\ ' 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 Ni-Zn fenites N C hau’, 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 eceived 20 A ugust 2008 A b s tra c t A m ong spinel ierriles, Cd and Zn ferrites are alw ays norm al ferrites w ith Cd and Zn ions locating only in tetrahedral sites This study presents effect o f Zn on the m agnetic and m aunctocaloric propcrlics o f the m ixed spinel ferrites N i).^Z nJ’C2 (x ^ 0.60, 0.65, 0.70, 0.75) T he prcscncc o f Zn affects lattice param eters, saturation m agnetization A/s, C urie tem perature, 7cand m agnelic entropy change A5„1 At hiuhcst Zn content, T, reduces to the tem perature low er than room teniỊXTature and m agnetic structure o f spins in the octahedral sublattice should be slroim lv caiued The m axim um m agnetic entropy change occurs in a large tem perature ranee from low lem perature to hundreds o fC e lc iu s dcẹrees K eyw ords: ferrite, m am ictocaloric cffect In tro d u ctio 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 half 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 NiFc 204 IS inverse spinel fcưiíe Among 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 widely used as soft magnetic materials suitable for high-frcquency applications Initial perm eability is maximum at 30 mol % N iFe 204 , 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 molecular-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: luongnh@vnu.cdu.vn 156 N C h a u et al / VN U Jo u rn a l o f Science M athem atics - P hysics 24 (2008) Ỉ5 -Ỉ6 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 paper we study magnetic 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 magnetocaloric cffect (MCH) in these feưites Experiments The polycrystalline feư ite samples Nii_xZn.^Fe204 (x = 0.60; 0.65; 0.70 and 0.75) were prepared by standard solid state reaction technique The mixed powders were presintercd at 900^c for hours and then reground to the fine particles, pressed into pellets and again heated at 0 T for hours Tlic second reground pow ders were pressed and sintered at 1300”C for hours The crystal structure of samples 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 measured by V ibrating Sample M agnetom eter DMS 880, Diiiital M easurem ent System Rcsisiivity m easurem ents w ere perform ed by four probe method Results and discussion 20 n Fig X -ray diffraction patterns o f ferrite sam ples Nii.xZrixFe 204 N C hau cĩ CIỈ / VN U J o u rn a ỉ o f Science, M aỉhem aíics - P hysics 24 (200S) Ị 55-162 157 The SI-M study showed that the microstructurc o f samples IS o f high hom ouencity and average particle size increases with Zn conlcnl in samples, namely 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 presents the XRD patterns o f sludied samples All samples have single phase f.c.c spinel structure and lattice param eters arc determ ined and listed in Tab 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 While the X-ray density is slightly decreased with increasing Zn content in sam ples (due to the extension of 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 Lattice parameters, X-ray density, real density and porosity of samples Nii_^Zn^Fe204 Sample Niu,Zn,Fe:04 x - a (A) V (Ả') D, (g/cm') D {2,/era) 8.4108 594.99 5.322 4.52 Ỉ5.1 p (%) x = 0.65 x = 0.70 x - 8.4149 595.86 5.321 4.65 8.4208 597.12 5.318 4.82 9.4 8.4251 598.03 5.317 4.92 7.6 12.6 In highest Zn content sample (x = 0.75) the crystal boundary became naưow 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 From the shape o f measured M (T) curves, we should approximately suppose that these values correspond to saturation magnetization o f samples at 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 formula unit could be determ ined by expression; ^ where Zn' = [2(1 - x ) /- h + 5(1 + ion is nonm agnetic ion, Ni"' has 2[Jjj, ] - 5(1 - x )/u ^ = (2 + ( 2) has 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 The saturation m agnetization Msi calculated with assum ing that exchange interaction A-ab is strongest therefore magnetic moments o f A and B sublattices are antiparallel to each other and Ms, increases with X In fact measured in experiment decreased with X It m eans that with 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 samples there is canting structure as illusfrated in Fig 2, where (p is the angle between direction o f magnetic moment o f A ions and magnetic moment o f B, and Bi ions Comparing Ms, and Mse from Tab 2, we could determine the canting angle betw een magnetic moments o f Bi and B ions in octahedral sublattice W e see that canting angle increases with increasing amount o f nonm agnetic ions Zr\* in ferrite which causes weakening exchange interactions 58 N C haii et al / VN U Jo u rn a l o f Science, M aiheiudiics - P hysics 24 (2()0S) Ị 5 -Ị6 B Fig Canting structure of NiZn ferrite when Xab- ^aa the same order Table Saturation maenctization and canting angle of ferrites N ii.^/nJ'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 samples, the lìcld-cooỉcd (I'C') and zero field-cooled (ZFC) m agnetization measurements were perform ed in magnetic field o f 20 Oc (I-Ig ) 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 magnetic frustration arising from the co-existencc o f competing antifeưomagnetic 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 detemiined based on Arrott plots and listed in Tab Table 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 0.84 0.98 0.88 |AS,J,„ax (J/kg.K) ') 0.88 is temperature at which |AS„i| reaches a maximum N C h au et aỉ / V N lỉ J o u rn a l o f Science, M aihernatics - P hysics 24 (2008) Ỉ55~ Ỉ62 159 T(K) Fig FC and ZFC thermomagnetic curves of feưite Nio.3Zno,7Fe204 Il is d early 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,3 Zno.7 F e 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 magnetic entropy change, ASni, is determ ined by M axw ell's fundamental relation [14]: ASiT^AH)^ Õ M ự , H y ÕT clH (3) H where Hpux is the final applied m agnetic field To study the M CE o f samples, a series o f isothermal magnetization curves around their respective T c has been measured in a m agnetic field up to 13.5 kOe Fig a shows these curves o f feưite NiojZrio.7Fe 204 When m agnetization is m easured in a small discrete field and tem perature interval, ASni could be determined from Eq (3) by expression: Z - T :i+i (4) 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.7Fe 204 is illustrated in Fig b and lASnil reached a maximum value o f 0.98 J/kg.K near Curie temperature Sim ilar behavior was observed for other samples investigated and the results are listed in Table The values o f |ASni|max in our samples are identify with that firstly exam ined by Chaudhary et al [15] for cobaltite perovskites Lai.^Sr^CoOs Thus Ni,.^Zn^Fe 204 (x = 0.60; 0.65; 0.70; 0.75) feưites could be considered as active magnetic refrigerant m aterials w orking in quite wide temperature range 160 N Chau et aỉ / VN U Jo u rn a l o f Science, M athem atics - P hysics 24 (2008) Ỉ 55-162 O) ễ H (kOe) , AS = J /k g K m max ^ ■ l\ 0 - l \ - / \ ■ \ d ) - ■ \ / E CO < - \ n \ - - ■ - L_, 270 , |285 , -T 300 ' 315 ' 330 345 T(K) Fig (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 samples has been measured in the tem perature region from 125 K to 300 K and the linear dependence o f Inp on 1/T for feưite Nio.3Zno,7Fe 204 has been obtained and Fig shows this result as an example N C h a u e ta l / V N V Jo u rn a l o f Science, M athem atics - P hysics 24 (2008) 155-162 22 16 - 20- c 18 - 16 - ,0 0 0 0 0 0 1/T (K ') Fig Dependence of Inp on 1/T for ferrite Nio.3Zno,7Fe204 Obviously, tem perature dependence o f resistivity o f fcưites follows the below expression [1,2 (5) From Fig vve could calculate activation energy Ep o f Nio,3Zno.7Fe 204 ferrite and the result showed to be 0.15 eV which coưesponds to electron conductivity o f ferrite [1] The sim ilar results are obtained for the rest studied feưites 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 magnetic moments in octahedral sublattice were approximately determ ined and that angle increases with Zn content in NiZn 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 Research 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 hau et aỉ / V N V Jo u rn a l o f Science, M athem atics - P hysics 24 (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 (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, M agn M agn M a ter, 2 (1 9 ) [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 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  ... has 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 The saturation m agnetization Msi... frustration arising from the co-existencc o f competing antifeưomagnetic and feưom agnetic interactions The separation o f FC and Z ¥C curvcs at low temperatures could be considered that the sample... showed that at this field the studied samples are nearly in saturation and saturation magnetization o f samples IS listed in Tab From the shape o f measured M (T) curves, we should approximately