Magnetic and electric properties of the a-axis-oriented orthorhombic HoMnO 3 thin films

In this section, we have successfully prepared the a-axis-oriented orthorhombic HoMnO3 (o-HMO) thin films by pulsed laser deposition on Nb-doped SrTiO3(110) single crystal substrates. The near perfectly aligned film growth orientation with the substrate allows us to study the magnetic transitions along the respective crystal orientation. The marked anisotropic behaviour of magnetic ordering probed along different crystallographic orientations appears to intimately relate to the respective ordering of moments arising from Mn and Ho ions.

4.4.1 Introduction

Multiferroics are scarce materials that can provide two or more switchable states, i.e.

polarization, magnetization, and strain [42] and be interdependent between the corresponding ordered phases. Among various multiferroic materials, the orthorhombic phase rare-earth manganites (RMnO₃ with R = Ho, Er, Tm, Yb, and Lu) are expected to exhibit an incommensurate antiferromagnetism (IC-AFM) to E-type AFM transition with an induced ferroelectric polarization to accompany the magnetic transition [6, 13, 36, 43].

However, direct demonstration of these fascinating phenomena has been hindered by the challenges in obtaining samples with the desired phase and, especially with definite crystallographic orientations, namely single crystals. Indeed, previous measurements performed on bulk polycrystalline orthorhombic HoMnO3 (o-HMO) revealed only a minute ferroelectric polarization (P ≈ 90 μC/m²) with evidences suggesting the possible involvement of Ho moments in the development of P [7, 9].

In trying to clarify some of these seemingly disputable issues, we have recently succeeded in preparing orientation-specified o-HMO thin films on various substrates [22, 33, 44]. These films, in addition to exhibiting the IC-AFM transition with TN ∼ 42 K and expected magnetoelectric effects, have displayed distinctive magnetic transition anisotropies at lower temperatures, which are believed to arise from the strain-induced effect on the IC-AFM to commensurate E-phase AFM transition. However, owing to the

from the temperature dependent magnetic susceptibility, χ(T), need further experimental supports. Moreover, as pointed out by Muñoz et al. [12], the smaller size of the rare-earth ion radius may also affect the magnetic ground states of RMnO3 (i.e. R= Y, Ho, Er etc.) manganites due to the tilting of the MnO6 octahedra and presence of the Jahn-Teller distortion.

Recently, it has also been suggested that epitaxial strain existing in substrate-stabilized orthorhombic YMnO3 (o-YMO) films may modify the magnetic structure, and hence the accompanying induced magnetoelectric effect [34, 45]. In previous reports [22, 33, 35, 44], the o-HMO films were fabricated with either a-, b- or c-axis perpendicular to the film surface. However, the analysis of the impact of the lattice distortion on the magnetic properties of these orientation-specified o-HMO thin films remains to be unraveled.

Another open question of current interest is how the introduction of the epitaxial strains would affect the magnetic lock-in transition temperature of the o-HMO system. On the other hand, it is also interesting to investigate the role of Ho moments, when coupled with the lattice strain, in shaping the eventual magnetic structure and the associated magnetoelectric effect in these multiferroic perovskite maganites.

In this paper, we have investigated the characteristics of the HMO films grown on 0.5

％-Nb:SrTiO₃(110) (Nb:STO(110)) substrates. The compressive in-plane stresses, resulting from the film/substrate epitaxial relations, have indeed led to marked anisotropic characteristics in magnetic transitions when probed by applying the field along various crystallographic orientations of these a-axis-oriented o-HMO films.

4.4.2 Results and Discussion

For rare-earth manganites with chemical composition RMnO3, it is known that the crystal structure of RMnO3 transforms form orthorhombic to hexagonal as the ionic size of the rare-earth elements gradually decreases from Nd to Ho. Unfortunately, HoMnO3 is located near boundary between hexagonal and orthorhombic structure. Thus, in order to stabilize the phase and epitaxially grow o-HMO films with controllable orientations, it is crucial to select the suitable substrates. In this study, we chose the Nb:STO(110) substrates.

The lattice constants along the [110] and [001] directions of Nb:STO are 5.523 Å and 7.810

Å, respectively, thus are quite suitable to accommodate the bc-plane of o-HMO. Fig. 4.11(a) shows the results of the θ-2θ scans for the o-HMO films grown on Nb:STO(110) substrates.

As is evident in figure 4.11(a), the diffraction pattern reveals pure (l00) peaks without discernible secondary phases, indicating that the films are perfectly epitaxial growth with a-axis perpendicular to the surface of substrates. The inset of Fig. 4.11(a) shows that a full width at half maximum in the rocking curve of the (200) peak is around 0.011°, indicating a reasonably good crystalline quality. Furthermore, as shown in Fig. 4.11(b), the φ-scans of the (110) and (020) reflections for the as-deposited o-HMO films and substrates exhibits a clear twofold symmetry, suggesting that the films were indeed of pure orthorhombic structure and well-aligned with the substrate. We note that the nearly perfect epitaxial relation between o-HMO(110) and Nb:STO (020) peaks shown in Fig. 4.11(b) is also consistent with the expected lattice mismatches of 5.75 % along the [001] direction and -5.66 % along the [110] direction for growing o-HMO on the Nb:STO(110) substrates. The lattice constants obtained here are a = 5.277 Å, b = 5.752 Å, and c = 7.478 Å, respectively.

Compare to a = 5.257 Å, b = 5.835 Å, and c = 7.361 Å for the bulk o-HMO, it is clear that the film is under tensile strain within the ac-plane and compressive strain along the b-axis, even when its thickness reaches 100 nm. The crystalline quality of the obtained o-HMO films thus allows us to directly probe the physical properties of the films along the three principal crystalline axes.

Figure 4.11 (a) The XRD θ-2θ scan of the o-HMO thin film grown on Nb:STO(110) substrate, showing that the film is single-phase a-axis-oriented orthorhombic perovskite manganites (in Pbnm space group settings). The inset in (a) shows a rocking curve measured around the o-HMO (200) peak. (b) The φ-scans of the same film displayed in (a), showing the nearly perfect in-plane alignments between film and substrate.

When the IC-AFM to CM-collinear E-phase transition takes place [12], the commensurate magnetic wave vector will propagate along the b-direction with the magnetic moments of Mn ions configuring anti-parallel to each other. Consequently, one expects that some extrinsic effects might arise due to the b-axis compressive strain. With this in mind, we first made careful characterizations on the magnetic properties of these a-axis-oriented o-HMO films.

As shown in Fig. 4.12, the results of temperature dependent magnetization [M(T)]

measured along the respective principal crystalline axis exhibit apparent anisotropic characteristics in terms of temperatures at which the magnetic structure transition occurs as well as the magnitude of susceptibility itself. We first note that, the expected Néel temperature at 44 K [12, 35] for the AFM transition can be clearly identified in each curve [see also the temperature derivative of magnetization, dM/dT, shown in the inset of Fig.

4.12]. Furthermore, we have pointed out that, for o-HMO films, the b-axis (in pbnm group symmetry setting) is the easy axis which explains the larger b-axis magnetization observed over the entire temperature range [22, 33, 44]. On the other hand, as is evident from the inset of Fig. 4.12, the weak lock-in-like magnetic transition (TMA ∼ 35 K) identified in the dM/dT of the c-axis was attributed to the compressive strain resulting from the epitaxial film growth [33, 44].

Finally, as can be seen in Fig. 4.12, we note that around 25 K there exists a broad but noticeable peak in the c-axis M(T) curve signifying another possible magnetic ordering which is undetectable when the field is applied either in the a-axis or along the b-axis. To better comprehend this broad anomaly, the c-axis dM(T)/dT curve probed by an applied magnetic field of 1000 Oe is obtained and displayed in the inset of Fig. 4.12. It is evident that, by increasing applied field strength from 100 Oe to 1000 Oe, the broad anomalous magnetic transition around 25 K remains essentially unchanged with the AFM ordering being kept around 44 K. According to the neutron diffraction results [12], at T ＝ 20~25 K, the magnetic moments of Ho³⁺ eventually come into play and appears ordered in the ab-plane. This observation explains the larger magnetization background along the b-axis over the entire temperature range probed. Moreover, perhaps the most significant implication of the present results is that the Ho atoms with parallel spins along the b-axis might have been drastically distorted by epitaxial strain, which has eventually led to a previously undisclosed magnetic ordering along the c-axis of o-HMO.

Figure 4.12 The zero-field-cooled (ZFC) χ(T) of the o-HMO films probed along the respective crystalline axis with an applied field of 100 Oe. The inset shows the

In order to further check whether or not the current c-axis ordering anomaly is indeed a result of similar canting effects, it is essential to delineate the magnetic moment configuration of the rare-earth ions. Fig. 4.13 shows the magnetization as a function of applied magnetic field (M-H) hysteresis loops of the o-HMO films measured at several temperatures below and above where the c-axis ordering occurs. As is evident from the results displayed in Fig. 4.13, the M-H loop becomes clearly hysteretic when T＜ 25 K, indicating the occurrence of a weak ferromagnetism after the Ho³⁺ moments start to participate in the AFM state. This is also consistent with the magnetic anomalous transition occurring at T~25 K when the magnetic moments of Ho atoms become ordered in the (110) planes. In this scenario, the observed second reordering (T_MA~35 K) along the c-axis may be relevant to the IC-commensurate AFM transition [33, 44], which has not been drastically affected by the distortion-induced canting. Therefore, it is indicative that the canted AFM-induced ferromagnetism is in fact more pronounced when the system is in the commensurated AFM state. The current results further suggest that the lattice mismatch induced strain can strongly alter the magnetic properties of the as-grown o-HMO films.

Figure 4.13 An enlarged vision of magnetic-field dependent magnetization (M-H) curves measured at 60, 35, 27, and 23.5 K. The inset shows a clear hysteretic behaviour displayed at 10 K indicating the presence of weak ferromagnetism.

Fig. 4.14 shows the temperature dependence of the dielectric constant directly measured along the a-axis by a gold pad as the top electrode and Nb:STO(110) substrate as the bottom electrode. It is clear that, when lowering across the AFM temperature (~ 44 K) there is no sign of ferroelectric transition occurring along the measuring a-axis. As the temperature is lowered across T~ 35K the relative permittivity starts to increase slowly, but no steep change is evident (see also the lower-left inset of Fig. 4.14(a)). Therefore, we associate the temperature of the gentle increase of εr(T) with TMA and we suggest that the weak second magnetic transition along the c-axis near 35 K might be more relevant to the distinct anomaly in the dielectric constant. The behavior is similar to that found in the c-axis-oriented o-HMO thin films reported previously [33]. Moreover, with further lowering of temperature, we note that the dielectric constant reaches maximum around T = 13.5 K and appears to be non-hysteretic in temperature. In order to establish the correlation of the magnetic order and dielectric anomalies and, in particular, the ferroelectric hysteresis observed below TMA, we have measured the polarization versus electric field (P-E) hysteresis loops with E parallel to a-axis between 13 and 40 K.

Figure 4.14 The dielectric constant (εr)as a function of temperature for o-HMO film evaluated from our measurements when cooling (closed symbols) and warming (open symbols) the sample. The left inset shows the temperature-dependent inverse relative

Nb:STO(110) Nb:STO(110) o-o-HMOHMO AuAu Silver Silver

glue glue Nb:STO(110) Nb:STO(110) o-o-HMOHMO AuAu Silver Silver

glue glue

Figure 4.15 The electric-field dependent polarization (P-E) hysteresis loops for the o-HMO/Nb:STO(110) measured at 40, and 13 K.

P-E characteristics measured by applying a field value of Emax~10 kV/cm with a frequency of 1 kHz at different temperatures are shown in Fig. 4.15. It can be clearly seen from Fig. 4.15 that a quasi-linear ferroelectric behavior displays below Néel temperature and which appears to increase this slope of P-E curves with decreasing temperature (not shown). However, when lowering across the dielectric anomalous temperature (TMA~35 K), there is none-hysteretic behavior in loop shape as expected ferroelectricity along the measuring a-axis, even when its temperature reaches 13 K. Perhaps the difference between the experiment and theoretic prediction can be explained in terms of restricted strain effect from XRD results discussed above. In this scenario, our current results maybe suggest that the relative permittivity anomaly could be relevant to the consequence of the presence of the unanticipated spontaneous polarization along the c-axis. Although commensurated AFM-induced ferroelectricity in polycrystalline samples of the E-type phase had also been observed recently[46], these spontaneous polarization involved, however, were all less ten times than the presupposed value [6, 13, 36, 43]. On the contrary, maybe of greater relevance here is that the strain induced by substrate, and the resulting change of bond angles and distances, could modify some magnetic interactions, eventually altering the orientation of ferroelectricity to fit the available result [46].

4.4.3 Summary

In summary, we have successfully grown the orthorhombic a-axis-oriented HoMnO₃ films on the Nb-doped SrTiO3(110) substrates. It is found that the behaviors of the orientation-dependent M(T) curves are significantly different. In addition to the 44 K AFM ordering, a weak lock-in transition around 35 K was evidently observed with the field parallel to the c-axis. Furthermore, the epitaxial strain inherent in these strain-stabilized films appears to introduce significant effects to the magnetic transitions. Several unprecedented observations have been revealed in the present study. In particular, it is argued that, due to the epitaxial strain-induced distortion on the Ho moments aligning along the b-axis, a previously undisclosed magnetic ordering near 25 K along the c-axis of o-HMO is introduced.

References

[1] T. Kimura, T. Goto, H. Shintani, K. Ishizaka, T. Arima, and Y. Tokura, Nature ( London) 426, 55 (2003).

[2] N. Hur, S. Park, P. A. Sharma, J. S. Ahn, S. Guha, and S. -W. Cheong, Nature (London) 429, 392 (2004).

[3] W. Eerenstein, N. D. Mathur, and J. F. Scott, Nature (London) 442, 759 (2006).

[4] D. I. Khomskii, J. Magn. Magn. Mater. 306, 1 (2006).

[5] S.-W. Cheong and M. Mostovoy, Nat. Mater. 6, 13 (2007).

[6] I. A. Sergienko, C. Sen, and E. Dagotto, Phys. Rev. Lett. 97, 227204 (2006).

[7] B. Lorenz, Y. -Q. Wang, and C. W. Chu, Phys. Rev. B 76, 104405 (2007).

[8] F. Ye, B. Lorenz, Q. Huang, Y. Q. Wang, Y. Y. Sun, C. W. Chu, J. A. Fernandez-Baca, P.

Dai, and H. A. Mook, Phys. Rev. B 76, 060402(R) (2007).

[9] B. Lorenz, Y. Q. Wang, Y. Y. Sun, and C. W. Chu, Phys. Rev. B 70, 212412 (2004).

[10] J.-S. Zhou and J. B. Goodenough, Phys. Rev. Lett. 96, 247202 (2006).

[11] H. W. Brinks, J. Rodríguez-Carvajal, H. Fjellvåg, A. Kjekshus, and B. C. Hauback, Phys. Rev. B 63, 094411 (2001).

[12] A. Muñoz, M. T. Casáis, J. A. Alonso, M. J. Martínez-Lope, J. L. Martínez, and M. T.

Fernández-Díaz, Inorg. Chem. 40, 1020 (2001).

[13] S. Picozzi, K. Yamauchi, B. Sanyal, I. A. Sergienko, and E. Dagotto, Phys. Rev. Lett.

99, 227201 (2007).

[14] J. A. Alonso, M. J. Martínez-Lope, M. T. Casais, and M. T. Fernández-Díaz, Inorg.

Chem. 39, 917 (2000).

[15] S. C. Chae, Y. J. Chang, S. S. A. Seo, T. W. Noh, D.-W. Kim, and C. U. Jung, Appl.

Phys. Lett. 89, 182512 (2006).

[16]A. Muñoz, J. A. Alonso, M. T. Casais, M. J. Martínez-Lope, J. L. Martínez, and M. T.

Fernández-Diaz, J. Phys.: Condens. Matter 14, 3285 (2002).

[17] T. Kimura, G. Lawes, T. Goto, Y. Tokura, and A. P. Ramirez, Phys. Rev. B 71, 22425 (2005).

[18] X. Martí, V. Skumryev, V. Laukhin, F. Sánchez, M.V. García-Cuenca, C. Ferrater, M.

Varela, and J. Fontcuberta, J. Mater. Res. 22, 2096 (2007).

[19] M. Mostovoy, Phys. Rev. Lett. 96, 067601 (2006).

[20] T. Goto, T. Kimura, G. Lawes, A.P. Ramirez, and Y. Tokura, Phys. Rev. Lett. 92, 257201 (2004).

[21] O. Prokhnenko, R. Feyerherm, E. Dudzik, S. Landsgesell, N. Aliouane, L.C. Chapon, and D.N. Argyriou, Phys. Rev. Lett. 98, 057206 (2007).

[22] T. H. Lin, C. C. Hsieh, H. C. Shih, C. W. Luo, T. M. Uen, K. H. Wu, J. Y. Juang, J.-Y.

Lin, C. H. Hsu, and S. J. Liu, Appl. Phys. Lett. 92, 132503 (2008).

[23] Steen Mørup, Daniel E Madsen, Cathrine Frandsen, Christian R H Bahl, and Mikkel F Hansen, J. Phys.: Condens. Matter 19, 213202 (2007).

[24] Airat Khasanov, Jian He, Jay Gaillard, Keqin Yang, Apparao M. Rao,_C. Michelle Cameron, J. M. Schmeltzer, John G. Stevens, and Amar Nath, Appl. Phys. Lett. 93, 013103 (2008).

[25] S. Picozzi, K. Yamauchi, G. Bihlmayer, and S. Blügel, Phys. Rev. B 74, 094402 (2006).

[26] B. Lorenz, A.P. Litvinchuk, M.M. Gospodinov, and C.W. Chu, Phys. Rev. Lett. 92, 087204 (2004).

[27] L. Néel, C. R. Acad. Sci. Paris 252, 4075 (1961).

[28] Our preliminary electric field dependent polarization measurements performed on these films, though still suffered from leakage current problem, have clearly shown ferroelectric polarization emerging at T ∼ 36.2 K (slightly higher than the ICM-CM transition temperature) and saturated for T < 20 K.

[29] H. Katsura, N. Nagaosa, and A. V. Balatsky, Phys. Rev. Lett. 95, 057205 (2005).

[30] C. Jia, S. Onoda, N. Nagaosa, and J. H. Han, Phys. Rev. B 74, 224444 (2006).

[31] D. Rubi, Sriram Venkatesan, B. J. Kooi, J. Th. M. De Hosson, T. T. M. Palstra, and B.

Noheda, Phys. Rev. B 78, 020408(R) (2008).

[32] M. Tachibana, T. Shimoyama, H. Kawaji, T. Atake, and E. Takayama-Muromachi, Phys. Rev. B 75, 144425 (2007).

[33] T. H. Lin, H. C. Shih, C. C. Hsieh, C. W. Luo, J.-Y. Lin, J. L. Her, H. D. Yang, C.-H.

Hsu, K. H. Wu, T. M. Uen, and J. Y. Juang, J. Phys.: Conden. Matter 21, 026013 (2009).

[34] C. C. Hsieh, T. H. Lin, H. C. Shih, C.-H. Hsu, C. W. Luo, J.-Y. Lin, K. H. Wu, T. M.

[35] T. C. Han, and J. G. Lin, Appl. Phys. Lett. 94, 082502 (2009).

[36] K. Yamauchi, F. Freimuth, S. Blügel, and S. Picozzi, Phys. Rev. B 78, 014403 (2008).

[37] D. Rubi, C. de Graaf, C. J. M. Daumont, D. Mannix, R. Broer, and B. Noheda, Phys.

Rev. B 79, 014416 (2009).

[38] René Meyer, Rainer Waser, Klaus Prume, Torsten Schmitz and Stephan Tiedke, Appl.

Phys. Lett. 86, 142907 (2005).

[39] The resistivity of the film increases from 10⁹ Ω-cm at room temperature to over 10¹⁰ Ω-cm near 13 K.

[40] M. A. Zurbuchen, T. Wu, S. Saha, J. Mitchell, and S. K. Streiffer, Appl. Phys. Lett. 87, 232908 (2005).

[41] Y. H. Chu, Q. Zhan, C.-H. Yang, M. P. Cruz, L. W. Martin, T. Zhao, P. Yu, R.

Ramesh, P. T. Joseph, I. N. Lin, W. Tian, and D. G. Schlom, Appl. Phys. Lett. 92, 102909 (2008); J. Li, H. Liang, B. Nagaraj, W. Cao, Chi H. Lee, and R. Ramesh, J. of Lightwave Technology 21, 3282 (2003).

[42] N. A. Spaldin, and M. Fiebig, Science 309, 391 (2005).

[43] C. Y. Ren, Phys. Rev. B 79, 125113 (2009).

[44] T. H. Lin, C. C. Hsieh, C. W. Luo, J.-Y. Lin, C. P. Sun, H. D. Yang, C.-H. Hsu, Y. H.

Chu, K. H. Wu, T. M. Uen, and J. Y. Juang, J. Appl. Phys. 106, 103923 (2009).=44

[45] X. Marti, I. Fina, V. Skumryev, C. Ferrater, M. Varela, L. Fábrega, F. Sánchez, and J.

Fontcuberta, Appl. Phys. Lett. 95, 142903 (2009).

[46] S. Ishiwata, Y. Kaneko, Y.Tokunaga, Y. Taguchi, T.-H. Arima, and Y. Tokura, Phys.

Rev. B 81, 100411(R) (2010).

Chapter 5