Appl. Phys. A 74 [Suppl.], S802–S804 (2002) / Digital Object Identifier (DOI) 10.1007/s003390201632
Applied Physics A Materials Science & Processing
Magnetic structure studies of ErMnO3 J. Park1 , U. Kong1 , S.I. Choi1 , J.-G. Park1,2,∗ , C. Lee3 , W. Jo4 1 Department of Physics, Inha University, Incheon 402-751, Korea 2 Department of Physics, SungKyunKwan University, Suwon 440-746, Korea 3 Neutron Physics Laboratory, Korea Atomic Energy Research Institute, Daejon 305-600, 4 Geballe Laboratory for Advanced Materials, Stanford University, CA 94305, USA
Korea
Received: 29 August 2001/Accepted: 11 December 2001 – Springer-Verlag 2002
Abstract. ErMnO3 forms in a hexagonal structure with P63 cm space group. Our neutron-diffraction studies from room temperature to 10 K show that ErMnO3 undergoes an antiferromagnetic transition at around 80 K, consistent with our bulk measurements. Magnetic group theoretical analysis and Rietveld refinements allow us to determine that the antiferromagnetic structure of ErMnO3 belongs to either the Γ2 or the Γ4 representation of the P63 cm space group.
for some earlier works in the 60s [1]. With renewed interests in YMnO3 , it is worthwhile and timely to re-investigate the magnetic properties of ErMnO3 with the same hexagonal structure as YMnO3 with better instrumentations than available in the 60s. In this study, we have measured bulk properties as well as neutron diffraction in order to understand better the magnetic properties of ErMnO3.
PACS: 25.40.Dn, 75.30.Kz
1 Experimental details
Rare-earth manganese oxides (RMnO3 ) have attracted much interests because of various interesting properties found in these compounds [1, 2]. For rare-earth elements with smaller ionic radius: R = Ho, Er, Tm, Yb, and Lu, the hexagonal structure of P63 cm space group is found [3]. Interestingly, some of these hexagonal manganese oxides have a ferroelectric transition at high temperature and an antiferromagnetic transition at low temperature. These two phases coexist at low temperature and thus the hexagonal manganese oxides are socalled ferroelectromagnets. There are few examples showing such kind of coexistence of the two phases, so it has become interesting to investigate a possible coupling between the two order parameters of the ferroelectric and antiferromagnetic phases in the hexagonal manganese oxides [4]. On the other hand, RMnO3 forms in the orthorhombic perovskite structure for rare-earth elements with larger ionic radius: R = La, Pr, Nd, Sm, Eu, Gd, and Tb. In these perovskite-type materials, colossal magnetoresistance behavior has been found and they have since been subject to intensive studies over the last few years. Among the hexagonal manganese oxides, YMnO3 is the most intensively studied system because of its potential importance as a future non-volatile memory device material. However, there are relatively few studies on ErMnO3 except ∗ Corresponding
author. (Fax: +82-31/290-7055, E-mail:
[email protected]) Also at Center for Strongly Correlated Materials Research, Seoul National University, Seoul 151-742, Korea
We used ErMnO3 of 99.9% purity from Superconductive Components, Inc. We measured magnetization from 300 to 2 K using a commercial magnetometer (PPMS9, Quantum Design), and thermal expansion from 300 to 10 K using a strain gauge method. In order to determine the temperature dependence of the lattice parameters and the magnetic structure, neutron diffraction data were taken from 300 to 10 K using the High Resolution Powder Diffractometer of the Korea Atomic Energy Research Institute with a neutron wavelength of λ = 1.834 Å. We refined our data using the Rietveld program FULLPROF and made a magnetic group theoretical analysis using the program MODY-2. 2 Experimental results and analysis Figure 1 shows the temperature dependence of the inverse susceptibility and susceptibility multiplied by temperature (χT ). As one can see, the inverse susceptibility follows the Curie-Weiss behavior from 300 to almost 100 K. Around 80 K, there is a small feature indicating some sort of weak transition. This feature is more noticeable in the χ T curve. Our magnetizaton measurements up to 9 Tesla show that the magnetization is linear above 100 K (not shown here). Below 80 K, however it begins to show some curvature that becomes pronounced with lowering temperature. The weak feature seen in χ is also reflected in our thermal expansion measurements. In Fig. 2, we show the temperature dependence of the thermal expansion data. A small, but clear shoulder-like structure is observed at around 80 K, where the
S803
a
b
Fig. 1. Temperature dependence of 1/χ and χT . The arrows indicate where a weak anomaly is found
Fig. 3a,b. Neutron diffraction data (symbols) taken at 10 K along with refinement results (line) due to nuclear and magnetic scattering. The line at the bottom shows the difference curve. The first row of bars shows the position of the nuclear Bragg peaks, while the second row of bars indicates that of the magnetic Bragg peaks for a the Γ2 and b Γ4 representations
Fig. 2. Temperature dependence of the thermal expansion results. The arrow indicates where an anomalous feature is found
susceptibility shows a weak feature too. This shoulder-like structure is not sensitive to magnetic fields up to 1 Tesla as shown in Fig. 2. Rietveld refinement of neutron diffraction data taken at room temperature confirms that ErMnO3 forms in the hexagonal structure of P63 cm space group with a = 6.1122(5) Å and c = 11.40181(1) Å. The room temperature data do not show any trace of an impurity phase testifying that our sample is indeed of high quality. With decreasing temperature, the a-axis contracts while the c-axis expands. Below around 80 K, new magnetic peaks begin to appear. From the low temperature data, we determine that propagation vector is k = 0 for ErMnO3. In order to analyze the magnetic peaks, we made a magnetic group theoretical analysis using the program MODY-2. According to this analysis, altogether twelve different magnetic structures are allowed for space group P63 cm with propagation vector k = 0 as in YMnO3 . Using all these twelve structures, we tried to refine the low temperature data, and found that, among the twelve structures, only two representations of P63 cm space group: Γ2 and Γ4 , explained the magnetic peaks much better than the rest, consistent with conclusions from the earlier works [1, 5] (see Fig. 3). A summary
Fig. 4. Temperature dependence of the refined magnetic moments for the Γ2 and Γ4 representations
of our refinement results for 10 K data is shown in Table 1. As one can see in the table, within the experimental resolution we could not distinguish between the two representations; Fig. 4 displays the temperature dependence of the ordered moments for the two representations. In both magnetic structures, the Mn magnetic moments are aligned on the basal plane. The only difference between the two structures lies in magnetic coupling along the c-axis. While the moments are coupled antiferromagnetically for Γ2 , they are coupled ferro-
S804 Table 1. Summary of the refinement results for the 10 K data Γ2
Er1 Er2 Mn O1 O2 O3 O4
a(Å) = 6.0922(1) c(Å) = 11.4117(3) V(Å3 ) = 367.27(1) Atomic coordinates x y 0.3333 0.6667 0.0000 0.0000 0.3288(12) 0.0000 0.3594(5) 0.0000 0.6930(7) 0.0000 0.3333 0.6667 0.0000 0.0000 Magnetic moment (µB ) Mn 3.23(2) Reliability factors RP = 3.16% RWP = 4.05% Rmag = 10.0% χ 2 = 1.89
Γ4
z 0.6381(5) 0.5971(5) 0.3665(7) 0.5298(7) 0.7028(7) 0.3534(5) 0.9005(8)
Er1 Er2 Mn O1 O2 O3 O4
4a(Å) = 6.0922(1) c(Å) = 11.4117(3) c(Å3 ) = 367.27(1) Atomic coordinates x y z 0.3333 0.6667 0.6388(8) 0.0000 0.0000 0.5958(6) 0.3352(15) 0.0000 0.3690(8) 0.3625(8) 0.0000 0.5319(9) 0.6930(6) 0.0000 0.7047(9) 0.3333 0.6667 0.3550(5) 0.0000 0.0000 0.8995(8) Magnetic moment (µB ) Mn 3.23(2) Reliability factors RP = 3.18% RWP = 4.06% Rmag = 9.72% χ 2 = 1.90
magnetically for Γ4 . The temperature dependence of the magnetic moments shows that there are no magnetic peaks above 80 K, suggesting that the anomalies in our bulk measurements should be due to the antiferromagnetic transition [6]. In order to investigate how the lattice changes at the antiferromagnetic transition, we have studied the temperature dependence of lattice parameters and of the total volume. With decreasing temperatures, the a-axis contracts while the c-axis expands: the total volume also decreases with cooling. Regarding the question of a possible coupling between the two order parameters of the ferroelectric and antiferromagnetic phases, we note that there is an anomaly in the lattice constants and the total volume at around 80 K. Therefore, together with the thermal expansion results this anomaly in the lattice constants at TN can be regarded as evidence for a probable coupling between the order parameters of the two phases. However, the anomaly is weaker in the lattice constants of ErMnO3 than of YMnO3 . It is also consistent with our bulk measurements that although both YMnO3 and ErMnO3 show anomalies at almost the same temperature, anomalies in the bulk properties near the antiferromagnetic transition are more pronounced for YMnO3 than for ErMnO3 [7]. Another interesting difference between YMnO3 and ErMnO3 is that while the former shows strong diffuse scattering at near the (002) nuclear peak position above TN , diffuse scattering is very weak, if any, for the latter. This difference, we think, is due to a probably drastic change in the spin dynamics of ErMnO3
caused by Er moments. In order to check this idea experimentally, inelastic neutron scattering experiments on YMnO3 and ErMnO3 are in progress. To summarize, we have found anomalies in magnetization and thermal expansion experiments of ErMnO3 near 80 K where our neutron diffraction studies show that it orders antiferromagnetically. From a magnetic group theoretical analysis, we conclude that the antiferromagnetic phase should belong to either the Γ2 or the Γ4 representation of space group P63 cm. Acknowledgements. We acknowledge Dr. A. Pirogov for helpful discussions. Work at Inha University and SungKyunKwan University was supported by the nuclear R & D program of the Ministry of Science and Technology.
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