�

J. M. D. Coey School of Physics and CRANN Trinity College Dublin, Ireland.

JSI 2-xii-2015

The zero-moment half metal – how can it change spintronics?

JSI 2-xii-2015

MRG

www.tcd.ie/Physics/Magnetism

1.  Half-metals and Heuslers

2.  Tetragonal and Cubic Mn-Ga films

3.  Mn2Ru0.5Ga

4.  What could it be good for?

MRG - A zero-moment half-metal K Rode, H. Kurt, D. Betto, Y. C. Lau, N. Thiyanarajah, P. Stamenov, J. M. D. Coey,

School of Physics and CRANN, Trinity College Dublin, Ireland

JSI 2-xii-2015

H Kurt et al PRL 112 027201 (2014) D Betto et al PRB 91 094410 (2015) N. Thiyanarajah APL 106 122402 (2015)

Coey-Stamenov Group 2014

Yong Chang Lau Gwenael Atkinson Karsten Rode Nivetha Thiyagarajah

Plamen Stamenov

JSI 2-xii-2015

1 Introduction

2 Magnetostatics

3 Magnetism of the electron

4 The many-electron atom

5 Ferromagnetism

6 Antiferromagnetism and other magnetic order

7 Micromagnetism

8 Nanoscale magnetism

9 Magnetic resonance

10 Experimental methods

11 Magnetic materials

12 Soft magnets

13 Hard magnets

14 Spin electronics and magnetic recording

15 Other topics

Appendices, conversion tables.

Cost ~ €68 Amazon.de

www.cambridge.org/9780521816144

1. Half-metals & Heuslers

JSI 2-xii-2015

What is a half-metal ?

↑ ↓

gap

semiconductor

↑ ↓

semimetal

↑ ↓

Fermi level

metal

↑ ↓

ferromagnetic metal ↑ ↓

spin gap

half metal

Spin Polarization P = (N↑ - N↓)/(N↑ + N↓)

JSI 2-xii-2015

What is its spin moment?

↑ ↓

spin gap

half metal

Spin Polarization P = (N↑ - N↓)/(N↑ + N↓) = 100%

Spin moment/formula m = (n↑ - n↓) µB Total electrons/formula n = (n↑ + n↓) – integer Spin↓ electrons/formula n↓ – integer Spin↑ electrons/formula n↑ = (n - n↓) – integer Spin moment/formula m = (n↑ - n↓) – integer

JSI 2-xii-2015

Why is spin polarization important for spintronics?

MR = (R↑↓- R↑↑)/R↑↑

af

I

TMR magnetic tunnel junction (MTJ)

free

pinned

B

GMR Spin valve sensor

af

I

free

pinned

B

Magnetoresistance depends on it.

JSI 2-xii-2015

Tunnel magnetoresistance (TMR)

I

Jullière formula: MR = 2P1P2/(1 - P1P2) If P1 = P2 MR = 2P2/(1 - P2) Taking P = 45%, MR = 51%

magnetic tunnel junction

Parallel ↑↑

Antiparallel ↑↓

An MgO tunnel barrier magnetic tunnel junction

Si/SiO2 Substrate Ta5

Ru30

Ta5 (Ni80Fe20)5

(Ir22Mn78)10

(Co90Fe10)2 Ru0.9

(Co40Fe40B20)3 MgO2.5

(Co40Fe40B20)3 Ta 5�

Top Contact Layer

Bottom Contact Layers

Pinned Layer

Free Layer

Pinning Layers

Ru5

Tunnel Barrier

Capping layers

Synthetic antiferromagnet

Ta 5�Cu 50

Structural guiding layer FCC (111) orientation

AFM with (111) orientation

JSI 2-xii-2015

Thin film stack deposition - Shamrock Tool

Base pressure < 3 x 10-8 Torr 2 Target Facing Target guns (MgO)

Base pressure < 3 x 10-7 Torr 6 Series-III S Guns (DC& RF)

Ø Chamber A Ø  Chamber B

Ø  Chamber C

Base pressure < 10-7 Torr

Ø Chamber D

Base pressure < 10-9 Torr

4-pocket e-beam source

Sputtering source

Vacuum annealing

Metals

Oxides

Wafer flip

UHV

E-beam

Chamber C

Chamber D

JSI 2-xii-2015

What is a Heusler?

JSI 2-xii-2015

Co

Si

Mn

Heusler Alloys X2YZ

Co2MnSi TC = 985 K

Co2FeSi TC = 1120 K

JSI 2-xii-2015

Cubic L21 structure

Half-metals m = 5 / 6 µB/fu

e.g. Co2CrAl 2 x 9 + 6 + 3 = 27 m = 3µB

JSI 2-xii-2015

Nv = 6 for Cr,7 for Mn, 8 for Fe, Ru, 9 for Co; 10 for Ni;3 for Al,Ga, 4 for Si,Ge

m = n↑ - n↓ ntot = n↑ + n↓ m = ntot - 2n↓ n↓ = 12 m = ntot - 24

m = 2n↑ - nv

m = nv - 2n↓ or

?

? H van Leuken and R A de Groot PRL 74 1171(1995)

↑ ↓

Spin gap

Mn3Ga; 3 x 7 + 3 = 24 m = 0µB ??

Ni

Sb

Mn

Half Heusler Alloys XYZ

NiMnSb TC = 730 K Half-metal m = 4µB/fu

JSI 2-xii-2015

m = ntot - 18

R A de Groot et al PRL 50 2024(1983)

Cubic C1b structure

JSI 2-xii-2015

D022 L10 D019

L21 D03 Mn3Ga C1b

Perfectly ordered crystal structures of (a) L21 full Heusler X2YZ (b) D03 X3Z and (c) C1b half-Heusler XYZ compounds. Red lines show the portion of the unit cell, which distorts to form the tetragonal unit cell of the D022 structure (d) and The L10 structure (two unit cells) (e). (f) shows the hexagonal D019 structure

Tetragonal Mn-based ‘Heusler’ Alloys

JSI 2-xii-2015

m = 1.1 µB/fu

K Rode et al PRB B87 184487 (2013)

Neutron Diffraction Ferrimagnetic Structure

2b 4d

Mn3Ga n = 24 D022 structure

2. Tetragonal and Cubic Mn-Ga films

JSI 2-xii-2015

JSI 2-xii-2015

Table 2 In-plane lattice spacings in Ångstroms for various substrates and seed layers.

InAs%†% 4.28% AlAs%†% 4.00%

V*% 4.28% Pt% 3.92%

MgO% 4.21% SrTiO3% 3.90%

%Cr*% 4.11% Pd% 3.89%

Au% 4.08% Ru% 3.82%%

Al% 4.05% Si%†% 3.84%

GaAs%†% 4.00% Cu% 3.61%

*a0√2 † a0/√2 Mn3Ga (D022) a = 3.92 Å;

Mn3Ga (D03) a0/√2 = 4.22 Å

Mn3-xGa thin film growth

Mn3Ga films grown on platinum; Tetragonal D022

Ms=110kAm-1Ku=0.89MJm-3

20 30 40 50 60

Mn3

Ga(

002)

2θ

MgO/Mn3Ga MgO/Pt/Mn3Ga

Mn3

Ga(

004)MgO(002)

Ts=350C

Pt(0

02)rms roughness 0.8 nm

Point contact Andreev reflection

H Kurt Phys Rev B 83, 020405 ( 2011) JSI 2-xii-2015

Ms=110kAm-1Ku=0.89MJm-3

2b 4d

P %

Fe 44

Co 45

Ni 33

Fe20Ni80 48

Fe50Co50 51

Point-contact Andreev reflection

ferromagnet

superconductor

P = (1/2) {1 – [G(0) - G(V>Δsc)]/[G(V>Δsc )]}

G

V

G

V Δsc//2e Δsc//2e

JSI 2-xii-2015

Δsc Δsc ↓

Mn2Ga films grown on 001 MgO; Tetragonal D022

Ms=(-)470kAm-1Ku=2.35MJm-3

JSI 2-xii-2015

series can occupy both X and Y sites. A rich family of materials including ferromagnets, antiferromagnets,

ferrimagnets, half-metals, semiconductors, superconductors, semi-metals, topological insulators and shape memory

alloys can be obtained16,17. The magnetization in Heusler compounds usually follow the Slater-Pauling rule18,19,

which predicts a net magnetic moment of m = Nv - 24 µB2,20 for a four atom X2YZ Heusler compound and m = Nv -

18 µB21 for a three-atom XYZ half-Heusler compound, where Nv is the number of valence electrons per formula.

For example, in Co2MnSi and Co2MnGe Heusler compounds Nv = 29, which predicts a magnetization of 5 µB per

formula for both, and the net magnetic moment determined by powder neutron diffraction are in good agreement

within few percents22. In half-Heuslers, the examples are NiMnSb (Nv = 22) and CoMnSb (Nv = 21), which have net

magnetic moments of approximately 4 and 3 µB per formula as predicted by Slater-Pauling rule and confirmed by

powder neutron diffraction23.

20 30 40 50 60 7010-2

100

102

104

106

(c)(b)(a)

C1 b (

004)

L21 (0

04)

D0 22

(004

)

Inte

nsity

2θ

D0 22

(002

)

L21 (

002)

C1 b (0

02)

MgO (002)

V (004)

(d)

Figure 1 Crystal structures of (a) tetragonal D022 full Heusler X2YZ with X, Y and Z occupying 4d, 2a and 2b positions respectively. (b) C1b

half-Heusler XYZ with X, Y and Z occupying 4c, 4a and 4b positions. (c) L21 full Heusler X2YZ with X, Y and Z occupying 8c, 4a and 4b

positions. (d) 2 X-ray diffraction scans of D022-Mn2Ga, C1b-Mn2Ga and L21-MnRuMnGa oriented thin films grown on MgO (001) substrates.

The C1b phase is obtained on (001) oriented vanadium seed layer.

The valence electron count predicts usually the correct magnetization in most cases22,24, provided the structure is

perfectly ordered. Deviations from the valence electron rules can arise from imperfect order and/or tetragonal

distortions. For example, Mn3Ga with 24 valence electrons should have no moment, but it goes through a tetragonal

transformation, which creates a ferrimagnetic order with small magnetisation as opposed to the zero net

magnetisation predicted by the valence electron rule. The alloys in the Mn3-xGa (0 ≤ x ≤ 1) series all undergo a

tetragonal transformation and crystallise in the D022 structure with a high c/a (~1.8) ratio, which increases the

uniaxial anisotropy. As a result, oriented films of these materials are potentially useful for high density non-volatile

memories14,25. For instance, Mn3Ga offers a unique combination of high spin polarisation, low magnetisation and

high uniaxial anisotropy making it an ideal material for spin torque memories down to 10 nm in size26,27, whereas

Growth of cubic Mn3Ga and Mn2Ga films

JSI 2-xii-2015

m(Mn3Ga) = 0.5µB/fu m(Mn2Ga) = (-)1.6µB/fu m(Mn2RuGa) = 0.6µB/fu

3. Mn2Ru0.5Ga

MRG

JSI 2-xii-2015

MRG

The Zero-moment Half Metal

(the compensated ferrimagnetic half-metal)

Crossing the spin gap with Ruthenium Cubic Mn-rich Heusler thin films

JSI 2-xii-2015

Note the spin gap 0.2 eV above EF

Electronic structure of cubic Mn2Ga films

JSI 2-xii-2015

Calculations by Mario Zic, Stefano Sanvito

20 30 40 50 60 7010-6

10-5

10-4

10-3

10-2

10-1

100

101

102

20 25 30 35 40 45 50 5510-6

10-5

10-4

10-3

10-2

10-1

100

101

102

300 325 350 375

0.6

0.8

1.0

0 50 100 150 200 250 300 3500

1

2

3

20 30 40 50 60 70 80

10-2

10-1

100

101

102

103

104

105

Cr (

002)

Mn 3G

a (0

02)

2θ (degrees)

Pt Cr MgO

Mn 3G

a (0

04)

MgO (002)

Pt (

001)

(a)

Cou

nts

(a.u

.)

Cou

nts

(a.u

.)

Cou

nts

(a.u

.)

2θ (degrees )

300°C 315°C 345°C 375°C

(b)

(d)

Cou

nts

(a.u

.)

φ (degrees)

MgO (202) Pt (202) Mn3Ga (202) Mn3Ga (101)

(c)

V (0

04)

D0 19

Mn 3G

a (0

002)

C1 b (

004)

C1 b (

002)

L21 (

004)

D0 22

(004

)

2θ (degrees)

L21 Mn2RuxGa D022 Mn2Ga C1b Mn2Ga D019 Mn3GaM

gO(0

02)

Si(0

04)

D0 22

(002

) L21 (

002)

Ru

(000

2)

JSI 2-xii-2015

Mn2RuxGa films; Cubic with biaxial strain

JSI 2-xii-2015

Atomic structure of cubic Mn2RuGa nv = 25 Ruthenium occupies half of the 4d sites

Atomic structure of cubic Mn2RuxGa MRG nv = 21 for x = 0.5

In-plane lattice parameter a = 5.956Å = √2MgO Out-of-plane lattice parameter c = 6.07Å – biaxial strain

JSI 2-xii-2015

Calculations by Mario Zic, Stefano Sanvito

0 100 200 300 400

0

50

100

150

200

250

0.0 0.2 0.4 0.6 0.8 1.0

600604

-4 -2 0 2 4

-200

-100

0

100

200

M (k

A m

-1)

T (K)

x = 0.00 x = 0.33 x = 0.48 x = 0.66 x = 0.83 x = 1.00

-1.5

-1.0

-0.5

0.0

0.5

m

m (µ

B f.

u-1)

200

300

400

500

600

700

800

900

TC

T C(K

)

17 18 19 20 21 22 23 24 25

a ⊥ (p

m)

x

Nv

(d)

(b)

(c)

Mn2Ru0.52Ga (⊥) Mn2Ru0.52Ga (//)M

(kA

m-1)

µ0H (T)

Mn2Ga (//) Mn2Ga (⊥)

T = 100K

T = 300K

(a)

JSI 2-xii-2015

(-)1.6µB 0 µB 0.6 µB

H Kurt et al PRL 112 027201 (2014)

MRG magnetic properties can be tuned by Ru concentration x

PMA

JSI 2-xii-2015

Mn2Ga Mn2RuxGa

JSI 2-xii-2015

Magnetotransport properties of cubic Mn2RuxGa films

x in the spin gap

Resistivity Anomalous Hall Effect Magneto-resistance

JSI 2-xii-2015

Anomalous Hall E�ect (AHE)

�10 �5 0 5 10

�2

�1

0

1

2

3

µ0H (T)

Rxy

(⌦)

400K350K300K200K100K

�2 �1 0 1 2

�2

�1

0

1

2

µ0H (T)

Rxy

(⌦)

x = 1.09x = 0.77x = 0.73x = 0.69x = 0.62

Anomalous Hall E�ect of MRG without-of-plane external field:

As a function of temperatureThe two Mn sublattices have di�erenttemperature dependencesTotal magnetisation = sum of the twoopposite sublattices magnetisationsVarying the temperature the sign ofthe AHE reverses

As a function of Ru concentration xRu addition shifts the electronic bandsChange in the sublatticesmagnetisationsChange in the Fermi level spinpolarisation

At compensation Mtot ≥ 0 æ noexternal field can influence it

The Hall resistivity is 10 times bigger thannormal 3d metalsæ signature of high spin polarisation

TCD - March 2015 D. Betto - 13/18

Anomalous Hall Effect

-  Used to characterise hysteresis loops

-  Used to probe the spin polarisation at the Fermi level as a function of an external field

Spin polarisation of the conduction electrons ≠ Mz

↑ ↓

spin gap

Mᶿz y

x

VH

I

JSI 2-xii-2015

Anomalous Hall E�ect (AHE)

�10 �5 0 5 10

�2

�1

0

1

2

3

µ0H (T)

Rxy

(⌦)

400K350K300K200K100K

�2 �1 0 1 2

�2

�1

0

1

2

µ0H (T)

Rxy

(⌦)

x = 1.09x = 0.77x = 0.73x = 0.69x = 0.62

Anomalous Hall E�ect of MRG without-of-plane external field:

As a function of temperatureThe two Mn sublattices have di�erenttemperature dependencesTotal magnetisation = sum of the twoopposite sublattices magnetisationsVarying the temperature the sign ofthe AHE reverses

As a function of Ru concentration xRu addition shifts the electronic bandsChange in the sublatticesmagnetisationsChange in the Fermi level spinpolarisation

At compensation Mtot ≥ 0 æ noexternal field can influence it

The Hall resistivity is 10 times bigger thannormal 3d metalsæ signature of high spin polarisation

TCD - March 2015 D. Betto - 13/18

Anomalous Hall Effect

Note: Huge coercivity when M è 0; Ha = 2K1/M Hc ~ 0.1Ha

The perpendicular anisotropy of the films is controlled by the biaxial strain on the film; K1 ≈ 30 kJm-3

MRG-based Tunnel Junctions Stack structure: MRG(40)/Al(0.6)/MgO(1.5)/CFB(1.0)/Ta(0.3)/CFB(0.9)/MgO(0.7)/Ta(2)/Ru(3)

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-1

0

1

2

3

4

5

6

7

TM

R (%

)

Field (T)

TMR_350C TMR_325C TMR_300C TMR_275C TMR_non

Heat treatments

Roughness ~ 0.2nm

MRG

CFB

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

0

10

20

30

40

TMR

(%)

Field (T)

10 K 50 K 100 K 150 K 200 K 250 K 300 K 350 K 400 K

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-18-16-14-12-10

-8-6-4-202

TMR

(%)

Field (T)

10 K 50 K 100 K 150 K 200 K 250 K 300 K

TMR at 10 mV (right), at -1V (left), annealed at 350°C, various temperatures

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-20

-10

0

10

20

30

40

50

TMR

(%)

U (V)

10 K 300 K

Not much temperature dependence

Big temperature dependence near zero bias . Can we fix it by EF engineering?

MRG

CFB

JSI 2-xii-2015

X-ray Magnetic Circular Dichroism (XMCD)

X-ray Absorption Spectroscopy (XAS):Synchrotron lightMn L-edges:

Electronic transition 2p æ 3dSpin-orbit coupling splits 2p1/2 and2p3/2Two peaks: L3 and L2

X-ray Magnetic Circular Dichroism(XMCD):

An undulator allows to have left orright circularly polarised lightExternal applied magnetic fieldDi�erent absorption for left/rightpolarisation (‡≠,‡+) with magneticsamplesXMCD signal = XAS(‡+) - XAS(‡≠)Sum rules are used to obtain themagnetic moment

TCD - March 2015 D. Betto - 14/18

X-ray magnetic circular dichroism XMCD

Information from surface layers (~ 5 nm) TEY

What we expect to see – sum of Mn 4a and 4c

JSI 2-xii-2015

AlOx or MgO 2 nm MRG 4 – 70 nm MgO substrate

JSI 2-xii-2015

XMCD signal flips near compensation as a function of the Ru composition and temperature

XAS and XMCD at Mn L2 and L3 edges All samples ~ 70nm

XAS shows there no change in structure or ionic state with temperature

X-ray Magnetic Circular Dichroism; Experiment

versus T

versus x

Site-specific magnetisation

0.6 0.7 0.8 0.9 1 1.1 1.20.4

0.6

0.8

1

1.2

1.4

Ru concentration x

2hS

zi(µ

B/Mn)

Mn 4c

Mn 4a

1.8 2 2.2 2.4 2.6 2.8

0.4

0.6

0.8

1

1.2

1.4

(

ca � 1)⇥ 100 (%)

2hS

zi(µ

B/Mn)

Mn 4c

Mn 4a

Sublattices magnetisations as a function ofRu concentration x :

Mn 4a is almost constant æ no Runearest neighboursMn 4c increases with two di�erentslopes:

x Æ 0.7: half metallic region æ onlyone spin band is filled with additionalelectronsx > 0.7: “normal” ferromagnet regionæ both spin bands are filled withadditional electrons

As a function of tetragonal distortion c/aMn 4c decreases linearly æcompressive strain empties themajority spin band by increasing itsenergyMn 4a first increases æ compressivestrain empties the minority spin bandby increasing its energy. . . then decreases æ deformation ofthe electronic bands at high strainvalues

TCD - March 2015 D. Betto - 16/18

•  XMCD flips at the compensation temperature or composition

•  From the simulation the site specific moments of the Mn in 4a and 4c sites are extracted

•  They are antiferromagnetically coupled •  Change in sign reflects the reversal of the

spin polarization at the Fermi level •  Moments are very sensitive to strain

XMCD; site-specific Mn moments

JSI 2-xii-2015

Site-specific magnetisation

0.6 0.7 0.8 0.9 1 1.1 1.20.4

0.6

0.8

1

1.2

1.4

Ru concentration x

2hS

zi(µ

B/Mn)

Mn 4c

Mn 4a

1.8 2 2.2 2.4 2.6 2.8

0.4

0.6

0.8

1

1.2

1.4

(

ca � 1)⇥ 100 (%)

2hS

zi(µ

B/Mn)

Mn 4c

Mn 4a

Sublattices magnetisations as a function ofRu concentration x :

Mn 4a is almost constant æ no Runearest neighboursMn 4c increases with two di�erentslopes:

x Æ 0.7: half metallic region æ onlyone spin band is filled with additionalelectronsx > 0.7: “normal” ferromagnet regionæ both spin bands are filled withadditional electrons

As a function of tetragonal distortion c/aMn 4c decreases linearly æcompressive strain empties themajority spin band by increasing itsenergyMn 4a first increases æ compressivestrain empties the minority spin bandby increasing its energy. . . then decreases æ deformation ofthe electronic bands at high strainvalues

TCD - March 2015 D. Betto - 16/18

DAVIDE BETTO et al. PHYSICAL REVIEW B 91, 094410 (2015)

The common way to circumvent this problem is to assumethat the values for the magnetic moments obtained by thesum rules have to be multiplied by a factor ∼1.5 in the caseof Mn [10], to obtain agreement with other magnetometrymethods. We address this differently. For a direct comparisonwith an explicit quantum mechanical core-hole correctedmultiplet calculation, we use the code initially written byCowan, and further developed by Thole [11], and calculatethe theoretical absorption and dichroism spectra for the twoMn crystallographic positions. The magnetic moments arethen given by the calculated expectation values of ⟨Sz⟩ and⟨Lz⟩. The experimentally observed magnetic moments aresubsequently obtained by scaling the calculated dichroic signalto the observed x-ray magnetic circular dichroism (XMCD).

In intermetallic systems, we expect a high degree ofcharge transfer between the different ions in the unit cell. Ithas, however, been reported that Mn in these alloys retainsa partially localized electronic configuration [12,13]. Wetherefore based our calculated spectra on a model whereMn is in a 3d5 ground configuration with charge transferby interaction with a 3d6L configuration, where L denotesa ligand hole. We used literature values for the transferintegrals Tt2g

= 0.9 eV and Teg= 2.0 eV. The difference

between the Udd and Upd Hubbard potentials was chosen tobe 1.12 eV [14]. In the Heusler structure, Mn occupies onesite that is octahedrally coordinated by Ga and another that istetrahedrally coordinated, as discussed above. We used 1.2 and0.5 eV for the respective crystal field parameters 10Dq. TheSlater integrals were reduced to 80% of their atomic values.The charge-transfer parameters ! were set to the values thatbest reproduce the experimental data: 4.0 and −4.0 eV forthe 4c and 4a positions, respectively. They are the only freeparameters in our model. The remarkably large difference incharge transfer from the ligands to the Mn of opposite spinsin the two different positions indicates the degree of chargedelocalization needed to displace sufficiently the states closeto the Fermi level, thereby producing high spin polarizationwhile retaining net moment compensation.

In Fig. 1 we show the experimental data along with thecalculated contribution from the two, antiferromagneticallycoupled, sites. We also identify a small (5%) contributionfrom Mn3+, which is likely to be an oxide in the grainboundaries in the film. An estimate of the grain surface-to-volume ratio can be obtained from the average grain sizeas measured by RSM. We find that the in-plane coherencelength is ∼150 nm, implying that ∼2.6% of the Mn ispresent in these boundaries, assuming the boundary is oneatomic layer thick. We calculated this contribution as a Mn3+

ion with 10Dq = 0.5 eV. The calculated and experimentalspectra show excellent agreement with each other. We findzero-temperature expectation values 2⟨Sz⟩ of 4.35µB and4.85µB with 3d occupation numbers of 5.65 and 5.15 forthe 4a and 4c positions, respectively. The orbital moment ⟨Lz⟩and the dipolar moment ⟨Tz⟩ are found to be ∼0 for bothpositions, as expected for Mn in the 3d5 configuration. In viewof recent neutron diffraction measurements [13] and densityfunctional theory (DFT) calculations [15] of Mn moments ofrelated compounds, the value of 2⟨Sz⟩ obtained from the fitsand the multiplet approximation are likely to be about 30%too high.

0

50

100

XA

S(a

rb.u

nit

) Exp.

Mn 4c

Mn 4a

Mn3+

Calc.

636 638 640 642 644 646 648 650 652 654 656 658 660

−10

0

10

20

Energy(eV)

XM

CD

(arb

.unit

)

FIG. 1. (Color online) The isotropic x-ray absorption and dichro-ism spectra for a typical Mn2RuxGa sample. The calculated con-tribution from each crystallographic position is shown in thindotted/dashed lines. The dichroic spectra for each site has oppositesign for the two positions 4a and 4c, confirming their antiferromag-netic ordering.

III. RESULTS AND DISCUSSION

In Fig. 2 we show the temperature dependence of the spinmoments for a typical sample. There is a clear variationof the magnitudes of the moments of MRG samples withdifferent Ru levels and c/a ratios, however, the general trend

0 50 100 150 200 250 300 350 400

−0.2

0

0.2

0.4

0.6

0.8

1

1.2

Temperature(K)2

Sz

(µB

f.u.−

1)

Mn 4cMn 4aTotal

FIG. 2. (Color online) Temperature dependence of the magneti-zation (absolute values) of a selected sample, with x = 0.98. Thechange of sign of the magnetization occurs at Tcomp ∼ 310 K, ingood agreement with the compensation temperature measured bythe spontaneous Hall effect. The lines are guides to the eye only.At T = 300 K, the sample is almost perfectly compensated, and wewere unable to achieve even partial magnetic saturation with themaximum applied field available (µ0H = 6.8 T), hence the deducedspin moments tend towards 0.

094410-2

TR-MOKE measurement

-100 0 100 200

0

5

10

Ker

r rot

atio

n (a

.u.)

Delay (ps)

Model ExpDec1

Equationy = A1*exp(-x/t1) + y0

Reduced Chi-Sqr

0.03953

Adj. R-Squ 0.98744Value Standard E

I=0A;Source=0

y0 0.6186 0.02437A1 12.515 0.24441t1 17.355 0.39

0 20 40 60

0

5

10

Ker

r rot

atio

n (a

.u.)

Delay (ps)

Model ExpDec1

Equationy = A1*exp(-x/t1) + y0

Reduced Chi-Sqr

0.1149

Adj. R-Square 0.97334Value Standard Error

I=0A;Source=0

y0 1.36612 0.06462A1 11.30823 0.16641t1 16.2464 0.47103

-1 0 1 2 3 4 5 6

0

5

10

Ker

r rot

atio

n (a

.u.)

Delay (ps)

Increased demagnetization time

G M. Müller et al. Nature Materials 8, 56 - 61 (2009)

JSI 2-xii-2015

4. What could it be good for?

JSI 2-xii-2015

Seed(MRG

Cu/TiN CFB

Cap(

uhMRG(MRG-1 Cu/TiN MRG-2

uhMRG(

This stack uses thermally assisted switching of the MRG-1 layer. Heating with a medium power current pulse reverses MRG-1, but not MRG-2. The ultra-hard uhMRG layers (or SAFs) should remain stable. Operation requires only unipolar pulsing and should have density advantages.

This stack uses STT switching of CFB, with a twist. The polarisation of the injected current from the MRG depends on temperature (current density). Bipolar writing is done with pulses of the same sign and different magnitude. There will be less commutation logic and therefore smaller footprint

JSI 2-xii-2015

New spintronic memory

JSI 2-xii-2015

A terahertz on-chip oscillator

Optical detection of an excited magnetisation mode with a frequency of 490 GHz (preliminary data, M. Gensch).

JSI 2-xii-2015

Ø Cubic Mn2Ru0.5Ga is the first example of the long-sought zero-moment half metal. It has 21 valence electrons

Ø Biaxially strained cubic films grow on MgO 001 at 350 °C Ø  They have perpendicular anisotropy (Ku ~ 30 kJm-3) and

huge coercivity. Ø Curie temperature is ~ 200 °C Ø  Fermi-level spin polarization at room temperature is ≈ 60% Ø MRG creates no stray field, and it is immune to external field Ø MRG may serve as the pinned layer in ultra-thin stacks with

no SAF (read heads); but better MTJs have to be built. Ø  It could serve as the storage layer in memory, but STT or

thermal switching switching needs to be demonstrated Ø Prospect of very high frequency operation for THz oscillators

Summary

JSI 2-xii-2015

!

Thank you !

MRG