NkB ln(2S+ 1)

-- a

0

E 4

. ---

�

---,

2

0 3

A

v

o p ⁼ 0 kbar ^D

,.

_{0.8 kbar} 0 h. 2.0 kbar ^•

.T(K) 6

2.7 kbar 3.2 kbar 4.6 kbar 6.5 kbar 7.8 kbar

9 b

c d

Figure 8.5: Temperature dependence of magnetic entropy

(Sm)

of F5PNN aL each pressure. The broken straight line shows the total magnetic entropy of

Nknln(2S

f

1)

with

S

=

1/2.

Four dotted curves (a-d) shows the Duffy et al. 's theoretical curves of

Sm

with such two parameters as a and

Jdks;

(a) a= 0,

J1/k8

= -3.1 K,

(b)

a

= 0.6,

Jdks

= -3^.9 K, (c) a= 0.8,

Jdks

= -4.4 K, and

(

d

)

a= 1.0,

Jdk8

= -1.5 K.

8.2. EXPERIMENTAL RESULT S AND DISCUSSION

70

•

60

^•

5� 2 4

p (kbar)

151

6 8

Figure 8.6: Pressure dependence of De bye temperature 80 of F 5PNN . The solid line is a guide for the eye.

152CHAPTER 8. PRESSURE EFFECT OF ONE-DIMENSIONAL ALTERNATING ANTIFJ

--0>

ro

o

5

.---�----�----�----�----�--�

1

0 3 T(K)

o p = 0 kbar ^A 2.7 kbar ... 0.8 kbar v 3.2 kbar

A 2.0 kbar ⁰ 4.6 kbar

a: ^a = 0, J1/k =

-

^{3.1 K}

b: ^{a =} 0.6, J1/k ^=- 3.9 K c : a ₌ o. 8, J1 I k =

-

^{4.4 K}

d ^{: a} = 1.0, J1/k =-4.3 K e : a ₌ 1 . 0, J1 I k =

-

^{4. 5 K}

c ...

d e

6

0 6.5 kbar

• 7.8 kbar

9

Figure 8. 7: Temperature dependence of

Cmag

of F 5PNN under the pressure up to 7.8 kbar. The broken lines

(

ar-e) express the Duffy et al. 's theoretical curves of

Cmag

[31]

up to 7.8 kbar with such two parameters ^{as a} and

Jifk8.

8.2.

EXPERIMENTAL RESULTS AND DISCUSSION

1

--

_"-.

�

...

...

'-..

'

0.9 �

� "

J '\ '\

ol

I

\

---..

�

� _0.8

�

0.7 0

\ I

' I

b I

\

_\

_I

I

/ "

\

/ ',

/ q

/

----o---0--0.5

a

"-.

153

I I I

I

4

I I

I ^..-₀

E

�

----J

....__...

o �

-- 3 1

Figure

8.8:

Alternating ratio (a)-dependence of the maximum of

Cmag (D)

and

Tmax (

^o

)

in the S =

1/2

Heisenberg antiferromagnetic chain with the infinite number of the spin

(31].

Duffy et al. have reported theoretically about the thermal and magnetic proper­

ties of the

S

=

1/2

alt

e

^r

n

ati

n

g H

e

i^s

en

b

e

^rg chain, i

n

cludi

n

g th

e

^{a =}

Jd J2 depen­

dence of the heat capacity

[31].

According to their study, ^as the alternating ratio a increases (i.e. the alternating chain approaches to the uniform one), the maximum value of

Cmag

decreases and the temperature

(Tmax)

giving the maximum value of

Cmag

increases a.s shown in Fig.

8.8,

which is the enlarged inset figure of Fig.l.

1

⁶

(

^A).

It is remarkable that our results of p =

2.0

^{kbar and 4.6} kbar are reproduced with the solution of a ⁼

0

.6 and

0.8,

respectively. Similar tendency ha.s been seen in th results of

Sm

^{a.s Fig.} 8.5. The experimentally determined relation among two interactions

(11, 12),

alternating ratio

(

^a

)

and pressure is shown in Fig.

8.9.

Figure

8.9

indicates that the magnetic state under p

�

6_.

5

kbar is of the uniform chain.

Both ]1 and

12 (

^<

Jd

are enhanced by the pressure, but the enhancement of

J2

^is

larger than that of

11.

l54CHAPTER 8. PRESSURE EFFECT OF ONE-DIMENSIONAL ALTERNATING ANTIFl

-0.8

a

1 0.6

0 0 2 4 6

P (kbar)

Figure

8.9:

Estimated pressure dependences of J1

(•),

¹²

(

^o

)

^{and a}

(•)

^{of F5PNN.}

The broken lines are guides for the eye.

8.3. CONCLUSION 155

8.3 Conclusion

We have confirmed the first pressure-induced crossover from the alternating to the uniform chain system which continuously occurs in one compow1d as a func­

tion of applied pressure by the measurement of heat capacity. The characteristic exponential behavior of the low-temperature heat capacity of F5PNN at ambient pressure gradually transforms to the linear dependence on the temperature with the increasing pressure, and eventually the lD spin system becomes to the uniform 1 D

Heisenberg system at p � 6.5 kbar.

The crystal structure of F 5PNN at room temperature and ambient pressure is of the uniform symmetry, but the magnetism at low temperatures is of the alternating chain. There must be the change of the structure from the uniform to the alter­

nating symmetry as the temperature is lowered. The applied pressure must work to suppress that structural transition , and therefore the uniform chain system will become stable at the pressurized state above 6.5 kbar.

.

...

^{. ·}

Chapter 9

Pressure Effect of Prototype FerroiTiagnetic CoiTipound Galvinoxyl

9.1 Introduction

Th galvinoxyl radical crystal * had played a great role in the study of mechanism of

� rromagnetic interaction in organic compounds before the discovery of the first bulk ferromagnet ,Bphase p NPNN. This was the first ferromagnetic compound studied by Mukai et al. in

1967 [4L

which shows the highest positive Weiss temperature e ⁼ + ¹¹ K and the change into a nonmagnetic state at

85

K (see the inset of Fig.

9. 1) [4].

However, the inspection of the ferromagnetic intermolecular interaction in galvinoxyl led K.Awaga and M.Kinoshita to the charge transfer mechanism as a possibility of the ferromagnetic interaction in organic radical crystals, as mentioned in chapter

1

(

ection

1. 1)

and chapter

6 [24,25].

On the other hand, the phase transition at

85

K was reconfirmed to be a first kind of phase transition by the heat capacity measurement

[94].

Hosokoshi et al.

m ntioned that the crystal structure af low temperatures loses the two-fold rotation symm try within a molecule at room temperature

[93].

The loss of the two-fold symmetry results in the permission of the dimerization between the molecules.

It has been reported that the transition at

85

K is suppressed by doping the impurity as shown in Fig.

9.1,

where the pure galvinoxyl is mixed with the precur­

sory clos d shell compound, hydrogalvinoxyl

[25,95].

It is obviously seen that the ferromagnetic f ature of the magnetic susceptibility XP is maintained in the impurity system down to low temperatures, without showing the abrupt change at

85

K.

•

4-[3,5-

bis(1, 1-dimethy lethy 1)-4-oxo-2 ,5-cyclohexadiene-1-ylidenemethyl]-2,6- bis(1 ,1-dimethy 1-ethyl)­

phenoxyl

157

158CHAPTER 9. PRESSURE EFFECT OF PROTOT)rp£ FERHOMAGNETIC COMPOUf\

4

'tlll 3

� "

8

^" "

� z"

� "

-�

0 1 ^I

rl "

"

�10

�e �

:::. ⁵

><

0

...

�

.·.

=\

I I I

100 100 )()()

T/K

0

"'--

... ..__.,._.,_• •mm[ID['IO_,m,... • •,. 11 :D:1I

0 100 zoo

T/K

300

·��-<}·

galvinoxyl

hydrogalvino�tyl

Figure

9.1:

T

�

mperature dependence of XP of the

6:1

mixed crystal of galvinoxy]

and hydrogalvmoxyl

[25].

Inset shows XP of the pure galvinoxyl crystal

[ 2

_5].

Recently, Hosokoshi et al. have observed that the application of high pressure also suppresses the phase transition at

85

K, and the ferromagnetic behavior is r main cl down to low-temperatures, by the measurements of magnetic susceptibility under pressure in the wider temperature region

(1.8

K � T � 300 K) (Fig.

9. 2) [39,93].

Furthermore they have observed that the rapid cooling of the crystal suppresse.<> the nonmagnetic transition as well, whereas in the slow cooling process the jump in the Xp was detected at

85

K. They suggested that the pressure enhanc d the energy barrier of the structural transition. In fact, under p =

7

kbar t.h transition was sufficiently suppressed, and the temperature dependence of magnetic susceptibility was reproduced by the S ⁼

1/2

Heisenberg ferromagnetic chain model

(J

DlfF) with

2] _/

_{kB =}

25

K, as shown in Fig.

9. 2.

Furthermore, the extrapolation of inverse slls­

ceptibility to the low temperature suggests the possibility of the bulk-ferromagnetic ordered state below

0. 05

K, if it occurs.

In these circumstances of the study about galvinoxyl, this chapter reports the magnetic properties of galvinoxyl in much lower temperature region

(T � 2.0!<)

ex­

amined by the measurements of heat capacity and ac-magnetic susceptibility under pressure. The detailed purposes are

(1)

to make the existence of the bulk-magnetic order clear and

(2)

to reconfirm the properties of the magnetic uniform

1D

system stabilized under pressure.

9.2. EXPERIMENTAL RESULTS AND DISCUSSION

....---.4

� 0 0 !23

h

/7 kbar

0.

h 2

X X 0.

159

Figure 9.2: Temperature dependences of Xac of galvinoxyl at p = Po and

7

^kbar

[39,93]. The solid curve expresses the theory of S = 1/2 1DHF with 2Jjk8 = 25 K.

9.2 Experimental Results and Discussion

9.2.1 Ac magnetic susceptibility

Figure 9.3 shows the temperature dependence of ac-magnetic susceptibility (Xac) (ffac = 0.1 Oe peak-to-peak and

f

= 200Hz) under the external field (H) at p = 6.4 kbar. The real hydrostatic pressure at low temperatures was estimated from the pressure dependences of the superconductive transition temperature of Sn [40].

The bulk-magnetic order in H = 0 kOe is detected at T = 0.

7

K ^as seen in Fig. 9.3, although Hosokoshi et al. suggested that the bulk-magnetic order would occur at T = 0.05 K. The extrapolated value of Xac down to the zero temperature is not far from about two thirds of the maximum value at T ⁼ 0. 7 K. Furthermore the application of external fields makes the maximum shift toward lower temperatures.

This field dependence is characteristic of the bulk-antiferromagnet [6] as is pointed out in Fig.6.15 and Fig.

7.3.

9.2.2 Heat capacity

Figure 9.4 shows the temperature dependence of magnetic heat capacity

( Cmag)

under external magnetic fields up to 10 kOe at p = 6.4 kbar. The magnetic heat capacity

Cmag

was estimated by subtracting the lattice contribution

(80

= 112 K, ^r

=

3)

from the total heat capacity in the same way ^as employed in previous chapters.

The sharp peak intrinsic of the bulk-magnetic order is detected around 0.

7

K, as seen in Xac· The application of external fields makes the peak shift toward lower

160 CHAPTER 9. PRES SURE EFFECT 0 F PROTOTYPE F ERROM A GN ETI C COM PO Uf\

[x1 o-6]

H = 500 Oe

1 galvinoxyl

Level: 0.2 V

f:

200Hz

,..-... Sens. : 3 1-1 V

>

:::t 0

p: 6.4 kbar

..._.., u ttl

�

H=OOe

-:-1 _��oo

o0o�

0 0

,.

••

0 1 ²

T(K)

Figure 9.3: Temperature dependence of Xac of galvinoxyl under external fields up to 500 Oe at p = 6.4 kbar.

9.2. EXPERIMENTAL RESULTS AND DISCUSSION

...-._

�

0

s

�

----1--,

...__, bO ro E

\.) 2

1

• •

o H ⁼

0

_Oe

_o

_H = 500 Oe ^A H ⁼ 5 kOe

o H ⁼ 200 Oe ^• H ⁼

1

_kOe • H ⁼

10

_kOe

1

61

Figure

9.4:

Temperature dependence of Cmag of galvinoxyl under external fields at p = 6.4 kbar. Each solid line expresses the theory of S = 1/2 lDHF

[15,48]

_with

2J /k8 =

18.0

_{K in} such magnetic fields ^as

II= 0, 5

_and

10

_kOe.

162CHAPTER 9. PRESSURE EFFECT OF PROTOTYPE F'ERHOMAGNETIC COMPOUl'

�

...-._

... CFJ

· �

� ^•

';:j

.n _{'1---4 -}

0

ro

'-'

bO

"'

\.)

E ₀ 0

- 0

0.4

I I I

I I

I

^• II= 1 kOc

I

^{• • • • • • • • • • •}

•

I

-I I

o 810o

/{ = 500 Oc

�

o

I

^o

C8 oo o o o

. o o O�oQ) 0 ooo

^CD

ooo

^•

0 0

0

I

0.6 ol I

I

^{o o o} II= 200 Oe

0

0

o o

cP

^o 00 ^{o o}

I

F

⁰

oo

lao

_<o ^()o _{II= 0 Oe}

I o ��oooo 6>oo

I ^{0 0}

I I

0.8 1

T(K)

0

-0

1.2

Figure

9.5:

Temperature dependence of Cmag of galvinoxyl under external fields up to 1 kOe at p

=

_6.4 kbar below 1.2 K.

temperatures, ^as shown in Fig.

9.5.

This behavior is characteristic of the bulk­

antiferromagnetic ordering, ^as seen also in the 1-phase

p-NPNN [6].

On the other hand, Cmag in the higher temperature region than

1.0

_{K shows}

the following feature of the S = 1/2 1DHF

[15,48]; (1)

the plateau in If ⁼

0

Oe, and (2) the appearance of the broad hump of Cmag under external fields

(II 2

1 kOe) that shifts toward higher temperatures with the increasing field. Here th ferromagnetic intrachain interaction is estimated to be 2] / k8 ⁼ 1 8.

0

K ^{by the com­}

parison with the theoretica

l calculation [15,48],

which is close to 2J jk8 ⁼

25

K determined by Hosokoshi et al. ^as in Fig. 9.2. These experimental results indicate that the pressurized state of galvinoxyl has the magnetically uniform chain structure.

This fact easily can be imaged from the crystal structure at room temperature ^as Fig.l.6, in which one-dimensional zig-zag interaction-paths accompanying the small SOMO-SOMO overlapping are expected. We may estimate the order of interchain interaction which brings about the bulk-antiferromagnetic orderin g at

Tc(P)

^�

0.

₇

K, based on the mean field theory ^as in eq.7.4. With the values of

Tc(p)

⁼ 0.7 K and 21/kc = 18.0 K, we have zJ'jkB ^rv 0.22 K ^{(lzJ'j2Jl rv 1.2 X}

10-2).

9.3. CONCDUSION 163

9.3 Conclusion

Galvinoxy l radical crystal has the highest Weiss temperature 8 = 11 K among all of genuine organic compounds. For this attractive value of large positive 8, this compound has provided many instructive informations, especially about the charge transfer mechanism of the ferromagnetic exchange interactions via overlapping of the molecular orbitals with the different symmetry. In reality, however, this compound undergoes a structural phase transition at 85 K, losing the ferromagnetic behavior below this temperature. Following the recent suggestion by Hosokoshi et al. that the magnetic susceptibility continues to behave ferromagnetically down to lower temper­

ature under certain pressure, we have confirmed the ferromagnetism of galvinoxyl is maintained down to 0. 7 K, higher temperature than 0.05 K predicted by them, by the measurements of heat capacity and magnetic susceptibility under pressure 6.4 kbar. We proposed a magnetic model system of galvinoxyl under pressure at 6.4 kbar as a quasi-one-dimensional ferromagnet with the intrachain interaction

2J j kB

� 18.0 K, weakly coupled by the interchain antiferromagnetic interaction

z]' /kB

^r-..J

0.22

K. This shows a very strong one-dimensionality of the order

lzJ' /211

'""' 1.2 x

10-2. Now a systematic experiment under pressure is being undertaken.

Chapter 10

ドキュメント内 純有機ラジカル結晶における磁性の実験的研究 (ページ 32-40)