Gross R., Sidorenko A., Tagirov L. Nanoscale Devices

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84 Rudolf Gross

applications and hence provides very valuable information from the

practical point of view.

Half-Metallic Ferromagnetic Oxides

Half-metallic ferromagnets are interesting for spintronics applications

because they provide fully spin-polarized charge carriers at the Fermi level.

Of course, from the application point of view not only their full spin

polarization is relevant, but also their Curie temperature, which should be

well above room temperature. There are several classes of potentially half-

metallic materials such as the doped manganites, the double perovskites,

the Heusler compounds, magnetite, CrO

or diluted magnetic

semiconductors (for recent reviews see [97, 152]). Most of them are

transition metal oxides, which we discuss here. In transition metal oxides

there is a strong hybridization of the outer electron shell (4s/4p) of the

transition metal ion with the oxygen 2p-states. This causes a wide s-p-gap,

into which the narrow 3d-band tends to fall. A half-metallic oxide is then

achieved, if only one of the spin polarized 3d-bands intersects the Fermi

level, whereas there is a gap in the DOS for the other spin direction. A

similar situation also applies for the Heusler alloys such as NiMnSb

[154, 180], PtMnSb or Co

MnSi [181], which are half-metals with high

Curie temperatures above about 600 K.

Chromium dioxide

CrO

is a ferromagnetic metal with tetragonal rutile structure (space group

/mmm) with Cr positions at (0,0,0); (½,½,½) and oxygen positions at

(±x, ±x,0); (½

± x, ½ m x, ½) with x = 0.302 (see Fig. 10a). Each oxygen has

three chromium neighbors and each Cr

(3d²) ion is octahedrally

coordinated by oxygen with two short apical bonds (0.189 nm) and four

longer equatorial bonds (0.191 nm) [151, 182]. The octahedra are sharing a

common edge and form ribbons along the c-axis. The lattice parameters are

a = 0.4422 nm and c = 0.2917 nm.

Due to the crystal field the five 3d-states are split (~2.5eV) into three t

and two e

states with the t

triplet further split into a nonbonding d

orbital in the equatorial plane of the octahedron and two bonding d

± d

orbitals with respect to the oxygen p-orbital directed perpendicular to the

0 triangles [183, 184]. According to Hund's rule the two electrons are

occupying two of the t

states with parallel spin. Ferromagnetism in CrO

arises from the exchange interaction between the Cr

ions. It is mainly

caused by a delocalization of one of the t

spin-up electrons. These

Magnetic Tunnel Junctions Based on Half-Metallic Oxides 85

electrons are hopping from site to site, while they are ferromagnetically

coupled to the localized S = 1/2 core spins by the on-site Hund's rule

exchange. Band structure calculations [185, 186] have indicated that CrO

is a type-I

half-metal (see Fig. 6a) with a saturation magnetization of 2µ

per formula unit, a spin gap ∆

↓

of about 1 eV and a spin-flip gap ∆

of a

few 100 meV (a more detailed analysis shows that there is a finite covalent

mixing of the Cr t

spin-up states and the O 2p states resulting actually in a

moment of 2.1µ

/f.u. [187, 188]). The Curie temperature of CrO

is T

≈

390 K.

Fig. 10. (a) - (d) Crystal structure, the effect of the crystal field on the one-electron

levels, and schematic density of states of some half-metallic oxide materials: (a)

CrO

, (b) Sr

FeMoO

, (c) La

2/3

1/3

MnO

, (d) Fe

. In (e) and (f) the crystal

structures of two ferromagnetic semiconductors are shown together with the

schematic density of states: (e) (Ga,Mn)As and (e) (Zn,Mn)O.

The spin polarization of CrO

has been measured using superconducting

tunnel junctions [173, 189, 190] and point contact Andreev spectroscopy

[167, 171]. For thin films values as high as 98% have been observed for

ballistic contacts at 4 K. However, the spin polarization was found to

86 Rudolf Gross

decrease rapidly with increasing temperature and becomes vanishingly

small at room temperature [151, 191, 192].

Doped manganites

Another class of transition metal oxides with a very rich phase diagram and

fascinating physics are the mixed valence manganites with composition

1-x

MnO

with A an alkaline earth [193, 194]. These oxides have

perovskite type structure where the Mn ions are surrounded by an oxygen

octahedra (see Fig. 10c). Due to the crystal field the five 3d-states are split

into three t

and two e

states. In the undoped case (x = 0)) only Mn

ions

(3d

) are present. Due to the strong on-site Hund's rule exchange the four d

electrons are filled in with parallel spin. The half-filled e

band is split by

the Jahn-Teller effect making this compound an antiferromagnetic

insulator. Doping with divalent alkaline earth ions introduces holes into the

band. If the hole density is sufficiently high, the holes can delocalize.

However, due to the strong on-site Hund's coupling this is only possible, if

the itinerant holes are ferromagnetically aligned with the local S = 3/2 core

spins of the t

electrons. That is, hopping provides a ferromagnetic double

exchange and hence a ferromagnetic ground state with a maximum Curie

temperature between 280 and 380 K for x = 0.3 and A = Ca, Ba, Sr.

For the compound La

0.7

0.3

MnO

with the maximum T

the spin

polarization has been discussed controversially, because the expected value

sensitively depends on the details of the band structure. From the

experimental point of view a high degree of spin polarization has been

derived from magnetic tunnel junctions (up to 95% at 4 K [101, 195-197]),

superconducting tunnel junctions (72%, [161]), and point contact Andreev

spectroscopy (80%, [172, 198]). However, these values have been obtained

interpreting experimental data on the basis of the oversimplifying Jullière

formula. At present it is still not fully established whether this compound is

a type-I

or type-III

half-metal. It may be that there are both itinerant e

spin-down states and localized t

spin-up states at the Fermi level, so that

0.7

0.3

MnO

is a type- III

half-metal. However, it also may be that the

spin-up band is above the Fermi level, so that we have a type-I

half-

metal.

For La

0.7

0.3

MnO

a spin polarization of about 80% at 4 K has been

derived from grain boundary tunnel junction experiments [100]. For all

studied

manganites, the spin polarization has been found to decrease

rapidly with increasing temperature and to become vanishingly small at

room

temperature. For grain boundaries this has been attributed to a strong

Magnetic Tunnel Junctions Based on Half-Metallic Oxides 87

increase of inelastic tunneling processes with increasing temperature

[85, 86]. For MTJs the detailed origin has not yet been clarified.

Double perovskites

Double perovskites of composition A

BB'O

with A=Ca, Sr, and Ba and

BB'= FeMo, CrW, FeRe, or CrRe recently hace attracted considerable

attention, since band structure calculations [199, 200] have predicted half-

metallic behavior and furthermore Curie temperatures as high as 420 K for

FeMoO

[201], 460 K for Sr

CrWO

[202, 203], and 630 K for

CrReO

[204, 205] have been observed. In the ideal case, the double

perovskites have an ordered NaCl-type array of B and B' cations on

octahedral sites (see Fig. 10b). However, in reality there is always a finite

amount of antisite defects in the cation order.

The magnetic exchange in double perovskites is explained within a

generalized double exchange or kinetic energy driven model proposed by

Sarma et al. [206] with subsequent extensions and generalizations by

Kanamori and Fang [207, 208]. The model can be easily understood

considering Sr

FeMoO

as an example. For the magnetic B ion (Fe

)

Hund's splitting is much larger than the crystal field splitting, and the

majority spin t

2g↑

and e

g↑

bands are filled with five electrons. In contrast, at

the 'non-magnetic' B' site (Mo

) the crystal field splitting between the t

and e

band is large and Hund's splitting is negligibly small. The Mo 4d t

band is filled with a single electron. As the majority spin bands at the

magnetic site are occupied, kinetic energy gain can only be obtained by

hybridization and the hopping of the minority spin electrons from the 'non-

magnetic' site into the empty minority t

2g↓

spin band of the magnetic ion.

This results in t

2g↑

electrons of mixed 3d/4d character at the Fermi level. By

shifting electrons from the majority spin band of the 'non-magnetic' site into

the minority spin band, the system can gain energy because the spin-down

electrons can delocalize. As a result, the charge carriers become strongly

half-metals.

Furthermore, at the 'non-magnetic' site a finite negative spin

moment is induced, which has been detected directly by XMCD

moments at the magnetic B site. Therefore, the

double perovskites

can be viewed as ferrimagnets.

Band

structure calculation predict a spin gap

∆

↑

of about 1 eV and a

spin-flip

gap

∆

of several 100 meV. The expected saturation

magnetization

is given by the difference of the spin moment of the

the double perovskites are type-I

measurements

magnetic

parallel to the loca-

lized

[209-211]. This induced spin moment is oriented anti-

gi ven above sification

polarized, in the extreme case even half-metallic. According to the clas-

88 Rudolf Gross

localized t

2g↑

states at the magnetic site (5µ

/f.u. for Fe

and 3µ

/f.u. for

) and the induced moment at the non-magnetic site (about -0.3 µ

/f.u.

both for Mo

and W

). The measured saturation magnetization is always

considerably smaller due to antisite defects. Measurements of the

magnetoresistance of granular samples and on tunnel junctions with Co

counter electrodes indicate a spin polarization above 50% at 5 K, which is

decreasing rapidly with increasing temperature [202, 203, 212].

Magnetite

Magnetite, Fe

, is a ferrimagnet with a very high ordering temperature of

860 K. Magnetite has the so-called cubic inverse spinel structure in which

the Fe cations occupy interstices of a face-centered-cubic (fcc) close packed

frame of oxygen ions (see Fig. 10d). The moments of Fe

are localized at

the eight tetrahedral (A) sites and are aligned anti-parallel to those of the

and Fe

ions equally sharing the 16 octahedral (B) sites [213]. The

expected saturation moment is 4µ

/f.u. Rapid hopping of electrons between

and Fe

ions in the B sites results in a room-temperature conductivity

of about 200 Ω

-1

. Upon cooling, bulk magnetite undergoes a metal to

insulator transition (Verway transition) at 120 K, where the electron

hopping is frozen and the conductivity decreases by several orders of

magnitude. This prevents the study of spin polarization by methods

employing superconductors and therefore requiring low temperatures.

Although it is generally accepted that the Verwey transition [214-218] is

due to the ordering of the Fe

and Fe

ions, the mechanism governing the

transport and magnetic properties of this material above and below the

transition is still not well understood.

Band structure calculations show that Fe

contains a gap in the

majority spin band at the Fermi level, but there is no gap in the minority

spin band [219-223]. This suggests that magnetite is a half-metal with the

Fermi level in the t

2g↓

band of the octahedral iron. However, the small

conductivity of magnetite indicates that the carriers are localized forming

polarons [224]. That is, magnetite is probably best classified as a type-II

half-metal. A large negative value of spin polarization of about up to -80%

has been measured in spin-polarized photoemission experiments [105, 225-

228]. Detailed magnetotransport measurements on Fe

have performed

only recently [229-232]. For both single crystals [229] and epitaxial films

[230] a large negative magnetoresistance of several 10% was found around

and below the Verwey transition. Coey et al. [231] have studied the

magnetoresistance of Fe

in polycrystalline thin film, powder compact,

and single-crystal form above the Verwey transition. A low-field negative

Magnetic Tunnel Junctions Based on Half-Metallic Oxides 89

MR effect has been observed in the polycrystalline thin film and powder

compacts, but not in the single crystal. MR effects of (1-3%) have been

measured, with the larger values appearing in the powder compacts. It has

been suggested that the effect is associated with intergranular transport of

spin-polarized electrons, with the MR arising from misaligned

magnetization in adjacent ferromagnetic grains that are exchange-

decoupled. In general, the MR values reported for polycrystalline samples

and tunnel junctions [233-240] are small and show wide spread. At 4 K

values of up to -25% have been reported by Hu et al. [233, 234], whereas a

value of +43% has been reported by Seneor et al. [235]. However, these

values drop to zero at room-temperature. Large MR has sometimes found

also in point contacts, from which a spin polarization as large as -72% has

been inferred [163]. The latter value agrees reasonably well with those

found in spin-polarized photo emission experiments [105]. The fact that for

magnetite based MTJs often low TMR values are obtained most likely is

related to the problem that controlling the stoichiometry and defect

structure at the interface between magnetite and the tunneling barrier is

challenging. However, recently magnetite based MTJs with Al

barriers

and Ni or Co counter electrodes have been fabricated, which show a room

interface [241]. In a complex crystal structure such as magnetite the surface

can have a number of nonidentical terminations and surface reconstructions

with completely different properties regarding spin-dependent transport.

For example, it has been shown that an oxygen layer deposited on a

magnetic electrode could change the properties of a MTJ drastically

including a sign reversal of the spin polarization [242, 243].

Ferromagnetic semiconductors

Ferromagnetic semiconductors are interesting regarding the integration of

spin electronics with conventional electronics. They should have fully spin-

polarized charge carriers and ferromagnetic ordering temperature well

above 300 K to allow for room temperature applications.

Oxide magnetic semiconductors have been studied intensively in the

1960s and 1970s and came into the focus again recently with the growing

interest in spin electronics. The textbook example certainly is EuO with T

= 69 K [37, 244]. In this material the Eu

level falls into the gap

between the oxygen 2p valence band and the Eu 5d/6s conduction band.

The

latter is occupied by n-type charge carrier due to oxygen deficiency or

observed for magnetite is related to disorder at the electrode/barrier

that the large scatter and even differing sign of the TMR effect

temperature TMR up to more than 20%. Recently, it has been suggested

90 Rudolf Gross

doping. The problem with EuO and related materials such as EuS or

EuB

is to achieve a high Curie temperature. The ferromagnetic cation-

cation superexchange can be traced back to the ferromagnetic s-f coupling,

and the presence of s-f hybridization. However, due to the local nature of

the 4f levels it is difficult to find a sufficiently strong exchange mechanism

delivering room temperature ferromagnetism [245].

The discovery of ferromagnetism in zinc-blende III-V [246, 247] and II-

IV [248, 249] Mn-based compounds strongly stimulated work exploring the

interesting combination of quantum structures and magnetism in

semiconductors. This was further stimulated by theoretical calculations

predicting room-temperature ferromagnetism in transition metal doped III-

V and II-VI semiconductors [260-262]. Recently, ordering temperatures

well above 100 K have been achieved with transition metal doped GaAs

(see Fig. 10e). Optimizing the deposition conditions allowed to raise T

to about 180 K for Ga

0.95

0.05

As [35, 247, 250-255]. The charge carriers

in this material are p-type in a spin-split 4p (As) valence band. For this

material a high spin polarization of 85% has been measured at low

temperature recently using Andreev reflection spectroscopy from high

transparency Ga

0.95

0.05

As/Ga point contacts [256]. A high spin

polarization also has been inferred from the large tunneling

magnetoresistance observed in MTJs from this material [257, 258]. This is

consistent with band structure calculations that predicted P ≈ 100% [259].

Much excitement has been generated by the observation of Curie

temperatures well above room temperature in various transparent oxides

such as ZnO [263] (see Fig. 10f), SnO

[264], TiO

(anatase) [265], and

HfO [266, 267] when doped with a few percent of transition metals such as

Co or Mn. Although there is still a controversial debate on these materials,

it is likely that the cobalt or manganese levels reside in the gap, with a spin-

split conduction band. They would then be n-type half-metallic

semiconductors, like EuO. The magnetic exchange may also be provided by

a spin-split impurity band [63, 157, 268]. Unfortunately, doubts persist as to

whether these materials are truly uniform and inherently semiconducting.

Very recently, several experiments suggested the existence of

ferromagnetism in the La

0.5

TiO

doped with Co [269, 270]. This has

been further confirmed by tunneling magnetoresistance measurements

showing a significant spin polarization in Co-doped (La,Sr)Ti

1-x

films

at low temperature [271, 272]. We note that the host material La

0.5

TiO

may not be considered as a semiconductor but rather as a strongly

correlated metal [273].

Magnetic Tunnel Junctions Based on Half-Metallic Oxides 91

Status

There is a large number of ferromagnetic materials that are predicted half-

metallic by band structure calculations. Most of these materials are

ferromagnetic oxides. These materials are interesting for studying basic

physics aspects of spin-dependent transport and single spin band effects.

Furthermore, they provide the basis for applications requiring enhanced

spin effects.

Among the oxides, chromium dioxide is probably the best studied

material. It shows full spin polarization at low temperatures. However, the

ferromagnetic ordering temperature (387K) is too low for room-temperature

applications. The same is true for the doped manganites with a maximum

Curie temperature of about 380 K. In this respect magnetite (T

= 860K)

and the double perovskites (T

≤ 630K) are more promising. However, so

far no reliable data exist regarding the spin polarization of the latter

materials at room temperature. Moreover, it is still unclear whether or not a

high degree of spin polarization can be achieved also at interfaces to

insulating barriers as required for MTJs because these materials usually

have complicated crystal structures. If these problems can be solved, the

half-metallic oxides offer good prospects for applications such as MTJs. Of

course, there is hope that new high Curie temperature half-metallic

ferromagnets will be discovered.

The new family of ferromagnetic oxide semiconductors is highly

interesting with respect to the underlying physics. In particular, the

clarification of the magnetic exchange in these materials is an important

task of future research. However, from the application point of view the

Curie temperatures in most cases still are too low and doubts remain

whether or not these materials are homogeneous ferromagnets with spin-

polarized charge carriers. Future work will show whether these materials

can enter spin electronics applications.

Finally, we would like to emphasize that ferromagnetic oxide materials

are promising with respect to combining half-metals and multiferroics [274-

276] in epitaxial heterostructures. Such structures are interesting for

spintronics, since they may allow to switch magnetization by electric fields

or design novel multi-functional materials systems [277, 278]. However, at

present this field is not yet well developed and first experiments mostly aim

at materials issues such as the heteroepitaxial growth of the relevant

materials [279-281].

92 Rudolf Gross

Magnetic Tunnel Junctions

In this section we discuss the fabrication of MTJs based on half-metallic

oxides. In particular we address questions regarding the degradation of the

expected tunnel magnetoresistance due to imperfections of the tunneling

barrier and the involved interfaces. In our discussion we will focus on MTJs

based on the doped manganites and magnetite. Tunnel junctions based on

chromium dioxide so far have reached only small TMR values of less than

10% even at at low temperature [190]. Experimental data on junctions

based on the double perovskites are hardly available [282].

Fabrication process

Magnetic tunnel junctions in most cases are fabricated by first depositing

the complete multilayer thin film structure with base electrode (sometimes

with additional buffer layer), tunneling barrier, top electrode, and wiring

layer. Then the multilayer is patterned ex-situ, e.g. by etching a mesa

structure into the multilayer film (see Fig. 11), to achieve the desired

junction geometry. This process has the advantage that the multilayer

structure can be fabricated in-situ avoiding contamination effects of the

sensitive interfaces. For thin film deposition of the oxide heterostructures

mostly pulsed laser deposition is used. Details about the laser deposition

process including in-situ reflection high energy electron diffraction

(RHEED) control of the growth process and laser substrate heating are

described elsewhere [283-286]. A critical point for the fabrication of MTJs

is the surface roughness of the base electrode. A rough surface usually

results in an inhomogeneous thickness of the tunneling barrier and even

pinholes.

Fig. 11. (a) Cross-sectional view of the multilayer structure used for planar MTJs.

(b) Optical micrograph of a chip containing for test junctions.

Magnetic Tunnel Junctions Based on Half-Metallic Oxides 93

MTJs based on doped manganites

Magnetic tunnel junctions based on the doped perovskite manganites have

been successfully fabricated using both planar type tunnel junctions as

shown in Fig. 11 [98-101, 287-294] as and artificial bicrystal grain

boundary junctions fabricated on SrTiO

bicrystal substrates [85-

87, 100, 295-299].

As shown by Fig. 12a, a large TMR of up to 300% at 4 K has been

achieved for manganite bicrystal junctions with almost ideal switching

behavior of the resistance versus applied magnetic field curves. According

Jullière's formula, the measured TMR value corresponds to a spin

polarization of about 60%. However, as shown in Fig. 12b, the large low-

temperature TMR effect was found to rapidly decrease with increasing T

due to a strong increase of the inelastic tunneling current [85-87, 100],

which is not spin-conserving. This was attributed to the large density of

defect states in the grain boundary barrier, which has been confirmed by

noise measurements [300].

Fig. 12. (a) Resistance vs. applied magnetic field curves for a La

2/3

1/3

MnO

grain

boundary junction. The direction of the field sweep is indicated by the arrows

(according Ref. [100]). (b) TMR (symbols) plotted versus bias voltage and

temperature. The lines show the expected decrease due to the measured increase of

the inelastic tunneling current (according Ref. [86]). The inset shows the sample

configuration.

Large TMR values up to more than 1800% corresponding to a spin

polarization of more than 90% also have been measured for planar type

junctions based on doped manganites [98-101, 195]. However, these large

values only have been achieved at low temperatures and a rapid decrease

Gross R., Sidorenko A., Tagirov L. Nanoscale Devices - Fundamentals and Applications

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