19 research outputs found
Landau theory of compressible magnets near a quantum critical point
Landau theory is used to investigate the behaviour of a metallic magnet
driven towards a quantum critical point by the application of pressure. The
observed dependence of the transition temperature with pressure is used to show
that the coupling of the magnetic order to the lattice diverges as the quantum
critical point is approached. This means that a first order transition will
occur in magnets (both ferromagnets and antiferromagnets) because of the
coupling to the lattice. The Landau equations are solved numerically without
further approximations. There are other mechanisms that can cause a first order
transition so the significance of this work is that it will enable us to
determine the extent to which any particular first order transition is driven
by coupling to the lattice or if other causes are responsible.Comment: 12 pages including 5 figures, to be presented at MMM-Intermag
conference and accepted for publication in Journal of Applied Physic
Theory of magnetism with temporal disorder applied to magnetically doped ZnO
A dynamic model of the asymmetric Ising glass is presented: an Ising model
with antiferromagnet bonds with probabilities q arranged at random in a
ferromagnetic matrix. The dynamics is introduced by changing the arrangement of
the antiferromagnetic bonds after n Monte Carlo steps but keeping the same
value of q and spin configuration. In the region where there is a second order
transition between the ferromagnetic and paramagnetic states the dynamic
behaviour follows that expected for motional narrowing and reverts to the
static behaviour only for large n. There is a different dynamic behaviour where
there is a first order transition between the ferromagnetic and spin glass
states where it shows no effects of motional narrowing. The implications of
this are discussed. This model is devised to explain the properties of doped
ZnO where the magnetisation is reduced when the exchange interactions change
with time.Comment: Paper was presented at MMM 2008 and is accepted for publication in
J.A.
Grain boundary ferromagnetism in vanadium-doped InO thin films
Room temperature ferromagnetism was observed in InO\,^{\circ}{\rm C}_2_3$ host lattice, thus excluding the existence of secondary phases
of vanadium compounds. Magnetic measurements based on SQUID magnetometry and
magnetic circular dichroism confirm that the magnetism is at grain boundaries
and also in the grains. The overall magnetization originates from the competing
effects between grains and grain boundaries.Comment: 12 pages, 7 figures, 1 table, accepted by Europhysics Letter
On Using Magnetic and optical methods to determine the size and characteristics of nanoparticles embedded in oxide semiconductors
Films of oxides doped with transition metals are frequently believed to have
magnetic inclusions. Magnetic methods to determine the amount of nanophases and
their magnetic characteristics are described. The amount of the sample that is
paramagnetic may also be measured. Optical methods are described and shown to
be very powerful to determine which defects are also magnetic.Comment: Manuscript of poster to be presented at MMM-Intermag 2010. Accepted
for publication in Magnetic Trans of IEE
Spin polarized transport current in n-type co-doped ZnO thin films measured by Andreev spectroscopy
We use point contact Andreev reflection measurements to determine the spin
polarization of the transport current in pulse laser deposited thin films of
ZnO with 1% Al and with and without 2%Mn. Only films with Mn are ferromagnetic
and show spin polarization of the transport current of up to 55 0.5% at
4.2 K, in sharp contrast to measurements of the nonmagnetic films without Mn
where the polarization is consistent with zero. Our results imply strongly that
ferromagnetism in these Al doped ZnO films requires the presence of Mn.Comment: Published versio
Magneto-optical properties of Co/ZnO multilayer films
Multilayer films of ZnO with Co were deposited on glass substrates then
annealed in a vacuum. The magnetisation of the films increased with annealing
but not the magnitude of the magneto-optical signals. The dielectric functions
for the films were calculated using the MCD spectra. A Maxwell Garnett theory
of a metallic Co/ZnO mixture is presented. The extent to which this explains
the MCD spectra taken on the films is discussed.Comment: This paper was presented at ICM (2009) and is accepted in this form
for the proceeding