SQUID magnetometry

Fundamentals. Movement of charged particles (e.g. an electronic current) causes a magnetic field. Very small fields can be detected with a SQUID. The small size of the sensitive elements in a SQUID allows spatially resolved measurements of magnetic fields down to a resolution of about 1 mm. 

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The study of these systems have become possible thanks to the development of various preparation routes, from sophisticated routes for the preparation of model materials with controlled nanostructures to industrial routes for the production of large quantity of materials. It has benefited as well from the development of new experimental techniques, allowing the properties of matter to be quantitatively examined at the nanometre scale. These include Hall micro-probe [3] or micro-SQUID magnetometry [4], XMCD at synchrotron radiation facilities [5] and scanning probe microscopes [6]. This is not the topic of this chapter to describe in detail these various techniques. They are only quoted in the following sections. The reader may find in the associated references the detailed technical descriptions that he may need. 

The superconducting fulleride (NH3) aK2C6o has been investigated using SQUID magnetometry and NMR in two differently doped samples. NMR relaxation measurements vahdate the interpretation of the spin susceptibility in terms of density of states and rule out the presence of strong antiferromagnetic correlations in the Fermi liquid. 

Complexes [MnX(PhBP3Pr)] (X = Cl, I) have been prepared and characterized by X-ray diffraction, SQUID magnetometry, and EPR spectroscopy. [MnX(PhBP 3Pr)] complexes serve as a precursor to the pseudotetrahedral [Mn(Y)(PhBP 3Pr)] (Y = N3, CH2Ph, Me, NH(2,6- Pr2C6H3), dbabh, l-Ph(isoindolate)). The two Mn(I) species [(PhBP /r)Tl-MnBr(CO)4] and [MnlPhBP llCN Buls] have been also reported.21... 

This outline of polymer-embedded cluster characterization is far from complete. Advances in nuclear magnetic resonance (NMR), SQUID-magnetometry, and many other characterization techniques are of considerable importance as well. In fact, the technological success of a nanostructured material depends strictly on the accurate characterization of its physical properties. 

Aru] SQUID magnetometry at 5 < T < 300 K and H up to 8 kOe. Magnetization, magnetic susceptibility and resistivity of Cr doped fiFeSi2... 

Som] Squid magnetometry and Faraday balance Magnetization and magnetoresistance... 

Many novel experiments based on this technique have been reported in the literature (e.g., Abulafia et al. 1995, 1996, Yeshurun et al. 1996, Doyle et al. 1997). For the purpose of this section, I wish to concentrate on two results, which are shown in fig. 9 (Werner 1997). First, the flux profile (fig. 9a) obtained at a certain external field ( 280 mT, 60 K) again shows all the characteristic Bean-like features and a (smeared-out) increase of the field at the sample edge as expected from demagnetisation. Second, the comparison between the local Hall-probe measurement and SQUID magnetometry (fig. 9b) is as favourable as in the case of torque magnetometry, i.e., the current densities deduced... 

To conclude this section, a few words on the experimental assessment of the irreversibility lines seem to be appropriate. As pointed out in much detail above, the timing ( time window ) of the experiment represents a deeisive factor for the apparent magnitude of J. Since we are now looking for methods to determine the exact location in the //.r plane, where becomes zero, the timing will again play a crucial role. In addition, the criterion Jc = 0 is extremely ill defined, since it entirely depends on the resolution of the experimental technique employed. I will choose two examples (SQUID magnetometry and ac susceptibility) to demonstrate the salient features of the problem. 

Fig. 14. Irreversibility temperatures as a fijnction of (top) frequency and (bottom) amplitude of the ac ripple field. The open symbols denote the quasistatic conditions of SQUID magnetometry for details cf the text (Frischherz et al. 1995).

Other techniques such as magnetic force microscopy or SQUID magnetometry [29] may also find application in assessing the quality of electrodeposited materials. However, limitations on the sensitivity and spatial resolution of these methods need to be assessed. 

SQUID magnetometry demonstrated that these crystalline materials exhibited superparamagnetism at room temperature whilst NMR spectroscopic relaxation studies revealed very high T1 and T2 relaxation times. These characteristics render these iron oxide/PAMAM dendrimer-based nanoparticles as promising prospects as a new class of MRI contrast agents. 

SQUID magnetometry experiments were also performed. The RUjCu complex was the most straightforward to interpret. Its magnetic data was modeled with parameters (g = 2.08,... 

Zaghib, K. etal. Optimized electrochemical performance of LiFeP04 at 60°C with purity controlled by SQUID magnetometry. J. Power Sources, 163,560 (2006). 

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