S. A. Friedberg

The magnetic susceptibilities of CsMnCl32H2O and CsMnCl32D2O

The magnetic susceptibilities of CsMnCl3·2H2O and CsMnCl3·2D2O have been measured at temperatures between 0.35 and 35 K. Above ∼9 K both salts show the broad susceptibility maximum of the one-dimensional Heisenberg antiferromagnet previously seen in the hydrate. In both cases the intrachain exchange parameter isJ/k=−3.00 K. AtTN (4.89 K for the hydrate) three-dimensional antiferromagnetic long-range order begins to develop due to weak interchain interactions with preferred alignment along theb axis. AtTN all susceptibility-temperature curves have their maximum slopes, those of χa and χc being negative. This is shown to be related to the unusual phase diagram of these systems. The limiting behavior of the susceptibilities asT → 0 K is established and interpreted. χ⊥(0) reflects the strong anisotropy of the spinwave spectrum due to the dominant intrachain interaction.

Low-dimensional magnetic behavior of α-CoC2O4.2H2O

Abstract The crystal structure of α-CoC 2 O 4 · 2H 2 O suggests a predominant one-dimensional magnetic interaction. The ac powder susceptibility was fitted with a 1D Ising model with g -factors determined by EPR. The magnetic heat capacity and χ p data delineate a strongly anisotropic 2D Ising system, with J/k ⋍ −30 K , |J′/J| ⋍ 3 × 10 −3 , and an antiferromagnetic transition at T N = 6.23 ± 0.02 K.

Theoretical studies on the isolated spin cluster complex, tetrameric cobalt (II) acetylacetonate Co4(C5H7O2)8: Effects of competing superexchange interactions

According to x‐ray studies, Co4(C5H7O2)8 is an assembly of linear, tetrameric Co2+ ion clusters, which are effectively magnetically isolated. An examination of superexchange paths suggests that this tetramer has two different kinds of nearest‐neighbor exchange constants plus a next‐nearest‐neighbor exchange constant. The only bulk experimental data available so far are the powder susceptibilities from room temperature down to about 0.35 K. This paper demonstrates that this information is apparently sufficient to infer that all three exchange constants are antiferromagnetic in type and have values in the region of Jnn1 ∼ 5 K, Jnn2 ∼ 10 K, and Jnnn3 ∼ 4 K. The interactions therefore compete; and they compete so as to give a paramagnetic character to the low temperature behavior. This is in interesting contrast to the behavior of a sister compound, trimeric nickel (II) acetylacetonate, where the paramagnetic character results from a dominant nearest‐neighbor ferromagnetic exchange constant. The exchange cons...

High-resolution measurements of the heat capacity of MnBr24H2O near its Nel temperature

The heat capacityCP of the antiferromagnet MnBr2 ·4H2O has been measured for polycrystalline and single-crystal samples nearTN(∼2.123 K) with temperature resolution of ∼ 1×10−6 K. Similar rounding of the lambda anomaly is found in both cases. For ε ≡ |1 −T/TN| ≲ 10−1 all data can be well fitted by assuming the samples to consist of many independent subsystems obeying the same power laws but with a Gaussian distribution ofTNs having a width of 1.1×10−3 K. ForT>TN, we findCP ∝ ε−0.12, essentially as predicted for three-dimensional Ising models in the critical region. ForT<TN and 10−3 ≲ ε ≲ 10−1,CP ∝ ln ε, which approximates Ising model behavior in this interval but is not expected to be valid for ε ≲ 10−4. ForT>TN and ε ≲ 2.5×10−1, CP agrees well with predictions for the classical Heisenberg model. This crossover at ε ≈ 10−1 is consistent with the known anisotropy of the salt and with present theory. The data forT>TN in the interval 10−4 ≲ ε ≲ 10−3, while not in the range of obvious rounding, appear to be strongly influenced by the mechanism responsible for that rounding.

Magnetic ordering in NiZrF6⋅6H20

NiZrF6⋅6H20 crystallizes in the trigonal NiSnCl6⋅6H20 structure with one axially distorted [Ni(H20)6]++ complex per unit cell. Low‐field susceptibilities, χ∥ and χ⊥, and Cp for several values of H∥ have been measured above ∠0.05 K in a dilution cryostat. χ∥ and Cp(H=0) exhibit large, sharp anomalies at Tc=0.164 K. The behavior of χ∥ and χ∥ suggests that the salt is a uniaxial ferromagnet with the easy direction along the trigonal axis. Cp(H=0) also shows a resolved Schottky anomaly with Cmax?0.24 R at Tmax∠1.5 K. This indicates that the 3A2g ground state of [Ni(H2O)6]++ is split into a lower doublet and an upper singlet separated by ∠3k. The spins are thus Ising‐like at Tc. For H∥≠0, the lower Cp anomaly is rounded and shifted to higher T. These observations are quite well described by an Oguchi model in which pairs of spins are treated exactly and their interaction with the rest of the crystal is represented by a mean field. The pair Hamiltonian is represented by a mean field. The pair Hamiltonian is H=D...

Singlet Ground State Effects in NiSnCl6·6H2O and Related Compounds

NiSnCl6·6H2O forms trigonal crystals containing one axially distorted [Ni(H2O)6]++ complex per unit cell. Above 0.4K its magnetic and thermal properties were shown to be described by the spin Hamiltonian H=DSz2+gμBS⇒.H.⇒ for S=1 with g=2.25 (isotropic) and D=+0.64k. The zero‐field splitting of the ground state thus leaves a lower singlet separated by 0.64k from an upper doublet. We have extended the measurement of the heat capacity Cp of a single crystal sample down to ∼0.06–0.04K in zero field as well as with a magnetic field parallel to the trigonal axis. For H =0 a Schottky anomaly is observed with Tmax∼0.2K. The absence of an additional cooperative peak down to 0.06K supports the conclusion that interaction among Ni++ ions is smaller than the critical value needed to give long‐ range spin order in H=0. With H∥ ∼4.2kOe, the singlet is crossed by the lower Zeeman component of the doublet and we observe a different Schottky anomaly (Tmax∼0.5K) produced by the artificially degenerate ground state and the ...

Singlet Ground State Effects in

NiSnCl6·6H2O forms trigonal crystals containing one axially distorted [Ni(H2O)6]++ complex per unit cell. Above 0.4K its magnetic and thermal properties were shown to be described by the spin Hamiltonian H=DSz2+gμBS⇒.H.⇒ for S=1 with g=2.25 (isotropic) and D=+0.64k. The zero‐field splitting of the ground state thus leaves a lower singlet separated by 0.64k from an upper doublet. We have extended the measurement of the heat capacity Cp of a single crystal sample down to ∼0.06–0.04K in zero field as well as with a magnetic field parallel to the trigonal axis. For H =0 a Schottky anomaly is observed with Tmax∼0.2K. The absence of an additional cooperative peak down to 0.06K supports the conclusion that interaction among Ni++ ions is smaller than the critical value needed to give long‐ range spin order in H=0. With H∥ ∼4.2kOe, the singlet is crossed by the lower Zeeman component of the doublet and we observe a different Schottky anomaly (Tmax∼0.5K) produced by the artificially degenerate ground state and the ...

UNUSUAL PHASE TRANSITION BEHAVIOR IN WEAKLY INTERACTING MAGNETIC CLUSTER SUBSTANCES: APPLICATION TO MAGNETIC COOLING

ABSTRACT The statistical mechanics of a variety of magnetic cluster substances is discussed, with particular reference to the effects of weak inter-cluster interactions. It is shown that in the case of clusters with an energy gap between the singlet ground state and the excited states an unusual high-field transition may occur. The uses of cluster materials of this and related types in antiferromagnetic cooling by adiabatic magnetization are noted. A novel and more powerful method of cooling by adiabatic demagnetization of antiferromagnets is presented.