Thickness and temperature dependence of magnetic anisotropies in ultrathin fcc Co(001) structures

Experimental exploration of dynamic phase transitions and associated metamagnetic fluctuations for materials with different Curie temperatures

We study dynamic magnetic behavior in the vicinity of the dynamic phase transition (DPT) for a suitable series of samples that have different Curie temperatures T_{C}, which thus enables us to experimentally explore the role of the reduced temperature T/T_{C} in the DPT. For this purpose, we fabricate Co_{1-x}Ru_{x} epitaxial thin films with uniaxial in-plane anisotropy by means of sputter deposition in the concentration range 0.0≤x≤0.26. All samples are ferromagnetic at room temperature, exhibit an abrupt magnetization reversal along their easy axis, and represent a unique T_{C} and thus T/T_{C} ratio according to their Ru concentration. The dynamic magnetic behavior is measured by using an ultrasensitive transverse magneto-optical detection method and the resulting dynamic states are explored as a function of the applied magnetic field amplitude H_{0} and period P, as well as an additional bias field H_{b}, which is the conjugate field of the dynamic order parameter Q. Our experimental results demonstrate that the qualitative behavior of the dynamic phase diagram is independent of the T/T_{C} ratio and that for all T/T_{C} values we observe metamagnetic anomalies in the dynamic paramagnetic state, which do not exist in the corresponding thermodynamic phase diagram. However, quantitatively, these metamagnetic anomalies are very strongly dependent on the T/T_{C} ratio, leading to an about 20-fold increase of large metamagnetic fluctuations in the paramagnetic regime as the T/T_{C} ratio increases from 0.37 to 0.68. Also, the phase space range in which these anomalous metamagnetic fluctuations occur extends closer and closer to the critical point as T/T_{C} increases.

Control of magnetocrystalline anisotropy by epitaxial strain in double perovskite Sr(2)FeMoO(6) films

We demonstrate tuning of magnetocrystalline anisotropy in high-quality Sr(2)FeMoO(6) epitaxial films over a range of several thousand Gauss using strain induced by epitaxial growth on substrates of varying lattice constants. Spectroscopic measurements reveal a striking, linear dependence of the out-of-plane anisotropy on the strain-induced tetragonal distortion of the Sr(2)FeMoO(6) lattice. This anisotropy can be tuned from +2000 to -3300 Oe, a range sufficient to rotate the easy axis from in plane to out of plane. Combined with its half-metallicity and high Curie temperature, this result implies a broad range of scientific and technological applications for this novel spintronic material.

The anomalous Hall effect in epitaxial face-centered-cubic cobalt films

The anomalous Hall effect in epitaxial face-centered-cubic Co film grown by molecular beam epitaxy on MgO(001) is investigated. The intrinsic anomalous Hall conductivity is separated from the scattering-related extrinsic contributions and determined to be 727 Ω(-1) cm(-1) and temperature independent. This result provides well defined experimental data for further theoretical justification.

Self-formed exchange bias of switchable conducting filaments in NiO resistive random access memory capacitors

We report on the ferromagnetism of conducting filaments formed in a NiO thin film, which exhibited a typical bistable resistive switching characteristic. The NiO thin film showed an antiferromagnetic hysteresis loop for a high resistive state (R(OFF)). However, for a low resistive state (R(ON)), the conducting filaments exhibited a ferromagnetic hysteresis loop for the field cooling. The ferromagnetic hysteresis behavior of the R(ON) state reveals switchable exchange coupling between the ferromagnetic Ni conducting filaments and the antiferromagnetic NiO layer.

Simulation of hysteresis in magnetic nanoparticles with Nosé thermostating

The magnetic hysteresis of a two-dimensional lattice of rotors with four-way anisotropy interaction and a Heisenberg exchange interaction is studied. The Hamiltonian dynamics of the lattice is thermostated using the Nosé thermostat, resulting in a system that approaches thermal equilibrium and which under certain conditions can remain in metastable states. Using physically realistic values for the interactions in a nanoparticle of monolayer thickness, we locate the Curie temperature of our lattice by determining the peak of the heat capacity curve. We then compare the coercive field of our two-dimensional lattice below this Curie temperature to the coercive field of an elliptical cobalt nanoparticle measured in experiment. We find an order of magnitude agreement between our lattice model and the experimental results, even though the value of the anisotropy used is more appropriate for a monolayer film than for the nanoparticle.

Anisotropic spin-orbit coupling and magnetocrystalline anisotropy in vicinal co films

The anisotropy of the spin-orbit interaction, <lambda(a)>, in vicinal Co films has been measured using x-ray magnetic linear dichroism (XMLD). A linear increase in <lambda(a)> with Co step density is found using a new sum rule and represents the first experimental confirmation that XMLD probes the magnetocrystalline anisotropy energy (MAE). X-ray magnetic circular dichroism is used to confirm that the XMLD arises from changes in the local step-edge electronic structure. The XMLD sum rule gives a larger MAE compared to macroscopic values and is discussed with respect to other local probes of the MAE.