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The effect of impurities on the magnetic structure of RbFeCl3
Journal of Magnetism
and Magnetic
Materials
54-57
THE EFFECT OF IMPURITIES A. HARRISON, Inorganrc
Chemlstp
D. VISSER, Lohorcrto~v,
ON THE MAGNETIC
P. DAY and K.R.A.
Unirvrsiry
1273
(1986) 127331274
STRUCTURE
ZIEBECK
OF RbFeCl,
+
of O.xJord. South Purks Rd., O.xJord. OXI
3QR,
UK
Single crystal neutron diffraction was performed on mixtures of the pseudo one-dimensional ferromagnet RhFeCl 1with the one-dimensional antiferromagnet RhFeBr, or the diamagnet RhMgCI 1. Less than 1% impurity was found to destroy magnetic long-range order. and displace the balance between interchain superexchange and the magnetic dipole-dipole interaction towards the latter.
1. Introduction
the (s, T) phase diagram of this system using single crystal neutron diffraction is presented here, as well as the behaviour of RbFeCl, doped with the RbMgCl,. Our findings for the mixed SGS-non-SGS system Rb (, , ,Cs, FeCl) are reported elsewhere [5].
The pseudo-one-dimensional insulating ferromagnet RbFeCl, displays a rich array of magnetic properties which have captured the attention of experimentalists and theorists recently [1,2]. A significant interchain antiferromagnetic exchange J’ induces three-dimensional magnetic long-range order at low temperatures. J’ competes with the magnetic dipole-dipole interaction to produce two successive incommensurate (IC) magnetic phases on cooling to 2.5 and 2.35 K, before a 120” antiferromagnetic array of moments forms in the basal plane at 1.95 K [2]. The IC phases are characterised in neutron scattering by satellite Bragg peaks centred around the K-point (Q = (l/3 l/3 0)). converging on the K-point as the commensurate phase forms. The magnetic character of this and isomorphous AFeX, compounds (A = Cs. Rb, Tl, and X = Cl, Br) is extremely sensitive to the nature of the superexchange pathways, and small distortions in the CsNiCl,-type crystal structure. RbFeBr, behaves as a pseudo one-dimensional antiferromagnet. CsFeCl, however has intrachain ferromagnetic superexchange as in RbFeCl?, but does not show long-range magnetic order down to 80 mK, and behaves as a true singlet ground state (SGS) material. The balance between SGS and non-SGS properties may be influenced in the opposite direction by the application of hydrostatic pressure [3]. One expects the balance of interactions in RbFeCI, to be strongly disturbed by impurities in both the magnetic and non-magnetic atoms. Furthermore, the mixed system RbFeCl+ \)Br, might possess spin glass properties at intermediate compositions and low temperatures if one considers the predictions of Fishman and Aharony [4] for a mixed ferro-antiferromagnet. It is conceivable, however, that SGS properties are found at intermediate compositions. There is the further complication of uncertainty in the nature of the magnetic exchange when there is a mixture of chlorine and bromine atoms bridging adjacent iron atoms. A study of
2. Experimental Single crystals of RbFeCl(, ~, )Br, and RbFe,, _ ~) Mg,CI,were grown by the Bridgman method, and their compositions determined by halide ion-exchange and X-ray microprobe analysis. Samples of size 10 X 3 X 3 mm3 were found to be homogeneous to within 1% chloride or magnesium ion. and compositions with s = 0.03, 0.06. 0.15 and 1 or y = 0.02 and 0.05 were chosen. The magnetic structure was studied down to 1.4 K at the Institut LaueeLangevin using the 2-axis diffractometer D15, which also has a tilting counter. Samples were mounted with the c-axis vertical. and the scattered neutron intensity mapped out in the a-b plane in the region of the K-point. 3. Results Samples of low dopant concentration gave a sharp peak in scattered neutron intensity at the K-point at low temperatures. As the temperature was raised, satellite peaks similar to those of the IC phases of the pure compound were seen. In all cases the peaks were best described by Lorentzian lineshapes convoluted with the Gaussian instrumental resolution function. The correlation length derived in this manner decreased with impurity concentration and appeared to pass through a maximum at about 2 K, decreasing on subsequent cooling. In addition to these relatively distinct features, a broad peak centred at the K-point was seen for all mixed samples, becoming more diffuse with temperature. This suggested the coexistence of magnetic order of much shorter range and hindered a rigorous assignment of lineshape to the main peaks. The magnitude of the scattered neutron intensity, as well as the temperature at which it converged on the K-point, fell with dopant concentration. Moreover, the
+ Institute Laue-Langevin, 156 X Centre de Tri, Crrenohle, Cedex 38042, France.
0304-8853/86/$03.50
0 Elsevier
Science Publishers
B.V
0.51 1
1.5 TM)
2
2.5
Fg 1. The dependence of the displacement of the scattered neutron intensity from the K-point on temperature and dopant concentration.
displacement of the satellite peaks from the K-point increased with impurity level, implying that the nearest-neighbour superexchange field is disrupted by the impurities more than the longer range dipoleedipole interaction (fig 1). At a higher impurity level (X = 1) no neutron intensity was observed on scanning through the I= 0 and I = 1 planes at (l/3 l/3 /), indicating destruction of ferro- or antiferromagnetic intrachain exchange.
4. Conclusions The effect on the NCel temperature of diluting the magnetic sites. or altering the exchange pathways of a
one-dimensional magnet was studied by Hone et al. [h]. who considered a collection of linear magnetic chains coupled by a molecular field. This approach is not very useful in the present case in which the magnetic correlations always appear finite. The Hone model does not include two of the peculiarities of RbFeCl, that may be important. Firstly, the closeness of RbFeCI, to SGS behaviour renders it particularily sensitive towards dilution. Secondly. the considerable frustration in the triangular antiferromagnetic array of moments may be partially relieved by impurities. This may also produce local canting of moments 171. Moreover. dilution with the bromide introduces the possibility of inserting antiferromagnetic links in the ferromagnetic chains. which one expects to be more disruptive than diluting the magnetic sites. This may account for the greater influence of the bromide on RbFeCl, than the magnesium compound. Further magnetic susceptibility and neutron scattering measurements are being performed to elucidate these observations. The authors wish to acknowledge the financial port of the UK S.E.R.C. for this project.
sup-
References [l] N. Wada. K. Ubukoshl and H. Hirakaha. J. Phys. Sot. Japan 51 (1982) 2833. (21 H. Shiba and N. Suzuki J. Phya. Sot. Japan, 51 (1982) 34Xx. [3] A. Harrison. D. Visser and C. Vettier. Annex to the ILL report (1984). [4] S. Fishman and .4. Aharony Phys. Rev. 821 (1980) 280. [5] A. Harrison. D. Visser, P. Day, W. Knop and M. Steiner. in preparation. [6] D. Hone. P.A. Montano. T. Tonegawa and Y. Imry, Phys. Rev. B12 (1975) 5741. [7] J. Villain. 2. Phys. B33 (1979) 31.
and Magnetic
Materials
54-57
THE EFFECT OF IMPURITIES A. HARRISON, Inorganrc
Chemlstp
D. VISSER, Lohorcrto~v,
ON THE MAGNETIC
P. DAY and K.R.A.
Unirvrsiry
1273
(1986) 127331274
STRUCTURE
ZIEBECK
OF RbFeCl,
+
of O.xJord. South Purks Rd., O.xJord. OXI
3QR,
UK
Single crystal neutron diffraction was performed on mixtures of the pseudo one-dimensional ferromagnet RhFeCl 1with the one-dimensional antiferromagnet RhFeBr, or the diamagnet RhMgCI 1. Less than 1% impurity was found to destroy magnetic long-range order. and displace the balance between interchain superexchange and the magnetic dipole-dipole interaction towards the latter.
1. Introduction
the (s, T) phase diagram of this system using single crystal neutron diffraction is presented here, as well as the behaviour of RbFeCl, doped with the RbMgCl,. Our findings for the mixed SGS-non-SGS system Rb (, , ,Cs, FeCl) are reported elsewhere [5].
The pseudo-one-dimensional insulating ferromagnet RbFeCl, displays a rich array of magnetic properties which have captured the attention of experimentalists and theorists recently [1,2]. A significant interchain antiferromagnetic exchange J’ induces three-dimensional magnetic long-range order at low temperatures. J’ competes with the magnetic dipole-dipole interaction to produce two successive incommensurate (IC) magnetic phases on cooling to 2.5 and 2.35 K, before a 120” antiferromagnetic array of moments forms in the basal plane at 1.95 K [2]. The IC phases are characterised in neutron scattering by satellite Bragg peaks centred around the K-point (Q = (l/3 l/3 0)). converging on the K-point as the commensurate phase forms. The magnetic character of this and isomorphous AFeX, compounds (A = Cs. Rb, Tl, and X = Cl, Br) is extremely sensitive to the nature of the superexchange pathways, and small distortions in the CsNiCl,-type crystal structure. RbFeBr, behaves as a pseudo one-dimensional antiferromagnet. CsFeCl, however has intrachain ferromagnetic superexchange as in RbFeCl?, but does not show long-range magnetic order down to 80 mK, and behaves as a true singlet ground state (SGS) material. The balance between SGS and non-SGS properties may be influenced in the opposite direction by the application of hydrostatic pressure [3]. One expects the balance of interactions in RbFeCI, to be strongly disturbed by impurities in both the magnetic and non-magnetic atoms. Furthermore, the mixed system RbFeCl+ \)Br, might possess spin glass properties at intermediate compositions and low temperatures if one considers the predictions of Fishman and Aharony [4] for a mixed ferro-antiferromagnet. It is conceivable, however, that SGS properties are found at intermediate compositions. There is the further complication of uncertainty in the nature of the magnetic exchange when there is a mixture of chlorine and bromine atoms bridging adjacent iron atoms. A study of
2. Experimental Single crystals of RbFeCl(, ~, )Br, and RbFe,, _ ~) Mg,CI,were grown by the Bridgman method, and their compositions determined by halide ion-exchange and X-ray microprobe analysis. Samples of size 10 X 3 X 3 mm3 were found to be homogeneous to within 1% chloride or magnesium ion. and compositions with s = 0.03, 0.06. 0.15 and 1 or y = 0.02 and 0.05 were chosen. The magnetic structure was studied down to 1.4 K at the Institut LaueeLangevin using the 2-axis diffractometer D15, which also has a tilting counter. Samples were mounted with the c-axis vertical. and the scattered neutron intensity mapped out in the a-b plane in the region of the K-point. 3. Results Samples of low dopant concentration gave a sharp peak in scattered neutron intensity at the K-point at low temperatures. As the temperature was raised, satellite peaks similar to those of the IC phases of the pure compound were seen. In all cases the peaks were best described by Lorentzian lineshapes convoluted with the Gaussian instrumental resolution function. The correlation length derived in this manner decreased with impurity concentration and appeared to pass through a maximum at about 2 K, decreasing on subsequent cooling. In addition to these relatively distinct features, a broad peak centred at the K-point was seen for all mixed samples, becoming more diffuse with temperature. This suggested the coexistence of magnetic order of much shorter range and hindered a rigorous assignment of lineshape to the main peaks. The magnitude of the scattered neutron intensity, as well as the temperature at which it converged on the K-point, fell with dopant concentration. Moreover, the
+ Institute Laue-Langevin, 156 X Centre de Tri, Crrenohle, Cedex 38042, France.
0304-8853/86/$03.50
0 Elsevier
Science Publishers
B.V
0.51 1
1.5 TM)
2
2.5
Fg 1. The dependence of the displacement of the scattered neutron intensity from the K-point on temperature and dopant concentration.
displacement of the satellite peaks from the K-point increased with impurity level, implying that the nearest-neighbour superexchange field is disrupted by the impurities more than the longer range dipoleedipole interaction (fig 1). At a higher impurity level (X = 1) no neutron intensity was observed on scanning through the I= 0 and I = 1 planes at (l/3 l/3 /), indicating destruction of ferro- or antiferromagnetic intrachain exchange.
4. Conclusions The effect on the NCel temperature of diluting the magnetic sites. or altering the exchange pathways of a
one-dimensional magnet was studied by Hone et al. [h]. who considered a collection of linear magnetic chains coupled by a molecular field. This approach is not very useful in the present case in which the magnetic correlations always appear finite. The Hone model does not include two of the peculiarities of RbFeCl, that may be important. Firstly, the closeness of RbFeCI, to SGS behaviour renders it particularily sensitive towards dilution. Secondly. the considerable frustration in the triangular antiferromagnetic array of moments may be partially relieved by impurities. This may also produce local canting of moments 171. Moreover. dilution with the bromide introduces the possibility of inserting antiferromagnetic links in the ferromagnetic chains. which one expects to be more disruptive than diluting the magnetic sites. This may account for the greater influence of the bromide on RbFeCl, than the magnesium compound. Further magnetic susceptibility and neutron scattering measurements are being performed to elucidate these observations. The authors wish to acknowledge the financial port of the UK S.E.R.C. for this project.
sup-
References [l] N. Wada. K. Ubukoshl and H. Hirakaha. J. Phys. Sot. Japan 51 (1982) 2833. (21 H. Shiba and N. Suzuki J. Phya. Sot. Japan, 51 (1982) 34Xx. [3] A. Harrison. D. Visser and C. Vettier. Annex to the ILL report (1984). [4] S. Fishman and .4. Aharony Phys. Rev. 821 (1980) 280. [5] A. Harrison. D. Visser, P. Day, W. Knop and M. Steiner. in preparation. [6] D. Hone. P.A. Montano. T. Tonegawa and Y. Imry, Phys. Rev. B12 (1975) 5741. [7] J. Villain. 2. Phys. B33 (1979) 31.