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IS487 Local Probe Studies in multiferroic Ag Cr O 2

IS487 Local Probe Studies in multiferroic Ag Cr O 2. A.M.L.Lopes, G. Oliveira Centro de Física Nuclear da Universidade de Lisboa, Portugal J.P. Araujo, T.M.Mendonça Instituto de Nanociência e Nanotecnologia, Universidade do Porto V.Amaral

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IS487 Local Probe Studies in multiferroic Ag Cr O 2

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  1. IS487 Local Probe Studies in multiferroicAgCrO2 A.M.L.Lopes, G. Oliveira Centro de Física Nuclear da Universidade de Lisboa, Portugal J.P. Araujo, T.M.Mendonça Instituto de Nanociência e Nanotecnologia, Universidade do Porto V.Amaral Departamento de Física e CICECO, Universidade de Aveiro J.G. Correia, Instituto Tecnológico e Nuclear, Portugal and ISOLDE CERN

  2. INDEX The ABO2 delafossite systems Synthesis and sample characterization Synthesis methods, structural, electric and magnetic studies AgCrO2 local probe studies Electrical field Gradient and Magnetic hyperfine field

  3. The Systems

  4. SYSTEM:BUILDING BLOCKS A B O Delafossite ABO2 Building Blocks • Rhombohedral lattice • Triangular A-atom layers • BO2 sandwich layers • B octahedrally coordinated • Linear O–A–O bonds

  5. SYSTEM: Materials and Properties Delafossite ABO2 • Prototype Materials • AgCrO2, CuCrO2, CuFeO2 • PdCrO2, PdCoO2, PtCoO2 • CuAlO2, CuGaO2, CuInO2 • Properties • semiconductors, AF interactions • very good metals, highly anisotropic • wide-gap semiconductors, p-type

  6. System:2D (spin frustration) spin frustration - + - +?- + - + A1+Cr3+O2-2(A=Ag, Cu, Li, Na) magnetic structure (and ferroelectrics) spins in a 2D triangular lattice antiferromagnet->spin frustration JNN>Jc <--->Cr-Crintraplane AF magnetic interactions stronger then Cr-O-Ag-O-Cr interplane AF ones ACrO2quasi-2D lattice--> highly frustrated systems At very low temperatures the system may presents a AF magnetic phase transition due to the interplane interactions. Above TN some authors claim that a 2D short range order exists

  7. System: Interests and Issues ACrO2 Known, since some time, due to potential applications as transparent semiconductors But very recently ACrO2 systems gain renew interest due to the discovery of ferroelectricity driven by magnetic ordering Boosting the search for multiferroicity in triangular lattice Revival the interest in geometrically frustrated spin systems • Open new (reopen!) Issues… • microscopic origin of magneto-electric coupling in triangular-lattice antiferromagnet • frustration vs. long-range order • 2D short range order

  8. AgCrO2 Synthesis and sample characterization

  9. Synthesis and sample characterization solid-state reaction different annealing conditions SEM images - sample morphology Homogeneous sample with plateaus/layers due to the highly asymmetric crystallographic structure (c>>a=b) single phase--Structural parameters: a=b= 2.986 and c=18.506 Ǻ X-ray, Rhombohedric str. (R-3m space group) sol-gel combustion synthesis different complexant agents/combustion initiators

  10. Synthesis and sample characterization Magnetization Clear transition @21K -> 3 dimensional magnetic order ->Para to antiferromagnetic phase transition Néel T-> TN=21K Curie-Weiss T-> qp=-150K -> Strong spin frustration as expected in triangular spin lattices |qp|/TN=7 Below ~120K -> c-1 deviates from the Curie-Weiss linear regime!! Real and imaginary dielectric constant Sharp increase in the dielectric constant @ 21K -> ferroelectric transition Ferroelectric transition simultaneously with the magnetic one. Below ~100K ->anomaly in e`

  11. 111In:AgCrO2 g-g PAC measurements

  12. 111In:AgCrO2g-g PAC measurements Ag Cr Ei g1 |Ii, mi L1 111In O m ,Q t E |I, m DE g2 L2 |If, mf Ef Hyperfine splitting Electricfield gradient / Magnetic hyperfine field Vzz- EFG principal component h- Asymmetry parameter Bhf- Magnetic hyperfine field Samples implanted with 111In @ ISOLDE/CERN E=60 keV, Dose < 1012 at/cm2 111In--111Cd: I=5/2 and t1/2= 85 ns Q=0.83 b and m=-0.766 mn Post-implant annealings @ T=700 ºC in oxygen Former studies in this and similar systems had proven that: 111In goes only to the Cr site 111mCd:111Cd 111Ag:111Cd 111In:111Cd 48Cr:48V

  13. 111In:AgCrO2g-g PAC measurements Ag wa1 wa2 wa3 Cr 111In O PAC data @ RT and @ 148K One single EFG corresponding to the Cr site Quadrupole frequency Q=131 MHz Asymmetry parameter ~0 From 500K to ~100k: Only one EFG with almost temperature independent parameters

  14. 111In:AgCrO2g-g PAC measurements wa1 wa2 wa3 wb1 wb2 wb3 From 500K to ~100k only one EFG Below 100k: A second EFG with similar quadrupole frequency but higher asymmetry parameter appears EFG1 --> Q1=131 MHz and 1~0 EFG2--> Q2=137 MHz and 2~0.5 Two Cr local environments Attenuation of both EFG --> D1~1% and D2~6% ~constant in all temperature range MHF is present at and below 21K

  15. 111In:AgCrO2g-g PAC measurements EFG parameters above TN Principal frequency and asymmetry parameters % of each local environment Second Efg: Local distortions? Below 100k a second Efg First order transition?? Temperature(K) % of each local environment varies ~ linearly with temperature (no single activation process involved) Below TN the process halts and the % of each LE ~ 50% This transition occurs at ~ the same temperature where the reciprocal magnetic susceptibility starts deviating from the Curie-Weiss linear regime.

  16. 111In:AgCrO2g-g PAC measurements Cr spin frustration - - + + - - +?- + + no frustration - - + + 111In 111In, non magnetic, in the Cr triangular lattice should locally release the spin frustration. 111In 1º neighbors can realize the antiferromagnetic order. --> No contribution from the 1º Cr to the MHF but ... PAC from 21K down to 12K Below the (antiferromagnetic) phase transition But…a relatively high MHF is observed!!! 294K MHF -->2º Cr neighbors i.e., inter-plane interactions 21K 20K two MHF two different EFG  different Cr-O-Ag-O-Cr pathways? 12K

  17. Discussion In the literature the non linear regime of the reciprocal susceptibility that is systematically observed in ACrO2 systems (Li, Ag, Cu, Pd…) is, up to now, ascribed to the development of 2D spin order. Note that above TNPAC shows: -Pure EFG´s (no MHF) -No significant changes in D1,2 when approach TN No evidences for any kind of spin order LiCrO2, local probe measurements just had shown that: Below 100K/above TN: We see no evidences for spin order and a 2º Cr local environment emerges. Same phenomenology as in LiCrO2? Ag atoms displacements? mSR--> no EFG PAC-- > EFG and MHF

  18. Conclusions -500K to ~100k a single Cr local environment -Below 100k, two Cr local environments (similar frequency but higher asymmetry) -Transition occurs @ the same T where the susceptibility deviate from the Curie-Weiss regime and an anomaly is observed in the dielectric constant -No evidences for any kind of spin order above TN -Anomalies observed in the dielectric and magnetic measurements maybe explained by the existence of local distortions as measured by PAC e.g. Ag atoms displacements - Two MHF observed below TN. MHF arises from inter-plane magnetic interactions

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