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Magnetization and magnetic resonance studies of ultrafine Ho3Fe5O12 and Yb3Fe5O12
Journal of Magnetism and Magnetic Materials 92 (1990) 217-227 North-Holland
217
Magnetization and magnetic resonance studies of ultrafine Ho,Fe,O,, and Yb,Fe,O,, V.K. Sankaranarayanan and N.S. Gajbhiye Department of Chemistry, Indian Institute of Technolom, Kanpur 208016, India Received 15 March 1990; in revised form 30 May 1990
Ultrafine particles of HosFe,O,z and Yb,F%O,, garnet materials from 1.0 to 35 nm crystallite in size are prepared by thermal decomposition of citrate precursors. The crystallites in the 1.0 to 1.5 nm size range exist in the disordered state as aggregates and exhibit superparamagnetism, whereas 10 to 35 nm crystallites are monoliths. The observed higher @-values and TC-values of 1.0 to 1.5 nm crystallites are attributed to the weakening of the anisotropic RF -02--F~+ coupling in the disordered state which allows the Fe sublattice magnetization to dominate. The g,,-values of > 2.0 in resonance studies and their inverse temperature dependence are the result of the low Hd+ and Yb3’ sublattice magnetization, the presence of an internal magnetic field produced by interparticle dipolar interactions and the distortion of Fe3+ sites in the garnet structure. The 10 to 35 nm crystallites have larger g,,,- values, as compared to single crystal or polycrystalline garnets due to the dipolar interactions; the temperature dependence of gcrr-values is comparable to the polycrystalline materials. The resonance line broadening in 1.0 to 1.5 nm crystallites is dominated by dipolar broadening AHdP whereas in 10 to 35 nm crystallites anisotropy broadening AH, and intrinsic broadening AHi are predominant.
1. Introduction The magnetism of ultrafine particles is of considerable recent importance from a scientific and practical point of view in ferrofluids and catalysts [ 1,2]. The saturation magnetization, an intrinsic magnetic prop :rty of the ferrimagnetic materials, varies in ultrafine particles unlike that in the bulk polycrystalline and single crystal materials due to the differences of spin structure between the surface and the bulk [3]. Therefore, ultrafine particles comprised of nanometer-size crystallites are expected to show very interesting properties as the particle would be effectively all surface. Thus it is worthwhile to explore magnetization characteristics in order to understand the magnetic interactions among various sublattices particularly in 3sublattice ferrimagnetic rare earth iron garnet (RIG), Ho,Fe& and Yb,Fe,O,,, materials. Magnetic resonance is often employed to study fine particles of magnetic materials in glass ceramics, ferrofluids and supported metal cata0304-8853/90/$03.50
lysts [4-S]. Ultrafine particles of ferro- and ferrimagnetic materials often exhibit superparamagnetic properties [9]. A few magnetic resonance investigations on superparamagnetic particles are reported which show comparatively narrow resonance linewidths [6-8,101. The resonance linewidth behaviour is not corroborated by the existing theories of magnetic resonance [ 1 l]. Resonance studies of fine particles have dealt with magnetic dipole interactions systems wherein among particles are very weak or negligible [12,13]. However, ultrafine particles comprising 1.0 to 35 nm size crystallites of the rare earth iron garnets. Ho,Fe,O,, and Yb,Fe,O,,, with strong dipolar interactions among particles have not been studied in the literature by using FMR technique. In the present study, the magnetization behaviour of u’rtraflne particles of and Yb,Fe,O,, (YbIG) having crystallite sizes in the range 1.0 tc 1.5 nm (X-ray amorphous. superparamagnetic) and 10 to 35 nm (crystalline state) are investigated in order to understand the various
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
V.K. Sankawnarayanan, N.S. Gajbhiye / Utrafine Ho>Fe,Ol
218
magnetic interactions e.g., superexchange interactions among the 3 sublattices of garnets and their relationship to &-values. These crystallites are of particular interest in FMR studies because of their existence as aggregates/ agglomerates wherein strong &polar interactions exist. Besides the 3sublattice magnetic structure of garnet has fast relaxing Ho3+ and Yb3 “-ions which are magnetically coupled to Fe 3+-ions. The effective g-factor and linewidth, AH, in FMR are sensitive to the effect of dipolar interactions, presence ‘of fast relaxing Ho3’ and Yb3+-ions, magnetocrystalline anisotropy and symmetry of Fe3+-sites; these aspects ar9 Investigated in this study.
2. Experimental procedure Ultrafine Ho,Fe,O,, and Yb,Fe,O,, were prepared by thermal decomposition of a citrate precursor, R,Fe,(Cit),, (36 + n)H,O where R = Ho and Yb. The method of preparation and thermal decomposition studies of the citrate precursor are described elsewhere [14]. The precursor on decomposition in an air atmosphere yields X-ray amorphous ultrafine particles at 450°C and crystalline ones above 6OOOC.The amorphous HoIG and YbIG materials were heat treated or annealed in air at various temperatures between 450 and 1000” C for 4 h in order to obtain crystallites of various size. The powder X-ray diffraction (XRD) patterns of the materials were recorded by using a Rich Seifert Isodebyflex, Model 2002, diffractometer. Cu K, radiation with Ni-filter was used in the 20 range of 10” to 120 O. The size of the primary crystalites was calculated from XRD-line broadening using the classical Scherrer relationsitip 1151, Dh$/ = kX/B cos 8 where Dhk, is particle diameter in A, k is a constant (shape factor) = 0.9, B is the half maximum linewidth and X is the wavelength Of the X-rays. Magnetization measurements were carried out by using a PAR Modei 15OA Vibrating sample magnetometer i-r,conjugation with a Varian Model V-7200 electromagnet assembly that provides a magnetic field up to 10 kG. The temperature variation of magnetization was studied using a furnace assembly which can provide tempera-l
and Yb3 FesO,,
tures up to 1000 K (Model 151) associated with
the magnetometer. The FMR-spectra were recorded using a Varian Model E-109 EPR spectrometer operating at the X-band frequency. The low temperature measurements were carried out in a home-made dewar from room temperature (RT) to liquid nitrogen ternperature (LNT).
3. Results and discussion 3.1. Nature of ultrafine HoIG and YbIG
Ultrafine particles of HoIG and YbIG were characterised in detail by XRD, TEM and BET surface area measurements and presented elsewhere [16]. In the temperature range 450 to 600 OC HoIG and YbIG materials exhibit a broad hump in the XRD pattern, d,, = 0.2 to 0.35 nm, which indicates the absence of crystalline periodicity and the existence of a highly strained disordered state having a crystallite size of 1.0 to 1.5 nm. Above 600° C, crystallites grow to 10 to 35 nm monoliths, db10 = 0.275 nm, due to crystallization with negligible lattice strain. The larger dazOspacing values for crystallites of size 1.0 to 1.5 nm compared to lo-35 nm corresponds to the relative increase of the specific volume of the HoIG and YbIG-lattice. The larger size-induced strain is suggested to be responsible for the distortion and instability of the garnet lattice. Such a lattice expansion with decreasing particle size has been reported for various metallic and non-metallic fine particle systems [17-191. The distortion may be attributed to crystal symmetry and surface effects or multiwinning in the structure of ultrafine crystallites. Ultrafine particles were identified with aggregate/ agglomerates. The crystallite aggregate forms a porous cluster-agglomerate with heat treatment. Clustering of crystallites (and aggregates) is undoubtedly due to the presence of the unsaturated bonds on the considerabiy extended, highly energetic surface of small particles. This means, the crystallites in aggregates are most probably held together by primary bonds. The rupture of intercrystallite bonds occurs during the crystallization process and eventually leads to the formation of monoliths.
V.K. Sankaranarayanan, N.S. Gajbhiye / Ultrafine IJo, Fe,O,, and Yb,Fe,O,,
3.2. Magnetization YbPG
studies of uitrafine HoIG and
The temperature dependence of magnetization of ultrafine HoIG and YbIG materials at field strength of 0.6 T (6 kG) is shown in figs. 1 and 2. The inset figures show the relative magnetization (M/M,) as a function of field/ temperature (H/T) quotient. There is a gradual decrease of magnetization with temperature for 1.0 to I.5 nm size crystallites with an amorphous nature. The individual particles order magnetically around 900 and 860 K which is much higher than the expected Curie temperature (T,) around 550 K for well crystallised materials (table 1). These transitions shift to 550 K due to crystallization when the materials are heat treated well above the crystallization temperatures. The magnetization approaches saturation in HoIG and YbIG materials having a crystallite size 10 to 35 nm at relatively low fields of 1.0 to 2.0 kG. The room temperature MS-values (saturation magnetization) are comparable to their polycrystalline values reported in the literature [20] and exhibit ferrimagnetic behaviour. The 1.0 to 1.5 nm crystallites show nearly superimposed M/M, vs. H/T curves (inset figs. 1
219
and 2) and do not show any hysteresis. The magnetization curves do not attain saturation even at 10 kG and resemb!e the langevin curves. These observations are characteristics ui’ aupt’~t ramagnetic behaviour of ultrafine particles [9.21]. The room temperature MS-values for these rrystallites are obtained by extrapolation of M t l/H curves to the limit l/Ii + 0 following literature reports for y-Fe,O, fine particles [22] and are given in table 1. In a superparamagnetic system. the magnetization curve can be described by a Langevin function. The insets of figs. 1 and 2 point out the deviation from Langevin curves, because of the distribution of particle size. The initial susceptibility is very sensitive to the larger particles present whereas the approach to saturation is governed by the smaller particles. Therefore, the average magnetic moment per particle and the magnetic particle size of HoIG and YbIG materials were determined from low field slopes of the magnetization curves by using the limit to the Langevin function. The relative difference between the magnetic particle size and crystallite size obtained from XRD (table 1) apparentiy suggests magnetic interactions in the crystallite aggregates (or magnetic
clusters) wherein
intercrys-
“O3Fe5q2 0
oso”c
A
6OO’C
0
7o0°c
H03Fe5Ol2 it 0
a 0 c
EC% 600°C 7Q0°C 7o0°c
6kG
(1 Gnmi (1 1 nm) (275nm) (n_! 6@G
tleld
4*e”3i
Temperature (K)
Fig. I. Temperature dependence of magnetization of ultrafine HoIG materials heat-treated at various temperatures. The XRD crystallite sizes are given in brackets. Insets show the plot of relative magnetization, M/M, vs. field temperature quotient. H/ 7:
K K. Sankaranarayanan, N.S. Gajbhiye / Ultrafine Ho3 FesOIz and Yb3Fees0,,
220
5
0
sQ
I
10
15
20
NIT.104ft
6
25
30
Yb,Fe5012 al 0
4
35
K”) 6kG tieid
A
46O*C 600°C
O.Onm) (l.lnm)
0 0
7OO*C 900°C
(31 nm) (34nm)
2
Temperature
(K)
Fig. 2. Temperature dependence of magnetization of ultrafine YbIG materials heat-treated at various temperatures. The XRD crystallite sizes are given in brackets. Insets show the plot of relative magnetization, M/M, vs. field temperature quotient, H/T.
crystallites having an intermediate particle size range, but slowly disappears around 900 and 850 K (figs. 1 and 2). On annealing at higher temperature for longer duration, these ultrafine crystallites show the disappearance of the 900 K transition
tallite bond exists. Such aggregates may lead to higher magnetization accompanied by an increase in Qvalues in ultrafine garnets. The magnetization does not vanish completely at/around 550 K for ultrafine HoIG and YbIG
Table 1 Particle size, magnetization and resonance parameters of ultrafine HoIG and YbIG materials Heat-treatment temp.
XRD crys-
Magnetic
47rlu,
Curie tem-
g, ff-value
talli te size
particle size
V%
perature
(“C)
(nm)
(nm)
at RT
at LNT
at RT
at LNT a)
450 600 700 900
1.0 1.1 27.5 33.0
11.8 12.6 14.8
1446 1276 987 850
900 895 555 555
2.15 2.08 2.16 2.12
2.81 3.08 3.24 2.09
962 930 1050 720
1620 2155 2450 2940 (168 K)
450
1.0
12.6
600
1.1 31.0 34.0
13.8 11.8
1100 1029 1544 1553
863 860 547 548
2.03 2.08 2.05 1.98
2.18 2.18 2.18 2.58
1290 735 1040 loo0
1590 1390 1480 2225 (145 K)
T,
AH-values (G)
(K)
WFe,O,,
700 900
‘) The temperature
(in K) are indicated in parentheses
wherever the corresponding
temperature
is not LNT.
I
accompanied by an enhancement of the 550 K transition. However, the magnetization vanishes completely around 550 K when the measurements were carried out at 60 G applied field. The curves do not show saturation even at 10 kG fields due to the presence of the ultrafine superparamagnetic fraction. These results may be attributed to the larger particles which only contribute to the magnetization at a 60 G field. The smaller superparamagnetic particles require a higher energy to overcome the thermal agitation and to align in the field direction, they make their presence felt only at higher applied fields (10 kG). Miissbauer spectra for intermediate crystallite sizes also change to two nearly pure sextets for the crystalline ferrimagnetic materials with larger crystallite size from a mixed situation which includes a broad paramagnetic doublet for smaller crystallites [16]. It is interesting to note from table 1 that ultrafine crystallites of 1.0 to 1.5 nm size of HoIG hav * larger M,-values compared to the bulk polycrystalline materials and lo-35 nm size ultrafine crystallites. This increase of the M,-value is related to the R3+-sublattice contribution which neutralizes the Fe3%ublattice magnetization i.e., Fe:+ -@2-Fe:+ or a-d interaction in crystalline garnets because of the antiparallel nature of the coupling in these two sublattices. There is a reduction of
R3,”-sublattice magnetization in ultrafine crystallites of 1.0 to 1.5 nm size and thus the a-d interactions or the Fe3+-sublattice magnetization dominates which leads to higher M,-values. In view of the observed increase of specific volume of HoIG and YbIG having 1.0 to 1.5 nm size crystallites existing in disordered state [16], a weak R3+ sublattice contribution is obviously expected. This affects the anisotropic R3,”-02--Fe:’ superexchange coupling. The line broadening and large quadrupole splitting in Miissbauer spectra of ultrafine 1.0 to 1.5 nm size crystallites clearly inclicate the considerably worse local ordering. In the garnet structure, the RF-02--Fei+ coupling of the molecular field of 2 X 10’ Oe, is weaker than the Fe3+-02--Fez+ coupling (molecular field of 2 x 10“6Oe) [23]. Further, the weak R3+-sublattice contribution in the disordered state of 1.0 to 1.5 nm crystallites leads to the domination of the Fez+ _..02-_Fe~+ superexchange coupling and thus explains the increase of A&-values as compared to crystalline garnets. In ultrafine crystallites of YbIG materials and also !n polycrystalline YbIG, the contribution of the Ry -sublattice magnetization in relatively small compared to HoIG materials and therefore, as expected, such an increase of the M,-value is not evident. Though there is a lack of knowledge on structural and
yh
F@s 012
45O’C
Field
221
(l.Onm)
(CtauSS)
Fig. 3. FMR spectra of YbIG materials heat-treated at 450 OC (crystallite size 1.0 run) in the temperature range RT to LNT.
V.K. Sunkaranarayman, N.S. Gajbhiye / Ultrafine Ho3Fe,U,, and Yb, Fe&
222
I
800
I
I
1800
I
I
I
I
3800
2800 Field
I
I
l800
2
“----do
(Gauss)
Fig. 4. FMR spectra of ultrafine YbIG materials halt-treated at 900 * C (~~stallite size 34 nm) in the temperature range RT to LNT.
configurational details of ultrafine garnets, the enhanced rC-values are suggested to be the result of the disordered state. The decrease of the Fe:‘O*--Fe:* distance and an increase of the included angle towards 180’ would strengthen the a-d superexchange interaction [24]. 3.3. Magnetic resonance studies of HolC and YblG The L~ltr~fine crystallites of HoIG and YbIG materials show broad asymmetric resonance in the FMR spectra which are represented in figs. 3 and 4 for 1.0-l .5 nm (amorphous/ disordered state) and lo-35 nm (crystalline state) crystallites for YbIG materials. At temperat~es between RT and LNT the observed resonance line is asymmetric with greater broadening on the high field side. The resonance line gradually broadens, and shifts to lower fields with decreasing temperature below RT for both 1.0 to 1.5 nm and lo-35 nm crystallites. The line becomes unmeasurably broad below 150 K in the case of larger crystallites. 3.3.1. Effective g-factor The variation of g-values of ultrafine crystallites of HoIG and YbIG as the temperature is lowered from RT to LNT arc shown in figs. 5 and 6. In the temperature range between RT and LNT the ultrafine crystallites of HoIG and YbIG have
g-values > 2.0 (figs. 5 and 6) which are greater than the reported single crystal garnet values, g < 2 [25,26]. In general, the increase of g-values for ultrafine crystallites may be attributed to: (i) the contribution of rare earth sublattice magnetization (ii) magnetic dipolar interactions among ultrafine particles and (iii) distortion of tetrahedral and octahedral Fe3’-sites. The g,, for rare earth iron garnets have a direct dependence on the magnetization of the sublattices and under the assumption of a large damping of the gyroscopic contributions of the rare earths, it is given by the relation [27]: g,,, = g&i& - MB)/MA, where MA is the magnetization for the resultant Fe3+-sublattices having a g,-value = 2.002 and MB is the magnetization of the R3“-sublattices. Therefore, as the contribution of the R3+ magnetization is reduced, g,, approaches the g,-value. Thus, in 1.0 to 1.5 nm crystallites of ultrafine WoIG ancl sfbfG, the weak contribution of the R3* sublattice magnetization could result in g,,-values approaching 2.002. The observed g-values for these ultrafine crystallites are greater than 2 which can be attributed to the presence of an internal magnetic field produced by magnetocrystalline anisotropy and interparticle dipolar interactions and distortion of Fe3+ sites. The resonance equation for ultrafine crystallites of I-M@ and YbIG materials therefore gets modified as: TV= y ( H, + Hi), where o is the resonance
K K. Sankaranarayanan, N.S. Gajbhiye / Ultra fine Ho3 Fe,O,, and Yb, Fe,O/ ,’
223
3.6 H”3Fe5012
3.2 -
0
t50°C
A
600°C
(1 lnm)
0
700%
f275nm)
Q 900°C
(33 nm)
(1Onm)
2.4 -
2.0‘ 0
I 50
I 100
I 150
I 200
Temperat
I 250
I 300
3’ 0
Jre (K)
Fig. 5. The temperature dependent behavior of gCF,-valuesin ultraV.ne HoIG materials of different XRD crystallite size (shown in bracbets).
observed to exist as aggregates/ agglomerates wherein crystallites are held together by primary bonds [16]. The interparticle dipolar interactions give rise to an internal magnetic field and the larger g,,rvalues. The marked effect of dipolar
frequency, y is the gyromagnetic ratio, He is the effective resonance field and Hi is the internal magnetic field due to magnetocrystalline or any other kind of anisotropies and dipolar interaction. The ultrafine crystallites of HoIG and YbIG are
2.7 Yb,Fe,%
2.5 -
al f 5 >
0
45OoC (1 Onm)
A
6OO’C
(1 1 nm)
Cl 700%
(31 nm)
Q
(34 nm)
900°C
Q
2.3 -
b
\
2.1 i
V
1.9. 0
I 50
I 100
I 150 Temperature
Fig. 6. The temperature dependent behavior of g,,+&ues
I 2’30
I 250
I 300
3 0
(K)
in ultrafine YbIG material:, -f v different XRD crystallite size (shown in brackets).
V.K. Sunkaranarayanan, N.S. Gujbhiye / Ultrafine Ho3FesO,z and Yb, Fe50,*
224
interactions is evident in the decrease of g-values from 2.58 fol powders to 2.12 for a dilute suspension in water observed in 11-12 nm sized Fe30, particles [7], since on dilution the particles get separated thereby decreasing the interparticle dipolar interactions. The greater distortion in octahedral and tetrahedral Fe3+ site in ultrafine crystallites of 1.0-1.5 nm size of HoIG and YbIG materials also contributes to an increase of the g,,+lues as has been reported in barium hexaferrite particles precipitated in a glass matrix [28]. Moreover g-values of 2,4,6 and 10 have been also observed for an Fe3%on in a glass matrix depending on the symmetry of the Fe-site [29,30]. The variation of g,,+alues with temperature depends on the relative magnitudes of dipo!ar interactions and R3+ magnetization. The initial gradual increase of g,,,-values from 1.95 to 2.0 in the temperature range RT to 200 K for 10 to 35 nm crystallites of HoIG and YbIG is predominantly due to increasing dipolar interactions. As the temperature is decreased below 200 K, in the case of HoIG, the substantial Ho3 + magnetization (&) predominate over the dipolar interactions
HWedh 4
2600
0
45O’C
0
600°C (1
(l.Onm)
V
700%
lllrn,
\
Tempe:oture
3.3.2. FMR linewidth AH The variation of FMR linewidth, AH, with decreasing temperature from RT to LNT is shown in figs. 7 and 8 for ultrafine crystallites of HoIG and YbIG materials. The FMR line broadening in ultrafine materials is caused by various factors such as:
(275nm)
A 900°C (33nm) t
leading to a decrease of the g,,+alue. The observation is similar to the single crystal garnets [20]. The sharp rise of g,,-values in ultrafine YbIG of 10 to 35 nm size is due to the combined effect of the relatively low Yb3 + magnetization and the vicinity of the compensation temperature where certain garnets show large g,,rvalues [25]. The 1.0 to 1.5 nm size crystallites show a gradual increase in g,,rvalues for HoIG and YbIG materials when the temperature is lowered from RT to 200 K because of stronger magnetic dipolar interactions resulting from larger overall magnetization in aggregates/agglomerates. Below 200 K down to LNT, these crystallites of HoIG show a steep increase of g,f,-values due to the presence of stronger dipolar interactions among the crystallites aggregates and lower Ho3+ sublattice magnetization effected by greater polyhedral distortion in disordered garnet lattice. The gradual increase of g,,+lues from RT to LNT in YbIG is attributed to the relatively lower Yb3’ magnetization.
(I9
Fig. 7. The temperature dependent behavior of Linewidth in ultrafine HoIG materials of different XRD crystallite size (shown in bracket ;).
AH,,, = AHe + AHi, + AHi + AH, + AHP + AH,,, where AH, = eddy current broadening, AHid = inhomogenous demagnetization broadening, AHi = intrinsic broadening of single crystals, AHa = anisotropy broadening, AHP = porosity broadening and AH+ = dipolar broadening. The eddy current contribution AH, does not increase the linewidth by any substantial amount because HoIG and YbIG materials have high resistivities of the order of lo9 SZcm.The inhomogenous demagnetization broadening, AHi~, may be considered negligible since equal amounts of samples were taken. In ultrafine crystallites of HoIG and YbIG, the major contribution to the line broadening comes from factors such as: porosity broadening, AHP, since the crystallites are identified with aggregates/ agglomerates; intrinsic
V.K. Sankaranarayanan,
N-S. Gajbhiye / Wtmfine Ho,Fe,O1z
and Yb,Fe,O,,
225
ywe5012 V V
0
lt50°C
(l.Onm)
A
600°C
(1.1nm)
7OO’C (31 nm) V 900°C Wnm) 0
2000 ‘si !! Q
\ g
1600-
2 .-E J
1200 -
8001 0
I 50
I 100
I 200
I 150 Temperature
I 250
I 300
3
(K)
Fig. 8. The temperature dependent behavior of linewidths in ultrafine YbIG materials of different XRD crystallite size (shown in brackets).
broadening, AHi 7 caused by the presence of the R3%on in garnets; anisotropy broadening, AH,, associated with magnetocrystalline anisotropic field and dipolar broadening, AHdP, caused due to the dipolar interactions among crystallites having high magnetization. The compromise of these various factors leads to the FMR line broadening in ultrafine crystallites of HoIG and YbIG materials. The line broadening in crystallites of 10 to 35 nm size is due to the major contribution from AHi, AH, and smaller contribution from AH, and AHdp. The AHi and AH, in these garnets are directly related to the presence of Ho3+ and Yb3+ ions [31]. Ow tlg to simultaneous coupling of the rare earth ions to the lattice (due to the orbital angular momentum of R3’ ions) and Fe3+ ions (through exchange), the R3+ ions provide a short circuit for the energy in the FMR mode to the lattice. This causes the intrinsic linewidth AHi. Now, the overall linewidth in the powder materials arises from the superposition of the individual angularly dependent resonances appropriately weighted to take into account the different orientations of the magnetization vector [32]. The magnetocrystalline an-
isotropy is responsible for the different orientations of the magnetization vector of ultrafine crystallites and the linebroadening is associated with the anisotropy field, 2&/M,: where K, is the first order magnetocrystalline anisotropy constant and 44, is the saturation magnetization. Qwing to the large single ion anisotropy of R”+ ions [33] the anisotropy field and linewidth would be larger at lower temperatures. The intercrystallite dipolar interactions in the aggregates/ agglomerates causes an internal magnetic field and results in dipolar line broadening AHdP. As the temperature is decreased below = 200 K, the R3+-_02- -Fe:+ coupling becomes stronger which in&eases AHi and due to the increase of R3+ single ion anisotropy considerable enhancement in &-values occurs. Thus. the extremely large AH, contributions results in the sharp increase in line-_ -_ 1 t,-n-m, ..IIM~~c...s.~F\l\, lclrna 85 b,he IUL 5” width Ziiu uc~u111c3 UIIIII~~~UIQV~~ temperature is lowered below 200 K. The gradual H-values in 1.0 to 1.5 nm size crystallites are due to the major factors such as AHP and AH+ and a relatively smaller contribution from AHi and AHa. The disordered nature of the garnet lattice in these crystallites leads to a weaker R”+-
226
V.K. Sankaranarayanan, N.S. Gajbhiye / Ultrafine Ho,, Fe,O,, and Yb3 Fe,Olz
coupling and substantial reduction of the antiparallel R3+-sublattice contribution and therefore larger overall magnetization. Due to very strong dipolar interactions among particles, the AH,, contribution is large. The weaker R”-02-Fe3+ coupling also reduces the AHi and AH,. 1.0 to 1.5 nm size crystallites of HoIG and YbIG materials show superparamagnetic behaviour in magnetization studies (figs. 1 and 2) and Mossbauer studies [16]. The direction of the magnetization fluctuates in superparamagnetic particles at a rate faster than the Larmour frequency, so the narrow resonance lines results due to an averaging effect of these fluctuations ou the magnetocrystalline anisotropy [7,34]. In ulirafine superparamagnetic particles of HoIG and YbIG, however, line broadening due to strong dipolar interaction dominate over the averaged anisotropy field and thus lead to relatively broad lines at RT. As the temperature is lowered below RT, the linewidth increase gradually in both HoIG and YbIG materials down to = 200 K due to the inverse temperature dependence of AH,,. Below 200 K the AH increases faster in HoIG in comparison with YbIG materials understandably due to the higher magnetization and anisotropy values of HoIG at lower temperature. (-p--Fe3’
. Conclusions
Ultrafine particles of HoIG and YbIG materials have been prepared by thermal decomposition of citrate precursors. Crystallites of 10 to 35 nm size are monoliths whereas 1.0 to 1.5 nm size crystallites exist in a disordered state as aggregates/ agglomerates with strong interparticle magnetic dipolar interactions and exhibit superparamagnetism. The observed bigher MS-values and higher T,-values are attributed to weak Ho3+, Yb3+ sublattice magnetization contributions due to the effect Of the disordered structure on the anisotropic R3,’-02- -Fez ’ exchange coupling in HoIG and YbIG and the antiparallel coupling with Fe3+ sublattice. The observed g,,rvalues 2 2.0 in FMR studies for 1.0 to 1.5 nm size crystallites are related to the Ho”+, Yb3+ sublattice magnetization contribution, the presence of an internal magnetic
field produced by magnetocrystalline anisotropy, dipolar interactions among particles and distortion of octahedral and tetrahedral symmetry for the Fe3+-site in a garnet structure. When the temperature is lowered from RT to LNT, the sharp increase of g,,,- values in HoIG is due to stronger dipolar interactions having inverse temperature dependence and the weaker Ho3+ sublattice magnetization. The gradual increase of g,ff-values in YbIG is attributed to small Yb3+ magnetiation. In 10 to 35 nm size crystallites, the Ho3+ sublattice contribution dominates over the dipolar interactions resulting in a decrease of geffvalues in HoIG and the compensation temperature causes sharp rise of g,ff-values in YbIG. The important factors for overall FMR-line broadening, AH, in ultrafine HoIG and YbIG are: porosity broadening AHP; intrinsic broadening A Hi; anisotropy broadening AHa; dipolar broadening AHdP. As the temperature is lowered from RT to LNT, a sharp increase in AH in 10 to 35 nm size crytallites arises from the large AHi and AH, components of AH due to the presence of Ho3+ and Yb3’ ions exchange coupled to the Fe3+ ions. In 1.0 to 1.5 nm size crystallites of HoIG and YbIG, which exist as superparamagnetic particles, the stronger dipolar interaction dominate over the averaged anisotropy field and lead to relatively broad lines. The temperature dependent variation of AH in these HoIG and YbIG materials is attributed to the different magnetization and anisotropy of HoIG and YbIG at lower temperatures.
References
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V.K. Sankaranarayanan, N.S. Gajbhje
M J. Dubowik
and J. Baszynski, J. Magn. Magn. Mat. 59 (1986) 161. 191 IS. Jacobs and C.P. Bean, in: Magnetism, vol. III, eds. G.T. Rado and H. Suhl (Academic Press, New York, 1963) p. 271. PI S.M. Aharoni and M.H. Litt, J. AppI. Phys. 42 (1971) 352. WI R.S. Gekht, V.A. Ignatchenko, Yu.L. Raikher and M.I. Shilomis, Sov. Phys. JETP 43 (1976) 677. [121 V.K. Sharma and A. Baiker, J. Chem. Phys. 75 (1981) 5596. [I31 R.S. Gekht and V.I. Ignatchenko, Sov. Phys. JETP 49 (1979) 84. and N.S. Gajbhiye, Thermochim. WI V.K. Sankaranarayanan Acta 153 (1989) 337. t151 H.P. KIug and L.P. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed. (John Wiley, New York, 1974) p. 618. WI V.K. Sankaranarayanan and N.S. Gajbhiye, J. Am. Ceram. sot. 73 (1990) 1301. WI M. Rappaz, C. Sollraird, A. Chatelain and L.A. Boatner, Phys. Rev. B 21 (1980) 906. WI H.J. Wasserman and J.S. Vermaak, Surface Sci. 32 (1972) 168. 1191 D. Schroeer and R.C. Nininger, Jr., Phys. Rev. Lett. 19 (1967) 632.
/ UItrafine Ho,Fe,O,,
and Yb,Fe,O,,
227
[20] P. Hansen, K. Enke and G. Winkler, Magnetic Oxides and Related Compounds, LB Series vol. 111/12a. ed. K.H. Hellwege (Springer Verlag, New York. 1978). [21] C.P. Bean and I.S. Jacobs, J. Appl. Phys. 27 (1956) 1448. i22] J.M.D. Coey and D. Khalafalla, Phys. Stat. Sol. (a) 11 (1972) 229. t231 F. Sayestat, J.X. Boucherle and F. Tcheon, J. Magn. Magn. Mat. 46 (1984) 219. t241 M.A. Gilleo, Phys. Rev. 109 (1958) 777. v51 V.H. Von Aulock, Handbook of Microwave Ferrite Materials (Academic Press, New York. 1965) p. 65. WI G.P. Rodrigue. H. Meyer and R.V. Jones, J. Appl. Phys. 31 (1960) 376s. 1271 C. Kittel, Phys. Rev. 115 (1959) 1587. WI S. Ram, J. Magn. Magn. Mat. 82 (1989) 129. v91 K.J. Rao and B.G. Rao, Proc. Ind. Acad. Sci. 95 (1985) 169. t301 3.L. Griscom, J. Non-Cryst. Solids 40 (1980) 211. 1311 P.G. de Gennes, C. Kittel and A.M. Portis, Phys. Rev. 116 (1959) 323. 1321 F.D. Tsay, S.L. Manatt and S.I. Chan, Geochim. Cosmochim. Acta 37 (1973) 1201. t331 R.F. Pearson, J. Appl. Phys. 33 (1962) 1236. [341 Y.G. Dorfman, Sov. Phys. JETP 21 (1965) 472.
217
Magnetization and magnetic resonance studies of ultrafine Ho,Fe,O,, and Yb,Fe,O,, V.K. Sankaranarayanan and N.S. Gajbhiye Department of Chemistry, Indian Institute of Technolom, Kanpur 208016, India Received 15 March 1990; in revised form 30 May 1990
Ultrafine particles of HosFe,O,z and Yb,F%O,, garnet materials from 1.0 to 35 nm crystallite in size are prepared by thermal decomposition of citrate precursors. The crystallites in the 1.0 to 1.5 nm size range exist in the disordered state as aggregates and exhibit superparamagnetism, whereas 10 to 35 nm crystallites are monoliths. The observed higher @-values and TC-values of 1.0 to 1.5 nm crystallites are attributed to the weakening of the anisotropic RF -02--F~+ coupling in the disordered state which allows the Fe sublattice magnetization to dominate. The g,,-values of > 2.0 in resonance studies and their inverse temperature dependence are the result of the low Hd+ and Yb3’ sublattice magnetization, the presence of an internal magnetic field produced by interparticle dipolar interactions and the distortion of Fe3+ sites in the garnet structure. The 10 to 35 nm crystallites have larger g,,,- values, as compared to single crystal or polycrystalline garnets due to the dipolar interactions; the temperature dependence of gcrr-values is comparable to the polycrystalline materials. The resonance line broadening in 1.0 to 1.5 nm crystallites is dominated by dipolar broadening AHdP whereas in 10 to 35 nm crystallites anisotropy broadening AH, and intrinsic broadening AHi are predominant.
1. Introduction The magnetism of ultrafine particles is of considerable recent importance from a scientific and practical point of view in ferrofluids and catalysts [ 1,2]. The saturation magnetization, an intrinsic magnetic prop :rty of the ferrimagnetic materials, varies in ultrafine particles unlike that in the bulk polycrystalline and single crystal materials due to the differences of spin structure between the surface and the bulk [3]. Therefore, ultrafine particles comprised of nanometer-size crystallites are expected to show very interesting properties as the particle would be effectively all surface. Thus it is worthwhile to explore magnetization characteristics in order to understand the magnetic interactions among various sublattices particularly in 3sublattice ferrimagnetic rare earth iron garnet (RIG), Ho,Fe& and Yb,Fe,O,,, materials. Magnetic resonance is often employed to study fine particles of magnetic materials in glass ceramics, ferrofluids and supported metal cata0304-8853/90/$03.50
lysts [4-S]. Ultrafine particles of ferro- and ferrimagnetic materials often exhibit superparamagnetic properties [9]. A few magnetic resonance investigations on superparamagnetic particles are reported which show comparatively narrow resonance linewidths [6-8,101. The resonance linewidth behaviour is not corroborated by the existing theories of magnetic resonance [ 1 l]. Resonance studies of fine particles have dealt with magnetic dipole interactions systems wherein among particles are very weak or negligible [12,13]. However, ultrafine particles comprising 1.0 to 35 nm size crystallites of the rare earth iron garnets. Ho,Fe,O,, and Yb,Fe,O,,, with strong dipolar interactions among particles have not been studied in the literature by using FMR technique. In the present study, the magnetization behaviour of u’rtraflne particles of and Yb,Fe,O,, (YbIG) having crystallite sizes in the range 1.0 tc 1.5 nm (X-ray amorphous. superparamagnetic) and 10 to 35 nm (crystalline state) are investigated in order to understand the various
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
V.K. Sankawnarayanan, N.S. Gajbhiye / Utrafine Ho>Fe,Ol
218
magnetic interactions e.g., superexchange interactions among the 3 sublattices of garnets and their relationship to &-values. These crystallites are of particular interest in FMR studies because of their existence as aggregates/ agglomerates wherein strong &polar interactions exist. Besides the 3sublattice magnetic structure of garnet has fast relaxing Ho3+ and Yb3 “-ions which are magnetically coupled to Fe 3+-ions. The effective g-factor and linewidth, AH, in FMR are sensitive to the effect of dipolar interactions, presence ‘of fast relaxing Ho3’ and Yb3+-ions, magnetocrystalline anisotropy and symmetry of Fe3+-sites; these aspects ar9 Investigated in this study.
2. Experimental procedure Ultrafine Ho,Fe,O,, and Yb,Fe,O,, were prepared by thermal decomposition of a citrate precursor, R,Fe,(Cit),, (36 + n)H,O where R = Ho and Yb. The method of preparation and thermal decomposition studies of the citrate precursor are described elsewhere [14]. The precursor on decomposition in an air atmosphere yields X-ray amorphous ultrafine particles at 450°C and crystalline ones above 6OOOC.The amorphous HoIG and YbIG materials were heat treated or annealed in air at various temperatures between 450 and 1000” C for 4 h in order to obtain crystallites of various size. The powder X-ray diffraction (XRD) patterns of the materials were recorded by using a Rich Seifert Isodebyflex, Model 2002, diffractometer. Cu K, radiation with Ni-filter was used in the 20 range of 10” to 120 O. The size of the primary crystalites was calculated from XRD-line broadening using the classical Scherrer relationsitip 1151, Dh$/ = kX/B cos 8 where Dhk, is particle diameter in A, k is a constant (shape factor) = 0.9, B is the half maximum linewidth and X is the wavelength Of the X-rays. Magnetization measurements were carried out by using a PAR Modei 15OA Vibrating sample magnetometer i-r,conjugation with a Varian Model V-7200 electromagnet assembly that provides a magnetic field up to 10 kG. The temperature variation of magnetization was studied using a furnace assembly which can provide tempera-l
and Yb3 FesO,,
tures up to 1000 K (Model 151) associated with
the magnetometer. The FMR-spectra were recorded using a Varian Model E-109 EPR spectrometer operating at the X-band frequency. The low temperature measurements were carried out in a home-made dewar from room temperature (RT) to liquid nitrogen ternperature (LNT).
3. Results and discussion 3.1. Nature of ultrafine HoIG and YbIG
Ultrafine particles of HoIG and YbIG were characterised in detail by XRD, TEM and BET surface area measurements and presented elsewhere [16]. In the temperature range 450 to 600 OC HoIG and YbIG materials exhibit a broad hump in the XRD pattern, d,, = 0.2 to 0.35 nm, which indicates the absence of crystalline periodicity and the existence of a highly strained disordered state having a crystallite size of 1.0 to 1.5 nm. Above 600° C, crystallites grow to 10 to 35 nm monoliths, db10 = 0.275 nm, due to crystallization with negligible lattice strain. The larger dazOspacing values for crystallites of size 1.0 to 1.5 nm compared to lo-35 nm corresponds to the relative increase of the specific volume of the HoIG and YbIG-lattice. The larger size-induced strain is suggested to be responsible for the distortion and instability of the garnet lattice. Such a lattice expansion with decreasing particle size has been reported for various metallic and non-metallic fine particle systems [17-191. The distortion may be attributed to crystal symmetry and surface effects or multiwinning in the structure of ultrafine crystallites. Ultrafine particles were identified with aggregate/ agglomerates. The crystallite aggregate forms a porous cluster-agglomerate with heat treatment. Clustering of crystallites (and aggregates) is undoubtedly due to the presence of the unsaturated bonds on the considerabiy extended, highly energetic surface of small particles. This means, the crystallites in aggregates are most probably held together by primary bonds. The rupture of intercrystallite bonds occurs during the crystallization process and eventually leads to the formation of monoliths.
V.K. Sankaranarayanan, N.S. Gajbhiye / Ultrafine IJo, Fe,O,, and Yb,Fe,O,,
3.2. Magnetization YbPG
studies of uitrafine HoIG and
The temperature dependence of magnetization of ultrafine HoIG and YbIG materials at field strength of 0.6 T (6 kG) is shown in figs. 1 and 2. The inset figures show the relative magnetization (M/M,) as a function of field/ temperature (H/T) quotient. There is a gradual decrease of magnetization with temperature for 1.0 to I.5 nm size crystallites with an amorphous nature. The individual particles order magnetically around 900 and 860 K which is much higher than the expected Curie temperature (T,) around 550 K for well crystallised materials (table 1). These transitions shift to 550 K due to crystallization when the materials are heat treated well above the crystallization temperatures. The magnetization approaches saturation in HoIG and YbIG materials having a crystallite size 10 to 35 nm at relatively low fields of 1.0 to 2.0 kG. The room temperature MS-values (saturation magnetization) are comparable to their polycrystalline values reported in the literature [20] and exhibit ferrimagnetic behaviour. The 1.0 to 1.5 nm crystallites show nearly superimposed M/M, vs. H/T curves (inset figs. 1
219
and 2) and do not show any hysteresis. The magnetization curves do not attain saturation even at 10 kG and resemb!e the langevin curves. These observations are characteristics ui’ aupt’~t ramagnetic behaviour of ultrafine particles [9.21]. The room temperature MS-values for these rrystallites are obtained by extrapolation of M t l/H curves to the limit l/Ii + 0 following literature reports for y-Fe,O, fine particles [22] and are given in table 1. In a superparamagnetic system. the magnetization curve can be described by a Langevin function. The insets of figs. 1 and 2 point out the deviation from Langevin curves, because of the distribution of particle size. The initial susceptibility is very sensitive to the larger particles present whereas the approach to saturation is governed by the smaller particles. Therefore, the average magnetic moment per particle and the magnetic particle size of HoIG and YbIG materials were determined from low field slopes of the magnetization curves by using the limit to the Langevin function. The relative difference between the magnetic particle size and crystallite size obtained from XRD (table 1) apparentiy suggests magnetic interactions in the crystallite aggregates (or magnetic
clusters) wherein
intercrys-
“O3Fe5q2 0
oso”c
A
6OO’C
0
7o0°c
H03Fe5Ol2 it 0
a 0 c
EC% 600°C 7Q0°C 7o0°c
6kG
(1 Gnmi (1 1 nm) (275nm) (n_! 6@G
tleld
4*e”3i
Temperature (K)
Fig. I. Temperature dependence of magnetization of ultrafine HoIG materials heat-treated at various temperatures. The XRD crystallite sizes are given in brackets. Insets show the plot of relative magnetization, M/M, vs. field temperature quotient. H/ 7:
K K. Sankaranarayanan, N.S. Gajbhiye / Ultrafine Ho3 FesOIz and Yb3Fees0,,
220
5
0
sQ
I
10
15
20
NIT.104ft
6
25
30
Yb,Fe5012 al 0
4
35
K”) 6kG tieid
A
46O*C 600°C
O.Onm) (l.lnm)
0 0
7OO*C 900°C
(31 nm) (34nm)
2
Temperature
(K)
Fig. 2. Temperature dependence of magnetization of ultrafine YbIG materials heat-treated at various temperatures. The XRD crystallite sizes are given in brackets. Insets show the plot of relative magnetization, M/M, vs. field temperature quotient, H/T.
crystallites having an intermediate particle size range, but slowly disappears around 900 and 850 K (figs. 1 and 2). On annealing at higher temperature for longer duration, these ultrafine crystallites show the disappearance of the 900 K transition
tallite bond exists. Such aggregates may lead to higher magnetization accompanied by an increase in Qvalues in ultrafine garnets. The magnetization does not vanish completely at/around 550 K for ultrafine HoIG and YbIG
Table 1 Particle size, magnetization and resonance parameters of ultrafine HoIG and YbIG materials Heat-treatment temp.
XRD crys-
Magnetic
47rlu,
Curie tem-
g, ff-value
talli te size
particle size
V%
perature
(“C)
(nm)
(nm)
at RT
at LNT
at RT
at LNT a)
450 600 700 900
1.0 1.1 27.5 33.0
11.8 12.6 14.8
1446 1276 987 850
900 895 555 555
2.15 2.08 2.16 2.12
2.81 3.08 3.24 2.09
962 930 1050 720
1620 2155 2450 2940 (168 K)
450
1.0
12.6
600
1.1 31.0 34.0
13.8 11.8
1100 1029 1544 1553
863 860 547 548
2.03 2.08 2.05 1.98
2.18 2.18 2.18 2.58
1290 735 1040 loo0
1590 1390 1480 2225 (145 K)
T,
AH-values (G)
(K)
WFe,O,,
700 900
‘) The temperature
(in K) are indicated in parentheses
wherever the corresponding
temperature
is not LNT.
I
accompanied by an enhancement of the 550 K transition. However, the magnetization vanishes completely around 550 K when the measurements were carried out at 60 G applied field. The curves do not show saturation even at 10 kG fields due to the presence of the ultrafine superparamagnetic fraction. These results may be attributed to the larger particles which only contribute to the magnetization at a 60 G field. The smaller superparamagnetic particles require a higher energy to overcome the thermal agitation and to align in the field direction, they make their presence felt only at higher applied fields (10 kG). Miissbauer spectra for intermediate crystallite sizes also change to two nearly pure sextets for the crystalline ferrimagnetic materials with larger crystallite size from a mixed situation which includes a broad paramagnetic doublet for smaller crystallites [16]. It is interesting to note from table 1 that ultrafine crystallites of 1.0 to 1.5 nm size of HoIG hav * larger M,-values compared to the bulk polycrystalline materials and lo-35 nm size ultrafine crystallites. This increase of the M,-value is related to the R3+-sublattice contribution which neutralizes the Fe3%ublattice magnetization i.e., Fe:+ -@2-Fe:+ or a-d interaction in crystalline garnets because of the antiparallel nature of the coupling in these two sublattices. There is a reduction of
R3,”-sublattice magnetization in ultrafine crystallites of 1.0 to 1.5 nm size and thus the a-d interactions or the Fe3+-sublattice magnetization dominates which leads to higher M,-values. In view of the observed increase of specific volume of HoIG and YbIG having 1.0 to 1.5 nm size crystallites existing in disordered state [16], a weak R3+ sublattice contribution is obviously expected. This affects the anisotropic R3,”-02--Fe:’ superexchange coupling. The line broadening and large quadrupole splitting in Miissbauer spectra of ultrafine 1.0 to 1.5 nm size crystallites clearly inclicate the considerably worse local ordering. In the garnet structure, the RF-02--Fei+ coupling of the molecular field of 2 X 10’ Oe, is weaker than the Fe3+-02--Fez+ coupling (molecular field of 2 x 10“6Oe) [23]. Further, the weak R3+-sublattice contribution in the disordered state of 1.0 to 1.5 nm crystallites leads to the domination of the Fez+ _..02-_Fe~+ superexchange coupling and thus explains the increase of A&-values as compared to crystalline garnets. In ultrafine crystallites of YbIG materials and also !n polycrystalline YbIG, the contribution of the Ry -sublattice magnetization in relatively small compared to HoIG materials and therefore, as expected, such an increase of the M,-value is not evident. Though there is a lack of knowledge on structural and
yh
F@s 012
45O’C
Field
221
(l.Onm)
(CtauSS)
Fig. 3. FMR spectra of YbIG materials heat-treated at 450 OC (crystallite size 1.0 run) in the temperature range RT to LNT.
V.K. Sunkaranarayman, N.S. Gajbhiye / Ultrafine Ho3Fe,U,, and Yb, Fe&
222
I
800
I
I
1800
I
I
I
I
3800
2800 Field
I
I
l800
2
“----do
(Gauss)
Fig. 4. FMR spectra of ultrafine YbIG materials halt-treated at 900 * C (~~stallite size 34 nm) in the temperature range RT to LNT.
configurational details of ultrafine garnets, the enhanced rC-values are suggested to be the result of the disordered state. The decrease of the Fe:‘O*--Fe:* distance and an increase of the included angle towards 180’ would strengthen the a-d superexchange interaction [24]. 3.3. Magnetic resonance studies of HolC and YblG The L~ltr~fine crystallites of HoIG and YbIG materials show broad asymmetric resonance in the FMR spectra which are represented in figs. 3 and 4 for 1.0-l .5 nm (amorphous/ disordered state) and lo-35 nm (crystalline state) crystallites for YbIG materials. At temperat~es between RT and LNT the observed resonance line is asymmetric with greater broadening on the high field side. The resonance line gradually broadens, and shifts to lower fields with decreasing temperature below RT for both 1.0 to 1.5 nm and lo-35 nm crystallites. The line becomes unmeasurably broad below 150 K in the case of larger crystallites. 3.3.1. Effective g-factor The variation of g-values of ultrafine crystallites of HoIG and YbIG as the temperature is lowered from RT to LNT arc shown in figs. 5 and 6. In the temperature range between RT and LNT the ultrafine crystallites of HoIG and YbIG have
g-values > 2.0 (figs. 5 and 6) which are greater than the reported single crystal garnet values, g < 2 [25,26]. In general, the increase of g-values for ultrafine crystallites may be attributed to: (i) the contribution of rare earth sublattice magnetization (ii) magnetic dipolar interactions among ultrafine particles and (iii) distortion of tetrahedral and octahedral Fe3’-sites. The g,, for rare earth iron garnets have a direct dependence on the magnetization of the sublattices and under the assumption of a large damping of the gyroscopic contributions of the rare earths, it is given by the relation [27]: g,,, = g&i& - MB)/MA, where MA is the magnetization for the resultant Fe3+-sublattices having a g,-value = 2.002 and MB is the magnetization of the R3“-sublattices. Therefore, as the contribution of the R3+ magnetization is reduced, g,, approaches the g,-value. Thus, in 1.0 to 1.5 nm crystallites of ultrafine WoIG ancl sfbfG, the weak contribution of the R3* sublattice magnetization could result in g,,-values approaching 2.002. The observed g-values for these ultrafine crystallites are greater than 2 which can be attributed to the presence of an internal magnetic field produced by magnetocrystalline anisotropy and interparticle dipolar interactions and distortion of Fe3+ sites. The resonance equation for ultrafine crystallites of I-M@ and YbIG materials therefore gets modified as: TV= y ( H, + Hi), where o is the resonance
K K. Sankaranarayanan, N.S. Gajbhiye / Ultra fine Ho3 Fe,O,, and Yb, Fe,O/ ,’
223
3.6 H”3Fe5012
3.2 -
0
t50°C
A
600°C
(1 lnm)
0
700%
f275nm)
Q 900°C
(33 nm)
(1Onm)
2.4 -
2.0‘ 0
I 50
I 100
I 150
I 200
Temperat
I 250
I 300
3’ 0
Jre (K)
Fig. 5. The temperature dependent behavior of gCF,-valuesin ultraV.ne HoIG materials of different XRD crystallite size (shown in bracbets).
observed to exist as aggregates/ agglomerates wherein crystallites are held together by primary bonds [16]. The interparticle dipolar interactions give rise to an internal magnetic field and the larger g,,rvalues. The marked effect of dipolar
frequency, y is the gyromagnetic ratio, He is the effective resonance field and Hi is the internal magnetic field due to magnetocrystalline or any other kind of anisotropies and dipolar interaction. The ultrafine crystallites of HoIG and YbIG are
2.7 Yb,Fe,%
2.5 -
al f 5 >
0
45OoC (1 Onm)
A
6OO’C
(1 1 nm)
Cl 700%
(31 nm)
Q
(34 nm)
900°C
Q
2.3 -
b
\
2.1 i
V
1.9. 0
I 50
I 100
I 150 Temperature
Fig. 6. The temperature dependent behavior of g,,+&ues
I 2’30
I 250
I 300
3 0
(K)
in ultrafine YbIG material:, -f v different XRD crystallite size (shown in brackets).
V.K. Sunkaranarayanan, N.S. Gujbhiye / Ultrafine Ho3FesO,z and Yb, Fe50,*
224
interactions is evident in the decrease of g-values from 2.58 fol powders to 2.12 for a dilute suspension in water observed in 11-12 nm sized Fe30, particles [7], since on dilution the particles get separated thereby decreasing the interparticle dipolar interactions. The greater distortion in octahedral and tetrahedral Fe3+ site in ultrafine crystallites of 1.0-1.5 nm size of HoIG and YbIG materials also contributes to an increase of the g,,+lues as has been reported in barium hexaferrite particles precipitated in a glass matrix [28]. Moreover g-values of 2,4,6 and 10 have been also observed for an Fe3%on in a glass matrix depending on the symmetry of the Fe-site [29,30]. The variation of g,,+alues with temperature depends on the relative magnitudes of dipo!ar interactions and R3+ magnetization. The initial gradual increase of g,,,-values from 1.95 to 2.0 in the temperature range RT to 200 K for 10 to 35 nm crystallites of HoIG and YbIG is predominantly due to increasing dipolar interactions. As the temperature is decreased below 200 K, in the case of HoIG, the substantial Ho3 + magnetization (&) predominate over the dipolar interactions
HWedh 4
2600
0
45O’C
0
600°C (1
(l.Onm)
V
700%
lllrn,
\
Tempe:oture
3.3.2. FMR linewidth AH The variation of FMR linewidth, AH, with decreasing temperature from RT to LNT is shown in figs. 7 and 8 for ultrafine crystallites of HoIG and YbIG materials. The FMR line broadening in ultrafine materials is caused by various factors such as:
(275nm)
A 900°C (33nm) t
leading to a decrease of the g,,+alue. The observation is similar to the single crystal garnets [20]. The sharp rise of g,,-values in ultrafine YbIG of 10 to 35 nm size is due to the combined effect of the relatively low Yb3 + magnetization and the vicinity of the compensation temperature where certain garnets show large g,,rvalues [25]. The 1.0 to 1.5 nm size crystallites show a gradual increase in g,,rvalues for HoIG and YbIG materials when the temperature is lowered from RT to 200 K because of stronger magnetic dipolar interactions resulting from larger overall magnetization in aggregates/agglomerates. Below 200 K down to LNT, these crystallites of HoIG show a steep increase of g,f,-values due to the presence of stronger dipolar interactions among the crystallites aggregates and lower Ho3+ sublattice magnetization effected by greater polyhedral distortion in disordered garnet lattice. The gradual increase of g,,+lues from RT to LNT in YbIG is attributed to the relatively lower Yb3’ magnetization.
(I9
Fig. 7. The temperature dependent behavior of Linewidth in ultrafine HoIG materials of different XRD crystallite size (shown in bracket ;).
AH,,, = AHe + AHi, + AHi + AH, + AHP + AH,,, where AH, = eddy current broadening, AHid = inhomogenous demagnetization broadening, AHi = intrinsic broadening of single crystals, AHa = anisotropy broadening, AHP = porosity broadening and AH+ = dipolar broadening. The eddy current contribution AH, does not increase the linewidth by any substantial amount because HoIG and YbIG materials have high resistivities of the order of lo9 SZcm.The inhomogenous demagnetization broadening, AHi~, may be considered negligible since equal amounts of samples were taken. In ultrafine crystallites of HoIG and YbIG, the major contribution to the line broadening comes from factors such as: porosity broadening, AHP, since the crystallites are identified with aggregates/ agglomerates; intrinsic
V.K. Sankaranarayanan,
N-S. Gajbhiye / Wtmfine Ho,Fe,O1z
and Yb,Fe,O,,
225
ywe5012 V V
0
lt50°C
(l.Onm)
A
600°C
(1.1nm)
7OO’C (31 nm) V 900°C Wnm) 0
2000 ‘si !! Q
\ g
1600-
2 .-E J
1200 -
8001 0
I 50
I 100
I 200
I 150 Temperature
I 250
I 300
3
(K)
Fig. 8. The temperature dependent behavior of linewidths in ultrafine YbIG materials of different XRD crystallite size (shown in brackets).
broadening, AHi 7 caused by the presence of the R3%on in garnets; anisotropy broadening, AH,, associated with magnetocrystalline anisotropic field and dipolar broadening, AHdP, caused due to the dipolar interactions among crystallites having high magnetization. The compromise of these various factors leads to the FMR line broadening in ultrafine crystallites of HoIG and YbIG materials. The line broadening in crystallites of 10 to 35 nm size is due to the major contribution from AHi, AH, and smaller contribution from AH, and AHdp. The AHi and AH, in these garnets are directly related to the presence of Ho3+ and Yb3+ ions [31]. Ow tlg to simultaneous coupling of the rare earth ions to the lattice (due to the orbital angular momentum of R3’ ions) and Fe3+ ions (through exchange), the R3+ ions provide a short circuit for the energy in the FMR mode to the lattice. This causes the intrinsic linewidth AHi. Now, the overall linewidth in the powder materials arises from the superposition of the individual angularly dependent resonances appropriately weighted to take into account the different orientations of the magnetization vector [32]. The magnetocrystalline an-
isotropy is responsible for the different orientations of the magnetization vector of ultrafine crystallites and the linebroadening is associated with the anisotropy field, 2&/M,: where K, is the first order magnetocrystalline anisotropy constant and 44, is the saturation magnetization. Qwing to the large single ion anisotropy of R”+ ions [33] the anisotropy field and linewidth would be larger at lower temperatures. The intercrystallite dipolar interactions in the aggregates/ agglomerates causes an internal magnetic field and results in dipolar line broadening AHdP. As the temperature is decreased below = 200 K, the R3+-_02- -Fe:+ coupling becomes stronger which in&eases AHi and due to the increase of R3+ single ion anisotropy considerable enhancement in &-values occurs. Thus. the extremely large AH, contributions results in the sharp increase in line-_ -_ 1 t,-n-m, ..IIM~~c...s.~F\l\, lclrna 85 b,he IUL 5” width Ziiu uc~u111c3 UIIIII~~~UIQV~~ temperature is lowered below 200 K. The gradual H-values in 1.0 to 1.5 nm size crystallites are due to the major factors such as AHP and AH+ and a relatively smaller contribution from AHi and AHa. The disordered nature of the garnet lattice in these crystallites leads to a weaker R”+-
226
V.K. Sankaranarayanan, N.S. Gajbhiye / Ultrafine Ho,, Fe,O,, and Yb3 Fe,Olz
coupling and substantial reduction of the antiparallel R3+-sublattice contribution and therefore larger overall magnetization. Due to very strong dipolar interactions among particles, the AH,, contribution is large. The weaker R”-02-Fe3+ coupling also reduces the AHi and AH,. 1.0 to 1.5 nm size crystallites of HoIG and YbIG materials show superparamagnetic behaviour in magnetization studies (figs. 1 and 2) and Mossbauer studies [16]. The direction of the magnetization fluctuates in superparamagnetic particles at a rate faster than the Larmour frequency, so the narrow resonance lines results due to an averaging effect of these fluctuations ou the magnetocrystalline anisotropy [7,34]. In ulirafine superparamagnetic particles of HoIG and YbIG, however, line broadening due to strong dipolar interaction dominate over the averaged anisotropy field and thus lead to relatively broad lines at RT. As the temperature is lowered below RT, the linewidth increase gradually in both HoIG and YbIG materials down to = 200 K due to the inverse temperature dependence of AH,,. Below 200 K the AH increases faster in HoIG in comparison with YbIG materials understandably due to the higher magnetization and anisotropy values of HoIG at lower temperature. (-p--Fe3’
. Conclusions
Ultrafine particles of HoIG and YbIG materials have been prepared by thermal decomposition of citrate precursors. Crystallites of 10 to 35 nm size are monoliths whereas 1.0 to 1.5 nm size crystallites exist in a disordered state as aggregates/ agglomerates with strong interparticle magnetic dipolar interactions and exhibit superparamagnetism. The observed bigher MS-values and higher T,-values are attributed to weak Ho3+, Yb3+ sublattice magnetization contributions due to the effect Of the disordered structure on the anisotropic R3,’-02- -Fez ’ exchange coupling in HoIG and YbIG and the antiparallel coupling with Fe3+ sublattice. The observed g,,rvalues 2 2.0 in FMR studies for 1.0 to 1.5 nm size crystallites are related to the Ho”+, Yb3+ sublattice magnetization contribution, the presence of an internal magnetic
field produced by magnetocrystalline anisotropy, dipolar interactions among particles and distortion of octahedral and tetrahedral symmetry for the Fe3+-site in a garnet structure. When the temperature is lowered from RT to LNT, the sharp increase of g,,,- values in HoIG is due to stronger dipolar interactions having inverse temperature dependence and the weaker Ho3+ sublattice magnetization. The gradual increase of g,ff-values in YbIG is attributed to small Yb3+ magnetiation. In 10 to 35 nm size crystallites, the Ho3+ sublattice contribution dominates over the dipolar interactions resulting in a decrease of geffvalues in HoIG and the compensation temperature causes sharp rise of g,ff-values in YbIG. The important factors for overall FMR-line broadening, AH, in ultrafine HoIG and YbIG are: porosity broadening AHP; intrinsic broadening A Hi; anisotropy broadening AHa; dipolar broadening AHdP. As the temperature is lowered from RT to LNT, a sharp increase in AH in 10 to 35 nm size crytallites arises from the large AHi and AH, components of AH due to the presence of Ho3+ and Yb3’ ions exchange coupled to the Fe3+ ions. In 1.0 to 1.5 nm size crystallites of HoIG and YbIG, which exist as superparamagnetic particles, the stronger dipolar interaction dominate over the averaged anisotropy field and lead to relatively broad lines. The temperature dependent variation of AH in these HoIG and YbIG materials is attributed to the different magnetization and anisotropy of HoIG and YbIG at lower temperatures.
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