Weak ferromagnetism in hexagonal orthoferrites RFeO3

APPLIED PHYSICS LETTERS 99, 122502 (2011)

Weak ferromagnetism in hexagonal orthoferrites RFeO3 (R 5 Lu, Er-Tb) A. R. Akbashev,1,a) A. S. Semisalova,2 N. S. Perov,2 and A. R. Kaul3 1

Department of Materials Science, Moscow State University, Moscow, Russia Physics Department, Moscow State University, Moscow, Russia 3 Chemistry Department, Moscow State University, Moscow, Russia 2

(Received 16 August 2011; accepted 31 August 2011; published online 20 September 2011) Hexagonal orthoferrites of rare earths RFeO3 (R ¼ Lu, Er-Tb) were grown epitaxially on (111)ZrO2(Y2O3) substrates using metal-organic chemical vapour deposition. Temperature and field dependences of magnetization were measured and analyzed for all samples and revealed weak ferromagnetic behavior below T ¼ 120–140 K. The difference in electronic structure along with a distinct similarity in the crystal structure of hexagonal manganites RMnO3 and hexagonal orthoferrites RFeO3 are brought into focus in order to explain the results. Hexagonal orthoferrites C 2011 American Institute of Physics. are regarded as a promising family of multiferroics. V [doi:10.1063/1.3643043]

In recent years much attention has been devoted to multiferroic materials owing to their potential use in modern electronics and spintronics.1,2 However, it turned out to be particularly difficult to design a proper multiferroic with both ferroelectric and ferromagnetic properties being acceptable enough for applications. Among well-studied magnetoelectrics are perovskite and hexagonal manganites RMnO3, which structure depends on the ionic radius of a rare-earth.3 Perovskite manganites were shown to exhibit magnetically induced ferroelectricity via the inverse Dzyaloshinskii-Moriya interaction or due to the E-type magnetic structure.4–6 On the other hand, hexagonal RMnO3 (R ¼ Ho-Lu) are magnetoelectrics with independent magnetic and ferroelectric orderings. Their structure is described by a P63cm space group and can be viewed as a dense packing of oxygen ions (ABCABC) with Mn3þ ions having a bipyramidal coordination and R3þ ions being in capped octahedra.3,7 Hexagonal manganites are improper ferroelectrics where polarization arises due to the cooperative tilting of bipyramids below the Curie temperature (typically, Tc 900 K).8 Antiferromagnetic spin ordering emerges from the superexchange interaction between manganese ions which form a planar triangular structure in the ab plane. As the result of it, spin frustration takes place in these compounds and promotes the instability of the magnetic structure, thus giving rise to their rich magnetic phase diagrams.9 Noteworthy, a metastable hexagonal modification of RMnO3 (R ¼ Sm-Dy), which normally are perovskites, can be obtained in thin-film state by means of epitaxial stabilization on a specific substrate.10 Notwithstanding a thorough exploration of hexagonal RMnO3, almost no results are published on hexagonal orthoferrites RFeO3 which possess the same structure despite having Fe3þ ions instead of Mn3þ ones.11,12 Orthoferrites RFeO3 are known as perovskites for all R3þ ions, leading to a conclusion that their hexagonal polymorphs, if any exists, are metastable. Today, few reports can be traced where hexagonal orthoferrites were synthesized as nanoparticles,13 thin-films14 or by a containerless processing.15 Similarly to the epitaxial stabilization of hexagonal RMnO3 a)

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(R ¼ Sm-Dy) in thin-film state,10 hexagonal orthoferrites can be grown on substrates such as (111)ZrO2(Y2O3), where crystallographic positions of oxygen coincide well with those of the hexagonal phase.14 Here, we report on the measurements of the temperature and field dependences of magnetization of hexagonal orthoferrites RFeO3 (R ¼ Lu, Er-Tb) that showed a weak ferromagnetic behavior markedly different from the antiferromagnetism in hexagonal RMnO3. Thin films (50–70 nm thick) of RFeO3 (R ¼ Lu, Ho-Tb) were grown by metal-organic chemical vapor deposition (MOCVD) on single-crystalline (111)ZrO2(Y2O3) substrates (YSZ) as described in the previous work of our group.14 The epitaxy of hexagonal RFeO3 on the (111)ZrO2(Y2O3) substrates was confirmed by detailed XRD (Rigaku Smartlab) studies. Characteristic reflections (00l) of the hexagonal phase are observed on 2h/x scans, indicating a c-oriented growth of thin films (Fig. 1). In-plane u scans evinced 6 reflections corresponding to six possible growth orientations of the hexagonal phase (Fig. 1). No impurity phases were found in the thin films by XRD study. A more detailed structural study of thin films of hexagonal orthoferrites will be published elsewhere.

FIG. 1. (a) 2h/x scan of the TbFeO3 thin film on the (111)ZrO2(Y2O3) substrate. (b) u scans of (300) peaks of the TbFeO3 thin film and (c) (220) peaks of the ZrO2(Y2O3) substrate. 99, 122502-1

C 2011 American Institute of Physics V

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Magnetization measurements of the samples were carried out using high-sensitive vibrating sample magnetometer (Lake Shore VSM). Fig. 2 shows the results of magnetization-versustemperature measurements of RFeO3 (R ¼ Lu, Er, Ho) under application of 8 kOe external magnetic field in in-plane geometry to eliminate the demagnetizing field. The curves on the left represent the temperature dependences of the relative magnetization M(T)/Mmax. To distinguish the paramagnetic part of the curves and possible deviations from the Curie-Weiss (CW) law, the reciprocal relative magnetization (M(T)/Mmax)1 is showed on the right in Fig. 2. The paramagnetic part of the magentization curves allowed us to determine Neel temperatures for the samples (see Table I). However, for DyFeO3 and TbFeO3, the temperature dependences of magnetization of the films, though clearly deviating from the CW law, were noisy due to smaller thickness of the films, making it impossible to extract precise temperatures of the magnetic ordering from our data for DyFeO3 and TbFeO3 (Fig. 2(b)). In this case, the deviation was taken as a possible Neel temperature of RFeO3 (R ¼ Dy, Tb). Noteworthy, such deviation was observed in thin films of metastable hexagonal TbMnO3 and was attributed to the antiferromagnetic ordering of the Mn3þspins in a triangular lattice.16 This estimation of Neel temperatures may not be sufficient. The theory describing a triangular planar spin structure predicts that, on introducing more heat in the system, a longrange antiferromagnetic interaction first becomes short-range and only then does a true paramagnetism appear.17 The gradual elongation of the range of antiferromagnetism can be partially responsible for a deviation from the Curie-Weiss law, blurring the transition point and making it difficult to be distinguished in our measurements. The doping of hexagonal LuMnO3 (TNeel 90 K) by Fe3þ ions was reported to lead to the increase of Neel temperature of LuMn1xFexO3 (up to x ¼ 0.2).18 As the a parameter of the hexagonal structure decreases, which is observed in LuMn1xFexO3 with increasing x, the value of the exchange integral J that characterizes the superexchange interaction between transition metal ions is expected to rise. When x ¼ 1, the compound is a pure LuFeO3 and the temperature of its magnetic transition is supposed to be higher than that of LuMn0.8Fe0.2O3, which

FIG. 2. (Color online) Temperature dependences of relative magnetization and its reciprocal value of the (a) LuFeO3 and (b) TbFeO3 thin films.

Appl. Phys. Lett. 99, 122502 (2011) TABLE I. Neel temperatures of hexagonal RFeO3 (R ¼ Lu, Er-Tb) derived from the magnetization measurement data. Thin film

LuFeO3

ErFeO3

HoFeO3

DyFeO3

TbFeO3

TNeela

120 K

100 K

130 K

70–80 K

120 K

a

The error is estimated to be 5 K.

correlates well with magnetic ordering temperature of LuFeO3 being near 120 K. The results of magnetization-versus-field measurements are depicted in Fig. 3 for LuFeO3 at T ¼ 20 K, where a saturation of magnetization is present and comparable to that of weak ferromagnets.19 Recent studies of the YbFeO3 thin films showed the same results, implying a weak ferromagnetic or ferrimagnetic behavior of YbFeO3 at least up to 50 K.20 The origin of the magnetic moment in this and previous studies is unclear and requires a more thorough investigation, especially by neutron diffraction. In principle, at least several reasons can account for weak ferromagnetism in hexagonal orthoferrites. Among hexagonal manganites, weak ferromagnetism is found only in ScMnO3, presumably arising from the DzyaloshinskiiMoriya interaction. Noteworthy, ScMnO3 has the highest Neel temperature among all hexagonal RMnO3 TN ¼ 130 K and the least a parameter of a unit cell.21 In general, the strength of the exchange interaction is determined by the magnetic moment of M3þ, the length of the M3þ O2 bonds, and the M3þ O2 M3þ bond angle. In the case of hexagonal orthoferrites, the a parameter is generally smaller than the same parameter of hexagonal manganites,11,14 and as long as Fe3þ has one additional unpaired electron compared to Mn3þ, we conclude that the spin structure of hexagonal RFeO3 is more robust and its ordering occurs at higher temperatures than in the case of hexagonal manganites (the bond angles are the same as in isostructural hexagonal RMnO3). We suppose that weak ferromagnetism appears as a result of spin canting due to the Dzyaloshinkii-Moriya interaction since it is very unlikely for d5-ions to order ferrimagnetically in the plane of a triagular lattice they form, especially when each Fe is in bipyramidal coordination. Small magnetic moment might originate as well from the complex exchange interaction between Fe3þ and magnetic R3þ ions, but the observation of weak ferromagnetism in LuFeO3 argues against it. As Lu3þ ions have a 4f14 electron configuration, they do not possess magnetic moment. Therefore, the magnetic order in LuFeO3, similarly to ScMnO3, is determined entirely by the transition metal ions and does not involve rare-earth ions. Hence, it is possible

FIG. 3. (Color online) Field dependence of relative magnetization of the LuFeO3 thin film at T ¼ 20 K.


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that the mechanism for the emergence of weak ferromagnetism in both LuFeO3 and ScMnO3 is the same. As to other rare-earth orthoferrites, the exchange interaction between a magnetic R3þ and Fe3þ ions is able to lead to the additional canting of the whole spin structure and the increase in magnetic moment. Recently, a ferroelectric order in the hexagonal YbFeO3 thin films has been found,20 which signifies that both ferroelectric and magnetic orderings co-exist in hexagonal orthoferrites of rare earth. To conclude, we synthesized epitaxial thin films of hexagonal orthoferrites RFeO3 (R ¼ Lu, Er-Tb) on (111)ZrO2(Y2O3) substrates by MOCVD. The temperature and field dependencies of magnetization of hexagonal RFeO3 (R ¼ Lu, Er-Tb) were measured and revealed a weak magnetic moment appearing in the thin films of hexagonal RFeO3 (R ¼ Lu, Er-Tb). It is assumed that weak ferromagnetism takes place in the thin films and its possible origins are discussed. By comparing hexagonal RFeO3 with RMnO3, we have demonstrated that a small difference between Mn3þand Fe3þ ions leads to such a remarkable difference in magnetic structure between the two phases. Hexagonal orthoferrites seem to be a family of multiferroics which closely resembles hexagonal manganites except RFeO3 being weak ferromagnets rather than antiferromagnets. This work was supported by the Russian Foundation of Basic Research (Project No. 10-03-00964). 1 2

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