AC susceptibility experiments

Figure 2.34 illustrates these serious experimental problems. In the upper part (a) the magnetic penetration depth of k-(ET)2Cu(NCS)2 measured by ac susceptibility [220] and in the lower part (b) the same quantity obtained by dc magnetization [227] is shown. The clear discrepancy of the behavior of A(T) at low temperatures is obvious. In the ac-susceptibility experiment consistent with [221, 222, 223] a variation is found. This non-exponential non-BCS dependence can be explained with energy-gap nodes of several different topologies [222]. On the other hand, the data of [227] could very well be described by conventional weak-coupUng theory in the clean limit as shown... 

Small deviations from equilibrium are assumed, which implies small changes in the applied magnetic field as in the case of ac magnetic susceptibility experiments. 

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. 

It should be noted that the data on the relaxation rates and spin dynamics of the diluted and mixed LiHopY pF4 (Reich et al. 1990), LiTbpYi pF4 (Lloyd and Mitchell 1990) and DyPpVi-p04 (Dirkmaat et al. 1987), obtained in the frequency dependent ac susceptibility measurements and in the inelastic neutron scattering experiments, demand special discussion. 

Metallic crystals Metallic systems, like Fe impurities in Au, are often cited as classical spin glasses, referring to the pioneering experiment by Cannella and Mydosh (1972) when a cusp in the ac susceptibility, a (T ), in low magnetic fields is observed in these dilute alloys. In such systems, however, the interactions are rather complicated and not precisely known ... 

For ac susceptibility, the measuring time is well defined, and this experiment is very useful for determining the parameters. Results obtained for ferritin, which also presents a broad peak, show that the model developed for isolated particles is verified with Tq clearly smaller than the expected value for ferromagnetic particles, in agreement with the above discussion (see Section H.l). We note that it is the model for isolated particles that has to be checked since the interparticle interactions are weak due to the small m value. 

Experiments using a standard electromagnet can be performed by measuring the strain modulated biased transverse susceptibility (Kraus 1989). In this measurement, a static bias field is applied in the film plane, now perpendicular to the film axis, while the ac driving field is parallel to that axis. The susceptibility (transverse to the bias field) is detected by the pick-up coils and its reciprocal value is then a linear function of both the dc bias field (tfdc) and the magnetoelastic field (Ha = 3As[Pg.109]

The techniques currently used to study the superparamagnetic relaxation are dc susceptibility is not well defined, estimated to be around 100 s, but it depends on the type of magnetometer and on the measuring procedure), ac susceptiblity (t , = 10 -10 s for experiments at very low frequencies t = 10 -10 s for classical experiments t = 10 —l0 s for measurements at very high frequencies, very difficult to realize, so far), Mossbauer spectroscopy (time window, 10 -10 s for Fe), ferromagnetic resonance (t , = 10 s), and neutron diffraction (time window, 10 -10 s, depending on the type of experiments). 

---