Pressure Dependance of Tc for TbNi2Mn

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

Compounds for Comparison: TbMn2

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