Critical RF losses in fine particles of La 1x Ag y MnO 3+d : Prospects for temperature-controlled hyperthermia V.A. Atsarkin a, , A.A. Generalov a , V.V. Demidov a , A.E. Mefed a , M.N. Markelova b , O.Yu. Gorbenko b , A.R. Kaul b , E.J. Roy c , B.M. Odintsov c a Kotel’nikov Institute of Radio Engineering and Electronics RAS, 11, Mokhovaya Str., 125009 Moscow, Russia b Moscow State Lomonosov University, Department of Chemistry,119992 Moscow, Russia c University of Illinois at Urbana-Champaign, 61801 IL, USA article info Article history: Received 11 January 2009 Received in revised form 14 May 2009 Available online 28 May 2009 PACS: 75.40.Gb 75.47.Lx 87.54.Br Keywords: Magnetic nanoparticle Manganite Dynamic susceptibility Curie point Magnetic hyperthermia Temperature stabilization abstract Temperature and frequency dependencies of the real (w 0 ) and imaginary (w 00 ) parts of the dynamic magnetic susceptibility were studied experimentally in fine particles of La–Ag manganites prepared by various methods. The samples under study have the Curie points in the range T C ¼ 42–48 1C, which is a medical hyperthermia range of interest. When approaching T C from below, a critical peak of w 00 was revealed, followed by a steep drop while passing to the paramagnetic phase. The experiments on the magnetic radio-frequency (RF) heating of the manganite aqueous suspensions demonstrated good autostabilization of the temperature near T C . Peculiar instability is found in the heating kinetics, caused by the observed critical behavior of the RF losses. The prospects of the La–Ag manganites as candidates for application in the temperature-controlled hyperthermia are discussed. & 2009 Elsevier B.V. All rights reserved. 1. Introduction A prospective branch of local medical hyperthermia, based on radio-frequency (RF) heating of fine magnetic particles introduced into diseased tissue (such as a tumor) has developed in the last few years [1–8]. The heating is produced by application of an alternating magnetic field H cos(ot), with the frequency o/2p0.1–1.0 MHz, being low enough to avoid the effect of inductive currents and provide RF absorption caused either by magnetic hysteresis or relaxation mechanisms. In many important applications, the permissible temperature variations are strictly limited by about 71K around the therapeutic value which commonly falls in the range 42–48 1C [2–5]. Thus accurate temperature control is crucial. To prevent overheating, magnetic materials are suggested which have the Curie temperature T C in the required temperature range. Since ferromagnetic RF losses vanish above T C , the autostabilization of the heating temperature near T C may be achieved. Recently, this idea was qualitatively confirmed by model experiments performed with aqueous suspensions of the manganites doped with Sr (LSMO) [9–11] and with Ag (LAMO) [12]. If the magnetic phase transition at T C could be infinitely narrow, the temperature stabilization would be absolute. In practice, however, the RF absorption does not disappear abruptly at T C ; instead, it is determined by more or less steep decreasing of the imaginary part of dynamic magnetic susceptibility, w 00 . Thus, the experimental study of w 00 (T) in the critical region is necessary to estimate the prospectives of materials as candidates for application in temperature-controlled hyperthermia. In the pre- sent work, such studies are performed on the La 1x Ag y MnO 3+d fine particles prepared by various methods and having T C within a medical hyperthermia range of interest. Both standard Q-meter and RF heating techniques were employed to investigate the critical behavior of the RF losses. Along with the critical decrease above T C , a strong peak in w 00 (T) just below T C was found, resulting in peculiar kinetics of the RF heating. 2. Material and methods Two different methods were used to prepare the La 1x Ag y MnO 3+d particles: (1) a version of chemical homogenization at 700–800 1C ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.05.043 Corresponding author. Tel.: +7495 6293440; fax: +7495 6293678. E-mail address: atsarkin@mail.cplire.ru (V.A. Atsarkin). Journal of Magnetism and Magnetic Materials 321 (2009) 3198–3202