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Scuola Di dottorato in fisica , astrofisica e fisica applicate. UNIVERSITÀ DEGLI STUDI DI MILANO. Magnetic Properties and spin dynamics in Antiferromagnetic Molecular RingS by 1 H NMR. Fatemeh Adelnia Universita’ degli studi di Milano Experiments performed at :

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slide1

Scuola Di dottorato in fisica, astrofisica e fisicaapplicate

UNIVERSITÀ DEGLI STUDI DI MILANO

Magnetic Properties and spin dynamics in Antiferromagnetic Molecular RingSby 1H NMR

Fatemeh Adelnia

Universita’ degli studi di Milano

Experimentsperformed at :

UNIVERSITA’ DEGLI STUDI DI PAVIA

slide2

Presentation outline

Molecularnanomagnetsasmilestonesfor the studyoflow-dimensional

magnetism: fundamental physics and applications

Wide-band solid-state NMR at a glance

Molecular spin dynamics vs temperature

Low temperature quantum level crossing

slide3

Molecular Nano Magnets (MNMs)

Promising candidates to study fundamental phenomena in physics

Quantum tunnelling of magnetization

Quantum information processing

Finite size effects in spin “chains”

slide4

Molecular Nano Magnets Applications

Possible applications of MNMs :

High density magnetic memory

Magneto- optical recording

Quantum computing

Spintronics

Magnetic sensors…

slide5

Antiferromagnetic (AFM) rings

Why Antiferromagnetic (AFM) rings?

As allmolecularclusters, studying bulk means

studying single moleculeasJinter-mol << Jintra-mol

Put just the equation.

“B” is at subscript

As allmolecularclusters, finite numberofions :

accurate spin Hamiltonian and exact

calculation of energy levels and eigenfunctions

Highly symmetric geometry

Ideal physical framework for low dimensional magnetism ( 0-D and/or 1-D)

slide6

Antiferromagnetic open rings: the Cr8Zn case

S=0

Finite size system Reduced number of spins

Discrete energy levels structure

Quantum phenomena

, S=3/2

  • Spin topology of a Quasi-Zero-Dimensional magnetic system......
  • “Open” molecularring : peculiarspindynamics
  • Interesting quantum behaviors due to “real” or anti- level crossing
slide7

Nuclear Magnetic Resonance (NMR) as a local probe

By NMR

we are measuring the response of nuclei but, through it, we are studying the physical properties of the whole system (electrons, nuclei & phonons)

: Spin-lattice relaxation rate

How is it possible ?

T1n

Nuclei

T1n

Nuclei are a local probe

But

in interaction with the whole system

: Spin-Spin relaxation rate

electron

phonon

NMR

absorption spectra

T1e

slide8

Nuclear Magnetic Resonance (NMR) : different local probes

Advancedtoolsformolecularspin dynamics investigation

  • 1HNMR
  • 19F NMR
  • 53Cr NMR

53CrNMR

  • 1HNMR
  • Abundance proton (High sensitivity )

19F NMR

Studyof NMR relaxationrates

and spectra

slide9

Spin dynamics vs temperature : NMR spectra

From1H NMR spectrumitispossibletoextract the

Full Width at HalfMaximum – FWHM, givenby :

Paramagnetic behaviour of

in the high temperature region (T>20K)

  • The temperature and magnetic field dependence of1H FWHM is similar to other antiferromagnetic molecularrings, but…….
slide10

Spin dynamics vs temperature: NMR spectra

Dramatic Increase!!!

At relatively high fields, the gap is reduced

and 0 and 1 states

are populated equally

… the gap….

;

First excited state

ST=1, Ms=+1

For T<20K, condensation in the G.S.

slide11

Spin dynamics vs temperature:

Spin-lattice Relaxation Rate (1/T1)

, , …

Homometallic rings (previous studies):

Two alternatives;

Current case (heterometallic Cr8Zn):

Theoretical calculation in progress…

slide12

Low temperature quantum level crossing:

NMR spectra

  • At low T (much less than the gap among =0 and =1, e.g. T=1.7K) molecular rings populate the ground state

Put e.g. insteadof i.e.

  • The local (at sites) magnetic field due to the contribution of electronic (molecular) magnetic moments, becomes:

…. field, due to the contributionof

electronic (molecular) magneticmoments, becomes:

approx.  M

=

slide13

Low temperature quantum level crossing:

NMR spectra

After first GS level crossing

  • NMR spectral broadening due to the increaseof the electronic magnetization value

Aftersecond GS level crossing

non-magnetic

Ground State ST = 0

magnetic

Ground State ST = 1

M(H) curve at T=2K

magnetic

Ground State ST = 2

Calculated energy levels in

external magnetic field

slide14

Low temperature quantum level crossing:

NMRspectra

Proton NMR spectra versus magnetic field on based on energy levels structure by using frequency sweep technique at the fixed temperature (T=1.7 K)

  • NMR spectra broadening by
  • passing of crossing level

1H NMR spectra before the first level crossing

( Non-magnetized system)

Wrong x-axislabel

Use1H insteadofporoton

Put 3° circlecorrectly

1H NMR spectra after the first level crossing

(

( Non-magnetized »»» Magnetized system)

Calculated energy levels in

an external magnetic field

1H NMR spectraafter the secondlevelcrossing

(ST = 1  ST = 2)

slide15

Low temperature quantum level crossing

Future investigation:

spin-lattice relaxation rate study of spin dynamics

(also level crossing problem details and mix of eigenfunctions)

Anti level crossing; Mixed functions

Real level crossing; Unmixed functions

slide16

Conclusions and future study

  • Conclusions:
  • Temperature spin dynamics of detected by “ 1H NMR 1/” is qualitatively
  • similar to homometallic rings; an exact calculation of correlation function is needed.
  • At low temperature 1H NMR spectra broadening reflects the effects of M increase when Quantum level crossing occur

Put “:” among NMR and 1/T1. “1” is at subscript

… effectsof M increasewhen quantum levelcrossingsoccur

  • Future issues :
  • Theoretical investigation of spin dynamics vs temperature
  • Quantum effects due to “Real ”/ Anti level crossing studied by means of
  • low-T 1H NMR spin-lattice relaxation rate
slide17

Scuola Di dottorato in fisica, astrofisica e fisicaapplicate

UNIVERSITÀ

DEGLI STUDI DI MILANO

Thank you

Special thanks ò

Pr. Lascialfari

January 15th 2013

Italy

slide18

NMR sequences

Spin-echo pulse sequences

:

:

This slide can alsobe cut

You can put at the end asanexample slide

T2relaxation curve

T1relaxation curve

NMR spectrum