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The positive ion mobility in superfluid He-3 has been measured under high magnetic fields up to 14.6 T over the wide pressure region. It increases rapidly below the transition temperatures in both superfluid A(1) and A(2) phases. The magnetic field dependence exhibits an anomalous behavior which is sensitive to the liquid pressure, especially above 20 bar. The behavior is similar to that observed recently in the normal phase at 3.2 mK. This fact indicates that the exchange spin scattering between the positive ion and the He-3 quasiparticles does exist both in the normal and superfluid He-3 phase.

The quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecule magnets (SMMs). SMMs are molecules that function as nanomagnets. Magnetization tunneling is described for two crystallographically different forms of [Mn12O12(O2CC6H4-p-Me)(16)(H2O)(4)].solvate. The two Mn-12 complexes are isomers, differing both in the positioning of the H2O and carboxylate ligands and also in the orientations of the Jahn-Teller elongation at the Mn-III ions. The magnetization vs magnetic field hysteresis loops are quite different for the two isomeric Mn-12 complexes. Frequency-dependent ac magnetic susceptibility and dc magnetization decay data are presented to characterize the magnetization relaxation rate vs temperature responses of two mixed-valence Mn-4 complexes. In both cases the Arrhenius plot of the logarithm of the magnetization relaxation rate vs the inverse absolute temperature shows a temperature-dependent region as well as a temperature-independent region. (C) 2002 American Institute of Physics.

The crystals of a Mn-12 cluster [Mn12O12(O2CC6H5)(16)(H2O)(4)].2C(6)H(5)CO(2)H are mosaic, at least from the magnetic point of view, involving two kinds of molecules with different relaxation rates. The ratios of the two components in the crystals are strongly batch dependent. We characterize the two by comparing the magnetic properties of two batches in which the ratios are very different. The angular dependence of the magnetic properties indicates that the magnetic easy axis of the slow relaxation molecule is parallel to the molecular axis, while that of the fast relaxation (FR) molecule is tilted by 12degrees. The high-frequency electron paramagnetic resonance for the FR molecule supports the tilted easy axis and suggests an anisotropy even in its hard plane. The x-ray analyses demonstrate that the FR molecule involves a Mn3+ site with an unusual distortion.

Resistivity of the Heusler-type paramagnetic intermetallic compound PrInAg2 is measured by the SQUID technique, down to 0.18 mK. The sample is cooled by an adiabatic Cu nuclear demagnetization refrigerator. No evidence of any phase transition is observed at any temperature. However, a '+' T-1/2 dependence of resistivity is observed below 100 mK. (C) 2002 Elsevier Science B.V. All rights reserved.

The magnetization of the polycrystalline manganese cluster Mn4Br was measured down to 0.13K under a magnetic field of +/-15 kOe. It shows a gradual drop at around 1.5K and the heat capacity measurement at 0kOe reveals a sharp anomaly at 1.33 K, indicating the occurrence of a second-order antiferromagnetic (AFM) phase transition. The M-H curve well below the transition temperature T-N shows a step around 2.5kOe, which probably corresponds to the transition from the AFM phase to metamagnetic (MM) phase. The observed field agrees with that estimated from T-N for the MM transition, using a simple molecular field theory. This is the first observation of a well-defined AFM and also a MM phase among the many manganese clusters with a high-spin ground state that function as molecular nanomagnets.

The crystals of a (formula presented) cluster (formula presented) are mosaic, at least from the magnetic point of view, involving two kinds of molecules with different relaxation rates. The ratios of the two components in the crystals are strongly batch dependent. We characterize the two by comparing the magnetic properties of two batches in which the ratios are very different. The angular dependence of the magnetic properties indicates that the magnetic easy axis of the slow relaxation molecule is parallel to the molecular axis, while that of the fast relaxation (FR) molecule is tilted by (formula presented) The high-frequency electron paramagnetic resonance for the FR molecule supports the tilted easy axis and suggests an anisotropy even in its hard plane. The x-ray analyses demonstrate that the FR molecule involves a (formula presented) site with an unusual distortion. © 2002 The American Physical Society.

Quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecule magnets (SMM's). Magnetization tunneling is described for two crystallographically different forms of [Mn12O12(O2CC6H4-p-Me)(16)(H2O)(4)] . solvate. The two Mn-12 complexes are isomers. The magnetization versus magnetic field hysteresis loops are quite different for the two isomeric Mn-12 complexes. One Mn-12 Complex exhibits a magnetization hysteresis loop that is characteristic of considerably faster magnetization tunneling than in the other Mn-12 isomer. The unusual orientation of Jahn-Teller elongation axes in one isomer leads to greater rhombic zero-field splitting that is the origin of the faster magnetization tunneling. In addition to data for Mn-12 complexes, magnetization relaxation rate versus temperature responses of three mixed-valence Mn-4 complexes are also reported. In all three cases the Arrhenius plot of the logarithm of the magnetization relaxation rate versus the inverse absolute temperature shows a temperature-independent region as well as a temperature-dependent region. The temperature-independent relaxation rate is definitive evidence of magnetization tunneling in the lowest-energy zero-field component of the ground state.

The behaviour of He-4 impurities has been investigated through positive ion mobility measurements in liquid He-3 at milliKelvin temperatures. In spite of the enormous surface area of the cell, the influence of He-4 lasted for hundreds of hours at temperatures of similar to 20 mK. However below 10 mK, the 4He impurities were quickly frozen out on the cell walls and we were free from the impurity problem. Nevertheless the multiple ion signal occasionally appeared even at a few mK, although it disappeared for a number of ionization pulses.

Positive ion mobility in normal liquid He-3 has been measured as a function of external magnetic fields up to 15 T at various pressures and temperatures down. to 3 mK. At 20 mK, a monotonic decrease of the mobility with increasing the magnetic field has been observed at the pressures above 10 bars. At 3.2 mK. the field dependence for pressures above 20 bars is found to exhibit a pressure dependent broad peak, followed by a big decrease. Possible origins for these anomalous behaviors are discussed.

The positive ion mobility in normal liquid He-3 has been measured as a function of external magnetic field up to 15 T at temperatures down to 3 mK. At 3.2 mK, the field dependence is found to exhibit a pressure-dependent broad peak followed by a large decrease at pressures above 20 bars. On the other hand, at 20 mK, a monotonic decrease with increasing the magnetic field has been observed in the same pressure region. Possible origins for these anomalous behaviors are discussed.

The preparations, X-ray structures, and detailed physical characterizations are presented for two new mixed-valence tetranuclear manganese complexes that function as single-molecule magnets (SMM's): [Mn(4)(hmp)(6)Br(2)-(H(2)O)(2)]Br(2). 4H(2)O (2) and [Mn(4)(6-me-hmp)(6)Cl(4)]. 4H(2)O (3), where hmp(-) is the anion of 2-hydroxymethylpyridine and 6-me-hmp(-) is the anion of 6-methyl-2-hydroxymethylpyridine. Complex 2 . 4H(2)O crystallizes in the space group P2(1)/c, with cell dimensions at -160 degreesC of a = 10.907(0) Angstrom, b = 15.788(0) Angstrom, c = 13.941(0) Angstrom, = 101.21(0)degrees, and Z = 2. The cation lies on an inversion center and consists of a planar Mn(4) rhombus that is mixed-valence, Mn(2)(III)Mn(2)(II). Them hmp(-) ligands function as bidentate ligands and as the only bridging ligands in 2 . 4H(2)O. Complex 3 . 4H(2)O crystallizes in the monoclinic space group C2/c, with cell dimensions at -160 degreesC of a = 17.0852(4) Angstrom, b = 20.8781(5) Angstrom, c = 14.835(3) Angstrom, beta = 90.5485(8)degrees, and Z = 4. This neutral complex also has a mixed-valence Mn(2)(III)Mn(2)(II) composition and is best described as having four manganese ions arranged in a bent chain. An mu (2)-oxygen atom of the 6-me-hmp(-) anion bridges between the manganese ions; the Cl(-) ligands: are terminal. Variable-field magnetization and high-frequency and -field EPR (HFEPR) data indicate that complex 2 . 4H(2)O has a S = 9 ground state whereas complex 3 . 4H(2)O has S = 0 ground state. Fine structure patterns are seen in the HFEPR spectra, and in the case of 2 . 4H(2)O it was possible to simulate the fine structure assuming S = 9 with the parameters g = 1.999, axial zero-field splitting of D/k(B) = -0.498 K, quartic longitudinal zero-field splitting of B(4 degrees)/k(B) = 1.72 x 10(-5) K, and rhombic zero-field splitting of E/k(B) = 0.124 K. Complex 2 . 4H(2)O exhibits a frequency-dependent out-of-phase AC magnetic susceptibility signal, clearly indicating that this complex functions as a SMM. The AC susceptibility data for complex 2 . 4H(2)O were measured in the 0.05-4.0 K range and when fit to the Arrhenius law, gave an activation energy of DeltaE = 15.8 K for the reversal of magnetization. This AE value is to be compared to the potential-energy barrier height of U/k(B) = \D (S) over cap (2)(Z)\ = 40.3 K calculated for 2 . 4H(2)O.

The quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecules magnets (SMM's). SMM's are molecules that function as nanomagnets. Magnetization tunneling is described for two crystallographically different forms of [Mn12O12(O2CC6H5-p-Me)(16)(H2O)(4)] solvale. The two Mn,, complexes are isomers that both differ in the positioning of the H2O and carboxylate ligands and also in the orientations of the Jahn-Teller elongation at the Mn-III ions. The magnetization versus magnetic field hysteresis loop is quite different for the two isomeric Mn-12 complexes. One Mn-12 complex exhibits a magnetization hysteresis loop that is characteristic of considerably faster magnetization tunneling than in the other Mn,, isomer. The lower symmetry and greater rhombic zero-field splitting are the origin of the faster magnetization tunneling. Frequency-dependent ac magnetic susceptibility and de magnetization decay data are presented to characterize the magnetization relaxation rate versus temperature responses of three mixed-valence Mn, complexes. In all three cases, the Arrhenius plot of the logarithm of the magnetization relaxation rate versus the inverse absolute temperature shows a temperature-dependent region as well as a temperature-independent region. The temperature-independent magnetization rate is definitive evidence of magnetization tunneling in the lowest-energy zero-field component of the ground state. (C) 2001 Elsevier Science Ltd. All rights reserved.

We report valence tautomerism in the spin-labeled complex, Co(nnbpy)(3,5-DTBSQ)(2), where nnbpy is a bipyridine substituted nitronyl nitroxide radical, 4-methyl-4'-(,4,4,5,5-tetramethyl-1-oxido-3-ylooxy-4,5-dihydro-3H-imidazol-2'-yl)-2,2'-bipyridine, and 3,5-DTBSQ is 3,5-di-tert-butylbenzosemiquinone. The temperature dependence of the magnetic susceptibility indicates a tautomeric conversion. Co(3+) (nnbpy)(3,5-DTBSQ(-.))(3,5-DTBSQ(2-)) --> Co(2+) (nnbpy)(3,5-DTBSQ(-.))(2), above 250 K with the thermodynamic parameters of DeltaH = 34 kJ mol(-1) and DeltaS = 98 J K(-1) mol(-1). The ligand radical, nnbpy, behaves as a Curie spin over the whole temperature region. The EPR spectra in the temperature range 150-380 K consist of a single absorption at g = 2.004 that gradually fades away above 300 K with a characteristic increase in linewidth.

The positive ion mobility in normal liquid He-3 has been measured as a function of external magnetic field up to 15 T at temperatures down to 3 mK. At 3.2 mK, the field dependence is found to exhibit a pressure-dependent broad peak for pressures above 20 bars. On the other hand, at 20 mK, a monotonic decrease with increasing magnetic field has been observed in the same pressure region. Possible origins for these anomalous behaviors are discussed.

The preparation, X-ray structure, and detailed physical characterization are presented for a new type of single-molecule magnet [Mn(4)(O(2)CMe)(2)(pdmH)(6)](ClO(4))(2) (1). Complex 1 . 2MeCN . Et(2)O crystallizes in the triclinic space group P1, with cell dimensions at 130 K of a = 11.914(3) Angstrom, b = 15.347(4) Angstrom, c = 9.660(3) Angstrom, alpha = 104.58(1)degrees, beta = 93.42(1)degrees, gamma = 106.06(1)degrees and Z = 1. The cation lies on an inversion center and consists of a planar Mn(4) rhombus that is mixed-valent, Mn(2)(III)Mn(2)(II) The pdmH(-) ligands (pdmH(2) is pyridine-2,6-dimethanol) function as either bidentate or tridentate ligands. The bridging between Mn atoms is established by either a deprotonated oxygen atom of a pdmH- ligand or an acetate ligand. The solvated complex readily loses all acetonitrile and ether solvate molecules to give complex 1, which with time becomes hydrated to give 1 . 2.5H(2)O. Direct current and alternating current magnetic susceptibility data are given for 1 and 1 2.5B20 and indicate that the desolvated complex has a S = 8 ground state, whereas the hydrated 1 . 2.5H(2)O has a S = 9 ground state. Ferromagnetic interactions between Mn(III)-Mn(II) and Mn(III)-Mn(III) pairs result in parallel spin alignments of the S = (5)/(2) Mn(II) and S = 2 Mn(III) ions. High-frequency EPR spectra were run for complex 1 . 2.5H(2)O at frequencies of 218, 328, and 436 GHz in the 4.5-30 K range. A magnetic-field-oriented polycrystallite sample was employed. Fine structure is clearly seen in this parallel-field EPR spectrum. The transition fields were least-squares-fit to give g = 1.99, D = -0.451 K, and B(4)degrees =2.94 x 10(-5) K for the S = 9 ground state of 1 . 2.5H(2)O. A molecule with a large-spin ground state with D < 0 can function as a single-molecule magnet, as detected by techniques such as ac magnetic susceptibility. Out-of-phase ac signals (chi''(M)) were seen for complexes 1 and 1 . 2.5H(2)O to show that these complexes are single-molecule magnets. A sample of 1 was studied by ac susceptibility in the 0.4-6.4 K range with the ac field oscillating at frequencies in the 1.1-1000 Hz range. A single peak in chi''(M) VS temperature plots was seen for each frequency; the temperature of the chi''(M) peak varies from 2.03 K at 995 Hz to 1.16 K at 1.1 Hz. Magnetization relaxation rates were evaluated in this way. An Arrhenius plot gave an activation energy of 17.3 K, which, as expected, is less than the 22.4 K value calculated for the thermodynamic barrier for magnetization direction reversal for an S = 8 complex with D = -0.35 K. The 1 . 2.5H(2)O complex with an S = 9 ground state has its chi"(M) peaks at higher temperatures.

Resistivity of a Heusler-type intermetallic paramagnetic compound PrInAg2 is measured precisely down to 50 mK. An empirical equation is well determined as Delta rho proportional to + log T between 50 mK and 1 K. The system is expected as an example of the quadrupolar Kondo system, since the Pr3+ (4f(2)) ion in the material has a non-magnetic doublet ground state Gamma(3). The results are not consistent with Delta rho proportional to +/- T-1/2 Which is expected from the single impurity quadrupolar Kondo model near the ground state. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Magnetization relaxation of [Mn-4(hpdm)(6)(OAc)(2)](ClO4)(2) with S = 8 ground state has been investigated with AC susceptibility and DC magnetization measurements below 4 K. The temperature dependence of the relaxation time follows the Arrhenius law with activation energy E = 17 K above 1 K. At lower temperatures, the relaxation deviates from the thermal activation formula and becomes temperature independent below 0.5 K with tau(sat) similar to 10(4) s. These observations are attributed to magnetic quantum tunneling. (C) 2000 Elsevier Science B.V. All rights reserved.

The magneto- (electrical-) resistivity of a Heusler-type intermetallic paramagnetic compound PrInAg2 is measured under high fields up to 12 T at 40 mK (down to 50 mK under zero held). No evidence of any transition is detected under zero field in our sample and deviations of the resistivity can well fit as proportional to + log T function below 1 K down to even 50 mK. The magnetoresistivity is positive at any measuring fields. It seems that the external magnetic field dissolves the anomaly at low temperature via breaking the cubic symmetry. Moreover, the magnetoresistivity varies with + log H from 0.01 T to 1 T and is scaleable to the electrical resistivity via the relation of T proportional to +\ H-n\(n approximate to 2/3). Then both of the anomalies between the magneto- and the electrical-resistivity must be originated from a common source. Thereon, the value of the exponent n does not take 1. i.e. the field effect on the resistivity is not Zeemanian, It has been already known that a Pr3+(4f(2)) ion in the compound is located in cubic symmetry and has a non-magnetic non-Kramers doublet Gamma(3) ground state. It is possible that the anomalies originate from the Gamma(3) ground state. (C) 2000 Elsevier Science B.V. All rights reserved.

The electron spin density distribution was investigated for p- and m-pyridyl nitronyl nitroxides (p-PYNN and m-PYNN) in the crystalline phase by the temperature dependence of the solid state high resolution H-1-MAS NMR spectrum. The results were compared with that of phenyl nitronyl nitroxide (PNN) for elucidating the effect of incorporation of a nitrogen atom into the aromatic group. For p-PYNN, the magnitude of the negative spin density at 3 and 5 positions of the pyridyl group was suppressed by 30% in comparison with that of PNN and the positive spin density at 2 and 6 positions was slightly enhanced by 10%. On the other hand, the positive spin density at 2, 4 and 6 positions of pyridyl group of m-PYNN was suppressed by 30% in average and the negative one at 5 was also suppressed by 20%. The DFT calculation at UBLYP/6-31G(d, p) level suggested that the molecular geometry largely contributed to the change of the spin density in addition to the effect of incorporation of the nitrogen atom. In fact, the spin density distribution of the aromatic ring of p-PYNN was remarkably reduced in solution compared with that in the crystalline phase.

Low temperature properties of the organic spin-1 Kagome' antiferromagnet, m-MPYNN . BF4, have been studied by measuring heat capacity and magnetic susceptibility down to 35 mK. The heat capacity maximum due to a magnetic short-range order was observed at 1.4 K, which is about half of the antiferromagnetic interaction 2\J\/k(B)=3.1 K in the Kagome' lattice. As temperature decreases, the susceptibility begins to decrease below 0.24 K. The susceptibilities both parallel and perpendicular to the Kagome' lattice collapse to almost zero at the lowest temperature, which indicates a nonmagnetic ground state. The temperature dependence suggests that the gap energy of the magnetic state is about 0.25 K.

We report unusual crystal structures and magnetic properties of nitronylnitroxide radical ions. (i) A nitronylnitroxide anion, p-(nitronyl nitroxide)ben to ate (p-NNBA), is found to crystallize with Mn2+ and water molecules in the form of Mn(p-NNBA)(2)(H2O)(2). In this crystal, the manganese(II) ion is in an octahedral environment, surrounded by two NO groups, two CO2 groups and two H2O. The magnetic properties are well interpreted in terms of the trimer model of [p-NNBA (S=1/2)][Mn2+ (S=5/2)]-[p-NNBA (S=1/2)]. (ii) A nitronylnitroxide cation, m-N-methylpyridinium nitronylnitroxide (m-MPYNN), crystallizes with I-, BF4-, or ClO4- into a trigonal space group, where the spin system is regarded as a spin-1 Kagome antiferromagnet with spin frustration. Low-temperature magnetic measurements reveal a spin-gap state below 0.24 K.

The complex p-NPNN . Cu(hfac)(2) (p - NPNN =p-nitrophenyl nitronylnitroxide and hfac=hexafluoroacetylacetonate) crystallizes into the triclinic P (1) over bar space group [a=12.515(3) Angstrom, b=12.540(2) Angstrom, c=11.610(3) Angstrom, alpha=106.18(2)degrees, beta=119.15(2)degrees, gamma=91.14(2)degrees, V=1501.7(6) Angstrom(3), Z=2]. The structure consists of an alternating chain of p-NPNN and Cu(hfac)(2) in which p-NPNN bridges two nonequivalent copper ions occupying the axial positions. The chains are connected by a pi-pi and head-to-tail overlap of the p-NPNN molecules, resulting in a two-dimensional network. The high-temperature magnetic susceptibilities can be interpreted in terms of a ferromagnetic intrachain interaction of J/k(B)=14.3 K and an antiferromagnetic interchain interaction of zJ'/k(B)=-2.6 K. The observed interchain interaction which would originate in the overlap of p-NPNN is much stronger than those in the related Cu(II) materials reported so far. The low-temperature magnetic susceptibilities indicate an antiferromagnetic order at T-N=1.22+/-0.01 K, followed by an enhancement of the susceptibility below T-c=0.6+/-0.1 K, probably due to a spin canting.

Organic radical cations, 4,4,5,5-tetramethyl-2-(1-methyl-3 or 4-pyridinio)-3-oxide-4,5-dihydro-1H-1-imidazolyloxyl (or, m- and p-N-methylpyridinium nitronyl nitroxides, abbreviated as m- and p-MPYNN(+), respectively), were found to crystallize with magnetic counter anions, MCl(4)(2-) (M = Mn2+ (S = 5/2) and Co2+ (S = 3/2)), although the MnCl42- salts were rather air-sensitive. X-Ray full-crystal analyses were carried out on the stable CoC1(4)(2-) salts. The structure of (m-MPYNN(+))(2)CoCl42- crystallizes in the monoclinic P2(1)/c space group, while that of (p-MPYNN(+))(2)CoCl42- belongs to the triclinic <P(1)over bar> space group. Although (m-MPYNN(+))(2)MnCl42- and (p-MPYNN(+))(2)MnCl42- were not stable enough for an X-ray full data collection, they were indicated to be isostructural to the corresponding CoCl42- salts, respectively, by their diffractions of 20 < theta <25 degrees. Variable-temperature magnetic susceptibility measurements reveal a clear contrast between the (m-MPYNN(+))(2)MCl(4)(2-) and (p-MPYNN(+))(2)MCl(4)(2-) salts. The two (m-MPYNN(+))(2)MCl(4)(2-) salts exhibit a ferromagnetic behavior independently of MCl(4)(2-), which is attributable to a m-MPYNN(+) dimer. However, the magnetic properties of two (p-MPYNN(+))(2)MCl(4)(2-) salts are strongly dependent on the MCl(4)(2-) anion: (p-MPYNN(+))(2)MnCl42- exhibits an antiferromagnetic interaction in opposition to a ferromagnetic one in (p-MPYNN(+))CoCl42-. The magnetic difference between them can be qualitatively understood in terms of a charge-transfer interaction between MCl(4)(2-) and p-MPYNN(+), in which the difference in the electronic structure between the Mn2+ and Co2+ ions is reflected.

EPR measurements were carried out on a single crystal of m-N-methylpyridinium nitronyl nitroxide perchlorate which consists of a 2-D bond-alternated hexagonal lattice. Angular dependence of the linewidth Delta H-pp shows minimums at the magic angles, and can be well interpreted with the theoretical equation for the 2-D magnetic system,Delta H-pp=A(3cos2 theta . 1)(2)+B. Temperature dependence of Delta H-pp shows an anomaly between 50 K and 100 K, which could be relevant to freezing of the orientational disorder of the perchlorate ion. Angular dependence of the g-value confirms the 2-D character of the magnetic system. Below 50 K, the EPR signal is affected by the demagnetizing effects.

Crystal structures and magnetic properties of m-N-methylpyridinium nitronyl nitroxide (abbreviated as m-MPYNN+) salts were studied. m-MPYNN+. ClO4-.(1/3)(acetone) is found to crystallize into the trigonal P3c1 space group, where the m-MPYNN+ molecules exist as a dimer and the dimer units form a 2-D triangular lattice. Temperature dependence of the magnetic susceptibility can be well interpreted in terms of a strong ferromagnetic intradimer interaction J(1) forming a triplet state and a weak antiferromagnetic interdimer interaction J(2). There is a possibility that this spin system can be characterized as a spin-1 Kagome antiferromagnet at very low temperatures. m-MPYNN+ makes salts with various anions, I-, Cl-, BF4-, ClO4- etc. and their mixtures. In an isostructural solid-solution system, m-MPYNN+.(ClO4-)(x) . l-(1-x).(1/3)(acetone) (0 less than or equal to x less than or equal to 1), J(2) quickly weakens with increasing the ratio of the ClO4- ion, in contrast to little dependence of J(1).

The magneto-structural correlation in some p-N-alkylpyridinium nitronyl nitroxide salts has been studied. In the iodide salts of p-N-R-pyridinium nitronyl nitroxides with R=methyl, ethyl, n-propyl, and n-butyl, the magnetic property varies from antiferromagnetic to ferromagnetic with the extension of the N-alkyl chain. The nearest-neighbor molecular arrangements of the nitroxides observed in the four crystals, can be classified into two groups: the radicals are connected by a intermolecular contact between the NO groups (Type I) or between the NO group and the pyridinium ring (Type II). The observed magnetic behaviors can be interpreted in terms of an antiferromagnetic intermolecular interaction for the Type I contact and a ferromagnetic for Type II. Effects of replacement of the I- ion by Br- or Cl-, have been also studied in the p-N-methyl derivative.

Magnetic measurements and X-ray crystal analyses were carried out on iodide salts of p-N-alkylpyridinium a-nitronyl nitroxides [4-(4,4,5,5-tetramethyl-1-oxido-3-oxyl-4,5-dihydro-3H-imidazol-2'yl)-1-R-pyridinium, with R = methyl (1(+)), ethyl (2(+)), n-propyl (3(+)) and n-butyl (4(+))]. The strongly antiferromagnetic crystal of 1(+).l(-) consists of a radical dimer and the iodide ion is out of the plane of the pyridinium ring. 2(+).l(-), which is weakly antiferromagnetic, includes two crystallographically independent molecules, 2A(+) and 2B(+), each of which forms a centrosymmetric dimer. in the pyridinium ring of 2A(+) the iodides are 'out-of-plane' while for 2B(+) they are 'in-plane'. The ferromagnetic 3(+).l(-) and 4(+).l(-) have similar structures: the crystal consists of a two-dimensional (2D) layer formed by a contact between the pyridinium ring and in-plane iodides. In this series, the iodide ion changes position from out-of-plane to in-plane and the magnetism varies from antiferromagnetic to ferromagnetic. It is found that the nitronyl nitroxide with an out-of-plane iodide has a short intermolecular contact between the NO groups (type I), while that with an in-plane iodide forms a contact between the NO group and the pyridinium ring (type II). The observed magnetic behaviour can be interpreted in terms of an antiferromagnetic interaction for the type I contact and a ferromagnetic interaction for type II.

In this paper we study an organic system of geometrical spin frustration. m-N-methylpyridinium alpha-nitronyl nitroxide (m-MPYNN+) is a spin-1/2 organic radical. The simple salt, m-MPYNN+.ClO4-.1/3 (acetone), crystallizes in a trigonal P3c1 space group, where the m-MPYNN+ molecules exist as a dimer and the dimer units form a two-dimensional (2D) triangular lattice. One-third of the ClO4- ions are in the organic layer, joining the m-MPYNN+ molecules, and the remainder is between the layers, compensating the excess of positive charge in the organic layers. The single-crystal EPR measurements clearly indicate a 2D Heisenberg character of the magnetic system in it. m-MPYNN+ makes a crystalline solid-solution system, m-MPYNN+.(ClO4-)x.I1-x-.1/3 (acetone) (0 less-than-or-equal-to x less-than-or-equal-to 1), which also belongs to the trigonal system. Both the a and c axes are slightly lengthened with increasing the ratio of the ClO4- ion, x, in the solid solution: The unit-cell volume is increased by 3.2% when x runs from 0 to 1. The temperature dependence of the magnetic susceptibilities of the solid solutions can be well interpreted in terms of a strong ferromagnetic intradimer interaction J1 forming a triplet state and a weak antiferromagnetic interdimer interaction J2 which is expected to give rise to spin frustration among the triplet spin species on each side of the triangles. It is found that J2 quickly weakens with an increase in x, while J1 shows little dependence. There is a possibility that this organic system can be characterized as a spin-1 kagome anti-ferromagnet at very low temperatures.

In this paper we study an organic system of geometrical spin frustration. m-N-methylpyridinium α-nitronyl nitroxide (m-MPYNN+) is a spin-1/2 organic radical. The simple salt, m-MPYNN+ClO4-1/3 (acetone), crystallizes in a trigonal P3c1 space group, where the m-MPYNN+ molecules exist as a dimer and the dimer units form a two-dimensional (2D) triangular lattice. One-third of the C1O4- ions are in the organic layer, joining the m-MPYNN+ molecules, and the remainder is between the layers, compensating the excess of positive charge in the organic layers. The single-crystal EPR measurements clearly indicate a 2D Heisenberg character of the magnetic system in it. m-MPYNN+ makes a crystalline solid-solution system, m-MPYNN+(ClO4-)xI1-x-1/3 (acetone) (0≤x≤1), which also belongs to the trigonal system. Both the a and c axes are slightly lengthened with increasing the ratio of the C1O4- ion, x, in the solid solution: The unit-cell volume is increased by 3.2% when x runs from 0 to 1. The temperature dependence of the magnetic susceptibilities of the solid solutions can be well interpreted in terms of a strong ferromagnetic intradimer interaction J1 forming a triplet state and a weak antiferromagnetic interdimer interaction J2 which is expected to give rise to spin frustration among the triplet spin species on each side of the triangles. It is found that J2 quickly weakens with an increase in x, while J1 shows little dependence. There is a possibility that this organic system can be characterized as a spin-1 kagomé antiferromagnet at very low temperatures. © 1994 The American Physical Society.