36 research outputs found
Towards the theory of ferrimagnetism
Two-sublattice ferrimagnet, with spin- operators at the
sublattice site and spin- operators at the sublattice
site, is considered. The magnon of the system, the transversal fluctuation
of the total magnetization, is a complicate mixture of the transversal
fluctuations of the sublattice and spins. As a result, the magnons'
fluctuations suppress in a different way the magnetic orders of the and
sublattices and one obtains two phases. At low temperature the
magnetic orders of the and spins contribute to the magnetization of the
system, while at the high temperature , the magnetic order of the
spins with a weaker intra-sublattice exchange is suppressed by magnon
fluctuations, and only the spins with stronger intra-sublattice exchange has
non-zero spontaneous magnetization. The transition is a transition
between two spin-ordered phases in contrast to the transition from spin-ordered
state to disordered state (-transition). There is no additional symmetry
breaking, and the Goldstone boson has a ferromagnetic dispersion in both
phases. A modified spin-wave theory is developed to describe the two phases.
All known Neel's anomalous curves are reproduced, in particular that
with "compensation point". The theoretical curves are compared with
experimental ones for sulpho-spinel and rare earth iron
garnets.Comment: 9 pages, 8 figure
On the rotational dynamics of the Rattleback
The Rattleback is a very popular science toy shown to students all over the
world to demonstrate the non-triviality of rotational motion. When spun on a
horizontal table, this boat-shaped object behaves in a peculiar way. Although
the object appears symmetric, the dynamics of its motion seem very asymmetric.
When spun in the preferred direction, it spins smoothly, whereas in the other
direction it starts to oscillate wildly. The oscillation soon dies out and the
rattleback starts to spin in the preferred way. We will construct and go
through an analytical model capable of explaining this behaviour in a simple
and intelligible way. Although we aim at a semi-pedagogical treatise, we will
study the details only when they are necessary to understand the calculation.
After presenting the calculations we will discuss the physical validity of our
assumptions and take a look at more sophisticated models requiring numerical
analysis. We will then improve our model by assuming a simple friction force.Comment: 17 pages and 2 figures, typos corrected, some minor additions and
rewording
Theory of ferromagnetic (III,Mn)V semiconductors
The body of research on (III,Mn)V diluted magnetic semiconductors initiated
during the 1990's has concentrated on three major fronts: i) the microscopic
origins and fundamental physics of the ferromagnetism that occurs in these
systems, ii) the materials science of growth and defects and iii) the
development of spintronic devices with new functionalities. This article
reviews the current status of the field, concentrating on the first two, more
mature research directions. From the fundamental point of view, (Ga,Mn)As and
several other (III,Mn)V DMSs are now regarded as textbook examples of a rare
class of robust ferromagnets with dilute magnetic moments coupled by
delocalized charge carriers. Both local moments and itinerant holes are
provided by Mn, which makes the systems particularly favorable for realizing
this unusual ordered state. Advances in growth and post-growth treatment
techniques have played a central role in the field, often pushing the limits of
dilute Mn moment densities and the uniformity and purity of materials far
beyond those allowed by equilibrium thermodynamics. In (III,Mn)V compounds,
material quality and magnetic properties are intimately connected. In the
review we focus on the theoretical understanding of the origins of
ferromagnetism and basic structural, magnetic, magneto-transport, and
magneto-optical characteristics of simple (III,Mn)V epilayers, with the main
emphasis on (Ga,Mn)As. The conclusions we arrive at are based on an extensive
literature covering results of complementary ab initio and effective
Hamiltonian computational techniques, and on comparisons between theory and
experiment.Comment: 58 pages, 49 figures Version accepted for publication in Rev. Mod.
Phys. Related webpage: http://unix12.fzu.cz/ms
MAGNETIC PROPERTIES OF Cu1/2In1/2Cr2S4 AND SOME RELATED COMPOUNDS
Les propriétés magnétiques des composés spinelles M1/2In1/2Cr2X4 où M = Cu ou Ag et X = S ou Se ont été examinées. Dans le cas des sulfures, les moments de Cr sont ordonnés antiferromagnétiquement avec TN = 40 °K dans le cas M = Cu et TN = 17 °K dans le cas M = Ag. Aucun ordre magnétique à grande distance n'est observé à 4 °K pour Cu1/2In1/2Cr2Se4, tandis que le composé M = Ag est ferromagnétique en dessous de Tc ≈ 60 °K. La structure magnétique de Cu1/2In1/2Cr2S4 a été déterminée par diffraction des neutrons. Elle est bâtie à partir de quatre sous-réseaux d'aimantation parallèles aux grandes diagonales du cube.The magnetic properties of the spinels M1/2In1/2Cr2X4 with M = Cu or Ag and X = S or Se were investigated. In the sulphides the Cr spins order antiferromagnetically at TN = 40 °K for M = Cu and at TN = 17 °K for M = Ag. No spin ordering above 4 °K was observed in Cu1/2In1/2Cr2Se4, while Ag1/2In1/2Cr2Se4 is ferromagnetic below Tc ≈ 60 °K. The antiferromagnetic spin structure of Cu1/2In1/2Cr2S4 as determined by neutron diffraction, consists of four spin sublattices with sublattice magnetizations along the cubic body diagonals
STRONG ANISOTROPY IN THE CUBIC FERRIMAGNET FeCr2 S4
On a mesuré l'aimantation des monocristaux de
FeCr2S4 en fonction du champ magnétique dans les
directions [100], [110] et [111] entre 4 °K et Tc = 195 °K ; [100]
est l'axe d'aimantation facile et la direction [111] est très difficile. A 4 °K
la rigidité magnétique dans la direction [100] correspond a K1 = 3 x
106 erg. cm-3. Des spectres Mössbauer du 57Fe
dans FeCr2S4 ont été mesurés à 4 et 77 °K. A 4 °K il y a
un dédoublement quadrupolaire non axial. Dans
Cd0.9857Fe0.02Cr2S4 à 4
°K, au contraire, le dédoublement quadrupolaire est axial. L'anisotropie
magnétique et les dédoublements quadrupolaires sont discutés partant d'un
schéma de niveaux d'énergie d'un ion Fe2+ isolé situé dans un site
tétraédrique.The magnetization of single crystals of
FeCr2S4 has been measured between 4 °K and Tc
= 195 °K with the field in the [100], [110] and [111] directions. [100] is the
easy axis of magnetization, [111] is very hard. At 4 °K the stiffness in the
[100] direction corresponds to K1 = 3 x 106
erg.cm-3. 57Fe Mössbauer spectra of
FeCr2S4 has been obtained at 77 and 4 °K. A non axial
quadrupole splitting is observed at 4 °K. A uniaxial quadrupole splitting is
found in Cd1-xFe57xCr2S4
(x = 0.02) at 4 °K. The magnetic anisotropy and the quadrupole splittings are
discussed on the basis of a single ion energy level scheme of tetrahedrally
coordinated Fe2+
MAGNETIC PROPERTIES OF THE INTERMETALLIC COMPOUNDS RFe2
Nous avons déterminé les aimantations à saturation à 4,2 °K, la température de Curie et les paramètres de la maille pour un certain nombre de composés intermétalliques du type RFe2 (R = Ce, Sm, Gd, Tb, Dy, Ho, Er, Y, Zr). Le comportement magnétique des composés RxY1-xFe2 (R = Gd, Tb, Er, Ce) a été étudié en fonction de x. On discute les moments variables du Fe dans les composés RFe2 et l'absence d'une température de compensation dans la variation thermique de l'aimantation des composés RFe2 (R = Gd, Tb, Dy, Ho).For a number of compounds RFe2 (R = Ce, Sm, Gd, Tb, Dy, Ho, Er, Y, Zr) the lattice constant, the saturation moment at 4.2 °K and the Curie temperature have been determined. The magnetic properties of the compounds RxY1-xFe2 for R = Gd, Tb, Er and Ce have been studied as a function of composition. The variable iron moment in the compounds RFe2 is discussed in terms of a rigid-band model with a non-saturated d-band magnetization. It is shown that the absence of a compensation point in the temperature dependence of the magnetization for R = Gd, Tb, Dy and Ho is a result of a relatively strong negative R-Fe interaction