1. Introduction
Magnetic Nanoparticles
Magnetic nanoparticles (MNPs) are playing a crucial role in an extensive number of potential applications and science fields. Nanotechnology industry is rapidly growing with the promise that it will lead to significant economic and scientific impacts on a wide range of areas, such as health care, nanoelectronics, environmental remediation. MNPs are mostly ferrites, i.e, transition metal oxides with ferric ions as main constituent. Although the magnetic properties of ferrites [1] are less intense than metal’s, especially saturation magnetization, ferrites possess a large chemical stability (corrosion resistance), high electrical resistivity, and extended applicability at high magnetic field frequencies.
The use of MNPs for biological and clinical applications [2] is undoubtedly one of the most challenging research areas in the field of nanomaterials, involving the organized collaboration of research teams formed by physicists, chemists, biologists, physiciens. The advantages of MNPs are based on their nanoscale size, large surface area, tailoring of magnetic properties and negligible side effects in living tissues. These applications include drug delivery [3], magnetic hyperthermia [4], magnetic resonance imaging [5], biosensors [6]. A field related with microwave absorption is electromagnetic interference EMI [7], as the number of electromagnetic radiation sources has growth at an exponential rate. MNPs have found applications also in environmental fields, such as soil remediation [8] and heavy metal removal [9], as MNPs provide high surface area and specific affinity for heavy metal adsorption from aqueous systems.
The reduction in scale leads to strong changes in macroscopic properties. The main reason can be attributed to the enhanced importance of the surface atom fraction as compared with core atom fraction, as the material becomes a nanoparticle. A simple estimate reveals that for a ~100 nm nanoparticle, surface atom fraction is about 6% of the total NP atoms, while for a ~5 nm NP this fraction can attain 78% [10]. Surface layer of materials exhibits different properties simply because these atoms have a very different structure than the core’s. Surface atoms, for instance, have a reduced coordination number (unsatisfied bonding), crystal defects and modified crystal planes (“broken symmetry”). In the case of magnetically ordered materials (ferro, ferri, antiferromagnetic phases), additionally, several magnetic properties critically change at the nanometric scale. These properties are, for instance, the change from multidomain to single domain magnetic structure, domain wall thickness, the decrease in anisotropy energy giving rise to superparamagnetic phenomena. MNPs can thus exhibit many property changes with the reduction in size. Last (but no least), MNPs can show important macroscopic effects of interparticle magnetic interactions, which can involve additive forces (exchange), or attraction/repulsion (dipole).
Synthesis of MNPs
The most common method to synthesize MNPs are based on coprecipitation and microemulsion [11]. The coprecipitation method produces NPs by a pH change in a solution containing the desired metals in the form of nitrates or chlorides. Average size and size distribution, as well as shape depend on the pH and the ionic strength of the precipitating solution. In the microemulsion method, an aqueous metal solution phase is dispersed (entrapped) as microdroplets in a continuous oil phase within a micellar assembly of stabilizing surfactants. The advantage is that the microdroplets provide a confined space which limits the growth and agglomeration of NPs.
An emerging method for preparation of uniform NPs is the polyol technique, where metallic salts (acetates, oxalates), dissolved in an alcohol (such as diethylenglycol) are directly precipitated by high temperature decomposition [12]. This method can produce metals; by addition of a controlled amount of water, it can lead to oxide MNPs.
Spray and laser pyrolysis, with great commercial scale-up potential have been reported [13]. In spray pyrolysis, a solution of a ferric salt (and a reducing agent) is sprayed through a reactor to produce evaporation of the solvent within each droplet. In laser pyrolysis, the laser energy is used to heat a flowing mixture of gases leading to a chemical reaction. Under the appropriate conditions, homogeneous nucleation occurs and NPs are produced.
A different method utilizes high-energy ultrasound waves to create acoustic cavitations resulting in extremely hot spots. The sound waves produced by these cavities can lead to particle size reduction and hence the formation of NPs [14]. Other methods are based on electrochemical deposition of metal in a cathode [11], and also the use of magnetotactic bacteria [15].
Microwave absorption in MNPs
In this brief review, some of the recent developments in microwave absorption in MNPs. The response of ferromagnetic resonance (FMR) of MNPs (under different conditions) is first reviewed. FMR results on consolidated materials by spark plasma sintering (SPS) techniques are included, as this method allows the preparation of nanostructured ferrites (grains under 100 nm in size) with high densities. The behavior of electron paramagnetic resonance (EPR) of some relevant magnetically disordered MNPs systems is also presented. We devote a part of this review to the emerging low field microwave absorption technique (LFMA), which is a non-resonant method providing valuable information on magnetically-ordered materials. Results on MNPs in different aggregation states, as well as SPS-sintered materials are briefly reviewed.
As FMR [16] and EPR [17] techniques are well known, so no additional treatment of them is included here, except for some references.
2. Ferromagnetic resonance (FMR) in ferrite nanoparticles
Temperature and size dependences
One of the most studied phenomena on MNP’s is the change of their FMR spectra as a function of temperature. The main parameters describing FMR signal are plotted versus temperature and eventually compared with the bulk counterpart, as an attempt to characterize the changes associated with the nanometric scale. Such parameters are generally peak-to-peak resonance linewidth, ∆
The resonance field behavior with temperature for bulk and MNP’s decreases with decreasing temperature, as a consequence of the enhancement of the contributions to the internal field associated with magnetic ordering (mainly exchange and anisotropy). This effect is stronger for small particles (figure 1) [25], revealing additional contributions to the internal field at low temperatures as a consequence of the size decreasing. These contributions can be assumed as an extra unidirectional internal field arising from surface disorder, where the magnetization processes are presumably to be isotropic and causes an extra shift of the resonance field. Therefore, as the surface area increases with decreasing the particle size, the isotropic effects on the magnetic resonance behavior are more pronounced and an additional distribution of energy barriers, promoted by surface isotropic disorder, must be assumed.
As observed in many works [23,25-27], the FMR spectra for MNPs at intermediate temperatures results to be a mixing of two lines: a broad component corresponding to typical anisotropic contributions and a narrow one, presumably corresponding to the surface isotropic contributions. This leads to a characteristic FMR shape for nanoparticulated systems. The general features include a broad component (becoming wider and shifting to lower fields upon cooling, see figure 2), a narrow component, and a large broadening and shifting as the particle size decreases. Figure 2 shows the FMR signal evolution with temperature variation from room temperature down to 100 K for a well diluted magnetite suspension [26]. A double component spectrum is well observed at high temperatures, while its two components seem to overlap in a single broad signal as temperature goes down. This can be attributed to an important decrease on isotropic contributions which causes the narrow component to disappear at low temperatures. In addition, decreasing temperature makes the broad component to widen, becoming more symmetric, and shifting to lower fields, thus revealing a random distribution of the anisotropy axis enhanced at low temperatures. Inter-particle interactions are negligible and do not contribute to this broadening since the high dilution of the studied suspension promotes isolation between particles.
Figure 3 shows the absorption signals for different particle distribution sizes of non-interacting maghemite (γ-Fe2O3) nanoparticle ferrofluid samples at room temperature [27]. The absorption signal changes drastically with changes on the particle size. Two limit cases are observed. The upper limit, for the largest size particles (sample 2,
Particle concentration dependence
Double component FMR spectra have also been reported for solid and liquid Fe3O4 suspensions [26]. Important changes were observed on the absorption signal depending on the particle concentration. Figure 4 shows the decrease of the narrow component as the concentration increases. Both components exhibit also a broadening as the concentration increases. This important dependence of the signal linewidth with concentration is due to an increase of particle dipolar interactions at mean distances and a consequence of aggregation [28-30].
Particle interactions always play an important role on the magnetic resonance absorption phenomena. FMR at different frequencies on dispersed KBr and then compressed NiFe2O4 commercial nanoparticles has recently been reported [31]. The resulting pellets showed strong shape anisotropy, as in-plane and out-of-plane analyzed measurements diverge with the increase of nanoparticle packing fraction (figure 5).
The resonance field decreases with increasing volume for in-plane measurements. In contrast, for out-of-plane measurements it increases with volume fraction. Both cases (in and out of plane), showed a resonance field shifted to a lower field in comparison with an ideal bulk as reported. As the packing fraction decreases, the in-plane and the out-of-plane curves converge to the same field value (≈ 0.04 T). This value can be identified as the average effective anisotropy field of the particles. A useful and powerful tool to estimate particle anisotropy can be based on these measurements.
Angular dependence
The anisotropy distributions and its FMR signal effects on MNP’s can be carried out by measuring the angle variation between the cooling field of field-cooled (FC) samples. By comparing the obtained spectra for different orientations of FC samples, it is possible to determine the contribution of the anisotropy distribution within each particle to the FMR signal. The corresponding resonance lines often lead to a double component signal. It is then possible to separate the contributions which have a strong dependence with the angle from the weakly dependent counterpart. The narrow component can be attributed to surface anisotropy, while the the broad component should be associated with internal anisotropy of the NPs. Recent works have demonstrated the isotropic/anisotropic nature of most common MNP’s, as described above. Figure 6 shows differences between FMR spectra for parallel and perpendicular configurations of FC samples. The samples consisted of maghemite nanoparticles with a particle diameter reported as 4.8 nm [27]. At the high temperature (top in figure 6), no variation as a function of θ is observed for the narrow component, while the broad component, on the contrary, shifts to higher resonance fields as the angle θ increases (bottom in figure 6). The narrow component is not affected by angular variations.
The resonance field of the anisotropic component generally is plotted against the angle θ, the dependences with θ use to be fitted, in good agreement, with sin2 θ or cos2 θ functions [24-32]. Its well established the axial symmetry nature for this kind of functions, as magnetization processes perform the higher energy absorption (energy improved by the resonance field) when the magnetizing field is perpendicular to the axial orientation, the resonance field also increases and finds a maximum at this position. The differences between the resonance field at θ = 0° and θ = 90°, are related to the anisotropy field, promoted by the respective axial anisotropy within the core of the nanoparticles revealing a mono-domain configuration on this region (figure 7).
The fitting of the angular resonance field dependence is useful to determine anisotropy parameters. The dependence can be given by [32]:
where
Magnetic phase transitions produce significant changes in |
Figure 8 shows the temperature dependence of magnetization of FC hematite NPs, parallel to the cooling field [24]. A change in the slope is observed in good agreement with observed transitions from a disordered phase to a magnetically ordered phase. In contrast with bulk hematite (antiferromagnetic above 260 K), these hematite NPs exhibited a superparamagnetic behavior, which can be explained in terms of a weakly ferromagnet below ~ 200 K, as shown in figure 8. The reported transition is also observed in plots of the resonance field against temperature (figure 9) and other FMR spectrum parameters, which suggest the FMR technique as a useful tool to investigate phase transitions on MNP’s.
Effects of the aggregation state
Ferrite NPs can show the effects of two extreme aggregation states, namely monodisperse NPs, and clusters of a few hundreds of NPs. Samples of composition Zn0.5Ni0.5Fe2O4 were prepared by the polyol method, and designated as “A” for monodisperse state, and “B” for clusters. FMR spectra exhibited significant differences, as shown in Fig. 10. The decrease in the effects of surface for sample B appear in the form of a lower resonance field and an increase in the linewidth, as compared with sample A. The former can be understood in terms of a larger internal field in the cluster sample as a consequence of the aggregation of NPs; simply, the NPs inside the cluster tend to behave as grains in a bulk material. The effects of surface (crystal defects, unsatisfied bonds, etc.) are relieved by the presence of other NPs. Magnetic exchange interactions among neighboring NPs can also be assumed, which increases the internal field and thereby decreases the applied magnetic field needed to fulfill the Larmor equation conditions. On the other hand, these interactions increase the linewidth, especially the random distribution of anisotropy axes.
At room temperature, as shown in Fig. 11, isolated NPs show a superparamagnetic phase and again a larger resonance field with a significant reduction in linewidth as the factors just mentioned, associated with an ordered magnetic structure, are absent. The cluster sample exhibits a reduced linewidth, but always larger than the superparamagnetic state.
3. Ferromagnetic resonance in nanostructured ferrites
Ferrite NPs have to be consolidated as a high density solid for many applications (in electronic devices, for instance), where a powder is unstable. Typical sintering processes needing high temperatures are difficult to apply, as NPs tend to grow very rapidly at temperatures above 500°C, losing the nanometric size range and thus the different properties associated with this size range. A particularly well suited method to consolidate NPs into a high density nanostructured solid preserving grains within the nanometric range is Spark Plasma Sintering (SPS for short) [33]. Also known as pulsed electric current sintering (PECS), in this technique the sample (typically in the form of a powder) is placed in a graphite die and pressed by two punches at pressures in the 200 MPa range, while a strong electric current goes through the system; the die is shown in Fig. 12. SPS therefore consolidates powders under the simultaneous action of pressure and electric current pulses (typically of a few milliseconds in duration [33]).
The electric current pulses result in a very rapid heating of the sample, at rates as high as 1000°C/min. If the sample is a good conductor, current goes through it and the heating is even more efficient. A significant point is that the electric current has a significant impact on the atomic diffusion during the process [34]. The sintering process can then reach high densities at very low temperatures and extremely short times [35]. Obviously, SPS can also be utilized for reactive sintering involving a chemical reaction [36].
SPS has been used to consolidate spinel [37], garnet [38], and hexagonal ferrites [39]. In the case of spinel Ni0.5Zn0.5Fe2O4 ferrites, samples prepared in the form of 6-8 nm nanoparticles by the forced hydrolysis in a polyol method [12], were consolidated by SPS at temperatures in the 350-500°C range by times as short as 5 min. Just for comparison, the typical conditions for sintering in the classic solid state reaction are 1200°C for at least 4 hours. NPs growth was controlled, as the final grain size in the consolidated ferrite was about 60 nm, even for the highest (500°C) SPS temperature [40].The densities reached values as high as 94% of the theoretical value. FMR spectra obtained at 77 and 300 K are shown in Fig 13, together with the FMR signal corresponding to the original NPs. The resonance field exhibited a decrease as the SPS temperature increased, which can be explained in terms of the components of the total field in the Larmor expression, ω = γ
All the samples exhibited a large broadening in the linewidth, generally interpreted by considering a random distribution of the anisotropy axis in single domain NPs [41]. The broadening decreases as consolidation increases, as the surface effects are diminished by formation of grain boundaries. At room temperature the linewidth decreases since all the samples approach the paramagnetic state where the internal field is eliminated and only the applied field is involved in the Larmor expression. The NPs signal exhibited the lowest linewidth as at room temperature these NPs are superparamagnetic; all the consolidated samples showed a ferrimagnetic behavior at 300 K as compared with the as-produced sample which has a blocking temperature about 90 K.
4. Paramagnetic resonance in ferrite nanoparticles
Zinc ferrite is a very good material to investigate the site occupancy by cations. In spite of a relatively large cation radius, Zn+2 has a tendency to occupy tetrahedral sites, while Fe3+ fill octahedral sites [1], in other words, a “normal” spinel. This arrangement leads to B-O-B superexchange interactions between iron cations and therefore to an antiferromagnetic structure with a Néel temperature about 9 K. In the case of nanosized Zn ferrite, however, the cation distribution can be significantly different; some degree of inversion occurs [1,42], with a fraction of Zn2+ on octahedral sites, and hence Fe3+ on both sites. The spinel becomes ferromagnetic, with a Curie temperature well above 9 K.
The variations in linewidth, Δ
Several factors can produce a distribution of local field, such as unresolved hyperfine structure, g-value anisotropy, strain distribution, crystal defects. The field strength on a particular spin is then modulated by local field distribution and leads to an additional linewidth broadening [44,45].
The strain distribution produced by a small average particle size can cause a small resonance signal [1]. As the particle size is increased, such small signal disappears and only the broad signal remains. This suggests that the particle size has a threshold value above which the strains are relieved.
In order to gain some insight into the relationship between internal structure and EPR spectra, it can be useful to compare the properties of several iron-based oxide NPs embedded in a polytethylene matrix, prepared by the same method: Fe2O3, BaFe2O4, and BaFe12O19 [46]. The experimental EPR spectra of the samples are presented in Figs. (14-20). At room temperature the EPR spectra of all the samples show a ‘‘two-line pattern’’ (Fig. 14) which is typical of superparamagnetic resonance (SPR) spectra. The relative intensity of these lines depends on the particle size and shape distribution function [47]. For the Fe2O3 and BaFe2O4 samples, the broad line predominates in the room temperature spectra; the opposite is observed for the BaFe12O19 sample, where the narrow line is more pronounced.
At room temperature, the spectra of all samples show a “two-line pattern” (Figure 15) which is typical of superparamagnetic resonance. These spectra can be considered as a broader line superimposed on a narrow line. The relative intensity of these lines depends on the particle size and shape distribution function, as well as on the magnitude of the magnetic anisotropy. For Fe2O3 and BaFe2O4 samples, the broad line predominates in the RT spectra. This line is characterized by a peak-to-peak linewidth of Δ
At low temperatures, the EPR spectra of Fe2O3 change significantly (Fig.16). On cooling below 100 K, the broad line S1 shows a monotonous increase of the linewidth ∆
EPR spectra of the BaFe2O4 nanoparticles at different temperatures are shown in Fig. 18. At all temperatures the spectra are broad and rather asymmetric. A significant shift of the line position to low magnetic fields and a marked spectrum broadening are observed at low temperatures.
On the other hand, the thermal variations of EPR spectra of the BaFe12O19 sample is typical of superparamagnetic resonance. The relatively narrow line that dominates at room temperature disappears as temperature decreases (Figs. 19 and 20). The BaFe12O19 nanoparticles reveal an EPR signal that is significantly narrowed at high temperatures by superparamagnetic fluctuations. This is evidence of the reduced magnetic anisotropy energy that may be due to the particle’s nanosize (effective diameter <10 nm).
These results show that EPR spectra at low temperatures are desirable for the correct identification of NPs and a comparison with high temperature experiments allows a better understanding of phenomena related with variations associated with nanosized materials.
5. Low Field Microwave Absorption (LFMA)
Low field microwave absorption (LFMA for short) refers to the non-resonant, hysteretical losses of a material subjected to a high frequency electromagnetic field. Recently, it has become a useful method to investigate magnetization processes [48], magnetoelastic effects [49], phase transitions [50], non-aligned ferromagnetic resonance [51,52], spin arrangements [53,54]. LFMA is similar to giant magnetoimpedance (GMI) [55], but physically different to ferromagnetic resonance (FMR) [56]). GMI, generally defined as the variations of impedance of a magnetic conductor carrying an alternate electrical current when subjected to an external magnetic field [57], extends into a very wide frequency range. Clearly, GMI is a non-resonant phenomenon as confirmed by two facts: GMI does not fulfill the resonant Larmor conditions, and exhibits magnetic hysteresis.
LFMA is associated with magnetization processes in magnetically ordered materials, in the process from the unmagnetized state to the magnetic saturation. In bulk ferro and ferrimagnetic materials, LFMA exhibits a flat response in the paramagnetic phase. To measure experimentally LFMA in a typical FMR/EPR facility, the applied field has to be cycled; usually between -1 kOe and +1kOe is enough. Also, a device to compensate for the remanent magnetization is needed in most electromagnets.
An important parameter is, of course, the total anisotropy field of the particular sample. In most cases, LFMA exhibits a critical behavior at the total anisotropy field in the form of a maximum and a minimum, leading to a characteristic signal as shown in Fig. 21 [59]. In bulk Ni-Zn ferrite, a correlation exists between the magnetocrystalline anisotropy and the half-peak-to-peak, measurement of LFMA. Figure 22 shows a comparison between the peak-to-peak LFMA field (divided by 2) [60] and a calculation from magnetocrystalline constant,
LFMA seems to be associated with spin structure. By convention, this signal can be assigned as “positive”, simply because it has the same shape than the FMR/EPR signal, i.e., a maximum and a minimum when observed from left to right. A positive LFMA sign has been observed in most insulator and semiconductor materials, while a “negative” signal appears for most metallic conductors, as shown in Fig. 23, for a Co-rich CoFeBSi amorphous microwire [59]. An interesting result was found in bulk Ni-Zn ferrites showing the Yafet-Kittel triangular arrangement [62], by measuring the LFMA as a function of temperature in the 150 K-240K temperature range [63]. The LFMA sign changed from negative at ~ 154 K to positive at
LFMA in NPs
LFMA can provide useful insights into the structure of NPs. When two different aggregation states are compared, i.e., monodisperse and clustered NPs with the same composition and NP diameter, clearly different spectra are obtained. By varying the synthesis conditions in the forced hydrolysis in a polyol method [12], Ni-Zn ferrites can be obtained as monodisperse, well crystallized ~ 6 nm NPs on one hand, and labeled as sample “A”; on the other, clusters about ~ 100 NPs constituted by NPs with the same composition and diameter [58], labeled as sample “B”. It is interesting to mention that high resolution transmission electron microscopy (HRTEM) showed some epitaxial arrangements within the clusters of sample B.
Sample A showed a superparamagnetic behavior at room temperature, and a blocking temperature about 50 K [58]. Sample B, in contrast, exhibited a ferromagnetic behavior up to 300 K (blocking temperature > 300 K), in spite of being constituted by NPs of the same composition and NP diameter. Clusters effectively decrease the effects of surface producing samples with a general behavior between that of NPs and bulk materials.
Figure 24 shows the LFMA signal from sample A at 77 and 300 K. AT the low temperature, a positive LFMA behavior is observed, corresponding to a non-conductor material. At 300 K, however, a flat response appears, associated with the superparamagnetic phase of these monodisperse NPs. Clearly, the superparamagnetic phase behaves like a paramagnetic signal. This flat signal with a small slope is associated with the microwave absorption of non-interacting dipoles [64].
Clusters of the same NPs, in contrast, showed a clear positive LFMA signal at room temperature, see Fig. 254. This is consistent, as this sample B behaved as a ferromagnetic phase close to the bulk properties.
For low temperatures, however, sample B exhibited a very different signal as shown in Fig. 26, with an important hysteresis and no similarity to either positive or negative character. As mentioned above, bulk Ni-Zn ferrites with Zn content
6. Conclusions
Microwave absorption (MA) is a very sensitive phenomenon and has become an extremely powerful characterization tool. MA accurately depends on all the factors surrounding unpaired electrons; it can play a significant role in the characterization of the complex and fascinating development of magnetic nanoparticles. In this brief review, recent results on the characterization of magnetic nanoparticles and consolidated spinel ferrites by means of ferromagnetic resonance, paramagnetic resonance, and low field microwave absorption have been presented.
Acknowledgement
Authors acknowledge partial support for this work from ANR-CONACyT grant 139292, as well as PAPIIT-UNAM grant IN141012.
References
- 1.
. Magnetic Ceramics. Cambridge University Press (Valenzuela R 2005 ) (0-52101-843-9 - 2.
Amiri S. Shokrollahi H. The role of cobalt ferrite magnetic nanoparticles in medical science Materials Science and engineering C1 EOF 8 EOF - 3.
Mahmoudi M. Sant S. Wang B. Laurent S. Sen T. Superparamagnetic iron oxide nanoparticles (SPIONs): development, surface modification and applications. Advanced Drug Delivery Reviews2011 - 4.
Kashevsky B. E. Agabekov B. E. Kashevsky S. B. Kekalo K. A. Manina E. Y. Prokhorov I. V. Ulashchik V. S. Study of cobalt ferrite nanosuspensions for low frequency ferromagnetic hyperthermia 2008 6 5 322 333 - 5.
Multifunctional magnetic resonance imaging probes. Recent Results in Cancer ResearchKliza E. Strijkers G. J. Nicolay K. 2013 187 151 - 6.
Electrochemical biosensors based on magnetic micro/nanoparticles. Electrochimica ActaXu Y. Wang E. 2012 84 1 62 73 - 7.
Conducting polymer embedded with nanoferrite and titanium dioxide nanoparticles for microwave absorption. Synthetic MetalsDahawan S. K. Singh K. Bakhshi A. K. Ohlan A. 2009 - 8.
. Nanoenhanced materials for reclamation of mine lands and other degrade soils: a review. Journal of Nanotechnology ,Liu R Lal R 2012 ; 461468-1-461468-18.doi: 10.1155/2012/461468. - 9.
Hua M. Zhang S. Pan B. Zang W. Lv L. Zhang Q. Heavy metal removal from water/wastewater by nanosized metal oxides. 2012 317 EOF 31 EOF - 10.
Hosokawa M. Nogi K. Naito M. Yokoyama T. Nanoparticle Technology Handbook Elsevier, Amsterdam)2007 - 11.
Tartaj P. Morales M. P. Gonzalez-carreño V. Veintemillas-verdaguer S. Serna C. J. Advances in magnetic nanoparticles for biotechnology applications 2005 290 1 28 34 - 12.
Ben Chaabane T., Smiri L.S., Ammar S., Fiévet F., Jouini N., and Grenèche J.M.Beji Z. Synthesis of nickel-zinc ferrite nanoparticles in polyol: morphological, structural and magnetic propert ies. physica status solidi (a)2006 203 3 504 512 - 13.
Tartaj P. Morales M. P. Gonzalez-carreño V. Veintemillas-verdaguer S. Serna C. J. The preparation of magnetic nanoparticles for applications in biomedicine 2003 R182 EOF R197 EOF - 14.
Dang F. Enomoto N. Hojo J. Enpuku K. A novel method to synthesize monodispersed magnetite nanoparticles Chemical Letters2008 37 5 530 531 - 15.
Matsunaga T. Okamura Y. Tanaka T. Biotechnological application of nano-scale engineered bacterial magnetic particles 2004 14 14 2099 2105 - 16.
. Magnetism and magnetic resonance in solids. Wiley VCH (Guimaraes AP 1998 ). (0-47119-774-2 - 17.
. Electron paramagnetic resonance: elementary theory and practical applications 2nd edition. John Wiley and Sons, ,Well JA Bolton JR 2007 . (ISBN: 047175496X); Brustolon MR. Electron paramagnetic resonance: a practitioner toolkit, 1st edition. John Wiley and Sons (2009) (0-47025-882-9 - 18.
Valenzuela R. Herbst F. Ammar S. Ferromagnetic resonance in Ni-Zn ferrite nanoparticles in different aggregation states 2012 324 21 3398 3401 - 19.
FMR study of ?-Fe2O3 agglomerated nanoparticles dispersed in glues. Reviews on Advanced Materials ScienceGuskos N. Zolnierkiewicz G. Typek J. Guskos A. Czech Z. 2007 14 57 - 20.
Thirupathi G. Singh R. Magnetic Properties of Zinc Ferrite Nanoparticles Institute of Electrical and Electronics Engineers Transactions on Magnetics2012 48 3630 - 21.
De Biasi E. Lima E. Ramos C A. Butera A. Zysler R D. Effect of thermal fluctuations in FMR experiments in uniaxial magnetic nanoparticles: Blocked vs superparamagnetic regimes. 2013 326 1 138 146 - 22.
Sobón M. Lipinski I E. Typek J. Guskos A. FMR Study of Carbon Coated Cobalt Nanoparticles Dispersed in a Paraffin Matrix Solid State Phenomena2007 128 193 - 23.
Nanometer size effects on magnetic order in La12xCaxMnO3 (x = 0.5 and 0.6) manganites, probed by ferromagnetic resonance. Journal of Applied PhysicsShames A I. Rozenberg E. Sominski E. Gedanken A. 2012 D701-1-07D701 2 - 24.
Owens F. Ferromagnetic resonance observation of a phase transition in magnetic field aligned Fe2O3 nanoparticles. 2009 321 15 2386 2391 - 25.
Raikher Yu L, Stepanov V I, Dubois E.Gazeau F. Bacri J C. Gendron F. Perzynski R. Magnetic resonance of ferrite nanoparticles: Evidence of surface effects 1998 186 2 175 187 - 26.
Noginov M. Noginova N. Amponsah O. Bah R. Rakhimov R. Atsarkin V A. Magnetic resonance in iron oxide nanoparticles: Quantum features and effect of size Journal of Magnetism and Magnetic Materials2008 320 18 2228 2232 - 27.
Raikher Yu L, Stepanov V I, Dubois E.Gazeau F. Bacri J C. Gendron F. Perzynski R. Magnetic resonance of nanoparticles in a ferrofluid: Evidence of thermofluctuational effects 1999 535 EOF 546 EOF - 28.
Edelman I. Petrakovskaja E. Petrov D. Zharkov S. Khaibullin R. Nuzhdin V. Stepanov A. FMR and TEM studies of Co and Ni nanoparticles implanted in the SiO2 matrix. 2011 40 363 - 29.
Balcells Ll, Rouanet A, Monty C. Low temperature surface spin-glass transition in gamma-Fe2O3 nanoparticles. Physical Review LettersMartinez B. Obradors X. 1998 80 181 - 30.
Winkler E. Zysler R D. Fiorani D. Surface and magnetic interaction effects in Mn3O4 nanoparticles 2004 - 31.
Song H. Mulley S. Coussens N. Dhagat P. Jander V. Effect of packing fraction on ferromagnetic resonance in NiFe2O4 nanocomposites Journal of Applied Physics2012 E348) 07E348_1 07 E348_3. - 32.
Anisotropy field of small magnetic particles as measured by resonance. Journal of Applied PhysicsDe Biasi R. Devezas T. 1978 49 2466 - 33.
Munir Z. A. andQuach D. V. Ohyanagi M. Electric current activation of sintering: a review of the pulsed electric current sintering process 2011 94 1 1 19 - 34.
Fundamental investigation on the spark plasma sintering/synthesis process III. Current effect on reactivity. Materilas Science and Engineering AAnselmi-tamburini U. andGaray J. E. Munir Z. A. 2005 407 24 - 35.
Transmission electron microscopy characterization of spark plasma sintered ZrB2 ceramicc. Ceramics InternationalMizuguchi T. andGuo S. Kagawa Y. 2010 36 3 943 946 - 36.
Regaieg Y. Delaizir G. Herbst F. Sicard L. Monnier J. Montero D. Villeroy B. Ammar-merah S. Cheikhrouhou A. Godart C. Koubaa M. Rapid solid state synthesis by spark plasma sintering and magnetic properties of LaMnO3 perovskite manganite 2012 80 1 195 198 - 37.
Valenzuela R. Ammar S. andNowak S. Vázquez G. Low field microwave absorption in nanostructured ferrite ceramics consolidated by spark plasma sintering 2012 25 7 2389 2393 - 38.
Combining soft chemistry and Spark Plasma Sintering to produce highly dense and finely grained soft ferrimagnetic Y3Fe5O12 (YIG) ceramics. (to be published).Gaudisson T. Acevedo U. Nowak S. Yaacoub N. Greneche J. M. Ammar S. Valenzuela R. - 39.
Nakamura T. Okano Y. Tabuchi M. Takeuchi T. Synthesis of hexagonal ferrite via spark plasma sintering technique. 2001 48 2 166 169 - 40.
Valenzuela R. Beji Z. andHerbst F. Ammar S. Ferromagnetic resonance behavior of spark plasma sintered Ni-Zn ferrite nanoparticles produced by a chemical route 2011 A329-1-07A329 3 - 41.
Sukhov A. andUsadel K. D. Nowak U. Ferromagnetic resonance in an ensemble of nanoparticles with randomly distributed anisotropy axes Journal of Magnetism and Magnetic Materials2008 320 1 31 33 - 42.
Magnetic properties of zin-ferrite nanoparticles synthesized by hydrolysis in a polyol medium. Journal of Physics: Condensed MatterAmmar S. Jouini N. Fièvet F. Beji Z. Smiri L. Moliné P. Danot M. Grenèche J. M. 2006 - 43.
Electrons spin resonance study of NiFe2O4 nanosolids compacted under high pressure. Journal of Applied PhysicsSui Y. Xu D. P. andZheng F. L. Su W. H. 1996 80 719 - 44.
andA. Abragam B. Bleaney Electron Paramagnetic Resonance of Transition Ions Clarendon Press, Oxford,1970 - 45.
, Relaxation in Magnetic Resonance (Academic Press, London, andC. P .Poole H. A .Farach 1971 ). - 46.
Koksharov Yu A, Pankratov D A., Gubin SP, Kosobudsky ID, Beltran M, Khodorkovsky Y and Tishin AM. Electron Paramagnetic Resonance of Ferrite Nanoparticles. Journal of Applied Physics2001 89 2293 - 47.
Size and Shape Distribution of Magnetic Nanoparticles in Disordered Systems: Computer Simulations of Superparamagnetic Resonance Spectra. Journal of Magnetism and Magnetic Materials andKliava J. Berger R. 1999 205 328 - 48.
Valenzuela R. Alvarez G. Montiel H. Gutiérrez M. P. Mata-zamora M. E. Barrón F. Sanchez A. Y. Betancourt I. Zamorano R. Characterization of magnetic materials by low-field microwave absorption techniques 2008 320 14 1961 1965 - 49.
Montiel H. Alvarez G. Gutiérrez M. P. andZamorano R. Valenzuela R. The effect of metal-to-glass ratio on the low field microwave absorption at 9.4 GHz of glass coated CoFeBSi microwires IEEE Transactions on Magnetics2006 42 10 3380 3382 - 50.
Montiel H. Alvarez G. Gutiérrez M. P. andZamorano R. Valenzuela R. Microwave absorption in Ni-Zn ferrites through the Curie transition 2004 369 1 141 143 - 51.
Magnetic properties of single-crystal {110} iron films grown on GaAs by molecular beam epitaxy. Journal of Applied PhysicsPrinz G. A. andRado G. T. Krebs J. J. 1982 53 3 2087 2091 - 52.
Li Yi, and Baberschke K.Gerhardter F. Temperature-dependent ferromagnetic-resonance study in ultrahigh vacuum: magnetic anisotropies of thin iron films 1993 47 17 11204 11210 - 53.
. The temperature behavior of resonant and non-resonant microwave absorption in Ni-n ferrites. In Electromagnetic Waves / Book 1, InTech Open Access Publisher, edited by Vitaliy Zhurbenko,Valenzuela R 387 402 (2011),978-9-53307-304-0 Online June 24,2011 at: http://www.intechopen.com/articles/show/title/the-temperature-behavior-of-resonant-and-non-resonant-microwave-absorption-in-ni-zn-ferrites. - 54.
Alvarez G. Montiel H. Barron J. F. Gutiérrez M. P. Zamorano R. Yafet-Kittel-type magnetic ordering in Ni0.35Zn0.65Fe2O4 ferrite detected by magnetosensitive microwave absorption measurements. 2009 322 3 348 352 - 55.
Correlation between low-field microwave absorption and magnetoimpedance in Co-based amorphous ribbons.Montiel H. Alvarez G. Betancourt I. andZamorano R. Valenzuela R. 2005 072503 EOF - 56.
Barandiarán M. andGarcía-arribas A. De Cos D. Transition from quasistatic to ferromagnetic resonance regime in giant magnetoimpedance 2006 103904 EOF - 57.
Magnetoimpedance. Handbook of Magnetic Materials, edited by K.H.J. Buschow, Elsevier AmsterdamKnobel M. andKraus L. Vázquez M. 2003 15 497 - 58.
Valenzuela R. Ammar S. Herbst F. Ortega-zempoalteca R. Low field microwave absorption in Ni-Zn ferrite nanoparticles in different aggregation states 2011 3 4 598 602 - 59.
Valenzuela R. Montiel H. andAlvarez G. Zamorano R. Low-field non-resonant microwave absorption in glass-coated Co-rich microwires Physica Status Solidi A2009 4 652 655 - 60.
Unpublished results)Valenzuela R. - 61.
Broese van Groenou A Schulkes J.A., and Annis D.A. Magnetic anisotropy in some nickel zinc ferrites. Journal of Applied Physics1967 - 62.
andYafet Y. Kittel C. Antiferromagnetic arrangements in ferrites 1952 87 2 290 294 - 63.
Yafet-Kittel-type magnetic ordering on NiZnFe2O4 ferrite detected by magnetosensitive microwave absorption measurements. Journal of Magnetism and Magnetic MaterialsAlvarez G. Montiel H. Barrón J. F. Gutiérrez M. P. Zamorano R. 2010 322 3 348 352 - 64.
Microwave power absorption as a function of temperature and magnetic field in the ferroelectromagnet Pb(Fe1/2Nb )O3. Journal of the Physics and Chemistry of SolidsAlvarez G. Font R. Portelles J. andZamorano R. Valenzuela R. 2007 68 7 1436 1442