pressure dependance of tc for tbni2mn
TRANSCRIPT
Pressure Dependance of the Curie temperature of TbNi2Mn,
Investigated Using Designer Diamond Anvils
Damon D JacksonScott K McCall
Sam T WeirLawrence Livermore Nat’l Lab
Wei QiuYogesh K. VohraUniv. Alabama, Birmingham
Dave P YoungLouisiana State UniversityUCRL-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.
Wei QiuYogesh K. VohraUniv. Alabama, Birmingham
Pressure Dependance of the Curie temperature of TbNi2Mn,
Investigated Using Designer Diamond Anvils
Damon D JacksonScott K McCall
Sam T WeirLawrence Livermore Nat’l Lab
Dave P YoungLouisiana State UniversityUCRL-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.
3d - 4f magnetic interactions
• Cubic laves structure (RT2) is useful for magnetic investigations
• T-T are nearest neighbors
• T-T distances similar to elemental T
• Vary band width or Fermi level by changing R
• R’s interact through RKKY super-exchange
Cubic Laves StructureMgCu2-type
R site - (green) diamond lattice
T site - (red) tetrahedra around R
RNi2Mn is Cubic Laves!
• Magnetization shows:
• FM phase
• ‘kink’ at lower temperatures
• Origin of low-T anomaly is unknown
Compounds for Comparison: RMn2
• Mn-Mn distance controls local moment formation
• local moment for light R (large ion)
• itinerant for heavy R (small ion)
• Goncharenko et al. investigated GdMn2 under pressure with neutrons
• destruction of Mn moment
• destruction of AFM
• stabilize Gd moment and FM
GdMnGdMn22 Phase Diagram Phase Diagram
Compounds for Comparison: RNi2
• Ni moment close to zero (0.16µB for GdNi2)
• TbNi2: Tc = 36 K
• Spin re-orientation at TR = 14 K
• subtle transition with 3 of 5 spins moving to to ⊥[111] direction
• Vacancies exist at Tb site, and vacancies order at sufficient T or P
TbNiTbNi22 Properties Properties
Motivation for TbNi2Mn
• Wang found 2 magnetic anomalies in RNi2Mn
• In GdMn2, 2 anomalies due to ordering of Mn and then Gd moments
• In TbNi2, anomalies due to Tb ordering, and spin reorientation
GdMn2
TbNi2
What are the anomalies What are the anomalies in TbNiin TbNi22Mn?Mn?
Magnetization of Magnetization of TbNiTbNi22MnMn
RNi2Mn - Cubic Laves FM
Wang et al. (PRB, 73, 094436, 2006) discovered a new cubic Laves material, RNi2Mn
• (R1-2xx yMn2x-y) (Ni1-xMnx)2
• for R=Tb, x=0.13, y=0.04
• RKKY mechanism (linear TC vs (g-1)2J(J+1)
• TC = 131 K (TbNi2 TC = 37 K)
Narrow domain walls cause large differences between ZFC and FC measurements
TTCC vs de Gennes vs de Gennes
TC = 151 K
ZFC MagnetizationZFC Magnetization
50 G
100 G
200 G
Magnetization, 400 GMagnetization, 400 G
FC
ZFC
Feature at T*≈37 K
• Local maximum in ZFC magnetization near 37 K
FC magnetization flattens out
No dependance on H
Observable for H ≤ 500 G
CoercivityCoercivityMagnetic Field Dependancies
• Coercivity decreases with temperature (as expected)
T* remains relatively constant
T*T* Anomaly Anomaly
Specific HeatSpecific HeatSpecific Heat of TbNi2Mn
• Anomaly at TC is much smaller than expected for J=6
entropy is gradually removed
No specific heat anomaly at T*
• γ = 65 mJ/mol K2
TbNi2: γ = 5 mJ/mol K2
TbMn2: γ = 44 mJ/mol K2
• large electron correlations
Investigate High Pressure Magnetic Phase Diagram
• Interested in effect of pressure on TC and T*
• Use Designer Diamond Anvils to measure χAC
Designer Diamond Anvils
• lithographically fabricated thin-film tungsten microprobes
• completely encased within epitaxial diamond
• embedded leads provide electrical insulation so that metal gaskets can still be used
• diamond-encapsulated probes remain functional to multi-Mbar pressures60-250 µm
4-12 µm
• Lithographic Fabrication of Microprobes
• laser pantography (electrical pads) and projection lithography (diamond flat)
• linewidths down to 1 µm
• Epitaxial Diamond Deposition
• Univ. of Alabama CVD process
• diamond film is typically 10-50 µm thick
• Final Polishing and Completion
• microprobes are now completely encapsulated in diamond, except for the exposed ends.
Designer Diamond Anvil Fabrication
300 µm
Representative Results
• Strong peak at TC
• Small peak at T*
• Performed both with and without a pressure medium (MEW)
MEW
AC SusceptibilityAC Susceptibility
no pressuremedium
Magnetic Phase Diagram
• T* has no P dependence
• TC is reduced with P
• dTC/dP = -1.96 K/GPa
• non-hydrostatic conditions flatten out TC(P)
• flattened regions are irreversible upon downloading
Magnetic Phase DiagramMagnetic Phase Diagram
P-mediumNo P-medium
Download
Magnetic Phase DiagramMagnetic Phase DiagramMagnetization of Magnetization of TbNiTbNi22MnMn
T*T* Anomaly Anomaly Conclusions
• TbNi2Mn forms in the cubic Laves phase
• Ferromagnetic at TC = 151 K
• Possible spin reorientation at T* = 37 K
• no evidence of Ni/Mn ordering
• dT*/dP ≈ 0 K/GPa
• dTC/dP = -1.96 K/GPa
Cubic Laves Cubic Laves Crystal Crystal
StructureStructure
(R1-2xx yMn2x-y)(Ni1-xMnx)2
Magnetic Phase DiagramMagnetic Phase DiagramQuestions Remain...
• What is the nature of T*?
• no specific heat anomaly
• no pressure dependence
• Why does the Curie temperature initially remain constant under non-hydrostatic conditions?
• This behavior is irreversible
•10-turn pick-up coil•coil lines are 5 µm wide and 0.35
µm thick•mechanically robust•high signal-to-noise sensitivity
(10-2emu/cm3)•no need for a compensation coil•AC Susceptibility can be
measured up to megabar pressures
•Temperature ranges: 15<T<300K
Designer Diamond Anvils for AC Magnetic Susceptibility
Magnetic Susceptibility Schematic
• Be-Cu DAC
• 50-turn excitation coil
• sensing coil very close to sample
• MP35N gasket is used due to its:
• temperature independent resistivity (RRR=9)
• hardness (yield strength≈20 kbar)
• low magnetization
• Designer diamond anvils are well suited for high sensitivity AC Magnetic Susceptibility
• AC Susceptibility can be measured up to megabar pressures
• Temperature ranges: 15<T<300K
Designer Diamond Anvils for AC Magnetic Susceptibility
Conventional Magnetic Susceptibility with a DAC
T. Timofeev, et al., Fizika I Tekhnika Vysokikh Davlenii, 16, 15 (1984)
low sensitivitylow signal to noise
Magnetic Susceptibility with Designer Diamond Anvils
• χac
experiments are difficult because
magnetic field decreases as 1/r3
• 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 (10-2emu/cm3)• no need for a compensation coil
Multiloop Designer Anvil Fabrication
Lithographic Fabrication Diamond Encapsulation
Diamond Polishing(reflected light)
Completed Anvil(transmitted light)
• Signal source is used to drive the excitation coil
• Lock-In is referenced to the signal source and measures voltage from microloop sensing coil
• Cryostat is used to cool DAC down to ≈15 K
• Automated kinematic fiber optic system to measure pressure as a function of temperature
Experimental Setup