1.3 Los valores expresivos como elementos fundamentales de la comunicación visual
1.3.2 Signo icónico
In section 6.2, I have studied the effect o f triv alen t ion doping on the transpo rt properties and m agnetoresistive behaviour o f N dSrM nO film s. The observed results can be explained by an increased overlap betw een B -site d orbitals and oxygen p orbitals, produced by the increased internal pressure at A -site in the presence of species with large ionic radii. W hen lots of work has been done to the A (divalent and trivalent ) site doped and their influence on the m agnetic and transport properties, little attention has been paid to the B-site doped by other 3d m etals, w here are at the heart o f double exchange (DE) interaction.
In this section, a study o f the Fe doped at B (M n) site is carried out. Both film s, Fe dop ed Lao.6oSro.4oM no.90 F eo .io0 3-ô (L S M F O ) and F e-free Lao.6oSro.4o M n03_8
(LSM O ) film s, have been purposely grow n on Si substrates under fixed preparation co ndition s using p ulsed laser depo sition (PLD ). U n do ped L S M O film s are the ferrom agnetic phase, and in this Fe doping range, a direct replacem ent o f Mn^+ by Fe^+ in the LSM FO films occurs [6.33]. M oreover the identical ion size of the Mn^+ and the Fe^+ ions could bypass the lattice effect, and the observed results may solely arise from the change in the electronic structure. The results presented in these film s dem onstrate that slight substitution o f M n by other 3d m etals in perovskite m anganese oxides, in a sim ilar way to that in the A -site substituted com pounds, can also produce a significant effects on the transport properties and m agnetoresistance behaviour.
6.3.1. Preparation and Characterisation
T he targ ets used h ad a nom inal co m p o sitio n o f Lao.55S ro .45M n i.o3 0 3_ô and Lao.55Sro.45M no.98F eo .i4 0 3_ô w ere m ade in-house usin g stand ard solid reaction techniques from high purity m etallic oxides and carbonates. Thin film s around 2000Â thick w ere grow n on Si (100) substrates using 4 ns pulse from a N d-Y A G laser operating at 266 nm, with a repetition frequency of 10 Hz, and a fluence around 3J/cm^. Full detailed preparation param eters can be found in Table 4.1. The structure, re sistiv ity , m a g n e to re sista n c e and m a g n e tisa tio n o f th e d ep o sited film s w as characterised by XRD, EPM A , com puter-controlled four point probe system and V SM operated at 0.75T.
Chapter Six The effect o f the lattice on the transport properties and m agnetoresistance
6.3.2. Experimental results
The chemical composition o f these two films is confirmed to be Lao.6oSro.4oMn0 3 _ 5 (LSMO) and Lao.6oSro.4oMno.9oFco.io03-§ (LSM FO) by EPM A. X-ray diffraction measurements, as shown in F ig.6.6 , revealed that as-deposited LSM FO films have a similar structure to that of La-Sr-Mn-O films grown as a perovskite-cubic structure with a (110) orientation and that no impurity phases existed in the films. The narrow FWHM of the (110) line indicates very good crystallinity. The Lao.6oSro.4oMno.9oFeo.io03-5 lattice parameter in the (110) orientation is around 3.884 Â, which is slightly larger than that of 3.883 Â for Lag ^oSro.aoMnOg.g films, indicating that the effect of Fe doping on the lattice expansion is negligible.
c 3 O _o c 60
20
30 40 50 70 28Fig. 6.6. XRD pattern o f an as-cleposited Lao,6oSro.40^ n o ,90^eo.io03.5
grown at 700°C on Si (100).
The temperature dependence of the resistivity with and without an applied magnetic field for LSMO and LSMFO films in the zero field cooled (ZFC) measurement is shown in Fig. 6.7. A maximum in the resistivity, as shown in Fig.6.7(a) and 6.7(b), is found at 185K and 230K for LSM FO and LSM O respectively, suggesting that Fe doped in the La-Sr-Mn-O can result in the peak temperature shift to low temperature. One can also observe that the largest resistivity value is increased when the Fe ions are doped, suggesting that the ferromagnetic exchange interaction between Mn^+ and Mn^+ is
C hapter Six The effect o f the lattice on the tran sport properties and m agnetoresistance 160 140
I 120
à E 100 > <u 50100
150 200 250 300 Temperature (K)Fig.6.7 The resistivity' vs temperature, at zero fie ld (a) Lao.6oSro.40^ n o .9oF(^o.loO3.5
film s and (b) Lcio,6oSro.40^ f ^0 3. s , and under a m agnetic fie ld o f 12T, (c) Lao.6oSro.4oM nO s.s and (d) Lao.6oSro.40^^no,9oFeo.wO3.5film s .
weakened [6.22]. Figure 6.7 also shows the effect of applied field on the temperature dependence of the resistivity in the field cooled (FC) measurement. It can be seen that the resistivity value for two films decreases throughout the temperature range and the resistivity maximum is shifted to high temperatures, namely at 250 and 225K for the Lao.6oSro.4oM n0 3 .g and Lao.6oSro,4oMno,9oFeo.io0 3 -5respectively, when a field of
12T is applied.
The temperature dependent magnetisation (M) for the Lao.6oSro.4oM n0 3_g and the Lao.6oSro.4oMno.9oFeo,io0 3 - 5 films is shown in F ig.
6
.8 . All measurements displayedare for the field cooled curve. The saturation magnetisation over the whole temperature range in LSM O films is higher than that of the LSM FO films. The ferromagnetic transition temperature is qualitatively observed to shift to lower temperature for the films with Fe doping. The maximum dM /dT have been used for determining the transition temperature (Tc). The Tc, as shown in Fig.6.8 (solid vertical line), is found to be 293 and 210 K for the LSMO and the LSMFO. Although the Curie temperature Tc does not coincide exactly with Tp^ax, the Tc shift to low temperature for L SM FO is highly correlated to the shifting of resistivity maximum temperature exhibited in F ig .6.8, suggesting a decrease of the overall ferromagnetic interaction between the Mn moments with Fe doping. The weakened ferromagnetic interaction and an insight into the spin structure can also be gained from measurement of the saturation magnetisation at lOOK
C hapter Six The effect o f the lattice on the tran sport pro p erties an d m agnetoresistance C O
I
100
80 60 4020
0
0 50 100 150 200 250 300 350 Tem perature (K)Fig.6.8. The temperature dependence o f magnetisation o f (a) LSM FO and
(b) LSM O film s deposited a t 700°C and an oxygen pressure o f 0.2 mbar.
as exhibited in F ig.
6
.8
. F or L SM O the saturated m om ent, with a m agnetic m om ent of3.0 |iB, is close to that expected for the theoretical value o f ~ 3.56 |Ib per M n atom.
The m agnetoresistance (M R), defined as 100% x {(p(0)-p(H )}/p(0), w here p(0) is the zero field resistivity and p(H) is the resistivity in the applied field H of 12T of these two film s as a function o f tem perature is shown in F ig.6.9. It is im portant to note that the peak of the m agnetoresistance versus tem perature relationship only appears in LSM FO films, while the m agnetoresistance is m onotonically increased for LSM O film s as the tem perature is decreased. The m agnetoresistance values obtained in the LSM FO films, how ever, are m ore than that o f the L SM O below room tem perature. E nhanced m agnetoresistance with 75% value (Ap/po under a field of 12T) at room tem perature is found in Lao.6oSro.4oMno.9oFeo.io03.5 film s.
F igure 6.10 shows the applied field dependence o f the m agnetoresistance of these two deposited films, w here it can be seen that there is saturation behaviour for either film at applied fields o f up to 7T. The m agnetoresistance of both film s are proportional to the applied magnetic field, and values o f 75% and 40% for LSM FO and LSM O respectively are obtained at room temperature.
C h apter Six The effect o f the lattice on the tran sport properties and m agnetoresistance
&
a: 100 80 60 4020
0
50 100 150 200 250 3000
Temperature (K)Fig. 6.9. The tempercitiire dependence o f magnetoresistance o f
(a) Lao.6oSro.40^no.9oFeo.io03-ôCind (h) Lao.ôoSroAO^nOs.s film s under a m agnetic fie ld o f 12 Tesla..
- 10 -20 ^ -30 - -40 S -50 -60 -70 -80 H (Tesla)
Fig. 6.10. The applied fie ld dependence o f magnetoresistance ratio at 2 9 0 K fo r (a) Lao,6oSro.40^ n O s.5 and (b) Lcio.6oSro.40^no.9()Feo.w03.ô film s. 6.3.3. Discussion
The lattice in the LSMFO, given by our XRD results, and correlated with the decreased Tc and Tp values, can not be used to explain the observed results, since a Tc and Tp shift to high temperatures should be expected when the lattice expands slightly. However, with Fe doping, there is a increased resistivity and a gradual trend toward
Chapter Six The effect o f the lattice on the tran sport p ro p erties a n d m agnetoresistance
reduced m agnetisation, indicating a gradual canting o f the m om ents w ith Fe doping. T he com petitio n betw een d o u ble ex ch an ge ferro m ag n etism and su perexch ang e antiferrom agnetism in layered antiferrom agnets can result in canted antiferrom agnetism as well as simple ferromagnetism .
The nature o f the spin coupling across the range of com position for La^.xMxMnO] (M = Ca, Sr and Ba) was first in troduced by G oodenough [6.23], w ith the hypothesis o f covalent and sem icovalent bond ing betw een the oxygen and m anganese, plus the m echanism of double exchange. In the perovskite-type m anganites, the Mn^+ with an outer-electron configuration d hybridises stable {dsp lattice oribitals and their lattice oribitals are square and coplanar w ith a single em pty d orbital, w hilst the Mn^+, with outer-electron configuration d ^ hybridises stable {d '^sp oribitals, and their lattice orbitals are octahedral with tw o em pty d orbitals and can therefore point simultaneously tow ard all six oxygen n ear neig hb ou rs if the Mn"^+ ion is in an octahedral site. Sem ivalence, exchange energy betw een the shared electron and the cation d shell electrons being greater than the energy difference betw een the em pty lattice orbital and the d-orbital energy level, can only occur below the Curie tem perature w here the net cation m agnetic m om ent is oriented. A bove the Curie tem perature there is norm al co ordinate covalence. A transition from sem icovalent to covalent bonding is accompanied by a m agnetic transition because o f the co-operative character o f m agnetic ordering. Thus the ferrom agnetic or antiferrom agnetic coupling in perovskite-type m anganites is determined by M n-O bonding type, i.e. covalent, sem icovalent and adm ixed bonding.
The M n-O bonding type in perovskite-type m anganites is highly influenced by the fraction o f Mn^+ ions, dependant on the com position. In our Lao.6oSro.4oMn0 3.§ film, the com position region is ju s t betw een the m odified type-A and the ferrom agnetic phases[6.24], and therefore a new phase w ith coexistence o f antiferrom agnetic A and ferrom agnetic A' type arrangem ents is considered. N eutron diffraction m easurem ents carried out by W olland and K oehler [6.25] have in fact shown that the two m agnetic phases can exist in this com position range. The LaFeOg, with a perovskite rhom boidal structure, is an antiferrom agnetic insulator with a G -type m agnetic cell where each ion atom presents a classical superexchange coupling with the six iron atom neighbours. It is m ost lik ely th at th e F e, rep la cin g th e M n site in L S M F O , m ain tain s the antiferrom agnetic cou pling w ith the M n via oxygen atom s, co nsidering that Fe possesses a strong d-p h yb rid isatio n and the bond w ith oxygen exhibits a typical covalent character. T he resu ltin g p hen om eno n could arise from the w eakened ferromagnetic coupling between two M n atoms as a result of a perturbation in the long-
C hapter Six The effect o f the lattice on the transport properties and m agnetoresistance
o rd er ferrom agn etic arran g em en t w hen the Fe is doped. T he m ag netisation m easurem ents in LSM O and LSM FO , as shown in F ig.6.8 (a) and (b), indicate a significant decrease in Tc and m agnetic mom ent. Our results together with Helm plt's recent experim ents [6.26] on Lao.80Sro.20M n i_xFex0 3 are highly consistent with G oodenough's theoretical prediction, indicating that other 3d metallic atoms introduced onto the Mn site favour a negative contribution to the double exchange mechanism.
It is well known that double exchange mediates ferromagnetic and metallic conduction. The transport and m agnetic results shown above clearly dem onstrate that partial replacem ent on Mn by Fe favours insulating and AF behaviour, opposing the effects of the double exchange. Since Fe doping is the direct replacem ent of Mn^+ by Fe^+, the experim ental results suggest that the sites that are now occupied by Fe^+ can no longer effectively participate in the double exchange process. The m echanism that Fe^+ terminates the double exchange process arises purely from the electronic structure of the materials as we describe in the following.
In the pervoskite oxides, the d electrons of the Mn and Fe ions are known to stay in the same spin state, due to the strong Hund's coupling. The vacant state with opposite spin lies higher in energy due to exchange interactions. Both occupied and unoccupied spin
Mn
1.71-1.86 eV
0.14-0.29 cV
Figure 6.10.1. Band structure o fF e and Mn in perovskite
states are further split into iig and eg orbitals, by the octahedral crystal field. In increasing energy, these states are t2gT, egt, t2gi and eg |. The electronic configuration are t2gî^ CgT^ for Fe^+. t2gT^ Sgf ' for Fe^+, and Mn^+, and t2gT^ for Mn^+,
C h apter Six The effect o f the lattice on the tran sport properties an d m agnetoresistance
respectively. In a solid, these states form bands. F or these ions, the t2g t bands are fully occupied, the t2gf and egj, bands are em pty, and the e g t bands, as show n in Fig.6.10.1, which can accom m odate a m axim um o f tw o electrons per ion, play a crucial role. In a m ixed system o f Fe and M n, the w idths and energies o f their eg t bands dictate the electron distribution of the Fe and M n ions.
Early study on the conductivity Lao.gsBao.isMni.xFcxOs by Jonker [6.27] has shown that for 0<x<0.85, Fe3+, Mn^+, and Mn^+ are present. The existence o f Fe^+, Mn^+, and Mn^+ in the range o f 0<x<0.85, indicates that the Fe egt band is full and the Mn egt band and a m ore than half-filled Fe egt band. From this, it can be inferred that the bottom of the M n egt band should be at the same level as, or higher than, the top o f the Fe egt band. Thereby, the Fe egt band rem ains com pletely filled as long as the M n egt band is partially filled.
My system of La0.60Sr0.40M n0.90F e0.10O3 is expected to have a sim ilar band structure as recent study on the La%.xCaxMnxFe %.xOg [6.28] The nom inal stoichiom etry is La^+o 6oSr^+0.40 ( Mn"^+xMn3+i_x.y)Fe3+yO'^3, for w hich both the M n t2gt and Fe t2gt bands are filled. For the all im portant eg t bands, the Fe eg t band is com pletely filled and (l-x -y )/2 (l-y ) o f the M n egt band is also filled. The latter filling factor is one half of the Mn eg band has been estim ated to be about 1 eV [6.29]. A ssum ing uniform filling for sim plicity, and that the overlapped width o f the Fe and M n egt bands is 3%, the Ferm i level w ould lie at [( 1 -x-y)/2( 1 -y)-0.03] eV above the top o f the Fe egt band. U sing the com position values of x and y for our series, the Ferm i level will lie 0.14 to 0.29 eV above the top o f the Fe egt band. This energy diagram clearly illustrates that electron hopping between Fe and M n is im peded by the lack o f available states in the Fe egt band. The only vacant states are in the Fe t2gf band, lying above the Fe eg t band, as shown in Fig 6.10.1. H ow ever, C hainani have show n [6.30] that LaFeOg is an insulator with an intrinsic gap of about 2.0 eV, which im plies that the Fe t2g i band is located about 2 eV above the top o f the Fe egt band, or 1.71-1.86 eV above the Ferm i surface for my system. It is clear that electron hopping from M n to Fe is energetically forbidd en even at roo m te m p eratu re. C o n seq u en tly , on the M n e g t band is electronically active, where electron hopping can occur between Mn^+ and Mn"^+. Since Fe^+ replaces Mn^+, doping with Fe causes a depletion o f the Mn3+/Mn'^+ ratio, the population o f the hopping electrons, and the num ber o f available hopping sites. Thus double exchange is suppressed, resulting in the reduction o f ferrom agnetism and metallic conduction.
C h apter Six The effect o f the lattice on the tran sport p ro p e rtie s an d m agnetoresistance