PACS. 75.45+j -Macroscopic quantum phenomena in magnetic systems. PACS. 75.60 Ej -Magnetisation curves, hysteresis, Barkhausen and related effects. PACS. 75.50 Tt -Fine-particle systems.Abstract. -Precise magnetic hysteresis measurements of small single crystals of Mn12 acetate of spin 10 have been conducted down to 0.4 K using a high sensitivity Hall magnetometer. At higher temperature (> 1.6 K) step-like changes in magnetization are observed at regularly spaced magnetic field intervals, as previously reported. However, on lowering the temperature the steps in magnetization shift to higher magnetic fields, initially gradually. These results are consistent with the presence of a second order uniaxial magnetic anisotropy, first observed by EPR spectroscopy, and thermally assisted tunnelling with tunnelling relaxation occurring from levels of progressively lower energy as the temperature is reduced. At lower temperature an abrupt shift in step positions is found. We suggest that this shift may be the first evidence of an abrupt, or first-order, transition between thermally assisted and pure quantum tunnelling, suggested by recent theory.Introduction. -The high spin (S = 10) molecular magnets Mn 12 acetate and Fe 8 have become prototypes for the study of the transition from classical superparamagnetism to quantum tunnelling of mesoscopic spins. Much of the recent interest in these materials has been stimulated by the observation of a remarkably regular series of steps and plateaus in the magnetic hysteresis loops of Mn 12 at low temperature (below a blocking temperature of 3 K), first in oriented powders [1] and shortly thereafter in single crystals [2]. These results indicate that the relaxation rate of the magnetization toward equilibrium is greatly enhanced at well-defined intervals of magnetic field. These observations have been interpreted within a simple effective spin Hamiltonian for these molecules and a model of thermally assisted tunnelling of the magnetization, first suggested in reference [3]. This model describes a regime intermediate between thermal activation over the anisotropy barrier (superparamagnetism)

The magnetoresistance (MR) associated with domain boundaries has been investigated in microfabricated bcc Fe (0.65 to 20 µm linewidth) wires with controlled stripe domains. Domain configurations have been characterized using MFM. MR measurements as a function of field angle, temperature and domain configuration are used to estimate MR contributions due to resistivity anisotropy and domain walls. Evidence is presented that domain boundaries enhance the conductivity in such microstructures over a broad range of temperatures (1.5 K to 80 K).

We report zero-field inelastic neutron scattering experiments on a deuter-

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We report point-by-point measurements below the blocking temperature of the magnetic relaxation of Mn 12 -acetate as a function of magnetic field applied along the easy axis of magnetization. Unexpectedly complex structure is observed which we attribute to the effect of higher-order terms of the spin Hamiltonian on the tunneling process. RAPID COMMUNICATIONS R9258PRB 62 ZHONG, SARACHIK, YOO, AND HENDRICKSON

We report Hall sensor measurements of the magnetic relaxation of Mn12-acetate as a function of magnetic field applied along the easy axis of magnetization. Data taken at a series of closely-spaced temperatures between 0.24 K and 1.4 K provide strong new evidence for an abrupt "first-order" transition between thermally-assisted relaxation and magnetic decay via quantum tunneling.

PACS. 75.45+j -Macroscopic quantum phenomena in magnetic systems. PACS. 75.60 Ej -Magnetisation curves, hysteresis, Barkhausen and related effects. PACS. 75.50 Tt -Fine-particle systems.Abstract. -Precise magnetic hysteresis measurements of small single crystals of Mn12 acetate of spin 10 have been conducted down to 0.4 K using a high sensitivity Hall magnetometer. At higher temperature (> 1.6 K) step-like changes in magnetization are observed at regularly spaced magnetic field intervals, as previously reported. However, on lowering the temperature the steps in magnetization shift to higher magnetic fields, initially gradually. These results are consistent with the presence of a second order uniaxial magnetic anisotropy, first observed by EPR spectroscopy, and thermally assisted tunnelling with tunnelling relaxation occurring from levels of progressively lower energy as the temperature is reduced. At lower temperature an abrupt shift in step positions is found. We suggest that this shift may be the first evidence of an abrupt, or first-order, transition between thermally assisted and pure quantum tunnelling, suggested by recent theory.Introduction. -The high spin (S = 10) molecular magnets Mn 12 acetate and Fe 8 have become prototypes for the study of the transition from classical superparamagnetism to quantum tunnelling of mesoscopic spins. Much of the recent interest in these materials has been stimulated by the observation of a remarkably regular series of steps and plateaus in the magnetic hysteresis loops of Mn 12 at low temperature (below a blocking temperature of 3 K), first in oriented powders [1] and shortly thereafter in single crystals [2]. These results indicate that the relaxation rate of the magnetization toward equilibrium is greatly enhanced at well-defined intervals of magnetic field. These observations have been interpreted within a simple effective spin Hamiltonian for these molecules and a model of thermally assisted tunnelling of the magnetization, first suggested in reference [3]. This model describes a regime intermediate between thermal activation over the anisotropy barrier (superparamagnetism)

No abstract