IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009 4265

Multiferroic Properties of Hexagonal YbMnO� Thin FilmsT. C. Han� and J. G. Lin�

Department of Applied Physics, National University of Kaohsiung, Nan-Tzu District, Kaohsiung 81148, Taiwan, R. O. C.Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan, R. O. C.

The hexagonal (HX) YbMnO� thin films were synthesized on the substrates of (111)Y�O�:ZrO� and (111)Pt/(0001)Al�O� by pulsedlaser deposition (PLD), and their structural, magnetic, and electrical properties are measured. The temperature-dependent magneti-zation clearly displays an antiferromagnetic ordering near 80 K with field perpendicular to the film surface. In the curve of polariza-tion-voltage, the HX-YbMnO� thin films show distinct ferroelectricity with the room-temperature remanent polarization and coercivefield being 1.1 C/cm� and 58 kV/cm, respectively. These results indicate that the HX-YbMnO� thin films are prominent candidates forthe applications of multiferroic devices.

Index Terms—Hexagonal, manganites, multiferroics, YbMnO�.

I. INTRODUCTION

R ECENTLY, multiferroic materials opened new avenuesfor the development of novel devices based on new

functionalities such as the control of the magnetic properties byelectrical fields and vice versa [1]. The strong coupling betweenelectrical and magnetic properties observed in some mangan-ites is of fundamental interest and of eminent significance forpotential applications [2]–[5], such as spintronic and memorydevices. However, the properties of these materials in the formof thin films need to be investigated for the application purpose.In particular, the anisotropy of polarization/magnetization isthe important factor for the design of devices. For example, thehexagonal (HX) manganite YbMnO exhibits a ferroelectric(FE) transition well above room temperature ( K), whilethe antiferromagnetic (AFM) ordering takes place at muchlower temperature (Néel temperature K). Structurewise, each Mn ion is surrounded by three in-plane and twoapical O ions, which exposes it to a crystal field of trigonalsymmetry. The magnetic ordering of Mn moments is deter-mined by the in-plane AFM Mn-O-Mn superexchange, whichis much stronger than the inter-plane Mn-O-Mn exchange [6].However, the Mn-O-Yb superexchange leads to the presenceof macroscopic magnetization along the -axis and affects thein-plane Mn-O-Mn AFM ordering. On the other hand, thedisplacement of ions along the -axis (perpendicular to thetriangular lattice plane) induces a FE polarization. Since theexperimental data of YbMnO films are still lacking, we makeefforts to synthesize and characterize the HX-YbMnO films.

II. EXPERIMENTS

The single-phased HX YbMnO target was prepared by solidstate reaction method using a stoichiometric high-purity Yb Oand MnO powders as the starting materials. YbMnO thin filmswere grown on the substrates of (111) yttrium-stabilized-zir-conia (YSZ) and (111)Pt/(0001)Al O at around 800 C byusing pulsed laser deposition (PLD) with a 248 nm KrF ex-cimer laser. The oxygen pressure is 50 mtorr and the frequency

Manuscript received March 04, 2009; revised May 01, 2009. Current ver-sion published September 18, 2009. Corresponding author: T. C. Han (e-mail:tchan@nuk.edu.tw).

Digital Object Identifier 10.1109/TMAG.2009.2023423

Fig. 1. X-ray diffraction patterns of the YbMnO thin films on (a) (111)Pt/(0001)Al O and (b) (111)-YSZ substrates.

is 5 Hz for the film growth. After deposition, films were in-situannealed in an oxygen pressure of 100 mtorr for 10 min atthe same temperature. The thickness of the films was around150 nm, controlled by the number of laser pulses. The struc-tural characteristics of the as-grown YbMnO thin films wereexamined by x-ray diffraction (XRD) with a monochromaticCu radiation. Microstructure of the samples was character-ized using a FEI Tecnai-F20-G2 field emission gun transmissionelectron microscope (FEG-TEM) operated at 200 kV. The mag-netic properties of the samples were measured with QuantumDesign superconducting quantum interference device (SQUID).Polarization was measured with a Sawyer-Tower circuit and aFE test system.

III. RESULTS AND DISCUSSION

Fig. 1(a) and (b) shows the XRD patterns of the YbMnOfilms deposited on (111)Pt/(0001)Al O and (111)YSZ, respec-tively. The patterns reveal the pure -oriented YbMnOreflections without a trace of any impurity or additional phases,

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4266 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009

Fig. 2. Panel (a) is the HRTEM image of YbMnO /YSZ(111) acquired alongthe [1–10] zone axis and (b) is the corresponding SAED pattern.

Fig. 3. Magnetization (M) versus temperature (T) for YbMnO /YSZ(111) withthe fields applied along two different directions: (a) In-plane and (b) out-of-plane.

suggesting that the crystal structure of these YbMnO thin filmsare hexagonally grown with -axis perpendicular to the surfaceof substrate.

The image of cross section high resolution transmissionelectron microscopy (HRTEM) and the selected-area-electron-diffraction (SAED) patterns of the HX-YbMnO /(111)YSZ areshown in Fig. 2(a) and (b), respectively. Based on the images inFig. 2(a), the in-plane lattice spacing is 0.58 nm, which is veryclose to the that in bulk HX-YbMnO ([7], [8]). It also confirms that the -axis of HX-YbMnO film isperpendicular to the surface of substrate.

Fig. 3(a) and (b) shows the magnetization (M) versus tem-perature (T) curves with an applied magnetic field 500 Oe forYbMnO /(111)YSZ with field parallel (in-plane) and perpen-dicular (out-of-plane) to the film surface, respectively. Opensymbols are the data with the zero-field-cooled (ZFC) mode,while the solid ones with the field-cooled (FC) mode. It clearly

Fig. 4. M-H curves of YbMnO /YSZ(111) measured with out-of-plane fieldat (a) 80, (b) 10, and (c) 2 K.

marks the AFM Néel temperature at 80 K with out-of-planemeasurement (along -axis), which is consistent with that forthe YbMnO single crystal reported by Katsufuji et al. [9].An important feature in Fig. 3(b) is the deviation of FC-M(T)from ZFC-M(T) measured at low magnetic field, which setsright below . The deviation temperature, at which twoM(T) curves starts setting apart, decreases with increasing theapplied magnetic field and finally diminishes at fields higherthan 1.5 kOe (not shown here). Such behavior is not a charac-teristic for an ideal AFM structure, for which the Mn momentsare supposed to compensate below . Unlike the data ofout-of-plane, no peculiarity at is observed in the in-planeM(T) curves. Instead, the FC and ZFC curves start to deviate ataround 5 K as seen in 3(a). The absent of in -plane can beattributed to the Mn-O-Yb superexchange as suggestion in [6].

Fig. 4 is the data of the field dependent magnetization M(H)of YbMnO /(111)-YSZ at 2, 10, and 80 K with out-of-planefield. The enhanced magnetic moment and coercivity are ob-served at low temperatures and high field as shown in Fig. 4(b)and (c). No coercivity is present above 80 K, suggesting that theobserved weak ferromagnetism is related to the Néel structurewith the triangular lattice. This behavior is reminiscent to thatfor LaMnO single crystal, which was attributed to the cantingof the AFM moments due to the antisymmetric Dzyaloshin-skii-Moria interaction [10]. On the other hand, when the mag-netic field is in-plane, the M-H hysteresis loop is absent evenat 2 K, suggesting that there is no ferromagnetic component in

HAN AND LIN: MULTIFERROIC PROPERTIES OF HEXAGONAL YBMNO THIN FILMS 4267

Fig. 5. P-V curve of YbMnO /(111)Pt/(0001)Al O measured at room tem-perature and at 1 kHz.

the -plane. Fig. 5 shows the polarization versus voltage (P-V)measured at room temperature and at a frequency of 1 kHz.The P-V hysteresis loop is observed with the remanent polar-ization and the coercive field are 1.1 C/cm and 58 kV/cm,respectively. Since the P-V curve is measured with voltage ap-plied along -axis, it is confirmed that both the FE ordering anda weak magnetization occur along -axis. However, it shouldbe further demonstrated in the near future that the P-V loopof YbMnO /YSZ(111) film can be effectively enhanced by ap-plying the magnetic field with a proper direction.

IV. CONCLUSION

In summary, the epitaxial HX-YbMnO thin films on the sub-strates of (111)YSZ and (111)Pt/(0001)Al O are successfullyprepared by pulsed laser deposition. Based on the results ofM(T), the HX-YbMnO /(111)YSZ film displays an AFM tran-sition at 80 K with magnetic field perpendicular to the filmsurface. Ferromagnetic behavior is observed below 80 K withthe applied magnetic field along the -axis of YbMnO . The

room temperature P-V hysteresis loop is obtained, indicatingthat the HX-YbMnO thin film has a distinct ferroelectricity.The overall results demonstrate that HX-YbMnO thin film canbe eventually utilized for the applications of multiferroic de-vices.

ACKNOWLEDGMENT

This work was supported by the National Science Councilof R.O.C. under Grants NSC96-2112-M-390-003-MY3 and96-2112-M-002-027-MY3.

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