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Register a new userBeating the superparamagnetic size limit of nanoparticles on a ferroelectric substrate
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When decreasing the size of nanoscale magnetic particles their magnetization becomes vulnerable to thermal fluctuations as approaching the superparamgnetic limit, hindering thus applications relying on a stable magnetization. Here, we show theoretically that a magnetoelectric coupling to a ferroelectric substrate renders possible the realization of substantially smaller nano clusters with thermally stable magnetization. For an estimate of cluster size we perform calculations with realistic material parameters for iron nano particles on ferroelectric BaTiO3 substrate. We find, steering the polarization of BaTiO3 with electric fields affects the magnetism of the deposited magnetic clusters. These findings point to a qualitatively new class of superparamagnetic composites.
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Most multiferroic materials with coexisting ferroelectric and magnetic order exhibit cycloidal antiferromagnetism with wavelength of several nanometers. The prototypical example is bismuth ferrite (BiFeO$_3$ or BFO), a room-temperature multiferroic considered for a number of technological applications. While most applications require small sizes such as nanoparticles, little is known about the state of these materials when their sizes are comparable to the cycloid wavelength. This work describes a microscopic theory of cycloidal magnetism in nanoparticles based on Hamiltonian calculations. It is demonstrated that magnetic anisotropy close to the surface has a huge impact on the multiferroic ground state. For certain nanoparticle sizes the modulus of the ferromagnetic and ferroelectric moments are bistable, an effect that may be used in the design of ideal memory bits that can be switched electrically and read out magnetically.
The results of density functional theory calculations and measurements using X-ray photoelectron spectroscopy of Co-nanoparticles dispersed on graphene/Cu are presented. It is found that for low cobalt thickness (0.02 nm - 0.06 nm) the Co forms islands distributed non-homogeneously which are strongly oxidized under exposure to air to form cobalt oxides. At greater thicknesses up to 2 nm the upper Co-layers are similarly oxidized whereas the lower layers contacting the graphene remain metallic. The measurements indicate a Co2+ oxidation state with no evidence of a 3+ state appearing at any Co thickness, consistent with CoO and Co[OH]2. The results show that thicker Co (2nm) coverage induces the formation of a protective oxide layer while providing the magnetic properties of Co nanoparticles.
We present X-ray diffraction (XRD), Mossbauer spectroscopy (MS) and d.c. magnetization measurements performed on ball-milled CuFe2O4 samples. The average particle size <d> was found to decrease to the nanometer range after t=15 min of milling. Room temperature Mossbauer data showed that the fraction of particles above the blocking temperature TB increases with milling time, and almost complete superparamagnetic samples are obtained for <d> = 7(2) nm. Magnetization measurements below TB suggest spin canting in milled samples. The values of saturation moment mu_S reveal that site populations are slightly affected by milling. Mossbauer resonant intensities are accounted for on the basis of local disorder of Fe3+ environments, and the development of sample inhomogeneities of CuxFe3-xO4 composition.
The possibility of reconciliation between seemingly mutually exclusive properties in one system can not only lead to theoretical breakthroughs but also potential novel applications. The research on the coexistence of two purportedly contra-indicated properties, ferroelectricity/polarity and conductivity, proposed by Anderson and Blount over 50 years ago was recently revitalized by the discovery of the first unambiguous polar metal LiOsO3 and further fueled by the demonstration of the first switchable ferroelectric metal WTe2. In this review, we first discuss the reasons why the coexistence of ferroelectricity/polarity and conductivity have been deemed incompatible, followed by a review on the history of ferroelectric/polar metals. Secondly, we review the important milestones along with the corresponding mechanisms for the ferroelectric/polar metallic phases in these materials. Thirdly, we summarize the design approaches for ferroelectric/polar metals. Finally, we discuss the future prospects and potential applications of ferroelectric/polar metals.
This paper reports the synthesis and detailed characterization of graphite thin films produced by thermal decomposition of the (0001) face of a 6H-SiC wafer, demonstrating the successful growth of single crystalline films down to approximately one graphene layer. The growth and characterization were carried out in ultrahigh vacuum (UHV) conditions. The growth process and sample quality were monitored by low-energy electron diffraction, and the thickness of the sample was determined by core level x-ray photoelectron spectroscopy. High-resolution angle-resolved photoemission spectroscopy shows constant energy map patterns, which are very sharp and fully momentum-resolved, but nonetheless not resolution limited. We discuss the implications of this observation in connection with scanning electron microscopy data, as well as with previous studies.
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