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Antiferromagnet ism in Intermetal lic Perovskite systems R.Rang anathan

Antiferromagnet ism in Intermetal lic Perovskite systems R.Rang anathan. Experimental Condensed Matter Physics Division. Saha Institute of Nuclear Physics 1/AF, Bidhannagar,Kolkata 700 064. Collaborators Intermetallic Perovskite : ABO 3 type Abhishek Pandey,C.Majumdar(SINP),

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Antiferromagnet ism in Intermetal lic Perovskite systems R.Rang anathan

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  1. Antiferromagnetism in Intermetallic Perovskite systemsR.Ranganathan Experimental Condensed Matter Physics Division Saha Institute of Nuclear Physics1/AF, Bidhannagar,Kolkata 700 064

  2. Collaborators • Intermetallic Perovskite : ABO3 type • Abhishek Pandey,C.Majumdar(SINP), • S.Dattagupta(IISER), • Molly Roychoudhuri, T. Saha Das gupta ( S. N. Bose center), • Dhananjai Pandey ( BHU)

  3. AFM in metals- significance application in spin polarized tunneling experiments. The tip made out of this material will not introduce any magnetic effects to the specimen under study unlike the ferromagnetic tips like the CrO2. Half-metallic materials are those which have a finite density of states in one spin direction and a gap in the DOS of the other spin direction. Currently, few examples of half-metallic ferromagnets. In an half-metallic system, if the integer spin moment is zero then it is termed as an half-metallic antiferromagnet. It possess no microscopic magnetization, yet their carriers are fully polarized. Anti-ferromagnetic half-metals are unique in their electronic property. They are not anti-ferromagnets in the usual sense. calculations have brought out a lot of prospective candidates for HM- AFM’s. No actual HM-AFM have been synthesized. Theoretically Nakao PRB 74 172404 (2006)

  4. Motivation -- AFM in metal ---- Choice of the systems : GdPd3BxC1-x ---

  5. Perovskite Systems Pervoskite comparsion Perovskite Oxides O2- Polarization Electron Phonon interaction strong Perovskite Inter-metallic systems (Few Works) GdPd3BxC1-x System Manganate, JT ion System High Tc Exclude oxygen: weak phonon coupling No crystal field effect for (Gd) : Wide band metal, ex Pd (this study) for tuning magnetism Possible AFM, Metal ? (This work) FM metal/ insulator AFM insulator Competition between KE of itinerant electrons and AFM interaction energy of core spins (Double exchange interaction) Eg , T2g Levels AFM metal not possible No core spins AFM not possible GdBaCuO ( example) Conduction itinerant electrons Magnetism : Gd Atoms Weak interaction Understanding AFM in metal

  6. Our Work Samples were prepared by arc melting process under Argon atmosphere followed by heat treatment under vacuum in a sealed quartz tube and characterized by X-ray diffraction. RE(TM)3 YPd3, LaPd3, CePd3, PrPd3, NdPd3, SmPd3, GdPd3, TbPd3, DyPd3, HoPd3, ErPd3, TmPd3, YbPd3, LuPd3 RE(TM)3B YPd3B, CePd3B, GdPd3B,TbPd3B DyPd3B, HoPd3B, ErPd3B (RE1)x(RE2)1-x(TM)3Bx Ce0.5Eu0.5Pd3, Ce0.5Eu0.5Pd3B0.5 Ce0.5Eu0.5Pd3B This talk:Focus RE(TM)3BxC1-x GdPd3, GdPd3B0.25C 0.75 GdPd3B0.5C0.5, GdPd3B0.75C 0.25 GdPd3B

  7. Choice of compoundsGdPd3BxC1-x - A Unique System • Perovskite metal- no oxygen. Limited work in comparison with perovskite oxides • Negative thermal expansion, Negative TCR • Study of negative pressure effect- : The size of B atom is >size of tetrahedral void present at the center of the unit cell- introduction of Boron causes negative pressure effect • Max TN ~ 7.5 K for GdPd3 from LaPd3 to LuPd3(Gardner J.Phy. F Met. Phys. 1972)

  8. Observation and discussion – this talk Observation and discussion

  9. Crystal Structure GdPd3 Lattice Parameter = 4.09Å Cubic Space group: Pm-3m Gd1a; Pd3c Pd Gd Atomic Percentage of B = 20% . Weight Percentage of B ~ 2% Small B can cause good effect GdPd3B (4.14Å) Cubic (Pervoskite) Space group: Pm-3m Gd1a; Pd3c; B1b B

  10. X-ray diffraction at room temperature Perovskite-like structure is maintained Defect structure has larger lattice volume GdPd3C does not form in single phase

  11. XRD at low temperature GdPd3B0.25C0.75

  12. XRD at low temperature GdPd3B0.75C0.25 Symmetry Change? GdPd3B0.75C0.25 (О) GdPd3B050C0.50 ΔGdPd3B0.25C0.75 FWHM Intensity reverses for (111) to (200) and (331) to (420) below 40K. Peak position same at RT and LT-suggest structure remains same- but symmetry slight change (space group): Example:ZrWO8 (430K)-Nature 2003

  13. Variation of lattice parameter of GdPd3BxC1-x with temperature For GdPd3B 0.75C 0.25 higher B stoichiometry and asymmetric is strong- lead to slight change in symmetry Intensity reverses for (111) to (200) , (331) to (420) below 40K. Negative lattice expansion observed in Some oxides- example:ZrWO8 (up to 1050’C): M-O-M link essential. For GdPd3B0.25C0.75 –XRD data at RT and at 12K preserve structure and symmetry -NTE is isotropic ForGdPd3B like normal metal.GdPd3B 0.50C 0.50 symmetric distribution of B and C –No negative lattice expansion and lattice parameter ” a “ remains almost same for 12-150K.

  14. Resistivity • R(T) linear in high temperatures • Temperature coefficient of resistivity (TCR) changes linearly with boron concentration (x) • Changes sign at x >0.5 • Anomaly below 40K, TCR: α(T) = 1/ρ(T)[dρ(T)/dT].

  15. Magnetization, AC susceptibility results • In REPd3 two possible hybridizations : • - Intra atomic RE(4f) –RE(5d) • - Inter atomic RE( 4f)-Pd(4d). • In REPd3 ordering hybridizations and easy polarizability of Pd are important for magnetic ordering. ( Elsenhans Z. Phy B Cond. Matter 82 61 (1991)) • Max TN ~ 7.5 K for GdPd3 from LaPd3 to LuPd3 • ( Gardner J.Phy. F Met. Phys. 1972) Rare earth intermetallic compounds: interesting a) RKKY magnetic interaction b) Crystalline electrical field effect c) Fermi surface effects- commonly present in these materials In RE series Gd3+ ions interesting : S state (s= 7/2 L=0)- No CEF effect. Present study purely reflects details of RKKY magnetic interaction and Fermi surface effects.

  16. χ (T)/ χ (TN) ~ 2/3 for an ideal AFM: Here GdPd3 ~ 0.579: GdPd3B0.25C0.75~ 0.776: GdPd3B0.50C0.50~ 0.670: GdPd3B0.75C0.25~ 0.881: GdPd3B ~ 0.871:

  17. M – H M-H data

  18. AC Susceptibility’ (dispersion) ” (absorption) ( Hysteresis- energy loss) ACS data

  19. AC Susceptibility ACS B 0.25 ” (absorption) ( Hysteresis- energy loss) AFM – at TN- No hysteresis- ” 0 True AFM order

  20. AC Susceptibility ACS data B(0.75) TN - indepedent of frequency- absence of relaxation- spin glass/cluster glass/ etc

  21. TN - Results Tn comparsion Nelis etal PRB (1974)- NpPd3 , PuPd3 High TN comparsion with our work TN - from Mdc magnetisation, AC Ssusceptibility data ’ (dispersion) ” (absorption) ( Hysteresis- energy loss) Note- the absence of ” (absorption) data For B(0.25), B(0.50)

  22. Observation and discussion

  23. Magnetic origin (invar alloys FeNi) Negative Thermal expansion in metals Valence fluctuations ( Sm2.75 Co60 ), YbGaGe Arvanitids et al , Salvador et al Nature (2003) Negative thermal expansion- Materials contraction upon heating Negative Thermal expansion in metals – Transverse vibrations driven lattice contraction at LT: (This work) Thermal expansion α = 1/a (da/dt) (present study- GdPd3B0.25C0.75 ) α12K = ~ - 1.5 X 10-5 K-1 α300K = ~ +1.13 X 10-5 K-1 Structure and symmetry preserve down to 12K – XRD data- so NTE is isotropic (ZrW2O8 system α = - 8.8 X 10-6 K-1 for temp. range 0.3-1050K)

  24. (b) (a) 2 3 1 4 6 7 5 8 Gd B Pd C Crystal structure and atomic arrangements inside unit cell (a) The cubic unit cell - The Gd atoms occupy the cube corner positions (0, 0, 0). The face centre positions (½, ½, 0) are occupied by Pd atoms. The body centre position (½, ½, ½ ) represented by green circle is occupied by either B or C. (b) The eight corner sharing octahedra surrounding the central Gd atoms. This configuration has been drawn for GdPd 3B 0.25 C 0.75. Depending upon the chemical stoichiometry, out of eight octahedral centres, six have been occupied by C and 2 have been occupied by B. For preserving the symmetry the B atoms would prefer the diagonal positions.

  25. B C Pd z z

  26. Pd Vibrations • Due to higher size of boron atom the B-Pd bond length > C-Pd bond length. • this will produce a stretching effect on the neighboring C-Pd bonds. • Due to the anharmonic pair potential the distance between atoms increases with increase in temperature • More stress along the bond direction C-Pd-C- the vibration of the Pd atoms along the bond direction would be pronounced. • This vibrational motion of Pd atom is perpendicular (transverse) to the unit cell face and reduces the effective lattice parameter of the unit cell. • In B-Pd-C arrangement not so effective because of asymmetry of the bond arrangement on both side of Pd atoms. In the case of GdPd3B0.50C0.50, we only have B-Pd-C arrangement and here the • transverse vibrations are not predominant and thus we do not observe any NTE in this compound

  27. Gd - + B - + Pd - + + - Magnetization results: Transverse vibrations driven lattice contraction at LT: To use same concept as in NTE data for AFM ordering GdPd3B0.25C0.75 -TN ~15K maximum The vibrational motion of Pd atom is perpendicular (transverse) to the unit cell face and would cause Gd atoms to move closer to each other. So Jex increases – higher TN ~15K for GdPd3B0.25C0.75. For other concentration B-Pd-C arrangement, lack of additional stress provided by B (increasing C concentration side) causes less vibrations of Pd atoms. as Jex is less so TN Consistency with Neutron data picture NpPd3 : Moment directions parallel to cube edge- positive and negative shows up and down spins

  28. Summary GdPd3BxC1-x - A Unique System • Perovskite metal- no oxygen. • Introduction of Boron causes negative pressure effect- • Negative thermal expansion (Transverse vibrations driven lattice contraction as reasons for NTE.Generally magnetic orgin, valeance fluctuations are reasons for NTE) • Negative TCR in the absence of Composition/chemical disordered. Not common for crystalline solid. • AFM in isotropic metallic systems • Max TN ~ 7.5 K for GdPd3 from LaPd3 to LuPd3

  29. Resistivity • R(T) linear in high temperatures • Temperature coefficient of resistivity (TCR) changes linearly with boron concentration (x) • Changes sign at x >0.5 • Anomaly below 40K, TCR: α(T) = 1/ρ(T)[dρ(T)/dT].

  30. Observation and discussion Observation and discussion

  31. ( T * ) Resistivity in magnetic field GdPd3B0.75C0.25 Note: (T*)- at ~45K in all samples- orgin? No Change in structure, No change in Magnetisation. No Change with Magnetic field in ρ(T) at 7 Tesala.

  32. Resistivity variation – Chemical- compositional disordered- A comparison Resistivity – chemical-compositional disorderd GdPd3 B 0.50 – compositionally is disordered ( half sites free) negative TCR GdPd3B 0.50C 0.50 – chemically disordered ( B and C 50% occupied)- positive TCR. Chemical disorder is not responsible for negative TCR. Note- Change in lattice parameter is high. By adding carbon- one more electron- more conducting – consistency with band structure GdPd3B 0.75 - compositionally disordered - negative TCR GdPd3B 0.75C 0.25 – chemically disordered- negative TCR. Here lattice parameter is same. So composition/chemical disordered is not a criteria for negative TCR. GdPd3B – negative TCR

  33. Effective mass concept- a proposal The local distortions of the lattice around the electrons make the electron motion slower and increase the electronic effective mass, meff. the introduction of B causes extra negative pressure – lattice expand. strain generated will effect the effective mass of the electrons in the lattice. electronic structure calculations also indicate the increment of meff with increasing B stoichiometry. we consider normal electron-phonon dynamics to be valid as system is crystalline and ρ(T)- linear ν = 1/τ Scattering frequency increases with T(K) for Crystalline solid. B0.75C0.25 For alkaline metals Na, k, Rb the ratio is 1.2 - 1.4 ~ 1.12 GdPd3B m0, ν0 are the lowest point of measurement of meff, ν

  34. Electronic Structure GdPd3 Nearly full Pd-d bands and nearly empty Gd-d bands (hybridized), resulting metallic behaviour. Inclusion of B  expands the lattice, but the band structure changes because of change of hybridization character. Partially filled B-p strongly hybridizes with Pd-d (small effect on Gd-d for large seperation)  responsible for hump-dip-hump behaviour Bars indicate the dominant band characters.

  35. Replacing B by C  C-p is closer to Pd-d; stronger hybridization  C-Pd bond more directional  one more C-electron; EF goes up All 3 compounds should be metallic For B(0.25), B(0.50)- Fermi level has crossed the dip- more available states - Positive TCR For B(0.75), B - Fermi level is in downhill way-less number of available states- so negative TCR- consistency with expt.

  36. Results – band structure More pronounced hump-dip structure in GdPd3C GdPd3 and GdPd3C  highly dispersive band at EF  high mobility and low effective mass of carrier  low resistivity GdPd3B  flat band at EF  low mobility and high effective mass  higher resistivity GdPd3B1-xCx  systematic effect of band filling and evolution of hump-dip hump structure with increasing boron concentration Positive TCR more available states and negative TCR less available states depending upon Fermi level at the “dip”

  37. Possible therotical approach -Perturbation theory– AFM stability of strongly correlated f electron system (M.Isoda etal J. Mag. Magn. Mater. 140-144 1385 (1995)) • Collection of singley occupied f – levels as a zeroth order – unperturbed state. • Effect of hybridization and exchange interaction pertubatively • Exchange coupling constant J between f and conduction electron – Positive • Hybridization Vij between localized f electrons and conduction (c-f ) mixing • In perturbation theory - • term O( V2 J)- FM coupling between f electron spins • term O( V4 )- AFM coupling • V is small FM ground state: • V increases AFM state is favoured • J ‘ (>0) nearest – neighbour AFM coupling between f spins

  38. The condition for metallic state is given J> ( J’ + 4Δ) (J’ + 4Δ + 4W) ( square root to be given) Where f levels with an energy εF lie within energy gap below the bottom of conduction band by an amount Δ , W energy dimensional quantities. Metallic AFM stabilizes for J’one order smaller than J Perturbation result shows even for optional value of J, i.e c-f mixing smaller than J contributes stability of metallic AFM. Positive pressure dependence of TN due to increase of c-f mixing by presseure

  39. Magnetism work on REPd3 - highest TN – exampleNpPd3 -PuPd3 Nelis etal PRB 9 1041 (1974) • NpPd3 - 4.095A - T N ~55K PuPd3 – 4.105A - T N ~24K • Magnetic transition is first order in NpPd3 • No sharp change in resistivity ρ(T) data at TN for PuPd3 unlike in NpPd3 • For cubic compound NpPd3 exchange interaction > crystal field effect. • Magnetic structure that consists of FM (111) palne coupled AFM cally . A large energy gap delta in the spin wave dispersion relation is observed -40K in resistivity data with Δ ~ 3k , TN ~ k ( Θ) ~ k(150K): Θ Debye temp. 150K from resistivity data.

  40. Half metal

  41. Electronic structure FCC co-ordination of Pd environment  t2g (xy; xz; yz) is above eg (x2 - y2, 3z2 - r2) 4 Gd+ 8 Pd environment  xy > xz+yz > x2 - y2 > 3z2-r2 Last 3 (including xz+yz) are nearly degenerate. Addition of B and C does not change the scheme signifcantly

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