Colossal magnetoresistance—a huge decrease in resistance in response to a magnetic field—has recently been observed in manganese oxides with perovskite structure. This effect is attracting considerable interest from both fundamental and practical points of view1. In the context of using this effect in practical devices, a noteworthy feature of these materials is the high degree of spin polarization of the charge carriers, caused by the half-metallic nature of these materials20,21; this in principle allows spin-dependent carrier scattering processes, and hence the resistance, to be strongly influenced by low magnetic fields. This type of field control has been demonstrated for charge-carrier scattering at tunnelling junctions2,3 and at crystal-twin or ceramic grain boundaries4,5, although the operating temperature of such structures is still too low (⩽150 K) for most applications. Here we report a material—Sr2FeMoO6, an ordered double perovskite6—exhibiting intrinsic tunnelling-type magnetoresistance at room temperature. We explain the origin of this behaviour with electronic-structure calculations that indicate the material to be half-metallic. Our results show promise for the development of ordered perovskite magnetoresistive devices that are operable at room temperature.

Room-temperature magnetoresistance in an oxide material with an ordered double-perovskite structure

We review recent experimental work falling under the broad classification of colossal magnetoresistance (CMR), which is magnetoresistance associated with a ferromagnetic-toparamagnetic phase transition. The prototypical CMR compound is derived from the parent compound, perovskite LaMnO 3. When hole doped at a concentration of 20–40% holes/Mn ion, for instance by Ca or Sr substitution for La, the material displays a transition from a high-temperature paramagnetic insulator to a low-temperature ferromagnetic metal. Near the phase transition temperature, which can exceed room temperature in some compositions, large magnetoresistance is observed and its possible application in magnetic recording has revived interest in these materials. In addition, unusual magneto-elastic effects and charge ordering have focused attention on strong electron–phonon coupling. This coupling, which is a type of dynamic extended-system version of the Jahn–Teller effect, in conjunction with the double-exchange interaction, is also viewed as essential for a microscopic description of CMR in the manganite perovskites. Large magnetoresistance is also seen in other systems, namely Tl 2Mn2O7 and some Cr chalcogenide spinels, compounds which differ greatly from the manganite perovskites. We describe the relevant points of contrast between the various CMR materials.

REVIEW ARTICLE: Colossal magnetoresistance

The nature and length scales of charge screening in complex oxides are fundamental to a wide range of systems, spanning ceramic voltage-dependent resistors (varistors), oxide tunnel junctions and charge ordering in mixed-valence compounds. There are wide variations in the degree of charge disproportionation, length scale, and orientation in the mixed-valence compounds: these have been the subject of intense theoretical study, but little is known about the microscopic electronic structure. Here we have fabricated an idealized structure to examine these issues by growing atomically abrupt layers of LaTi(3+)O(3) embedded in SrTi(4+)O(3). Using an atomic-scale electron beam, we have observed the spatial distribution of the extra electron on the titanium sites. This distribution results in metallic conductivity, even though the superlattice structure is based on two insulators. Despite the chemical abruptness of the interfaces, we find that a minimum thickness of five LaTiO(3) layers is required for the centre titanium site to recover bulk-like electronic properties. This represents a framework within which the short-length-scale electronic response can be probed and incorporated in thin-film oxide heterostructures.

Artificial charge-modulationin atomic-scale perovskite titanate superlattices

The manganese oxides of general formula RE1−xMxMnO3 (RE = rare earth, M = Ca, Sr, Ba, Pb) have remarkable interrelated structural, magnetic and transport properties induced by the mixed valence (3+–4+) of the Mn ions. In particular, they exhibit very large negative magnetoresistance, called colossal magnetoresistance (CMR), in the vicinity of metal–insulator transition for certain compositions. In this review paper, we summarize the most important features of the physics of the CMR manganites. The growth techniques for manganese oxide thin films, which are the basic material for potential applications, are reviewed and their structure and morphology examined in relation to growth parameters. The effects of epitaxial strains on the physical properties are discussed. Early works on superlattices and devices are presented.

http://iopscience.iop.org/article/10.1088/0022-3727/36/8/201/pdf

CMR manganites: physics, thin films and devices

Manganese perovskites based on the compound LaMnO3 are attracting considerable theoretical and technological interest by virtue of their unusual magnetic and electronic properties1–4. Most notable of these properties is the extremely large change in resistivity that accompanies the application of a magnetic field, an effect known as 'colossal' magnetoresistance. The origin of this effect has been attributed5–7 to the presence of magnetic polarons—charge carriers accompanied by a localized (and magnetically polarized) distortion of the surrounding crystal lattice8,9— but their existence and properties remains a matter of speculation. Here, using a combination of volume thermal expansion (with and without an applied field), magnetic susceptibility and small-angle neutron scattering measurements, we present evidence for the existence of magnetic polarons above the ferromagnetic ordering temperature, Tc. We detect the spontaneous formation of localized ∼12-A magnetic clusters above Tc which, on application of a magnetic field, grow in size but decrease in number. We argue that the response of these magnetic polarons to an applied magnetic field underlies the pronounced magnetoresistive properties in the compounds (La1–xAx)2/3Ca1/3MnO3 (where A is Y or Tb).

Evidence for magnetic polarons in the magnetoresistive perovskites

We report on the fabrication of a new class of trilayer epitaxial thin film devices based on the doped perovskite manganates La–Ca–Mn–O and La–Sr–Mn–O. We show that large resistance changes, up to a factor of 2, can be induced by a moderate applied magnetic field below 200 Oe in these trilayers supporting current‐perpendicular‐to‐plane transport. These results show that low‐field spin‐dependent transport in manganates can be accomplished, the magnitude of which is suitable for magnetoresistive field sensors.

Observation of large low‐field magnetoresistance in trilayer perpendicular transport devices made using doped manganate perovskites

We report results of quantitative magnetization studies on epitaxial thin films of La0.60Y0.07Ca0.33MnO3−δ and La1−x⧠xMnO3−δ, with ⧠ standing for vacancy on the La site. The magnetization in all samples shows significant field dependent broadening of the ferromagnetic transition. Classical mean‐field analysis of magnetization data in applied fields above a few Tesla leads to a magnetic cluster size of the order of a few unit cells. The density of such a magnetic cluster correlates with the doping concentration x in the compound series La1−x⧠xMnO3−δ. In thin films of La0.60Y0.07Ca0.33MnO3−δ, a magnetic field dependent, non‐Curie–Weiss susceptibility was found in the paramagnetic state, indicating local ferromagnetic cluster formation above macroscopic ferromagnetic ordering temperature.

Does magnetization in thin‐film manganates suggest the existence of magnetic clusters?

We report a large magnetoresistance in in situ grown La0.60Y0.07Ca0.33MnOx thin films made at a low substrate temperature of 600 °C without postdeposition annealing. The zero‐field resistivity peaks around 120 K. The resistivity peak is reduced by two orders of magnitude by a 6 T field. Comparison of transport and magnetization data reveals a correlation of conductivity versus magnetization below the ferromagnetic transition temperature, which may suggest a double‐exchange‐based conduction mechanism. Above the ferromagnetic transition at low fields, a quadratic dependence of conductivity on magnetization is found, consistent with a spin‐scattering‐limited conductivity.

Transport and magnetic properties of in situ grown thin‐film La–Y–Ca–Mn–O

Recent progress in oxide perovskite thin-film technology has led to the discovery of a large negative magnetoresistance at room temperature in doped manganate perovskite thin films. These films may have potentials for magnetic sensing applications. In this paper we review the basic phenomena and physics of magnetotransport in this class of materials. We also discuss our recent demonstration of a large low-field magnetoresistance effect, and the associated challenges that lie ahead.

Magnetotransport in doped manganate perovskites

Etude des caracteristiques de l'oxydation en phase vapeur de couches minces de WSi x riches en Si preparees par depot chimique en phase vapeur a basse pression directement sur SiO 2 (x=2,7 apres depot). On a signale que les depots minces (moins de 100 nm) en phase vapeur de siliciure de tungstene adherent bien a SiO 2 . Par spectroscopie Auger et retrodiffusion de Rutherford, on voit que le silicium depassant la proportion stœchiometrique de WSi 2 diffuse a travers le siliciure jusqu'a la surface pour former un revetement de passivation en SiO 2 . L'energie d'activation du processus d'oxydation est E a +1,2 eV, en accord avec la diffusion de l'oxygene dans SiO 2 . On trouve une valeur semblable pour WSi x depose sur du silicium polycristallin. Pendant le recuit, le degre de non-stœchiometrie x de WSi x decroit de facon monotone avec la temperature du recuit, atteignant x=2 apres 30 min a 900°C ou 20 min a 950°C. Des temps plus prolonges et des temperatures plus elevees produisent un appauvrissement en silicium, avec x=1,7 au bout de 30 min a 1000°C. Simultanement, la resistivite de WSi x decroit aussi de ∼90 Ω/carre pour 1500 A a 5 Ω/carre apres recuit a 1000°C. Interet

L. Krusin-Elbaum

Papers

Observation of large low‐field magnetoresistance in trilayer perpendicular transport devices made using doped manganate perovskites

Does magnetization in thin‐film manganates suggest the existence of magnetic clusters?

Transport and magnetic properties of in situ grown thin‐film La–Y–Ca–Mn–O

Magnetotransport in doped manganate perovskites

Oxidation of Si-rich chemical-vapor-deposited films of tungsten silicide