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Register a new userVisualizing Intramolecular Distortions as the Origin of Transverse Magnetic Anisotropy
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The magnetic properties of metal-organic complexes are strongly influenced by conformational changes in the ligand. The flexibility of Fe-tetra-pyridyl-porphyrin molecules leads to different adsorption configurations on a Au(111) surface. By combining low-temperature scanning tunneling spectroscopy and atomic force microscopy, we resolve a correlation of the molecular configuration with different spin states and magnitudes of magnetic anisotropy. When the macrocycle exhibits a laterally-undistorted saddle shape, the molecules lie in a S=1 state with axial anisotropy arising from a square-planar ligand field. If the symmetry in the molecular ligand field is reduced by a lateral distortion of the molecule, we find a finite contribution of transverse anisotropy. Some of the distorted molecules lie in a S=2 state, again exhibiting substantial transverse anisotropy.
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The problem of the role of transverse fields in Mn12-acetate, a molecular nanomagnet, is still open. We present structural evidences that the disorder of the acetic acid of crystallization indices sizeable distortion of the Mn(III) sites, giving rise to six different isomers, four of them with symmetry lower than tetragonal. Using a ligand field approach the effect of the structure modifications on the second order transverse magnetic anisotropy, forbidden in tetragonal symmetry, has been evaluated. The order of magnitude of the quadratic transverse anisotropies well agree with the values derived by the analysis of the field sweep dependence of the hysteresis loops performed by Mertes et al. (Phys. Rev. Lett 87, 227205 (2001)) and allows to better simulate the EPR spectra.
This paper discusses transport methods for the investigation of the (Ga,Mn)As magnetic anisotropy. Typical magnetoresistance behaviour for different anisotropy types is discussed, focusing on an in depth discussion of the anisotropy fingerprint technique and extending it to layers with primarily uniaxial magnetic anisotropy. We find that in all (Ga,Mn)As films studied, three anisotropy components are always present. The primary biaxial along ([100] and [010]) along with both uniaxial components along the [110] and [010] crystal directions which are often reported separately. Various fingerprints of typical (Ga,Mn)As transport samples at 4 K are included to illustrate the variation of the relative strength of these anisotropy terms. We further investigate the temperature dependence of the magnetic anisotropy and the domain wall nucleation energy with the help of the fingerprint method.
We theoretically investigate the effect of transverse magnetic anisotropy on spin-flip assisted tunneling through atomic spin chains. Using a phenomenological approach and first-order perturbation theory, we analytically calculate the inelastic tunneling current, differential conductance and atomic spin transition rates. We predict the appearance of additional steps in the differential conductance and a pronounced increase in the spin-flip transition rate which at low voltages scale quadratically with the ratio of the transverse anisotropy energy and the sum of the longitudinal anisotropy energy and the exchange energy. Our results provide intuitive quantitative insight in the role played by transverse anisotropy in inelastic tunneling spectroscopy of atomic chains and can be observed under realistic experimental conditions.
Recent analytical and numerical work on field driven domain wall propagation in nanowires has shown that for large transverse anisotropy and sufficiently large applied fields the Walker profile becomes unstable before the breakdown field, giving way to a slower stationary domain wall. We perform an asymptotic expansion of the Landau Lifshitz Gilbert equation for large transverse magnetic anisotropy and show that the asymptotic dynamics reproduces this behavior. At low applied field the speed increases linearly with the field and the profile is the classic Landau profile. Beyond a critical value of the applied field the domain wall slows down. The appearance of a slower domain wall profile in the asymptotic dynamics is due to a transition from a pushed to a pulled front of a reaction diffusion equation.
We obtain control of magnetic anisotropy in epitaxial (Ga,Mn)As by anisotropic strain relaxation in patterned structures. The strain in the structures is characterized using sophisticated X-ray techniques. The magnetic anisotropy before patterning of the layer, which shows biaxial easy axes along [100] and [010], is replaced by a hard axis in the direction of large elastic strain relaxation and a uniaxial easy axis in the direction where pseudomorphic conditions are retained. This strong anisotropy can not be explained by shape anisotropy and is attributed solely to lattice strain relaxation. Upon increasing the uniaxial strain anisotropy in the (Ga,Mn)As stripes, we also observe an increase in magnetic anisotropy.
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