Investigating the magnetic ground state of the skyrmion host material CuOSeO using long-wavelength neutron diffraction
We present long-wavelength neutron diffraction data measured on both single crystal and polycrystalline samples of the skyrmion host material CuOSeO. We observe magnetic satellites around the diffraction peak not accessible to other techniques, and distinguish helical from conical spin textures in reciprocal space. We confirm successive transitions from helical to conical to field polarised ordered spin textures as the external magnetic field is increased. The formation of a skyrmion lattice with propagation vectors perpendicular to the field direction is observed in a region of the field-temperature phase diagram that is consistent with previous reports. Our measurements show that not only the field-polarised phase but also the helical ground state are made up of ferrimagnetic clusters instead of individual spins. These clusters are distorted Cu tetrahedra, where the spin on one Cu ion is anti-aligned with the spin on the three other Cu ions.
Skyrmions are topologically protected, nano-sized swirls of spins found in a range of magnetic materials. In recent years a number of spectacular advances have demonstrated the existence, not only of magnetic skyrmions, but also their ordering into a skyrmion lattice (SL) in several magnetic materials with a chiral structure.Mühlbauer et al. (2009); Yu et al. (2010, 2011); Seki et al. (2012a); Tokunaga et al. (2015); Kézsmárki et al. (2015); Bordács et al. (2017); Kurumaji et al. (2017) This symmetry can lead to the formation of an antisymmetric Dzyaloshinskii-Moriya interaction (DMI)Dzyaloshinsky (1958); Moriya (1960) that favours canting between neighbouring spins. Competition between the symmetric exchange interaction promoting parallel alignment of spins and the DMI leads to a magnetic ground state that generally consists of helices, long period magnetic structures where the magnetisation rotates in a plane perpendicular to the propagation direction. On the application of a magnetic field a conical (C) structure forms, with a net magnetic moment along the propagation direction. The C structure exists over a wide range of applied magnetic fields with the cone angle decreasing upon increasing the field until it is reduced to zero and magnetic moments align with the field direction. Skyrmions are very close in energy to the C structure, and the SL phase is generally stabilised in a small temperature and field region of the phase diagram just below the critical temperature . Theoretical predictions show that for skyrmions to exist systems require an easy-axis ferromagnetism on top of a DMI.A. N. Bogdanov and Yablonskii (1989); Bogdanov and Hubert (1994)
The phase diagram of CuOSeO resembles that of other skyrmion hosting systems such as several B20 compounds,Yu et al. (2010, 2011) or -Mn-type CoZnMn alloys Tokunaga et al. (2015): It consists of a helical (H) ground state, a C phase in applied fields, a field polarised (FP) phase in large applied fields, and a SL phase in a pocket in applied field and temperature just below .Seki et al. (2012a) The helical ground state was determined by Lorentz TEM.Seki et al. (2012a); Adams et al. (2012); Seki et al. (2012b); Yang et al. (2012) Earlier neutron powder diffraction was unable to determine this structure due to the long helical wavelength of nm ( Å). Bos et al. (2008); Adams et al. (2012); Seki et al. (2012b) The phase diagram determined from Lorentz TEM has been confirmed by small angle neutron scattering (SANS) which finds that the ground state helices are oriented along directions, but cannot easily differentiate between a cone and a helix. Adams et al. (2012); Seki et al. (2012b) In SANS, the formation of a SL is observed as a sixfold diffraction pattern, consistent with the hexagonal close packing of skyrmions in a plane perpendicular to the applied field direction.
Previous investigations have either shown that CuOSeO is a ferrimagnet Kohn (1977); Bos et al. (2008); Belesi et al. (2010) without addressing the chiral nature of the magnetic ground state, or investigated the chirality without taking into account ferrimagnetism.Seki et al. (2012a); Adams et al. (2012); Seki et al. (2012b); Yang et al. (2012); Zhang et al. (2017) Here we investigate the magnetic microstructure of the magnetic phases of CuOSeO and address the question of whether the ground state H structure is also ferrimagnetic. Using time-of-flight long-wavelength neutron diffraction on a single crystal sample, we observe magnetic satellites around the diffraction peak not accessible to other techniques. Furthermore, our measurements uniquely allow us to distinguish helical from conical spin textures in reciprocal space. We show successive transitions from H to C to FP as the external magnetic field is increased in the magnetically ordered phases, and the formation of a skyrmion lattice with propagation vectors perpendicular to the field direction in a region of the field-temperature phase diagram that is consistent with previous reports.Seki et al. (2012a); Adams et al. (2012) We use powder diffraction measurements to determine the magnetic microstructure of the FP and H phases, and show that not only the FP phase but also the H ground state are made up of ferrimagnetic clusters instead of individual spins.
Polycrystalline samples of CuOSeO were synthesized as described previously.Bos et al. (2008) Phase purity was confirmed by powder x-ray diffraction. The same polycrystalline powder was also used for the growth of single crystals by the chemical vapour transport technique following the procedure described by Seki et al. (2012a)
DC magnetisation measurements were performed using SQUID magnetometry, in a Quantum Design MPMS. AC susceptibility measurements were performed on the same instrument at an excitation frequency of Hz with an amplitude of mT.
Neutron powder and single crystal diffraction experiments were performed on the time-of-flight long-wavelength neutron diffractometer WISH Chapon et al. (2011) at the ISIS Facility of the Rutherford Appleton Laboratory (UK) to determine the crystal and magnetic structures. The WISH detector system consists of pixelated He gas tubes covering scattering angles from to in the plane and out of the plane. Powder diffraction patterns are measured at fixed scattering angles as a function of the time-of-flight (which is related to -spacing). The highest resolution is offered at large scattering angles which require longer wavelength neutrons. This, however, limits the highest observable -spacing. For measuring powder diffraction patterns, we thus consider two detector banks, at and . For single crystal diffraction patterns we restricted the neutron wavelength to Å to observe the Bragg peak and its magnetic satellites.
Iii Results and Discussion
AC susceptibility measurements have proven powerful in determining the presence and location of the SL.Bauer and Pfleiderer (2012, 2016) Figure 1(a) shows the real part of the AC susceptibility from measurements on CuOSeO with the magnetic field applied along the direction. The SL is identified as a decrease in relative to the surrounding C phase. It is located in the range mT and K in agreement with previous observations.Seki et al. (2012a); Adams et al. (2012)
The inverse (DC) susceptibility presented in Figure 1(b) confirms a critical temperature K. Above , however, the temperature dependence of does not agree with the earlier report by Bos et al. (2008): in the PM phase, we observe a negative curvature (noted in an early paper by Kohn (1977)), instead of a linear increase with temperature as observed for ferromagnets and antiferromagnets. This dependence is characteristic of ferrimagnets,Kittel (1996) and is particularly clear with the log-log scale used here. Magnetisation measurements in the PM phase thus indicate a ferrimagnetic alignment of spins.
Single Crystal Diffraction:
For single crystal diffraction the sample was also mounted with the applied field along the direction. Data measured around the Bragg peak (yellow dot) is presented in Figure 2. The structural peak is removed by subtracting the structural diffraction pattern measured in zero field in the PM phase, thus showing only magnetic peaks. The technique is distinct from SANS, where the scattering around the transmitted neutron beam is investigated. The operating principle of our single crystal neutron diffraction is more similar to resonant elastic x-ray scattering,Langner et al. (2014); Zhang et al. (2016) in that periodically modulated spin textures are observed as satellite peaks around a structural Bragg reflection. As resonant elastic x-ray scattering relies on matching the photon energy (and thus wavelength) to a x-ray absorption edge, only the peak of CuOSeO is accessible at the Cu edge (in many other skyrmion hosts, such as FeGe or MnSi, no Bragg peaks are accessible at all).Zhang et al. (2016) Single crystal neutron diffraction has the advantage of allowing access to a wide range of structural diffraction peaks, such as the peak considered here. It furthermore allows distinguishing helices and cones as detailed below.
At K, in the absence of an applied magnetic field [Fig. 2(a)], satellite peaks (green dots) oriented along directions are observed as expected in the H phase Adams et al. (2012); Seki et al. (2012b): Two peaks observed to the left and right of the Bragg peak correspond to the direction. Two peaks above and below the Bragg peak correspond to the and directions (these satellites coincide due to the orientation of the sample in the instrument). We extract a helical of Å, in agreement with previous SANS measurements.Adams et al. (2012); Seki et al. (2012b) Upon the application of a magnetic field ( mT), the helices rotate towards the field direction [Fig. 2(b)]. No central peak is observed indicating the absence of a net magnetic moment as expected for a helix. A further increase of the magnetic field to mT [Fig. 2(c)] induces a spin canting and the structure becomes conical. This is observed as the appearance of intensity at the parent peak position indicating a net magnetic moment. When the applied field is increased further [Fig. 2(d)], all moments align with the field ( mT). Figure 2(e) shows the diffraction pattern measured at K in mT, at the location of the skyrmion phase [c.f. black dot in Figure 1(a)]. The resolution is too low to distinguish separate peaks, but a transfer of intensity from aligned along the applied field direction to the plane perpendicular to the applied field is observed, showing that the propagation vectors of the skyrmion lattice are in a plane perpendicular to the applied field. The lack of magnetic satellites above and below the parent Bragg peak precludes the coexistence of the skyrmion phase with the C phase in our sample.
The resolution of our single crystal diffraction data is not sufficient to easily distinguish the 6-fold SL satellites. However, it shows that the magnetic phase can be transformed from H to FP via the C phase by an increase in the applied magnetic field , and to the SL phase by a change in temperature. Single crystal neutron diffraction measurements agree with the phase diagram established by AC susceptibility measurements and the helical wavelength established in SANS measurements.Adams et al. (2012); Seki et al. (2012b)
Neutron powder diffraction data measured at K in the PM phase is presented in Figure 3. The pattern was measured at zero magnetic field and thus contains only structural information. The crystallographic parameters obtained from Rietveld refinement are reported in Table 1. The data is best fitted to the space group with lattice parameter Å in agreement with previous reports.Bos et al. (2008) The crystal structure of CuOSeO is shown in Figure 4(a). The sixteen Cu ions in the unit cell sit on two different Wyckoff sites, four Cu1 ions on the 4a and twelve Cu2 ions on the 12b site. Cu1 ions bond with oxygen ions to form bipyramidal CuO units, while Cu2 ions are bonded in distorted square based CuO pyramids. The Cu ions are arranged in a network of distorted tetrahedra consisting of one Cu1 and three Cu2 ions.
Data measured in the FP phase at K in an applied field mT is shown in Figure 5(a) for a scattering angle . The diffraction pattern containing only structural information and measured in the PM phase is shown for comparison (black line). Analogous to our approach for single crystal diffraction, the difference between both patterns is shown in Figure 5(b). should contain purely magnetic information, however, thermal contraction can lead to a small shift of structural peaks between the data measured at K and K. This can be identified by the shape of the difference peaks as a negative peak next to a positive peak (a shape resembling the derivative of a -function). This is for example observed at the reflection in Figure 5(b). A clear magnetic contribution to the diffraction pattern is observed at the reflection. The data is best fitted with a model with spins on Cu1 ions aligned antiparallel to the spins on Cu2 ions, as sketched in Figure 4(d), and a magnetic moment of for each Cu ion. The FP phase is thus ferrimagnetic, consisting of clusters of four Cu ions, where the spin on one Cu1 ion is anti-aligned with the spins on three Cu2 ions.
The H phase was investigated at K in zero field. The data measured at and is again compared to the data measured in the PM phase in Figures 5(c)-(f). In addition to the magnetic (110) peak already observed in the ferrimagnetic phase, additional purely magnetic peaks are observed at the position [as highlighted in the inset of Figure 5(c)] and the position. The higher resolution data at reveals the latter to be due to the presence of two magnetic satellite peaks, as shown in the inset of Figure 5(f). As for the single crystal diffraction data, the presence of these satellites around a structural Bragg reflection indicates the presence of a long-range periodically modulated spin texture. The data in the H phase was best fitted by a helical model, where the relative orientation of spins within the unit cell is fixed, but helically modulated between unit cells with a of Å. This corresponds to a helical state consisting of ferrimagnetic clusters, where we fit an average moment size of /Cu, and thus lower than the moment in the ferrimagnetic phase. We attribute this to the degenerate ground state, where the helical wavevector can point in any direction. Fluctuations can thus lead to a reduction in the observed magnetic moment.
While neutron powder diffraction patterns can be fitted by Rietveld refinement, a qualitative analysis is instructive in understanding the helical ground state of CuOSeO. To that end Figure 6 presents simulated powder diffraction patterns for (a) helices composed of individual spins, and (c) helices with ferrimagnetic alignment of spins within Cu tetrahedra. An exaggerated magnetic moment ( /Cu) and ( Å) have been used to better distinguish helical satellite peaks. Simulations reproducing the PM phase are included for comparison and the difference between patterns shown in Figures 6(b) & (d). Both models predict the absence of a magnetic peak, but the presence of magnetic satellites around it. They furthermore show a magnetic peak and satellites at , however these are more pronounced in the ferrimagnetic model. Finally, only the ferrimagnetic model indicates a magnetic peak at the Bragg peak position. Comparing the neutron diffraction data presented in Figure 5 with the simulations in Figure 6 it becomes apparent, that the magnetic ground state of CuOSeO does not only consist of helices, but that these are made up of ferrimagnetically aligned clusters of spins: the H satellites predicted around the structural peak are clearly observed in Figure 5(f). Magnetic satellites are not resolved around the (110) peak in the diffraction measurements, but a large magnetic peak is most likely consistent with the ferrimagnetic cluster model of the H phase. Finally, the magnetic peak observed at the position is only accounted for by the ferrimagnetic cluster model. We have thus established the magnetic ground state of CuOSeO to be composed of helices made from ferrimagnetically aligned spin clusters instead of individual spins.
Single crystal and powder long-wavelength neutron diffraction data was measured on the skyrmion host CuOSeO. We observe magnetic satellites around the diffraction peak not accessible by other techniques, and distinguish helical from conical spin textures in reciprocal space. Measurements show successive transitions from a helical to a conical to a field polarised magnetic phase as the external magnetic field is increased. While the resolution is too low to resolve the individual 6-fold skyrmion satellites, the formation of a skyrmion lattice with propagation vectors perpendicular to the field direction is observed. As our key result, we show that not only the field polarised phase but also the helical ground state are made up of ferrimagnetic clusters instead of individual spins. These clusters are distorted Cu tetrahedra, where the spin on the Cu1 ion is anti-aligned with the spin on three Cu2 ions. Supporting magnetometry in the paramagnetic phase confirms the proposed ferrimagnetism.
Acknowledgements.We are grateful for the provision of beamtime at the Science and Technology Facilities Council (STFC) ISIS Facility, Rutherford Appleton Laboratory, UK. We acknowledge the use of the MPMS on the I10 beamline of the Diamond Light Source. This work was supported by the EPSRC through grants EP/M028771/1 and EP/N032128/1 and a scholarship for P R D. Research data will be made available via Durham Collections.
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