J. Magn. Soc. Jpn., 41, 94-98 (2017)<Paper>Synthesis of non-magnetic-ion-substituted Ca-based M-typeferrite
M. Shigemura, K. Kakizaki, and K. Kamishima

Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama, Japan

We investigated the synthesis conditions and magnetic properties of non-magnetic-ion-substituted Ca-based
hexagonal ferrites, Ca0.8La0.2Fe8.0−2xMgxTixO19−σ (x = 0.1–3.5) and Ca0.8La0.2Fey−xAlxO19−σ (x = 1.0–5.0, y = 8.0–10). We
found that Mg-Ti-substituted Ca-based M-type ferrites formed at x = 0.1 and 0.2 at above 1225ºC. The sample at x =0.1 showed the crystallographic single phase of M-type ferrite. The minimum Curie temperature of theMg-Ti-substituted samples was 367ºC. Also, we found that the Al-substituted Ca-based M-type ferrites had the main
phase of M-type ferrite in all samples. As the amount of substituted Al increased, the saturation magnetization and
the Curie temperature decreased, and the coercivity increased. As the amount of substituted Al increased by Δx = 1,
the Curie temperature decreased by ΔTC = 50–70ºC. Al-substituted samples of x = 1.0 and 2.0 sintered at 1300ºC had
the single phase of M-type ferrite.
Keywords: hexagonal ferrites, M-type, calcium compound, non-magnetic ions, Mg-Ti, Al
1. Introduction
M-type ferrite is a type of hexagonal ferrite. Its
chemical formula is expressed as M2+Fe3+12O19(M2+=Ba2+, Sr2+). The M-type ferrite has high saturation
magnetization and high coercivity and is mainly used asa permanent magnet. The Curie temperatures ofBaFe12O19 and SrFe12O19 are 450°C and 460°C,respectively.1)–4) It is also possible to tune the magnetic
properties of the pure M-type ferrites by thesubstitution of non-magnetic ions such as Mg2+-Ti4+,Zn2+-Ti4+, and Al3+.5)–10)
On the other hand, we had investigated the synthesis
conditions of Ca-based M-type ferrite in our previousstudy. 11) In the study, the sintering temperature of thehighest magnetization sample was 1250°C and theanalyzed composition was approximately Ca:La:Fe =0.8:0.2:9.0. The saturation magnetization of the sample
was 68.0 Am2/kg at room temperature and 104 Am2/kg
at 5 K (−268°C). The Curie temperature of this sample
was about 400°C.
In this study, we investigated the synthesis conditions
andmagnetic
properties
ofnon-magnetic-ionsubstituted Ca-based M-type ferrites. The substitutionof non-magnetic ions for Fe3+ can decrease the Curie
temperature of a ferrite, which gives the opportunity for
new application. For instance, Shimizu et al. put forward

2. Experimental Procedure
Samples were prepared by a conventional ceramic
method. We used CaCO3, La2O3, α-Fe2O3, and (MgO,TiO2) or Al2O3 as starting materials. They were mixed
in a desired proportion, Ca0.8La0.2Fe8.0−2xMgxTixO19−σ (x= 0.1–3.5), or Ca0.8La0.2Fey−xAlxO19−σ (x = 1.0–4.0, y =8.0–10). The powder was ball-milled for 24 h. The mixed
powder was pressed into a pellet shape and pre-calcinedin air at 900°C. The sintered sample was pulverized in amortar and then milled into fine powder with aplanetary ball mill (Fritsch, P-7 Premium line with 1mmφ zirconia balls and a 45 ml zirconia container) for
10 min. at 1100 rpm. The processed powder was dried
and then pressed into disks. The disks were sintered at
TS = 1200–1300°C for 5 h. The crystal structure of thethe non-magnetic-ion-substituted spinel ferrites withrelatively low Curie temperatures as the implantmaterial for the magnetic induction hyperthermiatherapy.12)-13)

We employed Mg2+-Ti4+ and Al3+ as non-magnetic
substituting elements in the Ca-based M-type ferrite.
The resources of these elements are rich as well as thoseof Ca and Fe.14) Also, Ca, Mg, Ti, and Al are less toxic
than Ba, which is helpful in terms of producing a safer
material.

94

Fig.
1
X-ray
diffraction
patternsofCa0.8La0.2Fe8.0−2xMgxTixO19−σ (x = 0.0–0.4) sinteredat 1250°C.

Journal of the Magnetics Society of Japan Vol.41, No.5, 2017

sample was examined by powder X-ray diffraction(XRD)
analysis
with
Cu-Kαradiation.
The
magnetization was measured with a vibrating sample
magnetometer (Tamakawa TM-VSM2130HGC) and asuperconducting quantum interference device (SQUID)
magnetometer (Quantum Design MPMS-XL). The
composition was analyzed by the use of an Energy
Dispersive X-ray spectrometry (EDX). Here, thecomposition of oxygen was not examined because theanalyses of light elements are less accurate than thoseof heavy elements.
3. Results and discussion
3-1 Mg-Ti substituted Ca-based M-type ferrite
Figure 1 shows the X-ray diffraction patterns ofCa0.8La0.2Fe8.0−2xMgxTixO19−σ (x = 0.0–0.4) sintered at
1250°C. The main phase is M-type ferrite for thesamples with x ≤ 0.2. The sample at x = 0.2 also showed
the minority α-Fe2O3 phase. The sample at x = 0.3 wasmade up of the mixed phases of α-Fe2O3, M-type ferrite,MgFe2O4, and (CaLa)TiO3. The samples at x ≥ 0.4
consisted of α-Fe2O3, MgFe2O4, and (CaLa)TiO3 phases,
but didn’t contain M-type ferrite phase. The chemicalcomposition of the perovskite compound of (CaLa)TiO3
can be various forms such as Ca0.2La0.53TiO3,
Ca0.4La0.4TiO3, and Ca0.12La0.58TiO3.15, 16) It is difficult todetermine the chemical composition exactly because thelattice constant of (CaLa)TiO3 is similar to that ofCaTiO3. Since Ca and La cations became starting
materials to form the perovskite compounds, M-typeferrite was not formed.
Figure 2 shows the room-temperature saturation
magnetization of Ca0.8La0.2Fe8.0−2xMgxTixO19−σ (x =0.0–0.4) sintered at 1200–1300°C. Here, the saturation
magnetization of the sample at x = 0.0 is that of sample
prepared with the optimum preparation conditions in
the previous study.11) The saturation magnetization
decreased with increasing the amount of Mg-Ti
substitution. The gradual decrease of the magnetization
indicated the formation of Mg-Ti substituted M-typeferrite at x = 0.1 and 1225ºC ≤ TS ≤ 1250ºC. The
substitution of non-magnetic Mg2+ and Ti4+ for magnetic
Fe3+ can decrease total magnetic moment. Even at thesame substitution rate of x=0.1, however, the samples
with TS=1200ºC and 1300ºC showed low magnetizations.
The decrease of the magnetization was more obvious at
x=0.2 and TS≥1225ºC in spite that the main phase wasthe M-type phase. It should be noted that the synthesis
condition of Ca-based M-type ferrite is so delicate that
the M-type structure can easily be broken.11) Since thesample at x = 0.2 and TS = 1200°C didn’t contain M-typeferrite phase, the saturation magnetization becamemuch smaller than those of the x = 0.2 samples withhigher sintering temperatures.
Figure 3 shows the temperature dependence ofmagnetization of Ca0.8La0.2Fe8.0−2xMgxTixO19−σ at 0.0 ≤ x≤ 0.2. The Curie temperature decreased with increasing
the amount of Mg-Ti substitution. The non-magneticFig.
2 Saturation magnetization at room
temperature of Ca0.8La0.2Fe8.0−2xMgxTixO19−σ (x =0.0–0.4) sintered at 1200, 1225, 1250°C and 1300°C.

Fig. 3 Temperature dependence of magnetization ofCa0.8La0.2Fe8.0−2xMgxTixO19−σ (x = 0.1 and 0.2)
sintered at 1225°C and x = 0.0 sintered at 1250°C.
Mg2+ and Ti4+ ions decrease the number of theconnection between magnetic iron ions (Fe3+–O2-–Fe3+)
in Mg-Ti substituted Ca-based M-type ferrite at x ≤ 0.2.
3-2 Al substituted Ca-based M-type ferrite
Figure 4 shows X-ray diffraction patterns ofCa0.8La0.2Fe8.0−xAlxO19−σ (x = 1.0–4.0) sintered at 1250°C.
The main phase is M-type ferrite for these samples
although minority phases of CaFe4O7 and CaAl4O7 arealso observed. The formation of CaFe4O7 can be causedby the deficiency of Fe cations to form the M-typestructure. Therefore, we attempted to increase theamounts of (Fe+Al)/(Ca+La) to 10.
Figure 5 shows X-ray diffraction patterns ofJournal of the Magnetics Society of Japan Vol.41, No.5, 201795

Ca0.8La0.2Fe10−xAlxO19−σ (x = 1.0–4.0) sintered at 1300°C.
The samples of x = 1.0 and 2.0 are in the single phase ofM-type ferrite, but the samples of x = 3.0 and 4.0 havethe main M-type ferrite phase and the minority
CaFe4O7 phase. Here, it should be noted that hightemperature sintering is necessary for formation ofCaAl12O19.17) Actually, Nagumo et al. reported thatiron-substituted hibonite (CaAl12–xFexO19) samples weresynthesized at a relatively low sintering temperature of
1300ºC as the amount of Al was decreased.18) Therefore,it is possible that the substitution of Al may increase
the suitable sintering temperature to form the single
M-type phase. Also, the diffraction peaks of M-typephase shifted to the higher angle side as thesubstitution amount increased. Table 1 shows the ionicradii of Fe3+ and Al3+.19) Since Al3+ is smaller than Fe3+,
the lattice constants decrease with increasing theamount of Al3+, as shown in Fig. 6. Therefore, theAl-substituted M-type ferrite was formed successfully.
Figure 7 shows the saturation magnetization and thecoercivity
atroomtemperatureofFig.
4X-raydiffraction
patternsof
Ca0.8La0.2Fe8.0−xAlxO19−σ (x = 1.0–4.0) sintered at
1250°C.

Fig. 6 Lattice constant of Ca0.8La0.2Fe10−xAlxO19−σ (x= 1.0–4.0) sintered at 1300°C.

Fig.
5
X-ray
diffraction
patternsof
Ca0.8La0.2Fe10−xAlxO19−σ (x = 1.0–4.0) sintered at
1300°C.

Table 1 Ionic radii of each coordination number ofFe and Al.

Fig. 7 Saturation magnetization and coercivity atroom temperature of Ca0.8La0.2Fe10−xAlxO19−σ (x =1.0–4.0) sintered at 1200-1300°C.

96Journal of the Magnetics Society of Japan Vol.41, No.5, 2017

Ca0.8La0.2Fe10−xAlxO19−σ (x = 1.0–4.0) sintered at
1200–1300°C. The saturation magnetization decreased
and the coercivity increased with increasing amount ofAl. The decrease of the saturation magnetization atroom temperature is caused by the substitution of
non-magnetic ions (Al3+) for magnetic ions (Fe3+), which
can decrease the intrinsic magnetic moment and theCurie temperature.
In contrast, the Al substitution increased the coercivityof the Ca-based M-type ferrite although the coercivity ofthe starting sample without Al was small.11) TheAl-substitution effect on coercivity is similar to those ofother M-type ferrites. For instance, Haneda and Kojima
reported that BaAlxFe12–xO19 exhibited a large increasein coercivity and that the magnetocrystalline anisotropy
field contributed greatly to the increase of coercivity.20)
De Bitetto also found that the anisotropy field ofSrO·xAl2O3·(6–x)Fe2O3 increased as the substitution
amount of Al (x) was increased.21)
The Al-substitution effects on coercivity of M-typeferrites have been attributed to the variation ofmagnetocrystalline anisotropy under the assumption ofasingle-domainmodel(Stoner-Wohlfarthmodel).8,10,20–21) This assumption is plausible because
the single-domain structure can be stabilized by thereduction of magnetostatic energy if the decrease of thesaturation magnetization is more significant than thechange of the magnetocrystalline anisotropy.22–23)
According to the Stoner-Wohlfarth model, the coercivity
can be expressed as
H C ∝ K 1/ M S
where HC is the coercivity, K1 is the magnetocrystalline
uniaxial anisotropy constant, and MS is the saturationmagnetization. If the decrease of K1 is smaller than thedecrease of MS, HC can be increased. The studies of Al
substituted Sr-based M-type ferrite also showed thatthe saturation magnetization decreased and thecoercivity increased with increasing amount of Al.8–10, 21)This tendency is consistent with our result.

Figure 8 shows the temperature dependence ofmagnetization of Ca0.8La0.2Fe10−xAlxO19−σ (x = 1.0–4.0)
sintered at 1300°C. The Curie temperature of thesesamples decreased with increasing the amount of Al. As
the Al substitution increased by Δx = 1, the Curie
temperature decreased by ΔTC = 50–70ºC. The
substitution of the non-magnetic Al3+ for Fe3+ causes todecrease the links of Fe3+–O2−–Fe3+, and consequently
weakens the superexchange interaction in the M-typestructure.
Figure 9 shows the magnetization curves at 5 K(-268°C) of Ca0.8La0.2Fe10−xAlxO19−σ (x = 1.0 and 2.0)
sintered at 1300°C. The saturation magnetic moment
per molecule at x = 1.0 is estimated to be 12.5 µB/f.u.
from the magnetization at µ0H=7 T (85.0 Am2/kg). TheEDX analysis showed the chemical formula of thissample as Ca0.88La0.12Fe8.80Al1.01O15.8. Here, thecomposition ratio of oxygen is estimated from the chargebalance with the concentration of Ca2+, La3+, Fe3+, andAl3+ cations.
The saturation magnetic moment per molecule at x =2.0 is also estimated to be 7.86 µB/f.u. from themagnetization at µ0H=7 T (55.1 Am2/kg). The EDX
analysis of this sample showed the chemical formula of
Ca0.83La0.17Fe7.88Al1.83O15.7.
Comparing these two results, the difference of themagnetic moment between these two samples is 4.7 µB,
which is approximately close to the magnetic moment ofFe3+ (5 µB). In other words, as the substitution amountof Al3+ increased by 1, one Fe3+ located at the up spinsites decreased. In other words, Al3+ prefers to belocated at the up spin sites. Therefore, the saturation
magnetization was greatly decreased even by the small
amount of Al substitution.

Fig. 8 Temperature dependence of magnetization ofCa0.8La0.2Fe10−xAlxO19−σ (x = 1.0–4.0) sintered at1300°C.

Fig. 9 Magnetization curves at 5 K (−268°C) ofCa0.8La0.2Fe10−xAlxO19−σ (x = 1.0 and 2.0) sintered at
1300°C.

Journal of the Magnetics Society of Japan Vol.41, No.5, 201797

4. Conclusion
We have investigated the synthesis conditions andmagnetic properties of non-magnetic-ion-substituted
Ca-based M-type ferrites. In the Mg-Ti substitution caseof Ca0.8La0.2Fe8.0−2xMgxTixO19−σ, the single-phase sample
was formed at x = 0.1. As the amount of substitution
increased, both the saturation magnetization and theCurie temperature decreased. In the Al substitution
case of Ca0.8La0.2Fey−xAlxO19−σ, samples, the single
M-type phase samples were formed at x = 1.0 and 2.0
sintered at 1300°C. As the amount of substituted Al
increased, the saturation magnetization and the Curie
temperature decreased and the coercivity increased.
The Al substitution seems to change the distribution ofiron cations in the M-type structure and to cause tochange the magnetic properties.

1)
2)
3)
4)
5)
6)
7)
8)

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Received Apr. 19, 2017; Accepted Jul. 7, 2017

Journal of the Magnetics Society of Japan Vol.41, No.5, 2017