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The com position of film s deposited at 700°C in 0.2 m bar o f oxygen w as confirm ed to be Lao.6oSro.4oMn03.5 (LSM O ) by EPM A. X -ray diffraction m easurem ents revealed that these layers grew as a cubic structure with a (110) orientation, as show n in Fig. 5.1. The lattice param eter w as found to be a=3.883± 0.001 Â, slightly bigger than that of a=3.878± 0.001 Â of LC M O films. The FW H M value o f the (110) line for all the films grown was typically less than 0.24°, indicating a very good crystallinity.

The tem perature dependent characteristics of the resistivity with and w ithout an applied m agnetic field is show n in F ig .5.2 for the as-deposited LSM O thin film s. The magnetoresistance as a function o f temperature is also shown in the same figure. At

C hapter Five Growth o f La-Sr-M n-O and La-N d-Sr-M n -0 film s and

Fig.5 .1. XRD pattern o f L ao .o o ^ro jo ^n O s.sfilm grown on Si ( 100) at 700°C and an oxygen pressure o f 0.2 mbar.

E o à > a: 12 10 8 6 4 2 — 0 250 300 50 100 150

200

Fig.5. 2. Tem perature dependence o f the resistivity o f Lao.6oSro.4oMnOs.§ film s deposited at 700°C in an 0.2 m bar o f oxygen (a) without an applied m agnetic field, (b) with an applied fie ld o f 12T and (c)

magnetoresistance.

zero field a maximum in the resistivity versus temperature relationship is observed at 230K (Fig.5.2 (a) ), which is much higher than that, 157K, of the L a-C a-M n-0 films, with semiconducting behaviour (i.e., a negative dp/dT) above and metallic behaviour (a

C hapter Five G rowth o f La-Sr-M n-O an d L a-N d-S r-M n -0 film s a n d.

positive dp/dT ) being apparent below this peak tem perature. T he resistivity o f 10.6 m Q cm in L a-S r-M n-O is also m uch less than that, 25 O cm , o f L a-C a-M n -O , indicating an inherent difference in structure betw een L a-S r-M n-O and La-C a-M n-O materials. It is interesting to note that the suppression o f resistivity (p) in the m agnetic field persists ov er the w hole tem perature range, as show n in F ig .5.2 (b), and the m axim um resistivity tem perature is shifted upw ards as an external field is applied. The m agnetoresistance as a function o f tem perature is also show n in F ig .5.2 (c). The m agnetoresistance, defined as A p/po = p(H = 0)-p(H = 12T )/p(H = 0), is found to increase with a decrease in temperature, unlike the LCM O films on LaA10g[5.1], where a m axim um m agnetoresistance is exhibited at a certain tem perature. A value o f m ore than 37% for as-deposited LSM O film w as obtained at room tem perature under a field o f 12T.

F igure 5.3 show s the m agnetisation o f as-deposited LSM O film s as a function o f tem perature m easured at an applied field of 0.75T. The m agnetisation o f the LSM O film s is gradually increased w hen the tem perature is decreased to 330K , and then sharply increases in passing through the tem perature range from 330K to 270K. The Curie tem perature, defined as the m axim um in dM /dT versus T, is exhibited at 293 K (vertical line in Fig.5.3 ), w hich is consistent with the previous theoretical prediction [5.12]. The m agnetisation o f the LSM O film s quickly tends to saturation w hen the temperature reduces to lOOK.

100

20

Fig. 5. 3. Temperature dependence o f the magnetisation, M, under a 0.75 Tesla fie ld applied fo r the LSM O film s deposited at 700°C in

Chapter Five Growth o f La-Sr-M n-O and L a-N d-Sr-M n -0 film s and

Figure 5.4 shows the hysteresis loop of as-deposited LSM O films measured by VSM at a field up to 0.75T and at room tem perature. The films exhibited a fully ferrom agnetic state. The magnetic moment of LSM O films sharply increases with an applied field

M agnetic m o m en t (em u) 0.01 r 0.008 } 0.006 0.004 0.002 -8000 1. -4000 -2000 2000 4000 eooo 8000 A pp lied Held (O e) -0.002 -0.008 - -0.01 -

Fig. 5.4. H ysteresis loop o f a LSM O film m easured at room temperature under a m agnetic fie ld up toO. 75T.

H (Tesla)

Fig. 5. 5 . The fie ld dependence o f the magnetoresistance ratio at (a) 77K and (h) 300K fo r Lao,60^^o.40^ n O3.5 film s.

C hapter Five Growth o f La-Sr-M n-O and L a-N d-Sr-M n -0 film s and

below 0 .0 IT, and then increases rate slows down with the applied field up to 0.5T and tends to saturation gradually when the applied field is further increased above 0.5T.

The dependence of the m agnetoresistance at 300K on applied field for the as-deposited LSM O films grown at 700°C in an oxygen pressure of 0.2 m bar is shown in Fig. 5.5. The m agnetoresistance did not saturate under applied fields o f up to 12T, and M R values o f more than 37% w ere obtained at 300K. It is interesting to note, in contrast with F ig .5.4, that m agnetisation readily tends to saturation w hile m agnetoresistance saturation was not observed under magnetic fields up to 12T.