Low Temperature Ferroelectric and Magnetic Properties of Doped ...

Low Temperature Ferroelectric and Magnetic Properties of Doped Multiferroic Tb0.95Bi0.05MnO3 Natalia Vl. Andreeva1, Victoria A. Sanina2, Sergej B. Vakhrushev1,2, Alexey Vl. Filimonov1, Alexander E. Fotiadi1, and Andrey I. Rudskoy1 1

St.-Petersburg State Polytechnical University, St.-Petersburg, Russia [email protected], {filimonov,fotiadi}@rphf.spbstu.ru 2 Ioffe Physical Technical Institute, St.-Petersburg, Russia {sanina,s.vakhrushev}@mail.ioffe.ru

Abstract. Multiferroics are very promising materials for application in RF/microwave electronic devices. Electrical tuning of magnetism due to strong magnetoelectric coupling in these materials gives an opportunity to use them in reconfigurable microwave devices, ultra-low power electronics and magnetoelectric random access memories (MERAMs). Tb0.95Bi0.05MnO3 (TMNO) multiferroic is a solid solution of TbMnO3 and BiMnO3, the interest to investigation of this nanocomposite material is caused by the possibility to obtain the multiferroic with close temperatures of magnetic and ferroelectric ordering which are higher than in pure TbMnO3. Results of TMNO crystal investigations obtained using cryogenic magnetic force and piezopresponse force microscopy techniques are presented. An existence of ferroelectric and ferromagnetic ordering in TMNO at low temperatures was observed. Keywords: Multiferroics, Low temperature magnetic force microscopy, Low temperature piezoresponse force microscopy.

1

Introduction

A great potential of multiferroic materials in a voltage tunable and low power devices caused an increasing amount of works aimed at investigation of their properties and new material design. Strong magnetoelectric (ME) coupling in multiferroics provides opportunities for developing different multiferroic devices such as magnetic sensors, voltage and magnetic field tunable RF/microwave devices, including non-reciprocal tunable bandpass filters, low loss and high power handling phase shifters, etc. Nowadays ME coupling at room temperatures was demonstrated in multiferroic heterostructures [1]. Bulk multiferroics with strong ME coupling are considered as prospective materials for RF/microwave application, but nowadays the great challenge for material sciences is increasing temperatures of their magnetic and ferroelectric ordering. Tb0.95Bi0.05MnO3 is a solid solution of the initial compounds TbMnO3 and BiMnO3 and at room temperature has the rhombic perovskite structure with the space group Pnma (62) and the cell parameters a = 5.321 Е, b = 5.858 Е, and c = 7.429 Е [2]. S. Balandin et al. (Eds.): NEW2AN/ruSMART 2014, LNCS 8638, pp. 444–450, 2014. © Springer International Publishing Switzerland 2014

Low Temperature Ferroelectric and Magnetic Properties

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TbMnO3 has the structure of a rhombically distorted perovskite and is a multiferroic with ferroelectric and magnetic orders occur near 30 and 40 K respectively [3]. It was demonstrated that TbMnO3 has gigantic magnetoelectric and magnetocapacitance effects, which can be attributed to switching of the electric polarization induced by magnetic fields [herein]. Further theoretical and experimental studies reveal that the ferroelectricity in TbMnO3 and many other cycloidal-spin magnets is induced by the noncollinear Mn spiral spin order with inverse Dzyaloshinskii– Moriya (DM) interaction, which is the driving force of oxygen atom displacements [4]. BiMnO3 is a multiferroic with the ferromagnetic and ferroelectric Curie temperatures TC = 105 and 750–800 K, respectively, and has the monoclinic symmetry with the space group C2 [5]. The interest to the investigations of the solid solution of TbMnO3 and BiMnO3 is caused by the possibility to obtain the multiferroic with close temperatures of magnetic and ferroelectric ordering which are higher than in TbMnO3. Earlier it was shown that in the multiferroic compound, the doping and substituting with different types of magnetic ions could modify the properties of the compound, see, for example [6, 7, 8]. Studies of structural, magnetic and dielectric Tb0.95Bi0.05MnO3 properties revealed the existence of a state with a high dielectric constant (ε ~ 105) for temperatures above 165 K in the Tb0.95Bi0.05MnO3 + δ single crystals [9]. Relying on the obtained dispersion of the dielectric constant and the conductivity below 150 K in Tb0.95Bi0.05MnO3 + δ the conclusion about the presence of the inhomogeneous state of the crystal and the coexistence of the local domains of dipole correlations of different scales was made. It was shown that external magnetic field effects on the resistivity and capacitance of the crystal in all temperatures range. Thus it was supposed that Tb0.95Bi0.05MnO3 + δ crystal is an RF multiferroic at temperatures 5–500 K in which restricted domains of various sizes with simultaneous polar and magnetic correlations coexist. In [10] properties of Tb0.95Bi0.05MnO3 ceramics were investigated. It was shown that Bi occupies the Tb site and alters the multiferroicity of TBMO due to reduction in the exchange interactions JTb-Tb and JMn-Tb. The Bi partial substitution in TbBiMnO3 ceramics suppresses the Tb-spin ordering point (TTb) and ferroelectric ordering point (TC). Bi-doping significantly depressed the dielectric constant. At low temperature relaxorlike dielectric characteristics of Tb0.95Bi0.05MnO3 can be related to the dipolar effect induced by charge-carrier hopping motions [10]. The aim of the present study was an elucidation of the low temperature ferroelectric and magnetic states in TBMO using atomic-force microscopy (AFM) technique. Single crystal perovskite manganese oxides Tb1-xBixMnO3 with x=0.05 has been studied with magnetic force and atomic force microscopy techniques at low temperatures. The ferroelectric long-range order typical of TMO and the restricted domains with ferromagnetic correlations were revealed in TBMO at T = 8 K.

2

Material and Methods

All measurements were carried out on a TMBO single-crystal grown by the spontaneous crystallization technique, dimensions 2 Ч 1 Ч 0.5 mm3. Directions of crystallographic axes were determined by X-ray diffraction analysis using SuperNova diffractometer

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Fig. 1. TBMO crystal orientation. The surface for AFM measurements is pointed with the gray arrow.

(Oxford Diffraction, UK). The orientation of the crystal surface for ferroelectric and magnetic measurements is shown of a Fig.1. Ferroelectric and magnetic states of the TBMO crystal were measured using the cryogenic atomic-force microscope AttoAFM I (Attocube Systems, Germany) according previously developed procedures of measurements [11]. Both for magnetic and ferroelectric measurements cantilevers with Co coating were taken. The conductivity of the soft magnetic Co coated cantilever ensures the possibility to register magnetic properties with magnetic force microscopy (MFM) and ferroelectric properties of the surface with piezoresponse force microscopy (PFM) at the same place of the sample. We used MAGT cantilevers (Applied NanoStructures Inc., USA), with the resonant frequency of 62 kHz, k constant of 3 N/m and tip radius curvature of 40 nm. Taking into account the low value of TMO polarization (8Ч10-4 C·m-2 [3] at T ~10 K) comparing with the same value for classic ferroelectrics (for ex. 2.6Ч10-2 C·m-2 for BaTiO3 at 296 K [herein]), measurements of ferroelectric sample state were done under conditions of the tip-surface local contact resonance. This allowed to enhance the piezoresponse from the surface by the cantilever Q-factor times. All measurements were done in the temperature range of 8 – 30 K.

3

Experimental Results

The results of TMNO crystal measurements with MFM are shown on a Fig. 2 a,b. The distribution of magnetic properties on the TMNO surface revealed the existence of isolated ferromagnetic domains. The distribution of these domains doesn’t correlate with the surface topography. The autocorrelation analysis of MFM image was done (Fig. 2c). The shape of the spatial autocorrelation function corresponded to the isotropic spatial distribution of ferromagnetic properties. On this evidence the absence of the long-range ferromagnetic ordering in TBMO crystal was concluded. The absence of preferred direction in ferromagnetic domain distribution followed from the symmetry of the autocorrelation function peak. Determined autocorrelation length was 750 nm and evidenced of local ferromagnetic ordering in TBMO. The autocorrelation analysis of obtained topographic image didn’t reveal any correlation in surface topography of TBMO. This confirmed the fact that ferromagnetic ordering was not caused by topographic features. It should be noticed that the real size of ferromagnetic domains


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could be differed from that was determined in autocorrelation analysis. It could happen that visualized on MFM image ferromagnetic domains have a fine structure, but due to the finite size of the tip, fine structure wasn’t resolved.

Fig. 2. An autocorrelation analysis of ferromagnetic properties of TMNO crystal. a – autocorrelation of image of ferromagnetic properties distribution, b – autocorrelation of topography, c – profiles from autocorrelation of image of ferromagnetic properties distribution.

According to the results of PFM measurements, polar domains with weak piezoresponse were found (Fig. 3 c, d) on the TBMO surface at low temperatures. Ferroelectric domains had linear shape with 250 nm average thickness and length up to several microns. On some parts of the piezoresponse image it could be seen the union of linear domains resulted in a thickening effect of linear structures. Taking into account the fact that in TMO crystal polarization P is parallel to the crystallographic axe c (Pc >> Pa, Pb, Pc = 8*10-4 C/m2 [3]) it is reasonable to assume that obtained long-range ferroelectric ordering in TBMO originated from the long-range ordering in TMO. In the investigated TBMO crystal the axe c (Fig. 1) had a projection on the surface where out-of-plane PFM measurements were done, thus measured piezoresponse from polar domain structure in TBMO could have polarization in the

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direction of axe c. It means that the great part of TBMO crystal is mainly consist of TMO while the doped TMO occupied the small part of the crystal and didn’t reveal itself on the background of long-range ferroelectric ordering of TMO. a)

b)

c)

d)

Fig. 3. Results of TBMO single crystal measurements using PFM and MFM techniques in the temperature range of 8 – 30 K. a – topography of TBMO crystal surface; b – ferromagnetic properties distribution; c – out-of-plane amplitude PFM signal from the TBMO; d – out-ofplane phase PFM signal from the TBMO.

A comparison of ferroelectric domain structure (Fig. 3 c, d) with ferromagnetic ordering (Fig. 3 b) at low temperatures reveals the absence of correlation between ferroelectric and ferromagnetic properties of TBMO crystal.

4

Discussion

According to our experimental results it was found that in the temperature range of 8 – 30 K there are local ferromagnetic and long-range ferroelectric ordering in TBMO single crystal. We suppose that the main contribution in the ferromagnetic properties


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of TBMO crystal arise from the isolated ferromagnetic domains. The initial spiral magnetic ordering of TMO crystal isn’t revealed in the background of strong response from ferromagnetic domains. These isolated ferromagnetic domains could be formed in the process of crystal growth. TBMO single crystals were grown by the spontaneous crystallization method. Substitution of the bigger Bi3+ ions in Tb3+ positions causes local lattice distortions that bring to appearance of the smaller ions Mn4+ with changed valence. As a result of the TBMO contains both Mn3+ and Mn4+ ions, and free charge carries appear in it. Probably, charge carriers arise also due to the hybridization of the lone pair of 6s2-electrons of Bi3+ ions, 2p-oxygen orbitals and 3d-orbitals of Mn3+ ions, because their energies are close (see e.g. [12]), i.e. Bi3+ = Bi5+ +2 eg and 2 eg +2 Mn4+ = 2Mn3+. Another explanation of ferromagnetic domain formation could be formation of the conducting ferromagnetic domains inside the volume of TMO due to phase separation. The charge carriers and ferromagnetic Mn3+ - Mn4+ ion pairs originating from doping are not distributed in TBMO statistically. Due to the double exchange interaction resulting in the phase separation [13] they form conducting ferromagnetic domains inside the volume of TMO. As noted above, at low temperatures these domains occupy the small volume of TMO. But in accordance with our results it would be consider that isolated ferromagnetic domains have sufficiently large sizes to give rise to a long-range ferromagnetic ordering inside them. However, no long-range ordering occurs in the entire crystal. The ferroelectric domains structure of TBMO crystal corresponds to the ferroelectric ordering in TMO crystal. It proves that the main volume of TBMO is occupied by the initial TMO crystal. Ferroelectric long-range ordering from the structure of TMO crystal isn’t disturbed by doped parts of the TBMO crystal. Long range ferroelectric ordering doesn’t correlate with local ferromagnetic ordering in TBMO crystal. Acknowledgements. This work is supported by the Ministry of Education and Science Program “5-100-2020”.

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