Vector Algebra

If the volume of the parallelopiped formed by the vectors \(\hat{\mathbf{i}}+a \hat{\mathbf{j}}+\hat{\mathbf{k}}, \hat{\mathbf{j}}+a \hat{\mathbf{k}}\) and \(a \hat{\mathbf{i}}+\hat{\mathbf{k}}\) becomes minimum, then \(a\) is equal to