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94 Rudolf Gross

was found with increasing T resulting in a vanishingly small TMR at room-

temperature. The origin of this decay (inelastic tunneling, magnon

excitation, temperature variation of interface magnetization, etc.) has not

yet been clarified in detail. Furthermore, a strong voltage dependence of the

TMR was found, which in some cases could be attributed to the band

structure of doped manganites [195].

Most experiments on manganite based MTJs showed that the TMR and,

in turn, the derived value of the spin polarization strongly depend on the

chemical composition, the microstructure and the strain state of interfaces

and barriers. For La

1-x

MnO

(LSMO) based MTJs with SrTiO

barrier it

was found that there exists an instability towards the A-type (layered)

antiferromagnetic phase causing a suppression of ferromagnetism by spin

canting in the vicinity of the interface as x increases from 0.2 to 0.4. With

the proper choice of x at the interface, "atomically regulated''

LSMO/STO/LSMO MTJs could be fabricated, which showed a weaker

decrease of TMR with temperature and still a finite TMR value of about 1%

at 300 K [301, 302].

For MTJs based on La

2/3

1/3

MnO

/ SrTiO

/ La

2/3

1/3

MnO

(D = Ca or

Ba) trilayers large differences in the measured TMR values have been

observed, although the structural quality of the layer structure was almost

the same for the Ca and Ba doped material (see TEM micrograph in Fig.

13a). Whereas TMR high values above 1000% have been measured for the

2/3

1/3

MnO

(LBMO) based MTJs, only small TMR values typically

below 30% have been observed for the La

2/3

1/3

MnO

(LCMO) based

MTJs. This could be attributed to the different strain state of the manganite

films. Whereas LBMO growth almost strain-free on (001) SrTiO

, LCMO

is under tensile strain (-1.05%). During nucleation the LCMO film can

accommodate for the epitaxial coherency strain by incorporating less Ca

and/or Ca/La site swapping, thereby forming a Ca rich surface layer.

During growth of the LCMO base electrode this Ca rich surface layer is

swimming on top of the growing film and finally results in a Ca rich

interface layer between the LCMO base electrode and the STO barrier.

Correspondingly, a Ca deficient LCMO layer is expected at the interface

between the STO barrier and the LCMO top electrode. These Ca rich and

deficient interface layers have been indeed detected by an EELS analysis

(see Fig. 13c) [303]. Both Ca rich and deficient layers are expected to have

strongly reduced Curie temperatures (magnetically dead layers) or may

even

be antiferromagnetic. Note that the Ca enriched and deficient

interface layers have negligible influence on the microstructure but are

strongly affecting the magnetic properties of the interface. We further note

that

the tensile biaxial strain itself favors an A-type antiferromagnetic

order in LCMO. Therefore, strongly reduced TMR values are expected for

the LCMO/STO/LCMO MTJs grown on SrTiO

. In contrast, for MTJs

Magnetic Tunnel Junctions Based on Half-Metallic Oxides 95

based on almost strain free LCMO films grown on a lattice matched

NdGaO

substrate and barrier layer high TMR effects have been observed

[99]. In the same way, for LCMO grain boundary junctions large TMR

values have been measured as shown in Fig. 12 [100].

Fig. 13. (a) Cross-sectional TEM micrograph of a LBMO/STO/LBMO trilayer

structure for MTJs. (b) Illustration of the Ca enriched and deficient layers for a

LCMO/STO/LCMO heterostructure, which are absent for a LBMO/STO/LBMO

heterostructure. (c) Chemical profile (Ti, La, Ca, and Mn) obtained by EELS along

the spatial coordinate perpendicular to the layer structure of a LCMO/STO/LCMO

trilayer. In contrast to Ti, La, and Mn the Ca profile is asymmetric with excess Ca

at the top of the base electrode and Ca deficiency at the bottom of the counter

electrode. The nominal barrier thickness is 3.1 nm (according to [303]).

The

two examples discussed for manganite based MTJs clearly show

that a controlled engineering of interfaces on a nanometer scale is required

not only with respect the microstructure but also with respect to the

chemical composition and strain state.

96 Rudolf Gross

MTJs based on magnetite

Although large TMR values have been reported for junctions based on the

doped manganites, these large values are obtained only at low temperature.

They rapidly decrease with increasing temperature and become negligible

at room temperature. In the same way, for chromium dioxide a large value

of the spin polarization (up to 98%) has only been measured at low

temperature. However, MTJs with large TMR have not yet be fabricated.

Even at low temperature only small values below about 10% have been

achieved [190]. Furthermore, both chromium dioxide and the doped

manganites have Curie temperatures below 400 K what is insufficient for

room temperature applications. This has stimulated work on magnetite,

which has a very high Curie temperature of 860 K and is supposed to be a

half-metal.

There have been several attempts to fabricate magnetite based MTJs,

based MTJs are small and show large spread (even opposite sign) [233-

240]. Maximum values reported in literature are -25% for Fe

/CoCr

0.7

0.3

MnO

junctions at 4 K [233, 234] and +43% for Co/Al

/Fe

junctions [235] also at 4 K. Tunneling magnetoresistance also has been

studied for Fe

/MgO/Fe

MTJs with maximum TMR values of less

than 2%. The largest room-temperature TMR value of 10% has been

reported by Matsuda et al. for Fe

/Al O

/CoFe trilayer junctions [240].

The low values and large scatter of achieved TMR values of magnetite

based MTJs have been attributed to disorder at the electrode/barrier

interface and different surface terminations of the magnetite layer, which

are difficult to control during fabrication.

We recently have fabricated MTJs based on epitaxial magnetite thin

films deposited on (001) MgO single crystal substrates. We used magnetite

as the base electrode and Ni or Co for the counter electrode. For the

tunneling barrier five materials namely MgO, SrTiO

, NdGaO

, SiO

, and

3-x

have been used to study the effect of different barrier materials on

the magnetotransport properties of the MTJs. The whole thin film

multilayer structure was grown in-situ using pulsed laser deposition (PLD)

and electron beam evaporation in an ultra high vacuum system in order to

avoid the degradation of interfaces due to ex-situ processing [304].

above, the TMR values reported for various versions of magnetite

however, in most cases with limited success. As already discussed

Magnetic Tunnel Junctions Based on Half-Metallic Oxides 97

Fig. 14. (a) Magnetization versus applied magnetic field measured at room

temperature for multilayer films with two different barrier materials. Left panel:

/SiO

/Ni. Right panel: TiN/Fe

/Al

3-x

/Ni. (b) Resistance versus applied

magnetic field as well as measured TMR effect of a Fe

/Al

3-x

(2.5nm)/Ni MTJ

at T = 320 K.

Best results have been obtained for MTJs with SiO

and Al

3-x

barriers,

which provide magnetically decoupled junction electrodes as necessary for

sharp switching. This is shown in Fig. 14a, where we have plotted the

magnetization curves of a Fe

/SiO

/Ni and a Fe

/Al

3-x

/Ni trilayers

used for the fabrication of MTJs. These samples clearly show a fully

decoupled switching of the magnetization of the junction electrodes at all

temperatures. Fig. 14b shows the resistance versus applied magnetic field

curve as well as the TMR-effect of a Fe

/Al

3-x

(2.5nm)/Ni MTJ at T =

320 K. The MTJs with Al

3-x

barrier reproducibly showed a clear positive

TMR effect both for Ni and Co counter electrodes. Assuming a negative

spin polarization of Co and Ni as predicted by band structure calculation,

we can conclude that magnetite also has a negative spin polarization as

expected from band structure calculations.

Figure 14b shows that the measured R(H) curves have an almost ideal,

symmetric switching behavior. Furthermore, a well defined tunneling

magnetoresistance was observable in the whole measured temperature

range from 150 K to 350 K. Evaluating the true TMR effect of these

junction one has to take into account the large series resistance R

of the

magnetite electrodes due to the large resistivity of magnetite. The series

resistance R

is included in the measured resistance for both parallel and

anti-parallel magnetization configuration and is entering the expression for

the measured TMR as

98 Rudolf Gross

Fig. 15. True TMR effect and measured TMR effect plotted versus temperature for

a Fe

/Al

3-x

(2.5nm)/Ni magnetic tunnel junction.

)()(

ssp

eas

RRRR

TMR

∆

−

(48)

In order to obtain the true tunneling magnetoresistance we have to

correct for the series resistance R

and obtain the corrected value as

.1==













∆

TMR

easrue

(49)

Because the measured 4-point resistance in the parallel configuration is

eas

RRR +

= and hence

= RRR

eas

− , we can rewrite this equation

and obtain

.1==













−

∆

eas

easrue

TMR

(50)

TMR

true

is plotted in Fig. 15 versus temperature. In contrast to TMR

meas

shown in the inset, the corrected values TMR

true

show the expected

monotonic decrease with increasing temperature. True TMR values of

Magnetic Tunnel Junctions Based on Half-Metallic Oxides 99

above 20% at room temperature and about 50% at 150 K have been

obtained. Slightly larger values are obtained for MTJs with Co counter

electrodes not discussed here [305].

Acknowledgements

The author wish to thank Th. Brenninger, M. Bibes, A. Boger, J.M.D.

Coey, J. Fontcuberta, S. Geprägs, S. Gönnenwein, A. Gupta, D. Kölle, A.

Marx, P. Majewski, E. Menzel, K. Nielsen, M. Opel, J.B. Philipp, S.

Qureshi, M.S. Ramachandra Rao, and D. Reisinger for valuable and

stimulating discussions, disposal of experimental data and technical

support. Financial support by the German Science Foundation and the

Federal Ministry of Education and Research is gratefully acknowledged.

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