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Zr multilayers
ELSEVIER
Physica B 211 (1995) 335-337
Magnetic and transport properties of Ho/Zr multilayers B. Raquet a, A. Sdaq a, J.M. Broto a, H. Rakoto ~'*, J.C. Ousset a, S. Askenazy ~, A. Baudry b, P. Boyer b, M.C. Luche b, A. Khmou c a Laboratoire de Physique des Solides-SNCMP, Complexe Scientifique de Rangueil, 31077 Toulouse-Cedex, France b CEA/CENG, DRF MC, 85 X,, 38041 Grenoble-Cedex, France c Universitb Moulay lsmaiL Facultb des Sciences, B.P. 4010-Beni M'Hamed-Meknes, Morocco
Abstract We have elaborated on MBE, Ho/Zr multilayers in which holmium crystallographic parameters are considerably reduced compared to those for the bulk. We correlate magnetoresistance measurements with the magnetization under very high magnetic field. We point out a sharp magnetic transition due to the closing of the conicali or helical configurations as in bulk material. This effect disappears for the lower thicknesses of Ho layers and the transition fields changes with the magnetic coupling through the zirconium layers.
Up to now metallic multilayers composed with rare earths have not been so widely studied as those elaborated with noble and transition metals. Nevertheless, these systems present a great interest in the understanding of the relation between structure and magnetic properties which need very high magnetic fields to be understood. During the last years the whole studies underline two points: the low-dimensionality effect in the rare earth layers and the long-range magnetic order through the nonmagnetic metal as Yttrium or Lutetium [1-3]. In this paper we present magnetic and transport properties in high field of holmium/zirconium multilayers. Holmium displays an hexagonal close-packed (HCP) structure with a =3.57/~ and c = 5.61.A. Between T, = 133 K and Tc = 20 K, the magnetic moments are in the basal plane and exhibit an helical structure with a turn angle varying from 51 ° at T. and 33 ° at To. Below 20 K a magnetic transition appears giving a ferro conical
* Corresponding author.
structure with a ferromagnetic component along the c axis. Zirconium is a nonmagnetic 4d transltion metal with an H C P structure (a = 3.23 A, c = 5.14 A). The low miscibility between Zr and Ho is a favorab!e factor to elaborate multilayers with sharp interfacesi The mismatch (10%) compresses Ho in the basal plane. Multilayers [ H o ( X t ~ ) / Z r ( Y A ) with 10 < X , Y < 30 were prepared by an alternate vapour-deposition method in an ultra high vacuum chamber [4]. The samples were grown on a Si(1 00) surface after deposition ot~a 200/~ Zr buffer layer. They exhibit a high [0 0 2] texl~ure. In the basal plane the grain size is about 150A arid the Holmium lattice is contracted of about 1%. Thd zirconium lattice is unchanged. The interface roughness is about two monolayers. Measurements were performed in a high pulsed magnetic field up to 36 T. Magnetoresistance data were obtained by an AC technique (100 kHz) with ia selective amplifier and a digital storage triggered by tl~e field. The signal was recorded during increasing (70 ms) and decreasing ( -~ 1 s) times of the pulsed field ih order to avoid spurious signal such as temperature drift or transient effects. The magnetization is measured using two
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 1 0 5 3 - 6
336
B. Raquet et al. / Physica B 211 (1995) 335-337
concentric series-opposing pick-up coils. Two measurements are made: the first one with the sample outside the coils and the second one with the sample inside the coils. The former signal is subtracted from the latter to obtain the magnetization of the sample. Our first magnetization and magnetoresistance measurements on a multilayers [Ho(30/~)/Zr(30A)] x 160 were realized in a high perpendicular magnetic field at 4.2 K. Both results (Fig. 1) point out a large magnetic transition around 11 T. This transition would be related
1.5 M(n)
J.
R(I)
0.~~ T = 4.2K,,I1± o
o
H
(T)
Fig. 1. Typical magnetization M(H) and magnetoresistance MR(H) with a magnetic field perpendicular to the plane at 4.2 K for holmium/zirconium multilayers.
to the closing conical or helical magnetic configurations finally producing ferromagnetic alignment along the direction of the field. Down to 10 A of holmium, we show that the easy axis remains in the basal plane. By applying the field in plane, the magnetization curve tends to saturation in a monotonic way. At this stage, let us underline the high sensitivity of the magnetoresistance to magnetic transitions. So in Fig. 2, we present high-field magnetoresistance measurements over a large temperature range. Up to about 50 K (Fig. 2(a)); the sharp transition is observed. It is quite temperature independent. However in low fields, the negative magnetoresistance increases with temperature related to higher magnetic disorder. For higher temperature (Fig. 2(b)), the transition remains but is no more characterized by a disordered phase (the magnetoresistance does not exhibit a maximum). We think that such differences by varying temperature correspond to different states in zero field. Indeed resistance versus temperature measurements point out a smooth transition between 25 and 40 K. Only neutron diffraction data could allow to conclude about the initial magnetic state. The transition at about 12 T disappears for Holmium thicknesses, less than 15 A. The magnetic moments continuously turn from the basal plane to the c axis (field direction). In Fig. 3, we present the low-field magnetoresistance for various thicknesses of the zirconium layers. An oscillation is observed: this behaviour is of the same type of that of the squareness of the hysteresis loop. This effect suggest an oscillating indirect magnetic coupling through
o.5
i
AR
k (%)
IIl°(25A)/Zr(30A)tx30
..-'4.2K
0.5
/:; / ""':'~.'~<::.-...... •~,...,z0K / / :" .~'-'~,~.:..
-o.
-%. -• [11o(25A)1 Zr(30 A)lx30
~':k¢, "%%,.
-e~.. %.
-I
11±
" "
~"" "- 651(
a
-0,5
5
15
iO
H
(T)
20
25
b
-1 .E
....:"---.. 77K
ib
i~ H
2b
22
(T)
Fig. 2. Typical magnetoresistance for holmium/zirconium multilayers. (a) At low temperature showing a sharp magnetic transition. (b) At temperature higher than 50 K.
B. Raquet et al./Physica B 211 (1995) 335 337
09
0.2
~---(*/,)
.'""0 ~ 0.78
0.1
rt2 074
°;o ; ,'o ,', ~o ;, ;o tzr ( ' ~ )
o.o
-o. 1
O'..
...
I
~0
15
the zirconium layers as observed in noble transition metal multilayers [5]. New measurements 8re now in progress to confirm these indirect interaction effects. It was well-known that high magnetic field a r e needed to investigate the magnetic properties of rare-earth systems,
particularly m multllayer where interface amsotroples become preponderant. In this paper we show that magnetization i and magnetoresistance measurements in pulsed field can be precize enough: so they are a very efficient to~l to understand the physical properties of rare-earth mlaltilayers.
H=5T
-......"
-0.2
!
337
I
I
I
I
20
25
30
35
tZr(/~)
Fig. 3. Oscillation ofmagnetoresistance for [Ho(30.A)/Zr(Y,A)] x 20 multilayers. The inset shows the squareness of the hysteresis loops of the same samples•
References
I-1] R.W. Erwin, J.J. Rhyne, M.B. Salamon, J. Borchers, S. Sinha, R. Du, J.E. Cunningham and C.P. Flynrl, Phys. Rev. B 35 (1987) 6808. I-2] C.F. Majkrzak, J, Kwo, M. Hong, Y. Yafet, D.iGibbs, C.L. Chien and J. Bohr, Adv. Phys. 40 (1991) 99. 1-3] J.J. Rhyne, M.B. Salamon, C.P. Flynn, R.W. Er~vin and J.A. Borches, J. Magn. Magn. Mater 129 (1994) 3%46. I-4] M.C. Luche, Thesis, Grenoble (1994). 1-5] S.S.P. Parkin, R. Bhadra and K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152.
Physica B 211 (1995) 335-337
Magnetic and transport properties of Ho/Zr multilayers B. Raquet a, A. Sdaq a, J.M. Broto a, H. Rakoto ~'*, J.C. Ousset a, S. Askenazy ~, A. Baudry b, P. Boyer b, M.C. Luche b, A. Khmou c a Laboratoire de Physique des Solides-SNCMP, Complexe Scientifique de Rangueil, 31077 Toulouse-Cedex, France b CEA/CENG, DRF MC, 85 X,, 38041 Grenoble-Cedex, France c Universitb Moulay lsmaiL Facultb des Sciences, B.P. 4010-Beni M'Hamed-Meknes, Morocco
Abstract We have elaborated on MBE, Ho/Zr multilayers in which holmium crystallographic parameters are considerably reduced compared to those for the bulk. We correlate magnetoresistance measurements with the magnetization under very high magnetic field. We point out a sharp magnetic transition due to the closing of the conicali or helical configurations as in bulk material. This effect disappears for the lower thicknesses of Ho layers and the transition fields changes with the magnetic coupling through the zirconium layers.
Up to now metallic multilayers composed with rare earths have not been so widely studied as those elaborated with noble and transition metals. Nevertheless, these systems present a great interest in the understanding of the relation between structure and magnetic properties which need very high magnetic fields to be understood. During the last years the whole studies underline two points: the low-dimensionality effect in the rare earth layers and the long-range magnetic order through the nonmagnetic metal as Yttrium or Lutetium [1-3]. In this paper we present magnetic and transport properties in high field of holmium/zirconium multilayers. Holmium displays an hexagonal close-packed (HCP) structure with a =3.57/~ and c = 5.61.A. Between T, = 133 K and Tc = 20 K, the magnetic moments are in the basal plane and exhibit an helical structure with a turn angle varying from 51 ° at T. and 33 ° at To. Below 20 K a magnetic transition appears giving a ferro conical
* Corresponding author.
structure with a ferromagnetic component along the c axis. Zirconium is a nonmagnetic 4d transltion metal with an H C P structure (a = 3.23 A, c = 5.14 A). The low miscibility between Zr and Ho is a favorab!e factor to elaborate multilayers with sharp interfacesi The mismatch (10%) compresses Ho in the basal plane. Multilayers [ H o ( X t ~ ) / Z r ( Y A ) with 10 < X , Y < 30 were prepared by an alternate vapour-deposition method in an ultra high vacuum chamber [4]. The samples were grown on a Si(1 00) surface after deposition ot~a 200/~ Zr buffer layer. They exhibit a high [0 0 2] texl~ure. In the basal plane the grain size is about 150A arid the Holmium lattice is contracted of about 1%. Thd zirconium lattice is unchanged. The interface roughness is about two monolayers. Measurements were performed in a high pulsed magnetic field up to 36 T. Magnetoresistance data were obtained by an AC technique (100 kHz) with ia selective amplifier and a digital storage triggered by tl~e field. The signal was recorded during increasing (70 ms) and decreasing ( -~ 1 s) times of the pulsed field ih order to avoid spurious signal such as temperature drift or transient effects. The magnetization is measured using two
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 1 0 5 3 - 6
336
B. Raquet et al. / Physica B 211 (1995) 335-337
concentric series-opposing pick-up coils. Two measurements are made: the first one with the sample outside the coils and the second one with the sample inside the coils. The former signal is subtracted from the latter to obtain the magnetization of the sample. Our first magnetization and magnetoresistance measurements on a multilayers [Ho(30/~)/Zr(30A)] x 160 were realized in a high perpendicular magnetic field at 4.2 K. Both results (Fig. 1) point out a large magnetic transition around 11 T. This transition would be related
1.5 M(n)
J.
R(I)
0.~~ T = 4.2K,,I1± o
o
H
(T)
Fig. 1. Typical magnetization M(H) and magnetoresistance MR(H) with a magnetic field perpendicular to the plane at 4.2 K for holmium/zirconium multilayers.
to the closing conical or helical magnetic configurations finally producing ferromagnetic alignment along the direction of the field. Down to 10 A of holmium, we show that the easy axis remains in the basal plane. By applying the field in plane, the magnetization curve tends to saturation in a monotonic way. At this stage, let us underline the high sensitivity of the magnetoresistance to magnetic transitions. So in Fig. 2, we present high-field magnetoresistance measurements over a large temperature range. Up to about 50 K (Fig. 2(a)); the sharp transition is observed. It is quite temperature independent. However in low fields, the negative magnetoresistance increases with temperature related to higher magnetic disorder. For higher temperature (Fig. 2(b)), the transition remains but is no more characterized by a disordered phase (the magnetoresistance does not exhibit a maximum). We think that such differences by varying temperature correspond to different states in zero field. Indeed resistance versus temperature measurements point out a smooth transition between 25 and 40 K. Only neutron diffraction data could allow to conclude about the initial magnetic state. The transition at about 12 T disappears for Holmium thicknesses, less than 15 A. The magnetic moments continuously turn from the basal plane to the c axis (field direction). In Fig. 3, we present the low-field magnetoresistance for various thicknesses of the zirconium layers. An oscillation is observed: this behaviour is of the same type of that of the squareness of the hysteresis loop. This effect suggest an oscillating indirect magnetic coupling through
o.5
i
AR
k (%)
IIl°(25A)/Zr(30A)tx30
..-'4.2K
0.5
/:; / ""':'~.'~<::.-...... •~,...,z0K / / :" .~'-'~,~.:..
-o.
-%. -• [11o(25A)1 Zr(30 A)lx30
~':k¢, "%%,.
-e~.. %.
-I
11±
" "
~"" "- 651(
a
-0,5
5
15
iO
H
(T)
20
25
b
-1 .E
....:"---.. 77K
ib
i~ H
2b
22
(T)
Fig. 2. Typical magnetoresistance for holmium/zirconium multilayers. (a) At low temperature showing a sharp magnetic transition. (b) At temperature higher than 50 K.
B. Raquet et al./Physica B 211 (1995) 335 337
09
0.2
~---(*/,)
.'""0 ~ 0.78
0.1
rt2 074
°;o ; ,'o ,', ~o ;, ;o tzr ( ' ~ )
o.o
-o. 1
O'..
...
I
~0
15
the zirconium layers as observed in noble transition metal multilayers [5]. New measurements 8re now in progress to confirm these indirect interaction effects. It was well-known that high magnetic field a r e needed to investigate the magnetic properties of rare-earth systems,
particularly m multllayer where interface amsotroples become preponderant. In this paper we show that magnetization i and magnetoresistance measurements in pulsed field can be precize enough: so they are a very efficient to~l to understand the physical properties of rare-earth mlaltilayers.
H=5T
-......"
-0.2
!
337
I
I
I
I
20
25
30
35
tZr(/~)
Fig. 3. Oscillation ofmagnetoresistance for [Ho(30.A)/Zr(Y,A)] x 20 multilayers. The inset shows the squareness of the hysteresis loops of the same samples•
References
I-1] R.W. Erwin, J.J. Rhyne, M.B. Salamon, J. Borchers, S. Sinha, R. Du, J.E. Cunningham and C.P. Flynrl, Phys. Rev. B 35 (1987) 6808. I-2] C.F. Majkrzak, J, Kwo, M. Hong, Y. Yafet, D.iGibbs, C.L. Chien and J. Bohr, Adv. Phys. 40 (1991) 99. 1-3] J.J. Rhyne, M.B. Salamon, C.P. Flynn, R.W. Er~vin and J.A. Borches, J. Magn. Magn. Mater 129 (1994) 3%46. I-4] M.C. Luche, Thesis, Grenoble (1994). 1-5] S.S.P. Parkin, R. Bhadra and K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152.