Magnetodielectric coupling and multi-blocking effect in the Ising-chain magnet Sr2Ca2CoMn2O9

We have demonstrated magnetodielectric (MD) coupling in an Ising-chain magnet Sr2Ca2CoMn2O9, via detailed investigation of ac susceptibility and dielectric constant as a function of temperature, magnetic field and frequency. Sr2Ca2CoMn2O9 consists of spin-chains, made of the regular stacking of one CoO6 trigonal prism with two MnO6 octahedra. The (Co2+ Mn4+ Mn4+) unit stabilizes a (up-down-up) spin-state along the chains which are distributed on a triangular lattice. This compound undergoes a partially disordered antiferromagnetic transition at TN ~ 28 K. The dielectric constant exhibits a clear peak at TN only in presence of an external magnetic field (above 5 kOe), evidencing the presence of MD coupling, which is further confirmed by field-dependent dielectric measurements. We argue that spatial inversion symmetry can be broken as a result of exchange-striction along each spin chain, inducing uncompensated local dipoles. At low temperatures, a dipolar relaxation phenomenon is observed, bearing strong similarities with the blocking effect typical of the spin dynamics in this compound. Such a spin-dipole relationship is referred to as a multi-blocking effect, in relation with the concept of magnetodielectric multiglass previously introduced for related materials.Comment: Multiferroicity, Magnetoelectric coupling, Complex exchange interaction, spin-chain oxid

Investigations structurale et physique du système d'oxydes à chaînes de spins Ising (Sr, Ca)1+xCoxMn1-xO3

This manuscript presents the experimental study of structural and physical properties of a spin chain compounds Sr4-xCaxCoMn2O9, belonging to the large oxides family A1+XA'XB1-XO3. In this series, the 1D chains are made up of the octahedra MnO6 (Mn4+) and trigonal prisms CoO6 (Co2+) connected by the faces, and distributed over a triangular lattice with an antiferromagnetic coupling between them. These chains exhibit a strong Ising-type magnetic anisotropy, originating from the cation Co2+ (HS, 3d7, S = 3/2). First of all, the structural study of these chains reveals that they can be immcommensurate, due to a change in the degree of oxidation of cobalt depending on the synthesis conditions. Then, we were interested in the study of the two compounds Sr4CoMn2O9 (x=0) and Sr2Ca2CoMn2O9 (x=2). The x=0 compound showed the absence of long-range magnetic ordering (LRO) and dynamic spin relaxation responses, typical of Single-Ion Magnet (SIM) and Single-Chain Magnet (SCM), of which the amplitude of their characteristic peaks depends on the (in)commensurability. On the other hand, in x = 2, only the SIM response was observed at low temperature, and which coexists with the LRO at TN ~ 28 K. The neutron diffraction data show that this LRO is compatible with a partially disordered antiferromagnetic state (PDA). A particular pre-transitional regime was also observed between TN and T* (~ 32.5 K), which was considered to be a precursor effect of LRO. Furthermore, a magneto-electric (ME) coupling has also been demonstrated within this compound. The mechanism of this ME coupling has been discussed as a result of exchange-striction phenomenon. Finally, we studied the magnetic anisotropy in oriented samples, whose grain morphology was optimized by different heat treatments.Ce manuscrit présente l’étude expérimentale des propriétés structurales et physiques d’une série de composés à chaînes de spins Sr4−xCaxCoMn2O9, appartenant à la famille d’oxydes A1+X(A’XB1−X)O3. Dans cette série, les chaînes 1D sont constituées des octaèdres MnO6 et prismes trigonaux CoO6 reliés par les faces, et réparties sur un réseau triangulaire avec un couplage antiferromagnétique entre elles. Ces chaînes présentent une forte anisotropie magnétique de type Ising provenant du cation Co2+ (HS, 3d7, S = 3/2). D’abord, l'étude structurale de ces chaînes révèle qu’elles peuvent être incommensurables, à cause d’un changement de degré d'oxydation du cobalt en fonction des conditions de synthèse. Ensuite, nous nous sommes intéressés à l'étude des deux composés Sr4CoMn2O9 (x=0) et Sr2Ca2CoMn2O9 (x=2). Le composé x=0 commensurable a montré l'absence de mise en ordre magnétique à longue distance (LRO) et des réponses dynamiques de relaxation de spins, typiques des Single-Ion Magnet (SIM) et Single-Chain Magnet (SCM) dont l'amplitude de leurs pics caractéristiques dépendde la (in)commensurabilité. En revanche, dans x=2, seule la réponse SIM a été observée à basse température, et qui coexiste avec le LRO à TN ~ 28 K. Les données de diffraction des neutrons montrent que cet LRO est compatible avec un état antiferromagnétique partiellement désordonné (PDA). Un régime pré-transitionnel particulier a été aussi observé entre TN et T* (~ 32.5 K), qui a été considéré comme un effet précurseur de LRO. D'un autre côté, un couplage magnéto-électrique a été également mis en évidence au sein de ce composé, dont l'origine a été interprétée par un phénomène de striction d’échange. Finalement, nous avons étudié l'anisotropie magnétique dans des échantillons orientés, dont la morphologie des grains a été optimisée par différents traitements thermiques

Investigations structurale et physique du système d'oxydes à chaînes de spins Ising (Sr, Ca)1+xCoxMn1-xO3

This manuscript presents the experimental study of structural and physical properties of a spin chain compounds Sr4-xCaxCoMn2O9, belonging to the large oxides family A1+XA'XB1-XO3. In this series, the 1D chains are made up of the octahedra MnO6 (Mn4+) and trigonal prisms CoO6 (Co2+) connected by the faces, and distributed over a triangular lattice with an antiferromagnetic coupling between them. These chains exhibit a strong Ising-type magnetic anisotropy, originating from the cation Co2+ (HS, 3d7, S = 3/2). First of all, the structural study of these chains reveals that they can be immcommensurate, due to a change in the degree of oxidation of cobalt depending on the synthesis conditions. Then, we were interested in the study of the two compounds Sr4CoMn2O9 (x=0) and Sr2Ca2CoMn2O9 (x=2). The x=0 compound showed the absence of long-range magnetic ordering (LRO) and dynamic spin relaxation responses, typical of Single-Ion Magnet (SIM) and Single-Chain Magnet (SCM), of which the amplitude of their characteristic peaks depends on the (in)commensurability. On the other hand, in x = 2, only the SIM response was observed at low temperature, and which coexists with the LRO at TN ~ 28 K. The neutron diffraction data show that this LRO is compatible with a partially disordered antiferromagnetic state (PDA). A particular pre-transitional regime was also observed between TN and T* (~ 32.5 K), which was considered to be a precursor effect of LRO. Furthermore, a magneto-electric (ME) coupling has also been demonstrated within this compound. The mechanism of this ME coupling has been discussed as a result of exchange-striction phenomenon. Finally, we studied the magnetic anisotropy in oriented samples, whose grain morphology was optimized by different heat treatments.Ce manuscrit présente l’étude expérimentale des propriétés structurales et physiques d’une série de composés à chaînes de spins Sr4−xCaxCoMn2O9, appartenant à la famille d’oxydes A1+X(A’XB1−X)O3. Dans cette série, les chaînes 1D sont constituées des octaèdres MnO6 et prismes trigonaux CoO6 reliés par les faces, et réparties sur un réseau triangulaire avec un couplage antiferromagnétique entre elles. Ces chaînes présentent une forte anisotropie magnétique de type Ising provenant du cation Co2+ (HS, 3d7, S = 3/2). D’abord, l'étude structurale de ces chaînes révèle qu’elles peuvent être incommensurables, à cause d’un changement de degré d'oxydation du cobalt en fonction des conditions de synthèse. Ensuite, nous nous sommes intéressés à l'étude des deux composés Sr4CoMn2O9 (x=0) et Sr2Ca2CoMn2O9 (x=2). Le composé x=0 commensurable a montré l'absence de mise en ordre magnétique à longue distance (LRO) et des réponses dynamiques de relaxation de spins, typiques des Single-Ion Magnet (SIM) et Single-Chain Magnet (SCM) dont l'amplitude de leurs pics caractéristiques dépendde la (in)commensurabilité. En revanche, dans x=2, seule la réponse SIM a été observée à basse température, et qui coexiste avec le LRO à TN ~ 28 K. Les données de diffraction des neutrons montrent que cet LRO est compatible avec un état antiferromagnétique partiellement désordonné (PDA). Un régime pré-transitionnel particulier a été aussi observé entre TN et T* (~ 32.5 K), qui a été considéré comme un effet précurseur de LRO. D'un autre côté, un couplage magnéto-électrique a été également mis en évidence au sein de ce composé, dont l'origine a été interprétée par un phénomène de striction d’échange. Finalement, nous avons étudié l'anisotropie magnétique dans des échantillons orientés, dont la morphologie des grains a été optimisée par différents traitements thermiques

Structural and physical investigations of the Ising spin chain oxides (Sr, Ca)1+xCoxMn1-xO3

Ce manuscrit présente l’étude expérimentale des propriétés structurales et physiques d’une série de composés à chaînes de spins Sr4−xCaxCoMn2O9, appartenant à la famille d’oxydes A1+X(A’XB1−X)O3. Dans cette série, les chaînes 1D sont constituées des octaèdres MnO6 et prismes trigonaux CoO6 reliés par les faces, et réparties sur un réseau triangulaire avec un couplage antiferromagnétique entre elles. Ces chaînes présentent une forte anisotropie magnétique de type Ising provenant du cation Co2+ (HS, 3d7, S = 3/2). D’abord, l'étude structurale de ces chaînes révèle qu’elles peuvent être incommensurables, à cause d’un changement de degré d'oxydation du cobalt en fonction des conditions de synthèse. Ensuite, nous nous sommes intéressés à l'étude des deux composés Sr4CoMn2O9 (x=0) et Sr2Ca2CoMn2O9 (x=2). Le composé x=0 commensurable a montré l'absence de mise en ordre magnétique à longue distance (LRO) et des réponses dynamiques de relaxation de spins, typiques des Single-Ion Magnet (SIM) et Single-Chain Magnet (SCM) dont l'amplitude de leurs pics caractéristiques dépendde la (in)commensurabilité. En revanche, dans x=2, seule la réponse SIM a été observée à basse température, et qui coexiste avec le LRO à TN ~ 28 K. Les données de diffraction des neutrons montrent que cet LRO est compatible avec un état antiferromagnétique partiellement désordonné (PDA). Un régime pré-transitionnel particulier a été aussi observé entre TN et T* (~ 32.5 K), qui a été considéré comme un effet précurseur de LRO. D'un autre côté, un couplage magnéto-électrique a été également mis en évidence au sein de ce composé, dont l'origine a été interprétée par un phénomène de striction d’échange. Finalement, nous avons étudié l'anisotropie magnétique dans des échantillons orientés, dont la morphologie des grains a été optimisée par différents traitements thermiques.This manuscript presents the experimental study of structural and physical properties of a spin chain compounds Sr4-xCaxCoMn2O9, belonging to the large oxides family A1+XA'XB1-XO3. In this series, the 1D chains are made up of the octahedra MnO6 (Mn4+) and trigonal prisms CoO6 (Co2+) connected by the faces, and distributed over a triangular lattice with an antiferromagnetic coupling between them. These chains exhibit a strong Ising-type magnetic anisotropy, originating from the cation Co2+ (HS, 3d7, S = 3/2). First of all, the structural study of these chains reveals that they can be immcommensurate, due to a change in the degree of oxidation of cobalt depending on the synthesis conditions. Then, we were interested in the study of the two compounds Sr4CoMn2O9 (x=0) and Sr2Ca2CoMn2O9 (x=2). The x=0 compound showed the absence of long-range magnetic ordering (LRO) and dynamic spin relaxation responses, typical of Single-Ion Magnet (SIM) and Single-Chain Magnet (SCM), of which the amplitude of their characteristic peaks depends on the (in)commensurability. On the other hand, in x = 2, only the SIM response was observed at low temperature, and which coexists with the LRO at TN ~ 28 K. The neutron diffraction data show that this LRO is compatible with a partially disordered antiferromagnetic state (PDA). A particular pre-transitional regime was also observed between TN and T* (~ 32.5 K), which was considered to be a precursor effect of LRO. Furthermore, a magneto-electric (ME) coupling has also been demonstrated within this compound. The mechanism of this ME coupling has been discussed as a result of exchange-striction phenomenon. Finally, we studied the magnetic anisotropy in oriented samples, whose grain morphology was optimized by different heat treatments

Physical properties of a quasi-1D Ising S = ½ spin system: Ba4CoPt2O9+Δ

International audienceThe spin chain oxide Ba 4 CoPt 2 O 9+Δ gathers several features that make it close to a 1D Ising S = 1/2 system: (i) it contains only one type of spins, which are associated to a Kramers doublet with a substantial gap from the first excited state; (ii) these spins exhibit a pronounced uniaxial anisotropy; (iii) their easy-axis is perfectly oriented along the spin chain direction; (iv) the spin chains remain uncoupled down to at least 2 K. It is reported that the magnetic susceptibility, the isothermal magnetization and the specific heat in fields up to 9 T can be well accounted for by the theoretical predictions for 1D Ising S = 1/2 systems, yielding consistent set of values for the Landé factors and the intrachain coupling. The presence of missing spins associated to the oxygen overstoichiometry is taken into account, as well as the polycrystalline nature of the samples which requires to consider angular averages of the anisotropic physical properties

Effect of thermal treatment upon the structure incommensurability and magnetism of the spin chain oxide Sr3CaMn2CoO9+δ

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Effect of thermal treatment upon the structure incommensurability and magnetism of the spin chain oxide Sr3CaMn2CoO9+δ

International audienceSpin chain oxides with different incommensurate hexagonal composite structures have been obtained for a same cationic composition according to the formula Sr3CaMn2CoO9+δ by varying the thermal treatments in air. The cell parameters of these oxides, a∼9.5 Å, c1∼2.5 Å and c2∼3.8 Å, decrease as the deviation from commensurability increases, i.e. as γ = c1/c2 decreases and falls away from 2/3. All the investigated samples exhibit a signature typical of single-ion magnet (SIM), as well as a competition between single-chain magnet (SCM) and short-range ordering (SRO) responses. The balance between the latter two features turns out to be clearly affected by the thermal treatment. The sample quenched at 1450 °C, which structure is almost commensurate (γ = 0.6659, close to 2/3), exhibits a SCM behavior without sizeable indication of SRO, whereas, for the samples annealed at 1300 °C or furnace cooled from 1400 °C with an incommensurate structure (γ = 0.6622–0.6631), the SRO character is strongly reinforced. These changes in the magnetic response are assumed to originate from incorporation of extra oxygen into the commensurate stoichiometric Sr3CaMn2CoO9 oxide, leading to the replacement of some trimeric units built up of two octahedra and one trigonal prism (Oh2Tp) by tetrameric units (Oh3Tp) containing one additional octahedron, with a concomitant appearance of cationic vacancies in the trigonal prismatic sites

Oxygen over stoichiometry in the 2 H-perovskite related structure: The route to a large family of cation deficient Ising chain oxides Sr 1+y [(Mn 1-x Co x ) 1-z z ]O 3

International audienceThe realization of oxygen over stoichiometry through oxidation of Co 2+ into Co 3+ in the 2H-perovskite related oxides Sr1+x(Mn1-xCox)O3 is demonstrated, leading to a large family of cation deficient Ising chain oxides Sr1+y[(Mn1-xCox)1-z z]O3. In these aperiodic hexagonal structures also formulated Sr1+x(Mn1-xCox)O3+δ (a≈9.6 Å, c1≈2.6 Å, c2 ≈3.9 Å), and built up of face sharing octahedral and trigonal prismatic polyhedra, the observed modulation vector γ=c1/c2, (ranging from ~0.641 to ~0.668) allows the proportion of octahedral (NO) and prismatic sites (NP) to be obtained according to the relations: y=2γ-1= NP/(NP+NO). For these oxides, the rate of cationic vacancies (z) in the chains and the oxygen over-stoichiometry (δ) can be deduced from the relations: z =(x-y)/(1+x) and δ=3(x-y)/(1+y). The analysis of these results shows that the structure of the chains, i.e. the proportion of tetrameric units "M4O12" coexisting with trimeric units "M3O9" is highly sensitive to the oxygen over-stoichiometry for a fixed cationic composition. This is illustrated here by the structure of the oxide corresponding to x=0.375 with δ=0.09, which is commensurate with γ=2/3 similar to Sr4Mn2CoO9 but with a significant content of cationic vacancies in the trigonal prismatic sites according to the formula Sr4(Mn IV 1.818Co III 0.544 Co II 0.5480.09)O9. Thus, the presence in those chains of cationic vacancies due to oxygen overstoichiometry appears as a very important concept which opens the route to the synthesis of fragmented chain structures and may have a huge impact upon the magnetic and magneto-electric behavior of Ising chain oxides

Oxygen over stoichiometry in the 2 H-perovskite related structure: The route to a large family of cation deficient Ising chain oxides Sr 1+y [(Mn 1-x Co x ) 1-z z ]O 3

International audienceThe realization of oxygen over stoichiometry through oxidation of Co 2+ into Co 3+ in the 2H-perovskite related oxides Sr1+x(Mn1-xCox)O3 is demonstrated, leading to a large family of cation deficient Ising chain oxides Sr1+y[(Mn1-xCox)1-z z]O3. In these aperiodic hexagonal structures also formulated Sr1+x(Mn1-xCox)O3+δ (a≈9.6 Å, c1≈2.6 Å, c2 ≈3.9 Å), and built up of face sharing octahedral and trigonal prismatic polyhedra, the observed modulation vector γ=c1/c2, (ranging from ~0.641 to ~0.668) allows the proportion of octahedral (NO) and prismatic sites (NP) to be obtained according to the relations: y=2γ-1= NP/(NP+NO). For these oxides, the rate of cationic vacancies (z) in the chains and the oxygen over-stoichiometry (δ) can be deduced from the relations: z =(x-y)/(1+x) and δ=3(x-y)/(1+y). The analysis of these results shows that the structure of the chains, i.e. the proportion of tetrameric units "M4O12" coexisting with trimeric units "M3O9" is highly sensitive to the oxygen over-stoichiometry for a fixed cationic composition. This is illustrated here by the structure of the oxide corresponding to x=0.375 with δ=0.09, which is commensurate with γ=2/3 similar to Sr4Mn2CoO9 but with a significant content of cationic vacancies in the trigonal prismatic sites according to the formula Sr4(Mn IV 1.818Co III 0.544 Co II 0.5480.09)O9. Thus, the presence in those chains of cationic vacancies due to oxygen overstoichiometry appears as a very important concept which opens the route to the synthesis of fragmented chain structures and may have a huge impact upon the magnetic and magneto-electric behavior of Ising chain oxides