Available online at www.sciencedirect.com JOURNAL OF

ScienceDirect JOCRNAL OF RARE EARTHS 25 (2007) 79 - 83 www.elsevier .com/locate/jre

Structural Analysis and Magnetic Properties of Gd-Doped Li-Ni Ferrites Prepared Using Rheological Phase Reaction Method Jiang Jing ( % ( Department OJ Chemistri , Institute of Phlszcal Chemistry, Zhepang Vormul CTniversity, Jmhua 321004, China)

*), Li Liangchao ( * k&) * , Xu Feng ( j + '%)

Recelled 26 Ipnl 2006; reiised 20 October 2006

Abstract: A wries of Gd-doped Li-Ni ferrites with the formula of LiNio.sGd,Fe2-x04 where x = 0.00 - 0 .08 in steps of 0 . 0 2 , were pre'iared by thermolyis of oxalate precursors obtained by rheological phase reaction. The structure, morpholo- g y , and the ma :netic properties of the samples were characterized by powder X-ray diffraction (XRD) , atomic force mi- croscopy ( LIFM ) and a vibrating sample magnetometer (VSM) . A single spinel phase was obtained in the range of x = 0.00 - 0 .04 . The lattice parameters of the Gd-doped samples were larger than that of pure Li-Ni ferrite, and increased in the range of 0 . I )0 < s 0 .04 , then decreased up to x = 0 .08 , because of the formation of the secondary phase (Gd- FeO3) . All san ples were spheric particles with an average size of about 100 nm, but agglomerated to some extent. The h+eresis loops indicated that the saturation magnetization decreased gradually with increasing Gd content, while the van- ation of coercivi ty was related to the microstructure of the Gd-doped samples.

Key words : rheological phase ; Li-Ni ferrite ; Gd-doped ; structure ; magnetic properties ; rare earths CLC number: TM273 Article ID: 1002 - 0721 (2007)Ol- 0079 - 05 Document code: A

Lithium f t n i tes have attracted considerable atten- tion for their pl Itential applications in microwave devic- es such as is0 ators, circulators, and phase shifters, because of the r high resistivity, low dielectric losses, and high Curi: temperature. These are used as re- placements for expensive magnetic garnets because of their low costs ' . It is interesting that the electromag- netic propertiei of spinel ferrites can be tailored by controlling the type and amount of transition metallic substitutes. C ntil now, several investigations have been carried o i t to make further improvements on the dielectric and magnetic properties of substituted lithi- um ferrites-'.3- It is known that rare earth ( R E ) ions have unpaired 3f electrons, which have the role of originating maj;netic anisotropy because of their orbital

shape. The magnetocrystalline anisotropy in ferrite is related to the 4f-3d couplings between the transition metal and rare earth ions; thus doping rare earth ions into spinel lithium ferrite can improve their electrical and magnetic properties.

In this study, Gd-doped Li-Ni ferrite powders were obtained using thermolysis of oxalate precursors that prepared by the rheological phase r e a ~ t i o n ' ~ , and their structural and magnetic properties were investi- gated. The rheological phase reaction method is a pro- cess of preparing compounds or materials from solid- liquid rheological mixture, that is , the solid reactants are fully mixed in proper molar ratio, made by adding the required amount of water or other solvent to a sol- id-liquid rheological body in which the solid particles

- Corresponding author (E-mail: skv52@zjnu. c n ) Foundation item: Project supported by the Natural Science Foundation of China (Y405038) and Science and Technology Key Project of Zhejiang

Projince (2006C21080)

Biography : Jiang Jinp i 1982 - ) , Male. Master randidate

Cop!right j 2007, by Editorial Committee of Journal of the Chinese Rare Earths Society. Published by Elsevier B . V . All rights reserved.

80 JOURNAL OF RARE EARTHS, Vol. 25 , N o . 1 , Feb . 2007

and liquid substance are uniformly distributed. The precursors obtained using this method have the advan- tages of excellent stoichiometry and homogeneity, trace impurities, and relatively lower thermal-decom- position temperature. This method is inexpensive and simple.

1 Experimental Samples with chemical formula LiNi, Gd, Fez -

0, ( x = 0.00 - 0.08 ) were prepared using the rheo- logical phase method. Analytical grade chemical re- agents Li,CQ3, NiSO, * 6 H 2 0 , Gd203, Fe203, and H,C,O, * 2H,O were weighed in stoichiometric propor- tions and thoroughly mixed by being ground in an ag- ate mortar for 30 min; appropriate amount of ethanol was then added to form a mixture in rheological state. The mixture was sealed in a teflonlined stainless-steel autoclave and reacted at 120 'C for 48 h in an oven. The obtained precursors were washed for several times with deionized water and ethanol, dried at 60 "c for 12 h and sintered at 1000 "c for 2 h in air, followed by cooling with furnace to room temperature at a cooling rate of 5 'C * min ~ ' . Gd, Fez ~ 0, powders were obtained.

The X-ray diffraction patterns of samples were collected using a PW3040/60 diffractometer with a graphite monochromator and Cu Ka radiation ( A = 0.15418 nm) at a scanning rate of 4 (") srnin-' in the range of 20 from 20" to 80". The morphology and par- ticle sizes of the samples were examined using atomic force microscopy ( AFM , P47H-SPM-MDT) . Magneti- zation measurements were carried out using a vibrating sample magnetometer (VSM , Lakeshore 7404) under applied magnetic field at room temperature .

Polycrystalline LiNi,

2 Results and Discussion

2 .1 X-ray diffraction analysis The X-ray diffraction patterns of LiNio,sGd.Fez

0, ferrites are given in Fig. 1. All diffraction peaks of the samples correspond to the cubic spinel structure (ICDD PDF # 86-2267). A single spinel phase is obtained for the samples with x = 0.00 - 0.04, indi- cating that introducing an appropriate amount of Gd3+ ions into ferrite can replace the Fe3+ ions on the octa- hedral sites, which obeys Vegard's law'" . However, with the Gd content increasing further ( x > 0 . 0 4 ) , a small amount of secondary phase is formed with the diffraction peak (28 = 32.87") identified as the peak of the GdFe03 phase (ICDD PDF # 78-0451 ) , be- cause the ionic radius of Gd3 + ( 0 . 0 9 4 nm) is larger

than that of the Fe3+ ions (0.064 nm ) ; the amount of Fe3+ ions substituted by Gd3+ ions h a w a limit, there- fore redundant Gd" ions will aggregate on the grain boundaries forming the GdFeO? phase during the sin- tering process'6,7' .

The dependence of the lattice parameter on the Gd content x is shown in Fig. 2. The lattice parame- ters of all Gd-doped samples are larger than that of pure Li-Ni ferrite. This is because l a r y r Gd3 + ions re- place Fe3 + ions on the octahedral site5 , leading to the expansion of the spinel lattice. Howex er , when the Gd content x is beyond 0.04, the lattice parameter de- creases up to x = 0.08. A possible explanation for the decrease in the lattice parameter can be the compies- sion of the spinel lattice induced by vxondary phases because of the difference in the thermal expansion coefficients"' .

The X-ray density d , was calculated by the for- 8 M

mula d, = 7, where M , N , and a represent the Na

molecular weight, the Avogadro' s number, and the lattice parameter, respectively. It is Seen from Fig. 2 that the X-ray density increases linearly with the Gd content x. Both the molecular weight and the volume of the unit cell for the doped ferrite increase with

I i + Spinel 1 GdFeO, d x A ' ' A h

h n

1 1 b _ _ . . . A I n. . .

~ 0 . 0 6

u x=0.04

F0.02

.%=0.00

v

c .- yru 4

al c u L -

J c

c

30 40 50 60 70 80

2 a / ( " )

Fig. 1 XRD patterns of LiNio 5Gd,Fel. .Oq ferrites

0.8364 -

- 4.83

0.00 0.02 0.04 0.06 0.08

Gd content x

Fig.2 Dependence of lattice parameter (C and X-ray density (0 ) on Gd content x

Jiang J et al . Analysis and Properties of Gd-Doped Li-Ni Ferrites 81

the Gd content s , because of the replacement of larger Gd3+ for Fe'& , and the molecular weight has more in- crement when compared to the volume of the unit cell, therefore, tht X-ray density increases with the Cd content s.

2.2 Cation distribution

Energy calculations show that the size and valen- cy of cations md the oxygen parameters of the anions are the impo.-tant parameters in deciding the cation distribution i l l ferrites . The cation distribution for lithium ferritt has been suggested as ( Fe:+ ) [ Li& Fe:; 10, lo . [n general, Li + and hi'+ ions have pref- erence for tht. octahedral B sites; Fe3+ ions are dis- tributed betwtaen .A and B sites, and Gd3' ions with larger ionic radii replace Fe'+ ions on the octahedral B sites. Based in this and Neel's tWo sublattice model, the cation distribution of Cd-doped ferrites can be as- sumed as (Ft ; + ) [ Li;? + ,Yii',Gd: + Fe:I _I ] 0, , where Li0+5+ro 5 ! denotes that Li' ( 0 . 059 nni) replaces par- tially for F e ' ~ ( 0 . 0 6 4 nm) on the octahedral R sites, and fills partiaiiy into the octahedral interstices.

Accordirig to the cation distribution given by Gd- doped ferrite:. the average ionic radii of B sites ( r B ) and the oxygc 11 parameter 11 can be calculated by Eqs . ( 1 ) and ( 2 ) i ' :

r, =- - [0 .5r1 , - +0.5r,, '- + xrGdi+ + ( 1 - ,x)rFf3+] 1 2 ( 1 )

5 T B + Ro 8 u = - - - - -

where, u is he lattice parameter, and R , , is the radi- us of the os?-ren ion (0 .134 nni) .

The valiies of the oxygen parameter u , and the average radilri rB on the octahedral sites as a function of the Gd co itent Y are shown in Fig. 3 . The rB in- creases linea .l!~ with the Gd content .Y , which corre- sponds to Fe substituted by Gd'+ on the B sites. The

0.03882

c

C '=

0 0660 - 0.03876

0 0655

0.0650

0.0645

0.0610

0.03864

(1. u3 8 5: (.on 0.02 0.04 0.06 0.08

Gd contcnt I

Fig.3 .4\erag. radius r B 8 ~ 'I and oxygen parametrr u ( 0 1 as a functio I of Gd content .k

u values depend on the chemical composition, the preparation conditions, and the heating procedurerL2' , and the ideal u value is close to 0.0375 nm in spinel ferrite. For the investigated samples, the values of u are rnore than the ideal one, which indicates that the crystal lattice deviates from the ideal spinel ferrite be- cause of the difference in the preparation co- nditions '' .

The values of the tetrahedral and octahedral bondlength ( d,, and d,, ) , the tetrahedral edge ( d 4 F ) , and the shared and unshared octahedral edge (dBE and d,,,) can lie calculated according to Eqs. ( 3 ) - (7)'13,14' using the values of the lattice parame- ter a and the oxygen parameter u .

( 3 ) r 1

d,, = ao d 3 ( u - $

The edge and the bondlength of the tetrahedral and octahedral sites as a function of the Gd content x are shown in Table 1 , respectively. It is seen that d,, and d,, increase with the Gd content x , which may be because of the fact that Fe3+ ions are substituted by larger Gd3+ ions on the octahedral sites, resulting in the increase of d, , and dR1, along with the expansion of the B sublattice; meanwhile the A sublattice is com- pressed by the B sublattice, resulting in the decrease of dAL and dAE. dBE, exhibits similar behavior as the lattice parameter.

The distance between magnetic ions ( hopping length) in the tetrahedral and octahedral sites is given by Eqs . (8) - (9)i '4'L5':

4 A -

a J 3 L B = - 4

Table 1 Values of d A L , d s L , d A E , dBE. and dsEU as a function of Gd content x

Cd rontrnt 1

~~

0.00

0.02

0.04

0.06 0.08

Tetrahedral sites Octahedral sites

(1 4, i l l l l l d ,~llllll

0.1999 0.3264

0.1991 0.3251

0.1981 0.3234

0.1971 0.3219

0.1%1 0.3212

d H L l i i i i i dukinm C / I ( + , lnm

0.1986 0.2645 0.29630

0.1991 0 . m ) 0.29635

0.1W 0.2678 0.2%38

0.2002 0.2694 0.29628 0.2007 0.2710 0.2%16

a2 JOURNAL OF RARE EARTHS, Vol. 2 5 , N o . I , Feb . 2007

- 0.36218 0.29570 sion of the spinel lattice reaches its maximum. When x > 0.04, redundant Gd3+ ions form the GdFeO, phase along the grain boundaries to inhibit the grain growth. It is clear that the grain size of the sample with x = 0.08 is smaller than that of the sample with x = 0.04.

- E

- 0.29550 'g 2.4 Magnetic properties

E p 0.36211 4

- * -LA . 2 4 - L - The variation of the saturation magnetization as a

.C 0.36197

0.36190

-0.29540 0.00 0.02 0.04 0.06 0.08

Gd content x

Fig. 4 Dependence of hopping length for the A sites (0 ) and R sites (3) on Gd content x .

The dependence of the hopping length ( L A and L B ) for the A sites and the B sites on the Gd content x is shown in Fig. 4. It is observed that the variation of L A and LB exhibits similar behavior as the lattice parameter a . L A and L , increase at x = 0.04 because the Gd3+ ions replace the Fe3+ ions on the B sites in the spinel lattice, while these decrease with the Gd content x > 0.04, since the redundant Gd3 + ions form the GdFe03 phase along the grain boundaries and in- hibit the grain growth.

2.3 Morphologies of samples Fig. 5 shows the representative AFM photographs

of LiNio.5Gd,Fe2_z04( x = 0.00, 0.04, 0.08) fer- rites. It is observed that the samples are spheric parti- cles with average size of about 100 nm , but agglomer- ation occurs to some extent. The grain sizes of ferrites are related to the chemical composition, the prepara- tion conditions, the cell volume, the secondary phas- e s , and the glomeration behavior of particles. It is ob- served from Fig. 5 that the grain size of the sample with x = 0.04 is the largest among the three samples, because the replacement of limited amounts of Gd3+ ions for Fe3+ ions occurs at x = 0.04, and the expan-

function of the Gd content x is shown in Fig.6, which is in agreement with the literature"6' " . The satura- tion magnetization of the Gd-doped samples is less than that of pure ferrite, and decreases with the in- crease of the Gd content. The magnetic moments of rare earth ions generally originate from localized 4f electrons and these are characterized by lower magnet- ic ordering temperatures, i . e . , lower than 40 K""; their magnetic dipolar orientation exhibits disordered form at room temperature. Hence, it maj be reason- able that Gd3 ' ions are considered as non-magnetic ions, and make no contribution to the magnetization of doped ferrite at room temperature. Furthermore, the deformity of the spinel lattice produces due to Gd3+ substituted for Fe3 + , nonlinear antiferromagnetic cou- pling between A and B sublattice increases, and the magnetic dilution of secondary phase ( GdFe03 ) be- comes stronger with Gd content, consequently, satura- tion magnetization of samples decreases.

Fig. 6 also shows the variation of the coercivity as a function of the Gd content. The coercivity values of the Gd-doped samples are lower than that of pure Li- Ni ferrite. It is seen that the coercivitj decreases in the Gd content x = 0.00 - 0.04, then increases up to x = 0.06, and decreases thereafter. The coercivity is influenced by factors such as magnetocq stallinity , mi- crostrain , magnetic particle morphology and size dis- tribution, anisotropy , and the magnetit. domain S ~ F . The initial decrease may be related to the microstruc- ture of samples. The coercivity is inversely proportion-

. .

Fig. 5 AFM images of LiNio 5GdlFr2 - .04 samples (a) x =O.OO; (b) x = 0 . 0 4 ; ( c ) x =0.08

83 Jiang J et a1 . Analysis and Properties of Gd-Doped Li-Ni Fem'tes

Fig .6 Compositional \ariation nf saturation magnetization i 1 and coerc i \ i t \ 8, 1

14 a1 to the grain size . It is obsened by AFM images (F ig .5) that tl,e grain size of samples increases in the range of .T = 0 00 - 0 .04 . -1 larger grain size makes the motion of t ie domain walls easier, and thereb!- the coercivit!- decrl.ases. R-ith increasing Gd coiltent. re- dundant Gd3- ons ma!- reside at the grain boundaries and form secoi clan phase. The prrsrnce of secondary phase at the pa in boundaries ma!- not only inhibit the motion of the I iagnetic domain walls, h i t may also in- duce some dis ortion Mithin the grains leading to the initiation of th 1 internal stress ; moreor eI, w-ith niore Gd3 * ions iniroduced into the spinel ferrite, the strength of thr spin-orbital coupling that determines the magnetic ,inisotropy in the ferrites increases , and thus the ccercivity increases w-ith the Gd rontent x be!-ond 0.04.

?I 1

3 Conclusion L\ series t If LiNio 5Gd,Fe2 ~ 0, saniples were syn-

thesized by t h thrrniol!-sis of oxalate precursors oh- tained b!- the rheological phase reaction. It showed that all Gd-doled ferrites had a major spinel phase, and a single sliiiiel phase wab obtained in the range of x = 0.00 - 0. '91. The lattice parameters of Gd-doped samples were arger than those of pure Li-Ni ferrite, and increased \rith the Gd content .x = 0. 00 - 0. 04, and then decrt ased up to .Y = 0.08. because of the for- mation of the ,econdafi- phase. The oxygen parameter ~1 deviated fro] ti the ideal value (0.0375 nin) . The sat- uration magnei izatiori decreased gradually with the in- creasing Gd camtent. crhile the variation of coercivity ic'as related to the micrnstnir.tiirc~ nf the (Xciope(l sani- ples .

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