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This is paramagentism as I understand it:

Paramagnetic materials can be defined as having a low but positive magnetic susceptibility $\chi$. This low susceptibility is due to a lack of/ low interaction between neighbouring magnetic moments being unable to reinforce the effects of an applied field $\boldsymbol{H}$.

Without this reinforcing effect random thermal fluctuations can effectively misalign a majority of the moments in a paramagnetic material and thus the magnetisation of the material is drastically lowered.

If this is correct, would this mean that if the temperature of any paramagnetic material is sufficiently lowered it would display ferromagnetic properties?

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Some paramagnetic materials do become Ferromagnetic below the Curie temperature (or alternatively - ferromagnetic materials become paramagnetic when heated).

However, this is not a must: paramagnetism is due to the alignment of the magnetic moments along the magnetic field. The magnetic moments are effectively idnependent. Ferromagnetism, on the other hand, is due to the interaction between magnetic moments that aligns them in the same direction (which is not necessarily the direction of the external magnetic field - unless the latter is very strong). Higher temperature has potential for disrupting this interaction and converting a Ferromagnetic material into a paramagnetic one, but the converse is not true.

Magnetic susceptibility
Note that the difference between a paramagnet and a ferromagnet is not the strength of the magnetic response to the external field (i.e.,magnetic susceptibility), but the absence/presence of the spontaneous magnetisation - i.e., in Ferromagnet magnetization is present even in asence of an applied magnetic field, and persists, if the external field is not too strong.

The magnetic susceptibility of ferromagnets above the Curie temperature (i.e., in the paramagnetic phase) is described by Curie-Weiss law: $$\chi\propto \frac{1}{T-T_C},$$ where $T_C$ is the temperature of the ferromagnetic transition. As we see the susceptibility increases with lowering temperature, until a material becomes ferromagnetic. More precisely, near the transition the exponent in the law may be different: $$\chi\propto \frac{1}{(T-T_C)^\gamma}.$$

In paramagnetic materials, not exhibiting the ferromagnetic transition, this dependence is instead given by the Curie law: $$\chi\propto \frac{1}{T},$$ which also predicts that the susceptibility increases with lowering temperature, but that it remains finite - the material does not become ferromagnetic.

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  • $\begingroup$ Thanks! So what is it that stops the magnetic moments aligning more completely along an applied field? I'm given to understand it's thermal fluctuation but perhaps that isn't accurate? $\endgroup$
    – Connor
    Commented Dec 16, 2021 at 14:08
  • $\begingroup$ They align with magnetic field, but every moment does it on its own - they do not interact with each other. SO, if there is no field, they will not be ordered. Or, if the field changes, they will immediately follow it. $\endgroup$
    – Roger V.
    Commented Dec 16, 2021 at 14:12
  • $\begingroup$ Sorry, perhaps I'm not being clear, what I mean is why is the magnetisation inside of the material so low compared to the applied field? Or why do paramagnetic materials have low susceptibilities? $\endgroup$
    – Connor
    Commented Dec 16, 2021 at 14:19
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    $\begingroup$ @Connor I expanded the answer - perhaps now it clarifies your question now. $\endgroup$
    – Roger V.
    Commented Dec 16, 2021 at 14:34
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    $\begingroup$ A paramagnet has a finite density of magnetic moments, so there is a limit of how much magnetic field it can produce, even if all the moments are aligned. More recently one is able to produce materials exhibiting "giant response", although this is still in percentage points: e.g., arxiv.org/abs/1701.01964 $\endgroup$
    – Roger V.
    Commented Dec 16, 2021 at 15:18

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