Observation of spin glass state in weakly ferromagnetic Sr2FeCoO6 double perovskite R. Pradheesh, Harikrishnan S. Nair, C. M. N. Kumar, Jagat Lamsal, R. Nirmala et al. Citation: J. Appl. Phys. 111, 053905 (2012); doi: 10.1063/1.3686137 View online: http://dx.doi.org/10.1063/1.3686137 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v111/i5 Published by the American Institute of Physics. Additional information on J. Appl. Phys. Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors Downloaded 16 May 2013 to 134.94.122.141. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions JOURNAL OF APPLIED PHYSICS 111, 053905 (2012) Observation of spin glass state in weakly ferromagnetic Sr2FeCoO6 double perovskite R. Pradheesh,1 Harikrishnan S. Nair,2 C. M. N. Kumar,2 Jagat Lamsal,3 R. Nirmala,1 P. N. Santhosh,1 W. B. Yelon,4 S. K. Malik,5 V. Sankaranarayanan,1,a) and K. Sethupathi1,a) 1Low Temperature Physics Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India 2Ju¨lich Center for Neutron Sciences-2/Peter Gru¨nberg Institute-4, Forschungszentrum Ju¨lich, 52425 Ju¨lich, Germany 3Department of Physics and Astronomy, University of Missouri-Columbia, Columbia, Missouri 65211, USA 4Materials Research Center and Department of Chemistry, Missouri University of Science and Technology, Rolla, Missouri 65409, USA 5Departmento de Fisica Teo´rica e Experimental, Natal-RN, 59072-970, Brazil (Received 30 September 2011; accepted 18 January 2012; published online 1 March 2012) A spin glass state is observed in the double perovskite oxide Sr2FeCoO6 prepared through sol-gel technique. Initial structural studies using x rays reveal that the compound crystallizes in tetragonal I4/m structure with lattice parameters, a ¼ 5.4609(2) A˚ and c ¼ 7.7113(7) A˚ . The temperature dependent powder x ray diffraction data reveal no structural phase transition in the temperature range 10-300 K. However, the unit cell volume shows an anomaly coinciding with the magnetic transition temperature thereby suggesting a close connection between lattice and magnetism. Neutron diffraction studies and subsequent bond valence sums analysis show that in Sr2FeCoO6,theB site is randomly occupied by Fe and Co in the mixed valence states of Fe3 þ /Fe4þ and Co3þ/Co4þ. The random occupancy and mixed valence sets the stage for inhomogeneous magnetic exchange interactions and in turn, for the spin glass state in this double perovskite, which is observed as an irreversibility in temperature dependent dc magnetization at Tf 75 K. Dynamical scaling analysis of 0 v (T) yields a critical temperature Tct ¼ 75.14(8) K and an exponent z ¼ 6.2(2) typical for spin glasses. The signature of presence of mixed magnetic interactions is obtained from the thermal hysteresis in magnetization of Sr2FeCoO6. Combining the neutron and magnetization results of Sr2FeCoO6, we deduce that Fe is in low spin state while Co is in both low spin and intermediate spin states. VC 2012 American Institute of Physics. [doi:10.1063/1.3686137] I. INTRODUCTION that the Fe ions are separated far apart in these materials.2 However, a few exceptions (like Sr FeWO ) to this have been Ferromagnetic double perovskite oxides of general for- 2 6 reported with low magnetic transition temperatures 37 K.10 mula A BB0O (where A ¼ divalent alkaline earth; B and 2 6 The correlation between magnetoresistance and magnetism in B0 ¼ transition metal ions) have been extensively studied as these compounds and the site disorder is understood from the candidates for magnetoresistive materials.1,2 The crystallo- studies on ordered and disordered Sr FeMoO .11 The site dis- graphic ordering of the B cations in these compounds plays an 2 6 order can partially or completely destroy the B site ordering important role in realizing novel magnetic and transport prop- of cations and can influence the physical properties like mag- erties including magnetoresistance.2 Depending on the crys- nitude of magnetization, Curie temperature and low-field tallographic arrangement of B cations, double perovskites are magnetoresistance.12,13 It has been found that the site disorder classified as random, rock salt or layered.3 For example, can be influenced by preparative conditions such as heat treat- Ca FeReO has rock salt arrangement4 while PrBaCo O 2 6 2 5 þ d ment temperature,12,14 treatment time14 etc. Therefore, it is adopts layered structure.5 Though B site ordered double per- possible to tune the magnetic properties of A BB0O through ovskites are of interest for magnetoresistive and ferromagnetic 2 6 suitable selection of B/B0 cations and preparative conditions. (FM) properties, it has been observed that comparable ionic Though double perovskite oxides in the A BB0O family have radii and oxidation states of different cations at the B site can 2 6 been actively investigated, there are only a few reports on the lead to mixed crystallographic occupation and result in what Co based double perovskite, Sr FeCoO .15,16 Earlier studies is known as antisite disorder.3 Depending on the type, size 2 6 on the chemically similar single perovskite SrFe Co O and charge of the cations present at the B site, different mag- 1-x x 3 [x ¼ 0.5] reported a high T of340Kandsaturationmagnet- netic properties varying from antiferromagnetism6,7 (AFM) to c ization of 3 l .17,18 Meanwhile, Maignan et al., reported spin glass8,9 (SG) have been reported in double perovskites. B SrFe Co O to be ferromagnetic below T 200 K with a Most of the Fe based double perovskites display a high mag- 0.5 0.5 3 c saturation magnetization of 1.5 l at 5 K.15 Ab initio band netic transition temperature, which is surprising given the fact B structure calculations on Sr2FeCoO6 showed that both Co and Fe make comparable contributions to ferromagnetism of the 19 4þ 5 4þ 4 a)Electronic addresses: [email protected] and [email protected]. compound where Co (d L)andFe (d L)inhighspin ac.in. (HS) states can lead to metallicity and ferromagnetism. 0021-8979/2012/111(5)/053905/7/$30.00 111, 053905-1 VC 2012 American Institute of Physics Downloaded 16 May 2013 to 134.94.122.141. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions 053905-2 Pradheesh et al. J. Appl. Phys. 111, 053905 (2012) However, owing to the comparable ionic radii and valence goodness-of-fit (v2) of the refinement for I4/m and Pm3m states of Fe and Co (both in 4 þ state), it is surprising that Co space groups, and it is found that the former gives a better fit. doped SrFeO3 showed ferromagnetism. The absence of a lin- The temperature evolution of unit cell volume is presented in 0 0 ear B-O-B -O-B chain consequent to tilts in the BO6 and B O6 Fig. 2(b). The variation of cell volume was modeled using the octahedra can cause 90 and 180 superexchange interac- Gru¨neisen approximation,27 tions.20,21 Moreover, comparable ionic radii of the B site cati- ons combined with antisite disorder can lead to magnetic VðTÞ¼cUðTÞ=K0 þ V0; ð frustration in the double perovskites. Cluster glass phenom- HD=T UðTÞ¼9Nk TðT=H Þ3 x3=ðex À 1Þdx: enon in Sr2Mn1-xFexMoO6 (Ref. 20) was attributed to the B D 0 local magnetic frustration developed due to the competing nearest neighbor (NN) and next nearest neighbor (NNN) Here, U(T) is the specific internal energy, c is the Gru¨neisen superexchange interactions. Incompatible superexchange parameter, K0 is the incompressibility, V0 is the volume at interactions and magnetic frustration due to the site disorder T ¼ 0 K, and HD is the Debye temperature. The resulting fit 0 22 of the B and B cations were observed in Sr2FeTiO6-d. These is shown as a solid line in Fig. 2(b). Note that the fit deviates observations point to the fact that mixed valence state of the B from the data at T 75 K where the volume displays a and B0 ions and their crystallographic disorder can lead to a hump-like feature (a magnified low-temperature region spin glass state as it would create mixed interactions and ran- where the fit deviates from the data is shown in the inset 23 domness. In the present study on Sr2FeCoO6,through (1)). This is inferred as magnetoelastic or magnetovolume detailed structural and magnetic investigations, we observe an effect coinciding with the magnetic transition. In this respect, inhomogeneous magnetic state with the characteristics of a further studies aimed at investigating magneto-structural spin glass phase arising from incompatible magnetic interac- coupling and polarization measurements would be reward- 21 tions and crystallographic disorder. ing. The value of HD obtained from the fit is 463 K and ˚ 3 V0 ¼ 227.89 A . The HD of Sr2FeCoO6 lies in the range of II. EXPERIMENTAL DETAILS values estimated for double perovskites like Sr2FeTeO6 28 assuming two different HD’s for light and heavy atoms. In Polycrystalline samples of Sr2FeCoO6 were prepared by sol-gel method as described elsewhere.24 The gel was Fig. 2(c) we show the temperature variation of the lattice pa- dried at 120 C and the final sintering of the sample was rameters a and c. An important structural parameter charac- done at 1050 C for 36 h.