Spin fluctuations in MnGe chiral Magnet

Transcription

1 Spin fluctuations in MnGe chiral Magnet Isabelle Mirebeau, Nicolas Martin, Maxime Deutsch Laboratoire Léon Brillouin CE-Saclay Gif sur Yvette France Post-docs: Nicolas Martin ( ) Maxime Deutsch ( ) Main other contributors A. Amato, D. Andreica (PSI-Zurich) F. Bert (LPS) P. Bonville (SPEC) U. Roessler (Univ. Dresden) P. Bonfa (Univ. Firenze) A. Tsvyashchenko, L. Fomicheva ( Univ. Moscow) T. Fernandez-Diaz, T. Hansen (ILL)

2 Summary The B20 family A hierarchy of interactions Spin fluctuations, skyrmions, and quantum critical points MnGe : a case in point chiral fluctuations in MnGe the different probes MuSR Which type of fluctuations? Consequences on the elastic and electric properties

3 The B20 family non centrosymétric space group P2 1 3 MnSi cw ~J~30K itinerant magnetism, tuned by a hierarchy of interactions Ferro exchange D~Ja/ ~0.75K ~180Å a=4.5 Å DM Anisotropy MnSi, FeGe, Fe 1-x Co x Si,..,MnGe In bulk materials Quantum critical points Skyrmions E c =gµ B H c ~10-3 K H c =0.1T Crystal Anisotropy long period helixes Spin fluctuations 3

4 Skyrmions A. Fert review (2013) THE Bulk MnSi Schultz Nature (2012) Negative THE in MnSi Mulhbauer Science(2009) In bulk MnSi, the SK lattice is stable in a small (T, H) region : A phase

5 Skyrmions and spin fluctuations First order transition induced by strong fluctuations Chiral isotropic fluctuations MnSi T C T MF Bauer, Gast, Pfleiderer PRL(2013) Janoschek et al PRB(2014)

6 Quantum critical points and spin fluctuations Under pressure Partial ordered phase under magnetic field MnSi Pfleiderer et al, Nature (2004) No Cp or anomaly Ritz et al. PRB(2013)

7 Potential mechanisms Longitudinal fluctuations of the magnetic moment Spontaneous skyrmions (H=0) Skyrmion lattice in applied field Roessler Nature (2006) Mulhbauer Science (2009)

8 MnGe : a case in point (1) Two possible sources for the spin fluctuations Short pitch compound MnSi FeGe MnGe T N (K) L D (nm) P C (GPa) and ~13 Giant precursor phase O. Makarova et al PRB(2012) Kanazawa et al PRL(2011) T A D 1 L D 2 MnSi : T~4K FeGe 4K MnGe K! High skyrmion density? Giant THE Viennois et al, EPL (2015) Kanazawa, PRL 2011

9 pression MnGe : a case in point (2) Two possible sources for the spin fluctuations Two Quantum Critical points compound MnSi FeGe MnGe T N (K) L D (nm) P C (GPa) and ~13 Anomaly of the density of state HS et LS metastable spin states Slow «Invar-like» fluctuations? O. Makarova et al PRB(2012) Kanazawa et al PRL(2011) M. Deutsch et al PRB RC (2014) HS LS ZS ab initio prediction U. Roessler (2011)

10 MnGe : High pressure phase diagram M. Deutsch et al PRB 89, R, (2014)

11 low magnetic field MnGe :static susceptibility MnGe a=4.8å MnSi 4.5Å J~280K D~46K 30K 0.75K Ea~3.4K K Deutsch et al PRB (2014) In MnGe, spin fluctuations extend over a huge T range: T N 70K and exist below T N Sample dependence: the susceptibility peak scales the fluctuating region

12 Probing the spin fluctuations Which probes? Time-frequency scales length-q scales Static susceptibility ~1s MuSR ~10-7 _10-5 s Mössbauer effect ~10-8 s Resonant Neutron spin-echo ~10-11 _10-12 s a MuSR experiment Blundell, review (2002)

13 MnGe: MuSR T>T N T<T N Strong depolarization from static dipolar fields

14 MnGe: MuSR 2-fractions model f: fast ( f ) 1-f: slow ( s ) b: small fixed background Slow fluctuations in the ordered phase

15 MnGe: MuSR MuSR 2-fractions model Slow fast 10 6 Continuous distribution of relaxation rates Filtered at long times

16 MnGe: MuSR Continuous model Broad distribution of independent relaxation channels ( *, )

17 frequencies of the spin fluctuations? In the paramagnetic range T>> T N <B 2> =<B av2 > with B av =0.8T =2.5 Deep in the ordered phase T<<T N A very large range of frequencies

18 MnGe: MuSR, M.E, NSE MuSR 2-fractions picture Slow fast 10 6 Mössbauer frozen Paramagnetic 10 8 Neutron spin echo frozen fluctuating 10 11

19 57 Fe-doped MnGe: Mossbauer study P. Bonville M. Deutsch et al PRB (2014) Coexistence of fluctuating (seen as paramagnetic) and Frozen (seen as ordered) signals

20 MnGe: Neutron diffraction Probes the helical order and scales the ordered moment of the helix Intense zero satellite Position Intensity period Moment

21 MnGe: Resonant Neutron Spin echo Mieze technique (resonant spin Reseda (HMI, Munich) Fluctuating frozen Spin fluctuations at the Bragg peak position N. Martin et al unpublished

22 MnGe: Fluctuating fraction Comparison of the different probes Something new!

23 MnGe: Resonant Neutron Spin echo Quasielastic linewidth N. Martin et al unpublished Mieze (2015) The system becomes «static» ( =0) when the helical wave vector locks into a commensurate value 1/6

24 MnGe: Summary strong DM interaction Short helical pitch Huge extension of the fluctuating chiral phase Large distribution of spin fluctuating frequencies ( Hz ) New movements of the helixes seen by NSE in the LRO state Locking unlocking processes? Two possible origins for the spin fluctuations Influence on the anomalous elastic and electric properties Slow Specific to MnGe (2 QCP) HS and LS metastable states Fast present in the whole B20 family Enhanced in MnGe Invar-like behaviour N. Martin et al to appear Giant THE Kanazawa PRL 2011

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