83 research outputs found
Fluctuations and Instabilities of Ferromagnetic Domain Wall pairs in an External Magnetic Field
Soliton excitations and their stability in anisotropic quasi-1D ferromagnets
are analyzed analytically. In the presence of an external magnetic field, the
lowest lying topological excitations are shown to be either soliton-soliton or
soliton-antisoliton pairs. In ferromagnetic samples of macro- or mesoscopic
size, these configurations correspond to twisted or untwisted pairs of Bloch
walls. It is shown that the fluctuations around these configurations are
governed by the same set of operators. The soliton-antisoliton pair has exactly
one unstable mode and thus represents a critical nucleus for thermally
activated magnetization reversal in effectively one-dimensional systems. The
soliton-soliton pair is stable for small external fields but becomes unstable
for large magnetic fields. From the detailed expression of this instability
threshold and an analysis of nonlocal demagnetizing effects it is shown that
the relative chirality of domain walls can be detected experimentally in thin
ferromagnetic films. The static properties of the present model are equivalent
to those of a nonlinear sigma-model with anisotropies. In the limit of large
hard-axis anisotropy the model reduces to a double sine-Gordon model.Comment: 15 pages RevTex 3.0 (twocolumn), 9 figures available on request, to
appear in Phys Rev B, Dec (1994
Universal energy distribution for interfaces in a random field environment
We study the energy distribution function for interfaces in a
random field environment at zero temperature by summing the leading terms in
the perturbation expansion of in powers of the disorder strength,
and by taking into account the non perturbational effects of the disorder using
the functional renormalization group. We have found that the average and the
variance of the energy for one-dimensional interface of length behave as,
, , while the distribution
function of the energy tends for large to the Gumbel distribution of the
extreme value statistics.Comment: 4 pages, 2 figures, revtex4; the distribution function of the total
and the disorder energy is include
Statistical Mechanics of Nonuniform Magnetization Reversal
The magnetization reversal rate via thermal creation of soliton pairs in
quasi-1D ferromagnetic systems is calculated. Such a model describes e.g. the
time dependent coercivity of elongated particles as used in magnetic recording
media. The energy barrier that has to be overcome by thermal fluctuations
corresponds to a soliton-antisoliton pair whose size depends on the external
field. In contrast to other models of first order phase transitions such as the
phi^4 model, an analytical expression for this energy barrier is found for all
values of the external field. The magnetization reversal rate is calculated
using a functional Fokker-Planck description of the stochastic magnetization
dynamics. Analytical results are obtained in the limits of small fields and
fields close to the anisotropy field. In the former case the hard-axis
anisotropy becomes effectively strong and the magnetization reversal rate is
shown to reduce to the nucleation rate of soliton-antisoliton pairs in the
overdamped double sine-Gordon model. The present theory therefore includes the
nucleation rate of soliton-antisoliton pairs in the double sine-Gordon chain as
a special case. These results demonstrate that for elongated particles, the
experimentally observed coercivity is significantly lower than the value
predicted by the standard theories of N\'eel and Brown.Comment: 21 pages RevTex 3.0 (twocolumn), 6 figures available on request, to
appear in Phys Rev B, Dec (1994
Some recent radio talks.
Light land development in the South - By A. S. WILD, Assistant Superintendent of Wheat Farming, Department of Agriculture.
It is only since World War II ended that successful large scale development of light land near the south coast of this State has been undertaken, although the many millions of acres of treeless plain country throughout the agricultural areas have, for many years, presented a challenge to the pioneering ability of Western Australians.
The use of hormones in viticulture - By L. T. JONES, Senior Plant Research Officer
The discovery and proper use of plant hormones, or as they are more accurately termed, growth-regulating: substances, probably represents the greatest contribution which science has made to agriculture in the past few decades. It was the discovery by Kogl in 1934 that indole acetic acid was an active growth substance which led rapidly to the preparation and testing of large numbers of compounds of similar chemical structure and among these were found many substances which were very potent and also comparatively cheap to produce.
Recent trends in viticulture in Western Australia - By W. R. JAMIESON, Viticulturist, Department of Agriculture, Perth
Today I want to talk very briefly about some of the more recent trends in viticulture in this State. The commercial production of grapevines is centred mainly around the Swan Valley and as most of the suitable vineyard land in this area has been planted, growers are exploring avenues to increase the returns per acre.
Methods of irrigation - By G. GAUNTLETT, Assistant Officer-in-Charge, Irrigation
The success of an irrigation scheme depends on several factors. Important among these is the choice and design of the method to be adopted. There are two methods of irrigation, viz., surface and spray irrigation.
Canker disease of Red-flowered gum - By W. P. CASS SMITH, B.Sc. (Agric), Government Plant Pathologist
Of the many beautiful native plants in Western Australia, few are esteemed more than the red-flowered gum known botanically as Eucalyptus flcifolia. This shapely tree commences blossoming about Christmas time—and provides a magnificent display for several weeks—the blossom shades varying from dark red to pink.
Why many beans grow crooked - By M. HARDIE, Vegetable Instructor
Sometimes in the process of growing a crop, bean growers find a number of pods which, instead of being long and straight and suitable for marketing, are crooked or bent and quite unsuitable for sale unless beans are in very short supply. It is not unusual for a small number of pods to be affected in this way, generally at the tail-end of the crop, but occasionally the percentage may be high. When this happens the grower is often at a loss to account for his failure to grow a marketable crop. To the best of his ability he has followed methods which normally return a crop of first-grade pods and yet for some reason, his crop is an unpayable one and he is anxious to ascertain the cause.
Salt land programme for Autumn - By T. C. STONEMAN, Adviser, Soil Conservation Service
Now that summer is over and winter approaches, the time has come to decide what you\u27ll do with that salt problem on your farm. What you SHOULD do, will depend on how severely the area is affected. To make quite clear how the recommendations made by the Department of Agriculture can be applied I will deal with each degree of severity in turn.
Orchard cover crops - By J. CRIPPS, Horticultural Adviser
The sowing of a cover crop is one of those routine operations which the orchardist often undertakes without much thought, but it is an operation worthy of consideration.
Lupin seed - By B. J. QUINLIVAN, B.Sc. (Agric), Botanist, Weeds and Seeds Branch
The three types of lupins which we grow in this State are the New Zealand, the W.A. blue, and the yellow lupin. The first two types are very common, the New Zealand lupin being grown mainly in the South-West, while the W.A. lupin is grown over extensive areas of the central and northern wheatbelt, particularly on the sandier types of soil.
Cereal rye will grow on wind-eroded areas - By G. H. BURVILL, Chief Plant Research Officer.
Travelling around the wheatbelt, one often sees areas of sandy soil quite bare, A due to wind erosion. Surface soil blown from these patches has built up against fences, or as mounds in the nearby scrub. Many of the eroded places are quite small— perhaps less than an acre or only a few acres. But some are large, with one to three hundred acres of bare yellow sand. As much as two feet of soil has been blown from big areas.
Autumn-winter feeding for the fat lamb flock - By N. DAVENPORT, Senior Adviser, Meat Production
The autumn-winter period is the most important part of the year for the fat lamb crop from a feed point of view. It is important not only from the aspect of the wellbeing of the sheep, but also of the pasture. Pasture plants are living things, too, and they also require consideration and care for their development. When a new season pasture is kept hard grazed in those critical first few weeks of growth it cannot carry as many sheep over the season as one which is lightly stocked during that time.
Plant diseases in the home garden - By W. P. CASS-SMITH, B.Sc. (Agric), Government Plant Pathologist
Recently many home-gardeners have complained that plantings of vegetables or ornamentals are making unthrifty growth, in spite of liberal waterings and fertilisation. These symptoms are commonly caused by root-knot eelworm, a pest which has unfortunately become very widespread in the sandy soils adjacent to Perth, and in many other parts of this State. This eelworm parasite attacks the roots of many kinds of plants causing them to rot and to develop bead-like swellings or galls. As a result, the intake of plant food and water is restricted and stunting of the above-ground parts occurs.
Principles of commercial beekeeping - By R. S. COLEMAN, R.D.A., Government Apiculturist
We have many queries from small beekeepers, and from people who have never opened a hive, on how to break into full-time beekeeping. So it was thought that you would like to know just what are the principles of successful bee farming.
Grasshoppers and locusts - By C. F. H. JENKINS, M.A., Government Entomologist
Grasshoppers and locusts are among the most ancient enemies of mankind for they figured prominently in Biblical times, and periodically throughout the ages have devastated crops in all parts of the world.
Acetonaemia in dairy cattle - By P. M. A. HARWOOD, M.R.C.V.S., Veterinary Officer
Acetonaemia, more correctly known as ketosis is a metabolic disease. Unlike infectious diseases such as tuberculosis or mastitis, which are caused by microbes, metabolic diseases result from an upset in the normal bodily processes.
Molybdenum for subterranean clover - By T. C. DUNNE, B.Sc. (Agric), Ph.D. (Calif.), Chief Plant Nutrition and Research Officer
Subterranean clover is undoubtedly the most important pasture species in this State. Because it is a legume, it can make excellent growth on soils with a low nitrogen content. The nitrogen it needs is, of course, provided by bacteria which inhabit the nodules found on its roots. These bacteria provide nitrogen by taking it from the air and converting it to a form which can be used by the clover. However, it is now known that the bacteria can use the nitrogen of the air only if enough molybdenum is available to them.
Cobalt deficiency in sheep and cattle - By C. R. TOOP, B.V.SC, Chief Veterinary Officer
With the establishment of group settlements at Denmark after the first world war, it was observed that young cattle, although running on lush pastures, developed systems of unthriftness and wasting and usually died. Research conducted of Filmer and Underwood later revealed that this condition was caused by a deficiency of cobalt and that it could be cured and prevented by addition of this mineral to the diet. Cobalt deficiency has since been met with other parts of the State and was recently diagnosed in sheep at Mount Manypeaks
Recent radio talks
Recent Rural Radio Talks
Dairy Hygiene is Important; - D.C. Mickle
Farm Water Supplies; - K. Needham
Handling the Apple Crop; - K. Whitely
Renovation of Irrigated Pastures; - Dairying division
More About Lupinosis; - H. W. Bennetts
Progress in Doublegee Control; - Norman Halse
Changing Practices in Tobacco Growing in W.A.; - G. A. Pearce
Progress in doublegee control; - G.A. Pearce
Some Uses of Radio Isotopes in Agriculture; - E. N. Fitzpatrick
Guildford Grass—Sign of a Run-Down Pasture; - R. A. Bettenay
Silage and when to feed it; - R. Bettenay
Ants in the Apiary; - R. S. Coleman
Bacterial Canker of Stone Fruits; - Olga M. Goss
Poison Plants in the Home Garden; - R. D. Royce
The Cabbage White Butterfly; - C. F. H. Jenkins
Summer Treatments for San Jose Scale; - C. F. H. Jenkins
Preparations for Fodder Conservation; - H.G. Cariss
Sudan Grass Survived Dry Summer; - R.A. Bettenay
Root Maggot Flies; - J. A. Button
Fallowing for Cropping; - A.S. Wild
Feeding for Milk Quality; - L. C. Snook
The Rabbit— Friend or Foe. A. R. Tomlinso
Caracterização de colágenos tipos I e III no estroma do carcinoma de células escamosas cutâneo em cães
Fatty acid, triglyceride and tocopherol composition of Algerian Argan (Argania spinosa) fruit seed lipids
Superlinear convergence of affine scaling interior point newton method for linear inequality constrained minimization without strict complementarity
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