IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014 5201104 High-Frequency Speciﬁc Absorption Rate of Co x Fe 1−x Fe 2 O 4 Ferrite Nanoparticles for Hipertermia Applications Dan Durneata 1 , Rolf Hempelmann 1 , Ovidiu Caltun 2 , and Ioan Dumitru 2 1 Department of Physical Chemistry, University of Saarland, Saarbruecken 66123, Germany 2 Faculty of Physics, University Alexandru Ioan Cuza, Iasi 700506, Romania The heating processes in ferroﬂuids in ac magnetic ﬁeld depend on chemical composition, dimension, shape, magnetic properties of the nonoparticles, and rheological characteristics of the dispersing medium. By controlling these parameters, a maximum energy can be transferred to the medium. This paper was focused on determining the speciﬁc absorption rate (SAR) of a series of nanoparticles (NPs), with ferromagnetic properties at room temperature, dispersed in water to prove their possible use in medical applications. Co x Fe 3-x O 4 (with x = 0.2–1 in steps of 0.2) magnetic ﬂuids were synthesized by the coprecipitation method and subjected to an ac magnetic ﬁeld with different amplitudes and distinct frequencies. X-ray difractometry and transmission electron microscopy were used to characterize the phase and microstructure of NPs. Vibrating sample magnetometer measurements denoted a ferrimagnetic behavior of the particles at room temperature and expected superparamegnetic behavior for ferroﬂuids. The values of SAR obtained using a calorimetric method, at ﬁxed frequency, increased with strength of applied ﬁeld and Co content being higher at low frequencies. The results are explained in terms of relaxation times, and the experimental data were compared with theoretical predictions. Index Terms— Heating mechanisms, hyperthermia, magnetic particles. I. I NTRODUCTION M AGNETIC nanoparticles (MNPs) are intensively stud- ied due to their use in multiple applications as catalysts, such as contrast agents in the magnetic resonance imag- ing, drug delivery, magnetic hyperthermia (MHT), magnetic recording, and so on [1]–[4]. They can be dispersed in almost all liquid medium, and they may be used in MHT for industrial or medical reason [5]. This noninvasive method consists on injecting the magnetic seeds inside the tumor, where they are accumulated. To warm them, an external alternative (ac) magnetic ﬁeld [6] is applied for a period of time. Through some speciﬁc mechanisms, the particles will be heated up to an optimum temperature of 42 °C–46 °C at which the cancer cells will be destroyed without affecting the healthy ones. It is known that at temperatures above 41 °C, the cancerous cells are more sensitive than the healthy ones [7], and after the heating process, there are necrosed [8]. The heating mechanisms are inﬂuenced by the shape, size distribution, and magnetic anisotropy of MNPs [9], and adjust- ing these parameters, the heating capacity of the system could be modiﬁed. Speciﬁc absorption rate (SAR) is a physical parameter used to describe the interactions between the elec- tromagnetic ﬁeld and matter. SAR is deﬁned as the amount of heat released by a unit weight of material per unit of time during exposure to an alternating magnetic ﬁeld of the prescribed frequency and ﬁeld intensity values [10]. In the therapeutic process, the ﬁeld intensity ( H ) and frequency ( f ) must be chosen in such way as the product f × H should not exceed 5 × 10 9 Am −1 s −1 [11]. In the same time, the heat Manuscript received March 7, 2014; revised May 6, 2014; accepted May 6, 2014. Date of current version November 18, 2014. Corresponding author: I. Dumitru (e-mail: idumitru@stoner.phys.uaic.ro). 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.2014.2324011 dissipated by each particle in a right volume is preferred to be as high as possible to reduce the quantity of magnetic seeds that are injected into tumor on vivo studies and evitate the patient poisioning [12], [13]. In this paper, a series of Co x Fe 3−x O 4 , with x ranging between zero and one, ferrite nanoparticles (NPs) was synthesized by the coprecipitation method. The crystalline structure and spinel phase accomplishment during the synthesizing process were studied using X-ray difractometry (XRD). The NPs’ shape, average, and distribution of particles size were determinate by transmission electron microscopy (TEM). The magnetic properties as remanent and saturation magnetization and coercive ﬁeld strength were determined using vibrating sample magnetometer (VSM) at room temperature. Using a calorimetric method [14], the SAR was calculated from ferroﬂuid temperature dependence on time at different ﬁxed value of frequency and amplitude of the ac magnetic ﬁeld. The experimental data obtained were compared with the theoretical predictions. II. EXPERIMENT The MNs of Co x Fe 3−x O 4 ferrite were prepared by copre- cipitation method by mixing CoCl 2 and FeCl 3 solutions with a surplus of NaOH in different concentrations. The dark colored suspensions were kept under stirring and then left to cool at room temperature. To achieve more stable NPs, HNO 3 was added. The MNPs were washed three times in water using a permanent magnet. Fe(NO 3 ) was used to form a nonmagnetic shell on the NP surface and, thus, to ensure the stability of the particles dispersed in water. The coated particles were washed for several times to remove the salt compound. To achieve the colloidal dispersion of Co x Fe 3−x O 4 , the surface charge excess was neutralized by the addition of aqueous solutions. More details on the preparation method are described in [15]. 0018-9464 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.