Long-range Ferrimagnetic Order Research Articles

Tilting and Distortion in the Multiferroic Aurivillius Phase Bi6Ti3Fe1.5Mn0.5O18.

Aurivillius structured Bi6Ti3Fe1.5Mn0.5O18 (B6TFMO) has emerged as a rare room temperature multiferroic, exhibiting reversible magnetoelectric switching of ferroelectric domains under cycled magnetic fields. This layered oxide presents exceptional avenues for advancing data storage technologies owing to its distinctive ferroelectric and ferrimagnetic characteristics. Despite its immense potential, a comprehensive understanding of the underlying mechanisms driving multiferroic behavior remains elusive. Herein, we employ atomic resolution electron microscopy to elucidate the interplay of octahedral tilting and atomic-level structural distortions within B6TFMO, associating these phenomena with functional properties. Fundamental electronic features at varying bonding environments within this complex system are scrutinized using electron energy loss spectroscopy (EELS), revealing that the electronic nature of the Ti4+ cations within perovskite BO6 octahedra is influenced by position within the Aurivillius structure. Layer-by-layer EELS analysis shows an ascending crystal field splitting (Δ) trend from outer to center perovskite layers, with an average increase in Δ of 0.13 ± 0.06 eV. Density functional theory calculations, supported by atomic resolution polarization vector mapping of B-site cations, underscore the correlation between the evolving nature of Ti4+ cations, the extent of tetragonal distortion and ferroelectric behavior. Integrated differential phase contrast imaging unveils the position of light oxygen atoms in B6TFMO for the first time, exposing an escalating degree of octahedral tilting toward the center layers, which competes with the magnitude of BO6 tetragonal distortion. The observed octahedral tilting, influenced by B-site cation arrangement, is deemed crucial for juxtaposing magnetic cations and establishing long-range ferrimagnetic order in multiferroic B6TFMO.

Magnetic suppression for a possible Fe-poor organic–inorganic hybrid superconductor Fe14Se16(tepa)0.8 (tepa = tetraethylenepentamine) with a superconducting transition at ∼42 K

Composition/structure-dependent superconductivity for FeSe-based superconductors attracted great attention not only due to their high superconducting transition temperatures (TC), but also for understanding the origin of iron-based superconductivity. Here, we report a new Fe-poor organic-inorganic hybrid material Fe14Se16(tepa)0.8 with a paramagnetic-diamagnetic transition at ∼42 K grown by a high-temperature organic-solution-phase method with soluble iron/selenium sources in a tepa solution, alternative to previous intercalation strategies. The Fe14Se16(tepa)0.8 phase is in a tetragonal layered hybrid structure with a nanoplate shape. Composition analyses reveal a Fe-poor characteristic of the hybrid in contrast to previous FeSe-intercalated superconductor, and selected area electron diffraction pattern is featured by Fe3Se4 superstructures with a √2 × √2 of Fe vacancy order. Ab initio density functional calculations show that minus Fe3Se4 ions are stable in the hybrid and ∼0.25e−/Fe0.75Se is obviously larger than the reported values of approximately 0.2e−/FeSe in other FeSe-intercalated superconductors. Typical hysteresis loops and temperature dependence of dc/ac susceptibilities of the Fe14Se16(tepa)0.8 measured below ∼42 K suggest a presence of the Meissner effect in this material. Effects of synthesis conditions on structures and magnetic properties of the hybrids show a magnetic evolution from a long-range ferrimagnetic (FIM) order of Fe14Se16(tepa) to a coexistence of FIM and superconducting (SC) orders of Fe14Se16(tepa)0.9 and an SC order of Fe14Se16(tepa)0.8. X-ray absorption spectrum (XAS) confirms the presence of ferric/ferrous irons. Mössbauer studies reveal that the high-TC superconductivity originates from a suppression of the FIM order through tuning the spin states of irons from high-spin Fe3+ (S = 5/2) and Fe2+ (S = 2) in the Fe14Se16(tepa) to low-spin Fe3+ (S = 1/2) and Fe2+ (S = 0) in the Fe14Se16(tepa)0.8. Although no zero resistance is detected even at a temperature of 2 K, the resistivity at 2 K decreases by more than 1600 times compared to that in a normal state calculated by a variable range hopping (VRH) model, suggesting that the high-TC superconductivity of Fe14Se16(tepa)0.8 is possible.

A possible coexistence of spontaneous magnetic order and thermal entanglement in a spin-1/2 Ising–Heisenberg diamond-decorated square lattice

The spin-1/2 Ising–Heisenberg model on a diamond-decorated square lattice is exactly solved by the decoration-iteration transformation, which establishes an exact mapping correspondence with the spin-1/2 Ising model on a square lattice with an effective temperature-dependent interaction between the nearest-neighbor spins. Exact results for the pair correlation function and spontaneous magnetization of the Heisenberg spins gained from the exact mapping equivalence allow a straightforward implementation of the concurrence, which may serve as a measure of the bipartite entanglement within decorating Heisenberg spin pairs. It is evidenced that the concurrence gradually diminishes upon increase of temperature above a disordered monomer-dimer ground state until it completely vanishes at a threshold temperature, while the concurrence is either always zero above a spontaneously ordered ferrimagnetic ground state or it shows a peculiar thermally assisted reentrance accompanied with a mutual coexistence of the spontaneous ferrimagnetic long-range order and the thermal entanglement.

Interisland-Distance-Mediated Growth of Centimeter-Scale Two-Dimensional Magnetic Fe3O4 Arrays with Unidirectional Domain Orientations.

Two-dimensional (2D) nanosheet arrays with unidirectional orientations are of great significance for synthesizing wafer-scale single crystals. Although great efforts have been devoted, the growth of atomically thin magnetic nanosheet arrays and single crystals is still unaddressed. Here we design an interisland-distance-mediated chemical vapor deposition strategy to synthesize centimeter-scale atomically thin Fe3O4 arrays with unidirectional orientations on mica. The unidirectional alignment of nearly all the Fe3O4 nanosheets is driven by a dual-coupling-guided growth mechanism. The Fe3O4/mica interlayer interaction induces two preferred antiparallel orientations, whereas the interisland interaction of Fe3O4 breaks the energy degeneracy of antiparallel orientations. The room-temperature long-range ferrimagnetic order and thickness-tunable magnetic domain evolution are uncovered in atomically thin Fe3O4. This strategy to tune the orientations of nanosheets through the an interisland interaction can guide the synthesis of other 2D transition-metal oxides, thereby laying a solid foundation for future spintronic device applications at the integration level.

Ferrimagnetic Ordering and Spin-Glass State in Diluted GdFeO3-Type Perovskites (Lu0.5Mn0.5)(Mn1-xTix)O3 with x = 0.25, 0.50, and 0.75.

ABO3 perovskite materials with small cations at the A site, especially those with ordered cation arrangements, have attracted a great deal of interest because they show unusual physical properties and deviations from the general characteristics of perovskites. In this work, perovskite solid solutions (Lu0.5Mn0.5)(Mn1-xTix)O3 with x = 0.25, 0.50, and 0.75 were synthesized by means of a high-pressure, high-temperature method at approximately 6 GPa and approximately 1550 K. All the samples crystallize in the GdFeO3-type perovskite structure (space group Pnma) and have random distributions of the small Lu3+ and Mn2+ cations at the A site and Mn4+/3+/2+ and Ti4+ cations at the B site, as determined by Rietveld analysis of high-quality synchrotron X-ray powder diffraction data. Lattice parameters are a = 5.4431 Å, b = 7.4358 Å, c = 5.1872 Å (for x = 0.25); a = 5.4872 Å, b = 7.4863 Å, c = 5.2027 Å (for x = 0.50); and a = 5.4772 Å, b = 7.6027 Å, c = 5.2340 Å (for x = 0.75). Despite a significant dilution of the A and B sublattices by non-magnetic Ti4+ cations, the x = 0.25 and 0.50 samples show long-range ferrimagnetic order below TC = 89 K and 36 K, respectively. Mn cations at both A and B sublattices are involved in the long-range magnetic order. The x = 0.75 sample shows a spin-glass transition at TSG = 6 K and a large frustration index of approximately 22. A temperature-independent dielectric constant was observed for x = 0.50 (approximately 32 between 5 and 150 K) and for x = 0.75 (approximately 50 between 5 and 250 K).

Room-Temperature Magnetoelectric Coupling in Atomically Thin ε-Fe2 O3.

2D multiferroics with magnetoelectric coupling combine the magnetic order and electric polarization in a single phase, providing a cornerstone for constructing high-density information storages and low-energy-consumption spintronic devices. The strong interactions between various order parameters are crucial for realizing such multifunctional applications, nevertheless, this criterion is rarely met in classical 2D materials at room-temperature. Here an ingenious space-confined chemical vapor deposition strategy is designed to synthesize atomically thin non-layered ε-Fe2 O3 single crystals and disclose the room-temperature long-range ferrimagnetic order. Interestingly, the strong ferroelectricity and its switching behavior are unambiguously discovered in atomically thin ε-Fe2 O3 , accompanied with an anomalous thickness-dependent coercive voltage. More significantly, the robust room-temperature magnetoelectric coupling is uncovered by controlling the magnetism with electric field and verifies the multiferroic feature of atomically thin ε-Fe2 O3 . This work not only represents a substantial leap in terms of the controllable synthesis of 2D multiferroics with robust magnetoelectric coupling, but also provides a crucial step toward the practical applications in low-energy-consumption electric-writing/magnetic-reading devices.

Dielectric and Spin-Glass Magnetic Properties of the A-Site Columnar-Ordered Quadruple Perovskite Sm2CuMn(MnTi3)O12

Perovskite-type ABO3 oxides show a number of cation-ordered structures, which have significant effects on their properties. The rock-salt-type order is dominant for B cations, and the layered order for A cations. In this work, we prepared a new perovskite-type oxide, Sm2CuMn(MnTi3)O12, with a rare columnar A-site order using a high-pressure, high-temperature method at about 6 GPa and about 1700 K. Its crystal structure was studied with synchrotron powder X-ray diffraction. The compound crystallizes in space group P42/nmc (No. 137) at room temperature with a = 7.53477 Å and c = 7.69788 Å. The magnetic properties of the compound were studied with dc and ac magnetic susceptibility measurements and specific heat. Spin-glass (SG) magnetic properties were found with TSG = 7 K, while specific heat, in the form of Cp/T, showed a strong, very broad anomaly developing below 20 K and peaking at 4 K. The dielectric constant of Sm2CuMn(MnTi3)O12 was nearly frequency and temperature independent between 8 K and 200 K, with a value of about 50. Cu2+ doping drastically modified the magnetic and dielectric properties of Sm2CuMn(MnTi3)O12 in comparison with the parent compound Sm2MnMn(MnTi3)O12, which showed a long-range ferrimagnetic order at 34-40 K. The antisite disorder of Cu2+ and Mn2+ cations between square-planar and octahedral sites was responsible for the SG magnetic properties of Sm2CuMn(MnTi3)O12.

Spin-glass to long-range order to spin-glass evolution of magnetic properties with the composition in Sm2MnZnMn4−xTixO12 (x = 1, 2, and 3) perovskites

Sm2MnZnMn4−xTixO12 perovskites with x = 1, 2, and 3 were synthesized by a high-pressure, high-temperature method at 6 GPa and about 1700 K. They belong to a subfamily of perovskite materials with a general composition of A2A′A′′B4O12, where the A′ site has a square-planar coordination and the A′′ site has a tetrahedral coordination. The x = 1 and 3 samples were investigated by powder synchrotron X-ray diffraction, and it was found that triple A-site cation ordering was realized in the x = 1 sample (having only Mn3+ cations) with Zn2+ cations located in the A′′ site and with a very small degree of antisite disorder between the A and A′ sites. An additional antisite disorder takes place in the x = 3 sample (having only Mn2+ cations) between the A′′ and B sites. The three compounds crystallize in space group P42/nmc (No. 137) at room temperature with a = 7.39810 Å and c = 7.92180 Å (x = 1), a = 7.50656 Å and c = 7.79545 Å (x = 2), and a = 7.61311 Å and c = 7.68145 Å (x = 3). Unusual evolution of magnetic properties was found as spin-glass-like properties were observed at TSG = 21 K in the x = 1 sample with the largest Mn content and at TSG = 6.5 K in the x = 3 sample with the smallest Mn content. On the other hand, a long-range ferrimagnetic order takes place in the x = 2 sample at TC = 18 K with an intermediate Mn content. Zn doping significantly modifies magnetic properties of Sm2MnZnMn4−xTixO12 in comparison with the ‘parent’ compounds Sm2MnMnMn4−xTixO12.

Chemical Vapor Transport Synthesis of Cu(VO)2(AsO4)2 With Two Distinct Spin-1/2 Magnetic Ions.

A new copper vanadyl arsenate, Cu(VO)2(AsO4)2, was synthesized via the chemical vapor transport method. Cu(VO)2(AsO4)2 adopts an original structure type. It is characterized by layers formed by edge-sharing and corner-sharing V-centered octahedra resulting in a unique topology that was hitherto not reported for vanadates. Single CuO6 octahedra connect vanadate layers into a rigid framework. The thermal expansion of the framework studied by the single-crystal HT X-ray diffraction is reported. The magnetic behavior of Cu(VO)2(AsO4)2 shows an interplay of ferromagnetic V4+-V4+ and antiferromagnetic Cu2+-V4+ interactions that result in a ferrimagnetic long-range order below TC = 66 K.

Ferrimagnetic behavior in a naphthalene templated manganese(II) 1,1-cyclohexanediacetate compound

A new manganese(II) 1,1-cyclohexanediacetate compound of formula [Mn13(chda)12(H2O)4(OH)2]n·n(nap) [H2chda = 1,1-cyclohexanediacetic acid, nap = naphthalene] has been synthesized by a hydrothermal reaction and characterized through single crystal X-ray synchrotron diffraction, exhibiting a 3D architecture, where the nap molecules are weakly anchored in the cavities of the [Mn13(chda)12(H2O)4(OH)2]n coordination polymer through steric interactions. Magnetic analysis reveals a long-range ferrimagnetic order below 2.4 K, most likely due to non-compensation of the magnetic moments. The non-compensation in spin moments arises, for topological reasons, from the network of exchange couplings associated to the binding modes (carboxylate and µ-oxo). This compound is one of the few examples of homometallic topological ferrimagnets built up from carboxylate ligands.

Triple A-Site Cation Ordering in the Ferrimagnetic Y2CuGaMn4O12 Perovskite.

A new member of A-site columnar-ordered A2A′A″B4O12 quadruple perovskites with the composition of Y2CuGaMn4O12 was prepared by a high-pressure, high-temperature method at 6 GPa and about 1500 K. Its crystal structure and cation distributions were studied by powder synchrotron X-ray and neutron diffraction. There is a triple A-site cation ordering with some degrees of anti-site disorder among sites occupied by 3d transition metals: [Y2]A[Cu0.8Mn0.2]A′[Ga0.8Mn0.2]A″[Mn3.6Cu0.2Ga0.2]BO12. It has the space group P42/nmc (no. 137) between 1.5 and 873 K with a = 7.33884 Å and c = 7.66251 Å at 297 K. Despite anti-site disorder, it exhibits a long-range ferrimagnetic order at TC = 115 K with the ordered moment of 2.19 μB at each B site and 0.89 μB at the A′ or A″ site. Magnetic moments are aligned along the c axis; all moments are ordered ferromagnetically at the B sites, and the moments at the A′ or A″ site are ordered in the opposite direction. Cu2+ doping drastically changes magnetic properties as “parent” Y2MnGaMn4O12 just shows spin-glass magnetic properties without long-range ordering. Anisotropic thermal expansion was observed in Y2CuGaMn4O12: the lattice parameter a almost linearly decreases from 1.5 K to TC and then monotonically increases up to 873 K (almost linearly from 300 K); the parameter c monotonically increases from 1.5 to 300 K and then decreases up to 600 K.

Spontaneous magnetic order and spin and charge entanglement of a coupled spin-electron model on a decorated square lattice composed from trigonal bipyramids

A coupled spin-electron model on a decorated square lattice formed by interconnected trigonal bipyramids is exactly solved by imposing two mobile electrons per each triangular plaquette with the help of a generalized decoration-iteration transformation, which establishes a precise mapping correspondence with the effective Ising model on a square lattice with temperature-dependent interaction. The investigated spin-electron model exhibits two different macroscopically degenerate ground states. The residual entropy of the first ground state, which shows a spontaneous ferromagnetic or ferrimagnetic long-range order depending on character of the exchange coupling, arises from chiral degrees of freedom of the mobile electrons. In contrast, the second ground state is disordered due to a kinetically driven frustration of the localized Ising spins triggered by the hopping term of the mobile electrons. The outstanding reentrant phase transitions connected with temperature induced formation of the spontaneous ferromagnetic or ferrimagnetic order can be found if the spin-electron model is driven sufficiently close to the ground-state phase boundary, but the disordered frustrated phase is the respective ground state. It is verified that the bipartite fermionic entanglement between two mobile electrons within the spontaneously ordered ferromagnetic or ferrimagnetic phase predominantly comes from their charge degrees of freedom, while the one within the disordered frustrated phase comes from both charge and spin degrees of freedom of these particles.

Quantum Annealing Simulation of Out-of-Equilibrium Magnetization in a Spin-Chain Compound

Geometrically frustrated spin-chain compounds such as Ca3Co2O6 exhibit extremely slow relaxation under a changing magnetic field. Consequently, both low-temperature laboratory experiments and Monte Carlo simulations have shown peculiar out-of-equilibrium magnetization curves, which arise from trapping in metastable configurations. In this work we simulate this phenomenon in a superconducting quantum annealing processor, allowing us to probe the impact of quantum fluctuations on both equilibrium and dynamics of the system. Increasing the quantum fluctuations with a transverse field reduces the impact of metastable traps in out-of-equilibrium samples, and aids the development of three-sublattice ferrimagnetic (up-up-down) long-range order. At equilibrium we identify a finite-temperature shoulder in the 1/3-to-saturated phase transition, promoted by quantum fluctuations but with entropic origin. This work demonstrates the viability of dynamical as well as equilibrium studies of frustrated magnetism using large-scale programmable quantum systems, and is therefore an important step toward programmable simulation of dynamics in materials using quantum hardware.

An investigation of reentrant spin-glass behavior, magnetocaloric effect and critical behavior of MnCr2O4

The spin-glass (SG) behavior, magnetocaloric effect, and critical behavior of MnCr2O4 were investigated using dc/ac magnetic measurements. The strong magnetic anisotropy is observed from the dc magnetizations. The peaks of ac susceptibility display dependence on the frequency and magnetic field. Meanwhile, magnetic relaxation measurements show a slow decay of the remanent magnetization with time. These findings consistently demonstrate the coexistence of the SG component and long-range ferrimagnetic (FIM) order in MnCr2O4. The SG at low temperature is not conventional but has a reentrant character. Importantly, the reentrant SG (RSG) behavior in MnCr2O4 originates from the freezing and fluctuation of the short-range spiral order at low temperatures. On the other hand, the magnetocaloric effect (MCE) in spinel compounds MnCr2O4 was reported in this study. According to the isotherms measurements, the magnetic entropy change |ΔSM|=6.1 J kg−1 K−1 was calculated under applied field H = 50 kOe. A comprehensive study of the critical behavior of MnCr2O4 near the paramagnetic (PM) to FIM transition, might be helpful in understanding the interesting magnetic interaction of MnCr2O4. Besides, the critical exponents of MnCr2O4 was obtained [β = 0.488 and γ = 0.918 for MAP, β = 0.473 and γ = 0.909 for KF] by the different theoretical models. Interestingly, it is found that the critical phenomena of MnCr2O4 are close to the mean-field model.

Magnetic properties of alternating Hubbard ladders

We investigate the Hubbard Hamiltonian on ladders where the number of sites per rung alternates between two and three. These geometries are bipartite, with a non-equal number of sites on the two sublattices. Thus they share a key feature of the Hubbard model in a class of lattices which Lieb has shown analytically to exhibit long-range ferrimagnetic order, while being amenable to powerful numeric approaches developed for quasi-one-dimensional geometries. The Density Matrix Renormalization Group (DMRG) method is used to obtain the ground state properties, e.g. excitation gaps, charge and spin densities as well as their correlation functions at half-filling. We show the existence of long-range ferrimagnetic order in the one-dimensional ladder geometries. Our work provides detailed quantitative results which complement the general theorem of Lieb for generalized bipartite lattices. It also addresses the issue of how the alternation between quasi-long range order and spin liquid behavior for uniform ladders with odd and even numbers of legs might be affected by a regular alternation pattern.

A Rare 2D Framework Cu(atz)2 with Ferrimagnetic and High Proton Conductivity.

Materials combining proton conductivity and magnetism have attracted great attention in recent years due to their intriguing application in sensors and fuel cells. Herein a two-dimensional metal-organic framework, [Cu(atz)2 (H2 O)2 ]⋅H2 O (1) (Hatz=5-aminotetrazole), has been obtained in a green synthesis method. The single-crystal structure revealed that the atz- ligands as linkers coordinate with copper ions to sql networks, between which water molecules are immobilized through hydrogen bonds. The resulting complex 1 exhibits a high proton conductivity of 1.11×10-4 and 6.19×10-4 S cm-1 at room temperature and 333 K, respectively, under 98% RH with an activation energy of 0.56 eV. Upon dehydration, the proton conductivity of 1_dg drops by an order of magnitude. Furthermore, the magnetic behavior changes from long-range ferrimagnetic ordering of 1 to canted antiferromagnetic behaviour of 1_dg.

Exchange interactions and the nature of magnetic ordering in Zn0.6Mn2.4O4 particles

Reported here is the nature of magnetism of polycrystalline Zn0.6Mn2.4O4 (particle size ~ 125 nm) with non-magnetic Zn2+ ions occuping the tetrahedral sites as revealed by Rietveld refinement of the structural parameters. Analysis of magnetization vs.temperature (4 K–350 K) data showed long-range ferrimagnetic order below TC = 36 K and its analysis for T > TC using the Lotgering model modified by Srinivasan and Seehra (Phys. Rev. B 28, 1, 1983) yielded the exchange constants JBB = 13.4 K, JAB = 5.0 K and JAA = 3.8 K, which are weaker by about 30–50% than those reported in Mn3O4. The other effect of magnetic dilution is the observation of spin-glass state below TSG = 5.8 K characterized by its H2/3 decrease with increase in H. Thus Zn0.6Mn2.4O4 is a ferrimagnet with triangular Yafet-Kittel arrangement of spins having a reentrant spin-glass state with TSG < TC.

Reentrant spin-glass behaviour in highly frustrated Mn-rich spinel zinc manganate

The present work offers an insight into the magnetic properties of Mn-rich spinel zinc manganate. Rietveld refinement reveals the formation of Zn0.67Mn2.33O4, where Zn2+ and Mn2+ ions are randomly distributed in the tetrahedral sublattice. DC and AC susceptibility measurement of Zn0.67Mn2.33O4 infers the occurrence of two kinds of transition below 11 K. Paramagnetic to ferrimagnetic transition occur at 10.7 K and ferrimagnetic to spin glass-like transition occurs at 5.8 K. The long-range canted ferrimagnetic ordering is corroborated using modified Lotgering model and calculated the exchange interaction values (JAA = 5.2 K,JAB = 5.3 K, JBB = 14.8 K). Further, the observed shift in freezing temperature with DC magnetic field obeys Almeida–Thouless behaviour and frequency dependence of AC susceptibility follows Vogel–Fulcher law. However, the complete establishment of the canonical spin glass state is denied since the manganese ions occupied at the tetrahedral site is equal to the percolation threshold (=33%). Subsequently, the observed spin glass-like behaviour (5.8 K) below Curie temperature (10.7 K) evidences the reentrant spin glass nature. Similarly, the strong frustration in Zn0.67Mn2.33O4 lattice is observed through the substantial negative value of Curie–Weiss temperature (∼−599 K) and very high frustration factor (f = 56). Overall, the chosen Zn0.67Mn2.33O4 is a highly frustrated magnetic system revealing re-entrant spin-glass behaviour.

Jahn-Teller and geometric frustration effects on the structural and magnetic ground states of substituted spinels (Ni,A)Cr2O4 (A = Mn/Cu)

Comparative study of the effects of substitution (25%) of Jahn-Teller (JT) active Cu2+ and non JT active Mn2+ cations, at the Ni2+ site, on the geometric frustration (GF) and cooperative JT distortion (CJTD) in multiferroic NiCr2O4 has been carried out with the help of low-temperature magnetization, x-ray diffraction and time of flight (TOF) neutron powder diffraction (NPD) measurements. On Mn2+ substitution the CJTD, which causes elongation of the NiO4 tetrahedra in I41/amd phase of NiCr2O4, gets suppressed at room-temperature (RT) and stabilizes the Fd-3m phase in (Ni0.75Mn0.25)Cr2O4. On the contrary the Cu2+ substitution inverts the effect of CJTD and the tetrahedra get flattened at RT, which in turn oppositely distorts the I41/amd lattice in Ni0.75Cu0.25Cr2O4. The frustration index decreases on substitution. Intriguingly, the tetragonal to orthorhombic phase transition and the antiferromagnetic ordering, which are clearly evident in the pristine NiCr2O4 and the Cu2+ substituted sample respectively at 30 K and 70 K, have not been observed for the Mn2+ substituted sample. This has been attributed to the change in Cr3+- Cr3+ direct magnetic-interaction on structural transition. However, the long-range ferrimagnetic order remains intact for all the three compositions i.e. NiCr2O4, Ni0.75Mn0.25Cr2O4 and Ni0.75Cu0.25Cr2O4 occurring at 75 K, 70 K and 95 K, respectively. The Mn2+ substituted compound shows short-range magnetic correlation at low-temperatures, as evident from the diffuse and broad hump in the intensity-vs-d spacing plot of TOF NPD data. This short-range magnetic order has been attributed to the known spiral magnetic order.

Dynamic Susceptibility of Thiochromite FeCr2S4

The magnetic susceptibility of the ferrimagnet FeCr2S4 was studied by a dynamic method in order to highlight the nature of a magnetic transition in the region of T = 60 K. The properties of the compound were measured in zeroth DC magnetic field НDC = 0 in the temperature range 4–230 K at the modulation amplitudes НАC = 1 and 17 Oe and the AC frequencies ν = 10, 100, 1000, and 10 000 Hz. At T > 60 K, the properties of FeCr2S4 correspond to the long-range ferrimagnetic order with local spin disorder. Spin lattice distortions (microareas or magnetic clusters) strongly increase in the region of 150 K, where a local spin glass appears with the spin freezing temperature Tf = 155 K. The flat maximum of the impurity spin glass in FeCr2S4 at T = 155 K can be considered as a mere assembly or conglomerate of especially large magnetic clusters with the effective Curie temperature T = 155 K.