Ferromagnetic metal to cluster-glass insulator transition induced by A-site disorder in manganites

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Ferromagnetic metal to cluster-glass insulator transition induced by A-site disorder in manganites

K. F. Wang, Y. Wang, L. F. Wang, S. Dong, H. Yu, Q. C. Li, and J.-M. Liu^a^兲

Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China and International Center for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, China

Z. F. Ren

Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467

共Received 9 November 2005; accepted 15 March 2006; published online 12 April 2006兲

The magnetotransport behaviors of a series of rare earth manganites with the sameA-site cational mean radius and different A-site ionic radii variance 共A-site disorder兲 are investigated. It is found that the system’s ground state transforms from ferromagnetic metal to cluster-glass insulator with increasingA-site disorder. In the cluster-glass state, the magnetization shows the steplike behavior, indicating the existence of short-range magnetically ordered clusters. The significant effect of the A-site disorder on the electronic phase separation is revealed by detecting the cluster-glass ground state at low temperature. © 2006 American Institute of Physics.关DOI:10.1063/1.2194826兴

Perovskite manganites共AMnO3兲have been attracting at- tentions not only for the potential applications due to the colossal magnetoresistance^1–3共CMR兲that can be understood in the framework of double exchange共DE兲model,⁴but also for the fundamental understanding of the magnetic orders and the associated phase transitions. It has been recognized that the electronic phase diagram of CMR manganites is multicritical, involving competitions of spin, charge or orbital, and lattice orders,^5,6 leading to electronic phase separation and inhomogeneous electronic and magnetic ground states.^2,7At the same time, the significance of intrinsic disorder in manganites has also been recognized. For example, in Ln_0.5Ba_0.5MnO₃, rare earth Ln and Ba ions can form an ordered or disordered structure causing a significant disordered effect.⁸The disorder can result in glassy ground state and enhance the fluctuations of the order competitions, i.e., between the charge-ordered-orbital ordered 共CO–OO兲 state and ferromagnetic metal 共FMM兲state, near the original bi- critical point. Such fluctuations are amenable to an external magnetic field. Therefore, applying a field favors the FMM phase and produces the CMR effect.

It has been reported that the variance of theA-site ionic radii,␴²⁼兺ixir_i²−具rA典², wherexiandri are the atomic frac- tion and ionic radii ofi-type ions atA-site, respectively, is a key parameter to describe theA-site disorder and has significant influence on the magnetic and transport properties of manganites.^9,10TheA-site cational size mismatch can induce A-site disorder over a wide range but not causing distortion of the lattice structure. Therefore, an investigation of the A-site disorder can provide us additional clues to understand the CMR effect in manganites. Even though some earlier studies reported the relevance between ferromagnetic Curie pointT_Cand theA-site disorder in some manganites,^9,10there was not much work on the effect of the A-site disorder on neither the phase separation nor the inhomogeneity. It was postulated that theA-site disorder may lead to electronic and magnetic disordering effects, such as cluster-glass behavior, electronic localization, and so on. In this letter, we report the effects ofA-site disorder on the phase separation and particu-

larly the ground state transition from metal to glassy insulator.

In our experiments, we prepared a series of samples with the sameA-site cational mean radius具rA典= 1.20 Å but different variance ␴² from 0.003 to 0.015, as shown in Table I.

Both the 具r_A典 and ␴² were calculated using standard nine- coordinated cational radii.¹¹These samples were sintered by the conventional solid-state reaction in air. High-resolution x-ray diffraction 共XRD兲 with CuK␣ radiation was performed on these samples at room temperature. The transport measurements were performed using a standard four-probe method with temperature共T兲in 20– 300 K. The magnetiza- tions of zero-field cooling共ZFC兲and field cooling共FC兲were measured as a function ofT and magnetic field 共H兲 using a Quantum Design superconducting quantum interference device 共SQUID兲 magnetometer. The magnetic loops from H= 0 to 7 T were recorded at 3 K.

The XRD patterns of all the samples are presented in Fig. 1. All the peaks can be indexed with a single orthorhom- bic structure with space groupPbnm. There was no measur- able peak shift for samples with different␴², indicating es- sentially the same lattice parameters for all the samples. It is estimated that the volume change corresponding to the variation of␴²from 0.003 to 0.015 is less than 0.5%. The curves of zero-field resistivity 共␳兲 as a function of T for all the samples are plotted in Fig. 2共a兲. It is clearly shown that␳ ^is very much dependent on␴². In general, at a given temperature, ␳ increases with ␴², whereas at given ␴², different

␳⬃Tbehaviors were observed. For␴²^{= 0.003,}␳exhibited a

a兲Author to whom correspondence should be addressed; electronic mail:

[email protected]

TABLE I. Summary of chemical, structural, and physical data for the RE_0.55AE_0.45MnO₃ series with a constantA-site cation mean radius具rA典

= 1.20 Å.

Chemical composition

␴² 共Å²兲

T_MI 共K兲

TC

共K兲 Tf

共K兲

M共T= 3 K兲共␮B”f.u.兲 Nd_0.55共Ca_0.45Sr_0.55兲0.45MnO₃ 0.003 ⬃197 ⬃194 3.36

Sm_0.55共Ca_0.2Sr_0.8兲0.45MnO₃ 0.007 ⬃100 ⬃115 2.82 Nd_0.55共Ca_0.76Ba_0.24兲0.45MnO₃ 0.008 ⬃42 2.62

Gd_0.55Sr_0.45MnO₃ 0.009 ⬃42 1.51

Sm_0.55共CA_0.6Ba_0.4兲0.45MnO₃ 0.015 ⬃42 1.01 APPLIED PHYSICS LETTERS88, 152505

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2006

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metal-insulator transition共MIT兲atT=⬃197 K共TMI兲.²With

␴² increased to 0.007, the MIT occurred at⬃100 K. When

␴²艌0.008, there was no MIT observed down to 20 K, mean- ing that the samples remained to be insulating.

Figure 3 presents the T dependence of magnetization 共M兲. The samples with␴²= 0.003 and 0.007 exhibited a para- magnetic 共PM兲-FM transition at 共define TC兲 TC⬃194 and

⬃115 K, respectively, roughly in agreement with the MIT shown in Fig. 2. For ␴²⬎0.008, the M-T curves 共ZFC兲 showed a cusplike peak atT=Tf⬃42 K. The irreversibility between the ZFC and FC M-T curves is very clear. These phenomena allow us to argue that a cluster-glass transition is probably occurring at T_f. In fact, similar results for La_2/3Ca_1/3MnO₃doped with either Ga or Al or Fe were pre- viously reported.^12,13

The M-H curves at T= 3 K are shown in Fig. 4. For

␴²= 0.003 and 0.007, saturated M is reached at H= 1 T, whereas for ␴²^{= 0.008} ^M remains unsaturated till 3.5 T, at which a stepwise behavior occurred. For ␴²= 0.009 and 0.015,M remains unsaturated even at 6.0 T.

It is well known that if all the Mn ions in the samples were ferromagnetically aligned, the maximum spin-only mo- ment is 3.55␮B/ f.u. However, the measured M at 3 K and 3 T decreased from 3.36 to 1.01␮B as ␴² increased from

0.003 to 0.015, shown in Table I and Fig. 2共b兲, with an abrupt decrease occurred at␴²⬃0.008. Simultaneously, the zero field␳ increased significantly at␴²^{= 0.008}关Fig. 2共b兲兴, which is the reflection of a MIT induced by electron local-

FIG. 1. XRD␪-2␪spectra measured at room temperature for the samples with␴²= 0.003, 0.007, 0.008, 0.009, and 0.015, respectively.

FIG. 2.共a兲Measured␳^-Trelations for the samples with␴²from 0.003 to 0.015;共b兲␴²dependences ofMmeasured atT= 3 K underH= 3 T and␴² dependences of zero field␳measured atT= 50 K.

FIG. 3. Measured M-T relation under ZFC and FC conditions for the samples with共a兲␴²^{= 0.003,}共b兲0.007,共c兲0.008, and共d兲0.015, respectively.

The arrows in共c兲and共d兲indicate the cluster-glass transition point.

FIG. 4. Measured M-H curves at T= 3 K, for the samples with 共a兲

␴²= 0.003 and 0.007 and 共b兲 0.008 and 0.015, respectively. The arrows indicate the variation ofHduring measurements.

152505-2 Wanget al. Appl. Phys. Lett.88, 152505共2006兲

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ization. We argue that the increased␴² induced the electron localization and enabled ground state transition from FMM to cluster-glass-like insulator. The detailed properties of the cluster-glass state will be reported elsewhere.¹⁴ To understand the physics underlying the cluster-glassy transition, one may consider the scenario of phase separation in response to the A-site disorder and magnetic field. The DE interaction responsible for FMM state is scaled by single electron band- widthW.¹⁵ The FMM state is destabilized in distorted manganites, and often replaced by phases competing against FMM, such as CO–OO antiferromagnetic 共AFM兲 insulator 共AFI兲. A typical case is that the reduction of theA-site radius leads to ordered oxygen displacement and thus a CO–OO AFI. In addition, the size difference between the neighboring A-siteR³⁺ andM²⁺ ions around one oxygen ion共i.e., A-site size mismatch兲may enable the random oxygen displacement and consequently a local distortion of MnO₆octahedra.

Since all the samples in the present study have the same 具r_A典= 1.20 Å, very close to the ideal value, they have the same bandwidthW. The sample with small␴²共=0.003兲cer- tainly exhibits the typical MIT共FM-PM transition兲. As the A-site disorder increases 共␴²艋0.007兲, the oxygen displace- ments and the radial distortions of the MnO₆ octahedra are random because the A-site ions randomly distribute in the lattice, which keeps the macroscopic structure unchanged.

The Mn ions around the distorted MnO₆ octahedra may no longer be able to participate in the DE process. Moreover, considering the fact that the ground state in distorted manganites is often the CO–OO AFI rather than FM state, it is reasonable to argue that the distorted MnO₆ octahedra induced by A-site disorder prefers a locally short-ranged CO–OO AFI state. Although FM and CO–OO AFI phases coexist, the FM phase is dominant.

When ␴²⬃0.008, an intermediate disordered state, the system has more CO–OO AFI phase than the FM phase, and electron localization happens evidenced by the insulating behaviors over the wholeT range. In such a case, a magnetic field favors the FM phase expansion at the expense of the AFI phase. Subsequently, the expansion of the FM phase requiresHto be higher than a critical value in order for the Zeeman energy to overcome the strain energy. At this critical field, a sharp magnetization step is observed, as shown in Fig. 4共b兲共␴²= 0.008兲. This steplike effect indicates the coexistence of the long-range FM regions and the short-range CO–OO AFI regions in the sample.

As␴²⬎0.008, it is argued that the long-range FM order- ing is completely broken, and the short-range regions be- come dominant. This corresponds to the so-called cluster- glass state. The magnetization M is small 共M⬃1.01␮B at 4.0 T for the sample of␴²= 0.015兲. If this argument applies, the ␳^-T relation can be described by the variable-range- hopping共VRH兲model:¹⁶ ␳⁼␳i0exp关共T0/T兲^1/4兴, where␳i0 is the prefactor andT₀ is the characteristic temperature. Other- wise, the small polaron mechanism will apply aboveT_MI.²In fact, for ␴²艌0.007 the␳^-T relation aboveT_MI does follow the VRH model rather than the small polaron one. The good linear behavior apart from very low T indicates that the A-site disorder favors the electron localization and the cluster-glass state.

Earlier theoretical work dealt with the effect of quenched disorder on the electronic phase separation by first-order

transitions, which corresponds to a phase diagram with fea- tures resembling the quantum critical behavior. The low-T region consists of coexisting ordered clusters.^17,18Generally, in manganites, the DE interaction and AFM superexchange interaction favor the long-range FM and AFI orders, respectively. For a coexisting two-phase system, an intermediate disorder often brings forth a MIT transition. Our experimen- tal results seem to confirm the prediction of Tokura and Nagaosa,² Burgyet al.,¹⁷ and Senet al.¹⁸ that the competi- tion of two opposite interactions plus quenched disorder will favor a cluster-glass state, which may be induced by the enhanced quantum fluctuations between the competing interactions as the consequence of the quantum phase transition.

Therefore, the disorder-induced quantum fluctuation is probably one of the important ingredients of the CMR physics, although more direct and dynamic evidence is needed, which is being studied.

In conclusion, we have investigated the effect of the A-site disorder on the magnetic and transport behaviors of perovskite CMR manganites by changing the A-site cation size. It has been observed that the increasing ofA-site disorder results in the transition of the ground state from metal to insulator because of the electron localization. The long-range FM state preferred with␴²⬍0.007 is replaced by the cluster- glass state with␴²⬍0.009. At␴²⬃0.008, the coexistence of the two states has been revealed. Our results agree with the previous theoretical prediction and reveal the essential role of theA-site disorder in the CMR physics.

This work was supported by the Natural Science Foun- dation of China共50332020, 50528203, and 10021001兲 and the 973 Projects of China共2002CB613303兲. One of the au- thors 共J.M.L.兲 acknowledges the support of Hong Kong Polytechnic University through project共B-Q552兲.

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