G.C. DeFotis

MnCl2⋅H2O: A quasi‐one‐dimensional Heisenberg antiferromagnet

The magnetic behavior of NiCl2⋅H2O has been studied for the first time. A Curie–Weiss fit, χM=C/(T−θ), to the susceptibility between 90 and 300 K yields g=2.23±0.01(S=1) and θ=30.4±0.05 K. Systematic curvature in χ−1(T) is evident below 90 K. Despite the very positive θ, NiCl2⋅H2O appears to order antiferromagnetically at Tc=5.65±0.1 K, somewhat below a maximum in χ(T) at T(χmax)=8.4±0.1 K, with χmax=0.1297±0.0005 emu/mol. The ratios Tc/T(χmax)=0.67±0.01 and Tc/θ=0.186±0.005 suggest lower magnetic dimensionality, most likely one-dimensional character. Plausible looking fits to the low temperature susceptibility based on a one-dimensional antiferromagnetic Heisenberg model can be obtained. However, these presuppose antiferromagnetic intrachain exchange, and in NiCl2⋅H2O such exchange is almost certainly ferromagnetic, with weaker antiferromagnetic interchain interactions. Well above Tc the susceptibility can be accounted for assuming axial and rhombic crystal-field distortions, i.e., D[Ŝz2−S(S+1)/3] and E[...

Magnetic phase diagram and spin glass behavior of Fe1−xMnxCl2⋅2H2O

The magnetic phase diagram of Fe1−xMnxCl2⋅2H2O has been determined. A tetracritical point appears to be present at x≂0.74 and T≂2.8 K. The upper paramagnetic–antiferromagnetic phase boundaries exhibit an unusually rapid descent, and the mixed ordered region below the tetracritical point is quite small. Moreover, regions exhibiting spin glass character also occur in the diagram. The unusually complex behavior is presumably due to the unique constitution of this system: A mixture of two antiferromagnets with different periodicities, competing orthogonal spin anisotropies, competing ferromagnetic and antiferromagnetic near neighbor intrachain exchange interactions, and substantial next nearest neighbor antiferromagnetic interchain exchange interactions.

Quasi-one-dimensional antiferromagnetism in MnCl2•H2O

Recent magnetic susceptibility measurements on polycrystalline MnCl2⋅H2O indicate that it behaves like a quasi‐one‐dimensional antiferromagnet. The heat capacity of this compound has now been measured between ∼0.5 and 80 K in a liquid‐3He‐cooled vacuum calorimeter. Cp exhibits a large, sharp λ‐type peak at TN = 2.18 ± 0.01 K marking the onset of long‐range spin order. The critical entropy S(TN) is found to be ∼0.40R ln 6, confirming the short‐range, low‐dimensional nature of most of the spin ordering. An anisotropic elastic continuum model has been used to calculate the contribution of lattice vibrations to Cp. The data above ∼3 K are well represented by this model (with suitable choice of three parameters) plus the theoretical heat capacity of the 1D Heisenberg antiferromagnet with S =5/2 and exchange constant J/k=−0.45 K. Below ∼ 0.7TN, the magnetic part of Cp is well described by an anisotropic 3D spin‐wave model with Jz/k = J/k = −0.45 K, Jx/k = Jy/k = −0.015 K, and an anisotropy field HA ≊ 3.3 kOe. T...

Ferromagnetism of Ni(SCN)2(C2H5OH)2

The magnetic properties of Ni(SCN)2(C2H5OH)2 have been studied, the first Ni(II) system in the general family of compounds M(SCN)2(ROH)2 (where M=divalent Mn, Fe, Co, or Ni and R=CH3, C2H5, i‐ or n‐C3H7) to be examined at low temperatures. In contrast to previously studied Mn(II) and Co(II) members of this family, which exhibit predominant antiferromagnetism, the present compound is ferromagnetic. The susceptibility of a polycrystalline sample is of Curie–Weiss form only above 75 K, with g = 2.175 ± 0.01 and S=1 and with θ=24.1±1.0 K. The initial susceptibility is well accounted for by an asymptotic critical law, χ0 = Γ[T/Tc − 1]−γ, in the reduced temperature range 0.147–0.013, with Tc = 13.081 ± 0.01 K, γ=1.354±0.02, and Γ=0.0925±0.003 emu/mol. The γ value is between 3D‐XY and 3D‐Heisenberg model predictions. The susceptibility in the paramagnetic regime well above Tc is analyzed including the effects of axial and rhombic crystal field distortions, represented by D[S2z − S(S + 1)/3] and E[S2x − S2y] ...

Critical behavior of an unusual ferromagnet

Abstract The susceptibility near the critical point for the Ising ferromagnet, Fe(Cl)[S 2 CN(C 2 H 5 ) 2 ] 2 , is reported. Analysis of the data for three samples leads to values of the parameter γ substantially less than the value 1.25. Several possible explanations for this result are discussed, among them magnetic dipole-dipole interactions, randomly distributed exchange interactions and multi-spin exchange interactions. However, the existence of two ferromagnetic sublattices canted 42° apart, and the possibility of a non-diverging antiferromagnetic contribution to the susceptibility, seems the most plausible explanation.

Crystal Structures of Manganese and Cobalt Dichloride Monohydrate and Deuteration Effects on Magnetic Behavior

This work reports the long sought crystal structures of the title members of the intriguing series of 3d transition metal dichloride monohydrates. The double chain structure which results from rearrangement of the well-known pseudo-octahedral coordination geometry and single chains in the corresponding metal chloride dihydrate is extremely unusual. MnCl2·H2O and CoCl2·H2O each crystallize in orthorhombic space group Pnma with Z = 4 and lattice parameters a = 9.0339(1), 8.8207(3); b = 3.68751(5), 3.5435(1); c = 11.5385(2), 11.2944(4) all in Å and for Mn, Co, respectively. Results are reported also for both fully deuterated systems; the structures remain the same with lattice parameter changes typically much less than 0.1%. Various magnetic properties of MnCl2·D2O and CoCl2·D2O are reported. For the latter, there are no apparent differences, qualitatively or quantitatively, from the previously measured properties of CoCl2·H2O. Interestingly, for the former some differences with respect to MnCl2·H2O are apparent, principally a lower Tmax = 3.10(10) K about which a broad antiferromagnetic maximum is centered, and a larger value χmax = 0.336(3) emu/mol. However, antiferromagnetic ordering appears to occur at essentially the same 2.18(2) K. Results of fits to susceptibilities of MnCl2·D2O and CoCl2·D2O are compared with those obtained before for MnCl2·H2O and CoCl2·H2O. Structural considerations serve to rationalize the physical properties, especially the lower dimensional magnetism of monohydrates.

Magnetic anisotropy in quasi-2D Co(SCN)2(n-C3H7OH)2

Abstract The title system exhibits substantial magnetic anisotropy below 50 K, with only one principal crystal susceptibility of Curie-Weiss form and characterized by a large effective g ′ = 6.14. One of the other principal susceptibilities appears to diverge at lower temperatures, and conforms to a critical power law with γ = 1.75.

Ferromagnetic insulating Fe[S2CN(C2H5)2]2Cl: Critical exponents, scaling, and magnetic equation of state

Magnetization isotherms for the title system become coincident above Tc when the scaled magnetic field is plotted against the scaled magnetization. The associated critical exponents are β=0.24±0.01 and δ=5.65±0.15, with independently determined Tc =2.458±0.002 K. A separate measurement of the initial susceptibility is analyzed to yield γ=1.165±0.005. Both γ and β agree well with earlier determinations. δ, previously unknown, is consistent with γ and β in light of the scaling relation δ=γ/ β+1. Yet each exponent differs significantly from its theoretical value in the three‐dimensional Ising or any other familiar model. The magnetic equation of state is also determined, as an expansion about the critical isochore and as an expansion about the critical isotherm. Coefficients in each power series differ significantly from those available for any familiar theoretical model. Various explanations for the unusual observed behavior are discussed, including the possibility that this system may be an experimental re...

Heat capacity and magnetic properties of CoCl2⋅H2O

Recent magnetic studies of polycrystalline CoCl2⋅H2O showed an antiferromagnetic ordering transition at TN∼14 K and an apparent spin glass transition at ∼7 K. We now have extended the magnetic susceptibility measurements up to 300 K. The results above TN can be well described if the lowest electronic states of Co2+ in this salt are two Kramers doublets separated by ΔE/k=230 K, and if a predominantly ferromagnetic mean field interaction is introduced. The heat capacity of CoCl2⋅H2O over the range 0.6–80 K has also been measured. Below ∼10 K, Cp≊aT−2+bT3. The first term is the hyperfine contribution of the 59Co2+ nuclei. The second is the sum of lattice vibrational and electronic spin parts. Cp exhibits a λ‐type peak at TN1=15.0±0.05 K, and a second λ anomaly of comparable size at TN2=13.9±0.05 K, which appears to be associated with a spin reorientation transition. Correcting the observed Cp data for the nuclear spin contribution, one may calculate the sum of lattice vibration and electron spin entropies, a...

Co1-xMnxCl2·2H2O: A new kind of insulating spin glass

Abstract The title system is a mixture of isomorphous insulating antiferromagnets with different magnetic periodicities and competing ferromagnetic and antiferromagnetic exchange interactions. The magnetic phase diagram is of quite unusual structure, and spin glass behavior is observed. The time dependence of the thermoremanent magnetization is that of a “stretched” exponential.