C.A. Ramos

Size and anisotropy determination by ferromagnetic resonance in dispersed magnetic nanoparticle systems

Abstract We present results for the FMR line shape modelling of non-interacting magnetic nanoparticle systems. We compare the results of the Smit and Beljers formalism and the usual linear-model, where the effective anisotropy field, HAeff, in the superparamagnetic regime is considered as a perturbation to the Zeeman interaction and added to the applied field, H. While the difference between these approaches is negligible for small HAeff (high temperature regime), it becomes more pronounced when HAeff≈H. We show how these results influence the determination of the parameters characterizing an array of random particles.

Nanoscale magnetic structure and properties of solution-derived self-assembled La0.7Sr0.3MnO3 islands

Strain-induced self-assembled La0.7Sr0.3MnO3 nanoislands of lateral size 50−150 nm and height 10−40 nm have been grown on yttria-stabilized zirconia (001)-substrates from ultradiluted chemical solutions based on metal propionates. The nanoislands grow highly relaxed withstanding the epitaxial relation (001)LSMO[110]//(001)YSZ[010] and show bulk-like average magnetic properties in terms of Curie temperature and saturation magnetization. The interplay of the magnetocrystalline and shape anisotropy within the nanoisland ensemble results in an in-plane magnetic anisotropy with a magnetocrystalline constant K1(150  K)=-(5±1)  kJ/m3 and in-plane easy axis along the [110]-La0.7Sr0.3MnO3 direction as measured, for the first time, through ferromagnetic resonance experiments. Magnetic force microscopy studies reveal the correlation between nanoisland size and its magnetic domain structure in agreement with micromagnetic simulations. In particular, we have established the required geometric conditions for single dom...

Electron spin resonance and magnetization in perovskite and pyrochlore manganites

We have performed electron spin resonance (ESR) and dc susceptibility, χdc(T), measurements in La0.7Ca0.3MnO3 perovskite up to 1000 K. This is adequately described in the paramagnetic regime in the constant coupling approximation assuming an effective Heisenberg-like pair interaction. The ESR linewidth is given by ΔHpp(T)=ΔHpp(∞)[C/Tχ(T)]. The ESR intensity follows the T dependence of χdc(T). We conclude that the relaxation mechanism is essentially the same for the whole studied paramagnetic range. The observed behavior reflects the progressive importance of magnetic clustering on approaching Tc. We have found similar T dependence for La0.7Sr0.3MnO3 and La0.7Pb0.3MnO3 perovskites, and also for the pyrochlore In2Mn2O7.

Functional nanocomposites based on the infusion or in situ generation of nanoparticles into amphiphilic epoxy gels

The production of nanocomposites with functional properties via the infusion of preformed nanoparticles (NPs) or their in situ generation inside an amphiphilic epoxy gel is reported. The gel was synthesized by the reaction of a diepoxy monomer based on diglycidyl ether of bisphenol A with an n-alkylamine, followed by annealing the resulting linear polymers above their glass transition temperatures to produce physical gelation through tail-to-tail association of pendant alkyl chains. Some of the advantages of these polymer gels are: (a) they have a low crosslink density and can therefore be significantly swollen by several organic solvents, (b) the presence of pendant alkyl chains provides a convenient chemical environment for the stabilization of NPs coated with alkyl chains, (c) the presence of secondary hydroxyls and tertiary amine groups in the polar backbone of polymer chains can be used to coordinate and reduce different precursors of NPs. Preformed NPs could be successfully infused into the gels keeping their optical properties (e.g., CdSe quantum dots) or magnetic behavior (e.g., γ-Fe2O3@oleic acid NPs) in the resulting nanocomposite. In situ generation of Au and Ag NPs (average size close to 10 nm) inside the amphiphilic gels was produced by infusing HAuCl4 or AgNO3 followed by reduction to the corresponding metals with secondary alcohols present in the polymer backbone, at 100 °C. Amphiphilic gels were employed as hosts for the in situ precipitation of gold(I)-dodecanethiolate leading to films exhibiting a red emission (638 nm) when excited with UV light (300 nm).

Effect of porosity on FMR linewidth of Ln0.67A0.33MnO3 (Ln La, Pr; A Ca, Sr)

Abstract Magnetization and ferromagnetic resonance (FMR) measurements near the Curie temperature have been taken from sol—gel and ceramic samples of Ln 0.67 A 0.33 MnO 3 (Ln  Pr, La; A  Ca, Sr). We demonstrate that the demagnetizing fields arising from the pores in polycrystalline samples cause the FMR line broadening observed below 1.1 T C , with respect to the values of the equivalent single crystals.

Annealing effects on the magnetization of Co–Ni–B amorphous nanoparticles

Abstract Chemically prepared (Co x Ni 1− x ) 1− y B y ( x =0.5, 0.75, 1; y ≈0.4) amorphous fine particles were characterized by X-ray diffraction, DTA and TGA, and in situ magnetic measurement as a function of annealing temperature in an inert atmosphere. Magnetic measurement performed in as-prepared and ∼150°C annealed samples shows an increase of the saturation magnetization and magnetic moment after thermal treatment. Room temperature magnetization increases by factors of 3.5, 1.8, and 1.5, for x =0.5, 0.75, and 1, respectively. These measurements may indicate a local re-ordering of the amorphous phase at temperatures much lower than the full crystallization temperature.

Spontaneous magnetostriction in La2/3(Ca1−x Srx) 1/3MnO3 (x=0, 0.05, 0.15, 0.25 and 1.0) near TC and its field dependence

Abstract We report thermal expansion measurements in La 2/3 (Ca 1− x Sr x ) 1/3 MnO 3 perovskites. For x =0 and 0.05 a very narrow and symmetric cusp is observed at T C , indicating the transition is first order, changing to second order above x =0.15. The data of an x =1 single crystal was fitted in the critical region yielding α =−0.14±0.10 for the critical exponent and A + /A − =1.4±0.4 for the amplitude ratio, both characteristic of 3d-Heisenberg systems. The magnetic field dependence is observed to decrease with increasing x .

Magnetization and FMR in La0.7Pb0.3MnO3 single crystals

Abstract We report magnetization and FMR (Q- and X-bands) measurements of a single crystal of La 0.7 Pb 0.3 MnO 3 . In the critical region M ( T ) ∼ ( T c − T ) β with β = 0.33(3). FMR shows two main lines and an antiresonance whose T -dependence is qualitatively well described in terms of a non-uniform demagnetization tensor.

Experimental evidence of magnetic anisotropy induction by superconductivity in superlattices

We present a study of Nb∕Co superlattices by ferromagnetic resonance. We find that when the superconductor enters the Meissner state, an anisotropy field around 400Oe, parallel to the applied field, appears in the ferromagnetic layers, consistent with the superconducting flux expelled from the superconducting layers. This anisotropy field is an important key to understand previous results showing that the superconductor layers modify the magnetic state of the ferromagnetic layers.

Comment on ``generalized Stoner-Wohlfarth Model and the Non-Langevin Magnetism of Single-Domain Particles'' by M.A. Chuev

The model recently proposed by M.A. Chuev [JETP Lett.85, 611 (2007)] has been analyzed. It has been shown that the anisotropy corrections near the blocking temperature always lead to a magnetization lower than the equilibrium value, contrary to the findings in Chuev’s work. In addition, the asymptotic high-temperature limit of Chuev’s model is inconsistent with the expected thermodynamic limit. Moreover, even at a low temperature, only a careful implementation of this theory can guarantee arriving at the correct result, avoiding some wrong conclusions in Chuev’s work.