Magnetization plateau of two-dimensional antiferromagnetic solid 3He on a triangular lattice.

3He film adsorbed on a graphite surface offers an ideal two-dimensional antiferromagnetic S=1/2 quantum spin system on a triangular lattice. In addition, competition between various multiple spin exchange interactions makes it strongly frustrated. The ground state in such a frustrated quantum spin system is one of the most interesting issues in condensed matter physics. Recent experiments on the antiferromagnetic solid 3He in the second layer indicate the ground state is a spin liquid, whereas there still exists a controversy on whether or not the spin gap is finite. Here we report the first precise magnetization measurement below 1 mK over the wide magnetic field range up to 11 T. The magnetization curve is found to show a plateau at half of the saturation magnetization, being followed by the full saturation at the unexpectedly high field of around 10 T.

High temperature superconductor micro-superconducting-quantum-interference-device magnetometer for magnetization measurement of a microscale magnet.

We have developed a high temperature superconductor (HTS) micrometer-sized dc superconducting quantum interference device (SQUID) magnetometer for high field and high temperature operation. It was fabricated from YBa2Cu3O7-delta of 92 nm in thickness with photolithography techniques to have a hole of 4x9 microm2 and 2 microm wide grain boundary Josephson junctions. Combined with a three dimensional magnetic field coil system, the modulation patterns of critical current Ic were observed for three different field directions. They were successfully used to measure the magnetic properties of a molecular ferrimagnetic microcrystal (23x17x13 microm3), [Mn2(H2O)2(CH3COO)][W(CN)8]2H2O. The magnetization curve was obtained in magnetic field up to 0.12 T between 30 and 70 K. This is the first to measure the anisotropy of hysteresis curve in the field above 0.1 T with an accuracy of 10(-12) J T(-1) (10(-9) emu) with a HTS micro-SQUID magnetometer.

Fast magnetization tunneling in tetranickel(II) single-molecule magnets.

A series of Ni(4) cubane complexes with the composition [Ni(hmp)(ROH)Cl](4) complexes 1-4 where R= -CH(3) (complex 1), -CH(2)CH(3) (complex 2), -CH(2)CH(2)(C(4)H(9)) (complex 3), -CH(2)CH(2)CH(2)(C(6)H(11)) (complex 4), hmp(-) is the anion of 2-hydroxymethylpyridine, t-Buhmp(-) is the anion of 4-tert-butyl-2-hydroxymethylpyridine, and dmb is 3,3-dimethyl-1-butanol] and [Ni(hmp)(dmb)Br](4) (complex 5) and [Ni(t-Buhmp)(dmb)Cl](4) (complex 6) were prepared. All six complexes were characterized by dc magnetic susceptibility data to be ferromagnetically coupled to give an S = 4 ground state with significant magnetoanisotropy (D approximately equal to -0.6 cm(-1)). Magnetization hysteresis measurements carried out on single crystals of complexes 1-6 establish the single-molecule magnet (SMM) behavior of these complexes. The exchange bias observed in the magnetization hysteresis loops of complexes 1 and 2 is dramatically decreased to zero in complex 3, where the bulky dmb ligand is employed. Fast tunneling of magnetization is observed for the high-symmetry (S(4) site symmetry) Ni(4) complexes in the crystal of complex 3, and the tunneling rate can even be enhanced by destroying the S(4) site symmetry, as is the case for complex 4, where there are two crystallographically different Ni(4) molecules, one with C(2) and the other with C(1) site symmetry. Magnetic ordering temperatures due to intermolecular dipolar and magnetic exchange interactions were determined by means of very low-temperature ac susceptibility measurements; complex 1 orders at 1100 mK, complex 3 at 290 mK, complex 4 at approximately 80 mK, and complex 6 at <50 mK. This confirms that bulkier ligands correspond to more isolated molecules, and therefore, magnetic ordering occurs at lower temperatures for those complexes with the bulkiest ligands.

Possible one-dimensional 3He quantum fluid formed in nanopores.

Heat capacity measurements have been made down to 5 mK for 3He fluid films adsorbed in one-dimensional (1D) nanometer-scale pores, 28 A in diameter, preplated with 4He of 1.47 atomic layers. At low 3He density, the heat capacity shows a density-dependent, Schottky-like peak near 150 mK asymptoting to the value corresponding to a 2D Boltzmann gas at high temperatures. The peak behavior is attributed to the crossover from a 2D gas to a 1D state at low temperatures. The degenerate state of the 1D 3He fluid is indicated by a predominantly linear temperature dependence below about 30 mK.

A dinuclear MnIII-CuII single-molecule magnet.

The reaction of 1/3 equivalent of CuCl2.2H2O with MnCl2.4H2O and 5-bromo-2-salicylideneamino-1-propanol (H(2)5-Br-sap) in methanol gave dark brown crystals of [MnIIICuIICl(5-Br-sap)2(MeOH)] (1). Complex 1 has an alkoxo-bridged dinuclear core of MnIII and CuII ions, which have elongated octahedral and square-planar coordination geometries, respectively. In dc magnetic susceptibility measurements, chi(m)T values increased as the temperature was lowered, followed by a sudden decrease below 20 K. This behavior is indicative of the occurrence of intramolecular ferromagnetic interactions, and fitting gave an S=5/2 spin ground state with an exchange coupling constant J(MnCu) of +78 cm(-1). Magnetization data collected as a function of temperature and applied magnetic field were analyzed by using a spin Hamiltonian with isotropic Zeeman and axial zero-field splitting (ZFS) terms, and a negative D(5/2) value (-1.86 cm(-1)) was obtained. A high-field EPR (HFEPR) spectrum (342.0 GHz) at 4.2 K was composed of four peaks, and two additional peaks at higher magnetic field appeared as the temperature was increased. The temperature dependences in the HFEPR spectra are indicative of a negative D(5/2) value, and fitting of the data gave D(5/2)=-1.81 cm(-1). In the ac magnetic susceptibility measurements, frequency dependent in-phase (chi(m)') and out-of-phase (chi(m)'') signals with peak maxima at 0.7-1.5 K were observed and small peaks below 0.7 K appeared. The ac susceptibility data supports that 1 is a single-molecule magnet (SMM). Arrhenius plots for the chi(m)'' peaks from 0.7-1.5 K gave the re-orientation energy barrier (DeltaE) of 10.5 K with a pre-exponential factor of 8.2x10(-8) s.

Growth of a third ferromagnetic solid 3He layer on graphite.

We have measured the nuclear susceptibility of 3He in Grafoil filled with pure liquid 3He over the pressure region between 0.6 and 31.38 bars and at temperatures down to 0.5 mK with a cw NMR method. The nuclear magnetization corresponding to the adsorbed 3He layers on the Grafoil surface shows a strong ferromagnetic tendency with a periodic behavior as a function of liquid pressure. This observation is attributable to the growth of third and fourth solid 3He layers with the liquid pressure increase. The pressure dependence of the Weiss temperature indicates the third layer is completed at 19 bars and the fourth probably at 28 bars. The number of localized spins estimated from the solid magnetization is almost doubled from 0 to 28 bars, being consistent with this scenario.

Gapless spin liquid behavior in two-dimensional solid 3He.

Direct demagnetization has been made for two-dimensional solid 3He in both the paramagnetic and the antiferromagnetic phases. The lowest temperature is about 10 microK, judging from the observed magnetization for the paramagnetic solid 3He. The magnetization of the antiferromagnetic solid 3He shows a gradual increase to about 10 microK for the 4/7 phase adsorbed on both one layer of 4He and two layers of HD preplated graphite. This strongly suggests that the triangular antiferromagnet with the higher order multiple exchange has a quantum spin liquid ground state with nearly zero or extremely small spin gap less than 10 microK.

Quantum tunneling of magnetization in a new [Mn18]2+ single-molecule magnet with s = 13.

The reaction between 2-(hydroxyethyl)pyridine (hepH) and a 2:1 molar mixture of [Mn3O(O2CMe)6(py)3](ClO4) and [Mn3O(O2CMe)6(py)3](py) in MeCN leads to isolation of [Mn18O14(O2CMe)18(hep)4(hepH)2(H2O)2](ClO4)2 (1) in 10% yield. The complex is 2MnII,16MnIII and consists of a Mn4O6 central unit to either side of which is attached a Mn7O9 unit. Magnetization data collected in the 2.0-4.0 K and 20-50 kG ranges were fit to yield S = 13, g = 1.86, and D = -0.13 cm-1 = -0.19 K, where D is the axial zero-field splitting parameter. AC susceptibility studies in the 0.04-4.0 K range at frequencies up to 996 Hz display out-of-phase (chiM' ') signals, indicative of a single-molecule magnet (SMM). Magnetization vs applied DC field scans exhibit hysteresis at <1.0 K, confirming 1 to be a SMM. DC magnetization decay data were collected on both a microcrystalline sample and a single crystal, and the combined data were used to construct an Arrhenius plot. Between 3.50 and 0.50 K, the relaxation rate is temperature-dependent with an effective barrier to relaxation (Ueff) of 14.8 cm-1 = 21.3 K. Below ca. 0.25 K, the relaxation rate is temperature-independent at 1.3 x 10-8 s-1, indicative of quantum tunneling of magnetization (QTM) between the lowest energy Ms = +/-13 levels of the S = 13 state. Complex 1 is both the largest spin and highest nuclearity SMM to exhibit QTM.