Polymorphs, phase transitions and stability in BaM 2 (PO 4 ) 2 M = Mn, Fe, Co systems

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Synthesis, structure and magnetic behavior of iron arsenites with hierarchical magnetic units

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Multivalued Memory via Freezing of Super‐Hard Magnetic Domains in a Quasi 2D‐Magnet

International audienceAbstract The design of high‐density non‐volatile memories is a long‐standing dream, limited by conventional storage “0” or “1” bits. An alternative paradigm exists in which regions within candidate materials can be magnetized to intermediate values between the saturation limits. In principle, this paves the way to multivalued bits, vastly increasing storage density. Single‐molecule magnets, are good examples offering transitions between intramolecular quantum levels, but require ultra‐low temperatures and limited relaxation time between magnetization states. It is showed here that the quasi 2D‐Ising compound BaFe 2 (PO 4 ) 2 overcomes these limitations. The combination of giant magneto‐crystalline anisotropy, strong ferromagnetic exchange, and strong intrinsic pinning creates remarkably narrow magnetic domain walls, collectively freezing under T f ≈15 K. This results in a transition from a soft to a super‐hard magnet (coercive force > 14 T). Any magnetization can then be printed and robustly protected from external fields with an energy barrier >9T at 2 K

Fragile magnetic ordering between robust 2D-ferrimagnets in the AFe 3 (SeO 3 ) 2 F 6 (A=K, Rb, Cs) series

International audienceMixed anion compounds AFe3(SeO3)2F6 (A=K, Rb) have been synthesized by hydrothermal reactions. Prior results obtained for the isostructural Cs analog have revealed original 2D-ferrimagnetic (FI) blocks, antiferromagnetically (AFM) ordered around 120 K. Surprisingly, the whole series orders at a nearly constant TN temperature with similar field-induced magnetization steps, despite significant changes of the interlayer thickness. The metamagnetic transition from AFM to FI on oriented single crystals and Cp measurements prove an in-plane easy-magnetic axis confirmed by neutron powder diffraction (NPD). The ferrimagnetic alignment [Fe(1)↑−Fe(2)↓−Fe(1)↑] in each 2D subunit is robust, but the AFM-ordering between them is broken under weak field leaving field-aligned 2D-macrospins, with weak magnetocrystalline anisotropy. DFT+U calculations validate the exchange ratio Jinter/Jintra of 5.10-3 for all compounds and the relatively high TN value is well predicted by a modified random-phase approximation (RPA) like relation, established for Quasi-Low-Dimensional Heisenberg Antiferromagnets. However, our calculations of intralayer magnetic dipole-dipole (MDD) interactions indicate a non-neglectable values and suggest the latter to assist the in-plane spin orientation below TN. At TN, under field a 3D-Heisenberg model was deduced for the AFM →FI transition from the critical behavior using modified Arrott plots

Two isostructural oxalato-bridged dimetallic heptanuclear [Ba^II₃M^III₄] complexes (M = Cr; Fe) associated with 3-aminopyridinium cations: Synthesis, crystal structure and magnetic properties

Two heterometallic heptanuclear oxalato-bridged [Ba II 3M III 4] complexes, (Org-H) 6[Ba 3(H 2O) 5.1Cr 4(C 2O 4) 12]·5H 2O (1) and (Org-H) 6[Ba 3(H 2O) 5.3Fe 4(C 2O 4) 12]·5H 2O (2) (Org-H = C 5H 7N 2 +: 3-aminopyridinium cation), have been synthesized through an ion-exchange reaction strategy by combining {Ba 6(H 2O) 17[M III(C 2O 4) 3] 4}·7H 2O (M = Cr; Fe) with (C 5H 7N 2) 2C 2O 4 in a 1:3 M ratio. They have been characterized by elemental and thermal analyses, IR spectroscopy, single-crystal X-ray diffraction and variable temperature magnetic susceptibility measurements. The hybrid salts 1 and 2 are isostructural and they crystallize in the monoclinic space group C2/c. Their structures consist of [Ba 3(H 2O) 5M 4(C 2O 4) 12] 6− dimetallic heptanuclear units (M = Cr, Fe), six 3-aminopyridinium cations and five crystallization water molecules. The d-metal atom is located in a distorted (2 + 2 + 2) octahedral environment of six O atoms from three chelating oxalato(2−) ligands. In the crystal, intermolecular N[sbnd]H⋯O and O[sbnd]H⋯O hydrogen bonds link the anions and 3-aminopyridinium cations and lattice water molecules into a three-dimensional framework. In addition, π-π stacking interactions [centroid-centroid distances of 3.680 to 3.938 Å] between the pyridine rings contribute to the stabilization of the framework. The magnetic properties of the two salts have been investigated and they revealed weak antiferromagnetic coupling between d-metal atoms. </p

Tris(oxalato)chromate(III) hybrid salts templated by pyridinium and mixed pyridinium-ammonium cations: synthesis, structures and magnetism

By modifying the stoichiometric ratio of starting materials, two tris(oxalato)chromate(III) salts, (C7H11N2)3[Cr(C2O4)3] (1) and (C5H8N3)2(NH4)[Cr(C2O4)3]·2H2O (2) {(C7H11N2)+ = 2-amino-4,6-dimethylpyridinium, (C5H8N3)+ = 2,6-diaminopyridinium}, were synthesized and characterized by elemental and thermal analyses, single-crystal X-ray diffraction, IR and UV − Vis spectroscopies, EPR and SQUID measurements. Salt 1 exhibits a 3-D supramolecular framework based on [Cr(C2O4)3]3- and 2-amino-4,6-dimethylpyridinim cations, (C7H11N2)+, via N–H···O hydrogen bonds. Interestingly, π–π stacking interactions between pyridine rings contribute to the stabilization of the crystal packing. In contrast to salt 1, no π–π stacking interactions are observed in the mixed-cation salt 2 and its crystal packing is consolidated by N–H···O and O − H···O hydrogen bonds. EPR spectra of 1 and 2 are consistent with the oxidation state +3 of the chromium center in an octahedral environment. Temperature-dependence of the magnetic susceptibility data investigated from 2 to 300 K revealed the existence of zero-field splitting effects (ZFS) for Cr(III) ions in both compounds.</p

Multiferroic BaCoX 2 O 7 (X = P, As) Compounds with Incommensurate Structural Waves but Collinear Spin Ingredients

International audienceA new paradigm in multiferroics is observed in BaCoX 2 O 7 (X = As, P) compounds. They consist of one dimensional (1D) antiferromagnetic chains undulated by incommensurate structural modulations with unusually large atomic displacive waves, giving a mixed 1D/2D "real" magnetic topology. The magnetic ground state is antiferromagnetic (AFM) with k = [½ 0 0], leading to a nonmodulated collinear spin lattice despite the aperiodic atomic framework, and allows developing spin-induced multiferroicity below T N. Severe arguments against the identified mechanisms for type-II multiferroics, i.e., by inverse Dzyaloshinskii-Moriya, exchange striction and spin-dependent p-d hybridizations, suggest an original scenario in which the atomic waves, the collinear magnetic structure, and magnetic dipole-dipole interactions may interact as crucial ingredients of the spin-induced ferroelectric phase. Here, the specific role of the Co 2+ spin-orbit coupling in the magnetoelectric (ME) phase diagram is demonstrated by comparison with the novel Heisenberg BaFeP 2 O 7 isomorph, similarly structurally modulated. This compound shows a noncollinear modulated AFM ordering, while no ME coupling is detected in its case. Accordingly, both BaCoX 2 O 7 and BaFeP 2 O 7 also undergo metamagnetic transitions above 5-6 T promoted by the modulated distribution of spin exchanges, but the spin-flop progressive alignment of the spins in the noncollinear spin structure (Fe 2+ case) turns into an abrupt flip-like transition in the uniaxial spin structure (Co 2+ case)

Multiferroic BaCoX 2 O 7 (X = P, As) Compounds with Incommensurate Structural Waves but Collinear Spin Ingredients

International audienceA new paradigm in multiferroics is observed in BaCoX 2 O 7 (X = As, P) compounds. They consist of one dimensional (1D) antiferromagnetic chains undulated by incommensurate structural modulations with unusually large atomic displacive waves, giving a mixed 1D/2D "real" magnetic topology. The magnetic ground state is antiferromagnetic (AFM) with k = [½ 0 0], leading to a nonmodulated collinear spin lattice despite the aperiodic atomic framework, and allows developing spin-induced multiferroicity below T N. Severe arguments against the identified mechanisms for type-II multiferroics, i.e., by inverse Dzyaloshinskii-Moriya, exchange striction and spin-dependent p-d hybridizations, suggest an original scenario in which the atomic waves, the collinear magnetic structure, and magnetic dipole-dipole interactions may interact as crucial ingredients of the spin-induced ferroelectric phase. Here, the specific role of the Co 2+ spin-orbit coupling in the magnetoelectric (ME) phase diagram is demonstrated by comparison with the novel Heisenberg BaFeP 2 O 7 isomorph, similarly structurally modulated. This compound shows a noncollinear modulated AFM ordering, while no ME coupling is detected in its case. Accordingly, both BaCoX 2 O 7 and BaFeP 2 O 7 also undergo metamagnetic transitions above 5-6 T promoted by the modulated distribution of spin exchanges, but the spin-flop progressive alignment of the spins in the noncollinear spin structure (Fe 2+ case) turns into an abrupt flip-like transition in the uniaxial spin structure (Co 2+ case)

Cu3Te2O5(OH)4: A Frustrated Two-Dimensional Quantum “Magnetic Raft” as a Possible Pathway to a Spin Liquid

We report a combined experimental and theoretical study of the hitherto unknown compound Cu3Te2O5(OH)4 that comprises an original network of ferromagnetic (FM; J1 = −100 K with strong next-nearest neighbor exchanges JNNN = 50 K) chains and alternating antiferromagnetic (AFM; J2 ∼ 148 K, and J2′ ∼ 125 K) chains arranged in a so-called S = 1/2 two-dimensional “magnetic raft” spin-lattice. The two one-dimensional spin sublattices are interconnected by weaker exchanges (Jd ∼ 40 K), which create tetrahedral Cu4 knots between the cross-linked “raft” legs, bringing about strong magnetic frustration. The magnetic susceptibility and specific heat show the absence of magnetic ordering down to 1.8 K hampered by fully frustrated Cu1 spins. High-field magnetization reveals a one-third magnetization plateau that is stable up to 33 T, which conveys the fingerprint of the individual AFM and FM chains. Magnetic entropy shows a two-stage Schottky-like release, implying the thermal decoupling of magnetic sublattices. Our work establishes that Cu3Te2O5(OH)4 can serve as a prominent platform for discovering sought-after quantum spin liquids in chemistry and physics. © 2023 American Chemical Society.11Nsciescopu