Dimensionality crossover in the induced magnetization of Pd layers

Ising-like behaviour of mesoscopic magnetic chains

We demonstrate an experimental realization of the short range magnetic order in a one-dimensional Ising chain using fabricated mesospins. We confirm an excellent agreement between the experimental findings and simulations obtained using the original Ising model. In particular, we are able to show that the thermal behaviour of the mesoscopic Ising chain dominates over the thermal behaviour of the individual mesospins themselves, confirming that fabricated mesospins can be viewed as artificial magnetic atoms.

Thermally induced magnetic relaxation in square artificial spin ice

The properties of natural and artificial assemblies of interacting elements, ranging from Quarks to Galaxies, are at the heart of Physics. The collective response and dynamics of such assemblies are dictated by the intrinsic dynamical properties of the building blocks, the nature of their interactions and topological constraints. Here we report on the relaxation dynamics of the magnetization of artificial assemblies of mesoscopic spins. In our model nano-magnetic system - square artificial spin ice – we are able to control the geometrical arrangement and interaction strength between the magnetically interacting building blocks by means of nano-lithography. Using time resolved magnetometry we show that the relaxation process can be described using the Kohlrausch law and that the extracted temperature dependent relaxation times of the assemblies follow the Vogel-Fulcher law. The results provide insight into the relaxation dynamics of mesoscopic nano-magnetic model systems, with adjustable energy and time scales, and demonstrates that these can serve as an ideal playground for the studies of collective dynamics and relaxations.

Exploration of magnetic fluctuations in PdFe films

We investigated magnetic phase transitions, magnetic anisotropy, and magnetic domains in Pd1-xFex alloys with different Fe concentrations x = 2.2-7.2%. The Curie temperature depends linearly on the Fe concentration in the regime studied. The magnetization is dominantly in-plane with a small out-of-plane remanent contribution. Resonant magnetic small angle scattering with circularly polarized x-rays tuned to the L3 resonance edge of Fe revealed a small angle scattering ring corresponding to magnetic domain fluctuations on a length scale of 100 nm. These fluctuations are isotropically distributed in the film plane and appear to have an out-of-plane component. On increasing the transverse coherence of the incident beam, the scattering ring decomposes in a speckle pattern, indicative of magnetic correlations on a length scale smaller than the x-ray coherence length of about 4 μm.

Dimensionality and confinement effects in δ-doped Pd(Fe) layers

We address the dimensionality aspects of the magnetic ordering in δ-doped Pd(Fe) structures. The key property we investigate, via magneto-optic Kerr measurements, is the magnetization induced by iron in palladium, over a wide temperature range 5 K < T < 300 K. The dimensional crossover we observe cannot be rationalized on the basis of structural considerations alone, since we find the dimensionality of the low temperature and of the critical region can differ. We discuss the crossover in terms of the temperature dependence of the magnon modes, giving rise to lower dimensionality at low temperatures.

Universal window for two-dimensional critical exponents

Two-dimensional condensed matter is realized in increasingly diverse forms that are accessible to experiment and of potential technological value. The properties of these systems are influenced by many length scales and reflect both generic physics and chemical detail. To unify their physical description is therefore a complex and important challenge. Here we investigate the distribution of experimentally estimated critical exponents, β, that characterize the evolution of the order parameter through the ordering transition. The distribution is found to be bimodal and bounded within a window ∼0.1≤β≤0.25, facts that are only in partial agreement with the established theory of critical phenomena. In particular, the bounded nature of the distribution is impossible to reconcile with the existing theory for one of the major universality classes of two-dimensional behaviour-the XY model with four-fold crystal field-which predicts a spectrum of non-universal exponents bounded only from below. Through a combination of numerical and renormalization group arguments we resolve the contradiction between theory and experiment and demonstrate how the 'universal window' for critical exponents observed in experiment arises from a competition between marginal operators.