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We report measurements on magnetization reversal in the Fe$_8$ molecular magnet using fast pulsed magnetic fields of 1.5 kT/s and in the temperature range of 0.6-4.1 K. We observe and analyze the temperature dependence of the reversal process, which involves in some cases several resonances. Our experiments allow observation of resonant quantum tunneling of magnetization up to a temperature of $\sim$ 4 K. We also observe shifts of the resonance fields in temperature that suggest the emergence of a thermal instability---a combination of spin reversal and self-heating that may result in a magnetic deflagration process. The results are mainly understood in the framework of thermally-activated quantum tunneling transitions in combination with emergence of a thermal instability., Comment: 6 pages 5 figs

Driving a two-dimensional superconductor normal by applying a high magnetic field may lead to Cooper pair localization. In this case, there should be a quantum critical point associated with specific scaling laws. Such a transition has been evidenced in a number of low critical temperature superconducting thin films and has been suggested to occur also in high temperature cuprate superconductors. Here we show experimental evidence for two distinct quantum critical regimes when applying perpendicular magnetic fields to underdoped La2-xSrxCuO4 thin films. At intermediate values of the magnetic field (18T-20T), a "ghost" QCP is observed, for which the values of the related critical exponents point towards a fermionic -as opposed to bosonic- scenario. At higher (about 37 T) magnetic field, another QCP is observed, which suggests the existence of either a 2D/3D or a clean/dirty temperature crossover., Comment: arXiv admin note: substantial text overlap with arXiv:1204.3493

The dynamics of magnetic hysteresis, including the training effect and the field sweep rate dependence of the exchange bias, is experimentally investigated in exchange-coupled potassium split graphene nanoribbons (GNRs). We find that, at low field sweep rate, the pronounced absolute training effect is present over a large number of cycles. This is reflected in a gradual decrease of the exchange bias with the sequential field cycling. However, at high field sweep rate above 0.5 T/min, the training effect is not prominent. With the increase in field sweep rate, the average value of exchange bias field grows and is found to follow power law behavior. The response of the exchange bias field to the field sweep rate variation is linked to the difference in the time it takes to perform a hysteresis loop measurement compared with the relaxation time of the anti-ferromagnetically aligned spins. The present results may broaden our current understanding of magnetism of GNRs and would be helpful in establishing the GNRs based spintronic devices., Accepted Applied Physics Letters (In press)

Exchange bias (EB) and the training effects (TE) in an antiferromagnetically coupled La0.7Sr0.3MnO3 / SrRuO3 superlattices were studied in the temperature range 1.8 - 150 K. Strong antiferromagnetic (AFM) interlayer coupling is evidenced from AC - susceptibility measurements. Below 100 K, vertical magnetization shifts are present due to the two remanent states corresponding to the two ferromagnetic (FM) layers at FM and AFM coupling condition. After field cooling (FC), significant decrease in the exchange bias field (HEB) is observed when cycling the system through several consecutive hysteresis loops. Quantitative analysis for the variation of HEB vs. number of field cycles (n) indicates an excellent agreement between the theory, based on triggered relaxation phenomena, and our experimental observations. Nevertheless, the crucial fitting parameter K indicates smooth training effect upon repeated field cycling, in accordance with our observation., Accepted Europhysics Letters

We report on in-situ resistivity measurements on GaMnAs during post-growth annealing in different atmospheres. A drop in the resistivity is observed when the GaMnAs is exposed to oxygen, which indicates that the passivation of Mn interstitials (Mn_I) at the free surface occurs through oxidation. The presence of oxygen can therefore be an important annealing condition for the optimization of GaMnAs thin films, all the more since the oxidation appears to be limited to the sample surface. Annealing in an oxygen-free atmosphere leads to an increase in the resistivity indicating a second annealing mechanism besides the out-diffusion of Mn_I. According to our magnetization and Hall effect data, this mechanism reduces the amount of magnetically and electrically active Mn atoms., Comment: 3 pages, 3 figures

In type II hard superconductors the irreversible magnetization shows an impressive variety of different magnetic field dependencies. In this paper we will try to describe the M(H) relation at fixed temperature To by a numerical model in which we incorporate two features: the magnetic and electric field dependency of the local critical current density j(To,H,E). The electric field is determined by the magnetic field sweep rate; in effect E ~ dB/dt. We confront the model with experimental pulsed field magnetization measurements (PFMM) on a fast melt processed YBa2Cu3O7 sample., 4 pages, PDF-file

The normal state resistivity of high temperature superconductors can be probed in the region below Tc by suppressing the superconducting state in high magnetic fields. Here we present the normal state properties of YBa2Cu3Ox thin films in the underdoped regime and the normal state resistance of La2-xSrxCuO4 thin films under epitaxial strain, measured below Tc by applying pulsed fields up to 60 T. A universal rho(T) behaviour is reported. We interpret these data in terms of the recently proposed 1D quantum transport model with the 1D paths corresponding to the charge stripes., 5 pages, PDF and PS, including figures, presented at MOS99 and accepted for publication in J. of Low Temp. Phys

The temperature dependence of the resistivity r(T) of the novel Sr2.5Ca11.5Cu24O41 ladder compound under hydrostatic pressure of up to 8 GPa has been explained by assuming that the relevant length scale for electrical transport is given by the magnetic correlation length related to the opening of a spin-gap in a 1D even-chain spin-ladder (1D-SL). The pressure dependence of the gap was extracted by applying this model to r(T) data obtained at different pressures. The r(T) dependence of the underdoped cuprate YBa2Cu4O8 demonstrates a remarkable scaling with the r(T) of the 1D-SL compound Sr2.5Ca11.5Cu24O41. This scaling implies that underdoped cuprates at Tc < T < T* are in the 1D regime and their pseudo-gap below T* is the spin-gap in the even-chain 1D-SL formed at T < T* in these materials., 10 pages, all PDF, contribution to Europhys.Lett."

We have studied the influence of columnar defects, created by heavy-ion (Kr) irradiation with doses up to 6 10^11 Kr-ions/cm2, on the superconducting critical parameters of single crystalline (YxTm1-x)Ba2Cu3O7. Magnetisation measurements in pulsed fields up to 50 T in the temperature range 4.2 - 90 K revealed that: (i) in fields up to T the critical current Jc(H,T) is considerably enhanced and (ii) down to temperatures T ~ 40 K the irreversibility field Hirr(T) is strongly increased. The field range and magnitude of the Jc(H,T) and Hirr(T) enhancement increase with increasing irradiation dose. To interpret these observations, an effective matching field was defined. Moreover, introducing columnar defects also changes the pinning force fp qualitatively. Due to stronger pinning of flux lines by the amorphous defects, the superconducting critical parameters largely exceed those associated with the defect structures in the unirradiated as-grown material: Jc,irrad(77 K, 5 T) ^3 10* Jc,ref(77 K, 5 T)., Comment: 11 pages, all PDF, contribution to Physica C