long-range antiferromagnetic order of the Er1 ions is rapidly destroyed and replaced, in fields between 2 and 5 kOe, by a state with shorter-range correlations. The change in correlation length coincides with a fast increase in magnetisation during the metamagnetic transition above which a long-range order is reestablished and maintained into the high fields. The high-field ferromagnet-like order...
We have investigated the pressure effect on magnetic ordering of the ferromagnetic double perovsk... more We have investigated the pressure effect on magnetic ordering of the ferromagnetic double perovskite Lu 2 NiMnO 6 by magnetization, ac magnetic susceptibility, and neutron diffraction experiments up to 8.0 GPa in order to understand the ferromagnetic-to-antiferromagnetic phase transition by substitution of the A sites in A 2 NiMnO 6 from rare-earth to indium or scandium ions. Strong ferromagnetic spin correlation seen in the susceptibility at low pressure is significantly suppressed by increasing pressure. In a neutron diffraction experiment, the magnetic Bragg reflections associated with ferromagnetic ordering disappear above 4.5 GPa. For the high-pressure region above 4.5 GPa, an antiferromagnetic ordering with long-period incommensurate modulation appears, which is coexistent with ferromagnetic short-range order. From mean-field calculations, we infer that pressure modification of the delicate balance of the competing next-nearest-neighbor exchange interactions between Ni and Ni, or Mn and Mn, plays an important role in the phase transition from ferromagnetic to antiferromagnetic ordering in A 2 NiMnO 6 .
The multiferroic perovskite rare earth manganites RMnO 3 (R = Dy, Tb, Gd) are known as multiferro... more The multiferroic perovskite rare earth manganites RMnO 3 (R = Dy, Tb, Gd) are known as multiferroics exhibiting pressure-induced gigantic ferroelectric polarization. In this study, we have investigated the magnetic orderings in the pressure-induced phases for DyMnO 3 , by neutron diffraction and spherical neutron polarimetry (SNP) experiments up to 8.0 GPa. The magnetic ordering for Mn spins changes from the incommensurate bc-cycloid to the commensurate collinear E -type structure with k Mn = (0, 1 2 , 0) above 4.0 GPa, which is concomitant with the appearance of a giant ferroelectric polarization. The magnetic ordering for the Dy spins has been determined to be a noncollinear spin structure with a and b spin components and k Dy = (0, 1 2 , 0) for the low-and high-pressure phases. The magnetic field along the a axis, H a , affects the Dy ordering, which is seen in the changes in the k vector from k Dy = (0, 1 2 , 0) in H a 3 T to k Dy = (0, 0, 0) in H a 3 T. Considering the lattice distortion generated by the determined magnetic orderings through the exchange striction mechanism, we conclude that the exchange striction for rare earth and Mn bonds, which is added to the uniform polarization generated by the E -type Mn ordering, is strongly related to the significant magnetic field enhancement of ferroelectric polarization in the high-pressure phase of the rare earth manganites.
Multicomponent magnetic phase diagrams are a key property of functional materials for a variety o... more Multicomponent magnetic phase diagrams are a key property of functional materials for a variety of uses, such as manipulation of magnetization for energy efficient memory, data storage, and cooling applications. Strong spin‐lattice coupling extends this functionality further by allowing electric‐field‐control of magnetization via strain coupling with a piezoelectric. Here this work explores the magnetic phase diagram of piezomagnetic Mn3NiN thin films, with a frustrated noncollinear antiferromagnetic (AFM) structure, as a function of the growth induced biaxial strain. Under compressive strain, the films support a canted AFM state with large coercivity of the transverse anomalous Hall resistivity, ρxy, at low temperature, that transforms at a well‐defined Néel transition temperature (TN) into a soft ferrimagnetic‐like (FIM) state at high temperatures. In stark contrast, under tensile strain, the low temperature canted AFM phase transitions to a state where ρxy is an order of magnitud...
ND 4 ) 2 [FeCl 5 (D 2 O)] represents a promising example of the hybrid molecular/inorganic approa... more ND 4 ) 2 [FeCl 5 (D 2 O)] represents a promising example of the hybrid molecular/inorganic approach to create materials with strong magneto-electric coupling. Neutron spherical polarimetry, which is directly sensitive to the absolute magnetic configuration and domain population, has been used in this work to unambiguously prove the multiferroicity of this material. We demonstrate that the application of an electric field upon cooling results in the stabilization of a single-cycloidal magnetic domain below 6.9 K, while poling in the opposite electric field direction produces the full population of the domain with opposite magnetic chirality. We prove the complete switchability of the magnetic domains at low temperature by the applied electric field, which constitutes a direct proof of the strong magnetoelectric coupling. Additionally, we refine the magnetic structure of the ordered ground state, deducing the underlying magnetic space group consistent with the direction of the ferroelectric polarization, and we provide evidence of a collinear amplitude-modulated state with magnetic moments along the a-axis in the temperature region between 6.9 and 7.2 K. Unlike "type I" multiferroics, where electric and magnetic orders coexist but are weakly coupled, the magneto-electric (ME) coupling is very strong in "type II" (or "improper") multiferroics, where the magnetic ordering breaks spatial inversion symmetry and induces electric polarization 1-5 . This direct coupling leads to remarkable possibilities of manipulation of the electric order by magnetic fields and of the magnetic order by electric fields, which are at the origin of the significant attention devoted to this class of materials. The number of such spin-driven multiferroics is, however, still scarce, and mainly limited to transition metal oxides. Most of the examples involve non-collinear spiral spin arrangements, like in RMnO 3 (R = rare earth) 6,7 , MnWO 4 8,9 , CoCr 2 O 4 10 , CuFeO 2 11 and RbFe(MoO 4 ) 2 12 , although collinear or almost collinear magnetic structures can also give rise to spin-driven multiferroics, like in HoMnO 3 13 or in the RMn 2 O 5 family 14 . The difficult control of the delicate balance between exchange couplings in these systems impedes a rational design of these interesting materials. This fact highlights the interest for innovative approaches for obtaining magnetically driven multiferroic compounds. Important efforts have been made in this sense following metal-organic routes, which allow the combination of building blocks with the desired properties in a bottom-up approach . However, the cross coupling between magnetic and electric orders is absent or very weak in this type of systems . As an alternative, hybrid approaches inspired by the architectures of the successful organic-inorganic photovoltaic materials have been proposed 21 . For this research, microscopic techniques that give direct proof of the multiferroicity and give insight on the underlying magneto-electric coupling mechanism are crucial, since a good understanding of these mechanisms may help to design new improper multiferroic materials with improved functional properties. The family A 2 [FeX 5 (H 2 O)] 22 , where A stands for an alkali metal or ammonium ion and X for a halide ion, represents a new route to obtain materials with strong ME coupling. The occurrence of ME coupling in the alkali-based compounds K 2 [FeCl 5 (H 2 O)], Rb 2 [FeCl 5 (H 2 O)] and Cs 2 [FeCl 5 (H 2 O)] has been described in a thorough study of their macroscopic physical properties 23 , and understood from the analysis of their crystal and
The magnetic properties of the mixed compounds ͑Co x Ni 1−x ͒ 3 V 2 O 8 ͑CNVO͒ investigated by ma... more The magnetic properties of the mixed compounds ͑Co x Ni 1−x ͒ 3 V 2 O 8 ͑CNVO͒ investigated by magnetization and neutron diffraction measurements are presented. Unlike their parent compounds Ni 3 V 2 O 8 ͑NVO͒ and Co 3 V 2 O 8 ͑CVO͒, only one magnetic phase transition into an antiferromagnetic phase was detected for powder samples with x = 0.27, 0.52, and 0.76. The magnetic structures are modulated according to a propagation vector k ជ = ͑␦ ,0,0͒ with ␦ being dependent on the composition parameter x. Furthermore, magnetization data of a CVO single crystal is featured, which is qualitatively different from previous publications and exhibits a controversial aspect concerning the behavior of the curve under an applied magnetic field along the b axis.
The analysis of three-dimensional neutron spin polarization vectors, using a technique referred t... more The analysis of three-dimensional neutron spin polarization vectors, using a technique referred to as spherical neutron polarimetry (SNP), is a very powerful means of determining complex magnetic structures in magnetic materials. However, the requirement to maintain neutrons in a highly polarized state has made it difficult to use this technique in conjunction with extreme experimental conditions. We have developed a high pressure cell made completely of nonmagnetic materials and having no effect on neutron polarizations. Herein, we report the first SNP analyses under high pressure up to 4.0 GPa in the magnetoelectric multiferroic delafossite CuFeO2. This study also determined the complex spiral magnetic structures in these pressure-induced phases, by measuring the full neutron polarization matrix. The results presented herein demonstrate that the SNP measurements are feasible under high pressure conditions, and that this method is a useful approach to study pressure-induced physica...
The (010) plane of the C60·2CBrClH2 monoclinic (C2/m) co-crystal with both molecular entities, C6... more The (010) plane of the C60·2CBrClH2 monoclinic (C2/m) co-crystal with both molecular entities, C60 and CBrClH2, orientationally ordered.
The formation of co-crystals is often unexpected; however, the Buckminsterfullerene, for which ma... more The formation of co-crystals is often unexpected; however, the Buckminsterfullerene, for which many solvates are known, is an excellent system to study this tendency. In the present paper, C60 and...
We present a full characterization of the orientationally disordered co-crystal of C 60 with (1,1... more We present a full characterization of the orientationally disordered co-crystal of C 60 with (1,1,2)trichloroethane (C 2 H 3 Cl 3) by means of x-ray diffraction, Raman spectroscopy and broadband dielectric spectroscopy. Our results include the determination of molecular conformations, lattice structure, positional disorder, and molecular reorientational dynamics down to the microsecond timescale. We find that, while in the disordered solid phase of pure C 2 H 3 Cl 3 the molecules exist only in the gauche conformation, both gauche and transoid conformers are present in the solvate,
We demonstrate that solvates of fullerene C 60 form very predictable structures with finely tunab... more We demonstrate that solvates of fullerene C 60 form very predictable structures with finely tunable properties through the choice of the second component or solvent. Cubic co-crystals of C 60 •12CCl 2 Br 2 and C 60 •12CBr 2 (CH 3) 2 were grown at room temperature in saturated solutions of fcc C 60 and the respective solvents (with C 2v molecular symmetry) They are unstable in air and transform spontaneously into the hexagonal co-crystals C 60 •2CCl 2 Br 2 or C 60 •2CBr 2 (CH 3) 2. Whereas, the cubic co-crystals have positive excess volumes (+2% and +5%, respectively), the stable hexagonal crystals, for which structures are given for the first time, possess negative excess volumes (-5% and-4.3%, respectively). The unit-cell volumes for both cubic and hexagonal co-crystals depend exclusively on the van-der-Waals volumes of the constituents and this correlation has been confirmed using previously published data.
CuO is the only known binary multiferroic compound, and due to its high transition temperature in... more CuO is the only known binary multiferroic compound, and due to its high transition temperature into the multiferroic state, it has been extensively studied. In comparison to other prototype multiferroics, the nature and even the existence of the high-temperature incommensurate paraelectric phase (AF3) were strongly debated—both experimentally and theoretically—since it is stable for only a few tenths of a kelvin just below the Néel temperature. Until now, there is no proof by neutron diffraction techniques owing to its very small ordered Cu magnetic moment. Here, we demonstrate the potential of spherical neutron polarimetry, first, in detecting magnetic structure changes, which are not or weakly manifest in the peak intensity and, second, in deducing the spin arrangement of the so far hypothetic AF3 phase. Our findings suggest two coexisting spin density waves emerging from an accidental degeneracy of the respective states implying a delicate energy balance in the spin Hamiltonian.
We have investigated the magnetic structures of the Swedenborgite compound CaBaFe_44O_77 using ma... more We have investigated the magnetic structures of the Swedenborgite compound CaBaFe_44O_77 using magnetic susceptibility and neutron diffraction experiments on powder and single-crystal samples. Below T_{N1}TN1 = 274 K the system orders in a ferrimagnetic structure with spins along the c axis and an additional antiferromagnetic component within the kagome plane which obviously cannot satisfy all exchange interactions. Competing single-ion anisotropy and exchange interactions lead to a transition into a multi-\mathbf{q}𝐪 conical structure at T_{N2}TN2 = 202 K. The derivation of the complex ordering scheme below T_{N2}TN2 is an important step towards the understanding of the magnetoelectric effect under magnetic fields in this polar ferrimagnet.
We report an unusual in-field behaviour of SrEr_22O_44 for a magnetic field applied along two hig... more We report an unusual in-field behaviour of SrEr_22O_44 for a magnetic field applied along two high-symmetry directions, the a and c axes. This geometrically frustrated magnet hosts two crystallographically inequivalent Er ions, Er1 and Er2, that are both located on triangular zigzag ladders, but only one site, Er1, forms a long-range magnetic order at low temperatures in a zero field. We follow the sequence of peculiar field induced states in SrEr_22O_44 with detailed single-crystal magnetisation and neutron diffraction experiments. On application of an external field along the cc axis, the long-range antiferromagnetic order of the Er1 ions is rapidly destroyed and replaced, in fields between 2 and 5 kOe, by a state with shorter-range correlations. The change in correlation length coincides with a fast increase in magnetisation during the metamagnetic transition above which a long-range order is reestablished and maintained into the high fields. The high-field ferromagnet-like order...
We have investigated the pressure effect on magnetic ordering of the ferromagnetic double perovsk... more We have investigated the pressure effect on magnetic ordering of the ferromagnetic double perovskite Lu 2 NiMnO 6 by magnetization, ac magnetic susceptibility, and neutron diffraction experiments up to 8.0 GPa in order to understand the ferromagnetic-to-antiferromagnetic phase transition by substitution of the A sites in A 2 NiMnO 6 from rare-earth to indium or scandium ions. Strong ferromagnetic spin correlation seen in the susceptibility at low pressure is significantly suppressed by increasing pressure. In a neutron diffraction experiment, the magnetic Bragg reflections associated with ferromagnetic ordering disappear above 4.5 GPa. For the high-pressure region above 4.5 GPa, an antiferromagnetic ordering with long-period incommensurate modulation appears, which is coexistent with ferromagnetic short-range order. From mean-field calculations, we infer that pressure modification of the delicate balance of the competing next-nearest-neighbor exchange interactions between Ni and Ni, or Mn and Mn, plays an important role in the phase transition from ferromagnetic to antiferromagnetic ordering in A 2 NiMnO 6 .
The multiferroic perovskite rare earth manganites RMnO 3 (R = Dy, Tb, Gd) are known as multiferro... more The multiferroic perovskite rare earth manganites RMnO 3 (R = Dy, Tb, Gd) are known as multiferroics exhibiting pressure-induced gigantic ferroelectric polarization. In this study, we have investigated the magnetic orderings in the pressure-induced phases for DyMnO 3 , by neutron diffraction and spherical neutron polarimetry (SNP) experiments up to 8.0 GPa. The magnetic ordering for Mn spins changes from the incommensurate bc-cycloid to the commensurate collinear E -type structure with k Mn = (0, 1 2 , 0) above 4.0 GPa, which is concomitant with the appearance of a giant ferroelectric polarization. The magnetic ordering for the Dy spins has been determined to be a noncollinear spin structure with a and b spin components and k Dy = (0, 1 2 , 0) for the low-and high-pressure phases. The magnetic field along the a axis, H a , affects the Dy ordering, which is seen in the changes in the k vector from k Dy = (0, 1 2 , 0) in H a 3 T to k Dy = (0, 0, 0) in H a 3 T. Considering the lattice distortion generated by the determined magnetic orderings through the exchange striction mechanism, we conclude that the exchange striction for rare earth and Mn bonds, which is added to the uniform polarization generated by the E -type Mn ordering, is strongly related to the significant magnetic field enhancement of ferroelectric polarization in the high-pressure phase of the rare earth manganites.
Multicomponent magnetic phase diagrams are a key property of functional materials for a variety o... more Multicomponent magnetic phase diagrams are a key property of functional materials for a variety of uses, such as manipulation of magnetization for energy efficient memory, data storage, and cooling applications. Strong spin‐lattice coupling extends this functionality further by allowing electric‐field‐control of magnetization via strain coupling with a piezoelectric. Here this work explores the magnetic phase diagram of piezomagnetic Mn3NiN thin films, with a frustrated noncollinear antiferromagnetic (AFM) structure, as a function of the growth induced biaxial strain. Under compressive strain, the films support a canted AFM state with large coercivity of the transverse anomalous Hall resistivity, ρxy, at low temperature, that transforms at a well‐defined Néel transition temperature (TN) into a soft ferrimagnetic‐like (FIM) state at high temperatures. In stark contrast, under tensile strain, the low temperature canted AFM phase transitions to a state where ρxy is an order of magnitud...
ND 4 ) 2 [FeCl 5 (D 2 O)] represents a promising example of the hybrid molecular/inorganic approa... more ND 4 ) 2 [FeCl 5 (D 2 O)] represents a promising example of the hybrid molecular/inorganic approach to create materials with strong magneto-electric coupling. Neutron spherical polarimetry, which is directly sensitive to the absolute magnetic configuration and domain population, has been used in this work to unambiguously prove the multiferroicity of this material. We demonstrate that the application of an electric field upon cooling results in the stabilization of a single-cycloidal magnetic domain below 6.9 K, while poling in the opposite electric field direction produces the full population of the domain with opposite magnetic chirality. We prove the complete switchability of the magnetic domains at low temperature by the applied electric field, which constitutes a direct proof of the strong magnetoelectric coupling. Additionally, we refine the magnetic structure of the ordered ground state, deducing the underlying magnetic space group consistent with the direction of the ferroelectric polarization, and we provide evidence of a collinear amplitude-modulated state with magnetic moments along the a-axis in the temperature region between 6.9 and 7.2 K. Unlike "type I" multiferroics, where electric and magnetic orders coexist but are weakly coupled, the magneto-electric (ME) coupling is very strong in "type II" (or "improper") multiferroics, where the magnetic ordering breaks spatial inversion symmetry and induces electric polarization 1-5 . This direct coupling leads to remarkable possibilities of manipulation of the electric order by magnetic fields and of the magnetic order by electric fields, which are at the origin of the significant attention devoted to this class of materials. The number of such spin-driven multiferroics is, however, still scarce, and mainly limited to transition metal oxides. Most of the examples involve non-collinear spiral spin arrangements, like in RMnO 3 (R = rare earth) 6,7 , MnWO 4 8,9 , CoCr 2 O 4 10 , CuFeO 2 11 and RbFe(MoO 4 ) 2 12 , although collinear or almost collinear magnetic structures can also give rise to spin-driven multiferroics, like in HoMnO 3 13 or in the RMn 2 O 5 family 14 . The difficult control of the delicate balance between exchange couplings in these systems impedes a rational design of these interesting materials. This fact highlights the interest for innovative approaches for obtaining magnetically driven multiferroic compounds. Important efforts have been made in this sense following metal-organic routes, which allow the combination of building blocks with the desired properties in a bottom-up approach . However, the cross coupling between magnetic and electric orders is absent or very weak in this type of systems . As an alternative, hybrid approaches inspired by the architectures of the successful organic-inorganic photovoltaic materials have been proposed 21 . For this research, microscopic techniques that give direct proof of the multiferroicity and give insight on the underlying magneto-electric coupling mechanism are crucial, since a good understanding of these mechanisms may help to design new improper multiferroic materials with improved functional properties. The family A 2 [FeX 5 (H 2 O)] 22 , where A stands for an alkali metal or ammonium ion and X for a halide ion, represents a new route to obtain materials with strong ME coupling. The occurrence of ME coupling in the alkali-based compounds K 2 [FeCl 5 (H 2 O)], Rb 2 [FeCl 5 (H 2 O)] and Cs 2 [FeCl 5 (H 2 O)] has been described in a thorough study of their macroscopic physical properties 23 , and understood from the analysis of their crystal and
The magnetic properties of the mixed compounds ͑Co x Ni 1−x ͒ 3 V 2 O 8 ͑CNVO͒ investigated by ma... more The magnetic properties of the mixed compounds ͑Co x Ni 1−x ͒ 3 V 2 O 8 ͑CNVO͒ investigated by magnetization and neutron diffraction measurements are presented. Unlike their parent compounds Ni 3 V 2 O 8 ͑NVO͒ and Co 3 V 2 O 8 ͑CVO͒, only one magnetic phase transition into an antiferromagnetic phase was detected for powder samples with x = 0.27, 0.52, and 0.76. The magnetic structures are modulated according to a propagation vector k ជ = ͑␦ ,0,0͒ with ␦ being dependent on the composition parameter x. Furthermore, magnetization data of a CVO single crystal is featured, which is qualitatively different from previous publications and exhibits a controversial aspect concerning the behavior of the curve under an applied magnetic field along the b axis.
The analysis of three-dimensional neutron spin polarization vectors, using a technique referred t... more The analysis of three-dimensional neutron spin polarization vectors, using a technique referred to as spherical neutron polarimetry (SNP), is a very powerful means of determining complex magnetic structures in magnetic materials. However, the requirement to maintain neutrons in a highly polarized state has made it difficult to use this technique in conjunction with extreme experimental conditions. We have developed a high pressure cell made completely of nonmagnetic materials and having no effect on neutron polarizations. Herein, we report the first SNP analyses under high pressure up to 4.0 GPa in the magnetoelectric multiferroic delafossite CuFeO2. This study also determined the complex spiral magnetic structures in these pressure-induced phases, by measuring the full neutron polarization matrix. The results presented herein demonstrate that the SNP measurements are feasible under high pressure conditions, and that this method is a useful approach to study pressure-induced physica...
The (010) plane of the C60·2CBrClH2 monoclinic (C2/m) co-crystal with both molecular entities, C6... more The (010) plane of the C60·2CBrClH2 monoclinic (C2/m) co-crystal with both molecular entities, C60 and CBrClH2, orientationally ordered.
The formation of co-crystals is often unexpected; however, the Buckminsterfullerene, for which ma... more The formation of co-crystals is often unexpected; however, the Buckminsterfullerene, for which many solvates are known, is an excellent system to study this tendency. In the present paper, C60 and...
We present a full characterization of the orientationally disordered co-crystal of C 60 with (1,1... more We present a full characterization of the orientationally disordered co-crystal of C 60 with (1,1,2)trichloroethane (C 2 H 3 Cl 3) by means of x-ray diffraction, Raman spectroscopy and broadband dielectric spectroscopy. Our results include the determination of molecular conformations, lattice structure, positional disorder, and molecular reorientational dynamics down to the microsecond timescale. We find that, while in the disordered solid phase of pure C 2 H 3 Cl 3 the molecules exist only in the gauche conformation, both gauche and transoid conformers are present in the solvate,
We demonstrate that solvates of fullerene C 60 form very predictable structures with finely tunab... more We demonstrate that solvates of fullerene C 60 form very predictable structures with finely tunable properties through the choice of the second component or solvent. Cubic co-crystals of C 60 •12CCl 2 Br 2 and C 60 •12CBr 2 (CH 3) 2 were grown at room temperature in saturated solutions of fcc C 60 and the respective solvents (with C 2v molecular symmetry) They are unstable in air and transform spontaneously into the hexagonal co-crystals C 60 •2CCl 2 Br 2 or C 60 •2CBr 2 (CH 3) 2. Whereas, the cubic co-crystals have positive excess volumes (+2% and +5%, respectively), the stable hexagonal crystals, for which structures are given for the first time, possess negative excess volumes (-5% and-4.3%, respectively). The unit-cell volumes for both cubic and hexagonal co-crystals depend exclusively on the van-der-Waals volumes of the constituents and this correlation has been confirmed using previously published data.
CuO is the only known binary multiferroic compound, and due to its high transition temperature in... more CuO is the only known binary multiferroic compound, and due to its high transition temperature into the multiferroic state, it has been extensively studied. In comparison to other prototype multiferroics, the nature and even the existence of the high-temperature incommensurate paraelectric phase (AF3) were strongly debated—both experimentally and theoretically—since it is stable for only a few tenths of a kelvin just below the Néel temperature. Until now, there is no proof by neutron diffraction techniques owing to its very small ordered Cu magnetic moment. Here, we demonstrate the potential of spherical neutron polarimetry, first, in detecting magnetic structure changes, which are not or weakly manifest in the peak intensity and, second, in deducing the spin arrangement of the so far hypothetic AF3 phase. Our findings suggest two coexisting spin density waves emerging from an accidental degeneracy of the respective states implying a delicate energy balance in the spin Hamiltonian.
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Papers by Navid Qureshi