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An electron transport is studied in the system which consists of scanning tunneling microscopysingle molecule magnet-metal. Due to quantum tunneling of magnetization in single-molecule magnet, linear response conductance exhibits stepwise behavior with increasing longitudinal field and each step is maximized at a certain value of field sweeping speed. The conductance at each step oscillates as a function of the additional transverse magnetic field along the hard axis. Rigorous theory is presented that combines the exchange model with the Landau-Zener model. PACS numbers: 75.45.+j, 75.50.Xx, 75.50.Tt Recently high-spin molecular nanomagnets such as Mn 12 or Fe 8 attracted lots of attention due to observation of quantum tunneling of magnetization and possible applications in information storage and quantum computing [1,2,3,4,5,6]. These single-molecule magnets (SMMs) exhibit steps in the hysteresis loops at low temperature, which is attributed to resonant tunneling between degenerate quantum states or quantum tunneling of magnetization(QTM). These unique features of SMMs are the consequence of long-living metastable spin states due to the large spin and strong anisotropy of SMMs. QTM also made it possible to detect the interference effect of Berry's phase on the magnetization at each step while the transverse field along the hard axis is varied [5,6]. Novel features of quantum tunneling are expected to manifest themselves in, if any, other observables. Especially the effects of QTM on the electronic transport remain to be explored in both experiments [7] and theories.In this paper we study theoretically the effects of QTM on the transport properties of SMMs which are deposited on a metallic surface with monolayer coverage. Placing the scanning tunneling microscopy(STM) tip right above one SMM, we compute the electric current which flows through a SMM when the bias voltage is applied between the STM tip and the metallic substrate (Fig. 1). We find that the linear response conductance increases stepwise like the magnetization of a SMM as a longitudinal magnetic field is increased. The stepwise behavior of conductance results from the QTM in SMM. The conductance at each step oscillates periodically as a function of additional transverse magnetic field along the hard axis. Our theoretical predictions are not known in the literature as far as we know and can be tested experimentally.When a finite bias voltage is applied between the STM tip and the metallic substrate, the electrons will tunnel through a vacuum between the metal surface and the STM tip. Since the STM tip is placed right above the SMM in our model system, the tunneling electrons may well be scattered by the large spin of a SMM. Our model system can be considered as the conventional tunnel junction with a SMM sandwiched between two normal metallic electrodes. The metallic substrate and STM tip are conveniently called the left(p = L) and right(p = R) electrodes, respectively. Two electrodes are described by the featureless conduction bands with the energ...

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The effect of magnetoelastic dissipation upon the tunneling and coherence of the total magnetization in small (-100-A) single-domain ferromagnetic particles is investigated. Such tunneling would be an example of a macroscopic quantum phenomenon. It is shown that the dissipation has a super-Ohmic spectral density, J(c»)~(u^, and that the dissipation is weak for reasonable material parameters. Corrections to the rates, and the damping rate for coherent oscillations, are obtained. A key feature is the inclusion of the elastic waves in the medium surrounding the particle.PACS numbers: 75.60. Jp, 03.40.Dz, 03.65.Sq, 75.80.+q Consider a small, single-domain ferromagnetic particle at a temperature low enough to freeze out spin waves due to crystalline anisotropy. At such temperatures, the exchange interactions align the individual magnetic moments nearly perfectly, and the spontaneous magnetization, M, is close to its saturation value, Mo, in magnitude. The only interesting degree of freedom left is then the direction of M, and the question arises whether it is possible to see quantum-mechanical effects in its dynamical behavior. Since a sphere of radius a-50 A contains about 10^-10^ moments, such effects would provide another instance of macroscopic quantum phenomena, e.g., tunneling and coherence (MQT and MQC, respectively), which so far have been studied with any seriousness only for the rf SQUID and the closely related current-biased Josephson junction. ^'"^ The issue of macroscopic magnetization tunneling (MMT) was recently raised by Chudnovsky and Gun-ther^ (CG) (see also Refs. 6 and 7), who observed that both the barrier to tunneling and the mechanism for it would be provided by the magnetocrystalline anisotropy^ which is always present. Except for a few passing remarks, however, they have not considered the dissipative effect of the environment, and it is the purpose of this paper to do so. Dissipation generally suppresses quantum effects, and it is therefore clearly desirable to assess its importance for MMT before contemplating real experiments. The only source of dissipation that we shall consider in this paper is the magnetoelastic coupling of M to the phonons. This is perhaps the most obvious intrinsic source, and is also the one mentioned by CG.^The setup for MQT^ consists of a biaxial crystal, magnetized along an easy axis which we label x, and a field H opposite to M. '^ The Hamiltonian for M can be taken to be the experimentally determined anisotropy energy itself since this guarantees the correct semiclassical dynamics. In polar coordinates, the energy density can be taken to be Eie,(f>) = Kx cos^^+ (^1+^2) sin^^sin^^ -HMoil -sin^cos0) ,