Effect of a magnetic field on the long-range magnetic order in insulating Nd₂CuO₄ and nonsuperconducting and superconducting Nd_1.85Ce_0.15CuO₄

We have measured the effect of a c-axis-aligned magnetic field on the long-range magnetic order of insulating Nd₂CuO₄, as-grown nonsuperconducting and superconducting Nd_1.85Ce_0.15CuO₄. On cooling from room temperature, Nd₂CuO₄ goes through a series of antiferromagnetic (AF) phase transitions with different noncollinear spin structures. In all phases of Nd2CuO4, we find that the applied c-axis field induces a canting of the AF order but does not alter the basic zero-field noncollinear spin structures. A similar behavior is also found in as-grown nonsuperconducting Nd1.85Ce0.15CuO4. These results contrast dramatically with those of superconducting Nd_1.85Ce_0.15CuO₄, where the c-axis-aligned magnetic field induces a static, anomalously conducting, long-range ordered AF state. We confirm that the annealing process necessary to make superconducting Nd_1.85Ce_0.15CuO₄ also induces epitaxial, three-dimensional long-range-ordered cubic (Nd, Ce)₂O₃ as a small impurity phase. In addition, the annealing process makes a series of quasi-two-dimensional superlattice reflections associated with lattice distortions of Nd_1.85Ce_0.15CuO₄ in the CuO₂ plane. While the application of a magnetic field will induce a net moment in the impurity phase, we determine its magnitude and eliminate this as a possibility for the observed magnetic-field-induced effect in superconducting Nd_1.85Ce_0.15CuO₄. This is confirmed by measurements of the (1/2,1/2,3) peak, which is not lattice matched to the impurity phase.