A strong magneto-optical activity in rare-earth La 3 + substituted M-type strontium ferrites

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I. INTRODUCTION
M-type strontium hexaferrites are the most important ceramic permanent magnets and have attracted a great deal of interest.2][3][4] Strontium hexaferrites have been prepared by diverse techniques such as the hydrothermal process, 5 the self-propagating high-temperature synthesis, 6 the polymerizable complex method, 7 the microwave-assisted calcination route, 8 the sol-gel method, 9 and the ceramic process. 10In view of the large volume of the market, a considerable amount of work has been done to improve the performance of strontium hexaferrites by various methods such as doping, 11 heattreatment, 12 ion substitution, 13 etc.][16] The magneto-optical Kerr effect is important in magneto-optical recording technology. 17A strong interest has been focused on this subject in the past couple of decades as it is used for optical readout of magnetically stored information in erasable video and audio disks. 18Other applications of the magneto-optical Kerr effect, such as microscopy for domain observation, 19 magnetic characterization, 20 and magneto-optics, make it a very sensitive and nondestructive technique to investigate magnetism at the atomic scale as its origin relies on the spin-orbit coupling. 21,22For applications in high-density erasable magneto-optical memory devices, a large value of the Kerr rotation is required.The rare-earth elements in magneto-optical materials play an important role in the magneto-optical Kerr effect.Meanwhile, our previous works have shown that by introducing rare-earth La 3þ and transition Co 2þ , the magnetic properties of M-type strontium hexaferrites can be improved. 9,10However, no information is available on the magneto-optical Kerr effect studies of rareearth La 3þ substituted strontium hexaferrites in the literature.In this paper, the influence of the amount of substituted La 3þ on both the magneto-optical Kerr effect and the magnetic properties were systematically studied.

II. EXPERIMENTAL
The raw materials used in this study were SrCO 3 (99.0%purity), La 2 O 3 (99.0%),and Fe 2 O 3 (99.94%).Sr 1Àx La x Fe 12 O 19 ferrites were prepared by the conventional ceramic process, where x varies from 0 to 0.20 in 0.05 increments.The process consists of four main steps: (i) mixing the raw materials together, dry-milling the powder mixtures, presintering at 1000 C for 2 h in a muffle, then cooling in the muffle down to room temperature, (ii) dry-milling the presintered samples, pressing into disk-shaped compacts of 12Ø Â 1-2 mm h, (iii) sintering the pressed compacts in a muffle at 1290 C for 3 h with a heating rate of 300 C/h in air atmosphere, and cooling in the muffle down to room temperature, and (iv) finally, one side optical polishing of each disk-shaped compact for the investigation of the magnetooptical activity.
The powder x-ray diffraction (XRD) patterns were collected on a Bruker AXSD* Advanced Powder x-ray diffractometer by using Cu-Ka radiation (k ¼ 0.154 06 nm).The patterns were recorded in the 2-theta (2h) range from 20 to 80 .The magnetic parameters were measured using a Riken Denshi (BH-55 type) vibrating sample magnetometer (VSM) at room temperature with maximum applied magnetic field a) Author to whom correspondence should be addressed.Electronic mail: xiansongliu@yahoo.com.cn.
H of 800 kAÁm À1 .Magneto-optical ellipsometry measurements were carried out in the transverse Kerr configuration with a SOPRA GES E5 ellipsometer in the energy range 1.5-4.5 eV at an angle of incidence of 75 in order to maximize the effect under an in-plane magnetic field of 240 kAÁm À1 .The transverse Kerr effect taken from ellipsometry measurements was defined as , where tanW is the module in the well-known ellipsometric ratio q ¼ tanWÁe iD .The subindices þ and -account for the values obtained under positive and negative applied fields.

III. RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns of the sample Sr 0.8 La 0.2 Fe 12 O 19 sintered at 1000 C for 2 h and sintered at 1290 C for 3 h, respectively.As we can see, the desired M-type hexagonal structure (JCPDS 84-1531) was already formed at 1000 C.However, a-Fe 2 O 3 (JCPDS 89-0599) phase and trace amounts of LaFeO 3 (JCPDS 75-0541) were also found by XRD simultaneously, indicating an incomplete reaction of the powders.These impurity phases disappeared as the sample Sr 0.8 La 0.2 Fe 12 O 19 was sintered at 1290 C. Moreover with increasing sintering temperature, the relative intensity of the diffraction peaks became stronger and narrower, indicating a better structural quality of the materials.Compared to Sr 2þ ions and Fe 3þ ions, La 3þ ions have lower mobility, and their diffusion is expected to be slower.It is therefore concluded that in order to eliminate the impurity phases that can deteriorate the magnetic properties and to form single phase La-substituted samples Sr 1Àx La x Fe 12 O 19 , higher sintering temperatures and longer annealing time are required.In addition, we can obtain samples with a small coercivity (<160 kAÁm À1 ) by increasing the sintering temperature to a certain temperature range.
The XRD patterns corresponding to the samples Sr 1Àx La x For a deeper insight into the nature of the samples, the hysteresis loops of samples Sr 1Àx La x Fe 12 O 19 (x ¼ 0, 0.05, 0.10, 0.15, 0.20) in the powder form were taken by VSM at room temperature.With the La ionic addition, r s and H cJ of all the samples are different.For example, the saturation magnetization of SrFe 12 O 19 is 65.4 AÁm 2 Ákg À1 and its coercivity is 105.7 kAÁm À1 in Fig. 3(b) under the applied field 800 kAÁm À1 .It is expected that the coercivity of all the samples in the insert part of Fig. 3 need less than the applied field (240 kAÁm À1 ) of the magneto-optical measure.
Figure 4 illustrates the relationship between the magnetic properties of Sr 1Àx La x Fe 12 O 19 and the substituted amount x.It is found that the suitable amount of La 3þ substitution may remarkably influence the saturation magnetization r s and intrinsic coercivity H cj .Our results are very similar to La substituted barium hexaferrites Ba 1Àx La x- Fe 12 O 19 with the same range of substitution reported by S. Ounnunkad. 23In the hexagonal SrFe 12 O 19 ferrite, the Fe 3þ  ions are distributed over five different sites: three octahedral sites (12k, 4f 2 , and 2a), one tetrahedral site (4f 1 ), and one bipyramidal site (2b).In the La-substituted samples, La 3þ ions are expected to enter the Sr 2þ sites because of their compatibility in radius.It leads to a valence change of Fe 3þ to Fe 2þ at 2a or 4f 2 site to conserve the charge neutrality.
The changes of the saturation magnetization r s and intrinsic coercivity H cj shown in Fig. 4 are first decided by both a number of Fe 3þ ions with spin-up orientation and the superexchange interation. 24n order to determine the magneto-optical activity, two ellipsometry spectra were taken for each sample, one with a positive applied magnetic field and a second one with a negative applied magnetic field; 240 kAÁm À1 in both cases as stated in the Sect.I. From the ellipsometric ratios taken at positive and negative applied fields, tan(wþ) and tan(wÀ), the transverse Kerr effect is determined. 25The magneto-optical activity of these samples is shown in Fig. 5. Provided that the magneto-optical response of the samples is proportional to the magnetization, the transverse ellipsometric Kerr effect was multiplied for each sample by a factor M 800 /M 240 , where M 800 is the saturation magnetization of the samples (the magnetization measured at 800 kAÁm À1 ) and M 240 is the magnetization measured at 240 kAÁm À1 , that is, the magnetization of the sample under the applied field during the transverse ellipsometric Kerr measurements.When the hexaferrite is not doped [Fig.5(a)], its magneto-optical activity is very poor.When the substitution is low, x ¼ 0.05, the magneto-optical activity is still not relevant [Fig.5(b)].It has been shown in previous works that the magneto-optical activity of the M-type Sr and Ba hexaferrites is very low 26 and due to the Fe 3þ ions.The almost zero activity observed in this work at low La 3þ concentrations can, therefore, be both attributed to the moderately low magnetic fields applied and to the low intrinsic magneto-optic activity.A small magneto-optical activity appears for x ¼ 0.10 [Fig.5(c)].Two signatures are observed; one centered around 2 eV, whereas the second one is centered around 3 eV.When the La concentration is increased to 0.15 [Fig.5(d)], there still exists a minor magneto-optical activity around 2 eV, but the activity at 3 eV is clearly increased, and the peak gets better defined.This is an indication that the activity detected at low energy is intrinsic of the hexaferrite, in agreement with Ref. 26, whereas the activity at higher energies is induced by the doping.The reason why this activity had not been observed at the lower contents of La doping may be attributed to the small changes in the magnetization obtained with the 240 kAÁm À1 of applied magnetic field in the low La content samples (Fig. 3).This is indeed confirmed when the La doping is increased to x ¼ 0.20 [Fig.5(e)].In this case, the magnetooptical activity rises to almost 1% around 3 eV.The low energy magneto-optical activity is kept low, reinforcing the idea that it is due only to the Fe 3þ ions from the undoped hexaferrite.In Ref. 27, Co 2þ is substituted for Fe 3þ ; this leads to a decrease in the polar Kerr effect around 380 nm that is then attributed to fewer Fe 3þ ions or more specifically actually to a decrease in the amount of Fe atoms rather than to a chemical reduction.In the present work, in which La 3þ is substituted for Sr 2þ , only a minor amount of Fe atoms reduce, but there is no decrease in the total amount of

IV. CONCLUSIONS
Single-phase M-type strontium ferrites with substitution of Sr 2þ by rare-earth La 3þ , according to the formula Sr 1Àx La x Fe 12 O 19 (x ¼ 0, 0.05, 0.10, 0.15, 0.20), were prepared by the conventional ceramic technology.X-ray diffraction showed that a high treatment temperature and a long annealing time were required to form single phase Sr 1Àx La x Fe 12 O 19 .The magnetic properties were remarkably changed due to the valence change of Fe ions induced by the substitution of La ions.Most importantly, a large magnetooptical Kerr activity was induced in the La 3þ substituted M-type strontium ferrites, opening a route to fabricate hexaferrite based magneto-optical sensors.
Figure1shows the XRD patterns of the sample Sr 0.8 La 0.2 Fe 12 O 19 sintered at 1000 C for 2 h and sintered at 1290 C for 3 h, respectively.As we can see, the desired M-type hexagonal structure (JCPDS 84-1531) was already formed at 1000 C.However, a-Fe 2 O 3 (JCPDS 89-0599) phase and trace amounts of LaFeO 3 (JCPDS 75-0541) were also found by XRD simultaneously, indicating an incomplete reaction of the powders.These impurity phases disappeared as the sample Sr 0.8 La 0.2 Fe 12 O 19 was sintered at 1290 C. Moreover with increasing sintering temperature, the relative intensity of the diffraction peaks became stronger and narrower, indicating a better structural quality of the materials.Compared to Sr 2þ ions and Fe 3þ ions, La 3þ ions have lower mobility, and their diffusion is expected to be slower.It is therefore concluded that in order to eliminate the impurity phases that can deteriorate the magnetic properties and to form single phase La-substituted samples Sr 1Àx La x Fe 12 O 19 , higher sintering temperatures and longer annealing time are required.In addition, we can obtain samples with a small coercivity (<160 kAÁm À1 ) by increasing the sintering temperature to a certain temperature range.The XRD patterns corresponding to the samples Sr 1Àx La x Fe 12 O 19 (x ¼ 0, 0.05, 0.10, 0.15, 0.20) sintered at 1290 C for 3 h are shown in Fig.2.The positions of the diffraction peaks of each sample are in good agreement with

FIG. 1 .
FIG. 1. X-ray diffraction patterns of Sr 0.8 La 0.2 Fe 12 O 19 samples sintered at 1000 C for 2 h and at 1290 C for 3 h.
FIG. 3. Hysteresis loops of samples Sr 1Àx La x Fe 12 O 19 (x ¼ 0, 0.20) at room temperature.The samples were sintered at 129 C for 3 h.
the present work.These results indicate and rare earth doping of M-type hexaferrites is an interesting route to induce a strong magneto-optical activity in these systems.

TABLE I .
Lattice parameters a and c as a function of composition.