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Orientation control of interfacial magnetism at La0.67Sr0.33MnO3/SrTiO3 interfaces Er-Jia Guo, Timothy Charlton, Haile Ambaye, Ryan D. Desautels, Ho Nyung Lee, and Michael R. Fitzsimmons ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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ACS Applied Materials & Interfaces

Orientation Control of Interfacial Magnetism at La0.67Sr0.33MnO3/SrTiO3 Interfaces Er-Jia Guo†,*, Timothy Charlton†, Haile Ambaye‡, Ryan D. Desautels†, Ho Nyung Lee§, and Michael R. Fitzsimmons*†,ǁ

†Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA ‡ Instruments & Source Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA §Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA ǁ Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, USA

ABSTRACT. Understanding the magnetism at the interface between a ferromagnet and an insulator is essential because the commonly posited magnetic “dead” layer close to an interface can be problematic in magnetic tunnel junctions. Previously, degradation of the magnetic interface was attributed to charge discontinuity across the interface. Here, the interfacial magnetism was investigated using three identically prepared La0.67Sr0.33MnO3 (LSMO) thin films grown on different oriented SrTiO3 (STO) substrates by polarized neutron reflectometry. In all cases the magnetism at the LSMO/STO interface is larger than the film bulk. We show that the interfacial magnetism is largest across the LSMO/STO interfaces with (001) and (111) orientations, which have the largest net charge discontinuities across the interfaces. In contrast, the magnetization of LSMO/STO across the (110) interface, the orientation with no net charge discontinuity, is the smallest of the three orientations. We show that a magnetically degraded interface is not intrinsic to LSMO/STO heterostructures. The approach to use different crystallographic orientations provides a means to investigate the influence of charge discontinuity on the interfacial magnetism.

KEYWORDS. magnetic “dead” layer; polarized neutron reflectometry; interfacial magnetization; charge discontinuity

This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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TEXT. Perovskite oxide interfaces can exhibit novel phenomena which are not simple interpolations of the parent materials’ properties. Interface-induced discontinuities in the electronic state, atomic structure, and orbital occupancy play critical roles in controlling the physical properties and producing new functionalities.1-5 At oxide interfaces, charge discontinuity may promote charge transfer, which can be driven by a difference in chemical potential or by the screening of local dipoles. Charge transfer can alter the valence states of oxide thin films near the interface and enable the magnetic order that is distinct from the constituents, leading to ferromagnetism confined to the interface.6-12 For example, interfacial magnetization has been found at the interfaces between two antiferromagnetic insulators8, 9 or between a paramagnetic metal and an antiferromagnetic insulator.10-12 However, extra charges at the interfaces can also be problematic. For example, previous work found that the magnetization of the La1-xSrxMnO3 (LSMO) layers are greatly reduced at the interface close to SrTiO3 (STO).13-19 Although the origin of the “dead-layers” remains a topic of discussion, it is often cited that the polar discontinuity at the interface might be a possible mechanism.20-23 To test this scenario, interface engineering involving insertion of two unit cells (u. c.) non-doped LaMnO3 or a single La0.33Sr0.67O layer was used to compensate the valence changes across the interface, resulting in an improved interfacial magnetization.24,25 Recently, Huijben et al.,26 used polarized neutron reflectometry (PNR)27, 28 to measure the magnetization depth profile across the interface between a conventionally grown LSMO/STO superlattice and a similarly prepared superlattice, but with extra single La0.33Sr0.67O layers between LSMO (x = 0.33) and STO layers. The superlattice without and with the extra single La0.33Sr0.67O layers were called non-interface-engineered (non-IE) samples and IE

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samples, respectively. The magnetization of the LSMO interfacial layers in proximity to STO on the non-IE sample was suppressed by a factor of two compared with that of the film bulk. The nuclear and magnetic structures of the non-IE sample were incommensurate, i.e., differed by about one u. c. In the IE sample, the magnetization and nuclear scattering length density (SLD) profiles were commensurate, i.e., there was no magnetic dead layer, and the interfaces were sharper compared with the non-IE sample. They suggested that the magnetization of the interface could be improved by reducing the charge discontinuity across the interface from ∆ = 0.67e- (non-IE sample) to 0.33e- (IEsample), where ∆, the net charge discontinuity, is the sum of the charge of the two monolayers on either side of the interface. The previous result from Huijben et al.26 is somewhat counterintuitive, because the interface became more ferromagnetic when doped with a chemical composition (x = 0.67) which is in the antiferromagnetic part of the bulk phase diagram for LSMO. In this paper, we were motivated to test the hypothesis that charge discontinuity affects interfacial magnetism in LSMO/STO heterostructures by examining the influence of crystallographic orientation on the interfacial magnetism. As shown in Figure 1, the charge discontinuity across (100)- and (111)-oriented LSMO(x = 0.33)/STO is ∆ = 0.67e, while across the (110) interface, ∆ = 0. Thus, for samples of the same stoichiometry (x = 0.33) we can test the influence of charge discontinuity on the interfacial magnetism by simply varying the crystallographic orientation. High quality epitaxial LSMO films were grown on STO substrates with orientations of (100), (110), and (111) by pulsed laser deposition (PLD). Step-and-terrace



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topographies were observed for all LSMO thin films by atomic force microscopy (AFM). The orientation, thickness, and strain state of the films were characterized by high resolution x-ray diffractometry. Figure 2 shows x-ray diffraction θ-2θ scans (a-c) and reciprocal space maps (d-f) of LSMO films grown on STO substrates with different orientations. The thickness fringes from the main peaks of LSMO thin films were observed in Fig. 2, indicating a high crystalline quality of epitaxial thin films. All films are coherently strained to the STO substrates. Figure 3 shows the x-ray reflectivity (XRR) scans of LSMO films as a function of wave vector transfer Q (= 4πsinθ/λ, where θ is the angle subtended by the incident wave vector and its projection on the film surface, and λ is the wavelength). Models of the chemical depth profiles across the samples were obtained from the XRR.29, 30 The variations of the x-ray SLD profiles are shown in the insets of Figure 3. Analysis of the XRR provides the film thickness, LSMO/STO interface roughness, and LSMO surface roughness—parameters that were later used in analysis of the PNR data (Table 1). The magnetic properties of samples were measured with magnetic field applied along the in-plane direction using a superconducting quantum interference device (SQUID) magnetometry. Figure 4 shows the temperature and field dependent magnetization (M) from three LSMO films. The samples were measured during the warm-up cycle after field cooling at 1 kOe. The magnetic fields were applied along the in-plane direction (H parallel to the film surface) for all the samples. Temperature dependent magnetizations were measured at a field of 1 kOe. Values for the saturation M and remanent M averaged over the entire film thickness are lowest for the (100) oriented LSMO film and highest for the (111)-oriented LSMO film. The Curie temperature (Tc) is 4 ACS Paragon Plus Environment

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highest for the (111) orientation. We note that the Tc of all three LSMO films are much lower than that of the LSMO bulk with x = 0.33. These results are in consistence with previous findings that a fully strained LSMO film exhibits a reduced Tc.25,26 The parameters obtained from SQUID measurements are listed in the Table 1. PNR data were taken from the samples under the same conditions (field cooled at 1 kOe and measured in the same field). Briefly, the reflectivity data, R±, were recorded for neutron beam polarized up (+) and down (-) with respect to the applied field as a function of Q. The reflectivities normalized to the asymptotic value of the Fresnel reflectivity (RF=16π/Q4) are shown in Figure 5. The nuclear SLD profiles (constrained to parameters obtained from the x-ray SLD profiles) and the magnetization depth profiles were obtained from a least squares fitting routine that minimized the χ2 metric using the Parratt formulism.29, 30 The (1-σ) uncertainty of the magnetization depth profile is ± 3%. from PNR measurements were calculated by The thickness averaged magnetizations = ( ) , where τ is the film thickness. The difference between M measured obtained with PNR is comparable to the (1-σ) uncertainty. To with SQUID and the achieve a satisfactory low value of χ2, we require a M(z) that is largest at the LSMO/STO interface and smallest at the surface. We note the χ2 metrics increase by more than 50% if the interface magnetization is forced to be smaller than the film bulk. For convenience we represented the variation of M(z) as a sequence of steps; however, a constant decay of M(z) as a function of z is equally plausible. The three main results from our study are: firstly, the SQUID and PNR results are consistent, showing that Ms(111) > Ms(110) > Ms(100), where the (…) represents the orientation of the STO substrate. This fact combined with the somewhat more rounded 5 ACS Paragon Plus Environment


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hysteresis loop near remanence of the (100) and (110) samples compared to that of the (111) sample suggests a variation of anisotropy with [100] being harder than [111]. Secondly, the magnetization of the LSMO surface is consistently suppressed compared to the LSMO/STO interface. In contrast to previous studies,23-26 the LSMO/STO interfacial ferromagnetism is not reduced

compared to the film bulk. Typically, the magnetic

properties of LSMO are strongly modified close to the film-substrate interface. The degraded LSMO/STO magnetic interfaces are called magnetic “dead layers”. The thickness of magnetic “dead layer” varies from a few unit cells to more than ten unit cells, depending on many extrinsic parameters, such as strain, oxygen content, deposition conditions.23-26 Until now, the origin of this magnetic “dead layer” is still under debate. Our work shows that so-called degraded LSMO/STO magnetic interfaces are not intrinsic to the heterostructures, rather it is possible to grow LSMO/STO interfaces with a thickness over 10 unit cells with very favorable magnetic properties. Finally, the interface magnetism is highest for the (100) and (111) LSMO/STO interfaces, reaching ~ 583 and ~ 600 kA/m, respectively, corresponding to ~ 3.64 and ~ 3.75 µB/Mn. These magnetic moments at the interfaces are comparable with that of a bulk LSMO with x = 0.33 doping. However, the magnetic moment of LSMO for (110) orientation is about ~ 20% lower than the magnetic moments for the other two orientations (though its interfacial magnetization is still greater than its bulk value). The two orientations with the highest interface magnetism correspond to the interfaces with the largest charge discontinuity of ∆ = 0.67e-, whereas the lowest interfacial magnetism is found for the interface with no charge discontinuity, ∆ = 0. The difference between this work and previous work is that


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we measured the magnetization depth profile on single LSMO films interfacing with STO substrates, while that of Huijben et al.26 examined LSMO/STO superlattices. In order to neutralize the excess charge at the interfaces of LSMO/STO for the (100) and (111) orientations, electron doping is required. Electron doping at the interface perturbs the phase diagram of the interface towards a value equivalent to a small value of Sr concentration, i.e., hole depletion. Recently, the theoretical modeling from Hammouri et al.,16 and experimental work from Meyer et al.,31 and Herklotz et al.32 have shown that hole depletion (accumulation) of charge at the PZT/LSMO interface achieved by reversing the polarization of PZT increases (decreases) the interfacial magnetization. Our result is consistent with the prediction and recent observations. Namely, electron doping to compensate the positive charge imbalance plays the same role as hole depletion and leads to an increase of the interface magnetization compared to the case of the (110) orientation, in which no charge imbalance occurs. Our result is an example of how charge imbalance, if controlled, can alter the magnetization of LSMO locally at the interface. In summary, we measured the magnetization depth profile across identically grown epitaxial LSMO films on three different crystallographic orientations of STO. For all samples, the magnetization is largest at the LSMO/STO interface and is reduced towards the film surface. Further, we have demonstrated the possibility to grow an LSMO layer with interfacial magnetization no less than that of the film bulk. While the interfacial magnetism of the (110) orientation is still larger than the film bulk, it nevertheless has the lowest interfacial magnetization of the three orientations studied— the (100) and (111) orientations having comparable interfacial magnetism. This result correlates with the charge imbalance across the interface which is the same for the (100)



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and (111) orientation, ∆ = 0.67e-, and larger than that of ∆ = 0 for the (110) orientation. We suggest the positive charge imbalance can be compensated by electron doping which enhances the magnetism at the interface.

METHODS Thin Film Growth and Characterization LSMO thin films were grown on (001)-oriented STO single crystals by pulsed laser deposition (PLD). The STO substrates were etched in a NH4F buffered HF:H2O=1:10 (buffered oxide etch) solution (pH>4.5) and annealed in air at temperatures between 1100 and 1200°C to ensure well-defined TiO2-terminated surfaces. The substrate temperature and an oxygen pressure were maintained at 725 °C and 100 mTorr, respectively, during film growth. After deposition, the films were annealed in situ at an oxygen pressure of 760 Torr for 30 minutes at the growth temperature before cooling to room temperature. Structural characterization and XRR measurements were carried out with a Panalytical X’Pert MRD four-circle X-ray diffractometer. The total magnetization was measured with a Quantum Design SQUID-VSM instrument. The morphologies of the films were checked with a Nanoscope III AFM. Polarized Neutron and X-ray Reflectometry The Polarized Neutron Reflectivity (PNR) data was recorded at the BL-4A beamline of the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL). All data were collected at a temperature of 10 K, which is well below the Tc of LSMO. The measurements were carried out under field-cool conditions with an applied magnetic field of 1 T. The x-ray reflectivity (XRR) measurements were done at room temperature with a Panalytical X’Pert MRD diffractometer. The PNR and XRR data were simulated and fitted with GenX software. The authors declare no competing financial interest. ACKNOWLEDGEMENT This work was supported by the U.S. Department of Energy (DOE), Office of Science (OS), Basic Energy Sciences (BES), Materials Sciences and Engineering Division (sample design, fabrication, and physical property characterization) and by the Laboratory Directed Research and Development (LDRD) Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. DOE (PNR). The


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research at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, BES, U.S. DOE. AUTHOR INFORMATION Corresponding Author. *Corresponding author: [email protected]



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TABLE.

Thickness Film roughness (nm) (Å)

afilm (Å)

cfilm (Å)

(1) Tc, SQUID Tetragonality M(1 T)SQUID (c/a) (kA/m) (K) (kA/m)

LSMO (001) 21.8 ± 0.1

4.36 ± 0.07

3.905 3.874± 0.005

0.992± 0.001

381 ± 10

367 ± 8

150 ± 5

LSMO (110) 22.3 ± 0.2

4.88 ± 0.18

5.522 5.495± 0.005

0.995± 0.001

396 ± 10

384 ± 12

180 ± 5

LSMO (111) 20.9 ± 0.2

5.42 ± 0.32

6.763 6.711± 0.005

0.992± 0.001

421 ± 10

405 ± 10

240 ± 3

Table 1. List of parameters obtained from x-ray reflectivity (film thickness and roughness), reciprocal space mapping (in-plane lattice parameter, afilm, and out-of-plane lattice parameter, cfilm), magnetometry [M(1T)SQUID and Tc, SQUID], and the thickness averaged (1) ]. magnetization obtained with PNR measurements [


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FIGURES

Figure 1. Schematics of interface configurations in LSMO/STO heterostructures with (a) (100), (b) (110), and (c) (111) orientations. The numbers inside the brackets indicate the net charges at each layer.



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Figure 2. (a)-(c) XRD θ-2θ scans of LSMO thin films grown on STO substrates with (100), (110), and (111) orientations, respectively. (d)-(f) Reciprocal space maps (RSMs) around the STO substrate’s 103, 013, and 330 reflections for (100)-, (110)-, and (111)orientated heterostructures, respectively.


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-2

(a)

-5

0

10

-2

10

4.5 3.0

LSMO(100)

1.5 0.0

0

(b)

10

-2

10

(c)

10

-2

10

-4

0.0

25

20

25

0.0

5

10 15 Depth (nm)

4.5 3.0

LSMO(111)

1.5 0.0

0

10

20

LSMO(110)

1.5

-5

-2

X-ray SLD (10 Å )

0

25

3.0

0 -4

20

4.5

Exp. Fit

10

10 15 Depth (nm)

-5

-2

X-ray SLD (10 Å )

0

5

Exp. Fit

-4

10

Reflectivity (arb. unit)

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X-ray SLD (10 Å )

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5

10 15 Depth (nm)

0.3

0.4

Exp. Fit

0.1

0.2

0.5

-1

Q (Å ) Figure 3. X-ray reflectivity data from LSMO thin films grown on (a) (100), (b) (110), and (c) (111) oriented STO substrates. The solid lines are the fits to the experiment data (open symbols). Insets: x-ray scattering length density (SLD) depth profiles, which describe the chemical compositions of LSMO thin films.


450

(b) LSMO(111) LSMO(110) LSMO(100)

0 LSMO(001) -450 450

300

(c)

0 LSMO(110) -450 450

150

(d) µ0H // ab

µ0H // ab

0

0

100

0

T = 10 K

FC at 1 kOe

200

-0.2

300

0.0

LSMO(111) -450 0.2

M (kA/m)

(a)

M (kA/m)

450

M (kA/m)

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M (kA/m)


µ0H (T)

T (K)

Figure 4. (a) Comparison of temperature dependent magnetization for LSMO thin films with different orientations. The magnetic field was applied along the in-plane direction (H parallel to the film surface). All measurements were taken during the warm-up after field cooling at 1 kOe. (b)-(d) Magnetic hysteresis of LSMO thin films with (100), (110), and (111) orientations. All measurements were taken at 10 K.


10

-9

10

-10

10

-11

+

(a)

4.5 STO

R R

3.0

LSMO film

Vac.

600

(d) 300

1.5

10

-10

10

-11

10

-9

10

-10

10

-11

(c)

+

R R

-2

R R

LSMO (100)

0.0

+

3.0

-6

10

-9

(b)

nSLD (10 Å )

-4

R/RF (Å )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 600

(e)

300

1.5 LSMO (110) 0.0 3.0

0 600

(f)

1.5

0.00

0.05

0.10

0.15

-1

Q (Å )

0.0

M (kA/m)

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300

0

LSMO (111) 5 10 15

20

0

25

Distance from substrate (nm)

Figure 5. (a)-(c) Measured (solid and open dots) and fitted (solid lines) polarized neutron reflectivities of different oriented LSMO films as a function of wave vector Q [=4πsin(αi)/λ], where αi is the incident angle and λ is the neutron wavelength. The reflectivity values are normalized to the Fresnel reflectivity RF (=16π2/Q4). PNR measurements are taken at 10 K after cooling at 1 T. The correspondence nuclear scattering length density (nSLD) depth profiles (dash lines) and magnetization depth profiles (solid lines) of (100), (110), and (111) oriented LSMO films are shown in (d)-(f), respectively. Inset of (d) shows the schematic drawing of the sample geometry.



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For Table of Contents Only


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Figure 1. Schematics of interface configurations in LSMO/STO heterostructures with (a) (100), (b) (110), and (c) (111) orientations. The numbers inside the brackets indicate the net charges at each layer. 254x190mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. (a)-(c) XRD θ-2θ scans of LSMO thin films grown on STO substrates with (100), (110), and (111) orientations, respectively. (d)-(f) Reciprocal space maps (RSMs) around the STO substrate’s 103, 01 ̅3, and 330 reflections for (100)-, (110)-, and (111)- orientated heterostructures, respectively. 254x190mm (96 x 96 DPI)


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Figure 3. X-ray reflectivity data from LSMO thin films grown on (a) (100), (b) (110), and (c) (111) oriented STO substrates. The solid lines are the fits to the experiment data (open symbols). Insets: x-ray scattering length density (SLD) depth profiles, which describe the chemical compositions of LSMO thin films. 508x668mm (150 x 150 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. (a) Comparison of temperature dependent magnetization for LSMO thin films with different orientations. The magnetic field was applied along the in-plane direction (H parallel to the film surface). All measurements were taken during the warm-up after field cooling at 1 kOe. (b)-(d) Magnetic hysteresis of LSMO thin films with (100), (110), and (111) orientations. All measurements were taken at 10 K. 508x254mm (150 x 150 DPI)


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Figure 5. (a)-(c) Measured (solid and open dots) and fitted (solid lines) polarized neutron reflectivities of different oriented LSMO films as a function of wave vector Q [=4πsin(αi)/λ], where αi is the incident angle and λ is the neutron wavelength. The reflectivity values are normalized to the Fresnel reflectivity RF (=16π2/Q4). PNR measurements are taken at 10 K after cooling at 1 T. The correspondence nuclear scattering length density (nSLD) depth profiles (dash lines) and magnetization depth profiles (solid lines) of (100), (110), and (111) oriented LSMO films are shown in (d)-(f), respectively. Inset of (d) shows the schematic drawing of the sample geometry. 254x190mm (96 x 96 DPI)



Incident neutrons

H

αi

Reflected neutrons

αr

Thickness

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

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(100)

(110)

Magnetization ACS Paragon Plus Environment

(111)

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