45 research outputs found
Slater-Pauling Rule and Curie-Temperature of Co-based Heusler compounds
A concept is presented serving to guide in the search for new materials with
high spin polarization. It is shown that the magnetic moment of half-metallic
ferromagnets can be calculated from the generalized Slater-Pauling rule.
Further, it was found empirically that the Curie temperature of Co based
Heusler compounds can be estimated from a seemingly linear dependence on the
magnetic moment. As a successful application of these simple rules, it was
found that CoFeSi is, actually, the half-metallic ferromagnet exhibiting
the highest magnetic moment and the highest Curie temperature measured for a
Heusler compound
Correlation in the transition metal based Heusler compounds CoMnSi and CoFeSi
Half-metallic ferromagnets like the full Heusler compounds with formula
XYZ are supposed to show an integer value of the spin magnetic moment.
Calculations reveal in certain cases of X = Co based compounds non-integer
values, in contrast to experiments. In order to explain deviations of the
magnetic moment calculated for such compounds, the dependency of the electronic
structure on the lattice parameter was studied theoretically. In local density
approximation (LDA), the minimum total energy of CoFeSi is found for the
experimental lattice parameter, but the calculated magnetic moment is about 12%
too low. Half-metallic ferromagnetism and a magnetic moment equal to the
experimental value of are found, however, only after increasing the
lattice parameter by more than 6%.
To overcome this discrepancy, the LDA scheme was used to respect on-site
electron correlation in the calculations. Those calculations revealed for
CoFeSi that an effective Coulomb-exchange interaction in the
range of about 2eV to 5eV leads to half-metallic ferromagnetism and the
measured, integer magnetic moment at the measured lattice parameter. Finally,
it is shown in the case of CoMnSi that correlation may also serve to
destroy the half-metallic behavior if it becomes too strong (for CoMnSi
above 2eV and for CoFeSi above 5eV). These findings indicate that on-site
correlation may play an important role in the description of Heusler compounds
with localized moments.Comment: submitted to Phys. Rev.
Spectroscopic fingerprints for charge localization in the organic semiconductor (DOEO)4[HgBr4]·TCE
Changes of the electronic structure accompanied by charge localization and a transition to an antiferromagnetic ground state were observed in the organic semiconductor (DOEO)4 [HgBr4 ]·TCE. Localization starts in the temperature region of about 150 K and the antiferromagnetic state occurs below 60 K. The magnetic moment of the crystal contains contributions of inclusions (droplets), and individual paramagnetic centers formed by localized holes and free charge carriers at 2 K. Two types of inclusions of 100â400 nm and 2â5 nm sizes were revealed by transmission electron microscopy. Studying the temperature-and angular dependence of electron spin resonance (ESR) spectra revealed fingerprints of antiferromagnetic contributions as well as paramagnetic resonance spectra of individual localized charge carriers. The results point on coexistence of antiferromagnetic long and short range order as evident from a second ESR line. Photoelectron spectroscopy in the VUV, soft and hard X-ray range shows temperature-dependent effects upon crossing the critical temperatures around 60 K and 150 K. The substantially different probing depths of soft and hard X-ray photoelectron spectroscopy yield nformation on the surface termination. The combined investigation using complementary methods at the same sample eveals the close relation of changes in the transport properties and in the energy distribution of electronic states
High energy, high resolution photoelectron spectroscopy of Co2Mn(1-x)Fe(x)Si
This work reports on high resolution photoelectron spectroscopy for the
valence band of Co2Mn(1-x)Fe(x)Si (x=0,0.5,1) excited by photons of about 8 keV
energy. The measurements show a good agreement to calculations of the
electronic structure using the LDA+U scheme. It is shown that the high energy
spectra reveal the bulk electronic structure better compared to low energy XPS
spectra. The high resolution measurements of the valence band close to the
Fermi energy indicate the existence of the gap in the minority states for all
three alloys.Comment: 14 pages, 5 figures, submitted to J. Phys. D: Appl. Phy
Design of magnetic materials: CoCrFeAl
Doped Heusler compounds CoCrFeAl with varying Cr to Fe
ratio were investigated experimentally and theoretically. The electronic
structure of the ordered, doped Heusler compound CoCrFeAl
( was calculated using different types of band structure
calculations. The ordered compounds turned out to be ferromagnetic with small
Al magnetic moment being aligned anti-parallel to the 3d transition metal
moments. All compounds show a gap around the Fermi-energy in the minority
bands. The pure compounds exhibit an indirect minority gap, whereas the
ordered, doped compounds exhibit a direct gap. Magnetic circular dichroism
(MCD) in X-ray absorption spectra was measured at the edges of Co,
Fe, and Cr of the pure compounds and the alloy in order to determine
element specific magnetic moments. Calculations and measurements show an
increase of the magnetic moments with increasing iron content. The
experimentally observed reduction of the magnetic moment of Cr can be explained
by Co-Cr site-disorder. The presence of the gap in the minority bands of
CoCrAl can be attributed to the occurrence of pure Co and mixed CrAl
(001)-planes in the structure. It is retained in structures with
different order of the CrAl planes but vanishes in the -structure with
alternating CoCr and CoAl planes.Comment: corrected author lis
Electronic structure and spectroscopy of the quaternary Heusler alloy CoCrFeAl
Quaternary Heusler alloys CoCrFeAl with varying Cr to Fe
ratio were investigated experimentally and theoretically. The electronic
structure and spectroscopic properties were calculated using the full
relativistic Korringa-Kohn-Rostocker method with coherent potential
approximation to account for the random distribution of Cr and Fe atoms as well
as random disorder. Magnetic effects are included by the use of spin dependent
potentials in the local spin density approximation.
Magnetic circular dichroism in X-ray absorption was measured at the
edges of Co, Fe, and Cr of the pure compounds and the alloy in order to
determine element specific magnetic moments. Calculations and measurements show
an increase of the magnetic moments with increasing iron content. Resonant
(560eV - 800eV) soft X-ray as well as high resolution - high energy (keV) hard X-ray photo emission was used to probe the density of the
occupied states in CoCrFeAl.Comment: J.Phys.D_Appl.Phys. accepte
Magnetic dichroism in angular-resolved hard X-ray photoelectron spectroscopy from buried layers
This work reports the measurement of magnetic dichroism in angular-resolved
photoemission from in-plane magnetized buried thin films. The high bulk
sensitivity of hard X-ray photoelectron spectroscopy (HAXPES) in combination
with circularly polarized radiation enables the investigation of the magnetic
properties of buried layers. HAXPES experiments with an excitation energy of 8
keV were performed on exchange-biased magnetic layers covered by thin oxide
films. Two types of structures were investigated with the IrMn exchange-biasing
layer either above or below the ferromagnetic layer: one with a CoFe layer on
top and another with a CoFeAl layer buried beneath the IrMn layer. A
pronounced magnetic dichroism is found in the Co and Fe states of both
materials. The localization of the magnetic moments at the Fe site conditioning
the peculiar characteristics of the CoFeAl Heusler compound, predicted to
be a half-metallic ferromagnet, is revealed from the magnetic dichroism
detected in the Fe states