Structural, Magnetic and Magneto-Optical Propertiesof (YYbBi)3Fe5O12 Single Crystalfor High-Performance Magneto-Optical Applications

Min Huang1) (a) and Shouye Zhang (b)

(a) Physics Department, Zhejiang University, Yu-Quan, Hangzhou 310027,People’s Republic of China

(b) Materials Science and Engineering Department and State Key Laboratory of SiliconMaterials, Zhejiang University, Yu-Quan, Hangzhou 310027, People’s Republic of China

(Received by M. S. Brandt January 3, 2001; in revised form March 16, 2001;accepted April 3, 2001)

Subject classification: 78.20.Ls; 78.30.Hv; S11.2

A new composition of bismuth and ytterbium substituted yttrium iron garnet bulk single crystalðYYbBiÞ3Fe5O12 (YYbBiIG) was grown from a Bi2O3=B2O3 solvent using a spontaneous techni-que. The temperature derivative b of the Faraday rotation angle at 45� has been modified by substi-tuting Y3þ and Yb3þ ions in the dodecahedral site of iron garnet crystals. The structural and mag-neto-optical properties were investigated using XRD, VMS, near-infrared transmissionspectrophotometry and spectroscopic Faraday polarimetry. The Faraday rotation and optical ab-sorption coefficient of Y1:43Yb0:82Bi0:75Fe5O12 at 1550 nm were found to be �934 deg/cm and1.60 cm�1, respectively. The Faraday rotation temperature derivative b was 0.01 deg/�C at 1550 nmwavelength, the saturation magnetization 4pMs has been estimated to be about 1200 G. The largerFaraday rotation, low optical loss, low saturation magnetization and nearly zero temperature deri-vative b of the Faraday rotation of YYbBiIG crystal promise its use in high-performance magneto-optical application.

1. Introduction

Rare-earth iron garnet such as Y3Fe5O12 (YIG) is one of a well-known family of ferro-magnetic materials. It was first synthesized in the mid 1950s. Since then this materialhas been widely studied because of its excellent microwave properties including rela-tively low magnetization, extremely narrow linewidth and a low dielectric loss behavior.These properties make it suitable for microwave devices such as delay lines, tuningfilters, oscillators, isolators and phase shifters. YIG has also unique magneto-opticalproperties in the near-infrared (1–5 mm). One of the interesting YIG optical features isthe Faraday rotation effect: rotation of the polarization vector of the electromagneticwave propagating in magnetic media. At present, many attempts to use Faraday rota-tion effect for various applications as optical isolators, optical switches and modulatorsfor fiber-optic devices have been undertaken [1]. Unfortunately, the angle of the Fara-day rotation inherent to YIG crystals (about 200 deg/cm at 1300 nm) is not sufficientlylarge for the development of integrated magneto-optics. The bismuth-substituted irongarnet with enhanced Faraday rotation effect of one order of magnitude larger thanthat of YIG is much more promising for applications. However, Bi-substituted garnetshave large temperature derivatives b of the Faraday rotation angle at 45�, such that b

1) Corresponding author; Tel.: +86-571-8855139; e-mail: minhuang@mail.hz.zi.cn

phys. stat. sol. (a) 185, No. 2, 487–492 (2001)

of YIG is 0.04 deg/�C at 1300 nm wavelength [2], and b of the Bi-substituted garnets isbetween 0.04 and 0.11 deg/�C at 1300 nm wavelength [3, 4].Generally, substituting Y3þ or Yb3þ ions in the dodecahedral site of garnet crystals,

their contribution to the temperature derivative b of Faraday rotation shows the op-posite effects [5]. Further, the Faraday rotation increases with substituting Bi3þ intogarnet crystals. Using this promising method, we successfully prepared the newYYbBiIG bulk single crystal with larger Faraday rotation and small temperature deri-vative b of the Faraday rotation and described its structural, magnetic and magneto-optical properties.

2. Experimental Procedure

Single crystals of Yb3þ and Bi3þ substitued yttrium iron garnet were synthesized by theimproved flux method with taking Bi2O3�B2O3 as main flux while employing the Ac-celerated Crucible Rotation Technique (ACRT) and local cooling technique [6]. Theproper mixture of oxide powders of Y2O3 (99.999% purity), Yb2O3 (99.999% purity),Fe2O3 (99.99% purity) and Bi2O3 (99.99% purity) was mechanically ground. Then themixture of powders with weight of 500 g was put into a platinum crucible covered witha tight-fitting lid to minimize evaporative loss and heated up to about 1320 �C in thefurnace in which the crucible can be rotated and accelerated in positive and negativedirections. After being kept for many hours at the highest temperature, the melts wereslowly cooled down to 980 �C at the cooling rate of 0.8–3 K/h, then to room tempera-ture at a relatively quick cooling rate of over 20 K/h. During the growth period, thetemperature gradient was selected to be about 2 K/cm with a lower temperature at thebottom of the crucible than at the surface of the melt. Finally, the grown YYbBiIGbulk single crystals with a maximum size of 7� 6� 4 mm3 were separated from the fluxby diluted nitric acid. During the cooling, the melt was stirred or rotated to homogenizethe solute and the solvent. During the crystal growth, the maximum rotation speed ofthe platinum crucible is 60 rpm and the period of accelerated rotation is 8 min.The grown crystal was Bi3þ doped iron garnet monocrystal determined by directional

X-ray diffraction in h110i direction using a Rigaku D/max-3B diffractometer with Cutube. The lattice parameter was found to be 12.4021 �A calculated from (420), (422),(440), (521), (640), (633), (840) etc. Bragg reflection positions of powder X-ray diffrac-tion (XRD). The Y, Yb, Bi content was determined by electron probe microanalysis(EPMA) to within 2% accuracy, the formula of the as-grown single crystal wasY1:43Yb0:82Bi0:75Fe5O12.The as-grown bulk single crystals of YYbBiIG were cut into the shape of slices using

an annular saw with a diamond blade in h111i direction after being oriented by X-rays.The slices were carefully mechanically polished on both sides using a diamond paste ofgradually decreasing grain size down to <1 mm, the final thickness of the slices wasabout 200 mm. No antireflection coating was applied onto the slice surfaces.The Faraday rotation was measured by the polarization modulation method in a stea-

dy field up to 20 kOe at room temperature. The near-infrared transmission measure-ment was carried out on polished slices by a Hitachi U-3410 double-beam spectrophot-ometer. On the other hand, a vibrating sample magnetometer (VSM) was used tomeasure the saturation magnetization of the crystals at room temperature and to deter-mine the coercive force.

488 Min Huang and Shouye Zhang: Magneto-Optical Properties of YYbBiIG

3. Results and Discussion

3.1 VSM measurements

The magnetization MðHÞ in grown YYbBiIG single crystal was measured at room tem-perature in magnetic fields up to 3� 103 Oe in two geometrical arrangements byVSM. Figure 1 shows the hysteresis loop in parallel and perpendicular magnetic fieldswith respect to the slice surface. The saturation magnetization 4pMs has been estimatedto be about 1200 G for YYbBiIG crystal. In Hjj geometry, magnetization saturates atrather low magnetic field ð�500 OeÞ. In perpendicular magnetic field H?, the M–Hcurve saturates at higher fields 2 kOe. The coercive force did not exceed 50 Oe inboth the parallel and the perpendicular geometries.

3.2 Faraday rotation and optical absorption studies

The Faraday rotation spectrum was measured by the magneto-optically modulated dou-ble-frequency technique using a monochromator (Nikon G-250) and an iodine-tungstenlamp (250 W, 12 V) as the light source [7]. The sample was located at the center of theelectromagnet that contained holes in the pole pieces so that the plane-polarized radia-tion from the monochromator could propagate through the crystal parallel to the ap-plied magnetic field. The state of polarization of the radiation emerging from the crys-tal in the presence of both positive and negative magnetic field was determined usingan analyzer and germanium photodiode. Faraday rotation was calculated from the in-tensity changes of the photodiode.The Faraday rotation spectrum of the specimen in the near-infrared wavelength

range of 0.8 to 1.7 mm under saturation magnetic field at room temperature is plottedin Fig. 2. Due to the Bi3þ substitution, the Faraday rotation of the YYbBiIG singlecrystal is much larger than that of YIG in the wavelength range of 0.8 to 1.7 mm. Thespecific Faraday rotation for composition Y1:43Yb0:82Bi0:75Fe5O12 under saturationmagnetization was measured to be �1300 deg/cm at 1300 nm wavelength and�934 deg/cm at 1550 nm wavelength, which is about five times larger than that ofpure YIG.

phys. stat. sol. (a) 185, No. 2 (2001) 489

Fig. 1. Magnetization M vs. H inmagnetic field parallel Hjj and per-pendicular H? to the slice face ofY1:53Yb0:92Bi0:55Fe5O12 single crys-tal measured by a vibrating samplemagnetometer

The transmission spectrum measured in the near-infrared region of 0.8 to 2.6 mmhas been shown in Fig. 3. The ideal transparency of pure YIG in the infrared regionfor general multipath reflections is about 75% [8]. However, the YYbBiIG singlecrystal grown by the flux method shows good transparency, the transmittance in thenear-infrared being about 73% to 74.5%. The optical absorption coefficient of theYYbBiIG crystals was calculated from the transmittance and refractive index of Bi-substituted iron garnet [9]. The value of optical absorption coefficient in the transpar-ent region was about 1.41 to 2.30 cm�1 and the first absorption peak was found near0.9 mm which results from the 6A1g to 4T1g crystal field transition existing in the Fe3þ

ions. A principal source of extrinsic absorption in the iron garnets is the presence ofFe2þ and/or Fe4þ, which show broad absorption notably in the near-infrared wave-length of telecommunications interest. During the crystal growth, we adopted bismuthoxide as the main flux and added a small amount of boron oxide to decrease themelting point temperature. It is well known that the use of lead oxide-free flux iseffective to restrain Fe2þ=Fe4þ and reduce the absorption loss in the near-infraredregion.

490 Min Huang and Shouye Zhang: Magneto-Optical Properties of YYbBiIG

Fig. 2. Wavelength dependenceof the Faraday rotationfor Y1:43Yb0:82Bi0:75Fe5O12 singlecrystal at room temperature

Fig. 3. Near-infrared transmission spectrumof Y1:43Yb0:82Bi0:75Fe5O12 single crystal atroom temperature

3.3 Temperature properties of Faraday rotation

The temperature derivative b of the Faraday rotation plays an important role in themost magneto-optical applications. In order to get the temperature dependence of theFaraday rotation of the crystal, a variable-temperature device was added to the sampleholder. The sample was in contact with a thermal shield, the temperature was measuredby means of a thermocouple attached to the sample. Figure 4 shows the temperaturedependence of the Faraday rotation of YYbBiIG crystals at two wavelengths. The wa-velengths 1300 and 1550 nm are selected because of their importance in the long-dis-tance fiber communication systems. It is clear from the figure that the Faraday rotationvaries nearly linearly with temperature in the range from room temperature to 80 �C.The Faraday rotation temperature derivatives b are 0.008 deg/�C at 1300 nm wave-length and 0.01 deg/�C at 1550 nm wavelength for the Y1:43Yb0:82Bi0:75Fe5O12 crystal.Because of the positive contribution of Yb3þ ions and the negative contribution of Y3þ

and Bi3þ ions to the temperature dependence of Faraday rotation, the YYbBiIG singlecrystals show a negative temperature dependence of Faraday rotation absolute valuerather smaller than that of YIG, Yb3Fe5O12 and other Bi-substituted iron garnets underthe same conditions.

4. Conclusion

We have successfully grown a new Bi-substituted mixed iron garnet single crystal ofcomposition Y1:43Yb0:82Bi0:75Fe5O12. The temperature derivative of Faraday rotation ofthe grown crystal has been modified by substituting Y3þ and Yb3þ ions in the dodeca-hedral site of iron garnet crystals, which show the opposite contribution to the tempera-ture derivatives b of the Faraday rotation. The specific Faraday rotation, optical absorp-tion coefficient, Faraday rotation temperature derivative b and the saturationmagnetization 4pMs of Y1:43Yb0:82Bi0:75Fe5O12 at 1550 nm wavelength were found to be�934 deg/cm, 1.60 cm�1, 0.01 deg/�C and about 1200 G, respectively. These results sug-gest that bismuth and ytterbium substituted yttrium iron garnet is one of the promisingmaterials for the development of high-performance magneto-optical devices.

Acknowledgements The support of the National Natural Science Foundation Council ofPeople’s Republic of China (No. 69890230) on this project is gratefully acknowledged.

phys. stat. sol. (a) 185, No. 2 (2001) 491

Fig. 4. Temperature dependence ofthe Faraday rotation angle forY1:43Yb0:82Bi0:75Fe5O12 single crystalat optical wavelengths of 1300 and1550 nm. Faraday rotations are nor-malized so that they are put equal to45� at 25 �C

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492 Min Huang and Shouye Zhang: Magneto-Optical Properties of YYbBiIG