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Tuning the spin-alignment of interstitial electrons in two-dimensional YC electride via chemical pressure 2

Jongho Park, Jae-Yeol Hwang, Kyu Hyoung Lee, Seong-Gon Kim, Kimoon Lee, and Sung Wng Kim J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10338 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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Journal of the American Chemical Society

Tuning the spin-alignment of interstitial electrons in twodimensional Y2C electride via chemical pressure Jongho Park,1, 2 Jae-Yeol Hwang,1, 2 Kyu Hyoung Lee,3 Seong-Gon Kim,4 Kimoon Lee,5* and Sung Wng Kim1* 1

Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon 16419, Republic of Korea 3 Department of Nano Applied Engineering, Kangwon National University, Chuncheon 24341, Republic of Korea 4 Department of Physics & Astronomy and Center for Computational Sciences, Mississippi State University, Mississipi State, Mississipi 39762, USA 5 Department of Physics, Kunsan National University, Gunsan 54150, Republic of Korea 2

Supporting Information Placeholder ABSTRACT: We report that the spin-alignment of interstitial anionic electrons (IAEs) in two-dimensional (2D) interlayer spacing can be tuned by chemical pressure that controls the magnetic properties of 2D electrides. It was clarified from the isovalent Sc substitution on Y site in 2D Y2C electride that the localization degree of IAEs at interlayer becomes stronger as the unit cell volume and c-axis lattice parameter were systematically reduced by increasing the Sc contents, thus eventually enhancing superparamagnetic behavior originated from the increase in ferromagnetic particle concentration. It was also found that the spin-aligned localized IAEs dominated the electrical conduction of heavily Sc-substituted Y2C electride. These results indicate that the physcial properties of 2D electrides can be tailored by adjusting the localization of IAEs at interlayer spacing via structural modification that controls the spin instability as found in three-dimensional elemental electrides of pressurized potassium metals.

Electrides are unique form of ionic crystals in which electrons act as an anion.1, 2 Anionic electrons localized in the cavity space occupying crystallographic interstitial sites, not in atomic orbitals, behave like structurally coordinated constituent atoms. Depending on the geometrical topology and wavelength percolation of anionic electrons, diverse physical and/or chemical properties such as high electron mobility,3 low work-function3-5 and novel catalytic reactivity6-8 can be imparted to the non-active functional materials. In particular, two-dimensional (2D) electrides with interstitial anionic electrons (IAEs) at interlayer space have been attracted much attention due to the fact that the properties of material can be tuned by the degree of localization of IAEs, showing various functionalities beyond electronics and chemistry.9, 10 Recently, we have reported that 2D Y2C electride exhibits magnetic anisotropy originated from strongly localized IAEs at interlayer space.11 While the lattice structure of Y2C electride consists of paramagnetic elements of Y and C, the IAEs behave as spin-aligned elements which possess localized magnetic moments. However, it is

still unclear how the spin alignment of IAEs occurs in interlayer 2D space, leading to the unconventional magnetism free from atomic orbital electrons. When we compare with the isostructural non-magnetic 2D Ca2N electride, the distinct difference is the c-axis lattice parameter.3 From the structural analysis, c-axis parameter of Y2C electride is ~ 10% shorter than that from Ca2N, suggesting that the 2D interlayer space where the IAEs occupy is much more compressed for Y2C than that from Ca2N.3, 11 It is worthwhile to note that a similar phenomenon has been reported in the elemental electrides, which are theoretically predicted from the pressurized light elements.12-14 In a compressed potassium metal, the open structure is stabilized due to the strongly localized IAEs occupying crystallographic structural cavities, which is regarded as a generalized form of s- or p-block elements under an extreme pressure. While the Y2C electride showed a short-ranged magnetism encompassing ferromagnetic IAEs,15 the compressed potassium electrides exhibited a Stoner-type instability towards ferromagnetism.16-18 This variety in magnetism is ascribed to the dimensionality and geometry of IAEs determining the degree of localization at interstitial cavities. Indeed, the compressed potassium electride showed a 3D topological simple cubic structure of IAEs separated each other by 2.71 Å,17 which is much shorter than 3.62 Å for Y2C electride with 2D topology, restricting the interaction between spin aligned electrons along out-of-plane direction as a 2D magnet nature.19 In this report, we systematically study the effect of localization degree of IAEs on electrical and magnetic properties of 2D Y2C electride with respect to different c-axis parameters modulated by an internal chemical pressure. Isovalent Sc substitution on Y site results in the selective shrinkage on c-axis of 2D Y2C electride from 17.97 Å to 17.50 Å as confirmed by X-ray diffraction patterns analyzed by Rietveld refinement. Electrical measurements verify that the dominant scattering sources for carrier transport change from delocalized electrons to spin-aligned ones as c-axis decreases, indicating the increase in the number of magnetically ordered particles. From the magnetic moment measurements, we observed the enhancement of superparamagnetic behavior origi-

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nated from the increased concentration of ferromagnetic IAEs as the interlayer space of Y2C is reduced with chemical pressure. These results clearly demonstrate that the magnetism in 2D Y2C electride is essentially correlated with the degree of localization of IAEs as predicted in alkali electrides realized under high pressure.16-18 To synthesize a stoichiometric Y2-xScxC electride, we mixed Y, Sc, and graphite chips with 2 ∶ ∶ 1 ratio 0.6, then the mixed samples were melted using an arc melting furnace under high purity argon (99.999%) atmosphere. The melting process of the sample was repeated at least 5 times to improve structural and chemical homogeneity. Structural parameters of each sample were obtained by analyzing powder X-ray diffraction (PXRD) through the Rietveld method using GSAS code.20, 21 The electrical properties of Y2-xScxC electride were evaluated by fabricating a Hall bar configuration on the samples. (See figure S3 for the details) The temperature (T) and magnetic field (H) dependences of electrical and magnetic properties were measured by Physical Property Measurement System (PPMS) equipped with a vibrating sample magnetometer (VSM). All samples were prepared in argon-filled glovebox to prevent the surface degradation of Y2xScxC electride under an ambient atmosphere. Figure 1(a) shows the Rietveld refinement of PXRD pattern for representative Sc-substituted Y1.8Sc0.2C. All major reflections of Sc-substituted Y2-xScxC samples are well matched with those of pristine Y2C structure of anti-CdCl2 type structure with R 3 m space group as illustrated in the inset of Fig. 1(a) (See Fig. S1 in supplement information.). When the amount of the Sc substitution for Y is over 30 mol%, a secondary Sc (Space group: P63/mmc) phase starts to appear, thus the solubility limit of Sc to the Y site is estimated to be between 30 and 40 mol%. (See Fig. S2 in supplement information) From the Rietveld refinements, the detailed structural information including lattice parameters and the unit cell volume are obtained as listed in table S1 (Refer to supplement information.) As shown in Fig. 1(b) representing the change of lattice parameters with respect to the Sc contents (x), the c-axis lattice parameter monotonically decreases with increasing Sc content possibly due to the smaller ionic radius of Sc3+ (0.75 Å) compared to that of Y3+ (0.90 Å), whereas the a- and b-axis lattice parameter remains almost constant. It is noteworthy from the middle of Fig. 1(b) that the decrease of interlayer spacing occupied with the IAEs dominates the decrease of c-axis lattice parameter compared to the thickness change of layer. From the fact that the shrinkage of c-axis by Sc substitution mainly results in the compression of interlayer space where the IAEs exists, Sc substitution effectively induces internal chemical pressure on IAEs leading to an increase in degree of localization for IAEs. Figure 2(a) displays T dependence of electrical resistivity (ρ) for Y2-xScxC samples with various Sc doping ratio. As shown in Fig. 2(a), ρ values for all samples decrease as T decreases, indicating metallic transport behavior regardless of Sc doping contents. The T dependence of ρ for all samples can be well represented by a power law as obeying ρ ~ ATn with n = 1 especially in high T regime (> 100 K) as observed in a metal under a nearly free electron conduction with ns1 valence electron state.22 (See Fig. S3 in supplement information) It implies that electrical conduction of Y2-xScxC electrides in high T regime is dominated by a simple lattice vibration scattering like s-band metals such as alkali elements, indicating the conduction electrons are IAEs with snature character. However, the scattering mechanism changes at low T depending on Sc ratio.

Figure 1. (a) Powder X-ray diffraction patterns measured by Cu Kα radiation for the Y1.8Sc0.2C sample. The calculated pattern by Rietveld refinement is also displayed. Inset shows the schematic illustration for the crystal structure of Y2 xScxC electride. (b) Dependence of lattice parameters of a-, b-, c-axis (upper panel), interlayer distance (d), layer thickness (t) (middle panel), and volume (V) of unit cell (lower panel) on the ratio of scandium contents in Y2 xScxC electrides. All parameters are obtained from the Rietveld refinement as also listed in table S1. (Refer to supplement information) As the Sc contents increase, the suppression of residual resistivity ratio (RRR) becomes significant mainly due to the increased residual resistivity. This is probably attributed to the enhanced localization of IAEs and subsequently reinforced magnetic spinordering scattering as the chemical pressure is increased. In order to confirm the magnetic ordering enhancement, the transverse component of resistance (Rxy), which is perpendicular to the direction of excitation current, was measured as a function of H at 2 K. For pristine Y2C, Rxy exhibits linear dependence on H, while the linearity starts to deviate as the Sc contents increases. Since such a non-linear Rxy behavior is one of the hallmarks for carrier scattering associated with magnetic moments as observed in a magnetic metal,23 it is suggested that the spin-aligned elements are introduced as Sc substitution increases although Sc is also a paramagnetic element like Y and C.23 To examine the detailed magnetic properties of Y2-xScxC electrides, we measure the magnetic susceptibility (χ) characteristics. Figure 3(a) – 3(d) show χ curves as a function of T for different x values. The χ value for each sample is measured under zero field cooling (ZFC) and field cooling (FC) condition with H of 100 Oe. Under both ZFC and FC conditions, the increase in χ value is observed as Sc content increases. Consistently with Rxy results, it also implies that the spin-aligned components in Y2-xScxC electride are increased by decreasing the c-axis lattice parameter via


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Figure 2. (a) Resistivity (ρ) versus temperature (T) curves of Y2 xScxC electrides. Inset displays T dependence of residual resistivity ratio (RRR) for Y2 xScxC electrides ranged from 2 K to 300 K. (b) Applied magnetic field (H) dependence of Rxy ranged from 3 × 104 to +3 × 104 Oe at 2 K. Inset shows schematic illustration for the geometry of electrodes in Rxy measurements. paramagnetic Sc-substitution, as resulting in the improved magnetic moments due to the increased chemical pressure. In particular, an irreversibility between the FC and the ZFC curves appears for all samples. It is noteworthy that such a splitting of FC and ZFC for χ is often observed in the superparamagnetic system, which appears from ferromagnetic particles having a negligible interaction with paramagnetic matrix.23-25 In χ versus H curves as depicted in Figs. 3(e) – 3(h), all samples with different Sc substituted amount do not exhibit a hard saturation behavior with a negligible hysteresis, which is another characteristic signature of superparamagnetism.24-26 Since it was clarified from our previous density functional theory calculation that the dominant magnetic element in Y2C electride is the spin-aligned IAEs at interlayer space,11 we hypothesize that the IAEs serve as ferromagnetic particles responsible for the observed superparamagnetism. To confirm the origin of superparamagnetism and understand the effect of chemical pressure driven by Sc substitution on the enhanced superparamagnetism behavior of Y2-xScxC electride, we performed the quantitative analysis on χ depending on H. The χ H curves obtained at 2 K are well fitted with Langevin function (red-dashed lines in figure 3(e) – 3(h) based on below relation,24, 25, 28

L (H, µ) = coth (µH /kBT) - (kBT/µH) where µ is magnetic moment of IAEs and kB is Boltzmann constant. We assume that µ is Bohr magneton as an elementary atomic magnet due to the orbital free nature of IAEs.25 The resultant values of the number of ferromagnetic nanoparticles (nferro) and their radius (re) are plotted in Fig. 4(a) with respect to Sc contents in Y2-xScxC electride. Compared the calculated re value for pris-

Figure 3. (a) Magnetic susceptibility (χ) versus temperature (T) curves for Y2 xScxC samples under H = 100 Oe. (b) χ as a function of H ranges from 1 × 104 to +1 × 104 Oe at 2 K. Red dotted lines indicate the Langevin fitting curves. Goodness of fit (GoF) values are also listed in each curve. tine Y2C with theoretically predicted radius value of IAEs by electron localization function (ELF) calculation,11 it is nearly consistent with the value of 1.70 Å and 1.64 Å, respectively.11 The most striking result is the systematic decrease of re against increase of nferro as x value increases. This strongly supports that the localization of IAEs at 2D interlayer space is significantly enhanced and definitely tuned by the chemical pressure. As plotted in Fig. 4(a), nferro increases from 4.7 × 1017 to 1.1 × 1018 cm-3 as x increases, confirming the enhancement of magnetic moment in 2D interlayer space of Y2-xScxC electrides is responsible for the nferro increment. It should be noted that the strong localization of IAEs is predicted to exhibit a Stoner-type instability towards ferromagnetism.15 Since an electride can be regarded as a pressurized form of a simple metal in an early stage even under an ambient pressure,3, 16-18, 29 the present Sc-substituted Y2-xScxC electride can also be considered as an extended system of extremely compressed 2D metal realized by applying the uniaxial pressure in the c-axis direction. From the results that the most compressed region is the 2D interlayer space where the IAEs are located, it is concluded that the degree of localization for IAEs are selectively modulated by Sc substitution as illustrated in Fig. 4(b). As enhancing the degree of localization for IAEs by chemical pressure, spin-ordered state at the specified crystallographic site become more favorable, exhibiting not the delocalized states displaying nearly free electron conduction but the strongly localized character behaving like atoms as predicted from pressurized alkali metal, finally resulting in nferro increment.16-18 Consequently, the magnetic properties of 2D Y2-xScxC electride such as a magnetoelectrical transport and superparamagnetism are imparted by the localized IAEs at 2D interlayer space.



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AUTHOR INFORMATION 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

Corresponding Author *Corresponding author: [email protected], [email protected]

ACKNOWLEDGMENT This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2015M3D1A1070639).

REFERENCES

Figure 4. (a) Concentration of ferromagnetic IAEs (nferro) and their radius (re) for Y2 xScxC samples as a function of Sc contents calculated by Langevin fitting at 2 K. (b) The schematic illustration for the encouragement of magnetic ordering correlated with the degree of localization for IAEs at interlayer space in Y2C electride. Delocalized electrons between slabs (orange cloud) are integrated into the fixed crystallographic sites with a finite magnetic moment as c-axis decreases. In summary, chemically pressurized 2D Y2C electrides were synthesized by the substitution of isovalent Sc on Y site to induce the change in the interlayer space affecting the localization of anionic interstitial electrons. The systematic shrinkage of c-ais lattice parameter with increasing Sc content in Y2C host matrix, was verified by comprehensive PXRD analysis and its Rietveld refinement. From the electrical measurements, the dominant scattering source for carrier transport is confirmed to change from delocalized electron to localized one possessing magnetic moment as c-axis lattice parameter of Y2-xScxC electride decreases. The enhancement of superparamagnetic behaviour with increased nferro as well as reduced re through the chemically compressed Scsubstituted Y2C electride strongly demonstrates that the magnetism in 2D Y2C electride is critically correlated with the degree of localization for IAEs. From these results, we envisage that such a 2D electride can be a key material for realizing the modulation of the localization strength for a lone electron under low-dimensional, which is anticipated to emerge an exceptional functionality of materials such as a 2D electron magnet.

ASSOCIATED CONTENT Supporting Information Structural information of Y2C electride; Powder X-ray diffraction (PXRD) analysis; Rietveld refinement; The solubility limit of Y2xScxC; Logarithmic fitted resistivity. This material is available free of charge via the Internet at http://pubs.acs.org.

(1) Dye, J. L., Science 1990, 247, 663-668. (2) Dye, J. L., Acc. Chem. Res. 2009, 42, 1564-1572. (3) Lee, K.; Kim, S. W.; Toda, Y.; Matsuishi, S.; Hosono, H., Nature 2013, 494, 336-340. (4) Toda, Y.; Matsuishi, S.; Hayashi, K.; Ueda, K.; Kamiya, T.; Hirano, M.; Hosono, H., Adv. Mater. 2004, 16, 685-689. (5) Toda, Y.; Yanagi, H.; Ikenaga, E.; Kim, J. J.; Kobata, M.; Ueda, S.; Kamiya, T.; Hirano, M.; Kobayashi, K.; Hosono, H., Adv. Mater. 2007, 19, 3564-3569. (6) Kitano, M.; Inoue, Y.; Yamazaki, Y.; Hayashi, F.; Kanbara, S.; Matsuishi, S.; Yokoyama, T.; Kim, S.-W.; Hara, M.; Hosono, H., Nat. Chem. 2012, 4, 934-940. (7) Hayashi, F.; Toda, Y.; Kanie, Y.; Kitano, M.; Inoue, Y.; Yokoyama, T.; Hara, M.; Hosono, H., Chem. Sci. 2013, 4, 3124-3130. (8) Lu, Y.; Li, J.; Tada, T.; Toda, Y.; Ueda, S.; Yokoyama, T.; Ki-tano, M.; Hosono, H., J. Am. Chem. Soc. 2016, 138, 3970-3973. (9) Kim, S. W.; Matsuishi, S.; Nomura, T.; Kubota, Y.; Takata, M.; Hayashi, K.; Kamiya, T.; Hirano, M.; Hosono, H., Nano Lett. 2007, 7, 1138-1143. (10) Miyakawa, M.; Kim, S. W.; Hirano, M.; Kohama, Y.; Kawaji, H.; Atake, T.; Ikegami, H.; Kono, K.; Hosono, H., J. Am. Chem. Soc. 2007, 129, 7270-7271. (11) Park, J.; Lee, K.; Lee, S. Y.; Nandadasa, C. N.; Kim, S.; Lee, K. H.; Lee, Y. H.; Hosono, H.; Kim, S. G.; Kim, S. W., J. Am. Chem. Soc. 2017, 139, 615-618. (12) Seitz, F., Rev. Mod. Phys. 1946, 18, 384-408. (13) Seitz, F., Rev. Mod. Phys. 1954, 26, 7-94. (14) Markham, J. J., F-centers in Alkali Halides. Academic Press, UK, 1966 (15) Inoshita, T.; Hamada, N.; Hosono, H., Phys. Rev. B 2015, 92, 201109. (16) Pickard, C. J.; Needs, R., Phys. Rev. L. 2011, 107, 087201. (17) Dong, S.; Zhao, H., App. Phys. Lett. 2012, 100, 142404. (18) Dong, S.; Zhao, H., Phys. Status Solidi B 2014, 251, 527-532. (19) Huang, B.; Clark, G.; Navarro-Moratalla, E.; Klein, D. R.; Cheng, R.; Seyler, K. L.; Zhong, D.; Schmidgall, E.; McGuire, M. A.; Cobden, D. H.; Yao, W.; Xiao, D.; Jarillo-Herrero, P.; Xu, X., Nature 2017, 546, 270273. (20) Toby, B., J. Appl. Crystallogr. 2001, 34, 210-213. (21) Toby, B. H.; Von Dreele, R. B., J. Appl. Crystallogr. 2013, 46, 544-549. (22) Kasap, S. O., Principles of electronic materials and devices. McGraw-Hill New York: 2006; Vol. 2 (23) Blundell, S., Magnetism in condensed matter. Oxford University Press, UK, 2001. (24) Bean, C.; Livingston, u. D., J. Appl. Phys. 1959, 30, S120-S129. (25) Leslie-Pelecky, D. L.; Rieke, R. D., Chem. Mater. 1996, 8, 17701783. (26) Kodama, R. H., J. Magn. Magn. Mater 1999, 200, 359-372. (27) Mandel, K.; Hutter, F.; Gellermann, C.; Sextl, G., J. Magn. Magn. Mater 2013, 331, 269-275. (28) Goldfarb, R. B.; Patton, C. E., Phys. Rev. B 1981, 24, 1360. (29) Matsuishi, S.; Toda, Y.; Miyakawa, M.; Hayashi, K.; Kamiya, T.; Hirano, M.; Tanaka, I.; Hosono, H., Science 2003, 301, 626-629.


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Y1.8Sc0.2C

Intensity (arb. unit)

Observed Calculated Diﬀerence

d (Å)

a, b-axis (Å)

(b)

Y/Sc C d

c a

10

V (Å3)

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30

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t b

50 70 2θ (degree)

90

4.00

18.00

3.75

17.75

3.50 3.90

17.50 2.10

3.85

2.05

3.80

2.00

210 200 190 0

0.2 0.4 Sc contents (x)


0.6

c-axis (Å)

(a)


t (Å)

Figure 1

Figure Page 7 of 92


(a)

ρ/ρ300 K

1.0

10 ρ (mΩcm)

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Y2C Y1.8Sc0.2C Y1.6Sc0.4C Y1.4Sc0.6C

0.6 0.2 0

100 200 T (K)

1

0.1

(b)

5 0

Rxy (mΩ)

300

-5 -10 0 -5

10

T (K) 5

T = 2 K Y2C H I

+

I V+ Y1.6Sc0.4C

0

VI

-

100

300

Y1.8Sc0.2C

-5 -10 0

Y1.4Sc0.6C

-5

-10 -10 -3 -2 -1 0 ACS1 Paragon 2 Plus3 Environment -3 -2 -1 0 1 2 3 H (×104 Oe)

Figure 3

χ (emu/cm3)

χ (10-4 emu/cm3)

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400


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(b)

(c)

(d)

Y1.8Sc0.2C

Y1.6Sc0.4C

Y1.4Sc0.6C

300 H = 100 Oe ZFC FC

200 100 0

150 300 150 300 150 300 150 300 T (K)

0

0.5

(e)

(f)

(g)

(h)

Y2C

Y1.8Sc0.2C

Y1.6Sc0.4C

Y1.4Sc0.6C

2K ﬁ�ng

0

GoF 3.93 × 10-5 1.35 × 10-4 1.13 × 10-4 2.48 × 10-4

-0.5 -1

0

1

0

1

0

H (×104 Oe)


1

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nferro (×1017 cm-3)

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(b)

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1.8

T=2K

10

1.7

8

1.6

6

1.5

4

1.4

2

1.3

re (Å)

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0

0.2 0.4 Sc contents (x)

Y C

Loosely bound IAEs

0.6

Y/Sc

C

Chemical pressure Strongly localized IAEs ACS Paragon Plus Environment

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