# Why is the Susceptibility of Paramagnetic Materials so Low?

Qualitatively, what fundamental physical property stops a paramagnetic material from reaching a high magnetic saturation under an applied field? This question is similar to the one asked here: Why do paramagnetic materials require large fields to achieve magnetic saturation?.

I am given to understand that it's thermal fluctuations that prevent the magnetic moments of a paramagnetic material aligning, i.e. thermal fluctuations at room temperature have a significantly larger effect on electron spin orientation than an applied field. Is that accurate, or is there another physical principle that causes paramagnets to have such low susceptibility?

While there may be other effects I don't know about, the thermal effects are certainly sufficient to explain it. The energy associated with a spin flip is on the order of the Bohr magneton times the applied magnetic field. For a magnetic field of 1 T this works out to be on the order of $$\Delta E \approx 6 \times 10^{-5}$$ eV. But in a thermal environment at temperature $$T$$, energy fluctuations with $$\Delta E \ll k T$$ happen pretty much at will; and $$k T$$ for a room-temperature sample is about 0.025 eV ("one-fortieth of an eV" is the mnemonic I was taught as a young physicist.)
To approach saturation, you would need $$\Delta E \gtrsim k T$$, which would require either reducing the temperature (in Kelvin) by a factor of about 500, increasing the magnetic field by a factor of 500, or some combination thereof.