North-Holland. Coupling between antiferro and .... Phys. Rev. 136 (1963) A1641. [3] J. Baruchel, M. Schlenker and B. Barbara, J. Magn. Magn. Mater. 15-18 ...
Journal of Magnetism and Magnetic Materials 104-107 (1992) 350-352 North-Holland
Coupling between antiferro and ferromagnetic domains in hematite J. Sandonls ,,b, j. Baruchel b,c,d, B.K. T a n n e r ~, G. Fillion ~, V.V. Kvardakov and K.M. P o d u r e t s f " Dpto. CITTYM, Unit'. de Cantabria, Santander, Spain I, hist. Laue-Langecin, Grenoble, France ' Lab. L. N~el. CNRS-UJF, Grenoble, France a European Synchrotron Radiation Facility, Grenoble, France " Department of Physics, Durham Unicersity, UK /Kurchatot' htst. Atomic Ener~% Moscow. USSR
We observe a coupling between the weak ferromagnetic (WF) component and the antiferromagnettc (AF) vector by investigating 180° domains in the WF phase of hematite, in keeping with the symmetry of the Dzialoshinski interaction, and a memory effect associated with the coupling between the signs of the major type domains in the WF and AF phases when going through the Morin transition. We have studied the a n t i f e r r o - f e r r o m a g n e t i c domain structure on nearly perfect (111) platelet shaped, = 0.2 mm thick, crystals of h e m a t i t e ( a - F e = O 3) flux grown at the University of Crimea ( U S S R ) (sample No. 1, nearly perfect) and at the C l a r e n d o n Lab., Oxford (sample No. 2). T h e polarized n e u t r o n diffraction and topography experiments were p e r f o r m e d on the $20 diffractometer at ILL, and the m a g n e t i z a t i o n measurem e n t s were carried out on a S Q U I D m a g n e t o m e t e r at Lab. L. Ndel. H e m a t i t e displays two magnetically ord e r e d phases. For T > T M = 261 K it is a weak ferrom a g n e t (WF), the magnetic m o m e n t s of the two sublattices lying on the (111) trigonal basal plane and being slightly c a n t e d to produce a basal W F c o m p o n e n t . At T < T M it orders as a true A F with m o m e n t s p e r p e n dicular to the basal plane [1]. In addition to the 120 o trigonal domains h e m a t i t e exhibits 180 ° A F domains, which result from the fact that the sites c o r r e s p o n d i n g to the two sublattices are not related by a lattice translation [2]. T h e s e domains arc similar to those investigated in M n F z [3], but in the present case the existence of a W F c o m p o n e n t allows us to modify the d o m a i n configuration in the W F phase by just applying a small magnetic field. Polarized n e u t r o n s t o p o g r a p h s and flipping ratio m e a s u r e m e n t s give us information about the sign of the antiferromagnetic vector L = M t - M . inside the crystal, where M~ and M . are the sublattice magnetizations. I n d e e d the diffraction power of a given type of 180 ° A F domain is very different for the two signs of the n e u t r o n polarization directed along L for the 210 reflection, i.e. the flipping ratio ( F R ) is very different from 1. In o r d e r to act on the domains the m a g n e t i c field must be applied along the W F c o m p o n e n t on = M t + M e direction. This implies that the F R m e a s u r e m e n t s must be p e r f o r m e d in zero field to retain the a d e q u a t e L polarization of 0312-8853/92/$05.00 -~, 1992
the neutrons, and are in this way ' r e m a n c n t ' F R (RFR). Fig. 1 shows the hysteresis like b e h a v i o u r of the R F R and of the r e m a n e n t m a g n e t i z a t i o n of the nearly perfect sample No. 1 as a function of the last applied magnetic field. T h e two curves, r e c o r d e d in zero field, are very similar, indicating that t h e r e is a coupling b e t w e e n the W F c o m p o n e n t m of the major d o m a i n type, d e t e r m i n e d by the sign of the last applied field, and the sense of the c o r r e s p o n d i n g A F vector L, d e t e r m i n e d from the sign of the logarithm of the R F R . This is in keeping with the symmetry of Dzialoshinski interaction H D - - D ' ( M 1 × M ~ ) = ~t _ D ' ( L × m ) , because the two 180 ° domains are energetically equivalent, and the Dzialoshinski vector D is a constant: reversing the W F c o m p o n e n t must reverse the A F vector [11]. T o p o g r a p h s recorded with opposite polarizations on sample No. 1 in the W F phase show the whole crystal
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Elsevier Science Publishers B.V. All rights reserved
J. Sandon(s et aL / Antiferro- and ferromagnetic domains in hematite
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Fig. 2. F l i p p i n g r a t i o in t h e W F a n d A F p h a s e s as a f u n c t i o n o f t h e s a m p l e r o t a t i o n ( s e e text), a n d i n s e r t , s c h e m a t i c d r a w i n g o f t h e s a m p l e N o . 2; t h e t w o twin e l e m e n t s a r e i n d i c a t e d .
surface, but different defect images, suggesting that the 180° domain walls lie in (111) planes. Related extended basal planes walls were observed on hematite by synchrotron radiation topography [4]. This wall orientation is expected when taking into account the very weak anisotropy in the basal plane [5], which implies a small (111) wall energy. Sample No. 2, already used to investigate the Morin transition by neutron topography [6], is twinnned (rotation of 180° along z), and slightly curved, as indicated in the insert of fig. 2. This curvature leads to a widening of the 210 rocking curve (0.4 ° FWHM) and allows the observation of new features concerning the domains. Fig. 2 shows the evolution of the remanent flipping ratio in the two phases. The RFR was recorded after applying a 1.5 x 10 -2 T magnetic field along x. The neutron polarization was always directed along L, which is different for both phases. Fig. 2 shows that the Iog(RFR) reverses when rotating the sample in the WF phase. This implies that the remanent WF magnetization remained along the external magnetic field in both twins, which display, in this way, opposite antiferromagnetic domains. When going through the transition we obtain, a FR very different from one in the AF phase, even with no applied magnetic field during the transition, in contrast with ref. [2]. This effect is independent of the value, but not of the sign, of the RFR in the WF phase• This indicates a new coupling between the signs of the major type 180° domains in the WF and the AF phases• Fig. 2 shows that the twinned crystal (sample No. 2) exhibits the same sign of the RFR in the AF phase over the whole rocking curve. However contrast between the two twin elements was observed by polarized neutron topography in the AF phase when cooling from a WF state displaying a single AF domain, and consequently two opposite WF components, in each of the twin elements. Calculations of the structure factors
for the two elements of the twin in the AF phase indicate, rather surprisingly, that the same flipping ratio corresponds to two different 180° AF domains, one in each twin element. The reason lies in the fact that the diffraction vector is different for the two elements of the twin, 210 for one and 012 for the other. For these reflections the nuclear structure factor is only generated by the oxygen atoms and has a different sign for the two elements of the twin leaving unchanged the magnetic structure factor, which has only contribution from the iron atoms. If equivalent (different) flipping ratios occur in the two elements of the twin, i.e. if we have the same (opposite) phase relationship between the magnetic and nuclear parts of the structure factor on both sides of the twin boundary, this corresponds to different (equal) 180° AF domains. We thus observe in this sample No. 2 that a given major AF domain in the WF phase produces, in each twin element, always the same AF domain in the low temperature phase. The comparison between the two samples suggests, in addition, an explanation of the fact that sample No. 1 appears to be nearly single domain in the AF phase: the (111) wall is a very low energy one in the WF phase but not in the AF phase, and tends to disappear, whereas the twin boundary can constitute an AF wall in both phases. The crystal retains in addition the memory of the initial major type domain sign when heating back to the WF phase: indeed a multidomain state is reached, but such that the initial remanent magnetization, and consequently the AF domain, is at least partially recovered. Magnetization measurements show clearly this effect. The usual recovered magnetization is about one third of the saturated sample one. Precise measurements of this effect are difficult due to the high sensitivity of the domain structure, in the nearly perfect crystals we have investigated, to small parasitic magnetic fields (of the order of the oersted). This memory effect is different from the oscillating memory effect reported for more imperfect crystals [7] which is assumed to be produced by the existence of pinning centers inside the crystal [8]. The mechanism of the observed correlation between the sign of the WF and AF domains is still not clear for us. It could be related with the piezomagnetic properties of hematite [9]. The piezomagnetic tensor has the sign of L. A distortion associated with the difference in cell parameters between the two phases occurs in the neighbourhood of the interface, producing a piezomagnetic component in a volume of the AF phase which is close to the interface. The maximum magnitude of the piezomagnetic component, which decreases when moving apart from the (111) interface [10], is estimated to be of the order of the WF component when taking into account that topographic experiments indicate a difference in the cell parameters of the order of 10 -5 [4,10]. A correlation between do-
352
J. Sandon[s et al. / Antiferro- and ferromagnetic domains in hematite
mains in the W F and A F phases could result from a m e c h a n i s m w h e r e the sign of the piezomagnetic comp o n e n t is d e t e r m i n e d by the strain and the sign of the magnetic field seen by the c o n c e r n e d small volume of A F phase, which is in turn related to the sign of the W F c o m p o n e n t . F u r t h e r work is in progress in o r d e r to b e t t e r u n d e r s t a n d this correlation. References [1] C.G. Shull, W.A. Strauser and E.O. Wollan, Phys. Rev. 83 (1951) 333. [2] R. Nathans, S.I. Pickart, H.A. Alperin and P.J. Brown, Phys. Rev. 136 (1963) A1641. [3] J. Baruchel, M. Schlenker and B. Barbara, J. Magn. Magn. Mater. 15-18 (1980) 1510.
[4] B.K. Tanner, G.F. Clark, M. Safa, Philos. Mag. B 57 (1988) 361. [5] P.J. Besser, A.H. Morrish and C.W. Searle. Phys. Rev. 153 (1966) 632. [6] J. Baruchel, G.F. Clark, B.K. Tanner and B.E. Watts, ,I. Magn. Magn. Mater. 68 (1987) 374. [7] S.T. Lin, J. Appl. Phys., supplement to vol. 32 no. 3 (1961). [8] M. Yamamoto and T. Iwata, Proc. ICM (Nottingham) 581 (1964). [9] V.P. Andratskii and A.S. Borovik-Romanov, Sov. Phys. JETP 24 (1967) 687. [10] V.V. Kvardakov, J. Sandonls, K.M. Podurets, S.Sh. Shilstein and J. Baruchel, Physica B 168 (1991) 242. [11] P. Radhakrishna, J. de Phys. 43 (1982) C7-221.
