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Journal of Magnetism and Magnetic Materials 300 (2006) e111–e113 www.elsevier.com/locate/jmmm

Hall effect and conductivity in La1xBaxMnO3 single crystals N.G. Bebenina,, R.I. Zainullinaa, N.S. Chushevaa, V.V. Ustinova, Ya.M. Mukovskiib a

Institute of Metal Physics, Ural Division of RAS, 18 Kovalevskaya St., Ekaterinburg 620041, Russian Federation b Moscow State Steel & Alloys Institute, Leninskii prosp., 4, Moscow 117936, Russian Federation Available online 18 November 2005

Abstract We study the transport properties of La1xBaxMnO3 (x ¼ 0:15, 0.20, and 0.28) single crystals grown by the ﬂoating zone method. The magnetization, Hall effect, and resistivity r were measured in temperature range of 77–400 K in magnetic ﬁelds up to 15 kOe. It is shown that the band structure of the colossal magnetoresistance (CMR) manganites is determined by not only doping level but also the type of divalent ions. Near the Curie temperature TC, in all the samples ln r is linear in squared magnetization, which indicates the conductivity of semiconductor type with activation energy determined by the interaction with localized spins; the metal–insulator transition, if any, occurs below TC. r 2005 Elsevier B.V. All rights reserved. PACS: 75.30.Vn; 72.15.Gd Keywords: Colossal magnetoresistance; Manganites; Hall effect; La1xBaxMnO3

Ferromagnetic lanthanum manganites La1xDxMnO3, (D ¼ Ca, Sr, Ba) are known to exhibit the colossal magnetoresistance (CMR) near the Curie temperature TC. It is believed that the principal features of the band structure of these compounds are the same and the CMR effect is connected with the metal–insulator transition occurring near TC. It is to be noted that the electronic transport in La–Ba single crystals has been explored much weaker than in La–Sr manganites. In particular, the Hall effect, which is an effective tool for ﬁnding out the nature of electronic transport in rare-earth manganites, see Refs. [1–6] and references therein, was studied, to our knowledge, in the x ¼ 0:20 crystal only [7]. The purpose of this work is a comparative study of the Hall effect and conductivity in the La–Ba single crystals with x ¼ 0:15; 0.20; and 0.28. We report the temperature dependence of resistivity r, normal Hall coefﬁcient R0, and magnetoresistance Dr=r ¼ ½rðHÞ rð0Þ=rð0Þ, where H is a magnetic ﬁeld. In our analysis we use some experimental data published earlier [7]. We shall make conclusions about the evolution of band structure and conductivity mechanisms with increasing Ba doping. Corresponding author. Tel.: +7 343 3783890; fax: +7 343 3745 244.

E-mail address: [email protected]p.uran.ru (N.G. Bebenin). 0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.10.160

The La1xBaxMnO3 single crystals were grown by the ﬂoating-zone method. The resistivity and Hall effect experiments were carried out using the same sample in the form of a plate. The magnetization M was measured using a vibrating sample magnetometer. The resistivity was determined by a four-probe technique. The Hall resistivity rH was measured in two opposite directions of the ﬁeld and the electric current. In all the experiments, a magnetic ﬁeld was applied perpendicular to the plane of the sample. The Hall coefﬁcients are deﬁned by expression rH ¼ R0 B þ Rs M,

(1)

where B stands for the magnetic ﬁeld induction inside the sample; in our case (the sample in the form of a thin plate) the induction may be taken equal to the ﬁeld H. After measuring rH and M, we plotted rH =H versus M/H, determining thereby R0 and Rs. This method is obviously inapplicable in paramagnetic state. The crystals studied are ferromagnets. The Curie temperature evaluated through Arrott–Belov curves are 214, 252, and 310 K for x ¼ 0:15, 0.20, and 0.28, respectively. Fig. 1 shows the resistivity versus temperature at H ¼ 0. In the ferromagnetic phase, well below TC, the x ¼ 0:15

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Fig. 1. The resistivity of La1xBaxMnO3 single crystals. Inset: local activation energy in the x ¼ 0:15 manganite.

Fig. 2. Temperature dependence of magnetoresistance of La1xBaxMnO3 measured at H ¼ 10 kOe.

and 0.20 crystals are semiconductors while the x ¼ 0:28 compound is in a metallic state. Near TC there is the resistivity peak, which is typical to the CMR manganites. Above the Curie temperature, all the samples behave as a semiconductor. The peculiarity around T ¼ 284 K at the x ¼ 0:15 curve results from the structural phase transition between the low temperature orthorhombic and high temperature rhombohedral phases. Two other crystals also undergo such transition but it is not our aim here to discuss these peculiarities in detail. Let us ﬁrst analyze the rðTÞ dependence in the low temperature region. The x ¼ 0:15 crystal behaves as a semiconductor in the sense that dr=dTo0 but ln r is not a linear function of inverse temperature; therefore the activation energy for resistivity depends on temperature. The inset in Fig. 1 shows the local activation energy deﬁned as e ¼ d ln r=dðT 1 Þ as a function of T. The deep minimum at TE210 K is obviously due to the magnetic transition and the maximum at E293 K is due to the structural O-R transformation. Below E125 K the local activation energy increases with increasing temperature, which is an evidence for the variable range hopping (VRH). In the simplest version of VRH, the density of states NðEÞ is assumed to be independent of energy near the Fermi energy EF and the resistivity obeys the Mott’s 1/4 law [8,9] r ¼ ro exp½ðT o =TÞ1=4 . In our case T1/4 o E82 K and NðE F Þ is therefore very small. This suggests that the Fermi level lies in an energy gap and the Shklovskii–Efros’s model [9] in which NðEÞ is proportional to (EEF)2 seems to be more realistic. The resistivity of the x ¼ 0:20 crystal, which is also semiconductor below approximately 150 K, is essentially less than that in the x ¼ 0:15 crystal, so that NðE F Þ in La0.80Ba0.20MnO3 is probably signiﬁcantly greater than in La0.85Ba0.15MnO3. The resistivity of the x ¼ 0:28 crystal obeys the relation rðTÞ ¼ rð0Þ þ AT 2 at To180 K; if temperature is greater the resistivity increases with T much faster. This temperature dependence is typical for a ‘‘bad’’ metal and is observed in the single crystals of

La1xSrxMnO3 in metallic state [6,10]. It follows that in La0.72Ba0.28MnO3 the density of state at the Fermi level is large and close to the values found in La–Sr crystals. The effect of a magnetic ﬁeld on resistivity of the La–Ba crystals is shown in Fig. 2 where Dr=r measured at H ¼ 10 kOe is plotted against temperature. The absolute value of Dr=r is maximal practically at the Curie point and the peak value of jDr=rj is very high (‘‘colossal’’). Since dr=dT40 near TC and dr=dTo0 at higher temperatures, many authors believe that the metal–insulator transition occurs in a vicinity of the Curie point and the CMR effect is a consequence of this transition. It is to be noted however that estimates based on free-electron ideas suggest that in a metallic manganite, the mean free path approaches interatomic distance once the resistivity exceeds 1 mO cm [1]; in other words, 1 mO cm can be taken as a (rough) boundary between metallic and semiconductor regimes. Near TC the resistivity of all the crystals is signiﬁcantly larger 1mO cm, which suggests a semiconductor state. In Fig. 3 we plot ln r versus m2 ¼ ðM=M s Þ2 at H ¼ 10 kOe where Ms is saturation magnetization. One can see that ln r is linear in m2 if magnetization is not too large, i.e. just where the CMR effect is observed. Therefore, near the Curie temperature the change of the resistivity results from the change of activation energy. Fig. 4 shows the Hall mobility mH ¼ R0 =rðH ¼ 0Þ versus temperature. In the x ¼ 0:15 manganite the Hall mobility is negative, its absolute value is of order 0.1 cm2 V1 s1. The sign and small value of mH is fully consistent with the conclusion that the hopping conductivity dominates in this manganite. In the x ¼ 0:20 sample, mH is negative below 200 K and positive above this temperature, so that there are at least two competing conductivity mechanisms in this crystal. Since in the x ¼ 0:20 crystal the mH value near TC is close to 0.1 cm2 V1 s1, which is characteristic of the Hall mobility of the holes activated to the mobility edge, we may infer that such holes are responsible for the CMR effect in this crystal.

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as the mobility does not depend on T. We see that the metal–insulator transition occurs not in vicinity of the Curie temperature but well below TC. Notice that in the interval 170–295 K, mH in La0.72Ba0.28MnO3 (E0.1 cm2 V1 s1) is less than for example in La0.75Sr0.25MnO3 (E0.15–0.3 cm2 V1 s1). As is known, in Ge crystals the mobility decreases as the random potential of charged impurities becomes stronger [12]. We may thus suppose that in La–Ba manganites the random potential is essentially stronger than in the La–Sr crystals due to the fact that the difference between ionic radii of La and Ba is signiﬁcantly greater than the difference between the radii of La and Sr. Near (and perhaps above) TC the electrons play a key role again as it is evident from the change of sign of R0. Unfortunately our data are insufﬁcient to make a deﬁnite conclusion about conductivity mechanism(s) in the paramagnetic state. To summarize, our measurements of temperature dependence of resistivity and Hall effect show that the band structure of the CMR manganites is determined by not only doping level but also the type of divalent ions. The conductivity in the La–Ba manganites with different level of doping differs in nature. The formal reason for the CMR effect in La–Ba single crystal consists in the change of activation energy under application of a magnetic ﬁeld. The metal–insulator transition, if any, occurs well below TC.

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Acknowledgements

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The work was supported by grants RFBR 03-02-16065, SS-1380.2003.2, and Quantum Macrophysics Program of RAS.

Fig. 4. Hall mobility in La1xBaxMnO3 single crystals.

References Of particular interest is the behavior of mH in the x ¼ 0:28 crystal. The Hall mobility is negative at low temperatures, changes its sign at T ¼ 145 K, is approximately independent of temperature in the range of 170–295 K (mHE0.1 cm2 V1 s1), and again changes its sign near TC. In the La1xSrxMnO3 with xp0.4, the normal Hall coefﬁcient is always positive at T5TC [1–3,6], so that the holes dominate conductivity. In our La0.72Ba0.28MnO3 this coefﬁcient is negative at To145 K but the mobility value is small. It is reasonable to think that electrons dominate the kinetic properties but the number of holes is close to the number of electrons. It follows that the structure of the energy bands in the CMR manganites is sensitive to the type of dopant. As temperature increases, the role of electrons weakens because of localization since according to the band calculations [11] the effective mass of electrons is greater than the mass of the holes. In the range of 170–295 K the holes activated to the mobility edge dominate as long

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