A review of new developments in theoretical and experimental electronic-structure investigations of half-metallic ferromagnets (HMFs) is presented. Being semiconductors for one spin projection and metals for another, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMFs by the peculiarities of their electronic structure and chemical bonding is discussed. The effects of electron-magnon interaction in HMFs and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Special attention is paid to the appearance of nonquasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-the-art electronic calculations for correlated d-systems are discussed, and results for specific HMFs (Heusler alloys, zinc-blende structure compounds, CrO2, and Fe3O4) are reviewed.

/pdf/half-metallic-ferromagnets-from-band-structure-to-many-body-1hrl8mp5lm.pdf

Half-metallic ferromagnets: From band structure to many-body effects

Resistivity saturation is observed in many metallic systems with large resistivities---when the resistivity has reached a critical value, its further increase with temperature is substantially reduced. This typically happens when the apparent mean free path is comparable to the interatomic separations---the Ioffe-Regel condition. Recently, several exceptions to this rule have been found. This colloquium first reviews experimental results and early theories of resistivity saturation. It then describes more recent theoretical work, addressing cases both where the Ioffe-Regel condition is satisfied and where it is violated. In particular, the authors show how the (semiclassical) Ioffe-Regel condition can be derived quantum mechanically under certain assumptions about the system and why these assumptions are violated for high-${T}_{c}$ cuprates and alkali-doped fullerides.

/pdf/colloquium-saturation-of-electrical-resistivity-35kpresk7b.pdf

Colloquium : Saturation of electrical resistivity

Magnetotransport measurements were made on patterned, (110) oriented ${\mathrm{CrO}}_{2}$ thin films grown by the high-pressure, thermal decomposition of ${\mathrm{CrO}}_{3}$ onto rutile substrates. The low-temperature Hall effect exhibits a sign reversal from positive to negative as the magnetic field is increased above 1 T, which may be interpreted within a simple two-band model as indicating the presence of highly mobile $({\ensuremath{\mu}}_{h}=0.25{\mathrm{m}}^{2}/\mathrm{V}\mathrm{}\mathrm{s})$ holes as well as a much larger number of less mobile electrons $(n=0.4$ electrons/Cr). Between 50 and 100 K, the field at which the sign reversal occurs rapidly increases and a contribution from the anomalous Hall effect becomes significant, while the large, positive transverse magnetoresistance (MR) observed at low temperatures changes over to a predominantly negative MR. These changes correlate with a thermally activated dependence in the resistivity of the form ${T}^{2}{e}^{\ensuremath{-}\ensuremath{\Delta}/T}$ with $\ensuremath{\Delta}\ensuremath{\approx}80\mathrm{K},$ reflecting the lack of temperature dependence in the resistivity at low temperatures and a ${T}^{2}$ behavior above 100 K. The high mobilities at low temperature which result in the observed positive MR reflect the suppression of spin-flip scattering expected for a half-metallic system. However, the changes in magnetotransport above the temperature $\ensuremath{\Delta}$ must be due to the onset of spin-flip scattering, even though ${k}_{B}\ensuremath{\Delta}$ is much less than the expected energy gap in the minority spin density of states. The significance of $\ensuremath{\Delta}$ is discussed in terms of recent models for another half-metallic system, the perovskite manganites, and the possible formation of ``shadow bands.''

/pdf/evidence-for-two-band-magnetotransport-in-half-metallic-1icijjtaxj.pdf

Evidence for two-band magnetotransport in half-metallic chromium dioxide

Motivated by a collection of experimental results indicating the strongly correlated nature of the ferromagnetic metallic state of CrO 2 , we present results based on a combination of the actual band structure with dynamical-mean-field theory for the multiorbital case. In striking contrast to LSDA(+U) and model many-body approaches, much better semiquantitative agreement with (i) recent photoemission results, (ii) domain of applicability of the half-metal concept, and (iii) thermodynamic and dc transport data is obtained within a single picture. Our approach has broad applications for the detailed first-principles investigation of other transition metal oxide-based half-metallic ferromagnets.

Orbital Kondo effect in CrO2: a combined local-spin-density-approximation dynamical-mean-field-theory study.

Band-structure calculations predict a gap of 1.5 eV in the minority-spin density of states of the half-metallic ferromagnet ${\mathrm{CrO}}_{2}.$ The magnitude of ${T}_{C},$ the magnons, and the activation gap in the resistivity cannot be explained with this band picture. We propose a double-exchange mechanism for two electrons per Cr site invoking strong Hund's rule correlations and the distortion in the ${\mathrm{CrO}}_{6}$ octahedra, which localize one electron into the $\mathrm{xy}$ orbital at each Cr site, while the electrons in the $\mathrm{xz}$ and $\mathrm{yz}$ orbitals are itinerant. The effective exchange interaction naturally leads to a ferromagnetic ground state with low-energy magnon excitations and several branches of excitations separated by energy gaps from the ground state, reducing ${T}_{C}.$ Quantum fluctuations suppress the orbital long-range order, giving rise to an orbital resonant valence bond-type ground state.

Double-exchange mechanism for CrO 2

Infrared spectroscopy is used to investigate the electronic structure and charge carrier relaxation in crystalline films of ${\mathrm{CrO}}_{2}$ which is the simplest of all half-metallic ferromagnets. Chromium dioxide is a bad metal at room temperature but it has a remarkably low residual resistivity $(l5 \ensuremath{\mu}\ensuremath{\Omega}\mathrm{cm})$ despite the small spectral weight associated with free carrier absorption. The infrared measurements show that low residual resistivity is due to the collapse of the scattering rate at $\ensuremath{\omega}l2000 {\mathrm{cm}}^{\ensuremath{-}1}.$ The blocking of the relaxation channels at low $\ensuremath{\omega}$ and T can be attributed to the unique electronic structure of a half-metallic ferromagnet. In contrast to other ferromagnetic oxides, the intraband spectral weight is constant below the Curie temperature.

/pdf/charge-dynamics-in-the-half-metallic-ferromagnet-cro-2-514lvdxcr0.pdf

Charge dynamics in the half-metallic ferromagnet CrO 2

Handle everyday research tasks with reliable, citation-backed results

Your personal Research Agent to handle research tasks with citation-backed results

Popular Tasks used by Researchers

How can I help with your research?

Meet SciSpace

Get more enhanced response by uploading the PDFs you want me to reference.

No relevant PDFs in your library

SciSpace is the AI research assistant for academics. Run systematic literature reviews on 280M+ papers, and write papers with cited sources. Trusted by 1M+ students, PhDs & researchers.

SciSpace | AI for Research

Analyze PDFs

Code & Manuscripts

Funding & Grants

Literature & Patents

Medical & Clinical Data

Systematic Review

Visualize & Present

Web & Data

Build a Google Scholar-like website for your research.

Build a website

Create charts and images for your research

Create a Chart

Write a paper for submission to a journal

Draft a manuscript

Patent Search

Design eye-catching scientific posters in minutes.

Scientific Poster Generation

Systematic Literature Review

One task is running at the moment. Your messages will be shown right after.

Drag and drop or click here to browse

Loved by <highlight>1 million+</highlight> researchers

Extract a list of specific topics and their sources from unstructured text

Topics

Compare and analyze relevant papers that matches with your search

Papers

Get insights from PDFs and bookmarked papers from your library

My library

Recent searches

Try searching for:

Catch AI-generated content in scholarly and non-scholarly content

{ai} Detector

Ai Writer

Get PDF Summaries, highlighted text explanations 

Chat with PDF

Effortlessly create in-text citations and bibliographies in APA and 2,500 other formats

Citation generator

Get explanations, summaries, and answers on academic papers

Ease up your research workflow with {scispace}'s cohort of exciting AI tools

Elevate your academic writing skills and convey your ideas the way you want

Paraphraser

Explore our range of reading and writing tools

Your file is being prepared and should be ready in a few minutes. If it's a large file, it might take a bit longer. You can close this window, and we'll email you the file when it's done.

You have reached a maximum limit of <strong>{limit}</strong> columns in the table. Remove at least <strong>1</strong> column to add or create another one.

Christopher P. Weber

Author Tools

Chat about Author

Papers

Charge dynamics in the half-metallic ferromagnet CrO 2