Longitudinal relaxation and thermoactivation of quantum superparamagnets

The relaxation mechanisms of a quantum nanomagnet are discussed in the frame of linear response theory. We use a spin Hamiltonian with a uniaxial potential barrier plus a Zeeman term. The spin, having arbitrary $S$, is coupled to a bosonic environment. From the eigenstructure of the relaxation matrix, we identify two main mechanisms, namely, thermal activation over the barrier, with a time scale \eival_1^{-1}, and a faster dynamics inside the potential wells, with characteristic time \eivalW^{-1}. This allows to introduce a simple analytical formula for the response, which agrees well with the exact numerical results, and cover experiments even under moderate to strong fields in the superparamagnetic range. In passing, we generalize known classical results for a number of quantities (e.g., integral relaxation times, initial decay time, Kramers rate), results that are recovered in the limit $S\to\infty$.Comment: submitted to Phys. Rev.

Field-induced level crossings in spin clusters: Thermodynamics and magneto-elastic instability

Quantum spin clusters with dominant antiferromagnetic Heisenberg exchange interactions typically exhibit a sequence of field-induced level crossings in the ground state as function of magnetic field. For fields near a level crossing, the cluster can be approximated by a two-level Hamiltonian at low temperatures. Perturbations, such as magnetic anisotropy or spin-phonon coupling, sensitively affect the behavior at the level-crossing points. The general two-level Hamiltonian of the spin system is derived in first-order perturbation theory, and the thermodynamic functions magnetization, magnetic torque, and magnetic specific heat are calculated. Then a magneto-elastic coupling is introduced and the effective two-level Hamilitonian for the spin-lattice system derived in the adiabatic approximation of the phonons. At the level crossings the system becomes unconditionally unstable against lattice distortions due to the effects of magnetic anisotropy. The resultant magneto-elastic instabilities at the level crossings are discussed, as well as the magnetic behavior.Comment: 13 pages, 8 figures, REVTEX

Coherent Magnetization Precession in GaMnAs induced by Ultrafast Optical Excitation

We use femtosecond optical pulses to induce, control and monitor magnetization precession in ferromagnetic Ga0.965Mn0.035As. At temperatures below ~40 K we observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ~TC/2. We find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but largely independent on ambient temperature. We provide a physical interpretation of the observed light-induced collective Mn-spin relaxation and precession.Comment: 7 pages,3 figure

Solving spin quantum-master equations with matrix continued-fraction methods: application to superparamagnets

We implement continued-fraction techniques to solve exactly quantum master equations for a spin with arbitrary S coupled to a (bosonic) thermal bath. The full spin density matrix is obtained, so that along with relaxation and thermoactivation, coherent dynamics is included (precession, tunnel, etc.). The method is applied to study isotropic spins and spins in a bistable anisotropy potential (superparamagnets). We present examples of static response, the dynamical susceptibility including the contribution of the different relaxation modes, and of spin resonance in transverse fields.Comment: Resubmitted to J. Phys. A: Math. Gen. Some rewriting here and there. Discussion on positivity in App.D3 at request of one refere

13C NMR study of superconductivity near charge instability realized in beta"-(BEDT-TTF)4[(H3O)Ga(C2O4)3]C6H5NO2

To investigate the superconducting (SC) state near a charge instability, we performed ^{13}C NMR experiments on the molecular superconductor beta"-(BEDT-TTF)_{4}[(H_{3}O)Ga(C_{2}O_{4})_{3}]C_{6}H_{5}NO_{2}, which exhibits a charge anomaly at 100 K. The Knight shift which we measured in the SC state down to 1.5 K demonstrates that Cooper pairs are in spin-singlet state. Measurements of the nuclear spin-lattice relaxation time reveal strong electron-electron correlations in the normal state. The resistivity increase observed below 10 K indicates that the enhanced fluctuation has an electric origin. We discuss the possibility of charge-fluctuation-induced superconductivity.Comment: 5 pages, 4 figure

Equilibrium susceptibilities of superparamagnets: longitudinal & transverse, quantum & classical

The equilibrium susceptibility of uniaxial paramagnets is studied in a unified framework which permits to connect traditional results of the theory of quantum paramagnets, \Sm=1/2, 1, 3/2, ..., with molecular magnetic clusters, \Sm\sim5, 10, 20, all the way up, \Sm=30, 50, 100,... to the theory of classical superparamagnets. This is done using standard tools of quantum statistical mechanics and linear response theory (the Kubo correlator formalism). Several features of the temperature dependence of the susceptibility curves (crossovers, peaks, deviations from Curie law) are studied and their scalings with \Sm identified and characterized. Both the longitudinal and transverse susceptibilities are discussed, as well as the response of the ensemble with anisotropy axes oriented at random. For the latter case a simple approximate formula is derived too, and its range of validity assessed, so it could be used in modelization of experiments.Comment: 32 pages, 5 figures. Submitted to J.Phys.Condens.Matte