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Magnetisation study of the magnetic phase diagram in MnSi
Journal of Magnetism and Magnetic Materials 104-107 (1992) 689-690 North-Holland
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Magnetisation study of the magnetic phase diagram in MnSi C.I. G r e g o r y a, D.B. L a m b r i c k b and N.R. B e r n h o e f t a " Department of Physics, Uniuersity of Durham, Durham, UK Department of Physics and Mathematics, Manchester Polytechnic, Manchester, UK Magnetisation measurements have been made on a single crystal of MnSi in the temperature range 10-40 K using a vibrating sample magnetometer with the field applied in the [001] and [111] directions. The focus of attention is on the strong field dependence and anisotropic nature of the magnetic susceptibility in the vicinity of the magnetic ordering temperature. Manganese silicide (MnSi) belongs to a class of itinerant helimagnets with the cubic B20 crystal structure. Other materials in this class include the cubic polymorph of F e G e [1] and the random substituted F% xCoxSi alloys [2]. These materials exhibit a helical spin density wave with a long, incommensurable, spatial periodicity at low temperatures. In this brief report we complement previous studies [3-7] by detailed high sensitivity measurements of the magnetisation. The single crystal sample used in this study (2.5 mm diameter × 0.7 mm disc) has been carefully prepared to a high standard of purity and homogeneity. Preliminary characterisation has been made by X-ray crystallography a n d e l e c t r i c a l r e s i d u a l r e s i s t a n c e ratio (P293 K / P 4 . 2 K = 2 0 0 ) measurements. The samples have also been used in other experiments, for example Fermi surface studies by the de H a a s - v a n Alphen method [7]. To study the details of the phase transition to the magnetically ordered state we have made measurements of the bulk magnetic response with a high sensitivity vibrating sample magnetometer with a resolution of approximately 10 -4 emu. Temperatures were monitored with both a calibrated thermocouple and a carbon glass resistor. Standard field corrections and demagnetisation factors have been applied to all data sets. Preliminary studies revealed a richly structured and anisotropic magnetic phase diagram in the vicinity of the transition from the paramagnetic phase. We focus attention on the response obtained with the applied magnetic field parallel to two major symmetry directions [111] and [001] anticipated to be of major importance in the magnetic ordering of this class of materials [8,9]. The required temperature resolution and stability (typically +0.1 K at 30 K) made it necessary to collect data at a fixed temperature as a function of applied magnetic field. From these data we derive differential susceptibilities. The inverse susceptibilities for the major symmetry directions [001] and [111] are shown in fig. 1. At 30 K both directions exhibit a smooth, concave-upward, response indicative of incipient magnetic saturation. In the interval (29 _+ 0.2) K over a field sweep between
60
X45 30 75
<001> 29.1K
<111) 29.0K
<001) 28.8K
(111> 28.6K
(001;) 28.2K
<111;) 28.2K
(001) 27.6K
<1 11;) 27.6K
3 75 60
75 -- 6(:
7~ 60
3(
"
•
75 ( 0 0 1 ) 27.2K
( 1 1 1 ) 27.2K
60
30 B(GAUSS)
B(GAUSS)
Fig. 1. Displayed in the left-hand frame is the inverse magnetic susceptibility as measured in MnSi with an applied
magnetic field parallel to the [001] direction. Complementary values are given in the right-hand frame for the [111] direction. The solid line is an average through the data points. The spread in the data points for a given field value is a measure of the experimental uncertainty.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
691)
C.I. Gregory et al. / The magnetic phase diagram of MnSi 1
Mn Si [ 001 ]
H(kOe) \
2
\ o_o...o_..~
o I
'l
'~"--I 1
26
2J7
28
29
30
TIKI Fig. 2. The positions of field minima of the inverse susceptibility in the [001] direction displayed as a function of internal field and sample temperature. The dashed, roughly triangular, area in the centre of the figure represents the previously determined "A' phase [6].
+ 3 . 3 kOe we observe the start of the n o n - l i n e a r response with b o t h the [001] a n d [111] directions exhibiting four distinct r o u n d e d minima. T h e high field turning points may be associated with an induced ferromagnetic state [3]; on cooling below 29 K these turning points move out of the selected display window and are of no f u r t h e r interest to o u r discussion here. For this reason they are o m i t t e d as data points in fig. 2. Cooling below 28.8 K and down to 27.4 K we see f u r t h e r qualitative and quantitative changes in the magnetic response. Taking the [001] direction: at 28.8 K we have an onset of a pair (symmetric in positive and negative field) of t h r e e minima, a pair of low field r o u n d e d m i n i m a and two pairs of s h a r p discontinuities at higher fields. T h e s e m i n i m a r e m a i n to 27.4 K with the position of the lower s h a r p discontinuity evolving to higher fields. C o n t i n u e d cooling to 27.2 K and below leaves only the lower r o u n d e d minima. In the same t e m p e r a t u r e a n d field ranges the anisotropy of the response with respect to crystallographic direction becomes marked. In the [111] direction a pair of low field minima persist from 29 to 28.8 K w h e r e a new pair of s h a r p m i n i m a occur which t h e n d i s a p p e a r by 28.2 K
Icaving thc inverse susceptibility with a slow, monotonic, concave-down field d e p e n d e n c e . This unusual b e h a v i o u r indicative of a weak positive feedback in the m a g n e t i s a t i o n may be a vestige of the n o n - l i n c a r response observed in the interval 29 to 28.6 K. F r o m fig. 2 it is clear that our findings are broadly in a g r e e m e n t with previous work [6]. Since our technique selects information on the zero wavevcctor static m a g n e t i s a t i o n density whilst o t h e r techniques used in d e t e r m i n i n g the magnetic p h a s e diagram arc sensitive to a range of wavevector c o m p o n e n t s (possibly both static and dynamic) small differences may be anticipated. M o r e interesting is the discovery of a strong anisotropy and the possibility of a new phase near 29 K. This is of i m p o r t a n c e in the global u n d e r s t a n d i n g of magnetic p h a s e transitions in this class of c o m p o u n d s in the light of thc recent e x p e r i m e n t a l and theoretical findings in the cubic p o l y m o r p h of F e G c by Lebech et al. [1] and P l u m e r [8]. A more c o m p l e t e report covering the low t e m p e r a t u r e b e h a v i o r and f u r t h e r c o m p l e m e n tary m a g n e t i s a t i o n and n e u t r o n scattering studies currently in progress in b o t h materials will be p r e s e n t e d elsewhere. T h e authors wish to t h a n k S E R C for s u p p o r t i n g the M a n c h e s t e r V S M facility, S. Brown for the loan of his samples a n d B. Lebech for stimulating discussions. References
[1] B. Lebech, J. Bernhard and T. Freltoft, J. Phys.: Condens. Matter 2 (1989) 6105. [2] J. Beitle, J. Voiron, F. Towfig, M. Roth and Z.Y. Zhang, J. Phys. F 11 11981)2153. [3] S. Kusaka, K. Yamamoto, T. Komatsubara and Y. lshikawa, Solid State Commun. 2(1 (1976)925. [4] M. Date, K. Okuda and K. Kadowaki, J. Phys. Soc. Japan 42 (1977) 1555. [5] T. Sakahibara, H. Morimoto and M. Date, J. Phys. Soc. Japan 51 (1982) 2439. [6] Y. lshikawa and M. Arai, J. Phys. Soc. Japan 53 11984) 2726. [7] S. Brown, PhD Thesis University of Cambridge (1991)) (unpublished). [8] M.L. Plumer, J. Phys.: Condens. Matter 2 11991/) 75113. [9] P. Bak and M. Jensen, J. Phys. C 13 11980) L881.
/14=
Magnetisation study of the magnetic phase diagram in MnSi C.I. G r e g o r y a, D.B. L a m b r i c k b and N.R. B e r n h o e f t a " Department of Physics, Uniuersity of Durham, Durham, UK Department of Physics and Mathematics, Manchester Polytechnic, Manchester, UK Magnetisation measurements have been made on a single crystal of MnSi in the temperature range 10-40 K using a vibrating sample magnetometer with the field applied in the [001] and [111] directions. The focus of attention is on the strong field dependence and anisotropic nature of the magnetic susceptibility in the vicinity of the magnetic ordering temperature. Manganese silicide (MnSi) belongs to a class of itinerant helimagnets with the cubic B20 crystal structure. Other materials in this class include the cubic polymorph of F e G e [1] and the random substituted F% xCoxSi alloys [2]. These materials exhibit a helical spin density wave with a long, incommensurable, spatial periodicity at low temperatures. In this brief report we complement previous studies [3-7] by detailed high sensitivity measurements of the magnetisation. The single crystal sample used in this study (2.5 mm diameter × 0.7 mm disc) has been carefully prepared to a high standard of purity and homogeneity. Preliminary characterisation has been made by X-ray crystallography a n d e l e c t r i c a l r e s i d u a l r e s i s t a n c e ratio (P293 K / P 4 . 2 K = 2 0 0 ) measurements. The samples have also been used in other experiments, for example Fermi surface studies by the de H a a s - v a n Alphen method [7]. To study the details of the phase transition to the magnetically ordered state we have made measurements of the bulk magnetic response with a high sensitivity vibrating sample magnetometer with a resolution of approximately 10 -4 emu. Temperatures were monitored with both a calibrated thermocouple and a carbon glass resistor. Standard field corrections and demagnetisation factors have been applied to all data sets. Preliminary studies revealed a richly structured and anisotropic magnetic phase diagram in the vicinity of the transition from the paramagnetic phase. We focus attention on the response obtained with the applied magnetic field parallel to two major symmetry directions [111] and [001] anticipated to be of major importance in the magnetic ordering of this class of materials [8,9]. The required temperature resolution and stability (typically +0.1 K at 30 K) made it necessary to collect data at a fixed temperature as a function of applied magnetic field. From these data we derive differential susceptibilities. The inverse susceptibilities for the major symmetry directions [001] and [111] are shown in fig. 1. At 30 K both directions exhibit a smooth, concave-upward, response indicative of incipient magnetic saturation. In the interval (29 _+ 0.2) K over a field sweep between
60
X45 30 75
<001> 29.1K
<111) 29.0K
<001) 28.8K
(111> 28.6K
(001;) 28.2K
<111;) 28.2K
(001) 27.6K
<1 11;) 27.6K
3 75 60
75 -- 6(:
7~ 60
3(
"
•
75 ( 0 0 1 ) 27.2K
( 1 1 1 ) 27.2K
60
30 B(GAUSS)
B(GAUSS)
Fig. 1. Displayed in the left-hand frame is the inverse magnetic susceptibility as measured in MnSi with an applied
magnetic field parallel to the [001] direction. Complementary values are given in the right-hand frame for the [111] direction. The solid line is an average through the data points. The spread in the data points for a given field value is a measure of the experimental uncertainty.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
691)
C.I. Gregory et al. / The magnetic phase diagram of MnSi 1
Mn Si [ 001 ]
H(kOe) \
2
\ o_o...o_..~
o I
'l
'~"--I 1
26
2J7
28
29
30
TIKI Fig. 2. The positions of field minima of the inverse susceptibility in the [001] direction displayed as a function of internal field and sample temperature. The dashed, roughly triangular, area in the centre of the figure represents the previously determined "A' phase [6].
+ 3 . 3 kOe we observe the start of the n o n - l i n e a r response with b o t h the [001] a n d [111] directions exhibiting four distinct r o u n d e d minima. T h e high field turning points may be associated with an induced ferromagnetic state [3]; on cooling below 29 K these turning points move out of the selected display window and are of no f u r t h e r interest to o u r discussion here. For this reason they are o m i t t e d as data points in fig. 2. Cooling below 28.8 K and down to 27.4 K we see f u r t h e r qualitative and quantitative changes in the magnetic response. Taking the [001] direction: at 28.8 K we have an onset of a pair (symmetric in positive and negative field) of t h r e e minima, a pair of low field r o u n d e d m i n i m a and two pairs of s h a r p discontinuities at higher fields. T h e s e m i n i m a r e m a i n to 27.4 K with the position of the lower s h a r p discontinuity evolving to higher fields. C o n t i n u e d cooling to 27.2 K and below leaves only the lower r o u n d e d minima. In the same t e m p e r a t u r e a n d field ranges the anisotropy of the response with respect to crystallographic direction becomes marked. In the [111] direction a pair of low field minima persist from 29 to 28.8 K w h e r e a new pair of s h a r p m i n i m a occur which t h e n d i s a p p e a r by 28.2 K
Icaving thc inverse susceptibility with a slow, monotonic, concave-down field d e p e n d e n c e . This unusual b e h a v i o u r indicative of a weak positive feedback in the m a g n e t i s a t i o n may be a vestige of the n o n - l i n c a r response observed in the interval 29 to 28.6 K. F r o m fig. 2 it is clear that our findings are broadly in a g r e e m e n t with previous work [6]. Since our technique selects information on the zero wavevcctor static m a g n e t i s a t i o n density whilst o t h e r techniques used in d e t e r m i n i n g the magnetic p h a s e diagram arc sensitive to a range of wavevector c o m p o n e n t s (possibly both static and dynamic) small differences may be anticipated. M o r e interesting is the discovery of a strong anisotropy and the possibility of a new phase near 29 K. This is of i m p o r t a n c e in the global u n d e r s t a n d i n g of magnetic p h a s e transitions in this class of c o m p o u n d s in the light of thc recent e x p e r i m e n t a l and theoretical findings in the cubic p o l y m o r p h of F e G c by Lebech et al. [1] and P l u m e r [8]. A more c o m p l e t e report covering the low t e m p e r a t u r e b e h a v i o r and f u r t h e r c o m p l e m e n tary m a g n e t i s a t i o n and n e u t r o n scattering studies currently in progress in b o t h materials will be p r e s e n t e d elsewhere. T h e authors wish to t h a n k S E R C for s u p p o r t i n g the M a n c h e s t e r V S M facility, S. Brown for the loan of his samples a n d B. Lebech for stimulating discussions. References
[1] B. Lebech, J. Bernhard and T. Freltoft, J. Phys.: Condens. Matter 2 (1989) 6105. [2] J. Beitle, J. Voiron, F. Towfig, M. Roth and Z.Y. Zhang, J. Phys. F 11 11981)2153. [3] S. Kusaka, K. Yamamoto, T. Komatsubara and Y. lshikawa, Solid State Commun. 2(1 (1976)925. [4] M. Date, K. Okuda and K. Kadowaki, J. Phys. Soc. Japan 42 (1977) 1555. [5] T. Sakahibara, H. Morimoto and M. Date, J. Phys. Soc. Japan 51 (1982) 2439. [6] Y. lshikawa and M. Arai, J. Phys. Soc. Japan 53 11984) 2726. [7] S. Brown, PhD Thesis University of Cambridge (1991)) (unpublished). [8] M.L. Plumer, J. Phys.: Condens. Matter 2 11991/) 75113. [9] P. Bak and M. Jensen, J. Phys. C 13 11980) L881.