THE DIFFERENCE BETWEEN THE MAGNETO-CRYSTALLINE ANISOTROPY OF THE INTERMETALLIC ALLOY Pr2(Coo.5Feo.5)l7 AND INTERSTITIALLY MODIFIED Pr2(Coo.5Feo.5)l7N3-5
RAZLIKA MED MAGNETNO KRISTALNO ANIZOTROPIJO
Pr2(Coo.5Feo.5)l7 IN Pr2(Coo.5Feo.5)l7N3-5
MATEJ KOMELJ, S. KOBE
Institut Jožef Štefan Jamova 39, 1000 Ljubljana, Slovenija
Prejem rokopisa - received: 1997-10-0!; sprejem za objavo - accepted for publication: 1997-10-21
The al!oy vvith the composition PrjfCoo.sFeo.-On has an easy-axis magneto-crystaIline anisotropy. The anisotropy is changed to the easy-plane by introducing nitrogen on interstitial sites. We proved the difference betvveen the anisotropy of the basic alloy and the nitrided composition directly by the magnetic measurements and by observing the domain structure by means of optical and magnetic force microscopy. The transition from one to another type of anisotropy can be explained by a simple model based on crystal structure and shape of the electronic cloud of the Pr ion.
Key vvords: permanent magnet materials, magneto crstalline anisotropy, nitriding
Intermetalna zlitina Pr2(Coo.5Feo.5)i7 ima osno magneto-kristalno anizotropijo. Z uvajanjem dušika na intersticijska mesta anizotropija preide v ravninsko. Razliko med tipoma anizotropij med obema spojinama dokažemo neposredno z magnetnimi meritvami in opazovanjem domenske strukture z mikroskopom na magnetno silo (MFM). Prehod iz ene anizotropije v drugo kot posledico spremenjene sestave lahko razložimo s preprostim modelom, ki temelji na kristalni strukturi in obliki elektronskega oblaka iona Pr.
Ključne besede: trajno-magnetni materiali, magneto-kristalna anizotropija, nitriranje
1	INTRODUCTION
It is vvell knovvn that R2Tn, vvhere R stands for a rare earth and T for a transition metal, intermetallics exhibit interesting magnetic properties and represent important permanent magnet material1. To obtain high coercivity of the magnet it is necessary that the material has an easy-axis magneto-crysta!line anisotropy and that the value of the coefficient Ki, vvhich is a measure for the energy of the magneto-crystalline anisotropy, is as high as possible2. The magneto-crystalline anisotropy of the basic compound can be influenced by introducing nitrogen on interstitial sites in the crystal lattice3. The Pr2(Coo.5Feo.5)n compound crystallizes in the rhombohe-dral Tf^Znn type structure and it has easy-axis magneto-crystalline anisotropy4-5. But the anisotropy is too vveak and the magnet based on this material does not exhibit desired properties. In our vvork vve studied the influence of nitrogen in the Pr2(Coa5Feo.5)i7 alloy. In Sm2Fei7 inter-metallic alloy the magnetic properties vvere successfully improved by introducing nitrogen to the interstitial sites of the crystal lattice6.
2	EXPERIMENTAL
The samples vvere prepared from 99.9% pure ele-
ments by are melting. Excess of Pr vvas added to a nomi-
nal composition to offset Pr evaporation losses during
melting. After the fourth melting the samples vvere
vvrapped in a Ta foil and encapsulated after evacuation in quartz tubes and then annealed at 1000°C for 24 hours. Before and after the heat treatment their phase purity vvas determined by JEOL JXA 840 SEM/EPMA electron probe microanalysis. Nitriding, as a gas-solid reaction, vvas performed at 450°C for 10 hours in a high purity N2 1 bar gas atmosphere. The nitriding temperature vvas pre-viously determined by using DTA/TGA facilities (Netzch). The parent alloy and the nitrided povvder vvere characterized by XRD. The magnetization measurements on the povvdered and aligned samples vvere provided by the magnetometer-susceptometer (Manics) based on the Faraday principle. Magnetic domains vvere examined by magnetic force microscopy (MFM) using a Dimension 3000 scanning probe microscope (Digital Instruments, Inc.), vvhich allovvs separate atomic force and magnetic force images to be collected in the course of one scan. Surface topography vvas obtained using Tapping Mode7 AFM. High resolution MFM image vvas obtained by using Lift Mode softvvare7 and the ultra-soft Fe-Si02 tip. Images of the domain patterns vvere previously taken by Kerr microscopy using a Nikkon Optiphot XP-2 polariz-ing light microscope vvith a 150W Xe lamp.
3 RESULTS OF THE MEASUREMENTS
The magnetization versus applied field curves of the Pr2(Coo.5Feo.5)i7 bonded povvder measured parallel and perpendicular to the direction of the alignment are shovvn
in Figure 1. From these curves it is evident that the an-isotropy field is greater then 30 kA/cm vvhich implies the existence of vveak easy-axis magneto-crystalline anisot-ropy. The domain pattern of this alloy, observed by optical microscope in a polarized light using the magneto-optic Kerr effect is shovvn in Figure 2. A star shaped or "labyrinth" pattern in the grains cut perpendicular to the c-axis and a banded or "strip" structure vvithin the grains cut parallel to the c-axis are present. Such domain structure applies for materials vvith easy-axis type of mag-neto-crystalline anisotropy vvhere the crystallographic c-axis deftnes the easy direction8. Figure 3 shovvs the same area observed by the magnetic force microscope (MFM) vvhich appears to be in a good agreement vvith Kerr mi-croscopy. Both magnetization curves of the nitrided povvder, measured parallel and perpendicular to the direction of the alignment are essentially the same (Figure 4) that means that the nitrided material does not have an easy-axis magneto-crystalline anisotropy. This fact vvas confirmed also by observation of the domain structure. Figure 5 shovvs the Kerr image of a large (-100 pm) particle. Because of the slovv nitrogen bulk diffusion9 just a layer of vvidth -5 pm vvas nitrogenated. The core area represents the basic P^CoosFeos)]? material. The shell resolution is lovv, and no magnetic structure vvas observed in the nitrided layer. Only strip domains vvere found in the core region. On Figure 6 is shovvn a MFM detail image of the core-shell border area of the same particle vvith a sharp demarcation betvveen the strip domain pattern in the core area and the labyrinth domain structure in the nitrided shell vvith the star domain vvidth of -1-1.2 pm vvhich is significant for easy-plane mag-neto-crystalline anisotropy10.
Figure 2: The domain pattern of the alloy Pr2(Coo.5Feo.s)i7 observed by optical microscope in a polarized light using magneto-optic Kerr effect. Star shaped or "labyrinth" pattern in grains cut perpendicular to the c-axis and a banded or "strip" structure vvithin grains cut parallel to the c-axis are present
Slika 2: Domenska struktura zlitine Pr2(Coo.5Feo.5)i7, opazovana z optičnim mikroskopom v polarizirani svetlobi s pomočjo Kerrovega efekta. V zrnih, odrezanih pravokotno glede na os c, vidimo labirintno strukturo domen zvezdaste oblike. V zrnih, ki so odrezana vzporedno z osjo c pa so črtaste domene v obliki pasov
4 THEORETICAL
Undesired transition from the easy-axis to the plane magneto-crystalline anisotropy due to the introduced nitrogen in the čase of Pr2(Co0.5Fe0.5)i7 contrary to the čase of SmiFei?6 can be explained in the frame of the simplified crystal field theory". We assume that the energy of the magneto-crystalline anisotropy is given by the elec-trostatic interaction betvveen the 4f charge distribution of the rare earth atom and the non-4f charges present in the
Figure 3: MFM image of the same area as in Figure 2 Slika 3: MFM posnetek istega območja kot na sliki 2
Figure 1: Magnetization versus applied field of the P^tCoo.sFeo.s)^ bonded povvder measured parallel (upper curve) and perpendicular (lovver curve) to the direction of the aligment. The anisotropy field is greater then 30 kA/cm
Slika 1: Magnetizacija prašnega vzorca P^CoasFeosJn v odvisnosti od zunanjega magnetnega polja merjena vzporedno (zgornja krivulja), in pravokotno (spodnja krivulja), glede na smer predhodne namagnetenosti prahu
25um
Figure 4: Magnelization curves of nitrided powder measured parallel, and perpendicular, to the direction of the alignment are the same Slika 4: Krivulja magnetizacije nitriranega prahu, merjena pravokotno in vzporedno glede na smer predhodne namagnetenosti prahu
Iattice. The change of the coefficient Ki upon the inter-stitial modification in the first approximation depends:
•	on the angle 6 between the directions of the nitrogen atom and the magnetization relative to the position of the rare earth atom: Figure 7
•	and to the quadrapole contribution n2 to the 4f charge density.
One can schematically conclude that rare earth atoms with n2<0 (Pr, Nd, Dy etc.) have the oblate shape of the 4f electron cloud while in the čase of Sm with n2>0 it can be described as a prolate. To estimate vvhether the Ki is increased or decreased after the nitriding there is a simple rule":
sgn(AK,) = sgn(n2) sgn(9)
Figure 5: Kerr image of a large (-100 pm) particle. Just a layer of width -5 pm vvas nitrogenated. The core area represents the basic Pr2(Co().5Feo.5)i7 alloy
Slika 5: Posnetek velikega (-100 pm) delca s pomočjo Kerrovega efekta. Nitrirana je samo zunanja plast debeline -5 pm. Jedro predstavlja osnovna zlitina Pr2(Coo.5Feo.s)i7
Figure 6: MFM detail image of the core-shell border of the same particle as in Figure 5. A sharp demarcation betvveen the strip domain pattern in the corearea and the labyrinth domain structure in the nitrided shell is shovvn
Slika 6: MFM detajlni posnetek meje med nitriranim in nenitriranim območjem delca iz slike 5. Ostro se vidi razlika med črtastimi domenami v nenitriranem območju ter zvezdastimi domenami iz nitrirane plasti
where sgn(0 = 0°) = -1 and sgn(6 = 90°) = 1. It im-plies that in the čase of Pr (n2<0) we have to look for interstitial sites with the axial coordination (0 = 0°) vvhich is not the čase for the site 9e in Th2Zn17 type of crystal structure. It has in-plane interstitial coordination (6 = 90°) vvhich is favorable for the Sm (n2>0) based materials.
earth
Figure 7: Angle 6 betvveen the directions of the nitrogen atom and the magnetization relative to the position of rare earth atom
Slika 7: Kot 6 med zveznico atoma dušika ter atoma redke zemlje in smerjo magnetizacije
5 CONCLUSION
It was shown that besides the magneto-optic Kerr effect MFM can be used as an important technique for rapid and simple characterization of the magnetic structure of nevv magnetic materials. When the magnetic structure is too fine for optical microscopy, MFM can serve as a very convenient method in observing and char-acterizing the magnetic structure of different and un-knovvn magnetic materials. After the successful intersti-tial modification of the Sm2Fen 36 and some other intermetallics3 it was hoped that the same procedure could be applied also to the Pr2(Coo.5Feo.5)n alloy. In-stead of the increasing of magneto-crystalline anisotropy even a transition from the easy-axis to the undesired easy-plane type was observed. Therefore the nitrided al-loy Pr2(Co0.5Fe0.5)i7 can not be used as a permanent magnet material. Nevertheless it has very interesting properties from which we can learn much on domain structure and the effects of nitriding. Finally the experimental re-
sults obtained on the investigated alloy agree with the predictions of a model based on a crystal field theory".
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