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Ferromagnetic lanthanum manganites
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Ferromagnetic lanthanum manganites N.G. Bebenin n Institute of Metal Physics, Ural Division of RAS, Kovalevskaya Street 18, Ekaterinburg 620990, Russia
a r t i c l e i n f o
Keywords: Manganites Colossal magnetoresistance
abstract The short overview of the physical properties of the ferromagnetic manganites exhibiting colossal magnetoresistance is given. & 2012 Elsevier B.V. All rights reserved.
1. Introduction The strong interaction between charge carriers, localized magnetic moments and crystal lattice in ferromagnetic manganites La1 xDxMnO3 where D is a divalent ion gives rise to many interesting effects observed preferably in a vicinity of the Curie temperature TC [1–3]. Colossal magnetoresistance (CMR) is, perhaps, most popular but such phenomena as phase separation, giant magnetocaloric effect, etc., are also intensively studied. The aim of this work is to overview results on the physical properties of CMR manganites published mainly during last 5–10 years. We describe the crystal lattice, structural transitions, and magnetic phase diagrams for D¼Ca, Sr, Ba. Then we analyze the experimental data on resistivity, magnetoresistance, and Hall effect with special attention to their behavior near TC. The CMR effect in the materials with the first order transition is considered in close connection with the magnetic inhomogeneity. Optical properties are discussed from the viewpoint of the change of electronic band structure due to change in temperature. Finally we briefly review the magnetocaloric effect.
2. Crystal and magnetic structure The crystal structure of CMR manganites is called perovskite, which is not completely correct. In the true (cubic) perovskite cell the manganese ions would be located in the environment of six oxygen ions, which form a regular octahedron, and the Mn–O–Mn o bond angle would be equal to 180 % . In real lanthanum manganites the perovskite structure is always somewhat distorted, so that the symmetry of the lattice is lower than cubic [4–9]. The lattice can be orthorhombic (usually Pnma space group but in La0.7Ba0.3MnO3 it is Imma [10]), rhombohedral (R3c), or (in rare cases) monoclinic. It has been established in [11] that two orthorhombic Pnma phases can p beffiffiffi realized. The first of them (O’), with lattice parameters b=2 o c oa o b is characterized by strong Jahn– Teller distortions of the oxygen octahedra. The second phase, n
Corresponding author. Tel.: þ7 343 3783890; fax: þ7 343 3745244. E-mail address: [email protected]
O*, is called pseudocubic. In this phase pffiffiffithe distortions of the octahedra are considerably weaker and b=2 c a but the Mn– O–Mn bond angles, just as in the O* phase, noticeably differ from 1801. In the Imma structure the octahedra are not distorted. A transition between different crystal structures results in significant changes in elastic moduli ([12] and references therein). The resistivity is however weakly affected excepting the case when structural transition temperature coincides with the Curie temperature [13], so in what follows we shall deal preferably with the effects near TC. The compounds under consideration are ferromagnets if x lies between 0.1 and 0.5. Fig. 1 shows the Curie temperature as a function of the divalent concentration x for polycrystalline [4,5,13] (open symbols) and single-crystalline [14–18] (solid symbols) samples. One can see that the data for poly- and single-crystalline samples coincide for La1 xSrxMnO3 only while in case of La–Ba as well as La–Ca manganites the TC values for the single crystals are somewhat lower than for the polycrystals. The difference is likely to be due to uncontrolled defects created during the growth of single crystals by floating zone method. It is necessary to note that the magnetic transition is always smeared. The standard deviation for TC is the smallest (about 1 K) in La1 xSrxMnO3 single crystals, it is significantly greater in La1 xBaxMnO3, and in La1 xCaxMnO3 the deviation is highest (between 5 and 8 K) [19]. In La1 xSrxMnO3 and La1 xBaxMnO3 the ferromagnetic-to paramagnetic phase transition is always second order while in La1 xCaxMnO3 it is second order only if xo0.25, when x is greater, the magnetic transition is first order [16]. In La1 xCaxMnO3 with x around 1/3 the Curie temperature is increased with increasing magnetic field: TC(H) ¼TC(0)þBMHint where Hint is an internal field and BM E0.8 K/kOe [20,21].
3. Electronic transport In the ferromagnetic state well below TC the temperature dependence of resistivity r is of semiconductor type (dr/dTo0) if xoxc where xc is the critical concentration at which metal-semiconductor
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Fig. 1. Dependence of the Curie temperature on divalent element concentration.
increasing T. Above 200–250 K, however, mH practically does not depend on T and mH is between 0.15 and 0.3 cm2/(Vs). This means that the main contribution to the conductivity is given by charge carriers whose energy is near the mobility edge; in other words, when T becomes higher than 200–250 K, the growth of the resistivity is caused by the reduction of the concentration of charge carriers in extended states rather than the decrease of their mobility. It follows that the metal–semiconductor transition takes place in a wide temperature interval below the Curie temperature [25]. The experimental data for La1 xBaxMnO3 and La1 xCaxMnO3 single crystals were summarized in [23] and [26], respectively. Here it is sufficient to say that the mobility of the La0.7Ca0.3MnO3 crystal (TC(H¼0) ¼227 K) behaves as in a ‘‘bad’’ metal and that La0.7Ca0.3MnO3 is at the threshold of localization at T¼220 K when the rapid rise in resistivity begins. Now let us consider the resistivity in a vicinity of the Curie point. In the Inset (b) in Fig. 2 we show the temperature dependence of magnetoresistance Dr rðHÞrð0Þ measured in the magnetic field of 10 kOe [20,24,25]. The origin of the CMR effect depends on whether the magnetic transition is second or first order. If the first alternative is realized, i.e., in La–Sr and La– Ba manganites, the resistivity is determined by the dependence of activation energy on magnetization: E E m2 ð2Þ r ¼ r0 exp 0 1 kB T where r0 ¼const, m¼M/Ms, M(T,H) is magnetization, Ms stands for its saturation value, E0 and E1 weakly depend on temperature. If m is small we get
DrðHÞ
r
Fig. 2. Temperature dependence of resistivity. Inset (a): Hall mobility versus temperature; Inset (b): Temperature dependence of magnetoresistance.
transition takes place. In La1 xSrxMnO3 the value of xc is 0.17, in La1 xCaxMnO3 it is 0.225, and in La–Ba manganites xc is about 0.22–0.23 [17,22,23]. In the semiconductor state the resistivity is usually dominated by the variable range hopping; for example, in La0.85Sr0.15MnO3 (Sr15 in Fig. 2) the resistivity obeys relation r ¼ r0exp[(To/T)1/4 dW/(kBT)] with r0 ¼1.3 10 7 O cm, T1/4 0 ¼ 43 K1/4, dW/kB ¼34 K [24]. If x4xc the manganites are said to be in metallic state in the sense that dr/dT is positive. Below approximately 200 K the temperature dependence of resistivity is described by the relation [17] 2
rðTÞ ¼ rð0Þ þ AT :
¼
E1 2 m ðHÞm2 ð0Þ : kB T
It is worth noting that relation (2) was found to be valid not only in perovskite manganites but also in layered antiferromagnetic La0.5Sr1.5MnO4 [27]. The formula (2) means that just below TC the change of resistivity arises from change of the activation energy and therefore for the CMR effect in the manganites with the second order magnetic phase transition consists in the change of activation energy under application of a magnetic field. When the magnetic transition is first order as in La0.7Ca0.3MnO3 the CMR effect arises due to shift of the Curie temperature in a magnetic field [20]. The temperature dependence of magnetoresistance is then described by the relation:
DrðT,HÞ
r
¼
rðTDT C ðHÞÞrððTÞ rðTÞ
ð3Þ
where DTC ¼BMHint and r(T)¼ r(T, H¼0). Using (3) we calculated Dr/ r for La0.7Ca0.3MnO3; the result is shown in the Inset (b) in Fig. 2as solid line. One can see that in the transition region Dr/r is well described by relation (3).
ð1Þ 4
The residual resistivity r(0) is less than 10 O cm in La1 xSrxMnO3 when x ¼0.3 or greater [17]; in the La1 xBaxMnO3 and La1 xCaxMnO3 crystals with the same level of doping r(0) is significantly greater [22–24]. Important information on conductivity mechanism can be obtained from the Hall effect data. Inset (a) in Fig. 2 shows temperature dependence of the Hall mobility mH ¼R0/r with R0 being normal Hall coefficient. In La0.85Sr0.15MnO3 (TC ¼232 K) the variable range hopping dominates conductivity in the ferromagnetic state. The La0.80Sr0.20MnO3 (TC ¼308 K) and La0.75Sr0.25MnO3 (TC ¼341 K) crystals behave as ordinary bad metals in which charge carriers are holes and the mobility decreases with
4. Optical properties The optical and magnetooptical properties of the CMR manganites were recently reviewed by Gan’shina et al. [28] so in this section the focus is on the relation between optical and transport effects. The spectra of optical conductivity sopt(E) where E is photon energy are similar in all LaMnO3-based materials. In Fig. 3 we show the spectra for La0.75Ba0.25MnO3 (TC ¼300 K) as an example [29]. One can see two broad absorption bands, which are typical for the LaMnO3-based compounds. At T¼ 300 and 400 K, i.e., at TZTC, the low-energy band (band 1) has maxima at EE1.3 eV. The high-energy band (band 2) occupies the region above 2.5 eV
Please cite this article as: N.G. Bebenin, Journal of Magnetism and Magnetic Materials (2012), doi:10.1016/j.jmmm.2012.02.098
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magnetic entropy change DS(T,H), which can be expressed in terms of (@M(T,H)/@T)H: Z H @M DSðT,HÞ ¼ dH ð4Þ @T H 0
Fig. 3. Optical conductivity of La0.75Ba0.25MnO3 single crystal.
The maximum value of 9DS(T,H)9 is observed near TC in the manganites with the first order magnetic phase transition. Fig. 4 shows the temperature dependence of 9DS(T,H)9 obtained by Phan et al. [37] on the La0.7Ca0.3MnO3 single crystal at H¼ 30 kOe (solid circles) and reported by Szymczak et al. [38] for La0.7Ca0.3MnO3 polycrystalline sample prepared by a nonstandard ceramic method. We see that the polycrystalline sample shows greater 9DS9 (9DS9 E8.1 J/( kg K) in the field of 20 kOe) while in the case of the single crystal the maximum 9DS9 in the field of H¼30 kOe is only about 4.5 J/( kg K). Using Clapeyron equation and magnetization data one can easily show that the transition heat in La0.7Ca0.3MnO3 is about 9 J/( kg K). Therefore the 9DS9 reported in [38] is close to the maximum attainable one. Looking at Fig. 4 we also see that the width of the transition region in the single crystal under consideration is significantly greater than that in the polycrystal, therefore the field in which the maximum 9DS9 is achieved becomes lower when the magnetic transition region width is decreased.
Acknowledgment This work was supported by RFBR grant 09-02-00081 and a Program of Presidium of RAS.
Fig. 4. Entropy change in La0.7Ca0.3MnO3.
and has a maximum at E 4 eV. Decreasing T down to 95 K results in the shift of band 1 to lower energies while the low-energy wing of band 2 moves to higher energies. Band 1 is likely to be formed by intersite transitions across the Mott gap, i.e., transitions between eg states of neighboring Mn ions [30]. It is necessary to remember, however, that between two Mn ions there is an oxygen ion, and therefore transitions involving oxygen states may also give a contribution to band 1. As for band 2, it is commonly accepted that this band is due to the charge-transfer transitions from 2p states of oxygen to 3d states of manganese. If x 4xc i.e., if a manganites is in the metallic state in low temperature region the transition from paramagnetic to ferromagnetic phase gives rise to Drude-like peak in the low-frequency part of the spectrum (in Fig. 3 we see only beginning of this peak). When the magnetic transition is second order as in the La–Sr and La–Ba manganites, the Drude-like component appears and gradually increases at temperatures that are significantly less than TC [31,32], which confirms our conclusion that the metalsemiconductor transition takes place in a wide interval below the Curie temperature. On the contrary, if the magnetic transition is first order as in La0.7Ca0.3MnO3 the Drude-like contribution appears abruptly near TC [33]. Optical data in combination with those for resistivity were successfully used for studying the phase separation (coexistence of metallic and semiconductor state near TC) [29,34] and polaronic states [35] in the CMR compounds.
5. Giant magnetocaloric effect Magnetocaloric effect attracts attention because of possibility of practical application [36]. It can be characterized by the
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[23] N.G. Bebenin, R.I. Zainullina, N.S. Chusheva, V.V. Ustinov, Ya.M. Mukovskii, Journal of Magnetism and Magnetic Materials 300 (2006) e111. [24] R.I. Zainullina, N.G. Bebenin, V.V. Mashkautsan, V.V. Ustinov, Ya.M. Mukovskii, A.A. Arsenov, Physics of the Solid State 45 (2003) 1754. [25] N.G. Bebenin, R.I. Zainullina, V.V. Mashkautsan, V.V. Ustinov, Ya.M. Mukovskii, Physical Review B 69 (2004) 104434. [26] N.G. Bebenin, Physics of Metals and Metallography 111 (2011) 236. [27] M. Tokunaga, N. Miuri, Y. Moritomo, Y. Tokura, Physical Review B 59 (1999) 11151. [28] E. Gan’shina, N. Loshkareva, Yu. Sukhorukov, E. Mostovshchikova, A. Vinogradov, L. Nomerovannaya, Journal of Magnetism and Magnetic Materials 300 (2006) 62. [29] N.G. Bebenin, N.N. Loshkareva, A.A. Makhnev, E.V. Mostovshchikova, L.V. Nomerovannaya, E.A. Gan0 shina, A.N. Vinogradov, Ya.M. Mukovskii, Journal of Physics: Condensed Matter 22 (2010) 096003.
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Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Ferromagnetic lanthanum manganites N.G. Bebenin n Institute of Metal Physics, Ural Division of RAS, Kovalevskaya Street 18, Ekaterinburg 620990, Russia
a r t i c l e i n f o
Keywords: Manganites Colossal magnetoresistance
abstract The short overview of the physical properties of the ferromagnetic manganites exhibiting colossal magnetoresistance is given. & 2012 Elsevier B.V. All rights reserved.
1. Introduction The strong interaction between charge carriers, localized magnetic moments and crystal lattice in ferromagnetic manganites La1 xDxMnO3 where D is a divalent ion gives rise to many interesting effects observed preferably in a vicinity of the Curie temperature TC [1–3]. Colossal magnetoresistance (CMR) is, perhaps, most popular but such phenomena as phase separation, giant magnetocaloric effect, etc., are also intensively studied. The aim of this work is to overview results on the physical properties of CMR manganites published mainly during last 5–10 years. We describe the crystal lattice, structural transitions, and magnetic phase diagrams for D¼Ca, Sr, Ba. Then we analyze the experimental data on resistivity, magnetoresistance, and Hall effect with special attention to their behavior near TC. The CMR effect in the materials with the first order transition is considered in close connection with the magnetic inhomogeneity. Optical properties are discussed from the viewpoint of the change of electronic band structure due to change in temperature. Finally we briefly review the magnetocaloric effect.
2. Crystal and magnetic structure The crystal structure of CMR manganites is called perovskite, which is not completely correct. In the true (cubic) perovskite cell the manganese ions would be located in the environment of six oxygen ions, which form a regular octahedron, and the Mn–O–Mn o bond angle would be equal to 180 % . In real lanthanum manganites the perovskite structure is always somewhat distorted, so that the symmetry of the lattice is lower than cubic [4–9]. The lattice can be orthorhombic (usually Pnma space group but in La0.7Ba0.3MnO3 it is Imma [10]), rhombohedral (R3c), or (in rare cases) monoclinic. It has been established in [11] that two orthorhombic Pnma phases can p beffiffiffi realized. The first of them (O’), with lattice parameters b=2 o c oa o b is characterized by strong Jahn– Teller distortions of the oxygen octahedra. The second phase, n
Corresponding author. Tel.: þ7 343 3783890; fax: þ7 343 3745244. E-mail address: [email protected]
O*, is called pseudocubic. In this phase pffiffiffithe distortions of the octahedra are considerably weaker and b=2 c a but the Mn– O–Mn bond angles, just as in the O* phase, noticeably differ from 1801. In the Imma structure the octahedra are not distorted. A transition between different crystal structures results in significant changes in elastic moduli ([12] and references therein). The resistivity is however weakly affected excepting the case when structural transition temperature coincides with the Curie temperature [13], so in what follows we shall deal preferably with the effects near TC. The compounds under consideration are ferromagnets if x lies between 0.1 and 0.5. Fig. 1 shows the Curie temperature as a function of the divalent concentration x for polycrystalline [4,5,13] (open symbols) and single-crystalline [14–18] (solid symbols) samples. One can see that the data for poly- and single-crystalline samples coincide for La1 xSrxMnO3 only while in case of La–Ba as well as La–Ca manganites the TC values for the single crystals are somewhat lower than for the polycrystals. The difference is likely to be due to uncontrolled defects created during the growth of single crystals by floating zone method. It is necessary to note that the magnetic transition is always smeared. The standard deviation for TC is the smallest (about 1 K) in La1 xSrxMnO3 single crystals, it is significantly greater in La1 xBaxMnO3, and in La1 xCaxMnO3 the deviation is highest (between 5 and 8 K) [19]. In La1 xSrxMnO3 and La1 xBaxMnO3 the ferromagnetic-to paramagnetic phase transition is always second order while in La1 xCaxMnO3 it is second order only if xo0.25, when x is greater, the magnetic transition is first order [16]. In La1 xCaxMnO3 with x around 1/3 the Curie temperature is increased with increasing magnetic field: TC(H) ¼TC(0)þBMHint where Hint is an internal field and BM E0.8 K/kOe [20,21].
3. Electronic transport In the ferromagnetic state well below TC the temperature dependence of resistivity r is of semiconductor type (dr/dTo0) if xoxc where xc is the critical concentration at which metal-semiconductor
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Fig. 1. Dependence of the Curie temperature on divalent element concentration.
increasing T. Above 200–250 K, however, mH practically does not depend on T and mH is between 0.15 and 0.3 cm2/(Vs). This means that the main contribution to the conductivity is given by charge carriers whose energy is near the mobility edge; in other words, when T becomes higher than 200–250 K, the growth of the resistivity is caused by the reduction of the concentration of charge carriers in extended states rather than the decrease of their mobility. It follows that the metal–semiconductor transition takes place in a wide temperature interval below the Curie temperature [25]. The experimental data for La1 xBaxMnO3 and La1 xCaxMnO3 single crystals were summarized in [23] and [26], respectively. Here it is sufficient to say that the mobility of the La0.7Ca0.3MnO3 crystal (TC(H¼0) ¼227 K) behaves as in a ‘‘bad’’ metal and that La0.7Ca0.3MnO3 is at the threshold of localization at T¼220 K when the rapid rise in resistivity begins. Now let us consider the resistivity in a vicinity of the Curie point. In the Inset (b) in Fig. 2 we show the temperature dependence of magnetoresistance Dr rðHÞrð0Þ measured in the magnetic field of 10 kOe [20,24,25]. The origin of the CMR effect depends on whether the magnetic transition is second or first order. If the first alternative is realized, i.e., in La–Sr and La– Ba manganites, the resistivity is determined by the dependence of activation energy on magnetization: E E m2 ð2Þ r ¼ r0 exp 0 1 kB T where r0 ¼const, m¼M/Ms, M(T,H) is magnetization, Ms stands for its saturation value, E0 and E1 weakly depend on temperature. If m is small we get
DrðHÞ
r
Fig. 2. Temperature dependence of resistivity. Inset (a): Hall mobility versus temperature; Inset (b): Temperature dependence of magnetoresistance.
transition takes place. In La1 xSrxMnO3 the value of xc is 0.17, in La1 xCaxMnO3 it is 0.225, and in La–Ba manganites xc is about 0.22–0.23 [17,22,23]. In the semiconductor state the resistivity is usually dominated by the variable range hopping; for example, in La0.85Sr0.15MnO3 (Sr15 in Fig. 2) the resistivity obeys relation r ¼ r0exp[(To/T)1/4 dW/(kBT)] with r0 ¼1.3 10 7 O cm, T1/4 0 ¼ 43 K1/4, dW/kB ¼34 K [24]. If x4xc the manganites are said to be in metallic state in the sense that dr/dT is positive. Below approximately 200 K the temperature dependence of resistivity is described by the relation [17] 2
rðTÞ ¼ rð0Þ þ AT :
¼
E1 2 m ðHÞm2 ð0Þ : kB T
It is worth noting that relation (2) was found to be valid not only in perovskite manganites but also in layered antiferromagnetic La0.5Sr1.5MnO4 [27]. The formula (2) means that just below TC the change of resistivity arises from change of the activation energy and therefore for the CMR effect in the manganites with the second order magnetic phase transition consists in the change of activation energy under application of a magnetic field. When the magnetic transition is first order as in La0.7Ca0.3MnO3 the CMR effect arises due to shift of the Curie temperature in a magnetic field [20]. The temperature dependence of magnetoresistance is then described by the relation:
DrðT,HÞ
r
¼
rðTDT C ðHÞÞrððTÞ rðTÞ
ð3Þ
where DTC ¼BMHint and r(T)¼ r(T, H¼0). Using (3) we calculated Dr/ r for La0.7Ca0.3MnO3; the result is shown in the Inset (b) in Fig. 2as solid line. One can see that in the transition region Dr/r is well described by relation (3).
ð1Þ 4
The residual resistivity r(0) is less than 10 O cm in La1 xSrxMnO3 when x ¼0.3 or greater [17]; in the La1 xBaxMnO3 and La1 xCaxMnO3 crystals with the same level of doping r(0) is significantly greater [22–24]. Important information on conductivity mechanism can be obtained from the Hall effect data. Inset (a) in Fig. 2 shows temperature dependence of the Hall mobility mH ¼R0/r with R0 being normal Hall coefficient. In La0.85Sr0.15MnO3 (TC ¼232 K) the variable range hopping dominates conductivity in the ferromagnetic state. The La0.80Sr0.20MnO3 (TC ¼308 K) and La0.75Sr0.25MnO3 (TC ¼341 K) crystals behave as ordinary bad metals in which charge carriers are holes and the mobility decreases with
4. Optical properties The optical and magnetooptical properties of the CMR manganites were recently reviewed by Gan’shina et al. [28] so in this section the focus is on the relation between optical and transport effects. The spectra of optical conductivity sopt(E) where E is photon energy are similar in all LaMnO3-based materials. In Fig. 3 we show the spectra for La0.75Ba0.25MnO3 (TC ¼300 K) as an example [29]. One can see two broad absorption bands, which are typical for the LaMnO3-based compounds. At T¼ 300 and 400 K, i.e., at TZTC, the low-energy band (band 1) has maxima at EE1.3 eV. The high-energy band (band 2) occupies the region above 2.5 eV
Please cite this article as: N.G. Bebenin, Journal of Magnetism and Magnetic Materials (2012), doi:10.1016/j.jmmm.2012.02.098
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magnetic entropy change DS(T,H), which can be expressed in terms of (@M(T,H)/@T)H: Z H @M DSðT,HÞ ¼ dH ð4Þ @T H 0
Fig. 3. Optical conductivity of La0.75Ba0.25MnO3 single crystal.
The maximum value of 9DS(T,H)9 is observed near TC in the manganites with the first order magnetic phase transition. Fig. 4 shows the temperature dependence of 9DS(T,H)9 obtained by Phan et al. [37] on the La0.7Ca0.3MnO3 single crystal at H¼ 30 kOe (solid circles) and reported by Szymczak et al. [38] for La0.7Ca0.3MnO3 polycrystalline sample prepared by a nonstandard ceramic method. We see that the polycrystalline sample shows greater 9DS9 (9DS9 E8.1 J/( kg K) in the field of 20 kOe) while in the case of the single crystal the maximum 9DS9 in the field of H¼30 kOe is only about 4.5 J/( kg K). Using Clapeyron equation and magnetization data one can easily show that the transition heat in La0.7Ca0.3MnO3 is about 9 J/( kg K). Therefore the 9DS9 reported in [38] is close to the maximum attainable one. Looking at Fig. 4 we also see that the width of the transition region in the single crystal under consideration is significantly greater than that in the polycrystal, therefore the field in which the maximum 9DS9 is achieved becomes lower when the magnetic transition region width is decreased.
Acknowledgment This work was supported by RFBR grant 09-02-00081 and a Program of Presidium of RAS.
Fig. 4. Entropy change in La0.7Ca0.3MnO3.
and has a maximum at E 4 eV. Decreasing T down to 95 K results in the shift of band 1 to lower energies while the low-energy wing of band 2 moves to higher energies. Band 1 is likely to be formed by intersite transitions across the Mott gap, i.e., transitions between eg states of neighboring Mn ions [30]. It is necessary to remember, however, that between two Mn ions there is an oxygen ion, and therefore transitions involving oxygen states may also give a contribution to band 1. As for band 2, it is commonly accepted that this band is due to the charge-transfer transitions from 2p states of oxygen to 3d states of manganese. If x 4xc i.e., if a manganites is in the metallic state in low temperature region the transition from paramagnetic to ferromagnetic phase gives rise to Drude-like peak in the low-frequency part of the spectrum (in Fig. 3 we see only beginning of this peak). When the magnetic transition is second order as in the La–Sr and La–Ba manganites, the Drude-like component appears and gradually increases at temperatures that are significantly less than TC [31,32], which confirms our conclusion that the metalsemiconductor transition takes place in a wide interval below the Curie temperature. On the contrary, if the magnetic transition is first order as in La0.7Ca0.3MnO3 the Drude-like contribution appears abruptly near TC [33]. Optical data in combination with those for resistivity were successfully used for studying the phase separation (coexistence of metallic and semiconductor state near TC) [29,34] and polaronic states [35] in the CMR compounds.
5. Giant magnetocaloric effect Magnetocaloric effect attracts attention because of possibility of practical application [36]. It can be characterized by the
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