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Magnetocaloric properties of hydrogenated Gd, Tb and Dy
Journal of Magnetism and Magnetic Materials xxx (2017) xxx–xxx
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Magnetocaloric properties of hydrogenated Gd, Tb and Dy V.B. Chzhan a,b, I.S. Tereshina c,⇑, E.A. Tereshina-Chitrova d, G.S. Burkhanov a, G.A. Politova a, H. Drulis e a
Baikov Institute of Metallurgy and Materials Science RAS, 119334 Moscow, Russia National University of Science and Technology ‘‘MISIS”, 119049 Moscow, Russia c Lomonosov Moscow State University, Faculty of Physics, 119991 Moscow, Russia d Institute of Physics CAS, Prague 18221, Czech Republic e Institute of Low Temperature and Structural Research, 50-950 Wroclaw, Poland b
a r t i c l e
i n f o
Article history: Received 17 August 2017 Received in revised form 10 November 2017 Accepted 30 November 2017 Available online xxxx Keywords: Rare-earth metals Hydrogenation Magnetocaloric effect Curie temperature Néel temperature
a b s t r a c t The influence of hydrogen on magnetocaloric properties of three high purity heavy rare earths (R), gadolinium, terbium and dysprosium is studied. Hydrogenation procedure is performed using high purity hydrogen in a Sievert-type apparatus. In addition to the main a-RHx phase (where x is the hydrogen concentration), appearance of the second phase RH2 or its traces is detected in nearly all hydrogenated samples. In the samples where the main a-RHx phase prevails and only traces of the dihydride phase are found, the magnetocaloric effect (MCE) remains of the same magnitude as in the hydrogen-free samples. With the increase of hydrogen concentration (and with a corresponding increase of the RH2 phase content), the MCE values decrease. Only Gd samples demonstrate the increased Curie temperatures (TC = 291 and 296 K for Gd and GdH0.2, respectively) related to the increase of exchange interactions in the Gd-Gd pairs. At the same time, the Néel temperatures of Tb and Dy retain their values after hydrogenation. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Among various kinds of magnetocaloric materials [1–8], gadolinium and its alloys and compounds are most widely utilized as elements of magnetic refrigerator prototypes [9–11]. It is well known that gas-forming impurities are able to penetrate easily into metals and change their properties [12]. Influence of a certain impurity, hydrogen for instance, on a certain property (e.g. resistivity) of rare earth (R) metals was studied in detail in Ref. [13]. Vajda et al. [13] observed some regular patters in the properties of hydrogenated Rs such as ordering of hydrogen atoms (creation of H-H pairs chains) or decrease of the conduction electrons concentration (hydrogen captures a conduction electron to create an H ion) etc. With the current boost in the number of magnetocaloric effect (MCE) studies, the question of the influence of hydrogen on the MCE magnitude as well as on the temperature, at which the maximum MCE is observed in various rare-earth metals, is becoming very important [14]. It is known that [1–4,15] Gd is the only rare earth undergoing a para- to ferromagnetic transition near room temperature accompanied by a large magnetocaloric effect. The metals behind Gd in the Periodic table, Tb and Dy, demonstrate complex transitions from para- (PM) to the antiferromagnetic ⇑ Corresponding author. E-mail address: [email protected] (I.S. Tereshina).
(AFM) and then to the ferromagnetic (FM) state. The range of existence of the AFM phase in terbium is only 10 degrees while in dysprosium it is an order of magnitude wider, 100 K. Although magnetocaloric properties of Tb and Dy cannot find immediate application (due to low temperatures) the study of hydrogenation effects on the MCE and transition temperatures broadens fundamental scientific knowledge on these materials. Clearly, the study of the influence of impurities should only be conducted on samples subjected to deep purification [12,16]. The purpose of this work is to investigate the effects of gas-forming impurities, namely hydrogen, on the magnetocaloric properties of high purity rare-earth metals Gd, Tb, and Dy. Investigations of MCE in the hydrogenated rare-earths, especially in gadolinium, is utterly important when the material is functioning in a hydrogen-containing environment.
2. Experimental details Commercial Gd, Tb and Dy metals purified by vacuum distillation as described in Ref. [16] were used for the study. High purity rare earths are characterized by a reduced content of gas-forming impurities (10 2–10 3 wt%). The metals’ purity with respect to the content of 70 impurity elements are 99.92–99.96 wt%. The samples for hydrogenation and further studies were cut out of different portions of a distillate (see e.g. Refs. [17,18]) with
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Please cite this article in press as: V.B. Chzhan et al., Magnetocaloric properties of hydrogenated Gd, Tb and Dy, Journal of Magnetism and Magnetic Materials (2017), https://doi.org/10.1016/j.jmmm.2017.11.128
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V.B. Chzhan et al. / Journal of Magnetism and Magnetic Materials xxx (2017) xxx–xxx
dimensions 2 mm 4 mm 8 mm. The metals were hydrogenated using a Sievert-type apparatus using high purity hydrogen (impurities content <10 3–10 4 vol.%) under pressures up to 0.1 MPa. The samples of GdHx (with x = 0, 0.1, 0.15, 0.2, 0.4 and 1.0), TbHx (x = 0, 0.1, 0.2, 0.3, 0.5, 0.7, 0.9, 1.2) and DyHx (x = 0, 0.1, 0.13, 0.2, 0.5, 0.7, 0.9, 1.2) were obtained and investigated. The crystal structure of all samples was characterized by X-ray diffraction (XRD). The XRD patterns were recorded at a 0.02° scanning step on a Rigaku Ultima IV diffractometer (Japan) with a CuKa radiation from the samples surface. The patterns were analyzed using a program PDXL by Rigaku integrated with the international database ICDD. The samples magnetization M(H) was measured using a PPMS14 magnetometer (Quantum Design, USA) at a constant field step (0.05 T) and at various temperatures. Magnetocaloric effect was studied at 80–350 K and in magnetic fields up to 1.8 T by direct (DTad) and indirect ( DSM) methods [4]. The Maxwell’s relations were used in the latter case. 3. Results and discussion The XRD analysis showed that the parent samples of Gd, Tb and Dy purified by distillation and solid solutions of a-GdHx (x = 0.1, 0.15, 0.2) are single-phase and have a hexagonal close-packed crystal structure (space group P63/mcc) with structural characteristics close to those reported in literature [4,15,18]. The increase of lattice parameters of the materials was observed after hydrogenation Table 1 Lattice parameters and the unit cell volume of Gd and a-GdHx (x = 0.1, 0.15, 0.2). Sample
a, nm
c, nm
c/a
V, nm3
Gd a-GdH0.1 a-GdH0.15 a-GdH0.2
0.3633 0.3640 0.3643 0.3646
0.5770 0.5778 0.5780 0.5783
1.588 1.587 1.587 1.586
0.0660 0.0663 0.0664 0.0666
(see e.g. Table 1). The relative unit cell volume increase of Gd DV/V 1% is obtained in case of an alpha-hydride a-GdH0.2. Available literature data [13,19] shows that it is rather difficult to avoid formation of a b-phase RH2 when preparing solid solutions of hydrogen in rare earths. It is found that the samples TbHx and DyHx as well as GdHx with x > 0.2 at.H/at.R contain two phases. Apart from the main a-RHx phase, a cubic phase b-RH2 is formed. The amount of a dihydride b-RH2 phase in the samples corresponds to the data of the R-H diagram [13,20] and it increases upon the increase of hydrogen concentration. For instance, for the sample DyH1.2, the amount of b-DyH2 is 66% (detected by XRD on the samples’ surface). The Néel temperatures of the dihydride b-RH2 phases are 21, 17 and 3.5 K for Gd, Tb and Dy, respectively [13]. Therefore, the presence of the antiferromagnetic phase b-GdH2, b-TbH2 and b-DyH2 (which die out at rather low temperatures) does not influence the character of temperature dependence of magnetization M (T) and MCE at higher temperatures. Fig. 1(a and b) shows the M vs. H curves for Gd and a-GdH0.2 in fields up to 2 T near the Curie temperature (2 K step). The Curie temperatures of the parent and hydrided Gd were determined by means of the Arrott-Belov plot method (see Fig. 1(c and d)) based on thermodynamic theory [21]. The accuracy of the ArrottBelov’s method is the highest among other techniques for the TC determination. The second method used to evaluate the TC is by detecting the maximum of magnetocaloric effect (see Fig. 2). With the increase of hydrogen content in the gadolinium samples, we observed an increase of the Curie temperature. The latter indicates the enhancement of the Gd-Gd exchange interactions. Despite the fact that experimental evidence obtained previously on a Gd crystal under hydrostatic pressure gave an opposite result for TC [22,23], we were able to explain the increase of TC in our hydrogenated Gd from the first principles [18]. We showed that the interplay of characteristic features in the electronic structure of conduction band at the Fermi level in the high-temperature paramagnetic phase of Gd and ‘‘negative” pressure exerted by hydrogen are responsible for the observed effect.
Fig. 1. Field dependencies of magnetization in the vicinity of the Curie temperatures of Gd (a), a-GdH0.2 (b), Belov-Arrott plots for Gd (c) and a-GdH0.2 (d).
Please cite this article in press as: V.B. Chzhan et al., Magnetocaloric properties of hydrogenated Gd, Tb and Dy, Journal of Magnetism and Magnetic Materials (2017), https://doi.org/10.1016/j.jmmm.2017.11.128
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Fig. 2. Temperature dependencies of the change of magnetic entropy for solid solutions a-GdHx in the field up to 2 T. Inset: The concentration dependence of DTad(x) in a-GdHx.
The MCE of high purity Gd and its hydrogenated samples was studied in this work and in Refs. [17,18] on the samples cut out of different parts of a distillate, and using direct and indirect measurement methods. It was found that MCE retains its magnitude in the single-phase solid solutions (see inset in Fig. 2). However, once the second phase appears (this is confirmed by the XRD data), the value of MCE slightly decreases. For practical application of MCE in magnetic refrigeration, it is important to perform measurements under conditions as close to those of a real refrigerator. It was of interest to further investigate the influence of hydrogen on MCE in the vicinity of complex cascade transitions PM – AFM – FM. Such transitions are observed not only in pure rareearths [4,15], but in their alloys and compounds [24–30]. The DTad(T) curves obtained by direct method upon the variation of magnetic field from 0 to 1.8 T for hydrides TbH0.3 and TbH0.9 are shown in Fig. 3(a and b). We find that in low magnetic fields m0H < 0.03 T, the MCE of the studied samples TbHx is negative at 225–227 K (not shown here). The negative MCE is characteristic of antiferromagnets [4,15] for which the magnetic field destroys (first partly and then completely at higher fields) the antiferromagnetic structure. Therefore, at fields m0H > 0.03 T the maximum in the temperature dependence of MCE at around 231 K is already due to the magnetic phase transition from the ferromagnetic to the paramagnetic phase. The position of the maximum remains practically unaffected by hydrogen absorption. Fig. 3 shows that the increase of hydrogen concentration in the samples TbHx leads to the decrease of DTad. This fact is also confirmed by indirect calculations of magnetic entropy using the magnetization data. XRD data for these samples shows the presence of the second phase TbH2. The amount of the second phase in the samples increases linearly with the increase of absorbed hydrogen. The linear trend is also found in the decrease of MCE at high hydrogen concentrations. Unlike TbHx, DyHx demonstrates a noticeable magnetocaloric effect in weak and strong magnetic fields and not only in the vicinity of the disorder–order transition (PM AFM) at the Néel temperature TN but also at the order-order transition (AFM FM) at the temperature h [4,15,31]. Similarly to TbHx, we studied magnetocaloric effect in DyHx by direct method. Fig. 4(a and b) shows the temperature dependencies of DTad(T) for selected DyHx samples with x = 0, 0.1, 0.2, 0.5, 1.2. All curves demonstrate two peaks: a wide one (m0H > 0.5 T) at and above the temperature h and a
Fig. 3. Temperature dependencies of MCE of TbH0.3 (a), TbH0.9 (b) in various magnetic fields.
rather sharp peak at TN. The behavior of MCE vs. T at 100–150 K (see Fig. 4(a)) is typical of materials with a phase transition of the first order [4] when the application of magnetic field enlarges the temperature region where the maximum MCE is observed. In addition, it is clearly seen that MCE changes the sign to negative and back around the AFM FM transition. As it was mentioned above, negative MCE is characteristic of antiferromagnets. Higher applied magnetic field results in the shrinking of the region of negative MCE – the effect connected with the transformation of an antiferromagnetic structure under the magnetic field. However, as shown in the course of this experiment, the field of m0DH = 1. 8 T is not strong enough to completely remove negative MCE in the parent and hydrogenated samples. Direct measurements of MCE of Dy and its hydrogenated samples DyHx in fields up to 1.8 T showed that MCE decreases at the TN from DTad = 1.5 K to DTad = 0.9 K when x varies between 0 and 1.2 at.H/at.Dy. The reason for that is probably the same as in the case of Gd and Tb samples, namely, the increase of the amount of second phase in the samples. At the range of concentrations 0 < x 0.2 only traces of the dihydride phase are found in DyHx (according to XRD) and therefore, MCE remains of almost the same magnitude as in the hydrogen-free samples (inset in Fig. 4(b)). Hydrogenation does not increase the TN values and therefore does not change the Dy-Dy interactions. (Note that TN decreases under the hydrostatic pressure [23].) We earlier observed the decrease of MCE in the hydrogendoped single-crystalline samples R2Fe14B [14]. It was connected
Please cite this article in press as: V.B. Chzhan et al., Magnetocaloric properties of hydrogenated Gd, Tb and Dy, Journal of Magnetism and Magnetic Materials (2017), https://doi.org/10.1016/j.jmmm.2017.11.128
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MCE does not decrease with increasing x in single-phase samples while the temperature, at which the maximum of MCE it is observed is growing continuously. Dy demonstrates a considerable magnetocaloric effect in a wide temperature range that includes the transitions of the order-order and order–disorder types. In DyHx the magnitude of MCE is preserved at x 0.2 despite the presence of the second, dihydride phase. Hydrogenation does not change the TN values of both Dy and Tb. The increase of the amount of the second phase b-RH2 leads to a considerable decrease of MCE in Gd, Tb and Dy. Acknowledgements The work of I.S.T. is supported by the RFBR according to the research project № 16-03-00612. The work of E.A.T.Ch. was supported by Czech Science Foundation (Grant P16-03593S). The work of V.B.Ch. was carried out with financial support of the Ministry of Education and Science of Russian Federation in the framework of an increased Competitiveness Program of NUST ‘‘MISIS”, implemented by a governmental decree dated 16th of March 2013, № 211. References
Fig. 4. Temperature dependencies of MCE for DyH0.1 in various magnetic fields (a) and for Dy, DyH0.2, DyH0.5 and DyH1.2 in the fields up to 1.8 T (b). Inset: The concentration dependence of DTad(x) near TN in DyHx.
with strong influence of hydrogen on magnetocrystalline anisotropy. It is not unreasonable to assume that similar effect can be expected in the rare earth metals however the formation of the second phase is likely to conceal the effects related to the influence of hydrogenation on MCE of Gd, Tb and Dy. The structural state of samples (mono-, micro-, nanocrystalline) and relaxation processes in them also should be taken into account when studying magnetocaloric properties of the rare earth metals [18,32–36]. Investigation of the influence of hydrogen on the MCE of single-crystalline high purity rare earth samples is currently in progress.
4. Conclusions In this work, we studied the influence of hydrogenation on the magnitude of MCE and temperatures of magnetic phase transitions for the three high purity rare earth metals with the highest ordering temperatures, Gd, Tb and Dy. XRD studies of samples purified by distillation and then doped by hydrogen showed that hydrogen insertion not only changes the lattice parameters of the parent materials but leads to the appearance of the second crystallographic phase of dihydride b-RH2. The formation of the latter is avoided only in the samples of a-GdHx with x = 0.1, 0.15, 0.2. We observed the following important trends. For Gd the value of
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Please cite this article in press as: V.B. Chzhan et al., Magnetocaloric properties of hydrogenated Gd, Tb and Dy, Journal of Magnetism and Magnetic Materials (2017), https://doi.org/10.1016/j.jmmm.2017.11.128
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Research articles
Magnetocaloric properties of hydrogenated Gd, Tb and Dy V.B. Chzhan a,b, I.S. Tereshina c,⇑, E.A. Tereshina-Chitrova d, G.S. Burkhanov a, G.A. Politova a, H. Drulis e a
Baikov Institute of Metallurgy and Materials Science RAS, 119334 Moscow, Russia National University of Science and Technology ‘‘MISIS”, 119049 Moscow, Russia c Lomonosov Moscow State University, Faculty of Physics, 119991 Moscow, Russia d Institute of Physics CAS, Prague 18221, Czech Republic e Institute of Low Temperature and Structural Research, 50-950 Wroclaw, Poland b
a r t i c l e
i n f o
Article history: Received 17 August 2017 Received in revised form 10 November 2017 Accepted 30 November 2017 Available online xxxx Keywords: Rare-earth metals Hydrogenation Magnetocaloric effect Curie temperature Néel temperature
a b s t r a c t The influence of hydrogen on magnetocaloric properties of three high purity heavy rare earths (R), gadolinium, terbium and dysprosium is studied. Hydrogenation procedure is performed using high purity hydrogen in a Sievert-type apparatus. In addition to the main a-RHx phase (where x is the hydrogen concentration), appearance of the second phase RH2 or its traces is detected in nearly all hydrogenated samples. In the samples where the main a-RHx phase prevails and only traces of the dihydride phase are found, the magnetocaloric effect (MCE) remains of the same magnitude as in the hydrogen-free samples. With the increase of hydrogen concentration (and with a corresponding increase of the RH2 phase content), the MCE values decrease. Only Gd samples demonstrate the increased Curie temperatures (TC = 291 and 296 K for Gd and GdH0.2, respectively) related to the increase of exchange interactions in the Gd-Gd pairs. At the same time, the Néel temperatures of Tb and Dy retain their values after hydrogenation. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Among various kinds of magnetocaloric materials [1–8], gadolinium and its alloys and compounds are most widely utilized as elements of magnetic refrigerator prototypes [9–11]. It is well known that gas-forming impurities are able to penetrate easily into metals and change their properties [12]. Influence of a certain impurity, hydrogen for instance, on a certain property (e.g. resistivity) of rare earth (R) metals was studied in detail in Ref. [13]. Vajda et al. [13] observed some regular patters in the properties of hydrogenated Rs such as ordering of hydrogen atoms (creation of H-H pairs chains) or decrease of the conduction electrons concentration (hydrogen captures a conduction electron to create an H ion) etc. With the current boost in the number of magnetocaloric effect (MCE) studies, the question of the influence of hydrogen on the MCE magnitude as well as on the temperature, at which the maximum MCE is observed in various rare-earth metals, is becoming very important [14]. It is known that [1–4,15] Gd is the only rare earth undergoing a para- to ferromagnetic transition near room temperature accompanied by a large magnetocaloric effect. The metals behind Gd in the Periodic table, Tb and Dy, demonstrate complex transitions from para- (PM) to the antiferromagnetic ⇑ Corresponding author. E-mail address: [email protected] (I.S. Tereshina).
(AFM) and then to the ferromagnetic (FM) state. The range of existence of the AFM phase in terbium is only 10 degrees while in dysprosium it is an order of magnitude wider, 100 K. Although magnetocaloric properties of Tb and Dy cannot find immediate application (due to low temperatures) the study of hydrogenation effects on the MCE and transition temperatures broadens fundamental scientific knowledge on these materials. Clearly, the study of the influence of impurities should only be conducted on samples subjected to deep purification [12,16]. The purpose of this work is to investigate the effects of gas-forming impurities, namely hydrogen, on the magnetocaloric properties of high purity rare-earth metals Gd, Tb, and Dy. Investigations of MCE in the hydrogenated rare-earths, especially in gadolinium, is utterly important when the material is functioning in a hydrogen-containing environment.
2. Experimental details Commercial Gd, Tb and Dy metals purified by vacuum distillation as described in Ref. [16] were used for the study. High purity rare earths are characterized by a reduced content of gas-forming impurities (10 2–10 3 wt%). The metals’ purity with respect to the content of 70 impurity elements are 99.92–99.96 wt%. The samples for hydrogenation and further studies were cut out of different portions of a distillate (see e.g. Refs. [17,18]) with
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dimensions 2 mm 4 mm 8 mm. The metals were hydrogenated using a Sievert-type apparatus using high purity hydrogen (impurities content <10 3–10 4 vol.%) under pressures up to 0.1 MPa. The samples of GdHx (with x = 0, 0.1, 0.15, 0.2, 0.4 and 1.0), TbHx (x = 0, 0.1, 0.2, 0.3, 0.5, 0.7, 0.9, 1.2) and DyHx (x = 0, 0.1, 0.13, 0.2, 0.5, 0.7, 0.9, 1.2) were obtained and investigated. The crystal structure of all samples was characterized by X-ray diffraction (XRD). The XRD patterns were recorded at a 0.02° scanning step on a Rigaku Ultima IV diffractometer (Japan) with a CuKa radiation from the samples surface. The patterns were analyzed using a program PDXL by Rigaku integrated with the international database ICDD. The samples magnetization M(H) was measured using a PPMS14 magnetometer (Quantum Design, USA) at a constant field step (0.05 T) and at various temperatures. Magnetocaloric effect was studied at 80–350 K and in magnetic fields up to 1.8 T by direct (DTad) and indirect ( DSM) methods [4]. The Maxwell’s relations were used in the latter case. 3. Results and discussion The XRD analysis showed that the parent samples of Gd, Tb and Dy purified by distillation and solid solutions of a-GdHx (x = 0.1, 0.15, 0.2) are single-phase and have a hexagonal close-packed crystal structure (space group P63/mcc) with structural characteristics close to those reported in literature [4,15,18]. The increase of lattice parameters of the materials was observed after hydrogenation Table 1 Lattice parameters and the unit cell volume of Gd and a-GdHx (x = 0.1, 0.15, 0.2). Sample
a, nm
c, nm
c/a
V, nm3
Gd a-GdH0.1 a-GdH0.15 a-GdH0.2
0.3633 0.3640 0.3643 0.3646
0.5770 0.5778 0.5780 0.5783
1.588 1.587 1.587 1.586
0.0660 0.0663 0.0664 0.0666
(see e.g. Table 1). The relative unit cell volume increase of Gd DV/V 1% is obtained in case of an alpha-hydride a-GdH0.2. Available literature data [13,19] shows that it is rather difficult to avoid formation of a b-phase RH2 when preparing solid solutions of hydrogen in rare earths. It is found that the samples TbHx and DyHx as well as GdHx with x > 0.2 at.H/at.R contain two phases. Apart from the main a-RHx phase, a cubic phase b-RH2 is formed. The amount of a dihydride b-RH2 phase in the samples corresponds to the data of the R-H diagram [13,20] and it increases upon the increase of hydrogen concentration. For instance, for the sample DyH1.2, the amount of b-DyH2 is 66% (detected by XRD on the samples’ surface). The Néel temperatures of the dihydride b-RH2 phases are 21, 17 and 3.5 K for Gd, Tb and Dy, respectively [13]. Therefore, the presence of the antiferromagnetic phase b-GdH2, b-TbH2 and b-DyH2 (which die out at rather low temperatures) does not influence the character of temperature dependence of magnetization M (T) and MCE at higher temperatures. Fig. 1(a and b) shows the M vs. H curves for Gd and a-GdH0.2 in fields up to 2 T near the Curie temperature (2 K step). The Curie temperatures of the parent and hydrided Gd were determined by means of the Arrott-Belov plot method (see Fig. 1(c and d)) based on thermodynamic theory [21]. The accuracy of the ArrottBelov’s method is the highest among other techniques for the TC determination. The second method used to evaluate the TC is by detecting the maximum of magnetocaloric effect (see Fig. 2). With the increase of hydrogen content in the gadolinium samples, we observed an increase of the Curie temperature. The latter indicates the enhancement of the Gd-Gd exchange interactions. Despite the fact that experimental evidence obtained previously on a Gd crystal under hydrostatic pressure gave an opposite result for TC [22,23], we were able to explain the increase of TC in our hydrogenated Gd from the first principles [18]. We showed that the interplay of characteristic features in the electronic structure of conduction band at the Fermi level in the high-temperature paramagnetic phase of Gd and ‘‘negative” pressure exerted by hydrogen are responsible for the observed effect.
Fig. 1. Field dependencies of magnetization in the vicinity of the Curie temperatures of Gd (a), a-GdH0.2 (b), Belov-Arrott plots for Gd (c) and a-GdH0.2 (d).
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Fig. 2. Temperature dependencies of the change of magnetic entropy for solid solutions a-GdHx in the field up to 2 T. Inset: The concentration dependence of DTad(x) in a-GdHx.
The MCE of high purity Gd and its hydrogenated samples was studied in this work and in Refs. [17,18] on the samples cut out of different parts of a distillate, and using direct and indirect measurement methods. It was found that MCE retains its magnitude in the single-phase solid solutions (see inset in Fig. 2). However, once the second phase appears (this is confirmed by the XRD data), the value of MCE slightly decreases. For practical application of MCE in magnetic refrigeration, it is important to perform measurements under conditions as close to those of a real refrigerator. It was of interest to further investigate the influence of hydrogen on MCE in the vicinity of complex cascade transitions PM – AFM – FM. Such transitions are observed not only in pure rareearths [4,15], but in their alloys and compounds [24–30]. The DTad(T) curves obtained by direct method upon the variation of magnetic field from 0 to 1.8 T for hydrides TbH0.3 and TbH0.9 are shown in Fig. 3(a and b). We find that in low magnetic fields m0H < 0.03 T, the MCE of the studied samples TbHx is negative at 225–227 K (not shown here). The negative MCE is characteristic of antiferromagnets [4,15] for which the magnetic field destroys (first partly and then completely at higher fields) the antiferromagnetic structure. Therefore, at fields m0H > 0.03 T the maximum in the temperature dependence of MCE at around 231 K is already due to the magnetic phase transition from the ferromagnetic to the paramagnetic phase. The position of the maximum remains practically unaffected by hydrogen absorption. Fig. 3 shows that the increase of hydrogen concentration in the samples TbHx leads to the decrease of DTad. This fact is also confirmed by indirect calculations of magnetic entropy using the magnetization data. XRD data for these samples shows the presence of the second phase TbH2. The amount of the second phase in the samples increases linearly with the increase of absorbed hydrogen. The linear trend is also found in the decrease of MCE at high hydrogen concentrations. Unlike TbHx, DyHx demonstrates a noticeable magnetocaloric effect in weak and strong magnetic fields and not only in the vicinity of the disorder–order transition (PM AFM) at the Néel temperature TN but also at the order-order transition (AFM FM) at the temperature h [4,15,31]. Similarly to TbHx, we studied magnetocaloric effect in DyHx by direct method. Fig. 4(a and b) shows the temperature dependencies of DTad(T) for selected DyHx samples with x = 0, 0.1, 0.2, 0.5, 1.2. All curves demonstrate two peaks: a wide one (m0H > 0.5 T) at and above the temperature h and a
Fig. 3. Temperature dependencies of MCE of TbH0.3 (a), TbH0.9 (b) in various magnetic fields.
rather sharp peak at TN. The behavior of MCE vs. T at 100–150 K (see Fig. 4(a)) is typical of materials with a phase transition of the first order [4] when the application of magnetic field enlarges the temperature region where the maximum MCE is observed. In addition, it is clearly seen that MCE changes the sign to negative and back around the AFM FM transition. As it was mentioned above, negative MCE is characteristic of antiferromagnets. Higher applied magnetic field results in the shrinking of the region of negative MCE – the effect connected with the transformation of an antiferromagnetic structure under the magnetic field. However, as shown in the course of this experiment, the field of m0DH = 1. 8 T is not strong enough to completely remove negative MCE in the parent and hydrogenated samples. Direct measurements of MCE of Dy and its hydrogenated samples DyHx in fields up to 1.8 T showed that MCE decreases at the TN from DTad = 1.5 K to DTad = 0.9 K when x varies between 0 and 1.2 at.H/at.Dy. The reason for that is probably the same as in the case of Gd and Tb samples, namely, the increase of the amount of second phase in the samples. At the range of concentrations 0 < x 0.2 only traces of the dihydride phase are found in DyHx (according to XRD) and therefore, MCE remains of almost the same magnitude as in the hydrogen-free samples (inset in Fig. 4(b)). Hydrogenation does not increase the TN values and therefore does not change the Dy-Dy interactions. (Note that TN decreases under the hydrostatic pressure [23].) We earlier observed the decrease of MCE in the hydrogendoped single-crystalline samples R2Fe14B [14]. It was connected
Please cite this article in press as: V.B. Chzhan et al., Magnetocaloric properties of hydrogenated Gd, Tb and Dy, Journal of Magnetism and Magnetic Materials (2017), https://doi.org/10.1016/j.jmmm.2017.11.128
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MCE does not decrease with increasing x in single-phase samples while the temperature, at which the maximum of MCE it is observed is growing continuously. Dy demonstrates a considerable magnetocaloric effect in a wide temperature range that includes the transitions of the order-order and order–disorder types. In DyHx the magnitude of MCE is preserved at x 0.2 despite the presence of the second, dihydride phase. Hydrogenation does not change the TN values of both Dy and Tb. The increase of the amount of the second phase b-RH2 leads to a considerable decrease of MCE in Gd, Tb and Dy. Acknowledgements The work of I.S.T. is supported by the RFBR according to the research project № 16-03-00612. The work of E.A.T.Ch. was supported by Czech Science Foundation (Grant P16-03593S). The work of V.B.Ch. was carried out with financial support of the Ministry of Education and Science of Russian Federation in the framework of an increased Competitiveness Program of NUST ‘‘MISIS”, implemented by a governmental decree dated 16th of March 2013, № 211. References
Fig. 4. Temperature dependencies of MCE for DyH0.1 in various magnetic fields (a) and for Dy, DyH0.2, DyH0.5 and DyH1.2 in the fields up to 1.8 T (b). Inset: The concentration dependence of DTad(x) near TN in DyHx.
with strong influence of hydrogen on magnetocrystalline anisotropy. It is not unreasonable to assume that similar effect can be expected in the rare earth metals however the formation of the second phase is likely to conceal the effects related to the influence of hydrogenation on MCE of Gd, Tb and Dy. The structural state of samples (mono-, micro-, nanocrystalline) and relaxation processes in them also should be taken into account when studying magnetocaloric properties of the rare earth metals [18,32–36]. Investigation of the influence of hydrogen on the MCE of single-crystalline high purity rare earth samples is currently in progress.
4. Conclusions In this work, we studied the influence of hydrogenation on the magnitude of MCE and temperatures of magnetic phase transitions for the three high purity rare earth metals with the highest ordering temperatures, Gd, Tb and Dy. XRD studies of samples purified by distillation and then doped by hydrogen showed that hydrogen insertion not only changes the lattice parameters of the parent materials but leads to the appearance of the second crystallographic phase of dihydride b-RH2. The formation of the latter is avoided only in the samples of a-GdHx with x = 0.1, 0.15, 0.2. We observed the following important trends. For Gd the value of
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