C. Torres

Effect of Sn addition on the magnetic aftereffects of yttrium iron garnets

The magnetic aftereffect (MAE) phenomena in yttrium iron garnets (YIG) with Sn substitutions is presented. These studies provide very useful information about lattice symmetry and domain wall dynamics. This is specially interesting for a material as YIG, whose properties change significantly through the addition of a wide variety of dopants. The analysis of MAE is usually carried out by means of magnetic disaccommodation techniques, i.e., a time variation of domain wall mobility after a magnetic shock.

Magnetic aftereffects in Si-doped YIG

Magnetic aftereffect processes in yttrium iron garnet with silicon substitutions are analyzed by means of magnetic disaccommodation techniques. We have recorded the time evolution of the initial magnetic permeability after sample demagnetization for polycrystalline specimens with nominal composition Y/sub 3/Fe/sub 5-x/Si/sub x/O/sub 12/ (0/spl les/x/spl les/1) in the temperature range from 80 K to 500 K. We have found that silicon substitutions give rise to new relaxation and frequency dependent accommodation processes at high and low temperatures, respectively. The origins of both types of processes are finally discussed.

Magnetic aftereffects in hexagonal ferrites with BaO/spl middot/9Fe/sub 2/O/sub 3/ stoichiometric initial composition

We have measured the relaxation of the initial magnetic permeability after sample demagnetization, in the temperature range 80 K < T < 420 K, in polycrystalline barium hexaferrites with a Ba:Fe ratio in the initial composition of 1:18 corresponding to stoichiometric W phase. The materials were sintered under different temperature and oxygen pressure conditions. The results represented by isochronal curves show the existence of up to four relaxation processes. Additional thermogravimetric and X-ray diffraction studies reveal the successive formation of M, X, and W hexagonal phases with the increase of the sintering temperature. In this framework, we ascribe the relaxation processes to the progressive increase of ferrous cations in the different sites within the hexagonal lattice.

Magnetic aftereffects in magnesium ferrites

We report a study of aftereffect processes in magnetite with magnesium substitutions. The study observed the processes by means of disaccommodation techniques. We have recorded the time evolution of the initial magnetic susceptibility after sample demagnetization of polycrystalline specimens with nominal composition Mg/sub x/Fe/sub 3-x/O/sub 4/, 0/spl les//spl times//spl les/1 in the temperature range from 200 K to 500 K. We find that magnesium substitutions reveal interesting results about magnetic aftereffects in oxide spinels.

An Approach to the Magnetic Disaccommodation in Nd Doped Yttrium Iron Garnets

The magnetic disaccommodation on polycrystalline Nd doped YIG Y3-xNdxFe5O12 (0<x<2.5) is studied in this work. It reveals a very different behaviour with the sintering atmosphere. The results of magnetic disaccommodation for samples sintered in air show a relaxation peak with the maximum at a temperature changing from 120 K to 100 K with increasing doping rate. When the secondary perovskite phase appears with x=2.0, another relaxation process at around 300 K emerge, similar to the well-known III peak of polycrystalline magnetite. For CO2 sintered samples, the process at 120 K exhibit magnetic accommodation features (i.e. increase of apparent magnetic permeability after demagnetization), and vanishes with increasing Nd substitution, together with the appearance of the 300 K peak associated with the perovskite secondary phase which appears with x>1.2 doping rate. Introduction Yttrium iron garnets (YIG) are very interesting ferrimagnetic materials for use at high frequencies due to the low losses in this frequency range, according to their high electrical resistivity and narrow resonance linewidth [1]. These properties, together with the possibility of introducing a wide range of dopants for tailoring their properties [2], make them excellent candidates for microwave applications. It is usual the substitution of iron by metallic cations in tetrahedral or octahedral sites [3], but it is also possible the substitution of part of yttrium ions in dodecahedral sites. This latter type of substitution with a rare earth element has been studied so far [4,5], and it has been established that perovskite phase forms together with the magnetic garnet depending on the rare earth employed and on the doping rate. Recently, there is renewed interest in YIG films substituted with Nd [6,7] and Ce [8] due to the large Faraday effect and potential application in magneto-optic devices. The study of magnetic relaxations is important in order to minimize the losses. In addition, the study of this kind of processes provides information about the underlying mechanisms governing the dynamic behaviour of Bloch walls [9]. Among the different techniques available, the magnetic disaccommodation measurements are a powerful tool in the detection of magnetic relaxations in ferrites. It consists in the time variation of the mobility of domain walls after a magnetic shock, and is shown by a temporal evolution of the magnetic permeability after a demagnetization stage. This relaxation phenomenon has been observed in spinel ferrites [9], hexaferrites [10] and garnets [11]. Its origin has been attributed to either the rearrangement or the diffusion of anisotropic point defects (lattice vacancies, interstitials) within the Bloch walls, and the relaxation time, which characterizes each relaxation process, is strongly temperature-dependent. In this paper we study the effect of Nd doping on the magnetic disaccommodation in YIG. The results are compared with experimental results in undoped YIG [11] and interpreted under the theoretical background that our group has explained previously [12]. Materials Science Forum Online: 2004-05-15 ISSN: 1662-9752, Vols. 455-456, pp 143-147 doi:10.4028/www.scientific.net/MSF.455-456.143

Magnetoabsorption and magnetic hysteresis in Ni ferrite nanoparticles

Nickel ferrite nanoparticles were prepared by a modified sol-gel technique employing coconut oil, and then annealed at different temperatures in 400-1200 oC range. This route of preparation has revealed to be one efficient and cheap technique to obtain high quality nickel ferrite nanosized powder. Sample particles sizes obtained with XRD data and Scherrers formula lie in 13 nm to 138 nm, with increased size with annealing temperature. Hysteresis loops have been obtained at room temperature with an inductive method. Magnetic field induced microwave absorption in nanoscale ferrites is a recent an active area of research, in order to characterize and explore potential novel applications. In the present work microwave magnetoabsorption data of the annealed nickel ferrite nanoparticles are presented. These data have been obtained with a system based on a network analyzer that operates in the frequency range 0 - 8.5 GHz. At fields up to 400 mT we can observe a peak according to ferromagnetic resonance theory. Sample annealed at higher temperature exhibits different absorption, coercivity and saturation magnetization figures, revealing its multidomain character.

Analysis of Phase Transitions in La and Nd Substituted YIG with Magnetic Disaccommodation Measurement

The magnetic disaccommodation on polycrystalline Nd doped YIG NdxY3-xFe5O12 (0<x<2.5) and La doped YIG LaxY3-xFe5O12 (0<x<0.7) is studied in this work. A different behaviour with the sintering atmosphere and the dopant has been observed. For Nd doped garnets sintered in air the characteristic peak of disaccommodation in YIG shifts from 120 K to 100 K; for x=2.0 an additional 300 K peak is observed, due to the presence of magnetite in the sample, thus indicating that phase transition has been reached. In CO2 sintered samples, the process at 120 K exhibit magnetic accommodation features and the phase transition takes place at a lower doping rate. On the contrary, CO2 sintered La doped samples do not show magnetic accommodation as Nd doped YIG, but normal disaccommodation processes, with the solubility limit observed at x=0.3, whereas in air sintered samples the phase transition is not observed with disaccommodation measurements.