He Lun-Hua

Structure analysis of new superconductor MgB2

The superconductivity of a sintered pellet of the newly discovered MgB2 superconductor has been studied with magnetic measurements and its crystal structure was analysed using the Rietveld method of powder X-ray patterns. It has hexagonal symmetry (S.G. P6/mmm) with unit cell lattice parameters a=0.308136(14)nm and c=0.351782(17)nm.

Crystal structure and magnetic properties of La1.2Sr1.8-xCaxMn2O7

The effect of divalent cation substitution on the structure and magnetic properties of La1.2Sr1.8-xCaxMn2O7 (x = 0-0.900) is investigated in this paper. Partly replacing divalent cation Sr2+ by Ca2+ ions results in the weakening and then disappearance of long-range ferromagnetic ordering and the formation of spin canting and low-temperature spin-glass. Based on structural analysis by Rietveld profile fitting, we suggest that this variation of magnetic property be related to a Jahn-Teller-type lattice distortion of MnO6 octahedra due to the introduction of the smaller sized Ca2+ ions.

The structure and magnetic properties of La2-2xSr1Ca2xMn2O7 (x = 0.25-1.00)

In this paper, the effect of divalent cation substitution on the structure and magnetic properties in La2-2xSr1Ca2xMn2O7 have been investigated systematically using bulk samples with a wide doping concentration range 0.25 ≤ x ≤ 1.00. Replacing trivalent La ions by divalent Ca ions results in the weakening and then disappearance of the long-range ferromagnetic (FM) ordering, the formation of spin canting, antiferromagnetic (AFM) ordering and low-temperature spin-glass. These results show that increasing the hole-doping concentration significantly suppresses the FM state. We suggest that this variation of magnetic properties is related to the competition of the FM and AFM interactions resulting from the change of Mn3+/Mn4+ ratio and Jahn-Teller-type lattice distortion of MnO6 octahedra due to the introduction of Ca2+ ions.

Shape dependence of low-temperature magnetic relaxation of Mn(12)Ac

We report the discovery that the low-temperature magnetic relaxation in Mn(12)Ac single crystals strongly depends on the shape of the samples. The relaxation time exhibits a minimum at the phase transition point between ferromagnetic and antiferromagnetic phases. The shape dependence is attributed to the dipolar interaction between molecular magnets.We report the discovery that the low-temperature magnetic relaxation in Mn(12)Ac single crystals strongly depends on the shape of the samples. The relaxation time exhibits a minimum at the phase transition point between ferromagnetic and antiferromagnetic phases. The shape dependence is attributed to the dipolar interaction between molecular magnets.

The effect of doped ions on single-molecule magnets of the Mn12-Ac family

This paper investigates the single-molecule magnets of pure and Cr/Fe-doped Mn12-Ac. The components of the mixed crystals are identified by AC susceptibility technique. The ground-state spin and anisotropy parameters of doped Mn12-Ac are obtained: (i) Mn11Cr-Ac (S = 19/2, D = 0.62K, B = 0.0009 K, Δ = 63K), and (ii) Mn11Fe-Ac (S = 21/2, D = 0.39K, B = 0.001 K, Δ = 55K). The single-ion origin of the magnetic anisotropy is discussed.

Magnetic Quantum Tunnelling in Faster Relaxation Process in Mn12Ac Molecular Magnets

The field-tuned ac susceptibility of Mn12Ac single crystal has been measured as functions of temperature and frequency. Two relaxation processes appear in our measurement. One is related to the magnetic quantum tunnelling of a collective spin S = 12 by a 61 K magnetic barrier and it has been well studied, while the other is faster than the former and its mechanism is unclear. We find that the relaxation time for the faster process shows the minima at H = 0 and H = 3.4 kOe, indicating that the quantum tunnelling also takes place in this process while the resonant fields are different from what we have known before.

Resonant tunnelling of the magnetization in molecular crystal of [(Mn1-xCrx)12O12 (CH3COO)16(H2O)4] ·2CH3COOH·4H2O (x=0,0.03,0.04,0.05)

The quantum tunnelling of magnetization (QTM) in single crystals of the single molecule magnet (Mn1-xCrx)12-Ac (x=0, 0.03, 0.04, 0.05) has been investigated. In comparison with its parent Mn12-Ac, a greater rate of magnetization relaxation and a lower effective potential-energy barrier have been observed in Cr-doping samples. This modulation of QTM due to the Cr-doping could be attributed to the small change of Sz due to the smaller spin of Cr itself and additional intrinsic but distributed transverse and longitudinal anisotropy raised by a subtle change of the local environment in the magnetic Mn12 core.

Structure and magnetic properties of a new single-molecule magnet [Mn11Fe1O12 (CH3COO)16(H2O)4] .2CH3COOH.4H2O

A new single-molecule magnet [Mn11Fe1O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn11Fe1) has been synthesized. The structure has been studied by the single crystal x-ray diffraction. The difference of Jahn–Teller distortion between Fe3+ and Mn3+ ion reveals that Fe3+ ion substitutes for Mn3+ ion on the Mn(3) sites in the Mn12 skeleton. The temperature dependence of the magnetization gives a blocking temperature TB=1.9K for Mn11Fe1. Based on the magnetization process analysis of the crystal at T=2K, we suggest that Mn11Fe1 has the ground state with a total spin S= 11/2.