1. The first-order magnetostructural transition in Gd 5 Sn 4 D.H. Ryan Physics Department, McGill University, Montreal, QC, Canada, H3A 2T8 E-mail: dhryan@physics.mcgill.ca

2. The Gd 5 Si x Ge 4-x compound system exhibits a magnetostructural instability near x=2 that leads to a first- order change, on cooling, from a non-magnetic to a magnetic form accompanied by a large release of magnetic entropy: The “Giant Magnetocaloric effect”. These materials may be useful in magnetic refrigeration, so understanding their behaviour is important. Unfortunately, the method of choice for the study of magnetic and crystal structures − neutron diffraction − is very difficult to use with Gd- based materials as a result of the extremely large neutron absorption cross-section of natural gadolinium. A.O. Pecharsky et al., J. Alloys and Compounds 338 (2002) 126

3. Using 119 Sn Mössbauer spectroscopy we were able to show that Gd 5 Sn 4 exhibits a first order magnetic transition at 82 K. This is seen through the abrupt appearance of two sharp, well-split (B hf = 31.0 T and 38.5 T) magnetic sextets on cooling. This behaviour stands in clear contrast to that seen at a more common second order magnetic transition, where B hf grows continuously on crossing the critical temperature. We therefore turned to 119 Sn Mössbauer spectroscopy to obtain magnetic and structural information. Temperature dependence of the hyperfine field (B hf ) The combined area of the two high-field components shows an abrupt drop at 82 K where the first-order structural transition occurs.

4. Magnetisation data provide further evidence that the 82 K transition results from a magnetostructural instability − there is a marked increase in magnetisation when a large enough field is applied above the 82 K ordering temperature. Estimation of the magnetic entropy changes obtained using the Maxwell relation shows values comparable to those seen in the Gd 5 Si x Ge 4-x system, suggesting that Gd 5 Sn 4 may have significant potential for magnetic refrigeration. Selected magnetisation curves Magnetic entropy change derived from numerical integration of the Maxwell relation between indicated start and end fields.

5. A defining characteristic of the giant magnetocaloric effect is the reversal of the structural change by a magnetic field applied above the transition temperature. The applied field shifts the energy balance in favour of the low-temperature, magnetic form. This would most easily be followed by neutron diffraction in an applied field, however, as noted above, this is not easy to do with Gd-based materials. Here again, 119 Sn Mössbauer spectroscopy can be used. It is clear that by 3 T at 90 K (about 8 K above the transition) the well- split magnetic sextets have reappeared, and that further increases in the magnetic field lead to a growth of the magnetic components.

6. If we take the magnetisation as an indirect measure of the magnetic fraction, and compare it with the spectral area derived from fitting the 119 Sn Mössbauer data, we obtain excellent agreement. The broad coexistence regions in both temperature and field are typical of these giant magnetocaloric materials and derive from the first-order nature of the phase transition. Room temperature x-ray diffraction shows the structure of Gd 5 Sn 4 to be orthorhombic (the O(II) Sm 5 Ge 4 −type). Similarities in the hyperfine parameters with Si-doped Gd 5 Si x Sn 4-x which adopt the O(I) structure suggests that the low temperature structure is O(I), however low-T XRD is needed to confirm this. D.H. Ryan et al. Phys. Rev. Lett. 90 (2003) 117202(4)