Asymmetry of the latent heat signature in b -axis oriented single crystal Gd 5 Si 2 Ge 2
- PDF / 876,360 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 58 Downloads / 152 Views
Asymmetry of the latent heat signature in b-axis oriented single crystal Gd5Si2Ge2 K. Morrison1, V.K. Pecharsky2, K.A. Gschneidner, Jr.2, and L.F. Cohen1 1
Blackett Laboratory, Prince Consort Rd, London SW7 2BZ, UK Ames Laboratory of the U.S. Department of Energy, and Department of Materials Science and Engineering, Iowa State University, Ames, Iowa, 50011-3020, USA 2
ABSTRACT A 100 micron fragment of a b-axis oriented single crystal Gd5Si2Ge2 has been studied using microcalorimetry, enabling the separate measurement of the heat capacity and the latent heat. The sample was taken from the same crystal previously studied with Hall probe imaging, which showed that the phase transition is seeded by a second phase of Gd5Si1.5Ge1.5 nanoplatelets on the increasing field sweep direction only. The multiple transition features observed in the latent heat signature suggests a nucleation size of approximately 20 μm, consistent with the lengthscale suggested by Hall imaging. The difference in nucleation and growth process with field sweep direction is clearly identified in the latent heat. We show that the latent heat contribution to the entropy change is of the order of 50% of the total entropy change and unlike other systems studied, the transition does not broaden (and the latent heat contribution does not diminish significantly) as magnetic field and temperature are increased within the parameter range explored in these experiments. INTRODUCTION The magnetocaloric effect is observed as a change in temperature of a magnetic material in response to magnetic field; the result of conservation of entropy. Typically large changes in entropy occur when there is a first order phase transition, and this is often found in materials that exhibit a coincident structural phase transition or volume change. Associated with these large entropy changes is hysteresis, ΔH; a property that may limit the efficiency of the refrigerative cycle. A better understanding of the metamagnetic transitions and how they may be controlled to reduce the critical field, Hc, or ΔH, are important research directions. Here we relate the different nucleation and growth processes observed previously by Hall probe imaging to their manifestation in latent heat measurements. We also demonstrate that unlike previously studied systems such as CoMnSi or La(Fe,Si)13 the latent heat magnitude does not vary significantly with the critical field. The giant magnetocaloric effect was first reported in Gd5Si2Ge2 in 19971 and explained as the result of a co-incident orthorhombic (ferromagnetic FM) to monoclinic (paramagnetic PM) structural and magnetic phase transitions.2 For the Gd5Si2Ge2 composition the Tc is 274K. By varying the Si:Ge content, the Tc can easily be tuned and the phase transition moves from first to second order as the structural phase transition disappears.3 Studies suggest that the structural contribution to entropy change, ΔSstr, is of the order of 40-60% of the total entropy change, ΔSTotal.4,5,6 We use here the separation of the latent heat and the heat
Data Loading...
