4570 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012 Influence of Technologically Driven Disorder on Spin Dynamics in Manganites in Mid-to-Far Critical Range M. Auslender , A. I. Shames , and E. Rozenberg Department of Electrical and Computer Engineering BGU of the Negev, Beer-Sheva 84105, Israel Department of Physics BGU of the Negev, Beer-Sheva Israel Electron paramagnetic resonance measurements were used to probe spin dynamics over mid-to-far critical range for four samples of different structural forms of . As a result of different fabrication routes, these samples contain different imperfections which may have specific impacts on temperature dependence of the EPR linewidth. An analysis of critical spin relaxation speedup, man- ifested by sharp increase of the linewidth and Onsager kinetic coefficient when approaching to magnetic transition points, was carried out using two critical exponents derived from dynamic and static scaling laws. Deviations of these exponents from theoretical bounds were proposed to be hallmarks of the disorder types. Index Terms—Critical phenomena, magnetic resonance, manganese compounds, spin dynamics. I T IS generally accepted that the basic physics of doped mixed valence manganites ( and rare earths, , Sr, Ba, etc) is governed by the close inter- play between charge, spin, and lattice degrees of freedom [1]. It has also been realized that chemical atomic-scale disorder due to ion-radii mismatch between and strongly influences Curie temperature and entire phase diagram of the man- ganite system [2]. Less known is ‘technologically driven’ chem- ical disorder, i.e. spatial fluctuations of the dopant content [3], which appears in crystalline manganites due to a competition between the quenching and annealing effects when cooling the crystal from its high synthesis temperatures [4]. The meso-scale structural/chemical disorder, e.g. dislocations, phase boundaries/vacancies emerges at grains and crystallite boundaries in manganite ceramics and nano-crystal powder, re- spectively. Electron paramagnetic (PM) resonance (EPR) data, obtained in low-hole-doped series of manganite single crystals [4], [5] revealed visible hallmarks which evidence the techno- logically driven disorder. Also, the data for different samples of most stable optimally hole-doped manganite [6] indicate a strong impact of this disorder. All these findings motivated us to perform a study of critical spin dynamics in a series of samples using the EPR technique. Dynamical criticality is well known powerful tool for probing the magnetic order, see e.g. (1)–(8) below for details. We have managed to show that spin dynamics observed in mid-to-far critical -range differs for samples obtained using different fabrication routes [6]. It appears that in sam- ples studied, the meso-scale technologically driven disorder is probed mainly by the critical spin dynamics. It is specially worth noting that the reported analyses of the EPR data lie far beyond a routine spectroscopic study as it deals with the basic physical Manuscript received March 01, 2012; revised June 07, 2012; accepted June 11, 2012. Date of current version October 19, 2012. Corresponding author: M. Auslender (e-mail: marka@ee.bgu.ac.il). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2012.2204730 properties concerning magnetism of the studied manganese ox- ides. The (hereafter labeled as Ca0.1) is a typical low-hole-doped manganite with La-sites substitution below a critical level at which a crossover from localized- to itinerant-type of conduction occurs [1]. In this work, we have studied: a bulk single crystal (bcr-Ca0.1), grown by a radiative heating floating-zone method [3], nano-crystalline powder (ncr-Ca0.1) with the crystallite’s mean size of 24 4 nm, prepared by the sonication-assisted co-precipitation [7] and two ceramics—a stoichiometric (scer-Ca0.1) and a 1% La-Ca deficient (dcer-Ca0.1) synthesized in air using standard solid-state reaction at . The details of experi- mental techniques, samples’ structure, magnetic and resonance properties may be found in [8], [9]. In accordance with the literature data [10] below some , bcr-Ca0.1 transits to a mixed state comprising canted antiferromagnetic (AFM) matrix and meso-scale ferromagnetic (FM) clusters which order at . In a contrast, due to a weaker chemical and structural disorder in crystallite cores, ncr-Ca0.1 exhibits an in-core FM order [8]. The vacancies in Ca0.1 ceramics are randomly distributed over La-Ca sites and an inhomogeneous FM order is observed in both scer-Ca0.1 and dcer-Ca0.1 [9]. The magnetic transition temperatures for all samples are listed in Table I. The resonant field , linewidth and a doubly-in- tegrated intensity (DIN) were obtained in a course of X-band EPR measurements. It appears that in all cases, notably increases upon approaching magnetic transition temperatures , which is a clear indicator of the critical spin dynamics. An idea behind our analysis is to explore this spin relaxation speedup at PM state versus sample/disorder type. Due to Mori method, the spin-spin relaxation frequency is given by (for manganites, see e.g. in [11]) (1) where is the Bohr magneton, and are the Plank and the Boltzmann constants; is a transverse spin susceptibility, 0018-9464/$31.00 © 2012 IEEE