Magnetism of compounds with atomic magnetic buy (ˆ

Magnetism of compounds with atomic magnetic buy (ˆ

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MAGNETISM OF SUPERCONDUCTORS. The currents shield the body of the superconductor from ex boyfriendternal magnetic fields; therefore, in a massive superconductor the magnetic field is equal to zero when T < Tc (the Meissner effect).

Ferromagnetism exists in substances which have a confident exchange times (ˆ

int » ?B or ˆint » kT). FERROMAGNETISM. ex > 0): crystals of iron, cobalt, and nickel and a number of rare earths (gadolinium, terbium, dysprosium, holmium, erbium, thulium, and ytterbium) and alloys and compounds of these elements, as well as alloys of chromium and manganese and compounds of uranium. Spontaneous magnetization at temperatures below the Curie point ? is characteristic of ferromagnetism, and when T > ? ferromagnets pass into the paramagnetic or antiferromagnetic state (the latter case is observed, for example, in some rare earths). However, experiments have shown that ferromagnetic solids have no net magnetization in the absence of an external field (if the secondary phenomenon of residual magnetization is disregarded); this is because when H = 0 a ferromagnet breaks down into a large number of microscopic, spontaneously magnetized regions (domains). The magnetization vectors of the individual domains are oriented in such a way that the total magnetization of the ferromagnet is equal to zero. In an external field the domain structure changes, and the ferromagnet acquires net magnetization.

The latest magnetism of superconductors comes from electric currents flowing inside the a surface layer having a density of your own order from 10 -5 cm

ANTIFERROMAGNETISM. Antiferromagnetism exists in substances that have a negative exchange energy Santa Clara escortex < 0): crystals of chromium and manganese, a number of rare earths (cesium, praseodymium, neodymium, samarium, and europium), and numerous compounds and alloys in which elements of transition groups are found. In magnetic terms the crystal lattice of such substances breaks down into magnetic sublattices, whose spontaneous magnetization vectors Jki, are either antiparallel (collinear antiferromagnetic bond) or are opposed to each other at angles that differ from 0° and 180° (noncollinear bond). If the total moment of all magnetic sublattices in an antiferromagnet is equal to zero, then compensated antiferromagnetism is present, but uncompensated antiferromagnetism, or ferrimagnetism, which is found chiefly in crystals of metal oxides with a crystal lattice of the spinel type, such as garnet and perovskite (called ferrites), is observed in the case of nonzero differential spontaneous magnetization. These solids (usually semiconductors and insulators) are similar in magnetic properties to ordinary ferromagnets. When the compensation for magnetic moments is disturbed in antiferromagnets because of the weak interaction between the atomic carriers of magnetism, very slight spontaneous magnetization of the substances takes place (of the order of 0.1 percent of the ordinary values for ferromagnets and ferrimagnets). Such substances are called weak ferromagnets; examples are hematite, a-Fe2Ostep 3, and the carbonates of a number of metals and orthoferrites.

The magnetic state of a ferromagnet or antiferromagnet in an external magnetic field H is determined by the previous state of the magnet (the magnetic prehistory of the sample), in addition to the field strength. This phenomenon is called hysteresis. Magnetic hysteresis is manifested in the indeterminacy of the dependence of J on H (in the presence of a hysteresis loop). Because of hysteresis, elimination of the external field is insufficient for demagnetization of a specimen, since when H = 0 the specimen will retain its residual magnetization Jr. An opposite magnetic field Hc, which is called the coercive force, must be applied. A distinction is made between soft-magnetic materials (Hc < 800 amperes per meter [A/m], or 10 oersteds) and hard-magnetic, or high-coercivity, materials (Hc > 4 kA/m, or 50 oersteds), depending on the value of Hc. The quantities Jr and Hc are dependent on temperature and generally decline as the temperature increases, tending toward zero as T approaches ?.

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