# Relationship of the Demagnetizing Energy to the Demagnetizing Field

 P: 1 Hello, I'm currently reading material on micromagnetics. In these papers, authors define a quantity called the demagnetizing energy ($E_d$) as $$E_d = -\frac{1}{2} \int_V \vec{m} \cdot \vec{H}_d\;dV$$ where $\vec{m}$ is the internal magnetization of a material sample of volume $V$ and $\vec{H}_d$ is the demagnetising field. The demagnetizing field itself is defined as the negative derivative of the demagnetizing energy with respect to the material magnetisation, i.e. $$\vec{H}_d = -\frac{dE_d}{d\vec{m}}$$ My problem is that I would like to know how to derive $\vec{H}_d$ by taking the derivative of $E_d$ with respect to $\vec{m}$. This is as far as I have got (and I'm not too sure that this is correct) $$\frac{dE_d}{d\vec{m}} = \frac{d}{d\vec{m}} \left( -\frac{1}{2}\int_V \vec{m}\cdot\vec{H}_d\;dV \right)$$ $$\frac{dE_d}{d\vec{m}} = -\frac{1}{2}\int_V \frac{\partial}{\partial\vec{m}}\left(\vec{m}\cdot\vec{H}_d\right)\;dV=-\frac{1}{2}\int_V\frac{\partial\vec{m}}{\partial \vec{m}} \cdot \vec{H}_d + \vec{m}\cdot\frac{\partial \vec{H}_d}{\partial \vec{m}}\;dV=-\frac{1}{2} \int_V \vec{H}_d\;dV - \frac{1}{2}\int_V \vec{m}\cdot\frac{\partial \vec{H}_d}{\partial \vec{m}}\;dV$$ Could some kind soul please give me some pointers as to how to proceed and/or explain to me where I'm going wrong?