Magnetic Structure Refinement Using GSAS and EXPGUI

Transcription

1 Magnetic Structure Refinement Using GSAS and EXPGUI J. Cui, B. Toby, and Q. Huang Dept. of Materials Sci. & Eng., Univ. of Maryland, College Park, MD NCNR, NIST Center for Neutron Research, Gaithersburg, MD (Dated: April 7, 2004) Introduction This document is a practical guide on performing basic magnetic structure refinement using GSAS [1] and EXPGUI [2]. Example data used in this document is collected on YBa 2 Fe 3 O 8 compound at room temperature using the High-Resolution Neutron Powder Diffractometer BT-1 at NIST. Details of sample preparation and neutron diffraction measurements can be found in [3, 4]. The document includes two parts: 1) Magnetic structure modeling. 2) Structure refinement using GSAS and EXPGUI. Downloadable files are: Histogram data (YBAFEO.GSAS), Instrument file (YBAFEO.ins), GSAS EXP files (NEXP.bk1, M1EXP.bk1, M2EXP.bk2). GSAS has two ways to refine a magnetic structure, one is to set a phase both nuclear and magnetic, which usually involves complicated constraints setting if the nuclear structure is not simple, the other is to add an additional phase which is pure magnetic. The second way is easier in term of constraint setting, but it involves choosing a new space group among several possibilities, which can be a tricky exercise. In this document, we use the second way to describe how to refine magnetic structure using GSAS and EXPGUI. Part 1: Magnetic structure modeling 1. Nuclear structure The nuclear structure of YBa 2 Fe 3 O 8 is depicted by Figure 1. Note that in the figure, sizes of the atoms and lattice are distorted for clarity. The space group is PMMM (a=3.925, b=3.907, c=11.786). Fractional coordinates of the atoms are listed in Table 1. 1

2 Table 1: atoms and fractional coordinates of the nuclear lattice No. TYPE X Y Z Occupancy 1 Y BA FE FE O O O O O Figure 1: Nuclear and magnetic structures. 2. Magnetic structure YBa 2 Fe 3 O 8 compound is antiferromagnetic at room temperature. It is proposed that the magnetic moments carried by the Iron atoms are parallel to the X axis (a-axis), their directions are alternatively pointing to or away from each other as illustrated by the 2

3 arrows in Figure 1. This model can be described by several space group symmetries. Among them, the one with highest symmetry is a simple doubling along x, y, and z directions (a M =2a n, b M =2b n, c M =2c n.) The space group is FMM M, where the prime after the letter M refers the flip of magnetic moment direction after the mirror operation. The unique symmetry operations and the associated spin colors in this mode are: (Mx, My, Mz, Acen, Bcen, Ccen) = (Black, Red, Red, Black, Black, Black), where Mx, My, Mz refer to the mirror symmetries with respect to x, y, and z axes, respectively. Acen, Bcen, and Ccen refer to linear translation from origin to the face center of A, B, and C planes, respectively. Black spin color denotes that the symmetry operation does NOT involve flip of magnetization direction (ferromagnetic); red color denotes the operation requires flip of magnetization (antiferromagnetic). Only Iron atoms appear in the pure magnetic phase. The fractional coordinates of these atoms are listed in Table 2. Table 2: Atoms and fractional coordinates of the magnetic phase. The Z coordinate of the third Iron atom equals to half of the Z coordinates of the fourth atom in Table 1, which is z=0.34. No. TYPE X Y Z Moment 1 FE µ B 2 FE µ B 3 FE z/2-3.5µ B 4 FE z/2 3.5µ B Part 2: Magnetic structure refinement 1. Nuclear structure refinement. Use EXPGUI as GSAS interface to input the nuclear structures listed in Table 1. In addition, following setup need to be performed: 1. Set the histogram exclude region to 0~6, ~1000 (minimum d-spacing 0.781). 2. In the phase window of EXPGUI, turn on the refinement flag X and U for all atoms, and F for atoms O8 and O9 due to its partial occupancy. Turn on the flag for cell refinement. All damping factors used in this document are zero. 3. In the histogram window, turn on the flag for background refinement, and change the function type to 2, use 12 terms. In addition, the flag for Zero refinement need to be turned on. 3

4 4. In the Scaling window, input to the scale box, then turn on the flag for Scale refinement, leave the phase fraction at 1.0 unchanged and not refined. 5. In the profile window, change the function type to 2, use the peak cutoff value 0.005, turn on the flag for GU, GV, GW, LX, and asym. 6. In the Constraints window, set the temperature factor UISO of atoms O6 equals to O7 equal, and O8 equals to O9. Now, run powpref and genles a couple of times, the CHI*2 value should convert to about 7.5. (Exp file: NEXP.bk1) 2. Magnetic structure refinement Step 1: Input atoms parameter listed in Table 2 using EXPGUI. The lattice parameters are (FMMM, a= , b= , c= ). In addition, the correlation of the pure magnetic phase to the nuclear phase needs to be setup: 1. Set the Z fractional coordinate of atoms FE3 equals to half of that of the atom FE4 in the nuclear phase; adjust the fractional coordinate of atoms FE4 such that the sum of the Z fractional coordinate of FE3 and FE4 equals to 0.5. This relation needs to be maintained through out the refinement, thus has to be input as one constraint (Phase 1, FE4, Z, +1; Phase 2, FE3, Z, +0.5; Phase 2, FE4, Z, -0.5). Then turn on the X flag for all atoms in phase 2 to allow them follow the newly refined Z coordinates in nuclear phase. 2. Set the temperature factors (UISO) of atom FE1 and FE2 in phase 2 equal to that of FE3 in phase 1, and UISO of atom FE3 and FE4 in phase 2 equal to that of FE4 in phase 1. Similar to previous step, this relation need to be maintained through out the refinement thus two constraints need to be set (Phase 1, FE3, UISO, +1; Phase 2, FE1/FE2, UISO, +1) and (Phase 1, FE4, UISO, +1; Phase 2, FE3/FE4, UISO, +1). Then turn on the U flag for all atoms in phase 2 to allow them follow the newly refined UISO values of the nuclear phase. 3. Set the lattice parameters of the magnetic phase to be exactly twice of the nuclear phase. Then constrain the lattice parameters of the two phases together. This done by following: a. Run EXPEDT b. Type Y L O L K. c. Type I, then 1, RM11, 1, hit return key, then 2, RM11, 0.25, then hit return twice. d. Type I, then 1, RM22, 1, hit return, then 2, RM22, 0.25, hit return twice. c. Type I, then 1, RM33, 1, hit return, then 2, RM33, 0.25, hit return twice. 4

5 e, Type X X X X X, then hit return to exit EXPEDT. After the constraint is setup, the cell refinement flag can be turned on. Step 2: Label phase 2 to magnetic phase only. Run EXPEDT, type Y P P M 2, then C to turn the second phase to magnetic only. Keep typing X and hitting return key till EXPEDT is exited. Now, there should be a blue label Magnetic Only showing on the upper left corner in the second phase window of EXPGUI. Step 3: Setup the flip color. Run EXPEDT, type Y L A P 2, then M. GSAS will list all the unique symmetry operations and the associated spin colors. Change the color to (Mx, My, Mz, Acen, Bcen, Ccen)=(Black, Red, Red, Black, Black, Black). This can be done by typing S, then C n to change the n th color. For example, if we want to change the color of Acen, we need to type C 4. Step 4: Input magnetic moment. To input magnetic moment, type Y L A P 2 M in EXPEDT, then M to set atom FE1 s moment to 3.5µ B, then M M M to set the moment for the rest Iron atoms of the phase. After the moment is done, the moment refinement flag need to be turned on: type X, then V FE M. Now, we can exit EXPEDT by typing many many Xs. Step 5: Check the magnetic form factor (adjust it if necessary). The J0 magnetic form factor coeffs for Fe 3+ are: A1,B1,A2,B2 = A3,B3,A4,B4,C = Run EXPEDT, then type Y L F M, hit return, then type FE, if the listed J0 numbers do not match above, they have to be changed. Type C, then input the values. After all 9 parameters are input, type N, hit return key, then type U to update the entry. Notice that forgetting updates the entry is an easy mistake to make. The magnetic form factors of other atoms/ions can be accessed by running the sub routine ATOM in GSAS. Step 6: Histogram. In the histogram window of EXPGUI, make sure flags of both phase are checked and leave everything unchanged. Step 7: Setup the phase fraction. 5

6 In the Scaling window, keep the scale factor and its refinement flag unchanged, change the phase fraction of the second phase to 0.125, because the volume of the magnetic phase is 8 times higher than the nuclear phase. If the phase fraction of the magnetic phae is the same as the nuclear phase, then the magnetic intensity will be 8 times higher than the real value. We want to keep this ratio thus keep the refinement flag unchecked. Step 8: Profile In the profile window, change the parameter of the second phase to be exactly the same as the first phase. In addition, this relation needs to be maintained through out the refinement. This can be done by go to the constraint window, click the profile tab in the lower part of the window, and then input constraint. Step 9: Constraint on magnetic moment. We want the refined magnetic moments have the same absolute value for all Iron atoms. Thus the constraint is (Phase 2, FE1/FE4, MX, +1; Phase 2, FE2/FE3, MX, -1). Now, everything is set, run powpref and genles a couple of times, the CHI*2 value should convert to about The refined magnetic moment is (Exp file: M1EXP.bk1) 3. Impurity phase The sample contains a few percent impurity (BaFeO 3 ). Adding it as the third phase will further reduce the CHI*2 value down to The structure information of the impurity phase is Space group: P M 3 M (a=4.0875), fractional coordinates: (BA: 0 0 0) (FE: ), (O: ). The phase fraction is about 3%. Due to its small volume, expect the phase fraction and lattice parameter, all other variables should not be refined. (Exp file: M1EXP.bk2) 6