Colossal-magnetoresistance materials: manganites and conventional ferromagnetic semiconductors

The magnetic properties of the ground state of a low-density free-electron gas in three dimensions have been the subject of theoretical speculation and controversy for seven decades. Not only is this a difficult theoretical problem to solve, it is also a problem which has not hitherto been directly addressed experimentally. Here we report measurements on electron-doped calcium hexaboride (CaB6) which, we argue, show that-at a density of 7× 1019 electrons cm-3-the ground state is ferromagnetically polarized with a saturation moment of 0.07 µB per electron. Surprisingly, the magnetic ordering temperature of this itinerant ferromagnet is 600 K, of the order of the Fermi temperature of the electron gas.

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High-temperature weak ferromagnetism in a low-density free-electron gas

The perovskite manganites with generic formula RE1?xAExMnO3 (RE = rare earth, AE = Ca, Sr, Ba and Pb) have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). The most fundamental property of these materials is strong correlation between structure, transport and magnetic properties. They exhibit extraordinary large magnetoresistance named CMR in the vicinity of the insulator?metal/paramagnetic?ferromagnetic transition at relatively large applied magnetic fields. However, for applied aspects, occurrence of significant CMR at low applied magnetic fields would be required. This review consists of two sections: in the first section we have extensively reviewed the salient features, e.g.?structure, phase diagram, double-exchange mechanism, Jahn?Teller effect, different types of ordering and phase separation of CMR manganites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for various doped manganites having natural grain boundaries such as polycrystalline, nanocrystalline bulk and films, manganite-based composites and intrinsically layered manganites, and artificial grain boundaries such as bicrystal, step-edge and laser-patterned junctions. Some other potential magnetoresistive materials, e.g.?pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc, are also briefly dealt with. The review concludes with an overview of grain-boundary-induced low field magnetotransport behavior and prospects for possible applications.

https://iopscience.iop.org/article/10.1088/0953-8984/20/27/273201/pdf

Low field magnetotransport in manganites

The perovskite manganites of general formula RE_1-xAe_xMnO_3 (RE= rare earth,AE=Ca, Sr, Ba and Pb)have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). They exhibit extraordinary large magnetoresistance pronounced as CMR in the vicinity of insulator-metal/paramagnetic-ferromagnetic transition at a relatively large applied magnetic fields. However, for applied aspectes, occurence of significant CMR at low applied magnetic fields would be required. This review consists of of two sections: In the first section we have extensively reviewed the salient features e.g. structure, phase diagram, double exchange mechansim, Jahn Teller effect, different types of ordering and phase separation of CMR mangnaites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for doped manganites such as polycrystalline La_0.67Ca_0.33MnO_3 films, Ag admixed La_0.67Ca_0.33MnO_3 films, polycrystalline (La_0.7Ca_0.2Ba_0.1MnO_3)and epitaxial (La_0.67Ca_0.33MnO_3) films on different substrates, nanophasic La_0.7Ca_0.3MnO_3, mangnaite-polymer composites (La_0.7Ba_0.2Sr_0.1MnO_3-PMMA and La_0.67Ca_0.33MnO_3-PMMA)and double layered polycrystalline (La_1.4Ca_1.6-xBa_xMn_2O_7) and films (La_1.4Ca_1.6Mn_2O_7). Some other potential magnetoresistive materials e.g. pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc have aslo been briefly dealt with. The review concludes with the summary of results for low field magnetotransport behaviour and prospectes for applications.

Low Field Magnetotransport in Manganites

${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ crystallizes in the pyrochlore structure and undergoes a metal-insulator transition (MIT) near 226 K. We have characterized the MIT in ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ using x-ray diffraction, resistivity at ambient and high pressure, specific heat, magnetization, thermopower, Hall coefficient, and thermal conductivity. Both single crystals and polycrystalline material were examined. The MIT is accompanied by no change in crystal symmetry and a change in unit-cell volume of less than 0.05%. The resistivity shows little temperature dependence above 226 K, but increases by 3 orders of magnitude as the sample is cooled to 4 K. The specific heat anomaly resembles a mean-field transition and shows no hysteresis or latent heat. ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ orders magnetically at the MIT. The magnetization data are consistent with antiferromagnetic order, with a small parasitic ferromagnetic component. The Hall and Seebeck coefficients are consistent with a semiconducting gap opening at the Fermi energy at the MIT. We have also performed electronic structure calculations on ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}.$ These calculations indicate that ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ is metallic, with a sharp peak in the density of states at the Fermi energy. We interpret the data in terms of a Slater transition. In this scenario, the MIT is produced by a doubling of the unit cell due to the establishment of antiferromagnetic order. A Slater transition---unlike a Mott transition---is predicted to be continuous, with a semiconducting energy gap opening much like a BCS gap as the material is cooled below ${T}_{\mathrm{MIT}}.$

Continuous metal-insulator transition in the pyrochlore Cd 2 Os 2 O 7

We present a study of the structural and magnetic properties of single-crystalline EuB{sub 6}. Temperature-dependent x-ray diffraction found no significant anomalies at the onset of ferromagnetic order. The resistivity, dc susceptibility, magnetization, and specific heat prove the crystal to be of extraordinarily high quality. Two ferromagnetic transitions at T{sub c1}=15.3 K and T{sub c2}=12.5 K are observed in all investigated properties. The ordered state displays an unexpected anisotropy, and we derive the magnetic phase diagrams for the three principal cubic directions. We argue that the two magnetic phases are connected by spin reorientation, enabled by a reduction of the crystalline symmetry from cubic. An additional anomaly in the specific heat is observed at low temperatures, which we interpret as arising from the splitting of the Eu ground-state multiplet in internal magnetic fields. {copyright} {ital 1998} {ital The American Physical Society}

Structure and magnetic order of EuB 6

We present transport and thermodynamic measurements as a function of temperature (0.1{endash}300 K), pressure (1 bar{endash}16 kbars), and magnetic field (0{endash}600 kOe) on YbInNi{sub 4}. Ferromagnetism arises near 3 K out of a state with enhanced electronic specific-heat coefficient. Resistivity, specific-heat, and magnetization measurements imply that the ground state of YbInNi{sub 4} is a crystal-field doublet, whereas earlier neutron-scattering results suggest a ground-state quartet. We also compare YbInNi{sub 4} to YbInCu{sub 4} and intermediate alloys. {copyright} {ital 1998} {ital The American Physical Society}

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Ferromagnetism and crystal fields in ybinni4

Ferromagnetism and crystal fields in YbInNi_4

Growth of LaMnO{sub 3} films that exhibit colossal magnetoresistance (CMR) has concentrated heavily on Ca doped materials. However, since the 33% Sr doped films are ferromagnetic at room temperature, they are ideal candidates for dual growth-magnetic structure studies using scanned probe techniques. In this study, interest was focused on the relations between growth/processing parameters, film morphology, and electronic/magnetic properties. In addition, films were grown on both LaAlO{sub 3} (LAO) and SrTiO{sub 3} (STO) to examine the results of stress induced by different substrate mismatches. La{sub 0.67}Sr{sub 0.33}MnO{sub 3} (LSMO) was grown using pulsed laser deposition (PLD) at temperatures between 500 C and 800 C. The film microstructure, crystallinity, and magnetic and electrical properties were characterized by room temperature scanning tunneling microscopy (STM), atomic force microscopy (AFM), magnetic force microscopy (MFM), x-ray diffraction, and temperature dependent transport and magnetization measurements. The growth trends follow those previously reported for Ca doped films. Grains increase in size with increasing temperature and coalesce into extended layers after annealing. Although topographic contributions complicate interpretation of some MFM data, local magnetic structure observed here is generally associated with film defects.

/pdf/growth-and-magnetic-structure-of-la-sub-0-67-sr-sub-0-33-mno-6fhpezwko9.pdf

Growth and magnetic structure of La{sub 0.67}Sr{sub 0.33}MnO{sub 3} films

Growth of LaMnO3 films that exhibit colossal magnetoresistance (CMR) has concentrated heavily on Ca doped materials. However, since the 33% Sr doped films are ferromagnetic at room temperature, they are ideal candidates for dual growth-magnetic structure studies using scanned probe techniques. In this study, interest was focused on the relations between growth/processing parameters, film morphology, and electronic/magnetic properties. In addition, films were grown on both LaAIO3 (LAO) and SrTiO3 (STO) to examine the results of stress induced by different substrate mismatches. La0.67Sr0.33MnO3 (LSMO) was grown using pulsed laser deposition (PLD) at temperatures between 500 °C and 800 °C. The film microstructure, crystallinity, and magnetic and electrical properties were characterized by room temperature scanning tunneling microscopy (STM), atomic force microscopy (AFM), magnetic force microscopy (MFM), x-ray diffraction, and temperature dependent transport and magnetization measurements. The growth trends follow those previously reported for Ca doped films. Grains increase in size with increasing temperature and coalesce into extended layers after annealing. Although topographic contributions complicate interpretation of some MFM data, local magnetic structure observed here is generally associated with film defects.

D.K. Hristova

Papers

Structure and magnetic order of EuB 6

Ferromagnetism and crystal fields in ybinni4

Ferromagnetism and crystal fields in YbInNi_4

Growth and magnetic structure of La{sub 0.67}Sr{sub 0.33}MnO{sub 3} films

Growth and Magnetic Structure of La0.67Sr0.33MnO3 Films