What is the magnetic analog of resistivity?
I believe that "magnetic resistivity" is referred to as reluctance (inverse of permeance) and is given in units of \$A\cdot\text{turns}/Wb\$ (Amp-turns per weber) in an equivalent magnetic circuit. Reluctance can be calculated as:

$$\mathcal{R} = \frac{F}{\phi}$$

where \$F=N \cdot i\$ is the magnetomotive force and \$\phi=B\cdot A\$ is the flux, thus giving:

$$\text{reluctance} = \mathcal{R} = \frac{l}{\mu\cdot A}$$

Reluctance is defined by the magnetic permeability of the material (as well as its physical dimensions) which is given as:

$${\mu = \mu_0\mu_r}$$

Where \$\mu_0\$ is the magnetic constant and \$\mu_r\$ is the relative permeability of the material. Magnetic permeability is a measure of how easily a magnetic field is formed within the material and is affected by temperature, humidity, and magnetic field strength (among other things).

Hopefully someone can provide a more complete answer, as this is just off the top of my head.