Giant magnetoresistance in manganese oxides

32

Giant magnetoresistance in manganese oxides CNR Rao* and Rajappan Mahesh Giant magnetoresistance (GMR) was known to occur in metallic bilayers and granular materials. In 1993, GMR was also found to occur in thin films of manganates of the formula Lat_xAxMnO3 (A=alkaline earth metal). Since then, there has been intense research activity relating to the GMR of rare earth manganates and other oxides.

Addresses Solid State and Structural Chemistry Unit and CSIR Centre of Excellence in Chemistry, Indian Institute of Science, Bangalore 560 012, India "e-mail: [email protected] Current Opinion in Solid State & Materials Science 1997, 2:32-39 Electronic identifier: 1359-0286-002-00032 © Current Chemistry Ltd ISSN 1359-0286

Abbreviations CO charge ordering EXAFS extended X-ray absorption fine structure GMR giant magnetoresistance H magnetic field I--+M insulator--rmetal MR magnetoresistance Tc Curie temperature Tim insulator-->metal transition temperature TN Neel temperature

Introduction

Magnetoresistance (MR) is the relative change in the resistivity of a material caused by the application of a magnetic field as defined by the equation %MR=(pH -

po)x lO0/po

where PH and P0 represent the values of resistivity or resistance in the presence and absence of a magnetic field respectively. T h e sign of M R can be positive or negative. While most of the metals give a few % M R (-5%), giant magnctoresistance ( G M R ) was first observed in metallic muhilaycrs and granular materials [1]. In 1993, G M R was reported in thin films of perovskite oxides of the formula Lal_xAxMnO 3, where A is an alkaline earth metal [2,3]. Since the reports of G M R in these films, the p h e n o m e n o n has b e e n studied in extensive detail in a variety of manganates in thin film, single c u s t a l [4"] and poiycrystal [5"] forms. T h e intense interest in this field arises from potential applications in magnetic recording, actuators and sensors. We shall describe the main features of rare earth manganates and other oxide materials with respect to G M R and related properties and present an account of certain novel features of these solids, such as the high resistivity even in the metallic state and charge ordering.

Rare earth manganates GMR and related properties

T h e parent manganate, LaMnO3, is an orthorhombic perovskite with distorted MnO¢~ octahedra (with three different M n - O distances) arising from J a h n - T e l l e r distortion. L a M n O 3 is antiferromagnetic and its magnetic structure is well understood [6]. W h e n La in LaMnO3 is replaced by a divalent ion such as Ca, St, Ba or Pb, Mn 4+ ions are created. W h e n the Mn 4+ content is sufficiently large, the material becomes ferromagnetic and exhibits an insulator--+metal (I-->M) transition amt, nd the Curie temperature, T c, because of the Z e n e r double exchange mcchanism [71. In Figure 1 we show the magnetization and electrical resistivity behaviour of a sample of Lal_xCaxMnO 3 as an illustration of the Z e n e r double exchange mechanism. Application of a magnetic field decreases the resistivity m a r k e d l y around T c (see Fig. 1), the decrease being m a x i m u m around the Curie temperature. Around 30% Mn 4+ is the o p t i m u m for observing both ferromagnetism, and G M R effects in Lal_xAxMnO 3. T h e changes in the electronic and magnetic properties of these manganates with the change in composition have been worked out in several systems and we show typical electronic and magnetic phase diagrams of Lal_xAxMnO 3 in Figure 2 [4"°,8"]. We notice that when x < 0.2 the material is an antiferromagnetic insulator. Changes in structure are also noticed with a change in x and many of the manganates become rhombohedral when x >0.3. M e a s u r e m e n t s on Lal_xSrxMnO 3 [4 °° ] have shown that the resistivity scales with magnetization, M, as A p / p = C(M/Ms)2 for M/Ms <0.3 where M s is the saturation magnetization and C is a constant. G M R has been observed in a variety of rare earth manganates of the formula Lnl_xAxMnO 3 ( L n = r a r e earth, A = C a , St, Ba or Pb) [9-11]. In many of these systems, G M R reaches nearly 100%: however, the m a x i m u m G M R is generally observed below 300K. In Lal_xPbxMnO 3, high G M R is found around room temperature or above, but at high magnetic fields [11]. In Lal_xCaxMnO3, although G M R is found in con> positions with x <0.5, as described earlier, the x=0.5 composition exhibits a structural change involving a decrease in the b parameter and an increase in the a and c parameters below the ferromagnetic To: this change is accompanied by the onset of antiferromagnetism and charge ordering (ordering of Mn 3+ and Mn 4+ ions) [12]. W h e n x=0.2, a local structural distortion occurs due to the formation of lattice polarons which arise from the metal--,,insulator transition [13"]. E X A F S has revealed significant changes in the local structure in the x=0.3

Giant magnetoresistance in manganese oxides Rao and Mahesh

Figure 1

33

Figure 2

(a)

I

(a)

I

28OI-- O TN o Tc

Lao.sCao zMn03

PMI

Lal_xCa x M n O

3

M(?AFIMI

)

0

[3

[3

% % [3

7 0

(a)

(b

li

I

0 --

•

•

•

--

2

2

I

20

4O

6O "/o Ca

,OT

%

(b)

•

8O

IO0

5OC i I

o

PMI

•

2

J

6T'*

F~,4M

[~ I

400

Tc

:~;.Z.::.;:.:;:.~.~,..-""" ...... ""Q?...:.:: 300

0

;

I

(c)..; 6C

[

i

1

E 2OO

i

_,m"~

H : 6 Tesla

TN / I0C

_

\

--

~, ',

LO1_xSrX MnO 3

I I

cs]

i

t

I

I

0

100

200

300

'~

i

400

',FM: ,, 'd

10

(b) I

I

20

~

1

30 °/, Sr

i

[

40

L 50

60

T(K)

Temperature variation of the magnetization and electrical resistivity of Lao.sCao.2MnO3 illustrating the Zener double exchange mechanism. (a) Magnetization, M. (b) Resistivity, p, at H =OT and H=6T. (c) Magnetoresistance, 16p/p(o)l(0/o). Adapted with permission from [5"].

composition in the 80-300K range; these changes wcrc attributed to the formation of small polarons due to the J a h n - T e l l e r distortion when T > T c [14]. Lal_xSrxMnO 3 is somewhat different from Lal_xCaxMnO 3. Thus, the ferromagnetic-metallic regime in Lal_xSrxMnO 3 appears to extend over larger values of x and the Curie t e m p e r a t u r e s are also considerably higher (Fig. 2). T h e electronic transport and magnetic properties of the x = 0.3 compositions of both the Ca and Sr compositions have been investigated in thin films as well as in bulk materials [15]. While the magnetization decreases with T 2, the resistivity increases proportional to T 2. T h e spin and lattice dynamics of La0.7Sr0.3MnO 3 have been e x a m i n e d by neutron scattering [16]. Strangely the spin

Temperature-composition diagram of Lal_×AxMnO3. In (a) the alkaline earth metal is Ca, whereas in (b) it is Sr. CSI, canted-spin insulator; FMI, ferromagnetic insulator; FMM, ferromagnetic metal; AFMI, antiferromagnetic insulator; PMI, paramagnetic insulator; PMM, paramagnetic metal. Adapted with permission from [4",8"].

dynamics and the magnetic critical scattering sccm to be comparable to those of typical metallic fcrromagnets. In Lal_xSrxMnO,~ (x=0.17), the local spin moments and charge carriers couple strongly to changes in the structure, which can be altered by the application of a magnetic field d e p e n d i n g on the temperature [17l. For 0.1 < x < 0 . 1 7 , ferromagnetic ordering suppresses the lattice distortion at and below T c [18]. W h e n x=0.1 and 0.15, a polaron ordered phase is formed: thc polaron lattice has a t e n d e n c y to lock into a commensurate structure when x=0.125 [19"]. In Pro.7Ca0.3_xSrxMnO 3 a decrease in the J a h n - T e l l e r distortion has been noticed at the magnetic transition [201. We would expect a decrease in J a h n - T e l l e r interaction in the metallic ferromagnetic phasc since the d o u b l e - e x c h a n g e and J a h n - T e l l e r interactions c o m p e t e with each other in the rare earth manganates.

34

Electronic materials

Hydrostatic pressure stabilizes the ferromagnetic metallic state of Lal.xAxMnO 3 (i.e. it increases the T c) [21,22]. There is a direct relationship between T c and the average radius of the A-site cation ( [23°'], the Tc increasing with [23"°,24,25]. Increasing has the same effect as increasing pressure, indicating the importance of the Mn-Mn transfer integral and changes in the M n - O - M n bond angles. It should be noted that small distorts the MnO6 octahedra by bending the M n - O - M n bond, in turn causing the narrowing of the eg bandwidth to a greater extent. Accordingly, yttrium substitution in Lal-xAx MnO3 decreases T c and enhances the magnetoresistance [26-28]. By plotting the T c against , one obtains a phase diagram separating the ferromagnetic metal and paramagnetic insulator regimes [23°',29"]. In Figure 3 we show a typical phase diagram obtained in this manner. In the left hand bottom corner, we see the ferromagnetic insulator regime. GMR generally decreases with an increase in as does the peak resistivity at Tc or at the insulator--+metal transition point [29°']. Accordingly, GMR is favoured by a low T c and high peak resistivity [5°]. Enhanced GMR has been considered to arise from a reduced mobility of the doping holes and an increase in the coupling between the localized and itinerant electrons [25]. T h e effect of particle size on the electron transport and magnetic properties of La0.7Cao.3MnO 3 has been investigated. Although T c decreases with decreasing particle size, the magnetoresistance is insensitive to particle size [30°,31]. Samples of manganates with different particle sizes prepared with different heat treatments were found to possess large differences between their ferromagnetic transition temperatures and between the temperatures at which the I--+M transition took place [32]. In Lal.xCaxMnO 3 films, MR is strongly dependent on the film thickness and reaches a maximum around 100 nm [33]. T h e effect of dimensionality on G M R and properties relating to GMR have been studied by examining the (SrO) (Lal_xSrxMnO3) n family [34°,35]. The n = l member which has the two-dimensional K2NiF 4 structure, is an insulator, while the n =o~ member is the three-dimensional perovskite showing GMR and other properties. T h e n = 2 member exhibits the sharpest I--+M transition and high MR, suggesting that the in-plane antiferromagnetic interaction competes with the double-exchange interaction. T h e MR in rare earth manganates shows a magnetic field dependence with two distinct regimes in polycrystalline materials [5"]. T h e initial sharp change in MR is due to domain wall motion and the M R - H curve generally parallels the magnetization-H curve. Two such distinct regimes are not seen when T < T c . Single crystals do not seem to show two regimes in M R - H behaviour. Zero-field muon spin relaxation and resistivity experiments on La0.67Ca0.33MnO 3 [36] show that the sublattice magnetization x)ta(T) is well described for T_
where 1~=0.345+0.015. Below Tc, x)ta and the zero-field resistivity P0 are correlated, with a~gct-ln P0. Unusual relaxational dynamics suggest spatially inhomogeneous Mn-ion correlation times. A variety of investigations have been carried out on Lal.xAxMnO3 films prepared by, molecular beam epitaxy (MBE), pulsed laser deposition and other techniques. Block-by-block MBE has been employed to deposit La0.7Ca0.3MnO 3 films [37], while La-deficient LaMnO3 films have been obtained by laser deposition [38]. An interesting memory effect is found in Nd0.7Sr0.3MnO3 films [39]. T h e l/f resistance noise measurements of thin films of (La,Y)l_xCaxMnO3 show a l/f power law across the I--+M transition [40]. An enhancement of MR in Lao.TCao.3MnO3 and related systems is the result of the replacement of La by a smaller rare earth metal, as mentioned earlier. Bulk La0.33Nd0.33Ca0.33MnO 3 is found to exhibit 96% GMR at a low field of--0.7 T [41]. An improvement in MR response has also been obtained by sandwiching the manganates with soft ferromagnets [42°]. These observations suggest flexibility in the material design in order to obtain optimal G M R properties. What we require for applications are materials with high GMR at ordinary temperatures and low H. Certain unusual features

Electrical, magnetic and other measurements on Lal_xAxMnO 3 systems have revealed certain unusual features with respect to the charge carriers in these oxides. For example, the manganates exhibit very high resistivitics, particularly at low temperatures (<100K) [43°]. T h e values of resistivities are considerably higher than Mott's maximum value of resistivity [5°], which is a few million ohm cm, the resistivity reaching values as high as 103-104 ohm cm in the so-called metallic state. Optical conductivity measurements [44 °] on La0.825Sro.125MnO 3 in the range 0-10eV exhibit a band of 1.5eV and the spectral weight is transferred from this band with decreasing temperature. T h e 1.5eV band is due to interband transitions between the exchange split spin polarized eg bands. At T < Tc, intraband transitions in the eg band dominate the spectrum [44°]. A similar transfer of spectral weight from high energy to low energy occurs at T < T c in Nd0.7Sr0.3MnO 3 [45]. Photoemission and related studies of Lal_xSrxMnO 3 [46°°,47] have provided valuable information on the electronic structure of the manganates. There is a negligible density of states near the Fermi level, even in the metallic state in these systems. There is a transfer of spectral weight from the unoccupied states to occupied states with a decrease in hole doping. T h e redistribution of spectral weight in the photoemission spectra of La0.6Sr0.4MnO3 is attributed to a change in the electron correlation strength induced by the changing degree of ferromagnetic order with temperature [47]. T h e density of states at the Fermi level is much lower in

Giant magnetoresistance in manganese oxides Rao and Mahesh

35

Figure 3 Variation of T c (Tim) of Ln1.xAxMn03 with the weighted average radius of the A-site cations, . FMI, ferromagnetic insulator. Adapted with permission from 12g"].

0 • O •

L°Mn03 LOl-xAxMnO3 (A Ca,Sr,B%Pb)

/~. •

Lol_xAx_zYzMnO3 ( A =Ca ,Sr )

O l

Ndl_ x AxMnO3 (A - Co,Sr)

•

PARAMAGN ETIC INSULATOR f

0

O

f

FERROMAGNETIC METAL

/ / o/ /

FMI

/

I

OL1.16 ~

the insulator regime than in the metallic regime of the manganates [48]. Mn 2p resonant photoemission and O ls X-ray absorption studies have shown that the band gap collapses around Tc with an increase in the density of states near the Fermi level on cooling [49]. In the insulating state above Tc, small polaron effects seem to be present, T h e Seebeck coefficient in several rare earth manganates shows a positive peak in the composition range (x-0.3) and at a temperature where MR also peaks and becomes more negative with increase in Mn 4+ content [5°]. Seebeck coefficient measurements in Lal_xSrxMnO3 show a change in sign from positive to negative either at the I---~M transition boundary or at T < T c , depending on the composition [50]. A possible anomalous change in electronic structure with spin polarization may occur when x= 0.2-0.3. On the basis of the pressure dependence of the Seebeck coefficient it has been suggested that there is a cross-over from a polaronic to an extended state electronic conduction at Tc [51]. T h e occurrence of ferromagnetism, I---~M transition and G M R in the rare earth manganates has been considered to be a manifestation of double exchange by many workers, although, it appears necessary to involve electron-phonon interaction to explain the electrical resistivity behaviour in these materials [52°°]. Such lattice effects decrease with Tc and become maximum at T c, depending on the value of x [53]. T h e importance of electron-phonon

1.20

1.24

<© ci)

1.28,

1.32

1.36

interaction is underscored by the observation of the large oxygen isotope shift of the T c in La0.sCa0.zMnO 3 [54°]. Besides lattice effects, important correlation effects also yield a large distribution of spectral weights [55]. Clearly, it is difficult to understand the electrical resistivity of the rare earth manganates across the I---)M transition, especially at low temperatures, based on double exchange or electron-phonon interaction alone. Several other factors would have to be taken into account to fully explain the resistivity behaviour of the manganates. Charge ordering

T h e study of G M R in Lnl.xAxMnO 3 has brought forth novel features related to the charge and spin dynamics in these oxides. Charge ordering in the manganates is interesting because double exchange gives rise to metallicity along with ferromagnetism, while the charge-ordered state is associated with insulating and antiferromagnetic (or paramagnetic) behaviour. T h e charge-ordered state can be melted into a metallic ferromagnetic state by the application of a magnetic field. An examination of the observations on the rare earth manganates [56 °°] reveals two types of scenarios, as exemplified by Ndo.sSro.5MnO 3 and Prl.xCaxMnO 3 (0.3
36

Electronic materials

depicted in Figure 5, where we show the phase diagram for Prl_xCaxMnO 3 as well.

Figure 4

104

OT

._. 102 E

]'co

U

,1

6T lo o ul

n,

162 12T

O

1OO 200 Tern perut ure (K)

300

Temperature variation of resistivity of Prl_×CaxMnO3 (x=0.35) at H=0T, 6T and 12T. Adapted with permission from [59°].

to be of different types (CE and A respectively). The competition between the ferromagnetic double-exchange interaction and the antiferromagnetic charge ordering instabililty has been examined in (Nd,Sm)0.sSr0.sMnO 3 [61,62]. This system exhibits a large MR which is strongly coupled to the lattice restriction. The GMR phenomenon here, can be attributed solely to a first order I--+M transition induced by a magnetic field, accompanied by a metamagnetic transition and a structural change. In Prl_xCaxMnO 3 (x=0.35), an insulating charge-ordered state (Tco-200K) becomes antiferromagnetic around 140K (T N) and then undergoes a transformation to a cantedspin antiferromagnetic state at a still lower temperature ( - l l 0 K ) [59°]. T h e application of a magnetic field melts the charge-ordered state in all the charge-ordered manganates, giving rise to metallic behaviour (Fig. 4). The first-order I---~M transition induced by magnetic fields is generally accompanied by considerable hysteresis. The I--+M transitions in Ndo.sSro.sMnO 3 and Pr0.sSro.sMnO3, where the one-electron bandwidth is controlled by the composition, show interesting electronic phase diagrams [56"] in the temperature-magnetic field ( T - H ) plane, as

Charge ordering in the manganates is governed by the width of the eg band, which is directly determined by the weighted average radius of the A-site cations , or the tolerance factor. This is because a distortion of the M n - O - M n bond angle affects the transfer interaction of the eg conduction electrons (holes). We can describe the spin and charge ordering phenomena in the rare-earth manganates in terms of the generalized phase diagrams shown in Figure 6. The diagram shows that when is large (e.g. as in Lal_xSrxMnO3), only ferromagnetism and the associated I-+M transition occur, with no charge ordering. With decreasing , the ferromagnetic (charge-liquid) state transforms to the antiferromagnetic charge-ordered state on cooling (e.g. as in Nd0.sSr0.sMnO3). When is very small, as it is in Pr0.7Ca0.3MnO 3 and Nd0.sCa0.5MnO3, no ferromagnetism occurs, and only a charge-ordered state is found; the ferromagnetic metallic state can only be created by the application of the magnetic field in the charge-ordered state. Charge ordering occurs over a wide composition range in Ndl_xCaxMnO 3 [63]. A recent study of Nd0.sCa0.sMnO 3 [64] has shown a transition around 200K with a large change in volume, due to charge ordering; the charge-ordered state can be melted by the application of a magnetic field. In this manganate the resistivity showed an unusual field dependence and the magnetoresistance exhibited hysteresis [65]. T h e positive MR shown by this oxide was considered to originate from antiferromagnetic ordering in the presence of a ferromagnetic interaction. T h e regime between 1.18 and 1.22~. (the hatched region in Fig. 6) is interesting and one would expect to see both charge and ferromagnetic ordering. A study of Lal_xCaxMnO3 in this region has been carried out [66"]. Charge ordering in this system was accompanied by a dramatic increase in the sound velocity, suggesting the importance of the eleetron-phonon coupling. At the charge ordering temperature, these oxides exhibit an anomaly in d (ln9), AC/T (AC =heat capacity) and sound velocity. Charge ordering in the 2D compound La0.5Srl.sMnO 4 has received attention [67",68]. Superlattice reflections corresponding to alternating Mn3+ and Mn 4+ ions have been observed below the charge-ordering temperature (217K), while below the T N ( - l l 0 K ) the magnetic moments are ordered with a 2 ,2a×2#2a×2c cell. A structural phase transition related ~' to charge ordering and a metamagnetic transition has been observed by electron diffraction producing an orthorhombic unit cell at low temperatures.

GMR in other oxides GMR in oxides is not specific to either perovskite structure or the manganates. For example, G M R has been found in T12Mn207, which has the pyrochlore structure

Giant magnetoresistance in manganese oxides Rao and Mahesh

37

Figure 5 The electronic phase diagrams in the T-H plane of (a) Prl.xSrxMnO3 (x::O.5) (b) Ndl.xSrxMnO3 (x==0.5) and (c) Prt.xCaxMnO3 (x==0.35). Reproduced with permission from [56°'].

(a)

2501

2COI

'5Ol ~

IOO|

50t

.)(3(

Prl. x SIx MnO3 (x= 0.5)

•1•

/ / / / / /II - / / / //// / ///q

(No FIdM) / / / / / /

FM cool ~CO

FMM (No CO)

/ " / // / / // / / // / / / / / // / / / 4/ / / / J "//// /-- / ' / / / ' / / /--/'! / /////////--/ //1 // / // i I//.,I • / / l/t~ / ////I ,! / // / / -/ / / / I/ / /x/ / / / / / / / 4 u~. ¢ / / / / / / / ////I d~ll 0 rr) l /

.,,,/

=

//

/

c; o L / / _ / 0 0 ;/ / / /

z

/

//~.,,=

/_~',=o// /.. .-

t. , I / / / / / / / j

~

-

/ / .

/

/ //1

/ /i ~ x //I t~

~ ~n0

/(9 / / A _

l"//.':////J///~'v'/'.t 1.18 1.20

=

I I , 1.22

= I-°"

o

.,~////-//~/.'/Jz

k':;/..',~/v./ V///I I'_/ , ' / " / ~ ' , / } G " . / . ' I I

~

I

~1 I ~1 1.24

Ndt_xSrx MnO3 ( x= 0.5]

i °°

Metal

Figure 6

/ - t- / ~/./- - / / / ./- / / // / - - / / -

(b)

25O

(c)

2O0

~15O

t 1.26

Condusions Research on G M R of manganates and other oxidic materials will undoubtedly be pursued vigorously in the years

o

4 8 12 Magnetic f;eld (T)

0

0

4 8 12 Mognetic field(T)

R e f e r e n c e s and r e c o m m e n d e d r e a d i n g Papers of particular interest, published within the annual period of review, have been highlighted as:

W(eg)

[69,70°°]. This oxide has only Mn 4+, but shows G M R and ferromagnetic Tc. At Tc, there is a paramagnetic metal tO ferromagnetic metal transition rather than an I---)M transition. Since there cannot be any double exchange occurring in this system, the cause of GMR has to be different than in the Lanthanide systems described so far. Lnl.xAxCoO 3 is also a candidate for GMR because, at a particular value of x, the material becomes ferromagnetic and metallic. G M R has been found in some of these cobalt oxides in the insulating regime [71].

o

to come, not only because of their fascinating properties and phenomena but also because of their potential technological applications. There are many aspects of manganates that are yet to be properly understood, the unusual electronic properties across the metal ---)insulator (M--~I) transition being foremost amongst them. T h e r e is a need for careful structural and theoretical studies in this direction. T h e different types of charge-ordering and magnetic behaviours exhibited by manganates also require further investigation. It is necessary to establish beyond all doubt that TI2Mn207 has no Mn3÷ to be certain that the double exchange mechanism does not operate in this material.

• of special interest ee of outstanding interest

C) 1997 Current Opinion in Solid State & MaterielsScience

The phase diagram showing the prevalence of charge-ordered (CO) and ferromagnetic states in Lnl.xAxMnO3, depending on the weighted average radius of the A-site cation, , or eg bandwidth. The hatched region shows complex magnetic and other properties. FMM, ferromagnetic metal; FM, ferromagnetic; W, bandwidth.

50

5c

Non~

g;i'

Lo

oc

I I 4 8 12 Magnet;c field (T)

CU A

>.5(

1.

Levy PM: Giant magnetoresistance in magnetic layered and granular materials. Solid State Phys 1994, 47:367-462.

2.

Chahara K, Ohno T, Kasai M, Kozono Y: Magnetoresistance in manganese oxide with intrinsic antiferromagnetic spin structure. Appl Phys Lett 1993, 63:1990-1992.

3.

Jin S, Tiefel TH, McCormack M, Fastnacht R, Ramesh R, Chen LH: Thousandfold change in resistivity in magnetoresistive La-Ca-Mn-O films. Science 1994, 264:413-415.

4. ••

Urushibara A, Moritomo Y, Arima T, Asamitsu A, Kido G, Tokura Y: Insulator-metal transition and giant magnetoresistance in Lal.xSrxMnO3. Phys Rev B 1995, 51:14103-14109. Gives the electronic and magnetic phase diagrams for Lal.xSrxMnO3. 5. •

Mahendiran R, Tiwary SK, Raychaudhuri AK, Ramakrishnan TV, Mahesh R, Rangavittal N, Rao CNR: Structure, electron-transport properties and giant magnetoresistance of hole-doped LaMnO 3 systems. Phys Rev B 1996, 53:3348-3358. Discusses GMR and other properties of Lal.xAxMnO3 and defect LaMnO3 samples in the polycrystalline form. 6.

Solovyev I, Hamada N, Terakura K: Crucial role of the lattice distortion in the magnetism of LaMnO3. Phys Ray Lett 1996, 76:4875-4878.

7.

Zener C: Interaction between the d-shells in the transition metals II. Ferromagnetic compounds of manganese with perovskita structure. Phys Rev 1951, 82:403-405.

38

Electronic materials

8. •

Schiffer P, Ramirez AP, BaD W, Cheong SW: Low temperature magnetoresistance and the magnetic phase diagram of Lal.xCaxMnO3. Phys Rev Let/1995, 75:3336-3339. Gives the phase diagram for Lal.xCaxMnO3, 9.

MaignanA, Caignaert V, Simon Ch, Hervieu M, Raveau B: Giant magnetoresistance properties of polycrystalline praseodymium-based manganese perovskites: from Pro.75Sro.25MnO3.6 to Lao.75Sro.25MnO3. J Mater Chem 1995, 5:1089-1091.

10.

Caignaert V, Maignan A, Raveau B: Up to 50 0DO percent resistance variation in magnetoresistance polycrystalline perovskites Ln2/3Srl/3MnO 3 (Ln =Nd; Sm). Solid State Commun 1995, 95:357-359.

11.

MahendiranR, Mahesh R, Raychaudhuri AK, Rao CNR: Room temperature giant magnetoresistance in Lal.xPbxMnO3. J Phys D Appl Phys 1995, 28:1743-1745.

12.

RadaelliPG, Cox DE, Marezio M, Cheong SW, Schiffer PE, Ramirez AP: Simultaneous structural, magnetic and electronic transitions in Lal.xCaxMnO 3 with x=0.25 and 0.50. Phys Rev Let/1995, 75:4488-4491.

27.

MaignanA, Simon C, Caignaert V, Raveau B: Colossal magnetoresistance properties of the manganese perovskites Lao.7.xYxCa0.3MnO3.~. J Appl Phys 1996, 79:7891-7895.

28.

MahendiranR, Mahesh R, Raychaudhuri AK, Rao CNR: Effect of Y substitution in La-Ca-Mn-O perovskites showing giant magnetoresistance. Phys Rev B 1996, 53:12160-12165.

29. •,

MaheshR, Mahendiran R, Raychaudhuri AK, Rao CNR: Effect of the internal pressure due to the A-site cations on the giant magnetoresistance and related properties of doped rare earth manganates Lnl.xAxMnO3 (Ln=La, Nd, Gd, Y; A=,Ca, Sr, Ba, Pb). J Solid State Chem 1995, 120:204-207. Gives a phase diagram of Tc against the average radius of the A-site cation. 30. •

MaheshR, Mahendiran R, Raychaudhuri AK, Rao CNR: Effect of particle size on the giant magnetoresistance of La0.7Cao.3MnO3. Appl Phys Lett 1996, 68:2291-2293. Demonstrates that GMR is insensitive to particle size.

31.

Sanchez RD, Rivas J, Vazquez CV, Quintela AL, Causa MT, Tovar M, Oseroff S: Giant magnetoresistance in fine particle of La0.sTCa0.33MnO3 synthesized at low temperatures. Appl Phys Let/1996, 68:134-136.

Brillinge SJL, DiFrancesco RG, Kwei GH, Neumeier JJ, Thompson JD: Direct observation of lattice polaron formation in the local structure of Lal. x CaxMnO3. Phys Rev Let/1996, 77:715-718. This paper shows how the x=O.2 composition has a local distortion due to the formation of lattice polarons.

32.

MahendiranR, Mahesh R, Raychaudhuri AK, Rao CNR: Resistivity, giant magnetoresistance and thermopower in Lao.TSr0.3MnO 3

14.

TysonTA, Mustre de Leon J, Conradson SD, Bishop AR, Neumeier JJ, Roder H, Jun Zang: Evidence for a local lattice distortion in Ca-doped LaMnO 3. Phys Rev B 1996, 53:13985-13988.

33.

15.

Snyder JG, Hiskes R, DiCarolis S, Beasley MR, Geballe TH: Intrinsic electrical transport and magnetic properties of Lao.s7Cao.33MnO3 and Lao.sTSro.33MnO3 MOCVD thin films and bulk material. Phys Rev B 1996, 53:14434-14444.

34. •

16.

MartinMC, Shirane G, Endoh Y, Hirota K, Moritomo Y, Tokura Y: Magnetism and structural distortion in the Lao.TSro.3MnO3 metallic ferromagnet. Phys Rev B 1996, 53:14285-14290.

13.

•

showing a large difference in temperatures corresponding to

the ferromagnetic transition and the insulator-metal transition. Solid State Commun 1996, 99:149-152. Jin S, Tiefel TH, McCormack M, O'Bryan HM, Chen LH, Ramesh R, Schurig D: Thickness dependence of magnetoresistance in La-Ca-Mn-O epitaxial films. App/Phys Let/1995, 67:557-559.

Moritomo Y, Asamitsu A, Kuwahara H, Tokura Y: Giant magnetoresistance of manganese oxides with a layered perovskite structure. Nature 1996, 380:141-144. This paper shows that there is a competition between antiferromagnetic and doulbe exchange interactions, causing a sharp I-~M transition and high GMR. 35.

MaheshR, Mahendiran R, Raychaudhuri AK, Rao CNR: Effect of dimensionality on the giant magnetoresistance of the manganates: a study of the (La,Sr)n+1MnnO3n+t family. J Solid State Chem 1996, 122:448-450.

17.

Asamitsu A, Moritomo Y, Kumai R, Tomioka Y, Tokura Y: Magnetostructural phase transitions in Lal.xSrxMnO 3 with controlled carrier density. Phys Rev B 1996, 54:1716-1723.

36.

18.

Kawano H, Kajimoto R, Kubota M, Yoshizawa H: Ferromagnetisminduced reentrant structural transition and phase diagram of the highly doped insulator Lal. x SrxMnO 3 (x <0.17). Phys Rev B 1996, 53:14709-14712.

HeffnerRH, Le LP, Hundley MF, Neumeier JJ, Luke GM, Kojima K, Nachumi B, Uemura YJ, MacLaughlin DE, Cheong SW: Ferromagnetic ordering and unusual magnetic ion dynamics in Lao.s7Cao.33MnO3. Phys Rev Let/1996, 77:1869-1872.

37.

Vas'ko VA, Nordman CA, Kraus PA, Achutharaman VS, Ruosi AR, Goldman AM: Ultra-smooth, highly ordered, thin films of La0.s7Ca0.33MnO3. App/Phys Let/1996, 68:2571-2573.

38.

McGuire TR, Gupta A, Duncombe PR, Rupp M, Sun JZ, Laibowitz RB, Gallagher WJ, Xiao G: Magnetoresistance and magnetic properties of Lal.xxMnO3. 8. J App/Phys 1996, 79:4549-4551.

39.

Xiong GC, Li Q, Ju HL, Bhagat SM, Lofland SE, Greene R L, Venkatesan T: Giant magnetoresistive memory effect in Nd0.TSro.3MnOz films. App/Phys Let/1995, 67:3031-3033.

19. ••

Yamada Y, Hind O, Nohdo S, Kanao R, Inami T, Katano $: Polaron ordering in low-doping Lal.xSrxMnO3 . Phys Rev Let/1996, 77:904-907. Evidence for polaron ordering at low doping (x -0.1). 20.

Caignaert V, Suard E, Maignan A, Simon Ch, Raveau B: Neutron diffraction for an antiferromagnetic ordering in the CMR manganites Pr0.7Ca0.7.xSrxMnO3. J Magn Magn Mater 1996, 153:L260-L264.

21.

Moritomo Y, Asamitsu A, Tokura Y: Pressure effect on the double-exchange ferromagnet La1.xSrxMnO3 (0.15 < x <_0.5). Phys Rev B 1995, 51:16491-16494.

40.

Alers GB, Remirez AP, Jin S: 1/f resistance noise in the large magnetoresistance manganites. Appl Phys Let/1996, 68:3644-3646.

22.

Neumeier JJ, Hundley MF, Thompson JD, Heffner RH: Substantial pressure effects on the electrical resistivity and ferromagnetic transition temperature of Lat. x CaxMnO3, Phys Rev B 1995, 52:7006-7009.

41.

Rao GH, Sun JR, Liang JK, Zhou WY, Cheng XR: Giant magnetoresistance effect in bulk Lal/3Ndl/3Cal/3MnO3 at low field. App/Phys Let/1996, 69:424-426.

23. •,

Hwang HY, Cheong SW, Radaelli PG, Marezio M, Batlogg B: Lattice effects on the magnetoresistance in doped LaMnO 3. Phys Rev Let/1995, 75:914-917. Shows how pressure or the radius of the A-site cation is a good parameter to separate the ferromagnetic metal and paramagnetic insulator regimes. The study shows the importance of the Mn-Mn transfer integral. 24.

RaveauB, Maignan A, Caignaert V: Spectacular giant magnetoresistance effects in the polycrystalline perovskite Pro.7Sro.05Cao.25MnO3.& J Solid State Chem 1995, 117:424-426.

25.

Fontcuberta J, Martinez B, Seffar A, Pinol S, Garcia-Munoz JL, Obradors X: Colossal magnetoresistance of ferromagnetic manganites: structural tuning and mechanisms. Phys Rev Let/ 1996, 76:1122-1125.

26.

IbarraMR, Algarabel PA, Marquina C, Blasco J, Garcia J: Large magnetovolume effect in yttrium doped La-Ca-Mn-O perovskite. Phys Rev Let/1995, 75:3541-3544.

42. •

Hwang HY, Cheong SW, Batlogg B: Enhancing the low field magnetoresistive response in perovskite manganites. App/ Phys Let/1996, 68:3494-3496. Suggests a possible means of optimizing GMR. 43. •

Coey JMD, Viret M, Ranno L, Ounadjela K: Electron localization in mixed valence manganites. Phys Rev Lett 1995, 75:3910-3913. An explanation for high resistivity at low temperatures is given. 44. •

Okimoto Y, Katsufuji T, Ishikawa T, Urushibara A, Arima T, Tokura Y: Anomalous variation of optical spectra with spin polarization in double-exchange ferromagnet: Lal.xSrxMnO3. Phys Rev Let/ 1995, 75:109-112. Optical conductivity measurements show how the spectral weight is transferred on decreasing the temperature. 45.

Kaplan SG, O,uijada M, Drew HD, Tanner DB, Xiong GC, Ramesh R, Kwon C, Venkatesan T: Optical evidence for the dynamic Jahn-Teller effect in Ndo.TSro.3MnO3. Phys Rev Let/1996, 77:2081-2084.

Giant magnetoresistance in manganese oxides Rao and Mahesh

46. **

39

Sarma DD, Shanthi N, Krishnakumar SR, Saitoh T, Mizokawa T, Sekiyama A, Kobayashi K, Fujimori A, Weschke E, Meier Ret al.: Temperature-dependent photoemission spectral weight in La0.sSroAMnO3. Phys Ray B 1996, 53:6873-6876 These papers show how the density of states in manganates is negligible at the Fermi level.

Tomioka Y, Asamitsu A, Kuwahara H, Moritomo Y, Tokura Y: Magnetic-field-induced metal-insulator phenomena in Prl.xCaxMnO3 with controlled charge-ordering instability. Phys Rev B 1996, 53:1689-1692. This paper describes magnetic field induced melting of the charge-ordered state into a metallic.

47.

Saitoh 1", Bocquet AE, Mizokawa T, Namatame H, Fujimori A, Abbate M, Takeda Y, Takano M: Electronic structure of Lal.xSrxMnO3 studied by photoemission and X-ray-absorption spectroscopy. Phys Ray B 1995, 51:13942-13951.

60.

48.

Park JH, Chen CT, Cheong SW, Bao W, Meigs G, Chakarian V, Idzerda YU: Electron spectroscopic studies of colossal magnetoresistance material Lal.xCexMnO3. J Appl Phys 1996, 79:4558-4560.

49.

Park JH, Chen CT, Cheong SW, Bao W, Meigs G, Chakarian V, ldzerda YU: Electronic aspects of the ferromagneUc transition in manganese perovskites. Phys Rev Lett 1996, 76:4215-4218.

50.

Asemitsu A, Moritomo Y, Tokura Y: Thermoelectric effect in Lal.xSrxMnO3. Phys Ray B 1996, 53:2952-2955.

51.

Archibald W, Zhou .IS, Goodenough JB: First-order transition at Tc in the orthomanganites. Phys Rev B 1996, 53:14445-14449.

52. ,,

Millis AJ, Littlewood PB, Shralman Bh Double exchange alone does not explain the resistivity of Lat.xSrxMnO3. Phys Rev Lett 1995, 74:5144-5147. Shows the importance of electron-phonon interaction to explain the properties of the rare earth manganates. 53.

Rodar H, Jun Zang, Bishop AR: Lattice effects in the colossal magnetoresistance manganites. Ph,vs Rev Lett 1996, 76:1356-1359.

54. •

Zhao G, Conder K, Keller H, Muller KA: Giant oxygen isotope shift in the magnetoresistlve perovskite Lal.xCaxMnO3+y. Nature 1996, 381:676-678. An observation of the oxygen isotope effect on Tc suggests the importance of the electron-phonon interaction. 55.

Satpathy S, Popovic ZS, Vukajlovic FR: Electronic structure of the perovskita oxides: Lal.xCaxMnO3, Phys Rev Lett 1996, 76:960-963.

59. *

61.

62.

63.

64.

65.

Tomioka Y, Asamitsu A, Moritomo Y, Kuwshara H, Tokura Y: Collapse of a charge-ordered state under a magnetic field in Prl/2Srl/2MnO 3, Phys Ray Lett 1995, 74:5t 08-511 t. Tokura Y, Kuwahara H, Moritomo Y, Tomioka Y, Asamitsu A: Competing instabilities and metastable states in (Nd,Sm)l/2Srl/2MnO 3. Phys Rev Lett 1996, 76:3184-3187 Kuwahara H, Tomioka Y, Moritomo Y, Asamitsu A, Kseai M, Kumal R, Tokura Y: Striction-coupled magnetoresistance in perovskitetype manganateoxidea. Science 1996, 272:80-82. Liu K, Wu W, Ahn KH, Sulchek T, Chien C L, Xiao JQ: Chargeordering and magnetoresistance in Ndt.xCaxMnO3 due to reduced double exchange. Phys Rev B 1996, 54:3007-3010. Vogt T, Cheetham AK, Mahendizan R, Raychaudhuri AK, Mahesh R, Rao CNR: Charge-ordering and related effects in Nd0.sCa0.sMnO3. Phys Rev B 1996, 54:15303-1530'7. Mahendiran R, Mshseh R, Gundakaram R, Raychaudhuri AK, Rao CNR: Unusual field dependence of the resistivity end magnetoresistance in Nd0.sCa0..sMnO3. J Phys Condens Matter 1996, 8:L455-L459.

66. *

Ramirez AR Schiffer P, Cheong SW, Chen CH, Bao W, Palstra TTM, Gammel PL, Bishop DJ, Zegarski B: Thermodynamic and electron diffraction signatures of charge and spin ordering in La 1.xcaxMnO3. Phys Ray Lett 1996, 76:3188-3191. The characteristics of charge-ordering are given. 67. •

Stemlieb BJ, Hill JP, Wildgruber UC, Luke GM, Nachumi B, Moritomo Y, Tokura Y: Charge and magnetic order in Lao.sSrt.sMnO4. Phys Ray Lett 1996, 76:2169-2172. Changes in structure and magnetic properties due to charge-ordering are described. 68.

56. **

Tokura Y, Tomioka Y, Kuwshara H, Asamitsu A, Moritomo Y, Kasai M: Origins of colossal magnetoresistance in perovskita-type manganese oxides. J Appl Phys 1996, 79:5288-5291. Briefly reviews the work on charge ordering in rare earth manganates.

Moritomo Y, Tomioka Y, Asamitsu A, Tokura Y, Matsui Y: Magnetic and electronic properties in hole-doped manganese oxides with layered structures: Lat.xSrl+xMnO4. Phys Rev B 1995, 51:3297-3300.

69.

Shimakawa Y, Kubo Y, Manako T: Giant magnetoresistance in TI2Mn207 with the pyrochlore structure. Nature 1996, 379:53-55.

57. *

70. **

Kuwahara H, Tomioka Y, Asamitsu A, Moritomo Y, Tokura Y: A first-order phase transition induced by a magnetic field. Science 1995, 270:961-963. Gives an account of the magnetic field induced first-order electronic transition with large hysteresis. 58.

Lees MR, Barrett J, Balakrishnan G, McK Paul D, Yethiraj M: Influence of charge and magnetic ordering on the insulator-metal transition in Prl.xCaxMnO3. Phys Rev B 1995, 52:14303-14307.

Subramanian MA, Toby BH, Ramirez AP, Marshall WJ, Sleight AW, Kwei GH: Colossal magnetoresistance without Mn3+/Mn4+ double exchange in the stoichiometric pyrochlore TI2Mn207. Science 1996, 273:81-83. If TI2Mn20./ really has no Mn 3+ due to oxygen deficiency, then the mechanism of GMR in this material cannot invoke double exchange. 71.

Briceno G, Chang H, Sun X, Schultz PG, Xiang XD: A class of cobalt oxide magnetoresistance materials discovered with combinatorial synthesis. Science 1995, 270:273-275.