Growth of single-crystalline RFe2O4-Î´ (R=Y, Tm, and Yb) by floating

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Growth of single-crystalline RFe2O4-# (R=Y, Tm, and Yb) by floating zone melting method in a mixture of N2, H2 and CO2 gases and magnetic properties of the compounds Shinya Konishi, Kunihiko Oka, Hiroshi Eisaki, Katsuhisa Tanaka, and Taka-hisa Arima Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01393 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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Crystal Growth & Design

Growth of single-crystalline RFe2O4-δ (R=Y, Tm, and Yb) by floating zone melting method in a mixture of N2, H2 and CO2 gases and magnetic properties of the compounds Shinya Konishi †, Kunihiko Oka‡, Hiroshi Eisaki‡, Katsuhisa Tanaka†, Taka-hisa Arima§

†Department

of Material Chemistry, Graduate School of Engineering, Kyoto University,

Nishikyo-ku, Kyoto 615-8510, Japan

‡Low-Temperature

Physics Group, Nanoelectronics Research Institute, National Institute of

Advanced Industrial Science and Technology, 1-1-1 Central 2, Umezono, Tsukuba, Ibaraki, 305-8568, Japan

§Graduate

School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa,

Chiba 277-8561, Japan

*Corresponding

author: Shinya Konishi

Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan

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Tel: 81-75-383-2426

Fax: 81-75-383-2420 Email: [email protected]


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Abstract We have grown single crystals of RFe2O4-δ with R being Y, Tm, and Yb, proposed to be electronic ferroelectrics, by using floating-zone melting technique under a rather moderate condition. For the crystal growth, a mixture of 95 % N2 + 5 % H2 and CO2 gases was used, instead of a mixture of CO and CO2 usually utilized thus far, as an atmosphere.

The full width

at half maximum (FWHM) of the X-ray rocking curve was small enough to warrant that the quality of the resultant single crystals is rather high: 0.038, 0.024, and 0.041 for YFe2O4-δ, TmFe2O4-δ, and YbFe2O4-δ single crystals, respectively.

The single crystals of YbFe2O4-δ with

sufficient level of quality were grown at a growth rate as high as 18.80 mm/h.

Magnetic

properties have been examined to evaluate the nonstoichiometry of the present single crystals. Both magnetization and magnetic transition temperatures of single-crystalline YbFe2O4-δ are increased by annealing in a CO2/CO atmosphere, suggesting that the annealing contributes to a decrease in the concentration of oxygen vacancies, which much affects the magnetic functionality.

Introduction RFe2O4, where R is a rare-earth element with a smaller ionic radius or In3+, was first synthesized in the mid-1970s.1),2) The crystal structure belongs to the trigonal system.


The


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stoichiometric compounds should contain equivalent amounts of Fe2+ and Fe3+ ions.

In the

structure, the iron ions form double triangular-lattice layers stacked along the c-axis in the hexagonal setting.

The compounds have attracted considerable attention because of their

unique electrical and magnetic properties.

The electrical conduction around room temperature

is not metallic but semiconducting, and the conductivity at room temperature in the c-plane is larger by about two orders of magnitude than that along the c-axis.3),4) The semiconducting conduction was ascribed to a spatial ordering of Fe2+ and Fe3+ ions, as a short-range order of Fe2+ and Fe3+ with the propagation vector Q ~ (1/3,1/3,1/2) was confirmed by electron and X-ray diffraction studies at room temperature. a Mössbauer spectroscopy.5)

The Verwey-type transition was demonstrated also by

Because of the geometrical frustration in the arrangement of equal

number of divalent and trivalent Fe ions in the triangular lattice, one would expect unconventional charge distribution.

Yamada et al. pointed out that the charge arrangement

forming electric dipole along the c-axis should be stable in a double layer.6)

Ikeda et al.7),8)

suggested that the dielectric polarization due to the charge distribution among the iron ions can give rise to the ferroelectricity in RFe2O4. called an electronic ferroelectric.

From such a point of view, RFe2O4 has been often

However, the issue as to whether the compounds are

ferroelectric, antiferroelectric, or paraelectric is still controversial.9-15)


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In RFe2O4 not only the charge but also the spin and orbital degrees of freedom have attracted much attention. Both of the Fe2+ and Fe3+ ions are known to be of the high-spin states with a magnetic easy axis parallel to the c-axis.16) each Fe site behaves as an Ising spin.17)

As a consequence, the magnetic moment at

Previous studies of magnetic properties using neutron

diffraction as well as magnetization measurements have indicated that the magnetic properties of RFe2O4 are rather confusing mainly because the oxygen deficiencies, the amount of which depends on the synthesis condition, affect the correlation between magnetic moments of iron ions.

It seems that LuFe2O4 with the stoichiometric composition exhibits antiferromagnetic

transition at around 240 K, below which three-dimensional spin correlation is established.18) On the other hand, as the oxygen vacancy increases, the magnetic transition temperature monotonically decreases19) and the spin glass- or cluster glass-like behavior emerges.20)

At the

same time, spin correlation becomes restricted within the two-dimensional triangular iron ion layer.18) A neutron diffraction study has revealed that the spin ordering is not a long-range one, presumably because of the short range nature of the charge ordering and/or the geometrical spin frustration.21),22)

The magnetic structure is closely connected with the arrangement of Fe2+ and

Fe3+ since the superexchange interaction between two adjacent iron ions is affected by their valences. Such a strong correlation between magnetism and charge ordering in RFe2O4 is intriguing also from the viewpoint of the science of multiferroicity.



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A recent theoretical research has pointed out a possible effect of the Fe2+ orbital degree of freedom on low-temperature physical properties of RFe2O4.23） by five oxide ions has a local symmetry of C3v.

The iron ion site coordinated

The 3d orbitals are split into one

non-degenerate state (3z2-r2) and two sets of doubly degenerate states (approximately zx and yz as well as x2-y2 and xy) by the ligand field effect.

As the 3z2-r2 state is the highest energy level,

a high-spin Fe2+ ion with d6 configuration has doubly degenerate ground states. The orbital degree of freedom should almost always have a considerable impact on the transfer of an electron and hence the exchange interaction between adjacent transition-metal ions. Even a weak orbital effect could be a nontrivial perturbation in geometrically charge-frustrated RFe2O4

For further understanding of the interesting charge-spin-orbit coupling system, high-quality single crystals are inevitably required so that accurate measurements of physical properties can be performed.

In the synthesis of stoichiometric RFe2O4, the most important and troublesome

issue is to precisely control the valence state of iron ions and to avoid the possible occurrence of oxygen deficiencies.

It was reported that polycrystalline stoichiometric HoFe2O4, ErFe2O4,

TmFe2O4, YbFe2O4, and LuFe2O4 were prepared by sintering raw materials under an atmosphere of H2/CO2 or CO/CO2 mixed gas for 24 h at 1473 K.1)

The oxygen partial pressure during the

sintering was reported to be 10-11.15, 10-11.15, 10-10.65, 10-10.65, and 10-10.65 atm, respectively.

As

for the growth of single-crystalline RFe2O4, the floating zone melting method has been usually


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applied under an atmosphere of H2/CO2 or CO/CO2 since the first successful growth of single-crystalline YFe2O4.24)

The mosaicity was reported to be 0.4 for the single-crystalline

YFe2O4 grown at a rate of 2 mm/h.17) This technique was also applied to the growth of single-crystalline LuFe2O4.25)

An early study reported that the mosaicity of single-crystalline

ErFe2O4 and LuFe2O4 grown by the floating zone melting method was 0.15° and 0.3°, respectively.26) Recently, detailed investigation of growth of YFe2O4 single crystals was performed.

1 1 The peak width of the ( 2 2 13.5) superstructure reflection along [00l] is 0.034 r.l.u.

(reciprocal lattice units) at 120 K for the YFe2O4 single crystal grown in a CO/CO2 atmosphere at a growth rate of 1 mm/h, corresponding to a correlation length of 22 unit cells.27)

In the present study, we have utilized a mixture of N2, H2, and CO2 gases instead of the above-mentioned H2/CO2 or CO/CO2 mixed gas as an atmosphere for the growth of single crystals of YFe2O4, TmFe2O4, and YbFe2O4.

In other words, the crystal growth has been

performed under a rather moderate condition.

Also, it is worth noting that the growth of

TmFe2O4 single crystal has been reported less than other compounds, especially, LuFe2O4 and YFe2O4.

We demonstrate that the atmosphere used in the present experiments efficiently gives

rise to single crystals with sufficient level of quality from a point of view of the mosaicity. also show that the post-annealing of the as-grown single crystal in a CO/CO2 atmosphere is


We


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effective to increase both the magnetization and the magnetic transition temperatures of the compound.

Results and discussions Growth of single crystals and their quality

As mentioned above, the single crystals of RFe2O4 phases have been grown from their melts mainly by floating zone melting method. According to the phase diagrams of Fe-Fe2O3-R2O3 systems, RFe2O4- phases with  taking from about -0.05 to 0.1 are stable at 1200C but is absent at 1100C.2), 28)-30)

The atmosphere, in which the growth proceeds, as well

as the temperature is a crucial factor to form RFe2O4 phases.

For instance, the molar ratio of

CO2 to H2 in atmosphere should be in a range of 0.65 to 0.95 to obtain single phase of YFe2O4- from the melt.24)

Furthermore, recent study on the growth of single-crystalline YFe2O4-

revealed that the compound was decomposed into YFeO3- and FeO1- at the initial stage of growth, then the compound with stoichiometric composition was formed, and YFe2O4- with  > 0.04 appeared at the final stage of the growth process.27)

In the present study, an attempt was

made to grow single crystals in a mixture of N2, H2 and CO2 gases, different than the atmosphere utilized thus far, with the molar ratio of N2, H2, and CO2 being about 21:1:11.


The molar ratio

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of CO2 to H2 is 11, fairly higher than the value previously suggested for YFe2O4.24) shows a photographs of YFe2O4-, TmFe2O4-, and YbFe2O4- floating zone melting technique.

Figure 1

single crystals grown by the

The growth rate was 2.0 mm/h for YFe2O4-, and TmFe2O4- ,

while the growth rate for YbFe2O4-δ was 18.80 mm/h.

As seen in the figure, we prepared

large-sized single crystals, the diameter and the length of which are 0.5 and 12 cm, 0.5 and 9.5 cm, and 0.5 and 9.5 cm for YFe2O4-δ, TmFe2O4-δ , and YbFe2O4-δ, respectively. were easily cleaved along the c-plane as illustrated in Fig.2.

The crystals

Figures 3(a), (b), and (c) show

the X-ray rocking curves for (1 1 15) reflection of YFe2O4-δ, (0 1 10) reflection of TmFe2O4-δ, and (1 1 15) reflection of YbFe2O4-δ, respectively.

The FWHM of the rocking curve was

estimated by fitting a Gaussian curve to the experimental data.



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Figure 1. Photographs of (a) YFe2O4-δ, (b) TmFe2O4-δ, and (c) YbFe2O4-δ single crystals growth by the floating zone melting method

1cm (a)YFe2O4-δ

1cm (b)TmFe2O4-δ

1cm

(c)YbFe2O4-δ Figure 2. Photographs of cleaved surfaces for (a) YFe2O4-δ, (b) TmFe2O4-δ, and (c) YbFe2O4-δ single crystals

Figures 3(a), (b), and (c) show the X-ray rocking curves for (1 1 15) reflection of YFe2O4-δ, (0 1 10) reflection of TmFe2O4-δ, and (1 1 15) reflection of YbFe2O4-δ, respectively.

The

FWHM of the rocking curve was estimated by fitting a Gaussian curve to the experimental data. Table 1 summarizes values of FWHM or mosaicity evaluated for the obtained single crystals along with those reported thus far.17),26)

The values of FWHM are 0.038°, 0.024°, and 0.041°

for the obtained YFe2O4-, TmFe2O4-, and YbFe2O4-, respectively.

These values are smaller by

one order of magnitude than those previously obtained for single crystals of YFe2O4 (0.4), ErFe2O4 (0.15°), and LuFe2O4 (0.3°),17),26) although those values were evaluated from the linewidth of neutron diffraction lines.

It is unfair to simply compare the linewidth of diffraction


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lines between X-ray diffraction and neutron diffraction measurements because usually a single-crystalline specimen with a larger volume is required for the neutron diffraction than for the X-ray diffraction.

Nonetheless, at least, the rather small values of FWHM for the present

single-crystalline specimens suggest that their quality is good enough.

Furthermore, the use of

the values of FWHM evaluated from the different diffraction lines, i.e., (1 1 15) and (0 1 10) in the present case, is inadequate for a quantitative comparison of mosaicity or quality of the different crystals.

Hence, we do not compare the quality among the present single crystals of

YFe2O4-δ, TmFe2O4-δ, and YbFe2O4-δ.

The fact that the values of FWHM are rather small

clearly indicates that the composition of the mixed gas utilized as an atmosphere during the crystal growth in the present study is suitable to obtain single crystals of RFe2O4 with small mosaicty.

In general, the quality of a single crystal is reduced as the crystal growth rate is

increased.

Nonetheless, YbFe2O4-δ single crystal with sufficient level of quality was obtained

even when the growth rate was as high as 18.80 mm/h.

Our preliminary element analysis by

means of inductively coupled plasma-mass spectrometry (ICP-MS) reveals that the molar ratio of Fe to Tm in the present single-crystalline TmFe2O4- is 1.97 or so.

Also, the as-grown

crystals are non-stoichiometric as mentioned below in relation with their magnetic properties.

Table 1. Synthesis conditions and mosaicity or full width at half maximum (FWHM) of X-ray rocking curve for single-crystalline YFe2O4, ErFe2O4, TmFe2O4, YbFe2O4, and LuFe2O4



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Conditions for crystal growth Mosaicity Compound or FWHM ()

Speed of crystal growth (mm/h)

Rotational speed of feed rod Atmosphere and molar and seed ratio 　　　 crystal (rpm)

Ref.

Present work Present work

YFe2O4

0.038

2.0

25

95%N2+5%H2/CO2,2/1　

TmFe2O4

0.024

2.0

25

95%N2+5%H2/CO2,2/1

YbFe2O4

0.041

18.8

25

95%N2+5%H2/CO2,2/1

Present work

YFe2O4

0.4

2

-

H2/CO2, 5/4

17)

ErFe2O4

0.15

2

-

H2/CO2, 5/4

26)

LuFe2O4

0.3

2

-

H2/CO2, 5/4

26)


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Figure 3. Rocking curves for (a) YFe2O4-δ, (b) TmFe2O4-δ, and (c) YbFe2O4-δ single crystals.

Magnetic properties of RFe2O4-（R=Y, Tm, and Yb） A dc magnetic field was applied parallel to the c-axis in the magnetization measurements. For YFe2O4-, magnetization was also measured by applying a dc magnetic field perpendicular to the c-axis, and the result was compared with the data obtained by applying a dc field parallel to the c-axis.

Figures 4(a) and (b) depict the temperature dependence of magnetization for

as-grown YFe2O4-δ and TmFe2O4-δ single crystals, respectively.

Many studies have been carried

out on magnetic properties of both single-crystalline and polycrystalline RFe2O4 thus far.30)-43) Here, it should be noted that to our best knowledge, Fig.4(b) is the first report on magnetic properties of single-crystalline TmFe2O4-δ, although temperature dependence of magnetization has been already reported for polycrystalline TmFe2O4.31)

Previous researches indicate that the

magnetic structure and transition observed for RFe2O4 is rather complicated.

Phan et al.20)

examined magnetic properties of single-crystalline LuFe2O4 in detail and led to the magnetic phase diagram of the compound.

The phase diagram indicates that as the temperature is

decreased in the absence of a magnetic field, the phase transition from paramagnetic to ferrimagnetic state takes place at around 240 K, followed by the successive changes considered to be cluster glass transitions at around 225 and 150 K.

On the other hand, de Groot et al.18)

pointed out that the magnetic properties are different depending on samples of single-crystalline


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LuFe2O4 even if they stem from the same rod.

For example, one sample of the

single-crystalline LuFe2O4 exhibits the cluster glass- or spin glass-like behavior as reported by Phan et al.,20) whereas another sample manifests antiferromagnetic transition at around 240 K when the external field is zero, and the antiferromagnetic state undergoes metamagnetic transition into the ferrimagnetic state as the external field is increased at around the Néel temperature. They also suggested that the antiferromagnetic transition was more readily observed for a sample which had a higher magnetic phase transition temperature.

Thus,

magnetic properties are significantly affected by the chemical state, that is, presumably valence state of iron ions and defects relevant to oxide ion.

For single-crystalline YFe2O4, Sugihara et al.3) carried out measurements of magnetization as a function of temperature under several conditions.

They presented zero-field-cooled (ZFC)

and field-cooled (FC) magnetizations with the magnetic field applied parallel to a- or c-axis. The temperature dependence of magnetization shown in Fig. 4(a) is very similar to that obtained by Sugihara et al.;3) namely, when the magnetic field is parallel to the c-axis, the FC magnetization monotonically increases with a decrease in temperature accompanied by a rapid increase in the magnetization below about 220 K, and the ZFC magnetization manifests a broad cusp.

The temperature of the maximum ZFC magnetization is about 120 K for the present

YFe2O4-δ, while it is about 140 K for the sample reported by Sugihara et al.3)


Also, the


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phenomenon that the magnetization along the c-axis is much higher than that parallel to the c-plane is observed for both samples.

Based on the variation of magnetization with magnetic

field as well as temperature, Sugihara et al.3) concluded that the single-crystalline YFe2O4 was a parasitic ferrimagnet below 205 K, i.e., the Néel temperature.

On the other hand, according to

Inazumi et al.30) and Nakagawa et al.,32) single-crystalline and polycrystalline YFe2O4-δ with δ = 0.00 manifest two-step transitions in the temperature dependence of magnetization with the transition at higher temperature being antiferromagnetic ordering.

Also, the thermal hysteresis

is observed at these transitions, and whether the magnetic field is applied or not during the cooling process does not have any influence on the temperature dependence of magnetization. The behavior is in sharp contrast to the case of single-crystalline YFe2O4-δ with δ > 0.055, for which the temperature dependence of magnetization significantly depends on the cooling process; the FC magnetization gradually increases below about 220-230 K whereas the ZFC magnetization manifests a broad cusp at around 180-200 K.30)

Very similar phenomena were

reported by Mueller et al.27) for single crystalline YFe2O4-δ with δ  0.00 and δ > 0.05 recently. For the present single-crystalline YFe2O4-δ, as illustrated in Fig. 4(a), a drastic increase in magnetization is observed at around 210 K when the dc magnetic is applied parallel to the c-axis. Also, a maximum of ZFC magnetization is observed around 120 K, and the temperature dependence of the FC magnetization manifests a shoulder at the same temperature.


The

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behavior indicates that the present as-grown YFe2O4-δ single crystal includes oxygen vacancies and that the value of δ is presumably larger than 0.05.

Yoshii et al.31) reported FC and ZFC magnetizations for polycrystalline TmFe2O4.

They

observed that the FC magnetization manifested a rapid rise below 250 K and that the ZFC magnetization exhibited a broad cusp at around 240 K.

They considered that the increase in FC

magnetization along with the broad cusp of ZFC magnetization was ascribable to the development of ferrimagnetic ordering.

The temperature dependence of magnetizations for

single-crystalline TmFe2O4- shown in Fig. 4 (b) is similar to that for the polycrystalline TmFe2O4.

Namely, a drastic increase in FC magnetization is observed around 220 K and a

broad cusp appears at about 160 K in the temperature dependence of ZFC magnetization. Hence, the present single-crystalline TmFe2O4- seems to show the phase transition between paramagnetic and ferrimagnetic states at around 220 K.

In addition, supposing that the

magnetic phase diagram suggested by Phan et al.20) for LuFe2O4 can be qualitatively applied to the magnetic structures of TmFe2O4-δ, the broad cusp of the ZFC magnetization may correspond to the cluster spin glass-like transition.

However, as mentioned above, the temperature

dependence of both filed-cooled and ZFC magnetizations, which reflects the magnetic structure, transition, and relaxation as well, is very sensitive to the stoichiometry or the amount of oxygen vacancies.

Thus, more detailed investigation of magnetic properties is necessary for



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comprehensive clarification of the magnetic structure and ordering of TmFe2O4-. is in progress.

(a)

H // c ZFC H // c FC H ⊥c FC

Hdc=300Oe

(b)

H // c ZFC H // c FC

Hdc=300Oe


Such a study

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Figure 4. Temperature dependence of magnetization for (a) YFe2O4-δ and (b) TmFe2O4-δ single crystals

Figure 5 shows the temperature dependence of magnetization for YbFe2O4-δ single crystals before and after annealing in a CO2/CO atmosphere. for 17 h.

The annealing was performed at 1493 K

The molar ratio of CO2 to CO in the atmosphere was 2:1.

The as-grown YbFe2O4-δ

single crystal manifests a rapid increase in FC magnetization around 170 K.

Also, another

change in magnetization is observed around 140 K, where a maximum of ZFC magnetization appears.

The behavior of FC and ZFC magnetizations is similar to that reported by Yoshii et al.

recently, although they observed two maxima in the temperature dependence of FC magnetization.43)

They thought that difference between the FC and ZFC magnetizations was

rooted in the ferrimagnetic ordering.

Another probability for the occurrence of the maximum of

ZFC magnetization is the cluster spin glass-like transition as suggested by Phan et al. 20) for LuFe2O4.

In either case, the rapid increase in FC magnetization in Fig.5 seems to be ascribable

to ferrimagnetic ordering.

As shown in Fig. 5, the ferrimagnetic transition temperature is raised

from 170 to 250 K by the annealing in the CO2/CO atmosphere.

Furthermore, the

magnetization is significantly enhanced over a wide temperature range after the annealing. A



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similar phenomenon was observed for other RFe2O4-δ polycrystals as well.

Kishi et al.16)

revealed that the magnetization monotonically increased with an increase in the oxygen content at 77 K to room temperature for polycrystalline YbFe2O4-δ.

Yang et al.19) reported that the

ferrimagnetic transition temperature almost linearly decreased with an increase in oxygen deficiency for polycrystalline LuFe2O4-δ.

These facts including the present case indicate that

there exist oxygen vacancies accompanied with the Fe2+-rich composition in the as-grown single crystal of YbFe2O4-δ and that the annealing in the CO2/CO atmosphere results in oxidation of Fe2+ into Fe3+ ions.

The increase in Fe3+ content can lead to the intensified magnetization and

increased magnetic transition temperature, because both the magnetic moment and the superexchange interaction are larger for Fe3+ than for Fe2+.

In other words, the post-annealing

improves the crystal quality from the point of view of the reduced amount of oxygen vacancy as well as the magnetic functionality, although the broad peak observed in the temperature dependence of ZFC magnetization suggests that the present sample is still non-stoichiometric with some amounts of oxygen vacancies present.

A slight decrease in the magnetization with a decrease in temperature observed for the YbFe2O4-δ below about 50 K is ascribable to the contribution of magnetic moment of Yb3+ at low temperatures.37,38)

The magnetic moment of Yb3+ is affected by the internal magnetic field, Hi,

due to the magnetically coupled Fe3+ and Fe2+ ions as well as the external field, H, and the


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direction of Hi is antiparallel to H.

Hence, the magnetization gradually decreases with a

decrease in temperature at low temperatures.

2.0 H//c FC Before annealing H//c ZFC Before annealing

1.5

H//c FC After annealing

Hdc = 300 Oe 1.0

0.5

0.0 0

50

100

150

200

250

300

Temperature (K)

Figure 5. Temperature dependence of magnetization for YbFe2O4-δ single crystal before and after post-annealing

Conclusion Single crystals of RFe2O4 with R being Y, Tm, and Yb were grown by using the floating zone melting method under a rather moderate condition.

We used a mixture of 95% N2 + 5%

H2 and CO2 gasses instead of H2/CO2 or CO/CO2 usually used thus far as an atmosphere during



Page 22 of 35

the crystal growth and successfully obtained single crystals with small mosaicity, that is the FWHM of the X-ray diffraction rocking curve is 0.038°, 0.024°, and 0.041° for YFe2O4-, TmFe2O4-, and YbFe2O4-, respectively.

In particular, for YbFe2O4-δ, the quality of single

crystal obtained by the growth rate of 18.80 mm/h is almost the same as the quality when the growth rate is 2 mm/h.

Furthermore, the post-annealing in a CO/CO2 atmosphere makes the

magnetization and magnetic transition of YbFe2O4- increase, presumably because the post-annealing process reduces the concentration of oxygen vacancies.

Methods Rare-earth oxides, R2O3 with R＝Y, Tm, and Yb (99.9% purity, Furuuchi Chemical Co.), FeO (99.9% purity, Furuuchi Chemical Co.), and Fe3O4 (99.9% purity, Furuuchi Chemical Co.) were used as raw materials for the growth of single crystals.

The raw materials were

homogeneously mixed, and the resultant powder was pressed into a rod with a diameter of 8 mm by applying a hydrostatic pressure of 98 MPa. furnace (Crystal Systems Co.). source.

The rod was sintered in an optical floating zone

A halogen lamp with a power of 500 W was used as the heat

The atmosphere for the sintering was a mixture of 95% N2 + 5% H2 gas and CO2 gas

with 2:1 molar ratio, and the power of the halogen lamp as a light source was set to be about 250


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W so that the temperature was kept to be just below the melting point of the compounds.

The

rod was irradiated with the halogen lamp and slowly moved at a speed of about 10 mm/h.

Single crystals were grown in the same furnace under the same atmosphere.

The sintered

rod was melted by focusing the high-power light from the halogen lamp and the melt was cooled slowly by moving the rod. direction to each other.

The seed rod and growing crystal were rotated at 30 rpm in opposite

We tried to fabricate single crystals at growth rates of 2.00 and 18.80

mm/h.

X-ray diffraction measurements were carried out to characterize the grown RFe2O4- (R=Y, Tm, and Yb) single crystals.

In particular, X-ray rocking curves were obtained to evaluate the

mosaicity of the resultant single crystals.

The measurements were performed by X-ray with a

wavelength of 0.88795 Å on the Beam line 1A, Photon Factory (PF) at the High Energy Accelerator Research Organization (KEK), Tsukuba, Japan.

The single crystals were fixed to

six-axis diffractometer (Huber INC) equipped with Si (1 1 1) double crystal monochromator and cylindorically bended focusing mirror.

The diffractometer was tilted by the angle of θ.

X-ray was incident on the specific crystal planes, i.e., (1 1 15) for YFe2O4-δ, (0 1 10) for TmFe2O4-δ, and (1 1 15) for YbFe2O4-δ).

The measurements were carried out at room

temperature.



Page 24 of 35

Magnetization of RFe2O4-δ (R=Y, Tm, and Yb) single crystals was measured as a function of temperature.

A dc magnetic field of 300 Oe was applied parallel to the c-axis of the crystals,

and the measurements were performed via both field cooling and zero-field cooling processes with superconducting quantum interference device (SQUID, Quantum Design).


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For Table of Contents Use Only

Growth of single-crystalline RFe2O4-δ (R=Y, Tm, and Yb) by floating zone melting method in a mixture of N2, H2 and CO2 gases and magnetic properties of the compounds Shinya Konishi, Kunihiko Oka, Hiroshi Eisaki, Katsuhisa Tanaka, Taka-hisa Arima *Corresponding

author: Shinya Konishi

Table of Content Graphic



Page 34 of 35

Synopsis

We have grown single crystals of RFe2O4-δ with R being Y, Tm, and Yb, proposed to be electronic ferroelectrics, by using floating-zone melting technique under a rather moderate condition.

For the crystal growth, a mixture of 95 % N2 + 5 % H2 and CO2 gases was used,

instead of a mixture of CO and CO2 usually utilized thus far, as an atmosphere.

The full width

at half maximum (FWHM) of the X-ray rocking curve was small enough to warrant that the quality of the resultant single crystals is high: 0.038, 0.024, and 0.041 for YFe2O4-δ,


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TmFe2O4-δ, and YbFe2O4-δ single crystals, respectively.

The high-quality single crystals of

YbFe2O4-δ were grown at a growth rate as high as 18.80 mm/h.

Magnetic properties have been

examined to evaluate the nonstoichiometry of the present single crystals.

Both magnetization

and magnetic transition temperatures of single-crystalline YbFe2O4-δ are increased by annealing in a CO2/CO atmosphere, suggesting that the annealing contributes to a decrease in the concentration of oxygen vacancies, which much affects the magnetic functionality.


Growth of single-crystalline RFe2O4-Î´ (R=Y, Tm, and Yb) by floating

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