Magnetism, Electronic Transport and Magneto-Structural Coupling in Sr - PowerPoint PPT Presentation

Magnetism, Electronic Transport and Magneto-Structural Coupling in Sr 2 IrO 4 Ashim Kr. Pramanik School of Physical Sciences Jawaharlal Nehru University JNU, Frustrated Magnetism, Feb 13, 2015

Our ¡Group ¡at ¡JNU ¡ q Work in experimental condensed matter physics. q Most work in low temperature regime. q Do prepare materials, characterize them and study physical properties. q At present group of 7 people: 3 Ph.D students and 3 M.Sc students. q Transition metal oxides (4d & 5d) based materials.

Sr 2 IrO 4 ¡(214) ¡: ¡Overview ¡ q Sr 2 2+ Ir 4+ O 4 2- is 5 d based Transition Metal Oxide q Electronic configuration: Ir 4+ 4 ƒ 14 5 d 5 q Significant Crystal Field effect Low spin state e g t 2g q Half filled t 2g band. q Extended nature of 5 d orbitals. q Reduced electronic correlation effect than 3 d and 4 d counterpart. q Therefore, 5d oxides expected to be metallic. q Heavy character of Ir (77) atom. q Significant spin-orbit coupling (SOC) effect ( ∝ z 4 , atomic number) q Comparable energy scale of Coulomb interaction and SOC ∼ 0.5 eV

Sr 2 IrO 4 (214) : Structural Overview Importance ¡of ¡Structure ¡ q Member of Ruddlesden-Popper series Sr n+1 Ir n O 3n+1 with n = 1 . θ Oct ¡ q Layered (K 2 NiF 4 ) structure. q Crystalizes in tetragonal structure Space group : I4 1 /acd . q Alternate tilting of IrO 6 octahedra along c -axis ( θ Oct ∼ 11 o ) . q Iso-structural with La 2 CuO 4 and Sr 2 RuO 4 q Possible superconductivity !!! Wang, PRL, 106, 136402 (2011) Yang, PRB 89, 094518 (2014)

Sr 2 IrO 4 (214) : Magnetic Overview Ye, PRB, 87, 140406, (2013) Cao, PRB, 57, 11039, (1998) q Canted Antiferromagnet with T N ∼ 240 K. q Structural distortion induced Dzyaloshinsky-Moriya (DM) interaction. q Weak ferromagnetism with much lower moment than spin-only value (1 µ B /f.u) for S = ½

Sr 2 IrO 4 (214) : Electronic Transport Sr 2 IrO 4 Korneta, PRB, 82, 115117 (2010) Sr 2 RhO 4 Perry, JPCM, 8, 175 (2006)

Sr 2 IrO 4 (214) : Insulating behaviour J eff Mott Insulator : Electronic correlation driven q Interplay between SOC, W and U gives novel ground state. q SOC splits t 2g band onto J eff = 1/2 and J eff = 3/2 band. q J eff = 1/2 band is half-filled and narrow. q Moderate U can lead to Mott gap. B. J. Kim, PRL 101, 076402 (2008); Science, 323, 1329 (2009)

Sr 2 IrO 4 (214) : Insulating behaviour Slater Insulator : Magnetic Order driven Time resolved optical study STM/STS investigation. Hsieh, PRB, 86, 035128(2012) Li, Scientific Reports, 3, 3073 (2013)

Sr 2 IrO 4 (214) : Material Synthesis q Single-phase polycrystalline material prepared using solid state method. q Materials are characterized using XRD and allied Rietveld analysis. q Sample crystallizes in tetragonal structure with I4/acd symmetry. q Lattice parameters; a = 5.4980(2) Å and c = 25.779(1) Å. 1.5 T = 298 K 3 a. u.) Observed 1.0 Calculated Difference Intensity (10 0.5 0.0 10 20 30 40 50 60 70 80 90 2 θ (Deg) Bhatti, AKP , JPCM, 27, 016005 (2014)

Sr 2 IrO 4 (214) : Magnetic Properties 0.06 q Weak FM transition around 238 K (dM/dT). T = 5 K 0.04 q Steep decrease in M ZFC (T) below ∼ 95 K. M ( µ B /f.u) 0.02 0.00 q Curie-Weiss behaviour in limited temperature. -0.02 -0.04 q Estimated θ P = 233 K and µ eff = 0.56 µ B /f.u. -0.06 -80 -60 -40 -20 0 20 40 60 80 q Expected µ eff = 0.57 and µ H = 0.33 µ B /f.u ( g J = 2/3) H (kOe) 3 200 -3 T = 5 K × 10 H = 10 kOe 2 2 ( µ B /f.u) 2 ZFC 150 FC M (emu/mole) 1 Μ 1.5 100 1.0 3 ) 0 -1 (10 0 2 4 6 8 10 12 14 0.5 50 5 Oe f.u µ B -1 ) χ H/M (10 0.0 150 200 250 300 T (K) 0 0 50 100 150 200 250 300 T (K) Ge, ¡PRB, ¡84, ¡100402 ¡(2011). ¡

Sr 2 IrO 4 (214) : Temperature Dependent Structure q Representative XRD data in PM, FM and low temperature state. q No structural phase transition down to 20 K. 2.0 3 a.u.) 1.5 Intensity (10 1.0 298 K 0.5 200 K 20 K 0.0 10 20 30 40 50 60 70 80 90 2 θ (Deg)

Sr 2 IrO 4 (214) : Temperature Dependent Structure Temperature dependent changes in structural parameters. 5.500 160 (a) T M (a) T c T M T c <Ir-O2-Ir> (Deg) 5.495 159 0 ) 5.490 a (A 5.485 158 Unit Cell IrO 6 Octahedra 5.480 25.795 11.5 (b) (b) 25.790 θ Oct (Deg) 11.0 0 ) 25.785 c (A 10.5 25.780 25.775 10.0 (c) 4.705 (c) 1.980 0 ) d Ir-O2 (A 4.700 c/a 1.975 4.695 4.690 1.970 2.14 780 (d) 2.12 03 ) 0 ) 778 (d) d Ir-O1 (A V (A 2.10 2.08 776 2.06 774 0 50 100 150 200 250 300 0 50 100 150 200 250 300 T (K) T (K)

Sr 2 IrO 4 (214) : Prediction for Magneto-Structural correlation q Magnetism is linked to IrO 6 octahedra distorsion. q α - IrO 6 octahedra distortion angel. q Φ - spin canting angle. q θ - tetragonal distortion parameter. Jackeli, PRL, 102, 017205 (2009)

Sr 2 IrO 4 (214) : Electronic Transport q Resistivity shows insulating behaviour (d ρ /dT < 0). q Resistivity increases by five orders at low temperature. q Electronic transport can be understood using 2D Mott variable range hopping model. 5 10 10 1/3 ρ = ρ 0 exp(T 0 /T) 4 10 8 ρ (Ohm-cm) 3 10 6 ln ρ 2 10 4 T C 1 10 2 T M 0 10 0 0 50 100 150 200 250 300 0.2 0.3 0.4 0.5 0.6 T (K) -1/3 (K -1/3 ) T

Sr 2 IrO 4 (214) : Magneto-transport ( H ) ( 0 ) Δ ρ ρ − ρ Magnetoresistance (MR) = = ( 0 ) ( 0 ) ρ ρ q Positive MR (weak antilocalization) in strong SOC systems, Bi 2 Se 3 , Bi 2 Te 3 , Na 2 IrO 3 films. q Negative MR at low T in VRH regime – weak localization – quantum interference effect – Quadratic field dependence. 5 K 0.00 0.00 150 K 180 K ρ (H) - ρ (0) / ρ (0) -0.01 -0.01 ρ (H) - ρ (0) / ρ (0) -0.02 -0.02 -0.03 -0.03 -0.04 -0.04 -0.05 -0.05 0 1 2 3 4 5 6 7 0 10 20 30 40 50 60 70 80 2 (10 8 Oe 2 ) H H (kOe)

Sr 2 IrO 4 (214) : Critical Analysis q Magnetic isotherms across FM transition 0.4 Δ T = 2 K 0.4 T = 218 K 0.3 0.3 -1 ) -1 ) M (emu g M (emu g 0.2 0.2 PM FM T = 238 K 0.1 0.1 2.0 -1 ) 0.0 -1 Oe 0.0 0 50 100 150 200 250 300 0 10 20 30 40 50 1.5 226 K H (kOe) T (K) dM/dH (emu g 1.0 0.5 0.0 0 10 20 30 40 50 H (kOe)

Sr 2 IrO 4 (214) : Critical Analysis 0.09 3D Ising Model 3D Heisenberg Model 0.06 β = 0.325 1/ β β = 0.365 1/ β -1 ) γ = 1.24 0.06 -1 ) γ = 1.386 1/ β (emu g 1/ β (emu g 0.04 0.03 0.02 M M 0.00 0.00 0.0 0.2 0.4 0.6 0.0 0.5 1.0 1.5 1/ γ (10 4 Oe emu -1 g) 1/ γ (H/M) 1/ γ (10 4 Oe emu -1 g) 1/ γ (H/M) 0.16 Mean Field Model β = 0.5 β 0.12 -1 ) γ = 1.0 β (emu g 0.08 M 0.04 0.00 0 2 4 6 8 10 12 14 16 1/ γ (10 4 Oe emu -1 g) 1/ γ (H/M)

Sr 2 IrO 4 (214) : Critical Analysis Modified ¡ArroA ¡plot: ¡ 0.20 β = 0.55 0.16 γ = 1.15 1/ β -1 ) 0.12 1/ β (emu g 0.08 M 0.04 0.00 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 1/ γ (10 4 Oe emu -1 g) 1/ γ (H/M)

Sr 2 IrO 4 (214) : Critical Analysis Critical Plot Kouvel-Fisher Plot 8 0 0.12 12 -1 ) T C = 225 K -1 (K) -1 (K) T C 4 Oe emu g -3 10 6 0.10 β = 0.5517(1) -1 ) 8 M S (emu g -6 -1 /dT) M S (dM S /dT) 0.08 4 6 -9 -1 (d χ 0 − 1 (10 4 0.06 2 T c = 225 K T c = 225 K T C = 225 K -12 T C 2 β = 0.57(2) χ 0 γ = 1.147(1) 0.04 χ 0 γ = 1.14(3) 0 0 -15 216 220 224 228 232 236 240 216 220 224 228 232 236 240 T (K) T (K) 0.4 T = 226 K 0.3 -1 ) M (emu g Critical isotherm: M ∝ H 1/ δ -1.8 δ = 3.090(2) 0.2 -2.0 Log(M) -2.2 0.1 Log(H) -2.4 3.0 3.5 4.0 4.5 5.0 0.0 0 10 20 30 40 50 H (kOe)

Sr 2 IrO 4 (214) : Critical Analysis 8 218 K Scaling ¡analysis ¡ 220 K 6 222 K -1 ) 10 - β (emu g 224 K -1 ) 4 - β (emu g 226 K 1 M| ε | 228 K M| ε | 2 230 K -( γ + β ) (10 6 Oe) H| ε | 232 K 0.1 0.01 0.1 1 10 0 0 15 30 45 60 -( γ + β ) (10 6 Oe) H| ε | Exponents ¡ β ¡ γ ¡ δ ¡ This ¡Work ¡ 0.55 ¡ 1.15 ¡ 3.03 ¡ Mean ¡Field ¡ 0.5 ¡ 1.0 ¡ 3.0 ¡ 3D ¡Heisenberg ¡ 0.365 ¡ 1.386 ¡ 4.8 ¡ 3D ¡Ising ¡ 0.325 ¡ 1.241 ¡ 4.82 ¡

Sr 2 IrO 4 (214) : Thermal Demagnetization Thermal Demagnetization Δ M : M M ( T ) M ( 0 ) Δ − 3 / 2 Spin-wave (SW) excitation (Bloch) : BT = = M ( 0 ) M ( 0 ) M M ( T ) M ( 0 ) Δ − Δ 3 / 2 CT exp( ) Stoner single-particle (SP) excitation: = = − M ( 0 ) M ( 0 ) k T B Δ M Total = Δ M SW + Δ M SP

Sr 2 IrO 4 (214) : Magnetocaloric Effect (MCE) ( ) M T , H δ ⎛ ⎞ H Magnetic Entropy change: Δ S M (T,H) = S M (T,H) – S M (T,0) = dH ∫ ⎜ ⎟ 0 T δ ⎝ ⎠ H Relative cooling power: Δ S M (T,H) × δ T FWHM 0.8 6.0 7 T 5 K - 300 K 5 T 0.6 -1 ) 4.5 3 T -1 K 2 T M (emu/g) -2 Jkg 3.0 1 T 0.4 - Δ S (10 1.5 0.2 0.0 0.0 -1.5 0 20 40 60 80 0 50 100 150 200 250 300 T (K) H (kOe)

Sr 2 IrO 4 (214) : Field dependence of MCE Nice linear field dependence For both Δ S M and RCP . 0.060 Experimental data Linear dependence of -1 ) Linear fit of data 0.045 -1 K Δ S M and (H/T c ) 2/3 shows mean - Δ S M (J kg 0.030 Field nature. 0.015 0.000 0.02 0.04 0.06 0.08 0.10 2/3 (H/T C )