Observation of Spin Glass State in Weakly Ferromagnetic Sr2fecoo6 Double Perovskite R

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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.

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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

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 cations 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 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. Powder x ray diffraction patterns terizing the double perovskites, and having a bearing on the were recorded in the temperature range of 10-300 K using a magnetic properties is the cationic site disorder. However, Huber diffractometer (Mo Ka) in Guinier geometry. Neutron powder diffraction (NPD) measurement at 300 K was performed with neutron wavelength of 1.4789 A˚ at the Univer- sity of Missouri Research Reactor (MURR) employing position sensitive detector. The crystal structure was refined by Rietveld method25 using FULL-PROF program.26 dc and ac magnetic measurements were carried out in a commercial superconducting quantum interference device (SQUID) magnetometer and physical property measurement system (both M/s Quantum Design, USA). dc magnetization in field cooled (FC) and zero-field cooled (ZFC) cycles were performed at different applied fields from 100 Oe to 50 kOe. ac susceptibility was measured using SQUID magnetometer in an ac field of 3 Oe with frequencies of 33, 133, 337, 667, 967, and 1333 Hz.

III. RESULTS AND DISCUSSION Using the powder XRD data, the crystal structure of Sr2FeCoO6 was refined in the tetragonal space group I4/m ˚ (No. 87) with lattice constants a ¼ 5.4568(2) A and c ¼ 7.7082(4) A. A pictorial representation of the Sr2FeCoO6 double perovskite unit cell is shown in Fig. 1. Even though, following the earlier reports on SrFe1-xCoxO3 (Ref. 15)a refinement in cubic Pm3m space group was undertaken ini- tially, an improved fit to XRD data was achieved using the tetragonal space group. The results of the Rietveld analysis are presented in Fig. 2(a) where the observed XRD pattern at FIG. 1. (Color online) The tetragonal structure of the double perovskite Sr2FeCoO6. The CoO6 and FeO6 octahedra are shown with Fe (Co) at the 300 K, the calculated profile, difference profile, and the Bragg center surrounded by the oxygen. For succinctness the structure shown is of peaks are shown. The inset (1) of Fig. 2(a) compares the the B site ordered type.

atomic occupancies using structural models assuming ordered as well as disordered cationic arrangement at the B/B0 site. A better fit to the experimental data, in terms of profile-match and discrepancy factors, was achieved by assuming a disordered model with a random arrangement for the B/B0 cations. The oxygen content was quantified in the refinement process and the deviation from stoichiometry (d) was found to be 0.11. The structural parameters of Sr2FeCoO6 including atomic positions, lattice parameters and occupancies and the bond valence sums (BVS) calculated using VaList program31 are collected in Table I. A rock-salt arrangement of the cations in double perovskites normally occurs if the charge difference between the B and B0 is greater than 2.3 In the case of Sr2FeCoO6, the charge difference is close to unity as clear from Table I. The BVS calculations give an insight into the oxidation state of the ions and reasonably support random occupancy at the B/B0 site and mixed valence of Fe and Co. The

bond length Fe(2)-O(2) estimated from the structural analysis is 1.969 A and is comparable to the values found in the litera-

ture for double perovskites, especially Sr2Fe0.75Cr0.25MoO6 (Fe-O distance is 1.968 A).32 The bond lengths and bond FIG. 2. (Color online) (a) X-ray diffraction pattern of Sr2FeCoO6 at 300 K angles of Sr FeCoO obtained from the analysis of NPD data refined in tetragonal I4/m space group. Inset (1) compares the goodness-of- 2 6 fit of the refinement in two space groups. Quality measures of refinement are are given in Table II. According to Glazer’s notation, the 2 RwP ¼ 1.52%, RP ¼ 1.18%, v ¼ 1.23. (b) shows the analysis of unit cell space group I4/m has an antiphase tilting along c axis denoted volume using Gru¨neisen approximation. Inset (1) shows the enlarged view of as a0a0c-, which represents the small shifts of the in-plane ox- thepffiffiffiffiffi anomaly near 75 K. (c) shows temperature evolution of lattice parameters ygen atoms.33 The magnitude of tilting can be derived from 2a and c (the error bars were smaller than the size of the data markers). Fe-O-Co (/) angle as (180 - /)/2 ¼ 1.04. The average bond ˚ since the x-ray atomic structure factors of Fe and Co are length of hFe1-Oi and hFe2-Oi are 1.9147 A and 1.9573 A, comparable, an accurate evaluation of atomic occupancies where the latter bond length compare well with the expected 3þ was not possible from the present XRD data. value calculated as the sum of the ionic radii of Fe in low Fe 2- ˚ 34 In order to check the degree of B site ordering and to spin state (LS) (rLS ¼ 0.55 A)andO (rO ¼ 1.40 A). Simi- ˚ confirm the crystal structure of Sr FeCoO determined from larly the bond length values for hCo1-Oi (1.9147 A) and 2 6 ˚ XRD data, we performed neutron diffraction experiments hCo2-Oi (1.9573 A) are close to the values of the calculated (the neutron coherent scattering cross section of Fe and Co bond length if we assume cobalt has a low spin 4 þ valence -24 2 -24 2 29 Co ˚ are 11.22 10 cm and 0.779 10 cm , respectively). state (rLS ¼ 0.518 A) (Ref. 35) and an intermediate spin (IS) Co 36 In Fig. 3 we present the neutron diffraction pattern of 3 þ state (rIS ¼ 0.56 A) respectively. The magnetization of Sr FeCoO measured at 500 Oe in Sr2FeCoO6 collected at 300 K along with the results of struc- 2 6 ture refinement using I4/m space group. Our Mo¨ssbauer stud- the zero field-cooled (ZFC) as well as field-cooled cooling ies confirm that Fe occupies two distinct crystallographic (FC) and warming (FCW) cycles are shown in Fig. 4(a).A sites and hence the choice of I4/m space group was made in magnetic transition is evident at Tc 75 K. The low temper- our structural analysis. We carried out the refinement of ature magnetization data show a marked irreversibility below Tc indicating the presence of a weak component of ferromagnetism as suggested by Goodenough-Kanamori rules for the d4 – d5 cation-anion-cation superexchange37,38 or spin glass state. With an increase in applied field, the ZFC/FC curves tend to merge and the FC arm shows signs of saturation with broadening of the peak (see inset (1), Fig. 4(a)). This broadening is also evident in inset (2), Fig. 4(a) where the FC magnetization at higher fields is presented. It signifies the presence of majority FM phase since for an antiferromagnet the increase in the field would have had little effect on the sharpness of the transition.39 The inverse magnetic susceptibility in the temperature range 220- 350 K was fitted to Curie-Weiss law and the result is presented in Fig. 4(b).A deviation from the Curie-Weiss (CW) law can be clearly seen for temperatures significantly above Tc also. Such devi- FIG. 3. (Color online) (a) Observed (Iobs), calculated (Icalc) and difference ations from Curie- Weiss behavior above Tc attributed to (Idiff) profiles of neutron diffraction pattern of Sr2FeCoO6 at 300 K refined in tetragonal I4/m space group. Vertical markers correspond to Bragg peaks. antiferromagnetic spin fluctuations are commonly observed 2 40 Quality measures of refinement are RwP ¼ 17.6%, RP ¼ 14.6%, v ¼ 3.38. in spin glass systems. Previous reports show that Fe and

TABLE I. Occupancy and bond valence sums (BVS) and (x, y, z) values obtained after Rietveld refinement of the neutron data at 300 K. The unit cell parame- ˚ ˚ ters are a ¼ 5.4609(2) A, b ¼ 5.4609(2) A, and c ¼ 7.7113(7) A. The quality of the fit is indicated by the discrepancy factors, RWP ¼ 17.6%, RP ¼ 14.6%, and the goodness-of-fit, v2 ¼ 3.38.

Atom Site X y z Occupancy BVS

Sr 4d 0 0.5 0.25 0.25 2.336(9) Fe1/Co1 2a 0 0 0 0.0625 4.055(69)/3.650(62) Fe2/Co2 2b 0 0 0.5 0.0625 3.51(41)/2.967(54) O1 4e 0 0 0.24739(3) 0.25994(10) 1.970(38) O2 8h 0.26798(13) 0.21981(18) 0 0.47621(6) 1.985(15)

Co in SrFe1-xCoxO3 solid solutions can possess complex va- double perovskite oxides are also reported to show similar 41 lence states and that, Fe-O-Co superexchange path-ways thermal hysteresis below Tc as in the case of Sr2MReO6 can favor electronic transfer from Fe3þ high spin state (HS, (M ¼ Sc, Co etc).45 Drawing parallels, magnetic frustration 4þ 15 S ¼ 5/2) to Co low spin state (LS, S ¼ 1/2). Complex va- effects in Sr2FeCoO6 originating from different exchange lence states are observed in the case of Sr2FeCoO6 from the paths due to multiple valences of Co/Fe can also lead to such CW analysis of magnetic susceptibility and is described in effects. We also note that at higher applied fields like 500 Oe detail below. (Fig. 4(a)), the hysteresis is absent. These features indicate a The effective paramagnetic moment calculated from the weak first-order-like phase transition due to the presence of CW analysis is leff ¼ 3.9 lB. Keeping in mind that the system mixed magnetic phases. is randomly ordered, the effective magnetic moment can be The isothermal magnetization curve of Sr2FeCoO6 at calculated by assuming 50% of Fe and Co (in both 2a and 2b 2 K is shown in the main panel of Fig. 5(a). The inset (1) sites) in 4 þ and 3 þ spin states. The experimentally obtained presents the M(H) curves at 5 K, 10 K and the inset (2) those effective spin-only moment from the CW fit lies in between in the range 30-300 K. Hysteresis is observed below 50 K the theoretical spin-only moment values of leff ¼ 3.67 lB and indicating weak ferromagnetism at low temperature. A leff ¼ 4.12 lB (see, Table III). Comparing this with the bond decrease in remanance and coercivity is observed with an length values obtained we infer that Co3þ is in intermediate increase in temperature (Fig. 5(b)). As the temperature spin state while Fe3þ,Fe4þ and Co4þ are in low spin state. increases, the irreversibility reduces and becomes sigmoidal The positive value of Curie-Weiss temperature, HCW ¼ 99 K, at 50 K and above; whereas no saturation of magnetization is calculated from the fit confirms the presence of ferromagnetic attained even at an applied field of 50 kOe. A notable feature interactions in the system. This value is lower than that in the isothermal magnetization curves is the step like reported earlier for SrFe0.5Co0.5O3-d where the role of oxygen non-stoichiometry on ordering temperature was discussed.41 However, oxygen deficient double perovskites exhibit very low magnetoresistance (MR), which is not the case in Sr2FeCoO6 since we have observed 63% MR at 12 K at an applied field of 12 T.30 Figure 4(c) shows the plot of FCC and FCW magnetization curves measured at 100 Oe where a clear thermal hysteresis below Tc is discernible. Thermal hysteresis in magnetization cycles are related to the presence of first-order phase transitions with mixed magnetic phases as has been reported in the case of disordered manganites42,43 or rare earth double perovskites.44 The end compositions of Sr2FeCoO6, viz., SrFeO3 and SrCoO3 possess helical and collinear ferromagnetic magnetic structures, respectively. Hence, Sr2FeCoO6 could consist of AFM regions where Fe-O-Fe superexchange dominate and Fe-O-Co FM regions, which is an ideal scenario for the for- mation of spin glass phase. Magnetic frustration effects in

TABLE II. Selected bond angles and bond lengths of Sr2FeCoO6 obtained from the structural analysis.

Fe1/Co1-O1 1.9227(15) 2 FIG. 4. (Color online) (a) Zero field cooling (ZFC), field cooled cooling Fe1/Co1-O2 1.9107(15) 4 (FC), and warming (FCW) cycles of magnetization of Sr2FeCoO6 as a func- Fe2/Co2-O1 1.9329(15) 2 tion of temperature at 500 Oe. Inset (1) shows the ZFC/FC curves at 50 kOe Fe2/Co2-O2 1.9695(15) 4 and (2) FC curves at different applied fields. (b) Temperature dependence of Fe1-O2-Co2 177.92(1) inverse magnetic susceptibility along with Curie-Weiss fit. (c) Thermal hysteresis observed in FC data at 100 Oe.

2 2 2 TABLE III. Calculated spin only moments (lSO ¼ l2a þ l2b) for Sr2FeCoO6 data to the Arrhenius relation, f ¼ f0 exp[Ea/kBT], where Ea is assuming the possible spin state values. The calculated spin only moment 82 3 þ the activation energy, yielded unphysical values of f0 ¼ 10 (lso) lies in between the last two conﬁgurations. Note that Co HS S ¼ 2, LS 4þ 3þ Hz and E 14 301 eV. The failure of Arrhenius relation S ¼ 0, IS S ¼ 1; Co HS S ¼ 5/2, IS S ¼ 3/2, LS S ¼ 1/2; Fe HS S ¼ 5/2, LS a ¼ S ¼ 1/2; Fe4þ HS S ¼ 2, LS S ¼ 1. points to SG behavior, since a description of mere energy- barrier blocking and thermal activation will not suit the SG

2a 2b lSO(lB) transition. In order to conﬁrm the spin glass behavior, analy-

4þ 4þ 4þ 4 þ sis of the susceptibility data using the dynamical scaling rela- Co HS(S ¼ 5/2); Fe HS(S ¼ 2) Co HS(S ¼ 5/2); Fe HS(S ¼ 2) 7.68 3þ 4þ 4þ 4þ tion at was performed. For a spin glass, the maximum of Co (S ¼ 2); Fe (S ¼ 2) Co (S ¼ 5/2); Fe (S ¼ 5/2) 7.68 Tc HS HS HS HS 0 4þ 4þ 4þ 4þ v (T) (freezing temperature, T ) depends on the applied fre- Co HS(S ¼ 5/2); Fe LS(S ¼ 1) Co HS(S ¼ 5/2); Fe LS(S ¼ 1) 6.56 f 4þ 4þ 4þ 4þ Co LS(S ¼ 1/2); Fe HS(S ¼ 2) Co LS(S ¼ 1/2); Fe HS(S ¼ 2) 5.19 quency and the characteristic time, t ¼ 1/x, is equal to the 4þ 4þ 4þ 4þ Co LS(S ¼ 1/2); Fe LS(S ¼ 1) Co LS(S ¼ 1/2); Fe LS(S ¼ 1) 3.32 maximum relaxation time s. Thus, a spin glass phase can be 4þ 4þ 3þ 4þ Co LS(S ¼ 1/2); Fe LS(S ¼ 1) Co IS(S ¼ 1); Fe LS(S ¼ 1) 3.67 judged if s follow a power law divergence of the form 3þ 4þ 4þ 3þ Co IS(S ¼ 1); Fe LS(S ¼ 1) Co IS(S ¼ 3/2); Fe LS(S ¼ 1/2) 4.12 zv Tf s ¼ s0 ; Tct Tf behavior in magnetization at low fields as presented in the enlarged view in Fig. 5(c). Such a behavior was observed in where Tct is the critical temperature and zv and s0 are the 23 Sr2YRuO6 and has been attributed to spin flop transition tak- critical exponents and microscopic time scale, respectively. ing place at a critical field.39 However, the aptness of such an The best fit to the above equation, as shown in Fig. 6(c) explanation in the case of Sr2FeCoO6 cannot be tested until (inset presents the same graph in log-log scales), yields 12 the magnetic structure is ascertained. zv ¼ 6.2(2), Tct ¼ 75.14(8) K, and s0 10 s. The values of Sharpness of the peak in ac susceptibility curves and its s0 and zv are consistent with that of conventional 3 D spin shift to higher temperature with increasing frequency are glasses47 and Fe doped cobaltites.48 In order to probe typical features exhibited by SG systems.23,46 In order to whether the FM state coexist with the frozen spin glass state probe this, ac susceptibility measurements were performed as in reentrant spin glasses, we performed ac susceptibility in the temperature range 5–250 K, with a driving field of measurements with superimposed dc fields. The result is pre- 3 Oe. Figure 6(a) shows the temperature dependence of the sented in Fig. 6(d), which shows the out of phase susceptibil- real part of ac susceptibility, v0(T), at different applied fre- ity in an external dc field varying from 50–1000 Oe at a quencies in the range 33–1333 Hz (x). v0(T) attains a maxi- frequency of 33 Hz with an ac amplitude of 3 Oe. We do not mum at Tf, the freezing temperature that shifts to higher observe emergence of a second peak as observed in certain 49 temperature as the frequency is increased. The shift of the reentrant systems, instead, the intensity of the peak at Tc in peaks as a function of frequency is clear in Fig. 6(b) and is v’’(T) diminishes with increasing field indicating FM nature characteristic of spin glasses.23 A fit of the ac susceptibility of the compound. Thus ac susceptibility studies confirm the presence of canonical SG state in Sr2FeCoO6, which in turn suggests the presence of disorder.

FIG. 6. (Color online) (a) Temperature dependence of ac susceptibility of 0 Sr2FeCoO6 at different frequencies. (b) The peak in v (T) 75 K, which FIG. 5. (Color online) (a) Isothermal magnetization of Sr2FeCoO6 at 2 K. shifts to higher temperature as the frequency is increased, signifying glassy Insets (1) and (2) present isotherms at elevated temperatures. (b) Variation magnetism. (c) The fit using the power law for critical slowing down, where -12 of remnant magnetization, Mr (circles) and coercive field, Hc (squares) with the best fit gave z ¼ 6.2, s0 ¼ 10 s. The inset presents the same plot but in temperature. (c) An enlarged view of the magnetization isotherms at 2 K for log-log scale. (d) displays v’’(T) at different values of superimposed dc mag- low fields. netic fields.

To summarize the results, Sr2FeCoO6 crystallizes in tet- and Co and their mixed occupation in the lattice leads to com- ragonal I4/m structure where the B site is randomly occupied petition between NN and NNN exchange interactions, which by Fe and Co in 2a and 2b sites. The assignment of I4/m is also supported by the estimated bond angle values. dc mag- space group is also supported by the bond valence sums anal- netization measurements show irreversibility at 75 K where yses. The Fe and Co cations in Sr2FeCoO6 exist in mixed va- the spin glass phase sets in. The weak component of ferro- lence states of þ 3 and þ 4. The random crystallographic magnetism observed in magnetization can be due to the com- occupation and mixed valency sets the stage for mixed FM peting antiferromagnetic and ferromagnetic interactions and is and AFM interactions between the transition metal cations clear from the low values of remnance. The spin glass state in and in turn, lead to inhomogeneous magnetic behavior in Sr2FeCoO6 is further supported by the frequency dependence Sr2FeCoO6. In a perfectly ordered double perovskite, the of real component of ac susceptibility. Dynamical scaling magnetic exchange is predominantly governed by the ferro- analysis confirms the spin glass phase with Tct ¼ 75.14(8) K. 3þ 2– 4þ magnetic Fe -O Co . With the occurrence of site disor- We propose that spin glass behavior in Sr2FeCoO6 is due to der, additional antiferromagnetic exchange paths like Fe-Fe the magnetic frustration resulting from the competing NN and or Co-Co are introduced. The spin glass phase with a NNN interactions, which originate from mixed occupation of Tf 75 K that appears in Sr2FeCoO6 has its origin in the cations sites. Our work emphasizes that the cationic disorder multiple exchange paths that arise due to the mixed interac- leads to a spin glass state. The structural and magnetic studies tions. The magnetic moment of 0.39lB at 5 K suggests that also lead to the deduction of the spin states of Fe and Co in the transition metal ions in this compound are in a mixed va- Sr2FeCoO6. lence state unlike high-spin tetravalent state in SrFe0.5- 50 Co0.5O3. The magnetic ordering of double perovskites is a ACKNOWLEDGMENTS function of the strength of the NN and NNN interactions51 The authors acknowledge the Department of Science and the competition between these interactions is known to and Technology (DST), India for financial support for pro- lead to magnetic frustration effects in Ln LiRuO (Ln ¼ Pr, 2 6 viding the facilities used in this study (Grant Nos. SR/FST/ Nd, Eu, Gd, Tb) (Ref. 52) and LaBaCoNbO .53 Depending 6 PSII-002/2007 and SR/NM/NAT-02/2005). P.R wishes to on the strength of NN and NNN interactions materials are thank M. Angst and K. Balamurugan for fruitful discussions. classified as Type I (strength of NNN interaction is negligible), Type II (dominant NNN interaction) and Type III 1D. Serrate, J. M. De Teresa, and M. R. Ibarra, J. Phys.: Condens. Matter (strength of NNN interaction significant but less than NN 19, 023201 (2007). interaction). Type II antiferromagnetic structure leads to dis- 2D. D. Sarma, E. V. Sampathkumaran, S. Ray, R. Nagarajan, S. Majumdar, order and yields a ferromagnetic NN interaction along with A. Kumar, G. Nalini, and T. N. Guru Row, Solid State Commun. 114, 465 28 (2000). an antiferromagnetic NNN interaction. The presence of dis- 3M. T. Anderson, K. B. Greenwood, G. A. Taylor, and K. R. Poeppelmeier, order leading to spin glass behavior in Sr2FeCoO6 shows Prog. Solid State Chem. 22, 197 (1993). that the NNN interaction is not negligible in double perov- 4J. Gopalakrishnan, A. Chattopadhyay, S. B. Ogale, T. Venkatesan, R. L. skite systems. SG behavior due to incompatible superex- Greene, A. J. Mills, K. Ramesha, B. Hannoyer, and G. Marest, Phys. Rev. B 62, 9538 (2000). change interactions and magnetic frustration was observed in 5S. Streule, A. Podlesnyak, J. Mesot, M. Medarde, K. Conder, E. Pomja- double perovskites like Sr2FeTiO6 (Ref. 22) and Sr2Fe- kushina, E. Mitberg, and V. Kozhevnikov, J. Phys.: Condens. Matter 17, TaO .6 Since Fe3þ and Co4þ are isoelectronic, they interact 3317 (2005). 6 6 ferromagnetically through the NN Fe-O-Co superexchange E. J. Cussen, J. F. Vente, P. D. Battle, and T. C. Gibb, J. Mater. Chem. 7, 459 (1997). interaction. However, presence of like-pairs of Fe and Co 7K. -I. Kobayashi, T. Okuda, Y. Tomioka, T. Kimura, and Y. Tokura, leads to Fe-O-Co-O-Fe NNN antiferromagnetic superex- J. Magn. Magn. Mater. 218, 17 (2000). change interaction also.21 The competition between the NN 8P. D. Battle, T. C. Gibb, C. W. Jones, and F. Studer, J. 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