IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 2502804 Microwave Absorption in La1.5Sr0.5NiO4/CoFe2 O4 Nanocomposites (Revised Feb 2014) Chu T A Xuan1,2 , Pham T Tho1,2 , Doan M Quang1,3, Ta N Bach1, Tran D Thanh1, Ngo T H Le1 , Do H Manh1 , Nguyen X Phuc1 , and Dao N H Nam1 Institute of Materials Science, Vietnam Academy of Science and Technology, Hanoi, Vietnam of Sciences, Thai-Nguyen University, Thai-Nguyen 0084, Vietnam Department of Physics, Hanoi National University, Hanoi 10000, Vietnam College La1.5 Sr0.5 NiO4 is well known as a dielectric material that has a colossal permittivity (up to 107 ) and a weak paramagnet at room temperature The permeability is about 1.005, which is just slightly larger than that of air The weak magnetic moment together with the huge imbalance between permittivity and permeability seemed to negate La1.5 Sr0.5 NiO4 as a promising candidate for electromagnetic absorption due to the lack of magnetic losses However, we have found that La1.5 Sr0.5 NiO4 nanopowder indeed has a reasonably strong microwave absorption capability in the range of 4–18 GHz Apparently, impedance matching (|Z| = Z0 = 377 ) is found to be responsible for the absorption resonance that shifts to lower frequencies with increasing the absorber’s thickness To improve magnetic losses, as well as to balance out the dielectric and magnetic components, CoFe2 O4 nanoparticles are gradually added to the La1.5 Sr0.5 NiO4 /CoFe2 O4 composites The influence of adding magnetic nanoparticles on reflection loss, resonance frequency, and matching effects will be discussed Index Terms— Impedance matching, microwave absorption, phase matching, radar absorption I I NTRODUCTION B Y ABSORBING the incident wave and/or canceling out the reflected waves, microwave absorbers shield objects from microwave radiation with reduced or even zero reflection This characteristic makes microwave absorbers essential in applications, such as electromagnetic interference shielding and, particularly, stealth technology The performance of an absorber is commonly characterized by its strong absorption and weak reflection of electromagnetic radiation, which are in turn characterized by a large negative value of the reflection loss (RL) A perfect absorption is hard to achieve However, zero reflection could be reached when: 1) the impedance Z of the sample matches with that (Z ) of the incident media or 2) the reflected waves from two sides of a flat absorber cancel out each other according to the quarter-wavelength mechanism The two effects, also called Z -matching and phase matching, could practically result in a very large negative RL value and are commonly observed in many absorbers The RL values reaching down to less than −60 dB have been reported in several materials; most of them are ferrite/ferromagnetbased absorbers, such as carbonyl iron/BaTiO3 composites (RL = −64 dB) [1], Cux Ni0.4−x Zn0.6 Fe2 O4 ferrite (RL = −60 dB) [2], CoFe2 O4 /acrylated epoxy nanocomposite (RL = −60 dB) [3], La0.6 Sr0.4 MnO3 /polyaniline nanocomposites (RL = −64.6 dB) [4], ZnO/carbonyl-iron composite (RL = −61 dB) [5], and so on The popularity of ferrite- and ferromagnet-based absorbers that give record low RL values would apparently imply the dominant role of magnetic losses over the other possible losses La1.5 Sr0.5 NiO4 (LSNO) is a dielectric with colossal per- Manuscript received November 9, 2013; accepted January 14, 2014 Date of current version June 6, 2014 Corresponding author: C T A Xuan (e-mail: xuandhkh@gmail.com) 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.2014.2306693 mittivity (ε R > 107 ) [6], [7] Regarding the capability of microwave absorption, the material may have a negligibly small magnetic loss since it is in a paramagnetic state with a very small permeability (μ R = 1.006) at room temperature [8], [9] This scenario would be quite common for many other dielectrics where there exists a large imbalance between permittivity and permeability On the other hand, CoFe2 O4 (CFO) is a well-known ferrite that has a considerably large magnetization The CFO nanoparticles were found to strongly absorb microwave with the obtained RL of about −45 dB at 9.5 GHz for the sample’s thickness of mm [10] By using acrylated epoxy as the host matrix, the RL reached down to −60 dB but at a little higher frequency [3] Gradually mixing CFO with LSNO is thus expected to improve the magnetic loss and to balance out the dielectric and magnetic characteristics The microwave absorption properties of LSNO have not been reported so far In this paper, we show that doping CFO indeed significantly improves the microwave absorption capability of the (100 − x)LSNO/xCFO (x is in volume units) nanocomposites; the RL value decreases to minimum values for doping content x = and x = for samples unbacked or backed by an Al plate, respectively Our experiments also verify that, unlike the resonance caused by Z -matching, the phase-matching resonance can be observed only if the sample is short circuited by a metal plate II E XPERIMENT The LSNO and CFO nanoparticle powders were synthesized using a conventional solid-state reaction route combined with high-energy ball milling processes The structure, phase quality, and particle size of the nanopowders were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods The magnetic properties were characterized by a vibrating sample magnetometer To prepare the samples for microwave measurements, 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 2502804 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 Fig Magnetization loops, M(H ), of the LSNO and CFO nanoparticle powders at 300 K Fig XRD patterns of the (a) La1.5 Sr0.5 NiO4 and (b) CoFe2 O4 nanoparticle powders (100 − x)La1.5Sr0.5 NiO4 /xCoFe2 O4 (x = 0, 2, 4, 6, 8, and 10 in volume units) nanoparticle composites were mixed with paraffin in a 40/60 volume fraction, respectively, and finally pressed into flat plates of 2.5 mm thickness The free-space microwave measurement method in the frequency range of 4–18 GHz was utilized using a vector network analyzer (Anritsu MS2028B VNA Master) An Al plate was used as the reference material with 0% of attenuation or 100% of reflection The impedance and the RL can be calculated according to the transmission line theory [11] Z = Z μ R /ε R i (2π f t/c) (μ R ε R )1/2 (1) where the relative permittivity, ε R , and relative permeability, μ R , are calculated using an algorism proposed in [12] and [13] (hence called the NRW method), and RL = 20 log |(Z − Z )/(Z + Z )| (2) For the case of metal-backed samples, since Z = Z (1 + S11 )/(1 − S11 ) (3) the RL can be obtained directly from the complex reflection scattering coefficient S11 RL = 20 log |S11 | (4) III R ESULTS AND D ISCUSSION The XRD data (Fig 1) indicate that both of the nanoparticle powders are single phase; all the diffraction peaks could be indexed to the expected crystal structures of F4 K2 Ni perovskite-type tetragonal (I 4/mmm space group) for LSNO [8] and inverse spinel cubic for CoFe2 O4 Calculations of the particle size according to the Scherrer equation, d = K · λ/(β · cos θ ) (where K is the shape factor, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity, and θ is the Bragg angle), give the values of 50 and 46 nm for the LSNO and CFO nanoparticles, respectively However, these values in fact reflect the crystal domain/grain sizes inside the particles rather than the actual Fig Unbacked samples: RL of the (100−x)LSNO/xCFO absorbers within the frequency range of 4–18 GHz particle sizes In addition, the nanoparticles may have a layer of disordered surface that does not contribute to the XRD signal and therefore does not count toward the particle size The SEM images (not shown) indeed indicate that the particle sizes for the LSNO powder are in the range of 100–300 nm while those for the CFO are 200–400 nm The SEM images also show that the particles have quite a uniform quasi-spherical shape The magnetization loops, M(H ), for our LSNO and CFO nanoparticles are shown in Fig No magnetic hysteresis could be observed for the LSNO powder The CFO nanoparticles show typical hysteresis characteristics with the saturation magnetization comparable to previously reported data for CFO nanoparticles prepared by other methods [14], [15] Fig shows the RL( f ) curves for all the unbacked samples within 4–18 GHz frequency range All the samples show a resonance notch in the RL( f ) curves in the high frequency region near 14 GHz There seems to be a small random variation of the resonance frequency, probably due to the variation in the sample’s thickness and uniformity As expected, with doping CFO, the resonance notch in the RL( f ) curves becomes deeper; the minimum RL decreases from −12.8 dB for x = to −31.2 dB for x = Further increasing x leads to an abrupt increase of the minimum RL The decrease of RL with x for x ≤ should be attributed to the increase of magnetic loss and the balancing of permittivity and permeability caused by the substitution of CFO for LSNO nanoparticles Too high content of CFO would again cause an imbalance of permittivity and permeability leading to an abrupt increase of RL for x > The characteristic parameters such as the XUAN et al.: MICROWAVE ABSORPTION IN La1.5 Sr0.5 NiO4 /CoFe2 O4 NANOCOMPOSITES 2502804 TABLE I S UMMARY OF THE M ICROWAVE A BSORPTION C HARACTERISTICS a FOR THE PARAFFIN -M IXED (100 − x)La1.5 Sr 0.5 NiO4 + xCoFe2 O4 N ANOCOMPOSITES minimum RL value and the resonance frequency fr extracted from these measurements are listed in Table I Determining the absorption mechanism as well as the nature of the observed resonances would help to find the way to improve the absorption performance The quarter-wavelength (or phase-matching) and impedance matching (or Z -matching) resonances are the two phenomena commonly observed for microwave absorbers In the phase-matching model, the two reflected waves from both sides of the flat absorber cancel each other out if their phases differ by π; the matching frequency is √ written as f p = (2n + 1)c/(4t ε R μ R ) (where c is the speed of light in the incident medium and n = 0, 1, 2, ) Nonetheless, the calculations of matching frequency f p according to the phase-matching model, as listed in Table I, not match the observed fr values, which are near 14 GHz, but predict phase-matching resonances near GHz instead This implies that the phase-matching mechanism can be ruled out for the 14 GHz regime resonances The Z -matching condition is satisfied if the impedance Z of the sample matches that of the incident media Z In that case, there will be no reflection and complete energy dissipation of the propagating wave would occur within the sample Vice versa, a perfect absorption by the sample would result in zero reflection (S11 = 0) and therefore satisfy the impedance matching condition Z = Z In Fig 4, the resonance region shows a zoomed-in view and RL( f ) and |Z /Z | curves for each sample are plotted together for comparison It is quite clear that the resonance occurs near the minimum of |Z /Z | that is also close to The closer value of minimum |Z /Z | to gives a smaller value of RL minimum; the deepest RL notch is observed for x = [Fig 4(e)] where |Z /Z | is found equal to This provides evidence for the main role of the Z -matching mechanism in these resonances According to (2), when perfect energy absorption or reflection cancelation occurs, Z = Z = and therefore RL = −∞ For that Fig Unbacked samples: enlargements of the RL( f ) (right axis) and |Z /Z |( f ) (left axis) curves for all the samples in the resonance region near f = 14 GHz (a) x = 0, (b) x = 2, (c) x = 4, (d) x = 6, (e) x = 8, and (f) x = 10 to happen, the imaginary part Z must be zero A deviation of Z from zero will reduce RL to a finite value and could be considered as the mismatch of the Z -matching condition Non-zero Z not only shallows the resonance notch of RL( f ), but also shifts it away from impedance matching frequency f z where |Z /Z | = The Z -matching frequencies, f z , and the imaginary impedance Z ( f z ) are also listed in Table I The change in Z seems to be consistent with that in the RL minimum As mentioned above, although the calculations of f p suggest a phase matching of reflected waves at frequencies near GHz, no such resonance is observed The absence of the predicted resonance at the matching frequency could be caused by the fact that the samples are open circuited Without a metal backing plate, which is considered as a perfect reflector, the internal reflection wave on the reverse side would be much weaker than that on the incident side of the sample The cancelation of the two waves is therefore insignificant and phase-matching resonance may not be observed In order to further prove this assumption, reflection measurements for the corresponding samples with metal backing have been carried out; the results are shown in Fig Interestingly, the resonance around GHz is clearly shown by a sharp drop of |S11 | [Fig 5(a)] and a corresponding notch in the RL( f ) curve [Fig 5(b)] This observation not only proves the existence of the phase-matching resonance near GHz, but also suggests a method to differentiate between the phase-matching and Z -matching resonances, both of them give zero reflection and |Z /Z | = condition Our observation and argument seem to 2502804 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 both of which exhibit zero reflection (S11 = 0) and Z = Z , but true absorption only occurs in Z -matching resonances This paper suggests that nanocomposites based on LSNO could be developed into very high performance microwave absorbing materials by a pertinent addition of the magnetic component ACKNOWLEDGMENT This work was supported in part by the Vietnam National Foundation for Science and Technology Development under Grant 103.02-2012.58 and in part by the State Key Laboratory, Institute of Materials Sciences, Vietnam Academy of Science and Technology R EFERENCES Fig Al-backed samples (a) Absolute value of the reflection coefficient, |S11 | (b) RL of all the (100 − x)LSNO/xCFO absorbers within the frequency range of 4–18 GHz be in agreement with the results reported in [16], where the influence of metal backing was examined and phase-matching resonances were observed when the samples were backed by metal plates It is notable that with the presence of the Al-backing plate, all the Z -matching resonances in the high frequency regime seem to shift to even higher frequencies For an unbacked sample, the impedance is contributed by two components: 1) reflection and 2) transmission The attachment of the Al backing plate converts the transmission impedance and contributes to the reflection impedance This may cause a change in the total impedance, therefore causing a shift of the resonance in the high frequency region (near 14 GHz) to higher frequencies (near 16 GHz) and an impact to the RL minimum Another reason for the frequency shift of the resonance would be also the imperfect interface between the Al backing plate and the absorber’s surface Apart from the unexpected shift, the resonances are clear At both high- and low-frequency regimes, |S11 | is largely suppressed and is reduced to near zero for x = 6, where very large negative values of RL (less than −50 dB) are obtained IV C ONCLUSION We have found that by partially substituting CFO—a high permeability ferrite—for LSNO—a colossal permittivity dielectric, a substantial improvement of the absorption capability of the (100 − x)LSNO/xCFO nanocomposites is obtained Very low RL minima are obtained at x = and for Al-backed and unbacked samples, respectively The results also verify that phase-matching resonances can be observed only in metal backed samples; 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