Search

A Curie temperature enhancement in heterogeneous nanostructured materials with two Curie temperatures has been found. Two kinds of systems have been studied: nanocrystalline materials, consisting of α-Fe nanocrystals and an amorphous interphase, and multilayers composed of two different ferromagnetic metals. In FeBNbCu nanocrystalline alloy it has been observed that the Curie temperature of the intergranular amorphous region is greatly enhanced with respect to that of amorphous ribbons of the same composition, reaching this increase a value of 125 °C. Multilayers of NiCo and NiAg have been grown because of its simpler geometry. The Curie temperature of the Ni layers experiments a maximum increase of 75 °C, when Ni is located between Co layers. The effect depends mainly on the average separation between higher Curie temperature phases. A compositional gradient has been suggested to explain this temperature behaviour. The authors agree with its influence but consider it insufficient to account for such large temperature differences. We propose as a possible explanation a slight penetration of the exchange field of the higher Curie temperature phase. The great area occupied by ferromagnetic interfaces favours the induction of magnetization into the amorphous matrix or into the Ni layers, even above their Curie temperature.

We study the relation between the Roma and Zaragoza proposals for chiral fermions on the lattice. The fermion action in the Roma approach is shown to be equivalent to one of the Zaragoza type. This result is used to perform a mean-field study of the phase diagram for chiral Yukawa models based on the Roma action. The phase diagram is compared with the one based on the Zaragoza model with the most local choice for the fermion interactions., Comment: Version to appear in Nucl. Phys. B. Some general discussion added to the Introduction and Conclusion sections. Now 33 pages Latex; style files and (unchanged) PostScript figures again attached in uufiles format

We simulate the compact U(1)-Higgs model in a four-dimensional lattice. We present a numerical study for the behaviour around the region where the transition between the confining and Higgs phases disappears. The transition line is found to be first order and to end in a second-order point. We measure the critical exponents of the endpoint obtaining the mean field ones within errors.

Extraordinary magnetoresistance effect has been found in Ni/Co sputtered multilayers. Magnetoresistance values higher than 130% are achieved at room temperature with a saturation magnetic field of 35 Oe, using a particular arrangement of contacts. The large anisotropic magnetoresistance and the low coercivity of these samples are the origin of such an outstanding property. The results reported here seem to have promising applications in magnetoresistive devices.

The Roma and Zaragoza actions for chiral fermions on the lattice are shown to be essentially equivalent. The auxiliary fermion fields in the Roma model can be integrated out, and the resulting action is a special case of the Zaragoza approach. We use this result to perform a mean-field study of the phase diagram of chiral Yukawa models in the Roma formulation., Comment: Contribution to Lattice '94 by A.J. van der Sijs, 3 pages PostScript in uufiles format

Extraordinary magnetoresistance factors are reported in Co/Ni sputtered multilayers. Magnetic softness, large anisotropic magnetoresistance and a special contacts configuration give rise to magnetoresistance factors as large as 120% at room temperature and under an applied field of 40 Oe.

We stimulate the U(1)-Higgs lattice model with fixed Higgs modulus, in the region where the transition between the Higgs and Confining phases diappears. We find a first order transition line that ends in a second order point. Thermodynamic magnitudes and their associated critical exponents are introduced. The measured data allow us to fit the critical exponents finding values in agreement with the mean field prediction. The location of the critical point and the slope of the first order line are accurately given.