DSpace at VNU: Magnetocaloric effects in RCo2 compounds

Journal of Magnetism and Magnetic Materials 242–245 (2002) 873–875 Magnetocaloric effects in RCo2 compounds N.H Duc*, D.T Kim Anh Faculty of Physics, Cryogenic Laboratory, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam Abstract Magnetisation isotherms were measured for a number of (R; R0 )Co2 and (R, Y)Co2 (R, R0 =rare earths) compounds A metamagnetic transition is observed just above the Curie temperature (TC ) of compounds having a first-order phase transition, i.e ErCo2, HoCo2 and (Dy, Y)Co2 The magnetic entropy change DSm shows a largest value of À11.8 J/ mol K at 35 K for ErCo2 and it decreases exponentially with increasing temperature The obtained thermal variation of DSm is compared to that of RAl2 and other intermetallic compounds Giant magnetocaloric effects observed in RCo2based compounds are discussed in terms of the 4f(R)-localised spin, 3d(Co)-spin fluctuations as well as nature of the phase transition r 2002 Elsevier Science B.V All rights reserved Keywords: Rare earth–transition metal compounds; Magnetocaloric effects; Metamagnetic transition The best materials for magnetic refrigeration applications are compounds, which display first order or combined transitions [1] In this context, rare earth (R) intermetallics are one of the promising candidates because many of them (RCo2, RCo3, RCo5y) exhibit a metamagnetic transition (MMT) in an applied magnetic field and/or with a temperature variation [2,3 and Refs therein] The RCo2 compounds with R=Er, Ho, Dy show the first-order transition (FOT) at low temperatures (TC o178 K), whereas the RCo2 with higher TC (i.e TbCo2 and GdCo2) exhibit the secondorder transition (SOT) The formation of the 3d magnetic moments is combined with the volume expansion and the quenching of the spin fluctuations The size of the magnetocaloric effect (MCE) in these compounds, thus, depends not only on the number of (localised) 4f-spin, the nature of the transition, but also on the contribution of the 3d-itinerant electrons The aim of this paper is to investigate MMT and MCE in the vicinity of TC in (R,R0 )Co2 and (R,Lu(Y))Co2 compounds (R=Er, Ho, Dy, Tb, Gd) The (R0 ,R)Co2 and (R,Y)Co2 compounds were prepared by arc-melting the stoichiometric mixtures of rare earth (3N) and cobalt (4N8) in an argon atmosphere in a *Corresponding author E-mail address: duc@cryolab-hu.edu.vn (N.H Duc) water-cooled copper container The samples were annealed in an evacuated quartz tube at 9001C for 48 h Magnetisation were measured using the induction method in fields up to 10 T Magnetisation isotherms are illustrated in Fig 1(a–d) for RCo2 compounds (R=Er, Ho, Dy, Tb) MMT is observed in ErCo2, HoCo2 and DyCo2 This MMT exists only in a small range of temperature (DTE20 K) above the FOT In addition, the MMT is characterised by (i) the large hysteresis of magnetisation, (ii) the increase of the critical fields (Bc ) and (iii) the decrease of the magnetisation jump with increasing temperature The disappearance of the MMT is characterised by the disappearance of not only the magnetisation jump but also of the hysteresis For DyCo2, the MMT is weakly evident in the hysteresis of the magnetisation curves above TC : This behaviour has completely disappeared in TbCo2 The MMT is usually attributed to the 3d(Co) sublattice However, as can be seen from the magnetisation isotherms (e.g Fig 1a and b), the magnetisation jump at the MMT reaches a magnitude of DM > mB =f:u:; which is already over the contribution of 3d itinerant electrons This implies that, at the critical field of the MMT, not only the magnetic moment is formed at the Co sites, but also the rare earth moment becomes ordered suddenly In this magnetisation process, the total magnetisation of both 4f and 3d moments must be described as a proper thermodynamic variable 0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - 8 ( ) - N.H Duc, D.T.K Anh / Journal of Magnetism and Magnetic Materials 242–245 (2002) 873–875 36 K 40 K T = 34 K 5 48 K 45 K M (µB/f.u.) M (µB /f.u.) 874 10 B(T) (a) 85 K ErCo2 90 K 2 100 K 95 K HoCo2 T = 80 K 10 B(T) (b) 230 K 160 K 155 K 150 K 270 K 260 K 250 K DyCo2 (c) T=210K 145 K 220 K M (µB /f.u.) M (µB /f.u.) T= 140 K 240 K 10 B(T) (d) TbCo2 10 B(T) Fig Magnetisation isotherms of ErCo2, HoCo2, DyCo2 and TbCo2 The magnetic entropy change (DSm ) was calculated from magnetisation data MT ðBÞ by the Maxwell relation [4,5]:  Z B qMBịTav dB DSm Tav ịH ẳ qT H Z B ẵMTiỵ1 ; Bị MðTi ; Bފ dB: ð1Þ E DT Here, Tav ẳ Tiỵ1 ỵ Ti ị=2 is an average temperature and DT ẳ Tiỵ1 Ti is the temperature difference between the two magnetisation isotherms measured at Tiỵ1 and Ti with the magnetic field changing from to B: The obtained results of DSm are illustrated in Fig 2a and b for ErCo2 and TbCo2 Note that, DSm always shows its max maximum ðDSm Þ at TC : max For TbCo2, DSm ¼ À2:3 J/mol K in DB ¼ T In addition, DSm is almost symmetric with respect to TC : This is a general behaviour of the SOT For ErCo2, at max TC DSm reaches a huge value of À11.1 J/mol K in DB ¼ T DSm falls abruptly just below TC ; but it still has a rather large value in a small temperature range above TC ; where the MMT occurrs As regard applications of MCE, three samples of Gd0.4Tb0.6Co2, Gd0.7Lu0.3Co2 and Gd0.7Y0.3Co2 having the SOT around room temperature are prepared and investigated These intermetallic compounds show a max value of DSm E À 1:8 J/mol K in DB ¼ T Their entropy change is comparable with that of Gd metal (of À1.6 J/mol K), which is used as one of the working materials nowadays [4] max DSm is determined and collected in Fig for several (R,R0 )Co2 and (R,Y)Co2 compounds The data are in good agreement with those reported for RCo2 [4 and Refs therein] As the temperature decreases, an exmax ponential tendency to increase of DSm is found This temperature dependence of the entropy change was previously reported for several intermetallic systems such as RAl2, RNi2 [4] For a more detailed comparison, max those data of DSm are included in the same figure However, a similar variation is observed in the temperature range T > 200 K only At low temperatures, max DSm (RAl2) is smaller than that of the corresponding max max DSm (RCo2), e.g at TC ¼ 35 K, [DSm max (RCo2)ÀDSm (RAl2)]EÀ6 J/mol K This large difference may be related to (i) the nature of the FOT and (ii) the quenching of spin fluctuations observed in RCo2 It was indicated that the quenching of spin fluctuations reduced the electronic entropy DSe ¼ DgT; where g is the electronic specific heat constant From data of the change of the electronic specific heat Dg at the MMT, which was collected in Ref [3] for several Y(Lu)Co2 related compounds, it turns out that the electronic contribution to the entropy change is oÀ1 J/mol K Such a contribution is rather small with respect to the above mentioned entropy difference in RCo2 and other N.H Duc, D.T.K Anh / Journal of Magnetism and Magnetic Materials 242–245 (2002) 873–875 14 12.0 - ∆ Sm (J/mol.K) ErCo 10.0 -∆ S (J/mol.K) 875 B=2T B=4T 8.0 6.0 12 RRCo C o2 10 RRAl A l2 GGd d3Al2 2Al3 GGd d ∆B = T 4.0 0 2.0 100 200 300 400 TC (K) 0.0 20 40 T (K) (a) 60 max Fig DSm vs TC of RCo2 and other intermetallics lower cost Thus, they are rather promising for magnetic refrigeration applications 2.5 TbCo2 -∆ S (J/mol.K) 2.0 This work is supported by the National University of Hanoi, within project QG.TD.00.01 and by the project 420.301 of the Fundamental Research Program of Vietnam B=1T B=3T B=5T 1.5 1.0 0.5 References 0.0 200 (b) 220 240 260 280 300 T (K) Fig DSm vs T of ErCo2 (a) and TbCo2 (b) rare earth intermetallics Therefore, MCE observed in RCo2 with low ordering temperature can be attributed to the FOT In conclusion, MCE in RCo2 is mainly governed by the nature of the type of the magnetic phase transition The size of the effect observed in (R, R0 )Co2 compounds with the SOT at room temperature is comparable with that of pure Gd metal These materials, however, are of [1] M Foldeaki, A Giguere, R Chahine, T.K Bose, Adv Cryogenic Eng 43 (1998) 1533 [2] N.H Duc, T Goto, in: K.A Gschneirdner Jr., L Eyring (Eds.), Handbook on Physics and Chemistry of the Rare Earths, North-Holland, Amsterdam, 1999, Vol 26, Chapter 171, p 301 [3] N.H Duc, P.E Brommer, in: K.H.J Buschow (Ed.), Handbook on Magnetic Materials, North-Holland, Amsterdam, 1999, Vol 12, Chapter 3, p 259 [4] M.A Tishin, in: K.H.J Buschow (Ed.), Handbook on Magnetic Materials, North-Holland, Amsterdam, 1999, Vol 12, Chapter 4, p 395 [5] V.K Pecharsky, K.A Gschneidner Jr., J Appl Phys 86 (1999) 565  ... difference may be related to (i) the nature of the FOT and (ii) the quenching of spin fluctuations observed in RCo2 It was indicated that the quenching of spin fluctuations reduced the electronic entropy... (b) rare earth intermetallics Therefore, MCE observed in RCo2 with low ordering temperature can be attributed to the FOT In conclusion, MCE in RCo2 is mainly governed by the nature of the type... (R,Y)Co2 compounds The data are in good agreement with those reported for RCo2 [4 and Refs therein] As the temperature decreases, an exmax ponential tendency to increase of DSm is found This temperature