EPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler …ah566/core1/abstracts.pdfEPSRC-JSPS UK-Japan...

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EPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler Alloys 16 March 2016 Departments of Electronics and Physics University of York

Transcript of EPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler …ah566/core1/abstracts.pdfEPSRC-JSPS UK-Japan...

Page 1: EPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler …ah566/core1/abstracts.pdfEPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler Alloys 16 March 2016 Departments of Electronics

EPSRC-JSPS UK-Japan Core-to-Core

Workshop on Heusler Alloys

16 March 2016

Departments of Electronics and Physics

University of York

   

Page 2: EPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler …ah566/core1/abstracts.pdfEPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler Alloys 16 March 2016 Departments of Electronics

EPSRC-­‐‑JSPS  Core-­‐‑to-­‐‑Core  Workshop  on  Heusler  Alloys  

Wednesday,  16  March  2016  Departments  of  Electronics/Physics,  University  of  York  

 12:00   Lunch                                  G/N011  

12:50   Opening  remarks                              G/N120     Prof.  Kevin  O’Grady  (Dept.  of  Phys.,  Univ.  of  York)  

13:00   Session  I:  Magnetic  and  transport  properties  of  Hesuler  alloys                  G/N120  Chair:  Gonzalo  Vallejo-­‐‑Fernandez  

   13:00   Prof.  Masafumi  Shirai  (RIEC,  Tohoku  Univ.)  Electronic  structure  and  magnetic  properties  at  interfaces  between  Co-­‐‑based  Heusler  alloys  and  MgO  barrier  

   13:30  Miss  Chris  Yu  (Dept.  of  Phys.,  Univ.  of  York)     Exchange  bias  induced  at  a  Co2FeAl0.5Si0.5/Cr  interface      13:45  Mr  John  Sinclair  (Dept.  of  Phys.,  Univ.  of  York)     Electrical  characterisation  of  Néel  transitions      14:00  Dr  Gavin  Bell  (Dept.  of  Phys.,  Univ.  of  Warwick)     Polarised  neutron  reflectivity  of  half-­‐‑metallic  epitaxial  films      14:30  Mr  William  Frost  (Dept.  of  Elec.,  Univ.  of  York)     Development  of  a  CPP-­‐‑GMR  Heusler  Alloy  Junction      14:45  Mr  Haokaifeng  (Jason)  Wu  (Dept.  of  Phys.,  Univ.  of  York)  

Magnetic  and  structural  properties  of  Mn2VSi  alloy  grown  at  elevating  temperatures  

15:00   Coffee/Tea  break                              G/N011  

15:30   Session  II:  Structural  properties  of  Heusler  alloys                      G/N120  Chair:  Atsufumi  Hirohata  

   15:30  Dr  Masahito  Tsujikawa  (RIEC,  Tohoku  Univ.)  Perpendicular  magnetic  anisotropy  and  its  electric  field  modification  in  the  MgO/Co-­‐‑based  Heusler  alloy  interface  

   16:00  Mr  Teodor  Huminiuc  (Dept.  of  Phys.,  Univ.  of  York)     Novel  antiferromagnetic  materials  for  spintronic  devices      16:15  Dr  Barat  Kuerbanjiang  (Dept.  of  Phys.,  Univ.  of  York)  

The  role  of  chemical  structure  on  the  magnetic  and  electronic  properties  of  Co2FeAl0.5Si0.5/Si(111)  interface  

   16:45   Zlatko  Nedelkoski  (Dept.  of  Phys.,  Univ.  of  York)  The  role  of  interfaces  on  the  spin-­‐‑polarization  

17:00   Closing  remarks                              G/N120     Prof.  Masafumi  Shirai  (RIEC,  Tohoku  Univ.)  

19:00   Dinner                                                    Deans  Court      

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Session  I:  Magnetic  and  transport  properties  of  properties  of  Heusler  alloys  13:00~15:00                                                                                                                                                                        G/N  120  

Chair:  Gonzalo  Vallejo-­‐‑Fernandez  

       

Electronic  structure  and  magnetic  properties  at  interfaces  between                                                                                      Co-­‐‑based  Heusler  alloys  and  MgO  barrier  

Masafumi  Shirai  

Research  Institute  of  Electrical  Communications,  Tohoku  University  

Huge   tunneling   magnetoresistance   (TMR)   ratio   over   2,000%   was   observed   at   low  temperatures   for   magnetic   tunnel   junctions   (MTJ)   composed   of   Co-­‐‑based   Heusler   alloys   and  MgO   barrier.   However,   the   TMR   ratio   of   the   MTJ   remarkably   decreases   with   increasing  temperature.   It   was   proposed   that   the   spin-­‐‑flip   and/or   inelastic   tunneling   processes   via  interfacial  resonant  states  (IRS)  in  the  minority-­‐‑spin  gap  are  responsible  for  the  reduction  of  the  TMR  ratio  in  the  MTJ  with  half-­‐‑metallic  electrodes  [1].  Indeed,  we  found  that  the  IRS  appeared  at  the  Co2MnSi/MgO  junction  are  mainly  composed  of  Co  or  Mn  3dε  orbitals  having  the  Δ2  or  Δ5  symmetry   [2].   Since   the  Δ1   electrons  predominantly   transmit   the  MgO  barrier,   the   IRS  hardly  contribute  the  tunneling  conductance  in  the  MTJ  with  thicker  MgO  barrier.  On  the  contrary,   in  the  MTJ  with  thinner  MgO  barrier,   the  IRS  could  play  a  crucial  role   in  reducing  the  TMR  ratio.  We   proposed   that   the   harmful   IRS   can   be   eliminated   by   inserting   CrAl-­‐‑layer   into   the  Co2MnSi/MgO  interface  [3].  

Focusing   on   the   spin-­‐‑flip   tunneling   processes   caused   by   magnetic   excitations   in   the  ferromagnetic  electrodes,  we  evaluated  tunneling  conductance  of  Co2MnSi/MgO/  Co2MnSi  MTJ  by  tilting  the  direction  of  magnetic  moments  of  each  atomic  layer.  As  a  result,  we  conclude  that  the  thermal  fluctuation  of  Co  moments  in  the  interfacial  region  is  responsible  for  the  reduction  of   the  TMR  ratio  with   increasing  temperature  since  the  exchange  stiffness  of   the   interfacial  or  sub-­‐‑interfacial  Co-­‐‑layer  is  much  weaker  than  that  of  bulk  Co2MnSi  [4].  For  achieving  higher  TMR  ratio   at   room   temperature,   we   need   to   find   the   Heusler   alloys   possessing   higher   magnetic  anisotropy  and/or  stronger  exchange  stiffness  at  the  interface  of  the  MTJ.  

This  work  was  partly  supported  by  Grant-­‐‑in-­‐‑Aid  for  Scientific  Research  form  JSPS/MEXT  and  by   JSPS   Core-­‐‑to-­‐‑Core   Program,   A.   Advanced   Research   Networks   “New-­‐‑Concept   Spintronics  Devices”.  

References  [1]  P.  Mavropoulos,  M.  Lezaic,  and  S.  Blügel,  Phys.  Rev.  B  72,  174428  (2005).  [2]  Y.  Miura,  et  al.,  J.  Phys.:  Condens.  Matter  19,  365228  (2007).  [3]  Y.  Miura,  et  al.,  Phys.  Rev.  B  78,  064416  (2008).  [4]  Y.  Miura,  K.  Abe,  and  M.  Shirai,  Phys.  Rev.  B,  83,  214411  (2011).      

13:00

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 Exchange  bias  induced  at  a  Co2FeAl0.5Si0.5/Cr  interface  Nga  Tung  Chris  Yu,1  Andrew  Vick,1  Nobuhito  Inami,2  Kanta  Ono  2  and  Atsufumi  Hirohata  1  

1  Departments  of  Physics  and  Electronics,  University  of  York  2  Institute  of  Materials  Structure  Science,  High  Energy  Acceleration  Organization  (KEK)  

Among  HMFs,  Heusler  alloys  hold  greatest  potential  due  to  their  theoretically  predicted  half-­‐‑metallicity,   structural  matching  with   substrate  materials   and   high   Curie   temperatures   (Tc)   at  room  temperature  [1].  Co2FeAl0.5Si0.5  (CFAS)  is  an  attractive  half  metallic  material  because  of  its  high  Tc  (~1000K)  and  finite  band  gap  in  one  spin  channel  [2].  For  spintronics  device  application  large  remanent  magnetisation  is  one  of  the  requirements  and  it  can  be  achieved  by  exchanged  bias.  

In   order   to   engineer   the   strength   of   an   exchange   bias   to   introduce   anisotropy   in   a   cubic  Heusler   alloy   layer,   crystalline   strain   has   been   induced   at   a   ferromagnet/antiferromagnet  interface   by   lattice   mismatch   in   addition   to   the   conventional   interfacial   exchange   coupling.  [CFAS/Cr]3   structures   have   been  prepared  by   ultrahigh   vacuum  molecular   beam  epitaxy.   The  magnetic   and   structural   properties   have   been   characterised   to   investigate   the   exchange  interactions  at  the  CFAS/Cr  interfaces.  Due  to  the  lattice  mismatch  between  Cr  and  CFAS  layers  to  be  1.4%,  the  maximum  offset  of  18  Oe   in  a  magnetisation  curve  has  been  measured  for   the  case  of  a  2  nm  thick  CFAS/0.9  nm  Cr  interface  at  193K.  And  no  annealing  was  performed.  The  half-­‐‑metallic   property   of   CFAS   has   observed   to   remain   unchanged,   which   agrees   with   the  theoretical  prediction  by  Culbert  et  al.  [3]  Such  a  strain-­‐‑induced  exchange  bias  may  offer  insight  of  the  interfacial  interactions.  

Reference  [1]  I  Galanakis  et  al,  J.  Phys.  D:  Appl.  Phys.  39,  765  (2006).  [2]  N.  Tezuka  et  al,  Appl.  Phys.  Lett.,  vol  94,  162504  (2009).  [3]  C.  A.  Culbert  et  al,  J.  Appl.  Phys.  103,  07D707  (2008).  

   

13:30

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 Electrical  characterisation  of  Néel  transitions  John  Sinclair,  Teodor  Huminiuc,  Haokaifeng  Wu,  Gonzalo  Vallejo-­‐‑Fernandez                                                                              

and  Atsufumi  Hirohata  

Departments  of  Physics  and  Electronics,  University  of  York  

In   the   recent   years   there   has   been   a   large   increase   in   demand   for   Iridium   due   to   its  applications   in   magnetic   memory   and   storage.   This   is   due   to   its   ability   to   form   a   sheet  antiferromagnet   with   a   blocking   temperature   above   300K   when   combined   with   Manganese.  Unfortunately   as   a   consequence   of   this   increase   in   demand   for   Iridium,   the   price   has   soared  making   it   necessary   to   explore   other   options   for   creation   of   antiferromagnetic   layers   in  spintronic  devices.  

One  of  the  most  promising  groups  of  materials  for  this  application  are  Heusler  alloys,  with  a  number  predicted  to  exist  in  an  antiferromagnetic  state  [1].  To  achieve  this  it  will  be  necessary  to   characterise   the  magnetic   ordering   temperatures   of   the   alloys   of   interest,   namely  Ni2MnAl  and  Fe2VAl.    Sheet  resistance  measurements  are  achieved  using  4  point  measurement  with  Van  der   Pauw   geometry   in   an   Oxford   Instruments   cryostat.   After   measuring   twice   with   current  applied   in   perpendicular   directions   an   area   independent   resistivity   measurement   can   be  calculated.    To  ensure  that  this  measurement  technique  is  viable  initial  testing  has  been  carried  out  on  single  crystal  Chromium,  shown  in  figure  1.  

This   technique   will   allow   for   measurement   the   Néel   temperature   for   the   two   alloys  mentioned  above  and  also  exploration  of  the  dependence  of  the  Néel  temperature  on  annealing  time  and  crystallinity.  These  properties  are  controlled   through   the  use  of  a  vacuum  annealing  furnace  and  deposition  parameters  on  the  HiTUS  sputter  deposition  system  used  to  produce  the  films.  

240 250 260 270 280 290 300 310 320 330

9.0

9.5

10.0

10.5

11.0

Resistance  (µΩ)

Temperature  (K)

TN  =  311K

 Figure  1  -­‐‑  Electrical  resistivity  of  single  crystal  Chromium  with  Temperature.  

Reference  [1]  D.  J.  Singh  and  I.  I.  Mazin,  Phys.  Rev.  B  57,  14352  (1998).  

   

13:45

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Polarised  neutron  reflectivity  of  half-­‐‑metallic  epitaxial  films  

Gavin  Bell  

Departments  of  Physics,  University  of  Warwick  

Polarised   neutron   reflectivity   (PNR)   is   a   powerful   probe   of   magnetic   multilayers,   able   to  measure   depth-­‐‑resolved   magnetic   moments   among   other   properties.   The   technique   will   be  introduced   along  with   the   complementary   X-­‐‑ray   reflectivity   (XRR)  method   and   simultaneous  data  fitting  techniques.  Results  from  several  spintronic  multilayer  structures  will  be  discussed.      

14:00

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Development  of  a  CPP-­‐‑GMR  Heusler  Alloy  Junction  William  Frost  and  Atsufumi  Hirohata  

Department  of  Electronics,  University  of  York  

We  report  improvements  made  to  the  perpendicular  magnetic  anisotropy  of  the  Heusler  alloy  Co2FeSi   in   a   GMR   stack.     Perpendicular  magnetic   anisotropy   (PMA)   in  Heusler   alloys   is  most  commonly  induced  using  an  MgO  layer  where  hybridised  orbitals  induce  the  anisotropy.1  For  a  practical  GMR  device  this  is  not  possible  as  a  Silicon  substrate  is  used  and  the  barrier  layer  has  to  be  conductive.     In   this  work  Vanadium  has  been  used  as  a   seed   layer   to  produce  a  Co2FeSi  (110)   textured   film  with  a   rocking  curve  FWHM  of  5.4o.     Lattice  mismatch  between   the   (110)  lattice   directions   induces   a   strain   in   the   Co2FeSi   layer   shown   by   a   decrease   in   the   lattice  constant  by  0.5%  as  compared  to  the  bulk  value.2    This  strain  increases  the  PMA  in  the  Heusler  alloy   and   has   been   shown   to   be   interfacial   in   nature   by   varying   the   thickness   of   the   Co2FeSi  layer.  

A  top  interfacial  layer  of  Vanadium  was  then  added  to  induce  strain  at  both  interfaces  of  the  Co2FeSi  layer  in  order  to  increase  the  PMA,  as  would  be  present  in  a  GMR  device.    However  the  magnetic  anisotropy  remained  strongly   in  plane.    The   thickness  of   the  Co2FeSi   layer  has  been  reduced  showing  an  increase  in  the  perpendicular  anisotropy  with  decreasing  film  thickness.    At  a  Co2FeSi   layer  thickness  of  4  nm  a  much  higher   level  of  perpendicular  anisotropy  is  achieved  when  coupled  with  a  second  interfacial  layer  of  Vanadium.  

Alternative   seed   layers   have   also   been   used   in   an   attempt   to   decrease   the   thickness   of   the  seed   layer.   The   addition   of   an   Fe   buffer   between   the   V   and   CFS   radically   changes   the   crystal  structure  of  the  CFS  layer.  This  has  been  investigated  by  varying  the  thickness  of  the  V,  Fe  and  CFS  layers.  

Acknowledgement:    This  work  was  partly  funded  by  Seagate  Technology,  Derry.  

[1]  Z.Wen  et  al.,  Appl.  Phys.  Lett.  98,  242507  (2011).  [2]  W.  Frost  and  A.  Hirohata,  IEEE  Trans.  Mag.,  (in  press).  

 

 

Fig.1:  Perpendicular  M-­‐‑H  loops  showing  the  effect  of  the  second  Vanadium  interface.  

Fig.  2:  M-­‐‑H  loops  of  a  4  nm  Co2FeSi  layer  showing  an  improved  PMA.  

 

   

-­30 -­20 -­10 0 10 20 30

-­1.0

-­0.5

0.0

0.5

1.0 M/MS

H  (Oe)

 In  Plane  Perp.

14:30

-­500 -­250 0 250 500

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-­0.5

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0.5

1.0 M/MS

H  (Oe)

 V/Co2FeSi/V  V/Co2FeSi

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14:45 Magnetic  and  structural  properties  of  Mn2VSi  alloy  grown  at  elevating  temperatures  Haokaifeng  Wu,  Teodor  Huminiuc,  John  Sinclair,  Atsufumi  Hirohata,  Gonzalo  Vallejo  Fernandez  

and  Kevin  O’Grady  

Departments  of  Physics  and  Electronics,  University  of  York  

IrMn  has  currently  been  used  in  a  magnetic  read-­‐‑head  element  in  a  hard  disk  drive  (HDD)  due  to   its   high   thermal   stability   of   the   antiferromagnetism   corrosion   resistance.   Because   of   the  increasing  demand  for  the  HDD  and  the  scarcity  of   Ir,   this  study  aims  to  replace  IrMn  with  an  antiferromagnetic  Heusler  alloy.  100nm  thick  Mn2VSi  films  were  deposited  on  Silicon  substrates  with  a  18nm  silver  seed   layer  and  a  3nm  aluminium  capping   layer  using  a  HiTUS  system.  The  substrates  were  heated  at  above  250℃  during  the  deposition.  The  films  were  then  characterised  by  vibrating  sample  magnetometry  (VSM)  and  X-­‐‑ray  diffraction  (XRD).  

 Fig.  1.  XRD  θ-­‐‑2θ  scans  for  the  Mn2VSi  films  grown  at  different  temperatures.  

We  have  investigated  the  crystallization  and  surface  roughness  co-­‐‑relation  of  the  Mn2VSi  film.  Growth   at   higher   temperature   provides   highly   crystallized   films  which   show   a   sharp   peak   in  XRD  intensities  at  2θ  ≅  45°  indicating  the  (220)  Heusler  alloy  peak.  In  particular,  the  growth  at  488℃  provides   the   strongest   Heusler   peak   however   unfortunately   high   temperature   growth  also  increases  surface  roughness.    

In  order   to  verify   that   the  Mn2VSi   thin   film   is  antiferromagnetic,  a  5nm  ferromagnetic  CoFe  layer   was   deposited   on   the   top   of   the   Mn2VSi   layer.   When  materials   with   ferromagnetic   (F)  ordering  and  antiferromagnetic  (AF)  ordering  are  in  close  contact  and  are  field  cooled  through  the  Neel   temperature   (TN)  of  AF,  exchange  bias   is   induced   in   the  F   layer.  We  used   the  setting  temperature   of   65℃  for   30   minutes   to   make   the   AF   layer   becomes   paramagnetic   and   the  domains  are  aligned  along  the  direction  of  the  field  when  an  external  field  5000Oe  is  applied  for  sample  grown  at  250℃.  The  exchange  bias  about  10  Oe  was  observed  as  shown  in  Fig.2.  As  the  growth   temperature   was   increased   no   exchange   bias   was   observed.   This   may   indicate   the  intermixingbetween  the  Mn2VSi  and  CoFe  layers  occur  at  higher  temperature.    

 Fig.2.  Hysteresis  loop  for  Mn2VSi/CoFe  film  grown  at  250℃  measured  at  100K  and  room  temperature.  

-­45

-­35

-­25

-­15

-­5

5

15

25

35

45

-­1500 -­1000 -­500 0 500 1000 1500

measured  at  room  temperaturemeasured  at  100k

Magnetization  (emu/cm3)

Applied  field  (Oe)

\

Page 9: EPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler …ah566/core1/abstracts.pdfEPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler Alloys 16 March 2016 Departments of Electronics

Session  II:  Structural  properties  of  Heusler  alloys  15:30~17:00                                                                                                                                                                        G/N  120  

Chair:  Atsufumi  Hirohata  

       

Perpendicular  magnetic  anisotropy  and  its  electric  field  modification  in  the                            MgO/Co-­‐‑based  Heusler  alloy  interface  

Masahito  Tsujikawa  and  Masafumi  Shirai  

Research  Institute  of  Electrical  Communications,  Tohoku  University  

The  half-­‐‑metallic  Heusler   alloys   are   the  promising   candidates   for   the   application  of  spintronic   devices   due   to   the   excellent   physical   properties   such   as   a   high   spin-­‐‑polarization,   low   damping   constant.   For   the   application   of   magnetic   tunnel   junction  (MTJ),   large   perpendicular  magnetic   anisotropy   (PMA)   is   also   necessary   to   overcome  the  thermal   fluctuation.  Recently,   the   large  PMA  is  observed  for  the   interface  between  MgO   and   full-­‐‑Heusler   Co2FeAl.   In   this   study,   we   estimate   the   magnetic   anisotropy  energy   at   the   MgO/Co2XAl   (X   =   V,   Cr,   Mn,   Fe)   interface   by   using   first   principles  calculation,  and  discuss  the  physical  origin  of  interfacial  PMA  on  the  half-­‐‑metallic  alloys.  Furthermore,  we  also  discuss  the  potential  of  the  electrical  control  of  interfacial  PMA.      

15:30

Page 10: EPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler …ah566/core1/abstracts.pdfEPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler Alloys 16 March 2016 Departments of Electronics

Novel  antiferromagnetic  materials  for  spintronic  devices  Teodor  Huminiuc,1  Jan.  Balluff,2  Markus  Meinert,2    Günter  Reiss,2  Kevin  O’Grady  1                                                                        

and  Atsufumi  Hirohata  1  1  Departments  of  Physics  and  Electronics,  University  of  York  

2  Department  of  Physics,  Bielefeld  University  

Antiferromagnetic  (AF)  materials  are  used  as  pinning  layers  for  GMR  and  TMR  devices.    The  standard  alloy  used  for  exchange  bias  is  IrMn  but  Ir  is  a  scarce  element  hence  an  alternative  AF  is  required.  

Ni2MnAl  and  Ru2MnGe  are  Heusler  alloys  with  potential  to  form  an  AF  phase.    The  structural  and   magnetic   properties   of   these   alloys   have   been   studied.   It   is   predicted   that   the   partially  disordered  B2   phase   of   the  Heusler   alloys  will   develop   an   AF   structure   [1].  MnN   also   shows  good  AF  ordering  and  thermal  stability  with  high  values  of  Hex  at  room  temperature.  Epitaxial  and  polycrystalline  thin  films  of  these  materials  were  deposited  using  magnetron  co-­‐‑sputtering  and  an  IBD  system.  The  thickness  of  the  films  was  varied  from  10  to  (100  ±  2)  nm.    The  samples  were  annealed  or  grown  on  heated  substrates  at  temperatures  in  the  range  250  to  700oC.  

Prior   to   crystallisation   the   samples   exhibited   paramagnetic   behaviour.   The   degree   of   B2  ordering   increases   with   both   annealing   time   and   temperature.   A   shifted   loop   (Hex=45Oe)   is  observed  in  epitaxial  grown  samples  at  low  temperatures  (100K)  for  all  the  alloys  listed  as  seen  in  Figure  1.  They  have  been  used  as  pinning   layers   for  the  ferromagnetic  Fe,  Co2MnSi,  Co2FeSi  and  Co.  An  increase  in  coercivity  is  observed  after  field  cooling  the  polycrystalline  bilayers.  The  substrate/AF/F  interfaces  have  been  studied  using  cross  sectional  TEM  as  seen  in  Figure  2.  Seed  layers,  of  Cr/Ag  and  doping  with  Co/Al  have  been  investigated  to  promote  crystallisation  of  the  AF  and  an  increase  in  the  Néel  temperature.  

 

 

 

 

 

 

 

 

 

 

 

 

 

 [1]    Acet  et  al,  J.  Appl.  Phys,  92,  3867  (2002).  

   

16:00

Figure  1:  Hysteresis  loop  of  field  cooled  epitaxially  grown  Ru2MnGe/Fe  bilayer  on  MgO.    

Figure  2:  Cross  sectional  TEM  image  of  Ru2MnGe  deposited  on  a  Si/SiO2  substrate.  

Si

SiO2

Ru2MnGe

8 nm

2   n m2   n m Fe 2 nm

-­0.6 -­0.4 -­0.2 0.0 0.2 0.4 0.6-­60

-­40

-­20

0

20

40

60 m(µemu)

H(kOe)

T=100KH  cool=20kOe

Hex

Page 11: EPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler …ah566/core1/abstracts.pdfEPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler Alloys 16 March 2016 Departments of Electronics

The  role  of  chemical  structure  on  the  magnetic  and  electronic  properties                                                      of  Co2FeAl0.5Si0.5/Si(111)  interface  Barat  Kuerbanjiang  and  Vlado  Lazarov  

Department  of  Physics,  University  of  York  

We   study   high   quality   Co2FeAl0.5Si0.5/Si(111)   heterointerface   for   spintronic   applications.  Electron   energy   loss   spectroscopy   shows   that   in   a   narrow   interface   region   there   is   a  mutual  inter-­‐‑diffusion   dominated   by   Si   and   Co.   Aberration-­‐‑corrected   scanning   transmission   electron  microscopy  reveals  that  the  film  has  B2  ordering.  The  film  lattice  structure  is  unaltered  even  at  the   interface   due   to   the   substitutional   nature   of   the   intermixing.   First-­‐‑principles   calculations  performed  using  structural  models  based  on  the  microscopy  results,  show  that  the  increased  Si  incorporation   in   the   film   leads   to   a   gradual   decrease   of   the   magnetic   moment   as   well   as  significant   spin-­‐‑polarization   reduction.   These   effects   can   have   significant   detrimental   role   on  the   spin   injection   from   the   Co2FeAl0.5Si0.5   film   into   the   Si   substrate,   besides   the   structural  integrity  of  this  junction.  

   

16:15

Page 12: EPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler …ah566/core1/abstracts.pdfEPSRC-JSPS UK-Japan Core-to-Core Workshop on Heusler Alloys 16 March 2016 Departments of Electronics

The  role  of  interfaces  on  the  spin-­‐‑polarization  

Zlatko  Nedelkoski  and  Vlado  Lazarov  

Department  of  Physics,  University  of  York  

By   using   first-­‐‑principles   calculations   we   show   that   the   atomically   abrupt   Co2FeSi(111)/  Si(111)   interface   have   reversed   spin   polarisation   at   the   interface.   This   unfavourable   spin-­‐‑electronic  configuration  can  be  completely  avoided  by  introducing  a  monolayer  of  CoSi2  at  the  interface.  We  show  that  such  layer  is  energetically  favourable  to  exist  at  the  interface.  This  was  further   confirmed  by  direct   observations   of   CoSi2   nano-­‐‑islands   at   the   interface,   by   employing  atomic   resolution   scanning   transmission   electron   microscopy.   The   monolayer   of   CoSi2,   in  addition,  shifts  the  Fermi  level  from  the  valence  band  edge  (abrupt  interface  case)  towards  the  conduction   band   edge   of   Si.   The   significantly   improved   half-­‐‑metallicity   and   smaller   energy  difference   between   the   conduction   band   edges   of   the   film   and   Si   make   this   interface   highly  desirable  for  device  applications.  

16:45