Magnetocaloric effect of Sr-substituted BaFeO3 in the liquid nitrogen and natural gas temperature regions

nitrogen and natural gas temperature regions Author(s) Yoshii, Kenji; Hayashi, Naoaki; Mizumaki, Masaichiro; Takano, Mikio Citation AIP Advances (2017), 7(4) Issue Date 2017-04 URL http://hdl.handle.net/2433/225279 Right © Author(s) 2017. All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Type Journal Article Textversion publisher

Increasing demand for environmental protection has urged the development of refrigeration methods which are more efficient than the conventional gas compression-expansion cycle. 1,2One of such methods is magnetic refrigeration based on the magnetocaloric effect (MCE) due to the magnetic entropy change caused by application of magnetic field (H). 1,2A considerably large MCE has been found in several systems such as Gd 5 (Si 1x Ge x ) 4 , 3 MnAs, 4 La(Fe 1x Si x ) 13 , 5,6 HoAgGa, 7 and perovskite-type Mn-oxides. 8,9Generally, ferromagnets containing rare-earth elements with large atomic moments are advantageous because the field-induced entropy change near the Curie temperature is strongly enhanced.
Recently, we reported that a new perovskite, BaFeO 3 , is a candidate MCE material. 10This oxide contains heavily oxidized Fe ions, Fe 4+ (3d 4 , S=2), in the cubic perovskite structure (a∼3.97Å). 11 The localized moments are ordered in an antiferromagnetic helical spin structure of the A-type (denoted as AHM) with the propagation vector oriented along the cubic axis, k // <100>, below the Néel temperature of T N ∼111 K, but application of a small field of H = 3 kOe switches this complicated AHM spin structure to a simple ferromagnetic one (FM, 3.5 µ B /Fe). 11 A considerably large entropy change of ∼5.8 J kg -1 K -1 , comparable to those of manganites, has been observed at ∼116 K at H = 50 kOe, and the refrigerant capacity has been estimated to be ∼172 J kg -1 . 10Interestingly, the magnetic and thermal hysteretic losses are negligibly small contrary to expectations that the Fe 4+ :t 2g 3 e g 1 configuration with two-fold orbital degeneracy should bring about such losses caused by strong spin-lattice interactions.However, as revealed by photoelectron spectroscopic and X-ray absorption spectroscopic studies, 12,13 the real electronic configuration is close to Fe 3+ L (Fe 3+ : t 2g 3 e g 2 , L: ligand hole) in which the effect of orbital degeneracy is dispersed into the broad oxygen p hole band.It is also favorable that BaFeO 3 is rare-earth-free and stable against corrosion.This oxide may be suitable for the liquefaction of natural gas at ∼110 K 14 as well as cooling of cuprate superconductors.The purpose of the present work is to investigate the MCE of the solid solution of BaFeO 3 (BFO) and SrFeO 3 (SFO), Ba 1-x Sr x FeO 3 (BSFO).SFO is also an antiferromagnetic cubic perovskite with a helical spin structure of the G-type with k // <111> (denoted as GHM, T N =134 K), while the field-induced transition to ferromagnetism takes place at a very high field of ∼400 kOe. 16s revealed recently, 17 the type of helical spin structure changes from A to G at x 0.1.In comparison with the reversible field-induced AHM-FM transition, the GHM-FM transition is of the first order and is hysteretic.This makes the practical temperature-and field-dependence of magnetization complicated for x 0.1, especially at low temperatures and low fields, but we have believed that this solution system is worthy of being studied to clarify the chemical controllability of MCE.Refrigeration below 77K, mainly for the liquefaction of nitrogen, hydrogen and helium, is one of the two desired purposes of magnetic refrigeration, the other being operation at around room temperature. 18Quite remarkably a rare-earth-free spinel system, Fe-doped MnV 2 O 4 , has very recently been found to show a large entropy change of ∼5.8 J kg -1 K -1 at ∼77 K. 19 The performance of BSFO having lowered working temperatures will be described below.
The BSFO samples with x=0, 0.1 and 0.2 were prepared as reported previously, i.e. by high temperature solid-state reactions and subsequent low temperature treatment with ozone for the final oxidation. 17X-ray diffraction measurements showed that they were single-phase cubic perovskites (Pm-3m). 17The Sr substitution induced a shrinkage of the unit cell from a ∼3.97 Å (x=0) to ∼3.94 Å (x=0.2) because Sr 2+ is smaller than Ba 2+ .Magnetic measurements were carried out using a superconducting quantum interference device magnetometer (Quantum Design MPMS) below 300 K in a field range of H = ±50 kOe. 10 Temperature (T )-dependence of magnetization (M) was measured on heating after zero-field-cooling (ZFC), on field-cooling (FC), and also on heating after fieldcooling (HFC).Magnetization-field (MH) isotherm curves were measured in the protocol called the loop process noted later to avoid the thermal and magnetic histrories. 20The powder samples were pressed into a bar with a diameter of ∼1 mm and a height of ∼4-5 mm; considering this shape as a uniform magnetized cylinder, the demagnetizing factor is γ∼0.1. 21The demagnetizing field γM was assumed to be small (< ∼50 Oe) by using the saturation magnetization of M∼75-79 emu/g at low temperatures with H=50 kOe (For example, see Figs. 1b and 1c) and the density of ∼6.28 g/cm 3 (shown later).This field is fairy smaller than the applied fields used for the calculations of MCE (H > ∼10 kOe).Hence, the correction of the demagnetizing field was not performed.
The magnetization of Ba 0.8 Sr 0.2 FeO 3 (x=0.2) shows such strong T -and H-dependences as typically shown in Fig. 1(a).The low-field magnetization at H =1 kOe quickly increases/decreases at ∼70 K on cooling/heating, and the ZFC, FC, and HFC curves more or less deviate from each other below ∼50 K, while such an irreversibility is not seen at H =50 kOe.For this high field a well-defined anomaly in dM/dT suggestive of magnetic ordering appears at ∼90 K (inset).In parallel, as seen in Fig. 1b the isothermal MH curves become S-shaped below ∼90 K.This shape is definitely different from the convex and soon saturated type of BaFeO 3 , 10,11 indicating that the GHM-FM transition for x=0.2 proceeds more gradually over a wide field range than the AHM-FM transition for x=0 but is (almost) completed at H= 50 kOe.
The large difference in the temperature of rapidly changing M between H =1 kOe (50 K) and 50 kOe (90 K) should not be taken as indicating that the magnetic ordering temperature is strongly H-dependent.Mössbauer spectroscopic measurements in the absence of external field have revealed that the GHM-type ordering sets in at ∼90 K as will be reported elsewhere.The sharp change of M at ∼50 K for H = 1 kOe should rather be assigned to the field-induced generation of FM domains in the GHM matrix.To get additional information, we have constructed the Arrott plot (H/M vs. M 2 ) at 89 K, which is shown in the inset to Fig. 1(b).According to the criterion proposed, 22 the slope of this plot is negative for a first order magnetic transition (FOMT), while it is positive for a second order magnetic transition (SOMT).The present result shows that the transition for x=0.2 is a FOMT-type, while BFO is a SOMT-type. 10rdinal FOMT systems show both thermal and magnetic hystereses. 1 Compared in Figure 1(c) are the MH curves for x=0 and 0.2 at 4.5 K.The curve for x=0 is reversible within experimental error 10 but the curve for x=0.2 is hysteretic.The field-induced magnetization is saturated at ∼15 kOe for x=0 but a high field of ∼35 kOe is needed for x=0.2.The remarkable inconsistency between the 1st and 2nd loops for x=0.2 reflects complicated processes of the GHM-FM transition.However, the variation of saturation magnetization from 3.5 µ B /Fe for x=0 to 3.2 µ B /Fe for x=0.2 is small, indicating that the basic electronic structure of Fe 3+ L is preserved.This kind of hysteresis, which hinders efficient magnetic refrigeration, essentially disappears above ∼50 K fortunately.These results suggest that the MCE of BSFO at x = 0.2 should be considerably different from that of pure BFO in various aspects, not only in lowering of the working temperature.
Figure 2(a) shows the change of magnetic entropy (∆S) calculated from MH curves based on the Maxwell equation: 23,24 (1) Here, ∆T stands for an increment in measuring temperature.It is well known that a problem may arise for FOMT systems because ∂M/∂T becomes infinite at the T C , leading to a spike-like artifact of ∆S. 1 However, this problem does not apply to the present system showing mild field-dependences. 1,25oreover, note that the MH curves were measured in the loop process to avoid observing the artifact; each measurement was performed after resetting the sample by heating up to a temperature well above T C (nearly room temperature). 20Also, MH curves for FOMT should be averaged over the processes of increasing and decreasing field 25 because any artifact caused by hysteresis can be eliminated.However, from Fig. 2(a) as well as 1(b), such a correction is unnecessary in the temperature range of interest, T 1 ≤ T ≤ T 2 .Here, T 1 and T 2 are the temperatures where the value of -∆S is decreased to 50% of the maximum.
The T -and H-dependence of -∆S for x=0.2 is plotted in Fig. 2(a).At every fixed H, -∆S shows a peak at a certain temperature, T (-∆S max ), which becomes higher with increasing H.This tendency is similar to that of the temperature where dM/dT shows a negative peak, T ((dM/dT) min ), indicating that the entropy change is due to the field-induced change of magnetism.The peak value of -∆S max = ∼4.9J kg -1 K -1 for H = 50 kOe is considerably smaller than that of BFO (∼5.8 J kg -1 K -1 at T (-∆S max ) ∼ 116 K). 10 The Hand T-dependence of -∆S is also different.Firstly, the H-dependence of T (-∆S max ) is much stronger: For BFO T (-∆S max ) increases only by ∼4 K as the field is increased from 10kOe to 50kOe, 10 while it is raised by ∼20 K for x=0.2.Secondly, the temperature dependence of -∆S for x=0.2 is remarkably broadened below T (-∆S max ) though it is rather sharp for BFO.Thirdly, FIG. 2. (a) Magnetic entropy change (-∆S) plotted against temperature (T ) for x=0.2 with several H values (H: magnetic field).T 1 and T 2 are the temperatures of the cold and hot limit reservoirs for H=50 kOe (T 1 ∼51 K, T 2 ∼94 K), respectively, which provide the refrigeration temperature range used for the calculation of refrigerant capacity (RC; Fig. 2b).(b) RC values plotted against H, calculated from Fig. 2(a).T (-∆S max ) ∼ 77 K for H=50 kOe is lower than T ((dM/dT ) min ) ∼ 90 K (Fig. 1), while it is opposite for BFO: T (-∆S max ) ∼ 116 K is higher than the magnetic transition temperature of ∼110 K.These differences should have resulted from the difference in magnetism between GHM (x=0.2) and AHM (x=0).By the way, as seen in Figure 2(a) -∆S becomes negative below ∼20 K as observed for x=0 too. 10 This is because ∂M/∂T remains positive at relatively low fields due to the GHM state, as can be seen in Fig. 1(a).The refrigerant capacity (RC), which is the thermal energy transferred in a single refrigeration cycle, can be evaluated by integrating -∆S over the refrigeration range of T 1 ∼ T 2 .][10]19 For H=50 kOe, T 1 ∼51 K and T 2 ∼94 K and RC ∼167 J kg -1 , which is very close to that of BFO, ∼172 J kg -1 . 10The refrigeration range (T 1 -T 2 ) of ∼43 K is also close to that of BFO (∼39 K; T 1 ∼103 K, T 2 ∼142 K).The H-dependence of RC is plotted in Fig. 2(b).The similarity in RC between x=0 and 0.2 is in accord with the general understanding that the RC mainly depends on saturation magnetization. 26he refrigeration temperature can be controlled between ∼77 K and ∼116 K by changing x between 0.2 and 0 as shown below.Figure 3(a) demonstrates the results for x=0.1, which is the borderline composition showing the AHM-FM transition. 17For H = 50 kOe, -∆S max ∼5.8 J kg -1 K -1 at ∼98 K (Fig. 3(a)) and RC ∼160 J kg -1 with T 1 ∼80 K and T 2 ∼116 K (Fig. 3(b)).The performance was found to be both magnetically and thermally reversible, as seen for both x=0 and x=0.2.
In comparison to Fe-doped MnV 2 O 4 (Mn 1-x Fe x V 2 O 4 with x=0-0.3), 19its values of -∆S max ∼5-6 J kg -1 K -1 (H=40 kOe, at ∼77 K) are larger by ∼30% than that of Ba 0.8 Sr 0.2 FeO 3 .From the viewpoint of application to magnetic refrigerators, however, the magnitude of -∆S max should better be expressed in the unit of per volume than in the unit of per weight, although the latter unit is widely used. 1 The density of the present system is ∼6.28 g/cm 3 , which is ∼30% greater than that of Mn 1-x Fe x V 2 O 4 (∼4.75 g/cm 3 ; Mn 0.7 Fe 0.3 V 2 O 4 ).Thus, -∆S max in the unit of per volume is nearly the same for both systems.The peak temperature of -∆S max of Mn 1-x Fe x V 2 O 4 is tuned between ∼57 and ∼77 K, 19 while that of Ba 1-x Sr x FeO 3 can be varied between ∼77 K and ∼116 K; as noted, the present system covers the temperature regions for natural gas and cuprates as well as for N 2 .Since the presence or absence of magnetic hysteresis and also some parameters such as T 1 -T 2 were not explicitly reported for Mn 1-x Fe x V 2 O 4 , detailed comparison will be done in the future.Also, the other parameters such as the adiabatic temperature change (∆T ad ) should be determined.
This work was partially supported by JSPS KAKENHI (25620071).