LaFeO3 artificial lattices

PERGAMON

Solid State Communications 112 (1999) 201–205 www.elsevier.com/locate/ssc

Enhancement of magnetoresistance in spin frustrated (La,Sr)MnO3/LaFeO3 artificial lattices H. Tanaka*, T. Kawai Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Received 20 May 1999; accepted 1 July 1999 by C.N.R. Rao

Abstract We have formed ferromagnetic-[(La 0.8Sr0.2)MnO3]m/antiferromagnetic-[LaFeO 3]n superlattices with a stacking periodicity of 1 # m # 10 and 1 # n # 3 unit cells. It has been observed that resistivity is increased and Curie temperature is suppressed from 250 to 130 K by increasing the antiferromagnetic layer. Metal–insulator transition systematically occurred in the series of the superlattices …m ˆ 2† with increasing n from 1 to 3 unit cells. It is explained that spin fluctuation is induced by neighboring antiferromagnetic spin order. Their colossal magneto-resistance (CMR) effect is enhanced by up to 35% at the magnetic field of 1.0 T. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Magnetic films and multilayers; B. Epitaxy; D. Electronic transport; D. Exchange and superexchange

1. Introduction Recently, colossal magneto-resistance (CMR) on manganese perovskite oxides is attracting attention [1–9]. This phenomenon is not only interesting as science but also important for industrial applications. The magnetic and electrical properties have been explained within the framework of double exchange theory [10], which considers the transfer of electrons between neighboring Mn 31 –M 41 ions through Mn–O–Mn. The eg electron is hopping with 3 coupling with the localized t2g …St2g ˆ 3=2† spins ferromagnetically via strong Hund coupling. Strong magnetic field makes the localized spin aligned so that eg electron hopping is enhanced, and resistivity becomes small. Recently, Millis et al. [11,12] pointed out the importance of Jahn–Teller magnetic polaron, which enhances resistivity above TC in addition to double exchange mechanism. To control their magnetic and electrical transport properties, a lot of studies have been carried out in the R12x Ax MnO3 (R ˆ La; Nd, etc. A ˆ Ba; Sr, Ca, etc.) by applying high pressure [4–7], * Corresponding author. Tel.: 1 81-6-68798446; fax: 1 81-668752440. E-mail address: [email protected] (H. Tanaka)

cation [8], preparing films strained by substrate [9] to control electron transfer, via distorting the structure of MnO6 octahedron related to these theories. One novel way to obtain new insight and to control spin ordered state is the magnetic superlattice taking advantage of magnetic exchange interactions [13]. We propose and fabricate the new CMR superlattices to obtain higher MRratio on the basis of localized spin control by neighboring magnetic layer. The antiferromagnetic superexchange interaction competes with that of ferromagnetic double exchange. In the superlattices consisting of ferromagnetic (La,Sr)MnO3 and antiferromagnetic transition metal oxide, it is expected that the two interactions compete with each other via interface exchange interaction. If the magnitude of ferromagnetic double exchange and antiferromagnetic superexchange interactions is in the same level, spin alignment in (La,Sr)MnO3 layer is frustrated and readily controlled by weak magnetic field. This “spin frustrated magnetic superlattice” is the basic concept for the present CMR materials. LaFeO3 is a typical perovskite-type antiferromagnet with Ne´el temperature of 738 K. In this paper, we form spin frustrated [(La0.8Sr0.2)MnO3]m/[LaFeO3]n superlattices and demonstrate the control of the magnetic, electrical and CMR properties in the superlattices.

0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00331-2

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and resistivity–temperature …R–T† measurement by use of a four probe method under the magnetic field of 1.0 T.

3. Results and discussion

Fig. 1. (a) The RHEED intensity oscillation of the specular beam observed k100l azimuth during the growth of the [(La0.8Sr0.2)MnO3]4/[LaFeO3]2 superlattice. (b) The cross-sectional HRTEM image of the [(La0.8Sr0.2)MnO3]4/[LaFeO3]2 superlattice along the [001] direction.

2. Experimental [(La,Sr)MnO3]m/[LaFeO3]n superlattices were fabricated by a Laser Molecular Beam Epitaxy technique. The targets used were sintered La0.8Sr0.2MnO3 (LSMO) and LaFeO3 (LFO) pellets. The pulsed laser beam (ArF excimer laser: l ˆ 193 nm) was used for ablation. The LSMO and LFO were stacked alternately on the SrTiO3(001) single crystal substrate along k001l direction in NO2 ambient with a pressure of 1:0 × 1025 mbar; with in situ monitoring reflection high energy electron diffraction (RHEED). The films were formed at a substrate temperature of 5808C. After film formation, the substrate temperature was kept for 30 min in NO2 pressure of 1:0 × 1022 mbar: The superlattice structures were confirmed by X-ray diffraction and crosssectional high resolution transmission electron microscopy (HRTEM). Hereafter, [LSMO]m/[LFO]n superlattices are referred to as (m,n) superlattices. The numbers of each layer (m for LSMO and n for LFO) are changed in the range from 10 to 1 perovskite unit cells. The total thickness ˚ . Their magnetic, of the LSMO layer was fixed to 500 A electrical properties and CMR effect were measured by magnetization–temperature …M–T† and magnetization– magnetic field …M–H† measurements with the superconducting quantum interference device (SQUID) magnetometer,

Fig. 1 shows (a) the RHEED intensity oscillation and (b) the cross-sectional HRTEM image for (4,2) superlattice. The RHEED intensity oscillation indicates layer-by-layer growth with keeping an atomically flat surface. By in situ monitoring the number of these oscillations, we have achieved to control a number of each of the LSMO and the LFO layers accurately. In the HRTEM image, in spite of the difficulty in discriminating between the 3d transition metal ions that have almost the same scattering factor like Mn and Fe, we have observed the superlattice spot that came from the designated period of six perovskite units (four LSMO and two LFO units) and no defect occurs at the interface. For these good quality superlattice samples, we have measured the magnetic/electrical properties. Fig. 2 shows temperature dependence of magnetization and resistivity for the LSMO film, the LFO film and the (m,n) superlattices. The LSMO film shows ferromagnetism with a Curie temperature (TC) of 270 K same as that of the bulk materials. The LFO film shows little magnetization value because it is an antiferromagnet. The (m,1) superlattices with fixed number of LFO layers show ferromagnetism having almost the same Curie temperature of 250 K except the (1,1) superlattice as shown in Fig. 2(a). The (1,1) superlattice shows ferromagnetism with reduced a Curie temperature of 160 K. The reduction of TC can be interpreted by the competition between ferromagnetic and antiferromagnetic interactions [13]. The …m $ 2; 1† superlattices with bulk-like TC show metallic conductivity below TC as shown in Fig. 2(b). The (2,1) superlattice exhibits high resistivity and larger MR ratio (35%) than the LSMO film (18%) at 1.0 T. To confirm the antiferromagnetic layer effect in detail, the [LSMO]2/ [LFO]n superlattices with fixed number of LSMO have been investigated by changing the number of LFO layers …n ˆ 1–3†: Their TC is suppressed from 270 to 160 K with increasing LFO layer as shown in Fig. 2(c). This suppression is drastically observed in the electrical transport properties. As shown in Fig. 2(d), in the temperature dependence of resistivity, metal to insulator transition systematically occurs with increasing antiferromagnetic LFO layers …n ˆ 1–3†: Another evidence is observed for the coupling between ferromagnetic and antiferromagnetic layers in M–H curves as shown in the inset in Fig. 2(a) and (c). The (5,1), (3,1), (2,1), (2,2) superlattices with bulk-like TC have large saturated magnetization of ,3.2m B per LSMO unit same as single phase LSMO films corresponding to the full spin moment of LSMO …S ˆ 2 for Mn 31 and S ˆ 3=2 for Mn41). Its value agrees with the interpretation that spin moment in the LFO layer …S ˆ 5=2† is coupled antiferromagnetically and that in the LSMO layer is coupled ferromagnetically. In this case, all superlattices become soft magnets with a coercive field of 100 Oe same

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Fig. 2. (a) and (c): Temperature dependence of magnetization for the [(La0.8Sr0.2)MnO3]m/[LaFeO3]n superlattices taken under the field cooled (FC) condition at 1000 Oe (magnetization was normalized by numbers of (La,Sr)MnO3 unit cell). The insets show M–H curves. (b) and (d): Temperature dependence of their resistivity. Magnetic field was applied along the in-plane direction. Magnetoresistance was measured at H ˆ 1:0 T (dashed line). The MR ratio is defined as MR …%† ˆ ‰…R …H ˆ 0 T† 2 R …H ˆ 1 T††=R …H ˆ 1 T†Š × 100:

as the LSMO film. On the contrary, the saturated magnetizations of the (2,3), (1,2), (1,1) superlattices with reduced TC were reduced from 2.5 to 0.4m B and they became hard magnets with coercive fields from 500 to 700 Oe. It is explained that parallel spin alignment in the LSMO layer is disturbed by neighboring antiferromagnetic spin order via interface exchange interaction. Generally the [LSMO]m/[LFO]n superlattices show reduced ferromagnetism in the case of m # n and bulk-like metallic ferromagnetism in the case of m $ n: This competition between ferromagnetic interaction and antiferromagnetic interaction is suitable for CMR effect. The higher CMR effect (,35%) than that in the single phase LSMO film (,18%) is observed in the (2,2) and (2,1) superlattices as shown in Fig. 2(b) and (d). In the (10,1), (5,1), (3,1) superlattices, CMR effect is not enhanced. We have summarized the TC vs. numbers of layer …m=…m 1 n† ratio) diagram in Fig. 3. The ferromagnetic properties of superlattices are classified into four regions: (A) bulk-like ferromagnetic metal region; (A 0 ) magnetoresistance (MR) enhanced ferromagnetic metal region; (B) ferromagnetic insulator region with reduced TC; and (C)

ferromagnetic insulator region with strongly reduced TC and magnetization. By increasing the numbers of antiferromagnetic LFeO layer (n), TC of the LSMO/LFO superlattice becomes lower. That is, their double exchange ferromagnetism is systematically controlled by antiferromagnetic spin order in the neighboring LFO layer. The CMR effect is enhanced just above the border between the bulk-like ferromagnetic-metal region (A) and reduced TC ferromagnetic region (B), where spin frustration becomes most effective. We have interpreted the magnetic and electrical properties in spin frustrated superlattice as follows. The Hamiltonian is expressed as follows, which explains the superlattice consisting of double exchange ferromagnet LSMO and Heisenberg antiferromagnet LFO.   X t2g t2g u H ˆ 2tMn–Mn cos SMn SMn 2 KHund sSMn 2 Jt2g 2 LSMO k 2 Q …part A† 2 X t2g 2JFe–Mn SMn SFe 2 JFe–Fe SFe SSe …part B† 2 Adi1 Qdj 1

LFO

…1†

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localized spins in both Mn and Fe ion, and second term is the antiferromagnetic superexchange interaction between localized spins in the LFO layer. In the lattice polaron conduction mechanism, the resistivity behavior in the semiconductive region can be fitted by the following equation [9]:   aT Eb exp r…T† < 11G kB T   aT Eb0 exp ‰1 2 G…T†Š …2† < 11G kB T G…T† , M …T†=M …10 K†

Fig. 3. The relationship between variation of staking layers in the [(La0.8Sr0.2)MnO3]m/[LaFeO3]n superlattices and Curie temperatures: (A) Bulk-like ferromagnetic metal region; (A 0 ) MR enhanced ferromagnetic metal region; (B) ferromagnetic insulator region with reduced TC; (C) ferromagnetic insulator region with strongly reduced TC and magnetization.

For part A, the first and second terms are Anderson– Hasegawa Hamiltonian expressing double exchange ferromagnetism (tMn–Mn: transfer integral for eg electron, u : the angle between the neighboring localized spins in Mn ions [14,15], the third term is the one indicating the antiferromagnetic interaction between localized t2g spins in Mn ions, and fourth and fifth terms indicate dynamic Jahn–Teller effect proposed by Millis [11,12]. For part B, the first term is the interface superexchange interaction acting between

Fig. 4. A linear fit of ln ‰R…1 1 G…T††=TŠ vs. …1 2 G…T††=kB T for the [(La0.8Sr0.2)MnO3]m/[LaFeO3]n superlattices above Curie temperature.

…3†

where Eb is the polaron binding energy, a a constant, and temperature dependence of Eb is approximated by G(T). The function G(T) is closely related to the normal magnetization. Using Eqs. (2) and (3), the actual value of Eb0 is extracted from the fitting line to the resistivity and magnetization data. In Fig. 4, by increasing the numbers of antiferromagnetic layer, polaron binding energy, Eb0, becomes larger from 67 meV for the single phase LSMO film to 176 meV for the (2,3) superlattice. The neighboring antiferromagnetic spin order makes localized Mn t2g spin alignment fluctuated so that angle u in the term of 2tMn–Mn cos…u=2† becomes larger via interface exchange interaction term of JMn–Fe in Eq. (1) as observed in the suppression of magnetization. It is considered that the Jahn–Teller distortion mainly depends on the (3d) 4 electron configuration, not on the spin alignment of neighboring LFO. Therefore, the enhancement of Eb0 can be interpreted as the suppression of spin-dependent carrier hopping caused by localized spin fluctuation. As a carrier redistribution effect, doped holes prefer the Mn site to the Fe site because valence state of the Mn 31 ion has a higher energy level than that of the Fe 31 ion. The suppression of ferromagnetism was also observed in [(La0.8Sr0.2) MnO3]m/(La0.8Sr0.2)FeO3]n …m ˆ 1; 2, 3, 5 and n ˆ 1; 2) superlattices with the same filing of La/Sr. It suggests that phenomena observed here have their origin not in carrier redistribution from the LSMO layer to the LFO layer but in the variation of magnetic interactions. The strain effect in film which expands Mn ion–Mn ion distance is relatively small (misfit: 0.5%) so that change of transfer integral, tMn–Mn can be neglected [16]. It is reasonable to attribute the origin of changes in the magnetic, electrical and CMR properties to spin-dependent carrier hopping modulated by spin fluctuation with neighboring antiferromagnetic LFO layers. In conclusion, we have fabricated [La 0.8Sr0.2) MnO3]m/(LaFeO3]n spin frustrated superlattices and observed the strong coupling between the ferromagnetic (La0.8Sr0.2)MnO3 layer and the antiferromagnetic LaFeO3 layer in the increase of resistivity and the suppression of ferromagnetism. The coupling strength can be controlled by the arrangement of the number of antiferromagnetic or ferromagnetic layers. In the optimum superlattices, the enhancement of the CMR effect has been achieved up to

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35% at 1.0 T. The novel factor to enhance the CMR effect is found in addition to the well-known double exchange mechanism and the recently proposed dynamic Jahn–Teller mechanism, that is the localized spin fluctuation caused by neighboring antiferromagnetic spin order in spin frustrated magnetic superlattices. Acknowledgements We would like to thank to Prof. Y. Hirotsu and Dr Biab Bo for the HRTEM observation. References [1] R. von Helmolt et al., Phys. Lett. 71 (1993) 2331. [2] S. Jin et al., Science 264 (1994) 413. [3] Y. Tokura et al., J. Phys. Soc. Jpn 63 (1994) 3931.

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