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I was learning about proton NMR (Nuclear Magnetic Resonance Spectroscopy) which is where protons (with their own spins) are subjected to an external magnetic field and a radio frequency is applied to flip their spin states and the re-emitted frequencies are measured.

What I don't understand is why do some protons decide to align their spins against the magnetic field [Figure 2 - some are pointing the wrong way]. This seems ridiculous atleast from the everyday experience of compasses (a compass needle always points to North). enter image description here

I guess it seems weird that something would naturally assume a higher energy state. I would appreciate an explanation as to why they don't all point in the direction of the magnetic field.

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  • $\begingroup$ It is because protons have different spin configurations (left or right) which correspond to different energy state. Protons that align with the external magnetic field are in a lower energy state and protons that align against the external magnetic field are in a higher energy state. $\endgroup$ – AWanderingMind Jun 3 at 13:01
  • $\begingroup$ Have you read en.wikipedia.org/wiki/… ? $\endgroup$ – PM 2Ring Jun 3 at 13:36
  • $\begingroup$ @AWanderingMind That sounds like the beginning of an answer... $\endgroup$ – PM 2Ring Jun 3 at 13:37
  • $\begingroup$ Your doubts come from the inaccurate figure 1. Protons are bundled in nuclei and are aligned there in pairs due to their spins (spin "up" and spin "down"). The stronger the external magnetic field is, the stronger the protons are aligned, but some of them - "squeezed" between the aligned protons - are aligned antiparallel. $\endgroup$ – HolgerFiedler Jun 17 at 6:30
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The protons are not isolated but they are inside a material with a non-zero temperature. The thermal motion will provide the energy for the protons to be excited in a state with higher energy, corresponding to the alignment opposite to the field. This is a dynamic process, after some the times they go back to the ground state but other protons are excited all the time. For a specific temperature, there is an average fraction of protons aligned in the opposite direction. The same mechanism explains why the magnetization of a piece of iron in a magnetic field depends on temperature or how the magnetization of a paramagnetic material depends on temperature. See for example this typical curve of magnetization versus temperature: http://www.irm.umn.edu/hg2m/hg2m_b/Image9.gif

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