Torque magnetometry

Measurement of magnetic anisotropy can be performed by single crystal magnetometry methods. A particularly sensitive way to do this is by cantilever torque magnetometry where the crystal is mounted on an upper plate (the cantilever), fixed at one end, above a lower metallic plate. When a magnetic field is applied to an anisotropic substance it experiences a torque T given by the cross product T = M x H 

With any magnetisation measurements on single crystals it has to be remembered that it is the property of the crystal that is being measured. If the molecules are not all magnetically equivalent (with respect to the applied field) then analysis of the data to determine molecular properties, for example ZFSs, is not straightforward. Spectroscopic methods, where transitions between the Ms states are observed, give this information more directly. 

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This effect can in principle be measured by any technique that is sensitive to the magnetisation dynamics and in addition to the SQUID measurements above quantum tunnelling of magnetisation (QTM) has been observed for Mni2 by, for example, torque magnetometry, and Mn nuclear relaxation rates in and specific heat. ... 

As we have already noted, Tb and Dy have such large axial anisotropies that at low temperatures their moments can only be pulled a few degrees out of the basal plane. Conventional torque magnetometry techniques are thus unsuitable, but estimates of the axial anisotropy coefficients have been made from analyses of the magnetization measurements (Feron et al., 1970 Rhyne et al., 1968) and also from measurements of the zero torque position of a torque magnetometer with uncoupled servo (Rhyne and Clark, 1967). All of these experiments were actually interpreted with the aid of an expression for the macroscopic anisotropy energy of the form ... 

Fig. 7. Comparison of experimental data obtained by SQUID and by torque magnetometry on the same single...

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... 

Wu et al. (1993) have studied the canting model in more detail and have given both the calculated and experimental data as shown in fig. 53 for Tbjc(FeCo)i t alloys with A w 1800. It is seen clearly that there is an anomalous drop near the compensation point when using torque magnetometry with the field at 45". The figure also shows the experimental data and calculated ATu curve in terms of the extraordinary Hall effect. We notice that the torque technique produces a wider and deeper apparent dip and the Hall effect techniques produce a much narrower dip for the T-dominant case and a narrow peak for the R-dominant case. 

Siegfried P, Koo J-H, Pechan M. Torque characterization of functional magnetic polymers using torque magnetometry. Polym Test 2014 37 6-11. 

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