Thermal and magnetic studies of the nearly one-dimensional antiferromagnetic system CsNiBr3

Magnetic susceptibility, specific heat and ¹³³Cs magnetic resonance measurements in a single crystal of CsNiBr₃ are reported. The data reveal two magnetic transitions separating the paramagnetic phase from the antiferromagnetic ground state. At the higher transition temperature T_N2 = (14.25 ± 0.05)K a net magnetic moment is observed only along the hexagonal c-axis, while only below the lower transition temperature T_N1 = (11.75 ± 0.05)K a perpendicular component of the magnetic moment appears also. Above T_N2 CsNiBr₃ can be described as a one-dimensional antiferromagnet with intrachain exchange interaction $\text{J}\text{k}{}_{\mathrm{B}}\text{= ?(17.0 ± 0.2)}\text{K}$ and single-ion anisotropy constant $\text{D}\text{k}{}_{\mathrm{B}}\text{? ?1.5}\text{K}$. Below T_N1, the data are consistent with the non-colinear triangular structure of the Ni²⁺ moments proposed previously for the isomorphic crystal CsNiCl₃. A reduced value of the zero-temperature susceptibility over the classical value is found and atrributed to the zero point deviations.     

Magnetic study 2D-antiferromagnets Ba2NiF6 and Ba2FeF6 by neutron diffraction and mossbauer spectroscopy

The 2D-antiferromagnetism of tetragonal Ba₂NiF₆ and Ba₂FeF6 — whose structure derives from that of K₂NiF₄ — has been studied by magnetization measurements and by powder neutron diffraction. The magnetic cell is 2a, 2$\text{a}$,$\text{c}$ and the magnetic moments (μ_Ni²⁺ = 1.9μB and μ_Fe²⁺ = 3.46μB) lie along c. This has been confirmed by Mossbauer spectroscopy for Ba₂FeF₆.     

Critical behaviour of a uniaxial ferromagnet,ErCl3·6H2O with predominant dipolar interactions

The parallel magnetic susceptibility χ of a uniaxial ferromagnet ErCl₃·6H₂O has been measured between 0.3 and 4.2K and specially near T_c = 0.353 K. The predominant contribution to the Curie-Weiss temperature is due to the dipolar interactions. χ is proportional to ?^?γ with $\text{? =}\text{T}\text{T}{}_{\mathrm{c}}\text{?1}$ in the range 10^?3 < ? < 5 × 10^?2. The γ value, γ = 1.01 ±0.03 is consistent with the theoretical prediction for a uniaxial dipolar ferromagnet.     

Specific heat of (C6H11NH3) CuCl3 (CHAC), a system of ferromagnetic chains

The heat capacity of (C₆H₁₁NH₃) CuCl₃ (CHAC) has been measured for 0.45 < T < 60 K. Three-dimensional ordering is observed at T = 2.214 K. The data in the paramagnetic region can be described by a ferromagnetic $\text{S =}\text{1}\text{2}$ Heisenberg linear chain model system with J/k = +45 ± 5K.     

Etude de la structure magnetique et de la transition de typr “spin-flop” de Rb2FeF5

The nature of the magnetic interactions in the chain compound Rb₂FeF₅ has been investigated using neutron diffraction and magnetic measurements under high applied fields. Rb₂FeF₅ orders antiferromagnetically at T_N = 8.0 ± 0.5 K; the magnetic structure is of the A_Z + G_X mode and the moment of the Fe³⁺ ion extrapoled to 0K is 3.5 ± 0.2 μ_B, this low value being due to zero-point spin reduction. Within a chain the Fe³⁺ ions are antiferromagnetically coupled with an exchange constant of J/k = ?8.8 K. A spin-flop behavior has been observed and interpreted on the basis of the molecular field theory. The critical field was found to be H_C = 65 kOe at 1.7 K.     

Mössbauer study of the modified pyrochlores CsMFeF6 (M = Mn,Ni)

⁵⁷Fe Mössbauer spectra of the modified pyrochlores CsMFeF₆ (M = Mn, Ni) were obtained between 1.5 K and room temperature. The spectra for single crystal and powder absorbers are very similar, and demonstrate the randomness associated with the iron sites. Nevertheless, the presence of two distinct iron sites, recently reported by Varret and Courbion for powder samples, is confirmed. Both CsMnFeF₆ and CsNiFeF₆ are antiferromagnets with Néel temperatures, T_N, of 25 and 5.2 K, respectively. The hyperfine fields lie on the Brillouin function with $\text{S =}\text{5}\text{2}$. The hyperfine parameters show that the iron ions are ferric in the high-spin state.     

A Mössbauer study of amorphous Fe40Ni40B20

Amorphous Fe₄₀Ni₄₀B₂₀ (VITROVAC 0040) alloy has been investigated using ⁵⁷Fe Mössbauer Spectroscopy. The Curie temperature T_c is found to be well defined and is 695 ± 1 K. The quadrupole splitting just above T_c is 0.64 mm sec^?1. The crystallization temperature is 698 ± 2 K, close to but definitely above T_c. The average hyperfine field H_eff(T) of the glassy state shows a temperature dependence of $\text{H}{}_{\mathrm{<mtext>eff</mtext>}}\text{(0)[1 ? B}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{(T/T}{}_{\mathrm{c}}\text{)}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{? C}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{(T/T}{}_{\mathrm{c}}\text{)}{}^{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{? …]}$ indicative of the existence of spin wave excitations. The values of $\text{B}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{C}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ are found to be 0.40 and 0.06, respectively, for T/T_c ? 0.72. At temperatures close to T_c, H_eff(T) varies as (1 ? T/T_c)^β where β is one of the critical exponents and its value is found to be 0.29 ± 0.02.     

Magnetic moment of the 72+[404] ground state of 10.5 h 175Ta

The magnetic hyperfine splitting ν_M = |gμ_NB_HF/h| of ${}^{175}\text{Ta}\text{(J}{}^{\mathrm{\pi }}\text{=}\text{7}\text{2}{}^{\mathrm{+}}$; $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 10.5}\text{h}\text{)}$ in Fe has been measured with the technique of nuclear magnetic resonance on oriented nuclei as 320.4(1) MHz. With the known hyperfine field $\text{B}{}_{\mathrm{<mtext>HF</mtext>}}\text{(}\text{Ta}\text{Fe}\text{) = ?648(13)}\text{kG}$ the magnetic moment of the $\text{7}\text{2}{}^{\mathrm{+}}\text{[404]}$ ground state of ¹⁷⁵Ta is deduced to be $\text{¦μ¦ = 2.27(5)μ}{}_{\mathrm{<mtext>N</mtext>}}$.     

Decoupled magnetic subsystems in the random mixture FepCo1-p(C5H5NO)6(ClO4)2

Specific heat data on the random mixtures Fe_pCo_1-pL₆(ClO₄)₂, where L = C₅H₅NO, are presented. The Fe and Co magnetic atoms have competing anisotropies since the pure Fe and Co compounds are known to be good examples of the simple cubic, $\text{S =}\text{1}\text{2}$, Ising and XY magnet, respectively. The experimental data show the two magnetic subsystems in the mixtures to be almost completely decoupled, which is a consequence of the fact that the crystal field anisotropies of the Fe²⁺ and Co²⁺ ions, yielding g_∥ ? g_⊥ and g_⊥ ? g_∥, respectively, are very strong compared to the magnetic exchange interactions. Consequently the two magnetic subsystems experience one another as nonmagnetic impurities. A model is presented which explains these results, as well as those previously found for related random mixtures, in terms of two interpenetrating percolation clusters.     

Absorption spectra of Ni2+ in (NH4)2Mg(SO4)2·6H2O and Co2+ in Na2Zn(SO4)2·4H2O

Optical absorption spectra of Ni²⁺ in (NH₄)₂Mg(SO₄)₂·6H₂O and Co²⁺ in Na₂Zn(SO₄)₂·4H₂O single crystals have been studied at room and liquid nitrogen temperatures. From the nature and position of the observed bands, a successful interpretation could be made assuming octahedral symmetry for both the ions in the crystals. The splitting observed for ${}^3\text{T}{}_{\mathrm{1g}}\text{(F)}$ band in Ni²⁺ and ${}^4\text{T}{}_{\mathrm{2g}}\text{(F)}$ band in Co²⁺ at liquid nitrogen temperature have been explained as due to spin-orbit interaction. The extra band observed at 16,325 cm^-1 in the case of Ni²⁺ at low temperature has been interpreted to be the superposition of vibrational mode of SO^2-₄ radical on ${}^3\text{T}{}_{\mathrm{1g}}\text{(F)}$ band. The observed band positions in both the crystals have been fitted with four parameters B, C, Dq and ζ.     

Nuclear magnetic moment of the 3.9 s112− isomer 193mAu

The magnetic hyperfine splitting ν_M = |gμ_NB_HF/h| of ${}^{\mathrm{<mtext>193</mtext><mtext>m</mtext>}}\text{Au}\text{(j}{}^{\mathrm{\pi }}\text{=}\text{11}\text{2}{}^{\mathrm{?}}\text{, E = 290}\text{keV}$; $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 3.9}\text{s}\text{)}$ as a dilute impurity in Ni has been measured with nuclear magnetic resonance on oriented nuclei as 226.4(2) MHz. With the known hyperfine field B_HF = ?264.4(3.9) kG corrected for hyperfine anomalies the g-factor and magnetic moment of ^193mAu are deduced to be |g| = 1.123(17) and |μ| = 6.18(9) μ_N.     

Structure magnetique en champ applique nul de l'acetate de manganese tetrahydrate Mn(CH3COO)2, 4H2O

The magnetic structure of manganous acetate Mn(CH₃COO)₂, 4H₂O has been solved by neutron diffraction. Manganous acetate crystallizes in the space group $\text{P2}{}_1\text{c}$ with Z = 6. Manganese atoms (in position 2a and 4e) are located in (100) planes. Below T_N = 3.18 K this compound is antiferromagnetic in a zero applied field with the k vector [$\text{1}\text{2}$ 00]. The plane (100) is ferromagnetic. The magnetic group is $\text{P}{}_{\mathrm{2a}}\text{2}{}_1\text{c}$.     

Further neighbour interactions in the quasi b.c.c. Heisenberg ferromagnets M2 Cu X4 · 2H2 O (M = NH4, K,Rb or Cs; X = Cl or Br)

The Curie-Weiss constant θ of the quasi b.c.c., $\text{S =}\text{1}\text{2}$, Heisenberg ferromagnets, (NH₄)₂ Cu Br₄ · 2H₂ O and Rb₂ Cu Br₄ · 2H₂ O, have been derived from measurements of the susceptibility in the paramagnetic region (4.2 < T < 100 K) and of the high-field magnetization curves at $\text{T = 4.2}\text{K}\text{(}\text{T}\text{T}{}_{\mathrm{c}}\text{? 2.3)}$. The $\text{T}{}_{\mathrm{c}}\text{θ}$ values obtained point to the presence of further neighbour interactions considerably stronger than previously assumed. The effective number of equivalently interacting magnetic neighbours is estimated to be at least seventeen.     

Crystal growth,structure, electron paramagnetic resonance and magnetic properties of (NH4)2V3O8

A method to grow single crystals of ammonium vanadate (IV, V) (NH₄)₂V₃O₈ has been devised. The crystal structure is tetragonal P4bm; residual factor is R = 0.030. Cell parameters are $\text{a = 8.891 ± 0.004}\text{A}\text{?}$ and $\text{c = 5.582 + 0.002}\text{A}\text{?}$. The V⁵⁺ atom lies at the center of a triangular pyramide (VO₄ tetrahedron) while the V⁴⁺ atom is on A 4-fold rotation axis at the center of a square-based pyramide VO₅ whose symmetry point group is almost C_4v with the short V = O bond lying along the 4-fold axis parallel to the c edge of the tetragonal cell. Crystals are thin platlets with (001) cleavage planes. The platlets have very often a square or rectangular shape limited by {100} or {110} planes. Each single crystal was not large enough to record a good e.p.r. spectrum, but by sticking on the same quartz plate a score of them it was possible to gather enough crystals so to record correct spectra and by orienting the plate to obtain resonance lines separately for g_∥ = 1.9263 et g_τ = 1.9755. Measurements at 283 K on powder samples gave times for spin-spin relaxation T₂ = 0.4 × 10^?7s and for spin-lattice relaxation T₁ = 1.6 × 10^?7s. The magnetic structure is characterized by an exchange narrowing ω_e = 3 × 10¹⁰rad/s which corresponds to a transition temperature of about 0.5 K. Static susceptibility measurements at high magnetic field show a paramagnetic behaviour with an antiferromagnetic interaction which is interpreted in the magnetic space group P_2c4bm as the interaction between V⁴⁺ ions from consecutive planes parallel to (001).     

Magnetic structure of BaCaFe4O8-analysis of neutron diffraction measurements

The compound BaCaFe₄O₈ crystallizes in the trigonal space group P$\text{31}$m with one formula unit per unit cell with lattice constants $\text{a = 5.4059}\text{A}$ and $\text{c = 7.7023}\text{A}$ Neutron diffraction measurements carried out on a powder sample over the temperature range 300–900 K showed that the compound undergoes a magnetic transition to an antiferromagnetic state at a Néel temperature T_N = 680 ± 5 K. Analysis of the room temperature neutron diffraction pattern gave a magnetic unit cell that has the same periodicty as the crystallographic one. An antiferromagnetic model is proposed with the iron spin magnetic moments parallel to the c-axis of the unit cell. The magnetic moment of the Fe³⁺ ion was found to be (4.5 ± 0.1)μ_B     

Low temperature specific heats of nearly one-dimensional antiferromagnet: CuCl2·2NC5H5

Transition temperature to LRO state was found at T_N=1.14K for nearly one-dimensional antiferromagnet CuCl₂ · 2NC₅H₅. Intra- and inter-chain exchange constants J and J′ were estimated, $\text{kT}{}_{\mathrm{<mtext>N</mtext>}}\text{J}\text{=0.082}$ and $\text{J′}\text{J}\text{=3×10}{}^{\mathrm{?3}}$, respectively. Comparison with those of TMMC implies highly one-dimensional character.     

Electrical resistance and magnetoresistance of amorphous alloys Gd67Co33 and Pd80Si20

In search for structural contributions to the low temperature anomaly we report high resolution resistance and magnetoresistance measurements ($\text{0.02}\text{K}\text{? T ? 20}\text{K}$) of amorphous splats of Gd₆₇Co₃₃ and Pd₈₀Si₂₀. For both alloys, the resistivity ?(H = 0, T) has a minimum at T ～ 10 K and increases with decreasing T. The ferromagnetic Gd₆₇Co₃₃ shows a strong negative field dependence of $\text{Δ?}\text{?(0)}$, saturating at H ～ 2T for T = 4.2 K but no measurable change in $\text{??}\text{?}\text{T}$ below 10 K is observed.The diamagnetic Pd₈₀Si₂₀ exhibits a positive field dependent magnetoresistance $\text{[Δ?}\text{?(0)]}\text{(}\text{H}\text{)}$ at low temperatures. Additionally, a field dependent part in $\text{??}\text{?}\text{T}$ is found which is probably due to paramagnetic impurities (～ 1 ppm Fe). However, there is also a field independent contribution in the amorphous state of Pd₈₀Si₂₀, which vanishes after crystallization. We attribute this to non-magnetic scattering induced by the disordered structure.     

Magnetic phase transitions,anisotropic susceptibility and dipolar interactions in antiferromagnetic MnNb2O6

The magnetic properties of MnNb₂O₆ single crystals have been studied in the temperature range 1.6–300 K (T_N = 4.4 K), in fields up to 220 kOe. The high field saturation at low temperature, as well as the paramagnetic susceptibility at high temperature, agree well with a ${}^6\text{S}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ state for Mn²⁺ ions. From 1.6 to 3.8 K a spin flop is induced by fields ranging from 17 to 21 kOe, applied in the direction of the a-axis. Elements of the magnetic susceptibility tensors up to rank 12, measured below the spin flop field, are in accordance with a magnetic anisotropy originating mainly from magnetic dipolar interactions.     

NMR relaxation at low temperature in one dimensional antiferromagnets: Comparison between S = 12 and S = 52

The proton spin-lattice relaxation time T₁ has been measured at low temperatures in spin $\text{5}\text{2}$ (CH₃)₄NMnCl₃ (TMMC) and spin $\text{1}\text{2}$ CuCl₂2NC₅H₅ (CuPC). The two systems have very different temperature dependences which we attribute to the different spin values.     

Magnetic structures of PrCO2Si2 and TbCo2Si2 compounds

Neutron diffraction studies of polycrystalline PrCo₂Si₂ and TbCo₂Si₂ compounds were carried out at 4.2 and 293 K. Both samples have collinear antiferromagnetic order below T_N(31(1) and 46(1) K for Pr and Tb compound respectively), with their magnetic moments parallel to the c axis. The ordered magnetic moment values of Pr and Tb at 4.2 K (3.19 and 9.12 μ_B respectively), are close to the saturation value of the free ions. The corresponding magnetic space group P_l4/mnc (Sh⁴¹⁰₁₂₈) is body-anticentered ($\text{k =}\text{111}\text{222}$ refering to P_l cell).