smallmatrix using tikzpicture

Question

I wish to use tikzpicture instead of smallmatrix, because the former offers better control for dots via this post. However, the code

$\Z^4\!\xleftarrow{
\begin{tikzpicture}[baseline=(current bounding box.center)]
\matrix (m) [matrix of math nodes,nodes in empty cells,right delimiter={]},left delimiter={[} ]{
    & & & & & \\ 1& & -1& & 1& \\ 1& & -1& & 1& \\ & 1& & -1& & 1\\};
\end{tikzpicture} }\!\Z^6$

returns errors (unless tikzpicture is outside of \xleftarrow). How do I get the output of

$\Z^4\!\xleftarrow{ \left[\!\begin{smallmatrix}&&&&&\vspace{6pt}\\ 1&&-\!1&&1&\\1&&-\!1&&1&\\ &1&&-\!1&&1\\\end{smallmatrix}\!\right]}\!\Z^6$

(everything tight and small) using tikzpicture?

Answer 1

You need ampersand replacement to keep & unchanged.

\documentclass[border=7pt]{standalone}
\usepackage{mathtools,amssymb,tikz}
\usetikzlibrary{matrix}
\newcommand*{\Z}{\mathbb{Z}}
\begin{document}
  $\Z^4\!\xleftarrow{%
    \begin{tikzpicture}[baseline=(current bounding box.center)]
    \matrix (m) [
      matrix of math nodes,
      nodes in empty cells,
      right delimiter={]},
      left delimiter={[},
      ampersand replacement=\&, % <-- this is what you need
      nodes={scale=.5}]
      {
          \&   \&    \&    \&   \&   \\
        1 \&   \& -1 \&    \& 1 \&   \\
        1 \&   \& -1 \&    \& 1 \&   \\
          \& 1 \&    \& -1 \&   \& 1 \\
      };
    \end{tikzpicture}%
  }\!\Z^6$
\end{document}

Answer 2

This cries for an ampersand replacement.

\documentclass{article}
\usepackage{mathtools}
\usepackage{bbm}
\newcommand{\Z}{\mathbbm{Z}}
\usepackage{tikz}
\usetikzlibrary{matrix}
\begin{document}

$\Z^4\!\xleftarrow{\vcenter{\hbox{
\begin{tikzpicture}
\matrix (m) [matrix of math nodes,nodes in empty cells,right delimiter={]},
left delimiter={[},ampersand replacement=\&]{
    \& \& \& \& \& \\ 1\& \& -1\& \& 1\& \\ 1\& \& -1\& \& 1\& \\ \& 1\& \& -1\& \& 1\\};
\end{tikzpicture} }}}\!\Z^6$
\end{document}