|
NMR
This site is running |
NMR /
MagneticMnSpectroscopy< The physics of M hexaferrite | Index | Radio Frequency enhancement > The spin spectroscopy of the Mn ion in transition metal oxidesThe aim of this page is to illustrate what can be gathered on a typical magnetic transition metal ion in a magnetic material by NMR. Hyperfine couplings To understand the coupling between nuclei and electrons visit the NMR.Interactions and the NMR.HyperfineInteraction page, then come back here. Mn is a good NMR nucleus: it has a relatively large gyromagnetic ratio, i.e. above 1 T its Larmor frequency comfortably falls in the radio-wave range easily accessible to NMR spectrometers (>10 MHz), and nearly all nuclei have a spin.
The spin I>1/2 implies that the nucleus possesses a quadrupole moment. See NMR.Quadrupoles for details. For an ion in vacuum the total angular momentum is {$\mathbf{J}=\mathbf{L}+\mathbf{S}$}. The same ion in a crystalline environment is subject to a crystal potential (often called crystal field) that breaks the rotational symmetry. This tends to quench the orbital momentum {$\mathbf L$}, i.e to make it vanish, unless the relativistic spin-orbit effect brings it back. This is very often true for Mn in oxides, so that the magnetic moment of this ion is proportional just to {$\mathbf{S}$} (spin-only systems). Mn ion in cubic/tetragonal field According to the Hunds rules, valid for ions in a gas Magnetismo.ThreedIons, Mn ions have the following electronic configurations
The total spin S reported in the table follows from the assumption that the Mn ions is in the so called high spin (HS) state, that is, it follows strictly the Hund rules and the ground state is filled by as many parallel single electron spin states as possible. Another common assumption is that the orbital momentum is quenched by the crystal field: the linear combination of electron wave-functions that are eigenstates of the crystal fields have vanishing orbital angular momentum. The orbital contribution to the magnetic moment vanishes as well and we speak of spin-only systems. These two assumptions are generally, but not necessarily always, true. The HS condition implies that, from top to bottom in the table, the spin configuration corresponds to 5 parallel spins (half filled t'_2g' triplet and e'_g' doublet), 4 parallel spins (half filled triplet and singly occupied doublet), 3 parallel spins (half filled triplet). In ordered magnetic insulators an ionic picture applies: electron magnetic moments are localized on ions at lattice sites. With orbital momentum quenching we can think of the total spin S of an ion, that corresponds to an effective magnetic moment m=gγeħS, positioned at the lattice site. This is not necessarily true for a metals, like iron, where 3d moments may be partially localized, but their wave function overlaps with that of 4s electrons, leading to spin polarization (partial 3d character) of the conduction electrons. Intermediate cases exist both in nature and in artificial, nanostructured materials: metals on the verge of becoming insulating or insulators very close to becoming metallic. Controlling finely this condition leads to an insulator-metal transition. One way to obtain it is to dope a magnetic insulator (like it is done with a semiconductor) by substituting controlled fractions of an n valent ions with n+1 or n-1 valent ions having a similar radius. This may produce the so called mixed-valence of the magnetic ion. However it is not granted that mixed valence ions remain localized. Manganites are the prototype example. Cuprates are even more important examples, since high temperature superconductivity emerges close to the insulator-metal transition. Examples Lezione manganiti < The physics of M hexaferrite | Index | Radio Frequency enhancement > |
