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Large rotating magnetocaloric effect in the orthorhombic DyMnO3 single crystal
Author’s Accepted Manuscript Large rotating magnetocaloric effect in the orthorhombic DyMnO3 single crystal M. Balli, S. Mansouri, S. Jandl, P. Fournier, D.Z. Dimitrov www.elsevier.com/locate/ssc
PII: DOI: Reference:
S0038-1098(16)30025-4 http://dx.doi.org/10.1016/j.ssc.2016.04.002 SSC12909
To appear in: Solid State Communications Received date: 23 March 2016 Accepted date: 4 April 2016 Cite this article as: M. Balli, S. Mansouri, S. Jandl, P. Fournier and D.Z. Dimitrov, Large rotating magnetocaloric effect in the orthorhombic DyMnO single crystal, Solid State Communications, http://dx.doi.org/10.1016/j.ssc.2016.04.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Large rotating magnetocaloric effect in the orthorhombic DyMnO3 single crystal M. Balli1*, S. Mansouri1, S. Jandl1, P. Fournier1, 2, D. Z. Dimitrov3, 4 1
Regroupement québécois sur les matériaux de pointe, Département de physique, Université de
Sherbrooke, J1K 2R1, QC, Canada. 2
Canadian Institute for Advanced Research, Toronto, Ontario M5G 1Z8, Canada.
3
Institute of Solid State Physics, Bulgarian Academy of Science, Sofia 1184, Bulgaria.
4
Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria.
Abstract. A large magnetocaloric effect can be obtained around TN,
Dy
8 K simply by spinning the
orthorhombic DyMnO3 single crystal within the cb-plane in a constant magnetic field, instead of the standard magnetization-demagnetization process. Under 7 T, the maximum rotating entropy change (ΔSR, cb) and the associated adiabatic temperature change (ΔTad, cb) are 16.3 J/kg K and 11 K, respectively. The corresponding refrigerant capacity is 440 J/kg, with no thermal and field hysteresis losses. Our findings show that the orthorhombic DyMnO3 could be used as a good refrigerant in more compact and efficient cryomagnetocaloric refrigerators.
Keywords: Manganites; Single Crystals; Magnetocaloric effect, Anisotropy. *E-mail: [email protected]
I. INTRODUCTION During the last years, a great attention has been paid to the study of magnetocaloric materials due to their potential use as refrigerants in efficient and eco-friendly magnetic refrigerators [1-16]. In this context, the RMnO3 (R = rare earth) manganites emerge as promising materials for application in the temperature range below the liquid hydrogen temperature [16-21]. Additionally, the strong interplay between their magnetic and electric order parameters provides new routes for the development of spintronic devices [22, 23]. The RMnO3 magnetic and crystallographic properties are very sensitive to the atomic radius of the rare-earth R which results in fascinating physical properties [24-35]. The orthorhombic form of DyMnO3 (o-DyMnO3) reveals different magnetic transitions. Usually, an
1
incommensurate antiferromagnetic ordering of the Mn3+ moments oriented along the b axis takes place at around TN1 = 40 K [29, 30]. Bellow the locking temperature, T L = 18 K, the Mn3+ magnetic moments manifest an additional component along the c-axis which results in a spiral (cycloidal) magnetic structure. Below 15 K, the Dy3+ magnetic moments order also along the b-axis in a sinusoidal incommensurate state [27]. This structure transforms into a commensurate one with the propagation vector k = (0, ½, 0) below TN2, Dy = 6.5 K [27]. In this paper, we mainly investigate the rotating magnetocaloric effect (RMCE) [16] induced by the large magnetic anisotropy in orthorhombic DyMnO3 single crystals. Since o-DyMnO3 is an oxide with a high resistance against oxidation or corrosion phenomena and that it is an electrical insulator (multiferroic), it is an appealing candidate for magnetic refrigeration applications as these particular properties prevent gradual degradation of the magnetic materials and the energy losses caused by the eddy currents during the magnetization-demagnetization process.
II. EXPERIMENTAL The DyMnO3 orthorhombic perovskite single crystals were grown in a platinum crucible by the high temperature solution technique [33, 34]. The starting polycrystalline o-DyMnO3 prepared using standard solid state reaction method was mixed with Pb 3O4/PbF2/B2O3 (0.84/0.14/0.01) flux and subjected to a heat treatment for 48 h at 1280 °C. The synthesis process was achieved by decreasing the temperature from 1280 °C to 930 °C at a rate of 0.5 °C/h. The crystallographic symmetry of the crystal and its quality were checked with the help of Raman scattering. Micro-Raman spectra were collected using a Labram800 equipped with a microscope, He-Ne laser (6328 Å) and a nitrogen-cooled charge coupled device detector (CCD). The magnetization measurements were done using a superconducting quantum interference device (SQUID) magnetometer from Quantum Design (MPMS XL).
III. RESULTS AND DISCUSSION The study of Raman spectra at 5 K shown in Fig. 1 reveals that o-DyMnO3 single crystals exhibit a very high crystalline quality. All the observed excitations and the related impact from selection rules can be assigned to the expected signatures of DyMnO3 with an orthorhombic symmetry (Pbnm space group). Raman scattering can thus be used here to confirm the crystal symmetry, but most importantly the
2
orientation of the crystal axis with respect to the crystal edges as it is a key component of our interpretation of the magnetic properties and their comparison with previous reports. In Fig. 2-a, we present the 0.1 T-zero-field cooled (ZFC) and field-cooled (FC) thermomagnetic curves along the easy direction (b-axis), the intermediate direction (a-axis) and the hard direction (c-axis). The observed inflexion point around 8 K, corresponds to the ordering temperature of the Dy3+ magnetic moments which experience a sinusoidal incommensurate to commensurate phase transition [27]. The marked difference between magnetic isotherms (Fig.2-b) along the crystallographic directions reveals a gigantic magneto-crystalline anisotropy in o-DyMnO3, which is a common property of the RMnO3 orthorhombic manganites [21, 22, 24, 29, 33]. The 0.1 T-ZFC inverse susceptibility (1/χ) is reported in the inset of Fig. 2-a. From the linear fit of 1/χ at high temperatures, the paramagnetic Curie-Weiss temperatures Tθ are evaluated to be about 5.4 K, -12.2 K and -141 K for the b, a and c axes, respectively. These negative values of Tθ along the a and c axes demonstrate the presence of antiferromagnetic (AF) interactions in the orthorhombic DyMnO 3 single crystal. The low positive value of Tθ along the easy direction indicates the existence of a weak ferromagnetic (FM) character. The deduced effective magnetic moments are found to be 11.85 µB, 11.48 µB and 12.7 µB for the a, b and c axes, respectively, which are consistent
with
the
theoretical
eff ( eff ( Dy 3 )) 2 ( eff (Mn3 )) 2 11.72 B .
value
estimated
using
These results are in good agreement with
early works on o-DyMnO3 single crystals [21, 33]. However, the incommensurate transition of the Mn spins which takes place around 40 K and the lock-in transition occurring in the vicinity of 18 K are not visible in thermomagnetic curves, whereas both features can be clearly recognized from specific heat data reported in Ref. 35. On the other hand, the feature around 40 K is clearly visible in the temperature dependence of the Raman excitation energies corresponding to Ag (4) [out of phase MnO6 x rotations] phonons [36] shown in the inset of Fig.1. In this paper, our main objective is to investigate the magnetocaloric properties and we will particularly focus on the rotating MCE of o-DyMnO3 single crystals. In the absence of hysteresis phenomena, the entropy change ΔS which can be accurately deduced from magnetization measurements as in Figures 3-a and b using the Maxwell relation, is the widely used parameter for a fast characterization of magnetocaloric materials [1, 37]. As shown in Fig.3-a, the magnetization along the b-axis at low temperatures evolves almost linearly at low magnetic fields but is followed by a rapid enhancement to
3
reach the saturation state when the magnetic field reaches a critical value of roughly 1.4 T. The observed metamagnetic transition is first order in nature because the Arrott plots (H/M vs M2) [38] show negative slopes (not shown here). In addition, the magnetization behaviour is typical of materials showing fieldinduced AF-F transitions [39, 40]. This result markedly contrasts with the data of Harikrishnan et al [24]. In Ref. 24, the magnetization varies linearly with applied field along the b-axis even at 2 K evidencing the absence of any metamagnetic transition region. This discrepancy is more probably attributed to the fact that the isotherms reported in Ref. 24 for o-DyMnO3 are rather performed along the hard axis c and not following the easy axis b. In fact, following the hard axis c, the magnetization shows small values and remains linear in the whole temperature range as is shown in Fig.3-b. The temperature dependence of the isothermal entropy change for o-DyMnO3 under magnetic field variations up to 7 T along the b and c orientations is reported in Figs.3-c and d, respectively. As can be seen, -ΔS(T) exhibits a considerable anisotropy. For magnetic field variation from 0 to 7 T, the entropy change along the easy axis is about 22 times larger than along the hard axis. For T ≤ 6 K, o-DyMnO3 presents a negative MCE (along the axis b) which manifests as positive values of ΔS, revealing that the material cools down when subjected to an external magnetic field. This is because the applied field induces a change in the magnetic state from an ordered “antiferromagnetic” phase to a less-ordered “ferromagnetic” configuration, increasing consequently the material magnetic entropy. Along the easy axis, o-DyMnO3 reveals a large MCE distributed on a wide temperature range which is highly appreciated for applications. For field changes of 0-3, 0-5 and 0-7 T along the easy axis, the maximum values of –ΔS are 8.7, 14.6 and 17.25 J/kg K while –ΔS following the hard axis remains very low even under intense magnetic fields (about 0.8 J/kg K for a 0-7 T variation). The reported values of –ΔS along the easy axis are comparable with those recently reported in some RMnO3 manganites with similar working temperature range such as hexagonal DyMnO3 (19 J/kg K for 7 T) [18], TbMnO3 (18 J/kg K for 7 T) [17] and HoMnO3 (~ 18 J/kg K for 7 T) [21], while they exceed largely those exhibited by TmMnO 3 (9 J/kg K) [19]. However, if the reported entropy changes by Midya et al [21] in o-DyMnO3 along the easy direction perfectly agree with our results, their value of –ΔS along the hard direction (8 J/kg K for 7 T) is almost 10 times larger than our findings for –ΔS along the same direction (0.8 J/kg K for 7 T). This difference can be explained by the fact that the magnetic and magnetocaloric properties related to the hard direction as assumed by the authors in Ref.21 are rather corresponding to the intermediate a-axis.
4
As evidenced in Fig.3, the MCE in o-DyMnO3 single crystals represented by the entropy change shows a large anisotropy. Consequently, a large MCE could be obtained simply by rotating the crystal around the intermediate axis (a) in a constant magnetic field. This rotation scheme would represent a great gain for magnetic refrigeration compared to the standard magnetization-demagnetization process relying on the moving of a magnetocaloric material inside and outside of the magnetic field zone. In fact, the implementation of such effect could open the door for the design of a new category of magnetic cooling systems with several advantages such as the possibility to build efficient and compact systems working at high frequency with simplified design [16, 17, 19, 41]. The RMCE exhibited by o-DyMnO3 was firstly determined in terms of the entropy change. Considering the magnetic field initially parallel to the hard axis c, the resulting entropy change from a 90° rotation of the crystal around the a-axis with an applied magnetic field within the cb plane can be expressed by ΔSR,
cb
= ΔS(H//b)- ΔS(H//c). The
temperature dependence of -ΔSR,cb for different constant magnetic fields is plotted in Fig. 4-a. As shown, the o-DyMnO3 compound manifests a giant rotating magnetocaloric effect (GRMCE) thanks to its large magnetization and giant magnetic anisotropy. The associated maximum -ΔSR,cb reaches 8.4, 14.2 and 16.3 J/kg K in the field of 3, 5 and 7 T, respectively. Considering the restricted family of materials with RMCEs in a similar temperature range, the RMCE of o-DyMnO3 is much larger than that found in HoMn2O5 (12.3 J/kg K for 7 T)[16] and about two times larger than that exhibited by TbMnO3 (8.2 J/kg K for 7 T)[17] and TmMnO3 (8.73 J/kg K for 7 T)[19]. For 5 T, the rotating entropy change presented by o-DyMnO3 is about 3 times larger than that presented by TmMnO3 [19]. Also, it is worth noting that the o-DyMnO3 crystal reveals a meaningful RMCE under relatively low magnetic fields (≤3T). Under 3 T, ΔSR is only about 2 J/kg K for TmMnO3 [19], 3 J/kg K for TbMnO3 [17] and 5.6 J/kg K for HoMn2O5 [16] while it is 8.4 J/kg K for o-DyMnO3. This is highly appreciated from economical and energetic point of views, since the cooling process could be realized using permanent magnets [12] as a source of the magnetic field instead of superconducting magnets. From a practical point of view, it is recommended to search for materials that not only show high MCE levels but also have a large working temperature range. Both factors are taken into account in the refrigerant capacity (RC) parameter [42]. In Fig.4-b, we report the magnetic field dependence of the RC associated with the RMCE in the o-DyMnO3 single crystal (RCR,cb). Under a constant magnetic field of 7 T, the corresponding RCR value is about 440 J/kg which
5
should be compared to 300 J/kg for TbMnO3 [17], 205 J/kg for TmMnO3 [19] and 290 J/kg for HoMn2O5 [16]. In order to complete this study, the adiabatic temperature change ΔTad,cb associated with the rotation
motion
Tad ,cb
in
the
bc-plane
T S R ,cb CP ( H 0)
was
also
investigated.
ΔTad,cb
can
be
approached
by:
where Cp is the specific heat. For this purpose, the needed specific
heat data were taken from Ref. 35. Our estimation of ΔTad,cb is displayed in Fig.5-a as a function of temperature in several constant magnetic fields. ΔTad,cb reaches maximum values of about 5, 9 and 11 K, for 3, 5 and 7 T, respectively. This means that the exhibited rotating adiabatic temperature change is about 2 times larger than that found in TbMnO3[17] and HoMn2O5 [16], but remains lower when compared with that shown by TbMn2O5 [41] (14 K for 5 T). ΔTad,cb can be also determined from the temperature
dependence
of
the
full
entropy
curves
through
the
equation
Tad, cb (T, H) = [T(S)H//b - T (S)H//c ]S as shown in the inset of Fig. 5-a. The full entropy for a given field can be expressed by S (H, T) = S (0, T) + ΔS (H, T), with S (0, T) is the full entropy at 0 T that can
CP (0, T ' ) ' dT . Using this approach, the ΔTad, cb presents a maximum of T' 0
T
be calculated by S (0, T )
about 11 K under 7 T, which confirms the rough estimate above. Considering now the magnetic field initially parallel to the easy axis b, a negative MCE can be generated by rotating the o-DyMnO3 crystal around the intermediate axis a by 90 °. Thanks to the reversibility of the MCE, the associated ΔTad,bc curves could be built (not shown here) on the basis of the reported data in Fig.5-a. Furthermore, at about 26 K, the rotation motion from the b-axis to the c-axis (90 °) under 7 T reduces the crystal temperature to about 15 K. This process could be implemented in applications for the hydrogen liquefaction (for example). Aiming to determine the origin of the rotating magnetocaloric effect revealed by o-DyMnO3, preliminary calculations of the RMCE (-ΔSR) based on the coherent rotational model were carried out [17, 19, 43]. Considering initially the magnetic field parallel to the hard axis c (β = 90 °), the calculated ΔSR as a function of the angle β under 7 T is reported in Fig.5-b for T = 8 K. β is the angle between the magnetic field and the easy axis b. As shown, -ΔSR increases when β decreases to reach a maximum value of 14.54 J/kg K for β = 0, which is similar to the experimental value (ΔSR,
6
cb
= 14.77 J/kg K). This
preliminary result demonstrates the important role of the magnetocrystalline anisotropy in boosting the rotating magnetocaloric effect in o-DyMnO3 single crystal around 8 K. However, in RMnO3 materials, the magnetocrystalline anisotropy is usually more complex than assumed in our calculation. For this reason, additional measurements such as magnetization as a function of β must be performed. This will allow us to accurately determine the entropy change contributed from both the magnetocrystalline anisotropy as well as from the spin fluctuations [17, 19]. On the other hand, spin flop processes also should be taken into account [44].
IV. CONCLUSIONS In summary, a gigantic anisotropy of the magnetocaloric effect has been observed in the orthorhombic DyMnO3 around the ordering temperature of the Dy3+ magnetic moments. Along the easy axis b, the entropy change under a field variation of 5 T is about 34 times higher than that obtained along the hard axis c, which results in a large rotating magnetocaloric effect. Consequently, more compact, efficient and simplified magnetic liquefiers could be built by rotating continuously o-DyMnO3crystals in a constant magnetic field instead of the standard magnetization-demagnetization method. The presence of both reversible conventional and rotating MCEs combined with additional advantages such as the chemical stability and the electrical insulation indicate that the present crystal is an excellent candidate for low temperature magnetocaloric refrigeration.
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Acknowledgments
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The authors thank M. Castonguay and S. Pelletier for technical support. We acknowledge the financial support from NSERC (Canada), FQRNT (Québec), CFI , CIFAR and the Université de Sherbrooke.
Figure captions Figure 1: (a) Micro-Raman spectra at 5 K for the orthorhombic single crystal DyMnO3. Inset: Temperature dependence of the Raman-active phonon corresponding to 392 cm-1 [Ag (4)]. Figure 2: (a) ZFC and FC thermomagnetic curves under a field of 0.1 T along the b, a and c axes for oDyMnO3. Inset: 0.1 T- ZFC inverse magnetic susceptibility along the b, a and c axes. (b) Magnetic isotherms measured at 2 K for H//b, H//a and H//c. Figure 3: Isothermal magnetization curves of o-DyMnO3 for H//b (a) and H//c (b) in the temperature range 3-70 K with different steps. The increments of temperature are 1 K for 3-10 K, 2 K for 10-46 K and 4 K for 46-70 K. Isothermal entropy change of o-DyMnO3 as a function of temperature under different magnetic field variations for H//b (c) and H//c (d). Figure 4: (a) Entropy changes related to the rotation of the o-DyMnO3 single crystal around the intermediate axis a (within the cb-plane) by 90 ° in different constant magnetic fields. (b) Associated refrigerant capacity. Figure 5: (a) Adiabatic temperature change related to the rotation of the o-DyMnO3 single crystal around the intermediate axis a by 90 ° in different constant magnetic fields. Inset: Temperature dependence of the full entropy in a magnetic field of 7 T applied along the easy axis b and the hard axis c. (b) Rotating entropy change calculated from the coherent rotational model as a function of the angle between the easy axis and the magnetic field (β).
10
Figure 1
11
Figure 2
12
Figure 3
13
Figure 4
14
Figure 5
15
PII: DOI: Reference:
S0038-1098(16)30025-4 http://dx.doi.org/10.1016/j.ssc.2016.04.002 SSC12909
To appear in: Solid State Communications Received date: 23 March 2016 Accepted date: 4 April 2016 Cite this article as: M. Balli, S. Mansouri, S. Jandl, P. Fournier and D.Z. Dimitrov, Large rotating magnetocaloric effect in the orthorhombic DyMnO single crystal, Solid State Communications, http://dx.doi.org/10.1016/j.ssc.2016.04.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Large rotating magnetocaloric effect in the orthorhombic DyMnO3 single crystal M. Balli1*, S. Mansouri1, S. Jandl1, P. Fournier1, 2, D. Z. Dimitrov3, 4 1
Regroupement québécois sur les matériaux de pointe, Département de physique, Université de
Sherbrooke, J1K 2R1, QC, Canada. 2
Canadian Institute for Advanced Research, Toronto, Ontario M5G 1Z8, Canada.
3
Institute of Solid State Physics, Bulgarian Academy of Science, Sofia 1184, Bulgaria.
4
Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria.
Abstract. A large magnetocaloric effect can be obtained around TN,
Dy
8 K simply by spinning the
orthorhombic DyMnO3 single crystal within the cb-plane in a constant magnetic field, instead of the standard magnetization-demagnetization process. Under 7 T, the maximum rotating entropy change (ΔSR, cb) and the associated adiabatic temperature change (ΔTad, cb) are 16.3 J/kg K and 11 K, respectively. The corresponding refrigerant capacity is 440 J/kg, with no thermal and field hysteresis losses. Our findings show that the orthorhombic DyMnO3 could be used as a good refrigerant in more compact and efficient cryomagnetocaloric refrigerators.
Keywords: Manganites; Single Crystals; Magnetocaloric effect, Anisotropy. *E-mail: [email protected]
I. INTRODUCTION During the last years, a great attention has been paid to the study of magnetocaloric materials due to their potential use as refrigerants in efficient and eco-friendly magnetic refrigerators [1-16]. In this context, the RMnO3 (R = rare earth) manganites emerge as promising materials for application in the temperature range below the liquid hydrogen temperature [16-21]. Additionally, the strong interplay between their magnetic and electric order parameters provides new routes for the development of spintronic devices [22, 23]. The RMnO3 magnetic and crystallographic properties are very sensitive to the atomic radius of the rare-earth R which results in fascinating physical properties [24-35]. The orthorhombic form of DyMnO3 (o-DyMnO3) reveals different magnetic transitions. Usually, an
1
incommensurate antiferromagnetic ordering of the Mn3+ moments oriented along the b axis takes place at around TN1 = 40 K [29, 30]. Bellow the locking temperature, T L = 18 K, the Mn3+ magnetic moments manifest an additional component along the c-axis which results in a spiral (cycloidal) magnetic structure. Below 15 K, the Dy3+ magnetic moments order also along the b-axis in a sinusoidal incommensurate state [27]. This structure transforms into a commensurate one with the propagation vector k = (0, ½, 0) below TN2, Dy = 6.5 K [27]. In this paper, we mainly investigate the rotating magnetocaloric effect (RMCE) [16] induced by the large magnetic anisotropy in orthorhombic DyMnO3 single crystals. Since o-DyMnO3 is an oxide with a high resistance against oxidation or corrosion phenomena and that it is an electrical insulator (multiferroic), it is an appealing candidate for magnetic refrigeration applications as these particular properties prevent gradual degradation of the magnetic materials and the energy losses caused by the eddy currents during the magnetization-demagnetization process.
II. EXPERIMENTAL The DyMnO3 orthorhombic perovskite single crystals were grown in a platinum crucible by the high temperature solution technique [33, 34]. The starting polycrystalline o-DyMnO3 prepared using standard solid state reaction method was mixed with Pb 3O4/PbF2/B2O3 (0.84/0.14/0.01) flux and subjected to a heat treatment for 48 h at 1280 °C. The synthesis process was achieved by decreasing the temperature from 1280 °C to 930 °C at a rate of 0.5 °C/h. The crystallographic symmetry of the crystal and its quality were checked with the help of Raman scattering. Micro-Raman spectra were collected using a Labram800 equipped with a microscope, He-Ne laser (6328 Å) and a nitrogen-cooled charge coupled device detector (CCD). The magnetization measurements were done using a superconducting quantum interference device (SQUID) magnetometer from Quantum Design (MPMS XL).
III. RESULTS AND DISCUSSION The study of Raman spectra at 5 K shown in Fig. 1 reveals that o-DyMnO3 single crystals exhibit a very high crystalline quality. All the observed excitations and the related impact from selection rules can be assigned to the expected signatures of DyMnO3 with an orthorhombic symmetry (Pbnm space group). Raman scattering can thus be used here to confirm the crystal symmetry, but most importantly the
2
orientation of the crystal axis with respect to the crystal edges as it is a key component of our interpretation of the magnetic properties and their comparison with previous reports. In Fig. 2-a, we present the 0.1 T-zero-field cooled (ZFC) and field-cooled (FC) thermomagnetic curves along the easy direction (b-axis), the intermediate direction (a-axis) and the hard direction (c-axis). The observed inflexion point around 8 K, corresponds to the ordering temperature of the Dy3+ magnetic moments which experience a sinusoidal incommensurate to commensurate phase transition [27]. The marked difference between magnetic isotherms (Fig.2-b) along the crystallographic directions reveals a gigantic magneto-crystalline anisotropy in o-DyMnO3, which is a common property of the RMnO3 orthorhombic manganites [21, 22, 24, 29, 33]. The 0.1 T-ZFC inverse susceptibility (1/χ) is reported in the inset of Fig. 2-a. From the linear fit of 1/χ at high temperatures, the paramagnetic Curie-Weiss temperatures Tθ are evaluated to be about 5.4 K, -12.2 K and -141 K for the b, a and c axes, respectively. These negative values of Tθ along the a and c axes demonstrate the presence of antiferromagnetic (AF) interactions in the orthorhombic DyMnO 3 single crystal. The low positive value of Tθ along the easy direction indicates the existence of a weak ferromagnetic (FM) character. The deduced effective magnetic moments are found to be 11.85 µB, 11.48 µB and 12.7 µB for the a, b and c axes, respectively, which are consistent
with
the
theoretical
eff ( eff ( Dy 3 )) 2 ( eff (Mn3 )) 2 11.72 B .
value
estimated
using
These results are in good agreement with
early works on o-DyMnO3 single crystals [21, 33]. However, the incommensurate transition of the Mn spins which takes place around 40 K and the lock-in transition occurring in the vicinity of 18 K are not visible in thermomagnetic curves, whereas both features can be clearly recognized from specific heat data reported in Ref. 35. On the other hand, the feature around 40 K is clearly visible in the temperature dependence of the Raman excitation energies corresponding to Ag (4) [out of phase MnO6 x rotations] phonons [36] shown in the inset of Fig.1. In this paper, our main objective is to investigate the magnetocaloric properties and we will particularly focus on the rotating MCE of o-DyMnO3 single crystals. In the absence of hysteresis phenomena, the entropy change ΔS which can be accurately deduced from magnetization measurements as in Figures 3-a and b using the Maxwell relation, is the widely used parameter for a fast characterization of magnetocaloric materials [1, 37]. As shown in Fig.3-a, the magnetization along the b-axis at low temperatures evolves almost linearly at low magnetic fields but is followed by a rapid enhancement to
3
reach the saturation state when the magnetic field reaches a critical value of roughly 1.4 T. The observed metamagnetic transition is first order in nature because the Arrott plots (H/M vs M2) [38] show negative slopes (not shown here). In addition, the magnetization behaviour is typical of materials showing fieldinduced AF-F transitions [39, 40]. This result markedly contrasts with the data of Harikrishnan et al [24]. In Ref. 24, the magnetization varies linearly with applied field along the b-axis even at 2 K evidencing the absence of any metamagnetic transition region. This discrepancy is more probably attributed to the fact that the isotherms reported in Ref. 24 for o-DyMnO3 are rather performed along the hard axis c and not following the easy axis b. In fact, following the hard axis c, the magnetization shows small values and remains linear in the whole temperature range as is shown in Fig.3-b. The temperature dependence of the isothermal entropy change for o-DyMnO3 under magnetic field variations up to 7 T along the b and c orientations is reported in Figs.3-c and d, respectively. As can be seen, -ΔS(T) exhibits a considerable anisotropy. For magnetic field variation from 0 to 7 T, the entropy change along the easy axis is about 22 times larger than along the hard axis. For T ≤ 6 K, o-DyMnO3 presents a negative MCE (along the axis b) which manifests as positive values of ΔS, revealing that the material cools down when subjected to an external magnetic field. This is because the applied field induces a change in the magnetic state from an ordered “antiferromagnetic” phase to a less-ordered “ferromagnetic” configuration, increasing consequently the material magnetic entropy. Along the easy axis, o-DyMnO3 reveals a large MCE distributed on a wide temperature range which is highly appreciated for applications. For field changes of 0-3, 0-5 and 0-7 T along the easy axis, the maximum values of –ΔS are 8.7, 14.6 and 17.25 J/kg K while –ΔS following the hard axis remains very low even under intense magnetic fields (about 0.8 J/kg K for a 0-7 T variation). The reported values of –ΔS along the easy axis are comparable with those recently reported in some RMnO3 manganites with similar working temperature range such as hexagonal DyMnO3 (19 J/kg K for 7 T) [18], TbMnO3 (18 J/kg K for 7 T) [17] and HoMnO3 (~ 18 J/kg K for 7 T) [21], while they exceed largely those exhibited by TmMnO 3 (9 J/kg K) [19]. However, if the reported entropy changes by Midya et al [21] in o-DyMnO3 along the easy direction perfectly agree with our results, their value of –ΔS along the hard direction (8 J/kg K for 7 T) is almost 10 times larger than our findings for –ΔS along the same direction (0.8 J/kg K for 7 T). This difference can be explained by the fact that the magnetic and magnetocaloric properties related to the hard direction as assumed by the authors in Ref.21 are rather corresponding to the intermediate a-axis.
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As evidenced in Fig.3, the MCE in o-DyMnO3 single crystals represented by the entropy change shows a large anisotropy. Consequently, a large MCE could be obtained simply by rotating the crystal around the intermediate axis (a) in a constant magnetic field. This rotation scheme would represent a great gain for magnetic refrigeration compared to the standard magnetization-demagnetization process relying on the moving of a magnetocaloric material inside and outside of the magnetic field zone. In fact, the implementation of such effect could open the door for the design of a new category of magnetic cooling systems with several advantages such as the possibility to build efficient and compact systems working at high frequency with simplified design [16, 17, 19, 41]. The RMCE exhibited by o-DyMnO3 was firstly determined in terms of the entropy change. Considering the magnetic field initially parallel to the hard axis c, the resulting entropy change from a 90° rotation of the crystal around the a-axis with an applied magnetic field within the cb plane can be expressed by ΔSR,
cb
= ΔS(H//b)- ΔS(H//c). The
temperature dependence of -ΔSR,cb for different constant magnetic fields is plotted in Fig. 4-a. As shown, the o-DyMnO3 compound manifests a giant rotating magnetocaloric effect (GRMCE) thanks to its large magnetization and giant magnetic anisotropy. The associated maximum -ΔSR,cb reaches 8.4, 14.2 and 16.3 J/kg K in the field of 3, 5 and 7 T, respectively. Considering the restricted family of materials with RMCEs in a similar temperature range, the RMCE of o-DyMnO3 is much larger than that found in HoMn2O5 (12.3 J/kg K for 7 T)[16] and about two times larger than that exhibited by TbMnO3 (8.2 J/kg K for 7 T)[17] and TmMnO3 (8.73 J/kg K for 7 T)[19]. For 5 T, the rotating entropy change presented by o-DyMnO3 is about 3 times larger than that presented by TmMnO3 [19]. Also, it is worth noting that the o-DyMnO3 crystal reveals a meaningful RMCE under relatively low magnetic fields (≤3T). Under 3 T, ΔSR is only about 2 J/kg K for TmMnO3 [19], 3 J/kg K for TbMnO3 [17] and 5.6 J/kg K for HoMn2O5 [16] while it is 8.4 J/kg K for o-DyMnO3. This is highly appreciated from economical and energetic point of views, since the cooling process could be realized using permanent magnets [12] as a source of the magnetic field instead of superconducting magnets. From a practical point of view, it is recommended to search for materials that not only show high MCE levels but also have a large working temperature range. Both factors are taken into account in the refrigerant capacity (RC) parameter [42]. In Fig.4-b, we report the magnetic field dependence of the RC associated with the RMCE in the o-DyMnO3 single crystal (RCR,cb). Under a constant magnetic field of 7 T, the corresponding RCR value is about 440 J/kg which
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should be compared to 300 J/kg for TbMnO3 [17], 205 J/kg for TmMnO3 [19] and 290 J/kg for HoMn2O5 [16]. In order to complete this study, the adiabatic temperature change ΔTad,cb associated with the rotation
motion
Tad ,cb
in
the
bc-plane
T S R ,cb CP ( H 0)
was
also
investigated.
ΔTad,cb
can
be
approached
by:
where Cp is the specific heat. For this purpose, the needed specific
heat data were taken from Ref. 35. Our estimation of ΔTad,cb is displayed in Fig.5-a as a function of temperature in several constant magnetic fields. ΔTad,cb reaches maximum values of about 5, 9 and 11 K, for 3, 5 and 7 T, respectively. This means that the exhibited rotating adiabatic temperature change is about 2 times larger than that found in TbMnO3[17] and HoMn2O5 [16], but remains lower when compared with that shown by TbMn2O5 [41] (14 K for 5 T). ΔTad,cb can be also determined from the temperature
dependence
of
the
full
entropy
curves
through
the
equation
Tad, cb (T, H) = [T(S)H//b - T (S)H//c ]S as shown in the inset of Fig. 5-a. The full entropy for a given field can be expressed by S (H, T) = S (0, T) + ΔS (H, T), with S (0, T) is the full entropy at 0 T that can
CP (0, T ' ) ' dT . Using this approach, the ΔTad, cb presents a maximum of T' 0
T
be calculated by S (0, T )
about 11 K under 7 T, which confirms the rough estimate above. Considering now the magnetic field initially parallel to the easy axis b, a negative MCE can be generated by rotating the o-DyMnO3 crystal around the intermediate axis a by 90 °. Thanks to the reversibility of the MCE, the associated ΔTad,bc curves could be built (not shown here) on the basis of the reported data in Fig.5-a. Furthermore, at about 26 K, the rotation motion from the b-axis to the c-axis (90 °) under 7 T reduces the crystal temperature to about 15 K. This process could be implemented in applications for the hydrogen liquefaction (for example). Aiming to determine the origin of the rotating magnetocaloric effect revealed by o-DyMnO3, preliminary calculations of the RMCE (-ΔSR) based on the coherent rotational model were carried out [17, 19, 43]. Considering initially the magnetic field parallel to the hard axis c (β = 90 °), the calculated ΔSR as a function of the angle β under 7 T is reported in Fig.5-b for T = 8 K. β is the angle between the magnetic field and the easy axis b. As shown, -ΔSR increases when β decreases to reach a maximum value of 14.54 J/kg K for β = 0, which is similar to the experimental value (ΔSR,
6
cb
= 14.77 J/kg K). This
preliminary result demonstrates the important role of the magnetocrystalline anisotropy in boosting the rotating magnetocaloric effect in o-DyMnO3 single crystal around 8 K. However, in RMnO3 materials, the magnetocrystalline anisotropy is usually more complex than assumed in our calculation. For this reason, additional measurements such as magnetization as a function of β must be performed. This will allow us to accurately determine the entropy change contributed from both the magnetocrystalline anisotropy as well as from the spin fluctuations [17, 19]. On the other hand, spin flop processes also should be taken into account [44].
IV. CONCLUSIONS In summary, a gigantic anisotropy of the magnetocaloric effect has been observed in the orthorhombic DyMnO3 around the ordering temperature of the Dy3+ magnetic moments. Along the easy axis b, the entropy change under a field variation of 5 T is about 34 times higher than that obtained along the hard axis c, which results in a large rotating magnetocaloric effect. Consequently, more compact, efficient and simplified magnetic liquefiers could be built by rotating continuously o-DyMnO3crystals in a constant magnetic field instead of the standard magnetization-demagnetization method. The presence of both reversible conventional and rotating MCEs combined with additional advantages such as the chemical stability and the electrical insulation indicate that the present crystal is an excellent candidate for low temperature magnetocaloric refrigeration.
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Acknowledgments
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The authors thank M. Castonguay and S. Pelletier for technical support. We acknowledge the financial support from NSERC (Canada), FQRNT (Québec), CFI , CIFAR and the Université de Sherbrooke.
Figure captions Figure 1: (a) Micro-Raman spectra at 5 K for the orthorhombic single crystal DyMnO3. Inset: Temperature dependence of the Raman-active phonon corresponding to 392 cm-1 [Ag (4)]. Figure 2: (a) ZFC and FC thermomagnetic curves under a field of 0.1 T along the b, a and c axes for oDyMnO3. Inset: 0.1 T- ZFC inverse magnetic susceptibility along the b, a and c axes. (b) Magnetic isotherms measured at 2 K for H//b, H//a and H//c. Figure 3: Isothermal magnetization curves of o-DyMnO3 for H//b (a) and H//c (b) in the temperature range 3-70 K with different steps. The increments of temperature are 1 K for 3-10 K, 2 K for 10-46 K and 4 K for 46-70 K. Isothermal entropy change of o-DyMnO3 as a function of temperature under different magnetic field variations for H//b (c) and H//c (d). Figure 4: (a) Entropy changes related to the rotation of the o-DyMnO3 single crystal around the intermediate axis a (within the cb-plane) by 90 ° in different constant magnetic fields. (b) Associated refrigerant capacity. Figure 5: (a) Adiabatic temperature change related to the rotation of the o-DyMnO3 single crystal around the intermediate axis a by 90 ° in different constant magnetic fields. Inset: Temperature dependence of the full entropy in a magnetic field of 7 T applied along the easy axis b and the hard axis c. (b) Rotating entropy change calculated from the coherent rotational model as a function of the angle between the easy axis and the magnetic field (β).
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Figure 1
11
Figure 2
12
Figure 3
13
Figure 4
14
Figure 5
15