Memory effect in triglycine sulfate induced by a transverse electric field: specific heat measurement
M C Gallardo, J M Martin-Olalla, F J Romero, J del Cerro, B Fugiel
aa r X i v : . [ c ond - m a t . o t h e r] D ec doi: 10.1088/0953-8984/21/2/025902; J. Phys.: Condens. Matter (2009) 21 025902 Memory effect in triglycine sulfate induced by a transverse electric field: specific heatmeasurement
M C Gallardo, ∗ J M Mart´ın-Olalla, F J Romero, and J del Cerro
Departamento de F´ısica de la Materia Condensada. Instituto Ciencia MaterialesSevilla. Universidad de Sevilla-CSIC. PO Box 1065.E41080 Sevilla.Spain
B Fugiel
August Che lkowski Institute of Physics, Silesian University, Uniwersytecka 4, PL-40-007 Katowice, Poland (Dated: December 10, 2008.)The influence of a transverse electric field in the specific heat of triglycine sulphate (TGS) hasbeen studied. The specific heat of TGS has been measured heating the sample from ferroelectricto paraelectric phase after prolonged transverse electric field (i.e. perpendicular to the ferroelectricaxis). It is shown that the specific heat of TGS can remember the temperature T s at which thetransverse field was previously applied. PACS numbers: 77.84.Fa KDP- TGS-type crystals;65.40.Ba Heat capacity of crystalline solids; 77.80.DjDomain structure, hysteresis in ferroelectricity; 77.77.+a Pyroelectric and electrocaloric effect
Triglycine sulfate [(NH CH COOH) · H SO ] (TGS)has grown great importance for the development of in-frared sensors[1, 2] and flat-panel displays[3] due to itslarge pyroelectric coefficient[4] and large pyroelectric fig-ure of merit[5]. The crystal exhibits a ferroelectric phase(ferroelectric axis being parallel to axis b ) below T c =322 K. At this temperature the substance shows a con-tinuous phase transition to a paraelectric phase.An electric field E k parallel to the ferroelectric axisin TGS and its family has strong influence in the phys-ical properties close to the critical point as revealed bynumerous studies[4, 6, 7, 8, 9, 10, 11] and causes a single-domain (or almost single-domain) state. After field dis-connection regions with opposite polarization randomlygrows and the system tends to its equilibrium state for E = 0. The longitudinal field has then a non permanentinfluence in the state of the system.The scenario is quite different for transverse electricfield E ⊥ whose influence is reported to persist long timeafter switch-off although original properties can be eas-ily restored after annealing the sample above the criti-cal temperature. These properties are commonly named transverse field effect . Among them, the following list:reduced or even vanishing hysteresis loop[12] (the re-duction depends on how much time the field acted);the existence of temperature-dependent reversible trans-verse polarization[13, 14] and domain wall structure par-allel to c -axis[15]. Dielectric measurements also showeda discontinuity in the slope of the inverse dielectricsusceptibility[16] at the maximum temperature of heat-ing after transverse field application as well as a suppres-sion of the low frequency dispersion[16] observed fromthe temperature at which the transverse field was ap-plied to the critical temperature. Such behaviour can beconsidered as some kind of memory effect . ∗ Electronic address: [email protected]
The transverse field effect in TGS has always been re-ported in directional properties but it would be of greatinterest to ascertain if these effects can be observed inbulk properties such as specific heat which would also besignificant from the energetic point of view. It is also ofinterest for applications of this material since the specificheat plays a major role in determining the pyroelectricfigure of merit.In this letter, we present measurements of the specificheat after prolonged transverse electric field and reportthe existence of persistent memory effect in the specificheat of TGS.The specific heat measurements were performed usingconduction calorimeter previously described[17, 18]. Weshould quote its main features: able to provide specificheat absolute values, able to measure small changes ofenthalpy (some milli joules) by means of high resolutiondifferential thermal analysis (DTA) trace, able to mea-sure in electric field, small rate of temperature change(as low as few decikelvin per hour). These features havebeen applied for observing ferroelectric phase transitionsspecially in the influence of electric field and for measur-ing latent heat even in the neighborhood of a tricriticalpoint where it is expected to be comparatively small.[11, 19, 20]The TGS crystal used in this experiment was pro-duced by MolTech GmbH in Berlin. It was of theform of a rectangular parallelepiped with dimensions7 . × . × . b × c , b , c and 302 mgin mass. The electric field (2 kV cm − ) was applied per-pendicular to the ferroelectric axis b and parallel to c (see figure 1). Sides perpendicular to c were previouslypainted with a thin layer of a silver solution as an elec-trode.The following sequence was kept in our experiments:the sample was cooled from the paraelectric phase downto a given temperature T s in the ferroelectric phase wherethe transverse electric field (2 kV cm − ) was applied for atime t (typically 100 h) and then disconnected; the sam-Typeset by REVTEX FIG. 1: Sample geometry relative to electric field action. Asilver solution (gray) was painted on faces perpendicular to c axis. ple was cooled below 298 K and then heated above crit-ical temperature at a rate of 0 . − while recordingspecific heat data. As a reference we also recorded spe-cific heat data in a heating run after annealing in theparaelectric phase and without any kind of exposure totransverse electric field.Figure 2 shows data for the experiments. Light circlesalways apply the reference experiment (heating run andno exposure to electric field). Dark circles in figure 2(a)show data for T s = 310 . t = 100 h. Specific heatdata in the ferroelectric phase differ from reference dataabove T s showing a memory effect. It is also shown thatcritical temperature does not noticeably change but thepeak of the specific heat decreases. Figure 2(b) showsdata for T s = 315 . t = 100 h with analogousbehavior. Figure 2(c) shows data for T s = 319 . t = 100 h, now somehow close to the critical point, alsowith analogous behavior. The last experiment was alsorun for t = 50 h and t = 10 h with same results, inagreement with Ref [21]. It is also noteworthy to mentionthe persistence of the phenomenon: the memory effectis observed above T s = 319 . T = 315 . E ⊥ at T < T c alters permanently the dielectric prop-erties of a TGS crystal. We now show that it also altersthe thermal behavior of the sample. Having in mind allthe results illustrating the so called transverse field ef-fect in TGS we should ponder which of the previouslyreported experimental data obtained after the transversefield disconnection at T s differ from original data (i.e.not influenced by E ⊥ or rejuvenated) only for T > T s and may eventually have influence on the specific heat.On the basis of previous investigations we know thateach temperature increase from 300 K up to T c —after FIG. 2: Memory effect in specific heat of TGS. Light circlesshow the reference —no field— specific heat for a heatingrun. Dark circles show the specific heat (heating run) after(1) applying transverse field 2 kV cm − for 100 h at T s —vertical line—, (2) field disconnection and (3) cooling downto 298 K. the transverse field disconnection at T s ( < T c )— is fol-lowed by a special kind of electric charge excitation justabove T s as revealed, for instance, after warming of ashort circuited (by electrometer) transversely polarizedsample[13]. In these experiments, for T < T s , the trans-verse electric current density J ⊥ behaves as the pyroelec-tric current density flowing along the polar direction in FIG. 3: Rejuvenation of specific heat after annealing. Lightpoints show the reference data in figure 2. Dark points weretaken after: (1) experiment in figure 2(c) (dark points) , (2)annealing above T c , (3) cooling to T s = 319 . T s for three days (no field applied), (5)cooling the sample to 298 K and (6) heating the sample whilemeasuring specific heat (dark points). conventional pyroelectric measurements resembling sym-metry in cooling and heating. But above T s a secondtransverse electric current density is triggered. This com-ponent is characterized by an irretrievable outflow of freecharge carriers from the sample and it is only observedin heating experiment; on the contrary, the sample canbe cooled without observing any effect of this kind. Itshould be also mentioned that the influence of dP k /dT on conventional specific heat measurements of TGS crys-tals was previously reported (see for instance Ref. [22]and references therein). PSfrag replacements 0 J ⊥ ∆ J ⊥ J tp ⊥ TT s T r FIG. 4: Transverse current density J ⊥ (solid lines) in a TGScrystal after a transverse electric field disconnection at T s (