Ferromagnetism

Ferromagnetism The atomic moments in these materials exhibit very strong interactions, resulting in a parallel or antiparallel alignment of atomic moments. Exchange forces are very large, equivalent to a field on the order of 1000 Tesla, or approximately a 100 million times the strength of the earth's field. The exchange force is a quantum mechanical phenomenon due to the relative orientation of the spins of two electron.

Ferromagnetism Ferromagnetic materials exhibit parallel alignment of moments resulting in large net magnetization even in the absence of a magnetic field. The elements Fe, Ni, and Co and many of their alloys are typical ferromagnetic materials. Two distinct characteristics of ferromagnetic materials are their (1) spontaneous magnetization and the existence of (2) magnetic ordering temperature

Ferromagnetism

Origin of Ferromagnetism Hund’s rule Fe: [Ar]3d64s2

Ferromagnetism Ferromagnetic transition metals: Fe, Co, Ni i) Magnitude of Ms ii) Way to reach Ms M (ferro) >> M (para) : 1700 emu/cm3 for Fe >> 10-3 emu/cm3 Hs = 50 Oe

Weiss' Assumption Molecular field is acting in FM not only above Tc but also below Tc and this field is so strong that it could magnetize the substance to saturation even in the absence of an applied field. → spontaneously magnetized (Self-saturating) Magnetic domain : In demagnetized state, a ferromagnetic material is divided into a number of small regions called domains, each of which is spontaneously magnetized.

Magnetization process a) Unmagnetized specimen for random orientation of domains b) – c) Single domain process (motion of domain wall) d) Rotation of the domain along the field Question: Spontaneous magnetization? Division into domains?

Magnetic Domain (a) A single-domain sample with a large stray field. (b) A sample split into two domains in order to reduce the magnetostatic energy. (c) A sample divided into four domains. The closure domains at the ends of the sample make the magnetostatic energy zero.

Magnetic Domain

Domain wall motion Barkhausen effect

Magnetic Order Are ferromagnets already in an ordered state before a magnetic field is applied or is the order by the field?

Explanation of magnetic order in ferromagnets Weber (1852): The material could already have small atomic magnetic moments within the solid which are randomly aligned in the demagnetized but which became ordered under the action of a magnetic field. Poisson (1983) : The atomic magnetic moments may not exist at all in the demagnetized state but could be induced by a mangetic field.

Explanation of magnetic order in ferromagnets Ampère (1827): The origin of the atomic moments was suggested that they were due to electrical currents continually circulating within the atom. Ewing (1893): Followed Weber’s idea and interested in explaining hysteresis.

Weiss domain theory Atomic magnetic moments were in permanent existence (Weber’s hypothesis) Atomic magnetic moments were ordered even in the demagnetized state. It was the domains only which were randomly aligned in the demagnetized state. The magnetization process consisting of reorienting the domains so that more domains were aligned with field.

Magnetic Domain In order to minimize its magnetostatic energy, the magnetic material divides up into magnetic domains. Weiss (1907): concept of magnetic domains. A magnetic material consisted of a number of distinct regions termed ‘domains’ each of which was saturated in a different direction. The concept of domains is able to explain why ferromagnetic materials can be demagnetized even below their Curie temperature.

Weiss Mean Field Theory What is the origin of the alignment of the atomic magnetic moments? It is the Weiss mean field (later the “molecular field”, further later exchange coupling from quantum mechanics)

Curie-Weiss Law Curie's law: Individual carrier of magnetic moment (atoms or molecules) do not interact with one another Curie-Weiss law:  Under the consideration of interaction between electrons  Fictitious internal field Hm (“molecular field”) for interaction : molecular field constant

Molecular field theory Pierre Weiss introduced molecular field concept. Interaction between magnetic moments  Fictitious internal filed : molecular field constant For  > 0, Hm || M

Curie Temperature

Curie Temperature Even though electronic exchange forces in ferromagnets are very large, thermal energy eventually overcomes the exchange and produces a randomizing effect. This occurs at a particular temperature called the Curie temperature (TC). Below the Curie temperature, the ferromagnet is ordered and above it, disordered. The saturation magnetization goes to zero at the Curie temperature.

Curie temperature Saturation magnetization of Fe, Co, Ni as a function of temperature

Exchange Energy Exchange force depends on relative orientation of spins of two electrons due to Pauli's exclusion principle When two atoms, such as hydrogen atoms, are coming together, there are electrostatic attractive (e-↔p+) and repulsive (e-↔e-, p+↔p+) forces and exchange force. The internal field is produced by interactions between nearest-neighbor dipole moments. The interaction arises from the electrostatic electron-electron interaction, and is called the ”exchange interaction” or exchange force.

Exchange Energy: Heisenberg Model Si·Sj: spin angular momentum Je : a numerical quantity called exchange integral Relative orientation of two spins determines the energy states. ra/r3d Bethe-Slater curve (1) If Jex is positive, Eex is a minimum when the spins are parallel, leading to ferromagnetism (2) If Jex is negative, Eex is a minimum when the spins are antiparallel, leading to antiferromagnetism.

Band Theory of Ferromagnetism A simple extension of the band theory of paramagnetism by the introduction of an exchange coupling between the electrons. Source of magnetic moments: unpaired electrons In partially filled energy band, an imbalance of spins leads to a net magnetic moment per atom.

Band Theory When N atoms come together to form a solid, each level of the free atom must split into N levels. In transition metal elements, the outermost electrons are the 3d and 4s; these electron clouds are the first to overlap as the atoms are brought together, and the corresponding levels are the first to split.

Density of states