Let \({\mathcal S}\) denote the set of all univalent analytic functions \(f(z)=z+\sum _{n=2}^{\infty }a_n z^n\) on the unit disk \(\mathbb {D}\). In 1946, B. Friedman found that the set \(\mathcal S\) of those functions which have integer coefficients consists of only nine functions. In 1985, authors determine all functions in \({\mathcal S}\) whose coefficients are integers in complex quadratic fields. The first aim of this paper is to determine the class of univalent sense-preserving harmonic mappings \(f(z)=z+\sum _{n=2}^{\infty }a_n z^n+\sum _{n=1}^{\infty }\overline{b_n} \overline{z^n}\) with the coefficients \(a_n\), \(b_n\), \(n=2, 3, \cdots \), in a fixed complex quadratic field \(Q(\sqrt{d}i)\), where d is a positive square-free integer. Then, under the condition F is sense-preserving in \(\mathbb {D}\) and \(h'(z)\ne 0\) or F is sense-reversing in \(\mathbb {D}\) and \(g(z)\equiv 0\), we determine all univalent biharmonic mappings \(F(z)=|z|^2(h(z)+\overline{g(z)})=|z|^2(z+\sum _{n=2}^{\infty }a_n z^n+\sum _{n=1}^{\infty }\overline{b_n} \overline{z^n})\), where \(a_n, b_n\in Q(\sqrt{d}i)\), \(n=2, 3, \cdots \).