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Magnetic inhomogeneity and colossal magnetoresistance in manganese oxides
ELSEVIER
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journal of magnetism and magnetic materials
Journal of Magnetismand Magnetic Materials 167 (1997) 200-208
Magnetic inhomogeneity and colossal magnetoresistance in manganese oxides H.L. Ju a,b,*, Hyunchul Sohn b Center for Superconductivity Research, Department of Physics, University of Maryland, College Park, MD 20742, USA b Lawrence Berkeley Laboratory, Materials Science Division, Berkeley, CA 94720, USA
Received 19 August 1996
Abstract The magnetic and magnetotransport properties of the manganites, R 1_~AxMnOz (R = La, Pr, Nd; A = Ca, Sr, Ba), are strongly dependent on the doping level x and the oxygen stoichiometry z. For x > ~ 0.2, stoichiometric R 1_xAxMnO3 was found to have a small temperature independent coercivity of 5-30 Oe and the saturation magnetization ( ~ 90 emu/g) of fully aligned Mn ions. Oxygen deficient (or x < 0.2) R 1_xAxMnOz has a saturation magnetization smaller than that of fully aligned Mn spins and shows an enhancement and temperature dependence of the coercivity, and spin freezing behavior. The latter is characterized by a peak in zero field cooled (ZFC) magnetization. Our data suggest that oxygen deficient or low doped manganites are magnetically inhomogeneous. We discuss possible origins of the magnetic inhomogeneity and its influence on the colossal magnetoresistance. Keywords: Manganeseoxide; Magnetoresistance;Polaron; Magnetic inhomogeneity;Perovskite structure
1. I n t r o d u c t i o n Recently, there have been many studies of the manganese oxides, R l _ x A x M n O 3 (R = La, Pr, Nd; A = Ca, Sr, Ba, Pb), where a colossal magnetoresistance (CMR) effect has been reported [1-10]. The CMR effect in films of La0.67Ca0.33MnOz [2] and Nd0.vSr0.3MnOz [4] exceeds 99.9%, with the MR ratio defined here as [ R ( O ) - R ( H ) ] X 100/R(0), where R(0) is the resistance in zero field and R ( H ) is the resistance in a field. The parent insulator 3 1 (total spin R M n O 3 contains M n 3+ ions with a t2geg
* Correspondingauthor. Lawrence Berkeley laboratory, Materials Science Division, Berkeley, CA 94720, USA. Fax: + 1-510486-5888; email: honglyoul [email protected].
S = 2) electronic configuration. The substitution of R 3+ with a divalent element A 2+ results in a mixed valency of M n 3+ (t2geg. 3 1. S = 2) and Mn4+ (t32,: S = 3 / 2 ) ions. Their interaction is responsible for the metallic and ferromagnetic properties of R 1 _ x A x M n O 3 due to the 'double exchange (DE)' mechanism [11]. The concentration of M n 3+ and M n 4+ ions can be controlled by changing the A doping level or the oxygen stoichiometry [12]. One finds that the magnitude of the saturation magnetization ( M S) and the ferromagnetic transition temperature (Tc) depend on the A doping level, the oxygenation level, and the dopant used. The insulating x = 0 phase changes to a metallic and ferromagnetic phase for ~ 0 . 2 < x _ < ~ 0 . 5 [13,14]. The M s of as-prepared and oxygen reduced bulk samples is shown in
0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0304-8 853(96)00647-6
H.L. Ju, H. Sohn /Journal of Magnetism and Magnetic Materials 167 (1997) 200-208
Fig. 1, In the doping range 0.2 < x < 0.4, the as-prepared manganites have stoichiometric oxygen content (z = 3) [12,15] and a large M s close to the theoretical M s ( ~ 90 e m u / g ) for full alignment of the constituent Mn ions. That is M s = NglXBSa~ e, where N is the number of Mn ions per unit volume, g is the gyromagnetic ratio of 2, /x B is the Bohr magneton, and Save is the average spin of the constituent Mn 3+ (S = 2) and Mn 4+ (S = 3 / 2 ) . In this regime of composition one observes CMR effects for T ~ Tc. When the average oxidation state of the Mn ions is smaller than ~ 3.2, one sees from Fig. 1 that M~ is smaller than the theoretical M s. In this work, we focus on the magnetic properties of Mn oxides with a saturation magnetization less than that of the theoretical M s . Since an extremely large colossal magnetoresistance (CMR) effect has been o b s e r v e d in Nd0.7Sr0.3MnO ~ [4] and La0.67Ca0.33MnO z films [2], whose M s is smaller than the theoretical M s (or corresponding bulk Ms), and the transport and magnetic properties are intimately related to each other, it is important to understand the magnetic properties of samples with a relatively small M s. We have observed that thin film and bulk Mn oxides with a full theoretical magnetization show the typical magnetization behavior of a ferromagnet, i.e. a small and temperature independent coercivity and a low field magnetization with a sharp rise at Tc and a constant value below Tc. However, in samples where
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Fig. 1. The saturation magnetization vs. average manganese oxidation state for (C)) Lao.67Bao.33MnO~, ( 0 ) La l_xBaxMnO3 and ( A ) Lao.sCao.gMnOz). The bulk Laj _xBaxMnO3 data is taken from Ref. [ 13]. The solid line is the theoretical saturation magnetization (M S = Ngl~BSave) of La 1 xBaxMnO3.
201
the saturation magnetization is less than the full theoretical value, the magnetization behavior is anomalous and we find an enhancement and temperature dependence in the coercivity. In addition, we find evidence for spin freezing behavior as indicated by a peak in the zero field cooled (ZFC) magnetization. Our results are similar to those observed in materials with ferromagnetic grains in a non-ferromagnetic background, such as C o l _ x C u x or C o l _ x A g x [16,17]. Based on our experimental observations, we suggest the possible existence of ferromagnetic clusters in the manganites and discuss their effect on the CMR. 2.
Experimental
The bulk ceramic compounds were prepared using a conventional solid state reaction method. A stoichiometric amount of rare earth oxides (La203, Pr6Oll, Nd203) and carbonates (CaCO3, SrCO 3, BaCO3, MnCO 3) were mixed, ground and fired at 1100 to 1200°C repeatedly until reaching a single phase. The calcined materials were pressed into discs and fired at 1300 to 1450°C for 6 h to 3 days and then furnace cooled ( 2 0 0 - 3 0 0 ° C / h ) in air. The oxygen content was measured using the iodometric titration method [18]. Oxygen reduced La0.sCao.2MnO z samples were prepared by annealing the as-prepared La0.sCa0.zMnO3.01 at ~ 800°C in a N 2 atmosphere with a Ti getter. The grain size of La0.sCa0.zMnO3.01 was measured by wavelength dispersive spectroscopy (WDS) and found to be 10 _+ 2 IX. Thin films with thickness 200 to 500 nm were fabricated on (100) LaA103 single crystal substrates in 3 0 0 - 4 0 0 mTorr N 2 0 or 0 2 atmosphere using pulsed laser deposition. After the deposition the films were cooled to room temperature in 400 Torr O 2 atmosphere. The substrate temperatures we used are in the range of 600 to 700°C. Magnetic measurements were carried out with a SQUID magnetometer in fields of up to 5 T. For thin bulk samples the field was applied in the longest sample direction, and for films the field was applied along the film plane, to minimize the demagnetization field. In order to eliminate the background diamagnetic contribution from the LaA10 3 substrate, we measured the magnetization of the substrate with and without a film. The resistivity was measured by the usual four point
202
H.L. Ju, H. Sohn / Journal of Magnetism and Magnetic Materials 167 (1997) 200-208 100
method with the applied field perpendicular to the film plane and the current direction. The coercivity ( H c) was measured from the isothermal hysteresis curve ( - 1 T < H < I T) taking H~ as the field required to reduce the magnetization to zero. In the zero field cooled (ZFC) magnetization measurement, the samples were cooled to 5 K in zero field, and the magnetization was measured by warming the samples in an applied magnetic field. In the field cooled (FC) magnetization measurement, the samples were cooled to 5 K in the applied field and the magnetization was measured while warming the samples.
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T(K) Fig. 2. (a) Temperature dependence of the saturation magnetization for polycrystatline Lao.sCaonMnO z (2.96_< z -< 3.01). (b) Temperature dependence of the inverse susceptibility for polycrystalline Lao.aCao.2MnO z. Applied field is 1 T.
was measured in a field of 1 T (Fig. 2a) which is above the magnetic saturation field (2000-4000 G). T h e m a g n i t u d e o f the m a g n e t i z a t i o n for Lao.sCao.2MnO3.ol is 93 e m u / g at 5 K, i.e. essen-
H.L. Ju, H. Sohn / Journal of Magnetism and Magnetic Materials 167 (1997) 200-208
tially identical to the theoretical saturation magnetization. As seen in Fig. 2a, the magnitude of the saturation magnetization systematically decreases with increasing oxygen deficiency, as we found earlier for Lao.67Ba0.33MnO z [12]. Fig. 2b shows the temperature dependence of the inverse susceptibility for La0.sCa0.2MnO z. As seen in the figure, the susceptibility follows the Curie-Weiss law (X = C/(T 0)). From the Curie-Weiss fit, we find a Curie constant and Curie temperature for z = 2.96, 2.98 and 3.01 of 0.11 and 163 K, 0.12 and 176 K, and 0.14 and 212 K, respectively. Taking C = Ng2S(S + 1)/x2/3kB [19], S = Save, we find C = 0.06 K, which is about half of our observed value (0.11-0.14 K). The implications of this will be discussed in the next section. Fig. 3a shows the hysteresis loop for polycrystalline Lao.sCao.2MnO3.oa at 5 K. The saturation
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field for the sample is about ~ 2000 G, and above the magnetic saturation there is little change in the magnetization. The full theoretical M S of 93 e m u / g is achieved and a very small coercivity is observed ( H c = ~ 5 G). Fig. 3b shows the hysteresis loop for polycrystalline Lao.sCao. 2Mn2.96. The saturation field ( ~ 4000 G) for the sample is larger than that of Lao.sCao.2MnO3.0], and above the saturation there is a steady increase in the magnetization. This sample shows an enhanced coercivity ( H c = ~ 400 G) at 5 K. Fig. 3c summarizes the temperature dependence of the coercivity for bulk polycrystalline Lao.sCao.2MnOz. The parent Lao.sCao.2MnO3.ol has a very small, almost temperature independent coercivity ( ~ 5 G). With decreasing oxygen stoichiometry the magnitude of the coercivity increases and a temperature dependence appears. Fig. 4 shows the field dependence of the ZFC and FC magnetization for bulk Lao.sCao.2MnO3.01 and Lao.sCao.2MnO2:96.
H.L. Ju, H. Sohn / Journal of Magnetism and Magnetic Materials 167 (1997) 200-208
204
by changing the doping level. We find a similar magnetization behavior as found in our oxygen reduced manganites in manganite with reduced M S via A doping. Fig. 5a shows the temperature dependence of the ZFC magnetization for La0.67Ba0.33MnO2.99 which has the full theoretical M S. This sample also exhibits the typical ZFC magnetization behavior of a ferromagnet, i.e. a sharp increase at Tc and constant value below Tc. On the other hand, Fig. 5b shows the temperature dependence of the ZFC and FC magnetization for La0.95Bao.0sMnO3, whose saturation magnetization is ~ 40 e m u / g at 5 K. Here a spin freezing behavior is observed, as we found for oxygen depleted samples Fig. 6a shows the temperature dependence of the saturation magnetization for a Nd0.67 Sr0.33MnO3 bulk
As seen in Fig. 4a, the low field ZFC and FC magnetization of La0.sCao.2MnO3.01, which has full theoretical M s, sharply increases near Tc = 220 K. Below Tc the magnetization is more or less constant with the larger FC magnetization reflecting domain motion. Thus this sample shows the typical low field magnetization behavior of an ordinary ferromagnet. But, as seen in Fig. 4b, in the oxygen depleted La0.sCa0.2MnO2.96 with a reduced M s, the difference between the ZFC and FC magnetization increases with a decrease of the temperature. Moreover, the ZFC magnetization shows a peak, and below the peak the magnetization decreases with a decrease of the temperature, which suggests a spin freezing behavior. As seen in Fig. 1, it is also possible to reduce M s 100
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H.L. Ju, H. Sohn / Journal of Magnetism and Magnetic Materials 167 (1997) 200-208
sample (polycrystalline pellet) and a film (grown at a T~ of 600°C in 0 2 atmosphere and annealed in 1 atm 0 2 at 950°C for a day). The bulk Nd0.67Sr0.33MnO 3 has a large saturation magnetization of ~ 90 e m u / g at 5 K, while the Nd0.67Sr0.33MnO z film has a smaller magnetization of ~ 40 e m u / g at 5 K. The magnitude of the saturation magnetization for the bulk sample is close to the value for full ferromagnetic order of the constituent Mn 3+ and Mn 4+ ions. We found previously [12] that the magnitude of the magnetization was reduced from the full theoretical value in oxygen deficient polycrystalline La0.67Ba0,33MnO 3. Therefore, a smaller saturation magnetization for our Nd0.67Sr0.33MnO3 films suggests that the films have an oxygen deficiency. Fig. 6b shows the temperature dependence of the ZFC and FC magnetization for this film. Again the data suggest a spin freezing behavior which is characterized by a peak in the ZFC magnetization. Fig. 6c shows the temperature dependence of the coercivity ( H c) for a N d o . 6 7 S r o . 3 3 M n O z film. A sharp drop of Hc with increasing temperature is observed. The line in the figure represents a fit of the experimental data to the relation Ho=2K/Ms(1-(T/Tb) 1/2) [16,17]. The constants 2K/M s and Tb are found to be 1850 _+ 100 G and 25 -t- 2 K for Nd0.67Sr0.33MnO z. The relation for Ho is that used previously for superparamagnetic systems, where K is the anisotropy energy and Tb is the blocking temperature. We will discuss this further in the discussion section. Fig. 6d illustrates the typical temperature dependence of the resistivity for a similar film which shows a spin freezing behavior. Above the resistivity peak (Tp = 170 K) the resistivity fits the form p = Po exp(E/kBT), with an activation energy E = 53 _+ 3 meV at zero field. In an external magnetic field the resistivity near the peak is reduced, and the peak shifts to higher temperature, behavior typical of colossal magnetoresistance. At low temperature (T < 50 K), the temperature dependence of the resistivity is metallic, d p/dT > 0, but the resistivity of the film is very large ( ~ 40 mf~ cm at 4 K).
cient bulk samples and some R l _ x A x M n O 3 films is smaller than that of a fully spin polarized sample. For large x (x > 0.2), the experimental M~ of the as-prepared bulk materials is the same as the theoretical M s (=NgtzBSave). Traditionally, the reduced M S has been explained by the spin canting [11,20] of antiferromagnetic Mn sublattices (Fig. 7). The reduced M s may also be explained by a spin cluster model (Fig. 7) where local ferromagnetic regions exist. We have attempted to distinguish these two possibilities by low field magnetization and coercivity measurements. The spin canting model is expected to show the signature of an antiferromagnet in the paramagnetic susceptibility, i.e. X = C / ( T - O) with 0 = - 1 4 0 K s i n c e T N is = 1 4 0 K for x < 0 . 1 manganites [20]. However, we do not observe this antiferromagnetic signature for the manganites with reduced M s. In addition, when the M s is reduced from the full theoretical M s with doping or oxygen stoichiometry, we observe a spin freezing behavior as indicated by a peak in the ZFC magnetization, and an enhancement and unusual temperature dependence of the coercivity (Ho). We do not know any mechanism by which the spin canting model could give these magnetic behaviors. Since we would expect these behaviors in a spin cluster model, we believe that the spin cluster model is more applicable to these materials. The spin freezing behavior is often observed in inhomogeneous magnetic systems [16,17] such as granular films of Co-Cu, where ferromagnetic grains (grain size range 10-300 A) are embedded in a non-ferromagnetic background. The magnetohistory effects in these systems has been explained by a
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205
Ms = 0
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Our experimental results show that the saturation magnetization ( M s) of low x (x < 0.2), oxygen deft-
Fig. 7. Schematic diagram of the spin structure in the spin canting model and the spin cluster model.
206
H.L. Ju, H. Sohn / Journal of Magnetism and Magnetic Materials 167 (1997) 200-208
competition between positive and negative exchange interactions. The positive interaction comes from the ferromagnetic interaction between the grains while the negative interaction comes from the RKKY interaction or superexchange interaction between the grains. The competing interactions result in frustration of the clusters (ferromagnetic grains). As a result the clusters are frozen along a local easy direction below a certain temperature, i.e. the spin freezing temperature. Associated with the spin freezing behavior is a drop in the ZFC magnetization. This is manifested by a broad temperature dependent maximum in the ZFC magnetization. In addition to the spin freezing behavior, the coercivity of the manganites with reduced M S is enhanced from the low coercivity found in a fully oxygenated manganite with the full saturation magnetization. This enhancement in the coercivity is similar to that observed in inhomogeneous ferromagnetic systems [21,22]. The coercivity follows a temperature dependence 2K/Ms(1 -(T/Tb) 1/2) [16,17] (see Fig. 6c). This relation has been used to fit the temperature dependence of Ho for systems such as C o - C u and Co-Ag. Although the manganites with a small saturation magnetization show similar magnetic behavior to these systems, it is not certain that the properties originate from the same physical origin. It will require more detailed experimental work, such as inelastic neutron scattering, nuclear magnetic resonance, and the Mtissbauer effect in order to test this assumption. With this caveat in mind we assume here that the anomalous magnetic behavior in the manganites has the same physical origin, i.e. ferromagnetic grains exist in a nonferromagnetic background. It is not unreasonable that there might exist a local microscopic inhomogeneity in the dopant density and consequently a local variation of Mn oxidation. If the grains are ferromagnetic, they are composed of pairs of Mn 3+ and Mn 4+ ions interacting via the double exchange mechanism. The nonferromagnetic background could be composed of only Mn 3+ ions. The interaction between the grains is ferromagnetic and that between Mn 3+ ions is antiferromagnetic. These competing interactions may result in frustration. A similar conclusion about the existence of spin clusters in the manganites has been reached from other experiments [23], namely the field dependence of the
thermopower and the resistivity in samples with a small saturation magnetization. We attempted to estimate the size of the clusters from the temperature dependence of H c. For a uniaxial single domain particle the coercive field depends on temperature according to the law [15,16] Hc = (2K/Ms)(1 - ( T / T b ) I / 2 ) , where T b is the blocking temperature and M s is the saturation magnetization of the ferromagnetic grains. The blocking temperature Tb refers to the temperature above which the thermal energy overcomes the magnetic anisotropy energy of the single domain particles. From the fit in Fig. 6c we find a K of 5.5X105 e r g / c m 3 for Nd0.67Sr0.33MnOz which is close to the value (2-4 x 105 e r g / c m 3) estimated from ferromagnetic resonance experiments [24] in a similar Lao.67 Bao,33 MnO z film. The Tb can be written as KV/(25kB), where V is the grain volume. From this we estimate a ferromagnetic cluster diameter of 70 A for Nd0.67Sr0.33MnO z from taking V = ( 3 " r r / 4 ) ( d / 2 ) 3, where d is the diameter of the spherical grains (We do not have any direct information on the shape of the grains and for simplicity assume a spherical shape.) For a typical ferromagnet we know that long-range ferromagnetic order disappears at Tc and short-range magnetic order exists above Tc. As a result, the Curie constant C is enhanced due to the short-range order of the spins for T just above Tc, and C goes to the spin only value for T = Tc + ~ 150 K [10]. For the Mn oxides, we found that C is enhanced by a factor of 2 from the spin only value for T just above Tc. This suggests that small ferromagnetic regions exist even when T is just above Tc for the manganites. Outside the ferromagnetic grains are most probably Mn 3÷ ions with random spin directions. Above Tc, the carrier transfer from one grain to another is greatly restricted due to the randomized Mn 3+ spin between the grains (transfer integral t ~ cos(0/2) where 0 is the angle between Mn ions). As a result the carriers become immobile and stay inside the grains. When the sample is cooled below Tc the carriers are delocalized and all the Mn ions share the carriers and have effectively the same average oxidation states. The spin cluster for T > Tc may be identical to the magnetic polaron which has been assumed to exist for a long time. An applied magnetic field ( H ) suppresses the carrier localization by
H.L. Ju, H. Sohn /Journal of Magnetism and Magnetic Materials 167 (1997) 200-208
aligning the magnetic moments, thereby causing a large drop in the electrical resistivity, i.e. CMR. Although the MR is dominant near Tc, there exists a significant MR effect above Tc up to Tc + ~ 100 K, which is consistent with the idea of local ferromagnetic regions. In the samples where the saturation magnetization is reduced from the fully aligned value our results suggest that ferromagnetic grains exist below Tc. The smaller M s suggests that the ferromagnetic interaction between the grains is reduced and, therefore, Tc and the resistivity peak temperature decrease. As a result, the magnetic polaron can exist down to a much lower temperature and a large peak resistivity value can occur. Therefore, with a decrease of M s, an enhanced MR effect is expected since the zero field resistivity increases with decreasing M s, and the high field resistivity value remains more or less constant [6,9]. The fully oxygenated single crystals and films with theoretical M s do not exhibit any significant MR at low temperature (T < Tc/5) and reach a resistivity of 0.1 mf~ cm at low temperature. However, the manganites with reduced M s show significant MR at low temperature (50% of MR at 8 T of the film in Fig. 6d) and have a rather large resistivity magnitude. We believe these differences in the transport properties between manganites with full M s and reduced M s are due to magnetic inhomogeneity. However, it will require more detailed experimental and theoretical work to understand fully the correlation between magnetic and transport phenomena in these materials.
5. Conclusions We have measured the low field magnetization and the coercivity of the manganites Rl_xAxMnO 3. The manganites with a full saturation magnetization show similar magnetization behavior to that found in ordinary ferromagnets, i.e. small and temperature independent coercivity and a sharp rise in the low field magnetization at Tc and a constant value below Tc. The manganites with a small saturation magnetization show anomalous magnetic behavior, i.e. a spin freezing behavior, which is indicated by a peak in the zero field cooled (ZFC) magnetization and an
enhancement dence of the magnetization magnetic spin matrix.
207
and anomalous temperature depencoercivity. We interpret the reduced to result from the existence of ferroclusters embedded in a paramagnetic
Acknowledgements The authors thank NSF-DMR 9510474 for partial support of this work. The authors also thank Drs. A.K. Raychaudhuri and T. Venkatesan, and R.L. Greene, R. Ramesh, S. Bhagat, G.C. Xiong, T. Clinton, and Qi Li for useful discussions.
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[18] H. Kolthoff and X. Belcher, Volumetric Analysis (Interscience, New York, 1957). [19] C. Kittel, Introduction to Solid State Physics (Wiley, New York, t986). [20] G. Matsumoto, J. Phys. Soc. Jpn. 29 (1970) 615. [21] C.L. Chien, J.Q. Xiao and J.S. Jiang, J. Appl. Phys. 73 (1993) 5309.
[22] G. Xiao, S.H. Liou, A. Leu, J.N. Taylor and C.L. Chen, Phys. Rev. B 34 (1986) 7573. [23] M. Jaime, M.B. Salamon, K. Pettit, M. Rubinstein, R.E. Treece, J.S. Horwitz and D.B. Chrisey, unpublished. [24] S.E. Lofland, S.M. Bhagat, H.L. Ju, G.C. Xiong, T. Venkatesan and R.L. Greene, Phys. Rev. B 52 (1995) 11508.
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journal of magnetism and magnetic materials
Journal of Magnetismand Magnetic Materials 167 (1997) 200-208
Magnetic inhomogeneity and colossal magnetoresistance in manganese oxides H.L. Ju a,b,*, Hyunchul Sohn b Center for Superconductivity Research, Department of Physics, University of Maryland, College Park, MD 20742, USA b Lawrence Berkeley Laboratory, Materials Science Division, Berkeley, CA 94720, USA
Received 19 August 1996
Abstract The magnetic and magnetotransport properties of the manganites, R 1_~AxMnOz (R = La, Pr, Nd; A = Ca, Sr, Ba), are strongly dependent on the doping level x and the oxygen stoichiometry z. For x > ~ 0.2, stoichiometric R 1_xAxMnO3 was found to have a small temperature independent coercivity of 5-30 Oe and the saturation magnetization ( ~ 90 emu/g) of fully aligned Mn ions. Oxygen deficient (or x < 0.2) R 1_xAxMnOz has a saturation magnetization smaller than that of fully aligned Mn spins and shows an enhancement and temperature dependence of the coercivity, and spin freezing behavior. The latter is characterized by a peak in zero field cooled (ZFC) magnetization. Our data suggest that oxygen deficient or low doped manganites are magnetically inhomogeneous. We discuss possible origins of the magnetic inhomogeneity and its influence on the colossal magnetoresistance. Keywords: Manganeseoxide; Magnetoresistance;Polaron; Magnetic inhomogeneity;Perovskite structure
1. I n t r o d u c t i o n Recently, there have been many studies of the manganese oxides, R l _ x A x M n O 3 (R = La, Pr, Nd; A = Ca, Sr, Ba, Pb), where a colossal magnetoresistance (CMR) effect has been reported [1-10]. The CMR effect in films of La0.67Ca0.33MnOz [2] and Nd0.vSr0.3MnOz [4] exceeds 99.9%, with the MR ratio defined here as [ R ( O ) - R ( H ) ] X 100/R(0), where R(0) is the resistance in zero field and R ( H ) is the resistance in a field. The parent insulator 3 1 (total spin R M n O 3 contains M n 3+ ions with a t2geg
* Correspondingauthor. Lawrence Berkeley laboratory, Materials Science Division, Berkeley, CA 94720, USA. Fax: + 1-510486-5888; email: honglyoul [email protected].
S = 2) electronic configuration. The substitution of R 3+ with a divalent element A 2+ results in a mixed valency of M n 3+ (t2geg. 3 1. S = 2) and Mn4+ (t32,: S = 3 / 2 ) ions. Their interaction is responsible for the metallic and ferromagnetic properties of R 1 _ x A x M n O 3 due to the 'double exchange (DE)' mechanism [11]. The concentration of M n 3+ and M n 4+ ions can be controlled by changing the A doping level or the oxygen stoichiometry [12]. One finds that the magnitude of the saturation magnetization ( M S) and the ferromagnetic transition temperature (Tc) depend on the A doping level, the oxygenation level, and the dopant used. The insulating x = 0 phase changes to a metallic and ferromagnetic phase for ~ 0 . 2 < x _ < ~ 0 . 5 [13,14]. The M s of as-prepared and oxygen reduced bulk samples is shown in
0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0304-8 853(96)00647-6
H.L. Ju, H. Sohn /Journal of Magnetism and Magnetic Materials 167 (1997) 200-208
Fig. 1, In the doping range 0.2 < x < 0.4, the as-prepared manganites have stoichiometric oxygen content (z = 3) [12,15] and a large M s close to the theoretical M s ( ~ 90 e m u / g ) for full alignment of the constituent Mn ions. That is M s = NglXBSa~ e, where N is the number of Mn ions per unit volume, g is the gyromagnetic ratio of 2, /x B is the Bohr magneton, and Save is the average spin of the constituent Mn 3+ (S = 2) and Mn 4+ (S = 3 / 2 ) . In this regime of composition one observes CMR effects for T ~ Tc. When the average oxidation state of the Mn ions is smaller than ~ 3.2, one sees from Fig. 1 that M~ is smaller than the theoretical M s. In this work, we focus on the magnetic properties of Mn oxides with a saturation magnetization less than that of the theoretical M s . Since an extremely large colossal magnetoresistance (CMR) effect has been o b s e r v e d in Nd0.7Sr0.3MnO ~ [4] and La0.67Ca0.33MnO z films [2], whose M s is smaller than the theoretical M s (or corresponding bulk Ms), and the transport and magnetic properties are intimately related to each other, it is important to understand the magnetic properties of samples with a relatively small M s. We have observed that thin film and bulk Mn oxides with a full theoretical magnetization show the typical magnetization behavior of a ferromagnet, i.e. a small and temperature independent coercivity and a low field magnetization with a sharp rise at Tc and a constant value below Tc. However, in samples where
,~
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Fig. 1. The saturation magnetization vs. average manganese oxidation state for (C)) Lao.67Bao.33MnO~, ( 0 ) La l_xBaxMnO3 and ( A ) Lao.sCao.gMnOz). The bulk Laj _xBaxMnO3 data is taken from Ref. [ 13]. The solid line is the theoretical saturation magnetization (M S = Ngl~BSave) of La 1 xBaxMnO3.
201
the saturation magnetization is less than the full theoretical value, the magnetization behavior is anomalous and we find an enhancement and temperature dependence in the coercivity. In addition, we find evidence for spin freezing behavior as indicated by a peak in the zero field cooled (ZFC) magnetization. Our results are similar to those observed in materials with ferromagnetic grains in a non-ferromagnetic background, such as C o l _ x C u x or C o l _ x A g x [16,17]. Based on our experimental observations, we suggest the possible existence of ferromagnetic clusters in the manganites and discuss their effect on the CMR. 2.
Experimental
The bulk ceramic compounds were prepared using a conventional solid state reaction method. A stoichiometric amount of rare earth oxides (La203, Pr6Oll, Nd203) and carbonates (CaCO3, SrCO 3, BaCO3, MnCO 3) were mixed, ground and fired at 1100 to 1200°C repeatedly until reaching a single phase. The calcined materials were pressed into discs and fired at 1300 to 1450°C for 6 h to 3 days and then furnace cooled ( 2 0 0 - 3 0 0 ° C / h ) in air. The oxygen content was measured using the iodometric titration method [18]. Oxygen reduced La0.sCao.2MnO z samples were prepared by annealing the as-prepared La0.sCa0.zMnO3.01 at ~ 800°C in a N 2 atmosphere with a Ti getter. The grain size of La0.sCa0.zMnO3.01 was measured by wavelength dispersive spectroscopy (WDS) and found to be 10 _+ 2 IX. Thin films with thickness 200 to 500 nm were fabricated on (100) LaA103 single crystal substrates in 3 0 0 - 4 0 0 mTorr N 2 0 or 0 2 atmosphere using pulsed laser deposition. After the deposition the films were cooled to room temperature in 400 Torr O 2 atmosphere. The substrate temperatures we used are in the range of 600 to 700°C. Magnetic measurements were carried out with a SQUID magnetometer in fields of up to 5 T. For thin bulk samples the field was applied in the longest sample direction, and for films the field was applied along the film plane, to minimize the demagnetization field. In order to eliminate the background diamagnetic contribution from the LaA10 3 substrate, we measured the magnetization of the substrate with and without a film. The resistivity was measured by the usual four point
202
H.L. Ju, H. Sohn / Journal of Magnetism and Magnetic Materials 167 (1997) 200-208 100
method with the applied field perpendicular to the film plane and the current direction. The coercivity ( H c) was measured from the isothermal hysteresis curve ( - 1 T < H < I T) taking H~ as the field required to reduce the magnetization to zero. In the zero field cooled (ZFC) magnetization measurement, the samples were cooled to 5 K in zero field, and the magnetization was measured by warming the samples in an applied magnetic field. In the field cooled (FC) magnetization measurement, the samples were cooled to 5 K in the applied field and the magnetization was measured while warming the samples.
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T(K) Fig. 2. (a) Temperature dependence of the saturation magnetization for polycrystatline Lao.sCaonMnO z (2.96_< z -< 3.01). (b) Temperature dependence of the inverse susceptibility for polycrystalline Lao.aCao.2MnO z. Applied field is 1 T.
was measured in a field of 1 T (Fig. 2a) which is above the magnetic saturation field (2000-4000 G). T h e m a g n i t u d e o f the m a g n e t i z a t i o n for Lao.sCao.2MnO3.ol is 93 e m u / g at 5 K, i.e. essen-
H.L. Ju, H. Sohn / Journal of Magnetism and Magnetic Materials 167 (1997) 200-208
tially identical to the theoretical saturation magnetization. As seen in Fig. 2a, the magnitude of the saturation magnetization systematically decreases with increasing oxygen deficiency, as we found earlier for Lao.67Ba0.33MnO z [12]. Fig. 2b shows the temperature dependence of the inverse susceptibility for La0.sCa0.2MnO z. As seen in the figure, the susceptibility follows the Curie-Weiss law (X = C/(T 0)). From the Curie-Weiss fit, we find a Curie constant and Curie temperature for z = 2.96, 2.98 and 3.01 of 0.11 and 163 K, 0.12 and 176 K, and 0.14 and 212 K, respectively. Taking C = Ng2S(S + 1)/x2/3kB [19], S = Save, we find C = 0.06 K, which is about half of our observed value (0.11-0.14 K). The implications of this will be discussed in the next section. Fig. 3a shows the hysteresis loop for polycrystalline Lao.sCao.2MnO3.oa at 5 K. The saturation
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field for the sample is about ~ 2000 G, and above the magnetic saturation there is little change in the magnetization. The full theoretical M S of 93 e m u / g is achieved and a very small coercivity is observed ( H c = ~ 5 G). Fig. 3b shows the hysteresis loop for polycrystalline Lao.sCao. 2Mn2.96. The saturation field ( ~ 4000 G) for the sample is larger than that of Lao.sCao.2MnO3.0], and above the saturation there is a steady increase in the magnetization. This sample shows an enhanced coercivity ( H c = ~ 400 G) at 5 K. Fig. 3c summarizes the temperature dependence of the coercivity for bulk polycrystalline Lao.sCao.2MnOz. The parent Lao.sCao.2MnO3.ol has a very small, almost temperature independent coercivity ( ~ 5 G). With decreasing oxygen stoichiometry the magnitude of the coercivity increases and a temperature dependence appears. Fig. 4 shows the field dependence of the ZFC and FC magnetization for bulk Lao.sCao.2MnO3.01 and Lao.sCao.2MnO2:96.
H.L. Ju, H. Sohn / Journal of Magnetism and Magnetic Materials 167 (1997) 200-208
204
by changing the doping level. We find a similar magnetization behavior as found in our oxygen reduced manganites in manganite with reduced M S via A doping. Fig. 5a shows the temperature dependence of the ZFC magnetization for La0.67Ba0.33MnO2.99 which has the full theoretical M S. This sample also exhibits the typical ZFC magnetization behavior of a ferromagnet, i.e. a sharp increase at Tc and constant value below Tc. On the other hand, Fig. 5b shows the temperature dependence of the ZFC and FC magnetization for La0.95Bao.0sMnO3, whose saturation magnetization is ~ 40 e m u / g at 5 K. Here a spin freezing behavior is observed, as we found for oxygen depleted samples Fig. 6a shows the temperature dependence of the saturation magnetization for a Nd0.67 Sr0.33MnO3 bulk
As seen in Fig. 4a, the low field ZFC and FC magnetization of La0.sCao.2MnO3.01, which has full theoretical M s, sharply increases near Tc = 220 K. Below Tc the magnetization is more or less constant with the larger FC magnetization reflecting domain motion. Thus this sample shows the typical low field magnetization behavior of an ordinary ferromagnet. But, as seen in Fig. 4b, in the oxygen depleted La0.sCa0.2MnO2.96 with a reduced M s, the difference between the ZFC and FC magnetization increases with a decrease of the temperature. Moreover, the ZFC magnetization shows a peak, and below the peak the magnetization decreases with a decrease of the temperature, which suggests a spin freezing behavior. As seen in Fig. 1, it is also possible to reduce M s 100
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H.L. Ju, H. Sohn / Journal of Magnetism and Magnetic Materials 167 (1997) 200-208
sample (polycrystalline pellet) and a film (grown at a T~ of 600°C in 0 2 atmosphere and annealed in 1 atm 0 2 at 950°C for a day). The bulk Nd0.67Sr0.33MnO 3 has a large saturation magnetization of ~ 90 e m u / g at 5 K, while the Nd0.67Sr0.33MnO z film has a smaller magnetization of ~ 40 e m u / g at 5 K. The magnitude of the saturation magnetization for the bulk sample is close to the value for full ferromagnetic order of the constituent Mn 3+ and Mn 4+ ions. We found previously [12] that the magnitude of the magnetization was reduced from the full theoretical value in oxygen deficient polycrystalline La0.67Ba0,33MnO 3. Therefore, a smaller saturation magnetization for our Nd0.67Sr0.33MnO3 films suggests that the films have an oxygen deficiency. Fig. 6b shows the temperature dependence of the ZFC and FC magnetization for this film. Again the data suggest a spin freezing behavior which is characterized by a peak in the ZFC magnetization. Fig. 6c shows the temperature dependence of the coercivity ( H c) for a N d o . 6 7 S r o . 3 3 M n O z film. A sharp drop of Hc with increasing temperature is observed. The line in the figure represents a fit of the experimental data to the relation Ho=2K/Ms(1-(T/Tb) 1/2) [16,17]. The constants 2K/M s and Tb are found to be 1850 _+ 100 G and 25 -t- 2 K for Nd0.67Sr0.33MnO z. The relation for Ho is that used previously for superparamagnetic systems, where K is the anisotropy energy and Tb is the blocking temperature. We will discuss this further in the discussion section. Fig. 6d illustrates the typical temperature dependence of the resistivity for a similar film which shows a spin freezing behavior. Above the resistivity peak (Tp = 170 K) the resistivity fits the form p = Po exp(E/kBT), with an activation energy E = 53 _+ 3 meV at zero field. In an external magnetic field the resistivity near the peak is reduced, and the peak shifts to higher temperature, behavior typical of colossal magnetoresistance. At low temperature (T < 50 K), the temperature dependence of the resistivity is metallic, d p/dT > 0, but the resistivity of the film is very large ( ~ 40 mf~ cm at 4 K).
cient bulk samples and some R l _ x A x M n O 3 films is smaller than that of a fully spin polarized sample. For large x (x > 0.2), the experimental M~ of the as-prepared bulk materials is the same as the theoretical M s (=NgtzBSave). Traditionally, the reduced M S has been explained by the spin canting [11,20] of antiferromagnetic Mn sublattices (Fig. 7). The reduced M s may also be explained by a spin cluster model (Fig. 7) where local ferromagnetic regions exist. We have attempted to distinguish these two possibilities by low field magnetization and coercivity measurements. The spin canting model is expected to show the signature of an antiferromagnet in the paramagnetic susceptibility, i.e. X = C / ( T - O) with 0 = - 1 4 0 K s i n c e T N is = 1 4 0 K for x < 0 . 1 manganites [20]. However, we do not observe this antiferromagnetic signature for the manganites with reduced M s. In addition, when the M s is reduced from the full theoretical M s with doping or oxygen stoichiometry, we observe a spin freezing behavior as indicated by a peak in the ZFC magnetization, and an enhancement and unusual temperature dependence of the coercivity (Ho). We do not know any mechanism by which the spin canting model could give these magnetic behaviors. Since we would expect these behaviors in a spin cluster model, we believe that the spin cluster model is more applicable to these materials. The spin freezing behavior is often observed in inhomogeneous magnetic systems [16,17] such as granular films of Co-Cu, where ferromagnetic grains (grain size range 10-300 A) are embedded in a non-ferromagnetic background. The magnetohistory effects in these systems has been explained by a
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205
Ms = 0
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Our experimental results show that the saturation magnetization ( M s) of low x (x < 0.2), oxygen deft-
Fig. 7. Schematic diagram of the spin structure in the spin canting model and the spin cluster model.
206
H.L. Ju, H. Sohn / Journal of Magnetism and Magnetic Materials 167 (1997) 200-208
competition between positive and negative exchange interactions. The positive interaction comes from the ferromagnetic interaction between the grains while the negative interaction comes from the RKKY interaction or superexchange interaction between the grains. The competing interactions result in frustration of the clusters (ferromagnetic grains). As a result the clusters are frozen along a local easy direction below a certain temperature, i.e. the spin freezing temperature. Associated with the spin freezing behavior is a drop in the ZFC magnetization. This is manifested by a broad temperature dependent maximum in the ZFC magnetization. In addition to the spin freezing behavior, the coercivity of the manganites with reduced M S is enhanced from the low coercivity found in a fully oxygenated manganite with the full saturation magnetization. This enhancement in the coercivity is similar to that observed in inhomogeneous ferromagnetic systems [21,22]. The coercivity follows a temperature dependence 2K/Ms(1 -(T/Tb) 1/2) [16,17] (see Fig. 6c). This relation has been used to fit the temperature dependence of Ho for systems such as C o - C u and Co-Ag. Although the manganites with a small saturation magnetization show similar magnetic behavior to these systems, it is not certain that the properties originate from the same physical origin. It will require more detailed experimental work, such as inelastic neutron scattering, nuclear magnetic resonance, and the Mtissbauer effect in order to test this assumption. With this caveat in mind we assume here that the anomalous magnetic behavior in the manganites has the same physical origin, i.e. ferromagnetic grains exist in a nonferromagnetic background. It is not unreasonable that there might exist a local microscopic inhomogeneity in the dopant density and consequently a local variation of Mn oxidation. If the grains are ferromagnetic, they are composed of pairs of Mn 3+ and Mn 4+ ions interacting via the double exchange mechanism. The nonferromagnetic background could be composed of only Mn 3+ ions. The interaction between the grains is ferromagnetic and that between Mn 3+ ions is antiferromagnetic. These competing interactions may result in frustration. A similar conclusion about the existence of spin clusters in the manganites has been reached from other experiments [23], namely the field dependence of the
thermopower and the resistivity in samples with a small saturation magnetization. We attempted to estimate the size of the clusters from the temperature dependence of H c. For a uniaxial single domain particle the coercive field depends on temperature according to the law [15,16] Hc = (2K/Ms)(1 - ( T / T b ) I / 2 ) , where T b is the blocking temperature and M s is the saturation magnetization of the ferromagnetic grains. The blocking temperature Tb refers to the temperature above which the thermal energy overcomes the magnetic anisotropy energy of the single domain particles. From the fit in Fig. 6c we find a K of 5.5X105 e r g / c m 3 for Nd0.67Sr0.33MnOz which is close to the value (2-4 x 105 e r g / c m 3) estimated from ferromagnetic resonance experiments [24] in a similar Lao.67 Bao,33 MnO z film. The Tb can be written as KV/(25kB), where V is the grain volume. From this we estimate a ferromagnetic cluster diameter of 70 A for Nd0.67Sr0.33MnO z from taking V = ( 3 " r r / 4 ) ( d / 2 ) 3, where d is the diameter of the spherical grains (We do not have any direct information on the shape of the grains and for simplicity assume a spherical shape.) For a typical ferromagnet we know that long-range ferromagnetic order disappears at Tc and short-range magnetic order exists above Tc. As a result, the Curie constant C is enhanced due to the short-range order of the spins for T just above Tc, and C goes to the spin only value for T = Tc + ~ 150 K [10]. For the Mn oxides, we found that C is enhanced by a factor of 2 from the spin only value for T just above Tc. This suggests that small ferromagnetic regions exist even when T is just above Tc for the manganites. Outside the ferromagnetic grains are most probably Mn 3÷ ions with random spin directions. Above Tc, the carrier transfer from one grain to another is greatly restricted due to the randomized Mn 3+ spin between the grains (transfer integral t ~ cos(0/2) where 0 is the angle between Mn ions). As a result the carriers become immobile and stay inside the grains. When the sample is cooled below Tc the carriers are delocalized and all the Mn ions share the carriers and have effectively the same average oxidation states. The spin cluster for T > Tc may be identical to the magnetic polaron which has been assumed to exist for a long time. An applied magnetic field ( H ) suppresses the carrier localization by
H.L. Ju, H. Sohn /Journal of Magnetism and Magnetic Materials 167 (1997) 200-208
aligning the magnetic moments, thereby causing a large drop in the electrical resistivity, i.e. CMR. Although the MR is dominant near Tc, there exists a significant MR effect above Tc up to Tc + ~ 100 K, which is consistent with the idea of local ferromagnetic regions. In the samples where the saturation magnetization is reduced from the fully aligned value our results suggest that ferromagnetic grains exist below Tc. The smaller M s suggests that the ferromagnetic interaction between the grains is reduced and, therefore, Tc and the resistivity peak temperature decrease. As a result, the magnetic polaron can exist down to a much lower temperature and a large peak resistivity value can occur. Therefore, with a decrease of M s, an enhanced MR effect is expected since the zero field resistivity increases with decreasing M s, and the high field resistivity value remains more or less constant [6,9]. The fully oxygenated single crystals and films with theoretical M s do not exhibit any significant MR at low temperature (T < Tc/5) and reach a resistivity of 0.1 mf~ cm at low temperature. However, the manganites with reduced M s show significant MR at low temperature (50% of MR at 8 T of the film in Fig. 6d) and have a rather large resistivity magnitude. We believe these differences in the transport properties between manganites with full M s and reduced M s are due to magnetic inhomogeneity. However, it will require more detailed experimental and theoretical work to understand fully the correlation between magnetic and transport phenomena in these materials.
5. Conclusions We have measured the low field magnetization and the coercivity of the manganites Rl_xAxMnO 3. The manganites with a full saturation magnetization show similar magnetization behavior to that found in ordinary ferromagnets, i.e. small and temperature independent coercivity and a sharp rise in the low field magnetization at Tc and a constant value below Tc. The manganites with a small saturation magnetization show anomalous magnetic behavior, i.e. a spin freezing behavior, which is indicated by a peak in the zero field cooled (ZFC) magnetization and an
enhancement dence of the magnetization magnetic spin matrix.
207
and anomalous temperature depencoercivity. We interpret the reduced to result from the existence of ferroclusters embedded in a paramagnetic
Acknowledgements The authors thank NSF-DMR 9510474 for partial support of this work. The authors also thank Drs. A.K. Raychaudhuri and T. Venkatesan, and R.L. Greene, R. Ramesh, S. Bhagat, G.C. Xiong, T. Clinton, and Qi Li for useful discussions.
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