RESUMO
Using ac-susceptibility, dc-magnetization, and transmission electron microscopy, we have investigated the magnetic behavior of Mn(3)O(4) nanoparticle ensembles at temperatures below the paramagnetic-to-ferrimagnetic transition of the title material (T(N) approximately equal 41 K). Our data show no evidence of the complex magnetic ordering exhibited by bulk Mn(3)O(4), or of a magnetic behavior around T(N) that has a dynamic (relaxation) origin. Instead, we find a low-temperature (at approximately 11 K) magnetic anomaly that manifests itself as a peak in the out-of-phase component of the ac-susceptibility. Analysis of the frequency and average-particle-size dependence of the peak temperature demonstrates that this behavior is due to the onset of superparamagnetic relaxation, and not to a previously hinted at spin-glass-like transition. Indeed, the relative peak temperature variation per frequency decade DeltaT/TDeltalog(f) is 0.11, an order of magnitude larger than the value expected for collective spin freezing, but within the range of values observed for superparamagnetic blocking. Furthermore, attempts to fit the frequency f/observation time tau = 1/2pif dependence of the peak temperature by a power law led to parameter values unexpected for a spin-glass transition. On the other hand, a Vogel-Fulcher law tau = tau(0)exp[E(B)/k(B)(T - T(0))]-where E(B) is the energy barrier to magnetization reversal, k(B) is the Boltzmann constant, tau(0) and T(0) are constants related to the attempt frequency and the interparticle interaction strength-correctly describes the peak shift and yields values consistent with the superparamagnetic behavior of a slightly interacting system of nanoparticles. In addition, the peak temperature T is sensitive to minute changes in the average particle size (D), and scales as (T - T(0) is proportional to(D)3, another signature of superparamagnetic relaxation.
RESUMO
In this paper, we examine prospects for the manufacture of patient-specific biomedical implants replacing hard tissues (bone), particularly knee and hip stems and large bone (femoral) intramedullary rods, using additive manufacturing (AM) by electron beam melting (EBM). Of particular interest is the fabrication of complex functional (biocompatible) mesh arrays. Mesh elements or unit cells can be divided into different regions in order to use different cell designs in different areas of the component to produce various or continually varying (functionally graded) mesh densities. Numerous design elements have been used to fabricate prototypes by AM using EBM of Ti-6Al-4V powders, where the densities have been compared with the elastic (Young) moduli determined by resonant frequency and damping analysis. Density optimization at the bone-implant interface can allow for bone ingrowth and cementless implant components. Computerized tomography (CT) scans of metal (aluminium alloy) foam have also allowed for the building of Ti-6Al-4V foams by embedding the digital-layered scans in computer-aided design or software models for EBM. Variations in mesh complexity and especially strut (or truss) dimensions alter the cooling and solidification rate, which alters the alpha-phase (hexagonal close-packed) microstructure by creating mixtures of alpha/alpha' (martensite) observed by optical and electron metallography. Microindentation hardness measurements are characteristic of these microstructures and microstructure mixtures (alpha/alpha') and sizes.
