M.N. Baibich

Metastable states in the triangular-lattice Ising model studied by Monte Carlo simulations: Application to the spin-chain compound Ca 3 Co 2 O 6

It is known that the spin-chain compound Ca3Co2O6 exhibits very interesting plateaus in the magnetization as a function of the magnetic field at low temperatures. The origin of them is still controversial. In this paper we study the thermal behavior of this compound with a single-flip Monte Carlo simulation on a triangular lattice and demonstrate the decisive influence of metastable states in the splitting of the ferrimagnetic 1/3 plateau below 10 K. We consider the [Co2O6]n chains as giant magnetic moments described by large Ising spins on planar clusters with open boundary conditions. With this simple frozen-moment model we obtain stepped magnetization curves which agree quite well with the experimental results for different sweeping rates. We describe particularly the out-ofequilibrium states that split the low-temperature 1/3 plateau into three steps. They relax thermally to the 1/3 plateau, which has long-range order at the equilibrium. Such states are further analyzed with snapshots unveiling a domain-wall structure that is responsible for the observed behavior of the 1/3 plateau. A comparison is also given of the exact results in small triangular clusters with our Monte Carlo results, providing further support for our thermal description of this compound.

Magnetization dynamics as derived from magneto impedance measurements

In this work, the magnetization dynamics of soft magnetic materials is studied with the aid of transverse differential permeability μ(Iac,f,Hdc) spectra. Contributions to the magnetization processes from domain wall motion and rotation of the magnetization can be extracted from the transverse differential permeability data which are in turn obtained from impedance Z(Iac,f,Hdc) spectra. In particular, an iteration method is used to extract μ(Iac,f,Hdc) from Z(Iac,f,Hdc) data. The approach is tested in samples with a very well known domain structure, namely (110)[001]FeSi3%. Permeability spectra μ(Iac,f,Hdc) were obtained in the frequency range (100 Hz⩽f⩽100 kHz), probe current range (0.1⩽Iac⩽50 mA) and dc magnetizing field range (0⩽Hdc⩽500 Oe). It is shown that the method developed in this article can be efficiently used to identify and study different dynamic processes driven by the probe current and controlled by the external dc field. In particular, it is shown that the method provides the tools to sepa...

Co/Ag multilayer film: Role of annealing on the magnetic properties

Co(15 A)/Ag(60 A) multilayers produced by electron beam deposition on a 50 A chromium buffer layer over a Si(111) wafer have been studied by magnetoresistance, saturation magnetization, coercivity, and anisotropy. Annealing at various temperatures produces striking effects on the observed physical properties, such as an improvement on the value of the magnetoresistance associated to a ‘‘back‐diffusion’’ process in the Co/Ag interfaces. This leads one to believe that interface roughness is indeed most important to understanding the connection between giant magnetoresistance and antiferromagnetic coupling, as well as the behavior of coercivity vs interface anisotropy in these materials.

Annealing of Co/Ag multilayers

Abstract We have measured the magnetoresistance and magnetic properties of annealed Co/Ag multilayers. The results show an increase of the magnetoresistance associated to lower interface roughness for these immiscible elements. Higher temperature anneals lead to destruction of the multilayer, but still showing giant magnetoresistance for the composite formed.

Magnetization process and magnetoimpedance in (110)[001]FeSi3%

The present article investigates the relation between magnetoimpedance (MI) and the evolution of the magnetic domain structure along the magnetization curves of commercial FeSi3% sheets cut at different angles with respect to the easy magnetization axis [001]. The role of the magnetocrystalline anisotropy on MI is studied and the correlation between the basic features of the MI vs H and magnetization (M) vs H curves is established. From data obtained at appropriate frequency ranges, the frequency dependence of the effective transverse differential permeability is obtained. The frequency and field spectra of the impedance allow the identification of the rotation of the magnetization as the main contribution of the magnetization processes to the MI effects in the studied samples.

Electronic conductivity in 1D Co spin chain single crystal

We have measured the transport and magnetic properties of the model spin chain single crystal Ca3Co2O6 in high pulsed fields. A crossover between 1D and 3D transport is observed, with the opening of a Coulomb gap below the order temperature of the individual chains. The samples show changes in the variable range hopping transport dimensionality and gap; the magnetic field suppresses the gap, inducing a significant reduction of the resistance, while dimensionality seems governed by temperature. At fields associated with the levels of magnetization of the frustrated triangular lattice formed by the Co lines, and below the critical magnetic temperature, a further increase of the conductivity is observed. This effect is associated with planar magnetic states perpendicular to the lines.

Stress level in Finemet materials studied by impedanciometry

In this work, a study of the stress relief in Finemet ribbons, Fe73.5Cu1Nb3Si16.5B6, as a function of the annealing temperature is presented. The as melt-spun samples are amorphous and become partially crystallized after annealing at appropriate temperatures. For temperatures TA⩾480 °C the samples are nanocrystalline, with a microstructure composed by α-Fe1−xSix (x∼0.2) crystallites (10 nm average diameter) embedded in an amorphous matrix. Nanocrystallization, associated with stress relief effects, improves the soft magnetic properties of this kind of material. The stress level was quantified using magnetorestriction (measured by SAMR), magnetoelastic anisotropy, and domain wall energy data obtained from impedance spectra measurements. A reduction of the internal stress from 15 to 0.2 MPa was verified when comparing the as-cast to the sample annealed at 580 °C. Improvement of the magnetic softness of the samples was also followed by the increase of the domain wall and magnetization rotation contributions ...

Electronic noise in magnetic low-dimensional materials and nanostructures

Abstract After a brief review underlining the power of electronic noise studies to probe local magnetic instabilities, we present new data in three different systems in terms of dimensionality and magnetic ordering: the first results we present deal with low-frequency longitudinal and Hall resistance fluctuations in Ni nanostructures, where we address the questions of the inhomogeneous behavior of the noise at a nanometric scale. In order to get some insight into the atomic process, we report on noise generated in atomic contacts of Ni obtained by the break junction technique. And finally, we present the first electronic noise study in a one-dimensional frustrated magnetic Ca 3 Co 2 O 6 single crystal composed of parallel Co spin chains. From the magnetic noise analysis, we discuss the interplay between the low-temperature three-dimensional magnetic ordering and the spin-dependent hopping conductivity on the Co sites.

Anneal-induced transformations in the transport and magnetic properties of CoCu granulars

Abstract The electrical and magnetic properties of granular Co 5 Cu 95 , Co 10 Cu 90 and Co 15 Cu 85 ribbons treated with different annealing procedures were measured from helium to room temperatures. The isochronous anneals had fixed heating rate and the transformations were followed by “in situ” resistivity measurements. We found that the transformation occurs at nearly the same region of temperature for the three compositions. Besides, our results for magnetoresistance on Co 10 Cu 90 could be understood in the light of the re-dissolution of Co into the Cu matrix, as suggested in previous works (Miranda et al., J. Magn. Magn. Mater. 185 (1998) 331; da Silva et al., IEEE Trans. Mag., to appear).

A simple formulation for magnetoresistance in metal-insulator granular films with increased current

We studied the tunnel magnetoresistance in metal/insulator granular films when the applied current is varied. The tunnel magnetoresistance shows a strong modification related to a non-Ohmic behaviour of theses materials. It was verified that spin-dependent tunnelling is the main mechanism for magnetoresistance at low applied current. However, when the current is high, another mechanism gets to be important: it is independent of the magnetization and is associated to variable range hopping between metallic grains. In this work, we propose a simple modification of Inoue and Maekawas model for tunnelling magnetoresistance in granulars, rewriting the expression for resistance as a function of magnetic field and temperature, also taking into account the two different contributions.