Indian Journal of Pure & Applied Physics Vol. 40. January 2002. pp. 46-53

Studies of Mn0.5Cr0.5Fe20 4 ferrite by neutron diffraction at different temperatures in the range 768K > T > 13K

A K M Zakari a", M A Asgar", F U Ahmed", A K Azad", S M Yunus", S K Paranj pe" & A Das"

"Institute of Nuclear Science & Technology, Bangladesh Atomic Energy Commission, GPO Box 3787. Dhaka 1000. Bangladesh

"Department of Physics. Bangladesh Uni versity of Engineering & Technology. Dhaka I 000, Bangladesh

"Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085

Received 29 December 2000; revised 19 Apri l 200 I; accepted 25 November 200 I

Neutron diffraction studies of a polycrys ta ll ine manganese-chromium-ferrite with composition Mn0_5Cr05 Fe20 4 have

been performed at a number of temperatures in the range 768K 2 T 2 l3K. The .cati on distribut ions, oxygen position parameter (u ) and latti ce constant (a,) have been determined from the analysis of the highe r angle neutron diffraction data. The temperature response of the latt ice constant has also been investigated and a slight anomalous ex pansion has been found around the magneti c transiti on temperature. Sublatt ice as wel l as net ferrimagnetic moments of the specimen have been found out from the analys is of the neutron diffracti on data at different temperatures. A randoml y canted ordering of spi ns has been observed in the B sublattice, while the A sublattice moments appear to exhibit coll inear eel ty e ordering at all temperatures.

1 Introduction

Ferrites are considered to be the best magneti c materi a ls for high and very hi gh frequency c ircuits and are not like ly to be replaced by any othe r magneti c materi al 1.2. Ferri tes are wide ly used in microwaves, analog devices, transformer cores, high quality filters and radi o-frequency c ircuits2

, because they are re lati vely inexpensive, highl y res isti ve and easy to prepare . Because of the di vers ified uses of ferrites much interests have been observed in study ing the ferrites of di fferent compositions us ing various techniquesJ.?. Ferrites have the general fo rmul a AB20 4, where A and B represent divalent and tri valent metal ions respecti vely. The structure of the fe rrite is known to be spine l structure consisting of a cubic c lose-packed array of oxygen ions with two inte rstiti a l positions fo r the cations: tetrahedra l or A si te and octahedral or B site . The cations occupy these two sites depending on the ir ionic sizes to minimize the total crys talline energy. MnFe20 4 ferrites have been studied ex tensively by vari ous techniques inc luding neutron diffractionR·". Many fe rri te compounds substituted with Cr such as Zn,Cu 1_,FeCr04 (Ref. 12), CoFe2_xCr,0 4 (Ref. 13) ZnxCo 1_,FeCr04 (Ref. 14) and GaxFe 1_xNiCr 1.yAiy04

(Ref. 15) . In the present work, the authors report

the neutron di ffract ion studies on Mn0_5Cr05Fe20 4

fe rri te which is di fferent in composi tion due to the presence of Cr. No neutron di ffraction studies ex ist so far on thi s compositi on. The magnetic properties of fe rrites depend very sensiti ve ly on the type of cations and the nature of the ir distri bution present over the two sub latti ces 11

•15

•16

•

The aim of the present work is to dete rmine the exact cation di stribution and the temperatu re response of lat ti ce di mension and fin all y to f ind out the effect of substitution of Cr on the Tc as we ll as subl atti ce magneti zation.

2 Experimental Details

The manganese chromium ferrite sample with composition Mn0_5Cr0_5Fe20 4 was prepared by the standard ceramic sintering method. T he high puri ty

oxides (-·99.9%) Fe20 3, MnO and Cr20 3 were mixed intimate ly by ball milling in correct stoichi ometric proporti ons. A small amount of di still ed water was added to the mi xture for fin e milling. The mi xture was then dried in a ir and pressed into sma ll di scs . It was then heated at 900"C in a furnace for 8 hr and then cooled slowly. The sampl e so prepared was further crushed in to powder and the process of milling, drying and press ing were repeated. Fina lly,

the discs were sintered in air at 1200"C for 10 hr and subsequently cooled in the furn ace at a slow rate. The samples thus prepared were powdered fo r experimental measurements. The X-ray diffraction measurements on the sample Mn05Cr05Fe20 4 were carried out to check the quality and phase di stribution at the Solid State Phys ics Division (SSPD) of Bhabha Atomic Research Centre (BARC), Mumbai using a X-ray wavelength 1.54 A

of CuKa radiation . All the Bragg diffraction peaks in the X-ray pattern were found to be very sharp corresponding to the single-phase spinel structure as represented in Fig. I. Neutron di ffracti on measure­ments on the powdered sample were performed at a

number of temperatures in the range 13K :S T :S 768K at the medium flu x I 00 MW Dhru va reactor of BARC. The experiments were performed using the moderately high-reso lution pos iti on sensit ive detector (PSD) based neutron powder diffractometer. The neutron wave length used in the experiment was 1.09A corresponding to the take-off angle 28M= 60° from a fl at Si ( I 15) monochromator. The diffraction data were collected at an angul ar step of 0.03° at a reactor power level of 65 MW and

the neutron flux at the sample positi on was - 2 x 105

neutron cm·2 s·1•

10000 (311)

8000 -

~ c ~ 6000 c:-

-I! € .e.

4000 ~

- (220) fl) c .!l (111)

(400)

.5 2000 -

(222)

0 1 ___ l

10 20 30 40

47

The collected neutron diffrac tion data were analyzed by the Rietveld method17 using the program DBWS-9411 (Ref. 18) for determining crystal structure and the program HEW A T 19 fo r determining magnetic structure of the sample. Lattice constants, oxygen positi on parameters, atomic coordinates, ordering of the magnetic moments and its va lues at di ffe rent temperatures were determined through thi s analysis. The spinel structure exhibits the symmetry of the space group Fd3m and thi s space group was used in the data refinement process for generating the calculated profile. The crystall ographic, in strumental and profil e matching parameters were vari ed as free parameters during the data refinement process. The occupati on numbers of different ions were all owed to vary within the stoichi ometric constraints onl y.

3 Results and Discussion

The chemical structure of a spinel system is defined by the three crystall ographic parameters namely the lattice constant (a0 ) , oxygen position parameter (u) and cation di stributi on on the A and B sites. For the correct determinati on of these parameters speciall y the cation distributi on, diffraction data with nuclear contribution onl y is needed . Since the magnetic contribution in the

X-ray diffraction pattern of Mn

0_5Cr

0_5 Fe2 0 4 at RT

(442)

(511) (333)

(533) -(622) (731)

(422) (620) (444)

(553)

l (531)

L (642) (800)

~ • A

50 60 70 80 90 100

2 9 (degree)

Fig. I -X-ray diffraction pattern of Mn0_5Cr0_5Fe20 4 at room temperature

48 INDIAN 1 PURE & APPL PHYS, VOL 40, JANUARY 2002

2500 (440)

2000

~ 1500 ·;; "' I:' ~ (511)

:.e 1000 (333) (531)

-!!--'=' '<>I

"' ~ 500 ..s

0

-500

40

-----

(444) (551) (711)

50 60

Mn0 _5 c r 0 _5 Fc2 o 4 T = 300K

(840)

(911) (844) (753)

7 0

26 (degree)

80

Observed - Calculated -·- · D i f rerence

( 955)

90 100

Fig. 2- Fitted neutron diffraction pattern of Mn0.5Cr0 5Fe20 4 in the range of scattering angle 38° :::; 28 :::; 99° at room temperature. The bottom line indi cates the intensi ty differences between the observed and calcul ated pattern

~ ~ c e u

8.55

Mn0_5Cr 0.5Fe20 4

8.54

8.53

8.52

8.51

8.50

8.49

8.48 +----,--,----,--,--.--.--.--.--.--1 -100 0 100 200 300 400 500 600 700 800 900

Temperature, T(K)

Fig. 3-Dependence of the cell constants, a0 (A) on temperatures of Mn0.5Cr05Fe20 4 ferrite

diffraction pattern often introduces ambiguity in determining the correct cation distributions, room temperature neutron diffraction data at higher angles, where the magnetic contribution is negligible, were analyzed by the Rietveld method 17

using the computer program DBWS-9411 (Ref. 18),

for determi ning the cation distribut ion and other structural parameters. Fig. 2 shows the fitted neutron diffraction pattern at room temperature

limited to the higher scatteri ng angles (38":::; 28 :::; 99"), where only nuclear contribution exi sts. It is seen from the figure that the agreement between the observed and calculated data is reasonably good . The agreement factors 17 of the Rietve ld refinement are:

RN (RBragg) =7.0 I Rp==8.46

Rwr=ll. l5 RE=6.85 X2 =1.63.

The values of the observed and calcu lated integrated intensities for a ll the reflections found out from this refinement have been li sted in Tab le I .

The crystall ographic sites occupi ~d by var ious ions, their chemical occupation number and the overall cation distribution obtained from the analysis have been summarized in Tab le 2. It may be seen from the table that Mn ions in variab ly occupy the A site, while Fe ions are distributed over both A and B sites. The occupancy of Cr is found to be 0.04 on ly in the B si te, while all the rest occupy the A site. Fe mostly occupies the B site with its proportions being 0.04 and 1.96 on the A site and B site respecti vely. It is well known that usually in ferrites Cr is in trivalent state and Mn is in diva lent

ZAKARIA et a/.:Mn0 5Cr05Fe20 4 FERRITE 49

Table I - Comparison of the observed and calculated nuclear intensities for all the reflections from the

refinement of the higher angle data (3 8° ~ 29 ~ 99°) at room temperature for Mn0 5Cr0_5Fe20 4 ferrite

(h k I) Multi- 29 Nuclear intensity plicity

Ca lcu- Observed Ia ted

(5 I I) 24 39.090 106.25 94.9 1

(3 3 3) 8 39.090 0.78 0.70

(4 4 0) 12 42.718 322.35 340.03

(53 I) 48 44 .779 102.6 1 11 2.05

(5 3 3) 24 49 .948 23.29 29.57

(4 4 4) 8 52.985 136.79 137. 14

(7 I I) 24 54.750 7.30 6.92

(55 I ) 24 54.750 78 .94 74.79

(7 3 I ) 48 59.282 2.87 2.87

(55 3) 24 59.2S2 43.27 41.92

(8 0 0) 6 62.00S 7 1.42 75.65

(7 5 I) 48 67.782 22.52 26.S2

(55 5) s 67.782 23.07 27.47

(8 4 0) 24 70.326 220.40 232.38

(9 I I ) 24 71.832 46.92 51.9S

(7 53) 4S 71.832 ll .S4 13. 12

(9 3 I ) 48 75 .79 1 58 .34 55 .67

(S 4 4) 24 78.229 173.54 170.5S

(7 55) 24 79.6S2 14.10 16.18

(9 5 I) 48 83.525 90.75 93.74

(9 53) 48 S7.339 59.83 56.27 (8 s 0) 12 93 .51 9 60.58 5S.29 (9 55) 24 94.94S 48.46 4S.65

Table 2- Ionic distribution and crystallographic si te occupancies of diffe rent ions in Mn0_5Cr0_5Fe20 4 ferrite

Ions Crys- Chemi- Overall tallo- cal cation distribution graphic occupa-site ti on

number

Mn2+ Sa 0.50 Cr'+ Sa 0.46 Fe'+ Sa 0.04 Fc2+ 16d 0 .50 (Fe3+ omMn2+ o.sCr3+r,.4olA

[Fe2+ o.sFe3+ 1 .46c~+ o.04l o Fe·'+ 16d 1.46 c~+ 16d 0.04 o4· 32e 4

state . However, Fe can be present in both divalent and trivalent states. Furthermore, ferrites are ionic compounds to a very good approximation. Thus to satisfy the electro neutrality out of the 2.0 atoms per formula unit (fu ) of Fe, 0.5 atom should be Fe2

+ and the rest (1.5 atoms/fu) should be Fe3

+. The divalent Fe must invariably be in the B site20

• The values of lattice constants and oxygen position parameter obtained from our analysis of data at room temperature (300 K) are given in Table 3.

Table 3-Comparison of the cation distributions, lattice constants, oxygen position parameters of Mn0.5Cr0.5Fe20 4 ferrite

with that of cited in the refs 9 and I 0 at room temperature

Samples Lattice constant Oxygen position Refer

ao cAl parameter (u) ences

MnFe20 4 8.517 0.2596 ± 0.0003 9

MnFe20 4 8.524 ± 0.00 I 0.2600 ± 0.0006 10

M n0.5Cro.5Fez04 8.506 1 ± 0.0027 0.2609 ± 0.0006 Pre-sent study

The sample under study, differs in compositi on from the other compounds cited in Section I. Nevertheless, it is worthwhile to compare our results with those of MnFe20 4, since our sample resembles it to a considerable extent excepting the presence of 0.5 atom/fu of Cr. Conversely, such a difference in composition has provided us an opportunity to see the changes in crystallographic and magnetic properties induced by the presence of Cr. A comparison of cation di stribution and structural parameters of Mn05Cr0 5Fe20 4 is thus given with that cited in Refs 9, I 0 on MnFe20 4 in Tables 2 and 3. A little difference is observed for all the parameters of our sample with respect to the corresponding parameters of MnFe20 4• In Table 2, for both the samples refe rred , Mn mostly occupies the A site. A fraction of Mn has been reported to occupy the B site in the sample MnFe20 4 (Refs 9, I 0). The content of Mn in our sample is only half of that in MnFe20 4 . Therefore, from the considerati on of Mn content the occupation of A site by the total amount of Mn in our case is quite consistent. The occupation of Fe3+ in the B site in our sample is not expected to match with that of MnFe20 4 , s ince a little room is available in the A site due to presence of 0.46 atom/ftt of Cr3

+ in thi s site. However, in

50 INDIAN J PURE & APPL PHYS , VOL 40, JANUARY 2002

6000

5000

"E" ·a =- 4000 [:;> . 13K

(220)

(311)

(222)

IV.ln0

_5Cr

0_

5Fe

20

4 {400)

(331)

(511) (333)

.!; ~

'~--=3:__:0::...:~ __ .,.A..., - -·

~ 3000 ·,.; c: ~ ...9

2000

1000 563K

613K

0 668K

- - -- -,-·-------, 10 15 20 25 30 35 40

26 (decree)

Fig. 4- Neutron diffraction patterns of Mn0.5Cr0.5Fe20 4 ferrite at different temperatures in the range 13 to 768K

2500 {400)

!VIn0_~cr0.~Fc2o4 (440) T ~ 13K (111)

2000

Zl 1500 ... = e:-.s

:e 1000 -!!.-

-€ {3"1"1)

~ 500 ..s

0

-500 10 15 20 25 30 35

20 (degree)

{5"1 "1) {333)

40

{53"1)

45

· Ob!llerved - Calculated

--- - D i fference

50

{55"1) (7"1 "1)

<->

55

Fig. 5- Fitted observed and calculated neutron diffraction pattern of Mn05Cr0.5Fe20 4 ferrite at J3K between the range of scattering

angle 10° ~ 29 ~ 56o. The bottom line indicates the intensity differences between the observed and calculated pattern

respect of the occupancy of Fe3• ions in the B site,

our result is in close agreement with the result reported by Begum 10

• The values of lattice dimension and oxygen position parameter obtained from the analysis of room temperature neutron data are compared with those reported for room temperature data from MnFe20 /· 10

• It is fou nd that the lattice dimension obtained for our system is slightly smaller than the values reported for MnFe20 4. This may be due to the fact that our

system consists of Fe2• and Cr3

• whose dimensions are slightly smaller than the dimension of Mn2

•

present in MnFe20 4• However, the ~ it t le difference in cell dimension is not adequate enough to bring about any change in the oxygen parameter u. Therefore, our measured u value is quite close to the values reported in Refs 9 and 10. Fig. 3 shows the variation of the cell constant (a0) with temperature. It is observed that a0 increases almost linearly with the increase of temperature except

ZAKARIA et al.:Mn05CrosFe20 4 FERRITE 51

with a slight anomaly around 500 K.

1200 ~------------------------------,

1100 ·;; = ~ 1000 t :e -!-900 ~

~ ~ 800 f = ~ 700 ~ v; I y I ~ 600 l i I

500 ~ I I

I (Ill) peak

I

J.

I

J I I I

-100 0 100 200 300 400 500 600 700 800 900

Temperature,T(K)

Fig. 6- Temperature dependence of the magnetic structure facto r for ( Ill ) reflection in M n0_5Cr0_5Fe204 fe rrite

1 c 0 ll @, ~

E

'§ = - ~

~ c u g E

-~ ;; c .. ~

~

5

i 4

Mn.,Cr,_,Fe,o,

• A Site

3 • • BSite •

I

: 1 I

I

0 ~ • • •• • • ! I

I

-1 J I

-2 I I

I -3 !

~ +--.,--.--,,-or--r--.--,-~--~-i

-100 0 100 200 300 400 500 600 700 800 900

Temperature, T(K)

Fig. 7- Temperature dependence of the sublattices magnetization in Mn0_5Cr0_5Fe20 4 ferrite

In spinel compounds, the chemical and magnetic unit cell s are the same with identical symmetry relat ions and thus nuclear and magnetic Bragg peaks occur at the same scattering angles. Fig. 4

shows Bragg diffraction patterns confined to low scattering angles at a number of temperatures. The initi al Bragg peaks ( Ill ), (220) and (222) in this figure exhibit considerable enhancement with decreasing temperatures which is a clear indication of the presence of significant magnetic contributions in the di ffract ion patterns .

1.2

1.0

0.8 0

~ 0.6 < ~

0.4 .

0.2

0.0

0.2

1.2

1.0

0.8 0

~ = 0.6 ~

0.4

0.2

0.0

0.2

0.4 0.6

J. I

0.4 0.6

Mn0_5Cr0_5Fe20 4

A Site

(a)

I

I

0.8 1.0

M n0_5Cr 0_5F e2 0 4

B Site

(b)

I

0.8 1.0

1.2

1.2

Fig. 8 - Temperature dependence of the A s ite reduced moments for Mn0_5Cr05Fe20 4 ferrite : (a) for A site and (b) forB si te. The solid line represents the Brillouin function appropri ate to 1=312

The total neutron diffraction data below Tc containing both nuclear and magnetic contributions were analyzed by using the Rietve ld refinement program HEW AT in two different phases for nuclear and magnetic contributi ons respecti vely

52 INDIAN J PURE & APPL PHYS , VOL 40, JANUARY 2002

usin g the same space group Fd3m in both the phases . The occupation numbers of different cations at the two sites obta ined from the previous refinement (hi gher angle data) were kept fixed in thi s part of analys is. The symmetry operators for the magnetic atoms were suppli ed as ex ternal input to the program. All the vari able parameters were refined applying the con venti onal data refinement technique. The analysis gave reasonably good R­fac tors (Rr = 8.69, Rwr = I 0.50, Rexp = 6.80 , RN

(Rbragg) = 2.66, RM = 2.20, X2 = 1.54). One such f itted total neutron diffraction pattern at 13K in the

range I 0° ::; 28 ::; 56° is shown in Fig. 5 as fo r illustrati on. A compari son of several observed and calcul ated integrated intensities of reflections in the nuc lear and magnetic phases has been given in Table 4 . From Fig. 5 and Table 4 it is evident that the agreement between the observed and calculated Bragg intensities are al so qui te sati sfactory.

Fig. 6 shows the plot of the temperature vari ation of the ( Ill ) magneti c structure fac tor. From thi s plot, Tc for the sample appears to be 530K. However, for MnFe20 4, the reported values of Tc are 560 (Ref. I 0) and 550 (Ref. 11 ). Therefore, we may suggest that the Tc has been s lightly lowered as a result of Cr substituti on. At thi s point we may recall the little anomaly in the temperature dependence of ce ll constant around 500K as menti oned before. The thermal expans ion anomaly in the ce ll constant around the transition temperature may be exp lained as due to the spontaneous magneto-stri cti on that comes into play as a result of spontaneous magnetic ordering at Tc. Thi s magneto-stnct1 ve e ffect brings in a corresponding di storti on in the crystal lattice around Tc which may be the probable reason for the deviati on observed at thi s temperature in the curve fo r ce ll dimension pl otted as a fun ction of temperature in Fi g. 3. The observed sublattice magneti c moments at di fferen t temperatures are pl otted in Fig. 7. Magneti zation of both the subl attices exhibits smooth variation s with temperature.

The temperature dependence of the reduced magnetic moments of A and B sites for Mn0_5Cr0_5Fe20 4 a long with the Brillouin functi on app ropriate to 1= 312 is represented in Fig. 8. From Fig. 8(a) it is evident that the A site red uced

moments show reasonable agreement with the Brill ouin curve, while in the B site there is s ignificant deviati on of the reduced moments from the Bri ll ouin curve. In order to expl ain thi s it may be suggested that due to presence of four di fferent types of magnetic ions there is a distribution of ionic dimensions in the system, whi ch might have caused an interaction di sorder in the system. In spinels the AB (inter-subl attice) interacti on keeps the A and B site spins in anti-para lle l alignment resulting into long-range ferrimagnetic order. A decrea e in the AB inte racti on ari s ing from the vari ation of ionic dimensions may cause some of the B site spins to orient in random directi ons in stead of ori enting along the long-range order. Thi s randomly oriented B spin s do not con tribute to the Bragg peaks which may be a plausible reason of the devi ation of the reduced moments from the Brillou in curve in the B site. Simi lar results have also been reported fo r Zn-Mg-Ni ferrite~ . Any spati al ordering of the transverse spin components in the B site is, however, ruled out since no (200) super lattice refl ecti on was observed in our neutron data at a temperature as low as 13K. Thus the assumpti on of a random canting state of B site spin s appears to be more justified rather than the assumption of a Y-K canting s tate 1 ~ to account for the reduction of B site moments in our sample.

4 Conclusion

The present study gives a c lear picture of the cati onic di stribution in the system on the basis of the best fit conditions of the neutron di ffracti on data. The measured va lues of different parameters are in c lose agreement w ith those of M nFe20 4

with in the limi t of compositional d>fference. The anof.nalous the rmal expansion of the unit ce ll near the Tc may be a consequence of the spontaneous magneto-stri cti ve di stortion. Our assumpti on of the loca l an.\ random canting of the B sit e spin s due to variation of ionic dimensions is qua litati ve ly in agreement with the devi ati on in the B site moment from the theoretical temperature dependence curve. Furthermore, s ince most of the Cr substituted ferrite compounds were reported to be non-collinear12

-1\ it

may be thought that Cr possesses a sort of ani sotropic property and is re luctant in parti c ipating in the infinite long-range order. Thus the spins of Cr in the A site do not form strong bond with the spins of nearest neighbour B ions which forces some of

the B spins to deviate from collinear ordering. This could be another reason of the deviation observed in the B site moment in the present system.

5 Acknowledgments

The authors gratefully acknowledge the International Atomic Energy Agency (IAEA) for providing fellowships to two of the authors (AKMZ and AKA) and Bhabha Atomic Research Centre (BARC), Mumbai , India under the technica l cooperation project BGD/004. The authors like to thank Dr A S Sequeria, Ex-Head, Solid State Physics Division (SSPD) and Dr M Ramanadham, Head, SSPD, BARC, for providing necessary administrative support and making available their laboratory facilities for thi s work. The authors also thank Ms R Chitra of SSPD, BARC for the X-ray measurements .

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