F. L. A. Machado

A theoretical model for the giant magnetoimpedance in ribbons of amorphous soft‐ferromagnetic alloys

Giant magnetoimpedance (GMI) measured in ribbons of the soft ferromagnet Co75−xFexSi15B10 annealed in the presence of a transverse magnetic field exhibits peaks in its field dependence. The GMI is strongly dependent on the magnitude of the longitudinal field and on the frequency of the applied current. We present a theoretical model which explains the existence of the peaks and its frequency dependence. The model is based on the skin depth effect and on the domain‐wall motion due to the magnetic field and the ac current.

Giant ac magnetoresistance in the soft ferromagnet Co70.4Fe4.6Si15B10

ac magnetoresistance (MR) measurements performed in thin ribbons of the amorphous soft ferromagnet Co70.4Fe4.6Si15B10 are reported with a longitudinal dc applied magnetic field H varying from 0 to ±15 Oe. The dependence of the MR with the frequency f of the measuring ac current was investigated for 0.4≤f≤100 kHz. No significant frequency dependence in the resistivity ρ for H=0 and H=±15 Oe was found. For intermediate values of H, ρ(H) presents a peak in H which increases linearly in frequency. The MR peak in H at room temperature varies from typical values of 0.13% in the low‐frequency range to a giant value of 27% at 100 kHz. Two possible sources for the frequency dependence of the magnetoresistance in amorphous alloys are discussed.

Magnetoresistance of the random anisotropic Co70.4Fe4.6Si15B10 alloy

Longitudinal magnetoresistance measurements in Co70.4Fe4.6Si15B10 are reported for applied magnetic fields H varying from 0 to ±15 Oe. Two regimes were observed: one which increases with H2 and another which decreases with M2 for higher fields. The maximum in the magnetoresistance is observed for H=2.4 Oe for samples annealed for 15 min at 573 K in a transverse magnetic field of 2 kOe. The measured variation in R(H) from H=0 to 2.4 was about 0.2%. The value of H=2.4 is nearly the same field where a field‐induced transition in the magnetization was observed. For samples annealed in the absence of the magnetic field the H2 term is the only one observed. The present results can be accounted for by the rotational model for uniaxial transverse anisotropy and the results are also in agreement with the random anisotropy model used to discuss the irreversibility and the field‐induced transition.

Magnetic properties of nanoparticles of cobalt ferrite at high magnetic field

In this paper we report high magnetic field (− 140 ≤ H ≤ 140 kOe) magnetization data for cobalt ferrites (crystallites size ∼ 42 nm) for temperatures (T) varying from 5 to 340 K. The T-dependence for the cubic magnetocrystalline anisotropy constant K1 was determined by using the “law of approach” (LA) to saturation. The values of K1 were found to be substantially different from previously reported values obtained using the same procedure but with H up to 50 kOe. By properly choosing the quantum parameters we found a very good agreement between the values calculated by using the model proposed by Tachiki [Prog. Theor. Phys. 23, 1055 (1960)] and the K1 versus T data. For instance, the values of K1 measured (calculated) for 5 K and 340 K were 28.3 × 106 erg/cm3 (27.7 × 106 erg/cm3) and 7.4 × 106 erg/cm3 (6.8 × 106 erg/cm3), respectively. The values of the parameters used to fit the data in both magnetic field regimes were chosen based on cation distribution over the A and B-sites on the spinel structure of t...

Enhanced magnetic properties of Bi-substituted cobalt ferrites

In this paper we present a magnetic study of Co1−xBixFe2O4 nanoparticles obtained by applying magnetic fields up to 14 T and for temperatures in the range of 5 to 340 K. Hysteresis loops yield a saturation magnetization (Ms), coercive field (Hc), and remanent magnetization (Mr) that vary significantly with temperature and bismuth content. The T-dependence of Ms obtained for H = 5 T presents a maximum at 150 K and a minimum at 25 K that are also dependent on the value of x. However, for H = 14 T, this anomalous behavior disappears and the magnetization smoothly approaches saturation down to 5 K. The magnetic cubic anisotropy constant for different Bi contents, determined by a “law of approach” to saturation, was found to be smaller than those values for pure cobalt ferrite nanoparticles and strongly dependent on temperature. A discussion on the implications of the anomalous behavior in the determination of the anisotropy constant in these sample materials is also presented.

Enhanced GMI in ribbons of Co70.4Fe4.6Si15B10 alloy

Abstract We show that the longitudinal magnetoimpedance (GMI) in ribbons of amorphous alloys can be highly enhanced by varying the sample length, the annealing temperature, and the magnitude of the AC electrical current.

DOMAINS AND GIANT MAGNETO-IMPEDANCE IN AMORPHOUS RIBBONS BY MAGNETO-OPTICAL KERR EFFECT

By longitudinal magneto‐optical Kerr effect microscopy we analyzed the development of the domain structure in Co70.4Fe4.6Si15B10 amorphous ribbons for which giant magneto‐impedance had been observed. Using transverse and longitudinal magneto‐optical Kerr effect magnetometry we measured the magnetization direction around the hysteresis loop. These results allowed us to model the domain structure of the sample during the giant magneto‐impedance measurements.

Giant transversal magnetoimpedance and Hall‐effect measurements in Co70.4Fe4.6Si15B10

We report room‐temperature transversal magnetoimpedance (TMI) and Hall‐effect measurements performed in ribbons of the zero‐magnetostriction soft‐ferromagnet Co70.4Fe4.6Si15B10 alloy annealed at 587 K for 15 min at a dc magnetic field of 2 kOe. The annealing showed little effect on the magnetostriction of the samples. The frequency f and the amplitude Iac of the current used in the measurements, and the measuring magnetic field H were varied in the intervals 10≤f≤105 Hz, 2.5≤Iac≤25 mA, and −15≤H≤15 kOe, respectively. The magnetic field dependence of the TMI shows a peak which is strongly dependent on f and on Iac. It reaches the giant value of 28% at H=350 Oe for f=100 kHz and Iac=25 mA. This TMI giant value is of the same order of the giant longitudinal magnetoimpedance (GLMI) but its peak is positioned in a magnetic field two orders of magnitude larger than the value obtained for the GLMI. The Hall effect yielded a value of RS=0.43 μΩ cm kOe−1. Peaks which are also frequency dependent were observed at t...

Spin‐glass behavior in the Al:Mn quasicrystalline alloys (abstract)

Despite the large manganese concentration present in the quasicrystalline Al:Mn system (typically of the order of 20%), only a small fraction of unusually large magnetic moments are detected.1 Recently2 we performed a scaling analysis of the nonlinear magnetic susceptibility of the decagonal T‐Al78Mn22 quasicrystalline alloy. The critical exponents obtained (β=0.6 and γ=4.4) are identical to those obtained for canonical spin glasses.3 In this work, we present new scaling results for the icosahedral I‐Al80Mn20 alloy. A discussion on the dynamical scaling of the quasicrystalline alloys is also presented.

Scaling of the nonlinear susceptibility in the spin-glass TAl78Mn22 quasicrystalline alloy

Abstract It is shown for the first time that the non-linear susceptibility, XNL = X − M/H, of the quasicrystalline TAl78Mn22 alloy has a scaling behavior similar to canonical spin-glass systems. A good scaling is obtained for magnetic fields from 1. kOe up to 50. kOe and for temperatures above 14. K. The scaling yields the critical exponents γ = 4.4 and β = 0.6, and the critical temperature Tg = 7.6 K, consistent with values obtained in canonical spin-glasses.