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Magnetic structures and phase diagram of HCP-type heavy-light rare earth, Er-Y-La alloys
Physica B 156 & 157 (1989) 771-773 North-Holland, Amsterdam
MAGNETIC STRUCTURES AND PHASE DIAGRAM OF HCP-TYPE HEAVY-LIGHT RARE EARTH, Er-Y-La ALLOYS
Shinji KAWANO’ and Norio ACHIWA’ ‘Research Reactor Institute, Kyoto University, Kumatori, Sennan, Osaka 590-04, Japan 2Department of Physics, Kyushu University, Hakozaki, Fukuoka 812, Japan
The magnetic phase diagrams of hcp Er-Y, Er-La, Er,,Y,,_,La, and Er,,Y,,_,La, alloys determined by neutron diffraction are presented. Various oscillatory phases with the Ising-like character are found. The behaviour of the c-axis component of the moment is qualitatively discussed on the basis of the ANNNI model.
The magnetic structures of Er metal are a longitudinal sinusoidal order (LSO) below TN, a cycloidal order (CY) below Tcy and then, a conical structure (cone) below TB with decreasing temperature [ 1,2]. These structures are considered to have an Ising-like character. In the past decades Elliott [3] discussed the stability of the LSO phase and showed a transition to an antiphase domain (APD) phase and then to a ferromagnetic phase. Recently Bak and Boehm [4] have given a magnetic phase diagram from a mean field calculation of the Ising model with competing interactions (ANNNI model). They have found out various types of spatially modulated phases against competing interaction Table I Magnetic structures and magnetic ordering temperatures Alloys
TN (W
Er
84.4 75 62 75 72 63
%&a, Er,,La, Er,Y,,
Er,Y,d+.,
Er,Y,La,
Er,YJ+ 5 %J% Er80Y,o Er,,Y,,La, Er,Y,,La,, Er,Y,La,, Er,La,,
60 55 67 62 58 50 49
Magnetic structure LSO LSO LSO LSO LSO LSO LSO LSO LSO LSO LSO LSO LSO
ratios. The magnetic order in Er and Tm is an example of such cases. In Er-based alloys one can expect that the Ising-like character and competing interactions vary with Er concentration. The purpose of the present note is to examine a relation among magnetic phases, the Ising-like character and competing interactions in Er-based alloys with Y and La. We carried out neutron diffraction measurements of single crystals of hcp Er80YZO_XLaX alloys with a double-axis neutron diffractometer installed at the Kyoto University Reactor (KUR). Table I summarizes the results of hcp Er80YZO-xLax alloys, together with magnetic
in hcp Er-La,
Tc> Tc, (W TCY = 52.4 TcY = 38 TC = 34 TCy = 48 T,-- = 38 TcY = 25 T, = 33
Tc=40 TCY = 37 TCY = 18 T,, = 17 TC =21 Tc = 41
Er-Y and Er-Y-La
Magnetic structure CY CY CAF CY CY APD CAF CAF CY CY CY CAF CAF
T, (R) 18 25 18 _ 18.5 18.5 12.5 _ _ 12 -
LSO: longitudinal sinusoidal order; CY: cycloidal order; CAF: c-axis ferromagnetic APDC: antiphase domain cone.
0921-4526/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
alloys. Magnetic structure
Ref.
cone cone cone
151 i51
_ APDC cone cone _ _ cone _
order; APD: antiphase
111 PI 161 161 161 [5,61 present present present present 151 domain order;
772
S. Kawano
LSO
CAF
APD
and N. Achiwa
CY
Fig. 1. Rough sketch of magnetic present Er-Y-La alloys.
APDC
structures
I Magnetic phase diagram
of hcp Er-Y-La
alloys
Cone
appearing
in the
structural data of hcp Er-La [5], Er,,Y,,,-.La, alloys [6]. As for the detailed analysis for determination of magnetic structures see ref. [6]. Fig. 1 is a rough sketch of the magnetic structures in these Er-based alloys. Fig. 2 gives the magnetic phase diagrams of Er-Y and hcp Er-La alloys. In Er-Y alloys the c-axis components show the LSO phase down to low temperatures below T,, though below T,, oscillatory components appear perpendicularly to the c-axis. On the other hand, in hcp Er-La alloys only the c-axis components are ferromagnetically aligned (c-axis ferro, CAF) at intermediate temperatures. And then, below TB basal plane components order helically, where the caxis components keep the CAF arrangement. In spite of an existence of the RKKY exchange interaction of long-ranged oscillatory nature an occurrence of the simple ferromagnetic align-
-I
ErgOLalo Fig. 3. Magnetic
phase
Er90Yl0
Er9oY&% diagram
of hcp Er,,Y,,_,La,
alloys
[61. ment suggests that the Ising-like character is enhanced in hcp Er-La alloys. Fig. 3 is the magnetic phase diagram of hcp Er90Y1Gx La, alloys, after ref. [6]. In the La-rich side LSO, CAF, and conical phases are observed, whilst in the Y-rich side LSO and CY phases appear. Around the Y = La region LSO, APD and antiphase domain cone (APDC) phases occur. In fig. 4 the results for hcp ErXOY*O_xLax alloys are given. In the La-rich
PM
1,Y~~~\cy,J La
80
90
100
90
Er concentratton Fig. 2. Magnetic alloys [S, 61.
phase
diagrams
80’
0
01
(at.%)
of hcp Er-La
CY
TP“ Vane
y
and
Er-Y
Erd% Fig. 4. Magnetic
EreoYd% phase
diagram
EreoYzo
of hcp Er,,,Yz,,_,La,
alloys.
S. Kawano and N. Achiwa
I Magnetic phase diagram of hcp Er-Y-La
side the CAF phase persists down to low temperatures, and in the Y-rich side the CY phase extends to low temperatures. Around the x = 5 region, a conical phase appears. In the global magnetic phase diagram of hcp Er-Y-La alloys up to 20 at. % Er, La-rich alloys exhibit ferromagnetic behavior such as CAF, APD and conical phases. On the other hand, Y-rich alloys exhibit the CY phase with components perpendicular to the c-axis. The APD phase appears at a small region around the Er,,Y,La, alloy, while it disappears in 80 at.% Er. Thus, it is concluded from an extent of the CAF phase that La enhances and Y depresses the Ising-like character. Now, we should discuss the behavior of the c-axis components on the basis of the ANNNI model. In the ANNNI model an interaction between nearest neighbor layers is positive, 1, > 0 and an interaction between next-nearest neighbor layers is negative, J2 < 0. According to Bak and Boehm [4], a mean field calculation leads to various magnetic structures as a function of the parameter - J,lJ, . The calculation reveals that in the ground state a ferromagnetic phase is stable for - J21J, < 4 and modulated phases exist for - J,lJ, 3 1. At finite temperatures various modulated phases occur for - J2/J, > f . Since the APD phase is a kind of modulated phase, the present behavior of the c-axis moments is globally similar to this mean field phase diagram. With decreasing temperature for $ < - J21J, < 4 a
alloys
773
magnetic transition takes place from a paramagnetic (PM) phase to a modulated phase (LSO, APD) and then to a ferromagnetic phase (CAF). The plane produced by TN corresponds to the high-temperature transition line from PM to modulated phases in the calculated diagram. The plane arising from T, at the La-rich side corresponds to the transition line from modulated to ferromagnetic phases for the region of - J,lJ, < i. The appearance of the APD phase around the Er,,Y,La, alloy accounts for the region around - J,lJ, = 4. The variation of Y/La values is roughly equivalent to that of - J,lJ, values. In conclusion, La spreads the Ising-like character region, whilst the change in Y/La values expresses the change in importance of competing interactions in Er-Y-La alloys. The occurrence of perpendicular components may be due to higher order anisotropy effects, which are positive and incline the moment from the c-axis.
References [l] M. Habenschuss, C. Stassis, S.K. Sinha, H.W. Deckman and F.H. Spedding, Phys. Rev. B 10 (1974) 1020. [2] J. Jensen, J. Phys. F 6 (1976) 1145. [3] R.J. Elliott, Phys. Rev. 124 (1961) 346. [4] P. Bak and J. von Boehm, Phys. Rev. B 21 (1980) 297. [5] S. Kawano and N. Achiwa, J. Phys. Chem. Solids, to be submitted. [6] S. Kawano and N. Achiwa, J. Magn. Magn. Mat. 52 (1985) 464.
MAGNETIC STRUCTURES AND PHASE DIAGRAM OF HCP-TYPE HEAVY-LIGHT RARE EARTH, Er-Y-La ALLOYS
Shinji KAWANO’ and Norio ACHIWA’ ‘Research Reactor Institute, Kyoto University, Kumatori, Sennan, Osaka 590-04, Japan 2Department of Physics, Kyushu University, Hakozaki, Fukuoka 812, Japan
The magnetic phase diagrams of hcp Er-Y, Er-La, Er,,Y,,_,La, and Er,,Y,,_,La, alloys determined by neutron diffraction are presented. Various oscillatory phases with the Ising-like character are found. The behaviour of the c-axis component of the moment is qualitatively discussed on the basis of the ANNNI model.
The magnetic structures of Er metal are a longitudinal sinusoidal order (LSO) below TN, a cycloidal order (CY) below Tcy and then, a conical structure (cone) below TB with decreasing temperature [ 1,2]. These structures are considered to have an Ising-like character. In the past decades Elliott [3] discussed the stability of the LSO phase and showed a transition to an antiphase domain (APD) phase and then to a ferromagnetic phase. Recently Bak and Boehm [4] have given a magnetic phase diagram from a mean field calculation of the Ising model with competing interactions (ANNNI model). They have found out various types of spatially modulated phases against competing interaction Table I Magnetic structures and magnetic ordering temperatures Alloys
TN (W
Er
84.4 75 62 75 72 63
%&a, Er,,La, Er,Y,,
Er,Y,d+.,
Er,Y,La,
Er,YJ+ 5 %J% Er80Y,o Er,,Y,,La, Er,Y,,La,, Er,Y,La,, Er,La,,
60 55 67 62 58 50 49
Magnetic structure LSO LSO LSO LSO LSO LSO LSO LSO LSO LSO LSO LSO LSO
ratios. The magnetic order in Er and Tm is an example of such cases. In Er-based alloys one can expect that the Ising-like character and competing interactions vary with Er concentration. The purpose of the present note is to examine a relation among magnetic phases, the Ising-like character and competing interactions in Er-based alloys with Y and La. We carried out neutron diffraction measurements of single crystals of hcp Er80YZO_XLaX alloys with a double-axis neutron diffractometer installed at the Kyoto University Reactor (KUR). Table I summarizes the results of hcp Er80YZO-xLax alloys, together with magnetic
in hcp Er-La,
Tc> Tc, (W TCY = 52.4 TcY = 38 TC = 34 TCy = 48 T,-- = 38 TcY = 25 T, = 33
Tc=40 TCY = 37 TCY = 18 T,, = 17 TC =21 Tc = 41
Er-Y and Er-Y-La
Magnetic structure CY CY CAF CY CY APD CAF CAF CY CY CY CAF CAF
T, (R) 18 25 18 _ 18.5 18.5 12.5 _ _ 12 -
LSO: longitudinal sinusoidal order; CY: cycloidal order; CAF: c-axis ferromagnetic APDC: antiphase domain cone.
0921-4526/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
alloys. Magnetic structure
Ref.
cone cone cone
151 i51
_ APDC cone cone _ _ cone _
order; APD: antiphase
111 PI 161 161 161 [5,61 present present present present 151 domain order;
772
S. Kawano
LSO
CAF
APD
and N. Achiwa
CY
Fig. 1. Rough sketch of magnetic present Er-Y-La alloys.
APDC
structures
I Magnetic phase diagram
of hcp Er-Y-La
alloys
Cone
appearing
in the
structural data of hcp Er-La [5], Er,,Y,,,-.La, alloys [6]. As for the detailed analysis for determination of magnetic structures see ref. [6]. Fig. 1 is a rough sketch of the magnetic structures in these Er-based alloys. Fig. 2 gives the magnetic phase diagrams of Er-Y and hcp Er-La alloys. In Er-Y alloys the c-axis components show the LSO phase down to low temperatures below T,, though below T,, oscillatory components appear perpendicularly to the c-axis. On the other hand, in hcp Er-La alloys only the c-axis components are ferromagnetically aligned (c-axis ferro, CAF) at intermediate temperatures. And then, below TB basal plane components order helically, where the caxis components keep the CAF arrangement. In spite of an existence of the RKKY exchange interaction of long-ranged oscillatory nature an occurrence of the simple ferromagnetic align-
-I
ErgOLalo Fig. 3. Magnetic
phase
Er90Yl0
Er9oY&% diagram
of hcp Er,,Y,,_,La,
alloys
[61. ment suggests that the Ising-like character is enhanced in hcp Er-La alloys. Fig. 3 is the magnetic phase diagram of hcp Er90Y1Gx La, alloys, after ref. [6]. In the La-rich side LSO, CAF, and conical phases are observed, whilst in the Y-rich side LSO and CY phases appear. Around the Y = La region LSO, APD and antiphase domain cone (APDC) phases occur. In fig. 4 the results for hcp ErXOY*O_xLax alloys are given. In the La-rich
PM
1,Y~~~\cy,J La
80
90
100
90
Er concentratton Fig. 2. Magnetic alloys [S, 61.
phase
diagrams
80’
0
01
(at.%)
of hcp Er-La
CY
TP“ Vane
y
and
Er-Y
Erd% Fig. 4. Magnetic
EreoYd% phase
diagram
EreoYzo
of hcp Er,,,Yz,,_,La,
alloys.
S. Kawano and N. Achiwa
I Magnetic phase diagram of hcp Er-Y-La
side the CAF phase persists down to low temperatures, and in the Y-rich side the CY phase extends to low temperatures. Around the x = 5 region, a conical phase appears. In the global magnetic phase diagram of hcp Er-Y-La alloys up to 20 at. % Er, La-rich alloys exhibit ferromagnetic behavior such as CAF, APD and conical phases. On the other hand, Y-rich alloys exhibit the CY phase with components perpendicular to the c-axis. The APD phase appears at a small region around the Er,,Y,La, alloy, while it disappears in 80 at.% Er. Thus, it is concluded from an extent of the CAF phase that La enhances and Y depresses the Ising-like character. Now, we should discuss the behavior of the c-axis components on the basis of the ANNNI model. In the ANNNI model an interaction between nearest neighbor layers is positive, 1, > 0 and an interaction between next-nearest neighbor layers is negative, J2 < 0. According to Bak and Boehm [4], a mean field calculation leads to various magnetic structures as a function of the parameter - J,lJ, . The calculation reveals that in the ground state a ferromagnetic phase is stable for - J21J, < 4 and modulated phases exist for - J,lJ, 3 1. At finite temperatures various modulated phases occur for - J2/J, > f . Since the APD phase is a kind of modulated phase, the present behavior of the c-axis moments is globally similar to this mean field phase diagram. With decreasing temperature for $ < - J21J, < 4 a
alloys
773
magnetic transition takes place from a paramagnetic (PM) phase to a modulated phase (LSO, APD) and then to a ferromagnetic phase (CAF). The plane produced by TN corresponds to the high-temperature transition line from PM to modulated phases in the calculated diagram. The plane arising from T, at the La-rich side corresponds to the transition line from modulated to ferromagnetic phases for the region of - J,lJ, < i. The appearance of the APD phase around the Er,,Y,La, alloy accounts for the region around - J,lJ, = 4. The variation of Y/La values is roughly equivalent to that of - J,lJ, values. In conclusion, La spreads the Ising-like character region, whilst the change in Y/La values expresses the change in importance of competing interactions in Er-Y-La alloys. The occurrence of perpendicular components may be due to higher order anisotropy effects, which are positive and incline the moment from the c-axis.
References [l] M. Habenschuss, C. Stassis, S.K. Sinha, H.W. Deckman and F.H. Spedding, Phys. Rev. B 10 (1974) 1020. [2] J. Jensen, J. Phys. F 6 (1976) 1145. [3] R.J. Elliott, Phys. Rev. 124 (1961) 346. [4] P. Bak and J. von Boehm, Phys. Rev. B 21 (1980) 297. [5] S. Kawano and N. Achiwa, J. Phys. Chem. Solids, to be submitted. [6] S. Kawano and N. Achiwa, J. Magn. Magn. Mat. 52 (1985) 464.