Journal of Alloys and Compounds 488 (2009) 550–553 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Spontaneous generation of voltage in the magnetocaloric compound Tb 5 Si 2.2 Ge 1.8 and elemental Gd M. Zou a,b , V.K. Pecharsky a,b,∗ , K.A. Gschneidner Jr. a,b , D.L. Schlagel a , T.A. Lograsso a a Ames Laboratory of the US DOE, Iowa State University, Ames, IA 50011-3020, USA b Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011-2300, USA article info Article history: Received 5 June 2008 Accepted 6 September 2008 Available online 23 October 2008 Keywords: Intermetallics Electronic transport Magnetically ordered materials Magnetocaloric effect abstract The spontaneous generation of voltage (SGV) in single crystalline Tb 5 Si 2.2 Ge 1.8 and Gd has been studied. Temperature-induced SGVs were observed along the three principal crystallographic axes of Tb 5 Si 2.2 Ge 1.8 , but not in Gd. Field-induced SGVs were observed with magnetic fields less than 40 kOe applied along the a- axis of Tb 5 Si 2.2 Ge 1.8 , and the c-axis of Gd. The absence of the temperature-induced SGV in Gd indicates the key role first-order phase transformations play in the appearance of the effect when temperature varies. The anisotropy of the magnetic field-induced SGV in Tb 5 Si 2.2 Ge 1.8 and the existence of the field-induced SGV in Gd, highlight the importance of the magnetocaloric effect in bringing about the SGV. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Interesting physical properties, such as the giant magne- tocaloric effect (GMCE), giant magnetoresistance (GMR), colossal magnetostriction, and spontaneous generation of voltage (SGV) [1–3] are known to occur in rare earth intermetallic compounds R 5 (Si x Ge 1-x ) 4 , where R is a lanthanide element. All of these effects originate from first-order coupled magnetic and crystallographic phase transformations when R = Gd [1]. When 0 < x ≤ 0.3, the phase transformation is between a ferromagnetic orthorhombic phase [O(I)/FM] and a different orthorhombic phase in its antiferromag- netic state [O(II)/AFM]. When 0.4 < x ≤ 0.503, the transformation is between a ferromagnetic orthorhombic phase [O(I)/FM] and a para- magnetic monoclinic phase [M/PM]. Temperature, magnetic field, and/or pressure have been identified as triggers of these transfor- mations [1,4]. Different from Gd 5 (Si x Ge 1-x ) 4 , the magnetic and crystallo- graphic phase transformations of Tb 5 (Si x Ge 1-x ) 4 between O(I)/FM and M/PM are decoupled by ∼10 K and less than 5 K when x = 0.5 and 0.55, respectively [5–7]. These independent magnetic and crys- tallographic phase transformations can be recoupled by applying moderate hydrostatic pressures and/or magnetic fields as has been demonstrated for a polycrystalline sample with x = 0.5 and a single ∗ Corresponding author at: Ames Laboratory of the US DOE, Iowa State University, Ames, IA 50011-3020, USA. E-mail addresses: zoumin@iastate.edu (M. Zou), vitkp@ameslab.gov (V.K. Pecharsky). crystal with x = 0.55 [6,8]. The recoupling results in a 40% enhance- ment of the GMCE in polycrystalline Tb 5 Si 2 Ge 2 (x = 0.5) [8]. It is worth noting that in a single crystal of Tb 5 Si 2.2 Ge 1.8 (x = 0.55), the GMCE only occurs when the magnetic field is applied parallel to the a- and c-axes [6]. Interplay between decoupling and recou- pling of the magnetic and structural transformations indicates a complex energy landscape separating different magnetic and crys- tallographic phases, and the complexity is enhanced by the single ion anisotropy of Tb 3+ when compared to R = Gd 3+ [6]. The SGV has been observed in both polycrystalline and single crystalline Gd 5 (Si x Ge 1-x ) 4 in the vicinity of the Curie temperature (T C ) when either temperature was varied at constant field or when magnetic field was changed isothermally [2,3]. The origin of SGV in these compounds is believed to be thermoelectric power, i.e., See- beck effect [2,3]. Because a small compositional difference is always present between the two ends of a sample and because small tem- perature gradients are always present across a sample when its temperature is actively controlled, phase transformation typically starts at one end of the specimen and then propagates to the other. The heat release or absorption during this process gives rise to mea- surable temperature gradients necessary for the generation of the thermoelectric signal. The study of SGV in Tb 5 (Si x Ge 1-x ) 4 compounds is a continua- tion of previous studies of Gd 5 (Si x Ge 1-x ) 4 [2,3]. To gain a better understanding of the role that different factors play in the appear- ance of the SGV, a single crystal of Gd metal has also been examined with respect to both temperature and magnetic field-induced spon- taneous voltage. Among the materials undergoing second order magnetic order–disorder transformations, elemental Gd has one 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.032