The original paper uses Einstein summation notation as in the question title and I'm trying to write it using vector product notation. $\mathbf{m}$, $\mathbf{x}$ and $\mathbf{H}$ are all 3-component vectors.
My attempt is:
$$\frac{\partial\mathbf{m}}{\partial\mathbf{x}} = \mathbf{m} \cdot \nabla\mathbf{H}$$
Is this correct? If it is, what does a dot product with the gradient of a vector look like, given that is a tensor?