Permeability spectra of Co2Z hexaferrite compacts produced via a modified aqueous co-precipitation technique

Journal of Magnetism and Magnetic Materials 324 (2012) 3719–3722

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Permeability spectra of Co2Z hexaferrite compacts produced via a modified aqueous co-precipitation technique Andrew P. Daigle a,n, Michael Geiler a, Anton Geiler a, Eric DuPre´ b, Jacob Modest b, Yajie Chen b, Carmine Vittoria b, Vincent G Harris. a,b a b

Metamagnetics Inc., Canton, MA 02021, USA Northeastern University, Boston, MA 02115, USA

a r t i c l e i n f o

abstract

Article history: Received 13 February 2012 Received in revised form 31 May 2012 Available online 12 June 2012

Co2Z hexaferrite materials possess intrinsically high permeability, zero field ferromagnetic resonance values (  1 GHz), and have their magnetic orientation in the plane perpendicular to the c-axis. These characteristics make these materials practical for applications in low to mid ultra-high frequency and L-band microwave device designs. Due to the relatively large size of elements operating within these bands, it has become important to produce large amounts of Co2Z type hexaferrite materials. A modified co-precipitation method has been proposed to produce scalable quantities of high quality Co2Z hexaferrite particles, at  24 g/L. These particles have been thoroughly characterized by vibrating sample magnetometry (VSM) and X-ray diffraction (XRD) with regard to phase purity and magnetic properties. After formation and subsequent ball milling, to achieve single domain particles on the order of 0.5–2 um, particles were oriented and pressed into compacts inside a rotating field to ensure magnetization in plane. Samples then underwent VSM, XRD, and scanning electron microscopy to determine the orientation effect. In addition, the complex permittivity and permeability of these samples were measured as a function of applied field and processing conditions. The results show strong orientation in these compacts making them practical for a variety of device applications. & 2012 Elsevier B.V. All rights reserved.

Keywords: Z-type hexaferrite Textured ferrite Permeability Aqueous co-precipitation Permittivity

1. Introduction In today’s ultra-high frequency (UHF) and L-band communication devices there is a constant need for miniaturization without sacrifice to performance. One promising method to achieve this goal is utilizing high permeability and permittivity materials with low magnetic losses [1–6]. However, most materials, such as cubic spinels and garnets, that exhibit high permeability values, also exhibit relatively low cutoff frequencies that prevent their use at higher rf and microwave frequencies. Snoek’s Law states that the product of a material’s permeability and cutoff frequency is constant, which means that even with strong bias fields increasing the operating frequency of these materials, the resulting decrease in permeability would render them unpractical in the UHF band [7]. Some hexaferrite materials, such as cobalt substituted barium Y-type (Co2Y) and Z-type (Co2Z) have much higher natural resonance frequency of  1.0 GHz due to their high magnetocrystalline anisotropy fields. These materials also have moderate to high permeability values of  4 and  7, respectively,

n Correspondence to: 480 Neponset St., Building 12B, Canton, MA 02021, USA. Tel.: þ1 781 562 0756; fax: þ 781 253 3913. E-mail address: [email protected] (A.P. Daigle).

0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.06.002

making them attractive for device applications [8–12]. Additionally, doping these materials has shown to increase their initial permeabilities, while only slightly affecting their operating ranges [13]. Furthermore, it has been shown that texturing these materials can dramatically increase the permeability while simultaneously decreasing their magnetic loss tangents [14–15]. In this way, high permeability materials capable of operating in the UHF band as unbiased substrates can be produced. The operating range of such materials can be further expanded through the L-band by applying a relatively low bias field [14]. Due to the relatively large dimensions of substrates required at these frequencies because of the relatively long wavelengths, a scalable method for producing large batches (24 g/L) of these materials is strongly needed.

2. Synthesis method Due to higher permeability inherent to Co2Z hexaferrite materials relative to Co2Y hexaferrites, Co2Z was synthesized and used in the subsequent texturing studies. Similarly modified aqueous co-precipitation routines for Co2Y hexaferrites have been described in the past [16]. For this study, single phase powders of Co2Z were produced utilizing a modified co-precipitation of

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elements technique in 800 ml of de-ionized water. The reaction vessel was heated to 95 1C, and stirred at 150 rpm during the 2 h reaction time. Starting powders were mixed in appropriate stoichiometric ratios in the following amounts: 3.91 g of BaCl2, 25.95 g of Fe(III)Cl3, 3.8 g of CoCl2, 16 g of NaOH, and 8 g of Na2CO. Salts and chlorides were diluted independently in de-ionized water in 400 ml beakers, after which they were added simultaneously to a larger vessel (800 ml). 50 ml of tetra-ethylene glycol (TEG) was added during the mixing process to serve as a surfactant which aided in particle formation. This modification of the standard aqueous technique has also been shown to limit the size of the particles produced, allowing for lower sintering temperatures and fewer total sintering steps [7,16]. After precipitation, which took place over a period of 2 h, the resultant powders were filtered utilizing vacuum filtration to remove excess water and NaCl. These precipitated particles then underwent additional rinsing, including ultrasonic filtration and magnetic stirring before subsequently being dried and pressed. The goal of the thorough filtration process was to eliminate excess NaCl formed during the reaction. As with the other presented aqueous precipitation methods, the removal of NaCl was verified by energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) measurements [16]. After filtration, the powders were dried and pressed at 13.79 MPa in a 31.75 mm stainless steel die, and subsequently sintered at high temperatures for 14 h. Sintering temperatures ranged from 1100 to 13001 C. The effect of increased access to ambient oxygen due to increased airflow from the removal of the radiation blocks in the tube furnace can be seen in Fig. 1(a). Here, the XRD scans of a sintered sample at 12501 C with (closed) and without (open) the radiation blocks are compared to a reference standard powder diffraction pattern [17]. Results indicate that pure phase formation of the Co2Z ferrite is best achieved at a temperature of 12501 C with a single sintering step when the sample has free access to ambient oxygen (open). The increased phase purity of the open sample has also been verified via

vibrating sample magnetometry (VSM) measurements as shown in Fig. 1(b). Here, the 4pMs value of this material produced with free access to ambient oxygen was measured to be 3150 G, as compared to a reference value of 3100 G [9]. Scanning electron microscopy (SEM) images of the surface of the pressed compact taken after the single sintering step both with and without the radiation blocks are shown in Fig. 1(c and d). Here the hexagonal shape and size of the hexaferrite particles is readily apparent. The slight increase in grain size for the hexaferrite material sintered in a closed environment has been attributed to the more rapid rise and slower decline of temperature with the radiation blocks inserted in the tube furnace.

3. Permeability measurement After sintering, round compacts with an outer diameter of approximately 16 mm and thickness of approximately 3 mm had a concentric cylindrical hole with a diameter of approximately 8 mm drilled in the center using a diamond coring bit. In order to keep the edges of the toroids sharp during the drilling process, the compacts were mounted to glass slides prior to drilling utilizing TM Crystalbond( ) epoxy. After drilling, the toroids were removed from the glass slides and the extra bonding epoxy was removed by rinsing the samples with acetone. The complex m and e of the samples machined into either toroids (m) or circular plates (e) was measured using Agilent Material Analyzer A4991. Typical spectra for isotropically prepared samples are shown in Fig. 2. Here, complex m measurements of Co2Z hexaferrite samples have shown moderate real m values on the order of 6.8 and magnetic loss tangent values on the order of 0.01–0.1. The corresponding er values were  15, with a dielectric loss tangent of 0.01 over the majority of the operating range. The lack of orientation, or crystallographic texturing, of the Co2Z hexaferrite samples was deemed the reason for these relatively low values. These values are, however, in good

Fig. 1. (a) X-ray diffraction spectra verifying the pure phase nature of Co2Z hexaferrite powders synthesized through the modified aqueous co-precipitation method under different sintering conditions. (b) Magnetic moment as a function of applied field for Co2Z hexaferrite powders produced under different sintering conditions. (c,d) SEM images of hexaferrite particles produced under different sintering conditions.

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7500 Oe in the plane of the compact. The goal of the magnetic orientation process was to align the anisotropic ferrite particles with their crystallographic axes perpendicular to the compact plane to maximize m [14,15]. After orientation, the particles were sintered in a box furnace in air at 1250 1C for 10 h. The effects of the orientation of the particles can be seen in Fig. 3. Real permeability of 12.5 is shown, as 83% improvement over the non-oriented sample. In Fig. 4 the effect of small bias fields on the measured permeability value is shown. It is apparent that permeability values 45 can be achieved over the entire L-band (1–2 GHz) utilizing fields as low as 200–300 Oe. This allows device operation at both Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) frequencies. This data shows promise for the development of miniature and high performance devices at UHF and L-band frequency [5,18]. Furthermore, due to the fact that wave impedance (Z) of the Co2Z is equivalent to that of air (Eqs. (1) and (2)): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffi mo mr 4p  107 ¼ Zair ¼ ð1Þ ¼ 377 O eo er 8:854  1012

Fig. 2. Complex m and e spectra of isotropically prepared Co2Z hexaferrite samples are shown. Moderate real m values (a) on the order of 6.8 and magnetic loss tangent values (b) on the order of 0.01–0.1 are shown. Also shown is the e value (c) of  15 and the dielectric loss tangent (d) of 0.01 over the majority of the operating range.

Zco2z Ferrite ¼

rffiffiffiffiffiffiffiffiffiffiffi

mo mr ¼ eo er

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð4p  107 Þ  12:5 ð8:854  1012 Þ  12

¼ 384 O

ð2Þ

these materials can be used for low profile wideband antenna substrate applications where previously dielectric loading has been employed and bandwidth has suffered.

agreement with those in previously published works [8–11], allowing for the differences in density.

4. Orientation effect The first step in the orientation process was to grind compacts produced via the single sintering method to single magnetic domain particle size (0.5–2 mm). This reduction was achieved via ball milling in a Fritsch planetary mono mill. Prior to milling, the sample was hand ground till the powder passed through a 75 mm sieve in order to ensure a narrow particle size distribution in the final milling process. After hand grinding, the hexaferrite powders were placed in an 80 ml agate jar with agate grinding balls of varying sizes. The amount of media was selected after many trials to further refine the particle size distribution of the final powder. Prior to milling the 80 ml agate holder was filled with 30–40 ml of reagent alcohol to aid in milling process. The mill was run for 8 h at 400 rpm in 30 min intervals. In between the intervals, a 5 min pause was used to allow the slurry to settle, and each subsequent interval was run in an opposite direction to the previous. After grinding the particles exhibited a uniform size distribution verified via SEM with an average diameter on the order of 0.5–2 mm, consistent with single magnetic domain size. The next step in the texturing process involved the alignment of the single magnetic domain particles. This was achieved by using a rotating magnetic field apparatus [15]. In the process the particles were dispersed in water with a ratio of 2–4 g of powder to 30 ml of water or alcohol. This slurry was subsequently ultrasonically dispersed for 30 min to reduce the number of particle agglomerations. The dispersed particles were then loaded into a nonmagnetic steel die via a plastic 3 ml pipette. Excess water was removed from the cylinder after the particles had been allowed to settle in the rotating field. The rotating magnetic field was supplied in the plane perpendicular to the axis of the die by a permanent rotator around the die using an electric motor. The powder was then pressed with a hydraulic press at 1000–1500 psi while being subjected to a rotating magnetic field of  5000–

Fig. 3. Complex m and e spectra of magnetically oriented Co2Z hexaferrite samples are shown. Moderate real m values on the order of 12.5 and magnetic loss tangent values on the order of 0.01–0.1 are shown. Also shown is the e spectrum with an average value of  10, as well as the dielectric loss tangent of 0.01 over the majority of the operating range.

Fig. 4. Effect of varying magnetic bias field on the permeability of Co2Z hexaferrite toroids.

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5. Conclusion

References

In order to identify materials suitable for the development of next generation devices at UHF and L-band frequencies, the fabrication of Co2Z hexaferrites has been demonstrated through co-precipitation of elements technique. The phase purity of these materials formed through a single sintering step was verified via XRD measurements and the magnetic properties were measured with VSM and compared to bulk materials. The permeability spectra of isotropic hexaferrite compacts, as well as those textured by the application of a rotating magnetic field during pressing, were measured and compared. Optimization of process parameters was performed to maximize the permeability and minimize the magnetic losses below natural resonance frequency. Real permeability values for textured samples were measured to be 12.5, compared to 6.8 for isotropic samples. In addition, measurements have been performed utilizing bias fields to increase the operational ranges of these materials into the L-band and above with permeability 45. Co2Z has proven to be practical for device applications due to its intrinsically high permeability values, and its wave impedance value which is close to that of free space.

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