PHYSICAL REVIEW B 87, 064403 (2013) Effect of Ba and Ti doping on magnetic properties of multiferroic Pb(Fe 1/2 Nb 1/2 )O 3 V. V. Laguta, 1,2 M. D. Glinchuk, 2 M. Maryˇ sko, 1 R. O. Kuzian, 2 S. A. Prosandeev, 3,4 S. I. Raevskaya, 3 V. G. Smotrakov, 3 V. V. Eremkin, 3 and I. P. Raevski 3 1 Institute of Physics AS CR, 16253 Prague, Czech Republic 2 Institute for Problems of Materials Science, NASc of Ukraine, 03142 Kiev, Ukraine 3 Department of Physics and Research Institute of Physics, Southern Federal University, 344090 Rostov on Don, Russia 4 Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA (Received 22 November 2012; published 4 February 2013) On the basis of extensive experimental studies of Pb 1−x Ba x (Fe 1/2 Nb 1/2 )O 3 (PFN-BFN) and Pb(Fe 1/2 Nb 1/2 ) 1−x Ti x O 3 (PFN-PT) single crystals by several experimental methods, we have proposed phase diagrams describing the magnetic properties of these solid solutions. The comprehensive consideration of the magnetic properties of the PFN-based solid solutions has shown that these phase diagrams can be explained on the basis of a model suggested earlier for pure PFN [Phys. Rev. Lett. 105, 257202 (2010)]. This model assumes the coexistence in the crystal lattice of the long-range antiferromagnetic (AFM) cluster, which defines the N´ eel order parameter, with the finite-size mixed ferromagnetic-AFM clusters, responsible for the spin-glass order parameter. We state that one of these parameters, the N´ eel temperature, linearly decreases with the increasing dopant concentration and eventually disappears at some critical concentration as a result of the percolation phase transition. The other parameter survives until the maximal concentrations studied. We have also found a phase which can be related to the super-AFM order. These data can have important implications and provide the basis for the development of novel fundamental theory of multiferroics with the site, charge, and spin disorder. DOI: 10.1103/PhysRevB.87.064403 PACS number(s): 75.85.+t, 61.43.−j, 75.50.Lk, 77.80.Jk I. INTRODUCTION Double perovskite Pb(Fe 1/2 Nb 1/2 )O 3 (PFN) has been the center of attention in recent years, because of its extreme multiferroic properties. 1–7 Interestingly, these properties can become even better 2,4 , 6–8 if one tunes them by means of doping (see, e.g., Refs. 6 and 7, and references therein). However, the microscopic origin of this influence is still unclear. We believe the answer to this nontrivial question lies in some specialties of the magnetic phase diagram of PFN-based solid solutions, which have been studied only scarcely. 4,6,7 References 1–7 discuss the existence of the ferroelectric (FE), antiferromagnetic (AFM), and spin-glass (SG) phases in the PFN-based solid solutions and pure PFN. The SG phase 3 has been mostly studied in pure PFN. The μSR spectroscopy and neutron diffraction experiments 5 have shown that the magnetic ground state of PFN is of SG order, which coexists with the long-range AFM order, below T g ≈ 20 K. In a recent work, Kleemann et al. 3 suggested the occurrence in PFN below N´ eel temperature T N of the superantiferromagnetic (SAF) clusters, which coexist with the long-range AFM phase (note that, originally, the concept of superantiferromagnetism was introduced by N´ eel, when interpreting the experimental results on the fine AFM particles 9 ). Important results were also obtained by the nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) methods. 10–13 In particular, the 93 Nb NMR spectra in PFN 13 showed the coexistence of two different Nb sites which have different local magnetic fields originating from the Fe-rich (Nb-poor) and Fe-poor (Nb-rich) nanoregions. These data suggest that a SG state in PFN, below 20 K, might arise from the latter regions. 13 The information about the PFN-based solid solutions is more restricted. Valuable data were obtained in Refs. 4 and 7 concerning the influence of the Ba and Ca substitution for Pb on the ferroelectric and magnetic properties of PFN, from the point of view of a possible leading role of the Pb ions in both the ferroelectric and magnetic coupling. In particular, the decrease of T N with increase of the Ba doping was explained by possible involvement of the Pb ions in the superexchange of the iron ions. However, the ground magnetic state of the Ba-doped PFN remains unclear. Similar problems also still exist for the Ti-doped solid solutions of PFN. References 6 and 7 contain the phase diagram of the magnetic and ferroelectric properties of Ti- doped PFN, built on the basis of the dielectric, pyroelectric, piezoelectric, Mossbauer, structural, and magnetization data, for a Ti concentration up to x = 0.4. In these phase diagrams, the ferroelectric-to-paraelectric phase transition temperature was shown to increase approximately linearly with x , while the temperature of the transition between the ferroelectric rhombohedral (monoclinic) and tetragonal phases decreases with x nonlinearly. 6,7 However, the magnetic characteristics still need further measurements and understanding. The main aim of this paper is to report on experimental data about the magnetic properties of the Ti- and Ba-doped solid solutions of PFN, in order to utilize these data for plotting the concentration-temperature phase diagrams. We will show that these phase diagrams reveal fingerprint features of the percolation phase transitions of the AFM order in PFN on doping, which can help in the understanding of the changes of the properties of PFN with the concentration of the doping elements. The plan of our paper is the following. After a short description of the list of the experimental methods in use (Sec. II), we report on our experimental exploration of the magnetic susceptibility (Sec. III A) and magnetization hysteresis loops (Sec. III B). Sec. III C presents our EPR data. Then, in Sec. IV, we discuss the results obtained. 064403-1 1098-0121/2013/87(6)/064403(8) ©2013 American Physical Society