37 research outputs found
LiHoF: Cuboidal Demagnetizing Factor in an Ising Ferromagnet
The demagnetizing factor has an important effect on the physics of
ferromagnets. For cuboidal samples it depends on susceptibility and the
historic problem of determining this function continues to generate theoretical
and experimental challenges. To test a recent theory, we measure the magnetic
susceptibility of the Ising dipolar ferromagnet LiHoF, using samples of
varying aspect ratio, and we reconsider the demagnetizing transformation
necessary to obtain the intrinsic material susceptibility. Our experimental
results confirm that the microscopic details of the material significantly
affect the transformation, as predicted. In particular, we find that the
uniaxial Ising spins require a demagnetizing transformation that differs from
the one needed for Heisenberg spins and that use of the wrong demagnetizing
transformation would result in unacceptably large errors in the measured
physical properties of the system. Our results further shed light on the origin
of the mysterious `flat' susceptibility of ordered ferromagnets by
demonstrating that the intrinsic susceptibility of the ordered ferromagnetic
phase is infinite, regardless of sample shape.Comment: 8 pages, 4 figure
Spin correlations in Ca3Co2O6: A polarised-neutron diffraction and Monte Carlo study
We present polarised-neutron diffraction measurements of the Ising-like
spin-chain compound Ca3Co2O6 above and below the magnetic ordering temperature
TN. Below TN, a clear evolution from a single-phase spin-density wave (SDW)
structure to a mixture of SDW and commensurate antiferromagnet (CAFM)
structures is observed on cooling. For a rapidly-cooled sample, the majority
phase at low temperature is the SDW, while if the cooling is performed
sufficiently slowly, then the SDW and the CAFM structure coexist between 1.5
and 10 K. Above TN, we use Monte Carlo methods to analyse the magnetic diffuse
scattering data. We show that both intra- and inter-chain correlations persist
above TN, but are essentially decoupled. Intra-chain correlations resemble the
ferromagnetic Ising model, while inter-chain correlations resemble the
frustrated triangular-lattice antiferromagnet. Using previously-published bulk
property measurements and our neutron diffraction data, we obtain values of the
ferromagnetic and antiferromagnetic exchange interactions and the single-ion
anisotropy.Comment: 10 pages, 7 figure
Field-induced magnetic states in holmium tetraboride
A study of the zero field and field induced magnetic states of the frustrated rare earth tetraboride HoB4 has been carried out using single crystal neutron diffraction complemented by magnetization measurements. In zero field, HoB4 shows magnetic phase transitions at TN1 = 7.1 K to an incommensurate state with a propagation vector (delta, delta, delta'), where delta = 0.02 and delta' = 0.43 and at TN2 = 5.7 K to a noncollinear commensurate antiferromagnetic structure. Polarized neutron diffraction measurements in zero field have revealed that the incommensurate reflections, albeit much reduced in intensity, persist down to 1.5 K despite antiferromagnetic ordering at 5.7 K. At lower temperatures, application of a magnetic field along the c axis initially re-establishes the incommensurate phase as the dominant magnetic state in a narrow field range, just prior to HoB4 ordering with an up-up-down ferrimagnetic structure characterized by the (h k 1/3)-type reflections between 18 and 24 kOe. This field range is marked by the previously reported M/M-sat = 1 3 magnetization plateau, which we also see in our magnetization measurements. The region between 21 and 33 kOe is characterized by the increase in the intensity of the antiferromagnetic reflections, such as (100), the maximum of which coincides with the appearance of the narrow magnetization plateau with M/M-sat approximate to 3/5. Further increase of the magnetic field results in the stabilization of a polarized state above 33 kOe, while the incommensurate reflections are clearly present in all fields up to 59 kOe. We propose the H-T phase diagram of HoB4 for the H parallel to c containing both stationary and transitionary magnetic phases which overlap and show significant history dependence
Reducing Disorder in Artificial Kagome Ice
Artificial spin ice has become a valuable tool for understanding magnetic
interactions on a microscopic level. The strength in the approach lies in the
ability of a synthetic array of nanoscale magnets to mimic crystalline
materials, composed of atomic magnetic moments. Unfortunately, these nanoscale
magnets, patterned from metal alloys, can show substantial variation in
relevant quantities such as coercive field, with deviations up to 6%. By
carefully studying the reversal process of artificial kagome ice, we can
directly measure the distribution of coercivities, and by switching from
disconnected islands to a connected structure, we find that the coercivity
distribution can achieve a deviation of only 3.3%. These narrow deviations
should allow the observation of behavior that mimics canonical spin-ice
materials more closely
Order by quenched disorder in the model triangular antiferromagnet RbFe(MoO4)2
We observe a disappearance of the 1/3 magnetization plateau and a striking change of the magnetic configuration under a moderate doping of the model triangular antiferromagnet RbFe(MoO4)2. The reason is an effective lifting of degeneracy of mean-field ground states by a random potential of impurities, which compensates, in the low-temperature limit, the fluctuation contribution to free energy. These results provide a direct experimental confirmation of the fluctuation origin of the ground state in a real frustrated system. The change of the ground state to a least collinear configuration reveals an effective positive biquadratic exchange provided by the structural disorder. On heating, doped samples regain the structure of a pure compound, thus allowing for an investigation of the remarkable competition between thermal and structural disorder
Magnetic field driven 2D-3D crossover in the S=12 frustrated chain magnet LiCuVO4
We report on a heat-capacity study of high-quality single-crystal samples of LiCuVO4 — a frustrated spin S = 1/2 chain system—in a magnetic field amounting to 3/4 of the saturation field. A detailed examination of magnetic phase transitions observed in this field range shows that although the low-field helical state clearly has three-dimensional properties, the field-induced spin-modulated phase turns out to be quasi-two-dimensional. The model proposed in this paper allows one to qualitatively understand this crossover, thus eliminating the presently existing contradictions in the interpretations of NMR and neutron-scattering measurements
High-field magnetic structure of the triangular antiferromagnet RbFe(MoO4)2
The magnetic H − T phase diagram of a quasi-two-dimensional antiferromagnet RbFe(MoO4 )2 (S = 5/2) with an equilateral triangular lattice structure is studied with 87Rb NMR and neutron-diffraction techniques. This combination of experimental techniques allows us to determine the ordered components of the magnetic moments on the Fe3+ ions within various high-field phases—the Y, UUD, V, and fan structures, stabilized in the compound by the in-plane magnetic field. It is also established that the transition from the V to the fan phase is of first order, whereas the transition from the fan phase to the polarized paramagnetic phase is continuous. An analysis of the NMR spectra shows that the high-field fan phase of RbFe(MoO4 )2 can be successfully described by a periodic commensurate oscillation of the magnetic moments around the field direction in each Fe3+ layer combined with an incommensurate modulation of the magnetic structure perpendicular to the layers