Three dimensionality of field induced magnetism in a high temperature superconductor

Many physical properties of the high temperature high Tc superconductors are two dimensional phenomena derived from their square planar CuO2 building blocks. This is especially true of the magnetism from the copper ions electronically connected to each other via the p orbitals of intervening oxygen atoms. As mobile charge carriers enter the CuO2 layers, the antiferromagnetism of the parent insulators, where each copper spin is antiparallel to its nearest neighbours, evolves into a fluctuating state where the spins show tendencies towards magnetic order of a longer periodicity. For certain charge carrier densities, the quantum fluctuations are sufficiently suppressed to yield static long period order, and external fields also induce such order. Here we show that in contrast to the chemically controlled order in superconducting samples, the field induced order in these same samples is actually three dimensional, implying significant magnetic linkage between the CuO2 planes. The results are important because they show that there are three dimensional magnetic couplings which survive into the superconducting state, and coexist with the crucial inter layer couplings responsible for the three dimensional superconductivity. Both types of coupling will straighten the vortex lines, implying that we have finally established a direct link between technical superconductivity, which depends on the dynamics of the vortex lines, and the underlying antiferromagnetism of the cuprate