Pressure Dependance of the Curie temperature of TbNi 2 Mn, Investigated Using Designer Diamond Anvils Damon D Jackson Scot...
Pressure Dependance of the Curie temperature of TbNi 2 Mn, Investigated Using Designer Diamond Anvils Wei Qiu Yogesh K. Vo...
3 d - 4 f magnetic interactions <ul><li>Cubic laves structure ( RT 2 ) is useful for magnetic investigations </li></ul><...
R Ni 2 Mn is Cubic Laves! <ul><li>Magnetization shows: </li></ul><ul><ul><li>FM phase </li></ul></ul><ul><ul><li>‘ kink’ ...
Compounds for Comparison: R Mn 2 <ul><li>Mn-Mn distance controls local moment formation </li></ul><ul><ul><li>local momen...
Compounds for Comparison: R Ni 2 <ul><li>Ni moment close to zero (0.16µ B for GdNi 2 ) </li></ul><ul><li>TbNi 2 : T c ...
Compounds for Comparison: TbMn 2
Motivation for TbNi 2 Mn <ul><li>Wang found 2 magnetic anomalies in R Ni 2 Mn </li></ul><ul><li>In GdMn 2 , 2 anomalies d...
R Ni 2 Mn - Cubic Laves FM <ul><li>Wang et al. (PRB, 73 , 094436, 2006) discovered a new cubic Laves material, R Ni 2 ...
Feature at T* ≈37 K <ul><li>Local maximum in ZFC magnetization near 37 K </li></ul><ul><ul><li>FC magnetization flattens ...
Magnetic Field Dependancies <ul><li>Coercivity decreases with temperature (as expected) </li></ul><ul><li>T* remains rela...
Specific Heat of TbNi 2 Mn <ul><li>Anomaly at T C is much smaller than expected for J =6 </li></ul><ul><ul><li>entropy ...
Investigate High Pressure Magnetic Phase Diagram <ul><li>Interested in effect of pressure on T C and T* </li></ul><ul><...
Designer Diamond Anvils <ul><li>lithographically fabricated thin-film tungsten microprobes </li></ul><ul><li>completely en...
<ul><li>Lithographic Fabrication of Microprobes </li></ul><ul><ul><li>laser pantography (electrical pads) and projection l...
Representative Results <ul><li>Strong peak at T C </li></ul><ul><li>Small peak at T* </li></ul><ul><li>Performed both wi...
Magnetic Phase Diagram <ul><li>T* has no P dependence </li></ul><ul><li>T C is reduced with P </li></ul><ul><li>d T C /d...
Conclusions <ul><li>TbNi 2 Mn forms in the cubic Laves phase </li></ul><ul><li>Ferromagnetic at T C = 151 K </li></ul><u...
Questions Remain... <ul><li>What is the nature of T* ? </li></ul><ul><ul><li>no specific heat anomaly </li></ul></ul><ul>...
<ul><li>10-turn pick-up coil </li></ul><ul><li>coil lines are 5 µm wide and 0.35 µm thick </li></ul><ul><li>mechanically r...
Magnetic Susceptibility Schematic <ul><li>Be-Cu DAC </li></ul><ul><li>50-turn excitation coil </li></ul><ul><li>sensing co...
<ul><li>Designer diamond anvils are well suited for high sensitivity AC Magnetic Susceptibility </li></ul><ul><li>AC Susce...
Conventional Magnetic Susceptibility with a DAC T. Timofeev, et al., Fizika I Tekhnika Vysokikh Davlenii, 16 , 15 (1984) ...
Magnetic Susceptibility with Designer Diamond Anvils • χ ac experiments are difficult because magnetic field decreases a...
Multiloop Designer Anvil Fabrication Lithographic Fabrication Diamond Encapsulation Diamond Polishing (reflected light) Co...
<ul><li>Signal source is used to drive the excitation coil </li></ul><ul><li>Lock-In is referenced to the signal source an...
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Pressure Dependance of Tc for TbNi2Mn

The Curie temperature of the Cubic Laves material, TbNi2Mn, has been investigated through ambient pressure specific heat and magnetization, as well as high pressure AC magnetic susceptibility. The results show a pressure-independent spin reorientation below Tc, and a Curie temperature decrease of -1.96 K/GPa.
Published on: Mar 4, 2016
Published in: Technology      
Source: www.slideshare.net


Transcripts - Pressure Dependance of Tc for TbNi2Mn

  • 1. Pressure Dependance of the Curie temperature of TbNi 2 Mn, Investigated Using Designer Diamond Anvils Damon D Jackson Scott K McCall Sam T Weir Lawrence Livermore Nat’l Lab Wei Qiu Yogesh K. Vohra Univ. Alabama, Birmingham Dave P Young Louisiana State University UCRL-PRES-228374 ; This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
  • 2. Pressure Dependance of the Curie temperature of TbNi 2 Mn, Investigated Using Designer Diamond Anvils Wei Qiu Yogesh K. Vohra Univ. Alabama, Birmingham Damon D Jackson Scott K McCall Sam T Weir Lawrence Livermore Nat’l Lab Dave P Young Louisiana State University UCRL-PRES-228374 ; This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
  • 3. 3 d - 4 f magnetic interactions <ul><li>Cubic laves structure ( RT 2 ) is useful for magnetic investigations </li></ul><ul><ul><li>T-T are nearest neighbors </li></ul></ul><ul><ul><li>T-T distances similar to elemental T </li></ul></ul><ul><ul><li>Vary band width or Fermi level by changing R </li></ul></ul><ul><li>R ’s interact through RKKY super-exchange </li></ul>Cubic Laves Structure MgCu 2 -type R site - (green) diamond lattice T site - (red) tetrahedra around R
  • 4. R Ni 2 Mn is Cubic Laves! <ul><li>Magnetization shows: </li></ul><ul><ul><li>FM phase </li></ul></ul><ul><ul><li>‘ kink’ at lower temperatures </li></ul></ul><ul><li>Origin of low-T anomaly is unknown </li></ul>
  • 5. Compounds for Comparison: R Mn 2 <ul><li>Mn-Mn distance controls local moment formation </li></ul><ul><ul><li>local moment for light R (large ion) </li></ul></ul><ul><ul><li>itinerant for heavy R (small ion) </li></ul></ul><ul><li>Goncharenko et al. investigated GdMn 2 under pressure with neutrons </li></ul><ul><ul><li>destruction of Mn moment </li></ul></ul><ul><ul><li>destruction of AFM </li></ul></ul><ul><ul><li>stabilize Gd moment and FM </li></ul></ul>GdMn 2 Phase Diagram
  • 6. Compounds for Comparison: R Ni 2 <ul><li>Ni moment close to zero (0.16µ B for GdNi 2 ) </li></ul><ul><li>TbNi 2 : T c = 36 K </li></ul><ul><li>Spin re-orientation at T R = 14 K </li></ul><ul><ul><li>subtle transition with 3 of 5 spins moving to ⊥ to [111] direction </li></ul></ul><ul><li>Vacancies exist at Tb site, and vacancies order at sufficient T or P </li></ul>TbNi 2 Properties
  • 7. Compounds for Comparison: TbMn 2
  • 8. Motivation for TbNi 2 Mn <ul><li>Wang found 2 magnetic anomalies in R Ni 2 Mn </li></ul><ul><li>In GdMn 2 , 2 anomalies due to ordering of Mn and then Gd moments </li></ul><ul><li>In TbNi 2 , anomalies due to Tb ordering, and spin reorientation </li></ul>GdMn 2 TbNi 2 What are the anomalies in TbNi 2 Mn?
  • 9. R Ni 2 Mn - Cubic Laves FM <ul><li>Wang et al. (PRB, 73 , 094436, 2006) discovered a new cubic Laves material, R Ni 2 Mn </li></ul><ul><ul><li>( R 1-2 x ◻ y Mn 2 x - y ) (Ni 1- x Mn x ) 2 </li></ul></ul><ul><ul><li>for R =Tb, x=0.13, y=0.04 </li></ul></ul><ul><li>RKKY mechanism (linear T C vs ( g -1) 2 J ( J +1) </li></ul><ul><li>T C = 131 K (TbNi 2 T C = 37 K) </li></ul><ul><li>Narrow domain walls cause large differences between ZFC and FC measurements </li></ul>T C = 151 K Magnetization of TbNi 2 Mn T C vs de Gennes
  • 10. Feature at T* ≈37 K <ul><li>Local maximum in ZFC magnetization near 37 K </li></ul><ul><ul><li>FC magnetization flattens out </li></ul></ul><ul><li>No dependance on H </li></ul><ul><li>Observable for H ≤ 500 G </li></ul>ZFC Magnetization 50 G 100 G 200 G Magnetization, 400 G FC ZFC
  • 11. Magnetic Field Dependancies <ul><li>Coercivity decreases with temperature (as expected) </li></ul><ul><li>T* remains relatively constant </li></ul>Coercivity T* Anomaly
  • 12. Specific Heat of TbNi 2 Mn <ul><li>Anomaly at T C is much smaller than expected for J =6 </li></ul><ul><ul><li>entropy is gradually removed </li></ul></ul><ul><li>No specific heat anomaly at T* </li></ul><ul><li>γ = 65 mJ/mol K 2 </li></ul><ul><ul><li>TbNi 2 : γ = 5 mJ/mol K 2 </li></ul></ul><ul><ul><li>TbMn 2 : γ = 44 mJ/mol K 2 </li></ul></ul><ul><li>large electron correlations </li></ul>Specific Heat
  • 13. Investigate High Pressure Magnetic Phase Diagram <ul><li>Interested in effect of pressure on T C and T* </li></ul><ul><li>Use Designer Diamond Anvils to measure χ AC </li></ul>
  • 14. Designer Diamond Anvils <ul><li>lithographically fabricated thin-film tungsten microprobes </li></ul><ul><li>completely encased within epitaxial diamond </li></ul><ul><li>embedded leads provide electrical insulation so that metal gaskets can still be used </li></ul><ul><li>diamond-encapsulated probes remain functional to multi-Mbar pressures </li></ul>60-250 µm 4-12 µm
  • 15. <ul><li>Lithographic Fabrication of Microprobes </li></ul><ul><ul><li>laser pantography (electrical pads) and projection lithography (diamond flat) </li></ul></ul><ul><ul><li>linewidths down to 1 µm </li></ul></ul><ul><li>Epitaxial Diamond Deposition </li></ul><ul><ul><li>Univ. of Alabama CVD process </li></ul></ul><ul><ul><li>diamond film is typically 10-50 µm thick </li></ul></ul><ul><li>Final Polishing and Completion </li></ul><ul><ul><li>microprobes are now completely encapsulated in diamond, except for the exposed ends. </li></ul></ul>Designer Diamond Anvil Fabrication 300 µm
  • 16. Representative Results <ul><li>Strong peak at T C </li></ul><ul><li>Small peak at T* </li></ul><ul><li>Performed both with and without a pressure medium (MEW) </li></ul>MEW AC Susceptibility no pressure medium
  • 17. Magnetic Phase Diagram <ul><li>T* has no P dependence </li></ul><ul><li>T C is reduced with P </li></ul><ul><li>d T C /d P = -1.96 K/GPa </li></ul><ul><li>non-hydrostatic conditions flatten out T C ( P ) </li></ul><ul><li>flattened regions are irreversible upon downloading </li></ul>P-medium No P-medium Download Magnetic Phase Diagram
  • 18. Conclusions <ul><li>TbNi 2 Mn forms in the cubic Laves phase </li></ul><ul><li>Ferromagnetic at T C = 151 K </li></ul><ul><li>Possible spin reorientation at T* = 37 K </li></ul><ul><ul><li>no evidence of Ni/Mn ordering </li></ul></ul><ul><li>d T* /d P ≈ 0 K/GPa </li></ul><ul><li>d T C /d P = -1.96 K/GPa </li></ul>Magnetic Phase Diagram Magnetization of TbNi 2 Mn T* Anomaly Cubic Laves Crystal Structure ( R 1-2 x ◻ y Mn 2 x - y )(Ni 1- x Mn x ) 2
  • 19. Questions Remain... <ul><li>What is the nature of T* ? </li></ul><ul><ul><li>no specific heat anomaly </li></ul></ul><ul><ul><li>no pressure dependence </li></ul></ul><ul><li>Why does the Curie temperature initially remain constant under non-hydrostatic conditions? </li></ul><ul><ul><li>This behavior is irreversible </li></ul></ul>Magnetic Phase Diagram
  • 20. <ul><li>10-turn pick-up coil </li></ul><ul><li>coil lines are 5 µm wide and 0.35 µm thick </li></ul><ul><li>mechanically robust </li></ul><ul><li>high signal-to-noise sensitivity (10 -2 emu/cm 3 ) </li></ul><ul><li>no need for a compensation coil </li></ul><ul><li>AC Susceptibility can be measured up to megabar pressures </li></ul><ul><li>Temperature ranges: 15< T <300K </li></ul>Designer Diamond Anvils for AC Magnetic Susceptibility
  • 21. Magnetic Susceptibility Schematic <ul><li>Be-Cu DAC </li></ul><ul><li>50-turn excitation coil </li></ul><ul><li>sensing coil very close to sample </li></ul><ul><li>MP35N gasket is used due to its: </li></ul><ul><ul><li>temperature independent resistivity (RRR=9) </li></ul></ul><ul><ul><li>hardness (yield strength≈20 kbar) </li></ul></ul><ul><ul><li>low magnetization </li></ul></ul>
  • 22. <ul><li>Designer diamond anvils are well suited for high sensitivity AC Magnetic Susceptibility </li></ul><ul><li>AC Susceptibility can be measured up to megabar pressures </li></ul><ul><li>Temperature ranges: 15< T <300K </li></ul>Designer Diamond Anvils for AC Magnetic Susceptibility
  • 23. Conventional Magnetic Susceptibility with a DAC T. Timofeev, et al., Fizika I Tekhnika Vysokikh Davlenii, 16 , 15 (1984) low sensitivity low signal to noise
  • 24. Magnetic Susceptibility with Designer Diamond Anvils • χ ac experiments are difficult because magnetic field decreases as 1 /r 3 • Using Designer Diamond Anvils, “filling factor” is drastically increased due to embedded microloops • 10-turn pick-up coil • coil lines are 5 µm wide and 0.35 µm thick • mechanically robust • high signal-to-noise sensitivity (10 -2 emu/cm 3 ) • no need for a compensation coil
  • 25. Multiloop Designer Anvil Fabrication Lithographic Fabrication Diamond Encapsulation Diamond Polishing (reflected light) Completed Anvil (transmitted light)
  • 26. <ul><li>Signal source is used to drive the excitation coil </li></ul><ul><li>Lock-In is referenced to the signal source and measures voltage from microloop sensing coil </li></ul><ul><li>Cryostat is used to cool DAC down to ≈15 K </li></ul><ul><li>Automated kinematic fiber optic system to measure pressure as a function of temperature </li></ul>Experimental Setup

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