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MagneticallyOrderedMaterials

< New thoughts on FMuF | Index | Magnetic single crystal >

Let us assume for simplicity that the ordered moments {$ \hbar\gamma_e \mathbf{ S} $} are directed along a known, single crystallographic direction. This can be the case of a uniaxial ferromagnet in zero applied field. A uniaxial antiferromagnet may yield a similar situation.

The local field at the muon site is due to a spin Hamiltonian of the type described in Eq. (3) and (5), Interactions as

{$ \qquad\qquad \frac {\cal H} h = \frac {\gamma_\mu}{2\pi} \mathbf{ I}\cdot\left[\frac {\mu_0}{4\pi} \hbar\gamma_e \sum_i \frac {3\hat r_i(\mathbf{S}_i\cdot\hat r_i)- \mathbf{S}_i}{r_i^3} + (\delta\tilde A + A_0) \cdot \mathbf{ S}\right], $}

where, in fact, the electronic term in square brackets acts as an effective magnetic field, {$\mathbf{ B}_\mu $}, on the muon. However it is clear that {$\mathbf{B}_\mu $} is not necessarily parallel to {$\mathbf{S}$}, due to the tensorial character of the dipolar and pseudo-dipolar interactions.

This implies that crystallographically equivalent interstitial muon sites in the unit cell might experience different local fields, which give rise to magnetically inequivalent sites. This has important consequences:

• on the amplitudes of muon precessions in an oriented single crystal sample
• on the vector composition of the local field with externally applied magnetic fields

We shall briefly review the two topics.

< New thoughts on FMuF | Index | Magnetic single crystal >