The heat capacity of the layer compound tetrachlorobis (methylammonium) manganese—II (CH3NH3)2MnCl4, from 10 to 300k

The heat capacity of the layer compound, tetrachlorobis (methylammonium) manganese II, (CH₃NH₃)₂MnCl₄, has been measured over the range 10K <T<300K. In this region, two structural phase transitions have been observed previously by other techniques: one transition is from a monoclinic low temperature (MLT) phase to a tetragonal low temperature (TLT) phase, and the other is from TLT to an orthorhombic room temperature (ORT) phase. The present experiments have shown that the lower transition (MLT→TLT) occurs at T = 94.37±0.05K with ΔH_t = 727±5 J mol^?1 and ΔS_t = 7.76±0.05 J K^?1 mol^?1, and the upper transition (TLT→ORT) takes place at T = 257.02±0.07K with ΔH_t = 116±1J mol^?1 and ΔS_t = 0.451±0.004 J K^?1mol^?1. These results are discussed in the light of recent measurements on (CH₃NH₃)₂CdCl₄, and also with regard to a recent theoretical model of the structural phase transitions in compounds of this type.In addition to the structural phase transitions, (CH₃NH₃)₂MnCl₄ also undergoes magnetic ordering at T < 150K. The magnetic component to the heat capacity, as deduced from a corresponding states comparison of the heat capacity of the present compound with that of the Cd compound, is shown to be consistent with the behaviour expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Antiferromagnetic order and spin glass behavior in Dy2CuIn3

The magnetic properties of the intermetallic compound Dy₂CuIn₃ have been investigated. Ac and dc-susceptibility measurements indicate an onset of antiferromagnetic ordering at T_N=19.5 K and an additional frequency dependent transition at T_ds∼9 K. Neutron diffraction studies confirm the ordered transition at 19.5±1 K. The magnetic unit cell can be described by the propagation vector k=(0.25,0.25,0) with the magnetic moment μ=2.63(4)μ_B/Dy³⁺ parallel to the c-axis. Nevertheless, neutron diffraction reveals no additional magnetic phase transition around or below 9 K, which suggests that, at lower temperatures, a spin glass state may be formed in coexistence with the antiferromagnetic mode as a result of frustration and the antagonism between ferromagnetic and antiferromagnetic exchange interactions.     

Etude d'un nouveau type de fluorure antiferromagnetique a caractere bidimensionnel: NaFeF4

A new type of two-dimensional antiferromagnetic structure has been investigated by magnetic susceptibility measurements, neutron diffraction and Mössbauer resonance. The magnetic cell of NaFeF₄ is doubled along the a axis and the spins lie along the b axis. The κ^-1 vs T curve shows a sharp minimum at about 105 K close to the three-dimensional transition temperature determined by Mössbauer spectrometry (111.5 K). A calculation of z. snfc;J/kz. snfc; has been performed using a high temperature series expansions technique (J/k = -23 K). The variation of the hyperfine field in the range 0.6 ?T/T_N? 1 gave the value of the critical exponant β = 0.25.     

On the possible coexistence of spiral and collinear structures in antiferromagnetic KFe(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The static and resonance properties of a quasi-two-dimensional antiferromagnet phase on a distorted triangular lattice of KFe(MoO₄)₂ have been experimentally studied. Magnetization curves exhibit features corresponding to the spin-flop transition in a collinear biaxial antiferromagnet and simultaneously show a magnetization plateau characteristic of a triangular spin structure. The magnetic resonance spectra also display absorption lines corresponding to the spin structures of both types. The experimental data are described in terms of a model comprising alternating weakly bound magnetic layers, in which the main two exchange integrals have different values. Below the Néel temperature (T _N =2.5 K), some of these layers possess a collinear antiferromagnetic structure, while the other layers have a triangular or spiral structure.     

New phase transition in (CnH2n+1NH3)2MnCl4

(C_nH_2n+1NH₃)₂MnCl₄ consist of perovskite-type layers sandwiched between hydrocarbon radical layers. New phase transitions were found between liquid nitrogen and decomposition temperature in these compounds by differential thermal analysis and hot stage microscopy. A systematic research of the lattice dynamics, the transition mechanisms, their influence on physical properties and their dependence on the various possibilities of molecular engineering offered in these quasi two-dimensional perovskites was started.     

Magnetic anisotropy of the organic conductors (TMTTF)2X

The anisotropy in the static magnetic susceptibility of some tetramethyltetrathiafulvalene salts (TMTTF)₂X (where X^- = PF^-₆, ClO^-₄, SCN^- and Br^-) has been investigated with a torque method between 3 K and 300 K for magnetic fields up to 11 kG. The experimental results show that the salts with X^- = SCN^- and Br^- become antiferromagnetic at about 8 and 13 K respectively, whereas for the two other salts above 1 kG no field dependence of the magnetic susceptibility is observed down to 3 K.     

Structural and magnetic ordering in the two-dimensional coordination polymer Co(ox)(bpy-d8), (ox=C2O4, bpy-d8=4,4′-bipyridine-d8)

Neutron diffraction and magnetic susceptibility studies of the two-dimensional coordination polymer Co(ox)(bpy-d8) are presented, where ox=C₂O₄²⁻ and bpy-d8=4,4′-bipyridine-d8 (fully deuterated). The neutron powder diffraction data reveal a second-order crystallographic phase transition at 290 K. Above 290 K, a disordered structure, space group Immm, is observed that is closely related to the ordered structure previously proposed on the basis of single crystal X-ray diffraction. At low temperatures, the structure is an ordered variant of the high-temperature phase with space group I222. In both phases, the Co ions are linked by the oxalate forming infinite chains that are crosslinked by the bpy ligands.The magnetic susceptibility follows qualitatively a quasi one-dimensional chain behavior. It exhibits a broad maximum around 35 K, corresponding to a strong antiferromagnetic coupling through the oxalate bridges. A kink at 9 K marks the onset of long-range antiferromagnetic ordering due to much weaker interchain magnetic interactions.The magnetically ordered structure determined from the low-temperature neutron diffraction data can be described with the propagation vector (1/2, 1/2, 1/2), i.e. a doubling of the unit cell in each principal direction. It is concluded that a significant antiferromagnetic interaction is mediated through the bpy ligands, although the Co-Co distance along these bridges is 11.4 Å.     

The heat capacity of the layer compounds (CH3CH2CH2NH3)2MnCl4 and (CH3CH2CH2NH3)2CdCl4 from 10 K TO 300 K

The heat capacity of the layer compounds tetrachlorobis (n-propylammonium) manganese II and tetrachlorobis (n-propylammonium) cadmium II, (CH₃CH₂CH₂NH₃)₂MnCl₄ and (CH₃CH₂CH₂NH₃)₂CdCl₄ respectively, has been measured over the temperature range 10 K ?T ? 300 K.Two known structural phase transitions were observed for the Mn compound in this temperature region: at T = 112.8 ± 0.1 K (ΔH_t= 586 ± 2 J mol^?1; ΔS_t = 5.47 ± 0.02 J K^?1mol^?1) and at T =164.3 ± (ΔH_t = 496 ± 7 J mol^?1; ΔS_t =3.29 ± 0.05 J K^?1mol^?1). The lower transition is known to be from a monoclinic structure to a tetragonal structure, while the upper is from the tetragonal phase to an orthorhombic one. From comparison with the results for the corresponding methyl Mn compound it is deduced that the lower transition primarily involves changes in H-bonding while the upper transition involves motion in the propyl chain.A new structural phase transition was observed in the Cd compound at T= 105.5 ± 0.1 K (ΔH_t= 1472.3 ± 0.1 J mol^?1; ΔS_t = 13.956 ± 0.001 J K^?1mol^?1), in addition to two transitions that have been observed previously by other techniques. The higher of these transitions(T = 178.7 ± 0.3 K; ΔH_t = 982 ± 4 J mol^?1 ΔS_t = 6.16 ± 0.02 J K^? mol^?1) is known to be between two orthorhombic structures, while the structural changes at the lower transition (T= 156.8 ± 0.2 K; ΔH_t = 598 ± 5 J mol^?1, ΔS_t = 3.85 ± 0.03 J K^?1 mol^?1) and at the new transition are not known. It is proposed that these two transitions correspond respectively to the tetragonal to orthorhombic and monoclinic to tetragonal transitions in the propyl Mn compounds.In addition to the structural phase transitions (CH₃CH₂CH₂NH₃)₂MnCl₄ magnetically orders at t? 130 K. The magnetic contribution to the heat capacity is deduced from the heat capacity of the corresponding diamagnetic Cd compound and is of the form expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Magnetic resonance study of the two-dimensional spin diffusion in the Heisenberg paramaget (C18H37NH3)2MnCl4

¹H nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) techniques were employed to study the perovskite-type layered structure compound (C₁₈H₃₇NH₃)₂MnCl₄ undergoing structural phase transitions. The spin relaxation was found to sensitively reflect the two-dimensional electron spin diffusion.     

Magnetic transitions in K2MnCl4·2H2O

Results of (dM/dH) measurements on tetrahedral K₂MnCl₄·2H₂O as a function of temperature and magnetic field, are presented. An antiferromagnetic transition along the tetragonal axis is observed at T_N = (3.05±0.05) K. The H-T magnetic phase diagram was completely determined, and shows the usual characteristics of that of a low anisotropy antiferromagnet. The T = 0 critical fields are compatible with the values H_E = (29.2±0.3) kOe and H_A = (5.9±0.6) kOe for the exchange and anisotropy fields.     

High-pressure magnetic properties and <Emphasis Type="Italic">P-T</Emphasis> phase diagram of iron borate

The high-pressure magnetic states of iron borate ⁵⁷FeBO₃ single-crystal and powder samples have been investigated in diamond anvil cells by nuclear forward scattering (NFS) of synchrotron radiation at different temperatures. In the low-pressure (0 < P < 46 GPa) antiferromagnetic phase, an increase of the Neél temperature from 350 to 595 K induced by pressure was found. At pressures 46–49 GPa, a transition from the antiferromagnetic to a new magnetic state with a weak magnetic moment (magnetic collapse) was discovered. It is attributed to the electronic transition in Fe³⁺ ions from the high-spin 3d⁵ (S = 5/2, ⁶A_1g) to the low-spin (S = 1/2, ²T_2g) state (spin crossover) due to the insulator-semiconductor-type transition with extensive suppression of strong d-d electron correlations. At low temperatures, NFS spectra of the high-pressure phase indicate magnetic correlations in the low-spin system with a magnetic ordering temperature of about 50 K. A tentative magnetic P-T phase diagram of FeBO₃ is proposed. An important feature of this diagram is the presence of two triple points where magnetic and paramagnetic phases of the high-spin and low-spin states coexist.     

Magnetic phase transition in TbMn2Ge2

The crystal and magnetic stucture of TbMn₂Ge₂ are determined by neutron diffraction using a powder sample. The crystal structure of this compound is of the ThCr₂Si₂ type with small mixing of Mn and Ge atoms between 4(d) and 4(e) positions. At RT the antiferromagnetic collinear structure consist of a+?+? sequence of ferromagnetic layers of Mn atoms with the magnetic moment parallel to the c-axis. At 85 K, the ferromagnetic ordering within the Tb sublattice is observed. The magnetic moment (～7.7 μ_B) is parallel to the c-axis. At 4.2 K additional reflections are observed, which correspond to antiferromagnetic components in a monoclinic unit cell.     

Understanding the magnetic ground state of rare-earth intermetallic compound Ce4Sb3: Magnetization and neutron diffraction studies

Magnetization and neutron diffraction studies have been performed on Ce₄Sb₃ compound (cubic Th₃P₄-type, space group I4¯3d, no. 220). Magnetization of Ce₄Sb₃ reveals a ferromagnetic transition at ∼5 K, the temperature below which the zero-field-cooled and field-cooled magnetization bifurcate in low applied fields. However, a saturation magnetization (M_S) value of only ∼0.93μ_B/Ce³⁺ is observed at 1.8 K, suggesting possible presence of crystal field effects and a paramagnetic/antiferromagnetic Ce³⁺ moment. Magnetocaloric effect in this compound has been computed using the magnetization vs. field data obtained in the vicinity of the magnetic transition, and a maximum magnetic entropy change, −ΔS_M, of ∼8.9 J/kg/K is obtained at 5 K for a field change of 5 T. Inverse magnetocaloric effect occurs at ∼2 K in 5 T indicating the presence of antiferromagnetic component. This has been further confirmed by the neutron diffraction study that evidences commensurate antiferromagnetic ordering at 2 K in zero magnetic field. A magnetic moment of ∼1.24μ_B/Ce³⁺ is obtained at 2 K and the magnetic moments are directed along Z-axis.     

Spin-glass behavior and magnetic splitting in molecular magnetic materials {[N(n-C4H9)4][MIIFeIII(C2O4)3]}n (M = Co, Mn)

The mixed valence character and the antiferromagnetic coupling in the molecular magnetic materials {[N(n-C₄H₉)₄][M^IIFe^III(C₂O₄)₃]}_n (M = Co, Mn) were investigated by Mössbauer spectroscopy and magnetic measurements. In the material {[N(n-C₄H₉)₄][CoFe(C₂O₄)₃]}_n, the appearence of the spin-glass phase transition temperature and the magnetic phase transition under zero-field-cooled AC magnetic measurements were studied in the temperature range 5–100 K. In the Mössbauer spectra of {[N(n-C₄H₉)₄][MnFe(C₂O₄)₃]} a magnetic splitting was observed below 30 K. The appearance of the magnetic splitting indicates the occurrence of magnetic ordering in this complex. The hyperfine parameters show that the electronic state of iron is high-spin Fe³⁺.     

Magnetic structure of a CaMnO2.75 crystal with ordered oxygen vacancies

The magnetic state of a CaMnO_{3 ? δ} crystal with ordered oxygen vacancies (for δ = 0.25, when the numbers of Mn⁴⁺ and Mn³⁺ ions in the manganite are equal to each other) is studied using neutron diffraction. Magnetic scattering in the CaMnO_2.75 crystal in the ground state is determined by the wave vector (1/2, 1/2, 1/2)2π/a _c (G-type antiferromagnetic order). In the crystal, long-range magnetic order disappears at the temperature T _N = 116 K, whereas short-range magnetic order is retained up to 240 K. It is shown that the instability of the G-type structure in the temperature range 60 K < T < T _N is associated, in many respects, with the formation of the C′ antiferromagnetic phase in the bulk of the crystal. The structure of the C′ antiferromagnetic phase involves chains with Mn³⁺-Mn⁴⁺ ferromagnetic interaction. A comparison of the results of the neutron diffraction investigations with the experimental data on the magnetic characteristics and electrical resistivity demonstrates that the specific features revealed in the spin system of the CaMnO_2.75 crystal are governed directly by the competition of the Mn³⁺-Mn⁴⁺ ferromagnetic double exchange with the antiferromagnetic superexchange between manganese ions.     

Metamagnetic effects in epitaxial BaFe<Subscript>1.8</Subscript>Cr<Subscript>0.2</Subscript>As<Subscript>2</Subscript> thin films

Epitaxial BaFe_1.8Cr_0.2As₂ thin films with the tetragonal c-axis perpendicular to the thin film surface were grown on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSAT) single crystalline substrates using pulsed laser deposition (PLD). Resistive measurements indicate the existence of two transitions at temperatures of about 80 K and 40 K. The transition at 80 K is attributed to the structural transition from the high temperature tetragonal phase to the low temperature orthorhombic phase accompanied with the magnetic transition from a paramagnetic to an antiferromagnetic state as known for doped bulk systems. Below T ≈ 40 K the magnetization curves measured perpendicularly to the orthorhombic c-axis in fields up to 9 Tesla show two inflexion points indicating metamagnetic transitions.     

Low temperature differential magnetization of Ni(ClO4)26NH3 and Ni(BF4)26NH3

The differential magnetization of Ni(ClO₄)₂6NH₃ and Ni(BF₄)₂6NH₃ was measured as a function of temperature (20 to 0.3 K) and magnetic field (up to 40kOe). An antiferromagnetic transition was found at T_N = 0.45 K for the Ni(ClO₄)₂6NH₃ and T_N = 0.43 K for the Ni(BF₄)₂6NH₃, and a portion of the magnetic phase diagram was determined. The interpretation of the data in terms of a uniaxial model yielded (D/k) ～ 0.2 K for both salts.     

Thickness dependence of antiferromagnetic phase transition in Heisenberg-type MnPS3

The behavior of 2-dimensional (2D) van der Waals (vdW) layered magnetic materials in the 2D limit of the few-layer thickness is an important fundamental issue for the understanding of the magnetic ordering in lower dimensions. The antiferromagnetic transition temperature T_N of the Heisenberg-type 2D magnetic vdW material MnPS₃ was estimated as a function of the number of layers. The antiferromagnetic transition was identified by temperature-dependent Raman spectroscopy, from the broadening of a phonon peak at 155 cm⁻¹, accompanied by an abrupt redshift and an increase of its spectral weight. T_N is found to decrease only slightly from ~78 K for bulk to ~66 K for 3L. The small reduction of T_N in thin MnPS₃ approaching the 2D limit implies that the interlayer vdW interaction is playing an important role in stabilizing magnetic ordering in layered magnetic materials.     

Magnetic phase coexistence in Tb- and Sr-doped La0.7Ca0.3MnO3 manganite:A temperature-dependent neutron diffraction study

We report the results of the temperature-dependent neutron diffraction measurements on the nearly half-doped (La_0.325Tb_0.125)(Ca_0.3Sr_0.25)MnO₃ manganite sample. The simultaneous doping of magnetic Tb³⁺ and divalent Sr²⁺ in the La_0.7Ca_0.3MnO₃ system results into a large A-site size disorder. Rietveld refinement of neutron diffraction data reveal that the single phase sample crystallizes in a distorted orthorhombic structure. Increased 〈r_A〉 value affects the transport behavior that results into an insulating-like behavior of the sample. Under application of 1 T field sample exhibit insulating-like behavior while insulator-metal transition (T_IM) is exhibited under 5 and 8 T fields. Variable range hoping (VRH) mechanism of charge carriers is exhibited in the insulating region. Field cooled and zero field cooled magnetization measurement shows the Curie temperature (T_C)~47 K. The refinement of the ND data collected at various temperatures below 300 K shows that there is no structural phase transition in the compound. Around 100 K, a magnetic peak appears at lower angle that can be ascribed to the presence of the A-type antiferromagnetic (AFM) phase. Two more peaks are observed around 50 K at lower angles that can be fitted in CE-type antiferromagnetic phase. Splitting of the peaks at lower temperatures is the signature of orbital ordering in the presently studied nearly half-doped manganite system. Results of the detailed structural analysis of the temperature-dependent ND measurements on (LaTb)_0.45(CaSr)_0.55MnO₃ sample has been discussed in the light of coexisting A-type and CE-type antiferromagnetic phases present in the sample at low temperature.     

Triangular antiferromagnetic order in the honeycomb layer lattice of

ErCl₃ crystallizes in the AlCl₃-type layer structure. The crystal structure was refined in the paramagnetic state by powder neutron diffraction. The monoclinic lattice parameters at 1.5 K are a = 6.8040(3)?, b = 11.7456(5)?, c = 6.3187(3)? and . The space group is C2/m. Short-range, predominantly in-plane, magnetic ordering occurs above 350 mK up to several Kelvin. Below mK a three-dimensional antiferromagnetic order with a propagation vector of sets in. The magnetic structure of ErCl₃ was determined by powder and single-crystal neutron diffraction at temperatures down to 45 mK. The Er³⁺ ions are located on two-dimensional honeycomb layers in the a–b plane. There are two antiferromagnetically coupled triangular sublattices which form right- and left-handed helices along the c-axis. The magnetic moments are oriented in the a–b plane and amount to 3.3(1) at saturation. From the temperature dependence of the integrated neutron magnetic peak intensity a critical exponent (2) was derived for the magnetic phase transition. Received 1 December 1999 and Received in final form 21 July 2000