Magnetic Phase Competition in Off-Stoichiometric Heusler Alloys: The Case of Ni 50-x Co x Mn 25+y Sn 25-y

Transcription

1 Magnetic Phase Competition in Off-Stoichiometric Heusler Alloys: The Case of Ni 5-x Co x Mn 25+y Sn 25-y K. P. Bhatti, D.P. Phelan, C. Leighton Chemical Engineering and Materials Science, University of Minnesota V. Srivastava, R.D. James Aerospace Engineering and Mechanics, University of Minnesota S. El-Khatib Physics, American University of Sharjah NIST Center for Neutron Research S. Yuan, P.L. Kuhns, A.P. Reyes, J.S. Brooks, M.J.R. Hoch National High Magnetic Field Laboratory

2 Introduction: Martensitic (Magnetic) Alloys Martensite Austenite T M (first-order MPT) T Multiferroic alloys (ferroelastic + magnetic) > Magnetoelasticity, magnetic shape memory, field-induced phase transformations, magnetocaloric and barocaloric effects > Actuators/sensors, magnetic refrigeration, energy conversion, heat-assisted recording? Thermal hysteresis: Vital for applications, but fundamentals elusive. Krenke et al. Nat. Mater. (25) T (K)

3 Introduction: Minimization of Thermal Hysteresis Recent theoretical work by James et al.: Geometrical compatibility between austenite and martensite unit cells vital for T. An invariant plane occurs at their interface if, 2 = 1, where 1,2,3 are the ordered eigenvalues of the transformation stretch matrix, U. Experiment: Remarkable correlations between T and 2. Direct observation of ideal interface by HRTEM T << 1 K! Cui et al. Nat. Mater. (26) Zhang et al. Acta. Mat. (29)

4 Application to Magnetic Alloys: The Route to Ni 5-x Co x Mn 4 Sn 1 Step I Full Heusler alloys Ni 2 MnZ, Z = Sn, In, Ga, etc. (e.g. Ni 2 MnSn, F, T C = 35 K) [ ] [ ] [ ]

5 Application to Magnetic Alloys: The Route to Ni 5-x Co x Mn 4 Sn 1 Step I Full Heusler alloys Ni 2 MnZ, Z = Sn, In, Ga, etc. (e.g. Ni 2 MnSn, F, T C = 35 K) Step II Ni 5 Mn 25+y Z 25-y : Tunable magnetic interactions (F vs. AF)* MPT for y > [ ] [ ] [ ] (1) plane *Krenke, Acet, Wasserman, Moya, Manosa, Planes, PRB (25, 26) excess Mn

6 Application to Magnetic Alloys: The Route to Ni 5-x Co x Mn 4 Sn 1 Step I Full Heusler alloys Ni 2 MnZ, Z = Sn, In, Ga, etc. (e.g. Ni 2 MnSn, F, T C = 35 K) Step II Ni 5 Mn 25+y Z 25-y : Tunable magnetic interactions (F vs. AF)* MPT for y > ? *Krenke, Acet, Wasserman, Moya, Manosa, Planes, PRB (25, 26)

7 Application to Magnetic Alloys: The Route to Ni 5-x Co x Mn 4 Sn 1 Step I Full Heusler alloys Ni 2 MnZ, Z = Sn, In, Ga, etc. (e.g. Ni 2 MnSn, F, T C = 35 K) Step II Ni 5 Mn 25+y Z 25-y : Tunable magnetic interactions (F vs. AF)* MPT for y > Step III Ni 5-x Co x Mn 4 Sn 1 : Enhanced T C, M S Convenient T M 2 1 x = 6)? *Krenke, Acet, Wasserman, Moya, Manosa, Planes, PRB (25, 26)

8 Application to Magnetic Alloys: The Route to Ni 5-x Co x Mn 4 Sn 1 Step I Full Heusler alloys Ni 2 MnZ, Z = Sn, In, Ga, etc. (e.g. Ni 2 MnSn, F, T C = 35 K) Step II Ni 5 Mn 25+y Z 25-y : Tunable magnetic interactions (F vs. AF)* MPT for y > Step III Ni 5-x Co x Mn 4 Sn 1 : Enhanced T C, M S Convenient T M 2 1 x = 6)? Ni 46 Co 6 Mn 4 Sn 1 : T C 44 K T M 38 K M 1 emu/cm 3 T < 1 K *Krenke, Acet, Wasserman, Moya, Manosa, Planes, PRB (25, 26)

9 Magnetism in Ni 5-x Co x Mn 4 Sn 1 (and Related Systems) High T C, high M, convenient T M, low T F austenite, non-f martensite (AF or P?) Nominally non-f martensite reveals some significant surprises: > Superparamagnetic-like behavior at low T (in a bulk solid!) > Exchange bias in blocked low T state (in a single phase) > Collective freezing. Super-spin-glass? > ZFC exchange bias Nanoscale Magnetic Inhomogeneity? Acute sensitivity of magnetism to composition

10 Experimental Details Arc-melted polycrystalline bulk ingots [Srivastava et al APL (21); Bhatti et al PRB (212)] 4 3 Ni 44 Co 6 Mn 4 Sn 1 (a) T = 41K cubic, Fm-3m Characterized by: > DSC > T-dependent XRD > T-dependent Neutron Powder Diffraction (NPD) > Magnetometry > Small-Angle Neutron Scattering (SANS) > 55 Mn zero-field NMR Intensity [arb. units] (b) T = 3K 5M monoclinic, P2 1 Data Model Residual [deg]

11 Ni 5-x Co x Mn 4 Sn 1 Phase Diagram Ni 5 Mn 25+y Sn 25-y Ni 5-x Co x Mn 4 Sn 1 e/a (a) (b) T M P (Aust.) 6 5 P (Aust.) T M T C 5 4 F (Aust.) e/a T C F (Aust.) AF (Mart.)? 3 2 AF/SP (Mart.) y 1 Clustered (Mart.) T B EB Clustered (Mart.) x

12 Ni 5-x Co x Mn 4 Sn 1 Phase Diagram Ni 5 Mn 25+y Sn 25-y Ni 5-x Co x Mn 4 Sn 1 e/a (a) (b) T M P (Aust.) 6 5 P (Aust.) T M T C 5 4 F (Aust.) e/a T C F (Aust.) AF (Mart.)? 3 2 AF/SP (Mart.) y 1 Clustered (Mart.) T B EB Clustered (Mart.) x

13 Ni 5-x Co x Mn 4 Sn 1 Phase Diagram Ni 5 Mn 25+y Sn 25-y Ni 5-x Co x Mn 4 Sn 1 e/a (a) (b) T M P (Aust.) 6 5 P (Aust.) T M T C 5 4 F (Aust.) e/a T C F (Aust.) AF (Mart.)? 3 2 AF/SP (Mart.) y 1 Clustered (Mart.) T B EB Clustered (Mart.) x

14 Ni 5-x Co x Mn 4 Sn 1 Phase Diagram Ni 5 Mn 25+y Sn 25-y Ni 5-x Co x Mn 4 Sn 1 e/a (a) (b) T M P (Aust.) 6 5 P (Aust.) T M T C 5 4 F (Aust.) e/a T C F (Aust.) AF (Mart.)? 3 2 AF/SP (Mart.) y 1 Clustered (Mart.) T B EB Clustered (Mart.) x

15 Ni 5-x Co x Mn 4 Sn 1 Phase Diagram Ni 5 Mn 25+y Sn 25-y Ni 5-x Co x Mn 4 Sn 1 e/a (a) (b) T M P (Aust.) 6 5 P (Aust.) T M T C 5 4 F (Aust.) e/a T C F (Aust.) AF (Mart.)? 3 2 AF/SP (Mart.) y 1 Clustered (Mart.) T B EB Clustered (Mart.) x

16 M vs. T (x = 2) M [emu/cm 3 ] M [emu/cm 3 ] FCW FCC ZFCW Ni 48 Co 2 Mn 4 Sn 1 3 H = 5 Oe H = 1 Oe

17 M vs. T (x = 6) Ni 5 Mn 25+y Sn 25-y Ni 5-x Co x Mn 4 Sn 1 e/a (a) (b) T M P (Aust.) 6 5 P (Aust.) T M T C 5 4 F (Aust.) e/a T C F (Aust.) AF (Mart.)? 3 2 AF/SP (Mart.) y 1 Clustered (Mart.) T B EB Clustered (Mart.) x

18 M vs. T (x = 6) M [emu/cm 3 ] M [emu/cm 3 ] FCC ZFCW Ni 44 Co 6 Mn 4 Sn 1 H = 1 Oe

19 M vs. T (x = 14) Ni 5 Mn 25+y Sn 25-y Ni 5-x Co x Mn 4 Sn 1 e/a (a) (b) T M P (Aust.) 6 5 P (Aust.) T M T C 5 4 F (Aust.) e/a T C F (Aust.) AF (Mart.)? 3 2 AF/SP (Mart.) y 1 Clustered (Mart.) T B EB Clustered (Mart.) x

20 M vs. T (x = 14) M [emu/cm 3 ] M [emu/cm 3 ] K 3 K 55 K Ni 36 Co 14 Mn 4 Sn H [koe] FCC FCW ZFCW H = 1 Oe

21 M vs. H (x = 2) Ni 5 Mn 25+y Sn 25-y Ni 5-x Co x Mn 4 Sn 1 e/a (a) (b) T M P (Aust.) 6 5 P (Aust.) T M T C 5 4 F (Aust.) e/a T C F (Aust.) AF (Mart.)? 3 2 AF/SP (Mart.) y 1 Clustered (Mart.) T B EB Clustered (Mart.) x

22 M vs. H (x = 2) M [emu/cm 3 ] M [emu/cm 3 ] K 175 K 15 K 125 K Ni 48 Co 2 Mn 4 Sn 1 5 K 4 K K 5 K H [koe] (a) (b) Langevin fit (c)

23 M vs. H (x = 2) M [emu/cm 3 ] M [emu/cm 3 ] K 175 K 15 K 125 K Ni 48 Co 2 Mn 4 Sn 1 5 K 4 K K 5 K H [koe] (a) (b) Langevin fit (c) T > T M : T < T M : P AF/P

24 M vs. H (x = 2) M [emu/cm 3 ] M [emu/cm 3 ] K 4 K 2 K 175 K 15 K 125 K Ni 48 Co 2 Mn 4 Sn K 5 K H [koe] (a) (b) Langevin fit (c) T > T M : T < T M : 3-1 K: P AF/P Langevin-like M(H) C ( T) H k T B M ( H, T) nc ( T) C ( T) coth BG ( T) H kbt C ( T) H

25 M vs. H (x = 2) M [emu/cm 3 ] K 4 K 2 K 175 K 15 K 125 K Ni 48 Co 2 Mn 4 Sn 1 (a) (b) Langevin fit T > T M : T < T M : 3-1 K: P AF/P Langevin-like M(H) C ( T) H k T B M ( H, T) nc ( T) C ( T) coth BG ( T) H kbt C ( T) H M [emu/cm 3 ] K 5 K H [koe] (c) T < 1 K: M(H) gradually opens Static F ZFC exchange bias* *Wang et al PRL (211)

26 M vs. H (x = 6) Ni 5 Mn 25+y Sn 25-y Ni 5-x Co x Mn 4 Sn 1 e/a (a) (b) T M P (Aust.) 6 5 P (Aust.) T M T C 5 4 F (Aust.) e/a T C F (Aust.) AF (Mart.)? 3 2 AF/SP (Mart.) y 1 Clustered (Mart.) T B EB Clustered (Mart.) x

27 5 415K 55K (a) 2.8 M vs. H (x = 6) K 395K 39K (b) 415K M [emu/cm 3 ] K 1K 15K (c) 2K 3K 37K M [ B /f.u.] 1 5K 3K (d) H [koe]

28 K 55K 4K 395K 39K (a) (b) 415K T > T C : P T < T C : Soft F M vs. H (x = 6) M [emu/cm 3 ] K 1K 15K (c) 2K 3K 37K M [ B /f.u.] 1 5K 3K (d) H [koe]

29 M [emu/cm 3 ] K 55K 4K 395K 39K 8K 1K 15K (a) (b) (c) 415K K 3K 37K M [ B /f.u.] T > T C : P T < T C : Soft F 4-38 K: M vs. H (x = 6) Hysteresis region (H-induced MPT) 1 5K 3K (d) H [koe]

30 M [emu/cm 3 ] K 55K 4K 395K 39K 8K 1K 15K (a) (b) (c) 415K K 3K 37K M [ B /f.u.] T > T C : P T < T C : Soft F 4-38 K: 38-6 K: M vs. H (x = 6) Hysteresis region (H-induced MPT) Langevin-like M(H) 1 5K 3K (d) H [koe]

31 M [emu/cm 3 ] K 55K 4K 395K 39K 8K 1K 15K (a) (b) (c) 415K K 3K 37K M [ B /f.u.] T > T C : P T < T C : Soft F 4-38 K: 38-6 K: M vs. H (x = 6) Hysteresis region (H-induced MPT) Langevin-like M(H) 1 5K 3K (d) T < 6 K: M(H) gradually opens Static F H [koe]

32 Superparamagnetic-like Behavior C ( T ) H k T B M ( H, T ) nc ( T ) C ( T ) coth BG ( T ) H kbt C ( T ) H Ni 5-x Co x Mn 4 Sn 1 c [ B ] x = 2 x = 3 x = 4 x = 6 (a) n c [cm -3 ] (b) c 2-25 B n c 2-3 x 1 19 cm -3 Implication: d c 2-6 nm Dense ensemble of nm-scale spin clusters

33 Exchange Bias H c [Oe] 2 1 H FC = 7 koe x = 2: > SP blocking at 1-15 K > EB blocking at 6 K > Large H C peak H E [Oe] (a) 8 Ni 48 Co 2 Mn 4 Sn 1 6 Ni 44 Co 6 Mn 4 Sn (b) x = 6: > SP blocking at 6 K > EB blocking at 6 K > No H C peak

34 Phase Diagram

35 SANS (NIST) Neutron guide Reactor Cryostat/ magnet Detector chamber

36 q [Å -1 ] (a) 5 K d/d [cm -1 ] n P q T d d T q d d ) (, 2 2 ) ( 1 ) (, T q T d d T q d d L Low q (large length-scale) Porod Scattering High q (short length-scale) Lorentzian Scattering SANS (x = 6)

37 SANS: q Dependence d/d [cm -1 ] (a) 5 K (b) 425 K (c) 39 K (d) 38 K (e) 3 K.1.1 q [Å -1 ]

38 SANS: q Dependence Low q Porod scattering: > Increases below T C > Magnetic domain scattering > Proof of long-range F order (>> 1 nm) (a) 5 K (b) 425 K High q Lorentzian scattering: > Peaks at T C > F spin correlations Intermediate q scattering: d d > A peak develops! 2/q 12-2 Å > Structure factor peak. Liquid-like spatial distribution of magnetic clusters. Tot ( q, T) d d n q P ( T) d d G ( q qg ( T)) ( T)exp 2 2( T) 2 q d d 2 L ( T ) 1 ( T ) d/d [cm -1 ] (c) 39 K (d) 38 K (e) 3 K.1.1 q [Å -1 ]

39 SANS: T Dependence q =.5 Å -1 (12 Å): > Sharp onset of domain scattering at T C > Hysteretic loss at T M d/d [cm -1 ] (a) q =.5 Å -1 Heating Cooling Ni 44 Co 6 Mn 4 Sn 1 q =.1 Å -1 (6 Å): > Critical scattering peak at T C d/d [cm -1 ] d/d [cm -1 ] (b) q =.1 Å

40 SANS: T Dependence q =.5 Å -1 (12 Å): > Sharp onset of domain scattering at T C > Hysteretic loss at T M d/d [cm -1 ] (a) q =.5 Å -1 Heating Cooling Ni 44 Co 6 Mn 4 Sn 1 q =.1 Å -1 (6 Å): > Critical scattering peak at T C > 13 K transition (!), clearly associated with clusters > Blocking at 1-12 s time-scales! d/d [cm -1 ] d/d [cm -1 ] (b) q =.1 Å

41 SANS T Dependence: Porod (d/d) P [cm -1 ] n Scattering in martensite strong, T-dependent Paramagnetic, n = 4 (grains); Ferromagnetic, n = 2 (pinned domains); Martensitic, n = 3 (AF domains?) Additional (indirect) evidence for AF order in the martensite

42 Å SANS T Dependence: Gaussian Å (d/d) G [cm -1 ] 1 q G [cm -1 ] [cm -1 ] Gaussian peak intensity actually emerges at high T (even above T C?) Peak position weakly T-dependent. Peak width decreases on cooling Correlation length > 5 inter-particle spacings! Collective response*? *Cong et al APL (21); APL (21) and Wang et al PRL (211)

43 SANS T Dependence: Lorentzian (d/d) L [Å -1 ] [cm -1 ] Only present in significant intensities in paramagnetic phase Magnetic correlation length diverges as T T C +

44 Qualitative Model

45 Qualitative Model P

46 Qualitative Model P F

47 Qualitative Model P F AF?

48 Qualitative Model P F AF? AF?

49 AF in the Martensite: Neutron Diffraction Problems AF known at Ni 5 Mn 5, suspected in some form in Ni 5 Mn 25+y Sn 25-y Indirect evidence: > (T) > Exchange bias > SANS (Porod scattering) Why not just do neutron diffraction? > Texture an issue > Looking for the onset of AF order simultaneous with MPT to low symmetry state (very challenging refinement)

50 AF in the Martensite: Neutron Diffraction Problems AF known at Ni 5 Mn 5, suspected in some form in Ni 5 Mn 25+y Sn 25-y Indirect evidence: > (T) > Exchange bias > SANS (Porod scattering) Why not just do neutron diffraction? > Texture an issue > Looking for the onset of AF order simultaneous with MPT to low symmetry state (very challenging refinement)

51 AF in the Martensite: Neutron Diffraction Problems AF known at Ni 5 Mn 5, suspected in some form in Ni 5 Mn 25+y Sn 25-y Indirect evidence: > (T) > Exchange bias > SANS (Porod scattering) Why not just do neutron diffraction? > Texture an issue > Looking for the onset of AF order simultaneous with MPT to low symmetry state (very challenging refinement) Weak evidence for an AF order parameter? Polarized neutrons.

52 AF in the Martensite: 55 Mn Zero Field NMR (/2 = 1.5 MHz/T) 1 Ni 5-x Co x Mn 4 Sn 1 T = 1.6 K 225 AF F x= AF x.1 F x=7 F x=14 14 T Frequency f (MHz) NMR signal enhancement factor,, measured by calibration to 19 F Typically: High in Fs, low in AFs 48 T

53 AF in the Martensite: 55 Mn Zero Field NMR (/2 = 1.5 MHz/T) Ni 5 Mn 4 Sn 1 (x = ) -1.5 MHz / T Unambiguous F/AF assignment The martensite indeed has ordered AF, decreasing with Co substitution

54 AF in the Martensite: 55 Mn Zero Field NMR Ni 5 Mn 4 Sn 1 (x = ) (volume fraction) AF/F phase coexistence at low T T M Volume fractions << 1 at elevated T. Short-range AF order! AF more thermally stable than F. Simultaneous AF/F blocking at 1 K. Second F blocking at 1-2 K. AF transition at T M. T B when T 2 NMR (MHz). Solid lines: SP model, F cluster diameter = nm

55 Conclusions Ni 5-x Co x Mn 25+y Sn 25-y : Physically very interesting, potentially useful set of alloys High T C, convenient T M, exceptionally low T, high M Nanoscale magneto-electronic inhomogeneity: > SP-like freezing of nm-scale spin clusters > Exchange bias in a nominally single-phase system > First direct observation by SANS (liquid-like distribution, d c 2-6 nm, d c-c = 12 nm, strong interactions) > First observation by zero-field 55 Mn NMR (proof of short-range AF order, d c nm) New magnetic phase diagram Qualitative model based on (unavoidable) compositional fluctuations Bhatti, El-Khatib, Srivastava, James, Leighton, Phys. Rev. B. 85, (212) Bhatti, Srivastava, Phelan, James, Leighton, Heusler Alloys, Eds. Hirohata and Felser, Springer (215) Yuan, Kuhns, Reyes, Brooks, Hoch, Srivastava, James, El-Khatib, Leighton, Phys. Rev. B. 91, (215) Yuan, Kuhns, Reyes, Brooks, Hoch, Srivastava, James and Leighton, Phys. Rev. B., submitted (215)