  The development of short- and long-range magnetic order induced in a
frustrated zig-zag ladder compound SrDy2O4 by an applied field is studied using
neutron diffraction techniques. In zero field, SrDy2O4 lacks long-range
magnetic order down to temperatures as low as 60 mK, and the observed powder
neutron diffraction (PND) patterns are dominated by very broad diffuse
scattering peaks. Single crystal neutron diffraction reveals that the
zero-field magnetic structure consists of a collection of antiferromagnetic
chains running along the c axis and that there is very little correlation
between the chains in the ab plane. In an applied magnetic field, the broad
diffuse scattering features in PND are gradually replaced by much sharper
peaks, however, the pattern remains rather complex, reflecting the highly
anisotropic nature of SrDy2O4. Single crystal neutron diffraction shows that a
moderate field applied along the b axis induces an up-up-down magnetic order
associated with a 1/3-magnetisation plateau, in which magnetic correlation
length in the ab plane is significantly increased, but it nevertheless remains
finite. The resolution limited k = 0 peaks associated with a ferromagnetic
arrangement appear in powder and single crystal neutron diffraction patterns in
fields of 2.5 T and above.
