Ni – Mn – Ga thin films produced by pulsed laser deposition

Polycrystalline Ni–Mn–Ga thin films have been deposited by the pulsed laser deposition ~PLD! technique, using slices of a Ni–Mn–Ga single crystal as targets and onto Si ~100! substrates at temperatures ranging from 673 K up to 973 K. Off-stoichiometry thin films were deposited at a base pressure of 1310 Torr or in a 5 mTorr Ar atmosphere. Samples deposited in vacuum and temperatures above 823 K are magnetic at room temperature and show the austenitic $220% reflection in their x-ray diffraction patterns. The temperature dependences of both electrical resistance and magnetic susceptibility suggest that these samples exhibit a structural martensitic transition at around 260 K. The magnetoresistance ratio at low temperature can be as high as 1.3%, suggesting the existence of a granular structure in the films. © 2002 American Institute of Physics. @DOI: 10.1063/1.1452222 #


I. INTRODUCTION
In recent years considerable attention has been devoted to the Ni-Mn-Ga alloy system as a new material for actuating devices. 1,2A magnetic-field-induced strain of up to 6% has been reported in selected compositions of the Ni-Mn-Ga alloy system. 3These large field-induced strains have provided the possibility of using thin films of Ni-Mn-Ga as new materials for shape memory components in microelectromechanical systems ͑MEMS͒, as some reports have pointed out. 4,5he present work aims to contribute to this emerging field by reporting initial results concerning magnetic Ni-Mn-Ga thin films deposited by the pulsed laser deposition ͑PLD͒ technique.

II. EXPERIMENT
Ni-Mn-Ga thin films were deposited using a custom built vacuum chamber with a Nd-YAG laser, operating at 355 nm.The targets were sections of a Ni-Mn-Ga single crystal and were rotated during the ablation process.Substrates consisted in pieces of a 4 in.Si ͑100͒ wafer and were attached to a plate that could be heated up to 973 K. Samples were deposited at substrate temperatures ranging from 673 K up to 973 K. Depositions were undertaken at a base pressure of 1ϫ10 Ϫ6 Torr or in a 5 mTorr Ar atmosphere.Films were deposited during times varying between 1 and 3 h, yielding thicknesses between 0.5 and 2.5 m.The microstructure and composition of the films were checked using scanning electron microscopy ͑SEM͒, energy dispersive spectroscopy ͑EDS͒, and x-ray diffraction ͑XRD͒ measurements.
Magnetotransport measurements were made, using the conventional four-point probe method, in fields up to 1 T and temperatures ranging from room temperature to 4.2 K. Hysteresis loops and thermomagnetization measurements were performed, in fields up to 5 T and temperatures ranging from 120 K to 350 K, using a Quantum Design MPMS SQUID magnetometer.

III. RESULTS AND DISCUSSION
Figure 1 shows x-ray diffraction patterns corresponding to two Ni-Mn-Ga thin films deposited in vacuum and in a 5 mTorr Ar atmosphere, respectively.The austenitic ͕220͖ peak dominates the polycrystalline diffraction pattern of the films suggesting a strong texture, perhaps due a ͑110͒ growth a͒ Author to whom correspondence should be addressed; electronic mail: gfpcaalf@lg.ehu.esFIG. 1. XRD measurements corresponding to Ni-Mn-Ga thin film samples deposited at 723 K and 5 mTorr of Ar ͑a͒, and 823 K and 1ϫ10 Ϫ6 Torr ͑b͒, respectively.Each sample was grown during 3 h.mode.The ͕220͖ peak appeared only for samples deposited above 723 K ͑in Ar͒ and 823 K ͑in vacuum͒.Nevertheless only those samples produced in vacuum and at substrates temperature above 823 K were magnetic at room temperature.In the present work we will focus on a Ni-Mn-Ga thin film produced in vacuum and at a substrate temperature of 823 K.The diffraction pattern of this film ͓Fig.1͑b͔͒ shows a main peak at 2ϭ43.6°and what might be regarded as a shoulder at 2ϭ44.2°.The peak width suggests a polycrystalline structure with a grain size of about 12 nm, as estimated using the Scherrer formula.The shoulder is either due to the ͕202͖ reflection of untransformed ͑retained͒ martensite or a second unidentified phase.Ni-Mn-Ga thin films of coexisting austenite/martensite phases have been reported elsewhere. 5igure 2 shows three thermomagnetization zero field cooled ͑ZFC͒ measurements, performed at 50 Oe, 100 Oe, and 500 Oe, respectively.The appearance of a different magnetic phase below 260 K is evident in these measurements at applied fields below 1000 Oe.For temperature values around 260 K and low applied fields, the thermal dependence of the susceptibility differs considerably from that obtained on superimposing a Bloch law term ͓ M (T)ϭM (0)(1ϪBT Ϫ3/2 )͔ to the susceptibility curve of a magnetic phase with Curie temperature above 350 K. Nevertheless the susceptibility shoulder, manifested at low applied fields, might be regarded to the onset of ferromagnetism in a phase that would be paramagnetic at room temperature or to the transformation of some of the Ni-Mn-Ga austenite into martensite beginning at T mart ϳ260 K.
In order to further discuss this issue, Fig. 3 shows highlights of the magnetotransport properties of the Ni-Mn-Ga thin film investigated in the present work.Figure 3͑a͒ shows clear spin-dependent magnetoresistance, which may be ascribed to an inhomogeneous or granular structure. 6Figure 3͑b͒ shows that as temperature and resistivity decrease, an additional source of scattering appears below a T C of about 270 K, presumably due to spin scattering from the large moments on the randomly oriented grains.The experimental (T) curve shown here is in excellent agreement with those reported earlier, 7 for Ni-Mn-Ga martensites which had electron/atom ratios between 7.55 and 7.7 e/a and T mart below room temperature.The aforementioned experimental results ͑especially the broad XRD peaks and the large, granularlike MR ratio͒ strongly support the coexistence of granular phases and a martensitic transition starting around 260 K for the sample investigated.Recently performed energy dispersive spectroscopy ͑EDS͒ measurements revealed that the average composition of the film analyzed was Ni 50.99 Mn 29.28 Ga 19.72 ͑atomic %͒.It is interesting to locate this composition on a phase diagram corresponding to a Heusler alloy system.In the absence of published phase diagrams for Ni-Mn-Ga, here we refer to a Ni-Ti-Al phase diagram generated by Thermo-calc® 8 ͑this is the Heusler system for which the most complete set of experimental thermal data is available͒.Figure 4 shows this phase diagram.In this alloy system the L2 1 phase is clearly more stable than in Ni-Mn-Ga where it only appears below 1073 K.The extent of the L2 1 phase field shown here corresponds to the stable range of the Ni 2 MnGa Heusler phase at room temperature.The investigated film lies on the twophase region of this diagram.
In summary, we have shown that thin films of Ni-Mn-Ga can be produced on top of Si-͑100͒ substrate using pulsed laser deposition.According to our results, only those films produced in vacuum and at a substrate temperature above 823 K were magnetic at room temperature.Measure-ments of M ͑T͒ and ͑T͒ on one of these samples indicate the presence of a martensitic phase below 260 K.The film that shows a martensitic transition has an average composition of Ni 50.99 Mn 29.28 Ga 19.72 ͑at.%͒.Data presented suggest a film in which two granular phases coexist at room temperature.

FIG. 2 .FIG. 4 .
FIG. 2. Thermomagnetic measurements corresponding to the Ni-Mn-Ga thin film deposited at 823 K and 1ϫ10 Ϫ6 Torr.Measurements presented were performed on heating and under applied fields of 500 Oe and 100 Oe, respectively.The inset shows the normalized thermal dependence of the magnetization for an in-plane applied field of 50 Oe.Solid lines are shown as a guide to the eye.FIG. 3. ͑a͒ In-plane field magnetoresistance measurements performed at 23 K and corresponding to the Ni-Mn-Ga thin film deposited at 823 K and 1ϫ10 Ϫ6 Torr.͑b͒ Thermal dependence of the resistance measured on heating and cooling the sample.