Unfortunately you can’t bold symbols using the bold font, see this question on tex.stackexchange.

As the answer suggests, you could use `\boldsymbol`

to bold phi:

```
r'$\boldsymbol{\phi}$'
```

You’ll need to load `amsmath`

into the TeX preamble:

```
matplotlib.rc('text', usetex=True)
matplotlib.rcParams['text.latex.preamble']=[r"\usepackage{amsmath}"]
```