F.L.A. Machado

Exchange-spring behavior in nanopowders of CoFe2O4–CoFe2

The nature of the magnetic interactions among core-shell nanoparticles of CoFe2O4–CoFe2 having an average particle size of 32 nm were investigated by δm plots. It was found that the interactions are mainly demagnetizing in nature for nanoparticles with core diameters larger than 26 nm. For smaller values of core diameters, the exchange-spring phenomenon takes place due to the exchange-coupling at the core-shell interface. The critical thickness of the shell was estimated to be 7.8 nm. The overall results are in excellent agreement with current theoretical models.

Structural and magnetic properties of nanoparticles of La2/3Sr1/3MnO3

Nanoparticles of the La2/3Sr1/3MnO3La2/3Sr1/3MnO3 manganite were synthesized using the Pechini process and their structural properties were characterized by X-ray diffraction (XRD) and by transmission electron microscopy (TEM). The magnetization of compacted powders was measured for temperatures in the range 190⩽T⩽450K. The estimated average diameter (D) of the particles continuously increased from 20 to 95xa0nm when the calcination temperature was varied from 873 to 1273xa0K. From the magnetic data we found that the room temperature saturation magnetization scaled with 1/D1/D while the magnetic transition temperature follows a D2D2 power-law suggesting that the correlation length ξξ is mainly determined by the surface properties of the nanoparticles.

Giant magnetoimpedance in FeAg granular alloys

We have observed giant magnetoimpedance (GMI) in an FeAg granular alloy. Impedance and transverse susceptibility (TS) measurements as a function of the external dc magnetic field were made at room temperature. The variation of the magnetoimpedance ratio with magnetic field has been related to a corresponding TS ratio change as a function of the magnetic field. The peaks in the GMI and TS curves are in the same position and have been associated in both cases to effective anisotropy fields.

Giant magnetoimpedance effect in a thin-film multilayer meander-like sensor

A meander-like magnetic sensing element based on the giant magnetoimpedance (GMI) effect was prepared by using optical lithography and sputtering deposition techniques. The structure of the sensing element consists of layers of Permalloy (Pyu2009=u2009Ni81Fe19), titanium (Ti), and copper (Cu) with composition [Py(100u2009nm)/Ti(6u2009nm)]4/Cu(400u2009nm)/[Py(100u2009nm)/Ti(6u2009nm)]4. The GMI was investigated at room temperature under applied magnetic fields (H) varying in the range of ±4.0 kOe in both longitudinal and transversal geometries. The amplitude Iac and frequency f of the ac electrical current were varied in the range of 0.35–6.50u2009mA and 0.1–20u2009MHz, respectively. The overall dc electrical resistance of the sensing element was found to be 45.6 Ω. The sensing element yielded a GMI of 53.5% for H ≃ 5.0u2009Oe and f = 7.0u2009MHz, and the corresponding maximum average sensitivity of about 5 Ω/Oe. The sensing element was used for measuring the local Earth magnetic field ( H l o c a l = 0.26 ± 0.03 Oe) yielding a value close to the on...

GMI in the reentrant spin-glass Fe90Zr10 alloy: Investigation of the spin dynamics in the MHz frequency regime

Giant magnetoimpedance (GMI) in the reentrant spin-glass (SG) phase of ferromagnetic Fe90Zr10 is reported. The temperature (T) dependence of the GMI allows the investigation of the spin dynamics in the SG phase in the MHz frequency regime and thus very short relaxation times τ (∼10−8 s). The GMI shows a broad maximum around 150u2009K and diminishes with decreasing T below the glass temperature Tg of 15u2009K. The magnetic permeability data obtained from the GMI data show the general features observed in the ac magnetic susceptibility measured at lower frequencies (10u2009≤u2009fu2009≤u2009104 Hz), yielding values of Tf (=Tg(f)) that allow testing the validity of the power-law scaling used for describing the dynamics of SG-phases up to 15u2009MHz. A log-log plot of τ (=1/f) versus the reduced critical temperature shows two distinct regimes in the time-domain: (1) a critical slowing-down is observed for values τ > 3u2009×u200910−3 s; and (2) for 7u2009×u200910−8u2009≤u2009τu2009≤u20093u2009×u200910−3 s. In the latter case a fitting to the power-law yields the value 7.4 for ...

Giant Zeeman shifts in the optical transitions of yttrium iron garnet thin films

We report the observation of giant Zeeman shifts in the optical transitions of high-quality very thin films of yttrium iron garnet (YIG) grown by rf sputtering on gadolinium gallium garnet substrates. The optical absorption profile measured with magneto-optical absorption spectroscopy shows dual optical transition in the UV-visible frequency region attributed to transitions from the O-2p valence band to the Fe-3d conduction band and from the O-2p valence band to Fe-2p53d6 excitonic states at the Γ-symmetry point of the YIG band structure. The application of a static magnetic field of only 0.6u2009kOe produces giant Zeeman shifts of ∼100u2009meV in the YIG band structure and ∼60u2009meV in the excitonic states corresponding to effective g-factors on the order of 104. The giant Zeeman effects are attributed to changes in energy levels by the large exchange fields of the Fe-3d orbitals during the magnetization process.

On the dynamics of the reentrant spin-glass state of the Fe100−xZrx (x = 9, 10) amorphous alloys

Low temperature magnetization (M) and ac susceptibility (χac) data were used to investigate the reentrant spin-glass state in lengths of melt-spun ribbons of Fe100−xZrx (xu2009=u20099, 10) amorphous alloys. The temperature range investigated was from 2 to 300u2009K, while the applied magnetic field H was varied in the range of ±85 kOe. The magnetic properties were found to be strongly influenced by the sample composition despite the fact that the amount of Fe varies by 1 at.u2009%. For instance, the Curie temperature (TC) is reduced from 232.5u2009K to 213.0u2009K with decreasing Fe concentration, while M for the lower Fe concentration, measured at the highest applied magnetic field (Hu2009=u200985 kOe) was nearly double the value for that of the higher; the coercivity in the ferromagnetic regime is reduced by a factor close to five when x is increased from 9 at.u2009% to 10 at.u2009%. The ac susceptibility measured for frequencies f in the range of 10–104u2009Hz showed a sharp drop in the magnitude of the in-phase contribution ( χac′) and a peak a...

Specific heat of the dilute antiferromagnetic system FexZn1-xF2

The specific heat (cp) of the dilute antiferromagnet FexZn1-xF2 has been measured in absence of external magnetic fields for x = 0, 0.26, 0.31, 0.34, 0.36, 0.38, 0.41, 0.45, 0.56, 0.88, 0.97, and 1.0. For x > 0.45, a sharp peak associated to the antiferromagnetic (AF) phase transition at TN(x) is the only observed feature. For 0.31 ≤ x ≤ 0.41, this peak becomes smaller with decreasing x and a rounded bump appears at higher temperatures T. Closer to the percolation concentration (xp = 0.24), the peak characteristic of the AF phase transition disappears and the rounded bump becomes the only observed feature. The low-T cp behavior confirms a crossover from AF long range order at large x to a spin glass behavior close to xp. At intermediate x, the low-T joint signature of both phenomena indicates a coexistence of AF order and a cluster-glass phase. The x-dependence of the Neel temperature was accounted for using a simple phenomenological model.

Structural and magnetic properties of nanoparticles of La2/3Sr1/3MnO3La2/3Sr1/3MnO3

Nanoparticles of the La2/3Sr1/3MnO3La2/3Sr1/3MnO3 manganite were synthesized using the Pechini process and their structural properties were characterized by X-ray diffraction (XRD) and by transmission electron microscopy (TEM). The magnetization of compacted powders was measured for temperatures in the range 190⩽T⩽450K. The estimated average diameter (D) of the particles continuously increased from 20 to 95xa0nm when the calcination temperature was varied from 873 to 1273xa0K. From the magnetic data we found that the room temperature saturation magnetization scaled with 1/D1/D while the magnetic transition temperature follows a D2D2 power-law suggesting that the correlation length ξξ is mainly determined by the surface properties of the nanoparticles.