Effects o f Zn content on the magnetic and m agnetocaloric properties o f N i-Zn fen ites

\ ' N l Journal o f Sciciicc, M athem atics - Pliysics 24 (200S)

Effects o f Zn content on the magnetic and m agnetocaloric properties o f N i-Zn fen ites

N. C h a u ’, N.K. Thuan', D.L. Minh^ N.H. Luong' *

‘ Center for hỉaícnaỉs Scicnce, Coỉỉege o f Science, VNU, 334 Nguyen Trai, Hanoi, Vietnam Dcpartmcnl o f Solid State Physics. CoUege o f Science, VNƯ, 334 Nguyen Trai, Hanoi, Vietnam

R eceiv ed 20 A ugust 2008

A b s tr a c t. A m ong spinel ierriles, C d and Z n ferrites are alw ays norm al ferrites w ith C d and Zn ions locating only in tetrah ed ral sites. T his study presen ts effect o f Z n on the m agnetic and m aun cto calo ric p ro p crlics o f the m ixed spinel ferrites N i).^ Z n J’C2 0 4 (x ^ 0.60, 0.65, 0.70, 0.75).

T he p rcscncc o f Z n affects lattice param eters, saturation m ag n etizatio n A/s, C urie tem p eratu re, 7c- and m agnelic en tro p y change A5„1. A t hiuhcst Zn content, T,. reduces to the tem p eratu re low er than room teniỊXTature and m agnetic structure o f spins in the octahedral sublattice should be slroim lv caiued. T he m axim um m ag n etic entropy change occurs in a large tem p eratu re ra n ee from low lem perature to hu n d red s o fC e lc iu s dcẹrees.

K eyw ords: ferrite, m am icto calo ric cffect.

1. In tro d u c tio n

It IS well known that spinel f c r n t c s consist o f three types o f magnetic structures; normal, inverse and mixed spinel [1]. In normal spinel, the divalent ions locate al lelrahcdral sublaltice (A -site) and invalent ions Fc"'' locate at octahcdral sublatticc (B-sitc). In inverse spinel a h alf o f Fe"'' ions locates at A site and the rest Fe'^‘ ions together with divalent ions locate at B site. In mixed spinel ferrite, both Fe‘^' ions and divalent ions locate at A and B site. N iFc204 IS inverse spinel fcưiíe. A m ong spinel

f c i T i t c s , only Zn and Cd ferrites belong to pure normal structure. M ixed Ni-Zn feưitcs have extrem ely

high resistivity so that they are w idely used as soft magnetic m aterials suitable for high-frcquency applications. Initial perm eability is m axim um at 30 mol % N iFe204, 70 mol % Znl*C204 and this compositon has Curie tem perature, Tc, near room temperature [2]. For the theoretical exam ination o f properties o f fcrritcs it could be started from the param eters characterizing for superexchange interaction types A-A, B-B, A-B. Interaction A 'A belongs to neighbor magnetic ions in sublatticc A.

interaction B-B - betw een ions in sublattice B and interaction A-B - betw een ions o f sublattices A and B. We denote ^aa, ^ab correspond to m olecular-field constants o f exchange interaction A-A, B-B and A-B, respectively [3].

Exchange interactions between magnetic ions through oxygen ion are superexchange interaction with antiferromagnetic nature. These interactions depend on bond distance and bond length. Normally

Corresponding author. Fax: (84-4) 3858 9496 Email: l uo n g n h @v n u . c d u . v n

156 N .C h a u et al. / V N U J o u rn a l o f Science. M athem atics - P hysics 24 (2008) Ỉ5 5 -Ỉ6 2

l^bl ĩ^hhị > l^al, therefore magnetic moments o f A sublattice is antiparallel oriented with spins in B sublattice [4]. The increase o f Zn content in Ni-Zn ferrites makes weakening x,ab and could lead to canting structure in B site [3]. Usually canting structure o f ferrites was examined by neutron scattering.

The studies on spinel feirites were started long time ago but recently a large number o f publications dealing with them has been performed including nanoparticles and thin feirite films [5-12]. I-spccially, superparamagnetic properties o f Ni-Zn feirite for nano-bio fusion applications ware reported [ 13).

In this p ap er we study m agnetic and electric properties including cantinR structure o f Ni-Zn feưites w ith high Zn content and at the first time we attemped to observe m agnetocaloric cffect (MCH) in these feưites.

2. Experiments

The polycrystalline feư ite samples Nii_xZn.^Fe204 (x = 0.60; 0.65; 0.70 and 0.75) w ere prepared by standard solid state reaction technique. The mixed powders were presintercd at 900^c for 3 hours and then reground to the fine particles, pressed into pellets and again heated at 9 0 0 T for 3 hours. Tlic second reground pow ders w ere pressed and sintered at 1300”C for 3 hours. The crystal structure of sam ples w as checked by X -ray diffractom eter D5005, Bruker and the m icrostructure o f samples was exam ined by Scanning Electron M icroscope (SEM ) Jeol LV5410. M agnetic properties o f fcm tes were m easured by V ibrating Sam ple M agnetom eter DMS 880, Diiiital M easurem ent System. Rcsisiivity m easurem ents w ere perform ed by four probe method.

3. Results and discussion

20 n

Fig. 1. X -r a y diffraction patterns o f ferrite sam ples Nii.xZrixFe204.

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The SI-M study showed that the m icrostructurc o f samples IS o f high hom ouencity and average particle size increases with Zn conlcnl in samples, nam ely from 2.1 [im (x = 0.60) to 2.8 i-im (x ^ 0.65) to 2.9 |,im (x ^ 0.70) and to 3.1 i-im (x ^ 0.75). Fiii. 1 presents the XRD patterns o f sludied samples.

All sam ples have single phase f.c.c spinel structure and lattice param eters arc determ ined and listed in Tab. 1. It IS clear from this table that lattice constant and volume o f unit cell increase with Zn content m the samples due to larụcr ionic radius o f ion (0.82 Ả) substituted for N i“‘ ion (0.78 Ả).

The X-ray density, the real density as well as porosity o f ferrites also determ ined and illusfratcd in Tab. 1. W hile the X -ray density is slightly decreased with increasing Zn content in sam ples (due to the extension o f unit cell), the real density o f samples enhanced because o f reducing o f porosity from

15.1 % (X - 0.60) to 7.6 % (X = 0.75).

Table 1. Lattice parameters, X-ray density, real density and porosity of samples Nii_^Zn^Fe204

Sample Niu,Zn,Fe:04

x - 0 .6 0 x = 0.65 x = 0.70 x - 0 .7 5

a (A) 8.4108 8.4149 8.4208 8.4251

V (Ả') 594.99 595.86 597.12 598.03

D, (g/cm') 5.322 5.321 5.318 5.317

D {2,/era) 4.52 4.65 4.82 4.92

p (%) Ỉ5.1 12.6 9.4 7.6

In highest Zn content sample (x = 0.75) the crystal boundary became n aư o w er due to development o f particle size. Because ZnO has low melting temperature so in high ZnO contcnl sample, the liquid phase easy to perform at high sintering temperature and eliminalcs the porosity o f ferrite.

The magnetization curvcs o f all samples have been measured at 110 K in maximal applied field o f 13.5 kOe. The results showed that at this field the studied samples are nearly in saturation and saturation magnetization o f samples IS listed in Tab. 2. From the shape o f measured M (T) curves, we should approximately suppose that these values correspond to saturation m agnetization o f sam ples at 0 K.

As we known, NiZn ferrites are inverse spinel with following orientation o f spins Ị 1-3J:

N r ^ o r (1)

and according to Neel theory [4] saturation magnetization for form ula unit could be determ ined by expression;

^ = [

2(1

(2

2

where Zn' ion is n onm agnetic ion, Ni"' has 2[Jjj, has 5 ỊJb and X is Zn am ount containing in ferrite. Saturation m agnetization M.V, calculated from formula (2) and m easured for form ula unit are showed in Tab. 2.

The saturation m agnetization Msi calculated with assum ing that exchange interaction A-ab is strongest therefore m agnetic m om ents o f A and B sublattices are antiparallel to each other and Ms, increases with X. In fact m easured in experim ent decreased w ith X. It m eans that w ith increasing Zn content in fem te, A-B interaction became weakening so should be com pared w ith B-B interaction and we suppose in our studied sam ples there is canting structure as illusfrated in Fig. 2, w here (p is the angle between direction o f m agnetic moment o f A ions and m agnetic m om ent o f B, and Bi ions.

Comparing Ms, and Mse from Tab. 2, we could determ ine the canting angle betw een m agnetic moments o f Bi and B2 ions in octahedral sublattice. W e see that canting angle increases w ith increasing am ount o f nonm agnetic ions

Zr\*

58 N .C haii et al. / V N U J o u rn a l o f Science, M aiheiudiics - P h ysics 24 (2()0S) Ị 5 5 -Ị6 2

B

Fig. 2. Canting structure of NiZn ferrite when Xab- ^aa the same order.

Table 2. Saturation maenctization and canting angle of ferrites N ii.^/n J'c:>()4

0.60 0.65 0.70 0.75

Is (emu/g) 112.8 99.68 95.36 74,43

Mst (^n) 6.8 7.2 7.6 8.0

Mse (^b) ^{4 .59 4.26 4.08 3.19}

‘Pc (“) ^{41.5 47.8 52.1 61.3}

In order to study the spin order and magnetic behavior o f sam ples, the lìcld-cooỉcd (I'C') and zero field-cooled (ZFC) m agnetization m easurem ents were perform ed in magnetic field o f 20 Oc (I-Ig.

3 ). The FC and ZFC curves depart from each other below the freezing tem perature T(- indicating the onset o f blocking o f clusters. The sample settles into the frozen state below tem perature 1|-. I'his behavior is atừibuted to the m agnetic frustration arising from the co-existencc o f com peting antifeưom agnetic and feưom agnetic interactions. The separation o f FC and Z ¥ C curvcs at low temperatures could be considered that the sample exhibits cluster glass-likc slate. This behavior has been observed for all studied ferrites. From the data o f Fig. 3, the Curie lem peraturc I'l has been detem iined based on A rrott plots and listed in Tab. 3.

Table 3. Curie temperature, Tm and maximum value of magnetic entropy change of ferrites N i|.,ZnJ’C204

X 0.60 0.65 0.70 0.75

Tc 407 360 305 260

T,„* (K) 387 345 300 265

|AS,J,„ax (J/kg.K) 0.88 0.84 0.98 0.88

') is temperature at which |AS„i| reaches a maximum.

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T(K)

Fig. 3. FC and ZFC thermomagnetic curves of feưite Nio.3Zno,7Fe204.

Il is d e a rly seen from this table that Tc decreases with increasing Zn content substituted for Ni m fem tes and is around room temperature for ferrite Nio,3Zno.7F e2 0 4. The reduction o f T c here could be

explained by w eakening exchange interaction mainly between magnetic ions in sublattices A and B.

As wc w ell know n, the adiabatic m agnetic entropy change, ASni, is determ ined by M axw ell's fundam ental relation [14]:

...Õ M ự , H y A S i T ^ A H ) ^

ÕT

clH (3)

H

where Hpux is the final applied m agnetic field. To study the M CE o f sam ples, a series o f isothermal magnetization curves around their respective T c has been m easured in a m agnetic field up to 13.5 kOe.

Fig. 4 a shows these curves o f feưite NiojZrio.7Fe204.

W hen m agnetization is m easured in a small discrete field and tem perature interval, ASni could be determ ined from Eq. (3) by expression:

Z - T : (4)

i+i

where Mi and MiH are the experim ental values o f m agnetization at Ti and Tj+I, respectively, under magnetic field variation o f AH.

The |AS„,|(T) curve o f feưite Nio,3Zno.7Fe204 is illustrated in Fig. 4 b and lASnil reached a m aximum value o f 0.98 J/kg.K near Curie tem perature. Sim ilar behavior w as observed for other samples investigated and the results are listed in Table 3. The values o f |ASni|max in our samples are identify w ith that firstly exam ined by C haudhary et al. [15] for cobaltite perovskites Lai.^Sr^CoOs.

Thus Ni,.^Zn^Fe204 (x = 0.60; 0.65; 0.70; 0.75) feưites could be considered as active magnetic refrigerant m aterials w orking in quite wide tem perature range.

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O)

ễ

H (kOe)

1 . 0 0 -

1 ... ,

A S = 0 . 9 8 J / k g . K -

m max ^

■ l \ 0 . 9 5 -

l \

0 . 9 0 -

\

d )

0 . 8 5 -

■ /

■

\

\

E ^{0 . 8 0 -} 1

/

CO

< 0 . 7 5 -

\

0 . 7 0 -

0 . 6 5 - L_,--- 1 , |- ,

-

■

-T---1 ' 1 '

2 7 0 2 8 5 3 0 0 3 1 5

T(K)

3 3 0 3 4 5

Fig. 4. (a) A series of isothermal magnetization curves and (b) magnetic entropy change |ASm| versus temperature of sample Nio.3Zno.7Fe204.

N ote that large M CE in m anganite perovskites [16-19] and colossal M CE in am orphous alloys 20-23] have been exam ined by us.

The resistance o f sam ples has been m easured in the tem perature region from 125 K to 300 K and the linear dependence o f Inp on 1/T for feư ite Nio.3Zno,7Fe204 has been obtained and Fig. 5 shows this result as an example.

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c

2 2^-

2 0 -

1 8^-

1 6^-

0 , 0 0 3 0 . 0 0 4 0 . 0 0 5 0 . 0 0 6 0 . 0 0 7 0 . 0 0 8

1/ T ( K ' )

Fig. 5. Dependence of Inp on 1/T for ferrite Nio.3Zno,7Fe204.

O bviously, tem perature dependence o f resistivity o f fcưites follows the below expression [1,2 (5) From Fig. 3 vve could calculate activation energy Ep o f Nio,3Zno.7Fe204 ferrite and the result showed to be 0.15 eV w hich coưesponds to electron conductivity o f ferrite [1]. The sim ilar results are obtained for the rest studied feưites.

4. Conclusions

Single phase ferrites Nii.xZrixFe204 (x = 0.60; 0.65; 0.70 and 0.75) have been prepared with cluster glass-like state. The canting angles o f m agnetic m om ents in octahedral sublattice were approxim ately determ ined and that angle increases w ith Zn content in N iZn ferrite. At the first time we have exam ined the m agnetocaloric effect in ferrite generally and the obtained ỊASmlniax could be compared with that o f perovskite. M oreover the tem perature at which |ASni| reached a m axim um could be easily controlled by substitution effect.

Acknowlegements. The authors are grateful to the V ietnam N ational Fundam ental R esearch Program (Project 406006) for the financial support.

References

[1] J. Smit and H.P.J. Wijn, Ferrites (Wiley, New York, 1959).

[2] T. Tsushima, T. Tcranishi and K. Ohta, Handbook on the magnetic substances (ed. by s. Chikazumi et al., Akasura Publishing Co, Tokyo, 1975).

162 N .C h a u et aỉ. / V N V J o u rn a l o f Science, M athem atics - P h ysics 2 4 (2008) Ỉ 55-162

[3] B. Lax, K J. Button, Microwave ferrites and ĩeưimagnetics, McGraw-Hill Book Comp., inc., New York, San Francisco, Toronto, London 1962.

[4] L. Neel,/Í/Í/I. de Phys. 3 (1948) 137.

[5] A.S. Albuquerque, J.D. Ardisson, W.A.A. Macedo, J. Appl. Phys. 87 (2000) 4352.

[6] A. Verma, T .c. Goel, R.G. Mendisata, Mater. Sci. Tech. 16 (2000) 712.

[7] S.E. Jacobo, s. Duhaldc, H.R. Bertorello, J. Magn. Magn. Mater. 272-276 (2004) 2253.

[8] s .w . Lee, c .s . Kim,*/. Magn. Magn. Mater. 303 (2006) e315.

[9] J.H. Yin, J. Ding, J.s. Chen, x .s . Miao, / Magn. Magn. Maỉer. 303 (2006) e387.

[10] H.H. Nien, T.J. Liang, C.K. Huang, S.K. Changchien, J. Magn. Magn. Mater. 304 (2006) e204.

[11] H.H. Nien, T.J. Liang, C.K. Huang, S.K. Changchien, J. Magn. Magn. Mater. 304 (2006) 2409.

[12] H.T. Chan, Y.Y. Do, P.L. Huang, P.L. Chien, T.s. Chan, R.s. Liu, C.Y. Huang, S.Y. Yang, H.E. Horng, J. Magn.

Magn. Mater. 304 (2006) e415.

[13] s .w . Lee, c .s . Kim, J. Magn. Magn. Mater. 304 (2006) c418.

[14] A.H. Morish, The Physical Principles of Magnctics, Willey, New York, 1963 (Chapter 3).

[15] S. Chaudhary, v .s . Kumar, S.B. Roy, p. Chaddah, S.R. Krishnakumar, V.G. Sathc, A. Kumar, D-D. Sarma, 7.

M a g n . M a g n . M a ter, 2 0 2 ( 1 9 9 9 ) 4 7 .

[16] N. Chau, D.T. Hanh, N.D. Tho, N.H. Luong, J. Magn. Magn. Mater. 303 (2006) e335.

[17] N. Chau, N.D. Tho, N.H. Luong, B.H. Giang, B.T. Cong,}. Magn. Magn. Mater. 303 (2006) c402.

[18] D.T. Hanh, N. Chau, N.H. Luong, N.D. Tho, / Magn. Magn. Mater. 304 (2006) e325.

[19] D.T. Hanh, M.S. Islam, F.A. Khan, D.L. Minh, N. Chau, J. Magn. Magn. Maỉer. 3 10 (2007) 2826.

[20] N. Chau, P.Q. Thanh, N.Q. Hca, N.D. The, J. Magn. Magn. Mater. 304 (2006) 36.

[21] N. Chau, N.Q. Hoa, N.D. The, P.Q. Niem,y. Magn. Magn. Mater. 304 (2006) el79.

[22] N. Chau, N.D. The, N.Q. Hoa, c .x . Huu, N.D. Tho, S-C.Yu, Mater. Sci. Eng. A499-451 (2007) 360.

[23] N.Q. Hoa, N. Chau, S-C. Yu. T.M. Thang, N.D. The, N.D. Tho, Mater. Sci. Eng. A449-451 (2007) 364.