Spontaneous generation of voltage in single-crystal Gd5Si2Ge2 during magnetostructural phase transformations

Abstract

The spontaneous generation of voltage ͑SGV͒ in single-crystal and polycrystalline Gd 5 Si 2 Ge 2 during the coupled magnetostructural transformation has been examined. Our experiments show reversible, measurable, and repeatable SGV responses of the materials to the temperature and magnetic field. The parameters of the response and the magnitude of the signal are anisotropic and rate dependent. The magnitude of the SGV signal and the critical temperatures and critical magnetic fields at which the SGV occurs vary with the rate of temperature and magnetic-field changes. Since their rediscovery in 1997, 1,2 the R 5 ͑Si x Ge 1−x ͒ 4 intermetallic compounds, where R is a lanthanide metal, continue to attract considerable attention. This is because of their interesting physical properties, such as the giant magnetocaloric effect, 1-4 colossal magnetostriction, 5-8 giant magnetoresistance, 9-16 and spontaneous generation of voltage ͑SGV͒, 17 that have been observed during the firstorder magnetostructural, ferromagnetic-orthorhombic to paramagnetic-monoclinic ͑on heating͒, phase transformation ͑see Refs. 5,18͒. Among them, the SGV is especially intriguing because it may result in the development of sensors, which can respond not only to changes in temperature, pressure, and/or magnetic field but most importantly to the rates of their changes without the need for a complicated analysis of signals. Furthermore, all of this can be done by a single sensor requiring no standby power.The SGV in the R 5 ͑Si x Ge 1−x ͒ 4 system was reported by Levin et al. in several polycrystalline samples of Gd 5 ͑Si x Ge 1−x ͒ 4 . 17 Because a current was not supplied, the voltages observed across the samples were regarded as spontaneous. The SGV occurs near first-order magnetostructural phase transformations. Here we report on the SGV during the first-order phase transition in several single-crystal Gd 5 Si 2 Ge 2 samples. In addition to the SGV behavior as a function of temperature and magnetic field, we also examine the anisotropy of the signal and its dependence on the variable rates of change of these stimuli.The Gd 5 Si 2 Ge 2 single crystal was prepared using the Bridgman method. 19 Three parallelepiped-shaped samples were cut from a large grain by electric discharge machining. The longest side of each sample was parallel to one of the three major crystallographic directions ͓100͔, ͓010͔, or ͓001͔, and specimen dimensions were 3.66ϫ 1.82ϫ 0.49, 4.06 ϫ 1.03ϫ 1.00, and 5.6ϫ 2.0ϫ 0.92 mm 3 , respectively. The SGV behaviors of a polycrystalline Gd 5 Si 2 Ge 2 sample with the dimensions of 6.57ϫ 2.48ϫ 1.85 mm 3 were also measured in order to verify previous observations 17 and to compare them with the single-crystal samples.The dc voltages across the samples were measured by a standard two-probe method and recorded as functions of time by a Keithley 181 nanovoltmeter. Readouts from the nanovoltmeter were computer recorded every 0.25 s. The temperature and magnetic-field changes exerted on the samples were regulated by a LakeShore Model 7225...

Spontaneous generation of voltage in the magnetocaloric compound Tb5Si2.2Ge1.8 and elemental Gd

“…Contact resistance of the freshly prepared samples was below 1 . The SGV measuring method, instrument set up, and experiment errors were described elsewhere [3].…”

“…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., Seebeck effect [2,3].…”

“…The study of SGV in Tb 5 (Si x Ge 1−x ) 4 compounds is a continuation 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 appearance of the SGV, a single crystal of Gd metal has also been examined with respect to both temperature and magnetic field-induced spontaneous voltage.…”

“…The origin of SGV in these compounds is believed to be thermoelectric power, i.e., Seebeck effect [2,3]. Because a small compositional difference is always present between the two ends of a sample and because small temperature 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.…”

“…Interesting physical properties, such as the giant magnetocaloric effect (GMCE), giant magnetoresistance (GMR), colossal magnetostriction, and spontaneous generation of voltage (SGV) [1][2][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].…”