Mg assisted flux growth and characterization of single crystalline Sm 2 Co 17

This paper presents details of Mg-assisted flux growth of Sm2Co17 single crystals in a Ta crucible well below the melting temperature of binary Sm2Co17. Both the crushed single crystalline powder x-ray diffraction (XRD) and single crystalline XRD data revealed the Th2Zn17 type rhombohedral(R-3m) crystal structure. Ta atom is found to be statistically replacing the Co-Co dumbbell with its position being at the center of the dumbbell. The Curie temperature of our lightly Mg and Ta doped Sm2Co17 sample is determined to be ∼1100 K using method of generalized Bloch law fitting of easy axis spontaneous magnetization data. Disciplines Biological and Chemical Physics | Materials Chemistry Authors Tej Nath Lamichhane, Qisheng Lin, Valentin Taufour, Andriy Palasyuk, Tribhuwan Pandey, David Parker, Sergey L. Bud’ko, and Paul C. Canfield


I. INTRODUCTION
Despite of the wide application of Sm 2 Co 17 as a high performance magnet, its basic physical properties are not as extensively studied as other high flux commercial permanent magnets such as Nd 2 Fe 14 B, SmCo5, probably due to the lack of easily accessible single crystalline sample.][3] In case of SmCo5 and other high cobalt content binary R-Co compounds single crystals, the traditional self-flux growth technique out of binary melt is not readily accomplished.Most of the Sm-Co binary compounds are highly reactive with traditional ceramics crucibles and mostly peritectic with very high exposed liquidus temperature (>1300 ○ C).Single crystal growth by zone melting, Bridgman and Czochralski technique are also difficult because of high reactivity of Sm-Co compounds and high vapour pressure of Sm.The quality of Bridgman technique grown crystals strongly depend on the quality and type of the crystal growth crucibles and also needs an excess amount of Sm.The Bridgman technique for SmCo5 growth was successful only using the Ta crucible. 4For Sm 2 Co 17 , the pyrolitic sintered BN-crucible could not assist crystal growth.Additionally, the BN-coated recrystallized A1 2 O 3 crucible also produced a reacted complex layer containing Al, B, N and R when reached up to 1400 ○ C during the Bridgman crystal growth technique. 5In this work we use Mg-flux to reduce the melting temperature and allow for the use of a sealed Ta crucible to hold the melt to get single crystals.More importantly, this Mg assisted flux growth technique might open a route for the availability of most of R 2 T 17 single crystals which are surprisingly rare so far e.g.Sm 2 Fe 17 . 6

II. EXPERIMENTAL RESULTS AND DISCUSSIONS
A. Crystal growth and structural characterization Sm 9 Co 67 Mg 24 composition was loaded in a 3-capped Ta crucible 7 and growth profile similar to Ce 3−x MgxCo 9 8 was used for the crystal growth.The single crystals were separated from the flux at 1150 ○ C after cooling the ampoule from 1200 ○ C over 99 h.
As grown Sm 2 Co 17 single crystals are presented in Figure 1  plate as illustrated by backscattered Laue photograph with a hexagonal pattern as shown in Figure 1(b).
Initially, Sm 2 Co 17 single crystals were characterized using crushed single crystal powder x-ray diffraction (XRD) technique.These data were collected using Rigaku Miniflex II diffractormeter (Cu-Kα radiation).Finely ground Sm 2 Co 17 powder was spread over the silicon wafer sample holder with help of Dow Corning high vacuum grease and diffraction data were collected over 2θ range of 10 ○ to 100 ○ with a scan step of 0.01 ○ with a dwelling time of 5 sec.data were Rietveld refined using EXPGUI and GSAS software package. 9The Rietveld refined powder XRD pattern using Th 2 Zn 17 type structure with space group R-3m is presented in Figure 1(c) with Rp = 0.08.
Then the as grown single crystals of Sm 2 Co 17 were characterized using Scanning Electron Microscopy (SEM) technique.As grown crystals were mounted in a SEM sample-mountingepoxy both parallel and perpendicular to the plate to access both planar and cross sectional area of samples and finely polished to obtain the smooth surfaces as shown in Fig. 2. Figure 2  Then we got motivated to know the specific structural site of Mg and Ta in the structure and employed a single crystalline XRD analysis.
Single crystal XRD intensity data for an as grown single crystal were collected using Bruker smart Apex-II diffractometer (MoKαλ = 0.71073 Å) and analysed using SHELXTL structure solution software.In total, 5665 reflections were collected using 0.05 ○ scans in ω.The average exposure time was 10 sec and the crystal to detector distance was 60 mm.In the structure solution, Mg could be included in the specific Co site but the composition is never higher than error bar.Then we dropped Mg in the composition formula.Ta atoms were found to substitute the center of Co-Co dumbbell as shown in Fig. 3 Refined crystallographic information data and conditions are presented in Table I and II below.

B. Determination of Curie temperature
Temperature and field dependent magnetization data were obtained along easy axis of Sm 2 Co 16.69 Ta 0.31 single crystals using Quantum Design vibrating sample magnetometer (VSM: 300 K -1000 K).2][13] Getting the Curie tail of Sm 2 Co 16.69 Ta 0.31 to estimate   the transition temperature is not possible in commercially available magnetometers like QD VSM with oven option.In order to estimate the Curie temperature we used the method of generalized Bloch fitting of spontaneous magnetization data.Spontaneous magnetization data were obtained via the Y-intercept of linear fit of saturation magnetization part of M(H) data at various temperatures as shown in Fig. 4 for 300 K.The two tiny triangular vortexes in the M(H) loop might be the signature of ferrimagnetic coupling between Sm and Co demonstrated by domain wall movement at high field. 14,15Such spontaneous magnetization data were taken up to 1000 K for each interval of 50 K starting from the room temperature.To reduce the uncertainty, spontaneous magnetization data were measured using a well-polished sample which could be better aligned along the easy axis during the measurements.The generalized Bloch Law was fitted as shown in the inset of Figure 4 and the Curie temperature is inferred to be 1100 K. 16,17 Here MS(2K) and MS(T) are the base temperature and high temperature spontaneous magnetization data and TC is the Curie temperature.The fitted value of the α and β are found to be 2.53 ± 0.07 and 0.49 ± 0.02 respectively.

III. CONCLUSIONS
Single crystalline Sm 2 Co 17 samples were prepared using Mg assisted self-flux growth technique in a 3-caped Ta crucible.In case of Sm 2 Co 17 , Ta is found to be statistically distributed in the center of Co-Co dumbbell with an empirical formula Sm 2 Co 16.69 Ta 0.31 .The Curie temperature of Sm 2 Co 16.69 Ta 0.31 is determined to be ∼ 1100 K using the generalized Bloch law method.

FIG. 1 .
FIG. 1.(a) Sm 2 Co 16.69 Ta 0.31 single crystals over the millimeter grid (b) Laue pattern with beam direction [001].Although the crystals are not looking xexagonal, the back-scattered Laue photograph is hexagonal.(c) Rietveld refined powder XRD for Sm 2 Co 16.69 0.31 .I(Obs), I(Cal), I(Bkg) and I(Obs -Cal) are experimental, calculated, fitted background and differential diffractrogram data respectively.The vertical lines represents the various diffraction Bragg peaks.
FIG. 2. (a) SEM image of as grown Sm 2 Co 16.69 Ta 0.31 single crystal along the planar view (b) along the cross section.

FIG. 3 .
FIG. 3. (a) Crystal structure of Sm 2 Co 16.69 Ta 0.31 .The Ta atoms are statistically occupying the center of Co-Co dumbbell along the specific veritcal channel.(b) Crystal structure of Sm 2 Co 16.69 Ta 0.31 along the c-axis to demonstrate the dumbbells and Sm atoms channel.(c) Illustration of statistical replacement of Co-Co dumbbell by Ta atom.
(a).The structure of Sm 2 Co 16.69 Ta 0.31 (R-3m) features parallel hexagonal tunnels (defined by Co2-Co4 atoms) running along the c axis, cf. Figure 3(b).The tunnels are alternately filled by Co1-Co1 dimers and Sm atoms.In the present structure, Ta is statistically substituted for Co-Co dumbbell with Ta position being at the center of Co-Co dumbbell as shown in Figure 3(c), consistent with Zr atom position in Zr doped Sm 2 Co 17 magnet alloy predicted via lattice relaxation calculation, 10 with a refined occupancy of 2.6(2)%.

FIG. 4 .
FIG. 4. Determination of the spontaneous magnetization of Sm 2 Co 16.99 Mg 0.01 Ta 0.35 using [001] magnetization data at 300 K. See the text for a possible reason of tiny vortexes in the M(H) loop.

TABLE I .
Crystal data and structure refinement for Sm 2 Co 16.69 Ta 0.31 .

TABLE II .
Atomic coordinates and equivalent isotropic displacement parameters (A 2 ) for Sm 2 Co 16.69 Ta 0.31 .U(eq) is defined as one third of the trace of the orthogonalized U ij tensor.