N.H. Duc

Metamagnetism, giant magnetoresistance and magnetocaloric effects in RCo2-based compounds in the vicinity of the Curie temperature

Abstract Magnetisation and magnetoresistance isotherms were measured for a number of (R,R′)Co2, (R,Y)Co2 and R(Co,Si)2 (R,R′=rare earth) compounds. A metamagnetic transition is observed just above the Curie temperature (TC) of compounds having a first order phase transition, i.e. ErCo2-, HoCo2-, and DyCo2-based ones. Both 4f- and 3d-sublattice magnetic moments contribute to a sharp change of the magnetisation at this transition. The concurring suppression of the magnetoresistance can be considered to be due to quenching of spin fluctuations. In addition, the magnetic entropy change ΔSm is estimated from the magnetisation data by using a Maxwell equation. The resulting giant magnetocaloric effects are discussed in terms of the 4f(R)-localised spin and the 3d(Co)-spin fluctuations as well as the nature of the phase transition.

Exchange interactions in rare earth—transition metal compounds

Abstract Using the strenght of R—R interactions deduced from the ordering temperatures of RNi2 compounds, the R—T exchange-coupling parameter (ART) in RmTn ( R = rare earth, T = Co or Fe, m / n = 1 2 ; 1 3 ; 6 23 ; 1 5 ; 2 17 ; 1 12 ) and RmTnBk (m / n / k = 1/4/1; 2/14/1; 1/12/6) systems has been evaluated from analyses of Curie temperatures. Whereas ARR, as deduced from the RNi2 compounds, increases by almost an order of magnitude in a given series, ART is enhanced by a factor of only about three from the heavy to the light rare earth compounds. These phenomena, already reported in literature, are based on 4f–5d exchange interactions at R sites. In addition, for a given rare earth element, going from T-poor to T-rich compounds, a tendency to decrease is found for both ARCo and ARFe; this variation is discussed in terms of 3d–5d hybridization.

The magnetic behaviour of rare-earth—transition metal compounds

Abstract The basic exchange interactions will be discussed for a variety of (R, T) compounds, in particular RT 2 (R: rare earth, or Y; T: transition metal). A magnetic phase diagram for (R, Y)Co 2 compounds is derived. The combined effect of the volume and the molecular field caused by the rare-earth spins is demonstrated on R(Co, Fe) 2 , (R, Y)Co 2 , R(Co, Al) 2 and R(Co, Cu) 2 , with an emphasis on the metamagnetic transition in the 3d-subsystem.

Magnetism and magnetocaloric effect in La1−yNdy(Fe0.88Si0.12)13 compounds

Abstract Structural and magnetic properties of La1−yNdy(Fe0.88Si0.12)13 compounds have been investigated by means of X-ray diffraction and magnetization measurements. The single-phase NaZn13-type cubic structure is stabilized for the compounds with y=0, 0.1, 0.3 and 0.4. All the synthesized compounds are ferromagnetic. Their Curie temperature TC slightly increases with increasing Nd up to y=0.3. The most striking effect of the Nd substitution, however, is in their itinerant-electron metamagnetic behavior and the magnetocaloric effect in the vicinity of TC. The maximum entropy change decreases somewhat, but the relative cooling power increases with increasing Nd content (i.e. for y=0.3).

Preparation and characterization of magnetic nanoparticles coated with polyethylene glycol

Magnetic nanoparticles Fe3O4 were prepared in air environment by the coprecipitation method using molar ratios of Fe2+: Fe3+ = 1: 2. The surface of magnetic nanoparticles was coated with sodium oleate as the primary layer and polyethylene glycol 6000 (PEG-6000) as the second layer. The morphology of the particles was investigated by scanning electronic microscopy (SEM). X-ray diffraction (XRD) indicated the sole existence of inverse cubic spinel phase of Fe3O4 and an average size of about 25 nm. Fourier transform infrared spectroscopy (FTIR) analysis indicated existence of two distinct surfactants on the particle surface. In addition, the results of FT-IR indicated that the coated Fe3O4 particles improved the thermal stability due to the interaction between the Fe3O4 particles and protective layers.

Structural and magnetic properties of starch-coated magnetite nanoparticles

Magnetic Fe3O4 nanoparticles are prepared by the coprecipitation method and coated with starch as a surfactant. Their structural and magnetic behaviours are studied by means of X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Raman spectrum, Fourier Transform Infrared (FT-IR) as well as with a Vibrating Sample Magnetometer (VSM). The magnetic Fe3O4 nanoparticles under investigation have an average size of about 14 nm. The coated magnetic nanoparticles exhibit super-paramagnetic behaviours with a blocking temperature of about 170 K and saturation magnetisation ranging between 30 and 50 emu g−1. In addition, the results of FT-IR indicated that interactions between the Fe3O4 particles and starch layers are much improved.

Magnetic, Mössbauer and magnetostrictive studies of amorphous Tb(Fe0.55Co0.45)1.5 films

The Tb(Fe0.55Co0.45)1.5 films were fabricated by rf magnetron sputtering from a composite target. Samples were investigated by means of x-ray diffraction, vibrating sample magnetometer, conversion electron Mossbauer spectra, and magnetostriction measurements. The as-deposited film is an amorphous alloy with a perpendicular magnetic anisotropy and an intrinsic magnetostriction λ=1080×10−6 in an applied field of 0.7 T. In this state, it was determined that the hyperfine field Bhf=23.5 T and the cone-angle between the Fe moment direction and the film-normal direction β=12°. After annealing in the temperature range of TA=250–450 °C the amorphous structure still remained, however the anisotropy was changed to a parallel one. The soft magnetostrictive behavior has also been improved by these heat treatments: the parallel magnetostriction λ∥=465×10−6 was almost developed in low applied fields of less than 0.1 T and, especially, a huge magnetostrictive susceptibility χλ=dλ∥/d(μ0H)=1.8×10−2 T−1 was obtained at μ0H...

Magnetization and AC susceptibility of TbxY1-xCo2 compounds

Abstract Magnetization and ac susceptibility measurements have been performed on Tb x Y 1- x Co 2 compounds. Samples with x > 0.1 order ferromagnetically below room temperature. The cobalt magnetic moments in these compounds are induced by the internal magnetic field exerted by the terbium moments on the cobalt atoms. The ac susceptibility measurements indicate a change from second order to first order in the ferromagnetic transition for samples with x -values equal or smaller than 0.5.

Magnetic and electrical properties of the R(Co, Si)2 compounds (R = Gd, Tb, Dy) with invariable crystal unit cell parameters

Abstract The invariable crystal unit cell parameter compounds R(Co, Si) 2 (R = Gd, Tb and Dy) have been studied by mean of the magnetization, ac susceptibility and resistivity measurements. By partial substitution Co by Si, the ordering temperature is almost constant for all of the considered compounds, whereas the reduction of the 3d magnetic moment is observed. In the condition of fixed volume, these findings suggest the important effects of the hybridization between the 3d states of Co and the p states of Si. The anomalies of magnetization, susceptibility, resistivity and the character of the magnetic phase transition at T C are also discussed for the compounds with Dy.

Magnetic phase transitions in (Nd, Dy)Co2 and (Pr, Dy)Co2 compounds

Abstract In a series of (Nd, Dy)Co 2 and (Pr, Dy)Co 2 compounds, the type of the magnetic phase transition was studied by magnetization and electrical-resistivity measurements. In both systems, a cchange of type was observed from second order (for NdCo 2 and PrCo 2 ) to first order (for compounds containing more than about 20% Dy). The results are discussed in the Inoue-Shimizu model, generalized in such a way that the presence of both light and heavy rare-earth atoms is taken into account. The volume dependence of the parameter a 3 (i.e. of the free-energy contribution a 3 M 2 Co /4) is supposed to play a decisive role in the determination of the order of the magnetic transition.