Enhanced ferromagnetism and conductivity of ultrathin freestanding La0.7Sr0.3MnO3 membranes

Siqi Shan(单思齐), Yequan Chen(陈业全), Yongda Chen(陈勇达), Wenzhuo Zhuang(庄文卓),Ruxin Liu(刘汝新), Xu Zhang(张旭), Rong Zhang(张荣), and Xuefeng Wang(王学锋)

Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials,School of Electronic Science and Engineering,and Collaborative Innovation Center of Advanced Microstructures,Nanjing University,Nanjing 210093,China

Keywords: freestanding membranes,La0.7Sr0.3MnO3,ferromagnetism,magnetic anisotropy

The rapid development of flexible electronics successively provides new multifunctional smart wearable electronic devices, which are gradually changing daily life.[1-4]The miniaturization, flexibility, integration and low power consumption of wearable devices have increasingly required functional materials, especially correlated oxide materials,in which the d-orbital electrons are highly correlated with the charge, spin, orbital, and lattice degrees of freedom.[5-7]The critical interaction among these four degrees of freedom leads to novel quantum phenomena,such as high-temperature superconductivity,[8-10]colossal magnetoresistance,[11,12]metal-insulator phase transition,[13,14]and multiferroics.[15-18]In recent years, relying on the controllable film growth and effective transfer technologies, freestanding (FS)single-crystalline correlated oxide membranes have been obtained.[19-22]When membranes are stripped from the substrates,the intrinsic epitaxial strain of the films is fully relaxed,showing rich magnetism,[22-24]electrical properties[14,25]and ferroelectricity.[26-28]These FS membranes could find a wide range of applications in information storage, intelligent sensing,and energy,etc.

As a typical correlated oxide, La0.7Sr0.3MnO3(LSMO)has attracted great interest due to its high Curie temperature (TC) (～369 K), large spin polarization,[29]and colossal magnetoresistance.[11,30]FS-LSMO membranes were firstly reported by etching water-soluble Sr3Al2O6(SAO)sacrificial layer in 2016,[19]in which the hysteresis loop was observed in the 20-nm-thick membranes.Until now, ferromagnetism has been observed in FS-LSMO membranes with the thickness ranging from 20 to 88 nm, which were transferred via dissolving various sacrificial layers (e.g., SAO, SrRuO3, and SrCoO2.5).[18,23,31-33]However,the transport properties of ultrathin (less than 10 nm) FS-LSMO membranes still remain largely unexplored due to the extreme difficulty in obtaining the intact transferred sample.

In this work, we experimentally demonstrate a universal and effective methodology to transfer large-area and singlecrystalline FS-LSMO membranes onto any target substrates.The LSMO films and SAO sacrificial layers are deposited on SrTiO3(STO) substrates by pulsed laser deposition (PLD).Enhanced ferromagnetism and conductivity of the ultrathin FS-LSMO membranes are both observed.The saturated magnetic moment(MS)of 4-unit-cell(u.c.)-thick FS-LSMO membranes is as high as 2.19µB/Mn at 15 K.Moreover, it shows the out-of-plane magnetic anisotropy.This work provides a feasible route to fabricate flexible LSMO-based oxide spintronic devices compatible with conventional semiconductor technologies.

We firstly deposited 40-nm-thick SAO sacrificial layers and LSMO films with the thickness ranging from 4 to 120 u.c.on the (001)-STO substrates [as schematically shown in Fig.1(a)]by the PLD,which was monitored by reflected highenergy electron diffraction(RHEED)oscillations.[24,34,35]The SAO layers were deposited on the STO (001) substrates(5×5×0.5 mm3) at 750°C under the oxygen pressure of 10-5mbar.Then, LSMO films with varied thickness were deposited at 700°C under the oxygen pressure of 10-4mbar.The relative low partial oxygen pressure ensured the high quality of the grown SAO sacrificial layer.The deposition temperature of LSMO films was precisely controlled at 700°C to avoid the extra interfacial reaction between LSMO and SAO.[20]After growth,the samples were cooled down to room temperature at a rate of 10°C/min without changing the oxygen pressure.The surface morphology and thickness of the membranes were examined by means of an atomic force microscope(AFM)system(Asylum Cypher).The crystalline structure was examined by a high-resolution x-ray diffraction (XRD, Bruker D8 Discover).The incident x-ray was generated from CuKαemission and had a wavelength of 1.5418 °A.A Quantum Design superconducting quantum interference device magnetometer(SQUID)system of up to 9 T was employed to measure magnetic properties.A cryogen free measurement system(CFMS,Cryogenic) was employed to characterize the transport properties at temperatures from 1.6 K to 300 K in a magnetic field(up to 12 T).

The clear RHEED oscillations indicate the 2D layer-bylayer epitaxial growth of 4-and 12-u.c.-thick LSMO ultrathin films,respectively(see Fig.S1 in the supplementary material).A 120-u.c.-thick LSMO film was also prepared.Its thickness was estimated by the deposition time, as confirmed by AFM(see Fig.S2 in the supplementary material).To avoid the potential reaction at the interface between LSMO and SAO,the deposition temperature of the LSMO films was kept at 700°C.[36]

After growth, a poly-dimethylsiloxane (PDMS) supporting layer was stamped onto the clean surface of the heterostructure.The whole sample with PDMS was immersed in deionized water to etch the SAO sacrificial layer[Fig.1(b)].After the SAO sacrificial layer was fully etched, the LSMO film was exfoliated[Fig.1(c)].Subsequently,the LSMO film was transferred onto a clean silicon substrate [Fig.1(d)] and heated to 120°C to remove PDMS supporting layer[Fig.1(e)].Finally,the FS-LSMO membranes was attached on the silicon substrates,forming new strain-free heterostructures[Fig.1(f)].This transfer methodology has been applied to other correlated oxides and topological materials.[19,37]

Figure 1(g) displays the corner of a 12-u.c.-thick FSLSMO membrane, which manifests the integrality of the membrane.The XRD patterns of 12-u.c.-thick LSMO before (black curve) and after (red curve) transfer are shown in Fig.1(h).The diffraction peaks correspond to (002) planes,indicating the single-crystalline epitaxial growth along thecaxis.A slight shift of the diffraction peak towards the lower angle is observed after transfer, suggesting a decrease in inplane lattice constant of the LSMO membrane.According to the lattice constants of STO and LSMO(3.905 °A and 3.875 °A,respectively),the peak shift of the XRD pattern implies the relaxed tensile strain of the LSMO lattice in the FS-membrane.The XRD patterns of another 12-u.c.-thick LSMO sample are also provided (see Fig.S3 in the supplementary material),which is deposited on a (LaAlO3)0.3(Sr2AlTaO6)0.7(LSAT)substrate.The peak shift of the XRD patterns shows the opposite trend.Obviously, the in-plane lattice constant of LSMO increases after transfer, which is attributed to the smaller lattice constant of LSAT(3.868 °A).

Fig.1.(a)-(f)Schematic illustration of the release and transfer process of the ultrathin FS-LSMO membranes.(g)Photograph of transferred FS-LSMO membranes with thickness of 12 u.c.on Si.(h)XRD patterns of the LSMO(12 u.c.) before and after transfer,respectively.

Fig.2.The magnetic properties of LSMO films with different thicknesses before and after transfer.(a)-(c)Magnetic hysteresis loops of LSMO films before(black curve)and after(red curve)transfer at 15 K,respectively.(d)-(f)Temperature-dependent magnetization curves of LSMO films before (black curve) and after (red curve) transfer under 100 Oe, respectively.The magnetic field is applied perpendicular to the substrates.

T=15 K FS-LSMO As-grown 4 u.c.

Table 1.The TC of LSMO films and membranes with thicknesses of 120,12,and 4 u.c.,respectively.

Next, we investigate the magnetic properties of the LSMO films before and after transfer under the out-of-plane magnetic field.With the decreasing thickness of LSMO films,the ferromagnetism is gradually suppressed, as seen from the black curves in Fig.2.The 4-u.c.-thick LMSO film before transfer is generally considered to be less than the dead layer thickness.It shows the nonmagnetic/antiferromagnetic property,which is in line with the previous work.[24,38]After transfer, the 120- and 12-u.c.-thick FS-LSMO membranes maintain ferromagnetism at 15 K with the opposite trend.The saturation magnetic moment (MS) of 120-u.c.-thick LSMO membranes decreases from 4.31µB/Mn to 3.55µB/Mn while theMSof 12-u.c.-thick LSMO increases from 0.14µB/Mn to 3.44µB/Mn.This is ascribed to the different strain relaxation in the LSMO membranes.Specifically, the epitaxial strain of the 120-u.c.-thick LSMO is fully relaxed even before transfer.The degradation of theMSis due to the inevitable folding of the membranes during the transfer.On the other hand, the transfer process relaxes the epitaxial strain in the 12-u.c.-thick film, which leads to the obvious increase ofMS.TheTCvalues are determined by plotting the derivative of the temperature-dependent magnetization (M-T) curves under 100 Oe[Figs.2(d)-2(f)and Fig.S4 of the supplementary material], as summarized in Table 1.TheTCincreases about 81.3 K and 130.5 K for 120- and 12-u.c.-thick FS-LSMO membranes after transfer,respectively.Remarkably,a prominent transition to ferromagnetism withMSof 2.19µB/Mn andTCof 32.3 K is observed in the 4-u.c.-thick FS-LSMO membranes[Figs.2(c)and 2(f)], which is attributed to the full relaxation of the epitaxial strain.

The dead-layer effect has also led to the degradation of the conductivities in LSMO ultrathin films.[38,39]Here, the transport properties of the 4-u.c.-thick LSMO films before and after transfer were successfully measured via the standard six-probe method.The insulating property of the 4-u.c.-thick LSMO films is expected according to the temperaturedependent resistivity curves [black squares in Fig.3(a)].Remarkably, the 4-u.c.-thick intact FS-LSMO membranes show the well-defined metallic behavior after transfer[red circles in Fig.3(a)], which has never been seen before.However, the separation of the insulator-metal transition temperature (TIM,higher than 300 K) and theTCcan be attributed to the different impacts on the microstructures and Mn3+/Mn4+ratio during transfer process.[40]The magnetoresistance(MR)of 4-u.c.-thick FS-LSMO membranes under the out-of-plane magnetic field is further measured at 5 K and 10 K, which is defined as [Rxx(H)-Rxx(0)]/Rxx(0).The MR is negative and exhibits distinct symmetrical humps around origin at both 5 K and 10 K [Fig.3(b)].It suggests that the disordered structure of magnetic domains generates when the magnetic field is reduced to the coercivity (HC).Such magnetic domains in LSMO membranes are further aligned under a large magnetic field,which is generally considered as butterfly shaped behavior in previous studies.[23,24,41]Thus,the butterfly-shaped MR is manifested, further verifying the ferromagnetism of the 4-u.c.-thick FS-LSMO membranes.

Figure 4(a) shows the normalized hysteresis loops of 4-u.c.-thick FS-LSMO membranes at 15 K under in-plane and out-of-plane magnetic field,respectively.The magnetic easyaxis significantly lies towards in-plane alignment due to the smallerHCand the lower saturation field.The standard angledependent [Rxx(θ)-Rxx(0)]/Rxx(0) curves under 6 T at 5 K and 300 K are shown in Fig.4(b), whereθis defined as the angle between the external magnetic field and thec-axis of LSMO membranes.The magnetic easy axis can be directly distinguished atθ=120°at both 5 K and 300 K,which prefers to an in-plane alignment.The magnetic easy-axis of LSMO can be modulated from the out-of-plane to the in-plane direction according to the interfacial strain.[35,42]However, the canted easy-axis of ultrathin 4-u.c.FS-LSMO membrane may be attributed to the stress induced by the transfer process instead of epitaxial strain, which is fully released.The clear angular symmetry of the[Rxx(θ)-Rxx(0°)]/Rxx(0°)curves in the 4-u.c.-thick FS-LSMO membranes further manifests the high quality of the transferred FS-membranes.

Fig.3.(a)Temperature-dependent resistance curves of LSMO films before and after transfer,respectively.(b)The field-dependent MR curves of LSMO membranes measured at 5 K and 10 K,respectively.

Fig.4.Magnetic anisotropy of the 4-u.c.-thick FS-LSMO membranes.(a) In-plane and out-of-plane magnetic hysteresis loops at 15 K.(b)The angular dependence of[Rxx(θ)-Rxx(0)]/Rxx(0)curves at 5 K and 300 K,respectively.

In summary,we have demonstrated a universal method to transfer FS-LSMO membranes with various thicknesses onto silicon substrates, which maintains the high quality as well as the enhanced electrical and magnetic properties.Notably,the obvious ferromagnetism and metallicity are observed in the ultrathin 4-u.c.-thick FS-LSMO membranes,whose thickness is generally less than the thickness of dead layers.This anomalous behavior can be mostly ascribed to the fully released epitaxial strain after transfer.Our work paves a pathway to make ultrathin manganite membranes compatible with the relevant semiconductor technologies.This transfer technique has promising applications in flexible oxide electronic and spintronic devices.

Acknowledgments

This work was supported in part by the National Key R&D Program of China (Grant No.2022YFA1402404) and the National Natural Science Foundation of China (Grant Nos.62274085,11874203,and 61822403).